GEOLOGISCHE LEIDSE MEDEDELINGEN, Deel 48, Aflevering 2, pp. 275-379, 25-5-1972
Fluvial sediments of Cretaceous Age alongthe southern
borderof the CantabrianMountains, Spain
BY
Rob.K. Jonker
Abstract
This study deals with a sedimentological description and interpretation of a strip of continental deposits of Cretaceous age along the
southern border of the Cantabrian Mountains. In this work they are designatedas Voznuevo Formation (Evers, 1967). They belong to the
of have to type deposits which commonly been referred as ‘sediments in Wealden facies’.
As regards stratigraphy, palynological analyses (apart from pollen, the Voznuevo Formation proved completely barren) made it clear
that the transition between the Lower Cretaceous and the Upper Cretaceous runs obliquely through the formation. Hence, the Voznuevo
sediments diachronic of are deposits, decreasing in age in a westerly direction and forming the continental counterparts successively
shallow marine younger deposits towards the west.
In comparison with similar deposits in Wealden facies in other parts of Spain they represent one of the last vestiges of continental deposition in Upper Cretaceous times.
it clear of Sedimentological investigations made that the Voznuevo sediments are purely fluvial origin, derived from a granitic source
area, probably now partly hidden, which must have been situated on the arc of granitic rocks which runs from Galicia via northern
Sierra Portugal to the de Guadarrama, N ofMadrid. Especially paleocurrent directions indicate a transport from southwesterly directions. - - Typical final products of weathering of granites kaolinite and quartz form the main share of the sediments.
The independence of the Voznuevo deposits of the Paleozoic rocks which they overlie is striking; only the very lower part of the
formation testifies to a small contribution from the Cantabrian Mountains. In the extreme west of the area some supply could be
the there. into the shallow ascertained from Precambrian rocks present The transition overlying marine deposits is very gradual; a very
small amount of occurs in the of the formation. from sediments transitional a lagoonal deposits probably upper part Apart these, no to
marine environment, such as beach deposits, could be observed.
The fluvial sediments can be subdivided into braided and meanderingriver deposits, the former much coarser-grainedthan the latter. The
braided show such cut-and-fill river deposits characteristic features as channelling, structures, absence of distinct grading etc. The
meandering river deposits exhibit fining-upward grading cycles in which cross-bedding of the planar type with backflow phenomena in
their bottomsets is common, and sub-environments such as point bar, natural levee, crevasse-splay and backswamp deposits.
Towards the coastal side of the area of deposition (i. e. the eastern part of the area studied) meanderingriver deposits were found. They
presumably represent sediments of a coastal plain in which meanderingrivers, chiefly carrying sandy sediments, ran to the sea in the east.
At the towards same time, much coarser-grained braided river deposits formed further westwards in a mountain foreland. In shifting the
its this west in entirety, model has yielded the sediments as they can be observed today.
which be its its is The Voznuevo Formation clearly stands out against the deposits are to found at bottom and top. First of all, there the
complete absence of limestone, the predominance of quartz over quartzite and the abundance of kaolinite. Secondly, heavy minerals not
resistant to and in the chemical weathering, such as hornblende,epidote augite are completely lacking. Erosion of fresher rocks source area
the latter of is during part the sedimentation reflected by the increased appearance of metamorphic minerals such as andalusite and kyanite.
The abundance of authigenic kaolinite favours the supposition that weathering took place after deposition. The high percentage of
the the kaolinite, ubiquity of organic matter, the flatness and roundness of the pebbles and absence of any trace of laterization indicates under weathering temperate to warm, humid conditions with relatively strong leaching.
The Voznuevo sediments form a clearly foreign element in the depositional history of the sediments along the southern border of the
Cantabrian Mountains. Tertiary and Quaternary sediments, all having the Cantabrian Mountains as source area, have been partially
in supplied from the Voznuevo sediments. But hitherto enigmatical occurrencesof, for instance, staurolite and kaolinite Tertiary deposits
can also be explained by considering them as erosion products of the Voznuevo Formation.
Samenvatting
Dit onderzoek omvat een sedimentologische beschrijving en interpretatie van een strook continentale afzettingen van Krijt ouderdom, die gelegen zijn aan de zuidrand van het Cantabrisch gebergte. In dit proefschrift zullen zij Voznuevo Formatie (Evers, 1967) worden
in genoemd. Zij behoren tot het type afzettingen dat men gewoonlijk ‘sedimenten Wealden facies’ heeft genoemd. Wat de stratigrafie betreft, bleek uit palynologische analyses (behalve pollen is de Voznuevo Formatie volkomen fossielloos) dat de
Onder Boven zich scheef door de formatie De Voznuevo sedimenten diachrone overgang van Krijt naar Krijt beweegt. zijn dientengevolge die afzettingen naar het westen toe jonger worden en de continentale voortzetting vormen van geleidelijk naar het westen toe jonger wordende ondiep-mariene afzettingen.
Vergeleken met dergelijke afzettingen in Wealden facies in andere delen van Spanje vertegenwoordigen zij één der laatste resten van continentale afzettingen in het Boven Krijt.
Uit onderzoek bleek dat de Voznuevo sedimenten fluviatiele een sedimentologisch van puur oorsprong zijn, afkomstig van granitische — — wellicht Galicië via gesteenten die heden gedeeltelijk bedekt gelegen waren in de boog van granitische gesteenten die van Noord- 276
Portugal naar de Sierra de Guadarrama, ten N van Madrid, loopt. In het bijzonder aanvoerrichtingen, gemeten in scheef gelaagde
vanuit kaoliniet pakketten, wijzen op een aanvoer zuidwestelijkerichtingen. Typische eindproducten van verwering van granieten, zoals en
kwarts, vormen het leeuwendeel van het sediment.
De onafhankelijkheid der Voznuevo sedimenten van de Paleozoische gesteenten die zij bedekken is opvallend; slechts het onderste deel
invloed van de formatie getuigt van een geringe van het Cantabrisch gebergte. In het uiterste westen van het gebied kon enige aanvoervan
de daar Praecambrische worden de Formatie aanwezige gesteenten aangetoond. De overgang naar op de Voznuevo liggende ondiep mariene is afzettingen zeer geleidelijk; in het bovendeel van de formatie zijn vermoedelijk enige lagunaire afzettingen aanwezig. Afgezien hiervan
werden die de marien milieu — zoals strand nergens sedimenten, overgang vormen naar een afzettingen — aangetroffen. het De fluviatiele afzettingen kunnen worden onderverdeeld in sedimenten van verwilderde en van meanderende rivieren; die van
eerstgenoemde type zijn beduidend grofkorreliger. De afzettingen van het verwilderde rivier type vertonen specifieke kenmerken zoals ‘cut-and-fill’ geulvorming, structuren, afwezigheid van graderingenz. De afzettingen van meanderende rivieren vertonen cycli met graderin-
die boven toe het gen, naar fijner van korrel worden. In deze cycli komen pakketten met scheve gelaagdheid van type met vlakke In de - grensvlakken voor. onderste delen der scheef gelaagde eenheden werden vaak ‘backflow’-verschijnselen veroorzaakt door tegen-
circulatie - gestelde aan de lijzijde van een ribbel waargenomen. Gedetailleerd onderzoek in enkele goed ontsloten gedeelten van het
maakte in fluviatiele zoals gebied een indeling sub-milieus mogelijk, bijvoorbeeld kronkelwaard-, oeverwal-, doorbraak- en komgrond- afzettingen.
Aan de het kustzijde van sedimentatiegebied (het oostelijke gedeelte van het onderzochte gebied) werden afzettingen van meanderende
rivieren gevonden. Naar alle waarschijnlijkheid vertegenwoordigen zij de sedimenten van een kustvlakte, waarin kronkelende rivieren, die
zand zich voornamelijk met mee voerden, zeewaarts stroomden. Tegelijkertijd werden veel grovere afzettingen verder westelijk, in het
voorland van een gebergte, door verwilderde rivieren gevormd. Dit zich langzaam naar het westen verplaatsende model leverde de sedimenten zoals zij heden aangetroffen kunnen worden.
de De Voznuevo Formatie steekt schril af tegen boven- en onderliggende sedimenten. Ten eerste is er de totale afwezigheid vankalk, de
overheersing van kwarts over kwartsiet en de overvloed aan kaoliniet. Ten tweede moet de samenstelling der zware mineralen genoemd
worden, waarin mineralen die niet bestand zijn tegen chemische verwering, zoals hoornblende, epidoot en augiet, ontbreken. De erosie van
steeds versere gesteenten in het brongebied gedurende de latere stadia van de sedimentatie van de Voznuevo Formatie wordt weerspiegeld
toenemende in het in mate verschijnen van metamorfe mineralen zoals andalusiet en distheen.
De hoeveelheid kaoliniet het voorkomen grote authigene (die wijst op verwering na sedimentatie), veelvuldig van organisch materiaal, de
der rolstenen de ieder lateritisatie tot afplatting en afronding en afwezigheid van spoor van wijst op verwering onder gematigde warme,
vochtige omstandigheden met betrekkelijk sterke uitloging.
De Voznuevo afzettingen vormen een duidelijk vreemd element in de sedimentatie-geschiedenislangs de zuidrand van het Cantabrisch
gebergte. De samenstellingvan Tertiaire en Kwartaire sedimenten,allen met het Cantabrisch gebergte als herkomstgebied, werd gedeeltelijk
beïnvloed door Voznuevo materiaal. Ook de tot heden niet verklaarde op aanwezigheidvan bijvoorbeeld stauroliet enkaoliniet in Tertiaire worden afzettingen kan verklaard wanneermen ze beschouwt als erosie-productenvan de Voznuevo Formatie.
Résumé
Cette étude concerneune description et une interprétation sédimentologique d’une zône avec des sédiments continentaux du Crétacé le
de la bordure méridionale du Massif asturien. Dans ils Formation de Voznuevo Ces long cet ouvrage sont désignés par: (Evers, 1967). nommés faciès Wealdien’. dépôts appartiennent au groupe des sédiments habituellement ‘des sédiments à
à la des dans la Formation de Voznuevo des montré la Quant stratigraphie, analyses palynologiques (on ne trouve que pollen)ont que Crétacé inférieur Crétacé traverse sédiments sont transition du au supérieur obliquement la formation. Par conséquent, les du Voznuevo
des dépôts diachroniques, rajeunissant vers l’Ouest et formant les pendants continentaux de dépôts marins peu profonds, qui rajeunissent
successivement vers l’Ouest.
En comparaison avec des dépôts à faciès Wealdien semblables dans d’autres parties d’Espagne, ils représentent les derniers vestiges des dépôts continentaux du Crétacé supérieur.
Les recherches sédimentologiques ont montré que les dépôts du Voznuevo ont une origine strictement fluviale, et qu’ils sont dérivés
d’une région granitique, qui est à présent probablement partiellement recouverte et qui a dû avoir été située dans un arc de roches
située granitiques, s’étendant de la Galice, par le Nord du Portugal, vers la Sierra de Guadarrama, au Nord de Madrid. Particulièrement les
directions de paleocourants indiquent un transport de directions sud-ouest. Des produits d’altération typique des granites — le kaolin et le
quartz - représentent la majorité des sédiments.
ait la la Il est frappant qu’il n’y pas de relation entre les dépôts de Voznuevo et les roches Paléozoïques en dessous; seule partie plus
basse de la formation Massif de la montre qu’il y a eu une contribution, tant soit peu, de la part du des Asturies. Dans la partie région
située à influx Précambriennes. La marins tout l’Ouest, on a pu constater un peu important de roches transition aux dépôts qui les
recouvrent est fort graduelle; probablement une quantité minimale de dépôts lagunairs est présente dans la partie supérieure de la
ceux-ci formation. Hormis pas un seul des sédiments ne montre une transition à un milieu marin, comme par exemple des dépôts littoraux. sédiments Les fluviaux peuvent être subdivisés en dépôts de rivières anastomosées et en dépôts de rivières serpentantes, les premiers étant plus grossiers que les derniers. Les dépôts de rivières anastomosées montrent des propriétés caractéristiques comme des rigoles (channel-
ling), des chenaux d’érosion, l’absence de classement distinct etc. Les dépôts de rivières serpentantes montrent souvent des ‘fining-upward
de grading cycles’ dans lesquels on trouve une stratification entrecroisée à groupe planaire laminae (planar cross-stratification), avec des phénomènes de circulation inverse (backflow) dans leur couches basales. Des recherches détailléesdans quelques parties bien exposées de la ont montré des et région sous-milieux fluviaux comme par exemple des bourrelets arqués, des levées naturelles, des crevasses des dépôts de dépressions latérales.
Dans la partie côtière de la région de déposition (c’est à dire la partie orientale de la région étudiée), des dépôts dérivés de rivières serpentantes ont été trouvés. Ils représentent probablement les sédiments d’une plaine littorale dans laquelle les rivières serpentantes, sédiments rivières apportant principalement des sableux, descendaient vers la mer en direction orientale. En même temps, des dépôts de anastomosées consistants de sédiments bien plus grossiers se sont formés dans un promontoirevers l’Ouest. En se déplaçant intégralement vers modèle l’Ouest, ce a donné les sédiments du type comme ils peuvent être observés aujourd’hui. La Formation Premièrement de Voznuevo contraste avec les dépôts sous-jacents et les dépôts sus-jacents. il y a l’absence complète du
la du le et l’abondance il la lourds dans calcaire, prédominance quartz sur quartzite du kaolin. Secondement, y a composition des minéraux laquelle des minéraux non-résistants aux l’hornblende, et l’augite actions météoriques chimiques, comme par exemple l’epidote manquent 277
totalement. L’érosion de roches plus fraîches dans la région d’originependant les derniers stades de la sédimentation du Voznuevo, devient
évidente par l’apparition plus fréquente de minérauxmétamorphiques comme l’andalousite et le disthène.
L’abondance de kaolin renforce la des actions la Le authigène supposition que météoriques ont pris place après déposition. pourcentage élevé de kaolin authigène, la présence constante de matières organiques, l’aplatissement et l’émoussé des galets, et l’absence totale de toute
trace de latéritisation indiquent que les actions météoriques ont pris place sous des conditions climatologiques tempérées à chaudes et
humides avec un lessivage relativement fort. la bordure méridionale Les sédiments du Voznuevo représentent un élément clairement étranger dans l’histoire de la sédimentationdans
du Massif asturien. Les du Tertiaire du du Massif été fournis les sédiments et Quartaire, originaires asturien, ont partiellement par
sédiments du Voznuevo. Egalement la présence énigmatique du staurolite et du kaolin dans les dépôts tertiaires peut maintenant être
expliquée en considérant ces minéraux comme des produits d’érosion de la Formation de Voznuevo.
Resumen
la de edad Cretácea lo del Este trabajo trata descripción e interpretación sedimentológica de una faja de depósitos continentales a largo
han Voznuevo límite sur del macizo Cantábrico. En este trabajo sido designados estos depósitos como la Formación de (Evers, 1967).
Pertenecen a un tipo de depósitos comúnmente referidos como sedimentos en facies Wealdense.
análisis de Formación de es estéril) aclaró la En cuanto a la estratigrafía, el palinológico (aparte polen, la Voznuevo completamente que
transición de la formación. los sedimentos de Voznuevo entre el Cretáceo inferior y superior corre oblicuamente atravez De aquí que son
de edad dirección de marinos depósitos diacrónicos, aminorando en occidental, formando la parte continental complementaria depósitos aplacerados sucesivamente rejuveneciendohacia el occidente.
de los de En comparación a depósitos similares en facies Wealdense en otras regiones de España representan uno últimos vestigios
deposición continental del Cretáceo superior.
aclararon los sedimentos de Voznuevo de fluvial, derivados de un área de Investigaciones sedimentológicas que son puramente origen fuente granítica, probablemente ahora parcialmente cubierta, la cual debe haberse encontrado dentro del arco de rocas graníticas que se Sierra de al de Madrid. las direcciones de las extende desde Galicia por el norte de Portugal hasta la Guadarrama, norte Especialmente
paleocorrientes indican un transporte de direcciones sudoccidentales. Productos típicos de meteorización de granitos — caolinita y cuarzo - forman la mayor parte de los sedimentos.
solo de la La independencia de los depósitos de Voznuevo de las rocas Paleozoicas que cubren es sorprendente; una parte muy inferior área formación da testimonio de una pequeña contribución del macizo Cantábrico. En el extremo occidental del se puede observar una
contribución litológica de las rocas Precámbricas allí presentes. La transición a los depósitos marinos aplacerados suprayacentes es muy la la formación. de gradual; una pequeña cantidad de depósitos lacustres probablemente ocurre en parte superior de Aparte esta, ningún así sedimento transitorio a una región marina, como depósitos costales, pudo observarse. ríos más Los sedimentos fluviales pueden subdividirse en depósitos de ríos anóstomos y con meandros, los primeros de grano grueso que
los segundos. Los depósitos de ríos anóstomos muestran características tal como paleocanales, cauces de eroción rellenos, ausencia de
distinta río ciclos de hacia la cual clasificación, etc. Los depósitos de un que meandra, muestran clasificación con refinación arriba, en
estratificación entrecruzada basiplana con fenómenos de circulación inversa (‘backflow’) es común. Investigaciones detalladas en partes del área, así bar’, levee’, bien expuestas revelaron sub-regiones fluviales como depósitos ‘point ‘natural ‘crevasse-splay’ y ‘backswamp’ (Allen, 1965a). área ríos Hacia el lado costal del área de deposición (es decir la parte oriental del estudiada), se encontraron depósitos de con meandros. ríos, corrían Presumiblemente representan sedimentos de una llanura costal en la cual estos portando principalmente sedimentos arenosos,
el el Al ríos anóstomos el hacia mar en oriente. mismo tiempo, depósitos de de granulación gruesa se formaron más hacia occidente en un
montañoso. el este ha antepaís En su migración total hacia occidente, modelo cedido los sedimentos como se pueden observar hoy en día.
La Formación de Voznuevo se destaca claramente contra los depósitos Paleozoicos y post-Mesozoicos. Primero existe una ausencia
de completa de caliza, una predominancia de cuarzo sobre cuarcita y una abundancia de caolinita. En segundo lugar hay una composición
minerales pesados en cual minerales no resistentes a la desagregación química, tal como la hornblenda, epidota y augita, carecen com-
Erosión de frescas el área de durante la última fase de sedimentación de pletamente. rocas en origen es reflejada por un aumento aparición la la distena. de minerales metamórficos así como andalucita y
La abundancia de caolinita autígena, favorece la suposición que la meteorización tuvo lugar despues de la deposición. El alto porcentaje
de caolinita, la ubicuidad de materiaorgánica, el aplanamiento y la redondez de los guijarros y la ausencia de trazas de lateritización indican
relativamente meteorización bajo condiciones temperadas a calientes húmedas con lixiviación fuerte.
Los sedimentos de Voznuevo forman claramente un elemento extranjero en la historia de deposición de los sedimentos a lo largo del
Sedimentos del Tertiario ambos área de el macizo Cantábrico, han sido límite sur del macizo Cantábrico. y Cuaternario, con origen en
Mas hasta ahora ocurrencias estaurolita caolinita en abastecido parcialmente por los sedimentos de Voznuevo. enigmáticas, por ejemplo y erosión Formación depósitos Tertiarios, pueden explicarse también considerándoles como productos de de la de Voznuevo.
Contents
the I. Introduction 279 И. Mapping of area studied 282
General remarks 279 Comparison with mapping by previous authors 283
Characteristics in the field 279
of the Scope present study 280 III. Structure and contacts 284
of the Approach study 280 Structure in general 284
Methods and used techniques 281 General remarks 284
Investigations in the field 281 Strike-dip measurements in the Voznuevo
Laboratory examination of the samples 281 Formation 285
Economic use of the sediments 281 Faults and folds within the formation and
Remarks on the relief 281 their consequences 286
The drainagepattern 281 The thickness of the Voznuevo Formation 286
Acknowledgements 282 The fold thrust 287 278
Geophysical examination of the southern VII. The samples 335
thin sections border fault zone 287 The 335
The age of the deformation 287 The sandstones 335
The with the rocks contacts underlying . . . 288 The argillaceous sandstones 337
The contacts with the overlying rocks .... 290 The calcareous sandstones 338
Summary 290 Calcareous sediments within the Voznuevo
Formation 338
IV. Stratigraphy 291 Comparison with Paleozoic samples . . 338 The Lower Cretaceous in the Basco-Cantabrian The pebble composition 339
Pebbles area 291 consisting of'lydite' 339
The sediments in Wealden facies in other parts Concluding remarks regarding the radio-
of Spain 294 larites in the Voznuevo Formation . . . 340
The stratigraphical position of the Voznuevo Remarks on diagenesis 340
Formation 297 Grain-size distribution 341
Pollen analysis 301 General remarks 341
A comparison of the Voznuevo Formation with The results 341
other sediments in Wealden facies 302
Summary 302 VIII.The mineral associations 343
The heavy minerals 343
V. A. The sections 303 Introduction 343
Introduction 303 The results 343
Robla the The area between Riello and La . . . 303 Conclusions based on heavy-mineral
The areabetween La Robla and Cistierna . . 305 content 346
The area between Cistierna and Cervera de The influence of grain size on the heavy-
Pisuerga 309 mineral composition 347
B. Sedimentary structures outside sections A to T Comparison with the heavy-mineral com-
and I to III 313 position of other formations 347
Types of structures in general 313 Concluding remarks on the heavy-mineral
content of the Voznuevo Formation . . Channels between Boñar and Voznuevo ... 313 350
The mechanism of backflow 314 The light minerals 351
The excavation S of Grandoso 316 Introduction 351
The excavation E of Boñar 319 The results 351
and La and remarks The exposure of La Mata de la Riba Comparative concluding . . 352
Devesa de Boñar 321 The clay minerals 353 С. Paleocurrent directions 322 The clay minerals of the Voznuevo Forma-
Introduction and technique 322 tion 353
Results and differences 324 Clay minerals in comparable sediments 353 D. Interpretation of the characteristics of the Clay minerals in the formations overlying
Voznuevo Formation 324 the Voznuevo Formation 354
E. Correlation of the sections 328 Reflections the of kaolinite on presence
F. Paleogeography 329 and on paleoclimate 354
VI. Sedimentary petrography of components coarser IX. Remarks on inorganic and organic components of than 2 mm in diameter 331 the Voznuevo Formation 355
The quartz/quartzite/lydite ratio 331 Iron (hydr)oxides and pyrite 355
Introduction 331 Gypsum 356
Results 332 Sulphur 356
Local differences 332 Organic remains 356
Comparison with surrounding formations . 332
Measurements of flatness and roundness . . . 333 X. Discussion and conclusions 357
Introduction 333
Results and interpretation 333 References 359
Local differences in flatness and roundness
values 333 Plates I-VIII
Comparison with surrounding sediments . 334 Surface features of pebbles and cobbles Enclosures I—XI (in back flap)
and climatic conditions 334
Composition of pebbles of between 8 and 12 Drawing of text figures and enclosures by Mr. M. L.
mm in diameter 335 Brittijn
Summary 335 279
I. sketch of NW Area studied within Fig. Geological map Spain. rectangle.
CHAPTER I
INTRODUCTION
could there- GENERAL REMARKS metres wide (Fig. 1). The variations in area
fore only be studied in an E-W direction.
The Cretaceous of northern and central Spain contains The Voznuevo deposits directly overlie the Paleozoic continental, chiefly sandy deposits of considerable thick- (and locally the Precambrian) with an angular uncon- which have often been referred in and overlain followed ness, to, especially formity are by marine Cretaceous,
as 'Wealden' or 'sediments in Wealden — of older literature, — more to the south by the continental Tertiary facies'. The sedimentology of these so-called Wealden the Duero Basin. of Canta- beds occurring along the southern border the brian Mountains is the subject of the present study. the Voznuevo* Recently, Evers (1967) proposed name CHARACTERISTICS IN THE FIELD
Formation for these deposits, which name will be used in the present work. In the field the deposits of the Voznuevo Formation
Since the southern border of the Cantabrian Moun- show the following characteristics: tains is structurally a steep flexure (locally passing into a
Voznuevo in narrow buff with fault), the deposits are exposed a 1. They have a white to yellow or colour, and few kilo- S-dipping zone, over 130 km long only a occasional dark red and orange-red colours. The sedi-
ments are clearly lighter in colour than those of the
* 2.5 of Boñar. of Named after the village of Voznuevo, km east Vegaquemada Formation (Evers, 1967; Kuyp, 1969) 280
which colours of Lower Tertiary age in orange predo- deposited have been the subject various speculations. minate. Many uncertainties also surround the question where the from 2. They show highly varying grain sizes, gravel Voznuevo Formation has to be located stratigraphically
(rarely of cobble or boulder size) to clay. in the Cretaceous. The only reliable information was
3. There is obvious absence of limestone and Amerom an pebbles given by van (1965) who, by means of pollen
cobbles and limestone layers (another contrast to the determineda Cenomanian to Turanian for analyses, age
Vegaquemada Formation)" the formation near Boñar. It will be the subject of this
4. In general the sediments are not - or only slightly - study to contribute to the solution of various questions
consolidated. surrounding the Voznuevo Formation, the following of
which are the most important:
Natural exposures in the Voznuevo Formation are very
and informationhas been in sand and rare most gathered a. What is the textural and mineralogical composition
clay pits. These pits, many of them now abandoned, are of the sediments, both megascopically and micro-
commonly situated beside villages. They are now used scopically?
for dumping rubbish. Exposures other than in excava- b. By which agent and in which sedimentary environ- and tions can be found in brooklets, in river banks in ment has the deposition taken place?
railway and road cuttings. с Can a direction of supply be reconstructed?
In contrast to the mountainous area to the north, d. What is the possible source area of the sediments?
meseta border. summers are very dry near the Very e. Are there any differences in the characteristics of the and intensive precipitation takes place in spring autumn; sediment from east to west or from bottom to top? inaccessible in May and June many excavations are on f. Of what nature are the stratigraphical relationships of
account of the water accumulated in them. In summer the formation within the area studied and what is its
obliterate of occasional rain showers can in a matter stratigraphical position as compared with other deposits which minutes the sedimentary structures were minutely in a similar facies reported from other parts of Spain? of the been influenced carved by sun and wind. This is one reasons why g. How much have the sediments by Formation the Paleozoic rocks? the degree of exposure in the Voznuevo can underlying
h. What is the transition to the vary from day to day. Even more far-reaching changes overlying deposits like,
observed from to in ex- and to what extent has infiltration from these deposits could be year year recently taken cavated pits. Collapse of the pit walls, and rain wash and place? solifluction along them, followed by vegetation, com-
pletely obscure stratification and structures in the
sediments and make their study impossible. APPROACH OF THE STUDY
Vegetation along the southern border is in general very
scanty, neither of the Mesozoic or Tertiary formations is The description of the sediments of the Voznuevo For-
very fertile and agriculture is almost exclusively mation consists of four main parts.
restricted to the lowest parts of the valley. In many First (Chapters II and IV) a survey is given of the could be ob- locations examples of badland erosion investigations by previous authors in the present area as
served. far as mapping, descriptional and stratigraphical work similar A weathering phenomenon which can be observed on are concerned. The occurrences of deposits in of the surface of the sediments is the discolouration other parts of Spain, especially those in the provinces of sediments generally white or yellowish coloured into Santander, Burgos and Vizcaya and in the Sierra de los be will be purple to red coloured ones. This process must Cameros between Burgos and Soria, compared down- ascribed to surface waters rich in iron, running with the present sediments. New data on the age of the of sediment slope. The very superficial cementation the formation found by means of pollen analyses will be of Other disco- was probably also caused in this manner. great help in determining the stratigraphical position of formation and will lourations, especially in sediments of silt-size, occur as the within the Cretaceous contribute several deci- purple mottles with a diameter of up to to a better understanding of the paleogeographical of metres. Although their appearance reminds one a development along the southern border zone. also paleosol, they proved to have formed recently. The second part (Chapters III and V, VI) contains
the description of observations in the field with their conclusions. This forms of the sedimento- part the core
SCOPE OF THE PRESENT STUDY logical approach of this investigation.
The third part (Chapters VII, VIII and to a lesser
extent deals with the and Until recent years the sediments of the Voznuevo For- IX) petrological description submitted mation have been little known. The only information the interpretation of samples which have been of kaolinite given concerns the high content (the main to special treatments. of reason for the white colour) and the megascopical mono- Finally, as a general conclusion, a synthesis new of these tony of the components. The circumstances in which, facts contributing to a better understanding so far X. and the mechanism by which, the sediments were enigmatic deposits is presented in Chapter 281
METHODS AND TECHNIQUES USED carried out according to Cailleux's method (summarized
in 1959). Bed thickness was described according to
Investigations in the field McKee and Weir's nomenclature (1953). The classifica-
The areal extension of the Voznuevo Formation can in tion and terminology of cross-stratified units largely
fact only be mapped by delimiting the more resistant follows that proposed by McKee and Weir (1953), but of classification and better exposed rocks which limit the formation at wherever possible use was made the by Grain-size distribution cal- its bottom and its top. Allen (Allen, 1963a). was Of 34 sections of varying reliability and length, the culated according to Friedman's moment measures
best 24 sections have been selected to represent the (Friedman, 1962). will be dealt with Voznuevo Formation (Enclosures II to VIII). The The sediments in the present area
of the conventional from west to east, in some in- sections have been logged by means systematically except The is subdivided into techniques. stances. area arbitrarily a western, the have aimed a central and an eastern separated by Rio As to sampling we at obtaining represen- part,
statistical methods used. Since and the Rio Esla, respectively. tative samples; no were Bernesga
author had shown an introductory study by the present
that the composition of the sediments does not show ECONOMIC USE OF THE SEDIMENTS large differences over short horizontal or vertical
distances, spacing of the samples has been adapted The Voznuevo have been for accordingly. deposits dug many
The sands have and are still, used for the Paleocurrent directions have mainly been obtained purposes. been, manufacture of tiles and the clays - especially from the positions of foreset plains in cross-stratified bricks; - the black coloured are used for the same units and to a lesser extent from observations on im- clays purpose manufacture of and Small brication and in channels. Since paleocurrent analyses and for the ceramics pottery. works found the excavations. Most be achieved well brick are adjacent to can only in very exposed, mainly of the factories abandonedand recently excavated, pits, cross-stratified units were are now reorganization led small number of fac- measured wherever possible. The slightly consolidated to concentration into a large
condition of the sediments makes it possible to cut inter- tories. The sand pit south of the village of Grandoso is the with of several dozens secting planes in which the orientation of the foreset only excavation a production of the studied. The sand plane can exactly be determined. tons a day in area pits near
Other determinations which had to be carried out in Sorribos de Alba, Llanos de Alba, those 1.5 km E of
the field are those with regard to the composition of the Boñar and those N of La Ercina are only of local import-
pebble-sized sediments and to the geometrical properties ance. Large excavations of black clay can be found 2 km of Cervera de of the pebbles. E of Boñar, NE of Valmartino and S
Pisuerga, along the road to Herrera del Pisuerga. used abrasives Laboratory examination of the samples The fine-grained sands and silts are as of but their About 75 samples, among which a certain percentage for household purposes economic importance
to be well cemented be Paleozoic age, proved sufficiently can neglected. for preparing thin sections. A few voluminous samples It should finally be mentioned that the cobble con- could be studied of used with structures of macroscopical size glomerates which occur west La Magdalena are after them Less well cemented for roads and for tracks. mounting in plastic. as materials especially railway samples of fine gravel-, silt- and clay-size were treated in the sedimentological laboratory of the University of
Leiden for studying the grain-size distribution, the REMARKS ON THE RELIEF heavy- and light-mineral content of the sands and the composition of the clays. For the computation of grain- The drainagepattern. of The main rivers which drain the the size distribution use was made a PL/1 computer area belong to
The of northern of the Duero drainage basin. They all programme composed by Koldijk (1968b). study part and follow N-S heavy and light minerals was carried out by means of the come from the Cantabrian Mountains a the Voznuevo polarizing microscope with traversing light and by means directed course; they cross Formation In less the strike. All these rivers of the binocular with reflected light, respectively. more or perpendicular to obtained the result of have into the Paleozoic rocks of the both cases the percentages were cut deep valleys line counts. The fractions from 37 to 2 microns and Cantabrian Mountains.
Voznuevo smaller than 2 microns were subjected to X-ray analysis; In the depression coinciding with the For-
studied in situated between the mountain and the they were diffractograms. mation, range of The following classifications were applied. For the ridges of Cretaceous limestones, no rivers any drained small sub- description of grain size the Udden grade scale (Udden, importance are encountered; it is by
- — the border 1914) with the Wentworth modifications of size limits sequent brooklets parallel to mountain
(Wentworth, 1922) was used (see also Pettijohn, 1957). which run dry in summer.
Morphometrical analysis of pebbles and cobbles was The relief clearly reflects the geological structure as 282
expressed in differences in resistance to weathering of ACKNOWLEDGEMENTS the various rock units. From the massif to the south one
in The author wishes to his sincere thanks to the finds, order of decreasing age: express
following persons:
Professor Dr. J. D'. de the a. The Voznuevo Formation (Cretaceous). Jong, now at Agricultural for and b. A Mesozoic limestone complex, gradually decreasing University, Wageningen, his assistance
thickness and enthusiasm in the field and his of the manu- in wedging out in a westerly direction reading (Cretaceous). script.
с The Vegaquemada Formation (Paleogene) (Evers, Professor Dr. A. J. Pannekoek for his thorough 1967). criticism and discussion of the manuscript. d. The Candanedo Formation (Paleogene) (Evers, Dr. J. W. A. Bodenhausen for giving many fruitful
1967). ideas, both in the field and in studying the thin sections. the e. The red beds, beginning with the Vega de Riacos Miss E. van der Wilk for doing so much work on Formation (Neogene) (Evers, 1967). preparation and the treatment of the samples collected.
Dr. T. van der Hammen for providing one of the most the cemented and contributions, one of the pillars on which Units а, с and e are only slightly weather important the easily. Especially the sediments of the Voznuevo Forma- conclusions of the present study are based, viz. number of tion form depressions in topography. The Mesozoic determination of the age of a pollen-bearing of black limestones and the coarse conglomerates the clay samples. Mr. J. G. Zandstra of the of The Candanedo Formation are much more resistant; their Geological Survey generally south-dipping strata provide clear examples of Netherlands in Haarlem for the determination of the cuestas. The dip-slope of the Candanedo Formation composition of some gravel-sized samples. Mr. V. Zimmermann for his corrections in the gradually passes into the Spanish central table-land, the A. of 'meseta'. In the eastern part the area the transition English text.
must from the Cantabrian Mountains to the central plateau is Last but not least, warm-hearted thanks be
limestones the of D. Francisco Reina of La more gradual. There, the Mesozoic are the expressed to family
— showed, only rocks which stand out in topography. Herrera Valmaseda (Vizcaya, Spain) who when the author had accident the last of his In several locations the Tertiary sediments are in direct a car on day contact with the Paleozoic rocks and prevent out- fieldwork in Spain, that Spanish hospitality is not only a cropping of Cretaceous sediments; there, erosion was less phrase. that be intensive so topographically higher parts can found.
CHAPTER II
MAPPING OF THE AREA STUDIED
Cantabrian Mountains. Most investigators, who had the Cretaceous sediments southern and central part of the scale and under consideration as part of the area studied by them, The following maps, each on a 1:50,000
from W found useful: have dealt with them in a stepmotherly fashion. The enumerated to E, were reason for this is that the emphasis of their studies has always been laid on the Cantabrian Mountains them- 1. Sheet Luna-Sil(van den Bosch, 1969). 2. Sheet 1967). selves. The result is that mapping of the post-Paleozoic Luna-Bernesga-Torio (van Staalduinen, 3. Sheet rocks has usually been carried out in a hasty manner. Bernesga-Torfo-Curueño-Porma (Evers, 1967). For 4. Sheet Cea-Esla-Porma and Ciry (1939), the only one who concentrated on (Helmig Rupke, 1965). 5. The the theses Cretaceous deposits, the present area only forms a geological maps belonging to by
and Kanis for the area western offshoot, besides, the part W of the Rio Koopmans (1962) (1956) Curueño between Guardo and Cervera de was beyond the scope of his study. Pisuerga. An excellent of map the part between the Rio Almela from made of sheets Bernesga and the Río Cea was prepared by Apart these, use was two belonging scale the of the the de (1949), 1:50,000; easternmost part area to Mapa Geológico España. south of Cervera de As the of is not studied, Pisuerga, was mapped on preparation a geological map decidedly
the same scale by Ciry. the object of the present study, the general map (En- More closure scale. Differences recent work was carried out by investigators I) is presented on a 1:100,000
to workers will working on the mapping programme of the Department with respect the conceptions of previous of Structural Geology of the University of Leiden in the be discussed below. 283
COMPARISON WITH MAPPING BY PREVIOUS AUTHORS las Dueñas, to La Robla between km 2.5 and 3.5 and does not
form small 'islands' near the contact with the Paleozoic E of
westernmost Santiago de las Valles. This triangle-shaped area is a continuation The part of the area studied is situated in the south-
eastern den Bosch. of the narrow which runs from Llanos de Alba to Olleros de corner of the area investigated by van strip in which the Strangely enough the sediments outcropping between Riello and Alba, separated by a topographically higher part,
of Cretaceous overlying Vegaquemada Formation has not been eroded and is in Bobia were not recognized by him as being age, direct contact with the Paleozoic (at the in the road). The but were thought to belong to the Vega de Riacos Formation pass
the contact between the Voznuevo Formation and the (Miocene). This name was given to red beds in the Duero Vegaquemada
basin by Mabesoone (1959). Van den Bosch provisionally in- Formation willbe described separately in Chapter III.
W cluded the sediments in the Riello area in the Vega de Riacos 2. About 400 m of Olleros de Alba, the formation dips under
Formation 'The resemble the the contact Formation and not about 1 since (p. 183): outcrops ... closely Paleozoic-Vegaquemada
van Miocene deposits further to the south ... . They also resemble the km W of this village as proposed by Staalduinen.
Cretaceous Voznuevo Formation but their flat-lying 3. At the lower contact of the formation N of Sorribos de Alba, Upper ... ,
to not position on the ESF. dipping peneplain or pediplain, similar the Voznuevo deposits are clearly overturned and S-dipping.
further to the south, that of most of the Miocene deposits 4. No Cretaceous was found N of the village of Rabanal de It is suggests these deposits to be of a Miocene age.' abundantly Fcnar; the contact must be drawn through the village.
clear considered these to 5. found the S of Robledo that van den Bosch only deposits Mesozoic limestones were along path
to From the tens of metres N of the for this belong the Tertiary on purely structural grounds. de Fenar, some railway; reason
direction of cross-bedding and channels and from the terrestrial the boundary of the formation is shifted slightly to the N at this
properties of the sediment a mechanism of deposition by a fast location.
running river with a transport direction from WNW to ESE was To which Almela's covers only a small of van proposed. The sediments between Riello and Bobia are supposed map, very part Staalduinen's the last mentioned be to have been deposited in a basin (Riello basin) and to consist of area, only objection can
river made. an asymmetric former valley. The total thickness of the Sheet 129, La Robla, issued by the Instituto sediments in this basin is supposed to comprise 200 m. South- Geológico y
the of the Minero de with Pastor Gómez (1963), west of Oterico, coarse quartzite cobbles mark base España, description by
suffers from the same imperfections as listed above with the formation, whilst 'the uppermost deposits contain rather coarse of N of Rabanal de Kenar. A pebbles deposited in channels'. exception the structure conception
Bosch's shall first in which these Spanish investigators differ is the traceability of Before discussing van den findings we con-
issued the Instituto the Voznuevo Formation between La Robla and Brugos de sider the geological map, scale 1:50,000, by
Fenar. We tend to believe that Almela's (and van Staalduinen's) Geológico y Minero de España, Sheet 128, Riello, prepared and is much commented Pastor Gómez (1969). From the legend of proposition more likely. upon by East of the road from Otero de las Dueñas to León, km this map we may conclude that Pastor Gómez attributes a near of Mesozoic kaolinitic sands of the Riello In order 27 and opposite the path to Benllera, a small outcrop age to the area. and sands found between shed these statements, the Voznuevo sands Vegaquemada was to some light upon contradictory drawn Precambrian schists and sediments. At the western western border of the area studied by us was near Riello. post-Mesozoic
side of the some 500 W of km 25, the same situation is Apart from the lithologicalproperties, both macroscopical and road, m direction of de la microscopical, and the sedimentological features of these encountered. This exposure runs in the Tapia the deposits, which clearly show that they belong to the Voznuevo Ribera (both outcrops are represented on general map,
found Enclosure Both are not as Cretaceous Formation further to the east, a cast-iron proof could be I). exposures recognized
with overlying Eocene on the prepared by Pastor Gómez; he in the field. It showed that a plain division could be made into map
Almela (1951) on 48 of the two different formations with an east-west directed boundary, quotes page explanation accompany- las de la la the the ing the 'En inmediaciones Tapia de Ribera, base splitting area into a northern and a southern strip. Only map: de Miocene is remark the northern part shows the properties characteristic of the sedi- este terreno (the meant, by present
sobre el consiste unas ments of the Voznuevo Formation. In the southern strip sands author), que se apoya Precambriano, en
arcillas blancas rosadas, micáferas, arenas arcillosas and gravels were found with the typical dark red to orange-red y muy y de colours belonging to the Vegaquemada Formation, as described arenas con gravilla. Sobre ellas van unas arenas arcillosas,
de tamaños by Kuyp (1969) between the Rio Bcrnesga and the Rio Esla. The color salmón, con cantos bien rodados muy
Mesozoic limestones, which separate the Voznuevo Formation variables, que llegan hasta lo mas alto del páramo'. Both Almela
from Formation further the west, and Pastor Gómez attribute Miocene to these The the Vegaquemada to wedge a age deposits.
out between Llanos de Alba and Carrocera, as is proved in an resemblance in the field and the mineralogical composition
SE of Carrocera The sediments in the (especially the heavy-mineral content) leave no doubt, however, exposure (Chapter HI). be classed sediments of these the Voznuevo southern part of the Riello area should not only that the exposures belong to
the account of the but Formation and sands and overlain sedi- among Vegaquemada deposits on colour, (white pink clays), by
account content and the ments which to the Formation also on of the high of quartzite pebbles apparently belong Vegaquemada
of limestone Pastor Gomez's conception of the (salmon coloured clayey sands etc.). presence pebbles. both for the northern Other isolated indicated as Cretaceous on age therefore only proves correct strip. outcrops maps
with proved to consist of weathered Paleozoic rocks. Since we are dealing two gently south-dipping formations,
it Some corrections of the Almela were made we may not speak of a 'Riello basin'. Besides, appears that the map presented by Río the Rio Porma. quartzite-cobble conglomerate SW of Oterico is not situated at by Evers (1967) between the Torio and Unfortunately these corrections proved to be They refer the basal part of the formation but at the very top, whilst the wrong.
of the basin' in to the mention of 'Voznuevo islands' north of the thrust or coarse pebbles 'deposited in channels in the top
Voznuevo Formation! fault-line between and Boñar. Closer inspection fact lie in the upper part of the Apart Valdepiélago
showed that we are again dealing with weathering products of from this, it was deduced from a section near Bobia, that the Paleozoic rocks. N of the Voznuevo Formation alone has a thickness of more than 300 m. Besides, they are hardly explainable
flexure zone. Evers' alluvial fan conglomerate WNW of the village Regarding the geological map, it should be mentioned that the west of Boñar, consisting of 'rounded limestone and sandstone Voznuevo Formation proved to be present as far as of
cobbles', is also On his geological a lime- Riello. highly enigmatic. map
of stone is drawn at this location but not a Some objections have also to be made to the distribution conglomerate single could be limestone encountered, either in the field or the Voznuevo Formation as mapped by van Staalduinen (1967): fragment in thin sections of samples taken from this conglomerate;
soil rich in limestone was the ofthis 1. The formation is mappable just N of the road from Otero de apparently a pebbles reason 284
do with with the Mesozoic limestones E of Cistierna confusion. Finally we not agree the indication 'fossil- which runs mainly side fauna' in the southernmost anticline 1 km S of La Mata de la on the NE of the road between Cistierna and Valmartino. It
was finally found that the Voznuevo Riba, on the right bank of the Rio Porma. It became evident that deposits already disappear
several midway between Quintana de la Peña and Valmartino. in this location a huge block of cubic metres dislodged
the is The part of the area studied which is situated in the itself from overlying Mesozoic limestones, and now province
of Palencia was by Koopmans (1962). His ideas the situated in the core of the anticline. mapped on
A of Rio geographical distribution of the Voznuevo Formation cover geological map the Porma area was prepared by those of the author. and this the sediments present Helmig Rupke (1965). According to map, between The formation is traceable to the east as far as Be- in the area Boñar and Colle are cut out by a fault Traspeña. tween to Traspeña and Dehesa de Montejo no showing belonging the Sabero-Gordon fault zone(Rupke, 1965, N-S outcrops
Voznuevo sediments were found. The Cretaceous between sections 1-4). The area between the Sabero-Gordon fault zone Villanueva de la Peña and Cubillo de Kanis and the southern border fault is the westward extension of the Castrejón which
(1956) believes to Wealden Voznuevo so-called Peña Corada Unit. The Cretaceous and the overlying represent (= Formation) were not recognized as such in the present For the pre-Miocene deposits have been drapedanticlinally over the Peña investigation. easternmost the SW Corada Unit. part of area, of Vado de Cervera, Kanis
made use of the previously published geological of the Since the two S-dipping flanks have been preserved, a second map
Ligüerzana area (Ciry, 1939, Planche A). As a result of new strip of Voznuevo sediments is present, running continuously thanks to in this from the Rio Porma to the Rio Esla. exposures clay exploitation area, we propose to shift the western limit south of the to Minor corrections with this refer the railway some 100 m the regard to map to the which west. boundary with Paleozoic near Oceja de Valdellorma, runs through the village and not south of it and the boundary
CHAPTER III
STRUCTURE AND CONTACTS
STRUCTURE IN GENERAL
General remarks
Apparently Termier (1918) was the first to detect north-
below Paleozoic dipping strata of Cretaceous age the along the southern border of the Cantabrian Mountains. remarked Paleozoic massif has Ciry (1928) that the com- pressed the Cretaceous deposits into a syncline which is opened to the south. West of the village of Avifiante this
to in its syncline was said be visible entirety. Some years later, Ciry (1933c) returned to this subject and stated that: 'Le massif paléozoi'que Asturien apparaft done comme un pli de fond qui s'ennoie périclinalement vers l'Est en même temps qu'il montre une tendence générale Sud'. de déversement vers le Finally this author (Ciry,
1939) noted that the Voznuevo Formation and the
Mesozoic limestones have formed the normal cover of the Asturian massif which has locally been pushed into overturned direction. an position in a southward
The structures of the southern border are dealt with in three and parts: a. Between Cervera de Pisuerga Guardo, b. between Guardo and Cistierna and с between
Cistierna and Valdepiélago. Fig. 2. A: Schematical section near Aviñante (modified after
Ciry, 1939), showing asymmetrical syncline, open towards the the border of 'a' a. Along part two sections were south. B: Schematical section along the Río de las Cuevas (modified presented which are reproduced in a simplified form in after Ciry, 1939), showing asymmetrical syncline followed to the Figs. 2A and 2B. The first figure shows a section near south by an asymmetrical anticline. Aviñante, in which the asymmetrical syncline, opened to the northern flank and south, shows an overturned a
horizontal flank. 2B sediments nearly southern In Fig. a section the overturned position of the Cretaceous can
2 km W of Villanueva de la Peña is shown in which a be observed.
the of the more extensive deformation can be distinguished; с From Cistierna to the W the influence pressure
to of Paleozoic massif to the south seems gradually to syncline is followed the south by an equally asym- the metrical anticline. decrease. The San Adrian anticline, which causes the
sediments, is, ac- b. Part 'b' is a fragmentarily exposed area in which only duplication of exposed Cretaceous 285
to of directed and the Cretaceous in this section is certainly cording Ciry, a consequence an oppositely occurring tectonic force to the south. This supposition which not representative of other contacts in the area. The of limestones in this section would call for explanation by a very local process, will absence Mesozoic can be the between the be discussed below. The supposed exposures on the hardly explained by unconformity northern flank of the San Adrian anticline are somewhat Voznuevo Formation and the Tertiary conglomerates, of questionable. since only a fraction of the total thickness the
Voznuevo Formation proved to be present in this loca-
structural tion. from indications of West of the Rio Porma, Ciry reported a situa- Apart this, very strong com- faults tion comparable to that found between Guardo and pression could be found, so that one or more
Cistierna, with clearly overturned Cretaceous deposits. instead of an unconformity are, in our opinion, much
Observations by Almela ( 1949) for the most part cover more likely. which fault those by Ciry, although Almela's interest is mainly East of Guardo, the zone in movements or directed at the Mesozoic limestones. His finding that flexuring took place is apparently situated further to the
been indication of between north than the zone of Cretaceous outcrops (Koopmans, there has never any faulting the Paleozoic and the Cretaceous is important. 1962; Kanis, 1956). The Cretaceous sediments are
to overlie the Carboniferous strata with a small In the area W of La Robla, Almela (1951) noted that reported
massif de- in the studied the influence of pressure of the Paleozoic angular unconformity area by Koopmans, creases W of Carrocera. The sediments of the Voznuevo whilst the angular unconformity still further east, along
the road from Cervera de to Vado de Cervera Formation, which outcrop below the Miocene cover (red Pisuerga
be base of the section shows an of beds) near Tapia de la Ribera, were also supposed to (the present T), angle 30° Almela to assume intra- more than (cf. Kanis, 1956, Fig. 14). of Miocene age, which obliged miocene tectonic movements, since the red beds are
measurements in the Voznuevo Formation flat to the S less Strike-dip known to show a very dip (generally As a of a fold-thrust uplift (Berg, 1962; than 5°), while the dips recorded at this location are 15° consequence Evers, 1967), which will be explained below, the sedi- to the S (30° to 45° SE according to our measurements). of the ments of Cretaceous and Paleogene have been affec- Since these deposits proved to be Cretaceous age, age ted the Paleozoic massif in such a manner intramiocene movements of Almela must be abandoned. by upthrusting vertical overturned between the Paleozoic and that were into a or With respect to the contact they dragged the Voznuevo Formation Pastor Gómez (1963), who position. The deposits of the Voznuevo Formation, the first sediments after those of the likewise failed to notice a fault between them, proposed which represent
directed in this have suffered most, due to their the existence of a deeper situated E—W fault, or Stephanian area, of faults. One of these faults held close to the active The in- a system parallel was position tectonically region. responsible for the disappearance of the Mesozoic lime- fluence of this mountain flexuring on the Voznuevo For- the stones between Candanedo de Fenar and Rabanal de mation has, however, not been the same along entire of Fenar. southern border. Table I, which gives a listing some from Staalduinen's shows that the con- values of the stratification W to E, A glance at van map strike/dip planes have tact mentioned above is interpreted as an unconformity clearly shows the differences. Measurements always
fault between the been carried in the material with a probable fault. The supposed out finest-grained present, in Voznuevo Formation and the Mesozoic limestones is not generally in clays, in order to avoid misreadings sedi-
both below the Paleozoic- ments which could have been deposited on an inclined shown on that map; units dip surface. Vegaquemada Formation contact W of Brugos de Fenar.
the structural In the western part of the area, W of the Río Bernesga, Most information on relationships
clear is in the isolated area between the Paleozoic core and the sediments on its a uniformity recognizable From his between Riello and Bobia. The formation dips to the S southern border was given by Evers (1967). 26° structural sections learn that the structures with a gentle slope between and 36°, except for map with we Section С 2 where a slightly position could be S of Boñar (the San Adrian anticline, accompanied to steeper be observed. Apparently this area has not been influenced the south by the Las Bodas syncline) need not ex- much the which is not surprising plained by compression from the south as previously very by fold-thrust, formed since the sediments are bounded, also on their southern proposed by Ciry (1939): they were probably by flank, a Precambrian block. reactivation of the Sabero Fault zone and the Valde- by of the Between Carrocera and La Vetilla the Voznuevo piélago fault to the north and by an extension 1965, Formation has been influenced strongly by the fold-thrust near Cistierna (Rupke, Fig. 32). very southward Paleozoic massif: the strata dip more Viewed in this light, the rocks of Cretaceous age near pushing than 80° to the south (SSE), except at Solana de Fenar. Boñar have simply been draped anticlinally over an ex- At three locations (Carrocera, Sorribos de Alba and tension of the Pefia Corada Unit; the structure originated de Fenar) a clearly overturned position was as a result of mainly vertical movements. Rupke's Fig. Brugos
W of in which the observed. Between Boñar and Colle a very constant dip 32, a N-S section Cistierna, however, of 22° to 27° to the SW. As for the southern Paleozoic to the Miocene is shown, is slightly misleading occurs
it could be observed that the Voznuevo Formation since the low-angle unconformity between the Paleozoic strip, 286
Section A i 180/36 173/32 175/26 mation has only taken place in locations where the
Section В i 178/26 178/33 Cretaceous has been dragged into an overturned posi-
Section Cl i 175/35 tion.
Section C2 í 175/50 A much more drastic result of the pressure to which Section Ci : 175/36 the Voznuevo Formation has been exposed is the inten- Carrocera t 10/65 W8/80 sive of the sediments in where Section D pulverization places s 174/83 must have been This above Section В : 345/70 347/85 compression very strong. was all Section Г ï 0/65 encountered in Section О and Section P (Yugueros
Section 0 t 160/87 and Cistierna, respectively), and could frequently be
г Section H 154/55 observed east of Cistierna. In such locations it proved
Section I i 155/81 hardly possible (or even impossible) to obtain unbroken E of Boñar : 207/22 220/26 pebble specimens from the sediment. The tectonic in- La Devesa ' 185/45 fluence is not only limited to but S mega-components, of Oceja i 175/80 proved present in the thin sections as well (Chapter VII). Section 0 : 20/60 Near Solana de Fenar it was noticed that the forma- Section P í 45 overturned
tion turns into flatter towards the Cistierna ï 50/40 35/60 a S-dipping position
Prado : 55/90? top of Section H, (within 50 m). The Mesozoic lime-
Cerezal i 80 -90 overturned stones which constitute the southern limit have a much
W of Guardo: 315/80? than the Section that steeper position strata in H, so a
E of Guardoî 20/75 structure may be assumed comparable to the one des- Muñeca : 355/26! cribed by Ciry (1939) near Aviñante (Fig. 2A). Such a Section Q f 355/75 structure would easily explain the apparent thickness of SectionВ t 307/42
the Voznuevo Formation in this location on the geolo- Aviñante t 346/80 0/70
Section S t 350/55 gical map.
Villaverde t 355/85 In Section T near Vado de Cervera a syncline was
Section T i 116/39 125/30 115/30 35/45 ••• found, flanked to the south by an anticline. Both struc-
• . 65/21 100/30 130/30 tures have an E-plunging fold axis. Their consequence
for the thickness of the formation will be dealt with in
When more than one reading is is given, the first situated most Chapter V. to the north (i • e. contact with the Paleozoic)
Table I. Direction of dip and angle of dip of the stratification The thickness of the Voznuevo Formation plane from W to E (top to bottom) and N to S (left to right). Thicknesses authors reported by previous vary con- siderably. Ciry (1939) remarked that the thickness is
variable, but generally exceeds 100 m. Almela (1949)
turns into and overturned in again a more more position mentioned a thickness of 125 m in his description of the the direction of Cistierna. here Villaverde From on to de Cretaceous but the sections accompanying his work la Peña the steep, overturned northward dip remains, show an average thickness of 250 m, with an excessive
overturns with excessive near Mufleca and Santibañez de value near Solana de Fenar: 1000m. As we have stated la less Peña (dips than 45° N). Near Cervera de Pisuerga previously, folding within the formation probably plays the influence of the fold-thrust has the between Naredo de Fenar uplift obviously an important part in area found of decreased; a situation is comparable to that E and Rabanal de Fenar. Two exploratory drill-holes Boñar. (Zalofla and Sampelayo, 1943; Almela, 1949) near La
Mata de la Riba and Las Bodas, carried out in search of Faults and folds within the formation and their con- the continuation of the Carboniferous strata below the Mesozoic and sequences Tertiary cover, not only gave negative
Tectonic disturbances within the results to their to Zalona sediment, especially as original purpose: according be observed with and 352 of Cretaceous sand have been faults, can only great difficulty, on Sampelayo, m of account the unconsolidated state of the deposits. In penetrated near La Mata. Below this depth Paleozoic Section F de rocks hit. the of near Brugos Fenar, some normal faults were According to Almela, greater part could observed be with a stratigraphie throw of one or these sands belongs to what is now called the Vegaque- two metres. Larger faults of unknown dimensions were mada -Formation while only the last 87 m are supposed seen in Sections P (Cistierna) and R (Santibañez de la to be Cretaceous. It has been assumed that a fault-zone
Peña), both causing a sudden change in dip. For the rest was penetrated, since the Mesozoic limestones, which fault did most movements have taken place along planes were to be expected in a complete section, not show
to the of stratification. This Las Bodas was not parallel plane was deduced up. Unfortunately drilling near con- from the presence of slickensides which proved to be tinued as far as the Paleozoic and was stopped in rather frequent in the section near Cistierna and in all sediments of the Voznuevo Formation. For this reason sections E of Guardo, Section a minimum value can be 208 m. including T. It appears only given: that the formation of faults within the Voznuevo For- Koopmans (1962) did not give an explicit thickness, 287
thickness those between the Precambrian and the Paleozoic but it may be deduced from his sections that a are of 200-350 m was assumed. rocks and between the Paleozoic and the post-Paleozoic
sediments. Berg's observation that: 'The inverted section Rupke (1965) proposed a thickness of 70-350 m, of 600 be somewhat folded and while Helmig (1965) assumed a thickness m may relatively undisturbed, between Boñar and Colle and a 'normal' thickness of faulted, or highly deformed and jumbled' is important
250 m. for the situation in the present area.
Evers (1967) reported values fluctuating between 150 and 550 m. Geophysical examination of the southern border fault
The thickness of the sediments near Riello was zone
structures the contact estimated to be approximately 200 m (van den Bosch, New light was shed upon the near Paleozoic with sediments of 1969) but the proposed sedimentary explanation (i. е. a of the younger by means a
carried out the river valley), proved to be wrong (cf. Chapter II). gravity survey, by Geophysical Depart- result of which of the of Leiden in the 1964 and Incomplete outcropping as a a con- ment University years covered the between the Rio tinuous section is nowhere present causes that thick- 1965. This survey area
measured with Most Luna and the Río Cea. The results of this nesses can only be some reserve. investigation,
carried in were summarized and valuable are those measurements which were out presented unpublished reports, in locations where tectonic activity has been fairly weak. reinterpreted by de Bruyn and Evers (in prep.). As the and Notwithstanding these impediments, thicknesses could mentioned in Chapter I, Mesozoic Paleogene sediments the southern border in fact constitute be measured accurately in various locations, giving sur- along prisingly constant results. A maximum thickness of the the transition from the Paleozoic core, which forms the
Voznuevo Formation is found W of La Robla, where Cantabrian Mountains, to the Tertiary basin in the
— south. This transition has been the main of in- values exceeding 350 m with a maximum of 380 m object
- terest for the measurements near Llanos de Alba were encountered. There appears geophysicists. Gravity were towards the carried the of the main rivers W to be a gradual decrease in thickness east, to out along valleys (from and Villaverde E: Río Río Río Rio Porma approximately 250 m between Aviñante to Bernesga, Torio, Curueño,
(Section S) and S of Cervera de Pisuerga (Section T). and Rio Esla), and the gravity profiles consequently
this the structures This trend is clearly visible in the correlation chart(Encl. show a N—S orientation. In manner, favourable XI). It should be mentioned that the 250 m reported of the border zone are intersected at a angle. which constructed near Cervera is a minimum value since a hiatus of un- The interpreted gravity curves were
from contours show that known extent is present due to folding, but we certainly a steep gradient in gravity the transition from do not share the view of other workers (i. e. van de strong gravity changes are present at of Paleozoic rocks. The Graaff-Trouwborst, 1970) who estimated a thickness to younger hypothetical plane
which rocks with different densities is ex- more than 600 m. separates
pected to have the following orientation:
The fold-thrust Bernesga valley : 35° N-dipping (overturned) Torio The relationship between the Paleozoic massif and the valley : 60° N-dipping (overturned) found Curueño 75° post-Paleozoic (i. e. post-Jurassic) deposits, as at valley : N-dipping (overturned) the southern border of the Cantabrian Mountains, has Porma valley : 40° S-dipping 20° been compared by Evers (1967) with the situation in the Esla valley : N-dipping (overturned)
Rocky Mountain foreland as described by Berg (1962). the Considering Porma valley as a relatively stable There, the border structures are thought to have centre, a torsion of the contact plane in a western and originated from a combination of folding and thrusting; eastern direction is noticeable; towards the west the a process which has been referred to as 'fold-thrust overturn gradually increases. uplift'. According to Berg, a fold-thrust uplift can ex- Taking two different values for the density contrast plain a strongly overturned sequence beneath a fault between the Paleozoic and the younger rocks a throw plane, whereas uplift by thrusting alone cannot. Fig. 3 along a probable fault line of 1.75 or 2.1 km, of respec- shows a hypothesis such a fold-thrust uplift as pro- tively, was reconstructed. For further information on posed by Berg. In the Rocky Mountains several fault the reconstruction of the southward extension of the the most planes may occur, predominant ones, however, Tertiary basin the reader is referred to the above-
mentioned publication.
The age of the deformation
Investigations by Evers (1967) and Helmig (1965)
proved that the Stephanian strata, being the last deposits this before the in area deposition of the Cretaceous, has
been considerably folded before being covered. The Fig. 3. Diagram illustrating mountain flank deformation C: Paleozoic massif must have been in advanced state of according to Berg (1962). A: Block uplift; B: Thrust uplift; an
bold thrust uplift. peneplanization when Cretaceous sedimentation took 288
the with place. In this respect it is very questionable whether a. Stephanian conglomerates quartzite cobbles and
whole massif was covered at all; it seems much more boulders.
that of the southern border The cobbles and logical to assume only a part quartzite boulders, floating in a
ever received post-Paleozoic sediments. The uplift of the Voznuevo matrix, probably derived directly from the Carboniferous Paleozoic massif probably began after the deposition of underlying conglomerate. Since no
the Mesozoic limestones, at the beginning of the evidence of a fault could be found, it must be assumed
Paleogene. It presumably lasted until the end of the that we are dealing with an unconformable contact in this location. This is the Paleogene or the beginning of the Miocene. The presumption strengthened by fact that the thickness of the formation has the maximum influence of the uplift was probably reached expected
value and does not seem to be influenced faults. towards the later part of the Paleogene when very coarse by The which deserves alluvial fan deposits of the Candanedo Formation (Evers, next contact our attention is
situated at the bottom of Section de 1967; Kuyp, 1969) spread out from the Hercynian F, north of Brugos mountain chain towards the south. The red beds of Fenar. A few metres between the folded Carboniferous
at (Cuevas Formation) and the steeply N-dipping Voznuevo Miocene age have hardly been influenced all by
strata not the tectonic movements, since they are always less than 5° are exposed. Digging in arroyo brought a transition in the Paleozoic rocks from south-dipping. The main uplift can be correlated to to light: fresh via
Stille's Savian phase (Stille, 1924). Moreover, the uplift weathered rocks to greyish silts and clays with purple and red-brown discolourations. The latter seems to have been very gradual, since angular uncon- might small which often could be Paleozoic soil. the material formities have a angle not represent a Unfortunately, determined in the field. Besides, the phenomenon of 'in- was found to be in such a state of weathering that thin versed sedimentation'(Evers, 1967; Fig. 74), the partici- sections could not be prepared. It is very well possible pation of increasingly old material derived from the that the contact near Brugos de Fenar is unconformable mountain chain in increasingly young deposits, is in and has not been influenced by any faulting. favour of tectonic Near Solana de Fenar gradual movements. (Section H) an impressive angular unconformity between the Paleozoic and the
Cretaceous was encountered: Paleozoic strata dipping
THE CONTACTS WITH THE UNDERLYING ROCKS 35° to the northwest, while the Voznuevo Formation
dips 55° to the south. The direct contact is covered
In spite of the scarcity of well exposed contacts along everywhere, but the first silts which are reckoned among
the whole southern border, a systematic survey from the Voznuevo sediments show light yellow spots, over
east to west will be presented of a number of pheno- 10 cm in diameter, that might be weathered Paleozoic
mena which could be observed at the contacts in the material: the contact is assumed to be again unconform-
field. able.
the Riello In area, the contact between the Voznuevo The unconformity north of Naredo de Fenar (Section Formation and the Precambrian the northern side of the W of the Rib is never exposed. I) on road, Torio,
is clear. From bottom to E of Carrocera, the Cretaceous is found in contact very top we distinguished:
with the 80 of Paleozoic Stephanian but the contact itself is not ex- cm quartzites, intensively cut by joints;
posed. A gravitational sliding along the hill which occurs steeply N-dipping. in this 170 of cobbles and boulders with location, probably causes a duplication of the cm angular quartzite
contact in this between them the of the rocks. exposure. Both the Paleozoic and joint pattern underlying
Mesozoic rocks seem to have been folded into an over- ? cm of silts and clays belonging to the Voznuevo For-
turned position. It is a striking feature that the mation.
Stephanian in this location shows an abundance of The 170 cm thick unit with cobbles and boulders may quartzites (as conglomerates), while the quartz/ have originated: quartzite/lydite ratio in the Voznuevo Formation within
10 metres from the contact (cf. Chapter VI, Table II) 1. during a period of non-deposition between the Car- shows that 80% boniferous and over of the pebbles consist of quartz. the Cretaceous. Between these 2. after ten metres only clays and silts were found deposition of the Cretaceous, at the contact exposed, so that a lithological connection between the between consolidated and hardly consolidated sedi-
Voznuevo Formation and the underlying rocks does not ments. seem very likely. We must assume that at least slight 3. as a result of a combination of 1 and 2. fault movements took in this location. The best place exposed contact W of Boñar is the one in the
The contact W of Sorribos de Alba(Section E) is much arroyo N of La Valcueva which is presented in Fig. 24. better exposed (Fig. 23). Again Stephanian conglome- From left to right the following units could be dis- the rates outcrop at contact. The situation is as follows: tinguished:
f. consolidated Voznuevo sand.
with closely packed quartzite e. 40 cm of boulders, maximum diameter 3.50 of quartzite с m red clay 40 cobbles and boulders, over cm. b. 3.50 m of white silt d. 60 of 45 cm in diameter. cm partly consolidated Voznuevo sand. 289
20 of with it has been found so as in с. cm quartzite cobbles, poorly rounded, a although never conspicuously described above. maximum diameterof 25 cm. the exposure
In the southern between La b. 20 cm of red clay strip no exposures occur with Devesa de Boñar and La Ercina. Strike and a. Paleozoic quartzites in beds of 12 cm thickness, dip measure- in excavation in the of La Ercina a dip of approximately 50° to the north. ments an village proved be within short distance. Units b to f are vertical to steeply N-dipping. to highly diverging a They this The quartzite cobbles and boulders cannot have suggest that tectonic movements have been active in
derived directly from the underlying thinly bedded location. reduced thickness of the Voznuevo Formation N quartzites; yet the source area of the poorly rounded The makes It cobbles and boulders could have been very nearby, of Yugueros (Section O) faulting very probable. established whether this fault presumably the area which nowadays forms the Canta- could not be runs along elsewhere the both. At brian Mountains (cf. Chapter V, A; Section A). The the contact or in formation, or of five between the Paleozoic and the 20 cm red clay can be seen as a weathering product least metres
underlying Paleozoic deposits; it has been completely Voznuevo Formation are not exposed. the southeastern side of the homogenized and does not show any stratification. The In excavation on the soil of Cistiema clear indications found of red clay can be seen as a post-Carboniferous which village are one or
originated before the deposition of the Cretaceous. Also more faults; less than 60 metres are all that is left at the found without surface of the total thickness of the formation. At in this location a plain unconformity is some
hundreds of metres SE of this excavation, between any sign of a fault contact. encountered of Cistierna and the brick NW of By far the best exposed contact was N factory Valmartino,
the village of Voznuevo (Figs. 25 to 28), at both sides of twenty centimetres of very distorted black and grey
the Arroyo de Voznuevo. Here orthoquartzites of the clays and sands form the transition from Paleozoic lime-
Barrios Formation (Ordovician), which have been folded stones to the Voznuevo Formation. Fault movements 35° this into an overturned position and dip to the NW, are have apparently taken place along clayey horizon
covered by sedimentsof the Voznuevo Formation which which must have acted as a sliding plane. Opposite the
dip 27° to the SW. The latter, as could frequently be village of Valmartino the Paleozoic probably par- 40 thick limestone observed between Boñar and Colle, are more or less con- ticipated in the faulting, since a cm of breccia overlain 70 of white brown solidated. The unconformity is beautifully exposed W by cm to light clay,
the Arroyo de Voznuevo (Fig. 25, looking W). In this with a considerable quantity of limestone fragments, between with found the location no soil formation has been preserved together quartz pebbles, was near con-
the Paleozoic and the Cretaceous; the loosely con- tact. About 200 E of the border between the solidated gravels and sands of the Voznuevo Formation m provinces and 30 of km 35 of the immediately overlie the quartzites. The contact can even of León Palencia, m E recently constructed road from Puente be studied in hand specimens: in Fig. 26 the rare situa- Almuhey to Guardo, a sand small the northern side of the road tion is shown in which cross-stratified coarse is exposure on was
found which showed deposited on the unconformity surface. The paleo- (from top to bottom):
400 of from via coarse current direction indicates a supply from the SW, which с cm grading pebbles to clay coloured. coincides with the paleocurrent directions found in sand, fine sand and silt, yellow and white Imbrication b. 80 of silt and for the most channels in this area (Chapter V, B). in cm originally grey clay,
from the also to red discolouration. The is conglomerates within one metre contact part showing a purple top from the The coloured and contains and crushed indicates a supply SW. unconformity sur- lighter quartz pebbles
face itself, which is magnificently exposed in some loca- quartzites.
has ? of white silt and black with coal tions, a slightly undulating surface; locally remain- a. cm clay fragments
ders of consolidated Voznuevo material have been and yellowish sand layers.
Units and b the preserved on it (Fig. 27). It is strange to realize that the a possibly belong to Carboniferous,
old this form for the while unit с might represent the first Cretaceous erosion plane today reappears in second time. Another phenomenon which could be deposits. In this case unit b can be considered as a paleo-
sol with an eluvial zone at the There are no indicar observed on the unconformity surface can be seen in top. tions of this Fig. 28: a crack or fissure in the Barrios Formation has faulting near contact. until Santibañez de la Peña been filled with sand and gravel from the Voznuevo For- The contact is not exposed
mation. where, at the bottom of Section R, the following could
be observed (top to bottom):
to The composition of the pebbles in the conglomerates с 300 cm of yellow and pink sands belonging the strongly depends upon grain size: 62% of the pebbles Voznuevo Formation. of shales with coarser than 3 cm in diameter proved to be quartzite; in b. approximately 10 cm clayey purple to the sediment finer than 3 cm in diameter90% consist of red weathering colours. Frequent slickensides.
dark and shales quartz. a. grey sandy silty (Carboniferous). of and somewhat The unconformity as encountered N Voznuevo con- The contact between b с seems un- tinues along the entire border between Boñar and Colle, dulating. Although indications are present of small fault 290
movements (slickensides), their influence probably was directly overlain by sediments of the Vegaquemada
red shales of b could — not very great. The clayey unit as Formation (Fig. 29). Although the Cretaceous deposits
the — be the of fossil soil. in preceding case remnant a proved to be considerably weathered and have been dis- reduction thickness of the Voznuevo Formation The in turbed by recent vegetation, one can distinguish, from Santibañez de la Peña need be near not explained by top to bottom:
faulting along the contact, since clear indications could h. coarse-grained sand, brick-red with white speckles be observed offaulting within the section. (quartz grains).
North of the of Aviñante, the road to 2 as h, colour: lilac to village along g. cm in purple.
Villafria, a is The f. 7 of fine sand, rich muscovite, plain unconformity exposed. following cm bluish-grey very in units can be distinguished: with mm-thick red to purple clay stringers, especially at
d. silts and clays. the top.
с 14 m of fine-grained sand, partly consolidated by e. 4 cm of white silt with dark yellow weathering
iron oxides. colour.
b. about 80 cm of quartzite conglomerate; maximum d. 2 cm of white silt.
diameters of the cobbles 16 cm, nearly all broken. The с 12 cm as in b.
matrix consists of broken quartzite. b. 15 cm of silt, pink to brick-red, with clay at the top,
a. Paleozoic graywackes and black shales. dark purple and grey. Lower part: fine-grained sand.
The conglomerate mentioned under b seems to be a a. more than 150 cm of sand and clay, strongly
basal conglomerate of the Voznuevo Formation. The weathered. The coarser the grain size, the lighter the
same situation is encountered N of the village of Villa- colour.
verde where the conglomerate, which again consists Unit h shows the typical colour, peculiar of the of has reached entirely quartzite components, a thick- Vegaquemada Formation, while unit f can be compared
ness of 300 cm (maximum diameter of the boulders 23 with the sediments of the Voznuevo Formation. Unit g,
to cm). the very thin transition zone, belonging lithologically The basal also the has been discoloured due conglomerate proved to occur at the Vegaquemada Formation,
base of Section S between Avifiante and Villaverde, but to its position near the boundary. The contact is straight far could be exposure was rather poor. (as as determined in this exposure) and best of hiatus The angular unconformity is seen along the road gradual, no signs a in the sedimentary history
from Cervera de Pisuerga to Vado de Cervera where the being present.
conglomerate immediately overlies alternating sand- Very gradual transitions from the Voznuevo Forma-
stones and shales of Paleozoic age (cf. Kanis, 1956; Fig. tion to the Mesozoic limestones could be observed NW
14). In this location the conglomerate has reached a of Brugos de Fenar, E of Boflar, S of La Devesa de
thickness of 10 m. In contrast to the conglomerate Boñar, W of Muñeca and in Section T.
further to the W, thin intercalations of fine-grained In the field the first appearance of lime in the sands material do occur (see Section T). was taken as a criterion of separation. Observations in
thin sections from the transitional zone will be described
It find is surprising to that, in spite of strong movements in Chapter VII. along the border zone which have been able in many locations to push the Cretaceous sediments into vertical and overturned positions, and in spite of a considerable SUMMARY throw along a fault as assumed deep-seated by geo- physical the between the Paleo- survey, unconformity Retaining some main points of this chapter, we may say zoic and the Cretaceous has been preserved at so many that the sediments of the Voznuevo Formation have
The where fault could be influenced contacts. only area a contact been strongly by a tectonic uplift, at the end ascertained lies between Yugueros and Quintana de la of the Paleogene (Savian phase), of the Hercynian massif
Peña; slight fault movements probably took place near which now forms the Cantabrian Mountains. Carrocera and Santibañez de la Peña. In many locations the Voznuevo Formation has been
dragged into an overturned position. Where strong com-
pression has been active, the formation may be folded or
THE CONTACTS WITH THE OVERLYING ROCKS faulted; in extreme cases pulverization of the sediments took place.
for differences in Except one location, all contacts between "the During a geophysical survey density Voznuevo Formation and the overlying deposits suffer were found which led to the assumption of a dèep-seated from a very bad degree of exposure. The only contact fault with a throw of about 2 km between the Paleozoic which could be studied in greater detail is situated at and the Cretaceous. With the exception of results ob- km 3 on the northern side of the road Otero de las tained in the Porma valley, the fault plane was found to
— La Robla. this Dueñas In location a proof was found be dipping to the north. of the disappearance (by wedging out) of the Mesozoic Although fault movements near the contact are almost limestones (Chapter IV), the Voznuevo Formation being certainly to be expected in the subsurface, examination 291
of the contacts in the field showed that by far the most tion of the contact south of Carrocera, where the
contacts are unconformable and have not been in- Voznuevo Formation is in direct contact with the
No fault between these fluenced by strong faulting. Only between Yugueros and Vegaquemada Formation. two formations could be that the idea of fault Puente Almuhey, especially near Cistierna, has intensive observed, so a
faulting along the contact taken place. which might be the reason for the disappearance of the be The contact with the overlying Mesozoic limestones Mesozoic limestones in a westerly direction must
to be with the abandoned. proved very gradual everywhere, excep-
CHAPTER IV
STRATIGRAPHY
1. A northern with and or THE LOWER CRETACEOUS IN THE BASCO-CANTABRIAN zone, sandy marly lagoonal
AREA limnic and marine deposits. and 2. A southern zone, exclusively laguno-lacustrine
continental southern border of the Cantabrian The very comprehensive work by Ciry (1939) is ofgreat (the value of this to anyone studying Jurassic or Cretaceous Mountains is an extension zone).
sediments in the Basco-Cantabrian area. The reader is 3. A zone in which a transition from 1 to 2 is found. referred to him for a complete list of previous work of the northern carried out by investigators of these sediments. We shall Sections through the Cretaceous zone the of the of and confine ourselves here to a few remarks on the history of near boundary provinces Burgos Santander the the investigations in the area between Riello and Cervera give following general picture:
de Pisuerga. This area has been separately dealt with by border' Cenomanian Ciry, who referred to it as 'the southern
(bordure méridionale) of the Asturian Massif. 'Sus-Aptien' sandy complex, barren
As as the middle of the nineteenth the sands with Orbitohnas early century, с j ,. . , Bedouhan, Gargasian ( 1 , •„ ~ , . . sediments between Boñar and Colle had been recognized marls with Cephalopods
= in as belonging to the Cretaceous (Casiano de Prado, 1850; Neocomian, Barremian Wealden sandy complex
Pratt, 1850). These authors distinguished kaolinitic Wealden facies sands and more or less consolidated conglomerates, over- of lain by limestones and sands, both fossiliferous. By way simplification, we may say that the Lower Carrión Cretaceous of the northern of Oriol (1876a and b), who investigated the Río zone consists marine
Basin, assigned the kaolinitic sands to the Cenomanian deposits flanked by two mainly sandy deposits in and the overlying limestones to the Turonian. Wealden facies. Some later both kaolinitic years (Mallada, 1891), the sands and the limestones were considered to be In the southern zone the situation is different, and two
Turonian. Except for some short remarks by Mallada sequences can be distinguished (according to Ciry): a
(1900) and by Termier (1918), the southern border was lower sequence, of moderate thickness with lacustrine
of interest until when limestones and red' and not a subject 1933, Ciry published 'bright deposits, an upper some preliminary notes (Ciry, 1933a, b and c) on the sequence, with a maximum thickness of more than 1000 area which is roughly bounded by the triangle San- metres, consisting of exclusively detrital sediments, tander-Burgos-Boñar. mainly sands and conglomerates. Studying the contact with In 1934 Karrenberg reported on an area between the underlying sediments it became clear that the
two Bilbao, Burgos and Oviedo, covering the whole of Ciry's sequences are not present everywhere; locally the be direct area except for the southern border west of Aviñante. As upper sequence may in contact with pre- for the Upper Cretaceous, the Upper Cenomanian is Cretaceous deposits. According to Ciry, the absence of the lower is the result of differences found to be transgressive over the older deposits. The sequence not in kaolinitic sands of the southern border zone were sedimentation, but is due to abrasion between the thought to belong to it. deposition of the two sequences. These sediments of the
Ciry (1936) remarked that the Upper Cretaceous is upper group have also been referred to as 'Wealden', transgressive over the old Asturian Massif. The transgres- with the restriction that the entire Lower Cretaceous
to be sion starts in the E in the Cenomanianand continues may meant by this name, and not only the the W in an uninterrupted manner during the following Neocomian and Barremian. The name 'Wealden' there- stages. The maximum extension, as far as his study is fore has no timestratigraphical value and is only usable reached for concerned, is in the Santonian. the description of a facies type. Apart from some
The Cretaceous, as wood the Lower investigated by Ciry (1939) remains, upper sequence never yielded any in of the of and fossils. Columns 3 7 5 show parts provinces Santander, Burgos León, to in Fig. a simplification of can be divided into three zones: Ciry's schematic picture of the variations in facies in the 292
Lower Cretaceous, from NE to SW with corrections by
Rat (1962).
Summarizing Ciry's comments on the paleogeo- of graphical development the Lower Cretaceous, we
retain that after deposition of Callovian sediments a long of period stability set in during which no sediments have formed appear to on the continent. Influence by
subdesert-like climatic conditions led to the generation of hamada*-like red sediments. These found are again in
the first Cretaceous deposits as red clays and polygenetic pudding stones.** With the beginning of the Lower
Cretaceous, a transgression set in and a basin was formed
in the northern zone in which sandy detrital sediments
were deposited. This situation lasted until the Aptian. In the meantime, the subsidence of the basin was less
important in the southern zone, where first red clays and later lacustrine limestones were deposited. With the
ingression of the Bedoulian sea, which entered the area from the north, marine sedimentation took place in the
while continental basin in the south. 4. Schematic of the S of Santander, north, a originated Fig. geographical map area Almela and showing the position of the section in Fig. 5 (simplified after Ríos, Garrido (1945) gave a description of Rat, 1962). the Cretaceous in the area between Bilbao and Miranda
de Ebro, consisting of parts of the provinces of Burgos
and Alava, and to the north of small parts of the provin-
ces of Santander and Vizcaya. The Lower Cretaceous Neither does Rfos (1956), in another summarizing
deposits attain extraordinary thicknesses, from 5000 m article on the Cretaceous of the Pyrenees (« Pyrenees s.
to + in the north 1000 m in the south. There are no indica- s. Basco-Cantabrian Pyrenees), contribute many new of of values facts the tions an exaggeration these due to faulting to comprehension of our Voznuevo Formation.
or folding. The character is mainly continental, detrital The sediments of the southern border are indicated as
with carbonaceous intercalations. In the central part of being of the Iberian type, which is equivalent to Ciry's
the these found to Jurassic area deposits were cover southern zone. On a schematic map, on which the rocks. The limit upper was defined, as in many other Cretaceous along the northern and the southern borders
areas, by Orbitolinas. As a consequence of the thickness of the Asturian Massif is shown, the deposits of the area of the deposits, the authors believe all stages of the between Riello and Cervera de Pisuerga are included in Lower Cretaceous be to present (Wealden s. s. to the Upper Cretaceous. In the accompanying text, Rfos On the basis of the Albian). investigations by Ciry in notes that they may belong to the uppermost Lower of Santander fossils of provinces on the marine inter- Cretaceous or to the lowermost Upper Cretaceous. calations, this hypothesis seems correct. In the southern A very comprehensive work on Cretaceous sediments detrital sediments part exclusively are found, changing in the Basco-Cantabrian area, comparable to that by into towards the more clayey counterparts north; we see Ciry, was published by Rat (1959). The area studied by
that the situation is highly comparable to that in the him lies between San Sebastian and Santander and is
region studied by Ciry. One of the most important limited in the north by the Bay of Biscay and in the remarks of the authors the of the concerns origin con- south by the town of Vitoria. The essence of his in-
tinental sediments (p. 110): 'Hacia el Norte (read: Sur) vestigation is the marine Aptian — Lower Albian, the
nos al área elevada de denudación be acercamos que Urgonian Complex. This Urgonian Complex can seen suministraba el material de la red provinente probablemente as a marine wedge which separates the clayey and
masa granítica castellana (cuarzo blanco, abundancia de sandy complex in Wealden facies into two (Fig. 5). The The discussions these authors the of sediments of mica)'. by on strati- overlying deposits, which the the graphical conceptions by Schriel (1930), Sáenz (1933), southern border zone of the Cantabrian Mountains form
Karrenberg (1934) and Ciry (1939) does not shed any part, have been referred to as: 'sus-Aptien' or super- new light upon problems concerning our area. Aptian sands, (Ciry, 1939) or supra-Urgonian Complex (Rat, 1959). the between Callovian and three * In time Aptian, areas Schieferdecker (1959): hamada = rocky desert, rocky uplands
with a different of a desert, which have been swept clear of sand and dust by clearly sedimentological development
the is wind; rocky surface usually covered by coarse rock frag- can be distinguished: ments.
western formed eastern of the ** 1. A region, by the part stone term Pudding is a sometimes used for conglomerates of Santander, where calcareous, sandy beds of in which the matrix is preponderant (French: poudingue; province Spanish: Berriasian have been of marine pudinga). age deposited on top 293
Fig. 5. Correlations in the Lower Cretaceous S of Santander (simplified after Ciry, 1939 and Rat, 1962). For location of the section see
Fig. 4.
coloured Callovian. They are overlain by Valanginian limestones vividly erosion products originated on the land covered which later be carried and the which are in their turn by vividly coloured, were to away form mainly red fresh water sediments in Wealden facies. sediments of the Wealden facies. 2. central from where black 2. Return of the marine influence A part, near Bilbao, clays (Valanginian? ) with with brackish water features have been reported. neritic marine conditions and deposition of sandy lime-
Evaporites occur locally. stones and oolites.
3. An eastern part, formed by the province of 3. Active terrigenous sedimentation (Wealden s. s.) of material derived from the Guipúzcoa, in which exclusively limestones have been Asturian and the Castilian con- The has withdrawn formed (e. g. near Tolosa). tinents. sea towards the zone of Bilbao and Tolosa.
The correlation between these three areas as proposed by Rat (1959) is presented in Fig. 6, columns j, к and 1. In this connection it is worth noting that Rat proposed
The paleogeographical history in Rat's region is well changes in climatic conditions (apart from tectonic the for the differences comparable to the history further to the west as given by events) as main reason during
is Ciry. In Jurassic times fine-grained marly to calcareous these three periods. This process, which dealt with in a
called 'climatic sediments, associated with a stable flattened continent separate publication (Rat, 1963) is
(the Hercynian Massifs and the Meseta), were deposited marine transgressions and regressions'. in the Basco-Cantabrian area. The Callovian uplifts In the Urgonian (Aptian - Lower Albian) the coastline formed caused an increase in erosion and a reduction of the shifted towards the continent (still by the marine territory; continental sedimentation took place. Asturian and the Pyrenean Massifs and the Meseta). In
Until the return of marine conditions Rat distinguished the north of Old Castile the sedimentation in Wealden three periods: facies continued; the transition zone between the
terrigenous sediments and those of the marine Basco-
1. Emersion phase or brackish regimen (post-Callovian- Cantabrian gulf lies, as we have seen from the descrip-
Purbeckian) with the uplift of a land mass related to the tion by Ciry, in the province of Santander.
Asturian Massif and the Meseta together with the genera- The sandy supra-Urgonian Complex, comprising tion of the Basco-Cantabrian gulf. In the meantime sediments deposited before the Cenomanian transgres- 294
Cretaceous in northern Fig. 6. Approximate correlation between the Lower deposits Spain and the Lower Cretaceous in other parts of 1959 1962 1962 Europe (composed after data by Rat, (a, c, d, i, j, k, l); Rat, (a, c, d, e, f, g, h); Bartenstein, (a, b, c); Kneuper-Haack, 1969 1966 (b, c, d, m) and Rat & Salomon, (e).
- of Carboniferous sion (Upper Albian Cenomanian), is an extension Urgonian complex rests directly upon which still took the Wealden facies sedimentation place rocks; a 2 to 3 m thick, probably reworked, wine-red, in the southwestern part of Rat's area. The sandy and sandy and clayey unit is the only witness of the Wealden direc- clayey material was provided from southwesterly facies complex. north- tions, since it clearly decreases in grain size in a New facts on the position of Ciry's lower and upper
forward in recent Rat easterly direction. sequence are brought a paper by Three returned the and Salomon in connection with finds years later, Rat (1962) to problems (1969) new near connected with the Lower Cretaceous in the eastern part Vega de la Pas. Already in 1962, Rat noted the presence of limestones with Trocholinae and between of the province of Santander. In order to facilitate orien- Bryozoans outline of the the two 5; sections 2 and A of tation, a strongly simplified geographical sequences (Fig. 3). study of this limestone N of de la Pas situation is given in Fig. 4. In the southwestern corner of new exposures Vega showed that the transition from the lower the sketch the easternmost location can be seen of the sequence to
the limestones is due lateral It could area studied at present, Cervera de Pisuerga. The loca- to rapid changes. of be established whether these caused tions near the line А—В correspond to the sections not changes are by a
Fig. 5, simplified after Rat (1962). Sections 3 to 7 of difference in distribution of the marine sediments or by 5 almost identical the correlations differential erosion before of the Fig. are to proposed deposition upper of black shales by Ciry (1939; Fig. 73). The correlations introduced by sequence. The discovery of a horizon
Rat form the main part of his publication: they concern near the base of the upper sequence is more important. of which the correlation of Ciry's lower sequence (bottom These shales supplied Ostracods (Gomphocythere) sections 7 and 8) with section 3. Ciry had provisionally could be correlated to Wealden 4 of Germany (cf. Fig. of the connected his lower sequence to the Urgonian complex 6). As for the stratigraphical position boundary be said: (Bedoulian). between Ciry's two sequences, the following can
The ostracods (Iberina) in the beds directly overlying The fresh water and brackish water sediments of the the and marls Ramales of Jurassic limestones near (section lower sequence can be correlated to the Serpula zone 2, Fig. 5) could be correlated to the Serpulit of NW Germany and the Middle Purbeck of England (Rat,
Germany and the Middle Purbeck of SE England (cf. 1962). The sedimentationof the sandy upper sequence
New and in- the the base of Fig. 6). paleontological stratigraphical began as early as Berriasian, i.e. at very vestigations near Barcena de Ebro and Reocin de los the Cretaceous. The question whether the upper
Molinos to that the beds of the lower to the brought light sequence is continuous up Urgonian complex or
must be correlated to the Iberina beds of not still sequence is open. Ramales.
Considering the situation near the border of the THE SEDIMENTS IN WEALDEN FACIES IN OTHER PARTS of Asturian Massif, it was found that some 12m OF SPAIN lacustrine limestones occur between Jurassic and sands and in Wealden facies S of of that clays near Cabuerniga; Saja In anticipation an assumption by Ciry (1939) the (cf. Fig. 4) the latter directly overlie the marine sediments of the Voznuevo Formation do not have a Callovian. the of the the In a section in valley Rio Deva diachronic character due to their homogeneous appear- 295
believed to have commenced much towards the anee, it is the object of the following part of this chapter later, probably beginning of the to show that sediments in Wealden facies, reported from Aptian. From the northern part of the area two sections through pre- other regions in Spain, are distributed over a consider- Aptian sediments are described, deposited in Wealden facies able time interval, during the entire Lower Cretaceous the litho- some units of which show a strong resemblance to
and even during the early Upper Cretaceous. logical properties of our Voznuevo Formation as for instance
near Ahillas where '50 m de arenas caoliníferas sueltas, rojizas y cuarcita' Almela (1956) made a study of Cretaceous deposits in an area blancas, con cantos en la base de cuarzo lechoso y are
south of the Ebro Basin, bounded in the east by the Mediter- exposed (p. 222). The other beds described are less well com-
coast ranean near Barcelona and in the west by the meridian of parable, since they show many limestone and marl intercalations.
north this the Teruel. In the of area lignitic layers in deposits of One remark by the investigators concerning above mentioned
the attention Albian age have been object of mining activities, near Utrillas unit merits our special (p. 222).
amongst other places. These deposits are also reported from The facies of these 50 m is identical to that of the Utrillas Beds
other areas and have been referred to as 'Utrillas Beds' (Utrillas of Upper Albian age, yet according to their position they must
Schichten, Capas de Utrillas). For the most part the Cretaceous be older than Aptian. As no distinction can be made between
of these sediments and the Utrillas the unconformably overlies slightly folded rocks Jurassic age; Beds, stratigraphical position
where the Jurassic is absent (in the northern and the western of the latter (of which we know that they normally directly
underlie the when the of parts) the Cretaceous rests upon Triassic. The following deposi- Cenomanian) becomes uncertain, age the beds tional history was presented by Almela: towards the end of the surrounding them cannot be fixed with certainty.
Jurassic period, after the Kimmcridgian folding phase, a con- As a résumé, the authors mention that the Wealden facies
lacustrine tinental basin, bounded by the Ebro Massif in the north and by a represents conditions of fluviomarine or sedimentation
of Cordillera Catalana with times of littoral-marine sedimentation. spur the in the east, came into existence. non-deposition or
Continental deposits originated with a thickness of 200 m in the During the Aptian exclusively marine sedimentation took place,
the Albian. north and 50 m in the east. These sediments were deposited in showing a uniformity which lasted until In Albian
what facies'. moment of of times loose in is called a 'Wealden The cessation deposits consisting of sand Utrillas facies were
this sedimentation is hard to determine, since paleontological formed in isolated areas, apart from bathyal, neritic and littoral
is but to marine trans- is evidence lacking, according Almela a sediments. This sandy Utrillas facies best developed and
gression took place towards the end of the Neocomian reaches its largest extension in the northern part of the area.
(=Valanginian + Hauterivian) or the beginning of the Barremian Where both marine and continental Albian sediments are
this (=Urgonian), and the Urgo-Aptian sea began to rule over present, the latter will always be at a higher stratigraphical
area. Its neritic deposits attain a thickness of between 400 m and position since they precede the Cenomanian transgression.
With the Austrian took the 1000 m. first phase a regression place, Llopis Liado (1956) investigated Cretaceous in the sur-
continental which the westernmost and a new basin was formed in which mainly sandy roundings of Oviedo, are sediments of
of Albian with this north of the Cantabrian mountain chain. From well- deposits age (Utrillas Beds) were deposited a age contacts it could be deduced that maximum thickness of 600 m. The second Austrian phase finally exposed with the Paleozoic a
had before of Creta- was followed by the Cenomanian transgression in this area, peneplanation taken place deposition the
which definitely ended continental Cretaceous sedimentation. ceous. The Cretaceous does not always overlie Paleozoic rocks,
with Triassic Martínez Peña (1956) carried out investigations in an area little however; in several places a contact or Jurassic
could be of which was as yet known, viz. a part of the provinces of rocks observed. The area has been subdivided into three
Guadalajara, Ciudad Real and Cuenca. A well exposed road sec- parts:
1. tion between Mota del Cuervo and Belmonte shows (p. 168): Oviedo zone.
de с Arenas blancas y amarillas, areniscas blancas silíceas, duras, 2. Pola Siero zone, E of Oviedo.
blancas Arenas Aviles con esquistos de areniscas y duras. cuarzosas 3. zone, NNW of Oviedo.
From the sections the Cretaceous blancas y vinosas con granos de cuarzo. Arcillas rojas y violadas Oviedo zone several through the author. suelto rodado de are presented by In most of these sections a con grano y cuarzo. b. 15-20 thick found el,ow sand in Calizas rojovinosas con lentejones de areniscas silíceas. m unit was showing: 'y ' srl
facies with cross-stratification' far cross-stratifica- Calizas y carniolas irregulares, rojas, grises y violadas, con grano Wealden (so
tion has seldom been other de cuarzo. reported by any Spanish author).
a. Caliza gris, grisvioladas, cristalinas, finas, con Pentacrinus e According to Llopis Liado the Oviedo zone can be subdivided
Isocrinus. into three units (from top to bottom):
I: alternations marls and According to Martínez Peña, unit a belongs to the Liassic, Unit 145-150 m of of sandstones,
with while units b and с belong to the Eocretacéo (Lower Creta- sands, predominantly detrital, a marked continental
ceous), because of their position and their undeniable facies; unit character. Only in the lower part do limestones occur with a
b represents the 'Wealden-Aptian facies', unit с the typical small vertical extension and a marine character.
II: and Albian. In the western part of the area the combined thickness Unit 45-50 m of alternations of sands, marls limestones,
is intercalations. of units b and с very constant, 30-40 m, consisting mainly of mainly marine, with frequent detrital, continental
the varies unit с In eastern part the thickness from 40 to 100 m. Unit III: 25-30 m of molasses and sandy marls with a littoral
Unit с proved to be easily traceable, since 'el Albiense es siempre character.
arenisco de vivos The over- increase in limestone un grupo y silíceo, tonos y alegres'. It is clear that a progressive components all I I lying Cenomanian and younger deposits consist of an alterna- takes place from to II. In unit quartzite conglomerates occur and tion of limestones marls. with median diameters of the components between 10 and
de Lome and Sánchez Lozano described Creta- than coarsest in the Dupuy (1956) 60 cm (much coarser the sediments found
ceous sediments in the 'Levante Español', a 100 km broad strip area studied by the present author! ). These conglomeratesgrade northern of this between Valencia and Alicante. Only in the part into coarse-grained and fine-grained sands. They contain lenses
area have typical Wealden facies deposits developed. They are of kaolinite and of clays sometimes rich in lignite. Llopis Liado
not entirely continental and several marine intercalations occur. also realizes that the name 'Wealden' is not to be used here in a
not The lower boundary of these sediments could be studied in chronostratigraphical sense, but as a description of a certain type
detail with to its the observation of facies. On of the respect age; only following account frequent cross-stratification, rapid
could be made: changes in the size of quartz elements, and the clay lenses, Where the Cretaceous overlies Jurassic rocks, deposition began especially those containing ligniferous beds, it is almost certain
times (Neocomian); where the substratum is in early Cretaceous that we are dealing with continental deposits. formed by Triassic rocks ('Suprakeuper') the sedimentation is Unit II forms a transition zone to the littoral sediments of unit 296
is shown its Ill, which belong to the Upper Cretaceous as by 3. Urbión Beds
fossil As the indications content. to stratigraphical positions, Wealden 2. Oncala Beds were only found in unit III, in which an abundant Turonian 1. Тега Beds to to with strati' fauna proved occur. According a correlation Jurassic graphical determinations by Karrenberg (1934), the upper part
of unit II must belong to the Upper Bedoulian; a Lower Beds 1 and 2 are both limnic-fluvial sediments in
is the lower Bedoulian age probable for part ofunit II, while the which limestone intercalations such as lagoonal deposits sands and pudding stones in Wealden facies have not been dated, are very common. The Urbión Beds and the Utrillas Beds and therefore from the may occupy a stratigraphical position have so features in common with the sediments of Valanginianto the Aptian. many
the second unit I our Voznuevo Formation that deserve some In zone (E of Oviedo), proved to be entirely they special
continental while unit II, on the contrary, is more marine;which attention:
indicates differences in sedimentary conditions between E and The Urbión Beds resemble the two older beds, apart W. from the absence of limestone intercalations. As in beds In the third zone, near Aviles, the quartzose pudding stones in 1 and 2, sediment as deduced from measure- Wealden facies attain considerable thicknesses (the conglomerate transport ments on cross-stratification and decrease at the base measures more than 80 m). A division into two units in grain size
can be made have been from here, the lowermost sediments having mainly a must S to SW. The complex begins with detrital character (176 m thick), comparable to units I and II in conglomerates which clearly separate this unit from the the and area near Oviedo, stratigraphically to be located in the Oncala Beds. The components are well-rounded and have Aptian (or lower), and the uppermost sediments (90 m thick), a uniform 73% 25% belonging to the Cenomanian-Turonian (comparable with unit very composition: quartz, quartzite and 2% III). lydite. The quartz is glass-like and milky; the
In the between Luanco and Candas, exposures near coast at quartzites grey and brownish. Deviations from this least 20 of sand and dark overlie the lime- m yellowish clays locations pattern only occur in where very coarse gravel stones of Gargasian age. It is clear that the marine Aptian layers has been deposited; in that case consist of of Luanco are situated between two detrital layers, both they mainly
As On of these developed into a Wealden facies. was shown before, the quartzite. top conglomerates 'quartz sand-
'Wealden' have strati- (facies) developed in a much higher stones may containing feldspars' occur, sometimes with graphical position than in the 'Wealden s. s.' as used in Spain pebbly intercalations. Towards the northeast of the area (=Valanginian + Hauterivian + Barremian). the feldspar content decreases. Coal layers rich in pyrite, Summarizing we may say that there is an increase in thickness
W dark coloured from from to E, together with a change in facies from mainly accompanied by clays are reported
continental to mainly marine. In the eastern of the area several locations. As seatearths could part no be observed, Liado that studied by Llopis we see a complete sedimentary these were probably formed by floating plant remains. cycle takes place between the basal sediments in Wealden facies According to Beuther these Urbión Beds must be con- the and sediments on top of the Gargasian (also developed in a sidered fluvial of Wealden facies). Llopis Liado summarizes the paleogeographical as exclusively deposits in view their
events as follows: characteristics.
The Utrillas Beds, formerly regarded as belonging to with 1. Aptian transgression marine sediments near Luanco and the Cenomanian, to be of Albian proved Upper age in continental or paracontinental sedimentation near Oviedo and view of their and charac- Siero. paleontological stratigraphical teristics. overlie the Wealden 2. Limit (of the greatest extension) of the Aptian transgression They unconformably - sediments. The in the Upper Bedoulian Gargasian; marine limestones near lithological description of the Utrillas
Luanco, marls near Siero and limestones near Oviedo. Beds strongly resembles that of the Urbión Beds. The 3. Gargasian - Albian regression, deposition of the 'upper' of composition the conglomerates with 65—68% quartz, sands near Luanco. 30-35% and 5% of well- 4. Continental Albian, maximum extension of the regression. quartzites lydite, consisting
5. Cenomanian - Turonian transgression. rounded components, is almost identical, as is the
presence of coal rich in pyrite, carbonized wood and the the Alastrué (1956) reports on Cretaceous of Andalusian chopped plant remains. The paleocurrent direction from Cordillera Bética. Lower and Both Upper Cretaceous are entirely S to SW is also identical. The Utrillas Beds consist marine in higher this part of Spain, so that a comparison with our of alternations of fine to sometimes Voznuevo sediments cannot be made. coarse-grained, rather angular micaceous quartz sands with clays. The
maximum diameter of the pebbles in these sands is
Investigations by German geologists in the north- 16 cm. The quartz sands have white-grey to purplish-red
western Iberian Chain north of Soria - the so-called colours. The clays can be greyish-purple, dark red or
Sierra de los Cameros — with dark brown which began in 1953, were grey iron concretions. Grey coloured,
summarized in 1966 (Der Jura und Wealden in Nordost leafy claystones occur locally. In the upper part of the
of moderate lateral extension and Spanien Beuther, Dahm, Kneuper-Haack, Mensink, beds, lignite layers a maximum thickness of Near Tischer). 1 m may be present. the The of western part of the Sierra was investigated by village Fuentetoba, oil impregnations cause the forma- Beuther. The sediments between marine Jurassic and tion of this asphaltic layers; occurrence is, however, very marine Upper Cretaceous deposits have been subdivided local. The top of the Utrillas Beds is formed by greenish- into: brown sandy to pebbly calcareous oyster-beds with a Upper Cretaceous maximum thickness of 3 metres; they are often thinner Utrillas Beds or altogether absent. These beds mark the beginning of 297
the marine Upper Cretaceous. The author finally notes and one incomplete deltaic cycle, while in the area that the total thickness of the Utrillas Beds ranges described by Tischer two complete and one incomplete
have This difference is caused a between 300 and 350 m, with a maximum of 380 m W cycle developed. by
of Soria. gradual displacement of the deltaic system in a northerly
of the Voznuevo Formation direction. In view a comparison with
of the of a deltaic mechanism is sediments we shall now consider the source area and the One strongest 'proofs' fossils. mechanism of deposition, applicable to the Urbión Beds the presence of fresh-water and brackish water A the ostracod fauna of the and the Utrillas Beds. Following Beuther's train of paleontological study on
Wealden sediments was carried out thought we note: by Kneuper-Haack. Among the ostracods only brackish water types do
occur. In the column (m) of 6 the strati- 1. The composition of the gravel and the sand indicate right Fig. results of this are a derivation from Paleozoic and crystalline rocks. graphical paleontological investigation after the 2. A paleocurrent direction from S to SW. shown. In contrast to the sedimentation transi- tion from the Oxfordian the in the 3. A short distance of transport, concluded from the to Kimmeridgian intercalations found material involved. remainder of Europe, no marine are with the of Wealden in this area. Compared age type
sediments in other of Spain, the continental The source area of the Urbión Beds, as for the two parts
the Sierra de los Cameros set in at other Wealden Beds, is thought to be situated NW and N deposition in a very time and ceased after the transition from the of the present Sierra de Guadarrama, N of Madrid. A early just Jurassic to the Cretaceous. source area further to the E can be left out of account, Quintero Amador and Molina (1956), who since Jurassic sediments covered the Paleozoic. In the Trigueros
on the Cretaceous of an area of southern part of the Sierra de los Cameros, indications previously reported which the Sierra de los Cameros forms the northwestern are found of a nearby border of the basin, while the of also remarked that the Utrillas Beds strongly sediments probably continue below the Tertiary cover half, resemble those of the Wealden facies with the the Ebro Basin to the N. exception of the degree of cementation which is clearly less inten- For the Utrillas Beds the same source area must have beds which little consolidated. been active. The extension towards the south, however, sive, resulting in are very observed Beuther the This difference was also by (1966), as covers a larger area than do the Wealden sediments; he sandstones in the description of the Utrillas Beds extend as far as the northern border of the reports Urbión Beds and sands in the of the Sierra de Guadarrama. The size of the feldspar fragments description Utrillas Beds. and their good state of preservation would indicate the
presence of crystalline rocks over a broader surface and
at closer distance than Wealden times. a during THE STRATIGRAPHICAL POSITION OF THE VOZNUEVO
In combination with findings by Tischer in the eastern FORMATION
part of the Sierra de los Cameros, Beuther thinks that
the clastic units of the Wealden and the Utrillas Beds As we have seen from the foregoing, the Voznuevo of show characteristics of a deltaic environment. In the part Formation is a constituent part Ciry's 'upper of of the area studied by Beuther, especially in the south, sequence' the Lower Cretaceous, which is developed the Wealden the coarse clastic sediments are believed to represent in facies in the southern border zone of the
root of this delta. Cantabrian Mountains. From the general description of
eastern of the the The Cretaceous succession in the part upper sequence, we may already notice a strong subdivided resemblance the sediments the southern Sierra, as studied by Tischer, has been into to along border. the lowest three of which The fact the and five lithological units, are most striking is scarcity in this case known from the absence of limestones and marls already to us western part: even complete in com-
5. Olivan Beds clastic with the lower (top deposit) parison sequence, although a fairly
4. Enciso Beds (upper calcareous deposit) extensive lithological spectrum is present. According to
3. Urbión Beds (middle clastic deposit) Ciry (1939) there is (p. 78) 'a domination of coarse
2. Oncala Beds (lowest calcareous deposit) sands and gravel, showing cross-stratification; they con-
1. Tera Beds (basal clastic deposit) sist of well-rounded quartz pebbles with a median size of subdivision finer sands' This purely lithostratigraphical roughly a pill, drowned in siliceous (translation by subdivision. The from represents a time-stratigraphical descrip- the present author). Apart lignitic layers, which are
tion of the five beds will not be repeated in detail here; not uncommon in our sediments, Ciry mentions, in the
the three clastic units resemble those described of by general description the upper sequence, the appear-
Beuther. Red colours prove to be most intensive in the ance of azurite and malachite and impregnations of
lowest beds. Paleocurrent directions in this area pre- asphalt. Ciry's observation that in the upper part of the
dominantly indicate transport from SW or S, sometimes upper sequence exclusively fine-grained clayey sands are with reddish white from NW. The top of the Olivan Beds is absent, as are exposed, a or colour, accompanied
the Utrillas Beds in this area. by micaceous clay layers, often ligniferous or with plant
which overlain which In the part studied by Beuther we find one complete imprints, are by analogous deposits 298
contain a marine Cenomanianfauna is as An very important, interesting problem now arises, which was not it is also characteristic of the sediments a studied at solved by Ciry: If the present. Voznuevo deposits are considered as trans- Apart from these general properties of the part the upper gressée, onlapping upon basement, we may regard of the the sediments the southern upper sequence, along them as: border of the Asturian Massif (Cantabrian Mountains)
have been dealt with from other of separately deposits a. Belonging to the base of the Cenomanian along the southern border the zone in Ciry's work. The richness in whole border.
kaolinite to be the main for this b. Not appears reason special only belonging to the Cenomanian, but con- treatment. It is these which the nectible sediments, to name to the overlying transgressive stages (Ceno- 'Voznuevo Formation' has been given, that form the manian, Turonian etc.).
object of this dissertation.
We shall not deprive the reader of the of description Ciry lists the following objections against these two of the Voznuevo part Formation, as given by Ciry. The alternatives:
easternmost location where these kaolinitic sediments In case a the main transgression would begin in the appear is Cervera de Pisuerga (also the easternmost part Turonian, which seems in contradiction with the general- of the area studied) where 86): 'Below the marine (p. ly accepted history of the Cenomanian transgression.
Cenomanian a sandy to gravelly more or less complex, In case b one will have to explain: kaolinitic, can be observed, directly overlying the Paleo-
zoic. We are dealing with coarse detrital deposits, cross- 1. The remarkable equality in composition of the bedded, consisting of more or less consolidated gravel Voznuevo Formation all in outcrops. and coarse sand composed of siliceous, mainly 2. The independence in lithology of the overlying
elements which are rounded. quartzose, perfectly They stages (Cenomanian, Turonian etc.).
associated with kaolinitic sands and The terms are clays. 3. The absence in the overlying marine beds of pebbles
'destroyed granite' or 'arkose' which have been used by derived from the Voznuevo Formation. Casiano de Prado (1850) and by d'Archiac (1853),
to describe these are respectively, deposits, only locally Possibilities a and b are shown in Figs. 7 A and 7 B, applicable. Their colour is predominantly white because 'A' for the respectively; is most part a sketch of the of the content of but red and high kaolinite, yellow Upper Cretaceous transgression over the Asturian Massif colours were also found. The deposits have never yielded as originally proposed by Ciry, and referred to by many any fossils' (translated). other investigators on this subject. The sketch presented One of the observations most striking concerns here extended the far by Ciry was to west as as Brugos de Fenar the geographical distribution of the formation. Not only by Evers (1967). The second possibility is represented in does he these far recognize deposits as as La Robla, he 'B', which shows the diachronic character of the mentions the existence of these even sediments, sur- Voznuevo Formation.
rounded by Paleozoic (and older) rocks, near the village Before this let first giving our opinion on problem, us of Soto (=Soto where the bottom y Amío) 'they occupy consider some remarks by investigators in this area after of As have a valley'. we seen in Chapter II, this obser- the time of Ciry's publication. vation is absolutely correct. Almela (1949), in his study on the possible extension
of Carboniferous coal seams below younger deposits,
Since 1852 most have attributed a investigators accurately mapped the lines of outcrop of the lowest Cenomanian to the deposits of the Voznuevo Forma- age level of the Cretaceous deposits ('el Caolín') between La
tion. Karrenberg (1934) was the first to these compare Espina and the Rio Bernesga. One of his observations in sediments with the Utrillas Beds of the Celtiberian this area should not remain unmentioned(p. 424): Chain. The former were not considered to be of Albian 'En el camino de Boñar a Adrados, en el contacto con but located age, however, were at the base of the trans- el Paleozoico, se encuentra una curiosa formación gressée done Cenomanian, as was by other investigators. cretácea areniscas bastas muy localizada, ... integrada por The conclusions with respect to the of the Voznuevo age y duras, de color oscuro o rojo, de las que existen Formation have been based mainly on investigations muchos diseminados bloques grandes y pequeños por el near Cervera de where Pisuerga, a section through the suelo'. formation is indeed more complete than anywhere else. This is the same location where Evers (1967) erro-
on the contact with the lime- his Investigations overlying neously mapped limestone conglomerate. In part В stones further to the west, showed that the of however, Chapter V more attention will be paid to the pheno-
Voznuevo sands are covered by successively younger mena in this outcrop. Other facts on the Voznuevo stages to each other. As lateral Formation transgressive no passages as given by Almela are the disappearance of or interfingerings exist between them, the most obvious the overlying limestones between Rabanal de Fenar and conclusion would be that the Voznuevo sediments are Candanedo de Fenar, and the thickness of the formation
of the sediments which independent by they are in the area between La Devesa de Boñar and La Ercina: covered. 125 The of m. age the formation is still the main pro- 299
Fig. 7. Diagram of the Upper Cretaceous transgression along the southern border of the Cantabrian Mountains (modified after Ciry, 1939 and Evers, 1967).
A: Voznuevo Formation as synchronous deposits.
B: Voznuevo Formation as diachronous deposits.
Almela blem; observed 'analogous aspects' (= facies! ) to Puente Romano over the Porma River, poorly sorted the Albian of and accumulated deposits in the provinces Burgos, Soria gravel and sandstone lenses are in typical
Teruel. He is an advocate of Ciry's version of the deltaic fore-set beds showing inclined cross-stratification. Cenomanian i. with deltaic lower several allochthonous transgression, e. a slight uncon- In the part (200 m),
between the Voznuevo Formation and the over- streaks occur. The formity lignitic (10 cm) ... upper part lying stages, although this unconformity could not be (200—400 m) of the Voznuevo Formation is composed seen in the field. of finer-grained, micaceous sandstones with carbona-
Two years later (Almela, 1951), a study with a ceous clay lenses, but gravel streaks still occur.' for purpose similar to that mentioned above appeared Unfortunately we could not recover the evidence on of La Robla. sediments Riello the area W Again the near which these observations were based. the author were recognized as Cretaceous; according to Apart from these workers, all others refer to the in-
form the westernmost of Cretaceous 'they outcrop vestigations by Ciry (1939), and do not present any new sediments in the province of León and in Old Castile'. facts.
Almela did succeed Strangely enough not in measuring Continuing our discussion on the reconstruction of the the of the stratification of the Cretaceous Cenomanian first position plane transgression, we shall summarize the near Riello. The deposits near Tapia de Ribera were not evidence from which the schematic picture of Fig. 7 A regarded as Cretaceous (cf. Chapter III). has been obtained. As is to be expected, the determina- Pastor Gómez (1963) followed Almela with of of the been largely tion the age overlying complex has always Cretaceous respect to the between the Río Lunaand the carried out by determining the fossil content of the lime-
Río Torio. The Mesozoic limestones still disappear stones. Since the 'Wealden' of Fig. 7 proved to be between Rabanal de Fenar and Candanedo de Fenar, but unfossiliferous, its age could not be determined. Ciry's limited would be they are at the top by a fault, which study of the distribution (i. e. the lateral extent) of the of their accordance with the cause disappearance. In Upper Cretaceous stages along the southern border of
Ciry and Almela, the age was considered to be Albian. the Cantabrian Mountains results in the picture
By far the most important contribution to the strati- presented in Fig. 7 A. For the locations mentioned
of the Voznuevo Formation made below the reader is referred to graphy was by van the topographical map whose Amerom (1965), pollen analysis proved a (Enclosure I). i
Cenomanian to Turonian Boñar and age in deposits near
Cistierna. The Cenomanian
Evers who introduced the 'Voznuevo (1967), name As we have seen, the entire Voznuevo Formation has
made a number of brief remarks the Formation', on been considered as Cenomanian by various authors since formation, the of which is a bipartition quintessence 1853. This is in any case true of the deposits near into lower with deltaic and an a complex properties Cervera de Pisuerga. Here the Upper Cenomanian con- with He upper complex lagoonal properties. reported (p. sists of calcareous sands, alternating with sandy lime- 200 downstream from the 120): 'approximately m stones, both rich in iron concretions, lignitic clays and 300
sandy lignitic layers. Only in the very upper part of the lagoonal conditions (the so-called Garumnian facies),
Cenomanian are pure marly sediments found. As we constitute the end of the Cretaceous sedimentation.The
could ascertain in the field, the contact between the Campanian—Maastrichtian complex which, according to Voznuevo Formation and the Upper Cenomanian(which Almela (1949) and Pastor Gómez (1963), disappears established the field of the absence was in on account or between Candanedo de Fenar and Rabanal de Fenar, of presence calcite) proved to be very gradational. according to Evers (1967) and Kuyp (1969) NW of the marine took via de found be Probably transgression place a long Brugos Fenar, was to traceable as far as
period of brackish water conditions. Ciry found that Llanos de Alba, west of the Bemesga valley. few of this brackish-marine only a metres complex are Reviewing the lateral variations in these Upper Creta- Villaverde de la Peña. left near ceous stages, one important fact should be retained: all
deposits of the various stages conspicuously pass into
The Turonian more sandy equivalents in the direction in which they The Turonian with thickness of 12—15 limestones, a m wedge out. In the southern border zone this direction is
near Vado de Cervera, grow very detrital W of this loca- always to the west. tion; their thickness also diminishes in this direction. if first Ciry's assumption (cf. 'a', p. 298) concerning
Only sands and sandy limestones are found, inter- the relation of the Voznuevo Formation to the overlying limestones would mingling with those of the Cenomanian. Thanks to the is correct, we be dealing with a
is Voznuevo 300 thickness (scarce) presence of fossils, the Turonian traceable as deposit of, say, m deposited the of the Albian and the lower far as Guardo and possibly extends as far as Prado de la during upper part part of the before Guzpeña. Further to the west it is certainly not Cenomanian, just the transgression set in
from the east. the the developed. During following stages, up to
Maastrichtian, the marine influence moved a little
further The Coniacian to the west during each stage. This would mean that between the Cenomanian The Coniacian is clearly traceable from Vado de Cervera Upper and the Maastrich- tian, the sediments of the Voznuevo Formation in the to Prado de la Guzpeña, and is mainly deposited as white westernmost of the have been marls. From Valmartino, in the Esla valley, to La Ercina part area must exposed to erosion all this without influenced. Of sands, conglomerates and sandy marls represent a littoral time, being course
the sediments can have been eroded and in- facies. West of La Ercina the Coniacian is no longer partially into the facies of the present. The Coniacian is plainly a regressive stage, as corporated sandy overlying stages. In that should compared with the Turonian and the Santonian. case one expect a coarse-grained residue
and a diminished thickness of the formation in the
The Santonian western part of the area. Neither of these features could
be however. from should The Santonian consists at its base of white compact observed, Apart this, we expect
limestones, overlain by yellowish to reddish limestones to find an unconformity between the Voznuevo Forma- tion and the The of and again white limestones. Changes in this pattern transgressive stages. poor degree
the contacts does not shed occur W of the Esla valley; the reddish limestones grow exposure near any light on this indications of break in the more and more sandy towards the west. Near La Ercina problem either, no a have been found. a more or less clayey, micaceous sandy complex with a sedimentary history ever
rust-red colour has developed. The white limestones of Regarding the second possibility (cf. 'b', p. 298) in
the lower part have considerably diminishedin thickness which it was assumed that the Voznuevo Formation is in this location, and directly overlie the Voznuevo the direct continuation of each transgressive stage, we
Formation. Further to the west they only consist of shall have to refute the objections made by Ciry ('Г, '2' limestone — — '3' debris rich in quartz gradually passing and of p. 298). into sandy limestones and even into sands. The white limestones die out before reaching La Devesa de Boñar. 1. The following must be remarked with respect to the The which studied detail mainly sandy complex was in first objection, which questions the equality of the of still the by Ciry E Boñar is present near Valdepiélago, sediment during the whole of the Upper Cretaceous: As limit of the studied him. sediments western area by According to we have seen in the first parts of this chapter,
Evers (1967), the Santonian disappears E of Naredo de deposited in a Wealden facies have been reported from
Fenar. many locations in Spain, and from most diverging ages. the They are found to have been formed during entire
Campanian and Maastrichtian Lower Cretaceous, while in one location deposition in
sediments can As far west as Guardo the Campanian and Maastrichtian Upper Jurassic times could be proved. The reach marine intercala- are developed in the same facies as that east of Cervera enormous thicknesses; even when
tions occur a considerable of time, the con- de Pisuerga. West of the Esla valley, a considerable during span tinental still the Wealden change in facies takes place: the lower part of the com- deposits reappear in same facies 's Northern A must plex becomes sandy, while the upper part shows a marly (cf. Ciry Zone). source area have existed with of producing development. In the very top of the Maastrichtian, an enormous capability incredible of clastic material of a fairly sandy, green or variegated marls, deposited under quantities 301
the Voznuevo Formation be the constant composition. There is no reason whatsoever to zone must placed in lower of the assume that this state of affairs ceased during the Upper part Upper Cretaceous, possibly Cenomanian Turanian. This that the sediments Cretaceous in areas which had not been invaded by the to means than their extension N of sea. are younger eastern Burgos,
which Ciry reported to be of Albian age. Although the
2. An independence of the Voznuevo Formation of the diachronic character of the formation is clear from these determinations, be said about the overlying stages does not show up at all. On the con- nothing can yet strati- which the southern trary, in the very few locations where a contact, however graphical relationships prevail in indica- border between Riello and Cervera de poorly exposed, could be seen, there are more zone Pisuerga.
black to almost tions of a very gradual transition: nowhere could a sign Fortunately clays proved occur in observed. section W of La Robla. In the of of a break in the stratigraphical continuity be every hope finding
differences in the content from E to W, As Ciry noticed himself, all marine deposits grow palynological
shallow marine or lagoonal on approaching the contact samples were taken from practically all locations where found. with the underlying Voznuevo Formation, and sandy black clays could be Thirteen samples were examined with their counterparts develop. As could be observed in thin kindly a view to organic contents Dr. Th. der Hammen of the sections of samples taken near the contact, there is a by van Palynological Depart-
close textural and mineralogical resemblance between ment of the Municipal University of Amsterdam. The thirteen locations and the of samples from the top of the Voznuevo Formation and stratigraphical positions these stated in the correlation chart the lower parts of the overlying stages. samples are (Encl. XI).
3: The absence of pebbles derived from the Voznuevo No detailed study could be carried out in the time beds of led Formation in the overlying marine is course available, but a rough determination to a distinction
related to the gradational change-over mentioned under between:
2. Moreover, the solution of this problem is again given
by Ciry himself, when he mentions in the description of Lower Cretaceous samples devoid of pollen of Angio-
his upper sequence (p. 78): 'A la partie supérieure sperms containing Classopollis, Araucariacites,
cependant, et d'une façon très constante se montrent Abiaetineaepollinites, Cicatricosisporites/Plicatella
Cretaceous - rich in of exclusivement des grès argileux fins, roux ou blancs, Upper samples pollen Angio-
qu'accompagnent des lits d'argiles micacées, feuilletées, sperms belonging to and showing relationships to the
souvent lignitifères ou à empreintes végétales, et association described by van Amerom (1965): Turoni-
auxquels succèdent des dépôts tout à fait analogues, pollis, Conclavipollis, Classidites, several tricolporatae
mais à faune marine cénomanienne'. From the sections and triporatae, Cicatricosisporites, Plicatella etc.
through the Voznuevo Formation we shall see that the The Cretaceous have Cenoma- sediments are generally much finer-grained near the top Upper samples probably a Turonian This distinction made clear that of the formation. A possible cause for this fining nian to age.
upwards will be presented at the end of the next nine samples must be located in the Upper Cretaceous of sediments and three in the Lower Cretaceous. As chapter. Probably a very small quantity the samples a precau- the shallow Section located in the in the top was reworked and incorporated in tion, one sample (183, P) was Cretaceous' marine deposits. 'Middle (Albian to Turonian). The samples
belonging to the Lower Cretaceous form a 'time wedge', the correlation chart For these reasons, obtained from observations in bounded by line I in the (Encl. XI).
field and in thin sections, it becomes most likely that the This wedge widens towards the east (as is to be expected
Voznuevo Formation is indeed a diachronic deposit, and in a diachronic formation). The Lower Cretaceous/
hence corresponding to successively younger marine Upper Cretaceous boundary clearly runs through sec- direc- S and which could taken from the deposits belonging to various stages in a westerly tions T, in samples be
tion, representing their continental continuation. An bottom and the top of the section.
irrefutableproof of this hypothesis, however, was found Although, due to lack of time, a subdivision in stages
by pollen analysis. was, unfortunately, impossible, the time/formation
relationship is clear enough to show that the second
hypothesis proposed by Ciry (1939), mentioned on p.
must Voznuevo POLLEN ANALYSIS 298, be the only solution: The Forma-
towards the west. tion grows younger related the believe that the Voznuevo Forma- Part of the stratigraphical problems to As van Amerom, we Voznuevo Formation could be solved by palynological tion is approximately equivalent in time to the Upper
investigations. It was van Amerom (1965) who examined Cretaceous sediments in Wealden facies, deposited on from the of ENE of and pollen and spores of three black clay samples the Massif Oroz-Betelu Pamplona, locations Campohermoso (our Section J), S of La Mata described by Ciry, Amiot and Feuillée (1963). Here
de la Riba (Chapter V, В), and Soto de Valderrueda. Van approximately the same situation is encountered: the border continental sediments which Rat's Amerom found that in this part of the southern belong to (1959) 302
'complexe supra-urgonien' have been classified, after else in Europe. All sediments in Wealden fades seem to
correlation with nearby deposits, either in the lower have the following features in common:
Cenomanian or the upper Albian. Just as in our area,
both Jurassic and the remainder of the Lower Creta- 1. Their rather mineralogical composition has a
ceous sediments proved to be absent. The Cenomanian uniform character, quartz being the main constituent.
invaded massif east. transgression hardly the towards the 2. They are poor in fossils or even barren. took A regression obviously place in the Turonian to 3. Most organic remains are unidentifiable wood frag- Coniacian in this of this area, no deposits age having ments.
been encountered; the sea did not return before the 4. Red-coloured sediments always seem older than their
Santonian. white counterparts. There indications of such drastic are no a regression in 5. The predominant colours are white, yellow and red
our area, but as we have seen from Ciry's description of (in order of importance) and all gradations in between. the Mesozoic limestones between Cervera de Pisuerga 6. Paleocurrents, when reported, always indicate direc- and the Coniacian be Boñar, proved to clearly regressive tions from W, SW, S or SE.
as compared with the overlying and underlying stages.
The relationships between the Voznuevo Formationand A high kaolinite content has been reported locally.
the Mesozoic limestones will be still better understoodas When we compare the stratigraphical position of the sediments Wealden facies soon as a more precise determination of the palyno- in in various parts of Spain,
trend becomes clear: most logical contents of the black clays has been carried out. one general very in areas the continental sediments and are bipartite are separated by
of marine sedimentation of A COMPARISON OF THE VOZNUEVO FORMATION WITH products roughly Aptian age.
OTHER SEDIMENTS IN WEALDEN FACIES As for the Basco-Cantabrian area, to which the sedi-
ments of the Voznuevo Formation are geographically
Anticipating the lithological descriptions of the related, it was found that sediments in Wealden facies,
Voznuevo Formation in Chapter V, we may say that the with a generally unknown lower age limit, have been
Voznuevo Formation is distinguished from most other deposited until the beginning of the Cenomanian trans- Wealden facies of deposits in by the abundance kaoli- gression. In the southern and southwestern part of that
nite. area, continuous continental sedimentation took place,
Both the stratigraphical position and lithological while in the northern and northeastern part a Wealden s. and properties (complete absence of limestone, degree of s. a supra-Urgonian complex could be distinguished,
cementation, composition) suggest that the formation separated by the Urgonian complex (Aptian to Lower
can best be compared with the Utrillas Beds. The Albian).
description of the Utrillas Beds in the Sierra de los In other parts of Spain the supra-Urgonian complex
Cameros as given by Beuther (1966) could, with the has commonly been referred to as Utrillas Beds. The
exception of the formation of asphaltic layers, be a Utrillas Beds are completely identical to the older con-
description of the Voznuevo Formation. Even the transi- tinental deposits (Dupuy de Lome and Sánchez Lozano,
tion into the overlying marine Upper Cretaceous is iden- 1956). Other authors (Quintero Amador and Trigueros
tical, as is the total thickness of the complex. The dis- Molina, 1956; Beuther, 1966) only report a difference in Castilian tinctly lower feldspar content found in the Voznuevo cementation. As for the source area, the Massif Guadarrama have been Formation (cf. Chapter VIII), as compared with the and, more precisely, the Sierra de
Utrillas Beds, might be explained by weathering of feld- mentioned. The sediments of the Voznuevo Formation the spar into kaolinite. The question, whether this process along southern border of the Cantabrian Mountains have been took place in the source area or in the sediment, will be the of for considered later. object stratigraphical discussions many years, the lack of fossils absence of lower Stratigraphically and paleogeographically speaking, the owing to and the a Voznuevo Decrease in thickness and Formation shows a strong resemblance to the stratigraphical boundary.
the content of detrital material of the Ceno- transgressive Wealden facies deposits of Cenomanianage increase in and Turonian marls towards the W indicate described by Ciry, Amiot and Feuillée (1963) on the manian a
Massif of Oroz-Betelu NE of Pamplona. transgression from E to W during the Upper Cretaceous.
The problem as to whether the lower Upper Cretaceous
stages are absent towards the west or continue in
Wealden facies could be solved by palynological SUMMARY analyses. The Lower Cretaceous/Upper Cretaceous time
boundary was found to run obliquely through the Wealden 'time sediments, by which sediments in Wealden Voznuevo Formation, forming a Lower Cretaceous facies are always indicated, have been reported from wedge' open to the west (cf. End. XI). Ciry's (1939)
many parts of Spain. Their age is highly diverging; they objections to the connection of the sediments of the from may range the Kimmeridgian up to the Turanian, Voznuevo Formation to successively younger stages thus occupying a much larger time span than anywhere towards the west are not valid. 303
The sediments proved to be best comparable, as geographical history can be compared with the trans-
regards their lithological properties, with the Utrillas gressée Upper Cretaceous continental sediments of the
Beds, especially those of the Iberian Chain. The paleo- Massif ofOroz-Betelu.
CHAPTER V
A. THE SECTIONS
b. from to INTRODUCTION Sands, very coarse fine-grained sands, frequently found in combination with pebbles. Coarse
In the following chapter a number of lithological and sand with gravel and pebbles between 1 and 4 cm in
sedimentological features occurring in the Voznuevo diameter are by far the main constituent of the Formation will be described. Voznuevo Formation. The sands have often been An attempt to interpret the found will be in cross-stratified of the phenomena presented in part В of this deposited units, generally planar
chapter. type. Distinct longitudinal and vertical grading, both
As regards the lithological description of the forma- fining upwards (cf. part В of this chapter) could often be observed. found tion, we shall confine ourselves to the main points; for a Fine-grained sands were always ac-
more detailed outline the reader is referred to the ex- companied by muscovite. The main colour of the sand-
tensive sections (Sections A to T and Sections Г* to III; sized sediments is buff. Yellow and pinkish discoloura-
Enclosures II to VIII). Reference to the sections is made tions are frequent. Consolidation of the sands has taken
by mentioning the height in the sections in metres (m); place in gullies which fall dry seasonally. each in section the beginning has been marked m 0,
irrespective of the distance from the underlying forma- с. Silts, always accompanied by large quantities of tions. The will be dealt muscovite in the sand. area with in three parts. as fine-grained Never cross-
Before ourselves with the a bedded unless in combination with sand; silts occupying sections, pure are number of general lithological observations have to be always horizontally bedded or laminated. Colours are borne in mind: The Voznuevo Formation has white with discoloura- a very predominantly pink superficial uniform lithological character which remains constant tions. along the whole mountain border.
four d. almost and Schematically lithological types according to Clays, nearly accompanied by mixed grain size can be distinguished in the field: with silts. Heavy clays proved to be very rare. The clays often form the top of fining-upward cycles. They are
clearly more susceptible to discolouration than are
sediments of other sizes. in colour is a. Conglomerates, consisting of pebbles, cobbles and grain Variegation the main colours for fresh olive boulders, only exceptionally exceeding 10 cm in dia- very great, clays being: and all shades between and meter. The largest cobble ever encountered in the green, light green light grey
black. The turn Voznuevo Formation had a diameter of 25 cm. Where greenish clays into clays with lilac and these shades when to the conglomerates have for a long time been exposed purple exposed atmosphere. to the surface, they were found to be consolidated by cementation. Recently excavated conglomerates are THE AREA BETWEEN RIELLO AND LA ROBLA mainly unconsolidated, except for relatively thin layers
(less than 100 cm thick), which can also be consolidated. The isolated area between Riello and Bobia is intersected The conglomerates have an abundant matrix consisting by a number of N— S running small rivers, whose recent of sand, fine gravel and very white-coloured, powder-like alluvial deposits cover their valley bottoms. For this kaolinite. Sand-size and gravel-size particles have often in the Voznuevo Formation reason exposures can only been completely pulverized to silt-size. Channelling, cut- be found on the higher situated parts between the rivers. and-fill and festoon-like structures cross-bedding are the Three rather incomplete and poorly exposed sections most common structures which could be observed. The were made in this region. Between Carrocera and La colour of the conglomerates is mainly white, yellow and Robla one section through the upper part and one buff; only locally are brown, purple or pink discoloura- through the lower part of the formation are presented. tions encountered. In remarkable contrast to this generally light-coloured sediment are the dark grey to Section A. a section of the of Soto -, west village y black lydite pebbles. west Amio*, of the small pass in the road shows
* On the the road from previous maps village along La Magdalena to Amio. Riello is called By recent revision one village * I Roman numeral in sections indicated Г. 'Soto as by name y Amio' is used on the road signs. 304
30 metres of Voznuevo section at an unknown continuing presence of cobbles and clay balls, clay and
distance* above the Precambrian and some 100 metres silt participate in the sedimentation pattern.
below the location where the Voznuevo Formation
into the Formation. As have Sections С 1, С 2 and С 3. — measured north of the passes Vegaquemada we were
west of section seen in Chapter IV, the Mesozoic limestones are absent road near the pass Bobia. The first (C 1)
west of Llanos de Alba, and the Vegaquemada Forma- begins at about 5 metres distance above the Precam-
tion immediately overlies the Voznuevo Formation. brian. Approximately 11 metres, mainly pebbles and
The quantity of coarse components in this section is cobbles, are overlain by 10 metres of clays and silts,
striking, clays, silts and sands without gravel being apparently without any grading. The silts and clays,
almost absent. The lowermost ten metres of this section which have suffered from intensive discolouration,
strongly resemble those which will be described in originally had a grey to white colour. At the base of
detail in Section F de Fenar. section С 2 greater near Brugos They very closely packed quartzite conglomerates almost without matrix, with cobbles to 25 cm are composed of cross-stratified units of 60 to 100 cm occur, up in diameter. Between and sand thickness, containing pebbles to medium coarse sand. these, silty clay silty 30 40 thick. Their transition The units show a distinct grading, with clay balls in their layers were found, to cm of 12 into the is lower parts. The three metre thick layer clay (m to surrounding conglomerates always very sharp. of from and crushed m 15) apparently forms the top a sedimentary Apart some clay pebbles intensively to 92% of the than cycle** which is fining upwards. Between m 15 and quartz fragments, up pebbles coarser
3 cm diameter consist of As could be deduced m 20 another cycle begins, but its top is not exposed. quartzite. from imbrication, the have been supplied The top of the section shows very coarse deposits with quartzites of from northwesterly directions. After diameters of up to 20 cm. Clay balls attain diameters approximately
20 metres of sands and 40 cm. From m 32 onwards, grading no longer occurs clays, coarse deposits reappear
of 119 to m The intercalations are almost and dashes of pebbles and cobbles are characteristic (m 126). pebbly
of Some tens of metres the sedimentation pattern. These conglomerates are exclusively composed quartz. north of the С from closely packed and scarcely cemented. Channelling, cut- road, section 3 shows four cycles
coarse sand, fine sand and silt to and-fill structures and imbrication were frequently pebbles via clay. An
erosion surface at the of each marks the be- observed. Channel fills can consist of silt or clay. top cycle
Determination of the Quartz/Quartzite ratio (Chapter ginning of a next cycle. The pebble beds at the bottom of have often been consolidated iron VI) brought to light that the composition the coarse by (hydr)oxides.
dashes changes with changing paleocurrent directions; a Some 50 metres or more may cover the top of section С be related the total thickness of the Voznuevo Forma- high quartzite content seems to to a transport 3, bringing
direction from northern directions. The quartzites seem tion in this location to more than 350 metres.
to have been transported over a relatively short distance,
many of them being block-shaped with rounded edges. South of Benllera, where the Voznuevo Formation was The geometrical features of the quartzitic layers from recognized underlying the Vegaquemada Formation which they have been derived are still visible on these (Chapter II), fine-grained deposits form the top of the formation. quartzites. The coarsest material found is coarse sand,
grading into silts and clays. The mean thickness of the
Section B. - runs immediately E of Soto y Amío, both cycles is about 4 metres. north and south of the road. The oldest exposed beds lie West of Benllera, on the eastern side of the road from about Otero de las 30 metres above the contact with the Precam- Dueñas to León, the same situation was brian. Like the sediments in the top of section A, we are found, with the almost complete absence of gravel. Both here dealing with very coarse deposits: coarse sand and outcrops have strongly been affected by weathering. gravel with 30 cm thick pebble layers containing clay As we have seen from the description of the contact balls. From imbrication in pebbles and cobbles and between the Voznuevo Formation and the Vegaquemada the sediments cross-bedding (with set thicknesses of up to 135 cm) in Formation SE of Carrocera (Chapter III), the sands directional observations could be made. These in the vicinity of the contact also consist of sands and silts. coarse sediments, although less than 10 metres are exposed in the section, have a thickness of at least 20 Section D. - At the side of the in the road metres. On the southern side of the road some poorly eastern pass exposed cycles could be observed. Notwithstanding the from Otero de las Dueñas to La Robla, section D was 1250 E of measured in a clay pit approximately m the
* of Olleros de S of the road. In this section AU distances have been corrected for dip and topography. village Alba, information obtained from was the uppermost deposits
** to occur in several dimensions. A Grading proved grading of the Voznuevo Formation. As can be deduced from
of more than one cross-stratified unit will be called composed a the diameters exceed 5 The section, pebble never cm. cycle (cf. e. g. Allen, 1965a). Grading within a cross-stratified sediments in this section form a of unit 10 exposed sequence (from less than cm to more than 200 cm in thickness in the sediments with a maximum thickness of 5 metres. The studied) will simply be referred to as grading, cycles while grading within a foreset will be called vertical grading. proportion of elements coarser than sand-size probably 305
forms about tenth of the total thickness one (the Section С: Rather coarse character of the deposits in at
excavated part included). In the lowest part of the least the first 100 metres, especially in the bottom of
section (m 1 to m 3) laminationsbetween sand and silty section С 2. Sudden change in character of deposition in
clay occur. section С 3, where cycles occur with a maximum
thickness of 10 metres and with pebbles of moderate
Section E. west of the of Sorribos de sizes in their lower show a much more -, directly village parts. They Alba the of sedimentation on northern side the road, begins some regular pattern than the deposits en-
40 metres above the contact with the Paleozoic. countered in sections С 1 and С 2.
Although clays and silts with Paleozoic cobbles and Section D: Completely comparable to section С 3, with
boulders immediately overlie the contact, it is assumed a predominance of silts and clays.
that the majority of the 40 metres between the contact Section E: Indications of sedimentation under rather
and the section consists of sand and gravel. In section E quiet conditions throughout the main part of the
two cycles can be distinguished, the pebble diameters in section. First occurrence of black clays with organic
their respective lower parts seldomly exceeding 3 cm. In remains. Very thinly laminated alternations of fine-
the top of the first cycle two well-exposed parts (m 7.5 grained sand and silty clay.
to m 9 and m 13 to m 17, respectively) are presented in
detail. The first detailed section shows five gradings from
coarse sand to clay. The tops of the gradings are marked THE AREA BETWEEN LA ROBLA AND CISTIERNA alternations by of silt and clay, thinly laminated, with an
increase in towards the The clay stringers top. stringers In this part of the southern border, which is by far the
are a few millimetres with only thick, wavy, changing best exposed, a relatively close spacing of the sections thicknesses, but continuous. often generally They are makes correlation easier (cf. part E of this chapter). surrounded thin film of reddish iron by a oxide, by Between La Robla and Campohermoso the lowerpart of which accentuated. The with sand they are contact the the Voznuevo Formationexposed here, could be studied of the overlying grading is always sharp and erosive. The in sections F, H and I; the upper part in section G and in second detailed section shows the of black Section II. Section presence clay J near Campohermoso occupies an with and coal-like The black lignite fragments. clay intermediate position. Between Campohermoso and
exhibits a kind of efflorescence of which results sulphur, Boñar the formation proved to be traceable as far as La
in to dark foam-like a light yellow cover. Towards the Vecilla, coming from the west. From here onwards,
a laminatedalternation of sand red top very thinly (light Paleogene sediments cover the Voznuevo Formation as
to brick red) and (light grey) was found far clay (Fig. 30). as Boñar, except for a small exposure NW of La Mata The laminations are laterally traceable as far as the de la Riba. East of Boñar, the formation continues in Within of exposure permits. a thickness 5 centimetres, two branches; the most northern branch comes to a dead
62 alternations were counted. The transition into the end near the village of Colle, the other forms the con-
overlying sand is again very sharp and erosive. Apart tinuation along the southern border. Sections K, L, N from the second this and M all taken and measured between some grading in cycle in section, no were in the area
structures could be observed. Boñar and Colle; the first three in the lower part of the
The black horizon of this found formation. In the between La Devesa de Boñarand clay section was again strip
in an excavation between Sorribos de Alba and Llanos the Rio Esla section О could be made north of
de Alba, north of the road. South of the road a very Yugueros.
sandy limestone outcrops between the Voznuevo Forma-
Section F. — tion and the Vegaquemada Formation, the last witness represents a cross-section, more than
110 metres of the Mesozoic limestones further to the east. in length, through two arroyos (and partly sand NW of pits) Brugos de Fenar. The two exposures each other's When we summarize a number of typical features of the are complements, the change-over taking the W of La Robla the be 46. This deposits in area following must place at m transition happens to coincide stressed: approximately with a transition in sedimentation
pattern: In the stratigraphically oldest exposure very Section A: Cross-bedded and in the first sudden gradings clays changes in grain size take place; 1 to 2 metre
20 metres of the section, with a close resemblance to the thick concentrations pebble are present as dashes
deposits which will be described in section F. Very between sands, silts and clays. Grading is not ubiquitous; with sudden in size in the sometimes reversed coarse deposits changes grain grading (i. e. coarsening upwards)
top of the section. Dashes of imbricated, poorly rounded could be observed. The first 23 metres of the section
cobbles and The show considerable of pebbles. locally high quartzite content a amount clay. The youngest
seems be related to direction. Chan- much to paleocurrent exposure presents a more regular pattern, since at
cut-and-fili structures nelling and are common. least seven cycles could be discerned. Four of these are
Section B: Lowest of the section with part comparable presented in Section Г (Enclosure VIII). In order to the top of section A, finer-grained sediments in the facilitate reference, the units of this section have been
numbered upper part. in the left column. The four cycles comprise 306
1 37 55 81 and 82 show units to 36, to 54, to to 99, an erosive character with respect to the sediments
respectively. on which they were deposited. The gravel and sand such consists of bottom Schematically a cycle (from to layers are frequently cemented by iron (hydr)oxides,
top): showing two different patterns:
basal zone with in a coarse sand with a A pebbles matrix, a. Parallel to the stratification plane; this can have
maximum diameter of 6 cm, often orientated in an formed either before or after tectonical deformation of
imbricated position, and clay balls, obviously derived the Voznuevo Formation.
from the of b. Parallel clayey top an underlying cycle (or, thinking to the topographical surface (in this case environmental derived from in conceptions, nearby approximately perpendicular to the plane of stratifica- the natural swamps beyond levees). tion); a post-deformational feature.
Gravel and coarse sand in cross-stratified units about
finer — 35 cm in thickness, passing into units with sands Section H. In this section, north of the village of less than 10 thickness. The thickness of the of Solana de the lowermost cm in set Fenar, we return to part of the
cross-strata decreases with of Voznuevo Formation. The decreasing mean grain size exposure is schematically the material. In the cross-stratified units with section composing comparable F, since one can distinguish,
longitudinal and vertical grading (cf. part В of this from top to bottom:
chapter), both fining upwards, could frequently be 5. Black clays
observed. 4. Clays and silts (m 57 to m 82)
Silts, parallel laminated, occur near the top of a cycle. 3. Sands and gravel (m 18 to m 57)
They are homogeneous or alternate with more clayey 2. Clays and silts (ml to m 18)
layers. 1. Paleozoic rocks
Clays at the very top of the cycles; sometimes not Unfortunately, structures are generally obscured by
developed, always partly eroded. They are well homo- weathering, so that a direct comparison is very difficult
to genized; burrowing, however, could not be observed. make. Grading proved rather common, but transitions
in grain size are more abrupt. Clay pebbles and clay balls thickness The of such a cycle proved to be about are ubiquitous, even when the underlying unit does not
7 metres (Fig. 33). The contacts between the cycles are contain clay but silt or fine-grained sand. From this we found deduce that lateral always very sharp, large pebbles were to have sunk may changes in grain size occur over
into the underlying clay. Evidence from cross-stratifica- short distances. The maximum diameter of the material
be rather tion proved to poor. Theoretical models and deposited in this section was 11 cm.
classifications as proposed, for instance, by Allen
are based on observations on more than one Section I. the of Naredo de (1963a) —, opposite village Fenar, plane of intersection. Since in the present outcrop — as 300 m north of the road, is almost identical to the base - in many others only one plane can be studied, clas- of the previous section. About 19 metres of fine-grained sification such carried Allen is sediments found as out by hardly were on top of the Paleozoic, then a Allen's possible. Using descriptive terms we may say that sudden change in grain size takes place and gradings of the cross-stratified sets, as observed in the cycles of coarse pebbles into coarse sand could be observed. The Section Г, are grouped, large-scale (more than 5 cm in lowermost grading (m 20) contains some coaly frag- erosional with thickness), planar or slightly undulating ments. A conglomerate with a thickness of several lower boundary surfaces and homogeneous in com- metres forms the top of this section; the cobbles' and
to have most features in have position. They seem common pebbles' longest axes a very clear orientation Allen's with omicron-cross-stratification (Allen, 1963a). parallel to the plane of stratification, but their position
Near the of the section black horizon does not of to direction top a clay occurs permit any conclusions as the of
at m 110. transport.
— Section G. — shows a section along the La Robla to Section П. has been draughted according to data from
Boñar railway, between Rabanal de Fenar and the eastern border of the Rio Torio. Only during low
Candanedo de Fenar. The stratigraphical distance from water level is study of this section possible. The distance the the Mesozoic limestones outcrop to is some from the top of the section to the Mesozoic limestones is 100 that here metres or less, so we are dealing with about 75 metres, the first well discernible limestones are
from the of deposits upper part the Voznuevo Forma- exposed immediately S of the passage of the road over tion. For this find reason we are not surprised to the river. The sediments in this section strongly resemble which resemble those of С 3 deposits strongly sections those of section G, which are situated at a comparable and D. The the of limestones. only difference, however, is presence stratigraphical distance from the Mesozoic black at the of almost each Plant or grey clays top cycle. The most salient feature in this section is the abundance and wood the of remains are widespread throughout wood fragments, large trunks and branches, which section. bottoms sedimentswith diameters Only at two cycle were attain of more than 30 cm. Fining-upward diameter of 2 both these horizons could a over cm found; grading always be observed; the greater part of the 307
sand the bottom silt the grading consisting of clay and silt. Sediments coarser fine-grained at to or clay at top. Even the thinnest laminations could be traced the than coarse sand hardly occur. along of the 300 One Grading in the inorganic components is largely height exposure (approximately cm). such of: reflected in the organic components, since the following grading consists
to dark associations could be observed: с clay, light grey
b. white to coarse sand : trunks and branches silt, purplish sand : large wood remains a. fine-grained sand, white, locally cemented by iron silt : small woodremains and stems (hydr)oxides; in that case light brown The sand and silt clay : minor plant remains forming felt-like layers units are very rich in muscovite and small wood The contacts between the In a few locations cross-stratification indicates a paleo- fragments.
are The sand has often sunk current direction from W or SW, but measurements are gradings always very sharp: not accurate. mentioned black into the underlying clay causing an undulating very As before, many efflorescence of foam-like sometimes to loadcast-like structures. clays show an sulphur in boundary, leading bubbles. The high sulphur content is also reflected in the On top of the partly excavated part of the section
of wood (m 65 to m 78), grading units with sharp boundaries are complete pyritization many remains. Burrowing and be feature in the which can reach a maximum thickness of rooting seem to a fairly common exposed, metres. black of to the 12 Cross-stratification could not be observed in clays. An exposure a section parallel these but be plane of stratification (Fig. 31) shows extensive rooting gradings, might present. In the coarse
which have been caused dense deposits at the bottom of each grading, channels up might by vegetation. often for their The deposits of Section II can be traced as far as frequently occur. They are most part filled with balls. A distinct from to Robledo de la Valcueva, where they are exposed in a clay grading pebbles silt and could also be observed within the channels. brooklet at the eastern exit of the village, south of the clay railway. Immediately south of the railway, an older East of La Vecilla de Curueño the Voznuevo Formation black clay horizon was found; its stratigraphical height
black is not as far as Boñar, with the of can be compared with the position of the clays in exposed exception a sections F and G. very small exposure NW of La Mata de la Riba and an
exposure S of this village along the western bank of the
Rio Porma. The latter will be described in Section J. - is situated north of the village of separately В of this The former does Campohermoso. It begins some 50 metres above the con- part chapter. exposure not contribute tact with the Paleozoic. The first 32 metres consist much to our knowledge of the Voznuevo mainly of sand and gravel; concentrations of pebbles Formation (only poorly exposed sand with a black clay
has been but one can establish with cer- occur locally, sometimes as fillings of channels. Grading layer found), that the Voznuevo Formation below clear. from some channel-like is present is rare and not very Apart tainty structures, the sand-gravel complex proved to be rather the Paleogene cover. East of the structureless. From m 40 to m 79 the coarse character of Rio Porma two strips occur, in which sediments of the Voznuevo the sediments disappears completely; their place is Formation outcrop. The occupied by fine-grained sands with silts and clays. northern strip between Boñar and Colle will be the first of interest. In this About 26 metres of this fine-grained complex is object our area which, as was presented in detail in Section III. From this section the mentioned in Chapter III, has only been moderately affected and abundance of organic matter, such as black clays and by post-Cretaceous folding faulting, the plant and wood remains becomes clear. The plant strata have a uniform dip of 25° to 30° to the SW. Many with remains often have a peat-like character: fibrous a abandoned sand and clay pits provide a better exposure with of light brown colour. A jet-like organic product, a the Cretaceous than anywhere else and make tracing dark brown to black colour and a conchoidal fracture is of lithological units over a distance of several kilometres also frequent. In unit 14 of Section III clay pebbles, possible. Two sand excavations, one 1.5 km east of so-called armoured of coated with gravel and coarse sand, Boñar, north the road to Sabero, one south of the mud balls, could often be observed. Not only clay, but village of Grandoso, produced such well-exposed also wood fragments can be coated; in this case they sedimentary structures that they will be dealt with in could be referred 'armoured wood balls'. Desicca- detail in В of this 8 shows to as part chapter. Fig. a schema- ball tical tion of the central part of an armoured mud can correlation of some NE— SW sections in the area been observed between and Colle. form rattle stones, but these have never in Boñar From this correlation it
The black often becomes clear that the formation the Voznuevo Formation. clays are begins in this area with The veined by thin sand stringers (cf. Fig. 32). sandy two conglomerates separated by a clayey or silty layer. which The of these intercalations, are only mm-thick, are clearly lithological composition conglomerates is discontinuous and are only traceable in bundles. Sand different; the oldest conglomerate contains much more lenses with an originally circular cross-section have been quartzite pebbles and cobbles than the younger one (cf. compressed into oval-shaped lenses orientated parallel to Chapter VI). Some 100 to 150 metres from the contact with the Paleozoic first black the plane of stratification. Unit 39 is presented in Fig. a clay layer was found; a
36. This unit shows about 60 small-scale gradings from second at approximately 50 to 70 metres below the 308
tranquil conditions, the distinctly cross-bedded units
with diverging paleocurrent directions are clearly differ- the the lower ent to channelling in part of this section.
Section L. - lies less than 100 metres east of section K.
The second conglomerate, also about 14 metres thick in
this The section, is fully exposed. components are some-
what coarser than in the previous locality, reaching dia-
meters of more than 20 cm. Fig. 35 shows a detail of the
conglomerate in which the very sudden changes, in
lateral and vertical sense, can clearly be seen. The rest of
the sediments encountered in this section do not differ
K. very much from those of section
Section N. — West of the village of Colle, south of the
church, section N was taken. From m 2 to m 20 the
youngest conglomerate was again found. Especially
between m 15 and m 20, a closely packed conglomerate
was found in which a sandy or gravelly matrix is almost
absent. Below the conglomerate in this section, 2 metres of the silts and clays which separate the two con-
glomerates are just exposed. The oldest conglomerate
proved to occur east of section N in the village itself.
The upper part of the section consists of considerably weathered gravel and sand. Only between m 32 and could m 37 some cross-bedding and channelling be
observed.
8. Correlation of excavations and in the Fig. sections, exposures area between Boñar and Colle. Section M. — is a combination of three small sections
south of the road from Grandoso to Colle. Compared
Mesozoic limestones. Between this layer and the lime- with the sections from the lower part of the Voznuevo with small has stones a mainly sandy complex pebbles Formation, one notes a complete absence of pebbles been deposited. The two sand pits mentioned above larger than 5.5 cm in diameter. Sand and gravel, which of form part this complex. The conglomerates near the are by far the most important constituents of this contact with the Paleozoic best the are exposed in section, often show cross-stratification in which very excavations in which sections K, L and N were made. accurate paleocurrent measurements could be carried also in which They outcrop exposures are not man- out. The black clay horizon between m 19 and m 22, but in that have been such made, case they cemented, as which was found in one location, can be correlated with east of the road from Boñar to Adrados, where Almela the black clays of traverses 2, 3, 4 and 5 (Fig. 8). In the described the consolidated blocks (1949) large spread most easterly section this clay was found to be less dark; out the surface This over (p. 298). cementation con- its colour changes from light grey to purple. The tinues as far as the exposure of the contact north of the sediments of this section strongly resemble those of the of Voznuevo village (Chapter III). sand pits near Boñar and Grandoso (Part В of this chapter).
Section K. — is situated between Grandoso and Colle, north of the of of km 6 In the southern usable road, some tens metres west on strip only one exposure, some
the Boñar to Sabero road. Both conglomerates (m 2 to tens of metres below the Mesozoic limestones, was
7 and 18 A few found between Ercina. m m to m 32) are present. gradings La Devesa de Boñar and La Part could be observed between these conglomerates, but silts of this exposure, the black clays occurring in the and clays form the main constituent of these sediments. brooklet which flows in the bottom of the valley, will be
The maximum diameter of the cobbles in the older described further on. About 20 metres below the
conglomerate amounts to 16 cm, in the younger Mesozoic limestones (approximately corresponding to conglomerate to 13 cm. Imbrication is ubiquitous and the top of section M), the Voznuevo Formation can be has contributed considerably to the reconstruction of studied over a stratigraphical distance of about
paleocurrent directions. The conglomerates hardly show 20 metres. As was to be expected, the sediments show a
any grading; the pebbles and cobbles rarely touch, resemblance to those of section M. In contrast to the of and fine floating as they are in a matrix coarse sand situation in section M, however, lateral traceability is not gravel. The coarse sediments at the top of the section obscured by vegetation. The sediments mainly consist of
(Fig. 34) seem to have been deposited under more gravel and coarse sand, grading into fine sand, silt or 309
clay. The gradings have a thickness of between 70 and of the section a black clay horizon has formed. 100 cm; some beds contain cross-bedding with set thick- Section H: This section shows a likeness to section F, it of 60 beds nesses cm. The containing the coarsest shows the same schematical division in grain sizes. material have been cemented the by iron (hydr)oxides, they Unfortunately sediments in this exposure, especially
clearly stand out in cuesta-like in the with the topography, forming part grading sequences, show a very poor
ridges. Comparing the gradings of this exposure with degree ofexposure. those of first section M, the seem to be more complete. Section I: The same distribution in grain sizes as found The W and E of the of exposures village La Ercina in the lower parts of sections F and H. found be for detailed were to too decayed studies, so As in the area west of the Rio Bernesga, the sediments that taken this of only samples were in area. Besides, the upper part of the formation, as presented in tectonic have been different movements must here, since section G and Section II, have a clearly different
the strata have a of 15° 20° character. dip to to the SW in an depositional Both sections are highly compar- excavation the border of the while able and show on western village, a predominantly fine-grained, upward-
nearly vertical position was encountered everywhere grading cycles between 50 and 300 cm thickness. else. Throughout both sections and throughout each grading
organic remains proved to occur. The upper parts of the
Section О. - is situated N of the village ofYugueros. A sequences consist almost exclusively of clays.
surprisingly small thickness of the formation was found Section J: In this section, in which the contact with the this in location, the distance between the Paleozoic and Paleozoic is not exposed, the following division can be
the Mesozoic limestones being only approximately made:
127 metres. As the in exposure of the lower contact E of 1. A coarse-grained complex, structureless apart from
Carrocera, the Cretaceous is lithologically completely some channelling. of independent the underlying rocks: in this location the 2. A very fine-grained complex with black clays and Paleozoic of while consists layered quartzites, the sands, both with a very high organic content. Locally in the first 20 of laminated conglomerates metres the section are very thinly alternations between sand and almost of and fact small-scale exclusively composed quartz pebbles clay, in gradings, are exposed.
cobbles. Only the first metres of the section show 3. A complex with relatively coarse-grained grading-
the rest of the relatively coarse deposits, in section upward sequences.
pebble diameters to not exceed 2 to 3 cm. Sand and clay Sections K, L, and N, which have all been made in the
in are found approximately equal proportions. Grading sediments which directly overlie the contact with the obscured is locally somewhat by weathering. Between Paleozoic, are identical with the sediments in the same 63 and 73 and m m strongly pyritized plant remains stratigraphical position further to the west.
black clays testify that vegetation played an important Section M and О, section together with the exposure At 82 balls part during deposition. m armoured mud east of La Devesa de Boñar, all show the fining-upward
with black clay in their centres proved to be completely characteristic of the sediments the gradings in upper part balls, found at the base of pyritized. Clay predominantly of the formation. The only difference with respect to gradings, can reach diameters of 10 cm. section G and Section II is the scarcity of organic From to the Río Esla, the Voznuevo Forma- Yugueros material in the cycles; only in section О do about ten
tion can be traced loose material in the on only by soil; metres of the section contain a considerable amount of
the western side of the Esla the sediments lie in valley wood remains and plant remains.
situ; coarse sand and pebbles up to 6 cm in diameter could be observed location. in this These deposits are
completely cracked and disintegrated; their total thick-
ness does not exceed 50 metres. THE AREA BETWEEN CISTIERNA AND CERVERA DE
PISUERGA Summarizing the main points of the investigations of the sediments between the Rio Bernesga and the Rio Esla we In III of the part area studied the Cretaceous, and can say that: particularly the Voznuevo Formation, is only exposed in Section F: General in the section based far pattern on grain a fragmentary manner as as Guardo; from here to the - size, from bottom to fine - units east a continuous extends far the top: coarse grading strip as as village of
— fine. The of coarse part the section shows the same Traspeña. West of Guardo only one section could be
characteristics as those of sections А, В, С 1 and С 2. made, situated SE of Cistierna, east of the road to
Sudden in size and of Sections changes grain scarcity sedimentary Sahagun. Q, R, and S, all three representing the
structures (only channelling and imbrication occur). lower sediments of the formation, are located between
Clay layers are thin and show lateral thinning within Guardo and Villaverde de la Peña. Section T, in the short distances. In the part of the section which is extreme east of the area studied, provides information Г the covered by Section very well-developed fining-upward on sediments over the entire thickness of the forma-
grading sequences with distinct mega cross-stratification tion. This section runs from Cervera de Pisuerga to Vado
show an obvious regularity of deposition. In the very top de Cervera, parallel to the south bank of the Rio Rivera. 310
Section P. in the southeastern of the of the surface, but unsuitable for sedi- —, part village deposits on are
Cistierna, shows that only 61 metres of deposits mentological study.
belonging to the Voznuevo Formation outcrop in this North and south of the road from Guardo to Cervera
location. Only at the bottom of this section (m 3 to de Pisuerga, at the eastern exit of the village of Guardo,
the the Voznuevo Formation is found full m 10) were very fine-grained deposits encountered, again in its of the shows On the northern side of the few remaining part exposure predominantly extent. road, a metres
sediments the most from the contact with the 40 metres of very coarse-grained striking Paleozoic, some characteristic of which is the broken state of nearly all poorly exposed sedimentsshow coarse-grained sand with
to pebbles and cobbles; due to intensive weathering they pebbles up 6 cm in diameter(mean diameter 3 cm), of can be crushed by hand. At m 12 very large trunks, forming the main portion this exposure. They consist of thickness from often strongly pyritized, float in clayey sands. The gradings, varying in 50 cm to more of whole unit shows a light yellow colour in the field, due than 250 cm. The upper parts these gradings, varying surface. Some from The to efflorescence of sulphur at the large- in thickness 50 cm to more than 250 cm. be observed the section, silt scale grading can throughout upper parts of these gradings may contain fine sand,
since the upper 12 metres only produced pebbles with a or clay, while clay balls are frequent in the lower parts.
diameter of less than 3 cm. Between the conglomerates On the southern side of the road the relatively coarse- of silt sand indicate intercalations clay, or rapidly grained character foundon the northern side of the road
changing sedimentary conditions in a vertical sense. has completely vanished, only fine-grained sands, silts
and clays being exposed. At 90 metres south of the road
The Voznuevo Formation could also be studied behind the occurrence of Mesozoic limestones was observed.
the brick factory north of Valmartinoand in the arroyo North of the village of Muñeca, the first 30 metres of
between section P and the location mentionedabove. In the Voznuevo Formation are identical to the deposits
the arroyo about 15 metres of clays, silts and fine- near the contact in Guardo. Gradings could be dis-
grained sands overlie the contact with the Paleozoic, tinguished, about 2 metres in thickness, with silts, silty the before gravel occurs in considerable quantities. Near clays and clays at the top. Rather conspicuous is a of the base 15 thick of sand and in factory the fine-grained deposits are metre complex coarse gravel, thick. total thickness of which the diameter 3.5 while approximately 18 metres The mean pebble is cm,
the formation has increased to 120 metres in this loca- maxima of 8 cm were found. The pebbles are con-
tion. The components of pebble- and cobble-size in these centrated at certain horizons which form the lower part
two also crumbled and weathered. of cross-stratified sets. Paleocurrent directions, estimated exposures are highly
Opposite the village of Valmartino, on the northern from foreset dips, gave results with very diverging direc- side of the of black tions: From E and NE. On of these road, some metres clays are exposed SW, top mainly of at approximately 35 metres above the contact with the coarse sediments, about 10 metres predominantly of silts and have been A Paleozoic. These clays, containing a large proportion fine-grained sands, clays deposited. and nodule-like inclusions 70 thick black with wood remains of to pyrite, are very sandy contain cm clay layer up
and lenses, the thin sections of which will be dealt with 10 cm and stems shows the transition to conditions in an
in Chapter VII. The clays are intensively burrowed and environment of much lower energy.
comprise circular sand-filled tubes. The sediments show
of of with a Section — was surveyed on the western side of the a great deal folding. Crystals gypsum, length Q. often found of Las north of the road. The lowest part of up to several centimetres, were lying village Heras, of first characterized along the surface. Within the sediment gypsiferous layers this section (the 50 metres) is by a of silts and black horizons seem restricted to fillings of voids and along slickensides. predominance clays. Two clay
were found at m 37 and at m 41 to m 42.5, respectively.
A discussion the of The first black clay only contains a small proportion of on presence gypsum in this location black of will be presented in Chapter IX. sand, as compared, for instance, with the clays
The of Prado de la and north the section with those described exposures east Guzpeña near Campohermoso, or breadth and of Cerezal are both too fragmentarily exposed to draw in part В of this chapter. Stems 0.5 cm in
conclusions. It could be observed that burrows are the only macroscopical witnesses of many only - 41 and 42.5 dis- breakage of pebbles and cobbles and hence strong organisms. Between m m one can compression of the strata — has still taken place in both tinguish: of black with lenticular sand bodies localities. North of the railway near Cerezal, gradings g. 16 cm clay
two metres f. 30 of black with trunks and stems occur, about in thickness, from pebbles with cm clay diameter a longest axis of 5 cm to silts and clays. Thin coarse sand e. 10 cm of gravel with pebbles, maximum layers below the coarsest deposits at the bottom of some 6 cm d. 2 of black eroded gradings suggest the occurrence of backflow phenomena cm clay, laterally in 22 of medium sand with small and these deposits (cf. part В of this chapter). с cm coarse pebbles Between Puente and number of black Almuhey Guardo only very small, a clay stringers isolated of Cretaceous b. 16 of dark black with sand spots sediments occur. They cm grey to clay stringers suggest the presence of Cretaceous below the younger a. 15 cm oflight grey clay 311
As will be clear from the short sections E and W of Both occurrences of black clay are accompanied by nodules R the black horizon 33 pyritization of the wood fragments, small pyrite section (Enclosure VI), clay at m of second black is affected thickness. This is and efflorescence of sulphur. Unit e the by laterally rapidly changing
of of the few localities in which beds can be traced clay complex represents a sudden invasion very one and coarse-grained material. over more than merely a few metres, in which it of sand and in which becomes clear that the black bodies At m 51, sedimentation gravel set clays are lens-shaped
not lateral extent. will last until the end of the section. Especially the which probably do have a very large
between and 55 resemble those deposits m 51 m closely - of Section I' de Fenar. The of the Section S. lies on the northern side of the road near Brugos rest between and Villaverde de la about section shows alternations of coarse sand with gravel in Avifiante Peña, units between 25 cm and 100 cm thick. The coarse- 300 metres east of the railway station of a nameless,
abandoned The section with grained sand fraction is very rich in muscovite. In some mining village. begins badly
units cross-bedding could be observed and measured. exposed quartzite cobbles, belonging to the basal
conglomerate (cf. Chapter III). From m 10 to m 42 fine-
Section R. - is a section on the eastern side of the old grained sands, silts and thick layers of clay predominate.
of The sands village of Santibañez de la Peña, east the river on Upward-grading proved to occur everywhere.
in muscovite. either side of the railway. In the first 17 metres which and the silts are both very rich Only near of sand overlie the contact with the Paleozoic, sands, silts and m 36 does a 35 cm thick layer coarse-grained
At with and cobbles to 9 in diameter to clays are exposed in upward-grading complexes. m 5, gravel up cm testify of the brown to grey coloured fine-grained sand with small a temporary increase in capacity transporting sand From 42 with pebbles and plant remains was found locally. This is agents. m two gradings are exposed coarse
full of roots. From m 17 to m 52 coarse sand, pebbles pebbles and cobbles in their lowerparts. and and cobbles were deposited with very little fine sand
Section T. — is the section in the whole clay. Mega cross-stratification is very common. Grading only area lower well in these units is only longitudinal, but the boundary studied where exposures from the part as as balls from the of the formation This section between the units is always very sharp. Locally clay upper part occur. far 175 the southern side of could be observed with considerable dimensions (up to was surveyed as as m along
to Vado de Cervera 15 cm in diameter). The thickness of the cross-stratified the road from Cervera de Pisuerga
to further to south for the rest of the section. As sets varies from 70 cm in very coarse components and the sands. Formation has been covered about 15 cm in coarse- to medium-grained At the Voznuevo not by scree
isolated material from the mountains as is the case m 33 a 110 cm thick layer of black clay with everywhere such else between the Cantabrian Mountains sand lenses occurs in which organic remains as (the boundary
leaves, stems and black wood remains are frequent. and the Meseta is shifted to the north in this location), the surface - sand alternations the sediments have been Towards the top clay white were exposed to during a the encountered comparable to those in Fig. 32. The black considerable time. This is probably the reason why
clay unit lies between coarse-grained deposits and silts, sands and gravels in this section are superficially sudden in cir- consolidated iron represents a very change energetic by (hydr)oxides. first 10 of the section form the cumstances in the environment. From m 45.5 clear The metres eastern
of of the basal of the gradings are observable, usually with sedimentation continuation conglomerate previous Closer of this clays in their upper parts. These cycles have a maximum section. inspection conglomerate brought of thickness of 8 metres. They closely resemble those of to light that finer-grained intercalations, composed the sand and between the Section Г. Especially from m 60 to m 67, where clay, occur closely packed cobbles. These intercalations cannot be traced sediments are exposed in a gully polished by water, laterally gravel and sand with clear cross-stratification with over long distances, since erosion of the conglomerates discerned. their The transition between the obvious tangentiality could very well be As in precluded preservation. the thickness of the and the sands is the exposure near Brugos de Fenar, conglomerates or clays never gradual. downward sets decreases with decreasing grain size. The sets in the The fine-grained intercalations can have a bottom of the gradings have a mean thickness of 60 cm; convex lower boundary which can only have formed as of eroded channels towards the top they proved to be only 25 cm thick. fillings formerly (cut-and-fill
Clay balls mark the vicinity of clay not deposited in the structures); they always show a fining-upward grading. section exposed. The bottom of the grading at m 69 Sometimes clay has been preserved in the very top, rich material and with black colour. comprises pebbles 6 cm in diameter, clay balls con- in organic a light grey to material trunks. The latter remains of seatearth taining organic and large are In two fine-grained layers a are often filled with gravel and are strongly pyritized. At presumed: the sands or silty clays in which they occur
Tube-like structures with m 78, south of the railway, a fault with an unknown show intensive rooting up. diameters of 2 filled with blackish throw occurs. The rest of the section is not exposed up to cm are peaty material. until the road, where black clays were found at the
From mil to 47 at structureless sands northern side of the road, Mesozoic limestones at the m least, occur, alternate southern side. which with large gaps in the exposure. 312
From 68 112 cross-stra\ified units 500 The is divided m to m are exposed, cm. uppermost clay in two by a con- solidated sand which may reach thicknesses of more than 200 cm. The coarse-grained layer which shows an diameter of the cobbles 8 alternation of shale-like maximum is approximately to light grey clay and sand in its the diameter between 2 and 3 The latter contains brown and 9 cm, mean pebble cm. top. plant remains, leaves
Pebbles consisting of sand and gravel derived from the stems. The clay underlying these sandy intercalations
Voznuevo Formation itself were hardly encountered. does not contain any macroscopic plant remains; its
is low that not considered colour varies from to Their percentage so they were light grey light green. The over-
in the determinations of the pebble composition lying clay, again with a shale-like appearance, is thinly
(Chapter VI). A well-exposed part of the section near laminatedand has a light grey to brown-grey colour. It is
105 showed that backflow the form of rich in muscovite and shows abundance of wood m phenomena in very an
bottomset structures (cf. this chapter, part B) may remains, plant remains and stem fragments. Layers with
accompany some cross-stratified units. exclusively organic remains occur locally. Some tens of
From mill to m 203 a combination was made of a metres towards the west the intermediate sand layer reaches couple of excavations in which grading cross-stratified a thickness of 400 cm, thus forming a lens-like sand black units, generally not exceeding 100 cm in thickness, are body within the clay horizon. From m 175 to
225 slickensides indicate — common. The maximum pebble diameter has also m many the presence of
value of 8 small - faults. diminished; only in one case was a cm probably Approximately at m 252.5 the first of reached. Clays form the top of the gradings, but in many appearance Mesozoic limestones was observed. cases they will have been eroded and will only be On top of about 15 metres (estimated value) of lime- the form of balls the lower of 16 of preserved in clay in parts stones some metres deposits closely resembling the units. At 117 2 3 thick those of the Voznuevo overlying m a to metre Formation are exposed before able erode downward three the Mesozoic limestones of grading complex was to into reappear. At a distance
The horizon 100 from the of the first limestone underlying units. most important clay at cm top layer one 137 found m was to change laterally into a strongly may distinguish:
burrowed black clay rich in muscovite. An overgrown f. see e, calcareous. slope at m 174 apparently causes a hiatus of about е. 600 cm of medium to fine sand, cross-bedded, 200 metres in the of the section. Closer exposure locally accentuated by thin dark brown stringers examination of the structures in the Voznuevo Forma- consisting of muscovite and light brown wood remains. tion in this location by small-scale mapping revealed that d. presumably black clays. the formation has been folded into a syncline which runs с. 200 cm of medium to fine sand, consolidated, cross- approximately parallel to the road from Cervera de bedding common. Pisuerga to Vado de Cervera. This syncline is flanked to b. 150 cm not exposed. the south by an anticline. Both structures have an east- a. 400 cm of medium sand to silt, locally consolidated ward fold axis. these structures into dipping Taking and by iron (hydr)oxides very rich in muscovite. Large consideration we suggest that the slope must overgrown concentric concretion-like bodies several metres in width in the main follow the folded plane of stratification and and approximately 50 cm thick have formed in these that the hiatus in exposure comprises no more than only sands. Thin black clay layers are frequent. a few metres. This assumption was strengthened on finding a black clay horizon half-way the slope which We have drawn the boundary between the Voznuevo at may be correlated with the black clay m 223. Formation and the Mesozoic limestones between units e
From m 177 to m 189 mainly coarse-grained sand and and f. of buff colour form the of fine-grained gravel a cover a The sediments between the limestones are litho- hill which is shaped by the anticline. Strike measure- logically not distinguishable from the sediments at the ments in the strata exposed on the hill-top turn round of top Section T and in all probability belong to the through nearly 270°. Cross-stratified units not exceeding Voznuevo Formation. The only difference which can be 100 cm in thickness give rather diverging paleocurrent observed are the traces of intensive tectonic deformation silt directions. The units are sometimes topped by a thin which manifest themselves as folds, faults and slicken- or silty clay layer. By far the most important contribu- sides. An approximately 40 cm thick consolidated sand tion to the insight into the geometrical properties of layer which has been broken and compressed into these cross-bedded units is the exposure of a principal imbricated of scales constitutes evidence strong move- bedding plane, the only one in the area studied. From ments. The doubling of the contact between the this it became clear that the cross-stratified exposure Voznuevo Formation and the Mesozoic limestones was in units this part of the formation have straight to very of presumably caused by faulting in the upper part the crests (Fig. 37). On top of the slightly undulating ripple Voznuevo Formation in which the black clay horizons 190 part at m similar sands and gravels occur unexposed could have acted as lubricating layers. as those covering the top of the hill, but red to purple discolourations different number of features from sediments in the give them a appearance. A the area
From m 202.5 onwards fine-grained deposits prevail. between Cistierna and Cervera de Pisuerga can be
Thick reach thickness of than summarized follows: clay layers may a more as 313
Section P: Transition from sediments comparable to Section R: Sediments resembling those of, for instance, of the lower of the those sections K, L and N in part the upper part of Section F. Fining-upward grading units section to sediments with a certain cyclicity and of a are common. In this section it becomes clear that the smaller influence of fact bodies grain size. The tectonic movements black clay horizons are in lens-shaped with a this of This reflected of of in part the area is striking. is by a maximum extension only some tens metres. strongly reduced thickness of the formation in this Section S: Apart from a quartzitic basal conglomerate, location and by intensive crumbling, especially of the predominance of relatively fine-grained deposits in resemble the coarsest components. fining-upward grading sequences. They sediments encountered the of the forma- North of the villages of Guardo and Muñeca, gradings in upper part further the for those of Section D. from pebbles or even cobbles into silts or clays form by tion to west, instance far the main portion of the sediments exposed. Paleo- Section T: Fragmentary exposure of the entire forma- with the continuation of the basal current directions measured in cross-stratified units are tion, beginning
found Section S. From 68 to the very divergent. conglomerate in m end of the section 252.5 Section Q: This section is clearly bipartite, with a upper at m a fining-upward of and silts the lower of the trend of the formation whole could be observed. predominance clays in part as a section and grading units consisting of gravel and sand in The transition into the overlying Mesozoic limestones is the The sediments of this section and in the of the formation upper part. coarser very gradual. Faulting upper part of those of the excavations near Guardo and Muñeca are is probably responsible for doubling the contact. devoid of the rapid changes in grain size characteristic of the sediments in the lowerpart of the Voznuevo Forma- tion further to the west.
B. SEDIMENTARY STRUCTURES OUTSIDE SECTIONS A TO T AND I’ TO III
scribed TYPES OF STRUCTURES IN GENERAL separately in this part of the chapter, we shall
first consider the coarsest deposits. From the description The sedimentary structures occurring in the Voznuevo of the sections we have seen that E of La Robla the into Formation can be roughly subdivided three groups, of the deposits pebble and cobble size only occur in depending upon their grain sizes: lower part of the Voznuevo Formation. Unfortunately, a. Structures in fine sand, silt and clay, as described, for we usually obtain information from exposures in walls instance, in Section J and in this part of the chapter of excavations, in which only channels eroding each (locations La Mata de la Riba, La Devesa de Boñar and other can be recognized. The width:depth ratio, as well the of excavation near Grandoso), consisting mainly as the paleocurrent direction, can only be estimated, parallel lamination, with occasional small-scale cross- both with doubtful accuracy. Circumstances are much stratification in the coarsest constituents. This kind of favourable more between Boñar and Voznuevo, near the sediment is often burrowed very intensively by organic contact with the Paleozoic. Here the formation is locally black coloured activity. The clays are frequently grey to superficially consolidated on the SW dipping mountain due to organic remains. slope. As a result of differential weathering, channels b. Structures in medium sand to gravel (up to medium which can be studied in three dimensions have been 3 pebble size, cm in diameter), frequently showing mega weathered out. In two locations, NW of the village of cross-stratification, with tangential foresets. The Boñar (directly E of the tarmac road to Adrados) and N crestline of these current is pattern mega ripples straight of of of the village Voznuevo, the axes of a number to slightly undulating. Although transverse sections are channels, and hence the paleocurrent direction, could be
very rare, we believe that the type of mega cross- determined in the field. In both locations we are dealing stratification is intermediatebetween planar- and trough- with coarse deposits directly overlying the Paleozoic. towards the type cross-stratification, tending much more
planar type. Bottomset structures such as backflow
phenomena, and related structures, proved to be The results of the paleocurrent direction measurements
common. They will be extensively dealt with in this are given in Fig. 9 and on the general map (Enclosure I).
chapter. Distribution of current directions proved to be rather
с. Structures in coarse sand to pebbles and boulders; small. In the location Boñar all measurements lie within
of often found as eroding channels with a width of a few an angle 90°, at Voznuevo even within an angle of from 400 45°. The median direction for metres and a width:depth ratio (xlOO) varying the Boñar area was
to 600. approximately from the W (262°); for the Voznuevo
area from the WSW (249°). These data fit perfectly with the directions found from CHANNELS BETWEEN BOÑAR AND VOZNUEVO measurements on foreset-dips
in mega cross-stratification in comparable levels in the As the mentioned under and b will be de- structures a formation. 314
the while vertical has foresets, grading developed as in
type I. Subtype lib is similar to subtype Ha, but with silt
bands between the foresets and the bottomsets. The
angle between the foresets and the base of the unit is
generally very small (less than 10 degrees). The
bottomsets between the silt bands show an internal
small-scale cross-stratification, which can have a direc-
tion either opposite to the direction of the foreset in the
same unit or in the same direction as the foreset
(reported in fewer cases). These two directions can
appear together, the oppositely directed small-scale ripples always being closer to the adjoining foreset. Fig. 9. Compass-cards of paleocurrent measurements in channels
N NE of Boñar and of Voznuevo. Arrow indicates mean direc- to carried out Allen tion of supply (n= number of measurements). According experiments by (1965d),
vertical grading in the foresets, as found in all three
is caused of finer Two examples from the Boñar area have been chosen types, by filtering grains through
ones, when takes the lee- in order to illustrate these channels. Fig. 38 shows two coarser avalanching place on
side of a large-scale ripple. The coarser grains, moving on channels; the left one, just visible on the left hand side
the surface, overtake the finer ones and concentrate near of the figure, with a current direction from 280°, the
the toe of the foreset. This causes as right one with a direction from 295°. The photograph is longitudinal grading described in I. With increasing flow velocity, taken approximately in the direction of the channels, type becomes more there is insufficient looking downstream. On the principal bedding plane a avalanching frequent; channels time for and therefore slightly undulating boundary between the can filtering longitudinal grading be Differences becomes less well This that seen. in grain size are well-pronounced as developed. means relatively
of flow velocities over the crests a result weathering. higher large-scale ripple were involved in types Ha and lib than in type I. The silt
bands formed between the groups of foresets seem to be
in contradiction with increase in flow and THE MECHANISM OF BACKFLOW an velocity
must be explained in a different manner. This process
Uncommon of cross-stratification were could be visualized experimentally by the speed types mega varying encountered in the Voznuevo Formation. In order to of the flow. China clay was used to replace the silt and
marine conditions imitated salt to the compare these types with findings by other investigators, were by adding water, in order to accelerate the flocculation. a short introduction with a description of similar process by Allen and the three of phenomena will be presented here. These phenomena According to Narayan, types
cross-stratification as found in the Folkstone Beds have been described as having been generated by reverse mega
are thought to have been formed under shallow-tidal circulation or countercurrent or, more frequently, by conditions, the I and Ha under normal backflow. These three names are used for one and the types originated
tidal flow conditions, while type lib '... could have been same process. Following the use in recent literature formed from that travelled when (Boersma, 1967) we shall use the term backflow ripples only enhancing
(Jopling, 1961). storm currents arose, silt being deposited on the normal tidal conditions'. One of the first descriptions of backflow sedimenta- stationary ripples during
has as been said about the tion was given by Allen and Narayan (1964) in their Nothing yet process creating the small-scale in the bottomset layer. The sedimentological investigation of the Folkstone Beds ripples mechanism involved is best visualized in Fig. (Lower Albian) of the English Weald. The lithological 10, and according to Jopling (1964) and Boersma (1967). composition the presence of ammonites, together with wood indicate shallow Expansion of the water-body at the lee-side of the ripple remains, a relatively marine is environment. of cross-stratification leads to flow separation. The space filled by Here, two types remaining based foreset a reverse circulation system capable of on shape are distinguished: (backflow) forming oppositely directed small-scale ripples in the
bottomset The main flow also for Type I, with straight foresets which form an acute angle layer. is responsible the small-scale directed the the to the base of the unit. They show a concentrationof ripples in same sense as Between these coarse grains at the base of the layer (longitudinal mega ripple (coflow, see Fig. 10). two
small-scale at the where the main grading) and a grading from fine to coarse in the foreset ripple systems, place
flow the an 'erosion hollow' (Allen, layer, perpendicular to the stratification of the foresets impinges bed,
to (vertical grading). 1965d) is formed. The backflow is able sweep this and b. In Ha coarse the foreset surface; leads Type II, subdivided into subtypes a subtype relatively grains up the foresets meet the base of the layer tangentially. to the formation of upward directed wedges ('nose-like
Longitudinal grading is lacking, the median grain size at intrusions') between the foreset beds. More detailed information small-scale structures the base being even smaller than the median grain size in on 315
differentiation and lee-side and Fig. 10. Geometry of flow over a ripple with long stoss-side demonstrating topset-material area transport
to 1963, 1967). sedimentation under the action of a reverse circulation (backflow) (According Jopling, modified by Boersma,
cross-stratification These silt bands be accompanying mega was given by deposits. always seem to related to Boersma In this the backflow (1967). case, sedimentary environ- cross-stratification and are superimposed on
ment in which the structures found is known with the foresets were mega without interacting with them. the sediment studied subrecent certainty, being a deposit Boersma explains these phenomena, which are described
in a meander bend of the River Rhine. In this from the Folkstone exposure, Beds, as belonging to purely tidal weir-lock a large excavation near Amerongen (The water movements. The backflow ripples and the silt Boersma of bands both slackwater Netherlands), distinguished eight types mega originated during a period, the cross-stratification with bottomset structures, first short the latter arbitrarily during a one, during a long one,
chosen from constantly changing patterns. In one of the respectively. Boersma does not explain why this clay and best from which developed examples, a lacquer peel was silt cover is not eroded by the next main tidal phase. made, the foresets underlain large-scale are by: Cohesion of clay particles, as observed on tidal flats, is 4. subhorizontal coflow sets probably the cause. 3. transition irregular zone Though (a) climbing backflow ripples, (b) deposition 2. backflow climbing sets in areas expected to be erosional and (c) tangential 1. lamination foresets even are the three main phenomena indicating a large
Boersma's statement that vertical grading in the large- supply by suspension, Boersma's fourth indication that 'thick bottomset in make scale ripples may be from fine to coarse (from bottom to layers some instances up as 'reversed' much 50% of the total thickness' does top), but '...exceptionally was a distinct as set not seem reliable since unknown of the total vertical grading from coarse to fine upward encountered' quite to us, an part set have been eroded. is of great importance since this is in contradiction to might Other of backflow cross-stratification vertical grading as described by Allen and Narayan. properties were reported in relation to the cross-stratification of Characteristics of the bottomset layers are: climbing mega one and the same set, from the and tangential backflow cross-stratification, both again Amerongen excava- tion, by Boersma et al. (1968). Measurements indicating supply from suspension, and cross-stratifica- comparing crest orientation of both and backflow tion resembling wave-built ripples in the transitional ripple mega
ripples brought to light that the ripple crests are oblique zone, due to fluctuations in stream velocity in this zone. and sometimes to each other, the The supply of suspended material must have been perpendicular deviation from 10° than 90°. sufficiently strong to cause deposition in the transitional varying to slightly more
This divergence from the theoretical was ascribed zone and in the coflow zone, notwithstanding the fact picture to transverse, which is in that these zones are expected to be exposed to erosion. or radial, flow, common
According to Allen, tangentiality of the large-scale meandering rivers. Deviations in the Amerongen
all to same foresets was caused by a high rate of suspension plus a exposure proved have the direction, flow hand of the this high rate of velocity. The apparent connection indicating a right turn river. As in case between these two factors, however, is caused by the subrecent sediments are involved, the configuration of the river bend reconstructible. schematic deficiency in Allen's experiments. As Allen used a closed was easily A for his material from outside view of the interaction between main radial system experiments, no plan flow, available and the stoss-side of therefore and backflow in front of is was a ripple was flow, a mega ripple-front
et the only supplier of material deposited on the lee-side of presented in Fig. 11. Boersma al. propose to name the backflow with deviation of the same (or next) ripple. For this reason it is expected current a 90° 'secondary of flow'. In this much that a lower current velocity with a larger supply our opinion term might cause con- backflow itself flow! We suspended load may produce the same structures as a fusion, being a secondary higher current velocity with constant avalanching. therefore propose to use the term 'sideflow' in cases between flow and backflow The greatest difference between the backflow cross- where the angle main is less than 120°. stratification in a fluvial environment and in a tidal
With these facts backflow cross-stratification environment is the occurrence of silt bands in the latter on in 316
which never reach a thickness of more than 10 cm.
The lowest fine-grained layer consists of at least
300 cm of fine-grained sand, silt and clay. The clay is
rich in organic matter, and has a light grey to black colour. Cross-lamination rarely occurs in this layer, while lamination even is common. Only in a 20 cm thick clay
layer was. some cross-lamination encountered in sand
bands 3 mm in thickness. A closer look at these 300 cm
revealed them to consist of a number of units with a
grading fining upwards. Fine sand and silt contain finely
spun wavy clay stringers, generally traceable in bundles
and not individually. The maximum thickness of these
stringers is 1 cm. Concentration of iron (hydr)oxides
appears mainly to take place in the silt fraction, which results in light brown coloured, sometimes isolated
patches. Apart from this, some flow-like structures of
unknown origin occur in these deposits. The sediment
must have been soft at very the time of its deposition. The black coloured to grey clays are rich in sand lenses;
many tube-like sandy structures are formed by burrow-
ing organisms and/or plant roots. These small-scale
structures are again comparable to those found in
section J near Campohermoso. The 10 following metres, already mentioned above, are
poorly exposed, but probably do not differ very much from the 13 metres by which they are overlain (apart
from the iron content). The latter, mainly cross-bedded, 13 of sediment metres are chiefly composed of coarse to
medium-grained sand, never exceeding pebble-size, not 11. Fig. Strongly simplified plan view of the area between two even at the bottomsets of the cross-stratified units. fronts drawn with The local flow mega ripple straight crests. (R) Except for one case with a thickness of a cross-bedded is the resultant of radial flow and mainflow (or backflow) unit of 250 thicker than 150 (According to Boersma et al., 1968). cm, no units cm were
found; 15 cm proved to be the minimumthickness. mind we shall now take a closer look at two excavations Enclosure X gives a general view of the SW wall of the in the area between Boñar and Colle. pit; this wall is exposed in its full width. Here 7 sections
were made with a mutual distance of 10 metres (I—VII),
3 with a mutual distance of 20 metres (VIII—X).
THE EXCAVATION S OF GRANDOSO Measurements of paleocurrent direction were carried out
according to the technique described elsewhere in this
The The results subdivided four excavation situated directly on the southern border chapter. are according to of the of Grandoso the from 1 4 village is the only sand pit still in use. quadrants on compass-card, to (see the Enclosure X). From this it became clear that Consequently, appearance of the quarry changes paleo-
daily, especially in its south-eastern part. In the summer current directions vary between directions from NW to of 1970 the excavation was about 130 m long and some SW, except for the area between the dashed lines, where
40 m broad. The sand that is exploited is situated paleocurrent directions from SE to S prevail, together between two horizons, consisting mainly of clay and silt, with a number of cross-stratified units, laterally not which formed levels of which traceable further than with direc- probably two on top 15 metres, varying ground waters accumulated before tectonic deformation tions, some of which from the NE. The base of the area of the formation almost took place. On top of the lowest layer, with opposite directions is clearly elevated in the thickness of about 10 of SE of the of this a m the overlying sediments is part wall, the total thickness unit more or less consolidated by the formation of iron diminishing to 90 cm in section IX. crusts. These have the The cross-stratified Allen's generally developed parallel to mega units belong to planar stratification and show towards a preference towards pebble-size type, with sometimes a very slight tendency his sediments since these have the In difficult to highest permeability. trough type. It is, however, very pass a these found definite of addition, coarser parts are nearly always at judgement on a special type cross- the base of the section a grading sequence, on fine-grained top stratification, as a principal bedding-plane is of the underlying one. Sometimes pockets extending to never exposed. Using descriptive terms, these current
horizon In be an underlying coarse-grained may occur. ripples can designated as: large-scale, grouped, these first 10 metres thin clay horizons have developed erosional, mainly planar and homogeneous. It is very 317
surprising and highly peculiar that backflow phenomena 4. As oppositely directed small-scale ripples in mega
to be the rule rather foresets. appear than the exception. In many ripple
cases small-scale backflow ripples could be assumed from
the size in the bottomset of the grain layer cross- It is unfeasible and of little use to describe the wall in its
stratified could be studied in the 'natural' unit, or entirety in as detailed a manner as possible. A few
section (cleaned by wind action). In seven cases, as illustrative examples of sedimentary structures, indicated in Enclosure X by BF, the direction of the elucidated by figures, will therefore be selected. Fig. 39
backflow foresets could be determined accurately and an outline of gives a mega cross-stratified topset with compared to the direction of the foresets of the mega backflow structures in the foresets and of a bottomset
ripples belonging to them. The between the flow built backflow This be angle layer up by (section I). unit can direction of the and the small-scale mega ripple ripple traced laterally for about 20 metres. The bottomset
from 83° to ranged 180°, always in a clock-wise direc- structures consist of backflow crossbedding in two
tion. Boersma's relation between these two Applying channels, each 100 cm wide and with a maximum depth directions and the radial flow in all a meandering river, of 10 cm. The thickness of the backflow set in the mega structures these could originate in a river bend with a is 8 ripple cm, while the thickness of the mega ripple right hand turn. foreset varies between 5 and 8 cm. The sediment
Backñow structures manifest can themselves in various affected by backflow looks much finer-grained than its
ways: surrounding sediment. This apparent difference in grain
size has been ascertained by determining the grain-size As fine-grained bottomset layers, laterally traceable distribution of from backflow 1. a sample the zone (063) several metres. along and from the set From these data a sample mega (064). 2. As individual 'bottomset channels'. two cumulative curves were constructed (Fig. 12 A). As
3. visible as 'nose-like intrusions'. Only was to be expected, sample 063 is much finer-grained; it
Fig. 12. Diagram of size frequency distributions of samples collected in the mega cross-stratified units and in the backflow cross-stratified units in the excavation S of Grandoso (А, В and C) and in the excavation E of Boñar (D). 318
consists almost exclusively of sand; almost 50% of
sample 064, on the other hand, is composed of gravel.
Tangentiality is a common feature, both in the mega set
and in the small-scale set. Fig. 40, photographed from
nearer by, gives an impression of the longitudinal grading
in the backflow unit. The direction of the backflow
foreset is accentuated by the setting of small pebbles along the foreset plane. A channel with backflow
structures is exposed bodily laterally of section I (Fig.
with 41). Again two samples are compared respect to
their grain-size distribution. Though less well developed
than in the previous example, differences are still very clear(Fig. 12 B).
Fig. 42 represents the situation in section III, at the transition from the area where current directions from the SE predominate, to the area with directions from the
W. This transition is thought to be erosive, as is concluded from balls with the occurrence of big clay diameters of to 20 of Below the up cm (centre figure). horizon of changing current directions, two cross- stratified units are seen to replace (erode) each other.
A different, not very common structure, referred to as
'upward turned nose-like intrusions' (Boersma et al.,
1968), was encountered in section IV (Fig. 43). Samples from the gravel (068) and the medium to fine sand (069) are again compared in cumulative curves. In Fig. 12 С 13. Interaction between of the differences distribution Fig. avalanching coarse-grained great in grain-size are material along a mega foreset plane and fine-grained material visualized. The generation of these kinds of fine-grained from backflow ripples, leading to the formation of ‘upward noses is shown in 13 Boersma Fig. (after et al.). turned fine-grained noses’ consisting of cross-stratified material
In some cases it could be observed how a megaset unit in the mega foresets (According to Boersma et al., 1968). without bottomset structures passes laterally into a unit with a very distinct foreset layer and a bottomset layer bedding'. In the mega foresets vertical grading is very with latter well from bottom fine backflow, the not developed in channel-like developed, a coarse to a top. In bodies but as a continuous layer. An instance of such a the small-scale foresets of the internal cross-bedding a structure was found between sections III and IV (Fig. distinct longitudinal grading is present. Measurements in
The foreset at the left hand side, the centre the foresets and the small-scale foresets 45). layer in mega in gave a
is of the figure, seen to split up into a foreset and a difference in paleocurrent direction of only 8 degrees. bottomset Boersma's layer. With observations in mind, This value lies well within the accuracy which may be this be caused sudden the splitting may by a increase in expected with these measurements: both types of ripples of material quantity suspended or/and by an increase in thereforehave roughly the same direction. flow The unit which this noticed the foresets velocity. in phenomenon was As we before, vertical grading in encountered is overlain unit in which the stream of both and backflow is to by a mega ripples ripples opposite direction from to Enclosure the from fine to to found changes (transition j3 y, X). grading coarse (bottom top) In section V the transition boundary just mentioned is by Allen and Boersma. Fig. 46 clearly shows this clearly erosive (Fig. 44). Unit B, with a thickness of difference. 60 cm in the left part of the figure, diminishes con- Wood remains, probably trunks and branches, were siderably within a few metres and is finally cut off by frequently observed on the foreset planes. They may the which overlying unit A, belongs to direction unit y. reach considerable dimensions (up to a few decimetres). On the transition boundary clay balls and pebbles occur Probably they were left behind after having become with diameter 4 a up to cm. waterlogged. Near the of covered top section VI a mega cross-stratified unit All sections in the SW wall are by a con- 30 of cm in thickness is exposed, with a fine-grained spicuous pebble bed, 30 cm thick, consisting pebbles bottomset with a maximum thickness of 4 cm in which with a maximum diameter of 6 cm in a coarse sand indistinct backflow formed. of structures have been In the matrix. This layer is followed by 20 cm mainly coarse foresets irregularities occur, of a type which seems to be sand, fining upwards, with clay balls in the top. In this It could the SW very rare. concerns a phenomenon that be de- layer a paleocurrent direction from was scribed 'coflow in the as ripple'. Fig. 47 gives a general observed. In the final 300 cm, which are exposed on top view of this kind of 48 with of sediment structure; Fig. is a close-up, this sand, the character of the changes three (incomplete) foresets, showing this 'internal cross- completely. Grading alternations, consisting of sand, silt 319
with thickness with maximum and clay were found in layers a maximum by a relatively fine-grained layer a of 70 cm. Like the sediments that were encountered thickness of 2 cm. The filling of the channel is clearly
the base of the all sand-sized and and shows foresets, to the left of near excavation, asymmetrical dipping
sediments rich in iron and have dark the in which is coarser are very a figure, longitudinal grading perfectly red-brown to yellow-brown colour. The silts and clays developed. In this example, too, the finer beds stand out fractions in relief. are orange-red and light grey respectively. All do Unit E could be closer examined in the of are very rich in muscovite. The sands not display any summer 1970 result of sand activities in structures, the silts and clays on the other hand have as a new exploitation influenced of the Both ripple foreset-dips and back- been very intensively by organic activity, part pit. mega probably both by burrowing and by rooting up by flow ripple foreset-dips could accurately be determined.
to form vegetation. Some of the grey clays are very rich in Backflow directions proved an angle varying 171° 90° of flow organic material and have a seatearth-like appearance. from to to the direction the producing Concentrations of the foresets mega ripples. gravel along
underwent discolouration by iron (hydr)oxides, accen- cross-stratification. wood THE EXCAVATION E OF BOÑAR tuating the Large fragments,
armoured with gravel, were frequently found in the top
second sand of unit E. Along the road from Boñar to Sabero, a pit with remarkable structures is exposed at km 1.5. Some The transition from unit F to unit G is clearly
of of the Voznuevo Formation can erosional. Here a ripple backflow 26 m the upper part tripartite mega be observed here. The distance from the Mesozoic complex is exposed, composed of subunits a, b and с of limestones is approximately equal to that found near (compare Fig. 49). The total thickness the unit is 65
Grandoso. This pit is also one of the rare locations where to 75 cm.
well structures found. This very preserved were exposure the and the one S ofGrandoso are supposed to belong to 'a' is a 'normal' cross-bedded set, 40 to 45 cm thick, the same lithostratigraphical unit. In contrast to composed of medium to fine-grained sand to fine gravel, much lateral informa- Grandoso pit, we do not receive only very few fragments coarser than 1 cm in diameter.
5 metres of tion from the units exposed. Only the upper The median thickness of the foresets is 3.5 cm. Flow could be traced for of the sequence some tens metres, direction was from NE (47°). form the SW wall of the As in the of since they exposure. 'b' is a backflow zone, 0 to 15 cm thick, composed between and the stratification entire area Boñar Colle, coarse to fine-grained sand, gravel being practically 26° the SW. from plane dips, on an average, to absent. The flow giving rise to these ripples came The clayey and silty sediments, which form the SSW(197°). bottom and the of the Grandoso bottomset with channel-like top sequence, are not 'с' is a continuous layer exposed here. Clay is only found as thin layers in the thickenings, with a total thickness of 6 to 15 cm, nearly tops of fining-upward gradations. The main constituents exclusively composed of gravel. The coarsest fragments medium sand of this pit are again the fractions between are 1.5 cm in diameter. Flow direction proved to be the
sets with and fine gravel. Cross-bedding in a maximum same as that observed in 'a'. thickness of 100 cm is common; paleocurrent directions
be observed. be could frequently In minutely exposed The generation of this tripartite complex can ex- units, backflow phenomena could be observed and plained as follows: Subunit 'с' represents the fractions that slide down the foreset measured. Current directions change very rapidly in a immediately plane; they are
with to directions from NE. in almost constant contact with the bottom, and are vertical sense, a preference of the under- A section through this excavation is presented in capable eroding channel-like bodies into
Enclosure VIII. In order to facilitate reference, the units lying unit. This sedimentation is followed by the back-
flow front of to have been marked with letters (A to D') left of the ripple in the mega ripple, which appears will be be this of sediment. both section. In the following paragraphs attention common in type Finally features. subunits covered drawn to some well-developed are by large-scale ripples.
A foresets of Grain-size distribution of a from each subunit In unit mega cross-bedding are sample accentuated by differential weathering. It is worth was determined and represented in a cumulative curve of noting that finer-grained beds (i. e. very fine-grained (Fig. 12 D). Laboratory determination of grain size
to be more resistant to than the the sample does not differ much from the size sand) appear weathering very grain
less well estimated the field. to be the coarser beds. The latter are probably packed, in As was expected, grain
to foreset of the sediment intermediate and disintegrate more easily. Perpendicular the size mega rippled is planes vertical grading is well developed, again from a between the finer-grained backflow and the coarser- bottom material the bottom. coarse-grained to a fine-grained top. grained transported along
In unit В the plane of intersection exposes a number The next unit, which merits our special-attention, is
of channel-like bodies of relatively small dimensions. unit R. Here five gradations were distinguishable. Four and of them the The example of Fig. 53 has a width of 75 cm a contain angular clay fragments in top. channel covered have in the lowermost depth of 13 cm. The bottom of this is Besides, clay fragments developed 320
unit V the current direction changes into a direction
from the E; this situation lasts up to the top of the
section.
Unit V 14 thick and of is cm consists a 5 cm thick,
very coarse-grained, bottomset layer, with cobbles 8 cm
in diameter, which is overlain by cross-bedded coarse
sand and fine gravel (Fig. 50). Unit W of the is one most pronounced layers of this thanks excavation, to its coarseness. It consists of a
of concentration pebbles, with a maximum diameter of
14. Detail of small-scale units in the excavation E of 5 of Fig. grading cm, floating in a matrix coarse sand to fine gravel. Boñar with broken clay chips in their parts. This the upper unit belongs to same generation as unit X, and
that of the cross-bedded represents part mega set which gradation as armoured mud balls, distributed over the has been exclusively transported along the bottom.
but concentrated Some the left hand side entire gradation, more in two horizons, metres on of Fig. 50, finer- of them the The field sketch 14 shows affected found which one at top. in Fig. grained sand, by backflow, was in
detailed of R which is direction from the NW measured. a part unit in grading developed a current (304°) was between two clay pebble horizons. In the right of the The angle between the flow directions of mega cross- stratification backflow figure two gradations were visible, separated by a NW and of cross-stratification proved this be In the of dipping upward bent clay layer. Below this clay pebble in case to 135°. hope finding some horizon, small-scale cross-bedding is visible, with foresets regularity in the flow directions ofunits X, Y and Z in a lateral dipping to the SE. Yet another directional feature is sense (e. g. a river bend), measurements were show carried out in the locations marked Yl found in the angular clay fragments: they a clearly XI, X2, X3, etc. in The results follows: NW-dipping imbrication, thus indicating the same flow Fig. 51. are as direction as the sand. It is worth noting that these clay XI: from 79° X2: from 99° X3: from 108° flattened. fragments are scarcely rounded and still very Yl: from 95° Y2: from 80° Y3: from 106° The only possible conclusion as to these clay fragments Zl: from 85° Zl: from 96° Z3: from 76° is that the distance of transport must have been very short. It is seen that differences are very small and that there Unit the SE of the is T, exposed in part excavation, is no regularity in the changes in the values. We must of unit of and cross-bedded composed a megaripple consisting gravel therefore assume that replacement of units, sand. In the with maximum thickness result of coarse foresets, a as occurring in this wall, is not the a changing of 6 cm, vertical grading from coarse to fine is perfectly river configuration. One could also assume that the
wall intermediate between the developed. exposed is an section dip- which The showpiece of this pit is the SW wall, in section and the strike-section of a sequence of nearly D' considerable lateral units U to are exposed in a exten- identically directed cross-stratified sets. Erosional planar 52 and 50 show view of the sion. Figures 51, a general or slightly trough-shaped cross-stratification should then detail detail exposed wall, the easternmost part in and a be responsible for the successive replacement ofunits X of units V to A and Y. There much indications Y, respectively. cursory survey already are, however, more salient features brings some to light: pointing towards a third solution. This third possibility
A drastic in direction between units is based the fact that size in the cross-bedded 1. change current on grain
U and X, separated by two relatively coarse-grained units units decreases towards the right hand side of Fig. 51 (i.
V and W. e. to the NW). This is best demonstrated in two detailed 2. of A decrease in the number distinguishable cross- photographs (Figs. 54 and 55) from unit Y, taken some bedded wall: X units towards the NW of the unit proves 10 metres apart. Fig. 54 represents the situation in the
be which turn cross-stratification to replaced by the overlying unit Y, in its SE part of the wall: the mega is overlain and Z. The total On of is replaced by unit thickness, composed of coarse sand to fine gravel. top the however, thins very little. boundary with the underlying unit (marked by a pebble from the W in backflow In unit U a paleocurrent direction (and just above the ruler) a much finer-grained the from the found. Just below unit V thickness and with lateral extension of top NNW) was zone, 6 cm in a
to be 'out of 160 has In 55 the unit (Fig. 52) cross-bedding seems generated cm, developed. Fig. we see same material much nowhere'. A sudden supply of coarse-grained Y, further to the NW, composed of a finer-grained for between silt fine sand with seems to be the best explanation this curious sandy sediment, to layers a structure. Apart from this feature in the top part, the maximum thickness of 1 cm. Between the fine-grained unit looks rather structureless. This is, however, not the horizons, which are in fact very flat dipping foresets, least which case; a close examination brought to light that at backflow has developed. Obviously the flow, in thickness still seven cross-bedded units are present, varying in the second case carried finer material, was strong from not manifest backflow 12 to 21 cm. Several sets do clearly enough to whirl up the sediment into ripples. differences With Lack of themselves due to lack of in grain size. These were covered by a silty top. sediment, 321
however, brought this process to an end: the unit coarse sand, consolidated at the surface.
became thinner and finally disappeared. B. 80 cm of medium to coarse sand with light yellow to
Another interesting phenomenon was encountered on dark yellow discolourations due to iron (hydr)oxides and the boundary plane between units V and W. Pockets clay stringers, generally less than 1 mm thick, with
containing material ranging from fine gravel to very large an interweaving pattern. The stringers are traceable as
pebbles, have sunk into the underlying unit, locally bundles; individually they have a wavy appearance.
do not Isolated flasers Two sand-filledburrows tube- nearly replacing this unit. These pockets give any are rare. or directional and show like structures formed could be information, they are symmetrical by plant roots found,
vertical which both with a clear inclination to the S. In the 20 a grading is fining upwards. They may top cm,
either be pot-holes or load casts. small-scale cross-stratification in coarse sand with
N-dipping foresets were distinguished. Yet the clay
stringers have a slight tendency towards inclination to
the as with the unit. Small wood THE EXPOSURE OF LA MATA DE LA RIBA AND LA S, compared overlying abundant. DEVESA DE BOÑAR fragments are
С. In the southern of the 12.5 part exposure: a cm
A located S of the of thick alternation of medium sand to fine sand with some very interesting exposure is village
coarse and some and bands with a La Mata de la Riba, on the western bank of the Rio particles clay stringers
maximum thickness of 0.5 in This Porma. Here two anticlines, in their cores sediments cm equal proportions.
is overlain 5.5 of with sand linsen. In the belonging to the Voznuevo Formation, are cut by the by cm clay northern of the these two river. The small exposure in the northernmostanticline, part exposure layers
in which minute structures could be observed, merits amalgamate into 18 cm of chiefly black clay with sand
to detailed description (Fig. 15). lenses up to 1 cm in thickness, often dipping the N with and curious The We are dealing with a complex a stratigraphical frequently showing a S-shape. sand, number of thickness of 4.5 m, in which the main constituents are which is locally rather coarse, contains a large The dull the fractions from fine-grained sand to clay. clays small black coloured wood fragments, soft and or
and rich to in the are black to grey coloured are in plant remains. hard and lustrous. The unit thins the N;
the of this Amerom total thickness does not exceed From clay exposure van (1965) northernmost part the
took one of his samples for palynological analysis. 5 cm.
Hereafter, a brief description of the various units, as D. 105— 110 cm of sand and clay, clearly dipping to the
classified in Fig. 15, will be given: N with regard to the underlying clay horizon. The
influence of grain size on the inclination of the sedimentation is shown the circumstance that A. More than 100 cm of yellow coloured fine gravel to plane by
15. Section and situation in the field of the S of La Mata de la Riba. Fig. exposure 322
of the which richer in been filled of parts unit are relatively clay depression, which has by an alternation
approach the base of the unit at a much lower angle than mainly clay and sand. After deposition of unit C, the In the of the unit sand in inclined its coarser counterparts. top a sediments were placed an position, causing a channel-like clear lens was found with a shape, showing considerable angle (10°-20°) between unit С and the southern flank be consider- and asymmetry; the proved to overlying units D, E F. An angle of a few degrees than the northern flank. for have natural ably steeper Cross-bedding could, instance, represented a slope on a
shows that filling of this channel clearly took place from floodplain; the angle observed, however, is much too of this a northerly direction. Immediately on top large to have originated from other than tectonic causes.
channel the following unit is developed. A slight tectonic tilting probably caused the slump-like of alternation of sand E. 18 cm a very thinly layered structures in unit С The three overlying units are
and clay, comparable to the alternations found in parallel to each other. Cross-bedding in small channels burrowed. small section J. This unit is very strongly A indicates a supply from northerly directions. The plane width of filled with has channel with a only 10 cm, sand, of sedimentation apparently turned back into a position the described D. To C. the same asymmetry as one in unit comparable to that in unit
the N, unit E is in direct contact with unit C. In units G to M deposition of alternating sandy and
F. 70 cm of fine to sand, with a number directional indications, was coarse-grained clayey layers, without any of small, traceable, silty stringers, slightly discoloured by accompanied by intense vegetation. The cross-stratified
iron (hydr)oxides. This unit also gently dips to the N. gravels and sands, that erode units G to M successively to
G. 8 cm of black clay with small sand lenses. the south, definitely brought to an end the sedimenta-
H. 10 cm of medium sand, consolidated by iron tion under low energetic conditions, like the units G to
(hydr)oxides. M.
I. 8 cm, as G, clay partly replaced by felt-like
remains and of wood. In the between Palazuelo de Boñar and La fragmentary plant jet-like pieces area Ercina, of J. 7 cm of almost exclusively lignite with a number the Voznuevo Formation proved to be badly exposed.
small sand lenses. small in the brooklet NE of La Only in a very outcrop
K. 6 cm of white coloured very fine-grained sand Devesa de Boñar (the Arroyo de Valle de las Fuentes)
grading into: were some small-scale structures strongly resembling and of the L. 1 cm of very well traceable light grey clay. those mentioned near the bottom top
M. 8 cm of fine-grained white sand with plant remains Grandoso excavation and S of La Mata de la Riba,
of This latter The in layers with a maximum thickness 3 cm. exposed just above the water level (Fig. 56).
unit is eroded, in the southern part of the exposure, by sediment is composed of medium sand to fine sand and
fine gravel and coarse sand with cross-stratification. The of white and black clays. A striking fact is the scarcity or
foresets are inclined to the S, thus indicating currents absence of silt-sized components. The frequently traceable from northerly directions. mm-thick clay stringers are again not singly,
but only in bundles. Organic activity played an im-
Fig. 15 shows a schematical lithological section of the portant role; egg-shaped burrow-like sand lenses with a
exposure, together with a drawing of the situation in the maximum width of 2 cm and a maximum height of
field. A this is added in have in the All photograph illustrating exposure 1 cm, developed locally clay layers. clay
57. with thickness of less than 1 have the Fig. stringers a mm Strongly simplifying the situation in this exposure, we tendency to incline to the W (on the right hand side of
to Dark may say that there are two horizons (C and G M) in the figure), as compared with thicker clay layers. direction of which finer fractions are concentrated. Mainly sandy and organic remains clearly illustrate the easterly
silty sediments surround them. From the structures the flow that produced this kind of structures. little found in the latter, the following depositional history On top of these structures very clay was is caused may be deduced: deposited. The black colour in this part by material which rich in muscovite. In the units A and В a supply of material took place black organic is very caused small faults. from S to N, as demonstrated by small-scale cross- Slight disturbances are by
bedding in some sandy parts. Unit С represents a
С. PALEOCURRENT DIRECTIONS
stratification be fixed in when the inclination INTRODUCTION AND TECHNIQUE can space, of the plane is known in two measurable planes, or, in The rare when the foreset plane itself is exposed. Paleocurrent directions were mainly determined from cases, have be foreset that two other planes to orientation in mega cross-stratified units. Use was requirement difficulties in these occasionally made of the orientation of imbricated determined, already causes
sediments, on account of the planes flattened pebbles or cobbles. A foreset plane in cross- following property: 323
Fig. 16. Compass-cards of paleocurrent directions in the Voznuevo Formation (for exact position see Enclosure I). Figures refer to number of measurements.
been of foresets can often only be seen in walls that have used a polar Gnomonic projection net for the recon- influences wind of weathered out by atmospheric such as struction of the foreset plane. Some disadvantages his and water. Artificial planes of intersection seldom show technique are: be clear structures, and foreset planes can only foreset of between 7° a. The utility is limited to angles determined with great difficulty. Apart from this, the and 45°. material involved is in a semi-consolidated state; it is too b. The planes in which foreset angles are measured of hard to make sharp intersections. The creation a must be perpendicular to a third plane. second plane is altogether impossible in gullies polished с Corrections for tectonic changes must be carried out by water, where the sediment proved to be even more in a different net. consolidated than elsewhere. Any artificial changes will d. Extensive tectonic corrections give rise to mistakes of such cause destruction the structures. In exposures (e. (of to 40°). the of the foreset has been up g. in section F) position plane estimated. as we have Tangentiality, which, seen, proved In the excavations near Grandoso and Boñar, where to cross-stratified units, can be superabundant in mega the construction of perpendicular planes did not give be the cause of considerable misreadings in foreset dips, and where the tectonic correction is not any problem, when the construction of two measuring planes can only lilies' In order very great, technique proved satisfactory. be realized with distance between these In for all some planes. to have a comparable procedure measurements, this be carried the same of Schmidt which case, measurements must out at however, use was made a net in material of the height in the cross-stratified unit and in corrections for tectonics were also made. This somewhat the same grain size. cumbrous technique has the advantage that for A well-known technique processing measurements drawbacks of lilies' technique listed above are on foreset planes was proposed by lilies (1949). lilies eliminated. 324
The measurements that proved useful, either by was only found in the extreme E of the area. This is in
quality or by quantity, in the reconstruction of paleo- keeping with values found by van de Graaff-Trouwborst
and to current directions are presented in Fig. 16 in (1970), but due a very low number of measurements
Con- carried out her, directions from Enclosure I, the general map, in compass-cards. by S and SW, which the the number of direc- also did centric circles give, from centre, were frequently encountered, not appear. The observation tions observed (1, 3, 7, 10 or more). Where exact that scattering of paleoccurent directions measurements could be carried out, 16 directions were is more frequent in the upper part of the Voznuevo Formation than distinguished; less accurate readings (due to estimation in its lower part is of great As have from or to a low number of readings) are presented in eight importance. we seen the description of the and shall main directions. The directions near the northern border sections see in part D of this chapter, the sediments the base of the formation of the formation, between Boñar and Voznuevo, were near are con- than the sediments found by measuring channel axes (cf. part В of this siderably coarser at the top. chapter). Especially in the area between La Robla and Colle,
where our information on the deposits as a whole is
this trend relatively more extensive, is very conspicuous.
RESULTS AND DIFFERENCES In the channelling cobble-, pebble- and coarse sand-size
sediments, which form the lion's share of the lowerpart
With the exception of the results found in the excava- of the formation (clay layers or bodies are only tions of Grandoso and Boñar, and in sections T and M modestly represented), distribution of paleocurrent latter lesser directions (in the to a extent, as the number of proved to be relatively small; with a maximum of measurements is unfortunately very low), directions 180°, in most cases smaller, however.
from SW to NW prevail in all locations. The important The sediments from the top of the formation are in direction from the N in section A is caused by a local sharp contrast to the lower ones; they seldom exceed the
supply of 'foreign' material from the area which now coarse sand fraction. Multicoloured clays form an forms Cantabrian the Mountains. A review of the direc- important constituent of this part. Paleocurrent direc-
tions found in the Grandoso excavation diffuse tions and gives a diverge widely: up to 270°, change more with directions from and pattern, two prevailing the NW frequently and more suddenly. the SSE. This trend became clear from All already the these facts indicate that we are dealing with this The from sections in pit (Enclosure X). patterns the sediments which are the result of two different river excavation of Boflar and from T scattered section are as models; a mainly braiding pattern in the older history of
well, with NNE and SE respectively as main source the formation, changing into a mainly meandering
directions. The direction from the SE is very rare, and pattern towards the end of its deposition.
D. INTERPRETATION OF THE CHARACTERISTICS OF THE VOZNUEVO FORMATION
Even in the few localities in which the Voznuevo Forma- 5. Cross-stratification, mainly as mega cross-stratifica- be tion can studied with more precision than only to tion. Indicationsof eolian transport are absent.
establish be in its presence, sedimentary structures cannot 6. The ubiquity of backflow phenomena, especially
described and classified without certain reservations. the of the formation. upper part between Nevertheless, some criteria such as variations in grain size 7. The abundant matrix commonly present
(grading, abrupt transitions etc.) have been used as the conglomerates.
expedients in the reconstruction of the circumstances 8. Channelling and cut-and-fill structures, especially in under of which deposition of the sediments the the lower part of the formation.
Voznuevo As in Formation took place. we have already 9. The presence of fining-upward cycles every several fluvial ascribed mentioned times, a origin is to section.
the sediments of the present study. The most important 10. The paleocurrent directions, based on foreset dip,
grounds on which this assertion is based are: imbricationand channel direction.
of balls and the 11. The frequent occurrence clay armoured presence of mud balls. 12. The flatness and roundness of the pebbles (Chapter The of of sizes. 1. presence a large spectrum grain VI). 2. The of fossil for absence a fauna, except an 13. The grain-size distribution(Chapter VII).
abundance of worm tracks.
3. The of rich fossil such will be presence a flora, as trunks, In the following paragraphs some attention paid
well of etc. which stems, leaves, as as lignite, peat to the features described in part A of this chapter
4. The absence of limestones. in the which of are important solving question by type 325
river the sediments under consideration could have been might have formed on a natural levee, since their most falls class. deposited. important constituent within the silt Lateral
sediment movement could not be observed, crevasse-
- therefore do not seem while the Section A. In section A we are dealing with splays very likely,
in the lower of sediment is too coarse for The predominantly coarse deposits; only part pure backswamp deposits. the section do cross-stratified units, composed of mainly lower part of the second detailed section with its root- remind black and its abundance of remains coarse sand with pebbles in their lower parts, us bearing clays plant bars. The well the sediment of of deposition commonly found in point very may very represent a backswamp;
with the sand - alternations, laminated, coarse upper part of the section its rapidly changing silty clay very thinly alternations of pebbles and cobbles with coarse sand, which overlie the black clays may have been formed by of been seasonal inundations of the the giving the sediment the appearance having backswamp, perhaps as for the devoid of extreme ends of The laminations be supplied as 'dashes', are most part crevasse-splays. can well sedimentary structures. Large-scale cross-stratification exceptionally preserved since regular floods of can occasionally be found in the sands, but horizontal prevented growth vegetation.
well stratification is predominant. In some very exposed
Section F. — In the of section F parts imbrication was frequently observed, as were lower, coarse part silt characteristics of braided-river This channels with cut-and-fill structures. Clay and are occur deposits. is based the sudden in size only present as large clay galls or clay balls and as the supposition on changes grain features which could be observed and the of uppermost fillings of channels. All these scarcity sedimentary horizontal stratification could be indicate deposition by a braided river. structures (only observed).
of Section B. - The first tens of metres of this section The seven cycles which form the main part the
of the of the second excavation fine of alluvial show the same pattern deposition as top are examples cyclo-
and also thems. can be subdivided into four previous section were presumably deposited They parts:
under braided-river conditions. The top of this section is basal a. a pebbly part, interpreted as channel-lag hardly interpretable due to poor exposure. deposit,
b. a sandy, cross-stratified, fining-upward part, forming Section C. - This section is marked by the braided-river the of the bar characteristics of the sediment in its lower 100 metres, essence cycle, interpreted as typical point especially in the bottom of section С 2, in which fine- deposits,
с. a part with laminated silts, sometimes in grained sand to silt layers separate rather closely packed parallel diameter. alternation with silty clays, interpreted as levee deposits, conglomerates with pebbles up to 25 cm in d. the of the The picture which this part of the section presents in the a burrowed, homogenized silty clay at top field is exactly the same as that of the alternation cycle, interpreted as levee-top or backswamp deposits. between facies G and D, as described and shown by
Williams and Rust (1969; Fig. 20). The sudden change to Although point bars can be found in both braided and finer-grained deposits in cycles less than ten metres in meandering rivers, their occurrence in braided streams is
encountered in section С more and that thickness, as 3, suggests a apparently very rare (Williams Rust, 1969), so of much of regular mode of sedimentation by waters a we are most probably dealing with deposits found the lower smaller transporting capacity than in meandering streams in this part of the section. The of thickness of the bar - this 6 part of this section. A mechanism transport by a point deposits in case to
this — meandering river seems much more appropriate in 7 metres can give some indication of the dimensionof the river which these since the case. produced point bars,
thickness of the bar deposits is related to the depth of
Section D. - This section is highly comparable to the river channel (Allen, 1965b). For small rivers the section С 3, and shows an even more impressive point bar deposits have a thickness of approximately also be predominance of silts and clays. These cycles can 1 metre; very large rivers, like the Mississippi, have mean ascribed to deposition by meandering rivers. values of 15 metres, with maxima of 25 metres. The point bar deposits in the floodplain of the Niger delta
to 12 metres Section E. - Indications of sedimentation under quiet are 9 in thickness. A comparison with these
the of section E in which data that the bars in section F of the conditions are present in part suggests point indicate river of consider- the black clays are found. As the clays in the previous present study deposition by a vertical able dimensions. section, these clays were probably deposited by
assume with certainty that accretion. Although one may Section G. — The fluvial of section with their the deposits in the lower part of section E, especially cycles G, those of the two detailed sections, represent topstratum abundance of wood fragments and plant remains, are not rather different from the of sections С 3 and D. deposits, a further subdivision seems uncertain, as basically cycles
of the limited less than The content of remains, which indicates the a consequence exposure (often organic detailed of dense in this location, 50 cm wide). The gradings of the oldest section presence vegetation apparently 326
changes from place to place. The vegetation responsible units at the top of this cycle announce the return of
for the richness in organic in this section must calmer conditions. Nevertheless, the coarse sand beds
have been very persistent, since its traces are observed between the peaty beds in units 22 to 35
recognizable throughout the whole section. witness the presence of a nearby active stream channel. Unit 39 of Section III shows the typical rapid inter-
Section H. — In this section a transition from braided- bedding of fine-grained and coarse-grained sediments so
river deposits into meandering-river deposits can only be characteristic of levee deposits. Other characteristic
presumed on account of the clear transition from coarse- features such as burrowing and reddish to brown oxida-
grained sediments into fine-grained sediments at m 57 tion rims (limiting the sandy parts) are distinctly
and account of Local small-scale on the grain-size distribution on a large present. cross-stratification may occur scale, which the in the horizontal lamination thin is image of section F. The very sandy parts; or very muscovite-rich fine sands and the black clay in the bedding (classification of McKee and Weir, 1953) is, of the The thickness of uppermost part section suggest deposition in however, preponderant. mean a fining-
almost stagnant water. upward grading sand-clay bed is about 5 centimetres.
From unit 45 onwards, the sedimentationof topstratum
Section I. — The thick conglomerate at the top of this deposits comes to an end and clearly cross-stratified
section, devoid of other stratification than horizontal, point bar deposits take their place.
strengthens the assumption of the braided-river origin in
the oldest part of section H, since the stratigraphical 3. Although the fining-upward gradings in this section of this section that of the position is comparable to are somewhat coarser than the gradings in the previous section. previous sections (and for this reason also thicker), they still show
the regularity and graduality of the cycles commonly Section — The of the IL. upper part formation, as we reported from meandering rivers.
could already establish in the sections further to the
to west, is different the sediments the contact with - near The exposure S of La Mata de la Riba. For the sub- the Paleozoic. Section II with its fining-upward grading, division used below of the sediments encountered in this shows abund- distinctly fluvial, cycles an even greater exposure, the reader is referred to the lithological de- of than G. The thick black ance organic remains section scription (p. 321). As is emphasized there, units С and G
which covers each is the clay cycle presumably remnant to M are relatively fine-grained sediments amid more
of densely vegetated backswamps. of coarse sandy deposits (locally consisting very sand). The following depositional interpretations can be
Section J. — In section J, in which three parts were assumed:
distinguished, an attempt at an interpretation would be
as follows (numbering frombottom to top): A: Point bar sands; absence of remnants of vegetation
of the and burrows. 1. Braided-river deposits, assumed on account small-scale of horizontal stratification with scattered B: Upper point bar or natural-levee deposits; presence cross-stratification, burrowing and wood fragments channels and the absence of silts and clays. present. 2. Indistinct transition into the predominantly top- C: Crevasse-splay deposits; coarse-grained sand
stratum deposits of Section III. This section (cf. penetrated into fairly pure clays, small, drifted wood Enclosure VIII) shows rapid vertical changes in grain size remains. Silts almost absent.
along its entire length. Four to dark light grey grey clay D: Crevasse-splay deposits; laterally wedging out;
layers of considerable thickness (between 135 and presence of sand-filled channels. Silts almost absent. 275 cm) occur (units 4, 12, 38 and 43). All four units E: Natural-levee deposits; alternations comparable to
contain plant remains and burrows, sometimes wood those mentionedunder 2 in section J. Intensive burrow- These often remains. clays are silty or sandy and contain ing. number of thin sand lenses. a great They are fairly F: Comparable to E; somewhat coarser sediment. uniform and show distinct absence of no bedding due to G to M: Mainly floodbasin deposits with pure lignitic We therefore them lithological contrasts. assume to layers. Unit H, the only coarse-grained layer, might floodbasin The of represent deposits. occurrence a represent a crevasse-splay deposit. fining-upward grading 'cycle' (units 13 to 22), about 4.50 within these The - m thick, generally fine-grained exposure E of La Devesa de Boñar. The sand-clay sediments The lowermost 2.50 which alternations this resemblance is striking. m, is in small exposure show a
almost of is much too to units С and of the with exclusively composed gravel, D previous exposure regard to
coarse to have been deposited by vertical accretion as a the scarcity of silt. However, the sediments of this
topstratum and must be result of sedimenta- lack deposit, the exposure the coarse and sometimes gravelly sands
tion by an stream channel. The which active eroding capacity usually accompany crevasse-splay deposits. The of the stream is reflected by the presence of a large high content of organic remains and the organic activity
quantity of armoured mud balls. The cross-stratified expressed by remainders of roots and burrows advocate 327
natural levee. The intermittent character excavations do random selection of the deposition on a not present a of the sediment is also evidence in favour of this inter- sediments which can be found in the Voznuevo Forma- pretation. tion, but have been carried out with a certain (mainly
economical) purpose.
The excavations E ofBoñar and S of Grandoso. - By far Sections K, L andN. - With the exception of the upper- the majority of the sediments in these excavations most 15 metres of section К and the uppermost consist of sand. These sands are almost exclusively 20 metres of section N, in which sands and silts prevail, composed of cross-stratified units with thicknesses all phenomena characteristic of deposition under generally between 15 cm and 150 cm. Large-scale cross- braided-river conditions are present. The most important stratification proved to be commonly of the planar type, features on which this assumption is based are: but festoon-type cross-stratification occasionally occurred as well (e. g. in the excavation near Boñar, unit 1. The coarse character of the deposits with abrupt of fine sand This B). Gravel, only of small pebble size, is mainly con- changes into sediments to clay size. cross-stratified clear the sediments between 7 centrated in the bottomsets of the units. phenomenon is very in m
The ubiquity of backflow phenomena which accompany and m 22 in section L (Fig. 35). the cross-stratified units is extraordinary. Especially with 2. Scour-and-fill structures such as channels, trough the description of the subrecent meander-bend deposits cross-stratification and festoon cross-stratification. of the River Rhine (Boersma, 1967) in mind, an inter- 3. Very coarse-grained beds with horizontal stratifica- pretation of the sediments under consideration will lead tion in which imbrication is the only observable to the assumption of point bar sediments, deposited by a structure. meandering river. The divergency of paleocurrent 4. The fairly uniform paleocurrent directions. 5. The of thick horizon 15 directions had already suggested such an interpretation. scarcity clays (the clay at m in section К could swale-fill channel-fill The question may arise, whether the scarcity of clays represent or of and silts would not suggest an interpretation in favour deposits). distal the 6. The absence of fluvial with a braided-river mechanism of the type, cross- typical cycles gradational
formed the decrease in size in an direction (the stratified units being by deposition on grain upward grading
marks added to the left of the downstream side of transverse bars. One should bear in sections, lithological
of column, mark a trend; do not, mind, however, that topstratum deposits on top point only fining-upward they
however, indicate a smooth decrease in grain size). bar deposits can easily have been eroded after their deposition. This hypothesis is corroborated by the Sections M and O. - Sections M and О and the exposure at presence of clay balls of variable dimensions observed immediately below the contact with the Mesozoic lime- the boundaries between the cross-stratified units. Where stones E of La Devesa de Boñar show the - meanwhile direction takes in a drastic change in paleocurrent place familiar - picture of fining-upward cycles which seldom a vertical sense, these clay balls, up to 20 cm in diameter exceed sand size. As we have seen before, the organic in the Grandoso excavation, could be the remnants of content of these cycles is subject to lateral changes; such eroded deposits. topstratum M while black clays are practically absent in section As could be established in the Grandoso section, the (where they were found to change laterally into clays sand beds flanked their bottom and their are at top by about 10 without organic remains) and near La Devesa, layers of unknown thickness consisting of silts and clays. of and metres of section О show a large quantity plant The fine-grained beds which underlie the Grandoso sands wood remains. show the typical, fining-upward alternations between Especially the deposits of the first two exposures, sand, silt and clay interpreted in previous sections and far of containing a greater percentage sand, frequently natural levee The exposures as deposits. fine-grained display cross-stratification. The majority of the of the Grandoso sands much thicker layers on top are sediments of these exposures will have a meandering- bedded. Intensively burrowed clay beds with a between 18 river origin. Whether the sands and gravels m maximum thickness of 75 cm and almost purely organic, and m 27 in section О are thick channel-lag deposits or rather peaty layers are likely to be floodbasin deposits could be deter- for the most part point bar deposits not at than natural levee deposits. Pebbly sands the very top mined. could mark the return of point bar sediments, but P. - from thin of sediments evidence is very weak. Section Apart a layer silty
at the base, this section shows a from In the Boñar excavation topstratum deposits are only gradual change with transitions reflected as thin clay layers or as broken clay chips at coarse conglomeratic deposits abrupt into thin clayey and sandy layers to sediments with the top of grading units or as clay balls. The predo- towards minance of sand in these two excavations need not be smaller pebble sizes with a tendency cyclicity in
fining-upward cycles. Roughly speaking, a transition surprising as far as the interpretation which leads to from sediments with braided-river point bar deposits is concerned, since - as we have predominantly - characteristics into sediments with already stressed the information obtained from the predominantly the Voznuevo Formation be mis- characteristics take about at exposures in can meandering might place leading. This is caused by the circumstance that the m 30. 328
- Section In this section all braided-river features are Q. of the section the cycles become more complete, with absent. In the lowermost 50 metres of the completely coarse sediments in their lower parts. A meandering-river
silts and sands far the most section clays, are by origin is obvious for the sediments of this section. The of important lithological constituents. greater part of these sediments consists — possibly topstratum deposits Section T. This section can be described as consisting do further between which not merit a subdivision; only m of three parts: At the base of the section the cobble 50 could the have floodbasin The 35 and m clays a origin. conglomerate clearly distinguishes itself from the for instance the described thin pebbly layers, one near sediments lying on top of it. From mil to m 50 fine- cf. be with m 42 (unit e, p. 310), may a crevasse-splay grained sediments, generally not exceeding sand-size, which alien those of the flood- From 70 least 225 clear very coarse sediments, to prevail. m to at m there is a
basin, were carried into the backswamp. From m 50 decrease in mean grain size; the Voznuevo Formation reflected onwards, the more active part of the river is by shows a fining-upward grading on a large scale. between 51 and the sediments. Above all, the deposits m The basal conglomerate shows features highly com-
with their close resemblance to the those of section С 2 with the m 55, fining-upward parable to respect to very of cycles described in Section Г, are good examples abrupt changes in grain size existing between the conglo- of the point bar sediments. The rest section, too, meratic parts and the intercalated sandy, silty or even
exhibits sediments which were probably partly formed clayey parts. Cut-and-fill structures and plant remains, bar of as channel-lag deposits, partly as point deposits. some which probably in situ, are rather common. As
in section С 2, we must assume that the river which
produced these kind of sediments had a braided
Section R. Notwithstanding the coarse-grained character.
encountered in this character of the deposits section, Evidence from the fine-grained part of the section
features are not Typical fining- braided-river present. between m 11 and m 50 is very poor; a number of 8 in upward grading units generally not exceeding metres fining-upward units could be observed. thickness the entire section. occur throughout Especially From m 70 onwards, fining-upward grading is also the of the section shows conformities upper part many common, mainly in cross-stratified units in the coarser- with the bar sediments of section F. The point abrupt grained lower part and in units which grade into silt or
transition at m 32 from gravelly sands to silty clay may clay in the finer-grained upper part. Although the have been caused by a sudden diversion of the active scarcity of sediments of clay- and silt-size up to stream channel, for instance avulsion (Allen, 1965b). by approximately m 200 does not favour an interpretation thin form the Relatively clay layers commonly upper suggesting deposition by a meandering river, the of the In locations part cycles. some they have, however, regularity, the frequently changing paleocurrent direc-
been as balls. only preserved clay tions and the gradual changes in grain size are strongly in favour of a meandering-river origin. Especially between
m 110 and m 203, the sediments are highly comparable
Section S. — This section shows a more striking to those reported from the upper part of the Voznuevo of fines than other section. predominance any Fining- Formation between Boñar and Cistierna, as for instance
to The upward grading units proved occur everywhere. in the excavations near Boñar and near Grandoso. These
fineness of the sediment and the relatively thick clay predominantly sandy and gravelly sediments, almost that the layers suggest gradings originated at some without silts and clays, probably represent thick point
distance from the active of the stream channel. part bar accumulations. The presence of backflow bar and under conditions They represent upper point deposits topstratum phenomena also suggests deposition
deposits; a further subdivision does not seem warranted. comparable to those found in the excavations mentioned The limited and cobble between thin, sharply pebble layer at above. From m 203 onwards, and especially 36 could have Towards the m a crevasse-splay origin. top m 218 and m 225, topstratum deposits predominate.
E. CORRELATION OF THE SECTIONS
base of the formation K, L and N) be Enclosure XI gives an impression of sections A to T with the (sections correlated the field the between a very strong vertical exaggeration in order to improve in by tracing outcrops correlation between readability. We have already made reference to this the sections (cf. Fig. 8). The sections Q, S enclosure in order to show the Lower Cretaceous 'time sections F, H and I and that between R,
also since in wedge' which was found by means of pollen analysis and, to a lesser extent, T is fairly obvious, horizontal distances small. (Chapter IV). Correlation of the sections lands us in both cases the are relatively find other between the great difficulties since the sections, which themselves In order to relationships the sediments have been lithologically sub- suffer from lateral are sections, poor exposure in a sense, divided schematical to size generally separated by large unexposed areas. Only in in a manner according grain the area between Boñar and Colle could the sections at into: 329
clays and silts sections which are interpreted as having been deposited under conditions from the sediments of sands (grain diameter up to 2 mm) braiding typical 2 rivers. In the of the formation pebbles (diameter from 2 mm to cm) meandering western part
A and the to lie pebbles (diameter from 2 cm to 6 cm) (sections B) separation appears higher
pebbles, cobbles and boulders(diameter > 6 cm) up in the section; both irregularities in lines 11 and IV
must be sought in an intensified influence of tributaries
in the west, indications of which were found in the com-
The black clay horizons are indicated separately since position of the gravel and in divergent paleocurrent
they proved useful in an earlier study (Jonker, 1970) for directions. The thickness of the braided-river deposits
correlating sections E to N (cf. Fig. 8). In some sections clearly increases towards the west and proved to be zero of the of or parts sections, where cyclicity causes a great in sections east Guardo (sections Q to T). Since
braided-river diversity in grain sizes, the symbol 'cy' has been used to deposits are generally coarser-grained than their this difference indicate lithology. In these cases, however, the sediments meandering counterparts, may involved rarely exceed sand-size. explain the fact that the total thickness of the formation
From this simple subdivision it becomes clear that the increases in a westerly direction (line HI on the correla-
sediments whose components exceed 2 cm in diameter tion chart).
are almost exclusively limited to the lowermost The black clays cannot be used for the correlation over
150 metres of the formation's total thickness. Line IV great distances. From sections R and T, in which the
on the correlation chart approximately separates these black clay horizons could be traced laterally, we have
two grain-size fields. Only in the extreme west of the learned that the black clays are lens-shaped bodies, with
area (section A) can this line not be continued in a a thickness of a few metres and a width of some tens of
satisfactory manner. Nevertheless, there is a general metres. In all probability the black clay horizons
trend along the entire southern border of the Cantabrian distinguished in sections E to J and in the area between
Mountains towards decrease with Boñar and Colle do and the a in mean grain size (Fig. 8) not belong to one
transition from braided- but decreasing age. The apparent a same clay bodies, only indicate a level in the
to fact which Voznuevo in black river regime a meandering one, a we Formation which clay layers occur
previously assumed from the paleocurrent directions(p. frequently. Such a level could reflect certain topo-
324) is in close connection with a fining-upward grading graphical conditions in which extensive backswamps
on a large scale. This transition is marked by line II on could form behind the levees, as for instance in times of
the correlation chart which separates those parts of the small supply of clastic material.
F. PALEOGEOGRAPHY
Formation conclude that a river, or a Since braiding of a river is accompanied by a relatively one may only of the bed of had flowed from direc- strong gradient river or/and a high discharge system rivers, southwesterly
but tions to directions between Riello and (variable, periodically) of the river itself, we must northeasterly
Cervera de it does follow as a assume that one or both conditions mentioned above Pisuerga; not, however,
of that the which have been more important in the western part of the matter course, source area provided the studied than the and that the sediments was situated in the same direction as area in eastern part, they
to which measurements indicate. An appear to be of increasing influence towards the west area paleocurrent
(line II, Enclosure XI). attempt to make some concluding remarks concerning
will not be ventured, however, before we Other data important to the reconstruction of the the source area circumstances under which the sediments of the have considered the petrological composition of the
the silts and the VII and Voznuevo Formation have accumulated are the paleo- sands, clays (Chapters VIII). current directions. They could be reconstructed from Applying the division into proximal and distal braided- cross-stratification, imbrication and channel axes river deposits, the adjectives referring to the distance Smith (Chapter V, part C), and the history of the Cenomanian from the source area, as introduced by (1970), we with distal braided- transgression which is well known in the provinces of shall in the present case be dealing cross-stratification Vizcaya, Burgos and Santander (cf. Chapter IV). As for river deposits, since planar large-scale dominates horizontal stratification. A the paleocurrent directions, we have learned that clearly over of the sediments of the second feature in favour of distal is the transport Voznuevo Formation deposition took place from directions between west and south. In circumstance that the braided-river deposits are relation to this observation it should be stressed that this immediately overlain by meandering river deposits. This
to general trend in paleocurrent direction is only indicative argument does not apply, of course, the tributaries — from the Cantabrian — which introduced material of the direction of the transporting agent i. e. a river present for in the locations where they have been observed. In other Mountains (cf. Section A, instance); they have, most characteristics words, from the observations made in the Voznuevo probably, more proximal but they could 330 331
be only occasionally observed in the field so that no only thin clay layers, encountered in the excavations definite conclusions can be made. near Boñar and Grandoso, could well be presumed to A for the of such low possibility the paleogeographical conditions represent deposits a sinuosity stream. The which caused the deposition of the sediments of the strongly diverging paleocurrent directions, however, do favour Voznuevo Formation is presented in Figs. 17 A and 17 not such a hypothesis.
not difficult to B. These figures show a schematic representation of the Although it is very classify the findings
results of the investigations described in Chapter IV and concerning the sediments of the Voznuevo Formation this into theoretical alluvial the association of these in chapter. It is also a simplification of the correla- facies,
tion chart (Enclosure XI), in so far as use has been made facies apparently does not fit into the hypothetical
of lines models Allen since this author I, II and III. In Fig. 17 A a horizontal line proposed by (1965b),
represents the level where the transition from Paleozoic does not present a direct combination of a braided-river facies to Cretaceous sediments is situated. Fig. 17 В is in fact with a meandering one. Allen's association of identical alluvial fans with stream entirely to Fig. 17 A, but it has been tilted so a meandering (Model A, Fig.
that the time line becomes horizontal (line I). All lines 36), which coincides with the paleogeographical model
line of the Old Red Sandstone of parallel to I represent a moment in the history of Anglesey (Allen, 1965c)
the Cretaceous shows no resemblance to the situation in the transgression. Considering such a present since the of the moment in Upper Cretaceous times we may expect from study, coarse-grained deposits Voznuevo
east to west (numbers referring to Fig. 17 B): Formation lack the field relationships, the composition
and the texture of alluvial fan deposits. Allen's alluvial
1. Shallow marine deposits, becoming sandier towards facies association of the delta and the coastal plain type
the coastline, (Models В and C, Fig. 36) can both occur in the 2. of the formation. meandering-river deposits, meandering part 3. after The from braided-river deposits (only deposited the transition marine deposits via deposits of a
transition from the Lower Cretaceous to the Upper meandering river system to those of a braided river
Cretaceous). system from east to west, which must have occurred
during Cretaceous times, indicates that the coastal plain,
Comparing the findings in the present study with the considered at one moment, is replaced towards the west
hypothetical models as introduced by Allen (1965b), by a floodbasin in which slope and discharge — or one of both — only relationships with his braided stream model (Model gain much more importance. An increasingly small coastal and the foothills of B, Fig. 35) and his strongly meandering stream model plain steeper slopes at
(Model D, Fig. 35) could be ascertained. The inter- the massif, against which the Upper Cretaceous - dead mediate model the low sinuosity stream — is rather transgression came to a end, may be responsible for and could be the thickness of the braided-river rare not distinguished in the present in- greater deposits vestigation. The relatively thick sand accumulations with towards the west.
CHAPTER VI
SEDIMENTARY PETROGRAPHY OF COMPONENTS COARSER THAN 2 mm IN DIAMETER
after THE QUARTZ/QUARTZITE/LYDITERATIO Only a short field inspection did it become clear that there is of Introduction a higher percentage quartzites near the
base of the formation. This led to the determination of At first sight, the main constituent of the Voznuevo the quartz/quartzite/lydite ratio in several locations. As Formation appears to be quartz, with the exception of the seemed the quartzites coarser than the medium size of clays. Yet considerable quantities of quartzite and the limit chosen the gravel*, an arbitrary was at a longest lydite are present, latter quite eye-catching in these axis value of 3 cm, in order to compare the ratios in generally light coloured sediments. Some of the lydites both size classes. The taken far be samples were as as proved to carbonaceous quartz phyllites (cf. Chapter possible at random, not with 'eyes closed', which can will be referred VII), nevertheless, they to as lydites in lead of to a deceptively high proportion coarser material, this few sand chapter. Very pebbles, consisting of con- but means of frame by a in which samples were taken as solidated Voznuevo sand particles, were also found. close together as possible. A number of 25 measure-
Fig. 17. Model of the possible geographical situation in
Cretaceous times.
A: * With horizontal depositional base. 'Gravel' is used as equivalent for 'unconsolidated con-
B: With horizontal time line. and includes glomerate' all particles > 2 mm diameter. 332
> 3 cm < 3 его the diameterof the of 0 0 mean quartzites is larger than that
4 He Lyd « «e Lyd Location the quartzes.
«80 20 X 84 12 4 section A (ft/fte Al) 46 48 6 80 18 2 section A (ft/fte A2)
100 В - Bl) 84 16 X X section (ft/fte Local differences 100 X X 100 X X section В (ft/fte B2)
28 20 С 72 - 80 - section (ft/fte Cl) In the small isolated area between Boñar and Colle, a
68 28 section 8 92 - 4 С (ft/fte C2) relatively of the lowermost of the - - exposure 96 4 96 4 section С (Q/fte C3) good part 8 84 8 8 80 12 E of Carrocera formation is found. Here two gravel layers could be 72 16 12 100 - X Sorribos de Alba
80 ' 12 8 92 - 8 section P (ft/fte Pi) distinguished, both near the bottom of the formation - 68 20 12 92 8 section H (ft/fte Hi) In the fraction > 3 to be section (cf. Fig. 8). cm, which 92 4 4 96 - 4 H (ft/fte H2) proved
48 8 72 20 8 section I (ft/fte II) 44 more representative, there is a striking difference in 76 12 12 84 - 16 section J (ft/fte Jl)
8 8 section J between the The of 88 8 4 84 (ft/fte J2) composition two layers. percentage 36 62 2 90 10 X Voznuevo decreases with a to 8 12 Voznuevo-Grandoso decreasing (from b, 28 64 76 12 a quartzites age
32 60 8 68 20 12 Voznuevo-Grandosob these letters following the location name in Table II). 68 80 12 8 24 4 section К a (ft/fte Kl) 36 56 8 72 16 12 section К b (ft/fte K2) The oldest gravel layer is developed as a basal con-
76 12 12 84 8 8 section К (ft/fte КЗ)
28 in the 'Voznuevo contact' location, uncon- 28 68 4 60 12 section N a (ft/fte Nl) glomerate
46 52 2 76 24 - section N b (ft/fte N2) formably overlying the Paleozoic (i. e. Barrios Forma- 64 24 12 84 12 4 section N (ft/fte N3)
88 8 4 88 - 12 Yugueros tion). Still one third of the coarse fraction, and even
P 90 X 10 90 X 10 section (ft/fte PI,2) 90% of the fine fraction, consists of 85 - 15 Valmartino quartz.
80 16 4 88 4 8 Prado de la Guzpeña the of the basal In eastern part area a conglomerate, 50 50 - Frontier León - Palencîa
Guardo 100 x X 100 X x consisting exclusively of quartzites (in the coarse 92 X 8 96 X 4 Uufieca - contact - - found in four locations at the 100 100 X section ft (ft/fte ftl) fraction), was
84 X 16 84 X 16 section ft (ft/fte ft2) with the Paleozoic between Aviñante and 100 - 16 - Villaverde underlying - 84
84 - 16 section S (ft/fte Si) Cervera de Pisuerga. There is also a very clear increase in 36 56 8 72 24 4 section S (ft/fte S2)
- - of from from 100 section T (ft/fte Tl) thickness this basal conglomerate W to E,
90 7 3 100 X X section T (ft/fte T2) 80 cm N of Aviñante to 300 cm N of Villaverde and to
more than 1000 cm at the base of Section T.
Table II. The ratio from W E first tens Quartz/Quartzite/Lydite to (top to Generally speaking, we may say that only the bottom). Q/Qe + section letter and number refers to position in of metres of the Voznuevo Formation are relatively rich the sections (Enclosures II to VII). in quartzites, and that in the remainder of the formation
the quartzite percentage will not exceed 20%. The lydite considered be of the ments are to representative gravel ratio was never found to exceed 16%. The lydites are of each location. composition always in a more weathered state than the other con-
stituents, and are frequently broken. Results
The results of this investigation are listed in Table II, Comparison with surrounding formations
from the westernmost location (W of Soto y Armo) to Comparing the lithological composition of the Voznuevo
the easternmost location (Cervera de Pisuerga). The first Formation with the composition of the formations
name of the same location is always situated closer to surrounding it, we may say that, although quartz/ from the Paleozoic than the name below it. From the table it quartzite/lydite ratios have never been reported
became clear that the oldest sediments from the same elsewhere, the Voznuevo Formation has a very special found location have a relatively higher content of quartzites. aspect. Some features typical of the formation are
This does not hold for the locations W of La Magdalena, again in the Vegaquemada Formation, between La Robla studied This should where the differences in composition are due to other and Cistierna, by Kuyp (1969). not considerable of the sediments of causes (Chapter V). surprise us, since a part formation of the Voznuevo From Table II the following conclusions can be drawn: this are an erosion product Formation (cf. Chapter VIII and de Jong, 1971). Kuyp Number Size Quartz Quartzite Quartz both<50% well rounded pebbles, consisting of quartz, of >50% > 50% > 75% or both reports very
with from 2 to 6 and muscovite measure- 50% longest axes cm cm, ments flakes with a maximum diameter of 2 cm, which must
have been transported over a very short distance. 37 > 3cm 22 12 3 Erosion products of the Voznuevo Formation, however, 35 < 3cm 34 1 Forma- 35 28 are limited to the base of the < 3 cm only Vegaquemada
tion. The Voznuevo Formation must have been almost
The completely independent of the rocks which nowadays predominance of quartz is very clear, although this form the Cantabrian Mountains since the enumeration is not representative of the Voznuevo predominance localities of in the Voznuevo sediments is alien to any Formation as a whole, since relatively more quartz this rich will other erosion of the Paleozoic rocks in area. in quartzite were chosen. Absolute ratio values product of The remnants that to the presence of therefore be much higher in favour quartz. Comparing only testify erosion of nearby Paleozoic rocks are the quartzites. The the results in the two size classes, it is also evident that 333
of this erosion lasted for a difficulties, and cobbles to be predominance only relatively since many pebbles proved
short time. The quartzites were probably derived from broken. This was caused by: previously weathered Paleozoic rocks, which covered the 1. Tectonic forces.
before the laid down. area Cretaceous deposits were At 2. Weathering (both chemical and mechanical). that there little relief the of time must have been or no in 3. Taking the gravel out the wall.
Paleozoic mountain chain. Assuming a Paleozoic massif 4. A combinationof 1, 2 and 3.
with a strong relief, comparable to the present situation, Especially tectonic forces are very harmful to the con-
should from of unbroken much we expect that, apart quartzites, a con- servation material, probably more siderable of limestone would be No influences obvious quantity present. than the of weathering. This was very
limestone, however, either macroscopical or in the thin in the exposures along the Esla valley near Cistierna,
sections, proved to be present. where the Voznuevo Formation has been strongly
squeezed together and where, as a consequence, un- 25 broken pebbles are rare. In each location at least taken. MEASUREMENTS OF FLATNESS AND ROUNDNESS unbroken specimens were
Introduction Results and interpretation As be clear from the the of the whole might above, composition Measurements in 22 locations, covering area,
the gravels is rather monotonous, quartz pebbles being yielded indices of flatness of between 1.54 and 1.93, and the only constituent sufficiently abundant for the deter- indices of roundness of between 223 and 416.
minationof flatness and roundness indices. Nevertheless, Comparing these values with the values found by locations in three (in sections А, С and T) a comparison Cailleux (Cailleux and Tricart, 1959) from 12 different made between flatness and of was roundness quartz environments, the main cause of flattening and rounding gravel and of quartzite gravel (Table IV). of the gravel of the Voznuevo Formation must have
For the gravels the well-known formulae of Cailleux taken place in a fluvial environment. Since rivers were
(Cailleux and Tricart, 1959) were used for calculating responsible for the supply of material from south-
the index of flatness and the index of roundness. These reasonable that westerly directions, it seems to expect in
indices be the which has lateral of about appear to very popular, although many area studied, a extent
workers have proposed alternative solutions for the 150 km, approximately parallel to the paleocurrent
representation of morphometrical properties of pebbles direction, differences in the indices of flatness and and cobbles 1959; (see, e. g., Mabesoone, Koster, 1964). roundness might exist between W and E. Comparing the
The measurements in the field suffered from practical median index values for three regions, separated by the
rivers Bernesga and Esla (Table III), we find a distinct
decrease in the median index of flatness, together with
increase in the median index of roundness. This LOOATIONi NUMBER OF MEDIAN MEDIAN an
MEASUREMENTS 1 LONGEST AXISl INDEX VALUE1 contributes that to the many other indications X 25 sedimentary in the area studied took place region I transport
from W to E.
section A 2 6.416.2
Flatnessi 1.74 section В 1 6.0 Local differences in flatness and roundness values Roundness! 272 section С 1 4.4 As mentioned before, a comparison was made in three locations region II between two successive gravel beds with a different lithological composition; one consisting section F 4 4.9(5.2)3 7)4.3 of predominantly quartz, the other of quartzite. The section H 3 6.1И.Ц4 4 result of this is in Table IV. The section I 1 7.8 comparison given median index values of flatness La Val cueva 1 4.3 are in each case smaller Flatnessi 1.72 section Bofiar 2 4.5)4.0 for quartz than for for the index values of Roundneasi 323 quartzite;
section К 1 7.6 roundness the situation is reverse. Differences are much
section L 1 6.2
section N 2 6.0)5.2
region III MATERIALI INDEX LOCATIONi MEDIAN VALUE 1
Platnessl Roundness!
Prado de la Guzpeña 1 4.2 section A quartz gravel 1.82 240 Flatnessi 1.54 quartzite gravel 2.21 121 Muñeca 1 4.3
Roundnessi 388 1 section T 4.8 section С quartz gravel 1.63 «5 quartzite gravel 1.75 281
Il V of Río Bernesga section T quartz gravel 1.54 413 lit Between Bio Bernesga and Rio Esla quartzite gravel 1.67 399 lili E of Bit Eela
Table III. Median index values for flatness and roundness of Table IV. Comparison of flatness and roundness indices of gravel
of varying composition. quartz gravel calculated for three parts of the area. lithological 334
larger in section A than in the other two sections, expected, the limestone gravel is better rounded and less
probably because of the fact that the median longest flattened than the quartzitic gravel. for axis the gravel is 7.8 cm in section A and smaller in Even the very comprehensive morphometrical gravel
section С (6.3 cm) and section T (5.5 cm). The fact that analysis by Mabesoone (1959) in sediments of the Duero of the quartzites in section T are so much better rounded Basin in the province Palencia (including the Cuevas be may explained by: Formation, comparable to Kuyp's Candanedo Forma-
1. A greater distance from the source area. tion, and the overlying Vega de Riacos Formation)
2. More result of cannot be of intensive shaping as a more than one any help in our investigations, because in that and sedimentary cycle. case, too, only quartzite limestone gravels were
Using the previously mentioned assumption that the considered. The striking difference in lithological
quartzites were derived from the Paleozoic rocks of the composition as compared with the sediments of the
present Cantabrian Mountains, the quartzite gravels in Voznuevo Formation and part of the Permo-Triassic sections A and С could be derived from sediments shows that the latter directly us deposits occupy a
quartzite layers, the gravels of section T from a very special place in the post-hercynian history of the Paleozoic conglomerate. A second possibility is that the Cantabrian Mountains.
sediments in sections A and С originated in Paleozoic
conglomerates, while the sediments of section T, which
had the same source, underwent a second cycle in Surface features of pebbles and cobbles and climatic Permo-Triassic times (see discussion below). conditions
Little is known of surface textures of gravels since most
Comparison with surrounding sediments investigators use fractions between 0.3 and 2.0 mm
Measurements on gravels of underlying and overlying (Koster, 1964). Since comparison with findings by other sediments carried were out by Papa (1964) and Smit investigators is not possible, we shall confine ourselves to the Permo-Triassic NE of Cervera de few brief (1966) on Pisuerga, a remarks. Quartzites have in general a mat and and and Mabesoone by Kuyp (1969) (1959) on the dull appearance with a rough surface; they are never and the Paleogene younger deposits on southern and translucent or transparent. In contrast to the quartzites, southwestern border of the studied the area by present the quartzes generally show a greasy to mat lustre, they author. and Smit did whether made Papa not report they are nearly always transparent, sometimes translucent. distinction between and Discolourations any quartz quartzite pebbles in caused by iron (hydr)oxides occur Cailleux's indices. Yet mentioned calculating they an almost everywhere, giving the gravel a yellowish brown increase in the of towards the percentage quartz pebbles to pinkish aspect. Very intensive discolourations, such as of the Permo-Triassic top sediments, a accompanied by appear on the northern slope of the valley between decrease of the size. The flatness and round- Boñar and mean grain Colle, were caused by ferriferous waters indices of ness the of the older con- down from the Paleozoic components flowing rocks, which are glomerate of Permo-Triassic indicate derivation from age topographically higher. Although the quartz gravel is the Carboniferous Curavacas Conglomerates. According generally well-rounded and the surface looks rather
to the Papa, Permo-Triassic conglomerates had a smooth from it to younger a distance proves be very much
source area described as: 'quartz veins in the hinterland'. corroded. As the intensity of this corrosion differs from with the observation Together that the mean pebble size place to place, it is assumed that most of this corrosion decreases in an eastward direction, and consequently a took place after deposition of the material. Grooves and supply from the W must have taken place, it is very holes on the surface are filled with white-coloured likely that the of these Permo-Triassic sediments top kaolinitic material and pulverized sand particles. have the least similar same source area, or at a source It be clear that data obtained from flatness may gravel area, as the sediments of the Voznuevo Formation. The and contribute small roundness, only to a very extent to decrease in size the general upward grain throughout the reconstruction of paleoclimate. As no evidence of
Permo-Triassic can be - sequence interpreted according eolian transport was found on the surface of the gravel, to combination of distance of Papa as a a longer climate during the deposition of the Voznuevo Forma-
transport and an aplanation of the hinterland. Seen in not tion probably was very dry. This assumption is this the well-rounded T could light, quartzites in section supported by the strong chemical weathering to which have been derived fromeroded Permo-Triassic sediments the sediments must have been exposed (Chapter VIII). in this of the (only present part area studied!), mainly Other factors, such as the absence of limestone and the from the older in which of conglomerate quartzites are presence of kaolinite are not only in favour strong abundant. of chemical weathering, but are also indicative an acid An example of gravel-bearing formations overlying the environment. According to values found by Cailleux, Voznuevo Formation is the Candanedo Formation values from the Voznuevo morphometrical as reported studied by Kuyp (1969). This formation is rich in Formation can best be ascribed to deposition in a
and limestone since material derived The of quartzite gravel temperate or warm-humid climate. presence from the Voznuevo Formation forms small kaolinite also indicates humid climatic only a very warm and rather
part of these mountain debris sediments. As is to be conditions. 335
8-13 mm (n=300) II (n-300) III (n-300) Fractioni I our knowledge of the composition of the coarser frac-
1 white milky quartz 7*0 4.7 2.7 tions. The difference in the red quartz/yellow quartz 2 red quartz (+quartzite) 9.0 10.0 49.9 ratio in sample III as to I and II is 3 grey quartz (+quartzite) 4*0 6.0 4.2 compared samples 69.1 4 yellow quartz (+quartzite) 75.9 31.4 more striking. Yet the sum of the percentages of red and 5 crystalline 0.3 - 0.8
6 has the value in all lydite» hornfels, phtanite 2.7 0.7 1.1 yellow quartz approximately same 7 reeti miscellaneous red components 0.3 1.0 0.4 three and as the red colour is caused red ferriferous 0.3 - - samples, by 8 quartz 9 miscellaneous componentsf not red 7«3 1.7 9-5 secondary pigment, there is no obvious primary differ-
ence between the three 100.0 100.0 LOO.O samples.
We may assume that these three samples are represen- percentage of red components 16*9 11.0 5О.З
percentage of quartz and quartzite, tative of the composition of the small pebble fraction of 88.2 groups 7»8 and 9 excepted 89.1 96.6 the Voznuevo Formation (the very base of the forma-
vein tion with the restriction that the results of the 1 quartz s.s. (not translucent) excepted)
2 vein s.l. other ite resembling to quartz , quartz,and quartz countings are not altogether reliable, the material used quartz, red colour caused by secondary pigment
3 other quartz and quartzite resembling to quartz less being more or pulverized. 4 see 2
miscellaneous 5 crystalline - 6 lydite, hornfels, phtanite, light and dark coloured rocks, layered
restl ot to 7 miscellaneous red, quartzite-like (n resembling quartz) SUMMARY 8 rest) red ferriferous quartz
rest! not 9 miscellaneous, red; mainly grey quartzi tes, also
muscovite-rich sandstone and other compone nts Summarizing the main points of our investigations in the
Table V. Composition of the small pebbles (in percentages). Voznuevo Formation with respect to the composition of
the gravels and their morphometrical properties, we
must bear in mind a few striking facts. First of all there
is the predominance of quartz, only accompanied by
modest of and This com- COMPOSITION OF PEBBLES OF BETWEEN 8 AND 12 mm IN percentages quartzites lydites. formation DIAMETER position was never found in any other known the Cantabrian with the of the in area exception upper Since field differentiation made between of the Permo-Triassic sediments N of Cervera de in the no was part described and Smit. the various kinds of quartz, a number of samples of Pisuerga by Papa Quartzites, with small counted of the from which forms pebble size were in the laboratory certainty coming a region nowadays
Geological Survey of The Netherlands in Haarlem. the Cantabrian Mountains, are limited to the base of the
Counting was carried out in the fractions between 8 and Voznuevo Formation. Limestone gravel is absolutely
12 The result of the of various of absent in these sediments. Indices of flatness and round- mm. grouping types Table V. No difference made be- indicate fluvial mode of in quartz is given in was ness a transport a temperate
tween quartz and quartzite, as they strongly resemble or warm humid climate. For quartz gravel a decrease in
each other in this fraction. Three representative samples flatness and an increase in roundness from W to E are
from the locations of Grandoso (I), Campohermoso (II) obvious. In one and the same location, quartzite gravel
and Sorribos de Alba (III) were submitted to examina- proved to be flatter and less well-rounded than quartz
tion. gravel. Probably fluvial transport took place from W to all three about 90% of the material In samples proved E, the quartzes travelling over a greater distance than the
to be quartz (quartzite). This is not very surprising after quartzites.
CHAPTER VII
THE SAMPLES
THE THIN SECTIONS solidated. The conclusions from the study of thin
sections, which are summarized below, are based on thin The of following part this chapter is based on observa- sections of 32 samples. tions in thin sections of cemented sandstones and argilla- of ceous sandstones the Voznuevo Formation, of The sandstones pebbles collected in the Voznuevo Formation and, for When studying the sandstones of the Voznuevo Forma- of number comparison, a of sandstones of Paleozoic age. tion it becomes clear that in each of them detrital It should be mentioned that the thin sections of the components and a cement can be distinguished. cemented sediments are not representative of the
Voznuevo — Formation as a whole, since only a sub- The detrital components. The detrital components ordinate of the part formation to be con- be said to from to silt-size, often proved may range pebble-size 336
in and the thin section. stone one same Consequently, sorting layers as a consequence of faulting. In the field is very in the majority of the thin sections; and in the hand both have the poor specimen samples appear- moderate be rather of decalcified to good sorting appears to ex- ance a sandstone. Microscopically, the
ceptional. The original shape of most grains (i. e. the composition of the samples does not differ from that of of degree roundness at the time of their deposition) can other samples except for their content in feldspar. be hardly determined, corrosion having been very Values of 10% of feldspar for sample Va 2 and of 20% of extensive. Especially quartz grains on which indications feldspar for sample Va 4 were estimated in the respective both of and of solution — thin overgrowth sometimes on one sections. Staining of small cubes of these samples 3 and the — could be same grain observed, are not very (about 1 cm ) with hemateine and sodium cobaltinitrite
suitable for even a rough estimation of the degree of (see Chapter VIII) showed that we are dealing roundness. This indicates of phenomenon migration exclusively with alkali feldspars. The estimated per-
silica as described der Waals the these by van (1967) in uncon- centages in stained samples conform to the values
solidated sands of the of South The found in the thin sections. 58 shows of the Tertiary Limburg, Fig. a part Netherlands. There, it has been concluded that the stained surface. The quartz has remainedtranslucent and
mobilization of silica favoured it shows the was by a humid, tropical glass-like, up on photograph as darker climate. The to subtropical Quartzite grains are generally less grains. feldspars have a light yellow colour(white in
to be rounded to worn, they proved well-rounded. Fig. 58) and a dull surface; they are not translucent. The In number of the found surface of a samples grains were to be rough the sample under the microscope does intensively broken, which gives them a puzzle-like not the of Moreover, jigsaw permit counting points. many The cracks between the have been appearance. fragments quartz grains are partially or entirely covered by a thin
filled with cement; they reach a width of several skin. may feldspar These skins accumulate particularly in pits tens of microns. Unlike the sediments of and other which pebble-size, damage are to be found on the surface there is no between tectonic of the apparent relationship quartz grains. For this reason no more than a deformation observable in the field and the of estimate of the degree rough composition in percentages is of breakage the detrital grains in the thin sections. justified. Using the Travis' classification (1970), the In many instances the detrital grains float in the samples mentioned above just fall outside the 'arkose'
cement; thin of clay surround the area and have to be coatings occasionally designated as: 'feldspathic (quartz) without the grains filling entire space between the sandstone'. the sediment grains: is highly porous. Where contacts Part of the kaolinite in the Voznuevo Formation
occur between the almost grains they are exclusively proved to have a detrital origin since it is found as rolled- that there vermicular tangential or straight contacts, proving has up aggregates in which the grain shape is been intrastratal solution. no or hardly any accentuated by a thin iron skin (Fig. 59). The detrital of grains are mainly composed undulous Quartz phyllite grains were observed exclusively in the and of quartz quartzite. Chert grains, clay grains, samples from the oldest part of the formation. They also and flakes phyllitic quartz grains, feldspars muscovite are proved to occur as pebbles, especially in the western-
of minor As have learned from the most of the of importance. we part area, W La Magdalena. In all composition of the pebbles and the cobbles (Chapter probability this metamorphic material is derived from
VI), high of Precambrian rocks which western percentages quartzite, exceeding the per- outcrop in the part of of centages are restricted to the lowest the area. quartz, very part of the Voznuevo Formation. This also Detrital muscovite flakes tendency is are present in most of the reflected in the sand-sized sediments. much abundant Although quartzite samples, although they are more in grains are present in almost every thin section, they are samples of fine sand- to silt-size than in coarse-grained abundant more in the basal part of the formation. sands. Most of the muscovite flakes, however, probably the have Among quartz spheroidal and volcanic quartz could an authigenic origin. The detrital muscovite flakes sometimes be observed. Chert grains were encountered can be distinguished from the latter by their more in most in moderate corroded and the fact that samples quantities. appearance by they are often found Feldspars are rare in the average Voznuevo sandstone. crumpled and split between detrital grains (Fig.
They have commonly been altered into kaolinite or are 60).
In couldbe observed unrecognizably worn. one sample it A number of slightly pleochroitic flakes suggest that that the few few of the feldspar fragments present presumably muscovites are actually biotites which, under resisted weathering due to a coating of iron oxidizing conditions, have been altered into muscovite.
(hydr)oxides. Furthermore, it is evident that feldspars This could also explain the large quantity of iron are better in lime whose preserved samples containing or (hydr)oxides between the grains. Rutile skeletons the field position in suggests the ofsands with a which could some vicinity (sagenite), be observed in grains (e. g. certain lime content which will have given protection Fig. 61 ), prove that we are dealing with discoloured and of the altered biotites. against weathering feldspars. Examples of the Only in exceptional circumstances are latter Va 2 and biotites possibility are samples Va 4 which were preserved as surprisingly fresh, brown collected at the of section flakes. very top T, where the pleochroitic Fig. 62 shows a biotite flake Voznuevo sands are found between two Mesozoic lime- embedded in a large quartz grain along three sides which 337
flake is shut off at the other side by authigenic quartz. shall restrict ourselves to the description of this sample
In this manner the biotite fragment has escaped altera- only.
tion and could be preserved. In sands rich in organic remains, disseminated sulphur
minerals in the thin sections. besides iron One establish Heavy are relatively rare is present hydroxides. cannot
Tourmaline and zircon are present in most of the whether this sulphur replaces or has been replaced by fact crust-like samples, whereas staurolite, anatase and epidote were goethite, but the that the typical yellow
found inbut a few samples. sulphur overgrowths are only a superficial phenomenon
in the field suggests that the formation of sulphur was
— to all other events and limited to that of The cement. Much more characteristic than the posterior part the sediment which the detrital part of the samples is the cement. In all samples was recently exposed to
met with: atmosphere. the following sequence was
The first cement which precipitated between the grains of The sandstones had a quartzose composition, generally a crypto- argillaceous sandstones, crystalline to silt-size texture. The solution which The argillaceous representing fine-grained abundance of had destructive topstratum show an muscovite brought the quartz cement has a in- deposits,
which can amount to more than 25%. In all samples in fluence on some of the detrital grains (cf. Fig. 63), but which muscovite is abundant, a clear orientation of the also caused secondary overgrowth. The dust-rings, which
flakes was evident. The become visible after overgrowth of quartzose matter, are finer-grained samples generally contain a higher percentage of iron hydroxides, giving less abundant than in sandstones of Paleozoic age. The them brownish colour in the field. In these dissolved quartz is thought to have been derived from a samples also observe that is since the sediment itself. This conclusion is drawn from the one may sorting very poor,
of medium sand-size to fine silt-size were observation that a transition from weathered quartz(ite) grains very often encountered Detrital material other than fragments to a silt-sized cement occurs locally. together. of muscovite flakes and remains is Secondly, in the cement a high percentage kaolinite quartz grains, organic flakes could be observed in most of the samples (Fig. 64). This rare. Although many muscovite are supposed to
have detrital number of flakes — of kaolinite was found to replace the previous quartz a origin, a generally
- distorted of the cement and is assumed to be mainly derived from larger dimensions lack the appearance
flakes; seems more detrital feldspars. The authigenic kaolinite is - at least smaller an authigenic origin
partly - of early diagenetic origin, before compaction of appropriate. in the often the sediment took place. This can be concluded from Especially coarser grained parts, so encountered the black kaolinite crystals which were found compressed between accompanying clays, organic is showed detrital quartz grains (Fig. 65). In many cases kaolinite activity conspicuous. Microscopical analysis these of orientated has formed characteristic sheaf-like arrangements. that parts are composed ferruginous,
The third cement in the history of cementation is mica flakes with quartz grains of silt-size and organic the composed of iron (hydr)oxides which clearly replace the fragments. Small lens-shaped bodies, probably partly of filled with sand- previous cements. In the initial stage of infiltration by compressed remnants burrows, are iron (hydr)oxides small isolated specks, approximately and silt-sized quartz grains. The mica content is consider- dark lower in these kaolinization have 20 microns in diameter, form. They are generally ably lenses; some may
in the formation of these red to blood-red and in the description of the thin taken place them. As regards lenses the assumed: loose of sections have been referred to as hematite. X-ray following is a very complex
analyses of silt-sized samples (Chapter VIII) showed that muddy sediments — with a certain percentage of detrital most of the supposed hematite is in fact goethite. mica flakes — in which organic activity must have been be Orange-red iron in the thin sections is possibly limonite. abundant, was subjected to compression as can deduced from the of the sand- and silt-filled Some of the light grey, fine sand-sized to silt-sized shape sediments of the Voznuevo Formation show typical burrows. Pinching out of very thin sand layers, causing of of weathering with orange-red and dark purple discoloura- small sand lenses, may be the origin part these
sediment mottled the field. when as in tions, giving the a aspect in lenses, especially they are present strings
lenses connected horizons with a In a thin section it could be observed that the difference which the are by higher in colour is only caused by differences in cement. In the quartz concentration;other lenses, in which this associa- of lighter coloured part of the sample the original kaolinitic tion is not clear, are presumably organic origin. cement has been replaced by yellowish-brown iron Black stringers consisting of organic matter are hydroxide showing colloform, radial aggregates (Fig. frequent; in one sample the black stringers are not the orientation of the muscovite flakes but 66). In the deep purple part of the sample the iron parallel to
to perpendicular to the plane of hydroxides have changed into almost opaque dark arranged approximately could indicate brown-red hematite. Strong magnification shows that stratification (Fig. 67). This preservation most of this hematite cement is composed of small cir- of rootlets in situ. found in locations in cular specks, between 5 and 10 microns in diameter. Thin silt-clay' laminations, many under the of Other samples from comparable outcrops also proved to the field, proved microscope to consist alternations of: have the findings in this sample in common, so that we 338
a. Bands, approximately 1 mm thick, of loosely packed the formation of glauconite (see e. g. Pettijohn, 1957; of fine sand- silt-size quartz fragments to (occasionally MÜlot, 1964; Chilingar, 1956; Fairbridge, 1967; Muller, kaolinitic illitic formed of course sand-size) in a to ground mass. 1967) and that glauconite is mainly by halmyro-
A number of large authigenic muscovite flakes more lysis. The depth of its formation is thought to lie than 1 mm in length and about 5% of black lignitic between 20 and 700 metres below sea level. The
how the of these substance. questions now arise presence b. Bands, approximately 0.5 mm thick, with generally glauconite-bearing sands can be explained in the entirely of smaller quartz grains and a much higher content continental, fluvial deposits of the Voznuevo Formation, organic material. Many small muscovite-like flakes as and how it is possible that kaolinite and glauconite, well iron be observed between the as (hydr)oxide can which are known to be stable under diverging pH con- detrital A and marked orientation of grains. strong ditions, occur together. muscovite flakes and the organic stringers, parallel to As to the first problem we must remark that both is bedding plane, apparent. occurrences of glauconite are situated in the upper part
of the Voznuevo Formation, at a relatively short vertical
The thick and large muscovite flakes, particularly distance (some tens of metres) from the transition to the frequent in the coarse-grained laminae of the alterna- Mesozoic limestones. Keeping in mind that the fresh sediments the of the tions, are, judging from their appearance, more meandering-river in upper part likely to be of authigenic origin. formation probably formed on a coastal plain, it is very of likely that temporary invasions sea water took place for that The calcareous sandstones which became stagnant, instance in lagoons, so sediments the As was to be expected, the calcareous sands in the top of marine were subsequently incorporated in
the Voznuevo Formation are — also in the thin sections fluvial deposits.
— not basically different to the sediments which they The mechanism described above also leads to the of solution of the second It is well known that overlie. With the exception one sample, the calcite is question. clearly the result of precipitation from waters rich in glauconite is resistant to mechanical influences such as lime between the detrital grains. Detrital carbonate reworking, but susceptible to chemical changes. mentions in material such as limestone grains and remnants of fossils Chilingar (1956), however, two examples were observed, but form small of Russia and one in the United where associations they a very part the States, total detrital content. The exceptional composition of of glauconite with minerals characteristic of an acid of from one sample, consisting grainstone (sample 129, environment (e.g. authigenic chlorite) have been encoun- the contact near Brugos de Fenar) can be explained by tered. The associations have been explained by forma- the fact that tectonic deformation has been very active tion of glauconite in shallow marine water which has in the location where the sample was taken. For this been cut off and converted to lakes. The pH became reason the sample might not be representative of the much lower, giving rise to the formation of a chloritic sediments immediately overlying the non-calcareous clay cement. The low pH, which can reach values below sands of the Voznuevo Formation. 7.0, brought about bleaching of the glauconite grains
From the observed contacts of the various cementing (oxidation leads to alteration into goethite). To a certain minerals, it is clear that the cements first had a quartzose extent this is also applicable to the glauconite grains in of 140 to kaolinitic composition. The subsequent replacement the present samples; the glauconites sample are of cements corroded. which these by sparry calcite was of very great even clearly Concluding, the glauconite influence the detrital few locations the of on quartz grains which became proved to occur in a in upper part corroded. The last of the be forerunner strongly stage in replacement, Voznuevo Formation is suggested to a calcite by ferric calcite, is only of minor importance, of the Upper Cretaceous transgression. probably only taking place at the sediment's surface. The locally relative high feldspar content, encountered
It be both the basal and in must finally noted that the feldspars, if present, in part (sample 022; near Boñar) and have a fresher than the the of the formation Va 2 Va 4; remarkably appearance feldspars upper part (samples sediments in the samples which do not have a calcitic cement. They Section T) makes it clear that the fluvial were will have been protected from weathering since the not devoid of feldspar at the time of their deposition. calcite formed. This also be concluded from the of was can high percentage
newly formed kaolinite which may be viewed as an Calcareous sediments within the Voznuevo Formation alteration product of feldspars. Two samples, one from Section II (sample 140) and one from Valmartino (sample 201), are of special interest Comparison with Paleozoic samples since the calcareous sediments which collected both sides of the they are only were Although many samples on encountered within the Voznuevo Formation and not in contact with the Paleozoic proved to be unsuitable for the mineral very upper part. A is found, together with study in thin sections, the few remaining samples show which absent all other calcite, is in samples, viz. that Paleozoic samples can fairly easily be distinguished glauconite. from Cretaceous samples. In general the following It is generally accepted that sea water is required for characteristics could be observed: 339
the carbonaceous radiolarites radiolarian Paleozoic samples Cretaceous samples: among (black 'An cherts). As to these, Carozzi reports (p. 310): ...
content of argillaceous matter provides good sorting poor sorting increasing many transitional varieties toward black siliceous shales(Kiesel-
The carbonaceous radiolarites also known concavo-convex and sutured point and straight schiefer). are and the German contacts between the contacts between the under the French name of phtanites of detrital grains prevail detrital grains prevail designation Lydite (Correns, 1924; Cayeux, 1929;
Heritsch and Heritsch, It was 1943).' ... Cayeux (1929),
however, who made a clear distinction between dust rings common dust rings present lydiennes (lydite, quartz lydien, jaspe noir, pierre de and this cement: chlorite, illite, cement: first quartzose, touche) phtanites. According to author, of and this be kaolinite and sericite then kaolinite, sometimes lydienne is a variety jasper term can only
illite used to designate black jaspers with radiolaria. A high
content of carbonaceous material is characteristic.
and Creta- Phtanites siliceous stratified rocks with Figs. 68 and 69, representing a Paleozoic a are synonymous
German Kieselschiefer. are much harder than ceous sample, respectively, show most of the differences the They the listed above. In general no differences could be lydiennes. established in the heavy mineral content, since heavy minerals are only represented in very modest quantities; Microscopical analysis of the pebbles brought to light 021 and 044 besides, only the most resistant heavy minerals such as that only samples 112, P 1, are unchanged titaniferous minerals radiolarites. with carbonaceous radio- tourmaline, zircon and were We are dealing
Paleozoic and the Cretaceous larites of less than which have encountered in both the (with an iron content 1%)
black dark brown colour the field and samples. Only one Paleozoic sample showed a strikingly a to in a light high zircon and epidote content. As we shall find in brown to dark brown colour in plain light in the thin
Chapter VIII, the latter mineral is extremely rare in the sections. In this dark ground mass undeformed casts of
Voznuevo sediments. radiolarites stand out clearly. Veins filled with poly-
crystalline quartz are of minor importance in the
The pebble composition samples from the Vegamián Formation (Tournaisian) from basal of the Voznuevo A sample the conglomerate near Olleros de Alba, but are thick and abundant in
Formation east of Santibafiez de la Peña showed that the samples 021 and 044. cobbles pebbles and consist of almost pure quartzites The second group of samples, numbers 132, 157, 235, small of 269 with a amount clay (kaolinite? ) specks and 309, underwent the evolution from 305): very (p. ... between the grains. The contours of the individual grains 'a cement composed almost wholly of amorphous silica'
still but solution has led to concavo- to 'a of are visible, pressure ...... noncrystalline aggregate quartz granules' ... and sutured between them. In the thin convex contacts (Carozzi, 1960). The original amorphous ground mass section this quartzite shows a close resemblance to the has largely been replaced by a complex of interlocking
quartzites of the Barrios Formation between Boñar and quartz grains. The quartzes in the ground mass have the
Voznuevo. same undulatory extinction as the quartzes in the veins
From the composition of the pebbles in the con- which are also abundant in these sections. Radiolarites
in glomerates near Soto y Amío it became clear that part of occur all these thin sections, especially in parts where the still A the components of these conglomerates are strongly amorphous ground mass is present. high all of Precambrian of carbonaceous the folded quartz phyllites, in probability content matter (up to 5%) causes black colour origin, such rocks having been encountered in the in the hand specimen and under the micro- made clear that the Precambrian (Boelens, pers. comm.). From the geo- scope. Laboratory analyses
carbonaceous matter of logical map of northern Spain we learn that Precambrian is graphite. The samples this rocks border the Cantabrian Mountains in the northwest, group have undergone intensive tectonical deformation, the west and the southwest (cf. Fig. 1) and that they in contrast to the samples of the first group, which show presumably continue below the post-Cretaceous deposits parallel lamination or no structures at all. Following which form the present 'Castilian Meseta'. The pebbles Cayeux's classification, these samples are in fact the thin and cobbles in the conglomerates mentionedabove have phtanites, their appearance in section is covered of identical folded of not necessarily a long distance transport since to varieties Cayeux's 'phtanite the Cretaceous west of La Magdalena is surrounded by feuilleté (et amygdaloïde)' (Cayeux, 1929; Planche XXI,
Precambrian rocks. Figs. 80 and 81).
The third group, samples 093C, 093D and 094B, col- Pebbles consisting of ‘lydite’ lected in the conglomerates W and E of the village of
of - Following the subdivision and the description radio- Soto y Amío, does not in contrast to the samples
hitherto - casts of radiolarites. larites by Carozzi (I960), the samples, mentionedabove contain any It is described which collected the doubtful whether these folded as lydites, were among strongly and meta- pebbles in the Voznuevo Formation must be classified morphic rocks are completely recrystallized radiolarites 340
6. The of the second show (pseudoquartzites, Pantin, 1957) or quartz phyllites. samples group most
Samples 093C and 094B contain many orientated mica resemblance to the phtanites described by Cayeux flakes (Fig. 73). About 25% of graphitic matter (in the (1929). All examples of phtanites given by Cayeux are
thin section, 5% by means of quantitative analysis) of Precambrian age. causes the dark colour in the sample. These quartz of phyllites or pseudoquartzites closely resemble those In view of this, it is much more likely that by far the
the Precambrian Mora Formation. As we have majority of the dark coloured pebbles in the Voznuevo
mentioned before, the Voznuevo Formation is enclosed Formation was not derived from the radiolarites of the
W of La Magdalena by these Precambrian rocks which Vegamián Formation. The phtanites probably have a of coloured and could be the suppliers the dark pebbles source area identical to the area which supplied the
cobbles in this part of the area. Other indications such as other sediments of the Voznuevo Formation, while the directions and differences in paleocurrent pebble com- quartz phyllites probably came from the Precambrian led the conclusion that of the position previously to part strata which outcrop in the extreme west of the present
- - those of were sediments especially gravel-size area.
from rocks. Another feature which derived nearby Differences in the assumed lydites are clearly
makes of real more is a supply quartz phyllites probable visualized in Figs. 70, 71, 72 and 73 in which a radio-
the of muscovite flakes. These can also presence many larite of the Vegamián Formation, a radiolarite of the
but in that case a and accompany pseudoquartzites, only Voznuevo Formation, a phtanite a quartz phyllite, small of muscovite is 1960). quantity present (Carozzi, respectively, are presented in a thin section.
remarks the radiolarites in the Concluding regarding Remarks on diagenesis
Voznuevo Formation in the The diagenetic processes which have taken place
There is no reason to assume that the radiolarites, being sandstones of the Voznuevo Formation can be com-
of the Voznuevo such a characteristic component pared, for instance, with those in the Triassic Stuben- derived for small from the Formation, were not a part sandstein (Germany) as described by von Engelhardt
Vegamián Formation, although the typical interveining (1967). Von Engelhardt distinguishes two diagenetic encountered in the radiolarites of the of by quartz was not phases, first marked by decomposition feldspars de Alba. This difference be Olleros can explained by and garnet, with solution of quartz by pressure, and neo- differences the Formation local in Vegamián or by formation of kaolinite and quartz. The following after the Voznuevo Formation. be involved: changes deposition in reactions are thought to Such the a source is, however, only applicable to two
+ 3+ of the first 2 KA1S¡ 0 + 16 H 0—2 K + 2 Al + 80Н~+6 samples group. 3 8 2 + There that of the H Si0 -»Al + Si 0 + 4 Si0 + 2 К 2 0Н~ are reasons to suppose most (OH) + many 4 4 2 4 2 5 2
dark coloured and cobbles in the Voznuevo + 13Н 0 pebbles 2
Formation (i. e. those of the second and the third group)
were derived from a source other than the Vegamián This mechanism takes place preferably in acid environ-
Formation: ments, a condition which will also be responsible for the
solution of detrital garnet grains. The the 1. If all radiolarites had come from the Vegamián second diagenetic phase, always posterior to marked the of calcite and dolomite Formation, the question arises why they are present in a first, is by deposition
constant proportion throughout the entire formation, in the pore spaces of the sandstones. According to von
while other rocks of Paleozoic derivation are mainly Engelhardt, the results of both diagenetic stages can be
limited to the lowerpart of the formation. observed in one and the same bed. Chemically, the main difference 2. The thickness of the individualbeds in the Vegamián between the first and the second period is a
Formation rarely exceeds 2 cm. The strata are change in pH conditions. In the case of the Voznuevo
commonly laminated (less than 1 cm thick) or 'finely Formation a transition can be inferred from acid con-
laminated' of ditions, which values lie below 7.0, to alkaline (van Veen, 1965). A very large proportion in pH
the in of conditions at the of formation, the latter dark pebbles the conglomerates the Voznuevo very top the diameter which often caused influences after Formation, however, attains a by marine during deposition or lime- exceeds 4 cm. deposition of the overlying complex of Mesozoic 3. The samples of the second and the third group are stones. often whereas the intensively folded, isoclinally, As we already have concluded in this chapter, feldspars show all. of formation and better Vegamián samples no folding at in the top the are more frequent 4. levels of The samples of the second and the third group have preserved than in the samples from other the been recrystallized under the influence of meta- formation, with exception of sample 022. They must
morphism, in contrast to the Vegamián samples. have escaped alteration by local, more alkaline con-
5. The composition of the samples of the third group ditions. The acid environment in the remainder of the but also reflected of the suggests that they are not radiolarites, quartz formation is in the scarcity garnets phyllites. the minerals VIII). among heavy (cf. Chapter 341
and Mason and and The fact that both diagenetic stages have been acting Ward, 1957; Folk, 1958) moment Friedman 1966 and almost simultaneously in the top of the formation is measures according to (1961, 1962, proved by the incomplete alteration of feldspars before 1967). Since Trask's and Folk's measures often could cementation by calcite took place. not be calculated, only moment measures were used. Percentile have the that The diagenetic processes which have taken place in the measures disadvantage they
cannot sediments of the Voznuevo Formation can be classified be calculated when the weight percentage of the
finest coarsest according to the nomenclature proposed by Nagtegaal and the grain-size class exceeds a certain
(1969) in his excellent review of classifications used, for figure. When both graphic and moment measures are
and Siever available the latter are to be preferred since their instance, by many Russian authors by (1959), of Fairbridge (1967), Dapples (1967a and b) and Muller sensitivity is greater, especially the third and the and (1967). Nagtegaal distinguishes three stages in diagenesis, fourth moment (skewness kurtosis, respectively). viz. (a) Early Diagenesis (both subaerial and sub- The moment measure most sensitive to environment is aqueous), (b) Advanced Diagenesis and (c) Late Dia- the third moment, while the fourth moment proved to genesis. be less indicative (Friedman, 1961). Skewness, thus of the useful criteria for the Surveying the characteristics of each diagenetic stage being one most separation of sands from various is for and relating them to the findings in the present deposits, environments, negative beach sands and for river sands, for it is found that the second stage, that of advanced positive example. the the sub- diagenesis in which compaction, cementation, authi- For present grain-size study area was divided into four genesis and replacement are the main processes, is no- parts: I West of the Rio where exceeded; the fact that dickitization of kaolinites, : Bernesga (23 samples). Ha: Between the Rio and the Rio Porma a characteristic feature of late diagenesis (Dunoyer de Bernesga
Segonzac, 1970) has not developed, favours this assump- (47 samples); tion. lib: Between the Rio Porma and the Rio Esla
did exceed In general, temperatures probably not (51 samples). of burial less than 2000 Ill East of the Rio Esla 100°C, while the depth was : (36 samples). metres. The latter can easily be calculated from the estimated thicknesses of the formationswhich cover the The results
Voznuevo Formation. According to Mabesoone (1959), In Table VI the percentage values of the second, the
moment for more than 1600 metres of Upper Cretaceous and Paleo- third and the fourth are listed, each
moment for twelve value intervals. These gene sediments occur upon those of the Voznuevo percentages
Formation; according to Evers (1967), between 1500 were calculated for each individual part of the area and for the whole series of each and 2000 metres. The Neogene and younger deposits, area as a (lowermost
Before ourselves draw con- with a total thickness of 1200 metres, have never moment). permitting to clusions the TableVI should covered the sediments of the Voznuevo Formation concerning percentages in it studied in this investigation.
2.1 2.2 2.3 2-4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 О ф 2.1 278 2.2 2.3 2.4 2.5 2.6 2.7 279 37o 3.1
1 4 4 4 9 9 13 9 - 9 - 4 35 GRAIN-SIZE DISTRIBUTION НА 13 4 a; &5 '■■■: &5 ■■: as «5 4 ■■: 15
II » 25 4 14 10 12 17 8 4 6 - - - Generalremarks
- - III 32 3 22 14 14 3 6 3 - 3 Grain-size distributions of 176 samples of sand-sized
TOTAL 19 4 13 10 10 10 9 4 4 3 4 10 sediments from the Voznuevo Formation were deter-
-15 -10 -0.5 0 «5 +10 +15 +20 +25 +30 mined in the sedimentological laboratory of the осз» -15 +35 -Й5 5 +55 +10 +15 +20 +25 So +15 of University of Leiden. For a detailed description the
1 - - 4 - 9 31 39 13 4 - - - laboratory proceedings the reader is referred to Koldijk
- - - II* - - 11 - 13 (0 34 2 - (1968a), whose samples were processed in the same 10 l: 10 31 4 - - - III - - - 33 laboratory in a similar manner. Calculations on statistical III - - - - 3 31 41 14 8 3 - -
of the grain-size distributions were carried parameters 16 : - TOTAL - - 4 3 9 27 36 4 - out with the help of a PL/1 computer programme on 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 (1968b). K4« 1270 grain-size parameters, composed by Koldijk 270 з7о 470 5.0 67o 7.0 87o 9.0 10.0 11.0 13.0 The favourable circumstance that the processing of the 1 9 J9 30 9 9 4 and the calculations involved were identical samples II* 11 34 40 13 2
of between II» creates the possibility a comparison Koldijk's 16 12 17 14 25 6 6 4 - - - -
22 and the of - - - - - geologically recent samples present samples III 55 47 9 55 55 55
Nineteen to contain too Cretaceous samples proved TOTAL 3 - - - age. 9 21 33 15 12 4 2 1 much gravel, so that 157 samples are considered in the Table VI. Percentage values for various value intervals of following investigation. kurtosis standard deviation (σφ), skewness (азф) and (a4φ) for of the The PL/1 programme provides percentile III) and for the entire area output four parts of the area (I, IIa, IIb, Folk measures according to Trask (1932) and Folk (in: (TOTAL). 342
Fig. 18. Standard deviation (second moment) plotted against frequency in percentages for four parts of the Voznuevo Forma- tion.
be noted that the extreme columns of the second moment are a compilation of all values smaller than 2.10- and than absolute larger 3.10; they are not percentage Fig. 19: A: Standard deviation plotted against skewness (second values. against third moment) for Voznuevo samples, compared with results obtained by Koldijk (1968b). Three very significant trends can be observed from this B: Kurtosis plotted against skewness (fourth against third listing: from west to east the values of the standard moment) for Voznuevo samples, compared with results obtained deviation {cup) decrease, while those of skewness and by Koldijk (1968b). kurtosis of the frequency distribution increase. In
this of simpler words, means that sorting the sands that the the of the values improves in an easterly direction, frequency are approximate contours plotted by distribution shows an increasingly 'fine tail' in the same Koldijk. For the Voznuevo Formation highly
schematical fields which more than 10% direction and that the curves become sharper towards are given in in the formation of the values the east. Especially the better sorting in the same direc- (measured as a whole)
observed must be From this we must tion as was previously assumed, by means of paleo- ranged. comparison current measuments (Chapter V), to be the direction of conclude that: 1. The Voznuevo Formation consists of sands which transport is significant. The rigid gradation of improved
are of fluvial such as dunes, sorting from west to east is visualized in Fig. 18. origin (other possibilities
barrier sands, lake and desert sands are out of the Finally we have availed ourselves of the opportunity of
question), since the skewness of their distribution curves comparing the results obtained by grain-size separation of Voznuevo samples with those of recent beach and is positive. 2. The Voznuevo sands sorted, much river sediments from Galicia (northwestern Spain) are very poorly than river sands. studied by Koldijk (1968b). This is done in Fig. 19A, in poorer Koldijk's 3. The of the distribution which the standard deviation is plotted against skewness, shapes curves are more normal than those of the Galician samples, but there is and in Fig. 19B, in which kurtosis is plotted against still clear towards curves. skewness. The irregular fields sketched in these figures a tendency leptokurtic (sharp) 343
CHAPTER VIII
THE MINERAL ASSOCIATIONS
of which six mineral groups. Other mineral groups, only
THE HEAVY MINERALS titanite, epidote and topaz were observed, are com-
in pletely subordinate. They are listed Table VII among
Introduction 'others'.
the Five of the mineral In order to obtain a better insight into composition six groups are represented of sediments of the Voznuevo in 20, in order to examine and the provenance the graphically Fig. possible carried of trends differences from west to east. The Formation, a fairly extensive study was out and/or con- will be the the heavy-mineral content of the sands. Of a great clusions from this figure given in description material mineral number of samples slides were prepared of the of each individual group. of number of them between 50 and 500 microns, a large also fractionated slides of the fractions between 50 and Tourmaline. — is the most abundant heavy mineral
150 microns, between 150 and 300 microns and observed in the Voznuevo Formation. It occurs in all between 300 and 500 microns. Sixty-four samples were samples, its percentage generally fluctuating around selected as representing the heavy-mineral composition. 50%. Light to dark brown and yellowish brown of The position these samples is shown schematically in tourmalines were observed most, followed by the green which of the correlation chart Fig. 21, is a simplification variety. Black and blue tourmalines occur in most of Enclosure XI. In order to compare the samples as samples, but their share is always of subordinate objectively as possible (fractionating proved impossible importance. The tourmaline grains are generally fresh, for number of the the rounded or incomplete a great samples) and all transitions from prismatic to perfectly collective slides from 50 500 microns used of the to were specimens were observed. Most grains appear to of where possible. In samples in which the number be little rounded, however, while well-rounded grains are made of the heavy-mineral grains was too low, use was rare. One might assume that the rounded tourmalines are had been 150 to 300 micron fraction (which prepared multicycle, Paleozoic-derived grains, while the less the collective The results of of abraded have different A similar observa- prior to slides). countings grains a origin. the translucent listed Table grains are in VII, arranged tion was made by Leguey and Rodriguez (1970a), with from for three west to east (left to right) imaginary respect to zircons of recent fluvial sediments in the Esla levels middle and of the Voznuevo (lower, upper part and Pisuerga basins: they made a distinction between Formation). Paleozoic and post-Paleozoic zircons. of tourmaline remains The percentage constant over
The results the entire area (viz. from west to east and from the
Before taking a closer look at each of the minerals or lower part of the formation to the upper part). Anti- from the observations the mineral groups of Table VII, let us first retain the cipating the conclusions on fractionated bear mind that the following generalities: samples, we must in
tourmaline:zircon ratio is largely dependent upon the
of take this fact into 1. The heavy-mineral content of the sands of the grain size the sample. When we
Voznuevo Formation is low. consideration it is easily seen that irregularities in the
2. Many grains, several of which possibly belong to the representation of the percentages in Fig. 20 may be
light-mineral fraction, are coated with iron; under the directly attributed to grain-size differences. For this of of microscope they are completely opaque. The percentage reason the results of the grain-size analyses most be 90% of the total of opaque grains can very high: over the samples have been added in Table VII. Where the number In which differ of grains. a previous study, in only percentage ratios of grain-size distribution from the between Sorribos de Alba and from of the samples area distinctly the mean composition sands, i. e.
Colle were considered, a number of 300 grains had been less than 60% of sand, more than 50% of silt and more
counted in each alterites and than 20% of values chosen sample, opaque grains clay (these were at random), included. for alterites and The mean percentage opaque this is marked in the lower part of each column. Where
grains, calculated from 59 samples, proved to be 25% the grain-size distribution could have influenced the and 38%, respectively. smooth continuation of the percentage lines in Fig. 20
is S 4, Col 17, 180, 160 and Z 6), the result of 3. Since the number of translucent grains per sample (samples relatively low, the number of grains counted generally the heavy-mineral counting is represented by a dashed while solid line the values for the did not exceed one hundred. In some samples, in which line, the connects
of The of at the number countable grains was lower, the per- adjacent samples. position sample 240, a very short distance from the is held for centages were converted accordingly. Paleozoic, responsible of the extremely high percentage zircon, so that the
The fact which be from result of this will also have to be considered most conspicuous can gathered counting the heavy-mineral composition is the predominance of with some reserve. 344
5 ~ 5 17 71 12 - - - 0.4 3.0 «5 12 15 3 0.3 1.8 C5T 80.0 16. Va6 77.5 20.6
- - - " 0.0 15 73 12 1.0 4.5 23 67 10 77-5 11.3 10.6 C1T 81.6 13.0 313 extraordinary 2 - - ~ 0.7 3.3 38 40 12 10 0.0 9-9 269 31 35 18 16 68.4 26.3 259 63. 25.9 an
- X - - ~ 1 4 1 9.3 2.4 42 30 14 14 79 15 64.5 22.3 268 237 with
- X - 1 5 2 1 X 1 7 - 1-5 4-5 36 55 0.0 2.9 253 56 36 63.6 29.1 203 68.6 26.9
- " - - 2 - 2 X 20 38 16 24 4.1 14.9 70 28 0.1 26.2 13.1 244 61.1 I8.9 169 59.9 Samples
3 3 3 3 < 0 0.2 4.2 242 ш О 034 34 54 70.3 21.1 E.
b to - - - - 3 3 1 3 10 30 10 0.3 З.9 240 96 047 44 73.5 19-5 W
~ 2 - X 4 2 8 5 - 11 - 1.2 4.4 Z9 36 56 O.5 8.5 2.5 228 76 72.2 21.8 87.8 from
X 5 4 - - 0.0 6.7 14 13 0.0 " 227 64 27 65.9 26.6 Z6 47 26 34.7 46.2 19.I
X - - - b - - 3 0.2 З.4 46 14 18 22 0.3 4.6 57 40 8З.5 11.6 224 54.9 З9.9 059 arranged
X 4 - - - 0.2 6.2 8 20 0.0 1.0 236 35 23 21 21 70.8 21.2 057 68 72.0 25.8 area, " 5 - 1 1 4 - 2 0.0 3.2 36 57 0.0 4.З 210 43 12 29 10 70.4 26.4 083 78.7 17.8 the 3 3 8 - 6 1 FORMATION - 0.8 7.2 1.0 4-5 176 50 10 10 19 72.9 I8.9 080 34 37 19 79.2 15-4 of
X X X - - - - - X 2 1 9 3 2 0.2 5.0 42 0.4 1.6 95 38.0 55-7 46 71.7 23.З 180 137 parts - 5 5 - 3 3 - X 4 6 19 0.0 9.2 » X 30 56 0.1 9-9 3.1 Col 22 21 31 54.2 35.2 147 76.1
1 X X three 1 1 - 1 - 4 0.3 4.4 VEGAQUEMADA 0.6 7.5 6.8 21 Col 66 30 85.6 117 37 42 81.O 14.4 in * • 1 1 7 X - X - 0.0 X X 23 10 65 1.9 Col 17 33 25 35 28.8 49-1 21.3 124 75-6 10.6 11.7
X 7 1 5 8 - 3 5 0.8 9.7 3.9 17 16 013 57 22 86.1 122 30 29 samples
X - X 5 X 1 4 1 1.2 3.2 45 18 32 024 62 32 82.1 12.7 121 64
X - X - 4 3 1 4 1 0.0 7-5 1-5 of 020 56 38 91.5 135 44 49
3 2 5 1 50 X 3 X 2 0.6 9.4 1 1 3.4 Cl 42 69.4 20.1 101 63 27 73.1 10.1 14.9
1 - 4 5 6 - 7 5 2.4 8.1 5-9 46 12 24 4.7 X 40 50 84.5 S22 57.8 26.8 12.1 099 percentages
OC - - 4 6 9 5 3 3 4 4.0 4.4 59 31 0.8 8.1 81 10.4 SI 66.2 24.9 310 8O.9 1 - 3 - 0.1 X X - 9 0.8 З.4 S4 31 30 15 21 27.7 46.5 24.8 312 90 83.4 11.9 7 distribution - - - 8 0.0 32 11 18 0.0 4.7 S3 32 12 48 66.9 21.7 12.9 256 32 71.9 22.7 4 X - - - 2 X 0.0 7.3 56 11 6 23 0.2 4.7 Sb9 40 58 75.2 17.7 212 55-7 39.3 4 5 X - 3 - X X :.o 6.5 51 15 15 16 0.0 43 48 47.9 45-0 grain-size Sb8 28.5 60.4 12.7 160 - - - 5 8 1 1 4 1.1 3.4 95 0.0 5-7 33 14 19 72.2 22.9 108 - 84.4 10.4 033 and - - - - - 5 2 » 1 1.8 4-5 61 17 15 0.0 56.7 096 99 81.6 1З.7 025 31.1 11.4 S 1 X - 2 - 3 1.3 X 27 10 0.0 6.7 42 24 22 11 34.5 107 58 82.2 12.7 016 53.7 11. ‘x’. 1 3 i 2 - 3 3 2 composition 0.9 Sb 14 58 12 5.8 6.2 104 6i 30 74.4 12.4 13.4 24 66.1 22.7 by 1 00 г ас i и шi indicated clay silt sand clay ailt sand gravel Heavy-mineral are gravel sand silt clay 6O36 sand silt clay 20f, >50?S 60$ >20Jt >50)S NUMBED < NUMBER < > 0/ '° RUT.+ANAT.+BROOK. o/ '° BUT.+ANAT.+BROOK. VII. SAUPLE TOURMALINE ZIRCON GARNET STAUBOLITE KYANITE ANDALUSITE OTHEBS SAUPLE TOUEUALINE ZIBCON GARNET STAUBOLITE KÏANITE ANÏALUSITE OTHEBS Table composition 345 minerals for of Voznuevo Formation. Fig. 20. Frequency in percentages of the most common heavy three parts the Dashed lines: values uncertain due to extraordinary grain-size composition. - Zircon. is found in almost every sample, both as small, be explained by a temporary erosion of a garnet-rich egg-shaped grains and as hardly worn bipyramidal grains, rock and/or increased influence of a tributary. It is also that small-scale environmental so that two different sources, as mentioned above, are possible circumstances also applicable to the zircons. The first category is far have favoured their preservation in this location. All more frequent, but examples of both generations are garnets observed are of the colourless to pink variety. commonly found in one sample. As we stated before, the of Staurolite. — mineral characteristic of the Voznuevo percentages tourmaline and zircon are closely is a related, the zircons, generally smaller in size, being Formation, being the mineral second in importance. It is almost exclusively restricted to the grain size between 50 only absent in three samples of an extraordinary and 150 microns. The zircons are rarely strongly composition (Samples 096, 108 and 180) and in the corroded. From Fig. 20 it becomes clear that the per- lower part of the easternmost section. The staurolites centages of zircon decrease from east to west in both the proved to be susceptible to corrosion, which is not in view in lower and the upper part of the formation. surprising of the position of staurolite the in of heavy-mineral stability sequence and the order Rutile, Anatase and Brookite. — taken together as persistence (cf. Péttijohn, 1957, p. 504 and p. 506). titaniferous show obvious decolouration from brown minerals, are not plotted in Fig. 20, since They an via their does not exceed 20% four and to colourless percentage except in orange-red yellow grains with a very The titaniferous minerals show faint absent Thin samples. no special or pleochroism in many samples. characteristics, apart from the fact that two types of staurolite scales with low interference colours (grey) can rutile be observed: dark red and confusion with other minerals. The grains can a a yellow easily cause heavy variety. staurolites generally contain a large number of black 'concertina' From Table VII one can see that there is a plain (carbonaceous) inclusions. Grains with relationship between zircons and titaniferous minerals. boundaries have occasionally been observed. According This relationship is not only caused by them being to Milner (1962) this can probably be regarded as a solution From 20 that the related in grain size, but also by the fact that the percen- phenomenon. Fig. we can see of staurolite shows decrease from tage of titaniferous minerals decreases from east to west. percentage a slight the lower and middle of forma- It is also worth noting that the percentage of titaniferous west to east in part the and rather in the minerals shows a marked diminution from the lower part tion remains constant upper part. of the formation to the fact which could upper part, a not be observed in the case of the zircons. Kyanite. — is a very common heavy mineral but its percentage never exceeds 10%. It is, however, most Garnet. absent the is rare; only sample 256 shows a high per- remarkable that kyanite is in easternmost of minerals of the This Formation. centage garnet group (32%). may samples at all levels in the Voznuevo The 346 grains are often badly corroded, but they are still easily recognizable. Andalusite. — is the most remarkable heavy mineral in the present study, since it occurs in extremely diverging andalusite text. percentages. As kyanite, is absent in the eastern part of the formation; apparently both minerals see related their distribution Z are closely as to pattern. Very of andalusite limited to high percentages are to the area U west of the Rio Bernesga. The andalusites are generally found in large grains with a pleochroism from colourless lines to pink, occasionally to vivid red. As observed in the of of case the rutiles, two types of andalusite occur, one black with many graphite or carbonaceous inclusions, the other devoid of inclusions. In the exposure where explanation the direct transition from the Voznuevo Formation to the overlying Vegaquemada Formation was encountered, For the sample collected in the latter contains a much higher percentage of andalusite and a lower percentage of studied. tourmaline than the Voznuevo samples in this location 124 in Table This be (sample VII). may a coincidence, since heavy-mineral studies of the Vegaquemada Forma- samples tion (Kuyp, 1969) showed that the percentage of andalusite generally does not exceed 5%. combined of The percentages andalusite and kyanite, show marked decrease represented in Fig. 20, a in an heavy-mineral direction the of easterly (caused mainly by percentage andalusite; the percentages of kyanite are smaller and the more constant). Besides, the andalusite grains proved to of become conspicuously smaller towards the east. and T Conclusions based on the heavy-mineral content to Summarizing the distribution of the heavy minerals in A the Voznuevo Formation from west to east and from bottom to top we must state that: sections of a. The percentage of tourmaline is constant. b. The percentage of zircon increases towards the east, especially in the lower and the upper part of the forma- position tion. The of с percentage titaniferous minerals is constant with from west to east but decreases in a vertical sense (towards the top). d. The percentage of staurolite decreases towards the Formation east for except the upper part of the formation where it remains constant. The of e. percentage kyanite is constant, but zero in the eastern of formation. Voznuevo part the f. The percentage of andalusite decreases considerably the towards the east in three levels of the formation. of chart In 21 made relate Fig. an attempt is to the distribution of the heavy minerals to the diachronic character of the Voznuevo Formation. In this simplification of Enclosure Schematical XI we recognize the Lower Cretaceous — Upper Creta- which has been established ceous boundary (line U=I) by 21. pollen analysis (Chapter IV). Lines V to Z have the Fig. following significance: 347 the of zircon т 17 8 30 39 82 4 65 19 52 50 43 23 27 30 16 47 Line V separates the area where percentage 61 36 36 3 LO 48 2 9 45 18 exceeds 70% (not caused by small grain size) (right) z 50 9 2 9 30 49 В+А+В 12 35 u 8 9 - 26 16 12 10 4 20 5 38 9 12 from the area where it is less than 70% (left). 0 10 7 7 10 X * 1 - - - - Line W: Kyanite and andalusite absent (right) and - 6 s - - 11 7 - 24 - 19 5 4 40 15 19 17 (left). present - - - - 22 - - - X X X к - - 3 11 6 Line X: Staurolite less than 40% (right) and 8 exceeding And - - - - - 48 - 62 - - 3 2é 6 40% (left). Оч Оч Оч Оч ЧЭ ■«j- *r Оч Оч o\ см см 10% i^ u~> Line Y: Titaniferous minerals generally exceeding m \o 4) 43 ЧЭ líN р- о ЧО чо 43 Оч ON o\ Оч ON Оч os Оч Оч Оч Оч Оч Оч Оч оч o\ »H *Ч тЧ и «ч (right) and always lower than 10% (left). Ф Ф Я Я К Line Z: Andalusite less than 10% (right) and exceeding О ÍH о h я я я Я Я м * о О •Я ф M в ■ Ю ■p m g n Pi Pt • 10% (left). ft ft I К >» А Л ш 10 •H 1 Л i О 3 3 3 ■J о о о в 5 о о 14 H US M 1 я "* К И :--. i-. m Pt Pt 5з ь It is a most remarkable result that these separating lines a 3 » а а 3 э Ф Я Я я с о о to line the time •И 11 run approximately parallel U, boundary, л ф Ф 9 ф Ш a о 9 M :■ Я я al С А Pi а л л о о ф ■** which that in о 4» л ri d 1 « ■р ф ш te means changes mineralogical composition ■и Ф * о о 0) t. Ш n в £ « л te Ф Ф й h и h h h и О Pt № р« я я ■* я Р № е-> e< ь* í < í ' О О of the source area extended their influence almost synchronously over the area studied. Chronologically =Tourmal т ine Util. * Present in many г samples observe of associa- speaking, one may a predominance an X =2ire on Г t Si:-.'. R+A+B=Rutile+Anatase+Brookite gradei 50-150 micron tion of stable minerals rutile, anatase very heavy (zircon, G =Car net S=Stauroli te and brookite) which gradually passes into an association К =Kyanite of in which less stable heavy minerals metamorphic And=Andaluai te origin (staurolite, kyanite and especially andalusite) gain Table VIII. Heavy-mineral composition of representative samples in This is in accordance with the common importance. of rocks of various in the southern Cantabrian (in ages area observation that associations become heavy-mineral percentages). Size grade: 50 to 500 microns. trend more complex with decreasing age. A comparable in the alterites while others did not consider was observed, on a larger scale, heavy-mineral among 'others', assemblages in the sediments of the southern border of the alterites at all in calculating their total percentage. the Cantabrian Mountains in general, from Paleozoic to As we have noted previously, and as is clearly Tertiary (de Jong, 1971). illustrated in Table VIII, the heavy-mineral assemblage becomes more complex with decreasing age, due to the The influence of grain size on the heavy-mineral introduction of metamorphic minerals. The very composition resistant heavy minerals of the tourmaline, the zircon influences the and the titaniferous As we have remarked above, grain size group are generally present in heavy-mineral composition in a large measure. Calcula- throughout the sequences of formations percentages tion of the heavy-mineral percentages of some tens of which seem rather varying. Comparing the percentages fractionated samples showed that the following trends of Table VIII with the percentages found in the latter based are always found: an increase in grain size results in an Voznuevo Formation only, the on a large of observes that the increase in the percentages of tourmaline and especially number samples, one percentages of within formation much in the whole of andalusite, and in a decrease in the percentages vary one as as staurolite and The from Devonian The of zircon, titaniferous minerals, kyanite. sequence to Tertiary. percentage decreases with its corrections made in Fig. 20 show that the addition of garnet decreasing age, although has been all listed in grain-size distribution data (as added in Table VII) can presence reported by investigators Table VIII. The minerals in be of great help in understanding the differences in metamorphic only gain is to be after the transition from the Paleozoic to the heavy-mineral composition which may arise. It importance distribu- Mesozoic have been observed in percentages regretted that in many publications grain-size (they high the size studied have been omitted. in the Triassic Staurolite, however, tion or even grades upper deposits only). is also present in Westphalian deposits (Nossin, 1959; Comparison with the heavy-mineral composition of van Loon, 1972). other formations The behaviour and the trends shown by these seven of the results of mineral in accordance with the In Table VIII a comparison is presented groups are perfect minerals of all heavy-mineral counts obtained by investigators of the frequency of occurrence of heavy in rocks 1941, University of Leiden in rocks of various ages in the ages from different locations (Pettijohn, 1957) Cantabrian area. This table has been slightly modified from which frequency Pettijohn (1957) established his and extended in accordance with de Jong (1971). In order of persistence of heavy minerals. Other workers remarked that for order to make comparison more reliable, only the seven (Milner, 1962) andalusite, instance, listed, with back far as the Paleozoic, but that its most important mineral groups are omission can range as 'Others' the becomes with of the category so that remaining per- occurrence very rare increasing geological recalculation be said of and, to a lesser centages had to be recalculated. Moreover, age. The same may kyanite listed the of staurolite. proved necessary since some investigators only extent, 348 A sudden increase in metamorphic minerals, which kyanite and andalusite as a metamorphic association. To occurred in the Triassic makes a derivation from nearby what degree the Voznuevo Formation has contributed to Paleozoic rocks (i.e. from the Cantabrian Mountains) the supply of the Tertiary sediments will be the object of brief end of rather unlikely. Moreover, the minerals of the staurolite, a discussion at the this chapter. — and andalusite to Milner In the triangle Palencia Guardo — Cervera de kyanite group are, according which carried (1962), derived from: crystalline schists and contact Pisuerga, in a sedimentological study was metamorphic rocks (staurolite); metamorphic rocks, out by Mabesoone (1959), two distinct heavy-mineral could be tourmaline- especially mica schists and certain gneisses (kyanite); associations distinguished, a granites and contact metamorphic rocks (andalusite). A staurolite association in the northeastern part of his area with contact and zircon-rutile association in the southwestern provenance in a granitic area metamorphic a part. the the aureoles is therefore much more probable. As we have According to Mabesoone, various associations are stated before the with the (p. 334), the extensive investigations by result of different supply areas, group Papa (1964) in the Triassic rocks north of Cervera de resistant minerals being derived from Paleozoic showed with sediments with little influence from Cretaceous Pisuerga an increase in quartzose gravel deposits, The diameter of since the Mesozoic between Cistierna and decreasing age. mean the pebbles complex distant than the from Guardo is thin absent. The indicates a more source area one very or completely group which the oldest clastic Triassic deposits are thought to with the metamorphic minerals, found in the north- have been derived. Differences in pebble size indicate a eastern part, the drainage area of the Rio Pisuerga, shows which transport from west to east, so that the upper-Triassic a clear connection with the Mesozoic sediments much and thicker this location. sediments were probably supplied from an area which are more widespread in Mabesoone stated that the must have been situated in a westerly direction. Both the (1959) heavy-mineral composition and the heavy-mineral content of the association in the Quaternary sediments is closely con- nected with the with Triassic rocks and the Voznuevo Formation indicate a Tertiary association, a slightly content. common source area, different from that of the higher epidote in 1962 Mabesoone contributed another the Paleozoic rocks. This assumption can readily explain the paper on abrupt change in heavy-mineral content between the Tertiary in the northern part of the Duero basin, with Paleozoic and the post-Paleozoic sediments. the red beds (comparable to the Vega de Riacos Forma- The possibility that the sediments of the Voznuevo tion, Evers (1967), and the Riacos Formation, van den studied Bosch (1969)) between La Robla and Villadiego (32 km Formation, as in the present area, are produced by Triassic sediments being reworked must be rejected, NW of Burgos) as main subject. The presence of blue since Triassic known the tourmalinein of his led him to reconstruct deposits are only in very some samples of the which the eastern part of the area, while transport of the the ancient river course river deposited red beds. The sediments, as shown by paleocurrent measuments, has course runs in a southeasterly direction, taken from the southern border of place west to east. Besides, there is no approximately parallel to present the chain. have stated the evidence, that Triassic deposits ever covered the mountain As we above, Paleozoic - minerals rutile and west of the line Potes Guardo, although de resistant (zircon, tourmaline) are to have been Paleozoic rocks. Jong (1971) suggests that they may have extended for thought supplied by some distance to the W. According to Mabesoone, metamorphic minerals, The first sediments the Triassic post-Cretaceous in present area especially staurolite, were supplied by rocks, are the Lower Tertiary clastic wedge deposits (de Jong, while the Voznuevo sediments were considered to be the which becomes 1971) are thought to have been derived from the supplier of tourmaline. In this same study it uplifting Cantabrian massif. The Vegaquemada Forma- clear that the easternmost part of the area was in- tion and the Candanedo Formation(Evers, 1967; Kuyp, fluenced by the increasing importance of Mesozoic rocks in the of the towards the reflected in 1969) were distinguished western part area east, in an increase metamorphic of the Rio while Mabesoone made minerals. Mesozoic (west Esla), (1959) no Nevertheless, sediments, present as a subdivision flank and designated them as Cuevas facies in the narrow strip along the greater part of the southern had eastern area. of the Cantabrian mountains, will have some of According to most investigators, these deposits uncon- influence on the mineralogical composition the overlie Mabesoone faced with the formably the Mesozoic limestones, the angle of Tertiary red beds. (1962) was unconformity generally being too small to be observed problem from where the staurolites in the western part in the field. of he stated Other, mainly sedimentological, features the area studied by him were derived; (p. (see west of the Rio Evers, 1967, p. 121) are better indications of a 243): '... But here (the area Pisuerga is hiatus in the of sediments of sequence events. meant, remark by the present author) no The the have been the staurolite heavy-mineral composition in Vegaquemada Triassic age found, so that high Formation and the remains in Candanedo Formation shows no content, especially in the Porma region, un- significant differences to the assemblage observed in the explained. Although it is possible that the Triassic belt Voznuevo Formation. The only data available (Kuyp, could have extended somewhat farther to the W, this 1969) show that tourmaline, zircon and the titaniferous distance seems to be too great (more than 70 km) ...' In minerals are present as a resistant association, staurolite, search of a solution to this problem Mabesoone (1962) 349 proposed two different supply directions for the two mineral composition of the Rio Tera will be left out of consideration of irrelevance. heavy-mineral associations which he distinguished. The on account its zircon-rutile association was to have a northern The most salient feature of this enumeration is the rocks! the staurolite-tourmaline from west to east of andalusite, a provenance (Paleozoic ); decreasing importance Mabe- fact also observed in the Voznuevo Formation. The association a western provenance. Following high soone's train of thought, the staurolites in the red beds andalusite content of the latter is bound to have in- '... have been derived from Paleozoic deposits sedi- fluenced the mineralogical composition of its overlying ...' sediments and mented at the foot of the Galician block itself (p. (e. g. the Vegaquemada Formation) con- 244). The metamorphic erosion products would thus sequently also the composition of the recent alluvial have been deposited in the eastern part of the Galician sediments of the rivers which cut through these forma- area, in lower Paleozoic sediments, to be retransported tions. In this connection it should be kept in mind that further to the east in Tertiary times. Quoting Mabesoone the influence of Cretaceous heavy minerals on the recent solution fluvial of indirect nature since the (p. 245) '... Evidently they did not undergo any deposits is mainly an during that long period ...' (Previously he mentioned sediments in which eroded Voznuevo material is present thicker than the Voznuevo Formation that the staurolites in his samples had lost their are many times few of and that their influence the recent sediments pleochroism but show very signs solution itself, so upon Other facts which be corrosion). will be much greater. may con- and In our opinion this assumption is very debatable, since cluded from the findings by Leguey Rodríguez seem be contradiction the trends observed in the the staurolites of the Voznuevo Formation, which are to in to Voznuevo Formation. decrease in the thought to have a common or comparable source area, They are: a already show signs of solution and slight corrosion in importance of zircon towards the east and an increase in staurolite the this reason the percentages of tourmaline and in same many samples. For it is highly improbable solu- direction. and never that the staurolites escaped intensive or complete Unfortunately Leguey Rodriguez the size of the residues. tion and corrosion in the time-span from lower Paleozoic mention grade heavy-mineral influenced the zircon:tourmaline ratio to Tertiary. In this connection the Voznuevo Formation This could have Another fact which lead this can be considered as the 'missing link' between the considerably. may to difference from be caused the three Paleozoic and the Tertiary sediments, since we know west to east can by easternmost rivers Paleozoic rocks main that this formation is very rich in staurolite, and can crossing (the in easily be responsible for the high staurolite content in supplier of zircon, and scarce staurolite) along a very small of their it is worth some of the Tertiary red bed samples. This supposition is part courses only. Finally while strengthened by Mabesoone's observation that noting that tourmaline is always dominant, found of the staurolite is often but dominant. staurolites are especially in the western part abundant, never also the location where The fluvial show relative abundance area investigated by him. This is recent deposits a the Mesozoic limestone cover on the Voznuevo of tourmaline and especially of andalusite, with much Paleozoic and sediments was much thinner, so that Voznuevo material higher percentages than those found in could sediments. rocks. and ascribe this dif- more easily reach Tertiary Tertiary Leguey Rodriguez Leguey and Rodriguez (1969, 1970a, 1970b, 1971) ference to the grain-size distribution of the sand of the for have made an extensive study of the heavy-mineral recent deposits. Poor sorting can, example, cause a content of the recent river sediments in the northern high percentage of andalusite, since andalusite crystals — - much than other part of the Duero basin, subdivided into: (a) the Esla are on an average larger crystals. basin with, from west to east, the rivers Tera, Orbigo, The high percentage of tourmaline may have the same the Bernesga (with the Torîo as affluent), Esla (with cause. basin Staurolite is the lower of the Porma as affluent) and Cea, and (b) the Pisuerga only important in part with, from west to east, the rivers Carrion, Pisuerga, Rio Pisuerga, since only Tertiary deposits show a relative Arlanzón and Arlanza. The distribution of the heavy abundance in this mineral. Leguey and Rodriguez (1969) staurolite follows minerals in these rivers as found by the above-mentioned try to explain the supply of as (p. follows '... conclusión deducimos authors can be represented as (heavy-mineral 534): otra que es que como names abbreviated): este mineral no pudo llegar al Terciario procedente del de hacerlo de otras formaciones Paleozoico, tuvo que Dominant Frequent actualmente no existen en las proximidades ...'. As we Тега and., hbl., sillim. tourm., garnet have seen in the above, the Triassic rocks near Cervera de Orbigo and., zirc, tourm. rutile Pisuerga could have supplied the staurolites (Nossin, Berncsga tourm., zirc., and. staurol., rutile Esla zirc., tourm., and. rutile, hbl., garnet 1959), but their occurrence further to the west cannot rutile and., titanite Cea zirc, tourm., be explained. Mabesoone (1962) assumed the staurolites Carrion zirc, tourm., and. rutile, hbl., garnet in the red beds to have come from the Galician massif. Pisuerga tourm., zirc. rutile, garnet, and., staurol. have Cretaceous As we seen, however, deposits could Arlanzón tourm., diopsite garnet, zirc, staurol., and., rutile have the staurolite to Arlanza tourm., garnet rutile, staurol., zirc, and. easily supplied the Tertiary sediments and consequently also to the Quaternary In the following discussion on this result the heavy- sediments. This is already assumed by Leguey and 350 Rodríguez when they state (p. 235): '... de lo expuesto formations are found, points to a supply with at least a influencia northern The anteriormente se desprende una clara de los component. heavy-mineral assemblage ... sedimentos wealdenses en los diversos niveles de este recalls the continental Cretaceous Voznuevo Formation, tramo alto del Pisuerga ...'. from which it must have partly derived'. This derivation of does not for the of In the discussion on the results their investigations, cause many problems western part the Spanish authors arrive at the conclusion that the the area studied, since it has become clear from the tourmalines derived from Paleozoic that Mesozoic limestones rocks are generally present study have never been of between the Voznuevo Formation and the better developed (show a higher stage evolution) than present those from the Cretaceous and Tertiary rocks. As to the Vegaquemada Formation in the western part of the area. different could be observed But Mabesoone zircons, two cycles as well; (1959), too, reported layers consisting those derived from Paleozoic rocks (first cycle) which of conglomerates of quartz pebbles in alternation with limestone from that are subrounded, rounded and angular (in order of massive layers part of his area con- sidered the Grès importance) and a second cycle with an increase in the by Ciry (1939) as continuation of the de las Bodas the The absence of number of roundedand broken grains. to east. clastic limestone fragments is apparently not in accordance with the fact Concluding remarks on the heavy-mineral content of the that Voznuevo sediments supplied most of the material for Grès de las Bodas. In Voznuevo Formation the our opinion it is much more Summarizing the results obtained by heavy-mineral logical to assume that the oldest deposits of the that the of the Formation still had the counts, one may say composition heavy- Vegaquemada same source area the far detrital mineral assemblage reflects a development in the source as Voznuevo Formation as as the coarse of the concerned. The lime area. The stratigraphically oldest deposits show a higher parts cycles is high content of with the of the could percentage very resistant heavy minerals; in upper part Fining-upward sequences decreasing age an association comes to the fore in which have formed from waters rich in lime which had been in metamorphic minerals become more and more contact with the Mesozoic limestones. It is especially the latter association which The of the Formation inter- important. upper part Vegaquemada has had influence a strong upon all post-Cretaceous fingers with the conglomerates of the Candanedo Forma- sediments. tion, which for this reason represent the distal and the The which the material of the source area supplied proximal part of clastic wedges (de Jong, 1971); the Voznuevo Formation had been presumably already conglomerates, being the proximal parts, show quite a active in Triassic times, since the Triassic sediments N of different composition. Limestone pebbles, both of Cervera de described in Pisuerga by Papa (1964) show, Paleozoic and Mesozoic origin, are met with in these their close the in the of upper part, analogies to findings conglomerates. In the eastern part the area Voznuevo Formation. this became However, supply area Mabesoone (1959) found limestone conglomerates, with dominant Cretaceous in times. sandy intercalations, containing pebbles, mainly derived The and sediments, Paleogene, Neogene Quaternary from Mesozoic rocks, while Kuyp (1969) reported con- which further the were deposited successively to south, glomerates consisting of quartzites and 30% of Paleozoic bear of the stamp the Voznuevo Formation. Of course limestone pebbles in a sandy matrix. These findings are this influence gradually decreases with decreasing age, as in accordance with the observation that the Mesozoic increases. the staurolite and complexity Especially limestone cover gradually thins towards the west, being andalusite content of the formations post-Cretaceous even absent in the extreme west. The sandy inter- need no be the of unfounded longer object conjectures calations between the conglomerates in the east seem since the Cretaceous could supply part of their difficult to explain since their possible source, the sediments. Still the whether the problem remains Voznuevo Formation, must have been covered by Cretaceous sands could be eroded although they were Mesozoic limestones. In order to explain this de Jong covered by the Mesozoic limestones. proposes the following possibilities: Let for moment dwell of us a upon the provenance the sediments: with limestones Paleogene Deposition began relatively a. Valleys cutting through the Mesozoic de las Bodas' fine-grained deposits, Ciry's 'Grès (lower provided the sandy material. of the Vegaquemada Formation), which formed of Mesozoic part b. Sediments of Garumnian age, on top the with the of the Cantabrian beginning uplift mountain limestones, could have supplied the sands. chain. could be traced far east location of They as as a S с The sands were derived from areas not covered by the Villaverde-Tarilonte railway station (Mabesoone, Mesozoic limestones. 1959). The Grès de las Bodas complex consists of fining- upward grading fluvial cycles with a close resemblance to In this connection we should like to add three other those of the Voznuevo Formation. A distinct feature, possibilities: the of caliches however, is presence (Kuyp, 1969) or limnic limestone of d. The sands of layers (de Jong, 1971) at the top were supplied from the western part these to de 'The fact had been sequences. According Jong (p. 409) the area, where the Mesozoic limestone cover that so many elements known from the Cretaceous eroded. This assumption is based on Kuyp's statement 351 In later when the of the that a direction of transport from west to east could be a chapter, composition light observed in the Vegaquemada Formation. minerals and the clays will have been described, we shall how far these conditions have also affected the e. The sands are the erosion product of a part of the see Grès de las Bodas further to the west, which, as we sediments of the Voznuevo Formation. know, strongly resemble the sedimentsof the Voznuevo Formation. f. The mountain uplift, active since the beginning of THE LIGHT MINERALS the sedimentation of the Vegaquemada Formation and Introduction even more active at the time of deposition of the clastic of Forma- wedges of the Candanedo Formation,has dragged the The light-mineral assemblage the Voznuevo tion - is to be after of the Cretaceous sediments into an inclined position so that as expected study heavy- mineral of the formation — is the Voznuevo sediments could be exposed to the surface content mainly composed of minerals and rock resistant to chemical in a narrow strip, which is comparable to the present fragments alteration. situation, although with less steep dips. Consequently only quartz, quartzite, muscovite and feldspars were observed in the light frac- tion. The reader must have noticed that some heavy-mineral A number of thin sections, from uncon- groups, especially hornblende and augite, have not been prepared solidated mounted in showed that encountered in the Voznuevo Formation, while a samples Araldit, is the followed mineral such as epidote, frequently observed in the quartz predominant component, by Quaternary deposits of the southern border, is extremely quartzite, and that muscovite occurs in varying (0—15%). Since the muscovite rare. The absence or scarcity of these groups can be quantities quartz(ite) : size it will be explained by considering the stability scale for heavy ratio is highly dependent upon grain not and light minerals with regard to chemical and considered in detail, especially in the knowledge that of the micas dissolved mechanical attack as listed by Leguey and Rodriguez some were possibly during treatment (Mabesoone, 1959). (1970b, p. 176), composed after data by many previous preliminary authors this this order of For this have concentrated the on subject. From stability it reason we upon becomes clear that heavy minerals such as epidote and quartz:feldspar ratio with a supplementary distinction of augite may be compared, as regards their resistance to between alkali feldspars and plagioclases. For the sake fraction between 50 and 500 mechanical decomposition, with, for instance, garnet distinction the microns was of and sodium cobaltinitrite and staurolite and that they are much more resistant in stained by means hemateine of carried this respect than kyanite or andalusite. The values (Doeglas et al., 1965). Staining was out on two — of each a blue colouration for all their resistance to chemical attack as well as the value parts sample, yielding colouration for alkali — in one a yellow for hornblende are, however, many times lower than feldspars part, the other The is in- those of the metamorphic heavy minerals observed in feldspars only in part. quartz not the Voznuevo Formation. fluenced by colouring. The samples were studied under the binocular means of reflected The From this fact one must conclude that minerals such by light. quartz: and from the feldspar and the quartz:alkali feldspar ratios were as hornblende, augite epidote, reported of line for each sediments overlying the Voznuevo Formation determined by means counting; determination number of 200 was counted. A (Mabesoone, 1959; Nossin, 1959; Kuyp, 1969), are a grains total of 41 samples was studied, distributed over the absent in the Cretaceous sands as a consequence of entire formation. strong chemical weathering in these deposits. In this respect the sediments of the present study can The results be compared with the white Miocene sands of southern The results can be summarized as follows: Limburg (The Netherlands) (de Jong and van der Waals, Out of 41 1971) in which the following percentages of heavy samples: Nine contained no at all; in the other minerals were determined: tourmaline 20—55%, zircon+ samples feldspar the varies between 1 and rutile+anatase 0—32%, metamorphic minerals 2—64%. samples feldspar percentage The authors remark that 'less stable minerals(apparently 30%. samples showed equal of blue- and chemically less stable minerals, remark by the present Twenty percentages or a difference between the author) such as epidote, hornblende and garnet, yellow-stained grains per- centages not exceeding 2%. occasionally occur in very subordinate amounts but are showed of generally absent'. The heavy-mineral assemblage in Eight samples a higher percentage yellow stable minerals of the than of blue This indicates that the which only occur is one reasons grains grains. yellow is sensitive and why the sediments are thought to have occurred along staining more apparently more accurate. four had of the surface of a former peneplain. Strong leaching and Only samples a distinctly higher percentage climate under blue than This that chemical weathering in a subtropical grains (difference more 2%). means conditions in which sedimentation kept abreast of only four samples have a certain plagioclase content. and the weathering are supposed to have prevailed in these Comparing the quartz grains feldspars it is of much Miocene deposits. evident that the feldspars are, in general, 352 smaller than the and dimensions quartz. Many cracks in fissures the in quartz grains display a feldspar colouring. This is presumably caused by very fine-grained feldspar content accumulated 'dust' which in these locations. observed show that The percentages only 3 samples have total of 3 feldspar a feldspar content more than 20%; samples of between 10and 20%; 6 samples of between 5 of less alkali and 10% and 29 samples than 5%. It is clear that the feldspar content of the Voznuevo Formation is the rather low, generally lower than 5%. By far the majority of the feldspars are alkali feldspars, which are known to with be than more resistant plagioclases. The values have been plotted on the simplified correla- content tion chart (Fig. 22), the first figure referring to the total of the value brackets percentage feldspars, in to the per- centage of alkali feldspars. Exceptionally high percen- feldspar tages were found locally near the contact with the Mesozoic limestones, where they were probably alkaline milieu. The total protected by a relatively high per- south of I centage section (encircled in 22) was the Fig. presumably caused by marine influences (occurrence of to glauconite! ) which were observed in this location (cf. this draw line refer Chapter VII). Neglecting value, one may a 22 which which the in Fig. (line a) separates an area in does exceed 10% from Figures feldspar percentage not (left) an in which the to be area feldspar percentage appears in Line higher many samples. a runs approximately parallel to line U=I (the time line). Another striking determined. feature is the area of values less than 1% (bounded by line ¡5) which is limited to the lowest part of the forma- was tion as far as section E (coming from the east), but covers the entire formation further to the west. As the of attentive reader will remember, the area west section content E is the part of the Voznuevo Formation not covered by Mesozoic limestones! feldspar Comparative and concluding remarks the Comparing the results obtained from the present in- vestigation with those found by Mabesoone (1959, which 1962) in Tertiary and Quaternary deposits, we find that the feldspar content of the Voznuevo Formation is in higher than that of the younger deposits (between 2 and samples 4% in Tertiary deposits; generally absent in Quaternary difference deposits). The main reason for this is the presumed to be the source area. According to Mabe- the Cantabrian Mountains, the source area of the of soone, sediments studied by him, did not provide much feld- A position spar. second reason may be the different methods used for the determinationof feldspars (Mabesoone did refractive index not use a staining method but the with percentages). method). drawn from 21, Further-reaching conclusions may be Beuther's Sierra de los in (1966) observations in the Fig. Cameros (cf. Chapter IV). He reported a feldspar con- See (both tent generally exceeding 10% in his quartz sandstones arkosic lenses and a percentage of 25% of feldspar in the 22. brackets within the Utrillas Beds. Since these values coincide with Voznuevo Fig. the maximum percentages observed in the of Formation it may be suggested that the loss feldspar 353 of both alkali and in the Voznuevo Formation occurred in situ, the feld- Traces feldspars, feldspars plagio- observed 10 and spars being preserved in locations where they were clases, were in samples. They are rare protected against disappearance by alkaline conditions, limited to the extreme upper part of the formation in and being transformed into kaolinite in locations where the area W of Guardo; E of Guardo they are more abundant and the entire formation. no limestone cover was present (west of section E). spread over low In of nacrite be The feldspar content in the extreme lower part of two samples traces are thought to the formation is difficult to explain; alteration and/or present. dissolution the' of in lower part the complex, near the contact with of 32 of the fraction between 2 and 37 the Paleozoic rocks, may have played a Study samples part. microns, in which the iron (hydr)oxides were not all As foundby Beuther, part of the sediments in Wealden removed, showed that quartz is predominant in kaolinite facies in the Sierra de los Cameros have been eroded. samples, while muscovite and are ubiquitous. was found, 8 Although it is not very likely that the Voznuevo In 15 samples iron (hydr)oxide in samples sediments form of 7 the form of are an erosion product of the Lower Creta- in the goethite, in samples in hematite. hematite three ceous sediments of the Sierra (paleocurrent directions Of the samples containing only situated within the Voznuevo Formation. One suggest a source area further to the west), it is quite are of Paleozoic whereas the other three possible that the present sediments are a second cycle sample is origin, product, derived from an area which received its material samples were collected at the contact between the from Voznuevo Formation and the Formation a granitic mass in Lower Cretaceous times and was Vegaquemada of SE of Carrocera of which subsequently eroded to supply the area S the (Chapter III), one samples Cantabrian Mountains. Such an area could have been must be included in the Vegaquemada Formation. As we of material situated, for instance, N of Zamora, near the Portuguese have also seen from the composition the is border. The presence of detrital kaolinite grains corro- smaller than 2 microns, goethite much more abundant borates such a second cycle origin. than hematite. Of 5 samples containing alkali feldspars, only three belong to the Voznuevo Formation; they were all THE CLAY MINERALS collected in the very upper part of the formation. The clay minerals of the Voznuevo Formation Clay minerals in comparable sediments Samples of clay-sized materials (smaller than 2 microns) An investigation of the clay-mineral content of deposits similar of carried were studied by means of X-ray diffractograms. A total to those the Voznuevo Formation was of 80 samples, covering the entire Voznuevo Formation, out by López Aguado et al. (1971). It refers to a study of minerals from two of kaolinitic sands and of 9 Paleozoic samples near the contact with the clay exposures and of Cretaceous Wealdense Voznuevo Formation are taken to be representative of clays age ('facies y the finest-grained sediments. From this investigation it Utrillas'), underlying the marine Aptain in the province became clear that illite be of Valencia. A number of of this kaolinite, and quartz are to interesting points found in most of the samples, while mixed-layer clays investigation must be borne in mind. Interpretation of be observed 70% of of led the can in the samples. The clays were X-ray diffractograms eight samples to con- clusion of all distinguished according to their colour in the field; they that the most important constituent nearly comprise black, green and red or purple coloured clays. samples is quartz (unfortunately the authors do not give the fractions studied but The most conspicuous results are: they remark that a high per- centage of quartz is limited to the coarser-grained The second in fol- a. Kaolinite is predominant in by far the majority of samples). importance is kaolinite, the samples. lowed by illite. Mixed-layers are of minor importance b. than Quartz is never predominant but nearly always (0—10%). Only in one exposure were feldspars (less present. 10%) observed, both alkali feldspars and plagioclases. Illite dominant The decreases with of с. is hardly ever in Cretaceous samples, feldspar content decreasing age but always predominant in Paleozoic samples. the sediments. The kaolinite/illite ratio was found to be d. Black coloured clays show a higher illite content higher in the sandy samples than in the argillaceous than green and red coloured clays, and a lowerkaolinite samples. content. According to López Aguado et al., the sediments in Wealden facies in this derived from lateritic e. The kaolinite/illite ratio is higher in red clays than in area were a soil which eroded in its in the green clays. was entirety source area. The requirements for the formation of such a lateritic humid Although iron was for the most part removed by labora- soil are a warm and climate, or a warm climate in this there still of with and humid tory treatment fraction, were traces dry alternations, pluviosity being, in than 1500 Under iron (hydr)oxide in 14 samples. Except for one sample both cases, more mm/year. such con- ditions is absent in which hematite was observed, goethite proved to be organic matter or scarce (Loughnan, mineral The sediments in the Valencian the iron present. 1969). area are thought 354 to have formed from a kaolinitic laterite without iron 1962) showed that illite is dominant in all samples. Siliceous rocks and Kaolinite crusts. parent (such as granites occurs in varying quantities, from abundant to with sufficient and of medium Mabesoone's observation that gneisses) leaching acidity traces. kaolinite is more will favour kaolinization and form sediments with low abundant the of the a in western part area studied by him, iron content. Basic parent rocks will lead to the forma- can be related to the fact that this clay mineral could be tion of soils rich in iron. These generally accepted facts more easily derived from Mesozoic sediments such as the to those that in this the Voznuevo Formation in that of the than the suggest, according authors, area part area in sands derived from acid rocks. kaolinitic were López eastern part. al. that the decrease in the Aguado et presume feldspar Mabesoone (1962) came to the conclusion that the red simultaneous kaolinite content with a increase in the pigment of the red beds was due to red weathering in the with caused intensified content decreasing age is by an source area (the Cantabrian Mountains). This area was of the with alteration maturity source area together an believed to have been covered by thick red soils, in- of kaolinite. The of this feldspars into probability dicative of a warm climate with high precipitation. and the to how far the explanation question as Although latentes may form under these conditions, found these sediments be circumstances in can applied they apparently did not form in the Cantabrian area. Voznuevo will be closer to our deposits subjected to The area in which the sediments were deposited is X. had drier climate of examination in Chapter supposed to have a (occurrence gypsum layers! ), favouring the preservation of the red Clay minerals in the formations overlying the Voznuevo colour. Formation kaolinite It was Mabesoone (1959; 1962) who reported on the Reflections on the presence of and on paleo- clay-mineral associations in Tertiary and Quaternary climate climates with much sediments along the southern border of the Cantabrian It is generally accepted that warm Mountains. rainfall, moderate to strong leaching and low pH values Mabesoone's lead the formation of kaolinite the In Cuevas facies illite proved to be the to as most important mineral while kaolinite mineral. Eh conditions be predominant clay (approx. 55%), clay can oxidizing or reducing be to 35%. Measurements of the the of abundance of may present up pH (e. g. in case organic matter). value showed that the environment is alkaline. The Kaolinite is formed, since loss of soluble constituents of kaolinite such indicates presence in quantities a warm (alkalies, alkaline earths, some silica) leads to enrichment and rather humid climate. Since Mabesoone, from in alumina. Ferric oxide minerals (hematite and assumed and pebble analyses, a warm rather climate, become dominant when the rate of dry goethite) may the kaolinite have the Cuevas is thought to originated in destruction of organic matter is high. later alterations have been sediments during or to High permeability of loosely packed sands favours in- inherited 'from the source, that is weathering products filtration of rain-water and the mobility of ground of Mesozoic marine and brackish water limestoneswhich water; the texture of the sands of the Voznuevo Forma- in the where humid climate occur mountains, a more tion probably facilitated leaching. could have In this it should be prevailed'. connection On trying to place the Voznuevo sediments in relation remarked that the deposits of the Voznuevo Formation, of to one the great soil groups, most agreement appears rich in could also have of the so kaolinite, supplied part to be found with red-yellow podzolics (ultisols), typical now in the Cuevas facies. of clays present humid subtropical climates (such as a Ca- and, to a In the de Riacos facies associa- lesser Aw-climate Vega two clay-mineral extent, an according to the Koppen tions can be one found, with an equal proportion of Trewartha classification). These soils are formed in kaolinite and illite, one with a clearly higher illite per- warmer and wetter climates than other podzolic soils; (60 - A red in extend into such centage 70%). strong weathering a warm they may tropical areas. Although a and humid climate assumed climate also was again in the source area. may have prevailed at the time when the The de Vega Riacos sediments were cemented with Tertiary sediments formed in the Cantabrian area - calcite at later The local of laterization taken Cretaceous a stage. very high percentage having place in neither nor kaolinite — appears to be limited to locations where pH Tertiary sediments the striking difference in colour values - are low (5.0 5.5) as a result of the presence of between these deposits must be sought either in lignitic Here, a colour has in layers. change in taken place different source areas, or in differences the de- from red into white and grey. positional environment, or in both. As observed by The other facies Mabesoone Mabesoone Tertiary distinguished by (1959), a local occurrence of lignitic layers contain 75 - 90% of with the of the illite, exception caused a conspicuous decrease in the pH value together Zorita facies in which 30% of kaolinite be from up to may with a change in the colour of the sediments red to present. white and grey. In this connection it is worth noting that The Quaternary sediments are, in general, a reflection the Voznuevo Formation is relatively rich in organic of the Tertiary sediments as regards their clay-mineral matter, while it is almost absent in the overlying forma- associations. tions. According to Loughnan (1969), kaolinite with A later of the red beds hematite study Tertiary (Mabesoone, some illite, some and some organic matter can 355 be expected in temperate to warm climates, already with sediments, which is believed to have occurred in the moderate leaching and rainfall between 625 and 1300 source area (the Cantabrian Mountains). leached mm/year under acid pH and oxidizing to reducing Eh The parent material of strongly weatheredand of difficult to The conditions. On the other hand, hematite, goethite, sediments is, course, recover. com- of gibbsite and boehmite with some kaolinite, without position the heavy and light minerals, however, of rocks with organic matter are supposed to form in climates with suggests a granitic origin. Weathering a often leads sediments like those extensive leaching and rainfall over 1300 mm/year under granitic composition to On acid pH and exclusively oxidizing conditions. Of course encountered in the Voznuevo Formation. weathering of Oilier for remarked that: many transitional environments may be encountered. granites (1969), example, is common extreme leads The exceptional composition of the Voznuevo sediments 'deep weathering ... weathering of almost kaolin can be explained by the juxtaposition of oxidizing con- to china clay deposits consisting pure natural and and have in the ditions (point bars, levees) reducing con- quartz grains'. As we seen Chapter IV, ditions(backswamps). derivation from granitic rocks was assumed as early as From the predominance of authigenic kaolinite over 1850, when Casiano de Prado referred to the Voznuevo detrital kaolinite (Chapter VII) one may assume that sediments as 'destroyed granite'. Translation of the word kaolinization of the Voznuevo sediments occurred 'destroyed' into 'strongly chemically weathered' in the of sedimentation. This is in of involved. particularly area con- apparently gives a better picture the process trast to the formation of clay minerals in the Tertiary CHAPTER IX REMARKS ON INORGANIC AND ORGANIC COMPONENTS OF THE VOZNUEVO FORMATION IRON (HYDR)OXIDES AND PYRITE the juxtaposition of different Eh milieus in the sediments studied is typical of river flood plains. Iron ferrous may occur in sediments in a compound or The superficial discolouration in sediments of silt- and in the form of ferric Eh oxides, depending upon con- clay-size, giving rise to red and purple coloured mottles, ditions after The from during or deposition. change which was supposed to be the result of differences in divalent into trivalent iron occurs under the influence of iron led content, to a quantitative chemical analysis of 0 Reduction from ferric to ferrous com- the iron content of of different colours. atmospheric 2 - samples The takes result of bacterial which pounds place as a activity grey coloured samples were found to contain less than makes use of organic carbon. This reduction also causes 1% of ferric oxides, the lilac coloured samples less than of the transformation sulphates to sulphides. 2% and the, rarely found, purple to red coloured samples The formation of ferric of oxides, especially hematite, approximately 16 %. In none of the samples was ferrous requires oxidizing conditions with positive Eh values. oxide encountered. These conditions can only prevail if matter is The organic question may arise why there is such a striking absent or scarce, as for instance in semi-arid regions. difference in colour between the sediments of the aerobic bacterial Another possibility is that activity in Voznuevo Formation and those of the Vegaquemada the soil that matter is so rapid organic is destroyed Formation and the Red Beds. The content of ferric before which take in burial; a process can place warm oxides and hydroxides, responsible for this difference, is subtropical regions. In this case no reduction of the iron apparently lower in the Voznuevo sediments, although takes small place. changes in the percentage of iron are generally Since evidence of semi-arid climate there is no a during required to bring about considerable optical changes. of the Voznuevo sediments, concluded, for deposition as The lower content of ferric (hydr)-oxides is certainly not from the abundance of kaolinite, the second the result of instance, more acid conditions which prevailed much there possibility seems more applicable, although during deposition of the Voznuevo Formation, since is still considerable of remains. a amount vegetation ferric oxides do not become unstable before pH values These remnants of an extensive found in black flora, drop below 2 (Berner, 1970). The Eh m i 1 i e u ¡s more leaves and wood remains of small clay layers as important. It must have been highly oxidizing and free the sands wood dimensions, in as larger fragments, of organic matter during the formation of the red beds the occurrence of Eh conditions which were in suggest and less oxidizing, on an average, during the deposition contrast to the formation of hematite. those favouring of the Voznuevo Formation. Another possibility could the fields for hematite and be Although stability pyrite a stronger leaching of the Voznuevo sediments, in cannot precisely be delimited(well-crystallized hematite which iron removed in solution. process was This may under conditions in which is have been may persist pyrite stable), caused by a wetter climate during Cretaceous 356 Another the content is different times. reason why iron was already mentioned that the formation of pure be the As Mabesoone may source area. (1962) sulphur is limited to the very surface of the black clay demonstrated, the Red Beds are 'primary detrital' in the deposits. A few chemical reactions possibly involved in sense of (1949); were derived from red the formationof Krynine they pure sulphur are (Reeves, 1968): soils which originated in the source area (i.e. the Cantabrian Mountains). The sediments of the Voznuevo The oxidation of H S: 7 3 + + Formation have a different source area and could have 3H S + 2Fe 6H + 2FeS + S\ 2 been derived from soils in which yellow colours prevailed over red colours. The oxidation of FeS: 4FeS + 30 2Fe 0 + 4SV. 2 2 3 Many black clay horizons proved to be rich in pyrite; encountered than 1 concretions were more cm in The oxidation of thiosulphate: diameter. A number of wood fragments were also found 5Na S 0 + H 0 + 40 5Na S0 + H S0 + 4S 2 2 3 2 2 2 4 2 4 to be partially pyritized. The reduction of ferric oxides H S (formed by of matter by by 2 decomposition organic ORGANIC REMAINS reduction of sulphate) into ferrous ions which react with to form iron sulphide, is believed to hydrogen sulphide The only organic remains occurring in the Voznuevo take in two steps: formation of black iron unidentifiable place Formation are plant remains, varying monosulphides (FeS) with subsequent oxidation from trunks of several cubic decimetres to paper-thin (brought about by elemental sulphur as oxidizing agent) stems and leaves. By far the majority of the larger wood to pyrite. are allochthonous, found fragments they can, be as isolated pieces, coated with sand and gravel. The sand pit GYPSUM of south Grandoso (cf. Chapter V, part B) frequently contains such drifted wood fragments, incorporated in confronted A problem with which we are is the presence cross-stratified units. several of euhedral gypsum crystals, up to centrimetres All plant remains encountered in the Voznuevo in in the black Valmartino. It is length, clays near hardly Formation show a low degree of coalification; they must scattered likely that the crystals throughout the be the brown coals. The term gypsum ranged among 'lignite' may black clay are the product of the more so as evaporation, cause confusion, since it is synonymous with 'brown this climatic semi-arid requires abrupt changes (i. e. a or coal' in Britain, while '... in Germany and other parts of arid the wet climate which climate) in generally is the term brown coal restricted to Europe is mega- to have supposed prevailed. scopically compact structural varieties, while lignite is the formation of must have been the result of gypsum restricted to individual pieces of wood enclosed in the oxidation of the mineral so coal' pyrite, frequently brown (Tomkeieff, 1954). According to a North observed in the black Oxidation of leads to clays. pyrite American classification, brown coal is an unconsolidated the formation of sulphuric acid. The latter will react deposit, whereas lignite is consolidated (Williamson, with carbonate according to the following reaction: 1967). According to Tomkeieff 'one may subdivide brown » H S0 + CaC0 + 2H 0 CaS0 .2H 0 + H C0 coal inlo brown coal made of 2 4 3 2 4 2 2 3 (sensu Anglice) low-grade visible brown (Reeves, 1968) vegetable remains, and high-grade coal, a rock'. The compact, homogeneous and tough more It is highly probable that the supply of waters rich in recent classification by Williamson gives a subdivision carbonate took at late in the sediment's place a stage into brown lignites and black lignites (sub-bituminous of history in this location and that the formation coals in North America) which form the transition into gypsum in the black clays is only a superficial feature, bituminous coal. which assumed of its was already on account occurrence The brown coal in the Voznuevo Formation occurs in in cracks and fissures. This assumption is corroborated the form of: of at few by the presence Paleozoic limestones only a metres from the black Run-off of waters clay exposure. 1. Felt-like tissues of leaves and small stems; macro- rich lime have caused chemical in can easily a reaction scopically closely resembling paper coal (cf. Williamson, with the sulphuric acid in the black clays and have led to 1967; Plate 15b). the formation of gypsum. 2. Isolated small wood fragments with external charac- A similar process may have produced the gypsum teristics of recent wood. (1.94%) in the black clay horizon N of Las Heras 3. Black coloured, hard, jet-like fragments - clean to - (Section Q). the touch with a conchoidal fracture. 4. Fusain, macroscopically resembling charcoal, dirty to SULPHUR the touch and highly friable. In one thin section con- taining these charcoal fragments, the cellular structure of In the description of the thin sections (Chapter VII) it wood fibre is evident (fusite) (Fig. 74). 357 of remains shows coalification but coal The presence of these types organic to a stage, represents a constituent that coalification has not been very intensive in the which is not necessarily limited to bituminous hard coal, sediments of the Voznuevo Formation. This be but also be of brown coal may can a constituent (Kreulen, ascribed to low temperatures in the sediments and to a 1948). The occurrence of mineral charcoal is still relatively small amount of overburden; two facts somewhat obscure; forest fires are held responsible for previously concluded from the abundance of kaolinite its formation. (Chapter VII). Compared with the overlying Tertiary deposits, the felt-like form the brown The plant remains probably Voznuevo Formation is rich in vegetable matter, being a coal of and, with the wood of variety paper peat together remnant local luxuriant vegetation. One of the factors to brown is fragments mentioned in 2, belong the lignites. by which this difference controlled may be a wetter The jet-like material must be ranged among the black climate during Cretaceous times than during succeeding lignites. The charcoal mentioned in 4 cannot be related times. CHAPTER X DISCUSSION AND CONCLUSIONS the the of staurolite. The of the minerals is Surveying principal points yielded by present composition light investigation, we must first of all stress that all data, even simpler; they consist almost exclusively of quartz gathered both in the field and in laboratory-processed and muscovite. Feldspar may be present in some loca- its samples, indicate deposition of the Voznuevo sediments tions but proportion decreases with decreasing age. in an almost purely fluvial environment. This eliminates The clay-mineral assemblage makes it clear that the other environments which have been proposed in the predominant clay mineral is kaolinite, while illite is reddish of course of time, such as eolian, deltaic or limnic environ- generally of minor importance. The colour ments. the sediment in some locations must be ascribed to the of formation of hematite be The sediment shows a large spectrum grain sizes, goethite; proved to very rare. but of lens-like components exceeding about 6 cm in diameter Black clay layers, rich in organic remains, (pebble size) are very rare. The megascopical composi- shape, are fairly common. They contain many, some- wood and other tion is rather monotonous, showing a very impressive times pyritized, fragments organic of which do exceed the brown coal in predominance quartz and muscovite in the smaller remains not stage sizes. The absence of limestone coalification. In few locations oxidation of in grain components is one pyrite, of the most conspicuous aspects. Quartzite proved to be combination with superficial waters rich in lime, has led of of local importance only, while the content of black to the formation of gypsum crystals on the surface efflorescence of radiolarites (+ phtanites and quartz phyllites), probably the black clays. Oxidation also caused derived from the Precambrian strata which outcrop in sulphur. The fluvial of the sediments studied based the extreme W of the area studied, remains fairly con- origin was stant (generally 8—12%). The microscopical composi- on observations in the field in which many fluvial characteristics became evident. Two fluvial sedimenta- tion, too, shows a striking monotony, which is a reflec- could be tion of the megascopical composition. Moreover, the tion patterns distinguished, represented by braided-river sediments and sedi- detrital parts of the thin sections contain a small per- by meandering-river The of centage of detrital kaolinite grains and of detrital ments. importance the braided pattern clearly muscovite flakes. increases towards the west. Paleocurrent directions showed that of the sediments The Voznuevo sediments are cemented by crypto- incontrovertibly transport in this took from directions between S and W. crystalline quartz and kaolinite in the first generation area place and by iron (hydr)oxides in the second generation. Over- A clearly fluvial origin of the sediments may also be of but not deduced from flatness and roundness indices of the growth quartz on grains may occur, is very common. This feature is in clear contrast to overgrowth pebbles and from grain-size distribution features such as as observed in samples of Paleozoic origin. In the poor sorting and positive skewness. of latter the of the sediments still samples containing feldspars, alterations the As regards source area we are could faced with number of of into kaolinite frequently be observed. a problems. Some investigators An extensive study of the heavy-mineral composition lithologically similar deposits proposed that such resulted the observation that enrichment in sediments could have been derived from in an occurs granitic masses the composition of the heavy-mineral assemblage with which occur in a broad strip from Galicia to central decreasing age, as a result of the introduction of meta- Spain (cf. Fig. 1). One of these granitic masses, partly stable minerals morphic heavy minerals; yet only heavy hidden below Tertiary, is supposed to have supplied the those chemical such Voznuevo sediments well. The best occur, susceptible to weathering as as described and of the hornblende, epidote and augite being completely absent. most similar deposits are those Sierra de los sands believed have The Voznuevo contain a generally high percentage Cameros, which are to originated in the 358 northern of the Sierra de N of Madrid. of Cretaceous The of the part Guadarrama, Upper age. larger part Only feldspars are clearly more abundant and better Voznuevo deposits occupies a rather exceptional preserved in the sediments of the Sierra de los Cameros position in comparison with other locations in which of than in those the Voznuevo Formation. This might supra-Urgonian deposits occur, with the exception of the indicate a longer distance of transport of the latter. One transgressive continental sediments of Upper Cretaceous could that the sediments in the Cantabrian assume area age of the Massif of Oroz-Betelu NE of Pamplona. In for second been eroded still were deposited a time, having these two areas continental deposition apparently sediments which situated from were nearer to the continued, whereas other parts in which continental The original source area. relatively simple composition, sedimentation had taken place were already reached by of stable and the relative consisting constituents, scarcity the Cenomanian transgression. of is in favour of such a which dia- feldspar sedimentary history. The Voznuevo Formation, is a clearly al. the decrease According to López Aguado et (1971) chronic deposit along the southern border of the with found in the in feldspar decreasing age, as Cantabrian Mountains, must be the continental con- sediments in Wealden facies studied indicates by them, tinuation of the transgressive shallow marine deposits increased of the Two remarks an maturity source area. belonging to Upper Cretaceous stages. These deposits must be made, however, with respect to their reasoning: wedge out towards the west, where the Voznuevo For- mation can be connected with successively younger 1. Continued erosion will rather into cut increasingly stages. The transgression which began in the will lead fresh rocks, which to an increase in feldspar Cenomanian, slowly crept up along the southern border with decreasing age. of the Asturian Massif from an easterly direction, 2. Transformation of into kaolinite will take feldspar pushing ahead a strip of continental deposits. These in the levels of place particularly younger a porous continental deposits, as mentioned above, were supplied such sands Wealden complex as in facies; this trans- from southwesterly directions since they show paleo- formation therefore after from and S. taking place deposition. current directions with a supply between W the Voznuevo sediments Although are monotonous at which the In the Voznuevo Formation in feldspar first sight, there proved to exist many significant differ- also shows decrease with the content a decreasing age, ences in the deposits from E to W and from bottom to abundance of kaolinite the from these differ- authigenic suggests presence top (approximately N to S). Among of feldspar during sedimentation. We also see that the ences we shall mention: is in percentage of feldspar much higher the upper part of the where the of limestone made A decrease size from bottom formation, presence a. in grain to top, of the extreme preservation the feldspars possible. In observable in almost every section. west of the area, where the Mesozoic limestone cover is b. An increasing thickness towards the west of the lower encountered. this absent, no feldspar was For reason we part of the formation consisting of braided-river that transformation of and other assume feldspars deposits. In the eastern part of the area braided-river occurred after of the weathering phenomena deposition deposits are altogether absent. Voznuevo sediments and that formation's total they were not previously с A slight increase in the thickness subjected to intensive weathering. from 250 m in the east to more than 350 m in the The of stratigraphical position the Voznuevo Forma- extreme west. tion of the the posed a problem consisting two parts. First, d. A greater divergence of paleocurrent directions in of the with other of formation result of of the position deposits, as compared top the as a meandering sediments Wealden facies of in reported from other parts river(s). had be faced Spain, to determined; secondly we were e. A decrease in flatness and an increase in roundness of with the problem, whether the Voznuevo Formation is pebbles towards the east. of the independent overlying transgressive deposits, or f. A better sorting of the sands towards the east. had to be correlated laterally with these deposits. The introduction of g. new heavy-mineral assemblages Comparing the stratigraphical position (based on parallel to the transition from the Lower Cretaceous to of the Voznuevo sediments with the stable palynological studies) Upper Cretaceous, causing an extremely identical in and deposits, fades, in northern, eastern association in the oldest part of the formation in the east central it became clear that the sediments studied Spain, to a more diversified assemblage in the west. must be in the Lower Cretaceous in the ranged upper or h. The slight decrease in the feldspar content towards lower Upper Cretaceous. They must be included in the the west, supposed to have been caused by alteration supra-Urgonian complex and may be connected with into kaolinite after deposition. sediments commonly referred to as Utrillas Beds. to the Palynological analyses brought light that The contacts between the Paleozoic and the Mesozoic Voznuevo Formation in direc- grows younger a westerly proved to be unconformable in most locations. They the distribution of determinations showed that of the tion; age could not be observed in the central part area pollen samples of Lower Cretaceous age are limited to studied where fault movements of considerable the lowermost in the the remainder The in- deposits east, being importance are believed to have taken place. 359 fluence of the Paleozoic rocks the of the on composition from the Cantabrian area, of which the Cretaceous Voznuevo sediments has been however. very small, sediments formed a part. The introduction into the Cantabrian of Relief in the Cantabrian Mountains must have been very area heavy minerals such as staurolite and and of kaolinite the Voznuevo Forma- low, although there are no indications that Cretaceous andalusite, since covered of the tion be with erosion and deposits ever a large part present began to deposited, subsequent Cantabrian area. Relatively little material was supplied redeposition in younger deposits, can readily explain latter to the Voznuevo Formation. Apart from paleocurrent their occurrence in these sediments. directions and this occurred the heavy minerals, must also be con- Although some weathering must have in cluded from the fact limestone deduced from the of detrital that not a single frag- source area (as presence derived believe that has taken ment, from Paleozoic rocks, was ever en- kaolinite), we most weathering countered in the Voznuevo Formation. A relatively high place after deposition. of the lowest of The abundance of kaolinite and the of percentage quartzites in very part the ubiquity plant formation reflects a small influence of short durationof remains and other organic matter, as well as the flatness the Paleozoic. In one exposure it became clear that and roundness indices of the pebbles, are indicative of a tributaries, rich in quartzite, locally introduced materials temperate to warm climate with considerable pluviosity, from the rise Ca- Cantabrian Mountains into the formation. giving to relatively strong leaching (a or possibly Paleocurrent directions in these deposits indicate supply an Aw-climate according to the Köppen-Trewartha - from the N. classification). Both oxidizing and reducing conditions - From the degree of cementation in the thin sections in an acid environment must have prevailed at one and and the the characteristic of fluvial clay-mineral content, Paleozoic samples can same time, a phenomenon be from the first bars and natural easily distinguished Mesozoic samples, the environments, on point levees, between these sediments the latter in The absence of and boundary always being very backswamps. gibbsite distinct. other aluminium oxides shows that laterization did not A clearly gradual transition could be observed in the occur, so that formation of red-yellow podzolic soils of the formation where sediments must be assumed. The rather and humid climate top sandy pass into warm identical sediments with a cement of fits in with the, on an warm climates consisting sparry average, generally calcite. In these parts of the formation preservation of assumed for Cretaceous times (e. g. Schwarzbach, 1961). feldspars was observed. 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