LEIDSE GEOLOGISCHE MEDEDELINGEN, Deel 49, Aflevering 2, pp. 167-205, 24-5-1973
G eology of the area between the Luna and Torio rivers,
southern Cantabrian Mountains, NW Spain
BY
C.J. van Staalduinen
Abstract
Precambrian clastic rocks, deposited under unstable conditions, were folded before in a relatively stable environment shallow marine
the sedimentation spread out over the whole area. Silico-clastic sediments were deposited from Cambrian to Devonian, except from Lower-Middle Cambrian when carbonate deposition dominated.
With a hiatus in sedimentation during the Llanvirn to Llandovery the influence of a rising block, NNE of the present area, started.
During the Silurian this rise resulted in developmentof clastic sequences trending to thin towards the N.
From the Devonian to Upper Carboniferous sedimentation circumstances became less stable. As a result an alternation of clastic and
carbonate rocks developed.
Towards the end of the Devonian epeirogenetic uplift and tilting of the northern part of the area resulted in strong erosion and
sandstone of older consequently the uppermost transgressive Devonian rests on a variety deposits.
The Sabero-Gordón line separates the uplifted area in the north from the area where continued subsidence and sedimentation took
place during the Upper Devonian.
During the Lower Carboniferous differences in sedimentation circumstances were strongly reduced resulting in the deposition of the
Alba Formation all over the area.
Une its During the Namurian the Sabero-Gordón renewed function as a facies boundary between a northern and a southern area.
Together with the development of the progress of the maximal Carboniferous sedimentation towards the north the initial folding of
the Hercynian orogenesis started south of it. After the orogenesis oblong coal basins developed during the Stephanian B along normal
faults approximately parallel to the strike of the folding.
After folding of these coal basins a long period of non-deposition followed which ended in the Upper Cretaceous when sedimentation
took place along the southern border of the folded Palaeozoic. The Tertiary morphogenetic uplift of the Cantabrian Mountains is accompanied by continental deposits forming the border of the Duero basin.
During the Hercynian orogenesis major deformation took place in the Leoides (Fig. 3). The Sabero-Gordón line separates the Leónides in
in with thrust sheets north a strongly folded area the south and an area of it. The shape of folds and thrusts is mainly determined by the
lithologicalproperties of the Palaeozoic rocks. Table 2 shows the rocks units which are supposed to have their own tectonic-style.
In the southern area (Alba synclinorium) minor folding is an important feature. Based on a symmetry-concept most of these folds are
minor in in parasitic folds. In some places folds the folded area as well as the thrust area show that deformation took place by
gravity-stress.
The León line separating the Leonides from the Asturides seems to have no significance as a fundamental structural line in this area.
Resumen
Depósitos clásticos del Precámbrico, depositados en circunstancias inestables, se han plegado antes de que se esparcieran sedimentos
marinos toda la Sedimentos fueron durante el Cámbrico hasta el por región. siliciclásticos depositado Devónico, a excepción del periodo entre el Cámbrico inferior y el Cámbrico medio en el cual dominaba la sedimentación de carbonatos.
el La influencia de un levantamiento empezó con un hiato en la sedimentación durante el Llanvirniense y Llandoveriense y dejando
secuencias de menos espesor hacia el norte durante el Silúrico.
Del Carbonífero sedimentación Devónico hasta el superior las circunstancias de eran menos estables, resultando en una alternación de rocas clásticos y rocas carbonatos. Devónico Al final del el levantamiento epirogénico y la inclinación general de la parte del norte de la región originaron una fuerte
consecuencia erosión y como los areniscos transgresivos del Devónico superior se han depositado sobre varios yacimientos de una edad más antigua.
La falla de la de levantamiento el norte de de subsidencia Sabero-Gordón separa región en una región y sedimentacióncontinua en el las Durante el Carbonífero inferior diferencias en circunstancias de sedimentación desaparecieron y se formó en toda la región la Formación de Alba.
Durante el Namuriense la falla de Sabero-Gordón activó limite distintos facies el se como entre en norte y el sur. Al mismo tiempo en que la sedimentación máxima de depósitos carboníferos progresó hacia el norte, el plegamiento inicial de la
herciniana la el orogénesis empezó al sur. Se desarollaron después de orogénesis, durante Estefaniense B, cuencas oblongas junto a fallas normales aproximadamenteparalelas a la dirección del sistema orogénico.
Despues del plegamiento de estas cuencas terminó un periodo largo sin sedimentación con la sedimentación del Cretácico superior a lo largo de la zona de depósitos plegados del Paleozóico. El levantamiento morfogenético durante el Terciario acompañó la formación de depósitos continentales en la orilla de la Cuenca del Duero. 168
Sabero-Gordón Durante la orogénesis herciniana la mayor deformación tuvo lugar en los Leonides (Fig. 3). La falla de separa dentro de los
Leónides una región de plegamiento fuerte en el sur de una región de corrimientos en el norte. La forma de pliegues y corrimientos se han
determinado las de los Tabla 2 indica las unidades de de cada principálmente por propiedadeslitológicas depósitos paleozóicos. tipos rocas,
una de las cuales tiene su estilo tectónico propio.
En la región de plegamiento (synclinorio de Alba) un segundo plegamiento es de importancia. Los pliegues son parasíticos cuando se
les considera a base de propiedades simétricas perteneciendo a las estructuras primarias. En varias localidades de los Leonides los pliegues
secondarios indican deformaciónes causadas de unas por presión gravitación.
La falla de León, separando los Leonides de los Asturides, no tiene importancia ningunacomo falla fundamental en esta región.
Contents
I Introduction 168 IV Geologic history 200
II Stratigraphy 171 V Conclusions 201
Introduction 171
Lithologic characteristics of formations 173 References 203
III Structural geology 183 Appendices 1 -4 (in back flap)
Introduction 183 Geological map with three cross-sections
Narcea Anticlinorium 184 Columnar sections of the Lower Palaeozoic
Leónides 185 Columnar sections of the Piedrasecha, Nocedo and Ermita
Central Asturian Coal Basin 198 Formations
Stephanian Coal Basins 198 Structural cross-sections
The Mountain Border Area 199
CHAPTER I
INTRODUCTION
General remarks The present map is the result of a geological study of part of the southern slope of the Cantabrian Mountains, carried out as part of a programme of systematic map- ping by the Department of Structural Geology of the University of Leiden. Evers (1967) and van den Bosch (1969) published the maps east and west of the present area. The surrounding areas have also been mapped by others and on various scales, Fig. 1. In 1962 de Sitter published a provisional map (1 : 100,000) of the area between the Pisuerga and Luna rivers. The present area is situated in the western part of that map in the province of León and the province of Oviedo. Its geographical limitations are: Latitudes 42°47'01" and 43°01'38"N, longitudes 2°11'48" and 1°50'00"W of Madrid, which correspond with ca 5° 18' and s°4l'W of Greenwich. out Fieldwork was carried on topographic maps 1 : 25,000 enlarged and simplified from topographic of maps (1 : 50,000) the Instituto Geográfico y Catas- Fig. 1. Map showing existing geological maps in the SW part of tral, Madrid. The present area is covered by the follow- the Cantabrian Mountains on different scales. I. García-Fuente ing sheets: 77 (La Plaza), 78 (La Pola de Lena), 102 (1959); II Marcos (1968); III Llopis Lladó (1955); IV Van den
(Barrios de Luna), 103 (La Pola de Gordon), 128 Bosch (1969); V Pastor Gómez (1963); VI Evers (1967); VII
with this thesis. (Riello)and 129 (La Robla). map presented
Mapping was facilitated by the availability of aereal
photographs. The area is covered by seven runs of photo-
graphs with the following numbers: 45260-45271, interpreted from these aerial photographs. Field map-
52976-52964, 43178-43167, 43330-43338, ping was carried out on a scale of 1 : 25,000 and the
43512-43504, 43722-43713 and 53596-53589. The photo-interpretation was adjusted to this scale with a
northern 'Rocks Sketchmaster'. area of Carboniferous rocks has been mainly 169
Hydrological remarks number of small landslides and V-shaped valleys. The
In the present area the main watershed of the E-W south draining rivers generally have rather U-shaped val-
running Cantabrian Mountains separates the province of leys. from of León the province Oviedo. North of it drainage Drinking-water problems do not exist in this area
is to the Bay of Biscay by the Valgrande river and since the mountain hills supply abundant water for the
further north by the Pajares river. South of the water- villages around.
shed three rivers flow south towards the Meseta, where
feed the Duero From Remarks they into river system. west to east on glacial phenomena the and Torio The drain- North of Celleros they are Luna, Bernesga rivers. a cirque with a moraine bow is shown
run at to the on the age systems approximately right angles geological map. Throughout the northern area
mountain range. They are separated by N-S running small cirques can be found, many of which have been watersheds. secondary deformed by further erosion and scree-building. In the Luna river dam has been built form North of Casares a to a a moraine, mainly composed of reservoir for the water of the Lake Luna (Embalse de Barrios quartzite boulders and alternating cross-bedded be used for the electric Mora de shales Luna) to plant near sandy point to a fluvio-glacial origin. The lowest Luna. In the Páramo the Meseta the of level of these about region on water deposits is 1,350 m above mean this used for sealevel. A small of the Casares reservoir is irrigation purposes. tributary river now has off In the Casares river, an affluent of the Bemesga river, cut its own accumulation terrace. West of Folledo
two artificial lakes are planned, one in the Casares Val- comparable phenomena have been found. A small river the ley, the other near Beberino, flooding the lower course cut through moraine material, mostly consisting of San Pedro boulders. Furthermore accumulation of the Casares river. Both reservoirs are meant to serve as terraces
the river and tributaries reserves for the electric plant which has been built for occur along Bernesga its towards
Rodiezmo. The terraces have thickness here of industrial purposes in the Bemesga river near La Robla. a 3-7 m. River also the southern border of Another plan now investigated is the building of an terraces occur along
artificial lake in the Torio Valley for which the water of the mountains in the Luna, Bernesga and Torio river valleys. and however the upper Bffrnesga river would have to be diverted from Vegetation agricultural cultivation, have often obliterated Villanueva de la Tercia to the Torio Valley. heights and the kinds of terraces Where noticeable thinner Lithology greatly affects the courses of the main actually occurring. they are than rivers. Nevertheless the four principal rivers Luna, Ber- 2 m.
nesga, Torio and Valgrande are consequent rivers on the southern and northern slopes of the Cantabrian Moun- Acknowledgments
The author wishes to his sincere thanks tains. The affluents are mainly subsequent rivers, run- express to: Professor Dr. L. U. de Sitter for his assistance ning generally in an E-W direction parallel to the general during my strike of the folded Palaeozoic. graduate work and at the start of the present study;
Professor Dr. H. J. Zwart for the critical of the Most watersheds are rounded except for the areas reading Dr. J. F. for the discussions where limestonesare the separating rocks. manuscript; Savage inspiring The often and in the field and for his criticism and discussion main riversystems are asymmetric thorough of the Dr. D. Boschma and dendritic, especially in their upper courses. manuscript; Dr. W. J. van den Bosch for fruitful ideas the There is a great difference in length between the giving many during fieldtrips;
Valgrande river and the others. Within 100 km the Prof. Dr. A. Brouwer for his advice in stratigraphic
Valgrande river reaches the Bay of Biscay, whereas the questions and critical reading of the manuscript; all
southbound rivers reach the Atlantic Ocean after more members of the staff of the Geological and Mineralogical
than 500 km. These differences in length have caused Institute for their helpful discussions; Miss Cor Roest for
the to be off the Val- drawing the geological and the cross-sections; Mr. upper Bernesga course cut by map grande riversystem. The main watershed became deform- R. M. Metten and A. Walkeuter for drawing the text- W. ed and now shows an E-W split. Llopis Liado (1954a) figures; Mr. C. Laurijssen for preparing the photo-
already mentioned the capturing of the upper Bernesga, graphs; Mrs. W. H. P. M. van Es-Jacobs for correcting the
Mrs. - that the had course to English text; M. A. Woensdregt Bermúdez-Fernán- supposing original Bernesga its up dez for the the Telledo at a level of 1300—1400 m. This seems to be an correcting Resumen; people of the village of Geras de who showed exaggeration. Gordon, their great hospitality and the fieldwork. The Valgrande precipices are steep and have caused a friendship during 170
Fig. 2. Correlation between chronostratigraphic units and lithostratigraphic units in the Luna-Torío area. 171
CHAPTER II
STRATIGRAPHY
INTRODUCTION Formigoso Formation. Comte (1959) assumed that
during part of the Llanvirnian, the Caradocian, the Ash-
gillian and part of the Llandoverian no sedimentation The rocks exposed in the area of the present map range took in age from Precambrian to Tertiary. These rocks have place.
an between the Famennian been subdivided into formations as listed in Fig. 2. The c) unconformity chiefly Ermita Formation and older Devonian rocks. From succession does not represent a continuous sedimenta- tion. Angular unconformities, disconformities and hia- south to north the eroded part of the older Devonian tuses occur. rocks increase with a maximum in the Bodón unit
The following major breaks in sedimentation have (Fig. 3) where the Ermita Formation covers the La Vid been observed: Formation which is possibly of Siegenian age there.
d) an angular unconformity between the Stephanian a) an angular unconformity between the Mora Forma- rocks of the Prado Formation and older Palaeozoic tionand the Herrería Formation; rocks. b) a hiatus between the Barrios Formation and the e) an unconformity between the Upper Cretaceous Voz-
Luna-Torío Fig. 3. Major structural elements in the area and subdivision of the Leonides into structural units. 172
nuevo Formation and Palaeozoic rocks, giving evidence and (f) are chiefly due to tilting before deposition, of the absence of almost the entire Mesozoic. which also holds for the disconformity of (c). The origin the f) an assumed unconformity between Vegaquemada of the hiatusof (b) is questionable.
Formation (Eocene-Oligocene) and the Upper Creta-
ceous Voznuevo Formation. Previous work
The angular unconformities of (a) and (d) were Many French, Spanish and German geologists studied
caused by previous folding. The unconformities of (e) the southern Cantabrian Mountains before Comte de-
Thickness
Comte's subdivision (1959): in Age Formations
meters
Conglomeráis Riacos
de Castille Miocene Candanedo
Argües - (lacune?) - Vegaquemada
Marnes de Boñar
Crétacé
Graviers et Sables kaoliniques ± 200 Voznuevo - lacune -
Schistes houillers et Gres de Tineo Prado
Stephanien
Schistes houillers de Sabero > 150 - - lacune
Gres et Schistes de Sama
Westphalien
Calcaire et Schistes de Lena >1000 Lena
Calcaire et Schistes de Villanueva San Emiliano
Cuevas
Calcaire des Cañons 200-800 Namurien(?) 'Caliza de Montaña'
Griotte de Puente de Alba 25-40 Viséen Alba
- (lacune?) -
'Couches de Vegamián' ± 15 Tournaisien(?) Vegamián - lacune -
Gres de l'Ermitage 0-1000 Strunien Ermita
- lacune -
Schistes ± de Fueyo 100 Piedrasecha
Famennien
Gres de Nocedo (Calcaires de Valdoré) ± 500 Nocedo
- (lacune?) -
Frasnien
Calcaires de la Portilla 50-80 Portilla
Givetien
Gres et Schistes de Huergas 220-300 Huergas
Eifelien Caldas
Calcaires de Santa Lucia 100-250 Santa Lucía
Emsien
Calcschistes et Calcaires de La Vid 180-500 La Vid
Siegenien
Gedinnien
Gres de San Pedro rouges 70-170 San Pedro
Silurien
Schistes du Formigoso 50-100 Formigoso
- lacune -
Arenig
Quartzites de Barrios 160-480 Barrios
Schistes et Gres d'Oville 120-240 Tremadoc & Potsdamien Oville
- (lacune?) -
Griotte de Lancara 12-25
Acadien
Dolomites de Lancara 45-80 Lancara Géorgien
Gres de La Herreria >1400 Herrería
- (lacune?) - Mora
Précambrien
Table 1. Slightly modified after Evers (1967). 173
scribed this area. Most of them did so in view of the nate facies of the Santa Lucía Formation and concluded
that Caldas should be included the mineral resources and rich fossil localities. Their publica- the Formation in Lucía Formation. tions include: C, de Prado (1850, 1852); de Vernieul Santa
(1850); Barrois (1882); Mallada (1887); Oehlert (1897); Reyers (1972) and Mohanti (1972) made a detailed
Kegel (1929); P. H. Sampelayo (1934); Ciry (1939); study of the Portilla Formation.
Delépine (1943) and Almela (1949). Van Ginkel (1965) redefined the Alba and Vegamián
For the lithostratigraphic subdivision of the Palaeo- Formations and described the San Emiliano Formation. chose He also the Lena Formation. zoic deposits of the present area Comte (1959) a designated
type-section along the Bernesga river between Busdongo Boschma & van Staalduinen (1968) introduced the
and La Robla. Although the chosen rockunits (Table 1) Cuevas Formation and the Prado Formation. Brouwer & Ginkel introduced the coincide to a great extent with formations in the sense van (1964) Escapa of the Caliza Formation of Hedberg (1961, 1972) Comte's nomenclature differs Formation instead de Montaña and other Winkler Prins made detail- from Hedberg's. In the type-section it was Comte who synonyms. (1968) a ed of the Formation and its defined the Palaeozoic formations, but the names of the study Escapa brachiopod
fauna. pre-Devonian formations and those of the Devonian Por- Van Adrichem Boogaert (1967) reviewed the Lower tilla Formation were derived from reference sections. Carboniferous stratigraphy and studied the conodonts of The names of these formations have now been accepted the and Alba Formations. on the basis of priority rules. Later publications either Vegamián Liado described the Lena Formation complete the lithostratigraphic subdivision of this area Llopis (1954b) of the southern border of the 'Cuenca Central' of Astu- or detail the pre-existing units. rias. The Precambrian rocks in this area remained un-
first described The Ciñera-Matallanaand Magdalena Coal Basins have known for a long time. De Sitter (1962) been intensively studied by Wagner (1962-1964). Van an angular unconformity in what had until then been Amerom & described and called the Herrería Formation. He suggested the rocks van Dillewijn (1963) mapped the western part of the Ciñera-Matallana Basin. underlying the unconformity to be of Precambrian age Evers defined the rocks of the and called them Mora slates and greywackes, now for- (1967) post-Palaeozoic
Formation. area. mally named Mora £ adjacent data have been Oele (1964) studied the sedimentology of the Her- Biostratigraphic published by Lotze & of rería, Láncara, Oville and Barrios Formations. Van der Sdzuy (1961), who studied tribobites the Láncara
Meer Mohr (1969) investigated the Láncara Formation Formation and part of the Oville Formation. studied of the on the basis of carbonate sedimentology. Cramer (1964, 1966) micro-organisms
Backer (1959) studied the San Pedro Formation with Formigoso Formation, the San Pedro Formationand the
reference to the Silurian-Devonian boundary in the La Vid Formation. fossils have been studied Krans Asturo-Leonese area. Devonian by (1964)
Westbroek (Rhynchonellida); and Brouwer (1962, 1964, 1967, 1968a and b) paid (spirifers); (1967) Sleumer (stromatoporoids). special attention to the development of Devonian strati- (1969) The Lower Carboniferous biostratigraphy has been graphy in the Leonese facies area. Van Adrichem
in dealt with by Kullmann, 1961, 1963; Wagner-Gentis, Boogaert (1967) reviewed the Devonian stratigraphy 1963 Adrichem general and the Upper Devonian and Lower Carboni- 1963; Higgens et al, (cephalopods); van
et 1963; & 1964; van ferous in particular. Boogaert al., Budinger Kullmann, Adrichem 1967 Graduate students from the Department of Structural Boogaert, (conodonts). To Carboniferous contributed Geology of the University of Leiden produced a number Upper stratigraphy fusulinids bivalves of sheets on a scale of 1 to studies on (van Ginkel, 1965), (Wink- map : 50,000 contributing ler and land the regional stratigraphy of the southern Cantabrian Prins, 1968), algae (Rácz, 1964) plants Gomez de Mountains. These include Kanis (1956), Nederlof (Jongmans, 1951; Wagner, 1962-1964; 1950 and Stockmans & (1959), Koopmans (1962), de Sitter & Boschma (1966), Llarena, Willière, 1965).
Frets (1965) and Boschma (1968) in the Palentian area.
In the Leonese area map sheets have been published by LITHOLOGIC CHARACTERISTICS OF Rupke (1965), Helmig (1965), Sjerp (1967), van Veen den FORMATIONS (1965), Evers (1967), Savage (1967), de Sitter & van Bosch Bosch (1968) and van den (1969). Mora Formation. - The Mora Formation contains main- The official Spanish edition of the geological memoir Pastor Gómez ly greenish slates, greywackes, quartzites and shales. The and map sheet La Robla as prepared by slates have been observed south of de (1963) also deals with the Palaeozoic stratigraphy of the green only Vega los Caballeros, with laminar stratification and small-scale present area. into Smits (1965) described an aberrant development of cross-bedding. Sandy layers generally grade upwards
which are foliated with a Towards Couvinian deposits in the Caldas de Luna area (see map) pelites slaty cleavage. and named this the Caldas Formation. De Coo the north and probably upwards in the sequence the sequence increases and and et al. (1971) studied the palaeogeography and the carbo- grain size greywackes some quartzites 174
have been deposited. In the coarse part of the formation limestone member in the Rozo unit is partly of second-
is absent, but a fracture slaty cleavage slightly developed ary origin because here dolomitization is irregular and
cleavage may occur. not parallel to stratification. Near the unconformity, (Fig. 4) the rocks show little The griotte member is important for its outstanding evidence of metamorphism, which is partly due to the red colour and the limestone nodules, which serve as
markers for mapping and stratigraphic interpretation. In where areas the thrust-plane follows the basal part of the dolomite the member, e.g. in Rozo unit, ores have accumulated within the formation: barite, together with
azurite and malachite. In the area of the Sierra del Cueto
Negro the dolomite is often intraformationally brecciat-
ed.
- Oville Formation (150-400 m). The formation can be
subdivided into lower shale-siltstone two parts: a mem-
ber and an upper sandstone member. The lower member is composed of glauconitic shales and siltstones with limestone nodules in its lower part.
Upwards the number of limestone nodules gradually
decreases. The shales in this member are fossiliferous.
The member upper is mainly composed of green Fig. 4. Unconformity between the Precambrian Mora Formation sandstones and white quartzites alternating with the Formation green (below left) and Herrería near Irede. shale beds. Weathered coloured orange glauconite grains differentiate the quartzites of the Oville Formation from absence of pelites and possibly to a diminishing grade of the overlying Barrios quartzites. metamorphism in this direction (Matte, 1968) as well. In the northern Bodón unit intrusives occur in con-
The of the Mora Formation is uppermost part usually trast to the rest of the area. They are found in both the
red coloured and the stratification has been obliterated Oville and the Barrios Formations. On the overlying map due before the of the Herrería to weathering deposition only the larger ones have been indicated. Apart from Formation. these dolerite sills tuffite beds parallel to general bedding
are present in this area.
Herrería Formation - In the Luna area (700-1000 m). The thinnest development of the formationoccurs in
the starts with to 1 of sequence up m quartz-pebble the Aralla zone, where the upper member was deposited followed brown and shales alter- conglomerate by green under conditions which allow erosion, as is shown by with sandstones. The shales and sandstones nating quartz several eroded bedding planes.
are overlain by the bulk of the formation with very
coarse sandstones and gravels. Barrios Formation (200-400 m). — The lithology of the and in bedded Cross-bedding graded bedding parallel formation is rather monotonous and composed ofwhite rocks the most structures in are striking sedimentary this to coloured and thin of pink quartzites, beds green to part of the formation. Near the of the formation top black shales in between. Van den Bosch (1969) based a
green and brown shale beds reappear. subdivision into five members the upon occurrence of In the Luna area in the lower of the shale and part two shale beds between the quartzites in the southern sandstone sequence a cleavage has developed of this the this locally. part area. In present area subdivision is The of the Mora Formation cleavage here seems to useful. In there not some sections, however, are one or continue in the direction the same into Herrería shales. two often thin (20-50 cm) carbonaceous black shale
Near Mora de Luna brown dolomites occur in the beds between the These shale beds quartzites. may be shales at about 100 m above the base. Dolomite beds the shale members referred comparable to to by van den have also been found the northern in in area a sandstone Bosch (1969). sequence. These beds were mostly disturbed by synsedi- Van den Bosch also stated that the upper part of the mentary slumping. formation is not developed in the Aralla zone.
Láncara Formation (50-180 m). All authors subdivide Formigoso Formation (90-200 m). - The formation the formation into three parts, a lower dolomite mem- be subdivided can into two members, a basal black shale ber, a middle limestone member and an upper griotte member and an upper shale/sandstone member.
member. In the area the three members could be present The black shales at the base are the most indicative
recognized south of the Lake Luna and in the Rozo unit sediments of the formation. There are, however, very (App. 2). In the areas north of the Rozo unit and the few exposures where these shales are not covered by
Aralla zone no limestones occur between the (Fig. 3) scree from the Barrios Formation. In the well-exposed dolomites and the The dolomitization of the griottes. sections north of Villasimpliz (App. 2, Pajares section), 175
and north of Geras de Gordon in the Rozo section the
shales overlie the Barrios quartzites with a sharp contact.
In both sections after about 1 m of shale coarser deposits of with occur, mainly composed quartzites alternating
thin mudstone beds. The latter always have a high con-
centration of Fe. Maybe this sandy part in the shale
member represents the transitional beds, mentioned by
van den Bosch (1969). In general this sandy sequence
does not exceed a thickness of 15 m.
The black shales are rather fossiliferous, especially in
finest the lower parts with the grainsizes.
The shale member becomes more sandy towards the
the member top. The transition to upper shale/sandstone sandstone beds this member is very gradual. The in dominate the shale beds. In general these sandstones
and disturbed bio- layers are very irregularly-bedded by
turbation.
San Pedro Formation (100-170 m). - Van den Bosch
(1969) and Backer (1959) investigated the San Pedro
sandstones in detail and subdivided the formation into
three members which could also be recognized in the
present area. These members are:
of a) a basal member, composed thick red channelling,
sandstone beds, often with hematite ooids,
of b) a middle member, composed of an alternation
green shales and red and greenish sandstones with hema-
tite and chamosite ooids resp.,
c) an upper member, composed of an alternation of Fig. 5. Stratigraphic position and lithology of the Devonian white quartzites and black shales. formations in the Luna-Torío area.
The lower member often contains a high percentage of
hematite. White to yellowish coloured phosphorite peb- The transition of sandstones of the San Pedro Forma-
bles often concentrated in beds of the coarsest fer- are tion into limestones of the La Vid Formation is gradual. sandstones. shales and sandstones ruginous Alternating At the top of the San Pedro Formation the sandstone of as well as quartzites the member have no upper percentage decreases, while the carbonate percentage significant hematite content. Brown and black shales (cement) increases. The La Vid Formation starts arbi- become more abundant near the top of the member. trarily, where the first carbonate bed appears (Brouwer, Shales occupy nearly 70% of the whole upper part, and 1968). The lower part of the limestone member has for the cement in the coarser parts grows more calcareous. the greater part developed as dolomite beds (Fig. 5). Due to gradual increase of the often dolomitic calcarous Upwards blue nodular limestones and fossiliferous components there is a transitional zone between the San (mainly brachiopods) dark gray limestones occur. In Pedro Formation and the overlying La Vid limestones. general the limestones change abruptly into carbona- structures abundant in the Sedimentary are especially shales. On of shale is ceous top a sequence, which often middle member. Current ripples, burrows, linsen and disturbed by faulting and folding, thin limestones occur.
flaser structures as well as loadcast and structures slump These limestones are mainly composed of reddish have been frequently recognized. crinoid stems. The transition into the overlying Santa In general the formation has developed rather uni- Lucía Formation is abrupt in many locations. In other formly throughout the area. In the Aralla however, zone, places, e.g. N of Mirantes, there is a gradual transition erosion removed parts of the and middle member. upper from red crinoidal La Vid limestones into more massive In the thickness of the Pedro Formation general San limestones of the Santa Lucía Formation.
as well as that of the underlying Formigoso Formation In the southern of the Alba the lower part syncline decreases towards the north. limestone member is poorly developed. N of Los Barrios
de Luna a thickness of 50 m has been measured. East-
La Vid Formation - In the area (130-400 m). present wards the lower member becomes even thinner. In the
the La Vid Formation could be subdivided into two northern areas the formation becomes thinner. Comte members: lower limestone member and a an assumed that of the shale upper (1959) part upper member was shale/limestone member. eroded. This may be true for the Bernesga section near 176
well-bedded limestone Camplongo, where the Ermita Formation uncon- member, and an upper member formably covers the La Vid Formation, but north of with red coarse crinoidal limestone beds in its uppermost member has Caldas de Luna the upper shale the same part.
without unconformable contact at the the shale all thickness an top. Although percentage in sections is very
Therefore it is still questionable if erosion strongly af- small, it shows a remarkable increase from S to N. In the fected the thickness of the formation in the Bemesga present area no noticeable variations could be observed valley. from W to E.
Lucia Formation - The Lucía Santa (75-240 m). Santa Caldas Formation (0-360 m). — Two members couldbe Formation is almost of limestones this formation: lower entirely composed recognized in a argillaceous lime- and dolomites. Therefore often difficult to sub- it is stone member, characterized by an alternation of grey divide the subdivision of sequence. Microscopically a limestones and calcareous shales; a limestone member
could be made the reference two seven units in section (de containing dark grey limestones with marker beds of
Coo, in prep.). A subdivision based upon morphological red nodular limestone. features three members lower well-bed- gives (Fig. 6): a The most complete sections occur in the western part ded limestone member, a middle massive limestone of the area. To the E the Caldas Formation has been
Lucía Fig. 6. Santa limestones, south of Geras de Gordón in vertical and slightly overturned position.
7. Relations Fig. between the Santa Lucía and Caldas Formations and their differences in lithologicproperties. 177
eroded. Therefore near Casares de Arbas only the lower shows knoll-like features where it borders the overlying of member occurs. Further east even the lower member has Nocedo Formation (Fig. 8). The shales the Nocedo the lime- been eroded. Formation are regularly layered upon massive
The southernmost outcrop of the formation has been
observed near the Collada de Alongó (Fig. 7). Here the
small. The limestone shale content is very argillaceous
member could not be recognized there. Due to intensive
folding it is impossible to reconstruct the original posi-
tion of the sequence, although it is significant that it
of limestone For this it is consists a sequence. reason
more likely to belong to the Santa Lucía Formation
of the Caldas Formation. The than to any other section typical subdivision of members of the Santa Lucía is
missing and therefore this outcrop is classified with the Caldas Formation.
Compared with the type-locality (N of Caldas de
Luna, Smits (1965)) the shale content of the argilla-
ceous limestone member along the northern boundary of
the Tesa syncline is greatest. Even thin sandstone beds
occur in this sequence. The limestone member too is
thicker, suggesting that erosion here had not such a great
influence as it had farther east.
- Huergas Formation (200-300 mj. The bulk of the
sediments of the Huergas Formation consists of mica- shales. ceous and decalcified silty Sandstone beds, how-
ever, occur everywhere in the upper part of the forma-
tion and in the lower part of the sequences in the
northernmost sections. Near Ciñera the formation is Fig. 8. Portilla Formation near Beberino; a lower limestone
member (right-side of photograph), a middle shale/limestone almost entirely composed of brown weathered sand- member and limestone member with knoll-like an upper stones. In the area between Vega de Gordon and N of structures at the top. Mirantes de Luna shales are strongly predominant with
small limestone concretions, often with specimens of
Buchiola spec, in their centre. E and S of Mirantes de stones and there are no indications of talus deposits at
Luna even limestone lenses are intercalated with the the sides of the limestones. Therefore it is assumed that
shales. These limestone lenses often have a coral fauna erosion caused the formation of knolls.
resembling that of the overlying Portilla Formation. Along the southern flank of the Alba syncline the
Along the southern limb of the Alba syncline the limestones thin out to the E. Near Saguera only small of shales of the formation are thinly developed in the area remnants the Portilla limestones have been found.
W of Saguera; E of Saguera the identity of the Huergas In the hinge of the Alba syncline the limestones are
Formation becomes vague owing to the absence of over- extraordinarily thick. lying limestones. After the deposition of the Santa Lucía Mohanti in Struve and Mohanti (1970) regards the facies of the formation 'reef facies with Formation here a clastic series developed during the rest as: "a develop-
of the Devonian, which is called the Piedrasecha Forma- ments of 'lagoonal' facies, biostromal 'reef barrier
tion. facies. Slow and prolonged differentialsubsidence of the
depositional basin with possible emergence of marginal - Portilla Formation (0-200 mj. The Portilla Forma- areas is believed to be one of the main factors in control- ofthree different the thickness and facies in the Portilla tion consists parts: ling changes lower limestone Formation of the Alba a. a massive, sometimes platy, member; syncline." Reyers (1972) con-
b. a middle member, composed of an admixture of firms this statement.
thin-layered limestones, shales and even sandstone beds;
c. an upper massive limestone member. Nocedo Formation (30-600 mj. - Calcareous sand-
In the steeply inclined sections the middle member stones are the main components of the basal part of the formation. forms a depression between the limestone members A limestone lens occurs in the type-locality
(Fig. 8). The subdivision into three members could be but one km E and W of that locality no limestones are
in all sections W of the river. The limestones followed recognized nearly Bernesga present. are by a sandstone/
E of the Bernesga and S of the MatallanaCoal Basin this shale sequence, changing upwards into a black shale unit difficult with sandstone lenses. This subdivision is to maintain. unit represents the 'Schistes
In the Beberino section the upper limestone member de Fueyo' of Comte. Its small lateral extension causes 178
the latter be included the of the the Alba Formation N of unit now to as upper part village up to the village, the Noce do Formation. thickness has been chosen as type-locality. The decreases
The general aspect of the Nocedo is that of a calcare- to the E as can be observed from map and sections.
ous sandstone sequence with diminishing grain size to- Intense faulting, especially near the contact with the
wards the top of the formation. Cross-laminations are Santa Lucía Formation in the area between Piedrasecha and Villas of the most striking sedimentary structures. Santiago de las causes repetition the basal the beds. With respect to sandstone/mudstone ratio of the Pieters The ratio the Nocedo Formation (1967, int. rep.) observed a sandstone/mudstone in type-section is
constant 1:1 ratio in the northern limb of the Alba 1 : 4 (Fig. 9). Compared with the ratio in the area north
syncline (Fig. 9). of the Alba syncline, where Nocedo and Ermita Forma-
In the southern bank of the Alba syncline the Nocedo tions occur, this ratio is much smaller. This diminishing
Formation could not be recognized. The gradual lateral tendency continues from Piedrasecha eastwards.
change into the Piedrasecha Formation there is to be The lithology of the type-section near Piedrasecha
seen around the nose of the Alba syncline and an arbi- and of a section NE of Santiago de las Villas is compared
trary cut is proposed along the axis of this structure, with the type-section of the Nocedo and Ermita Forma-
separating the Nocedo from the Piedrasecha Formation. tions in the Bernesga valley (App. 3).
The type-section starts with about 200 m of brown
Piedrasecha Formation (0-600 m¡. — The Piedrasecha shales in which concretions of claystone and limestone The limestone concretions often fossili- Formation is introduced here for the clastic sequence of are common. are in the basal of the mainly Devonian rocks deposited on top of the Santa ferous, especially part sequence. from the Lucía Formation along the southern limb of the Alba Apart concretions thin ferruginous beds occur in this lower of the formation. Then follows syncline. This clastic sequence is present from Portilla de part a
monotonous of of Luna to the Bernesga valley. The western delimitationof series ca 160 m dark brown to black
Piedrasecha, Nocedo and Ermita Formations is purely shales with thin ferruginous sandstone beds. The follow-
of about 100 is of arbitrary. It is assumed on the map that there exists a ing sequence m composed thin-layer- lateral facies between these three formations. change ed sandstones alternating with shale beds. In this part of The Piedrasecha S of section near (App. 3), starting the sequence animal tracks, loadcasts and slump struc-
9. Fig. Sandstone/mudstone ratio and diagrams showing the thickness of the Piedrasecha, Nocedo and Ermita Formations. 179
could be The 20 of this the base of the formation is of thin tures recognized. upper m composed a con-
sequence also contains concretions. White quartzites and glomeratic sandstone layer (Wagner, 1963, p. 41) resting of and sandstones form the following 10 m the section, with a sharp contact on the Piedrasecha Formation.
30 m of badly exposed brown shales cover the quartzites The contact with the overlying Alba Formation is and sandstones. The upper 50 m of the sequence consists sharp in the Santiago and Olleros sections, but in general of alternating brown sandstones and mudstones with the contact is gradual. cross-bedding in the sandstones of the uppermost 10 m.
Casts of brachiopods have been found in the mudstone Alba Formation (5-40 m). - The Alba Formation is beds. composed of red and green nodular limestones with fine
Compared with the type-section (App. 3) of the crystalline wavy limestones in the basal part, red cherty the Piedra- clastic Upper Devonian in the Bernesga valley shales in the middle part, and red shales between the secha Formation shows more shales and mudstones. The nodules in the upper part of the formation. Winkler fossiliferous lower part of the section with the limestone Prins (1968) defined these subdivisions as members, concretions is likely to be the equivalent of the Huergas naming them Gete Member, Valdehuesa Member and La of the Formation elsewhere. The upper part formation, Venta Member. should be correlated The with the showing more sandy layers, perhaps contact underlying Vegamián Forma- of the The the Ermita with the Ermita Formation Bernesga section. tion or Formation is sometimes sharp (dis- of this Ermita-like should be the conformable with the Ermita but lower limit sequence Formation) generally contact between the quartzitic sandstones and the gradual with the Vegamián Formation. North of the Sabero-Gordón line the Alba Formation overlain underlying sandstone/shale sequence. However, a sharp is by
contact could not be observed in contrast to a compa- the Caliza de Montaña Formation and southof that line
rable de las Villas The the Cuevas Formation. sequence near Santiago (App. 3). by sand- brown shales which have been deposited upon the
different from the — stones there, are very coarse transgres- Caliza de Montaña Formation (50-750 m). The Caliza of the Ermita Formation the sive sandstones in Bernesga de Montaña Formation can be divided into two parts: a
section. lower part, mainly composed of thin-bedded fetid lime-
stone in which calcite veins are common phenomena,
- of the forma- Ermita Formation (1-140 mj. The base and an upper part, consisting of more competent often often which tion is typified by microconglomerates reefoid limestones, which have been dolomitized in sev-
decalcified sandstones and eral Winkler grade upwards into coarse places. Prins (1968) named the two parts
ferruginous quartz sandstones. This part of the forma- Vegacervera Micrite Member and Valdeteja Biosparite
tion often has a reddidh colour, mainly caused by the Member respectively.
high iron content of the sandstones. In the area of the Pedroso syncline only the lower of the formation often of The upper part is composed part of the formation has been developed. North of the There from arenaceous limestones. is a gradual transition Rozo unit and the Aralla zone both members are pre- the underlying sandstones to the limestones. sent.
The Sabero-Gordón line in this thick of lower separates area a The thickness the part varies between 200 of the Ermita Formation in the S from and whereas the thickness of the development a 400 m, upper part is
thin sequence north of it. South of the Sabero-Gordón difficult to establish as the top of the formation inter-
Une a gradual contact between the Nocedo Formation fingers with the overlying San Emiliano Formation.
and the Ermita Formation makes the boundary an arbi-
North of the line the of Cuevas Formation 400 — The lies trary one. transgressive nature (> mj. formation con-
the Ermita Formation is marked by thin microconglo- formably upon the Alba Formation and can be sub-
merates deposited upon Devonian deposits, which are divided into two members; a shale/greywacke member
older when situated more to the N. Near Villar del and a limestone member (Fig. 10).
Puerto the Ermita sandstones overlie Huergas shales and In the lower shale/greywacke member the beds are
sandstones. Near Camplongo the Ermita Formation alternating shales and greywackes. The sequence has at
covers la Vid shales, showing an increasing gap below the its base first thinly laminated shales in which some
in Ermita Formation (Fig. 5). limestone nodules still occur. Higher the sequence the limestone nodules disappear and thin sandstone beds
Vegamián Formation (5-10 m). - The formation is occur. The sandstone beds show sharp basal contacts and
ofblack shales with of mainly composed cherty phosphatic gradation to their top. Some these sandstone beds are
and limestone nodules at several levels. Thin-bedded cross-bedded. Upwards the greywacke beds become
cross-laminated sandstones occur in the middle part. thicker and coarser. Gradation is less pronounced and
Generally the uppermost part of the formation is com- internal structures have not been found. In the valley of with thin Villas posed of reddish-brown mudstones alternating Santiago de las loadcasts and groove castst are
limestone beds. present incidentally at the base of greywacke beds.
The contact with the underlying Devonian rocks is North of Cuevas a nice slump structure is exposed in the de las Villas generally sharp. In the section near Santiago upper part of this member. Fig. 10. Stratigraphic position and lithology of Carboniferous formations in the Leonides (Prado Formation excluded).
between the There is a gradual contact shale/ The conglomerates are likely to point to the rapid
greywacke member and the limestone member, i.e. thin change in depositional conditions and reflect the initial
beds of laminated limestones between the shales and movements of the basin, resulting in a shift of shore line
greywackes are overlain by a vast deposit of limestones. and the return to predominantly clastic sedimentation.
The limestone member consists of thin-bedded, dark-
which calcite — grey, slightly fetid limestones in veins are San Emiliano Formation (500-2500 m). The lower
abundant. part of the formation is mainly composed of brownish
deformed shales and mudstones with thin sub- The limestones are strongly by folding, grey alternating
difficult establish their thickness. In beds. the central mudstones alternate making it to original greywacke In part Staalduinen with and fossiliferous the section as given by Boschma & Van argillaceous pure limestones and a therefore somewhat few beds of thick the (1968) their thickness seems to be (10—15 m) subgreywackes. In
exaggerated. upper part greywackes, mudstones and carbonaceous
The limestone member strongly resembles the lower dark-colouredshales are the dominant rock types.
member of the Caliza de Montaña Formation (p. 179). In the Pedroso and Tesa synclines only the lower part of On top of the limestone member another clastic the formation is exposed. E of the Bernesga in the
This of the Cuevas Bodón unit the limestone beds mainly occur in the sequence was deposited. upper part
here are restricted the central Formation generally starts with a fine-grained conglom- upper part. Greywackes to Thin coal beds have been found the erate with subrounded grains. The grains are chiefly part. only in section N of Villamanin. composed of quartz with angular chert fragments. The
conglomerates are followed by a shale/greywacke
limestone lenses. This Lena Formation - The formation is sequence with thin upper part (1400-3400 mj.
shows a striking resemblance with greywacke/shale series composed of shales which are often carbonaceous, mud-
below the limestone member, and is considered to be stones, sandstones, greywackes, thin limestone beds, thin beds and part of the same clastic member. conglomerate some poor coal-seams. Evers the three members: The sedimentological structures found in the shale/ (1967) recognized in adjacent area a basal greywacke member point to turbiditic depositional con- marine to paralic mudstone member, a paralic limestone and wacke ditions. Typically deltaic features such as cross-bedding, member, a paralic to limnic
ripple marks and bioturbation as well as fossils are greywacke member.
absent in this member. These subdivisions could not be mapped in the whole
For the member of the The oldest of the Lena Forma- limestone euxinix conditions, pos- present area. part from this Corallón sibly in an area that was sheltered the open sea, are tion in area is present in the anticline, which the has been indicated suggested. on map erroneously as a
The of the The becomes the west upper clastic series Cuevas Formation, syncline. sequence younger to turbiditic with the clastic north of however, were deposited in a environment. its top in deposits Busdongo. 181
Ginkel the Corallón bands and beds also The According to van (pers. comm.) lignitic clay gravel are present. limestones the subzone B and sandstones of well-rounded belong to Profusulinella are mainly composed quartz of the coal seams north Busdongo to the uppermost grains coated with hematite, micaflakes and felspars.
Westfalian C (Wagner, 1962). These data lead to the Due to chemical weathering most of the felspars have
to conclusion that the Corallón limestones belong the changed into kaolinite. Contacts are indicative of a of limestone and wacke member and the rocks north source rock in which quartz grains and felspars are the
the member. Further main constituents. of Busdongo to upper greywacke Only once a felspar content 11 difficult which member west, however, it is to ascertain percent has been observed in the Herrería Formation
the rocks belong to. It seems appropriate to place the (Oele, 1964). None of the other Palaeozoic formations limestones San del Rio within the limestone near Miguel has a felspar content of more than 4 percent. Conse- and wacke member. quently the Palaeozoic rocks did not supply the material
for the Voznuevo Formation. Due to previous peneplani-
Prado Formation (> 1000 m). — The main characteris- of sation the Palaeozoic (Evers, 1967) hardly any elastics tics of the formation basal of are: a part composed could have supplied to the Voznuevo Formation.
a of sub- conglomerates; younger part composed The numerous granites as described by Pannekoek
greywackes, mudstones and coal seams. Lenticular con- from the Castillian-Galician have the (1968) zone proper
occur locally. In the Cifiera-Matallana Basin glomerates composition. The assumption of a western source area is the first unconformable limestone deposits are conglom- supported by the occurence of inclined stratifications
erates sorted with very coarse and badly angular boul- which suggest currents running from west to east. ders and smaller better rounded elements. The In the of the small band of quartzite upper part formation a limestone the limestone boul- conglomerates, especially arenaceous limestones occurs especially near Brugos de
ders, seem to derive from the Palaeozoic limestones in Fenar and south of Candanedo and Solana de Fenar.
the immediate whereas the boulders environs, quartzite These limestones represent the Boñar Formation as have been distance. named Evers of this may transported over a long by (1967) east area. They are yellow The with the Palaeozoic and contact underlying is irregu- are more resistant to weathering than the accom-
lar. In some places red earth occurs just below the panying sandstones.
conglomerates, pointing to subaqueous weathering be-
fore deposition. Vegaquemada Formation. — Reddishbrown argillaceous
The basal conglomerates are followed by a cyclic sandstones, thin conglomerate beds, claybeds and mica- flakes series of greywackes, siltstones and coal seams. In the all over the sequence are the most characteristic features of western part of the basin the coal seams are thicker than the formation. The lower part of the formation in the eastern part. Coal seams are more frequent in the is mainly composed of argillaceous sand-
eastern and southern parts of the basin. stones. Only when unconformable contacts with the
The Matallana Basin is an asymmetric basin as is underlying Voznuevo Formation or the limestones of illustrated the of the by occurence conglomerates through- Boñar Formation are present could the sandstones
out the basin. In most places the original southern be distinguished from the sandstones of the Voznuevo
border is obscured Formation. The now by faulting. upper part of the formation is character- the sedimentation started In Magdalena Basin on a ized by increasing grainsize of the sandstones and
weathered surface of lower Palaeozoic or Precambrian of channelling coarse grained sandstones in finer sand- rocks. the Luna river north of Garaño this Along stones. Channels cut with erosive base through well- weathered surface is red coloured. The basal in deposits sorted sandstones. The gravels of the channel fill do not this basin well-rounded are quartzite conglomerates. show any sorting. The thickness decreases from the
The thickness of these conglomerates never exceeds border of the Tertiary basin towards the centre of the of 50 m. The conglomerates are followed by a series basin and varies due to the lateral interfingering with the
siltstones and coal seams as in the Matallana greywackes, Candanedo Formation. The sandstones and the con- There fewer coal in this basin and the Basin. are seams glomerate material have presumably been supplied by less than in the seams are poorer. As they are productive two manners of transport. The fine sandstones, which Cifiera-Matallana Basin actual coal has been mining generally show a good sorting, were transported by rivers. stopped. The poorly rounded conglomerate beds of the upper of the formation part may represent mountain flood
Voznuevo Formation (0-400 m). — The rocksequence debris, supplied by vertical mountainerosion.
starts unconformably upon the Cuevas Formation Mabesoone (1959) investigated the conglomerates of the fades (Fig. 11) with thin fluvial sandstone beds alternating Cuevas in Palencia, an equivalent of the
with clay beds. Consequently linsen and flaser structures Vegaquemada Formation. From the pebble analysis he could are the most striking sedimentary structures. In some too distinguish two types of pebbles, one derived
cases the flasers have been deformed to clayballs. Most from slopes, another of fluviatile origin.
sandstones are white but upwards they become multi-
coloured. Within the sandstones quartz pebbles of Candanedo Formation. - The Candanedo Formation is of formation of limestone > 2 cm are common. In the upper part the composed conglomerate and coarse sand- 182
Voznuevo north of Fig. 11. Unconformity between the Carboniferous Cuevas Formation (upper side left) and the Cretaceous Formation,
Brugos de Fenar.
base of the formation stones with calcite cement. At the The uppermost part of the formation contains quartzite diameter of the conglomerate cobbles have a mean cobbles which may have derived from older Palaeozoic the 15 cm. In general they are subrounded. Upwards in conglomeratic rocks. the sequence the grainsize diminishes and in between The present author is not entirely satisfied with the of his conglomerate beds thin layers of coarse sandstone occur. suggested modes transport. In opinion the
The cement in these sandstone layers also contains rounded quartzite cobbles together with small channel beds. features calcite though less than in the conglomerate Near suggest river transport rather than flash floods.
the top the proportion of limestone cobbles decreases
rapidly to less than 10 percent of the total composition. Riacos Formation. - Coarse boulders of quartzites
Most cobbles here are well rounded quartzites. The alternating with red sandstones and sandy clays are the
limestone cobbles are the typical black bituminous most characteristic rocks of the Riacos Formation. The
the in limestones with a high content of calcite veins, from beds lie an almost horizontal position resting slightly from the Carboniferous limestones of the Cuevas or unconformable upon the underlying rocks. The uncon- Caliza de Montaña Formations. formity is largest between the Riacos Formation and the
With regard to the origin of the Candanedo Forma- Mora Formation in the west of the map sheet. cloudbursts tion Evers (1967) suggests that intermittent The sandy clays and the red sandstones dominate in loaded flash floods in a semi-arid climate caused heavily the areas outside the river valleys, whereas the quartzite which carried the limestone cobbles from the mountains. cobbles and boulders are abundant in the valleys. The East of the the cobbles the base of present area at conglomerates and the clays here represent the latest the formation derived from the Cretaceous lime- were stage of the filling up of the Duero Basin. The sediments the also Palaeozoic stones, near top quartzites were are continental and were transported by rivers and
deposited. This 'inverted' sedimentation was also possibly by floods. The flash floods carried the conglom- noticed in the The of the which distributed the present area. composition erates lay over Duero Basin,
limestone conglomerates in this area reflect the outcrops depositing them fan-wise around the river mouths at the
in the adjoining mountains (e.g. the Cuevas Formation). border of the mountain chain. 183
CHAPTER III
STRUCTURAL GEOLOGY
INTRODUCTION
West-Asturian-Leonese internal zone. The rocks form
Lotze and subdivided the Iberian Her- Sdzuy (1961) part of the Narcea Anticlinorium as mentioned by cynian Orogene into a Galician-Castillian axial zone, a Julivert and Martínez Garcia (1967) and Matte (1968),
West-Asturian-Leonese internal zone and a Cantabrian Fig. 3. external zone, the last being roughly the present De Sitter (1964, p. 432) distinguished two subzones morphological mountain chain. This mountain chain within the external zone; a central subzone, the Astur- owes its morphological prominence to Alpine move- ides, and a southern subzone, the Leónides. These ments rather than to any special effect of the Hercynian subzones are supposed to have been separated by a sharp orogenesis (de Sitter, 1964, p. 432). line, the León line, which acted as a structural as well as facies The Luna-Bernesga-Torío area is situated in the SW a boundary. In the present area this line separates the Carboniferous rocks of the part of the Cantabrian Mountain chain, thus forming Lena Formation, which of the Central Coal from part of the external zone the Hercynian orogenesis. belong to Asturian Basin (Fig. 3), the Lower Palaeozoic of the However, a small part of the Precambrian rocks in the rocks Leónides.
SW of our area is regarded as belonging to the The present study describes structures in the
Lena Formation Heterogenoussequence with shales, sandstones and limestones. Irregular folding
San Emiliano Formation specially governed by the rapid lateral facies change of these sediments
Limestone member of the Cuevas Formation Platy limestones, concentric folding, largely parasitic folds
Caliza de Montaña Formation
Shale member of the Cuevas Formation Incompetent beds, disharmonie concentric folding
Alba Formation
Vegamián Formation
Piedrasecha Formation
Ermita Formation
sandstones ... Competentl . e Nocedo Formation "\_. concentric folding „ „ , ■ Competent limestones Portilla Formation
Formation Huergas Incompetent sequence, small-scale folding. Sequence causing disharmony between Santa Lucía limestones and overlying Portilla limestones
Santa Lucía/Caldas Formation Competent limestones, strike-faulting, large-scale box-folding, often with
thrust anticlines
Shale member of the La Vid Formation Incompetent and mobile shales causing considerable disharmony between underlying and overlying Palaeozoic rocks, small-scale folding, fracture cleavage,
core ofbox-folds
Limestone member of the La Vid Formation Competent sequence, intensive minor folding,long strike-faults between the lime-
San Pedro Formation stones and the San Pedro sandstones
Formigoso Formation Black shales, incompetent sequence, minor folding allowing disharmony between
the Barrios and San Pedro Formations
Barrios Formation Competent sandstones and quartzites, controlling shape and width of most folds,
Upper part of the Oville Formation characteristic cross-faulting
Lower of the Oville Formation and Láncara part Shales limestones, incompetent sequence, base of the Formation
Lancara Formation served as 'detachment' plane of thrusts
in Herrería Formation Competent sandstones, not involved thrust structures, causing faults in core of
anticlines, functioningas 'basement' to overlying thrust structures
Mora Formation Slates and greywackes, slaty cleavage and fracture cleavage, intensely folded
Table 2. 184
NARCEA ANTICLINORIUM Leónides and íhe interpretation of the León line as a structural phenomenon. The Leónides are typically thin The folded Precambrian rocks of the Mora skinned supra-structures as described by de Sitter(1962, strongly which the southwestern of the 1965), Evers (1967), Rupke (1965) and van den Bosch Formation occur in part
and are of de los Caballeros, are of an arc (1969). The dominant folds are concentric map area, S Vega part of Precambrian the Asturian controlled by a number of competent horizons (Table 2) rocks, starting at coast at den S de then as have been specially worked out by van Bosch Cudillero, trending through Cangas Narcea,
E to the E. The (1969, p. 191). bending near Villablino, continuing Incompetent formationsallow disharmony to develop Palaeozoic strata also follow this arc on either side of the of between structures of various parts of the stratigraphic Precambrian, giving rise to the concept the Narcea and sequence. Complete decollement developing into thrust- Anticlinorium (Julivert-Martínez-García (1967) ing is a typical feature of the Leonide structures. Other Matte (1968).
the Precambrian rocks form the most faults also developed in relation to folding. Their In map area of the Narcea here detailed behaviour is clearly dictated by the competence easterly part Anticlinorium, partly of the individual strata cut. covered by Carboniferous and Tertiary deposits. Gener-
of anticlinorium to the The major geological regions — the Narcea Anti- ally the outcrops the disappear
Major Periods Stille classification Results
The Alpine Period Savian morphogenetic phase Cantabrian Mts. obtained their present configuration, started in the Palaeogene.
Saalic phase Folding of the Stephanian Coal Basins. Activity in Post-
Stephanian and Pré-Triassic times. Angular unconformity
between Triassic and Palaeozoic rocks. Gentle folding.
Asturian orogenetic phase Folding in the Leónides, started at the end of the
Westfalian and continued into the earlier Upper Period Hercynian Stephanian. Angular unconformity of overlying Prado
rocks of Stephanian B age. Main folding.
Sudetic initial folding phase Thrusting and folding of the Leónides.
It started in the Namurian and continued in the
Westfalian.
Bretonnic epeirogenetic phase Uplifts started at the end pf the Devonian and continued
in the Lower-Carboniferous (base of the Visean). A structural stratigraphical hiatus between the Ermita
Formation and older Palaeozoic rocks is the result of
these uplifts.
Precambrian Period Orogenetic phase which folded Precambrian rocks of
the Narcea Anticlinorium before the deposition of
Cambrian sediments. Angular unconformity between Mora
rocks and Herrería sandstones.
Table 3.
east of clinorium, Central Asturian Coal Basin, Stephanian Coal under a cover Tertiary deposits at the border of
— the Meseta to Basins and Leónides are shown in Fig. 3, which also (Miocene-Pliocene deposits according Pastor Gómez The ofCarboniferous rocks shows the subdivision within the Leónides, used in this (1963). cover
study. in the map is bounded by faults to be discussed under the Mountain Border Area Tectonic activity ranging from Precambrian to Ter- heading (p. 199).
tiary, has been proved here. The various orogenetic According to Matte (1968) the Cambrian in the southern flank of the anticlinorium phases classified according to the system of Stille (1924) rests unconformably the Precambrian but Juli and Martínez- are shown in Table 3. Further details are given in the on rocks, vert García consider the be conformable. discussion of the appropriate units. (1967) contact to
Three coloured sections at the left side of the In the northern flank of the E-W trending Narcea
Anticlinorium the contact is unconformable geological map reflect the general structural pattern of cleary (de den Bosch the investigated area. In App. 4, (back pocket) 13 Sitter, (1963), van (1969).
cross-sections show the relations between the folded About one km east of Vega de los Caballeros near the
structures of the Carboniferous rocks in the southern main road along the Luna river an affluent from Portilla
de Luna into the main river. There E-W folded map area. runs 185
of Mora beds the Prado Formation. This have Precambrian deposits are abruptly cut off by micro- upon may deformation which also conglomerates at the base of the Herrería deposits. The occurred during a late stage may the Mora slates the overlying Herrería deposits, here slightly overturned, dip have caused the kinkbanding in along
steeply to the south. The angle between. Mora and southern thrustfault.
Herrería deposits is different due to folding of the Mora
Formation near the unconformity. LEONIDES Near Vega de los Caballeros the unconformity is also between slates of the Mora exposed. Here it occurs Alba Formation and the Herrerian shales. The beds of the synclinorium The Alba forms the southernmost Herrería Formation are subvertical, whereas the Mora synclinorium (Fig. 3)
the slates Palaeozoic structure of the Cantabrian Mountain beds dip about '20° to the north. In a cleavage slope of This here and is succeeded farther south by the uncon- occurs with a southward dip 15-30°. cleavage formable Mesozoic of the Duero Basin. with a WSW strike occurs not only in the Mora deposits sequence In the Alba the north flank is formed but is also present with equal direction and inclination in synclinorium Namurian whereas in the the lowermost shaly beds of the Herrería Formation. by Devonian to formations, south flank the is from Namurian as Matte (1968) made observations in the area west of sequence complete results. far down the Precambrian basement. the river Sil which yielded the same His as of The Lower Palaeozoic rocks the south flank conclusion was that the cleavage is Hercynian age. In along
con- show different structural elements as compared with the the present area, however, the cleavage generally
of Carboniferous rocks in the core of the tinues to about 30 m above the base of the Herrería structures the
will therefore be dealt with Formation. Therefore the present author would suggest synclinorium. They separate-
the cleavage in the Herrería beds to be a reactivation of a iy-
pre-Hercynian cleavage during the Hercynian deforma-
tion. The pre-Carboniferous southern flank. - The southern
flank shows of faults related the a system clearly to
of the Strike faults A comparable phenomenon has been observed in the development major structure. are considerable of the strati- Central Pyrenees, where Devonian cleavage seems to common, repeating parts south of Barrios de Luna and continue into unconformable Triassic. Mey (1969) de- graphic succession, e.g. south of Portilla de Luna 4). These monstrated that post-Triassic folding reactivated move- (section 2, App. SE-NW faults have downthrown the ments along pre-Triassic cleavages planes, resulting in running sequences its southern side. parallelism ofnon-contemporaneous structures. at
Near to and northof Portilla de Luna the Santa Lucía Just outside the present map area, near Irede, lies the
limestone is cut several times by faults slightly oblique best known exposure of the unconformable contact (de
to the strike but the same sense of downthrow Sitter, 1963; Julivert y Martínez-García, 1967 and van retaining the south. be traced further in the den Bosch, 1969), Fig. 4. to They cannot shales above below this formation. The In the three localities where the unconformities are incompetent or
exposed, the angle between Mora and Herrería beds is fault-planes of the strike-faults and the oblique strike- folded faults vertical north. different, suggesting that orogenetic forces the are or dip steeply
Mora deposits before the deposition of the rocks of the Cross-faults also show a very peculiar development
Herrería Formation. More to the west, however, the especially in the zone between Los Barrios de Luna and
angle between the two formations is more or less Vinayo. The fault-planes appear to be more or less invariable (van den Bosch, 1969) affording proof of vertical, trending approximately NE-SW. Most of these
tilting movements rather than folding. faults extend to, though apparently not through, the of the Otero de las what be In the southern part area, near competent upper Herrería, contrary to might fault Carboniferous Dueñas, an E-W trending separates expected in view of their very clear expression in the rocks of the Prado Formation from the Mora deposits other competent formations such as Barrios and Oville.
south of them. This fault extends to the west over a long The NE-SW trend of the fault-planes (mean: N35E) direction. distance (Pastor Gómez, 1963, van den Bosch, 1969) roughly indicates the stress Assuming that the and forms the southern border of the Magdalena Coal direction perpendicular to the axial plane of the main
Basin. The fault-plane dips steeply to the south and has syncline in this area (30-35°) also points to the main
thrust the Mora deposits onto the Prado beds. deformation stress direction, the suggestion is that the from South of Vega de los Caballeros another E-W trending cross-faults and the folds originate the same
fault also forms the boundary of the Carboniferous stressfield. rocks. This time the northern block has been upthrown. the the Both faults are strike faults in the Narcea Anticlinorium, Structural elements in Carboniferous part of and caused depressions within the Mora Formation, Alba synclinorium. — Although the central core of the looks rather it is of number which afterwards were filled during the Stephanian by structure simple composed a of smaller and anticlines. The of continental, coal bearing deposits. At a later stage synclines complexity reactivation of the fault movements caused overthrusting the structure is probably connected with the irregular 186
mentioned earlier sedimentationpattern as (p. 180). the oblique strike-faults in the pre-Carboniferous rocks
The average trend of the syncline varies from WNW- along the southern flank of the Alba syncline s.s. Its
east of ESE in the west to WSW-ENE Santiago de las presence affects in the same way the shape and the
Villas. As shown in the cross-sections belonging to the stratigraphic level of the exposed major syncline.
southern the The fault from Alcedo to the east is strike- map area (App. 4) western part of the main running a has fault the Alba level. structure no irregular pattern (sections 1—4). It was expressed at griotte It indicates a and that here especially in section 4 by means of broken anticlinal structure possibly related to the methods for folds Southern Border which will be discussed construction concentric (de Sitter, Fault, on
the model of the Alba 199. 1941) general syncline s.s. was p.
obtained. This simple pattern only holds for the general East of the Bernesga the north side of the exposed
model because the limestones of the Cuevas Formation Carboniferous rocks have been deformed by normal
here have been strongly affected by minor folding. The faulting. This change actually occurs east of Llombera,
structure is to where the Matallana nearest to the evidently a single major syncline up Basin is Alba Here the Nocedo Forma- Cuevas, but to the east secondary structures develop on syncline. generally competent flank tion is also It that both these the southern en echelon with the main syncline thinning. is supposed factors involved in the this of (sections 5—8). Up to the Bernesga river the southern were cross-faulting in part
structures become dominant and the former major the area. So far the of the external deforma- syncline SE of Nocedo is even lost in two minor outcrop pattern synclines and anticlines. tion as a whole has been discussed. The limestones and
East of the Bernesga river the folded structure is the greywackes, however, also react differently to the
on lower the clastic folding. This internal deformation will be dealt with exposed a stratigraphic level, e.g. these sediments also lower member of the Cuevas Formation, (sections now. In it is clearly demonstrated
10—13). Due to the rapid lateral change of facies at this that the manner in which individual beds are layered, level the size of in disharmonie stratigraphic the structures are irregular and dis- governs folds, resulting harmonie. structures at various levels.
the of Robla In area north La two faults are present. Minor north Piedrasecha. — The The fault running from about 1 km east of Sorribos de folds of thin-layered limestones thrown Alba to the NE acts partly as a wrench fault in that it rocks, in particular, were into a large number of minor folds the of the Alba exposes the Piedrasecha Formation more to the N along in core syncline.
of these folds are the the eastern side of the fault-plane, partly as a normal Many asymmetrical to major and thus related it. Most of them could be fault, lowering the stratigraphic level on that same side structure to of the fault-plane. This fault is possibly comparable with determined as parasitic folds in the sense of de Sitter
Fig. 12. Synclinal north model of Cuevas limestones of Piedrasecha with types of minor folds as observed from north to south. 187
folds (1958). In the present author's view other minor rather than tectonic seem to be due to gravity to a
stressfield, and hence should indeed be regarded as The cascade cascade folds (de Sitter, 1964, p. 240). of the four classes of folds, belonging to one collapse
structures as defined by Harrison and Falcon (1934),
develop by the gliding of competent beds into incom-
petent deposits. of A good impression of the minor folds in the core
the Carboniferous rocks within the Alba syncline is given
the limestone member of the by a section through
Cuevas Formation north of Piedrasecha, Fig. 12.
The first of minor folds is illustrated in 12a. type Fig. Fig. 13. Position of two sets of minor folds (p1 and p2) along
north of These folds occur in a rather uniform limestone se- the northern limb of the limestone syncline
60—70° south. The limestones Piedrasecha. quence, dipping to the thick beds with are composed of 40—60 cm alternating of the Al- 5 cm beds, apparently same composition. lated plunge of the major syncline, make it plausible to though differences in competency between folded and include these accordeon folds with parasitic folds. The unfolded beds are not noticeable the difference in relation between the two sets of minor folds (pi small difference in existing thickness of the limestones point to a and and the Alba is given in 13. there difference p2) major syncline Fig. competency. However, is not enough to of Near the centre of the main syncline a sequence cause flattening in the foldflanks or stretching in the
about 70 m of unfolded limestone beds occur (Fig. 12d). foldhinges. The southern limbs of antiforms are long of the The beds are nearly vertical in this part syncline flanks in comparison with the almost horizontal north- 60°N. and the limestones themselves are very thin-bedded ern flanks. The axial plane dips ca The fold-type
described can be regarded as parasitic to the major (5-10 cm). Further south but still on the northern side of the syncline because: folds have been observed in of the axial fits with the major syncline again minor a. the direction plane concept The axial of fold the northern limb of the still vertically bedded limestones (Fig. 12e). a parasitic belonging to
are also almost vertical or steeply northwards, Alba syncline; planes dip the fold-axes plunging 1-5°E. These folds have sharp b. the southern limb is many times longer than the and often broken in the without northern limb, which would be expected of flankfolds of hinges are cores, loosing, however, the connection between beds along the major syncline. of the folds both sides of the fault-plane. The asymmetry of these Southwards the aspects minor change of folds. folds suggests that they are parasitic. slowly. Fig. 12b shows one of this new type south and close the axial of the found limestone Further to trace The folds have been in a sequence major syncline minor folds have the following properties comparable to that described above. There is no ap- between the different (Fig. 12f): parent difference in competence
a. northern flanks of antiforms are flat and north- layers. Antiforms of these folds have flat south-dipping dipping (10-30°); northern flanks and nearly vertical southern flanks. The b. the northern flanks are long in comparison with the axial planes of these folds are flatter than those southern folded limbs; described above and foldaxes plunge 5°E. These folds the set of folds of the southern limb have the author considers be intermediatebetween c. secondery present to axial planes, dipping steeply north (60-80°). Fig. 14 parasitic and cascade folds because they have the
shows some of these folds. Near the bottom of the expected asymmetry of parasitic folds, though some of which photograph starts a simple concentric fold-hinge the hinges show small scale faulting, causing the hinges continued anticlinal of higher in the sequence is as two to be downthrown to the inner part the syncline. folds. that lack of evident- Further southwards of minor folds is accordeon This proves space, a composite set ly when the folding is only concentric, is found. There is a type of folds with long vertical limbs occuring the development of two accordeon folds alternating with small nearly horizontal limbs. The axial anticipated by
on the of the original fold. There is no of planes dip 50-60°N. This type corresponds to the hinge sign thickening in the hinges of the folds. The axial planes of description given of the parasitic folds of Fig. 12a. The
the first and second set of minor folds here are horizontal limbs of these folds are here strongly folded. parallel and northwards. These small folds show an irregular pattern but nearly all dip what of fold foldaxes 5 - It is difficult to decide as to type is the axial planes are vertical. All plunge 10°E. of flanks The small faults have straight flanks and sharply curved found here. The asymmetry the and the resemblance of these folds with folds described hinges and are therefore accordeon folds (de Sitter, parasitic
The such vertical earlier suggest that they belong to them. However, their 1964, p. 294). geometric properties, as indicates that these folds to the axial planes and plunging axes agreeing with the calcu- asymmetry belong 188
the north of Fig. 14. Minor folds in the core of limestone syncline of Piedrasecha (model f. Fig. 12).
the limestones which southern limb of the main structure, whereas they still folds occur in slightly coarser are bedded limestones. Their occur in the hinge of the syncline. This may be an in contact with more thinly indication that the forms due the small differences during folding major synclinal axis irregular are possibly to towards the south. between the limestones. Therefore migrated in competency it is South of the all minor this of the major synclinal hinge nearly assumed that in part synclinal structure folds have axial The well cascade folds north-dipping planes. (Fig. 12g). parasitic as as occur. low both fold-plunges are (< 5 ) eastwards and west- Near the southern border of the limestone member of
wards. The folds because: minor are parasitic the Cuevas Formation the fold pattern becomes simple the axial than the a. planes dip more steeply strata; (Fig. 12k). The limestones are still thin-bedded and are b. the axial from planes are slightly fanning out the almost vertical or dipping steeply to the north. The of the centre main syncline; fold-hinges of synforms are often rounded and directed
c. the limbs are the limbs Antiforms asymmetric, long coinciding to the central part of the major syncline. are 40—60°N with the general dip and the short limbs dipping often broken in the hinges. The synforms have long 60 -80°N. The short limbs of of the folds some are northern limbs dipping steeply north and generally flat
with those in the core of the The limbs. This comparable syncline. lying north dipping southern gives an axial of set of in the folds rather than planes a secondary folds, occuring asymmetry that characterizes cascade short limbs of the first of set minor folds, are parallel to parasitic folds.
the axial planes of the first set of minor folds. The
secondary set of folds, however, have flanks of equal Minor folds north of Olleros de Alba. - In limestones of
the of the southern minor folds have length. core syncline many Further southwards a first and second set of folds been observed. Especially north of Olleros de Alba both be The folds cannot distinguished. minor occuring here synclinal flanks are folded by minor folds. On the
are intermediate in size between the two sets of folds southern flank the folds generally have steeply south-
described above. Their axial planes dip to the north dipping axial planes, and plunge gently west and east with have (70—90°) axes gently plunging to the west. The (Fig. 15, uppermost diagrams). The small anticlines minor folds here the flanks which flat and are asymmetric, longer limbs of short southern are south-dipping.
antiforms dipping gently to the north and the short The longer northern flanks dip steeply north. The hinges
limbs dipping steeply south (Fig. 12h). The short limb is of the folds are sharp and show no stretching of any
now straight in contrast to the folds in the centre of the importance in the flanks. The style of folding is
syncline. These folds are regarded as parasitic to the accordeon folding and resembles that of the Piedrasecha
major syncline. section. The geometric properties of most of the folds in Further south an irregular minor fold pattern occurs relation to the main syncline classify them as parasitic
(Fig. 12j,k). Folds with horizontal axial planes change folds. However, a conjugate set of minor folds has also into folds with vertical axial nearly planes and vice versa. been observed in this part of the syncline (Fig. 16). The There be of axial seems to no regular pattern plane dips. fold-system in the photograph indicates extension in a Some of the folds show with an asymmetry coinciding horizontal sense and shortening in a vertical sense. The that of folds described earlier. Most of the parasitic as strain ellipsoid for this deformation then has a long fold forms with flat axial are irregular generally planes horizontal axis and a shorter vertical axis. This implies of dipping towards the centre the major syncline. These that the main stress here has been a vertical one. Fig. 15. Stereograms of minor folds in the Alba Syncline north of Olleros de Alba and near Pico Fontañon. 190
folds in the synclines north of Olleros and Piedrasecha. The folds almost well are symmetrical, matching very
with the form expected for minor folds on hinges of
anticlines (de Sitter, 1964,p. 282). of Minor folds north of Alcedo may be called typical folds. the synclinal hinge. The folds are accordeon The
axial planes dip steeply north (75-85°) along the
northern synclinal flank. Fold-axes plunge east with a
dip of 15-25°.
South of the synclinal axis minor folds occur in a thick limestone sequence that is interbedded between
greywacke layers. The fold-pattern is irregular as the
axial planes have various dips from horizontal to vertical. folds They are perhaps comparable to the conjugate as and described above. In addition there occur parasitic
cascade folds as well.
Plunge pattern of minorfolds in the Alba syncline (Fig. - folds the 17). The plunges of axes of minor in entire
Alba synclinorium are generally about 5 to 10° west or SE east. They are parallel to the general strike, trending
from Portilla de Luna to Santiago de las Villas, EW from
Santiago to the Bernesga and WSW east of the Bernesga.
In the western and southern parts of the synclinorium
the folds plunge E, whereas east of the Fontañón most have folds plunge W. The southern syncline seems to a there for horizontal axis because is no preference any
special plunge direction. In a small area west of the
fault-contact in the valley of Santiago de las Villas fold-
axes, plunging 50—70°E, occur.
Sabero-Gordón line
Rupke (1965, p. 39) and de Sitter (1965, p. 376)
mentioned the Sabero-Gordón line as an important fault Fig. 16. Conjugate fold set of minor folds north of Olleros de facies the Alba. which represents a boundary during Upper
Devonian. Rupke (1965, fig. 14) traced the line from
Valderrueda in the east to La Vecilla in the west and
mentioned its western prolongation into the Bernesga
region where it is considered to border the asymmetric from basin the south. Therefore I suggest that these folds originated a Matallana to the south of the fault-zone late stage of the folding, when the synclinal structure In area a complete
already existed. Devonian succession is present whereas north of it the Ermita Formation the Por- Along the northern flank of the syncline the minor uppermost Devonian covers
folds also show E-W trending axial planes. They dip tilla Formation but leaving a hiatus. The difference in due the absence of the Nocedo northwards and are notably uniform, thus showing a thickness is mainly to the northern This absence clear point maximum in the stereograms(in the middle of Formation in area (Fig. 5). could have been caused differential subsidence. This Fig. 15). The folds are symmetric but symmetry is often by fit with the fact that disturbed by small faults. Most of them seem to assumption is strengthened by near folds for of the Portilla Formation indications of the geometric properties of parasitic this part Beberino on top of the fold-axes with bend have been found The limestones there syncline. The plunge a (5 ) in erosion (Fig. 8). well direction show The erosion out of an easterly as as westerly (Fig. 15). gulley-like structures. remnants reef the gullies formerly used to be interpreted as — Minor folds near Pico Fontañón. In the flat anticlinal structures (Evers, 1967, p. 96). Sleumer (1969, p. 8) the the results of hinge of the limestones near Pico Fontañón, in already recognized the limestone humps as
furthermore: cannot be inter- geological map erroneously indicated as Fontun, minor erosion, stating "They folds have bioherms of normal biomicro- axial planes, dipping steeply (60—80°) north, preted as as they consist
while the fold-axes plunge gently (5—10°) west (Fig. dites and not of rigid coral and stromatoporoid skele-
15). The folds here are again of the accordeon type of tons".
folding but most of them are twice as large as the minor The development of the stratigraphic sequence upon 191
Fig. 17. Alba synclinorium with axial plunges of secondary folds; SE plunging axes in the western part of the structures and west plunging axes in the eastern part.
rather in to the the Alba Formation also varies on the two sides of the mass, rigid comparison surrounding fault-zone fades shales of the Formation. In the the (Fig. 10) so that a boundary seems to Huergas east exist. fault-zone cuts the topographic surface at the level Vid Formation reacted The exposed structures in this fault-zone are strongly where the very incompetent La small scale of the related to the Iithologic properties of the formations to faulting by folding, especially shale member. Near Lake Luna, north of exposed. The Devonian formations are much broken up upper just the Santa Lucía Formation is also by strike-faults. The multiple faults are quite clear in the Mirantes, strongly folded. These folds have above the main west; east of Los Barrios de Gordon, however, only a may developed Sabero-Gordón fault and faults have been single fault can be traced along the contact with the although no fault drawn the small movements have taken Prado Formation. Further east even this disappears, on map, may and folded probably below the Prado rocks (sections 1-13, App. place within the vertically dipping isoclinally
4). A similar behaviour for this fault-zone has been Santa Lucía Formation. East of the Bernesga river the fault is exposed at the recorded in the areas to the east by Evers (1967, p. 127) contact of La Vid shales and Prado conglomerates. Part and Rupke (1965, p. 55). The differences between the the of fault of the Prado Formation as presented on map west zone in the west and the single in the east are San Mateo is considered to be a marine succession mainly due to the different levels reached by erosion. In now of older Carboniferous comparable with the facies the west the zone is exposed where it is a fault-contact age, of the Cuevas and the San Emiliano Formations. The to the Santa Lucía Formation, a competent limestone
Fig. 18. Map-detailof the San Mateo structure. 192
and Artieda This fold to have in connection with map as given by Wagner (1970, p. 32) gives seems developed of the formations in this the in strike noticeable north of the Pedroso an impression exposed area. swing
The area around San Mateo is also reinterpreted as syncline.
shown by Fig. 18. La Vid shales have been observed at
the base of the Santa Lucía limestones in the hinge of Beberino fault
the synclinal structure. In the north the limestone con- The Beberino fault is the E-W strike-fault that runs along
glomerate ofthe Padro Formation covers La Vid shales. the northern limb of the Pedroso syncline from Lake
The Santa Lucía lense-shaped exposure on the geologi- Luna to Beberino near the Bernesga valley and
cal map is now looked upon as Caliza de Montaña lime- continues further east along the north of the Santa
of of the Mateo Mountain the stones or even Cuevas limestones. The presence the Lucía limestones San into
Carboniferous limestones suggests that the Sabero- Ciñera-Matallana Basin. The Beberino fault is strongly
Gordón fault the north side ofthe Mateo controlled the of the Santa Lucía Forma- runs along San by presence from limestones. The San Mateo structure itself represents an tion. Its structural aspects change west to east. the the fault hidden the but asymmetrical syncline in the crest of a box-fold (see Near Lake Luna is to eye kilometres farther the shales in section 9, App. 4) with La Vid shales in its core. To the two east Huergas are fault-contact with the calcareous shales of the west of the limestone structure a mass of limestones upper
member of the La Vid Formation. north- have slid down to the valley. The sliding has resulted in a Irregulary
La Vid shales are of the block layer of brecciated limestones around the glided mass. dipping part upthrown shales form On the geological map this breccia has led to the misinter- whereas moderately south-dipping Huergas
the fault-contactof the Sabero-Gordón the downthrown block. East of the Pedroso Mountain pretation as being
fault. South of the SW-NE fault (Fig. 18) also Prado rocks the apparent throw becomes zero. It increases again to
that these rocks and the slid the east, however, now with a downthrown northern are exposed. It is supposed
ofthe same mass are related to each other in that they are part.
Further east the Beberino fault maintains its course age.
Conclusions about the Sabero-Gordón line are now as along the Santa Lucía Formation cutting it obliquely
follows: The has acted facies from the base the of the limestones distance zone as a boundary during to top at a
parts of the Devonian and Carboniferous sedimentation of a few kilometres from Pedroso to a point about one
period. The fault-zone is therefore a fundamentalline as kilometre east of Cabornera. From that point onwards
mentioned by Rupke (1965) and Evers (1967). The the northern part becomes the downthrown part and the
the related the shales of the southern block eastwards contact structures in present area are to lithologic Huergas the the Alba and Caliza de Montaña properties of the formations exposed and not directly Portilla, Ermita, influenced fundamental fault the The Formations The fault attains its maximum by a in depth. respectively. place of the Sabero-Gordón line could therefore be displacement east of Beberino where Huergas shales are in with Caliza de Montaña limestones. East of anywhere in the area between the Alba syncline and the contact
Pedroso syncline. the Bernesga the fault becomes involved in the complex
structures around the San Mateo Mountain. A continua-
Pedroso syncline tion of the Beberino fault is postulated below the
The Pedroso syncline is bordered in the south by the unconformablePrado Formation of the Ciñera-Matallana
Sabero-Gordón faults and in the north by the Beberino Basin and further east it probably joins the Sabero-
fault. Between Lake Luna and the Pedroso the nose Gordón line.
of the syncline is clearly expressed by the Ermita, Alba
and Caliza de Montaña Formations. North of Los Barrios Bregón unit
de Gordon the synclinal structure is partly covered by The Bregón unit comprises the southernmostthrust-unit
the westernmost deposits of the Prado Formation in the of the Leónides. It trends E-W starting one km E of
Cifiera-MatallanaCoal Basin and most probably loses its Ciñera and continues westwards to Lake Luna. Rocks
identity in the complicated folding between La Pola de ranging from the Láncara to the Santa Lucía Formations
Gordon and San Mateo Mountain. are incorporated in this structural unit. East of the
The is off the northern asymmetric syncline has an overturned or vertical Bernesga river it obliquely cut by
northern limb. The asymmetry is accentuated by dif- boundary-fault along the Ciñera-Matallana Coal Basin.
ferences of thickness in the Ermita and Caliza de Evers (1967, p. 128) assumes that east of this basin the
Montaña Formations (sections 1, 2 and 5, App. 4) in unit reappears in the Montuerto syncline from below the both flanks. Prado beds.
has East of the Pedroso a small isoclinal anticline is In the present area the thrust-unit its greatest
developed along the steeply north-dipping north flank displacement in the east, where Cambrian limestones of
(App. 4, sections 3 and 4). The anticlinal axis trends the Láncara Formation contact with Devonian shales of almost SE-NW and forms an angle with the major the Huergas Formation. To the west the displacement
ESE-WNW Pedroso syncline. The smaller folds plunge to along the fault is diminishing rapidly, partly through the the west so that a closure of key horizons such as the upward cutting of the thrust through Láncara limestones Alba fact that Griotte can clearly be seen near the Beberino fault. to San Pedro sandstones and partly through the 193
westwards the covered sequences become older when Puerto. From this point eastwards the Láncara dolomites
changing from the Huergas Formation to the La Vid are exposed along the fault-contact. In the Bernesga
Formation. valley the thrust sequences are almost vertical but from
From the Bregón Mountain to the area west of Villar del Puerto to the east they are strongly overturned
Lake Luna, where the fault enters the strongly faulted to the north. northern flank of the From Pico del Pozo onwards direction Abelgas syncline (van den Bosch, in a westerly the fault the the be far the La Vid 1969), is structurally unimportant near structure can recognized as as
surface. shales. There it disappears in the shales which are north of In the railway cutting near Ciñera minor folds are exposed Buiza. The main feature of the unit is anticlinal exposed in the Oville and Barrios Formations just south an structure of the main thrust (Fig. 19). Despite the proximity of plunging slightly to the east. The shape of the structure is controlled the of the Barrios Formation. the thrust-front the majority of these folds do not seem by folding
in Fig. 19. Minor folds in the Oville Formation near Ciñera the Bernesga valley.
to have been actively involved in the thrusting. The East of Villar del Puerto small secondary strike-faults thrust from typical parasitic symmetry to a originating a run almost parallel to the main thrust-fault. Just north fold should have minor folds with flat short given lying of Pico del Pozo a small secondary syncline developed in
limbs and steeply dipping long limbs. The exposed folds, the Barrios and Formigoso Formations. Its axial plane flanks north and however, have short dipping steeply trends to the north and the fold-axis plunges strongly flanks south. In the long dipping gently my opinion northwards (45°) and disappears near the thrust-front. folds shown 19 due as on Fig. are to gravity at a stage Along the southern side of the unit synclinal and
when the had an inclined anticlinal folds stratigraphic sequence already occur in the competent Santa Lucía The horizontal of the position. present nearly position Formation (section 3). These folds disappear as soon as axial could have been caused of the planes by refolding they reach into the incompetent La Vid Formation near beds. original south-dipping Buiza. Generally the strata in this unit are north-dipping North of Geras de Gordon in the valley of the river and overturned with the exeption of the noses of the folds have also been observed. Casares minor They only fold structures.
occur in the immediate neighbourhood of the Bregón
where have deformed the San Pedro fault-contact, they Rozo unit
sandstones. do not show favourite orientation They any The Rozo unit, situated north of the Pozo unit in the in their axial planes. east and north of the Bregón unit further west, consists
of quite complicated structures west of the Bernesga. Pozo unit the The sequence generally gets younger to south. It has The Pozo unit the thrust anticlinal structure comprises been thrust northwards over the youngest rocks of the which has its anticlinal in the Barrios of nose quartzites San Emiliano Formation. East of Rodiezmo two thrusts
Pico del Pozo, west of the river between Bernesga developed, the Rozo thrust to the north and the of Villasimpliz and La Vid. On the map the core the Correcilla thrust cutting obliquely through the Rozo anticline is erroneously indicated as the San Pedro sequence in a SE direction (cf. Evers, 1967). The between these thrusts form Formation, this should be the Oville Formation (Evers, complications an extension
1967). Rocks from Láncara to Huergas Formations have of the intricately folded Correcilla unit of Evers (1967). been found in this unit. These are most striking in the Láncara Formation south of Rodiezmo The front of the Pozo unit is a thrust-fault with the (Fig. 20).
The cross-sections of Fig. 21 an of the Oville sandstones as the oldest sequence against the give impression far Villar del in the structures from east to west. The fault-contact in the Bernesga valley as as change thrusts, 194
Rodiezmo.
of south unit, thrust Rozo
the
of Map-detail
20.
Fig. 21. Cross-sections of the Rozo thrust unit south of the of 20. Fig. Rodiezmo; sections are indicated on map fig. 196
became overturned originally probably south-dipping, at youngest rocks of the Aralla zone. The thrust-front is as advanced of the and an stage orogenesis are now a rule composed of Láncara dolomites but in some
generally north-dipping. In the east the north-dipping places the Láncara Formation is entirely absent and
thrust-front is composed of faulted and folded deposits of Oville shales form the contact with the San Emiliano
the Láncara and Oville Formations. In the the rocks. either side of the thrust west same Dips on are steep, trend- thrust-front is without north. It that south-dipping complicated struc- ing is supposed present occurrences of the
tures. It can be followed as far as the Casares river. The deposits of the Peña de la Plaza are the result of down- followed two major thrusts can be through sections 1 —6 faulting of the original thrust-front. This idea is strength-
(Fig. 21). The complicated structures south of the front ened by the fact that the Láncara deposits in this area
in in it are illustrated Fig. 20 and my opinion is evident show a very irregular folding pattern, whereas such
that they must be interpreted as refolding and simul- irregular folding is completely absent in the original
taneous faulting of already existing thrust structures. thrust-front. The area north of Aralla is also strongly
This faulting has also obliterated the original thrust faulted. The Láncara Formation is imbricated so that the The N-S cross-fault phenomena in some places. (Fig. 20) dolomite member is repeated three times in its contact
between the older Palaeozoic and the Carboniferous San with Carboniferous deposits.
Emiliano deposits is perhaps connected with the second- The igneous rocks in the Carboniferousjust north of the thrust-front also indicate that the shales ary folding and faulting, but the antiformindicated on the were the Emiliano geological map in San deposits is now broken to an extent enough to permit intrusions here.
considered to be nothing else but small scale crumpling Moreover the inlier of Devonian deposits in the Collada
along the fault-contact. de Alonga can only be explained by assuming faulting.
It is obvious that the northern blocks along the The Barrios Formation also plays its special role in thrust-faults downthrown. This well are agrees quite controlling the structures in the Aralla zone. In the
with the idea that from the south the of the formation upthrusting was western part area, where the is relativ-
first tectonic movement, followed by folding and accom- ely thin it conforms to the shape of the structure in the
panying faulting. Faulting probably took place contem- other formations. East of Peña de la Plaza the formation
with poraneously folding. becomes thicker and cross-faulting testifies to the rel- It difficult for the southernmost is to account ative rigidity of the quartzites. Most of the cross-faults
isolated Láncara outcrops in the Oville Formation. In die out in the La Vid Formation. In Pico del Cueto the
these be accounted for my opinion outcrops may by thick Barrios is also folded but even here two faults have
the of faults from assuming occurrence originating developed to enable the much more extreme con-
refolding. It is quite possible for one of these faults to be volutions of the neighbouring beds to be accommodated.
the eastern prolongation of the fault which broke the
the southwest Barrios quartzites more to (see geological Bodón unit It follow the fault- map). was impossible, however, to The Bodón unit was first described by de Sitter (1961) line the Oville between through deposits the Barrios as a thrust-faulted sequence in the area north of and the isolated quartzites Láncara outcrops. Carmenes in the Torio valley. Evers (1967, p. 131) used third thrust level has been A recognized in the Rozo the name for the eastern extension of that area, while
Unit to the south of the others. The displacement is the Bodón unit in this area is the western extension of
Láncara smaller; deposits are in contact with Barrios the original. quartzites. East of the Bernesga it includes all formationsfrom the The geological pattern along the western boundary of Herrería as far as and including the San Emiliano Forma- the Rozo unit along the Casares river is similar to the tion, bounded in the south by the Rozo thrust-fault and of Rodiezmo fault from pattern south (compare section 2, geolo- in the north by the that separates Leónides gical map and section 6, Fig. 21). Here the two Asturides. West of the Bernesga the unit loses its sig- of phenomena, thrusting and refaulting the thrust-front, nificance as a single unit; the area therewill be discussed are clearly expressed. later under the headings Cueto Negro unit and Tesa-Bar-
radal unit.
Aralla zone The front of the unit coincides with the Léon line as
The Aralla the thrust zone is strip north of the Bregón described by de Sitter(1961) and is here a fault contact-
unit in the western part of the area, forming in essence ing the Herrería Formation against the Lena Formation.
the western prolongation of the Correcilla, Rozo and A displacement of about 3.5 km may be estimated, this
Pozo units together. They are represented by a thin being the thickness of the stratigraphic sequence here.
thrust slice, about 2 km wide, less than half the thick- The fault-line between the Herrería and Lena Forma-
ness of the thrusts in the for Beresga valley. tions is badly exposed except a few places where The Aralla first mentioned de isolated zone was by Sitter measurements suggest a steeply north-dipping
(1962, p. 261), who included all deposits between the fault-plane. The front of the Bodón unit is further thrust-front and the Pedroso syncline. Now its southern complicated by small secondary folds near Millaró and is part considered to be part of the Bregón unit. The La longitudinal faults near Busdongo.
Vid limestones and shales form the southernmost The San Emiliano Formation is folded in E-W trend- 197
den The northern ing infraformational folds which are parallel to the main ing WSW (van Bosch, 1960, p. 210). trend. Near Fontun and Barrio small faults have been flank of the anticline apparently has been upthrust by
normal the fault. This have occurred under the found cutting off thin limestone beds. Some are Grajos may
faults and others longitudinal faults, possibly originating influence of the differences in thickness of the Barrios
from the Gayo thrust-front (Evers, 1967, p. 132). Formation on either flank. The Tesa syncline is the most complicated of the four
Cueto Negro unit structures. Its irregular shape is partly due to strati-
The Cueto Negro unit comprises the Celleros anticli- graphic differences on either side of the structure. In the
norium of Herrería deposits west of the main road southern limb the Caldas Formation is missing and La Vid shales underlie the Alba and the between Busdongo and Puerto Pajares. Just north of the directly griotte Ermita sandstones. On the northern Sierra del Cueto Negro, Láncara, Oville and Barrios rocks limb, however, a which faulted outliers. Caldas Formation is The Barrios are exposed are interpreted as well-developed exposed. flank of The faults in the centre of the unit could not be traced Formation, too, is thicker on the northern the
the due The faults and the Caliza de Montaña Formation shows throughout area to poor exposures. syncline, differences in thickness. The unusual thickness of are extensions of the faults in the Babia Baja unit (van great shales here be due den the of the Robledo the La Vid are likely to to minor Bosch, 1969, p. 208), e.g. core and Normal has also deform- anticline and the eastern prolongation of the Grajos fault folding crumpling. faulting ed the these faults. This in the Villasecino anticline, (see also section 1, geological syncline as it was displaced by have the small-scale map). In the Cueto Negro Unit the Robledo fault is faulting might accompanied sec-
which took on the northern limb of splayed into three or more E-W running longitudinal ondary folding place the north of Pico Tesa. The axial planes of faults. From the Serronal syncline only outliers of the structure,
left. these folds are N-S orientated. youngest deposits in the Herrería Formation are
In the southern limb of the structure small-scale The section through the Celleros area (geological the limestones of the Caliza de has been constructed from measure- folding is found in map, section 2) of these folds S-N line Los Celleros Montaña Formation. The on ments along a running across as asymmetry
the limb of the structure to folds. well as from measurements in the railway tunnel east of points parasitic
Marcos it. In contrast to the concept presented here,
(1968) assumes that the Láncara, Oville and Barrios León line
Formations in the centre of the Cueto Negro unit are De Sitter (1962, p. 255) defined the León line as a autochthonous and tectonic window. Thus exposed as a lineament separating the Asturides from the Leónides. Herrería assumed to be of a deposits are part nappe The most characteristic function seems to be that of the thrust from south to north over the Cueto Negro unit. divide between the distinctive stratigraphic sequences of
In this he derives a horizontal thrust-movement over way the two blocks. In de Sitter's opinion the present fault which for a distance of nearly 10 km, is exceptional between the Lena Formation and Lower Palaeozoic
thrust-faults in the Leónides. The largest ever occurring rocks represents the León line in this area and we have recorded have of not more than 4 km. a displacement to consider whether the definition is fulfilled.
Marcos' fails to an for the concept provide explanation The fault separates Westfalian C deposits of the between this and the autoch- connection nappe probable Asturian Coal Basin from Lower Palaeozoic deposits in thonous nature of the Babia unit. Therefore this fault has Baja the southern part. a minimum
throw equal to the thickness of the stratigraphic Tesa-Barradalunit between the Herrería Formation and the Lena sequence unit of The Tesa-Barradal unit is the most northwesterly Formation, estimated at about 3.5 km.
of and two the Leónides and consists two synclines In the field the fault was often difficult to follow anticlines and section Its (geological map 1). eastern except where the Láncara Formation occurs next to it. boundary is the Herrería anticlinorium, its southern In the area between Millaró and Puerto Pajares the fault border the Rozo thrust-fault and its northern border the between the Lena and Herrería Formations was always The fault between the Láncara and Lena Formations. found to occur in small topographic depressions in the their structural elements have been described in continu- ridges between the N-S valleys. The mapping indicated the den Bosch ation to west by van (1969). that the fault-plane is vertical nearly everywhere near the anticline the south The Robledo in is symmetrical surface (geological map, sections 1—3 ). Since the fault is with axial The axial a WSW-ENE trending plane. nearly always found between the Lena and Láncara
Robledo fault has clearly been caused by lack of space in Formations the structure is similar to that found in all
the of the anticline. To the north the Serronal would reasonable core the thrusts of the Leónides and it seem syncline also trends ESE-WNW and plunges west. Its to include it with them. axial from the plane starts to crumple Formigoso nose In the Leónides the thrust blocks were in most cases
onwards to the east. The axial plane is north-dipping. related to broken anticlinal structures which developed
North of the Serronal syncline (section 1) lies the during folding. This relation seems to be confirmed here
den Villasecino anticline which is broken in its core by the (van Bosch, 1969, p. 217).
wrench fault trend- noticed that the fault was not Grajos fault, originally a left lateral Marcos, (1968, p. 82) 198
only a thrust-fault but was later active as a normal fault trending Corallón anticline takes a central place. The as well. Near Millaró the fault cuts through the Lower smaller structures change from an E-W direction in the
Palaeozoic rocks far the Barrios Formation. Here it as as east to a NW-SE direction between Millaró and would seem that the fault was active even after the Camplongo, while further west near Pajares two inter-
of The movements caused folding the Lena rocks. by the fingering folds could be found with N-S and E-W trends normal led this faulting possibly to upcutting near (Fig. 22).
Millaró and may even have caused small-scale upthrust- ing from the Carboniferous deposits onto the Lower
Palaeozoic succession. If the fault is a thrust-fault the age of thrusting can be derived by comparing the ages of the youngest rocks on either side of the fault. In the thrust block the youngest deposits are those of the San
Emiliano Formation in the area west of Villamanin.
Moore et al. (1971, p. 307) concluded that the age of the youngest San Emiliano Formation is Westfalian C.
Llopis Liado (1955) and Wagner (1962) on the other hand decided upon a Westfalian C and possibly West- falian D age for the Lena rocks just north of the fault-contact. This implies that thrusting took place after the Westfalian C or even D.
The normal faulting cuts off the folded structures of the Carboniferous in the area between Millaró and
Camplongo which must have taken place after the fold- of these least the This ing rocks or at at same time. folding is generally considered to be a result of the
Asturian folding phase, which is of pre-Stephanian B time in this area.
The faulting along the León line has also been thought of of to relate to the presence the Stephanian B-C basins in the areas to the east (Evers, 1967 and Rupke, 1965).
In the stratigraphic successions in the northern
Leónides major differences in the facies of the San
Emiliano and the Lena Formations do not exist. A small difference is that in some places in the Lena Formation Fig. 22. Model of interfingering structures in limestones of the the facies is continental as is the locally proved by Lena Formation near San Miguel del Rio. presence of coal measures north of Arbas del Puerto, whereas in the San Emiliano Formation all deposits are marine. In the older Palaeozoic rocks there is a sudden STEPHANIAN COAL BASINS change in the development of the Caldas Formation into the Tesa-Barradal South of the Villasecino unit. anti- The Stephanian Coal Basins occur in two segments of cline and the northern limb of the Tesa the in syncline the southern part of the map area. They are the Cinera-
Caldas rocks are well-developed, whereas between the Matallana Coal Basin in the east and the Magdalena Coal these rocks absent. This could be two structures are an Basin in the west. The western part of the former is indication that there used to be a facies boundary due to represented by the Prado deposits roughly between the differences in the sedimentary basin. North and southof Bernesga and Torio rivers, but also includes isolated the Herrería anticlinorium there indications of are no outcrops near Valle de Vegacervera, north of Vega de facies differences. Gordon and between Pola de Gordon and Barrios de
structural indications The stratigraphic and seem to Gordon. An intensive study of the stratigraphy and the that the León line in this is of little detailed structural of this coal suggest area sig- pattern basin has been nificance. If it the Villasecino made and present, may run through by Wagner (1963 a b, 1964, 1972) and Wagner anticline. & Artieda (1970). Van Amerom & van Dillewijn (1965)
and Evers (1967) also dealt with the structural pattern
of this coal basin.
CENTRAL The coal basin lies folded ASTURIAN COAL BASIN unconformably upon a
substratum of pre-Westfalian rocks. It was gently folded
The Central Asturian Coal Basin lies mainly north of the during a post-Stephanian and pre-Triassic (Table 3)
but small of southern- The coal basin is E-W present map area a section the orogenetic phase. an trending most part is represented by the rocks of the Lena synclinorium of which the synclines are well-developed, Formation. of whereas the anticlines deformed due In the eastern part this area the E-W are strongly to 199
the of the and has been faulting. The synclines occur in depressions simple; no complicated folding faulting south beds horizontal but pre-Stephanian surface which developed after the folding observed. In the extreme are of the Leónides. These structures are shown in the to the north this changes into vertical and even overturn-
the Palaeozoic rocks. sections 10-13 of App. 4. Details of such N-S sections ed beds against the of the between the are presented by Wagner & Artieda (1970, figs. 34-38). In eastern part area Bernesga and Torio the is fault and The northern boundary is often an unconformity rivers contact partly a partly followed The fault between Robledo and by a mass of quartzite and limestone conglom- an unconformity. occurs
for the of La Mountain and Candanedo de Fenar with a subvertical to north- erates except part Campa steeply 12 and Near the river Bemesga, where a fault-contact is undeniable. dipping fault-plane, (sections 13, App. 4). Solana the vertical throws Along the southern border no conglomerates have been fault-plane north-dipping of Cuevas rocks Voznuevo beds. The observed. This may be an indication that the supply against south-dipping between north- and coarse sediments came from the north, where the con- straight contact south-dipping strata glomerates have been found, whereas in the south fine on both sides of the contact-plane suggests a fault-plane. The in the Voznuevo beds between material was deposited. The fining of the material to the dips strongly varying overturned north and 40° south short south may indicate that there was a certain asymmetry dips dips over a
that not restricted to in the basin. distance, also suggest faulting was
of but that move- The structural pattern within the basin is one one movement only post-Cretaceous took several concentrically folded synclines with E-W trending sub- ments place probably during periods.
Candanedo and de Fenar the contact vertical axial planes. The shape and width of the syn- Between Brugos
in to be which has clinal structures are governed by the depressions the is supposed an unconformity (Fig. 11), been the observations of fossil weathered pre-Stephanian surface which on their part depend on proved by a faults the the of the Carboniferous imme- the rock type exposed and the present in zone at top deposits The of the Lower Palaeozoic rocks. It is supposed that these main diately near the contact. present shape struc- de Fenar lie faultsof the substratum continueinto the Prado rocks at ture near Brugos is supposed originally to as
fold-thrust Here the Car- places where the present anticlinal structures are a uplift system (Berg, 1962). the Voznuevo exposed. boniferous deposits were thrust upon rotated overturned During the detailed mapping of Wagner and Artieda Formation, the latter was into an of faults These the (1970) another set was recognized. position by same event. The the of de Fenar generally south-dipping, small thrust-faults are only sig- straight contact to west Brugos the Formation and the Palaeo- nificant near the present surface. Apparently they do between Vegaquemada
considered to be fault. The contact between not reach the base of the coal basin (Wagner & Artieda, zoic is a
1970, cf. figs. 34-38). conglomerates of the Prado Formation and Voznuevo
The Magdalena Coal Basin is largely comparable with beds west of Llanos de Alba is probably the original The relation- the Ciñera-Matallana Coal Basin. An unconformable con- unconformity which was faulted later on.
northern between the two faults which strike into the tact has been found in many places along the ship basal sedi- from and west is not since border and conglomerates are usually the Bernesga valley east clear,
means of their trends. ments of the Prado rocks. From Olleros on to the east there is no obvious connection by
of the Prado rocks. the in the Carboniferous and Devonian conglomerates are the only remnants Since structures Robla-Alcedo-Llanos de Alba West of the Luna river the nature of the northern con- rocks in the La triangle are
fault it be that other complexities tact is questionable but a along an original uncon- complex can only expected concealed beneath the recent deposits of the formable contact seems to be the most appropriate must lie
is To the west as far as Santiago de las explanation. A small part of the southern border Bernesga river. Villas the Southern Border Fault is well-exposed in exposed south of Carrocera which is undeniably faulted. In the absence of Cainozoic sediments Mora deposits can be seen to have been thrust upon the various places. further definite evidence for this of fault- Prado rocks. The basin itself is a syncline with a sub- west no type
could be In fact no at all couldbe vertical axial plane in the area east of the Luna river. ing expected. faulting
into detected in the Prado Formation and it is thought that West of the Luna river the basin is split up two fault taken the subbasins separated by Mora rocks. The subbasins show the relative movement was up again along the Prado and Mora rocks properties symmetric to the Mora rocks in the centre. thrust-fault between more
to the south. The movement of this Both basins have an unconformable contact with the than one kilometre
lifted the southern block in contrast Mora rocks in the centre and both basins have a faulted thrust has upwards
movements further east. border at their outer side. On both occasions the Mora to the
the Prado rocks due to that Other evidence as to the nature of the faulting has rocks have been thrust upon 1 and from observations. A faulting (geological map, sections 2). been gathered geophysical gravity
carried out a team from the Geological survey by of the of Leiden has yielded inter- THE MOUNTAIN BORDER AREA Institute University pretable results owing to the density contrast between
the Palaeozoic the hand and rocks on The Mountain Border Area includes the post-Carbonifer- on one younger The sediments in the south. The is the other (int. rep. 1967). interpretation implies a ous structural pattern 200
structural hinge near Carrocera, east of which fault (p. 177). The calculated fault-plane in the Torio valley blocks tended down the while of much to drop to south, west dips more steeply (± 60°) and the fault con- fault blocks down the north. The surface almost the it dropped to sequently outcrops in same place as is given of the Precambrian rocks is supposed to dip quite by the gravity interpretation (Matallana Estación, Evers, strongly (20—30°? ) to the east. In the Bernesga valley 1969, p. 146). The fault between Llanos and Brugos is fault the interpretation yields the major near Alcedo therefore considered a minor feature, perhaps a splay or with 35 the north. a fault-plane dipping to This is a synthetic fault to the major border fault somewhat to further confirmation of the fold-thrust type of structure the north. of suggested from the area Brugos and Candanedo
CHAPTER IV
GEOLOGIC HISTORY
The Precambrian Mora rocks were deposited under South of the Alba syncline the fades of the Huergas unstable conditions den and (van Bosch, 1969, p. 143) and the Portilla Formations varies from west to east, of of the Galician Pre- which makes difficult their are composed erosion products it to recognize presence. A cambrian The structural of the affected the of the (Matte, 1968b). pattern comparable change deposition
Mora rocks differs from the structures of the Palaeozoic Nocedo Formation, so that the clastic sequence resting
between the two the Santa Lucía Formation in this sequence. An angular unconformity unconformably upon demonstrates that the Mora rocks folded has been renamed Piedrasecha Formation. sequences were area before the deposition of the Palaeozoic rocks. Along the north flank of the Alba syncline the
During the Lower Palaeozoic sedimentation took Nocedo Formation is fully developed but north of the
place under shallow marine conditions throughout the Sabero-Gordón line it is absent. This means that at least
whole area. Silico-clastic material formed the Herrería, at the time of deposition of the Nocedo Formation Oville and and Barrios Formations, only during the (approximately Frasnian-Famennian) the northern area of the Láncara Formation deposition (Lower-Middle was uplifted. At the end of the Devonian the irreg- did carbonate dominate. the Cambrian) deposition In ularities causes by differentialsubsidence had been level- Leónides the do show stratigraphic successions not great led off by the deposits. The sea then transgressed again, differences in thickness. flooding the entire area and allowing deposition of the After the of the Formation deposition Formigoso Ermita Formation upon the Nocedo Formation in the differences in the further (Middle Silurian) stratigraphic develop- south, upon the Portilla Formation north near
ment be detected in that thicknesses of individual can Pola de Gordon and Beberino, upon the Huergas Forma-
units to thin towards the N. This vary trending probably tion north of Villar and upon the Caldas and La Vid
means that the rate of sedimentation in the S was greater Formations in the northern Leónides. due to more rapid subsidence. Deposition could still While north of the Sabero-Gordón line differential
match the subsidence that sedimentation could take so subsidence followed by a transgression and sedimenta- under shallow marine conditions. place tion of the Ermita Formation provides an adequate the of the Santa Lucía Formation During deposition explanation of the variations in the stratigraphic succes-
the differences between the southern and northern areas when this sion, there remains a problem area is com- became accentuated. the south the more In Santa Lucía pared with adjacent regions. Evers (1967) and Rupke limestones under shallow were deposited marine con- (1965) described the deposition of the transgressive ditions. the the of In north, however, proximity a Ermita Formation not only upon Devonian deposits but coastline resulted increased of clastic in an supply also on older Palaeozoic rocks, e.g. the Oville Formation. material forming the Caldas Formation, which is partly As the facies and the thickness of these older rocks do
the of differ from south differential subsidence equivalent the Santa Lucía Formation. There was not to north, could be the of the this even an area of non-deposition in the NE part. not cause disconformity in case.
The marine conditions did not change during the Therefore it is suggested that uplift and erosion of the older Palaeozoic had taken in the north before the following part of the Devonian Period. Alternations of place
the supply of silico-clastic material and carbonates deposition of the Ermita Formation. De Sitter (1962)
continued in the of the called this the to exist, resulting deposition uplift Bretonic uplift, suggesting a
Huergas shales and the Portilla limestones. Later struc- relation with the uplift at the end of the Devonian in
tural movements caused the absence of these rocks in France. It is suggested here that both phenomena, dif- the northern of the part area. Huergas rocks occur as ferential subsidence and emergence, took place in this
north as the Correcilla unit (NE of Villar del Puerto). area.
The Portilla Formation is exposed as far north as The Sabero-Gordón line apparently acted as a hinge-
Beberino. line these the strati- during movements, as no gaps in 201
graphic succession can be found further south. As we occur in a subsiding basin in which the unstable condi- basical structural line shall see below, this also strongly tions governed the kind of sediments deposited. Rapidly influenced the kinds of that conditions also the structure were to develop at changing are proved by great variety
a later stage of deformation. in fossil assemblages. This subsiding basin in the north- the In the Tournaisian Vegamián Formation was ern part of the area ends the sedimentary history of the
deposited in depressions under shallow marine con- Leónides.
ditions as a condensed sequence with synsedimentary In the Asturides the Central Asturian Coal Basin had
erosion and chert deposition. already started to develop from the Namurian C onwards
During the Visean the Alba Formation was deposited (Sjerp, 1967; Julivert, 1957). Part of the deposits in this all and the sedimentation conditions were the same over basin have been named Lena Formation. The rate of
differences the area. In the Namurian, however, again in sedimentation increased rapidly after the Westfalian A,
facies developed and once more the Sabero-Gordón line comparable to what happened in the Leónides. In both
acted as a hinge-line. areas a minor hiatus represents the lower part of the and the thickness after- South of it the Cuevas Formation begins with a Westfalian, greatest developed
clastic sequence, followed by black bituminous, thin- wards. The deposition of the Lena Formation, which, as
layered limestones. After the deposition of the black far as the present area is concerned, continued till the
to end of the Westfalian D; a of limestones, which may point quiet conditions, coarse period non-deposition
clastic detritus the onset of unstable followed in which the Leónides and the Asturides prove conditions, were folded possibly due to oncoming tectonic activity. The succes- (Asturian folding phase). After this continental intramontane coal sion of the Cuevas Formation ends with deposits com- folding basins B time. The strike of parable with the basal clastic part. developed during Stephanian
North of the Sabero-Gordón line black limestones these basins agrees with the general trend of the folded
comparable with those of the Cuevas Formation overlie belt, e.g. an EW orientated longitudinal axis of the
the Alba Formation. They are followed by a more basins. With a continental facies of the Prado Formation,
limestone All these lime- unconformably older Palaeozoic, sedi- massive and coarser sequence. resting upon
mentation came to an end in this area. A stones are included in the Caliza de Montaña Formation. folding phase
Clastic sediments, considered to belong to the San after the Stephanian B and before the Triassic folded the of folds. EmilianoFormation, are found on top the limestones. Prado rocks into regular concentric establish the of the the of Cretaceous It was impossible to age various At end the Upper a marine trans- clastic sequences upon the limestones in the Alba and gression approached this area along the folded belt from
Pedroso synclines. As they are the youngest deposits in the east. The sandstones of the Voznuevo Formation
these structures their original thickness is unknownbut a form the continental counterparts of marine deposits,
minimumof 200 could be measured. towards the m successively younger west. In this area only North of the Rozo unit the San Emiliano Formation small limestone beds have been found marine exposed as details. The of the clastic provides more age youngest far west as Brugos de Fenar. After sedimentation of the rocks here established al. is by Moore et (1971) as West- sandstones and thin limestone a subsequent regression
falian C, whereas the top of the Caliza de Montaña changed the deposition conditions eventually resulting in limestones is of Namurian B age. During the Westfalian the continental facies of the Vegaquemada Formation.
A these authors suggest a break in the sedimentation. At that time the morphogenetic uplift of the Canta- The relation of between Namurian B deposition-time brian Mountains started. This uplift and the consequent and Westfalian and the of about of C tickness 2,500 m Southern Border Fault resulted in the Vegaquemada, the San Emiliano Formation gives us a sedimentation Candanedo and Riacos Formations due to fluvial and rate of approximately 0.25 mm/year. This could only torrential activity along the hilly southern border.
CHAPTER V
CONCLUSIONS
Most of the structures in the area of the present map Herrería. The fold structures in the Mora deposits are sheet originated in the Hercynian orogenic Period cut off by this unconformity in several places. Thus a
(Table 1), which mainly caused the fold belt of the pre-Hercynian deformation must have caused these which differ from the Palaeozoic rocks in this area. The Cantabrian Mountains structures, moreover Hercynian which deformation in that now expose this folded belt are due to a morpho- they are accompanied by slaty genetic uplift during the Alpine Period. cleavage in the fine grained sequences and by fracture The Narcea Anticlinorium in the In the is is one of the Hercynian cleavage greywackes. most cases cleavage axial folds off the fold-structures in which we find the Precambrian Mora an plane cleavage belonging to cut by of the rocks in the core of the anticlinorium separated by an unconformity. The folds in our part area mainly from Palaeozoic unconformity the possibly Palaeozoic rocks of the trend E-W parallel to the structures, giving no 202
their fold the indication as to Hercynian or pre-Hercynian origin. mental line. On the contrary the structures and
Near the unconformity greywackes are mostly exposed, faulting here are strongly related to the lithologic often showing fracture cleavage. The same direction of properties of the formations exposed, resulting in box- cleavage has been observed in the Herrería sediments on folding of the Santa Lucía Formation (e.g. San Mateo the other side of the unconformity. Within 30 m from structure) and strike-faulting along the limestones of the the unconformable contact the fracture cleavage of the Portilla and Santa Lucía Formations. Due to their
Herrería Formation generally ends. The fracture cleavage incompetency the la Vid shales almost everywhere form in the Herrería could therefore be regarded as a reactiva- the core of the box-folds. tion of the pre-Hercynian cleavage during the Hercynian The deformation of the Leónides north of the deformation, especially because the cleavage has only Sabero-Gordón line started with the thrusting of the been observed near the unconformablecontact. units discussed above during the Sudetic initial folding
The Hercynian deformation is mainly expressed in phase. The main folding during the Asturian folding the structures of the Leónides. Within the Leónides phase brought the thrust units in their generally over-
turned In the different structural patterns can be distinguished. The steep north-dipping position. present area there indications that the worked Sabero-Gordón line separates intensely folded rocks in are no two phases the that the south from folded thrust-structures in the north. separately. On contrary it seems quite likely
South of the Sabero-Gordón line a subsiding basin thrusting and folding are progressing developments,
filled the Palaeozoic the Namurian it for them to be in one was up during up to making possible put together or even Westfalian. A well-developed syncline, the Alba particular orogenetic phase. Post-thrusting structures are syncline, resulted from the Hercynian orogenesis. In the observed in the minor folds within the thrust units. They also core of the syncline Cuevas limestones have been de- probably developed due to gravity-stress. Gravity of thrust-front of the formed by intensive minor folding. From the asymmetry influenced the deformation the of these minor folds to the main structure most of them Aralla Zone, giving rise to imbricated structures of the of seem to be parasitic, though not parasitic in the sense as Lower Palaeozoic rocks with shales and greywackes from given by de Sitter (1964, p. 282) as they originate the San Emiliano Formation.
steeply dipping (>45°) strata. The fault that separated the Lena Formation from of limestone northof the northern of the The core the synclinal structure the Lower Palaeozoic in part area is
Piedrasecha contains minor folds which show an asym- considered to be the boundary between Leónides and
for folds south of the This of this fault offers metry expected core. may Asturides. The structural aspect no the the south fundamental León line imply that synclinal axis migrated to reason to relate it to the as during folding. The migration of the fold-axis could have defined by de Sitter (1962). caused of the fundamental Sabe- been through activity The structural pattern of the Asturian Coal Basin ro-Gordón line at the northern border of the basin. This shows an interference of E-W trending and N-S trending line caused differential subsidence the which due already during structures (Fig. 22), is probably to a general and the of the Upper Devonian even during deposition bend of the Palaeozoic basin from an E-W direction to a Renewed the sheet Cuevas Formation. activity during folding N-S direction west of the present map area could easily have been the cause of tilting of the north- (Asturian Knee). border of the basin and the of Leónides N-S the fold ern consequent migration Structures in the trending (e.g. the fold-axis to the south as well. North of Olleros de structures in the Rozo unit, Fig. 20) are probably also
fold indicates that due the of this basin. Alba a conjugate set (Fig. 16) during to bending
the folding minor folds developed which are obviously Intramontane coal basins developed in the lower parts due of the folded Leónides and are folded concentrically to a vertical stress-field, e.g. gravity. The migrating synclinal axis of the major syncline and during the post-Stephanian-pre-Triassic (Saalic) phase. the of fold it be The of the Cantabrian Moun- occurrence conjugate sets prove to present configuration obtained the dangerous for the conclusion of folds being parasitic to tains was finally during morphogenetic be based the Savian which the Palaeozoic belt. on symmetry concept. phase, uplifted Along unconformable Cainozoic The structures of the Sabero-Gordón fault-zone do the Southern Border Fault
rocks are partly thrust during this uplift. not show any influence of movements along a funda- 203
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