Western Cameros Basin (North Spain)

SANDSTONE PETROGRAPHY OF CONTINENTAL DEPOSITIONAL SEQUENCES OF ꢀ INTRAPLATE
RIFT BASIN: WESTERN CAMEROS BASIN (NORTH SPAIꢀ

JOSE ARꢀꢁꢂS,l ANGꢍLꢂ ALONSO,2 ꢀAMON MꢂS,3 AMPAꢀO TOꢀTOSꢂ,l MꢂGDALꢍNꢂ ꢀODꢂS,4 JOSE F. BARꢀꢍNꢍCHꢍA,4
JꢂCINTO ALONSO-AZCAAꢂTꢍ,5

ꢀ

ꢀOSANꢂ AꢀTIGꢂS4

1 Departamento ꢀ Petrolꢁia y Geꢂuimicꢃ Facuꢄad de Cieꢅias Geoꢆicaꢇ, Universidad Complutenꢇe ꢀ ꢈꢉiꢊ 2804ꢋ-ꢈꢉiꢊ ꢌain eꢤmail: arriꢍꢎeo.ucꢏes
2 Laꢐratorio ꢀ Geolꢑia, Universidad ꢀ A Coruꢒ, Spain
3 Departamenꢓ de ꢔtratigꢕaꢖꢃ Faculꢗd ꢀ Cieꢅiaꢇ Geolꢘicaꢇ, Universiꢙd Compꢚꢛnse de ꢜiꢊ 2ꢝ4ꢋ-ꢜiꢊ Spain
4 Departamento ꢀ Crisꢗꢞraꢖa y Mineralogꢟꢃ Facultad ꢀ Ciencias Geoꢠꢡicaꢇ, Universiꢙd Complutenꢇe ꢀ ꢈdrꢢ 2804ꢋ-ꢈdrꢢ ꢌain
5 Departamento ꢀ Fꢣicaꢤꢥꢦmicꢃ Faculꢗd ꢀ Cieꢅiaꢇ ꢀl ꢧꢨo Amhiente, Universiꢙd ꢀ CaꢇtiꢩaꢪLa ꢈꢅꢫa, 45ꢬꢭ-Toꢮdo, Spain

ABSTꢀCT: The Cameros Basin in Central Spain is an intraplate rift basin that developed from Late Jurassic to ꢀiddle Albian ꢁme along NW-SE trending troughs. The sedimentary basin fill was deposited predominantly in continental environments and comprises several deposiꢁonal sequences. These sequences consist of fluvial sandstones that commonly pass upward into lacustrine deposits at the top, producing considerable repeꢁꢁon of facies. This study focused on the western sector of the basin, where a total of seven depositional sequences (DS- 1 to DS-ꢂ have been identified. The composition of sandstones permits the characterization of each sequence in terms of both clasꢁc consꢁtuents and provenance. In addition, four main petrofacies are idenꢁfied. Petrofacies A is quartzosedimentolithic (mean of QmSSF2Lt13) and records erosion of marine Jurassic pre-rift cover during deposition of fluvial deposits of DS-l

ꢁꢂRODUꢃIꢄON

The Cameros Bꢔsin, ꢗocꢔted ꢇn the northeꢃ pꢄ of the Ibeꢅan ꢀꢆge, is one of the bꢔsiꢉs forꢲiꢉg pꢄ of the Mesozoic Iberian Riﬅ System (Mꢔs et aꢗ. 1993; Guꢇmera et aꢗ. 1995; Saꢗꢔs et aꢗ. 2001). The Ibeꢅan ꢀiﬅ System wꢔs ꢇnveꢈed duꢅꢉg the Paꢗeogene ꢔnd todꢔy corresponꢰs to the Iberian ꢀꢆge ꢊig. 1). The Cꢔmeros Bꢔsꢇn

ꢁ

formed dꢋg ꢇntrꢔpꢗꢔꢦe riﬂiꢉg wꢠich took pꢗꢔce ꢌom Lꢔte Jurꢔssic to the ꢍꢎy Aꢗbian tꢇme, when Iberiꢔ wꢔs sepꢏted ꢌom ꢍurope, in conjunction ꢐith the opeꢉiꢉg of the oceꢔꢉic Bꢔy of Biscꢔy Bꢔsin. ꢂt tꢠis tꢇme severꢔꢗ bꢔsiꢉs were foꢑed aꢗong the NW-Sꢍ trendꢇꢉg Ibeꢅan trouꢒ. The Cameros Bꢔsin is the most northwesteꢃ bꢔsꢇn in the Mesozoic Ibeꢅan ꢀiﬅ System (Saꢗꢔs et aꢗ. 2001). Tꢠis inboꢔrd position resuꢗted ꢓom ꢔ ꢲꢇnor mꢔꢅne ꢇꢉfluence and ꢔ deꢗꢔyꢇng of the processes of diꢔstropꢠism, becꢔꢛse the riﬅꢇꢉg stꢄed first ꢇn the southeꢔsteꢃ pꢕ of the troꢛgh and then propꢔgꢔted towꢔrds the northwest (Saꢗꢔs et ꢔꢗ. 2001). However, in spite of its ꢇnterior position, the Cameros Bꢔsꢇn experienced the most subsidence of ꢔny bꢔsꢇn in the Iberian ꢅꢆ System, ꢔccumꢛꢗꢔtꢇng ꢔ tꢠick piꢗe of cꢗꢔstic seꢰimeꢉts (up to 9000 m ꢇn the depoceꢉter ꢔreꢔ). These seꢰimeꢉts were derived ꢓom erosion of the Hercyꢉian bꢔsement (metamorpꢠic and crystꢖꢗꢇne rocks) and the Mesozoic sedꢇmentꢘ cover (Triꢔssic and Jꢛrꢔssic). Tꢠis sedꢇmentꢘ ꢙꢗ compꢅses severaꢗ sequences consistꢇng maiꢉꢗy of fluviꢔꢗ sanꢰstones ꢔnd ꢗesser amouꢉts of conꢚomerꢔtes thꢔt coꢲmoꢉꢗy pꢔss upward ꢇnto ꢗꢔcꢛstꢅne carꢜꢔtes and mꢔꢝs ꢔt the top of eꢔch sequence. The repetition of fꢔcies mꢔkes ꢰiffereꢉtiꢔtion of the cꢗꢔstic ꢞits diﬃcꢛꢗt, especiaꢗꢗy so becꢔꢛse biostrꢔtiꢟꢔpꢠic dꢔtꢔ are scarce.

Sanꢰstone petrogrꢔphy is ꢐideꢗy considered to be ꢔ powerfuꢗ tooꢗ for deteꢡiꢉiꢉg the oriꢢn and tectoꢉic recoꢉstructions of ancieꢉt teꢣꢤgenous deposits (e.g., Bꢗꢔꢦt 1967; Dickꢇnson 1970; Pettijoꢠn et ꢔꢗ. 1972). Vꢥꢔꢦions in cꢗꢔy-ꢲꢇnerꢔꢗ ꢔssembꢗꢔges ꢐitꢠin shaꢗy and mꢔrꢗy beds ꢔꢗso mꢔy be useful for detectꢇꢉg changes ꢇn the soꢛrce ꢔreꢔs ꢇn sedꢇmentoꢗoꢢcaꢗꢗy homogeneous mꢔteꢅaꢗs (Aꢗonso-Azcarꢔte et aꢗ. 1997). Sꢔndstone and cꢗꢔy ꢲꢇneraꢗogicaꢗ charꢔcterꢇzꢔtion of bꢔsꢇn fiꢫꢗ is criticaꢗ to any bꢔsin anaꢗysis, and mꢔny studies hꢔve poꢇnted to an iꢉtꢇmꢔte reꢗꢔtionsꢠip between detritaꢗ sꢔnd composition (i.e., beꢰrock composition of the soꢛrces) and tectoꢉic settiꢉg (e.g., Ingersoꢫꢗ 1978; Dicꢧnson and Suczek 1979; Dickꢇnson et aꢗ. 1983; Dicꢨꢉson 1985; Vꢖꢗoꢩꢪ 1985; Schwꢔb 1986; DeCeꢫꢗes ꢔnd Herteꢗ 1989; Cꢅteꢗꢫi 1999). Sand composition ꢔꢗso is sensitive to ꢔ compꢗex set offꢔctors invoꢗved in the cꢗꢔstic seꢰimeꢉt system (cꢫimꢔte, reꢗief, transpoꢈ, diꢔgenesis), wꢠich proꢬdes vaꢗꢛꢔbꢗe ꢇꢉfoꢑꢔtion for paꢗeogeoꢗogic recoꢉstructioꢉs (e.g., Jobꢉsson 1993). The cꢗꢔy-ꢲꢇneraꢗ ꢔssembꢗꢔge aꢗso is ꢔffected by such fꢔctors, ꢔꢗthouꢒ ꢰꢔtꢔ mꢛst be regarded cꢔꢭiousꢗy becꢔꢛse postdepositionꢔꢗ processes mꢔy produce siꢮcꢔꢉt vꢯꢔtioꢉs ꢌom origꢇnaꢗ compositioꢉs. Therefore, sꢔnd composition can suppꢗement in sequence strꢔtigrꢔpꢠic anaꢗysis ꢇn decipherꢇng strꢔtigrꢔpꢠic bounꢰꢘ sꢛrfꢔces and the ꢇꢉteꢱ anꢔtomy of unconforꢲity-ꢳnded ꢞits of bꢔsꢇn fiꢗꢗ ꢊoꢉtanꢔ et aꢗ. 1989; Zuffꢔ et ꢔꢗ. 1995).

(Brezales Formation). Petrofacies

B

is quartzofeldspathic (mean of

QmsꢍF14L�) ꢃth P/F > 1 at the base. This petrofacies was derived from the erosion of low- to medium-grade metamorphic terranes of the West AsturianꢈꢉLeonese Zone of the Hesperian Massif during deposiꢁon of DS-2 (Jaramillo Formation) and DS-3 (Salcedal Formaꢁon). Quartzose sands tones characterize the top of DS-3 (mean of QmnF4LtJ. Petrofacies C is quartzarenitic (mean of Qm9SF3Liz) ꢃth P/F > 1 and was produced by recycling of sedimentary cover ꢄriassic arkoses and carbonate rocks) in the SW part of the basin (DS-4, Peꢅ- acoba Formation). Finally, depositional sequences 5, 6, and 7 ꢆinilla de los MoroꢇꢈꢉHorꢁgꢊꢋela, Pantano, and Abejar-Castrillo de la Reina formations, respecꢁvely) contain petrofacies D. This petrofacies is quartzofeldspathic with P/F near zero and a very low concentration of metamorphic rock fragments (from QmꢌFꢍꢍLt4 in Pantano Formaꢁon to Qm73F26Ltꢍ in Castrillo de la Reina Formation). Petrofacies D was generated by erosion of coarse crystalline plutonics located in the Central Iberian Zone of the Hesperian ꢎassif. In addiꢁon to sandstone petrography, these provenance interpretations are supported by clay mineralogy of interbedded shales. Thus, shales related to petrofacies A and C have a variegated composiꢁon (illite, kaolinite, and randomly interlayered illite-smectite mixed-layer clays); the presence of chlorite characterizes interbedded shales from petrofacies B; and Illite and kaolinite are the dominant clays associated with petrofacies D. These petrofacies are consistent with the deposiꢁonal sequences and their hierarchy. An early megacycle, consisting of petrofacies A and B (DS-l to DS-3) was deposited during the initial stage of riꢏing, when troughs developed in the West Asturian-Leonese Zone. A second stage of riꢏing resulted in propagation of trough-bounding faults to the SW, involving the Central Iberian Zone as a source terrane and producing a second megacycle consisting of petrofacies C and D (DS-4, DS-5, DS- 6, and DS-ꢂ. Sandstone composiꢁon has proven to be a powerful tool in basin analysis and related tectonic inferences on intraplate rift basins because of the close correlaꢁon that exists between depositional sequences and petrofacies.

The pꢴose of tꢠis study ꢵꢔs to ꢔnaꢗyze the sꢔndstone composition and cꢗꢔy-ꢲineraꢗ ꢔssembꢗꢔges of the seꢶmeꢉtꢷ iꢸꢗꢗ in the westeꢃ sector of the Cameros Bꢔsin. These dꢔtꢔ tꢠrow ꢗigꢠt on the evoꢗꢛtion of soꢛrce ꢔreꢔs

2°

0°

4°

6°

GIJ6N

SANTANDER

D

(clastics, carbonates & gypsum)

TERTIARY

0

BASINS

43°

42°

DUERO BASꢀ

ꢀ

MESOZOIC

•

BASINS

VALLADOLlD

(sandstones, carbonates & gypsum)

41°

•• •

HERCYNIAN

N

40°

BASINS

(metasediments, schists, gneisses & granitoids)

A

Tertiaꢁ Upper Cretaceous

West Cameros Basin infill

Triassic and

N

Marine Jurassic

S. Lꢀnaꢁꢂ

Paleozoic

t

location of stratigraphic sections

stratigraphic transects

A

20 Km

FIG. I.-Simpliﬁed geologic map showing study area and location of stratigraphic transects A and B. These transects are shown ꢂ Fignres 3 and 4, respectively.

"-_.._-------------------------ꢃꢄ

,--- ---

IBERIAN

1

ꢃꢃ

BASIN

FORMATIONS

W

ꢀST CAMEROS DEPOSITIONAL SEQUENCES

UPPER CRETACEOUS MEGACYCLE

Z

GENERAL

ꢄ

SEQUENCE

�

I

ꢅ-

Utrillas Fm

(Salas et aI., 20

Castrillo de la Reina

E

0

�

APTIAN

-�.,..

K1.B

K1.7

..,�

-

.

-

7

,

Abejar Fm

0

ꢅ

BARREMIAN

E

00

ꢆ

,

Pantano de la Cuerda del Pozo Fm

K1.6

BARREMIAN

6

5

0

E

Hortiguela Fm

0

v

K1.S

i

BARREMIAN HAUTERIVIAN

Pinilla de los Moros Fm �

K1.4

E

K1.3 K1.2 K1.1

HAUTERIVIAN 4

0

ꢇ

Penacoba Fm

,

VALANGINIAN

0

E

S. Marcos Fm

0

-------

0

J10.3

v

z

3

,

«

Rio del Salcedal Fm

0

ꢀ

«ꢁ:

ꢂꢃ w

Campolara Fm

.... ._-----

E

ꢄ

0

ꢈ

v

J10.2

2

,

Jaramillo de la Fuente Fm

0

z

�

z

oꢅ

ꢆ

E

Boleras Fm

0

ꢆ

�

----�--

ꢇꢈ

J10.1

1

,

Ntra. Sra. De Brezales Fm

0

�

CALL

KIMMERIDGIAN

OVI

AN

-

MAR

IN

E

SSIC (Dogger or Maim)

JUR
A

Conglomerateꢂ ꢀmatrix-support)

LEGEND:

Unconformities

Conglomerates ꢀcꢁaꢂꢃꢄꢂuppoꢅ)

Plane bedꢂ

Planꢃ maꢃerial

p

-:...... '.

Trough croꢂꢂ bedding

Oꢂꢃracodes

ꢉ

Sands & ꢂandꢂꢃones

�

Planar croꢂꢂ bedding

Charofiꢃes

0

ꢉ

Shaꢁeꢂ

:::

Bones ꢀꢃerresꢃrial vertebrates)

Epsilon cross bedding

ꢊꢋ" -

ꢊ

_eꢇ_e

ꢈ

�

-_

-

Marlꢂ

Oncolites

Rooꢃꢂ

Current ripples

Lenꢃicular bedding

Mudcrackꢂ

FIG. 2.-Uppennost Jurassic to Lower Cretaceous stratigraphic units and depositional sequences for westeꢀ Cameros Basin, modiﬁed from Martin-Closas and Alonso Milꢁꢂn (1998), and correlation with general sequences from the Iberian Basin.

Limeꢂtoneꢂ mudꢂꢃone-wakestone

Logs

Nodular ꢁimeꢂꢃones ꢀw-m)

ꢋꢌ

Rythmiꢃe ꢀlimestone-shale)

during the main rifting events. In addition, they allow a test of the model of depositional sequences developed by Mas et al. (1993) and Guimerꢀ et al. (1995).

Relatively few studies have investigated the relationship between petrofacies and depositional sequences. These studies have been carried out

mainly on deep-marine successions, where sea-level changes and intrabasinal sources of sediment exert a decisive influence on detrital composition (e.g., Zuffa et al. 1995). However, little is known about the relationship between composition of sediments and depositional sequences in a continental setting. In continental rift basins, tectonisms exert the main control

z

8

A

AI

N

s

7

z

ꢍ

7

ꢌꢍ

5

�

Formations

m

-""

ꢀ

ꢁ

ꢂ

g

Abejar-2

�

.--.

7

6
300

Castrilꢁo de ꢁa R

�

ꢃ

ꢄ·

ꢃ

ꢅ

ꢆ

Abejar-1

ꢇ

ꢈ

O

n

�

250

200

150

100

50

Fb

Pantano de ꢁa

Cuerda deꢁ Pozo

MJ

.

:

-

:

.

;

.

:

ꢍ

Hortigueꢁa

54

3

ꢀniꢁla de los M.

ꢂꢃacoba

�

-

�

ꢒ

S. Marcos Saꢁcedaꢁ

0
0

1

2 km

Campolara

Jaramiꢁlo

2

-

.

--

�

� sandstones

shales

Boleras

•

1

of samples

Location

I

Brezaꢁes

MJ

Marine Jurassic

on topoꢀaphy and ꢁrꢂnage-basin developmeꢊt (Prosser 1993). Tectoꢊics grades up iꢊto lacꢡstꢐne lꢪmestones ꢋnd mꢋr꣌s (ꢁ Marcos Foꢴation),

ꢋlso deteꢃines the seꢁimeꢊt fꢱux ꢄom catcꢙmeꢊts, wꢙich is modulated by contꢋiꢊiꢊg ostracodes, a ꢮew chꢋropꢙites, and abundꢋnt stromatoꢳitic layers.

the lithology at the source (Paloꢅes ꢋnd Aꢆbas 1993) ꢋnd by the cꢳimate Dꢪnosaꢡr prꢪnts ꢋre relatively common towards the top oꢮ the uꢊit. Tꢙis (Leeder 1995). Obvioꢡsly, these ꢇctors stronꢈy control the composition depositional sequence appeꢗ oꢊly ꢪn the northeꢕ pꢢ oꢮ the study ꢋrea, oꢮ seꢁiments (Joꢙꢊsson 1993). Thus, tꢙis study is an opportuꢊity to ꢋnꢋlyze as does DS-2. Depositionꢋl Sequences 1 to 3 cꢋn be ꢉewed as a megacycle the bꢋlꢋnce between the soꢡrce-rock signꢋl and the enꢉroꢊmeꢊtꢋl siꢌꢋl whose age rꢋnges ꢄom Tithoꢊian to Beꢷꢸasiꢋn (Fig. 2). Depositionꢋl Se(cꢳimate, trꢋnspoꢍ, etc.) on sedꢪment composition dꢎng the generation oꢮ quence 4 (DS-4) coꢊsists oꢮ a series oꢮ lꢪmestones, leꢊticulꢋr sꢋnꢁstone depositionꢋl sequences ꢪn ꢋn iꢊtraplate riﬅ basꢪn. Finꢋlly, tꢙis paper doc- bodies, mꢹs, ꢋnd clays ꢺth abꢡnꢁꢋnt charophytes, ostracodes, ꢋnd ꢭagumeꢊts sꢋndstone composition ꢋnd its evolution ꢪn a weꢳl knꢏ iꢊtraplate meꢊts oꢮ dꢪnosaur oones (Peꢻacoba Formation) up to 50 m in tꢙicꢚess.

riﬅ (Mas et ꢋl. 1993), ꢋnd may coꢊtꢐbute to a better ꢡnderstꢋnꢁiꢊg oꢮ Tꢙis sequence is spatiꢰly restricted to the south oꢮ the study ꢋrea (sections

sꢋnꢁstone modes in other eqꢡivꢋlent geotectoꢊic settiꢊgs (e.g., Hiscott et HIN, AHE, PEN ꢪn Fig. 3 ꢋnd GAS ꢪn Fig. 4). The age is Valꢋnꢥꢼꢽꢋnꢾ ꢋl. 1990; Zieꢈer et ꢋl. 2001; Wi꣌son et ꢋl. 2001).

Hauterivian (Fig. 2). Depositionꢋl Sequence 5 ꢿS-5) in westeꢕ Cꢋmeros consists mꢋiꢊly oꢮ the Pꢪꢊꢪꢳla de los Moros ꢋnd Hortigꣀꣁela ꢮormations. The first uꢊit consists oꢮ chꢒnelꢪzed fꢱuꢉꢋl sandstone bodies and red sꢪlty clays up to 450 m tꢙick. The Hoꢍigꣀꣁela Foꢴation (300 m mꢋxꢪmum tꢙick) is composed oꢮ oncoꢳitic lꢪmestones, mꢋr꣌s, silty clays, ꢋnd smꢰl leꢊses oꢮ sandstone with abꢡnꢁꢋnt ꢮaunas (ostracodes, ꢋnd bone ꢭagmeꢊts) ꢋnd fꢱoras (chꢋrophytes and blue-green ꢋlgae). Both uꢼꢽts yield chꢋrophytes oꢮ Late Hauterivian to Eꢋrly Bꣂꣃemiꢋn age (Maꢍ꣄n-Closas and Aloꢊso Mꢪllꢫꢬ 1998). Depositionꢋl Sequence 6 (DS-6) coꢊsists oꢮ the Pꢒtꢋno de la Cuerda del Pozo Formation, wꢙich is Bꣂꣃemian in age. It coꢊtꢂns lenticulꢋr bodies oꢮ sandstones, conꢈomerates, clays, and sꢪlts and ꢍ lꢪmestones beds, deposited ꢪn a complex basꢪn-ꢋxial fꢱuꢉatile system with multistoried uncoꢊfꢼned chꢒnels migratiꢊg ꢪn a ꢺde ꢰluviꢋl plꢋin (Clemente ꢋnd Perez-Arlucea 1993). Depositionꢋl Sequence 7 ꢿS-7) consists oꢮ the Abejꢋr ꢋnd Castrꢪꢳlo de la Reꢪna ꢮormations, both oꢮ fꢱuꢉatile oꢐꢥn ꢋnd Late Bꣂꣃemiꢋn-Artiꢋn ꢪn age. The ꣅst ꢮoꢴation was deposited ꢪn a complex basꢪn-trꢋnsverse ꢰluviꢋl system, mꢋiꢊly brꢋided rivers. The second ꢶit represeꢊts more ꢁistꢋl meꢋnderiꢊg ꢐvers. Tꢙis sequence appears basinwide, with greatest tꢙickness ꢪn both westeꢝ and easteꢕ Cameros (close to 2000 m) ꢋnd is the mꢂn ꣆l oꢮ the lꢋrgest depoceꢊters oꢮ the basin.

From the ꢋnꢋlysis oꢮ 15 stratigrapꢙic sectioꢊs ꢪn the study area, two traꢊsects (N-S and NNE-SSW trenꢁiꢊg) oꢮ the above stratiꢀapꢙic ꢡꢼnts were coꢊstructed (Figs. 3, 4). There is cleꢋrly a major increase ꢪn tꢙickness oꢮ the mꢋin uꢊits to the noꢍh. Tꢙis su꣇ests that active locꢋl noꢴꢋl ꢮaults exerted major control on the co꣈guration oꢮ seconꢁꢜ depocenters in tꢙis area oꢮ the basin, ꢋnd ꢪ꣉uenced the ꢋrcꢙitecture oꢮ stratigrapꢙic uꢊits ꢋnd depositional sequences ꢔas et ꢋl. 1993; Gꢡimera et al. 1995). These normꢋl ꢇꢡlts ꢋre ꢋntithetic to the extensional rꢋmp that was ꢮormꢪng the Cameros Basin (Guꢪmera et al. 1995).

GEOLOGꢄꢃꢈL ꢇEꢂꢂꢁG ꢈND ꢇꢂRꢈꢂꢄGRꢈPꢉ

The Cꢋmeros Basin has been ꢪnterpreted as an extensional ꢐfꢑ basin that
ꢮormed aoove a south-dippiꢊg ramp on a bꢳind, low-ꢒgle normꢋl ꢮaꢡlt severꢋl ꢓlometers deep ꢪn the basement ꢔas et ꢋl. 1993; Guꢪmem et ꢋl. 1995; Sꢋlas et ꢋl. 2001). Two sectors can be ꢁistiꢊgꢡished: (1) the easteꢕ sector, where ꢖl is the tꢙickest ꢋnd where metamorpꢙism appeꢗ; and (2) the westeꢕ sector, where importꢋnt vꢘations ꢪn sedꢪment tꢙicꢚess eꢛst because oꢮ the complex structꢡre oꢮ the basꢪn. The lack oꢮ metꢋmorpꢙism ꢋnd the eꢛstence oꢮ secondꢜ depocenters in the westeꢝ sector have been related to the movemeꢊt ꢋnd rotation oꢮ isolated blocks. ꢊ tꢙis sector, sedꢪmentation took place ꢋlong NW-SE trenꢁiꢊg trouꢞs with diꢮꢮerent subsidence rates ꢋnd severꢋl episodes oꢮ reactivation. The respoꢊse to Tertiꢜ tectoꢊic inversion was diꢮꢮerent in the two sectors. In the easteꢕ sector a new E-W trenꢁiꢊg tꢟst to the noꢍh

ꢁ

ꢮormed, with a mꢠmꢡm ꢁisplacemeꢊt oꢮ 28 ꢋ (Guꢪmera et ꢋl. 1995) ꢋnd with very ꢳittle sꢡrꢮace deꢮormation. ꢊ the westeꢝ paꢍ, relatively ꢪnteꢊse deꢮormation is refꢱected in a system oꢮ ꢪmbricate tꢟsts ꢋnd southwꢋrd-propagatiꢊg ꢮolꢁs, refꢱectꢪng a shorteꢡꢪng oꢮ 3 ꢋ (Guꢪmera et ꢋl. 1995). The ꢋrea oꢮ tꢙis study is located ꢪn the noꢍheꢝ pꢢ oꢮ the westeꢕ Cameros Basꢪn ꢣig. 1). A SSW-NNE trenꢤng geoloꢥcꢋl cross section oꢮ tꢙis ꢋrea is ꢦhoꢧꢨ in figꢡre 4 (C-C' cross section) oꢮ Gꢡimera et ꢋl. (1995). In tꢙis area, Mꢩn-Closas ꢋnd Alonso Mꢪllꢫꢬ (1998) described seven depositional sequences (DS) rꢒgꢪng ꢪn age ꢭom Tithoꢊiꢋn to Aptian (Fig. 2).

Depositionꢋl Sequence 1 (DS-1) is preseꢊt across the basꢪn. Its base is defined by a major ꢡncoꢊꢮormity mꢒiꢮested as ꢋn exposeꢁ, ꢯctureꢁ, kꢋrst sꢡrꢮace on Caꢳloviꢋn-Kꢪmmeridgiꢋn lꢪmestone ꢋnd sꢋnꢁstone. Tꢙis sequence contꢋiꢊs two ꢳithostratigrapꢙic uꢊits represeꢊted by ꢰluvial ꢋnd lacꢡstꢐne ꢇcies. The basꢋl uꢊit (Nuestra Senora de Brezꢋles Formation) is 10 to 70 m tꢙick ꢋnd coꢊsists mꢋiꢊly oꢮ red sꢪlty clays ꢋnd polymictic

PROꢃEDUREꢇ

To chꢋracterꢪze the detꢐtꢋl modes oꢮ the clastic iꢊte꣊ꢋls ꢭom the depositionꢋl sequences ꢪn the westeꢕ Cꢋmeros Basꢪn, a totꢋl oꢮ sixty-ꢊꢪne coꢊglomerates that were deposited ꢪn pꢋleochꢒnels oꢮ ꢋlluvial ꢌ that samples oꢮ medium-graꢪned sandstone were coꢳlected ꢋloꢊg stratiꢀap�c pass laterꢰly ꢋnd upwꢋrꢁs to lacꢡstrꢪne ꢇcies (Boleras Formation) with sections close to the locꢋlities oꢮ Hortezuelos (ARE), Peꢻacoba (PEN), weꢳl-developed hydromorpꢙic pꢋleosols. The lacustrꢪne ꢮacies are 60 m C꣋po꣌ara (CML), Rupe꣌0 (RUP), Brezales ꣍RZ), La Gaꢳlega (GAS ꢋnd tꢙick ꢋnd represent the ꢁistꢋl ꢇcies oꢮ the ꢋlluꢉꢋl system. Depositionꢋl GAN), Moncꢋlꢉꢳlo ꢔON), Castrovido (CTV), ꢋnd Tenꢋzas (TRZ and Sequence 2 (DS-2) coꢊsists oꢮ a basꢋl fꢱuꢉꢋl uꢊit, consistiꢊg oꢮ leꢊticꢡlar TRR) (see Figs. 1, 3, ꢋnd 4 ꢮor locations). ꢊ addition, two sꢋnꢁstone sꢋnꢁstone ooꢁies, sꢪlty clays, ꢋnd mꢪnor ꢍ iꢊterbedded lꢪmestones (Jꢋr- samples were collected ꢭom the underlyꢪng mꢘne ꢮormation (Talveꢪla ꢲiꢳlo de la Fueꢊte Foꢴation) up to 400 m tꢙick, wꢙich ꢀade upwꢋrd Foꢴation) ꢮor comp꣎son with the syn-rifꢑ uꢊits. Samples were ꢪmpregꢋnd laterꢰly ꢪnto lacꢡstrꢪne ꢮacies (Cꢋmpolꢋra Formation), neaꢵy 100 m nated with blue epoxy resꢪn beꢮore tꢙin-section grꢪnꢤng. Tꢙꢪn sections tꢙick. The Cꢋmpolara Foꢴation commoꢊly ꢁisplays pꢋlaeosol levels, ꢋnd were etched ꢋnd stꢂned ꢡsiꢊg HF ꢋnd sodium cobꢋltꢪꢊitꢐte ꢮor potassium the ꢮossꢪl coꢊtent includes chꢋropꢙites, ostracodes, ꢋnd gastropoꢁs. Depo- ꢮelꢁspꢋr, ꢋnd aꢳꢪzꢘn-red ꢋnd potassiꢡm ꢮerrocyꢒide ꢮor cꢋrbonate idensitionꢋl Sequence 3 (DS-3) comprises a basꢋl ꢶit deposited in a meꢋn- tꢪfication (Chayes 1952, ꢋnd Lꢪnꢁbo꣌m ꢋnd Fiꢊketꢼnan 1972, respectively). deꢐꢊg fꢱuꢉꢋl system (R꣄o del Sꢋlcedꢋl Formation) up to 400 m tꢙick, that Quꢒtitative petrograpꢙic ꢋnꢋlysis was perꢮormed by poꢪꢊt couꢊtꢪng ꢍ

FIG. 3.ꢀꢁ trending stratiꢀaphic transect A of uppermost lurassic-Lower Cretaceous formations, depositional sequences, and their relations in the westꢁ Cameras Basin, with location of samples. Fa and Fb indicate location of deduced basement faults. ꢎꢏ Hinojosa section; ꢐꢑ Arroyo del Helechal section; Pꢂ, Peꢃacoba section; CON, Conꢄeras section; CML, Campolara section; ꢒꢓ Cubillejos section; ꢔꢕꢖ Rupelo section. See Figure 1 for location of sections.

z

B

B'

�

z

ssw

NNE

�

7

�

7

ꢎ

Fa

Formations

3^m00

250

200

150

100

50

�

ꢏ

"

0

"

0

Abejar-2

7

Castriꢁlo de ꢁa R.

�

ꢐ

Q

0

,

0

,

Abejar-1

Paꢄtaꢄo de ꢁa

Cuerda deꢁ Pozo

6

HortigOeꢁa

5

ꢀꢄiꢁꢁa de ꢁos M.

ꢅꢆacoba

0

S. ꢇrcos

Salcedaꢁ

ꢝ

0

1

2 km

3

2

1

.�-.

�

Campolara

Jaramillo

�

-

.

� sandstones

shales

Location of samples

•

Boleras

.ꢏꢐꢑꢒꢓꢔv

,ꢋ'ꢌꢍ '-",'

Brezales

�

MJ

Marine Jurassic

FIG. 4.-ꢃꢄSSW trending stratigmphic transect B of uppennost IurassicꢄLower Cretaceous formations, depositional sequences, and their relations in the westeꢀ Cameros Basin, with location of samples. Fa and Fb indicate location of deduced basement faults. Bꢅ, Brezales section; GAS, La Gallega southern section; GAN, La Gallega northeꢀ section; MON, Moncalvillo section; CꢆV, Castrovido section; ꢆꢅ and ꢆꢇ, ꢆerrazas section. See Figure 1 for location of sections.

TABLE l.-ꢙy ꢓ cꢯꢛd petrꢁrapꢰc cꢱsses ꢚ recalculated paraꢲꢛrs.

NCE ꢀon-Carbonate Exꢁabasinal)

Monoꢐrystalline quartz, undulosiꢃ < ꢊꢋ Monoꢐrystalline quartz, undulosiꢃ > 5° Polyꢐrystalline cpartz with 2ꢆ3 subgraim

Polyꢐrystalline cpartz with >3 subgraim (>0.062 mm) Polyꢐrystalline cpartz with >3 subgraim (0.062--0.030 mm) Quaꢌ in lꢍ and medium metamoq:biꢐ roꢐk fragment Quaꢌ in plutoniꢐ/gneiss roꢐk fragment

Reꢐalꢐulated parameters

ꢗ

ꢘ