fisii’’i¡¡I,i/Ibeiis¡,í (hs’Ií-i,-t tSSN: 0378—l02X 2052 29. 1128 Wave-dominated siliciclastie and carbonate sedimentation in a Lower Cretaceous lake (Cameros basin, northern Spain) Sedimenuu.ión siliciclóstica dominada por la acción del oleaje>’ carbonatada en un lago del Cretácico Inferior (cuenca de Cameros, norte de España) S. DotmLtst, Ji’. GARCÍA. M. OLtIRAUD, A. MÉNARD ABSTRACT Aceording to struetural, sedimentary and biostratigraphic data, the north- eastern Cameros rift basin (northern Spain) has recorded almost 8 000 meters of nonmarine Late Jurassie to Early Cretaeeous deposits with minor marine íncursions. Our purpose is: (1) to document within this basin both carbonate and silicielastie facies of the Enciso Group; (2) to illustrate a depositional model and (3) to diseuss palacoenvironmental reconstruction of a large lacus- trine system. Our depositional model is based on an integrated approaeh involv- mg both rnapping and stratigraphic section analysis. Three sedimentary envi- ronínents are recognized on six stratigraphie sections and are outlined by vaílous outcrop observations in tbe Enciso Group. They are: 1) open lacustrine environment; 2) siliciclastie marginal lacustrine environment (distributary channel, mouth-bar delta, shoreface lake-fringing marsb facies assoeiations and 3) carbonate lake-margin facies environment (storm-dominated, desiecated opeP lacustrine. palustrine facies assocíations). Two types of lake margin set- tings alternate through time: a wave-dorninated siliciclastie vs a low-gradient carbonate ramp setting. A elimatie controJ on the onset of the two types of lacustrine depositional settings is suggested. Keywords: Cameros basin, Lower Cretaceous, lakes, palaeoenvironments, pal aeoe íi mate. LJMR/CNR5 5561 Biogésusciences. Ceíítrc des Scieííces de la Terre, Université de Bs,urgognc. 6 beulevard Gabriel. 1<21 (1(1(1 [)ijsu¡í, FRANCE. e—mail ( 5correspoíiding aushor): StefaíiDoublct@u—bourgogíiebr II
Wave-dominatedsiliciclastieandcarbonatesedimentationin a Lower Cretaceouslake
(Camerosbasin, northern Spain)
Sedimenuu.iónsiliciclóstica dominadapor la acción
deloleaje>’ carbonatadaen un lago del CretácicoInferior
(cuencade Cameros,norte de España)
S. DotmLtst, Ji’. GARCÍA. M. OLtIRAUD, A. MÉNARD
ABSTRACT
Aceording to struetural, sedimentary and biostratigraphic data, the north-eastern Cameros rift basin (northern Spain) has recorded almost 8 000 metersof nonmarine Late Jurassie to Early Cretaeeous deposits with minor marineíncursions. Our purpose is: (1) to document within this basin both carbonateand silicielastie facies of the Enciso Group; (2) to illustrate a depositionalmodel and (3) to diseuss palacoenvironmental reconstruction of a large lacus-trine system. Ourdepositional model is based on an integrated approaeh involv-mg both rnapping and stratigraphic section analysis. Three sedimentary envi-ronínents are recognized on six stratigraphie sections and are outlined byvaílous outcrop observations in tbe Enciso Group. They are: 1) open lacustrineenvironment; 2) siliciclastie marginal lacustrine environment (distributarychannel, mouth-bar delta, shoreface lake-fringing marsb facies assoeiations and3) carbonate lake-margin facies environment (storm-dominated, desiecatedopeP lacustrine. palustrine facies assocíations). Two types of lake margin set-tings alternate through time: a wave-dorninated siliciclastie vs a low-gradientcarbonate ramp setting. A elimatie controJ on the onset of the two types oflacustrine depositional settings is suggested.
Keywords: Cameros basin, Lower Cretaceous, lakes, palaeoenvironments,palaeoe í i mate.
LJMR/CNR5 5561 Biogésusciences. Ceíítrc des Scieííces de la Terre, Université de Bs,urgognc. 6 beulevardGabriel. 1<21 (1(1(1 [)ijsu¡í, FRANCE. e—mail (
De acueído etin los datos estructurales. sedimentológicos y bioestratigráftcosdisponibles, la cuenca de rift del noreste de Cameros (norte de España) presenta elregistro de unos 800<.) metros (le depósitos (le sedimentos cotítinentales de edadJurásico Superiora Cretácico Inferior, con inetirsiones menores de origen marino. Elpí-opósito dcl pí-esente. trabaío es: (1) documentar la presencia en esta cuenca de lasfacies caí-bonatadas y sil icicláslicas del Grupo Enciso: (2) proponer cín modelodeposíciotíal : y (3) realizar la í-econsti-uecion palcoambiental (le un gran sistemalacustre. N tíestí-o modelo cleposicional está basado en la integración de datos car-tografícos con los procedentes del análisis de secciones eslratiorálicas. 1 “‘1A pwír .4ccsttidio cíe seis secciones estratigí-álicas. se han 1)odido iclentificaiz- tres asociacionescíe facies di lerentes. reconocidas a sví vez en olios afloramientos del Grupo Enciso.Estas asociaciones (le fúcies soil: 1) asociación cíe facies (le ambiente lacustre abier-(o: 2) asoe ación (le facies tic ambiente 1 acustie marcí nal siliciclastico (canal clistii—
y facíbtítaiit,. baila cíe cleseiil1x)c-~t(ltIra es patítanosas de shorefiu -e Iake—/Pingi¡ígí ) y 3>ambie ile 1 actístre íríaiei nal de facies cai-bonatada ( asOo Ltc 10flC5 cíe facies pa Itístresdom nadas por tormeíítas y lactí st re abieí4a coíi desooatu in> A lo largo del tiempo sealteuíian dois t pos de íííaigen 1 actístre: uno sil e chistit o dom inado por la accion deloleaje. y otro carbonatado secun un modelo tic rampa suax omente- inclinada. Se sug—íeíe que el establecíni íe u lo cíe estos dos li pos de dc posítos latusties estuvo contiola—dii por factores cli ni-atict)s.
Dtiíí ng Mesoíoic ti mes. intíaplaíe ri 1) ing stages related to the open ing of theBiscay Gímíl arO íc sponsible for the emplaceiríení of a set of extensional basins with—iii the lbu II It] Doinjí íi( AIx—aro el al. - 1979; Salas and Casas, 1993; Casas—Sainz ancíGil—ln~sí¡ IsiOMa ¡ [su 1 ¡mor-nc h=íeh,~í-sí-uísuíi-.íí~vl ,iiiI~n¡, tVe. m-in rWir,,,t•fl.it•tLi¡IttI¡tft¿4
(I.Áítc [dr íssí 1 ti lx ( itt it cotís) and corresponcis to the t.hickest westcrn depoceíítre.Accoi clín~’ lo stí tiultíl ttl scdi.mcnurv antí biostratigraphio- data (Tisher, 1966;Salonion l98~ Gtií r itíd 1983: Martín—Closas V Alonso—MIlán. 1998) the north—castuní ( imo í os b tsi n i utorcíecí almosí 8 (RiO níeteis cd noninarine Late J urassic to[-LadyCi et9ccous tluposíts wit.h íníííor inaííne incursions (Alonso y Mas, 199.3;Cómo ¡ í un inclez and Msluríck z 994: Alonso—Azcárate et aL. 1995). Tisher
1966) u.st ibí ished ihe iíuilial stí ítí~~r íphie li-amework stíbdivicling íhe synrift deposi—tion tito fíve liihosír;íiigr~iph it •‘i ocíps ( naincly, Tira. (jincala, Urbión Enciso.(1)1 iy lo) liii-, inc<¡leí was ¡ti 1 huí di sí. ussed 1w aii thors leadí nc to several svnri it sed—mcml tí \ oid setítíemíce strali~’i aphíu íííteipretaticíns (Salomon, 1982; Gcíiraud. 1983:
Gui í ~íuclíí íd Su gtíret. 985 Nl is al . 1993>. ( )ur purpose ¡ s: (1) to docuiuent bothcaí-bonatu tnd sil i e iclastie t~íoícs of the Enciso Group (late Baíi-emian (o Aptian): (2)
u. 1 1 12
5. Doubleí, .í-P Can-ja. M. Cuire,ud. A. Ménard
Enciso Group
Legend
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t.ions were cairied c)uf usíne facies analvsis entena (grain ....~, ¡¡~¡uwu~y, sossíis, sedi-mentaí-y and biological sti-uctures). Thin seetions and hand samples are also examí—necí for i nterpretafion of carbonate facies. The. geometry of sedimentary bodies wascharacteí-ízed físííii ficíd observalions and c)titcrop photogíaphís. As biostratigraphicdata mm (he Enciso (liíoup reinain veíy searce (Martín—Glosas y Alonso—Millán.1998; for a í-cview ) ccí-rclat ions are basal on 1:10.0(X) vnapping of maí-ker beds, i. e.latcíal extent ¿md fhcies vaí-iations as well as geometiy of sedimentaíy intervalsbetwecíí each marker (Hg. 3).
SEDIMENTARY ENVIRONMENTSAND FACIESASSOCIATIONS
[-hesecli ¡ncnts of the Enciso Group are alTangecí i uto carbonate and sil iciclastietíoi ts aocI diV cíes] i nIc open—lacustrine. sil iciclastie marci ial Iaeustri nc. and carbon-ate 1 aL—mare i n cmvi míí mcml s.
II liS LuX iionment is dominatecí by bovh si liciclasties aud carbonates arranged intoíbice dist¡nct t icies associations.
Sílícicí tstíc facies is clominated by blue-gíeenish bioturbated micaceous silt—stcncs (Fi” 4). These honíoceneous silistones are massive and frequently exhibitsoil scclímcnt m icrotaulti ng relates] lo compaction (Guiraud and Ségcííet, 1987).Sedíincníu} stttictures as well as fossils are virtually absent.The low-cliversity tíacelossil íssctnbl ige is domínated by abundant simple horizontal to vertical btírrowsthat can lic scxcral tetis of centilvietí-es deep.
C u lion tc uníts display mid— to dark-grey miente interbeáded with grey níarís¿md calr acolAs claystones (Fig. 5). They often contain laminated ostracod—shellskelutal dcbí is Vetiebrate bomie fragmencs including ve.rtebrae, crocodile teeth atdinotí. fícqucntly fish seales and tecth have been noticed. Bioturbation when observecíis dilitisí..
Hí.tcí olahic secliínentaí-y units consist of alternating marís. liniestone beds andcastraí.”d r’ch ínclstonús-~-lÁmestones--appear -as-single-o~racodal c’aicarenite bcdslaterallx pelsistcnt over kilometcrs. They are sharp-bascd ¿¡os] dispí íy a upper undu-latina stiíj¿íce Intemal slntictuies ale featuied bs- erosive suri ice prodticinc hurn—i1]OckS ncl SXX tIsis íííantlcd by ostracoclal laminae. ~l’hesestructurcs así. distinct.ive ofhu nim it kv ci 5sss—strati ficzíí ions 1 Fie- 6—1). Sandstones Occur as ~raclcdcení i mcteí—cii ic-k bí. ch ¡u Xx í s cusís o cx tetis] - ()cculic:oc-í.’ of ineipiení síaivucí xx ixe í ípples attcstof ac f Ivc xx ix c rc-woí-ki hg. S¿ínclstone becís ¿ii-e intei-preted as tcmpcstilc líke deposits(Mii -uí A í o ncí 1985. lis x-ertical sccti on, (bey u c scackccl i rito ti Iii ng—u pwarclscc~tíc ticc s A híc 1] a tu-mate xviii carbí mate beds arid mas Is ([1 c~. 5).
1 ac u’ s s lcsc ri bes] stigaest ti nc—graí t]ed seclí íricnLitíon lo an cixygenates] openla.cií sú inc scttiííg. Abscní.-c s )f scdi mentaí-y- fc¿ttures diagnostíí. of subaeí-i al exposurenduc c s a pci ííuincnt w-aler c:oIcii nn - l3oth sil iciclastic md c¿trbon:tte sc’chimentary
uíí i ts 1 & í. tu cl ¾sxv —md ay secli iíícíítation - lhev comirasí xxi th heteroli th e tín its where
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Wave-dominated sedimentation in a Lower Cretaceous lake
Fis u 4.- ve-tical scctis¡í¡ líos Llaisos sectislís) shsswing siliciclastic marginal laciustrine succession. See figures 1 aísd2 for locatisnís.Fis;. ‘k— Sección cstratigu<ilica (Lsss Llanos) que muiesira ¡una sucesión lacustre si liciclástica nsarginal. Ver figuras 1y 2
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Opon ¡acusurinoenvironmonu
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2003. 29, 11-28
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occurrence of tempestites aud HCS suggest storrn-dominated open lake eonditions.
SiLICIcLÁsTIC MARGINAl.. LACUSrRtNls ENVIRONMENT
The sijiciclastie marginal lacustrine environínení is composed of silístonesané very fine fo medium sandstones. This environment is represented by distrib-ulary channel, tnouth-bar delta, shoreface and maíginal lacustrine níarsh faciesassociatíons.
TI-lE 13151RIBLFIARY CIJANNI-Il. FACIES ASSOCIATiON
Ris facies association is present within single fining-upward channel sand-stones, 2.5 to 4 ni thick. Grain size decreases upward from medium- to very fines]-gíained sandstories. In cíoss section, sandbodies show fIat top and lJ-shaped en-siotiaJ base soíiíetimes with thin lag conglotnerate containing logs (Fig. 6-2). Theyare simple although several display internal bounding suífaces. Commonly wedge-shapes] massive silty sandstones known as wings extend lateíally and thin overmeters into silty deposits. Sandbody intUí is dominated by latera! aceretion (epsilon)cross-bes]ding associates] with plan¿u- cíoss-beds.
On the basis of the W/T í-atio. channel sandstones are reported as ribbon san&stones or lateíallv isistífleted channel sandstotes (Friend et al, 1986). Epsilon cross-sti-atifications suggest lateral migration. Ribbon sandstones are inteíyí-eied ¿ts isol¿tt—ecl epheuiíer¿íl channels cutting down intc) lacustrine deposits.
½>1>111—BAR iii-II [11½1 -ACIES A55OCIAF ION
Mouth—bars form single sir stackecl individual lobate sandstones, 0.1 (o 4 ¡u thick.lii strike section. single bedset extension considerably vary frora 25 ni to more than100(1ni, but facies arrangcmcnl ané lobate geometí-v remain constant. Ir cross see—tion, lobes consisí in cc)nvex típ leus shaped sandbodies. They frequently overlie fines]eposits (open lactístrine sudíínents) xvith sharp scouring Iower boundino stíí-faces5, -(l--ig. 6-3>, but ití soíue cascs transítion frorn fine s]eposits is gíadual ané corrcspond
u I:i,~. .1tc pal a¡ cl — ¡¿ti ¡u’ ¡ ¡aseo sí ¡ LSIS<i cx ~ ci íu al sallusloilí. lJ5>í.11C5 lii spiÉtY irocígh cross—stí-at—itications which laterally p tss nito Ltngentiaí ¿md planar cross—stratifications. Sinalí—sealed huínsnocky cross—strat¡fícs] sands may occur inteíbedded with single lenticu-lar bes]sets. Reworking of cross becís is loeallv piesent ¿xx tíndtílatiííg ciosive suitaces(Hg. 4).
Geoíretry and facies í-ecogíiízes] in sans]stones suggest lobate delta systems.Parallel—lairtinated siltstones recors] prodelta cleposits, while sans]stone bes]s chaí-ac—teíize clic proxiníal p¿trt of delta. The alTatigement of sedimentary struclures withinsingle bbc alloxv us to define several doínains. ‘[-he proxirnal delta which recordsi]]lgration of dcínes (tíough cross becís) 1] a chanííelized flow is reportes] here as dicclistribut¿try movíth-ban l~ateraIly, as the fiow speed decreases, sandwaVes (tabularcross beds) can tbrm on the delta front <l2igs. 4 and 6-4). Towards bbc margins, cro-s¡ons as weIl as lICS sandsiones tetW tu í-ecord a more cffective w¿tve activity.
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2<553. 29. II 25 18
Wave-dominaced sedimentation in a Lower Cretaceous lake..
-- - wave reworked della-ironí
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sandalone reworlsod lrough croas-suratiltcauion
Fío. 5.- Veilical section wiilsin <líe Vadillos ng showing veutical alternances between open acusnine, siliciclasticand castasnate lacustri¡ie s¡¡¡ils. See figures 1 asid 2 [sir locathsn.Fis;. 5.- Sección estraiigrsii)ca dc detalle (Vadillos) en la que se muestra la alternanciavertical entre las tres unida-des lacustres tabierta. silicicláslica y carbonatada)- ver figuras 1 y 2 para su lucatizución.
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Sl.¡ORvzI2ÁCE FÁCIl 5 ASS~X IATION
Dcposits íecognizccl ín this Iheisis ¿tssociation ¿u-e ari-aííged units 1.5 to 3.5 ni thickand few kilomctcí s in 1 atcral extencí.
The lower p trt is s]onímnated by silty u> verx- fine grained rnicaeeous sandslones.They eonsist in 1 ítcí ally ¡íoníogeneous bedfonns which nonnally gíade froin openl¿ícustíinc deposíts Scdíineníaíy structurcs s]isplayed coniprise fiat to low ¿ingle ci-osslannnations intí. ícli~ít <lcd with synímctrical ripples or symínctrical ripple—foí-ííí setslcss thaíí 1 cm thíck Xciv sniall—scalcd huníníocky cross—stratiticatic)n liave been als()observes]. Thesc. sans]~ dcposits show- diagííostic feavt~tres of vv-ave action (Harrus etiii., 1982). Occuí ícnce of even lamination associ¿tied xvitlí wave ripples ¿aid sníall—scaled lICS sug”ests í lower shorefiíce settinc.
Ití verlical scctíon lowcr shorefacc deposits ate overlain by stacked letítícul¿trsandstoíic beds Tliickncss as welf as lateral extend of these well sorted fine gi-ainedsans]stones are similar te ihose cff sníal 1 single delta Ictes. Amalgamatecí small—sc¿íles]HCS aíid xx-avc-liflplc bedseís ¿nc ¡líe ilotí.— coíníííon fealtíres buí laiger HCS sand—siones ancí ci ndsí fatíng Iow—aííglc fiat beds also occur 1. Fig. 7—1>. Numerous vertebratetí-acks pí-escí-ved over large íippled svírfaces ¿íttest of occasional s2niergence.
]Vlost sif i íítcí-naf strtlctíli-es of leraicular sans]stoííes show teafures cluíracteri stic c)fvaxe aocI sierro ¿íctic¡n in the upper shorcface. Fíat becís indicate oscillatory uppertiow-í-eciíne ccínclítioíís and suggesí high tlow velocity ¿md sh¿tlloxx- w-ater depfh(Harmos ct cii., l98~) 1 u ce— as wel 1 as stnall—scalecf HCS beds icecíré storni eVcntsthe cippcr síícaí. lace 5 íoí ní—influetices] tnaígi ial l¿tcctstí-ine setíiíígs ¿ire both exten—si vely decumeuifccl u í tic ídeííí ¿tos] anciení. lakcs (e. y Eylcs ¿iris] Clark. 1986;Gt-eeiíxx-ood ancí Slícrínaí 1986: D¿íní ancí Surlyk, 1991; Martel and Gibling. 1991).Modein ex¿tmplc of htuíííííecky cross—slratiticat ions frc>íii Otítario lake similar tcthose ebseixed Ls píc íll~ forní i u thesurf zoííe ¿it ver>- shallow clepth (Greenwoed ¿mclSheí-man. 198<)>
lii Vcrtic¿tl scctíon loxx-er aticí cípper shoreface are ¿ín-an”ed iii coarscning—upw¿trd5,successíons sí iii 1 ¿tr lo those observes] o rnocíth—b:tr cmvi ronííicíit (Fi g. 7—2). Bothcmvirc>íi nícnts clisplay a gencía 1 shal low i ng—típxv-¿ti-d tendency íuiaí-kecl by sedimenta—rv.slí-clclttícs.and.tp.inií. ~“-5i2-¡O sandcentent-andgrasn-sízc. Upper slíoreface- are oftenasseei¿ítccl xvi <li distri butary í íiovith h¿trs or delta frení deposus ( Fig. 4>. Loc¿íl píes—erice el i-c>xvc>rkc-d Ireuglí cross bedding atid lenticu lar geometíy of shoref¿tce sancí—siones aíícl Ihe ir close assec ation lo s]clt¿t lobes implv that shorclaí.-e formes] at wave—iii ti cieíiccd river sicauths -
Sí 1 .1(It i.A5tIC 1 A kl-SIR INC INC MARSI lIS 1 KACI 125 .AS5OCIATION
it i s fhc ¡es i s coíuíposed of slark rsurv si It s ancí s¿tnclstoiíe becís. Their níost strikyfeattií-c is cccurreuice of root níouls]s up to 5 cm in diarocter (Fig. 7—3). Single rootedhoí-i¡ons can be stacl-¿ed lo ioí-m sancístone tínits up to 4 ní thick. Sandsrones pre—serVecí fícm rooting display l¿unt wave íipples ¿md parallel lamlnatlc)ns.
Althectgh root ¡ypifies pes]cgenic modifícation. pedoturbated units lack of other
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Ware-dominated sedimentationin a Lower Cretaceous lake
characteristie features sueb as nodules or mottling. mese pedoturbated units areinterpreted as immature paleosol formed in lake-fringing forested-marsh setting(Moore, 1987).Laterally, paleosois are associated wiÚi distributary ehannels. Weakpedogenetic features documentes] in paleosols eould mflect poor drainage related totbe lake water level.
CARBONATE LAKE-MARGIN ENVIRONMENT
This environment comprises mudstone and waekestone carbonates andmarís. Carbonate fhcies typieally outerop as tabular, sbeet-like beds extendingboth in dip and strike sections over tens of kilometers. Limestone beds are fre-quently stacked te form stratigraphie successíons several meters in thicknessintercalated with open lacustrine facies. Wc distinguish storm-dominated car-bonate, palustrine carbonate and desiecated open lacustrine limestone faciesassociations.
SrORM-tNFLUENCEU CARBONATE FACIES ASSOCiATION
These carbonales consist in tabular silty to sandy mientes as thiek as 1 to 3 mwith very searce desiceation eracks. Fossil content is low and mostly representes] byostracod shells although occurrence gyrogonites has been documented in one case.Liínestones develop individual near symmetric furrows, 5 cm deep and 30 to 45 cmlong. Furrows can amalgamate to forni ondulating bed surfaces visible along sever-al kilorneters (Fig. 7-4). They are intilled with miente containing low to nioderateamounts of intraclatie pebbles and quartz sand grains.The clastie particles are fre-quently disperses] in the matrix to form homogenous sandymiente. But wherethepourcentage of s¿tnd is greater, particles are arranged into draping laminae alternat-ing with featureless micnitie laminaeto form small-sealed hummoekycross-stratifi-c¿ttion5-
These facies are interpreted as stoníí-influenced carbonates whose sedimentarystructures reeord stoms events in perínanent water column aboye storm-weatherwave base.
DiISIcCArED OPEN [ACUSTRINE LIMESTONE FACIES ASSOCIATION
Rese limestoiíes occur as massive monotonous units composed of stacked tab-ular uniform beds (Eig. 8-1). Miente are homogeneous and display faint millimetresc¿tle silty laminae. In thin-section miente display rhythmieally laminae laterally dis-rupted by bioturbation (Fig. 8-2). Sand content is low and eonsist in millimeter- toeentimeter-thiek hoí-izontal laminae. Thin te very thin breecia occur within lime-stone beds over large arcas (bUs of meters). Breeciated limestones contain well-reunded intraelasís associates] with elastie particles and ostracodal shell fragments.Bes] tops are planar ¿md frequently exhibit preserves] dinosaur foot prints with des-iccation craeks (Hg. 8-3).
23 Jos¿íísiu( of I/srruríís (kolog}’2003. 29. /1-28
Wave-dominated sedimentation in a Lower Cretaceous lake
Facies ¿md fossil eontent suggest low-energy open lacustrine sedimentation overlong periods without subaerial exposure. Breccia are interpreted as wave-reworkeddesiecated miente (Freytet, 1984). Together with vertebrate tracks ¿md desiceationeracks, brecciated limestones are indicative of oceasional emergenee events.
PALI ISTRINE CARBONATE FACtES ASS(X?IATION
They occur as rooted marís and yellow dolostones extensive over tens of kilome-tres among the Enciso Group. They consist in nodular limestones showing littledevelopment. Subvertieal nodules are weakly develop around roots (Figs. 8-1 ¿md 8-4). In te matrix, abundanee of ostraeods attest of the lacustrine origin of lime. Thisfacies is quite rare ir the group ¿md intewreted as palustrine carbonate indicative ofmarginal carbonates] swamps.
DISCUSSIONAND CONCLUSIONS
Lithological ¿md sedimentological data suggest that sedimentation took place ina lacustrine setting. Emersive tabular limestone beds ¿md sheet-like sandstones canbe traced over the entire basin. They allow both dip ¿md strike stratigraphic architee-ture of te Enciso lacustrine system tobe documented ata high time resolution (10to 50 kyr).
Siliciclastie units inelude Iittoral deposits ¿md open lacustnine siltstones. Theproximal part of te littoral zone is dominated by distributaiy ehanneis ¿md lake-fringing marshes. Isolated ehannel sandbodies documentes] in the Enciso group aredifferent as compares] to thicker ¿md wider níultistorey channel belts located in non-lacustrine alluvial settings (Ryder eta!., 1976). They are frequently interealated inboth open lacustrine ¿md shorefaee deposits reeording major terrigeneous sedimentinputs into the lake basin. Laterally to channeis, occasional oceurrence of weaklydeveloped paleosol sequenees is eonsistent with lake-shoreline fluetuations.Lakeward sedinient supplied by distributaries fon lobate deltaie s¿mdbodies.Intemal organization as well as geometry of delta lobes resemble lobate delta mod-els reported in modem and aneient lakes (e.1g. Ayers, 1986; Dani and Surkyk, 1991;(Slover ¿md O’Breine, 1994; Seholz, 1995; Lemons and Chan, 1999). In strike see-tion, active mouth-b¿trs are bordered by inactive delta lobes which display abundantwave-generated structures typieal of shoreface conditions. Both shorefaee ¿md deltadeposits are arranged in sheet s¿mdstone which grade into open lacustnine deposits indip section.
Limestones form laterally persistent sheet-like stacked beds mostly indicative ofopen lacustrine conditions. In dip seetion, tabular desiecated limestones pass gradu-alIy into storm-dominated carbonates. In tabular limestone poonly preserved rhyth-mícal lamination and intense bioturbation activity record sedimentation from paúl-cíe settling in a well-oxygenated low-energy environment. Although frequent,features indicative of emergenee oiíly eonsist in desiccation surfaces (desiceationeracks, dinosaur traeks, breeciated limestones) typical of a Iow to very low pedoge-
25 Jouírría< of llar/sun Gíologv2tX33, 29. 11-28
S. f)oublel .ar-ia, 44. Cr,j,-aud, A. Méí-uíí,il
netie moditícation. Intensity of pedogenetie everprint is known to mercase with timeexposure (Góníez-Fernández and Meléndez, 1991; Alonso-Zarza eta!., 1992). Thusthe weakness of pedogenesis documentes] in tabular limestones suggests open lakeconditions with only occasional subaerial exposín-es during sI-iort-lived Iow-Iake peri-ods. Very extensive subaerial exposures at top of beds suggest low-gradient lake mar-gin similar to the ramp-type lake margin as discussed by Platt (1 989a), Platt andWright( 1991). Owingto dic low-gradients, lake-lcvel fluctuations cause likely expo-stíre of l¿trge arcas of open lake. even in distal parts.
Almost lacking of palustrine facies and continuous lacustrine flejes deciphered inthe Enciso group iníply the existence of one single lake, variable iii time ¿md reachedat least 50() kní. Presence of a permanent fltactuating lake s]uring the Enciso groupin tlíe castení Cameros basin is likely te result of te regional extensive tectonies ina cc)ntext cif continental rifting.
Laterally persistent sheet-like geometry of deposits document absence of lateraltransition between carbonate and siliciclastie nnits. According te the sedimentarycoltínín maximurn thickness (f 500 m) and the duration of the Enciso group (lateB¿trremian toAptian; N4artín-Closas yAlonso-MiII-án, 1998) we can stíspectthatcar-bonate and siliciclastie units (lO te 20 m thick) alteinated at a high time resolution(roughly lO toSO kyr).
Tlíis implies tlíat dursng some periods,carbonate sedirnentation extended over thewbele area reflecting reduces] clastic supply fi-em te alluvial dotnain. Variable lacus-trine clastie input may reflcct fluett¡ating drainage cons]itions. It suggests a climatiecontlc)l (roughtly wet/dry- coííditions) en the onset of the two types of lacustrinedepositioiíal settíngs.
ACKNOWLEDGEMENTS
Authors wish te thank i.M. Molina ¿md tui anonymous reviewer fér their sugges-tions and helpfttll criticisrn en the papen Wc are also gratefíjí te F. Pérez Lorentedirector of the Fundación Pairirnonio Paleontologic-o de la Rioja for his help on theficíd. This work is supported by l4rench ‘Instilul des Seicuces de 1 ‘Univers’ ¿md‘Centre National de la Recherche Scientifique’ as part of the ‘Projet ECLiPSE’. It isa contnbution ti-el]] (¡MR CNRS5561Biogéosc-ienú-es, ‘Partear déterminanís et con-traintes da sigua! .vtratigraphi¿
1ue’ of UniY--ersity of Burgundy, France.
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