Guerrero_The Stratigraphy of the W Side of the Cretaceous Colombian Basin in the Upper Magdalena...

7/27/2019 Guerrero_The Stratigraphy of the W Side of the Cretaceous Colombian Basin in the Upper Magdalena Valley

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Geologa Colombiana No. 25, Diciembre, 2000

The Stratigraphy of the W Side of the Cretaceous Colombian Basin in theUpper Magdalena Valley. Reevaluation of Selected Areas and TypeLocalities Including Aipe, Guaduas, Ortega, and Piedras.JAVIER GUERRERODepartamento de Geociencias, Universidad Nacional de Colombia. Apartado Areo 14490, Bogot, Colombia .E-mail: jaguer@ciencias .ciencias.unal.edu.coGUSTAVO SARMIENTODepartamento de Geociencias, Universidad Nacional de Colombia. Apartado Areo 14490, Bogot, Colombia .E-mail: [email protected] NAVARRETEPaleosedes Ltda. Apartado Areo 58686, Bogot, Colombia.E-mail: [email protected]

GUERRERO J, SARMIENTO G, NAVARRETE R (2000) : The Stratigraphy 01 the W Side 01 the Cretaceous ColombianBasin in the Upper MagdalenaValley. Reevaluation 01 Selected Areas and Type Localities Including Aipe, Guaduas, Ortega,and Piedras. GEOLOGIA COLOMBIANA 25, Pgs. 45-110, 12 PI., 5 Figs., 6 Tabl. , 1 Microp. App.: 6 PI.

ABSTRAeTMarine strata composed 01 mixtures 01 calcareous and terrigenous particles dominate theCretaceous succession 01 the Upper Magdalena Valley (UMV).These include mainly biomicrites,biosparites, mudstones, sandstones and mixed calcareous/terrigenous rocks 01 a lithareniticaffinity. Sedimentary rocks deposited in Iluvial environments , and including granule and pebbleconglomerates, are present only in the lowermost and uppermost parts 01 the section.The succession was deposited in a ramp, which had an inclination 01 about 0.06toward the

E and reached marine depths 01 200 to 250 m on the axis 01 the basin. The whole Cretaceoussection 01 the UMV was sourced lrom the W by the ancestral Central Cordillera metamorphic /volcanic are, as indicated by the abundant content 01 metamorphic and volcanic rock Iragments.The new term CRETACEOUS COLOMBIAN BASIN is proposed to make emphasis in asingle elongated back-arc basin which opened N to the Caribbean and connected intermittentlyto the S with other back-arc and loreland basins E 01 the volcanic are, along Ecuador, Per,Bolivia, Chile , and Argentina.The basin reached a width 01 400 to 500 km during times 01 high sea

levels and had two sources 01 sediment, one W in the Central Cordillera and another E in theGuyana Shield. The strata in the W side 01 the basin have a calcareous/litharenitic nature, in contrast with the ones in the opposite E side that have a quartzarenitic nature, because the last oneswere sourced mainly by the Precambrian and Paleozoic sedimentary rocks 01 the Guyana Shield.

Sedimentary environments in the UMV were control!ed mainly by eustatic sea level changesthat affected the whole intracratonic basin, so that sea level pulses are synchronous in both , Eand W sides 01 the basin. Since the sequence stratigraphy 01 sea level changes in the UMV issynchronous with the one in the Eastern Cordillera and Llanos Foothills , a Iramework 01allostratigraphic units is lormal ly proposed lor the whole basin.

Type localities and selected areas lrom the UMV including Aipe, Guaduas, Ortega, and Piedraswere sampled lor analyses 01 sedimentary petrography and micropaleontology 01 loraminilersand palynomorphs , allowing precise characterization 01 the units and appropriate dating . Theoldest known ages 01 the Cretaceous sedimentary succession in this part 01 the basin are Aptian;however, the ancestral Central Cordillera also sourced older Cretaceous strata 01 known Berriasianto Barremian age in adjacent areas.

Key Words: Allostratigraphy, Back-Arc Basin, Colombia, Cretaceous, Foraminifers, Hydrocarbon Exploration, Lithostratigraphy, Paleogeography, Palynomorphs, Sedimentology, Sedimen-tary Petrography, Sequence Stratigraphy, and Upper Magdalena Valley.

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Guerrero et al.: The Stratigraphy of the W Side of the Cretaceous Colombian Basin .

INTRODUCTION

RESUMENLa sucesin Cretcica del Valle Superior del Magdalena (VSM) est dominada por estratosmarinos, compuestos por una mezcla de partculas calcreas y terrgenas. Estos incluyen principalmente biomicritas,bioesparitas, lodolitas,arenitas y rocas mixtas calcreo/terrgenas de afinidadlitoarentica. Rocas sedimentarias depositadas en ambientes fluvitiles, incluyendo conglomeradosde grnulos y guijos, solamente estn presentes en la base y en el tope de la seccin.La sucesin se deposit en una rampa, que tuvo inclinacin de alrededor de 0.06hacia el Ey alcanz profundidades marinas de 200 a 250 m en el eje de la cuenca.La seccin Cretcica delVSM tuvo como fuente de detritos una cadena montaosa localizada al W y correspondiente a laancestral Cordillera Central , como lo indica el abundante contenido de fragmentos de rocametamrf ica y volcnica.Se propone el trmino CUENCA CRETCICA COLOMBIANA, para hacer nfasis en elconcepto de un nica cuenca de back-arc. Esta se abra al N hacia el Caribe y se conectaba alS de manera intermitente con otras cuencas de back- arc y de foreland al E del arco volcnico, alo largo de Ecuador, Per , Bolivia, Chile y Argentina. La cuenca alcanz un ancho de 400 a 500km durante intervalos de ascenso del nivel del mar y tuvo dos fuentes de detritos, una al W en laancestral Cordillera Central y otra al E en el Escudo de Guyana. Los estratos en el lado W de lacuenca tienen afinidad calcreo/litarentica, en contraste con los del lado opuesto de la cuenca al

E que tienen una afinidad cuarzoarentica, debido a la proveniencia de detritos originadosprincipalmente de la erosin de las rocas sedimentarias del Precmbrico y Paleozoico del Escudo de Guyana.Los ambientes sedimentarios del VSM fueron controlados principalmente por cambioseustticos del nivel del mar, que afectaron toda la cuenca intracratnica, de manera que lospulsos de ascenso y descenso del nivel del mar son sincrnicos en los dos lados E y W de lacuenca. Teniendo en cuenta que las secuencias estratigrficas del VSM son sincrnicas con lasde la Cordillera Oriental y el Piedemonte Llanero, se propone un esquema formal de unidadesaloestratigrficas para toda la cuenca.Se tomaron muestras para anlisis de petrografa sedimentaria y micropaleontologa deforaminferos y palinomorfos en varias localidades tipo y reas seleccionadas , incluyendo Aipe,Guaduas, Ortega y Piedras, para lograr una caracterizacin litolgica ms precisa, as como

dataciones adecuadas. Las edades ms antiguas que se conocen para esta parte de la cuencason del Aptiano, no obstante que la ancestral Cordillera Central sirvi tambin de area fuente aestratos del Berriasiano al Barremiano en reas adyacentes .

Most of the names currently used in the stratigraphicnomenclature of the Cretaceous of the UMV are derivedfrom Piedras and Ortega, two areas closely located toeach other, on the N side of this part of the valley (Fig. 1).The Caballos (CORRIGAN 1967) and the e t u ~ n (ALLEN 1989;BARR Io & COFFIELD 1992) units were first studied and namedin the area of Ortega . The Hondita, Lomagorda, Olini, andLa Tabla units (PORTA 1965, 1966) have their type localityin the area of Piedras . The Buscavida (CORRIGAN 1967)and Seca (PORTA 1965, 1966) units were also named fromareas nearby Piedras. Based on these early works, a stratigraphic nomenclature (Table 1) including all or so me ofthose units has been extended to the S of the UMV forseveral authors (e .g. BELTRN & GALLO 1968; BARRIo &COFFIELD 1992; ETAYO & FLREZ 1994; VERGARA 1994, 1997;VILLAMIL 1998).

UMV comes originally from the stratigraphy of the GirardotNario Section of BRGL & DUMIT (1954) , the stratigraphyof the Ortega area of BRGL (1961), and the stratigraphyand geological map (Plate L9-Girardot) of ~ A A S V E L D T et al.(1956). This geological map covered the UMV ares froma latitude around Ortega-Saldaa on the SW, to PiedrasJerusaln on the NE.

The Cretaceous succession of the UMV was dividedby VERGARA (1994, 1997) in the Yav, Caballos, Hondita,and Lomagorda Formations, the Olini Group, the "nivel delutitas y arenas", and the La Tabla Formation . However, adirect comparison with thE: lithology and biostratigraphy ofthe type locality near Piedras was not performed by him,due to the poor state of preservation and advanced weathering of the strata. Because of that, and although Vergaraestablished that most of the units could be identified inthe whole U ~ V , doubts arise regarding some of theirboundaries a n ~ t h e i r correlation .

The subdivision of the Cretaceous succession of the

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Since the lithology and biostratigraphy of thetype locality are essential to properly correlate andidentify the units in the UMV, we have oone an effort to obtain a better characterization of them .Several excellent sections (from creeks) exposedin the Piedras area have been reevaluated , including field measuring and description , additionally tosampling for laboratory analysis, mainly of sedimentary petrography and micropaleontology. AIthough emphasis is placed in the stratigraphy ofthe Piedras area, other sections from the Aipe,Guaduas, and Ortega areas where al so revisedand sampled.

Due to the fact that most of the Cretaceoussedimentary record of the UMV is composed of amixture of very fine grained calcareous and terrigenous partic les (mud to very fine sand size), rocksare difficult to classify properly in the outcrops witha hand lens and common field techniques. Commonly, sandstones with carbonate cement and minor amounts of fossils are erroneously classifiedas "Iimestones", or true biomicrites of planktic foraminifers are classified as terrigenous mudstones.Because of that, we believe that sedimentary petrography (Plates 1 to 11) of well exposed and nonweathered sections (Plate 12) is essential to properly characterize these rocks. The procedure ofcareful field collection has been also applied to thesamples for micropaleontological analysis , resulting in a much better recovery of well preservedspecimens of palynomorphs (App. Plate 1) and foraminifers (App. Plates 11 to VI) , that in road and otherweathered outcrop samples

.{ With a better description and dating of the unitsin their type locality, we hope to solve some of theconfusion that has been a constant in the stratigraphy of the Cretaceous of Colombia, due to thelack of appropriate procedures according to international standards of stratigraphic codes andguides. Units with similar lithology but very different stratigraphic position have been given the samename, and there is also the contrary and most common case, that a single unit has been given differe"t names in nearby localities. A mixture of formaland non-formal names is used in unpublished thesis, field trips, and company reports, that makesvery difficult the communication for the purposesof characterization of the units, geological mappingand exploration of resources.

The boundary between the Upper and Middle

Geologa Colombiana No.25, Diciembre, 2000

Magdalena Valley is located near the town of Dorada. The uppercourse of the Magdalena River is characterized by a narrowvalley in which the river has an alternative sedimentary and erosive nature over rock units as old as Paleozoic. To the S, in thearea of Neiva and Tatacoa Desert , the valley ex poses mainly theCretaceous and Tertiary strata that rest with angular unconformity

"

N" - iGIRARDOT

" -

PRAGA 2\ C;O Ion

} ' -

YAGUARA

Fig, 1, Location of Cretaceous Sections from the. UMV,Crdoba (1), Honda (2), Guaduas (3), Piedras (4), El Cobre(5), Ortega (6), OIini (7), Ataco (8), Aipe (9), and Yaguar (10),

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Guerrero et al.: The Stratigraphy 01 the W Side 01 the Cretaceous Colombian Basin .

on the Jurassic basement of the Saldaa Formation.Between Girardot, Piedras, and Dorada, the river is eroding Late Cretaceous and Middle Miocene strata. Due tothe differential resistance of these strata , several rapidsthat make navigation difficult are found near Honda. Theerosive nature of the Magdalena River in its upper courseis due to the continued uplift of the valley along with theuplift of Central and Eastern Cordilleras. The climate isdry in the UMV, being in the rain shadow of the two cordilleras; the transition to more wet regimes is located inDorada, were the valley becomes wider. From Dorada tothe N, the Magdalena River develops a more permanentmeandering to anastomosed pattern in a less confinedva 1ey, with sedimentation dominating over erosiono

The stratigraphy of the Cretaceous succession alongthe UMV, as far N as Dorada, is very comparable betweendifferent localities. Reverse faults that today control thevalley (e .g. the Cambrs and the Bituima Fault systems) ,were sinsedimentary normal faults parallel to the cretaceous shoreline (see LUNOBERG et al. 1998; MARTINEZ &VERGARA 1999). Notorious variations in thickness and lithology are found only in an E direction , toward thedepocenter of a back arc basin. The E boundary of theUMV stratigraphy is located in the Bituima Fault; the opposite W boundary is located in the E foothills of the Central Cordillera.METHOOS ANO CLASSIFICATIONS

For the procedures of naming , reevaluating and proposing stratigraphic units, we follow the North AmericanCode of Stratigraphic Nomenclature (NORTH AMERICAN COMMISSION ON STRATIGRAPHIC NOM ENCLATURE 1983) published bythe American Association of Petroleum Geologists. TheInternational Stratigraphic Guide (SALVADOR 1994), published by the International Union of Geological Sciencesand the Geological Society of America , was also used. Inthose cases in which there are differences between thetwo publications, the specific norm is discussed.

Regarding the nomenclature to refer the thickness ofstrata, the ones of 1 cm or less are designated as laminae and those thicker as beds, according to INGRAM (1954) .The laminae are subdivided in very thin (Iess than 0.3mm), thin (0.3-1 .0 mm), medium (1-3 mm) and thick (3-10 mm) . The beds are subdivided in very thin (1-3 cm) ,thin (3-10 cm) , medium (10-30 cm) , thick (30-100 cm)and very thick (more than 1 m) .

The compositional and textural classifications are thoseof FOLK (1959, 1962, 1974) for the terrigenous and calcareous rocks (and their mixtures) . Because in the terrig-

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enous rocks it is very important the analysis of provenance(source area), a modification was introduced to place allthe polycrystalline quartz among the rock fragments, including it in one of the three RF poles , metamorphic, sedimentary or volcanic. For the calcareous rocks, additionally to the textural and compositional classifications of FOLK(1959, 1962), the textural classification of DUNHAM (1962)was also used.

To express the texture and porosity of the rocks underthe petrographic microscope, the results of counting 250to 400 points (at 1 mm intervals, over lines perpendicularto stratification and spaced 3 mm) are indicated in percentages of framework, matrix, cement, and pores. In thematrix, were included the particles with an average diameter 1/10 smaller than the average size of the particlesof the framework. In the fossiliferous muddy sandstonesand impure sandy biomicrites (fine and very fine sandstone range), the muddy matrix is usually composed of amixture of calcareous and terrigenous particles smallerthan 8 covered with organic matter of marine origin thatdarkens them and makes difficult the identification of individual particles. The matrix appears as a paste of pale todark brown color in the microscope and dark grey to blackcolor on hand sample .

The relative percentages of particles of the framework,that can be more easily identified than those of the matrix, always determined if the rock had a more terrigenousor a more calcareous nature. The proportion of calcareous and terrigenous particles of the matrix in the clay andvery fine silt size (Iess than 8 ~ ) , is usually the same oneof the larger particles of the framework . When doubts didarise in the petrographic microscope, other complementary laboratory methods, such as dissolving the sample inHCI to weight the remaining particles were used.

Following Folk's guidelines for classification of finegrained rocks composed of mixtures of terrigenous andcalcareous particles, a fossiliferous mudstone is an impureterrigenous rock composed of a mixture of terrigenousand calcareous particles in the clay and silt size range.The matrix is composed of clay minerals and otherterrigenous particles , mix'ed with minor amounts ofcalcareous particles. The composition of the frameworkparticles above is predominantly terrigenous (Q, F, andRF), with lesser amounts of fossils and other calcareousparticles (>10% and


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TABLE 1. HISTORICAL OEVELOPMENT OF THE SlHATlGRAPHIC CLASSIRCATION OF THE CflETACEOUS SUCCESSION FROM THE UMV,INVAUO SYNONYMS ANO HOMONYMS, ALONG WITH MISUSEO OR INFORMAL NAMES ARE INOICATEO IN 'QUOTATlON MARKS',

RAASVEl.DT El AL , 19561 PORTA, 1965, 1966 CORR1GAN, 1967 BELlRN&GAlLO, 1966 1BAAA10&COFAB.D, 19921 ElAYO & LREZ, 1994 1 VERGARA, 1994, 1997 VlLLAMIL, 1998

KO SECA FORMATION 'GUAOOAS FORMATION' 'GlJAOlIAlAGROUP' 'GUAOOAS FORMATION' SECA FORMAnON 'GlJAlJUAS FORMATION'

K1 LA TABlA FORMA TION ' G l J A D . l J . . ~ 'BUSCAVIDAFORMATI SHAlE FACIES" 'MONSERPATE FORMATION' 'MONSERPATE FORMATION LA TABlA FORMAnON 'GUAOAlUPE FORMAnON'

K2 'NIVEL DE LUTlTAS yARENAS" 'MONSERPATE I 'BUSCAVIDA SHAl.P ~ I V E L DE LUTlTAS Y RENAS" 'UNNAMED'O

I 'UPPER CHERl O OK3 L L1D1TA SUPERIOR V 'UPPERCHERl FORMATlON' L1DITASUPERIOR L L1DITA SUPERIOR L 'UPPER CHERlI I I IN L N N f - - - - - - -I L I i Fl r.oRRF ;ANOSl' I IK4 'NIVEL DE LUTITAS" E L 1 'AK:O SHAlE' 'NIVEL DE LUTITAS' 'MIDOLE SHAlE'G T L , G GR A E R RO T O OI U L1D1TA INFERIOR A 1 L1D ITA INFERIOR I 'LOWER CHERl5 'LOWER CHER1' I 'LOWER CHERl L101TA INFERIORP F iO iK6 I LOMAGORDA FORMATION I R I I 'VILLETA I R IM ' 'LA LUNA L1MESTONE' 1 LOMAGORDA FORMA TIONATK7 HONOITA FORMA TION I I I FORMATION' I "fiAMBLC SHALE" ' v 'UNNAMED UNIl'O L UN 1 'HONDITA FORMAnON' E PT 'K8 'TETUN L1MESTONE' i A 'HIL FORMATION'

UPPER K9 I UPPER MEfolBER' I 'CABALLOS FORMATION' I 'CABALLOS FORMATION' I I 'SEGMENT 5' I I UPPER CABALLOS"MIDOLE K9 CABALLOS IA I MIDOLE folEMSER' I " T 1 : T I 1 1 ~ I U ~ C : : T f ' I N ~ I 'EL OCAL FORMATION' I I 'SEGMENT 4' I ILOWER K9 FORMATlON I I 1 N EGMENTS 1-3' N 'LOWER CABALLOS"

'LOWER SANDSTONE' 'ALPUJARPA FORMATlON' iKJ YAv FORMAnoN

J3 1PRE-CRETACEOIJS BASEMENT IPRE-CRETACEOUS BASEfolEj PRE-CRET ACEOIJS BASEMENTI SALDAA FORMA TION SALDAA FORMAT ION

GUERRERO El AL ,200l

SECA FORMATON

LA TABlA FORMATION

HONDITA FORMATIONG>roTETUN FORMAnON oo(Qj-

I IUPPER SANOSTONE MEt.IlER V :::::::::::: ooo3crI IM1UULt ~ ~ 4 t J CUMlvMfi!Ni!fl:OOM j':::JQlZ"LOWER SANDSTONE MEt.IlER 1\).tno,,'YAvf FORMAnON ;'3cr.o

PRE-CRETACEOUS BASEMENT- 1\)ooo


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Guerrero et al.: The Stratigraphy 01 h e W Side 01 h e Cretaceous Colombian Basin.

particles in the clay and silt size range. The matrix iscomposed 01 calcareous particles , mixed with minoramounts 01 clay minerals and other terrigenous particles.The composition 01 the Iramework particles aboye 8iJ ispredominantly calcareous (Iossils) , with lesser amounts01 terrigenous particles (> 10% and


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-f J ,rr '

CALCAAEOUS : Md W k Pk Grll instone. Aud tesTERRIGENO US C Md S t vfS f5 mS eS veS Gr Pebb

_9 @,' c

Odh phc

O' ? s rs

( O phtS l \:;."'3-


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Guerrero el al.: The Stratigraphy 01 h e W Side 01 h e Cretaceous Colombia n Bas in .

exposes the Lower Terti ary, and th e Seca, La Tabla , andBuscavida Formations, along with part of the LiditaSuperior and El Cobre Formations. The second sectionexposes most of the El Cobre Formation and the lowerpart of the Lidita Superior Formation .

On the E flank of the syncline, the Guaduas Section(Fig. 1) is located approximately 11 km S40E of Guaduasand 1 km SSW of the "Balastera" open pit mine. It isreached from the main road Guaduas - Bogot, taking adeviation to the S from a small village named La Cabaa.This deviation goes in an unpaved road for about 7 km , upto the La Bolsa Creek, where excellent exposures of theGuaduas, La Tabla, Buscavida, Lidita Superior, El Cobre,Lidita Inferior, and Lomagorda Formations are found.

The El Cobre Creek is reached from Payand. Its upper course is located approximately 3 km SW, taking anunpaved road to the Chical Creek, a tributary of the ElCobre. The other way to reach it is taking the road to Vallede San Juan, toward the S for approximately 8 km , up to adeviation to the NNW that goes for about 1.5 km to theconfluence of the El Cobre Creek with the Luisa River.The section has very good exposures of the three members of the Caballos Formation .

The Aipe Section in the Palmarosa and BambucCreeks is reached on the W deviation to Praga, located Nof the Aipe River, from the main road Bogot - Neiva. Theexposures located approximately 12 km from the maindeviation are part of the W flank of the Media Luna Syncline. The Palmarosa Creek exposes the Yav Formationand part of the Caballos Formation . The Bambuc Creekexposes the rest of the succession , up to the top of the LaTabla Formation .

STRATIGRAPHYThe Cretaceous Succession of the UMV (Table 1) is

divided in the Yav , Caballos, Tetun , Hondita, Lomagorda,Lidita Inferior, El Cobre, Lidita Superior, Buscavida , LaTabla, and Seca Formations (Plates 1 - 12). Some of theseunits that were used fo r many years in an informal andimprecise way, are here formally defined and characterized. The new units include the Tetun Formation , the ElCobre Formation, and the Buscavida Formation .YAv FORMATION

The Yav Formation (MOJICA & MACiA 1983) constitutesthe basal unit of the cretaceous succession in some 10-calities of the UMV. It rests with angular unconformity on

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older units, and has a variable thickness that reaches amaximum of approximately 300 m.

The Yav Formation is recognized by the alternance ofreddish and multicolored sandstones and mudstones invery thick beds; it includes in some localities a basal conglomeratic sandstone or a pebble and co bbl e conglomerate up to 35-40 m thick. Its type locality is the Yav River,near Prado and Dolores. According to the data of MOJICA& MACiA (1983) , the sandstones include significant amountsof volcanic ro ck fragments, along with quartz and feldspars , that would classify them as lithic (volcanic) arenites.

The thickness of the Yav Formation decreases as muchas the thickness of the lower sandstone segment of theCaballos Formation increases, so that. the basal transgressive sandstone beds of the Cretaceous in the UMVinclude the Yav Formation and the Lower SandstoneMember of the Caballos Formation . In those places wherethe basal cretaceous section does not include reddish andmulticolored mudstones interlayered with the sandstonesand conglomerates, it is considered that the Yav Fo rmation do es not ex ist, and the basal strata are included inthe Lower Sandstone Member of the Caballos Formation.

A section of the Yav Formation with a thickness ofapproximately 300 m was measured in the PalmarosaBambuc Creek (Aipe Section) , where the lower contactwith the Saldaa Formation is well exposed. The composition of the sandstones of the unit classifies them as lithic(volcanic) arenites (Plate 1) .

PLATE 1. YAv FORMATION, Palmarosa Creek (AipeArea). Thin section photographs of lithic (volcanic)arenites including quartz, feldspar, and abundant vol -canic rock fragments with phenocrysts in a microc-rystalline groundmass. Note calcareous (A, B) andferruginous cement (G , H), the absence of fossils, andthe alteration of feldspars and volcanic rock fragments(C, D, E, F).A, B: Crossed nicols. Samples from 20 and 25 m aboyethe base of the unit.C, D: Crossed and parallel nicols. Sample from 40 maboye the base of the unit.E, F: Crossed and parallel nicols. Sample from 140 maboye the base of the unit.G, H: Crossed nicols. Sample from 210 m aboye thebase of the unit. H is an enlargement (rotated 90 tothe left) of the central portion of G.


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G

E

e

A

EE'"

]l


H

F

D

B

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Guerre ro el al.: The Stratigraphy 01 he W Side 01 he Cretaceous Colombian Ba s in .

Although the Yav Formation and the Lower SandstoneMember of the Caballos Formation are composed predominantiy of sandstone, the accompanying finer grainedlithology is very important to establish the boundary between the two units. The upper contact of the Yav Formation is placed in the uppermost bed of reddish mudstone,considering the fact that at some point of the section theinterbeded mudstones are of dark grey to black color andcontain marine fossils. This contact is transitional in manylocalities, where an interval of a few meters could includeinterbeddings of reddish and black mudstones.

Palynomorphs of late Early Aptian age (VERGARA &PROSSL 1994) were collected from a part of the sectionthat could be interpreted either as the uppermost part ofthe Yav Formation or lowermost part of the Caballos Formation in the Palmarosa-Bambuc section. These are theoldest dated fossils of the cretaceous succession of theUMV.CABALLOS FORMATION

The type locality of the Caballos Formation (CORRIGAN1967) is the one including the vicinities of Cerro Caballosnear Ortega. The author originally proposed it to refer tothe "basal sandstone unit of the Aptian-Albian, which iswidespread in the Upper Magdalena Basin ". According toCorrigan, the thickness of the formation ranges from 100to 450 m; the lower contact is transgressive, with slight tostrong unconformity over pre-Cretaceous rocks; its uppercontact was placed "at the top of the first massive sandstone unit beneath the vi leta shales". Later on , BELTRN &GALLO (1968) divided the Caballos Formation in threelithological units, including two sandstone segments separated by a finer grained segment that included black shales .

The proposal of FLREZ & CARR ILLO (1994) to subdividethe Caballos Formation was discussed by VERGAR A et al.(1995), who indicated that the new names "alpujarra" and"el ocal" should be avoided because they were applied tostrata of variable lithology and stratigraphic position . Because of these variation in thickness, lithology and stratigraphic position of the three subdivisions from one to another locality, the upper contact of the "el cal formation"is in some areas with the "caballos formation" and in others with the "vi lleta formation" (FLREZ & CARRILLO, 1994: p.11-16) . Since the subdivision of the Caballos Formationwas not attempted in a single section and the proposedunits do not have a clearly defined lithology and stratigraphic position , their boundaries are very confusing . Webelieve that the originally published definition (Article 4 ofthe NASC) of the Caballos Formation by CORRIGAN (1967)is clear enough, has priority (Article 7c of the NASC) over

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other proposed names, and should be maintained. If newnames for the three segments of the Caballos Formationintroduced by BELTRN & GALLO are going to be proposed,it should be done at the type area of the Caballos Formation of CORRIGAN (1967) .The Caballos Formation is mainly of marine nature ,especially from the middle segment which includes ammonites (Ra/abaceras sp., Epiche/aniceras sp.) and foraminifers of Late Aptian age (VERGARA et a/., 1995). Theformation is approximately 300 m thick in the QuebradaPalmarosa-Bambuc (Aipe Section), according to VERGARA(1994). The minor interbeddings of mudstone containedin the Lower and Upper Sandstone Members are of blackcolor, in contrast with the reddish and multicolored onesof the underlying Yav Formation.The lower contact of the Caballos Formation is con formable on top of the Yav Formation or unconformableon pre-cretaceous units, in the areas where the Yav Formation was not deposited.In this study, the section of the El Cobre Creek NNE ofOrtega, was measured and sampled (Fig . 1, Plate 2) . Thecomposition of the sandstones from the Caballos Formation classifies them in a range that goes from lithic arenitesand sublitharenites of volcanic nature, to quartz arenites.An small percentage of limestones and impure limestones

that includes biosparite and sandy biosparite is alsopresent in several sections.The textural and compositionalmaturity of the sandstones increases toward the top ofthe formation , where some quartz sandstone is present.

PLATE 2. CABALLOS FORMATION. Thin section photographs of lithic -yolcanic- arenite (A, B), recrystallized biomicrite (C, O), fossiliferous quartz arenite (E,F), and fish-bone sandy biosparite (G, H).A, B: Crossed and parallel nicols. Sample from 30 maboye the base of the Lower Sandstone Member. ElCobre Creek (Ortega Area).C, O: Crossed and parallel nicols. Sample from 5 maboye the base of the Middle Mudstone/BiomicriteMember. El Cobre Creek (Ortega Area).E, F: Crossed and parallel nicols. Sample from 20 maboye the base of the Upper Sandstone Member. ElCobre Creek (Ortega Area).G, H: Crossed and parallel nicols. Top of the UpperSandstone Member. Bambuc Creek (Aipe Area).


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TETUN FORMATION (new unit)VERGARA (1994, 1997) ineluded the strata below theOlini Group in the Hondita and Lomagorda Formations,following the original proposal of PORTA (1965, 1966).

VERGARA (1994) dated the whole Cretaceous section withforaminifers for the first time , and extended the lowerboundary of the Hondita Formation of PORTA to inelude thestrata overlying the sandstones of the Caballos Formation. However, the lithology of the Hondita Formation in itstype locality (fossiliferous terrigenous shales and very finegrained sandstones) , its age (essentially Cenomanian) ,and stratigraphic position , do not correspond with the stratathat rest on top of the Caballos Formation. These stratathat inelude marlstones and biomicrites of late Early,Middle, and Late Albian age must be excluded from thelower part of the "hondita formation" of Vergara.The name Tetun Formation is formally proposed forthe approximately 200 m of marlstones and biomicritesthat rest on the sandstones of the Caballos Formationand underlie the coarsening-upward terrigenous mudstones, sandy mudstones, and sandstones of the HonditaFormation in the UMV. The age of such strata is late Early,Middle and Late Albian, according to VERGARA (1997) andVILLAMIL (1998) . The name is apparentiy derived from theTetun River, so that we formally propose the type localityof the unit in the area of Ortega. Good sections are 10-

cated on the Chicuambe Anticline, 6 km NE of Ortegaand 10 km N of the Tetun River. The Tetun Formationineludes the lower 150 to 200 m of the "villeta formation",that rest on the Caballos Formation at the ChicuambeAnticline Section of BARRIO & COFFIELD (1992: fig. 8) .The Tetun Formation corresponds to the K8 unit ofRAASVELDT et al. (1956) , who had already mapped anddescribed it as clayish limestones, alternating with calcareous and bituminous black shales ("Calizas arcillosas

alternando con esquistos negros calcreos ybituminosos").The name "tetun limestone" has been previously usedin an imprecise and informal way to refer strata in different stratigraphic positions. BARRIO & COFFIELD (1992: fig. 2and p. 126) used the name in an informal way to ineludestrata underlying the Upper Sandstone Member of theCaballos Formation. They also indicated in their correlation chart (BARR IO & COFF IELD, 1992: fig. 2) that the namehad been previously used to refer to strata aboye theCaballos Formation . ETAYO & FLREZ (1994) and ETAYO

(1 994) used the name informally to refer to the strata overIying the Upper Sandstone Member of the Caballos Formation. PEA & ANNICCHARICO (1999) placed the unit aboye

56

the Caballos Formation and indicated that it attained amaximum thickness of 145 m in oil wells, and measuredfield sections up to 80 m thick in the lower part of the unit.According to them, the most common lithology is the onecorresponding to sample APO-16, which in thin section(PEA & ANNICCHARICO, 1999: Anexo 2.1) is a biomicrite offoraminifers.The Tetun Formation is redefined here to inelude allthe fine-grained calcareous strata aboye the CaballosFormation and below the Hondita Formation. The contactbetween the Caballos and Tetun Formations is placedon top of the last massive, thick to very thick san dstone(or sandy biosparite) bed of the Caballos Formation, aboyewhich the predominant lithology is not any more of a sandynature but of a marlstone or biomicrite nature (Fig. 2) . Thecontact between the Tetun and the Hondita Formatian is

PLATE 3. TETUN FORMATION. Thin section photographs of biomicrites and fossiliferous shales fromthe Aipe Area (A, B, e) and from the Ortega Area (O, E,F, G, H). Note partial recrystallization of framework(foraminifers) and calcareous mud matrix in the Aipesamples.A: Parallel nicols. Sample from 5 m aboye the base ofthe unit. Packstone-texture biomicrite of foraminifers.Most of the framework fossils are recrystallized. AipeArea.B: Parallel nicols. Sample from 15 m aboye the baseof the unit. Note silt size glauconite (green) particleand so me foraminifers partially recrystallized andpyritized, floating in a calcareous mud matrix. AipeArea.e: Parallel nicols. Sample from 80 m aboye the baseof the unit. Foraminifers and matrix partially recrystallized and pyritized floating in a calcareous mudmatrix. Aipe AreaO: Parallel nicols. Sample from 10m aboye the baseof the unit. Fossiliferous mudstone with very thin siltstone lamina at the bottom. Ortega Area.E, F: erossed and parallel nicols: Very fossiliferousshale. SiIt size terrigenous particles in the frameworkare slightly more abundant than foraminifers. Samplefrom 30 m aboye the base of the unit. Ortega Area.G, H: erossed and parallel nicols. Wackestone-texturebiomicrite of foraminifers from 90 m aboye the baseof the unit. Ortega Area. Note the absence of terrigenous particles in the framework.


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placed on top of the last foraminiferal marlstone orbiomicrite bed of the Tetun Formation, where the stratachange in nature to the predominantly terrigenous onesof the overlying Hondita Formation .

Good reference sections of the Tetun Formation arethe ones from Ataco (VERGARA, 1997) and from theQuebrada Olini (VILLAML , 1998). According to VERGARA(1997: p. 114 and fig. 4) , it would be composed of "blacklaminated marlstones interbeded with biomicrite beds andabundant carbonate concretions". According to VILLAM IL(1998: p. 210, and append. fig . 1) it would be composed ofpelagic limestones and organic-rich calcareous shales.

Several authors (e.g. ETAYO, 1994; ETAYO & FLREZ , 1994;VILLAM IL , 1998) have also divided the strata that overlie theCaballos Formation and underlie the Olini Group in threelithostratigraphic units. We do agree with the presence ofthree well-defined units (Table 1) , but we can not followthe usage of several formations and informal units thatinclude a mixture of names from the Eastern Cordilleraand the UMV.

ETAYO & FLREZ (1994) included the strata above thesandstones of the Caballos Formation and below thecherts of the Olini Group in the informal units "tetun limestone", "bambuc shale" and "la luna limestone". ETAYO(1994: fig . 2) divided the "villeta group" in the informal units"tetun limestone" and "la frontera limestone", withoutnaming the intermediate shales and very fine-grainedsandstones. Neither these names were formally proposednor the lithology, thickness and boundaries clearly stated,so that they have signif icant differences between successive publications of these authors.

VILLAMIL (1998) proposed the subdivision of the "villetagroup" of the UMV in the "hil formation", the "unnamedunit" and the "la frontera formation". Villamil also provideda biostratigraphic framework, based mainly in bivalves andammonites, and recognized the presence of three well defined stratigraphic units between the Caballos Formation and the Olini Group. However, the "hil formation" andthe "la frontera formation" are improperly used in the UMV,because their lithology and stratigraphic position do notcorrespond with the characteristics of the units in theirtype localities on the Eastern Cordillera. The three unitsrecognized by VILLAMIL (1998) correspond to the TetunFormation of this work and to the Hondita and LomagordaFormations of PORTA (1965, 1966).

In conclusion, the units "vi leta", "bambuc", "hil", "lafrontera", and "la luna", can not be recognized in the UMV,because they include non-valid homonyms and synonyms

58

of the units formally proposed by PORTA (1965, 1966). Several authors that studied the Vi lleta Group stratigraphy,paleogeography and correlations (e.g. HUBACH, 1957;JULlVERT, 1968; VERGARA 1994, 1997; VERGARA et al., 1995;GUERRERO & SARMIENTO , 1996; LUNDBERG et al ., 1998 ;MARTIN EZ & VERGARA, 1999) indicated that the use of theterm Vi leta Group is restricted to the Eastern Cordillera.

It is inadequate to include in the "vi lleta group" the stratabetween the Caballos Formation and the Olini Group inthe UMV, because the Vi leta Group in its type locality onthe Eastern Cordillera is a unit of different lithology andstratigraphic position . The Villeta Group is composed ofterrigenous mudstones and sandstones with aquartzarenite affinity and encompasses strata ofBarremian to Santonian age. Includes the Fmeque, Uneand Chipaque Formations above the Cqueza Group onthe E flank of the Eastern Cordillera, and the Trincheras,Socot, Capotes, Hil, Limolitas de Pacho, La Fronteraand Conejo Formations above the La Naveta Formationon the W flan k of the Eastern Cordillera. The Villeta Grouphas a thickness of about 3.000 to 4.000 m, and differentsource area, mostly in the Guyana Shield on the E side ofthe basin . In contrast, the succession erroneously named"vi lleta group" in the UMV is mainly of calcareous andmetamorphic/volcanic nature including biomicrites ,biosparites and fossiliferous litharenites. It encompassesonly strata of late Early Albian to Coniacian age , has athickness of approximately 600 m, and has a differentsource area on the opposite, W side of the basin , on ametamorphic-volcanic ancestral Central Cordillera arc (seeLUNDBERG et al, 1998).

Regarding the use of the term "villeta formation ",CORRIGAN (1967: p. 232) stated: "an unfortunate term ... it isoften used to designate little studied rocks that may en-compass the entire Cretaceous, and locally may includepre-Cretaceous and even Tertiary sediments". We shouldadd that still today the term is loosely and improperly usedin the Magdalena Valley, Eastern Llanos, and Putumayo,to refer to almost any fine grained, dark-grey to black colored cretaceous strata.

HONDITA FORMATION

The Hondita Formation was the name given by PORTA(1965, 1966) to the lowermost unit of the Cretaceous typesection from the area of Piedras. According to his description (PORTA, 1965: p. 12 and fig. 2; PORTA , 1966: p. 31 andplate 1), the unit has 90 m of thickness , and is composedmainly by "sandy limestones" in beds of a few cm to 1 m,that alternate with mudstones and shales.


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CALCAREOUS Md Wk Pk Gr . nstones Rud ltes Md Wk Pk Gr. instones Rud ltesTERRtGENOUS. e Md Slt vfS fS mS eS veS Gr Pebb e Md S il ., S IS mS eS veS G, Pebb

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According to PORTA, the formation corresponds to theunit K7 of RAASVELDT et al. (1956) , that in the legend of thegeological map indicates "calcareous-clayish shales,scarce beds of impure limestones". In Spanish: "esquistoscalcreo-arcillosos, escasos bancos de calizas impuras ".This would suggest that the predominant lithology is theshale, contrary to Porta 's descriptions and drawings ofthe sections that would suggest that the "sandy limestones"predominate.

Our field and laboratory results from the Piedras areaindicate that the Hondita Formation has a thickness of atleast 140 m, and in fact includes "sandy" lithologies. Veryfine grained, clean and also muddy sandstones (Fig. 3,Plate 4) have been identified; they contain calcareous cement, abundant foraminifers, a few fish bones, and someglauconite. Although there is a fare amount of sandstone,terrigenous mudstones are also very common; they arefrequently sandy (with minor amounts of very fine sandterrigenous particles) and fossiliferous . The terrigenousnature of the Hondita Formation has a very sharp contrast with the overlying foraminiferal biomicrites of theLomagorda Formation.

The sandy mudstone has a soft aspect in the field,that is interrupted by the thin to medium (occasionally thick)beds of very fine sandstones, which are harder and moreprominent than the mudstones. The internal stratificationis mostly in lamina sets and thin .beds. The sandstonesexhibit ripple and wavy bedding. Bioturbation is occasionally observed.

The terrigenous component of the sandstones classifies them as lithic arenites. The rock fragment (RF pole)component being mainly metamorphic and volcanic .Muscovitic quartzites are the most common metamorphicrock fragment, followed by more altered volcanic fragmentscomposed of a mixture of microcrystalline quartz and feldspars with some phenocrysts, in the dacite to andesitecompositional range. The monocrystalline quartz is frequently strained and clearly of nietamorphic origin; somequartz can al so be identified as of volcanic originoSomeof the sandstones have important amounts of feldsparsand are classified as feldspathic litharenites.

The terrigenous "impure limestone" or "sandy limestone" character of the Hondita Formation indicated byPORTA and RAASVELDT et al. is notorious enough to establish a lithological difference with the units below (the TetunFormation) and aboye (the Lomagorda Formation) , whichare composed mainly of biomicrites. In the Piedras area,the lower boundary of the Hondita Formation is not observed due to a major reverse fault that rises the creta-

60

ceous block E of Piedras and places the unit in contactwith the Middle Miocene Honda Group. Other minor re-ve rse faults repeat parts of the unit , so that ~ e a s u r e dthickness is approximate. The contact with the overlyingLomagorda Formation is a relatively sharp boundaryplaced where the terrigenous nature of the strata disappear, to change to the foraminiferal biomicrites of theLomagorda Formation .

Other sections of the Hondita FormationThe "unnamed" Cenomanian strata in the Olini Creek

near Chaparral (100 km to the SSW of Piedras) are hereincluded in the Hondita Formation . According to VILLAML(1998: p. 205-206, p. 210-212, figs. 11, 27, 29, 30) theyinclude claystones and siltstones in the lower part withfine grained sandstones toward the upper par. In theYaguar Section further S, the upper part of these stratawould include 20 m of coarse grained sandstones in thickto very thick beds, interlayered with claystones (VILLAMIL1998: p. 205-206, p. 210-212, fig. 26) . The lower boundaryof the Hondita Formation in the Olini Section would be an

PLATE 4. HONDITA FORMATION. Thin section photographs of fossiliferous mudstones and very finegrained Iithic (metamorphic I volcanic) arenites.Hondita Creek, Piedras Area.A, B: Crossed and parallel nicols. Very fine grainedsandstone sample from 129 m below the top of theunit. Note abundant feldspars (twinned), altered volcanic rock fragments, calcite cement, and calcareousfossil fragments.C, O: Crossed and parallel nicols. Fossiliferous mudstone from 92 m below the top of the unit. Largestterrigenous particles have average diameters of 40-50 The calcareous (in part pyritized) particles arefossils of foraminifers. Most of the framework (medium and coarse silt) and matrix (clay and very finesilt) particles are of terrigenous nature.E, F: Crossed and parallel nicols. Laminae of matrixfree very fine grained sandstone (above) and sandyfossiliferous mudstone (below). Sample from 81 m below the top of the unit. Note foraminifers in the mudstone lamina. Glauconite in the center of photo.G, H: Crossed and parallel nicols. Calcite-cemented,very fine grained, fossiliferous sandstone. Samplefrom 75 m below the top of the unit. Note irregularfestooned shapes and corroded borders of terrigenousparticles, because of partial replacement by calcite.


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abrupt one over the calcareous shales and pelagic limestones of the underlying ("hil formation") uni!. The HonditaFormation would be approximately 200 m thick in the OliniSection (from m 260 to m 460 in fig. 27 and append. fig. 1of VILLAMIL 1998).The approximately 95 m of Cenomanian strata fromthe Quebrada Calamb Section that ETAYO & FLREZ (1994)included in the "bambuc shale" are also included here inthe Hondita Formation . They documented the section as

severely faulted , with two repetitions of the unit, in whichincluded muddy-clay'sh shales, with minor thin beds ofmicrite; toward the upper part, medium beds of fine grainedsandstones with a mud matrix of terrigenous and calcareous particles.Age

The Hondita Formation is essentially of Cenomanianage , according to its foraminiferal content (Tables 2, 3,Appendix Plates 11 , 111) in its type section, in the area ofPiedras. The data of ETAYO & FLREZ (1994) and VILLAMIL(1998) also indicate a Cenomanian age .ETAYO & FLR EZ reported the presence of several ammonite genera in the "bambuc shale" from the CalambSection, including Sharpeiceras, Schloenbachia, Acomp-

soceras , Mariella , Metoicoceras , Carthaginites,Neostlingoceras, and Stonohamites .VILLAMIL (1998: p. 189) indicated that the resolution ofbiostratigraphic correlation of the Cenomanian stage in

Colombia remains relatively low. He proposed severalzones of bivalves and ammonites, including in the ammo-nites the genera Parengonoceras , Metoicoceras ,Desmoceras and Wrightoceras . The upper part of his "unnamed unit" included the zone of the bivalve Exogyrasquamata and the ammonite Wrightoceras munieri, indicating that the zone is gene rally represented by coarsegrained sandstones with thick Thalassinoides burrows andshallow water bivalves.This sandstones correspond to theuppermost part of the Hondita Formation of this work.

We do agree with a boundary between the Honditaand Lomagorda Formations very close to the CenomanianTuronian boundary, but acknowledge that this lithologicalboundary could be slightly diachronic between sections.In the other hand, the diachronism might be created oroveremphasized as a result of different biostratigraphicinterpretations; for example, VILLAMIL (1998: p. 189) indicated that Wrightoceras munieri had been proposed asan Early Turonian fossil by ZABORSKY (1990) , but in Colombia it occurs in Late Cenomanian strata.

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Our ages from the upper part of the Hondita Formation indicate clearly a Cenomanian . The uppermost partof the formation includes strata broadly dated in the transitio n Late Cenomanian / Early Turonian. The planktic foraminifers (Table 3) include Rotalipora brotzeni (Plate 11) andPraeglobotruncana delrioensis, two forms that accordingto CARON (1985) have thei r last appearance in the LateCenomanian.They coexist with Whiteinella baltica that hasits first appearance also during the Late Cenomanian .

Other planktic species present are Guembelitriacenomana and Helvetoglobotruncana praehelvetica, whichcoexist in the latest Cenomanian - earliest Turonian. Inthe other hand , there are abundant specimens ofWhiteinella archaeocretacea and inornata, that have theirfirst appearance in the Early Turonian.

In contrast with the very scarce planktic recovery, thebenthic foraminifers (Table 3) are very abundant in theHondita Formation . Important forms are Gavelinellacenomanica, Praebulimina subcretacea, Praebuliminaprimitiva, Lenticulina gaultiana, and Pyramidina minima.

The palynological content (Appendix Plate 1) of the uppermost part of the Hondita Formation in the Piedras areaalso indicates an age range of latest Cenomanian to earliest Turonian. Dichastopollenites sp. is a pollen known fromthe Late Cenomanian of Egypt (IBRAHIM 1996) . Classopollissp. is a pollen known to have its last appearance in theearliest Turonian of Senegal (JARDINE & MAGLOIRE 1965).Appendicisporites auritus is a spore reported to have itslast appearance in the Turonian of California (JAMEOSSANAIE& LINDSLEY-GRIFFIN 1993) . The marine dinoflagellateDinogymnium sp . has been reported as having his firstappearance in the Turonian of Venezuela (HELENES et al.1998), but ROBASZINSKY et al. (1982) have reported it fromthe latest Cenomanian of France.

LOMAGORDA FORMATIONAccording to PORTA (1966: p. 34), the Lomagorda Formation has a thickness of 167 m; it is composed predominantly by mudstones and shales, with large elliptical calcareous concretions of up to 1 m of diameter, that alternate with minor proportions of sandstone beds and somechert beds.The Lomagorda Formation overlies the HonditaFormation and underlies the Olini Group.The Lomagorda Formation corresponds to the K6 unitof RAASVELDT et al. (1956), that was described as "calcare

ous-clayish mudstones with beds of sandy limestone andsome "Iidita" toward the base. The same calcareous clay-


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TABLE 2. PLANKTIC FORAMINIFERS (IN NUMBER OF SPECIMENS) FROM THE HONDITA , LOMAGORDA ,ANO LIDITA INFERIOR FORMATIONS. HONDITA ANO CARDONA CREEKS, PIEDRAS AREA.

HONDITAFM LOMAGORDA FORMATION LlDITA INF FM LlTHOSTRATIGRAPHYCENOMAN CIT TURONIAN CONIACIAN EARLY SANTONIAN SANTONIAN GEOCHRONOLOGYI I I I I I I I I I :I: :I: I :I: :I: :I: I :I: I I I I :I: :I: I I I ( ) ( ) ( )

N '" ." . '" '" ..., O> '" o - W v. m c;; ) N N N N N N N N N '" PLANKTiCFORAMINIFERSN ..., o N '" .". '" '" ..., O> '" oc;, m (JI o Hedbergella p/anispira

N N c;, (JI w o


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ish mudstones with large calcareous concretions in tllemiddle part (wagon wheel horizon) , and very finely laminated shales toward the topo In Spanish : "esquistoscalcreo-arcillosos con bancos de caliza silicosa y algunasliditas en la base ; los mismos esquistos con grandesconcreciones -ruedas de carreta- en la parte media yesquistos pizarrosos muy delgados -hojosos- en la partesuperior.

To B RGL & DUMIT (1954: p. 32-33 and Plates 2-3), thelower K6b horizon is composed of sandy marlstones with"Iiditas" and limestones. The upper K6a horizon is composed of sandy marlstones with large calcareous concre tions up to 2 m in diameter, and finely laminated calcareous shales (K6b: margas arenosas con lid itas y calizas;K6a : margas arenosas con grandes concrecionescalcreas de hasta 2 m de dimetro y esquistos margososhojosos.

AII the descriptions make emphasis in the la rge calcareous concretions present in the medium to upper partof the un it, and in the presence of "lid tas" or "cherts" toward the base of it. However, the predominant lithology issomewhat different to these authors. To R AASVELDT et al.(1956), it is calcareous-clayish mudstone; to P ORTA (1966),it is mudstone and shale; to B URGL & D UMIT (1954), it is"marlstone". It is certainly difficult to differentiate in thefield between terrigenous mudstones with minor amountof fossils and impure biomicrites with minor amounts ofterrigenous mud, especially if the samples are weathered.They are both very fine grained (clay and silt size partic es) and react to HCI ; the only observable particles witha hand lens are probably the foraminifers, with the additional problem that some of them are diagenetically silicified and might appear like terrigenous quartz particles inweathered samples.

The non weathered exposures of the Hondita Creekand our petrographic analyses indicate that the predominant lithology of the Lomagorda Forination is laminatedforaminiferal biomicrite of packstone texture. The rocksare almost entirely made of foraminifers and minor amounts of small bivalves, containing very minor amounts ofterrigenous particles (up to coarse silt) around 1-2%. Someof them reach up to 15% of te rr igenous particles and areclassified as impure biomicrites. Minor loraminileralbiomicrites 01 wackestone texture are also present (les8than 20% 01 the lormation). The large calcareous concretions 01 up to 2 m 01 diameter are a very notorious leaturein the upper part of the Lomagorda Formation . These calcareous concretions are the result 01 early cementation01 the biomicrite beca use the strata surrounding are morecompacted and bend around the concretions.

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Close to the base of the Lomagorda Formation thereare scarce occurrences of soft, ve ry thin and thin beds ofgreenish-grey colo red bentonites that contrast very sharplywith the dark grey and black colors 01 the predominantbiomicrites . Fo r a thickness of about 5 m, some of thebiomicrite strata associated with the bentonite beds areIractured horizontally and vertically to produce the cubicshapes 01 5-10 cm side, that are referred as "Iiditas". Ne ither sandstones nor sandy lithologies where detected inthe unit.

PLATE 5. LOMAGORDA FORMATION. Thin sectionphotographs of packstone biomicrites of foraminifers.So me of them (D, F) include small percentages (Iessthan 10%) of terrigenous particles in the mud size fraction. Others, from the lower part of the unit (A and B),include 10% to 15% terrigenous particles and are classified as impure (muddy) biomicrites. Hondita Creek,Piedras Area.A: Crossed nicols. Largest terrigenous particle isabout 80 ~ (very fine sand), but most of them are. smaller than 60 in the clay to coarse silt range.Sample from 8 m aboye the base of the unit.B: Crossed nicols. Large terrigenous part icles are 20-30 medium silt size. Note large silicified foraminifers (100 and 150 long) in the lower part of photo.Sample from 42 m aboye the base of the unit.C: Parallel nicols. Phosphatized fish spines in thelower right and upper lett. Sample from 72 m belowthe top of the unit.D: Crossed nicols . Scarce terrigenous particles ofcoarse silt size. Sample from 51 m below the top ofthe unit.E: Parallel nicols. Horizontal fractures filled with oil.Sample from 38 m below the top of the unit.F: Crossed nicols. The isolated fine sand quartz particle (150 ~ long) in the center of photo is among thelargest terrigenous particles observed in the unit, andconstitutes an exception. Sample from 30 m belowthe top of the unit.G: Parallel nicols. Subhorizontal fractures filled withoil. Sample from 15 m below the top of the unit.H: Parallel nicols. Green 90 glauconite in the upperlett, and some phosphatized particles including fishspine in the lower right. Sample from 3 m below thetop of the unit.


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The Lomagorda Formation measures 238 m in theQuebrada Hondita. Toward the lower part of the section,there are several minor folds and reverse faults that dip inthe same direction of the beds (SE) , and repeat them. Webelieve that these faults have minor displacements of lessthan 10m , because there are no significant changes inlithology or biostratigraphic contents between the faultedblocks. Measurement of the Lomagorda Formation in nonfaulted sections of nearby locations, indicates an averagethickness of 210m .

Other sections of t e Lomagorda FormationVERGARA (1997: p. 116) designated the Ataco Section

(110 km SSW of Piedras, and also very near to the OliniCreek) , as the reference section of the Lomagorda Formation . Over there, it would be a 121 m thick succession ... "with a basal 15 m set of fine chert-mudstone intercalations" .. and consists "predominantly of dark calcareous shales with carbonate banks tenths of meters thick.The carbonates are foraminiferal wackestones (biomicrites)containing up to 33% of planktic foraminifers".

The lithology of the Ataco Section awaits revision andpetrographic characterization , because no shales areknown in the type loca ity of the unit nearby Piedras . Inthe fig. 5 of VERGARA (1997), the section is about 103 mthick (instead of the 121 given in his text) and "cherts" goup to about m 25 (instead of m 15). Most of the remainingof the section has the symbol of "black marls" (instead of"dark calcareous shales"). We believe that the "calcareous shales" or "black marls" of VERGARA could well be thebiomicrites found in the type locality of the unit near Piedras.The only published photo of a thin section of theLomagorda Formation in Ataco (VERGARA 1994: plate 8)indicates that lithology is very similar to the one at Piedras.

The strata from the Olini Creek that Villamil included inthe "la frontera formation " h e r e included in theLomagorda Formation.According tO"vILLAMIL (1998:p.212) ,the unit is characterized by "rhythmically interbeded limestones and very dark, laminated shales". These "dark, laminated shales" appear to be calcareous shales, accordingwith the symbol used in his stratigraphic section (append.fig . 1 of VILLAMIL 1998).

AgeAs indicated by its very rich content of planktic

foraminifers (Table 2) , the Lomagorda Formation includesthe Turonian , the Coniacian, and the earliest Santonian.

66

The most diverse assemblages with more abundantspecimens are present in the lower part of the LomagordaFormation. Toward the upper part of the unit , diversity andnumber of specimens decreases . Benthic foraminifers(Table 3) are scarce in the unit, in contrast with the veryrich content of them in the underlying Hondita Formation .

The lower part of the formation contains abundantspecimens of Whiteinella archaeocretacea and inornata,that according to CARON (1985), have their first appearance in the Early Turonian. Hedbergella delrioensis, a formnot any younger than earliest Santonian is present troughthe whole formation . These three species would narrowthe age determination to a range no older than EarlyTuronian, and no younger than the earliest Santonian .

Other species that support the age range include thefi rst appearances of Archaeoglobigerina cretacea, blowiand bosquensis during the Coniacian. Pseudogembelinacostulata has its first appearance in the upper part of theformation , indicating the late Coniacian . Hedbergellaflandrini , a relatively common species of the unit, has arange of Late Turonian to earliest Santonian . The firstappearance of Contusotruncana fornicata indicates thelate Coniacian to earliest Santonian .

In the Guaduas Section, on the E side of the GuaduasSyncline , the age range of the Lomagorda Formationincludes also the Turonian, the Coniacian, and the earliestSantonian . The lower part of the unit includes Heterohelixmoremani, Whiteinella archaeocretacea, and Heterohelixreussi. The upper part of the unit includes Marginotruncanasinuosa , Heterohelix reussi , Archaeoglobigerinabosquensis, and Archaeoglobigerina blowi.

OLlNI GROUPThe Olini Group (PORTA 1965, 1966) is composed of

the Lidita Inferior, the "nivel de lutitas" and the Lidita Superior. The "lid itas" have been intervals of correlationsince BURGL & DUMIT (1954) and PETTERS (1954), corresponding to strata in thin beds (5-10 cm) that are foundgenerally fractured orthogonally in cubic shapes. However,its lithology has been little studied and there is very littlepetrographic documentation published. As stated by PORTA(1965: p.14 and 1966: p.37) , "the stratigraphic names efthe units of the Olini Group, not always correspond withtheir petrography (Iithology)" . The middle level of the OliniGroup, or "nivel de lutitas" includes coarser grain size thanany one of the liditas aboye and below. In several localities it is constituted not by mudstones (Iutitas), but by sandstones that vary of very fine to medium grain size.


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LlDITA INFERIOR FORMATIONAccording to PORTA(1965, 1966), the unit is composedby 30-35 m of interlayering of "cherts" of yellowish whitecolor, "Iiditas", mudstones and black shales. Several foldsaffect the section . The "cherts" in thin and sporadicallymedium beds prevail over the other lithologies. Petrographically, they would be "porcelanites and calcareous cherts".In the Quebrada Hondita, the succession assigned to

the Lidita Inferior Formation is composed predominantlyby finely laminated wackestone biomicrites, in part silicified , that in some levels are fractured in orthogonal prismsof 5-10 cm of side. There are also minor laminae and verythin beds of muddy packstone biomicrites, with concentrations of phosphatic particles. The Lidita Inferior Formation corresponds to the K5 unit of RAASVE OT et al. (1956).

The rocks of the Olini Group, that in field have beenassigned traditionally to "cherts" or to "Iiditas", correspondin a more detailed examination in thin section, with veryfinely laminated wackestone biomicrites, exhibiting partialsilicification in laminae and irregular lenses. The replacement for silica involves mostly the tests of the foraminifersthat originally are made of calcite, and sometimes do involve sectors of the calcareous matrix. These biomicritesof wackestone texture have a relationship of calcareousmud matrix that is around 75% to 90%, and framework ofplanktic and benthic foraminifers around 10% to 25% ofthe total of the rock. Vertical microfractures in two perpendicular directions produce the known pattern of lithologyin orthogonal prisms 5-10 cm of side.In the micropaleontologic analysis, there have been

reported some radiolarians in very small and negligibleamounts of less than 1%. The bioturbation in these rocksis almost totally absent and the foraminifers are as a rulesmaller (diameters generally around 20 than those ofunits above and below. They lay in a perfectly horizontalway that is made very notorious by the presence of specimens with relatively long tests . The partially silicifiedwackestone biomicrites constitute beds of diagenetic, butnot biogenic or primary cherts.

The laminae and very thin beds of muddy packstonebiomicrites, with concentrations of phosphatic particles,are present in very minor amounts and constitute lessthan 5% of the unit. The content of phosphatized particlesis not enough to show up in the gamma ray logs, in contrast with the Lidita Superior Formation , that is very phosphatic.

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Other sections of the Lidita Inferior FormationAccording to BARRIO& COFFlE O 1992:p. 132), the "Iower

chert" includes lOto 20 m of "thin bedded, finely laminated brown siliceous shales" .. "A few beds of black lenticular and laminated chert are also present ".VERGARA (1997) measured 64.4 m of the Lidita Inferior

Formation in the Ataco area. He indicated that "the.lithology consists of blocky, fractured, dark cherts that weatherto a beige color, exhibit rhythmic bedded layers 1 to 15 cmthick and alternate with laminated shale".

PLATE 6. LlDITA INFERIOR FORMATION. Thin sectionphotographs of fossiliferous micrites and wackestonebiomicrites of foraminifers. The fossiliferous micritesare the most typical lithology of the formation, beingcomposed of calcareous mud in the clay and fine siltsize range, with less than 10% of larger (more than 16J.I.) identifiable calcareous particles (fossils in thiscase). The wackestone biomicrites have percentagesof i.cIentifiable fossil particles larger than fine silt, thatnever reach more than 15% of the samples. There isreplacement of calcite by quartz, so that parts of thesame sample, and some times of the same thin section, range from micrite, to partially silicified micrite,to diagenetic chert. Hondita and Cardona Creeks,Piedras Area.A, B: Crossed and parallel nicols. Two silicified foraminifers (almost transparent in parallel nicols), in thelower center (160 J.I. long) and upper left (100 J.I. long).The other foraminifers retained their original calcitetest, although some of them are recrystallized and losttheir original texture. Sample from 7 m aboye the baseof the unit.C, D: Crossed and parallel nicols. Silicified foraminiter (200 J.I. long) in the center ot photo. Phosphatizedfish spine in the lower left. Sample trom 10 m aboyethe base of the unit.E, F: Crossed and parallel nicols. Two silicified foraminiters (100 J.I. long) in the upper left and upper right.The shades of pale to dark brown colors show differential concentration ot organic matter in the calcareous mud matrix. Black spots are pyrite crystals.Sample trom 17 m aboye the base of the unit.G, H: Crossed and parallel nicols. Large (120 J.I. long)toraminiter in the upper right retained its original calcite test. Most others are replaced by quartz. Blackspots are pyrite crystals. Sample trom 27 m aboyethe base of the unit.


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On the E flank of the Guaduas Syncline (GuaduasSection) , the Lidita Inferior Formation has 43 m of thickness. It is composed predominantly of thin beds of finelylaminated wackestone biomicrites, in part silicified , thatare usually fractured in orthogonal prisms of 5-10 cm ofside. There are also some thin beds of muddy packstonebiomicrites, with notorious concentrations of phosphatizedparticles.Age

The Lidita Inferior Formation is of Santonian age, asindicated by its strat igraphic position between theLomagorda Formation (which includes the earl ies tSantonian) and the El Cobre Formation (which in somelocalities includes the latest Santonian) .The Lidita InferiorFormation represents only part of the approximately 3 .5Ma time span of the Santonian. The samples from theHondita and Cardona Creeks (Table 4) include Heterohelixreussi, Hastigerinoides sp., Archaeoglobigerina cretacea,and A. blowi.EL COBRE FORMATION (new unit)

According to PORTA (1965, 1966), the intermediate levelof the Olini Group or "nivel de lutitas" (Ievel of mudstones)has 65 m of thickness, being composed predominantly ofmudstones, locally sandy, with minor intercalations of somebeds of sandstones. His section from the Piedras - LaTabla walkway is very generalized, and it is not indicatedwhich type of sandstones they are, neither from the compositional nor textural point of view. The Plate 1 of PORTA(1966) has a grain size curve that assigns the term "finedetritics" to the sandstone interbeddings, suggesting thatthey would be fine or very fine grained sandstones .

BAR RIO & COFFIELD (1992: fig . 2 and p. 133) designatedthe uppermost 20 m of the "unit between the two cherts"in an informal way as "El Cobre Sandstone". BARRO &COFFIELD did not explain the origin of the "el cobre" name,but it could not have been taken from the Quebrada ElCobre beca use over there the outcrops are from theCaballos Formation instead of the Olini Group. To theseauthors, the "unit between the two cherts" in the La Favorita- La Manga section (10 km SSW of Payand) is 380 mthick and composed of "brown micritic limestones and greyto fine-grained light green calcareous sandstones withconcretions, bivalves , and ammonites". In the QuebradaEl Loro (30 km SW of Payand), the "section between thetwo cherts" is composed of "medium- to fine-grained greensandstones with abundant lenticular and flaser bedding ".In the Ortega area (40 km SSW of Payand) , the "sectionbetween the cherts consists of finely laminated black and

70

brown limestones with abundant microfauna. The top isdominated by laminated yellowish marls with thin coquinabeds and thin beds of calcareous sandstones .. This section ranges in thickness from 90 to 120 m".

Most of the authors that have published sections ofthe Olini Group recognize the presence of sandstones inthe unit between the Lidita Inferior and Lidita SuperiorFormations. However, they also recognize lithological differences between the sections and include along with thesandstones, other lithologies such as mudstones (PORTA1965), micrites (BARRIO & COFFIELD1992) or shales (VERGARA1997). Unfortunately, most of the sections have little detail and no petrographic documentation, making it difficultto recognize how much sandstone do the unit containsand what would it be the accompanying lithology.

Although the unit between the Lidita Inferior and LiditaSuperior Formations has not been formally named, the"El Cobre Sandstone" is a known term for oil geologists,that some times use it not only for the uppermost part ofthese strata, but for all of it. For these reason , we believethat. the El Cobre Formation is an appropriated name forthe unit. We do not name it "El Cobre Sandstone Formation", because the unit is not entirely made of sandstone.

In all the sections known to us, sandstone is abundant. If mudstones or limestones are present, they arealways sandy or at least have a notorious terrigenous component, a fact that contrasts very sharply with the LiditaInferior and Lidita Superior Formations, that are composedof biomicrites of wackestone texture, almost entirely barren of terrigenous particles.

The El Cobre Formation is formally proposed to referto the strata between the Lidita Inferior and Lidita Superior Formations of the Olini Group. It replaces the informalterm "Ievel of mudstones" of PORTA (1965, 1966). The typelocality of the El Cobre Formation is the same one of theother two formations of the Olini Group, in the area ofPiedras.

Our th in sections and field observations from thePiedras - La Tabla walkway and Talora Creek, indicate thatthe unit between the Lidita Inferior and the Lidita SuperiorFormations of the Olini Group has 140 m of thickness .Glauconitic, fossiliferous sandstones of very fine grain size,with a lower proportion of sandy biosparites compose it.The foraminifers are again the most common fossil type,along with phosphatic fish remains . Some of the sandstones reach fine grain size. The El Cobre Formation corresponds in stratigraphic position to the K4 unit of RAASVELDTet al. (1956).


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eALCAREOUS: Md W k Pk Gr. n.tones Rud tnTERRtGENOUS : el Md Sil vfS fS mS eS veS Gr Pebb

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Geologa Colombiana No.25, Diciembre,2000

Md Wk Pk Grain.tonn Rud tese l Md Slt vfS fS mS eS veS Gr Pebb

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- T-20

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Fig. 4, Latest Santonian to early Late Maastrichtian Section. Talora Creek, Piedras Area.

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In the Talora Creek outcrops are excellent, being exposed the uppermost 123 m of the unit. A polygonal measured in the Piedras - La Tabla walk way indicates that theunit is 140 m thick. The contact with the underlying LiditaInferior Formation is relatively sharp, with a transition ofless than 5 m in which the finely laminated wackestonebiomicrites of the Lidita Inferior Formation change to thebioturbated fossiliferous sandstones and sandy biosparitesof the El Cobre Formation.

Three segments are distinguished in the unit. The lowersegment (41 m thick) is composed of an alternan ce ofmedium to very thick beds of sandstones and limestones,that have a soft (friable) and hard expression in the field .The friable strata predominate , being bioturbated, veryfine grained, fossiliferous sandstones in thick and very thickbeds, with calcareous concretions 20-40 cm in diameter.The more resistant and prominent strata correspond tosandy biosparites in medium and thick beds, al so totallyhomogenized by bioturbation . Benthic foraminifers (veryfine to medium sand size) are the most common fossil inboth lithologies.

In the middle segment (49 m thick) predominate theclean, calcite cemented, glauconitic and fossiliferous, veryfine grained sandstones. The segment is notorious for itsstratification in laminae and very thin beds present in setsup to 30 cm thick that monotonously repeat. Bioturbationis notoriously absent and predominate the hydrodynamicover the biogenic structures, including very low angle wavyand lenticular strata. There are sporadic but notorious thinand medium phosphatic beds that have gamma ray values slightly aboye the ones of the rest of the unit. Thesehorizons include phosphatic and terrigenous particles that

PLATE 7. EL COBRE FORMATION. Thin section photographs of matrix-free, calcite-cemented, fossilifer ous, very fine to fine grained, lithic (metamorphic/volcanic) arenites, and biosparites of foraminifers andbivalves. AII of them with crossed nicols. Note irregular festooned shapes and corroded borders of terrigenous particles, beca use of partial replacement bycalcite. Talora Cree k, Piedras Area.A: Very fine grained, fossiliferous sandstone. Twotwinned feldspars in the lower left and right corners.Large (120 .t) foraminifer to the left of the feldspar onthe lower right cornero Sample is composed of 57%framework particles and 43% calcite cemento Theframework particles are divided in 57% terrigenousparticles (41% monocrystalline quartz, 6% metamorphic rock fragments, 5% volcanic rock fragments, 4%feldspars, 1% glauconite) and 43% calcareous par-

72

ticles (benthic foraminifers). Sample from 110m below the top of the unit.B: Fine grained, fossili ferous sandstone. Twinned feldspar in the center of photo. Abundant bivalve fragments. Sample is composed of 65% framework particles and 35% calcite cemento The framework is composed of 72% terrigenous particles and 28% calcareous particles. Sample from 97 m below the top of theunit.C: Impure (sandy) biosparite of foraminifers. Abun-dant specimens of Buliminella sp. in the center andupper right side. Large specimen of Sipho-generinoides sp. in the lower part is 1.1 mm long. Largest terrigenous particles (center right) are around 100.t in diameter. Sample is composed of 61 % frameworkparticles and 39% calcite cement: The framework iscomposed of 18% terrigenous particles and 82% calcareous particles. Note brownish colored, phos-phatized tests of foraminifers. Sample from 65 m below the top of the unit.o: Very fine grained, fossiliferous sandstone. Sampleis composed of 62% framework particles and 38%calcite cemento The framework is composed of 76%terrigenous particles and 24% calcareous particles.Sample from 50 m below the top of the unit.E: Very fine grained, fossil iferous sandstone, with verysimilar percentages to the sample on D. Large (200 .t)twinned feldspar in the center is partially replaced bycalcite, at the middle part and borders of the particle.Other terrigenous particles are very reduced in sizeand have angular, corroded, and festooned shapes dueto extensive replacement by calcite. Sample from 45m below the top of the unit.F: Very fine grained, fossiliferous sandstone. Sampleis composed of 63% framework particles and 37%calcite cement. The framework is composed of 53%terrigenous particles and 47% calcareous particles.Original 80 .t size of twinned feldspar in the centerhas reduced to 40 .t., due to partial replacement bycalcite. Sample from 25 m below the top of the unit.G: Very fine to fine grained, glauconitic, fossiliferoussandstone. Sample is composed of 65% frameworkparticles and 35% cal cite cemento The framework iscomposed of 84% terrigenous particles and 16% calcareous particles (foraminifers and fish spines).Sample from 10 m below the top of the unit.H: Very fine grained, glauconitic, fossiliferous sandstone, with very similar percentages to the sample onG. Green glauconite particle (300 .t long) at the center, probably corresponds to a fecal pellet. Samplefrom 5 m below the top of the unit.


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reach coarse sand and granule size ; some of them arefriable due to the weathering of phosphates.

The upper segment (33 m thick) includes greenishgrey, very fine and fine , friable sandstones. They are alsofossiliferous and calcite cemented , but very glauconitic.Stratified in laminae and very thin beds, that composesets up to 3 m thick, alternating with very thick bioturbatedbeds of about the same thickness . The bioturbation ismoderated, caused by small organisms that did not displaced vertically, so that original stratification can be partlyappreciated . Grain size is slightly coarser than in the lowerparts of the formation . Some beds are harder due to largeramounts of calcite cemen!.Other sections of the El Cobre Formation

BERMUDEZ & MORCOTE (1995) studied several sectionsof the unit ("formacin nivel de lutitas") in the vicinities ofPayand. According to them, it would be approximately165 m thick, and composed of fossiliferous, glauconitic,bioturbated, calcite cemented, fine and very fine grainedsandstones that toward the top of the unit change gradually to conglomeratic sandstones including granule sizeparticles. They also reported calcite concretions in thesandstones of the lower part of the uni!. The most common fossils include benthic foraminifers, bivalves and phosphatic fish bones. Along with the calcite cemented, veryfine grained sandstones that constitute the most commonlithology, they reported "calcareous sandstones" that probably are the sandy biosparites recognized in the type 10-cality nearby Piedras.

In the southern portion of the UMV, in the proximity ofAipe (Quebrada Bambuc), VERGARA (1997: p. 118) indicated that the "unit between the two liditas" of the OliniGroup consisted of 52 m of fine and very fine grainedsandstones. VERGARA also indicated that the most common lithology of the unit in the Ataco Section was shale ,but over there the section is faulted (VER


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TABLE 4. PLANKTIC FORAMINIFERS (IN NUMBER OF SPECIMENS) FROM THE EL COBRE,BUSCAVIDA, ANO LA TABLA FORMATIONS. TALORA CREEK, PIEDRAS AREA.EL COBRE LlDITA SUP BUSCAVI LA TABLA SECA LlTHOSTRATIGRAPHY

EARLY CAMP LATE CAMPANIAN EARLy MAASTRICHTIAN LMAA GEOCHRONOLOGYen

- t - t - t - t - t - t - t - t - t - t - t - t - t - t - t - t - t - t :l>, . ~ PLANKTIC FORAMINIFERSc.> 01 (J) ... CD


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In the middle (16 m) segment, predominate the veryfinely laminated wackestone biomicrites, in part silicified ,that give origin to diagenetic cherts. The verticalmicrofractures in two perpendicular directions produce theknown standard lithology in orthogonal prisms of 5-10 cmof side that have been referred traditionally as "Iiditas" .Calcareous concretions and len ses up to 30 cm thick, area product of early diagenesis beca use the surroundingstrata bend around them , to depart from the usual pattern of planar and parallel bedding. Phosphatic beds arenegligible and present only in laminae that separate thethin beds of micrite and chert. These laminae of phosphates are recognized because of their soft nature andweathering to yellowish colors. Cherts are black, very brittleand are easily recognized in the field because of their conchoidal fracture ; their aspect is rugged in contrast with therounded borders developed on the micrites.

The upper (24 m) segment is similar to the basal segment and again composed of foraminiferal biomicrites ofpackstone texture, in finely laminated thin to medium beds,with sporadic thin and medium beds of phosphorites thatinclude bio -, pel - and intra - micrites of packstone texture. These phosphatic beds are present in hard and softbeds; the last ones probably correspond to the "mudstonesand black shales" of PORTA (1966), from the Piedras - LaTabla walkway.

The highest gamma ray reading of the sucession (230CPS) , corresponds to the beds of phosphorites of the lowerand upper segments and of the Lidita Superior Formation , in contrast with the chert beds of the middle segment, that have much lower readings of about 50 CPS.

The Lidita Superior Formation corresponds to the K3unit of RAASVELDT et al. (1956).Other sections of the Lidita Superior Formation

BARRIO & COFFIELD (1992: p. 133) indicated that the "upper chert" in the Ortega area ranged in thickness from 40

PLATE 8. LlDITA SUPERiOR FORMATION. Thin section photographs of fossiliferous micrites, wackestonebiomicrites, and phosphatized packstone intrabiomicrites of foraminifers. The fossiliferous micritesare the most typical lithology of the middle part of theformation, being composed of calcareous mud in theclay and fine silt size range, with less than 10% oflarger (more than 16 identifiable calcareous particles (fossils in this case). The wackestone biomicr iteshave percentages of identifiable fossil particles larger

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than fine silt, that rarely reach more than 15% of thesamples. There is replacement of calcite by quartz, sothat parts of the same sample range from micrite, topartially silicified micrite, to diagenetic chert. Thepackstone intra-biomicrites are present in scarce thinto medium beds that are thicker and more common inthe lower and upper parts of the formation. TaloraCreek, Piedras Area.A, B: Crossed and parallel nicols. Silicified, fossiliferous micrite (diagenetic chert). Most foraminifers andmatrix are replaced by quartz. Diagenetic cherts arerecognized in thin section because almost all foraminifers are replaced by quartz. Although the mineralogical change is not as evident in the matrix because it is darkened by organic matter, the handsamples reveal the nature of these rocks, that do notreact to HCI, are black colored, and have a brittle conchoidal fracture. Sample from 20 m aboye the base ofthe unit.C: Parallel nicols. Wackestone biomicrite of foraminifers. Calcite-filled vertical fracture (typical of the"liditas") in the left. Glauconite (60 in the center.Sample from 24 m aboye the base of the unit.D: Parallel nicols. Medium lamina (2 mm thick) ofpackstone intra-biomicrite from the same thin sectionshown in C. Light brown colored particles are phosphatized foraminifers, intraclasts, and fish bones.Black spots are pyrite crystals. The test of foraminiferat the center is replaced by pyrite. The specimen between two phosphatized particles in the upper rightcorner is Buliminella colonensis. Very large specimensin the lower and upper part are Siphogenerinoides sp.E, F: Crossed and parallel nicols. Partially silicifiedwackestone biomicrite of foraminifers. Large specimens (up to 300 long) with their original calcite filling and test replaced by quartz (gray, black, and whitespeckled in crossed nicols, and almost transparentwith parallel nicols). Note that some foraminifers retained their original calcite composition. Phosphatizedparticles (on the left), are black in crossed nicols andlight brown in parallel nicols. Sample from 32 m aboyethe base of the unit.G: Parallel nicols. Phosphatized bio-intramicrite. Lightbrown colored intraclasts reach coarse sand size.Sample from 43 m aboye the base of the unit.H: Parallel nicols. Wackestone biomicrite of foraminifers. Note dark brown, almost black colored calcareous mud matrix, due to large concentrations of organic matter. Most specimens are of Buliminellacolonensis. Sample from 2 m below the top of the unit.


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to 90 m. According to them , the unit consists of "intercal

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