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An Integrated Calcareous Microfossil Biostratigraphic andCarbon-Isotope Stratigraphic Framework for the La Luna

Formation, Western Venezuela

LINDA M. DE ROMERODepartment of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599,

E-mail: [email protected]

IRENE M. TRUSKOWSKIPetroleos de Venezuela, PDVSA, Exploracion, Apdo. 829, Caracas 1010A, Venezuela

TIMOTHY J. BRALOWER*Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599

JAMES A. BERGEN**Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC

OSCAR ODREMANEscuela de Ingenierıa Geologica, Universidad de Los Andes, Merida, Venezuela

JAMES C. ZACHOSEarth Sciences Department, University of California, Santa Cruz, CA 95064

FRANCIA A. GALEA-ALVAREZPetroleos de Venezuela, PDVSA, Exploracion, Apdo. 829, Caracas 1010A, Venezuela

PALAIOS, 2003, V. 18, p. 349–366

The biostratigraphy of the La Luna Formation has longbeen in dispute, despite the importance of this unit as themost important source rock for hydrocarbons in Venezuela.In this paper, calcareous microfossil biostratigraphy com-bined with carbon-isotope stratigraphy provides a strati-graphic framework for the formation, permitting revision oftemporal and spatial patterns of deposition of organic-richsediments. Detailed studies were conducted on a cored bore-hole and five outcrop sections distributed across the Mara-caibo Basin of western Venezuela.

Planktic foraminifera have fair to good preservation, andnannofossils are poorly preserved. Many of the Cenomani-an to Campanian planktic foraminiferal marker speciesare present, permitting the application of a traditional zon-al scheme. An informal nannofossil biostratigraphic zonalscheme, based primarily on dissolution-resistant species,has been developed. Integration of these zonal schemes hasenabled the correlation of changes in carbon-isotope ratiosto the global C-isotope stratigraphy. The results have beenused to estimate temporal variation in sedimentation ratesas well as to reconstruct depositional patterns across theMaracaibo Basin. Deposition began in the eastern andnorthwestern part of the Maracaibo Basin in the middle

* Current address: Department of Geosciences, Pennsylvania StateUniversity, University Park, PA 16802.** Current address: BP America, Inc., 501 Westlake Park Blvd., Hous-ton, TX 77079.

Cenomanian and progressed towards the south and west,reaching the southwestern corner by the middle Coniacian.Although the uppermost part of the formation could not bedated, deposition in the eastern basin continued at leastinto the Coniacian and in the western basin at least untilthe middle–late Santonian. Sedimentation rates were high-ly variable with a period of condensed sedimentation at theCenomanian–Turonian boundary.

INTRODUCTION

The Upper Cretaceous La Luna Formation, distributedthroughout western Venezuela and northeastern Colom-bia, is the source of most of the oil and gas in the Maracai-bo Basin and adjacent areas (Talukdar and Marcano,1994). The formation contains possibly the most prolificsource rocks in the world (Brooks, 1990; West, 1996). Ageequivalents of the La Luna Formation are also the princi-pal petroleum source rock in eastern Venezuela, Colom-bia, Ecuador, and northern Peru (Talukdar et al., 1985;Mello et al., 1995; Schenk et al., 2000). Despite its signifi-cance, the age of the formation has not been precisely de-termined. The purpose of this investigation is to provide atemporal framework for the La Luna Formation. Nanno-fossil biostratigraphy has been integrated with plankticforaminiferal biostratigraphy and carbon isotope stratig-raphy to produce a workable stratigraphic framework,providing the necessary age control for further studies ofthe formation.

350 DE ROMERO ET AL.

FIGURE 1—Map showing the location of outcrop and well sections.

FIGURE 2—Lithostratigraphy of the La Luna Formation in differentregions. The Totumo-3 Well is in Perija, the San Pedro del Rıo Sectionin Tachira, the Rıo Loro Section in Tachira at the border of Merida,and the La Pena/San Felipe Section in Trujillo–Lara. Age control takenfrom Ford and Houbolt (1963), Boesi et al. (1988), de Romero (1991),De Romero and Galea-Alvarez (1995), de Romero and Odreman(1996), Lorente et al. (1996), and Farias et al. (2000). The base ofthe formation reflects results from the present study. The time scaleis according to Gradstein et al. (1995).

Lithostratigraphy

The formal naming of the La Luna Formation and des-ignation of the type section were first published by Garner(1926). Prior to formal publication, Caribbean PetroleumCompany geologists in internal unpublished reports ap-plied the term ‘‘La Luna Formation’’ to the sequences ofblack calcareous shale and limestone found in the Mara-caibo Basin (Hedberg and Sass, 1937). The type section islocated on Quebrada La Luna, in the Perija foothills, 26miles northwest of La Villa del Rosario, in the state of Zu-lia (Fig. 1). The formation is characterized by alternatingbeds of laminated gray bituminous limestone and blackcalcareous shale with large discoidal limestone concre-tions (Hedberg and Sass, 1937).

The base of the La Luna Formation is defined through-out western Venezuela by a sharp contact with an under-lying non-laminated, macrofossiliferous limestone, calledthe upper ‘‘La Puya’’ bed of the Penas Altas Formation inthe states of Lara and part of Trujillo. In the state of Me-rida and part of Trujillo, the underlying unit is called theGuayacan Member of the Capacho Formation, and in Per-ija and Lake Maracaibo, it is called the Maraca Formation.The top of the La Luna Formation is defined by the contactbetween the glauconitic-phosphatic Tres Esquinas Mem-ber and the shale of the Colon Formation. Although thebase of the Colon Formation frequently contains reworkedphosphate particles, the shale has a typical concoidal frac-ture, which is absent in the Tres Esquinas Member. InPerija, contacts have been reported between the Tres Es-quinas Member and the Socuy Member of the Colon For-mation, a light-gray, massive limestone. Erlich et al.

(1999) included the Socuy Member in the La Luna For-mation.

Renz (1959) divided the La Luna Formation from bot-tom to top into the La Aguada, Chejende, and TimbetesMembers in the states of Trujillo and Lara (Fig. 2). Allthree members are composed of alternating beds of lami-nated gray bituminous limestone and black calcareousshale. The Chejende Member is distinguished by commonintervals of black shale. Black shale may also be a promi-nent component in some intervals in areas other than Tru-jillo and Lara. The Timbetes Member is characterized bylaminated lenticular limestone beds and frequent concre-tions. These limestones are known as ‘‘stretched beds’’ or‘‘capas estiradas.’’ They appear to be horizons of large, co-alescing concretions, but possibly were formed by differ-ential dissolution of limestone beds (Galea Alvarez, 1989,citing pers. comm. J.F. Stephan, 1988). Occasional lentic-ular chert layers are also found in the Timbetes Member.The La Aguada Member is identified by its stratigraphic

CALCAREOUS MICROFOSSIL BIOSTRATIGRAPHY, VENEZUELAN LA LUNA FORMATION 351

position and lack of distinguishing lithologic characteris-tics.

A glauconitic-phosphatic bed occurs immediately belowthe Colon Formation wherever the La Luna Formation ispresent (Sellier de Civrieux, 1952) (Fig. 2). Stainforth(1962) formalized this unit as the Tres Esquinas Forma-tion, designating an outcrop in the Rıo Guaruries as thetype section. However, de Romero and Odreman (1996)questioned the continuity of this member. In the phos-phate mining industry in the state of Tachira, this glau-conitic-phosphatic horizon is referred to informally as‘‘Capa I’’ while a non-glauconitic layer of similar thicknessbelow the Ftanita de Tachira Member is designated as‘‘Capa II’’ (Odreman et al., 1990; Odreman and Rojas,1992). Additional centimeter- to decimeter-thick, non-glauconitic phosphatic horizons are found frequently inthe upper part of the La Luna Formation in the state ofTachira and occasionally in the state of Merida.

In the state of Tachira, the Ftanita de Tachira Memberis recognized in the upper part of the formation (ComiteInterfilial de Estratigrafıa y Nomenclatura, 2000) (Fig. 2),sandwiched between ‘‘Capa I’’ and ‘‘Capa II.’’ Black chertbeds, 5 to 20 cm thick, are the major component of thismember (Renz, 1959). Individual beds of chert that varyfrom 1 to 15 cm in thickness often are lenticular, and fre-quently are found in the upper part of the formation below‘‘Capa II’’ (Galea Alvarez, 1989), as well as in the upperpart of the formation in other areas.

Biostratigraphy

The age of the La Luna Formation originally was deter-mined from ammonite biostratigraphy (Liddle, 1928; Sut-ton, 1946; Renz, 1959, 1977, 1982). Sutton (1946) cau-tioned that many ammonites used in the biostratigraphyprior to 1946 often were not found in situ, or were misiden-tified. Most of the ammonite collections studied are pri-vate and many age determinations are unpublished. Evenwith the sparse published data, global correlation of theLa Luna Formation with ammonite biostratigraphy canbe difficult. Ages determined by Renz (1959, 1977, 1982)have proved reliable, however, ammonites generally arerare and are only found in a few horizons. They are notsuitable for biostratigraphy in borehole cores. Inoceramidsalso have been found in the La Luna Formation, but sufferfrom the same problems as ammonites. Fish, small bi-valves, and rare saurian remains also occur, but are notstratigraphically useful.

The difficulty of macrofossil biostratigraphy in the LaLuna Formation has led modern investigators to concen-trate on microfossils for biostratigraphic purposes. Al-though several foraminiferal and calcareous nannofossilbiostratigraphic studies have been conducted in the lastdecade, most published results are abstracts (Farias et al.,1995, 1996, 2000; Truskowski et al., 1995, 1996; de Rome-ro and Odreman, 1996; Lorente et al., 1996; de Romero etal., 2000). Only three detailed papers based on microfossilbiostratigraphy have been published, with each study geo-graphically restricted. Boesi et al. (1988) examined nan-nofossils and foraminifera in the La Luna Formation infour sections in the southwestern part of the MaracaiboBasin. De Romero and Galea-Alvarez (1995) restrictedtheir study of foraminifera to the phosphate horizons

‘‘Capa I,’’ ‘‘Capa II,’’ and the Tres Esquinas Member in thesouthern part of the Maracaibo Basin. Finally, Davis et al.(1999) studied the Alpuf-6 well core from northwesternMaracaibo Basin using planktic foraminiferal biostratig-raphy.

Carbon Isotope Stratigraphy

In a classic paper, Scholle and Arthur (1980) proposedthe potential of carbon-isotope fluctuations as a strati-graphic tool. They noted large positive excursions in d13Cin carbonates at the Aptian–Albian and Cenomanian–Tu-ronian boundaries, with smaller excursions in other timeperiods found in several sections. The applicability of theCenomanian–Turonian boundary excursion in global cor-relation has been demonstrated by a number of workers(e. g., Pratt and Threlkeld, 1984; Gale et al., 1993; Jenkynset al., 1994; Voigt and Hilbrecht, 1997; Huber et al., 1999;Nederbragt and Fiorentino, 1999).

Carbon-isotope analyses of the La Luna sections andwell cores have been performed by several investigators,however, results primarily have been published as ab-stracts without presentation of the data (Olivares et al.,1996). Davis (1995) presented a d13Corg curve for the Alpuf-6 borehole core from the northwestern Maracaibo Basin(Fig. 1). Carbon isotopes were included in the study con-ducted by Perez-Infante et al. (1996) on the Rıo MaracaSection west of the Alpuf-6 well (Fig. 1). Zapata et al.(2000; this volume) constructed d13Corg curves for the MesaBolıvar Section in the south-central part of the MaracaiboBasin and the Las Hernandez Section to the west (RıoLoro Section of this paper) (Fig. 1). Rangel et al. (2000)published a low-resolution bitumen and aromatic d13C iso-tope curve without biostratigraphic control for the La Sor-da Creek Section in the Middle Magdalena Valley, Colom-bia, approximately 150 km southwest of the San Pedro delRıo Section studied here.

In this investigation, carbon isotopes of the organic-matter fraction of samples from five sections locatedthroughout the Maracaibo Basin have been studied. Re-sults have been integrated with detailed calcareous nan-nofossil and foraminiferal biostratigraphy to produce acomprehensive stratigraphic framework for the La LunaFormation. The framework developed has enabled an es-timation of sedimentation rates and patterns of organic-rich sediment deposition in the Maracaibo Basin. Selecteddata sets from this study are archived at the National Geo-physical Data Center (http://www.ngdc.noaa.gov/mgg/sepm/archive/).

METHODS

Sampling

Five outcrop sections and one cored borehole were stud-ied (Fig. 1). These sections provide a broad geographic dis-tribution, include all members of the La Luna Formation,and have good stratigraphic continuity. Fieldwork wascarried out in January and February of 1999 and 2000.Outcrops were logged in detail and samples taken in theCarora, La Pena, San Felipe, Rıo Loro, and San Pedro delRıo Sections, as well as Totumo-3 Well core stored in thePetroleos de Venezuela, S.A. (PDVSA) core repository inLa Concepcion, Zulia.


Shale and calcareous black shale were sampled prefer-entially for calcareous nannofossil biostratigraphy andcarbon-isotope stratigraphy, whereas laminated lime-stone and hard calcareous black shale were sampled pref-erentially for planktic foraminiferal biostratigraphy.Sam-ples of unlaminated limestone and chert were taken onlywhere no other lithology was available. Where possible,the sampling interval was 50 cm for calcareous nannofos-sil and foraminiferal biostratigraphy, and 1 m for carbon-isotope stratigraphy.

Calcareous Nannofossil Smear Slide Analysis

Smear slides were prepared using the technique forhard samples developed by Monechi and Thierstein(1985). Slides were examined at 1250x under a light mi-croscope using polarized light, natural light, phase con-trast, and a gypsum wedge. Standard taxonomy followsPerch-Nielsen (1985), Varol (1992), and Varol and Girgis(1994). Semi-quantitative counts were made where: rare(R)5fewer than 1 specimen per 10 fields of view; few(F)51–9 specimens per 10 fields; common (C)51–10 spec-imens per field; and abundant (A)5more than 10 speci-mens per field. All calcareous nannofossil determinationsfor biostratigraphic analyses were performed at the Uni-versity of North Carolina at Chapel Hill.

Foraminiferal Analysis

Samples for foraminiferal studies were prepared andanalyzed by standard techniques in the laboratory ofPDVSA in Caracas, Venezuela. Thin sections were em-ployed primarily for foraminiferal biostratigraphy, al-though washed samples of soft material also were exam-ined. The foraminiferal taxonomy utilized Caron (1985)and Sliter (1989).

Carbon-Isotope Analysis

The organic fraction of samples from the San Pedro delRıo and Rıo Loro Sections and the Chejende and the Agua-da Members in the La Pena/San Felipe Sections was sep-arated by double acidification with 1 M HCl. The residuewas rinsed twice with distilled water and oven dried.These samples were analyzed using an Autocarb devicecoupled to a Fisons Prism mass spectrometer at the Uni-versity of California, Santa Cruz. Samples from the Tim-betes Member of the La Pena Section were acidified in 2 MHCl, rinsed in 1 M HCl, and oven dried. The Totumo-3core samples were acidified in a 4 M HCl solution, triplerinsed in buffered distilled water, and oven dried. Resi-dues from the Totumo-3 Well and the Timbetes Member ofthe La Pena Section were analyzed on a Carlo Erba ele-mental analyzer coupled to a Finnigan MAT-252 isotoperatio mass spectrometer using a conflo interface in theMarine Sciences Department at the University of NorthCarolina at Chapel Hill. All values are reported as d13 Cper mil (‰) relative to the Vienna Pee Dee belemnite (V-PDB) standard.

Although three methods were used to prepare samples,little variation in results is observed. Two samples fromthe Totumo-3 Well were prepared with and without rins-ing with buffered distilled water. The results varied by a

standard deviation of less than 0.09 ‰. An external ana-lytical precision better than 6 0.2 ‰ was based on acet-anilide (an in-house standard) at the University of NorthCarolina and 60.13 ‰ was based on NBS-19 at the Uni-versity of California.

LITHOSTRATIGRAPHY OF SECTIONS

Carora Section

The Carora Section is located on the Lara-Zulia High-way approximately 2 km west of the town of Carora, Lara(Fig. 1). The 100-m-thick outcrop is completely exposedwith no covered sections. Intensive weathering has causedleaching and the oxidation of Fe and organic material,leaving a friable calcareous claystone that varies in colorbetween tan, pink, white, and gray.

La Pena / San Felipe Sections

The La Pena Section is located in the eastern MaracaiboBasin on the eastern edge of the village of Chejende, Tru-jillo (Fig. 1). The San Felipe Section is 3.5 km from LaPena on the Chejende–Mitton road.

The base of the La Aguada Member of the La Luna For-mation crops out in the San Felipe Section. Only the lower5 m of the 60-m-thick member are exposed (Renz, 1959).The contact with the underlying Penas Altas Formation isplaced above the uppermost limestone bed containinglarge bivalves, primarily oysters. The lower La AguadaMember is represented by 20–40 cm dark-gray limestonebeds separated by beds of brownish gray shale less than 15cm thick (Fig. 3A).

The Chejende and Timbetes Members, and the contactbetween them, are well exposed in the La Pena Section.The Chejende Member crops out on a narrow path alongthe bottom of the La Pena hillside. The base of this 80-m-thick member (Renz, 1959) is not exposed. The ChejendeMember is at least 61 m thick here. A fault visible in theupper portion of the hill may indicate that the lowermostpart of the member is missing, however this cannot be de-termined because the contact with the underlying rock iscovered by a landslide. The Chejende Member is distin-guished by thick beds of dark-gray shale alternating withlimestone and calcareous shale that are typical of the LaLuna Formation (Fig. 3A). The contact with the overlyingTimbetes Member is placed at the base of the first‘‘stretched’’ limestone bed, above which shales becomerare and limestone concretions increase in size and fre-quency (Renz, 1959). The two members appear to be con-formable.

Although the Timbetes Member is present along thehillside, it is better exposed at the top of the hill where thecontact with the Chejende Member is also visible. The low-ermost beds of black chert are found 1.5 m above the con-tact. Small to large concretions, some at least a meter indiameter, are frequent throughout the Timbetes Member.Exposure of the Timbetes Member was almost continuousbelow 166.5 m. Above 166.5 m is ;10 m of covered section.Two samples were obtained above the covered section.Thirty-seven m of the 90 m Timbetes Member (Renz,1959) were measured. The top of the member has beeneroded in the La Pena Section.


FIGURE 3—Stratigraphic columns for the sections studied. (A) La Pena/San Felipe Section. (B) Rıo Loro Section. (C) San Pedro del RıoSection. (D) Totumo-3 Well. See Figure 5 for reference numbers and letters for nannofossil intervals and foraminifera zones. In (A), ForaminiferaZone C is by necessity divided into an upper (u.) and lower (l.) subzone. Numbers on the carbon isotope curves are referred to in text.


Rıo Loro Section

The Rıo Loro Section is located between the La Penaand San Pedro del Rıo Sections, 7.2 km from San Simon,Tachira (Fig. 1). It crops out along the San Simon–El Sal-ado road just past the Rıo Loro Bridge. This section is lessthan two kilometers from the Tachira–Merida state line.

Except for the base, the La Luna Formation (Fig. 3B) isexceptionally well exposed at Rıo Loro. The Tres EsquinasMember, the only member recognized in Merida State, ispresent. However, the Ftanita de Tachira Member, pre-sent in outcrops farther to the west in Tachira State, is notfound in this section.

Highly resistant bioclastic limestones of the underlyingCapacho Formation are clearly exposed, and the 18.75-mcovered section immediately above these beds is assumedto be the La Luna Formation. The lithology is typical of theLa Luna Formation, with limestone concretions concen-trated towards the base and occasional thin black chertbeds towards the top. The upper contact with the ColonFormation is placed at the top of the 1.1 m glauconitic-phosphatic bed that forms the Tres Esquinas Member.The soft shale of the overlying formation is assumed to beeroded. Total thickness of the La Luna Formation at thislocality is estimated to be 56.7 m.

San Pedro del Rıo Section

This section is located in the southwestern part of theMaracaibo Basin, 400 m south of San Pedro del Rıo, Tach-ira (Fig. 1). The La Luna Formation is exposed as a seriesof inclined beds along the length of a prominent hill justacross Quebrada Chirirıa at the edge of the town. The baseof the formation is poorly exposed, with the lowermost LaLuna bed about 10 m above the top of the underlying Ca-pacho Formation. Several small slides cover 5-m intervalsof the outcrop in the 89.25 m-thick section. The thicknessof ‘‘Capa I’’ at the top of the section is known to be 1.3 mfrom previous measurements, however, all but 0.5 m arecovered by a slide.

Dark-gray to black shales dominate the section up to 35m (Fig. 3C). The lowest bed of black chert lies at 37.8 m.Chert is not common until 43.7 m, where it alternates withlimestone, occasional black calcareous shale, and thinshale beds. The non-glauconitic, phosphatic sediments of‘‘Capa II’’ extend from 70 to 71.5 m, but the top of the bedappears to be covered. Between ‘‘Capa II’’ and the glauco-nitic-phosphatic sediments of ‘‘Capa I’’, whose base lies at88 m, the Ftanita de Tachira Member is almost entirelycomposed of black chert with occasional thin beds of lime-stone and black calcareous shale.

Totumo-3 Well

The Totumo-3 Well is located in the northwestern partof the Maracaibo Basin, less than 30 km from the type sec-tion of the La Luna Formation (Fig. 1). Coring commencedat 129059 (3933.4 m). The cored interval includes the con-tact between the La Luna Formation and the underlyingMaraca Formation, located at 131999 2’’ (4023.1 m), not at132109 (4026.4 m) as reported in the (former) Lagoven,S.A. well log. This contact is placed at the top of the upper-most light-gray bioclastic limestone bed.

Hard, laminated, black calcareous shale comprises mostof the core (Fig. 3D). The black calcareous shale commonlyalternates with gray calcareous shale and laminated andmassive limestone. The change between lithologies maybe abrupt or gradational and appears to be cyclic. Lime-stone beds are more frequent and increase in thicknessup-core. Several cm-thick beds with wavy lamination,abundant fish remains and thin, broken bivalves lie nearthe base of the formation. These beds are interpreted asstorm layers.

Between 13.3 m and 22.8 m above the base of the for-mation (4009.8–4000.3 m depth) the lithology is dominat-ed by black shale, with elemental sulfur prominent in thelowermost bed of this interval from 13.3 m to 14.1 m(4009.8–4009.0 m depth). Davis et al. (1999) reported vol-canic ash in a similar sulfur-rich shale horizon in the Al-puf-6 well (Fig. 1). They linked this layer to the Cenoman-ian–Turonian activity of a volcanic arc located to the westof Colombia (Parnaud et al., 1995). Except for a black-shale bed between 80.2 m and 80.9 m above the base(3942.9–4942.7 m depth), there is no other non-calcareousshale.

Based on electric and gamma ray logs, the upper 8.5 mof the La Luna Formation were not cored (Oscar Quintero,pers. comm., 2002). This includes the phosphatic Tres Es-quinas Member.

BIOSTRATIGRAPHY

The standard planktic foraminiferal zonal scheme es-tablished by Caron (1985) and modified by Sliter (1989)and Premoli Silva and Sliter (1999) has been applied.Ranges of foraminiferal genera and species are from Pre-moli Silva and Sliter (1999), unless otherwise noted. Age-diagnostic keeled species are rare and confined to a fewlaminae in the La Luna Formation. Foraminiferal faunasare dominated by opportunistic r-selected and r/K-selectedtaxa, with a predominance of planktic species in the loweranoxic part of the formation, and benthic species in the up-per dysoxic part. The globular genera Hedbergella, Whitei-nella, Globigerinelloides, Archaeoglobigerina, and Hetero-helix comprise most of the planktic foraminifera. These as-semblages indicate an expanded oxygen-minimum zone,probably associated with upwelling, and possibly episodic(Leckie, 1987; Galea Alvarez, 1989; Davis, 1995; Trus-kowski et al., 1996; Davis et al., 1999; Premoli Silva andSliter, 1999).

A number of different Upper Cretaceous calcareousnannofossil zonal schemes have been proposed over thelast thirty years. The majority of these schemes are basedon individual sections, or only cover restricted time peri-ods (Verbeek, 1977; Crux, 1982; Mortimer, 1987; Watkinset al., 1996; Bralower and Bergen, 1998). Others havebeen modified and incorporated into newer zonal schemes(Cepek and Hay, 1969; Thierstein, 1976). The most widelyapplied scheme for outcrop sections, developed by Sissingh(1977) and modified by Perch-Nielsen (1985), was basedon land sections mainly in Europe and northern Africa(Fig. 4). The zonal scheme of Roth (1978), traditionallyused in deep-sea sections, is based mainly on Deep SeaDrilling Project (DSDP) sites. This scheme was modifiedby Bralower et al. (1995), who also developed an integrat-ed nannofossil-foraminiferal zonal scheme. The most re-


FIGURE 4—Traditional and recent calcareous nannofossil zonal schemes. Nannofossils useful as markers in the La Luna Formation are inbold. Timescale is according to Gradstein (1995). Stage divisions follow Rawson et al. (1996).

cent and detailed zonations have been published by Bur-nett (1998) and Bergen and Sikora (1999).

Application of standard calcareous nannofossil zonalschemes to the La Luna Formation is difficult due to poorpreservation. Dissolution, overgrowth, and recrystalliza-tion hinder identification of important markers, and onlythe most dissolution-resistant species are commonly pre-served. The original moderate diversity of the nanno-plankton assemblages is demonstrated by the very rare

occurrence of a considerable number of species, 100 spe-cies all total with as many as 30 in the best-preserved sam-ples. Most of the marker species of traditional zonalschemes are not present or are exceedingly rare (Fig. 4),due to dissolution or the preference of some species forhigh latitudes or an open-ocean environment. In thisstudy, an informal zonal scheme has been developed forthe La Luna Formation (Fig. 5). Marker species have beenchosen that are morphologically distinctive, present in all


FIGURE 5—Biostratigraphic schemes applied to the La Luna Formation in this investigation. Microfossil datums from Perch-Nielsen (1985),Bralower (1988), Bralower et al. (1995), Bralower and Bergen (1998), Bergen and Sikora (1999), and Premoli Silva and Sliter (1999). Numbersand letters assigned to intervals and zones are referred to in Figures 3, 7, and 8. Timescale and stage divisions follow Gradstein (1995) andRawson et al. (1996).

sections, have first occurrences (FOs) or last occurrences(LOs) that are stratigraphically useful, and whose occur-rence in most cases is continuous or semi-continuous with-in their range (Fig. 6). Most of the chosen species areknown to be resistant to dissolution (Thierstein, 1980;Roth and Krumbach, 1986). First and last occurrences ofmarker species have been assigned to stages according toPerch-Nielsen (1985), Bralower (1988), Varol (1992), Bra-lower et al. (1995), Bralower and Bergen (1998), Bown etal. (1998), Burnett (1998), and Bergen and Sikora (1999).All numerical ages were determined using the scheme ofGradstein et al. (1995). At the international meeting onCretaceous stage boundaries in Brussels in 1995, Creta-ceous stages (except for the Maastrichtian) were dividedinto upper, middle, and lower substages (Rawson et al.,1996). Although not all Cretaceous substage boundarystratotypes have been chosen, the tendency in many of themost recent publications is to apply this threefold stage di-vision (e.g., Gradstein et al., 1995; Burnett, 1998; Bergenand Sikora, 1999). With the exception of the Campanian,the threefold division is used in this investigation, as it

simplifies correlation between the Gradstein et al. (1995)timescale and the most recent nannofossil publications.

BIOSTRATIGRAPHIC INTERVALS

Eiffellithus turriseiffelii–Gartnerago obliquum Interval

Definition: Interval from the first occurrence (FO) of Eif-fellithus turriseiffelii to the FO of Gartnerago obliquum.

Range: Uppermost Albian to middle Cenomanian.Locality: This interval was identified at the base of the

La Aguada Member in the San Felipe Section.Remarks: The accepted FO of E. turriseiffelii is upper-

most Albian (Perch-Nielsen, 1985; Bown et al., 1998).Gartnerago obliquum traditionally is distinguished fromG. segmentatum by the presence of pores in the centralplate. This feature is difficult to observe in poorly pre-served material (Bralower, 1988; Burnett, 1998). To avoidpossible misidentification, they are grouped here under G.obliquum.

The age of the FO of G. obliquum is uncertain. Bralower


FIGURE 6—Light micrographs of nannofossil marker species. All magnifications 30003. (A) Braarudosphaera africana, LAG-99-05. (B) Eif-fellithus turriseiffelii, LP-99-79. (C) Gartnerago obliquum, LPT-00-28. (D) Eprolithus floralis, LP-99-19. (E) Eprolithus octopetalus, LPT-99-79.(F) Eprolithus eptapetalus, LP-99-79. (G) Marthasterites furcatus, RL-99-51. (H) Lithastrinus moratus, RL-99-45. (I) Lithastrinus grillii, RL-99-45. (J) Micula staurophora, RL-99-51. (K) Micula concava, RL-99-45. (L) Quadrum gartneri, LPT-00-36.

(1988) and Burnett (1998) reported G. obliquum in the up-per Cenomanian, halfway through the middle Cenomani-an and upper lower Cenomanian, respectively. Bralower(1988) and Burnett (1998) also identified Braarudos-phaera africana in the upper Cenomanian, however, oneof the authors of this study (Bergen) has not found thistaxon above the middle Cenomanian. The LOs of Hemipo-dorhabdus biforatus and Zygodiscus xenotus are in themiddle Cenomanian. Burnett (1998) considered the LO ofZ. xenotus to lie in the lower Cenomanian. Immediatelyabove the top of the E. turriseiffelii–G. obliquum intervalin the San Felipe Section, the co-occurrence of B. africana,G. obliquum, Zygodiscus xenotus, and H. biforatus sug-gests that the Gartnerago obliquum–Axopodorhabdus al-bianus interval extends down into the middle Cenomani-an. No planktic foraminiferal marker species have been

identified in the Eiffellithus turriseiffelii–Gartnerago obli-quum interval in the San Felipe Section.

Gartnerago obliquum–Axopodorhabdus albianus Interval

Definition: Interval from the FO of Gartnerago obliquumto the LO of Axopodorhabdus albianus.

Range: Middle to upper Cenomanian.Locality: This interval was identified in both the La

Aguada Member in the San Felipe Section and at the baseof the La Luna Formation in the Totumo-3 Well.

Remarks: The base of this interval is discussed above.The LO of A. albianus is slightly below the Cenomanian/Turonian boundary, just above the LO of the foraminiferRotalipora cushmani (Bralower, 1988; Bralower et al.,1995; Bergen and Sikora, 1999). Although A. albianus is


rare in the La Luna Formation, a positive identificationcan be made from a fragment of the shield with a smallpiece of the cross attached. Microstaurus chiastius andRhagodiscus asper, the other two marker species for thistime period (Bralower, 1988; Bralower et al., 1995; Bergenand Sikora, 1999), also are present, but are equally rare.Microstaurus chiastius is difficult to identify when over-grown, and R. asper can be difficult to distinguish fromRhagodiscus splendens. The FO of Quadrum lies above theFO of Eprolithus eptapetalus, making it unsuitable as amarker.

Axopodorhabdus albianus–Eprolithus eptapetalusInterval

Definition: Interval from the LO of Axopodorhabdus al-bianus to the FO of Eprolithus eptapetalus.

Range: Uppermost Cenomanian to lower lower Turoni-an (Bralower and Bergen, 1998; Bergen and Sikora, 1999).

Locality: This interval was identified in the Totumo-3Well and in the Chejende Member of the La Pena Section.

Remarks: Eprolithus eptapetalus has a continuousrange in the La Pena Section and a semi-continuous rangein the Totumo-3 Well.

Eprolithus eptapetalus–Eprolithus octopetalus Interval(Eprolithus I Interval)

Definition: Interval from the FO of Eprolithus eptapetal-us to the LO of Eprolithus octopetalus.

Range: Upper lower to lower middle Turonian.Locality: The Eprolithus eptapetalus–Eprolithus octope-

talus interval was identified in the Totumo-3 Well and inthe Chejende Member of the La Pena Section.

Remarks: Although rare, the presence of Eprolithus oc-topetalus is relatively continuous within its range, and theLO of this species is a good marker in the lower middle Tu-ronian (Bralower and Bergen, 1998; Bergen and Sikora,1999).

Eprolithus octopetalus–Eprolithus eptapetalus Interval(Eprolithus II Interval)

Definition: Interval from the LO of Eprolithus octopetal-us to the LO of Eprolithus eptapetalus.

Range: Upper middle to upper Turonian.Locality: This interval was identified in the Totumo-3

Well and in the Chejende Member of the La Pena Section.Remarks: In the La Luna Formation, the LO of E. epta-

petalus and the FOs of Marthasterites furcatus and Lith-astrinus moratus lie close together. The FO of M. furcatusshould occur slightly below the LO of E. eptapetalus, andthe FO of L. moratus should be below the FO of M. furcatus(Bralower, 1988; Bralower and Bergen, 1998; Bergen andSikora, 1999). In the Totumo-3 Well, the FO of L. moratusis below the LO of E. eptapetalus; however, the FO of M.furcatus occurs slightly above the latter datum. The posi-tion of the FOs of both L. moratus and M. furcatus is abovethe LO of E. eptapetalus in the La Pena Section. Thisanomaly is probably a result of poor preservation. The LOof E. eptapetalus was chosen to define the top of the inter-val because it is found within 1 m of the Turonian/Conia-cian boundary as defined in the Chalk sections of Eng-

land, and its occurrence seems to be more consistent thanthe other two markers. The FOs of both M. furcatus and L.moratus could be used as proxies for the LO of E. eptape-talus.

Eprolithus eptapetalus–Micula staurophora Interval

Definition: Interval from the LO of Eprolithus eptapetal-us to the FO of Micula staurophora.

Range: Lower Coniacian.Locality: This interval was recognized in the Totumo-3

Well, in the Timbetes Member of the La Pena Section, andin the Rıo Loro Section.

Remarks: The presence of L. moratus and M. furcatuscombined with the absence of E. eptapetalus and M. stau-rophora is characteristic of this interval. Micula cubifor-mis also is observed in the upper part of the interval. Theplanktic foraminifer Dicarinella concavata also is present.

Micula staurophora–Micula concava Interval

Definition: Interval from the FO of Micula staurophorato the FO of Micula concava.

Range: Middle Coniacian to lowermost Santonian.Locality: This interval was identified in the Rıo Loro

and the San Pedro del Rıo Sections.Remarks: The FO of M. staurophora marks the base of

the middle Coniacian in the stratotype section (Rawson etal., 1996; Bergen and Sikora, 1999). Differentiating be-tween M. staurophora and M. concava can be difficult, par-ticularly in poorly preserved material. It may be necessaryin some instances to combine this interval with the Miculaconcava–Lithastrinus moratus interval, however, the iden-tification of M. concava helps divide a long stratigraphicinterval that would include the middle and upper Conia-cian, as well as the lower Santonian (Perch-Nielsen, 1985recalibrated by Burnett, 1998). The FO of the planktic fo-raminifera Dicarinella asymetrica lies just above the FO ofM. concava and could be used as a proxy for the top of theinterval (Premoli Silva and Sliter, 1999).

Micula concava–Lithastrinus moratus Interval

Definition: Interval from the FO of Micula concava to theLO of Lithastrinus moratus.

Range: Lower Santonian.Locality: This interval was recognized in the Rıo Loro

and the San Pedro del Rıo Sections.Remarks: See remarks for the Micula staurophora–Mi-

cula concava interval. Lithastrinus moratus has a semi-continuous range in all of the sections. The LO of this spe-cies is considered to define the top of the lower Santonian(Burnett, 1998; Bergen and Sikora, 1999).

Lithastrinus moratus–Lithastrinus grillii Interval

Definition: Interval from the LO of Lithastrinus moratusto the LO of Lithastrinus grillii.

Age: Middle Santonian to lower Campanian.Remarks: Lithastrinus grillii can be confused with Ru-

cinolithus magnus, which has tear-shaped arms and doesnot have a central platform. There is a lapse in time be-tween the LO of L. grillii at the top of the lower Campani-


TABLE 1—First and last occurrences of major microfossil marker species in each section. Figures represent the location of the event measuredin meters above the base of the La Luna Formation.

Microfossil event Totumo-3La Pena/

San FelipeRio

LoroSan Pedro

del Rio

LO Lithastrinus grilliiLO Marthasterites furcatusLO Dicarinella asymetricaLO Lithastrinus moratusFO Dicarinella asymetricaFO Micula concavaFO Lithastrinus grilliiFO Micula staurophoraFO Marthasterites furcatusLO Eprolithus eptapetalus

85.8*88.9*

85.288.9

85.8

68.067.1

164.4140.4131.3*

52.3*51.3*51.3*41.927.927.927.920.924.6

66.6*65.2*67.5*66.3*54.325.921.510.0*11.7*

FO Lithastrinus moratusFO Dicarinella concavataFO Dicarinella primitivaFO Marginotruncana renziLO Epropetalus octopetalusFO Eprolithus eptapetalusLO Axopodorhabdus albianusLO Rotalipora cushmaniFO Gartnerago obliquumFO Whiteinella baltica

65.967.159.656.328.120.813.511.20.90.0

141.1126.1

118.079.0

3.9

24.618.7*

10.0*12.4*

* Apparent due to covered or barren intervals.Planktonic foraminifera are in bold type.

an (Bralower et al., 1995; Bergen and Sikora, 1999) andthe FO of R. magnus in the upper Campanian which canbe of help differentiating the two species in poorly pre-served material. It is possible that the two species havebeen combined in zonal schemes that have an upper Cam-panian LO of L. grillii (e.g., Perch-Nielsen). The LO of M.furcatus lies slightly below the top of the lower Campanian(Perch-Nielsen, 1985, recalibrated by Premoli-Silva andSliter, 1999) and can be used as a proxy for the top of theinterval. The FO of the foraminifer Rugoglobigerina rugo-sa is close to the LO of L. grillii.

Biostratigraphy of Sections

Of the 523 samples examined, 7% were barren of nan-nofossils, except the Carora Section where 100% are bar-ren. Calcareous nannofossils are poorly preserved. Infor-mal remarks concerning the criteria used to identify nan-nofossil marker species are found in de Romero (2001, Ap-pendix III). With the exception of the Carora Section,foraminiferal preservation is fair to good. At Carora, onlya few unidentifiable ghosts of foraminifera were observed.Most of the planktic foraminifera present are withoutkeels. The FOs and LOs of the major foraminiferal andnannofossil markers in each section are illustrated in Ta-ble 1.

La Pena/San Felipe Sections: Although nannofossils arecommon in the majority of samples in the San Felipe Sec-tion, diversity is low. Presence of the nannofossils Eiffelli-thus turriseiffelii, Microstaurus chiastius, and Braarudos-phaera africana, and absence of Gartnerago obliquum sug-gest correlation of the base of the La Aguada Member inthe San Felipe Section to the Eiffellithus turriseiffelii–Gartnerago obliquum interval with an age no older thanlatest Albian and probably no younger than middle Ceno-manian (Figs. 3A, 5). The interval from 3.7 m to 5.0 m, the

top of the lower segment of the measured section, is as-signed to the Gartnerago obliquum–Axopodorhabdus albi-anus interval based on the FO of G. obliquum. The co-oc-currence of Braarudosphaera africana, G. obliquum, Zyg-odiscus xenotus, and Hemipodorhabdus biforatus at 3.9 msuggests an age of middle to late Cenomanian. No diag-nostic foraminifera were observed.

The top of the Gartnerago obliquum–Axopodorhabdusalbianus interval and the Axopodorhabdus albianus–Eprolithus eptapetalus interval were not identified be-cause the middle and upper La Aguada Member do notcrop out in the San Felipe Section and the base of the Che-jende Member is not exposed at La Pena.

In the La Pena Section, calcareous nannofossils are fre-quent to abundant in the Chejende Member, and frequentto common in the Timbetes Member. Taking into accountthe measured thickness and the total thickness of eachmember according to Renz (1959), the lowest sample inthe Chejende Member is estimated to be 19 m above thebase of the member (79 m above the base of the formation).Based on the simultaneous occurrence of the nannofossilsEprolithus eptapetalus and E. octopetalus this sample isassigned to the Eprolithus I interval. This interval contin-ues to the LO of E. octopetalus at 118.0 m and representsthe upper lower–lower middle Turonian. Based on thecontinuing presence of E. eptapetalus, the upper middle–upper Turonian Eprolithus II interval extends from 118.0m to at least 131.5 m. The FOs of the planktic foraminiferaDicarinella concavata and Hastigerinoides alexanderi at126.8 m mark the base of the D. concavata Zone (Caron,1985; Premoli Silva and Sliter, 1999). The presence of thenannofossil Corollithion achylosum at 131.5 m, the last LOof which is considered to be substantially lower than thatof E. eptapetalus (Bralower and Bergen, 1998; Bergen andSikora, 1999), suggests correlation to the lowermost D.concavata Zone (upper upper Turonian). The interval from


131.5 m to the base of the Timbetes Member at 140 m can-not be assigned to a calcareous nannofossil zonal intervaldue to poor exposure and poor preservation of samplesthat contain no primary or secondary marker species.

The base of the Timbetes Member is assigned to the low-er Coniacian Eprolithus eptapetalus–Micula staurophorainterval based on the presence of Marthasterites furcatusand the absence of Micula staurophora. This interval ex-tends to the FO of Micula staurophora at 164.0 m. Thepresence of M. staurophora and Lithastrinus moratus andthe absence of M. concava suggest that the uppermost partof the section (164.0 m to 177.0 m) belongs to the middle–upper Coniacian Micula staurophora–M. concava interval.With one exception, M. furcatus and L. moratus do not oc-cur in the same sample, pointing to a possible environ-mental or preservational control on their occurrence. Thepresence of the planktic foraminifer Dicarinella concavataand the absence of D. asymetrica are consistent with thisconclusion.

Rıo Loro Section: Nannofossils generally are frequent tocommon in the Rıo Loro Section, which has only three bar-ren samples. The lower 18.5 m of the formation are cov-ered (Fig. 3B). The interval from 18.5 m to 27.9 m is as-signed to the upper upper Turonian–lowermost SantonianDicarinella concavata Zone on the basis of the presence ofthe planktic foraminifer D. concavata and the nannofossilEprolithus floralis, in the absence of Dicarinella asymetri-ca and Eprolithus eptapetalus from the lowermost exposedsample (Fig. 5). The absence of D. concavata in this lowersample is attributed to the rarity of carinate species in theformation. The lowermost exposed sample is also assignedto the lower Coniacian E. eptapetalus–Micula staurophorainterval based on the absence of these two nannofossils.Lithastrinus moratus and Marthasterites furcatus shouldbe present throughout this interval, however the FOs at24.6 m seem to be offset relative to both the FO of M. stau-rophora and the LO of E. eptapetalus (Table 1; Perch-Niel-sen, 1985; Bralower et al., 1995; Burnett, 1998; Bergenand Sikora, 1999). This might be attributed to dissolution.The FO of M. staurophora at 20.9 m marks the base of themiddle Coniacian to lowermost Santonian M. stauropho-ra–M. concava interval, which continues to the FO of M.concava at 27.9 m. The FO of the foraminifer D. asymetricaat the same level as the FOs of M. concava and Lithastri-nus grillii suggests a minor unconformity, condensed sed-imentation, or that the actual FOs of M. concava and L.grillii are lower (Perch-Nielsen, 1985; Bergen and Sikora,1999). The FO of D. asymetrica marks the base of the up-per lower Santonian–lowermost Campanian D. asymetricaZone.

The lower Santonian M. concava–L. moratus intervallies between 27.9 m and 41.9 m. The section from 41.9 m to52.3 m is assigned to the middle Santonian–lower Cam-panian L. moratus–L. grillii interval based on the absenceof L. moratus and the semi-continuous presence of M. fur-catus and L. grillii. The presence of Dicarinella asymetricaat 52.3 m suggests an age no younger than earliest Cam-panian for the top of this interval, while the absence ofBroinsonia parca suggests that it may be no younger thanlatest Santonian. Above 52.3 m, no marker foraminiferawere observed.

The absence of B. parca between 52.3 m and 55.5 m in-dicates that this interval is below the uppermost Santoni-

an to lowermost Campanian first occurrence of this spe-cies. The presence of other species of Broinsonia in the RıoLoro Section suggests that this absence is meaningful(Perch-Nielsen, 1985; Bralower et al., 1995; Bergen andSikora, 1999). Additional evidence that the interval be-tween 52.3 and 55.5 m lies below the FO of B. parca is thecontinuing presence of M. staurophora, M. concava, andWatznaueria barnesae, as well as rare specimens of othernannofossils. Thierstein (1980) found that B. parca was asequally susceptible to solution as W. barnesae, thereforeboth species should be found under similar degrees of dis-solution. Thus, the L. moratus–L. grillii interval is tenta-tively extended to 55.5 m. The two samples above 55.5 min the Tres Esquinas Member are barren of nannofossils.

San Pedro del Rıo Section: Nannofossils are rare to com-mon in the San Pedro del Rıo Section. Several samplesnear the middle of the section and the top two samples arebarren of nannofossils. The lowermost sample, about 10 mabove the base of the formation, is assigned to the middle–upper Coniacian Micula staurophora–Micula concava in-terval based on the co-occurrence of M. staurophora andLithastrinus moratus and the absence of M. concava (Figs.3C, 5). The FO of the nannofossil Marthasterites furcatusabove that of M. staurophora might be explained by thesporadic occurrence of M. furcatus. Although the plankticforaminifer Dicarinella concavata does not appear until12.4 m, its absence below this might be explained by therarity of keeled forms. Thus, the lower part of the sectionis tentatively assigned to the D. concavata Zone. The M.staurophora–M. concava interval continues to the FO of M.concava at 27.0 m. The section between 27.0 m and 66.3 mis assigned to the lower Santonian M. concava–Lithastri-nus moratus interval based on the presence of L. moratus.Midway in this interval at 54.3 m, the FO of the plankticforaminifer Dicarinella asymetrica marks the base of theD. asymetrica Zone. The LO of the foraminifer Hastigeri-noides alexanderi, a secondary marker, marks the top ofthe D. asymetrica Zone at 67.5 m. It should be noted thatthe range and distribution of Hastigerinoides alexanderi isvariable, dependent on unusual environmental conditionsand possibly associated with an oxygen-deficient watercolumn (Premoli Silva and Sliter, 1999). However, anoxicbottom waters prevailed in the lower part of the forma-tion, with a gradual change to dysoxia in the upper part(Galea Alvarz, 1989; de Romero, 1991; De Romero and Ga-lea-Alvarez, 1995; Truskowski et al.; 1996; Davis, 1995;Davis, et al., 1999).

Above 66.3 m, strong dissolution of nannofossils is indi-cated by the presence of only those taxa most resistant todissolution—several species of Micula and very rare Watz-naueria barnesae (Thierstein, 1980; Roth and Krumbach,1986) plus the occurrence of very rare Lithastrinus sp. inthe sample at 70.6 m. Based on the occurrence of Lithas-trinus sp. above the LO of Hastigerinoides alexanderi, thesection between 66.3 m and 70.6 m is tentatively assignedto the middle Santonian–early Campanian Lithastrinusmoratus–Lithastrinus grillii interval. The proposed age isin agreement with de Romero (1991) and De Romero andGalea-Alvarez (1995), who found Rugoglobigerina rugosain ‘‘Capa II’’ in sections with better preservation. The ab-sence of Broinsonia parca is accounted for by the poor nan-nofossil preservation in this interval in which even W. bar-


nesae is very rare. The absence of microfossil markers pre-cludes the determination of the age of the upper 18.6 m.

Totumo-3 Well: Nannofossils are rare to common in theTotumo-3 Well core. Only seven samples are barren ofnannofossils, including the lowermost sample 0.4 m abovethe contact with the underlying Maraca Formation.

The interval from the contact with the Maraca Forma-tion to 11.2 m is assigned to the middle to upper Cenoman-ian Rotalipora cushmani Zone (Dicarinella algeriana Sub-zone) based on the presence of the planktic foraminiferaWhiteinella baltica, R. cushmani, and R. greenhornensis(Figs. 3D, 5). The lower 0.9 m is tentatively assigned to theuppermost part of the upper Albian to lower middle Cen-omanian Eiffellithus turriseiffelii–Gartnerago obliquuminterval based on the absence of the nannofossil G. obli-quum. The barren lowermost sample, the very rare occur-rence of species other than Watznaueria barnesae below0.9 m, and the semi-continuous rather than continuous oc-currence of G. obliquum throughout the core section ac-count for the uncertainty. Coincidence of the foraminiferalzone and the nannofossil zone indicates an age possibly asold as early middle Cenomanian for the base of the for-mation.

The section between 0.9 m and 13.5 m is assigned to theupper-middle to upper Cenomanian G. obliquum–Axopo-dorhabdus albianus interval based on the FO of the nan-nofossil G. obliquum and the presence of A. albianus andRhagodiscus asper at 1.6 m. Beginning slightly below thetop of this interval at 11.2 m and extending to 56.3 m, thestrata contain no planktic foraminiferal marker species,and are tentatively assigned to the Whiteinella archaeo-cretacea–lower Helvetoglobotruncana helvetica Zones.

The uppermost Cenomanian to lower lower TuronianAxopodorhabdus albianus–Eprolithus eptapetalus inter-val between 13.5 m and 20.8 m is based on the absence ofA. albianus and E. eptapetalus. Despite the sporadic occur-rence of A. albianus, the position of the LO of this speciesat 13.5 m is supported by the LO of R. asper at 12.3 m (Bra-lower, 1988; Bergen and Sikora, 1999) and the LO of theforaminifer R. cushmani at 11.2 m.

The co-occurrence of Eprolithus octopetalus and E. ep-tapetalus between 20.8 m and 28.1 m defines the upperlower–lower middle Turonian Eprolithus I interval. Thepresence of E. eptapetalus and the absence of E. octopetal-us from 28.1 m to 67.1 m define the lower-middle to upperTuronian Eprolithus II interval. The LO of E. eptapetalusis in agreement with the FOs of Lithastrinus moratus at65.9 m and of Marthasterites furcatus at 68.0 m. Withinthe Eprolithus II nannofossil interval, the upper H. helve-tica Zone is defined by the FOs of the planktic foraminif-era Marginotruncana renzi at 56.3 m and Dicarinellaprimitiva at 59.9 m. The FO of M. renzi is delayed with re-spect to the LO of the nannofossil E. octopetalus. Alsowithin this nannofossil interval is the upper-middle tolower-upper Turonian planktic foraminiferal Margino-truncana sigali–D. primitiva Zone that begins at 59.9 mand extends to the FO of Dicarinella concavata at 67.1 m.The co-occurrence of the FO of D. concavata and the LO ofE. eptapetalus possibly indicates a small hiatus in the up-permost Turonian or reflects the paucity of keeled fora-minifera. The upper upper Turonian–lower lower Santon-ian D. concavata Zone extends from 67.1 m to 88.9 m.

The base of the lower Coniacian E. eptapetalus–Micula

staurophora interval is clearly defined by the LO of E. ep-tapetalus; however, the top of this interval is problematic.The only specimen of M. staurophora observed at 65.6 m isfar too low in the core relative to the LO of E. eptapetalusat 67.1 m and the FO of L. moratus at 65.9 m (Perch-Niel-sen, 1985, Bralower and Bergen, 1998; Burnett, 1998; Ber-gen and Sikora, 1999). This is attributed to downhole con-tamination. Micula staurophora was considered by Thier-stein (1980) to have the highest resistance of all nannofos-sils to dissolution, which is apparent in the other sections.Considering that other more fragile nannofossils are pre-sent, the absence of M. staurophora suggests that the FOof that species is above the core section and that the E. ep-tapetalus–M. staurophora interval continues to the top ofthe section. However, the presence of the foraminifer Di-carinella asymetrica 0.5 m from the top of the core seemsto contradict this conclusion, indicating that the upperportion is at least as young as the Santonian/Coniacianboundary. The presence of one specimen of Lithastrinusgrillii at 85.8 m, 3.9 m below the top of the well core, wouldtend to support this. Further work needs to be done to de-fine the age of the top portion of the core. The upper 9 m ofthe La Luna Formation, including the Tres EsquinasMember, were not cored (Oscar Quintero, pers. comm.,2000).

CARBON ISOTOPE STRATIGRAPHY

Scholle and Arthur (1980) demonstrated the potential ofcarbon-isotope fluctuations in stratigraphic correlation ona global basis. The Albian–Campanian C-isotopic record isnow well established (Jenkyns et al., 1994). Positive excur-sions in the global d13C curve are generally believed tohave been caused largely by burial of large quantities of12C-enriched organic matter during Oceanic AnoxicEvents (OAEs). The global nature of the Cenomanian/Tu-ronian (C/T) boundary (OAE2) d13C excursion has beendemonstrated (Pratt and Threlkeld, 1984; Gale et al.,1993; Jenkyns et al., 1994; Voigt and Hilbrecht, 1997;Huber et al., 1999; Nederbragt and Fiorentino, 1999). Inthe Totumo-3 core, the C/T boundary excursion is pro-nounced, with a positive 6.23‰ shift in d13C values (Fig.3D). This correlates with a similar positive excursion atthe C/T boundary in the nearby Alpuf-6 Well core (Davis,1995; Fig. 1). However, the excursion peak in the Totumo-3 Well is almost 5.5 m higher with respect to the contactwith the underlying Maraca Formation than in the Alpuf-6 Well. This supports the theory that the contact is an un-even, karstic surface (Truskowski et al., 1995).

The C/T boundary excursion in the Totumo-3 Well con-sists of a single peak, similar to that found in condensedsections such as the Ponca State Park section in Nebraska(Pratt and Threlkeld, 1984). The detailed stratigraphy ofthe C/T boundary C-isotope curve described in expandedsections in the U.S. Western Interior and England (Prattand Thelkeld, 1984; Gale et al., 1993) cannot be recognizedin the Totumo-3 data. This suggests that the C/T bound-ary interval in the Totumo-3 Well is condensed.

Correlation of the C-isotope curves above the C/Tboundary in the sections investigated is subjective, espe-cially in the Totumo-3 Well where numerous high-ampli-tude fluctuations occur (Fig. 3D). A tentative correlation isprovided of two positive excursions (excursions 2 and 3 in


FIGURE 7—Tentative correlation of the sections studied. See Figure 5 for reference numbers and letters for the nannofossil intervals andforaminifera zones.

Fig. 3) that occur in the Coniacian to lowermost Santonianpart of the section. Although these excursions are definedby few data points, the proposed correlation is consistentwith the biostratigraphy. Excursions 2 or 3 may be asso-ciated with Coniacian/Santonian OAE3 (Jenkyns, 1980),an event that has not been well constrained stratigraphi-cally. Thus, C-isotope stratigraphy provides a means ofcorrelation of the C/T boundary interval in the La LunaFormation and perhaps of levels within the Coniacian.

RATES AND PATTERNS OF DEPOSITION

The stratigraphic framework established above (Fig. 3)and the correlation of the sections (Fig. 7) allow us to in-terpret the depositional history of the La Luna Formationin the Maracaibo Basin in more detail than has been pos-sible previously. Sedimentation of the La Luna Formationbegan in the eastern part of the Maracaibo Basin. In theSan Felipe area, deposition began possibly as early as thelate Albian, but probably not until the middle to late-mid-dle Cenomanian. Davis et al. (1999) noted that depositioncommenced slightly later, in the late middle Cenomanianin the area of the Totumo-3 Well in the northwestern Ma-racaibo Basin. Sedimentation expanded towards thesouthwest, reaching the Rıo Loro area by the early Conia-cian and the San Pedro del Rıo area in the middle Conia-cian. Data presented here for the onset of deposition in thestate of Tachira agree with the stratigraphy of Renz(1982), but not with the late Cenomanian age proposed byBoesi et al. (1993) for this event. In the east of the basin

near the La Pena Section, deposition of the La Luna For-mation continued during the Turonian and Coniacian.

The La Luna Formation and the overlying uppermostCampanian–lower Maastrichtian Colon Formation areseparated by an unconformity throughout Venezuela (deRomero and Odreman, 1996; Lorente et al., 1996). In theLa Pena Section, the full extent of the unconformity couldnot be assessed, therefore the age of the top of the La Lunain this region could not be determined. Deposition of theLa Luna in the southwest of the Maracaibo Basin contin-ued until at least the middle-late Santonian. Previouswork by De Romero and Galea-Alvarez (1995) indicatesthat the La Luna ranges up into the uppermost lowerCampanian in this area. An age no younger than late San-tonian for the interval immediately below ‘‘Capa II’’ sup-ports the conclusions of De Romero and Galea-Alvarez(1995), and differs with the Campanian interpretation ofBoesi et al. (1993).

The application of the informal nannofossil biostratig-raphy has enabled calculation of sedimentation rates inthe sections investigated using the Gradstein et al. (1995)time scale. Data from this study suggest that sedimenta-tion was highly variable (Fig. 8). Pulses of rapid depositionalternated with periods of slow accumulation. Periods ofintensified sedimentation occurred in the Eprolithus ep-tapetalus–Micula staurophora interval from 89.0–88.9 Ma(early Coniacian) and again in the Micula concava–Lith-astrinus moratus interval from 85.6–84.8 Ma (late earlySantonian). Peak rates were approximately 4.8 cm/kyr


FIGURE 8—Histogram of calculated sedimentation rates of each nan-nofossil interval (Fig. 3).

compared to 1.9 to 0.7 cm/kyr for other intervals. Periodsof rapid sedimentation correlate to the shortest durationnannofossil intervals. Although sedimentation rates pos-sibly are affected by errors in the time scale, the differenc-es between high- and low-sedimentation rates are too con-siderable to be explained by problems such as uncertain-ties in absolute dates or stratigraphic imprecision. Thelowest sedimentation rates in any interval are found inthe Rıo Loro Section. This is to be expected because RıoLoro lies on the Merida Arch, a structural high where thetotal thickness of Cretaceous sediments is reduced consid-erably. In El Valle, just outside of the town of Merida, all ofthe Cretaceous formations are condensed to a total thick-ness of approximately 3 m. The highest sedimentationrates are in the eastern part of the Maracaibo Basin dur-ing the early Coniacian and in the southwestern part ofthe basin during the late early Santonian. Relatively highrates are also found in the eastern basin during the mid-Cenomanian to the mid-Turonian.

CONCLUSIONS

Poor preservation of calcareous nannofossils and thescarcity of carinate planktic foraminifera in washed sam-ples and thin sections have plagued the application of mi-crofossil biostratigraphy in the La Luna Formation. Inthis study, integration of a traditional planktic foraminif-eral zonal scheme and an informal nannofossil zonalscheme is based on dissolution-resistant species. The ap-plication of the combined biostratigraphy, complementedby C-isotope stratigraphy, has enabled establishment ofrates and patterns of sedimentation of the La Luna For-mation throughout the Maracaibo Basin.

Deposition of the La Luna Formation began in the mid-dle to late-middle Cenomanian in the northwestern part ofthe Maracaibo Basin, and probably no earlier than this inthe eastern part. Deposition progressively expanded west-ward and southward, reaching the southwestern basin noearlier than the middle Coniacian. In the eastern basin,deposition of the La Luna continued into the Coniacian; inthe western basin, deposition continued at least until themiddle-late Santonian. Previous work indicates that de-position in the western basin continued at least into theuppermost lower Campanian (De Romero and Galea-Al-varez, 1995) and possibly into the upper Campanian (Far-ias et al., 2000). In the different parts of the basin, sedi-mentation rates were highly variable, with short pulses ofrapid deposition, and a period of condensed sedimentationoccurring at the Cenomanian–Turonian boundary.

ACKNOWLEDGEMENTS

This project has been financed by the Petroleum Re-search Fund of the American Chemical Society (GrantAC8-33403). We would like to thank Petroleos de Vene-zuela, S.A. (PDVSA) for permission to sample the Totumo-3 Well core and the personnel in the core repository in LaConcepcion for their generous assistance, with specialthanks to Oscar Quintero and Sr. Freddy. Thin sectionswere prepared by the personnel in the PDVSA laboratoryin Caracas. Many thanks go to Ovidio Lizcano and CarlosRomero for their help in the field, Howard Mendlovitz forrunning carbon isotope samples, Kate Sowder and PaulWinberry for laboratory work, and Karin Peyer for assis-tance with developing micrographs. We are indebted tothe reviewers Brian Huber, Ian Jarvis, and Isabella Pre-moli Silva for their many helpful suggestions for improv-ing the manuscript.

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