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ELSEVIER Palaeogeography, Palaeoclimatology, Palaeoecology 153 (1999) 203–238

Palaeoecology, palaeogeography and depositional environments of Upper Cretaceous rocks of western Venezuela

R.N. Erlich a,Ł, O. Macsotay I. b, A.J. Nederbragt c,1, M. Antonieta Lorente d

a Amoco Exploration and Production, 501 Westlake Park Blvd., P.O. Box 3092, Houston, TX 77079, USAb Apartado Postal 62262, Caracas, Venezuela

c Vrije Universiteit, Faculteit der Aardwetenschappen, de Boelelaan 1085, 1081 HV Amsterdam, Netherlandsd PDVSA Exploracion y Produccion, Apartado 829, Caracas 1010A, Venezuela

Received 18 September 1998; accepted 1 April 1999

Abstract

The end of the Early Cretaceous in northern South America was marked by regional palaeoceanographic change. In

western Venezuela, this change was highlighted near the end of the Albian by drowning of the Maraca Formation shal-

low-water carbonate platform. Regional marine transgression continued during the Cenomanian and Turonian in western

Venezuela with drowning of the more southerly Guayacan Member (Capacho and Escandalosa formations) carbonate

platform. Deposition of organic carbon-rich intervals of the La Luna and Navay formations occurred unconformably on

Maraca Formation and Guayacan Member shallow-water carbonates and continued through the early Santonian. During

this interval, the Maracaibo and Barinas=Apure basins were characterized by low-oxygen or anoxic bottom-water condi-tions away from the basin margins. Deposition of organic carbon-lean upper La Luna and Navay formation strata show

that bottom-water oxygen content increased from the late Santonian through the end of the Cretaceous. The siliceous

and phosphatic late Santonian to early Maastrichtian Tres Esquinas Member, and the glauconitic late Campanian to early

Maastrichtian Socuy Member (both of the La Luna Formation) represent the final phase of La Luna deposition in the

Maracaibo Basin. Tectonic uplift in eastern Colombia during the Campanian and Maastrichtian caused progradation of

the Colon, Mito Juan, and Burguita Formation deltas, and eventual infilling of the Maracaibo and Barinas=Apure basins.

© 1999 Elsevier Science B.V. All rights reserved.

Keywords: Upper Cretaceous; La Luna Formation; palaeo-oceanography; organic carbon

1. Introduction

The origin of Cretaceous organic carbon-richsediments has become the focus of much recent

Ł Corresponding author. Present address: Burlington Resources

International, 1001 Fannin Street, Suite 4500, Houston, Texas

77002-6712, USA; Fax: C1 713 336 6713; E-mail:

rnerlich@ pdq.net1 Present address: Dept. of Geological Sciences, University Col-

lege London, Gower Street, London WC1E 6BT, UK.

study, due at least in part to the large hydrocar-bon resources generated from them (Klemme andUlmishek, 1991). Characterizing the depositional en-vironments of these units has often been done us-ing highly generalized regional or continent-widepalaeogeographic and palaeoclimatic models (e.g.,Barron, 1985; Barron and Peterson, 1990; Kruijs and

Barron, 1990; Barron et al., 1995; Slingerland et al.,1996), or has centered on the organic geochemicalcharacteristics of selected units (Blaser and White,

0031-0182/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.

P I I : S 0 0 3 1 - 0 1 8 2 ( 9 9 ) 0 0 0 7 2 - 3



204 R.N. Erlich et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 153 (1999) 203–238

1984; Talukdar and Marcano, 1994). Such modelsoffer important insights into global or continent-wide phenomena, but may sacrifice details regardingspecific depositional systems.

One area where organic carbon-rich strata can beexamined in detail, through outcrop and subsurfacecontrol, is in western Venezuela (Fig. 1). Previousstudies by Ghosh (1984), Macellari and De Vries(1987), and Martınez and Hernandez (1992) exam-ined some of the factors surrounding anoxia andthe accumulation of organic carbon-rich and phos-phatic sediments in the Maracaibo and Barinas=Apure basins. In this paper, we document the palaeo-geographic evolution from shallow-water carbonates

Fig. 1. Location map of western Venezuela showing all wells (circles) and outcrops (triangles) used in the study, and important geological

features.

to deep-water organic carbon-rich sediments in west-ern Venezuela using detailed ecologic and sedimen-tologic analyses. Subsequent papers (Erlich et al.,1999a,b) identify the influence of palaeoceanogra-

phy and palaeoclimate on the establishment andtermination of anoxia in the Maracaibo and Barinas=Apure basins.

1.1. Study area and summary of the structural

evolution of western Venezuela

The Maracaibo and Barinas=Apure basins are lo-cated in northwestern Venezuela, and are separatedby the northeast–southwest trending Merida Andes



R.N. Erlich et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 153 (1999) 203–238 205

Fig. 2. Major tectonic elements of the Maracaibo and Barinas=Apure basins (stippled pattern denotes basement uplifts, lined pattern

denotes half grabens).

Mountains (Fig. 1). The Maracaibo Basin is bor-dered on the north by the Gulf of Venezuela andon the west by the Sierra Perija Mountains. TheBarinas=Apure Basin is bordered on the east by thePrecambrian Guayana Shield, on the north by thePalaeozoic El Baul arch, and in the south continues

into Colombia as the Llanos Basin.

The Mesozoic structural evolution of these areasbegan during the Late Jurassic with formation of large rift grabens (Machiques, Uribante, and Bar-quisimeto troughs), oriented roughly parallel to andunderlying the present-day Merida Andes and SierraPerija (Fig. 2). The grabens were filled by lacus-trine and fluvial red beds and volcanics and later by

Neocomian alluvial fans and braided-stream deposits(Feo-Codecido et al., 1984; Maze, 1984; Lugo andMann, 1995).

Lower Aptian to Cenomanian marine rocks weredeposited along a passive continental margin (Zam-brano et al., 1971; Gonzalez de Juana et al.,1980; Bartok et al., 1981; Parnaud et al., 1995).Granitic and metamorphic basement blocks that bor-dered the Sierra Perija (Santa Marta and Santandermassifs) and the Merida Andes (Paraguana Block,

and Arauca, Merida, and El Baul arches) acted aspalaeotopographic and palaeobathymetric features,with significant thinning of Cretaceous sediments(Renz, 1959, 1982; Lugo, 1994; Lugo and Mann,1995). Reactivation of Jurassic normal faults duringthe Cenomanian and Turonian produced differentialsubsidence in the central and southern MaracaiboBasin, with a relatively stable to slowly subsid-ing eastern margin (proto-Merida Andes; Macellari,1988; Cooper et al., 1995; Vergara, 1997a).




The deposition of marine rocks continued throughthe end of the Cretaceous (Gonzalez de Juana et al.,1980; Parnaud et al., 1995). However, tectonic upliftin northern and eastern Colombia during the Cam-

panian through Maastrichtian provided a sedimentsource for upper deltaic and non-marine siliciclastics,which eventually filled the Maracaibo and Barinas=Apure basins (Kellogg, 1984; Shagam et al., 1984;Macellari, 1988; Pindell and Barrett, 1990; Aude-mard, 1991; Sanchez Nunez et al., 1994; Cooper etal., 1995; Parnaud et al., 1995; Villamil, 1998).

Southward-directed compression and transpres-sion between the Caribbean and South Ameri-can plates caused initial uplift (inversion of theMachiques Trough) of the Sierra Perija in the Mid-

dle Eocene, and subsequent development of thenortheastern Maracaibo foreland basin. Northward-directed compression and transpression between thePacific, Caribbean, and South American plates inthe Late Miocene to Early Pliocene caused up-lift and inversion of the northeastern Maracaiboforeland basin and Merida Andes (BarquisimetoTrough), and segmentation of the Maracaibo andBarinas=Apure basins (Macellari, 1984; Giegengack,1984; Stephan, 1985; James, 1990; Audemard, 1991;Sanchez Nunez et al., 1994; Colletta et al., 1997).

1.2. Previous work

Previous studies of western Venezuela have em-phasized the general aspects of Upper Cretaceousstratigraphy within the Maracaibo Basin (Hedbergand Sass, 1937; Sutton, 1946; Rod and Maync,1954; O. Renz, 1959, 1982; H.H. Renz, 1961; Stain-forth, 1962; Gonzalez de Juana et al., 1980; Ma-cellari, 1988; De Romero and Galea-Alvarez, 1995;Lugo and Mann, 1995; Parnaud et al., 1995). Sim-ilar studies in the Barinas=Apure Basin (Rod and

Maync, 1954; O. Renz, 1959; Kiser, 1961, 1989;H.H. Renz, 1961; Gaenslen, 1962; Chigne, 1985;Escamilla et al., 1994; Helenes et al., 1998), and ineastern Colombia (Burgl, 1961; Ward et al., 1973;Macellari and De Vries, 1987; Martınez, 1989; Bar-rio and Coffield, 1992; Follmi et al., 1992; Vergara,1997a,b; Villamil, 1998) have established the re-gional framework of the Upper Cretaceous unitsover northwestern South America.

Work by Ford and Houbolt (1963), Ghosh (1984),

and Martınez and Hernandez (1992) dealt primarilywith detailed facies relationships within and be-tween the various units, while Macellari and DeVries (1987), Tribovillard et al. (1991), Barrio and

Coffield (1992), Follmi et al. (1992), and Villamil(1998) emphasized their palaeoclimatic and palaeo-ceanographic significance. While this study buildson these earlier efforts, it is also in part a revi-sion of the established lithostratigraphy of westernVenezuela. This study also emphasizes the impor-tance of specific lithostratigraphic units as aids ininterpreting the Late Cretaceous palaeoclimatic andpalaeoceanographic framework of the region.

2. Field methods, petrographic analyses, and datasources

Outcrops examined in this study were sampledextensively for planktic foraminiferal biostratigra-phy, nannofossils, palynology, organic and inorganicgeochemistry, and thin-section petrography. Sampleswere taken at intervals ranging from 1 to 5 m,or as needed in order to characterize rapid litho-logic changes. Each outcrop was measured in one-meter increments with corresponding gamma logmeasurements (via hand-held scintillometer); litho-

logic changes or evidence of condensed sections orunconformities were measured in 10-cm incrementswhere appropriate.

Outcrop lithologies were characterized by bulk X-ray diffraction analysis (XRD). Inorganic and or-ganic geochemical data are presented here only insummary form; a detailed discussion of the geo-chemical data can be found in Erlich et al. (1999b).Thin-section descriptions of rock fabrics and con-stituent grains were made for 290 samples in orderto check bulk mineralogical results, and provide ad-

ditional information on depositional environments.A total of 155 wells and 62 outcrops (Fig. 1)

were used to construct the lithostratigraphic frame-work presented in this study. Well data were obtainedfrom proprietary (Maraven, S.A. and Amoco Explo-ration and Production files) and published sources,and consist of wireline logs, biostratigraphy (plank-tic and benthic foraminiferal, nannofossil, and paly-nological analyses), and lithology (as indicated byconventional and sidewall cores and cuttings). Corre-




lations between wells and between wells and outcropswere done using standard sequence stratigraphic tech-niques, and were verified by biostratigraphically de-termined ages and interpretations of depositional en-

vironments (e.g., data of Fuenmayor, 1989).

3. Biostratigraphic and palaeoecologic analyses

The chronostratigraphic framework of upper Al-bian through Maastrichtian strata has been es-tablished by planktic foraminiferal biostratigraphy(Fuenmayor, 1989; Canache et al., 1994; Galea-

Alvarez et al., 1994; De Romero and Galea-Alvarez,1995; Truskowski et al., 1995; Lorente et al., 1996;

Truskowski et al., 1996) and ammonite and mol-luscan biostratigraphy (see for summaries of char-acteristic Late Cretaceous molluscan and ammonitefauna of Venezuela: Sutton, 1946; Rod and Maync,1954; Renz, 1959; Macsotay, 1975, 1980; Renz,1982). Revisions and refinements to this framework were concentrated primarily at formation contacts.It should be noted that ammonite species diversitywas not adequate enough to determine zonationssuch as those used in other areas (e.g., Berthou andLauverjat, 1974). Nevertheless, assemblages of age-diagnostic ammonites were developed for western

Venezuela and show that the Cenomanian throughConiacian is complete in northeastern and centralMaracaibo (Renz, 1982).

In this study, calcareous nannofossils were alsoused to date late Campanian through Maastrichtianparts of the Las Delicias and Las Vegas outcropsections ( Figs. 3–6; data point location names arepresented in Table 1). Spores, pollen, and dinoflagel-lates were used to verify the ages of other sections inthe Merida Andes, such as Rıo Guaruries (Boesi etal., 1988) and Rıo Sto. Domingo (Tribovillard et al.,

1991).

3.1. Comments on the completeness of planktic

foraminiferal zones of western Venezuela

In the Sierra Perija area (Fig. 1), part of the lateAlbian and all of the early and middle Cenomanianare missing, as evidenced by the absence of part of the Rotalipora appenninica zone and the absence of the R. brotzeni and R. reicheli zones (Canache et al.,

1994; Truskowski et al., 1995). However, the lateAlbian is recorded in the southern part of Maracaiboand in the Barinas=Apure Basin by the presence of

Ticinella spp. in the Rotalipora appenninica zone

(this study).The Turonian is complete in the Maracaibo Basin

(Galea-Alvarez et al., 1994); however, the upper partof the Turonian (above the Heterohelix helvetica

zone) is apparently missing from the Barinas=ApureBasin (see also Escamilla et al., 1994). The pres-ence of the Dicarinella concavata and D. asymetrica

zones throughout both basins (Lorente et al., 1996;Truskowski et al., 1996; this study) shows that theSantonian is complete in western Venezuela.

The Campanian is represented by varying

amounts of stratigraphic condensation in the Mara-caibo and Barinas=Apure basins. For example, the

Globotruncana ventricosa and Globotruncanita ele-

vata zones have been recorded in the Sierra Perija(Fig. 1), while the Globotruncanita calcarata zoneis rare or absent (Sellier De Civrieux, 1952; Fordand Houbolt, 1963; Gonzalez de Juana et al., 1980;Ghosh, 1984; Fuenmayor, 1989; Lorente et al., 1996;this study). The Globotruncanita calcarata zone andpart of the Globotruncana ventricosa zone are alsomissing from the western part of the Barinas=ApureBasin and the southern Maracaibo Basin, while the

Globotruncanita elevata zone is still present (DeRomero and Galea-Alvarez, 1995; this study).

Part of the early Maastrichtian is missing insouthwestern Venezuela, as illustrated by the ab-sence of the early part of the Globotruncanella

havanensis zone (Fuenmayor, 1989; Truskowski etal., 1996; this study). The Globotruncana aegypti-

aca zone is apparently complete (De Romero andGalea-Alvarez, 1995). While the Late MaastrichtianGansserina gansseri zone is preserved, the Abathom-

phalus mayaroensis zone is absent from the Colon

and part of the Mito Juan formations, and indicates ahiatus prior to the end of the Cretaceous (Lorente etal., 1996).

3.2. Analytical methods

Foraminiferal data from this study are almostentirely based on recognition of species or morpho-types in thin section (Postuma, 1971; Sliter, 1989;Premoli Silva and Sliter, 1995), as few samples




Fig. 3. Location map showing key wells and outcrops used to construct lithostratigraphic cross-sections (Figs. 4–6).

yielded results from conventional washed residues.Additionally, surface-water conditions in the studyarea were apparently environmentally unfavorable tothe extent that the keeled species that provide goodbiostratigraphic resolution were only sporadically

present.Contributions of planktic foraminifera to the

regional biostratigraphic framework are therefore

mostly based on globular forms with well-estab-lished biostratigraphic ranges (Fig. 7; Caron, 1985;Nederbragt, 1991). In addition, changes in the rela-tive abundance of biserial heterohelicids within theplanktic population were used to subdivide the Al-

bian and Cenomanian (biserial planktic foraminiferafirst appeared in the late Albian; Bandy, 1967; Neder-bragt, 1991). Bandy (1967) noted that heterohelicids



R.N. Erlich et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 153 (1999) 203–238

Fig. 4. Lithostratigraphic cross-section A–A0 , from the northwestern Maracaibo Basin to the southeastern Barinas=Apure Basin.

Horizontal scale is shown; no vertical scale is implied in the cross-sections.



pp. 211–212

Fig. 5. Lithostratigraphic cross-section B–B0, from the southwestern to the northeastern Merida Andes.



Fig. 6. Lithostratigraphic cross-section C–C0, from the east-central Maracaibo Basin to the east-central Bari




Table 1

Names and locations of wells and outcrops shown in Figs. 3–6

(1) La Paz Field (19) Quebrada Escandalosa outcrop (37) Chejende outcrop

(2) Totumos-5 well (20) Rıo Caparo=La Vueltosa outcrops (38) Barbacoas outcrop

(3) Quebrada La Ge outcrop (21) Yaure-2 well (39) Curarigua outcrop(4) Quebrada La Luna outcrop (22) Agua Linda-1 well (40) La Ceiba-1X well

(5) Rıo Aponcito outcrop (23) Rosalia-1 well (41) Rıo Sto. Domingo outcrop

(6) Rıo Negro outcrop (24) Apure-2 well (42) Maporal Field

(7) Rıo Yasa outcrop (25) La Quinta=La Frıa outcrops (43) Paez Field

(8) Alturitas-6 well (26) Rıo Omuquena outcrop (44) Conso-3 well

(9) Rosario-12 well (27) Rıo Guaruries=Zea outcrops (45) Machiques-2X well

(10) Rıo de Oro-18 well (28) Rıo Chama outcrop (46) Paso Andino outcrop

(11) West Tarra-6 well (29) Carbonera area outcrop (47) Las Vegas outcrop

(12) West Tarra-51 well (30) DUA-1X well (48) Burgua-3 well

(13) Mucurera-3 well (31) Rıo Torondoy outcrop (49) Rıo Lora outcrop

(14) Mucujun=San Pedro del Rıo outcrops (32) Rıo Mimbos outcrop (50) La Villa del Rosario Quarry

(15) Quebrada Zorca=Independencia outcrops (33) Monte Carmelo area outcrops (51) Rıo Higuerones outcrop

(16) Quebrada Ortiza outcrop (34) Conquistado-1 well (52) Rıo Sucumo outcrop

(17) Las Delicias outcrop (35) Arbol Redondo outcrops

(18) Rıo Uribante outcrop (36) Butaque-1S well

became more abundant during the Cenomanian; our

material suggests that two stepwise increases in het-erohelicid percentages, from rare to common, andfrom common to abundant, can be used to approx-imate the Albian–Cenomanian and middle to lateCenomanian boundaries, respectively.

Palaeoecological interpretations are based on

quantitative analysis of foraminiferal faunas inthin section, as well as benthic foraminiferal and

macrofossil (primarily mollusc and gastropod) as-semblages. The number of foraminifera per unitsurface area was estimated, as well as ratios(expressed as percentages) between benthic andplanktic foraminifera, keeled and globular plankticforaminifera, and between spiral and biserial plankticforaminifera. A more detailed quantitative analysisof planktic foraminiferal faunal composition was notfeasible, as too few specimens could be positively

identified. Therefore, species and genus distributionpatterns were compiled from qualitative presence=absence data.

Analysis of benthic foraminiferal faunas in thinsection was limited to recognition of the morpho-types that are abundant in some of the samples.These include representatives of the genus Or-

thokarstenia, Siphogenerinoides, Praebulimina, and Neobulimina; the last two groups may include rep-resentatives of other genera with similar morpholo-

gies. ‘Low trochospiral forms’ are common in somesamples, and cover a number of morphotypes (e.g.,Gavelinella; but others cannot be determined re-liably at the genus or even family level). Fordand Houbolt (1963) and De Romero and Galea-Alvarez (1995) also described Lenticulina sp. and Bolivina sp. as abundant in some levels. All mor-

photypes listed above have been classified as low-oxygen opportunists (e.g., Bernhard, 1986; Ly andKuhnt, 1994). In addition, low-oxygen molluscangenera such as Inoceramus, Mytilopsis, and Didymo-

tis (Sutton, 1946; Renz, 1959), along with juvenileammonites and terebratulid brachiopods occur inshallower-water depositional environments of the LaLuna Formation, and show that the basin marginswere at least intermittently oxic during the Ceno-manian and Turonian (Aguasuelos Engineers, 1991).Post-Turonian bivalves such as Pseudocucullae, Pli-

catula, Trigonarca, and Veniella of the Merida Andesoccurred in middle to outer shelf environments, andare indicative of low-oxygen conditions (Macsotay,1980).

Under ‘normal’ open marine conditions, ben-thic faunal composition is a useful indicator of palaeobathymetry, as in the ratio of benthic=plankticforaminifera (Wright, 1977; Van Marle et al., 1987).In the Venezuelan units, however, variation in thepercentage of benthic foraminifera is also a function




Fig. 7. Albian to Maastrichtian planktic foraminiferal biostratigraphic framework used in this study.

of variation in surface water productivity and extentof bottom oxygenation. Shifts in benthic=plankticratios within a section therefore reflect changingconditions in the water column as well as changesin water depth. Geographic variations between age-equivalent samples are interpreted to represent apalaeobathymetric signal (Fig. 8), in that lower per-

centages of benthic foraminifera would occur deeper

in an oxygen minimum zone (OMZ).

3.3. Age distribution of benthic and planktic

foraminifera and other important microfossils

Viewed by age, foraminiferal concentrationsin Albian to Cenomanian parts of the section

are variable, but consist mostly of faunas com-posed of globular planktic foraminifera, with rarebenthic foraminifera. In these intervals, benthic

foraminifera are generally more diverse than plank-tic foraminifera; however, preservation is poor. Inthe type section of Quebrada La Luna (Fig. 3) and inthe northern part of the Maracaibo Basin in general(Figs. 4 and 5), La Luna Formation rocks are mostlyfine-grained planktic foraminiferal oozes. There arerare or no occurrences of shallow-water fossils in

these areas; rotaliporids and globotruncanids are

only occasionally present (Fig. 7; Truskowski etal., 1996).

Globular forms ( Heterohelix globulosa, Hed-

bergella sp., Globigerinelloides spp., and White-

inella sp.) dominate planktic foraminiferal faunasin Turonian to early Santonian strata. Keeled forms( Dicarinella= Marginotruncana spp.) are absent or

very rare eastwards away from the Sierra Perija andcentral Lake Maracaibo (Fig. 7; also see Lorenteand Duran, 1995; Lorente et al., 1996). Benthic




Fig. 8. Regional palaeobathymetric interpretation of Upper Cretaceous rocks from outcrops in the Sierra Perija and Merida Andes

mountains. Each circled location reflects the average palaeo-environment for multiple nearby outcrop locations, and also represents an

average for the late Albian through early Maastrichtian.

foraminifera, if present, are dominated by Neobulim-

ina sp., with low trochospiral morphotypes.In the southern part of the Maracaibo Basin and in

the Barinas=Apure Basin (Figs. 3 and 4), fauna aregenerally dominated by planktic foraminifera sim-ilar to those described from the northwestern partof the Maracaibo Basin, with the addition of morecommon Rotalipora sp. The benthic foraminifera

Siphogenerinoides sp., Orthokarstenia sp., Lentic-

ulina sp., Bolivina sp., and Praebulimina sp. arevery common in the upper parts of the La LunaFormation in these areas (see De Romero and Galea-Alvarez, 1995), and can be the dominant fauna insome sections. Neobulimina sp. occurs commonly inthe older (Cenomanian) parts of the section. The LaMorita Member of the Navay Formation (Fig. 4) has

a similar faunal composition to lower La Luna For-mation units in northern Maracaibo. The QuevedoMember contains phosphatic pellets, fish bones, and

Siphogenerinoides sp., commonly concentrated inthin layers.

Benthic foraminiferal faunas in Santonian to earlyCampanian strata have very low diversity, or aremonospecific. In certain parts of the La Luna For-mation, Orthokarstenia sp. or Praebulimina sp. mayform the bulk of all foraminiferal faunas (De Romeroand Galea-Alvarez, 1995). Neobulimina sp. appearsto be more abundant in sections that appear to transi-

tion into deeper (more pelagic) environments domi-nated by planktic foraminifera. Species composition,as well as the low diversity in general are indicativeof intermittent low-oxygen conditions in the watercolumn.

Planktic foraminifera are generally rare in theSantonian to early Campanian parts of the LaLuna Formation; usually less than 30% of the to-tal foraminifera in any given sample. In coarser-grained units, this could be attributed to size sorting




through selective removal of planktic foraminifera.However, concentrations of planktic foraminifera (innumbers of individuals per surface area in thin sec-tion) in more fine-grained samples are also much

lower than in the older units. Palaeobathymetricshallowing, resulting in lower numbers of plankticforaminifera, may explain the shift from the olderplanktic foraminiferal faunas to faunas dominatedby benthic foraminifera. This decrease in plank-tic foraminifera may also be due to environmentalfactors, such as changing productivity and oxygena-tion of the water column. A decrease in plankticforaminiferal productivity could occur if nutrientlevels became too high or too variable.

Foraminifera are generally rare within Campa-

nian- to Maastrichtian-aged strata. Faunas (plank-tic and benthic) appear more diverse than in olderrocks, but diversity is still low enough to classify thefaunas as opportunistic. Concentrations of plankticforaminifera are relatively high in the Socuy Mem-ber of the La Luna Formation, and in the La LunaFormation from the northern Merida Andes area(Figs. 4 and 5). The Socuy Member contains thelate Campanian age-diagnostic Globotruncanita cal-

carata (Sellier De Civrieux, 1952; Ford and Houbolt,1963; Gonzalez de Juana et al., 1980; Ghosh, 1984),though many individuals are fragmented or partially

dissolved. Benthic foraminifera are rare in most sam-ples, but faunas appear to be moderately diverse.

Benthic foraminifera such as Cibicides spp., Marginulina spp., and Trochammina spp. are im-portant tools for the identification of specific lithos-tratigraphic units (Maastrichtian) in the northernand central Maracaibo Basin (Fig. 4; Fuenmayor,1989). In the southern Maracaibo and Barinas=Apurebasins, palynological as well as foraminiferal as-semblages are used to determine lithostratigraphy.For example, characterization of the Maastrichtian

Colon and Mito Juan formations is based on a com-bination of faunal and lithologic criteria (Figs. 4 and5; Kuyl et al., 1955). An early Maastrichtian agewas established for the Colon Formation using theGansserina gansseri and Globotruncana aegyptiaca

planktic foraminiferal zones, and the base of theProteacidites dehaani spore assemblage (De Romeroand Galea-Alvarez, 1995; Lorente and Duran, 1995).The Mito Juan Formation was assigned to the lateMaastrichtian to early Danian based on the presence

of the top of the Proteacidites dehaani spore assem-blage, the Carapatella cornuta dinoflagellate zone,and the Morozovella pseudobulloides foraminiferalzone (Lorente and Duran, 1995).

4. Depositional environments and

palaeogeographies of upper Albian to

Maastrichtian rocks

4.1. Palinspastic reconstructions

Data used to construct regional lithostratigraphiccross-sections of late Albian to Maastrichtian forma-tions (Figs. 4–6) were compiled from published and

proprietary sources (Appendix A). These data werethen combined with the biostratigraphic and palaeoe-cological analyses to construct regional palaeogeo-graphic maps for the late Albian, Cenomanian, Tur-onian, Coniacian=Santonian, early Campanian, lateCampanian to early Maastrichtian, and late Maas-trichtian (Figs. 9–15).

Palinspastic reconstructions of crustal shorteningor lateral (fault) displacements were done prior toconstruction of the palaeogeographic maps. Esti-mates of lateral (fault) displacement and crustal (foldand thrust-related) shortening used in this study rep-

resent our best estimates from published sources.This is due to the high degree of uncertainty sur-rounding relative amounts of displacement and short-ening of the major faults and fold belts in westernVenezuela.

For example, estimates of the amount of dextraloffset along the Bocono Fault (Fig. 2) vary frommore than 100 km (e.g., Stephan, 1985; Pindelland Barrett, 1990) to a cumulative 10–70 km alongthe entire fault system (Rod et al., 1958; Lugo,1994). Sinistral offset along the Santa Marta Fault

has been calculated to be up to 100 km (Kellogg,1984), although Salvador (1986) derived a value formaximum combined offset along the Bocono andSanta Marta faults of 100 km. Dextral offset of 65–90 km along the Oca Fault system (Kellogg, 1984;Pindell and Barrett, 1990) was also considered in thepalaeogeographic reconstructions.

Estimates of crustal shortening across the SierraPerija, Merida Andes, and Lara Nappes fold andthrust belts (Fig. 2) also vary. For example, shorten-




Fig. 9. Palaeogeographic map for the late Albian. ‘Eroded’ areas were eroded post-Cretaceous unless indicated otherwise; cross-hatching

shows the location of subsurface basement highs where the section is thin or otherwise attenuated. All palaeogeographic maps have been

structurally restored (see text for explanation). LS D limestone; SS D sandstone; SLTST D siltstone; SH D shale; PHOS D phosphatic;

SIL D siliceous; GLAUC D glauconitic.

ing across the Sierra Perija thrust belt has been esti-mated to be between 16 and 26 km (Kellogg, 1984),

while shortening across the northern and southern

Merida Andes thrust belt may total no more than60 km (Colletta et al., 1997). Additionally, directestimates of shortening across the Lara Nappes canonly be inferred from regional studies (Beck, 1978;Stephan, 1985; Pindell and Barrett, 1990), but mayvary from 40 to 150 km.

Based on the estimates of fault offset and crustal

shortening cited previously, our reconstructions re-flect dextral offset along the Bocono Fault system of 75 km, sinistral offset along the Santa Marta Fault

system of 90 km, and dextral offset along the OcaFault system of 90 km (Fig. 2). Our palaeogeo-

graphic maps also reflect 25 km of shortening in the

Sierra Perija thrust belt, 60 km of shortening acrossthe Merida Andes, and 100 km of shortening acrossthe Lara Nappes. The reconstructed positions of im-portant faults, rift grabens, and basement uplifts areshown in Fig. 16.

4.2. Calculation of sedimentation rates

Biostratigraphic data from three wells and elevenoutcrop sections from this study, and well docu-




Fig. 10. Palaeogeographic map of the Cenomanian. LS D limestone; SS D sandstone; SLTST D siltstone; SH D shale; PHOS D

phosphatic; SIL D siliceous; GLAUC D glauconitic.

mented published studies (Rod and Maync, 1954;Renz, 1959; Boesi et al., 1988; Tribovillard et al.,1991; Helenes et al., 1998) were used to estab-lish average, minimum, and maximum sedimenta-tion rates for late Albian through Maastrichtian for-

mations (Table 2). Sedimentation rates (expressed

in mm=1000 yr) were calculated by dividing thethickness of specific units by the amount of timerepresented (time scale from Gradstein et al., 1995);unit thicknesses were not corrected for compactionor erosion, and therefore represent minimum esti-mates. In addition, we recognize that the amountof time represented for incomplete lithostratigraphic

sections (e.g., rocks of early Maastrichtian age) maynot have been determined correctly. Nevertheless,the sedimentation rates shown in Table 2 (and Ap-

pendix B) represent reasonable estimates of sedi-mentation during the Late Cretaceous of westernVenezuela.

4.3. Late Albian

The late Albian of western Venezuela was charac-terized by the widespread deposition of shallow ma-rine sediments in the Maracaibo and Barinas=Apurebasins (Zambrano et al., 1971; Gonzalez de Juanaet al., 1980; Bartok et al., 1981; Macellari, 1988;Mendez, 1989; Martınez and Hernandez, 1992; He-lenes et al., 1998). Large subaerial or shallow sub-

marine basement uplifts influenced sedimentationpatterns in northern Maracaibo (Santa Marta Massif,Paraguana Block), southwestern Maracaibo (San-




Table 2

Minimum, average, and maximum sedimentation rates for key

time intervals (late Albian through Maastrichtian), formations,

and members

Rate: min=avg=max(mm=1000 yr)

Age

Maastrichtian (total) 83.3 =118.3=170.3

Early Maastrichtian 4.8 = 5.8= 7.5

Campanian 0.1 = 2.7= 11.1

Santonian 1.3 = 16.6= 58.7

Coniacian 0.3 = 4.5= 8.8

Turonian 0.03= 2 = 7.1

Santonian-Cenomanian 5.4 = 9.9= 13.3

Cenomanian 2 = 15.9= 28.1

Late Albian 4.8 = 21.4= 36.4

Formation= Member

Colon=Mito Juan (total) 83.3 =118.3=170.3Lower Colon (only) 4 = 6.7= 10

Socuy 2.4 = 3.1= 4.9

Tres Esquinas 0.5 = 1 = 1.5

La Luna*=Navay 0.6 = 7.2= 15.2

Guayacan 24 = 34 = 54

Seboruco 13.1

Maraca=Penas Altas 6.8 = 17.4= 36.4

Aguardiente 22.7?

A more certain sedimentation rate for the Aguardiente Formation

could not be derived from the data in this study due to the lack

of a complete measured section; however, the data of Helenes

et al. (1998) suggest that the rate presented here is a reasonable

estimate. A detailed set of sedimentation rates for important

outcrops and wells is shown in Appendix B.

tander Massif), and the central Merida Andes to ElBaul area (Fig. 9). The expression of these featuresas positive palaeotopographic or palaeobathymet-ric highs is reflected in the thinning of Cretaceousrocks over each of these areas, or their depositionunconformably on basement (Renz, 1959, 1982; Ro-

drıguez, 1989; Lugo, 1994; Lugo and Mann, 1995;

Helenes et al., 1998).In the northern and central Maracaibo Basin, inner

to middle shelf carbonates of the Lisure and Maracaformations graded southwards into inner shelf lime-stones and shales of the Capacho Formation, andinner shelf sandstones and shales of the Aguardi-ente Formation (Figs. 4 and 9). This facies change

is also reflected in sedimentation rates (Table 2);shallow-water carbonates of the Maraca and PenasAltas formations were deposited at an average rate of

17.4 mm=1000 yr, while late Albian portions of theSeboruco Member (Capacho Formation) and Aguar-diente Formation were deposited at average rates of 4.8 mm=1000 yr and 22.7 mm=1000 yr, respectively

(Rıo Guaruries and Rıo Sto. Domingo, Appendix B).Sedimentation rates for the shallow carbonates aregreater than or equal to growth rates of modernreefs and carbonate platforms (Schlager, 1981), andsimilar to rates documented for the same rocks ineastern Colombia (24 mm=1000 yr; Martınez andHernandez, 1992).

In the Barinas=Apure Basin, the Aguardiente For-mation contains more massive sandstones, whichwere deposited in a deltaic to nearshore marine depo-sitional environment (Fig. 9); fluvial sandstones are

dominant in the subsurface of southeastern Barinas=Apure (Notestein et al., 1944; Rod and Maync, 1954;Renz, 1959, 1982; Gaenslen, 1962; Garcia Jarpa etal., 1980; Chigne, 1985; Kiser, 1989; Galea-Alvarezet al., 1994; Parnaud et al., 1995). Helenes et al.(1998) reported sedimentation rates of up to 15.2mm=1000 yr for mixed siliciclastic and carbonatesrocks in the west central Barinas Basin. Palaeotopo-graphic highs in the central Merida Andes and ElBaul area acted as a sediment source for feldspathicgravels that occur within the Penas Altas Formation.

4.4. Cenomanian

Sedimentation patterns changed rapidly near theend of the Albian. Drowning of the Maraca=LisureFormation carbonate platform occurred as sea leveltransgressed the shallow marine Capacho=Aguardi-ente Formation shelf (Bartok et al., 1981; Macellariand De Vries, 1987; Macellari, 1988; Martınez andHernandez, 1992; Parnaud et al., 1995; Helenes et al.,1998). Flooding of the platform and the end of shal-low-water carbonate sedimentation marked the first

in a series of four marine transgressions that affectedwestern Venezuela during the Late Cretaceous. Thefirst transgression (late Albian to early Cenomanian)may be due in part to a change in subsidence ratesin western Venezuela (Lugo and Mann, 1995). Cir-cumstantial support for an increase in the rate of sub-sidence is reflected in an increase in the sedimenta-tion rate of Seboruco Member (Capacho Formation)shales, from 4.8 mm=1000 yr in the late Albian to22.3 mm=1000 yr during the Cenomanian (derived




Fig. 11. Palaeogeographic map of the Turonian. For legend see Fig. 10.

from data in Appendix B). This compares favorablywith a maximum sedimentation rate of 33.9 mm=1000yr derived from the data of Helenes et al. (1998).

An unconformity (in some places, the Type 3sequence boundary of Schlager, 1999) between theMaraca and overlying La Luna formations is easily

recognized biostratigraphically in the northwestern

and central parts of the Maracaibo Basin (Canacheet al., 1994; Truskowski et al., 1995). There, upperCenomanian rocks immediately overlie upper Albianrocks (Fig. 4). The precise cause of the unconformityis unclear; while drowning appears to be a signif-icant factor over most of Maracaibo, the frequentoccurrence of palaeokarst at the top of the upper

Maraca Formation in western Maracaibo indicatessome period of subaerial exposure (Truskowski etal., 1995). In northwestern and central Maracaibo,

the drowning unconformity is expressed as a subma-rine hardground (Fig. 17). However, a major hiatusbetween Penas Altas Formation shallow-water lime-stones and La Luna Formation shales and marls inthe rest of northern and northeastern Maracaibo isnot indicated from biostratigraphic data (e.g., Tri-bovillard et al., 1991). Nevertheless, the abrupt shift

from shallow-water to ‘deeper’-water deposits overa lithostratigraphic thickness of 1 m suggests that ahiatus may be present.

In addition to containing a combination of skele-tal constituents from the Maraca and La Luna forma-tions (Fig. 17), ‘transitional’ facies also have lowerTOC values than ‘typical’ (microlaminated plank-tic foraminiferal ooze) La Luna Formation organiccarbon-rich rocks (Table 3). In this context, it isimportant to note that evidence of bioturbation is




Fig. 12. Palaeogeographic map of the Coniacian–Santonian. For legend see Fig. 10.

Table 3

Average corrected TOC content (see Erlich et al., 1999b) of selected Upper Cretaceous formations

Unit Avg. TOCcorr (wt.%) No. of analyses

All La Luna=Navay a 2.26 159

La Luna=Navay (only b) 2.59 122

La Luna transitional facies c 1.43 11

Socuy 1.2 2

La Luna=Navay (Campanian only d) 0.94 36

All Tres Esquinas 0.92 34Tres Esquinas >5% phosphate 0.87 11

Age equivalent to Tres Esquinas e 1.79 85

a Includes all La Luna=Navay Formation samples (analyses include all Tres Esquinas, Tachira Chert, and Socuy Member samples; no

other formations).b Includes only La Luna and Navay Formation samples (a subset of all samples, without Tres Esquinas, Tachira Chert, and Socuy

members).c Includes only ‘transition zone’ samples 94-2-8, 8A through 11, 31-1-16 through 19, LL-1, and 95-1-2.d Includes only Campanian La Luna=Navay Formation samples (without Tres Esquinas, Tachira Chert, and Socuy members).e Includes only analyses of Santonian to early Maastrichtian formations equivalent to Tres Esquinas Member (includes all La Luna=

Navay, Colon, Burguita Formation, and Tachira Chert and Socuy Member samples; excludes Tres Esquinas Member samples).




Fig. 13. Palaeogeographic map of the early to latest Campanian. For legend see Fig. 10.

common throughout the lower La Luna Formation.Organic carbon-rich La Luna Formation rocks im-mediately overlie ‘transitional’ facies in the northernMaracaibo Basin, and have two primary facies de-pending on the amount of shale present (less thanor more than 50% shale). In the central part of the

basin, the La Luna Formation contains generally

more than 50% shale, while in the rest of the basin,the La Luna Formation is generally more than 50%limestone (Fig. 10).

The reason for the localized dominance of shaleover limestone is unclear, but could be related to thelack of preservation of limestone below an OMZ,or a local source (Santa Marta Massif or Paraguana

Block?) of shaly turbidites. It is difficult to provewhich case may be more likely, since the overallthickness of ‘shale-rich’ versus ‘limestone-rich’ sec-

tions is similar (the presence of an active OMZ mighttend to create thin shaly units and thicker carbonateunits). Nevertheless, lower upper Cenomanian rocksin the northwestern Maracaibo Basin still containan admixture of in-situ fecal pellets and Inocera-

mus spp. fragments, indicating at least the proximityof oxygenated bottom waters. However, this facies

quickly grades upward into microlaminated plankticforaminiferal oozes typical of the La Luna Formationthroughout western Venezuela.

A late Albian to early or early-middle Cenoma-nian unconformity is also present in the subsurfaceof the Barinas=Apure Basin (Helenes et al., 1998),where basal Escandalosa Formation transgressivemarine shales overlie mostly non-marine Aguardi-ente Formation sandstones (Fig. 4). Deltaic and in-ner shelf Aguardiente sandstones were reworked into




Fig. 14. Palaeogeographic map, end Campanian to early Maastrichtian. For legend see Fig. 10.

Escandalosa marine shelf sandstones and CapachoFormation shales and siltstones in the southern partof the Barinas=Apure Basin (Figs. 4 and 10). At theRıo Guaruries outcrop (Figs. 3 and 5), this trans-gressive marine facies is represented by 115 m of upper Albian to upper Cenomanian dark-gray and

siliceous shales of the Seboruco Member. These are

in turn overlain by 36 m of massive, dense fossilifer-ous middle shelf limestones of the late CenomanianGuayacan Member.

Shallow-water carbonate sedimentation shiftedfrom the northwestern and central Maracaibo Basinsouthward to the Merida Andes during the lateCenomanian to early Turonian (Figs. 10 and 11).

Shallow-water limestones and siliciclastics were alsodeposited in the northern part of the Barinas=ApureBasin (Fig. 6). Gravel-bearing calcareous sandstones

and sandy limestones containing small bivalve banksare interpreted as storm deposits. Skolithos trace fos-sils are common in the siliciclastic rocks, which fre-quently overlie basement or late Albian non-marinestrata.

Carbonate platform sediments of the upper Guay-acan Member (Capacho Formation) closely resem-

ble upper Maraca Formation platform sediments,and suggest a similar termination history. GuayacanMember rocks contain an admixture of large mi-crobored mollusc fragments, planktic foraminifera,and crustacean pellets (Ford and Houbolt, 1963).Guayacan limestones were deposited at an averagerate of 34 mm=1000 yr (and up to 54 mm=1000 yr;Table 2) and suggest another rapid increase in therate of subsidence in the Maracaibo Basin (Lugo andMann, 1995). La Luna Formation microlaminated




Fig. 15. Palaeogeographic map of the late Maastrichtian. For legend see Fig. 10.

and siliceous limestones were deposited in the south-western Merida Andes (Paso Andino outcrop, Figs. 3and 4) and later in the central and northern MeridaAndes following drowning of the Guayacan Memberplatform. By the end of the Cenomanian, the last re-maining basement highs of the central Merida Andes

were rapidly onlapped by Capacho Formation shales

and limestones before being completely buried.

4.5. Turonian

The end of the Cenomanian and onset of the Tur-onian signaled another major change in depositionalpatterns in western Venezuela. The second marine

transgression to affect the region (Villamil, 1998)enhanced the establishment of anoxic bottom-waterconditions in the Maracaibo and western Barinas=

Apure basins, and resulted in the preservation of large amounts of organic matter in the La Luna andNavay formations. Microlaminated shales and lime-stones were deposited throughout Maracaibo, whileinner and middle shelf Navay Formation shales andEscandalosa Formation nearshore sandstone facieswere deposited to the south and east (De Monroy

and Van Erve, 1988; Fig. 11). However, the Turoniansection is generally thin in the eastern and southeast-ern parts of the Maracaibo Basin, ranging from 0.5 mat Rıo Sto. Domingo in the north (Fig. 18B) to 6 m atPaso Andino in the south (Figs. 3 and 4). Based on itslimited thickness and exclusively pelagic biofacies,the Turonian section in this part of the MaracaiboBasin is interpreted as a condensed section (similarto the interpretation of Villamil and Arango, 1998,for the Turonian of Colombia). This interpretation is




Fig. 16. Tectonic reconstruction of the major tectonic features shown in Fig. 2 (see text for a detailed description of the displacements

used in the reconstruction). Syndepositional bathymetric or topographic highs are shown in stippled pattern.

Fig. 17. (A) Contact between the Maraca ( M ) and La Luna ( LL) formations, La Villa del Rosario quarry. DS D drowning sequence. Scale

D 5 m. Arrow marks the location of (B). (B) Contact between the Maraca and La Luna, upper transition zone (submarine hardground),

and samples CG-1–CG-3. Hammer is 25 cm. (C) Thin section photomicrograph of upper transition zone sequence, upper Albian, sample

CG-1. MO D bivalve or ammonite fragment. All photographs are in plane polarized light unless noted otherwise. Scale is 250 µm in

all photomicrographs. (D) Thin section photomicrograph of upper transition zone sequence, upper Albian, sample CG-2. PF D planktic

foraminifera; F D phosphatic fish bone. (E) Thin section photomicrograph of upper transition zone sequence, upper Albian, sample CG-3.




Fig. 18. (A) Mega-fossiliferous Guayacan Member limestone, upper Cenomanian, Rıo Sto. Domingo. Hammer is 25 cm. (B)

Cenomanian=Turonian boundary in the Rıo Sto. Domingo. N D Navay Formation; GY D Guayacan Member; T D Turonian. Scale is 2

m. (C) Arrow marks the contact between the Coniacian=Santonian limestone section (C =S) and mixed carbonate–siliciclastic Campanian

section (C ), Rıo Sto. Domingo. Scale is 1 m. (D) Arrow marks the contact between massive Maastrichtian Burguita Formation sandstones

( B) and the Campanian section (C ), Rıo Sto. Domingo. MT D Maastrichtian. Scale is 3 m.

supported by sedimentation rates for Turonian rocksthat average 2 mm=1000 yr (as low as 0.03 mm=1000yr; Table 2), an order of magnitude smaller than dur-ing the Cenomanian. Good agreement also existswith maximum rates (6.4 mm=1000 yr) derived from

the data of Helenes et al. (1998).The lateral facies transition between the La Luna

Formation limestones of the Maracaibo Basin and

the Navay Formation organic carbon-rich shales of the Barinas=Apure Basin occurs along the orien-tation of the present-day Merida Andes (Fig. 4).Navay shales and limestones were deposited in a nar-row band within a similarly restricted, but shallower

shelf setting than La Luna sediments in Maracaibo(Fig. 11). Siliceous La Luna Formation limestonesand phosphatic limestones and cherts occur from the




central Merida Andes southwestward into Colom-bia (see Las Delicias, Paso Andino, Las Vegas, RıoGuaruries, and Rıo Chama outcrop sections, Figs. 3–5, and Villamil, 1998). The coincidence of these fa-

cies and palaeobathymetric changes may be relatedto incipient deformation of the Eastern Cordilleraof Colombia (Martınez, 1989; Cooper et al., 1995;Parnaud et al., 1995; Villamil, 1998). However, theprecise timing and impact of this event in westernVenezuela is still uncertain.

The eastward and southeastward extent of organiccarbon-rich facies was dependent upon the palaeo-bathymetric location of wave base on the shelf, andapparently did not continue to the palaeo-shoreface(Vergara, 1997a,b; also see discussion in Tribovil-

lard et al., 1991). The Merida Andes, Santander, andSanta Marta palaeobathymetric highs caused poorcirculation and enhanced the conditions necessaryfor the deposition of organic carbon-rich facies, andthe subsequent development of anoxia. A more com-plete discussion of the origin of anoxia in westernVenezuela can be found in Erlich et al. (1999a).

4.6. Coniacian–Santonian

The third marine transgression to affect the Mara-caibo and Barinas=Apure basins occurred within the

Coniacian to early Santonian (Parnaud et al., 1995;Helenes et al., 1998). Initially low sedimentationrates in the Coniacian (average 4.5 mm=1000 yr;Table 2) dramatically increased in the Santonian (av-erage 16.6 mm=1000 yr), in part due to increasedregional subsidence (Lugo and Mann, 1995). How-ever, reactivation of Jurassic(?) normal faults mayhave had a significant impact on local sedimentationrates (see data in Appendix B).

The Coniacian–Santonian transgression resultedin another shift in the position of the palaeoshoreface

into extreme southeastern Barinas=Apure (Fig. 12).Minor ‘Guayacan-type’ carbonate buildups nucle-ated near buried palaeotopographic highs locatedaround the Maporal and Paez oil fields in centralBarinas=Apure (Fig. 3; Table 1). These represent thelast vestiges of shallow-water carbonate sedimenta-tion in the Late Cretaceous of the Barinas=ApureBasin (Kiser, 1989; Aquino et al., 1995).

Navay Formation shales and limestones were de-posited in inner to middle shelf environments (Figs. 8

and 12). The ‘ancestral’ Merida Andes continued toact as a bathymetric barrier, which helped to main-tain poor bottom-water circulation and the preser-vation of organic matter in areas east of the uplift.

In the Maracaibo Basin, microlaminated organic car-bon-rich La Luna Formation limestones and shaleswere deposited in proximal slope depositional en-vironments in the north to outer shelf depositionalenvironments in the south (Figs. 8 and 12). In thenortheast, La Luna Formation limestones and shaleschanged facies to Timbetes Member marls, shales,and limestones.

The inner to middle shelf palaeobathymetric highcreated by uplift of the ‘ancestral’ Merida Andesbecame increasingly important as a focus for the

deposition of siliceous and phosphatic sediments(Tachira Chert and Tres Esquinas members, early-late Santonian; see Erlich et al., 1999a). The rapidinflux of quartz silt (confirmed through thin-sectionpetrography; Fig. 19) at the end of the Santoniansignaled a change in climatic and oceanographicconditions in the region (Erlich et al., 1999b). Thischange was manifested, in part, by the early advanceof the Campanian to Maastrichtian Colon=Burguita=Guaduas Formation deltas (also see Villamil, 1998).

4.7. Early–latest Campanian

A profound change in sedimentation patterns inwestern Venezuela occurred in the early Campa-nian, exemplified by the return of intermittently oxy-genated bottom waters to the entire Maracaibo Basin.In northwestern Maracaibo and northeastern Colom-bia, deposition of Socuy Member (La Luna For-mation) limestones apparently occurred under oxy-genated conditions within a hemipelagic setting sim-ilar to the upper La Luna Formation (as interpretedfrom planktic foraminiferal and trace fossil assem-

blages of the Zoophycos ichnocoenose; Macsotay andVivas, 1993). The Socuy Member is highly biotur-bated and has a TOC content lower than that of aver-age La Luna and Navay Formation rocks (Table 3).

In the southwestern Maracaibo Basin, La LunaFormation microlaminated limestones and shaleswere deposited in an outer shelf environment(Fig. 13; De Romero and Galea-Alvarez, 1995). LaLuna Formation and Socuy Member outer shelf lime-stones grade northeastward into outer shelf siliceous




Fig. 19. (A) Thin section photomicrograph of coarse-grained phosphatic Tres Esquinas Member, sample 94-2-32, upper Santonian,

Las Delicias. (B) Thin section photomicrograph of quartz-rich Navay Formation phosphatic bed, sample 95-10-15, upper Campanian,

Rıo Sto. Domingo. (C) Thin section photomicrograph of shaly, phosphatic, quartz-rich Tres Esquinas Member, sample 94-2-65, lower

Maastrichtian, Las Vegas. (D) Thin section photomicrograph of glauconitic, phosphatic, quartz-rich Tres Esquinas Member, sample

31-1-30, lower Maastrichtian, Rıo Guaruries. Note rare occurrence of oolitically coated phosphatic grains ( O).

shales of the lower Colon Formation. In northeast-ern Maracaibo, Colon Formation shales represent thedistal parts of deltas that prograded north from theBarinas=Apure Basin.

Expansion of inner to middle shelf environments

towards the southwest were associated with the ‘an-cestral’ Merida Andes palaeobathymetric high (be-tween the Maracaibo and Barinas=Apure basins).

This expansion was due to uplift in Colombia or rel-ative sea level low-stand (Haq et al., 1987; Martınez,1989; Cooper et al., 1995; Parnaud et al., 1995; Vil-lamil, 1998). This area continued to be the focusof deposition of condensed siliceous and phosphatic

Tres Esquinas Member and upper Navay Formationsediments. Sedimentation rates during the Campa-nian averaged 2.7 mm=1000 yr (Table 2); however,




sedimentation rates for sites along the central andsouthern ‘ancestral’ Merida Andes averaged only 0.3mm=1000 yr (calculated from data in Appendix B).This value is very similar to the sedimentation rate

for Tres Esquinas rocks in Colombia (less than 1mm=1000 yr) calculated by Martınez and Hernan-dez (1992), and suggests that inner to middle shelf environments were subjected to frequent sedimenterosion and re-distribution.

Deposition of Navay Formation sediments overthe remainder of the Barinas=Apure Basin was alsorestricted to inner to middle shelf siliciclastics (DeMonroy and Van Erve, 1988; Figs. 4 and 13). How-ever, rapid deposition and soft sediment deformationof upper Navay rocks is indicated in some areas by

the presence of numerous sandstone dikes (Vivas etal., 1988), and locally higher sediment sedimentationrates (up to 11.1 mm=1000 yr at Rıo Sto. Domingo,Appendix B, and 5 mm=1000 yr 65 km east of RıoSto. Domingo, derived from subsurface data of He-lenes et al., 1998). Variations in sedimentation ratesmay reflect differential normal fault motion alongthe margins of the ‘ancestral’ Merida Andes. Finally,in the northeastern Merida Andes (Arbol Redondoarea; Fig. 3; Table 1), Timbetes Member shales andlimestones were deposited through the end of theCampanian.

4.8. End Campanian–early Maastrichtian

The fourth marine transgression to affect west-ern Venezuela during the Late Cretaceous beganduring the Campanian, and reached its maximumexpression at the end of the Campanian and earliestMaastrichtian. The start of the transgression in thispart of northern South America occurred just priorto the eustatic transgression shown by Haq et al.(1987). This difference was due, in part, to uplift of

the Central Cordillera of Colombia and subsequentsubsidence of the Maracaibo foreland basin (Macel-lari, 1988; Martınez, 1989; Martınez and Hernandez,1992; Vergara, 1994; Cooper et al., 1995; Parnaud etal., 1995; Villamil, 1998).

Although earliest Maastrichtian subsidence rateswere low (Lugo and Mann, 1995), subsidence ratesincreased rapidly during the late Maastrichtian. Forexample, the sedimentation rate of early Maas-trichtian Colon Formation shales was lower (average

5.8 mm=1000 yr; Table 2) than that of the com-plete Maastrichtian Colon=Mito Juan section (av-erage 118.3 mm=1000 yr in western Venezuela;50 mm=1000 yr for the Colon Formation in east-

ern Colombia; 52.3 mm=1000 yr for the La TablaFormation in central Colombia; Martınez and Her-nandez, 1992; Vergara, 1994). Uplift of the CentralCordillera of Colombia also coincided with initialprogradation of prodeltaic Colon Formation shales.The northeast–southwest deepening of the new fore-land basin caused the deposition of outer shelf toslope Colon Formation shales from Acarigua to thesouthwestern Merida Andes (Fig. 14).

Deposition of Socuy Member limestones (LaLuna Formation) continued in the northwestern

Maracaibo Basin (at the relatively low average sed-imentation rate of 3.1 mm=1000 yr; Table 2), whileprogradation of Colon Formation shales limited car-bonate sedimentation to a narrow zone near north-eastern Colombia. Socuy Member carbonate sedi-mentation finally ceased in the early Maastrichtian,and was buried by prodelta and shelf shales of theColon=Molino Formation delta system (Martınez,1989; Canache et al., 1994; Parnaud et al., 1995;Guerrero and Sarmiento, 1996).

A narrow band of outer shelf glauconitic siltstonesand shales extended south from the central-western

part of the Maracaibo Basin to the southwesternMerida Andes and into Colombia (Fig. 13). Middleand outer shelf Tres Esquinas Member phosphaticand siliceous sediments alternated with dark gray,siliceous outer shelf to slope Colon Formation shalesat the southwestern margin of the ‘ancestral’ MeridaAndes palaeobathymetric high (Figs. 14 and 20;De Romero and Galea-Alvarez, 1995). Mito JuanFormation deltaic sediments subsequently buried thisfeature in the late part of the early Maastrichtian.

Local delta progradation along the northern

Merida Andes caused the influx of fine- to medium-grained arkosic sandstones onto the Colon Forma-tion outer shelf. The Cujisal Member (Renz, 1959)sands are interpreted as having been deposited in anorth-northwest-oriented submarine canyon (Agua-suelos Engineers, 1991), which later became a locusfor the initiation of submarine slumps, slides, andsandstone dikes (Fig. 14). Outer shelf Colon shalespassed to the southeast into Navay Formation mid-dle shelf shales and siltstones. Further southeast,




Fig. 20. (A) La Luna to Colon Formation transition, Las Delicias. C D Colon Formation; LL D La Luna Formation. Scale is 2 m. (B)

Tres Esquinas Member phosphatic beds, Las Delicias. Arrows show the locations of the three units. MT D Maastrichtian; TE D Tres

Esquinas Member; C D Campanian. Scale is 1 m.

Navay Formation sediments changed facies laterallyto shallow-water deposits derived from the incipientBurguita Formation delta system (Fig. 14). Evidenceof the proximity of the Burguita Formation delta sys-

tem can be seen in increasing amounts of eolian andturbidite-deposited quartz silt in upper Tres Esquinasand Navay rocks (Fig. 19).

The last remnants of Tres Esquinas Member phos-phatic sediments were reworked and re-depositedalong a narrow band from the area of Rıo Sto.

Domingo and south of the Burgua-3 well (Fig. 3;Table 1). Only rare occurrences of relict phosphatic

and glauconitic sediments are known from post-lower Maastrichtian rocks in the subsurface of thesouthern Barinas=Apure Basin (Bejarano and Zor-rilla, 1994), Las Delicias, and the Rıo Guaruries area(Fig. 19; De Romero and Galea-Alvarez, 1995).

4.9. Late Maastrichtian

Late Maastrichtian depositional systems reflect

the influence of continued structural deformation ineastern Colombia coupled with changing palaeo-climate near the end of the Cretaceous. Devel-opment of the northeast–southwest-oriented Mara-caibo foreland basin continued through the end of the Maastrichtian and into the early Palaeogene(Martınez, 1989; Lorente and Duran, 1995; Guer-

rero and Sarmiento, 1996; Villamil, 1998). Thebasin was progressively infilled by deltaic sedimentsfrom eastern Colombia and the southern Barinas=

Apure Basin (Fig. 15). Submarine palaeobathymet-ric highs in eastern and southeastern Maracaibo (asdefined by outcrop and proprietary seismic data)probably developed during differential foreland sub-sidence. These may be related to ongoing basement-involved normal faulting in the eastern Maracaiboand Barinas=Apure basins (Lugo and Mann, 1995).

Mixed carbonate and siliciclastic sediments weredeposited in a zone extending from the central-west-ern Maracaibo Basin south into Colombia, and are

associated with episodes of delta abandonment andlocal marine transgression (Fig. 15). These facies areobserved exclusively from well data and are confinedto the subsurface of Maracaibo.

Guaduas=Mito Juan=Burguita Formation deltaicsediments prograded from eastern Colombia and theBarinas=Apure Basin towards the north-northeast,and passed from inner and middle shelf to outer shelf and slope depositional environments (Fig. 15). Initialprogradation was punctuated by shelf turbidites andreworking of previously deposited units.

Mito Juan Formation facies are not recognized inthe eastern parts of the Maracaibo Basin; in theseareas, Colon Formation shale facies were depositedthrough the late Maastrichtian, and directly overlieLa Luna Formation limestones and shales (Fig. 15).The two formations are also not easily differentiatedin the southwestern Merida Andes (Fig. 4), where theentire siltstone=shale sequence has been given thecolloquial name ‘Colon=Mito Juan’ (Galea-Alvarezet al., 1994).




5. Conclusions

The late Albian to early Cenomanian drowningof the Maraca Formation shallow-water carbon-

ate platform marked the onset of deeper water,hemipelagic sedimentation in northwestern SouthAmerica. As a result, shallow-water carbonate sed-imentation shifted to the southeastern MaracaiboBasin during deposition of the Guayacan Memberplatform. This platform was subsequently drownedduring the late Cenomanian to early Turonian.Temporary re-establishment of Guayacan Memberlimestone deposition in the eastern Maracaibo andcentral Barinas=Apure basins during the Turonianmarked the end of widespread shallow-water car-

bonate sedimentation in the Cretaceous of westernVenezuela.Progressive uplift of the Central Cordillera of

Colombia during the Turonian through Campaniancaused the development of the ‘ancestral’ MeridaAndes palaeobathymetric high. The rapid uplift of this feature, combined with the presence of the pos-itive Santa Marta and Santander massifs, coincidedwith the development of low-oxygen or anoxic con-ditions during deposition of the La Luna and Navayformations. Deposition of the Socuy and Tres Es-quinas members of the La Luna Formation reflects

a change in palaeoclimate and palaeoceanographyin western Venezuela. Although upper La LunaFormation and Socuy Member depositional envi-ronments were similar, an increase in bottom-wateroxygenation facilitated infaunal activity, which ulti-mately greatly reduced the preservation of organicmatter.

Acknowledgements

The authors would like to thank D. Pocknall, D.Loureiro, C. Yeilding, F. Yoris, C. Giraldo, M. Ostos,M. Steinbis, and J. Blickwede for their valuable as-sistance in the field from 1990 to 1996. D. Pocknalland J. Bergen contributed important palaeontologicalanalyses to this study, and C. Yeilding contributedphotographs and thin sections for analysis. We alsothank S. Palmer-Koleman, L. Pratt, Z. Sofer, and G.Young for contributing ideas on the development of low-oxygen conditions in the Cretaceous of western

Venezuela. Special thanks are due to D.B. Felio andH. Krause, who supported the idea for this work and the formation of this diverse study team. W.Schlager, J. Van Hinte, and reviewers K. Follmi and

P. De Deckker made significant improvements to themanuscript. R. Squyres and E. Kreger drafted thefigures for this report. Finally, we wish to thank PDV Exploration and Production and Amoco Explo-ration and Production for permission to publish thisreport.

Appendix A. References used in combination

with proprietary data to construct chrono-

lithostratigraphic cross-sections (Figs. 4–6).

Aquino et al. (1995) Lugo (1994)

Baptista and Scherer (1996) Lugo and Mann (1995)

Bartok et al. (1981) Macellari (1988)

Bejarano and Zorrilla (1994) Macellari and De Vries (1987)

Boesi et al. (1988) Martinez (1989)

Canache et al. (1994) Martinez and Hernandez (1992)

Cardenas (1977) Notestein et al. (1944)

Chigne (1985) Parnaud et al. (1995)

De Monroy and Van Erve (1988) Renz (1959; 1982)

Escamilla et al. (1994) Renz (1961)

Ford and Houbolt (1963) Rod and Maync (1954)

Gaenslen (1962) Rodrıguez (1989)

Galea-Alvarez et al. (1994) Sanchez and Lorente (1977)Gamarra et al. (1994) Schubert (1968)

Garcia Jarpa et al. (1980) Sellier De Civrieux (1952)

Ghosh (1984) Stainforth (1962)

Gonzalez De Juana et al. (1980) Sutton (1946)

Hedberg (1931) Tribovillard et al. (1990)

Hedberg and Sass (1937) Tribovillard et al. (1991)

Kiser (1961, 1989) Trump and Salvador (1964)

Krause and James Truskowski et al. (1995)

Kuyl et al. (1955) (1989) Truskowski et al. (1996)

Lorente and Duran (1995) Von Der Osten (1966)

Lorente et al. (1996) Zambrano et al. (1971)

Appendix B. Sedimentation rates for selected

outcrop localities and key wells shown in Fig. 3

Rates were derived by dividing the amount of measured sec-

tion for a specific lithostratigraphic unit into the total years (as

taken from the scale of (Gradstein et al., 1995) represented by

the unit. Rates were not corrected for compaction. The duration

of key unconformities were estimated for each lithostratigraphic

section, however, the sedimentation rates shown here and sum-

marized in Table 2 must be regarded as minimum values only.




Thickness Time Rate

(m) (m.y.) (mm=1000 yr)

Las Delicias (Locality 17)

AgeEarly Maastrichtian 15 2? 7.5

Campanian 3.6 12.2 0.3

Late Santonian 60 1.15 52.2

Early Santonian 75 1.15 65.2

Coniacian 20 3.2 6.3

Turonian 3 4.5 0.7

Late Cenomanian 3.4 0.5? 2.3

Formation

Colon Fm. 10 1 10

Tres Esquinas Fm. 7 13.2 0.5

La Luna* Fm. 160 10.5 15.2

Guayacan Fm. 1.5 0.063? 24

Paso Andino (Locality 46)

Age

Turonian (faulted) 5.8 4 1.5

Late Cenomanian 21 1.8? 11.7

Formation

La Luna Fm. 26.8 5.8 4.6

Las Vegas (Locality 47)

Age

Early Maastrichtian 9.5 2? 4.8

Campanian 1.5 12.2 0.1

Santonian 3 2.3 1.3

Coniacian 12 3.2 3.8Turonian 4 4.5 0.9

Formation

Colon Fm. 4 1 4


La Luna* Fm. 16 7.7 2.1

Chejende (Locality 37)

Formation

Colon (Maas.–Camp.) Fm. 180 18.5 9.7

La Luna (Sant.–Cen.) Fm. 205 15.4 13.3

Penas Altas (l. Alb.) Fm. 30 4.4? 6.8

Rio Guaruries (Locality 27)

Age

Early Maastrichtian 5 1? 5

Campanian 5 12.2 0.4


Early Santonian 9 1.15 7.8


Turonian 5 4.5 1.1

Late Cenomanian 90 1.8 50

Middle Cenomanian 21 1.8 11.7

Early Cenomanian 41 1.8 22.8

Late Albian 21 4.4? 4.8

Santonian–Ceno. 185 15.4 12

Thickness Time Rate

(m) (m.y.) (mm=1000 yr)

Rio Guaruries (Locality 27) (continued)

FormationTres Esquinas Fm. 20 13.3 1.5

La Luna* Fm. 27 8.7 3.1

Guayacan Fm. 54 1? 54

Seboruco Fm. 115 8.8 13.1

Rio Chama (Locality 28)

Age

Early Maastrichtian 12 2? 6


Late Santonian 7 1.15 6

Early Santonian 7 1.15 6

Coniacian 1 3.2 0.3

Turonian 4 4.5 0.9

Formation

Colon Fm. 12 2 6


La Luna* Fm. 5 7.7 0.6

Quebrada La Luna=Perija Front (Localities 4–7)

Formation

Colon=Mito Juan Fm. 800 (est.) 6.3 127

Socuy (Campanian) Fm. 60 (avg.) 12.2 4.9

La Luna*(Sant.–Cen.) Fm. 160 (avg.) 15.4 10.4

Maraca (l. Albian) Fm. 50 1.8? 28

Rio Sto. Domingo (Locality 41)

AgeMaastrichtian 17 3? 5.7

Campanian 136 12.2 11.1


Early Santonian 11.5 1.15 10


Turonian 0.15 4.5 0.03

Cenomanian 130 5.4 24.1

Late Albian 10 4.4? 22.7?

Santonian-Ceno. 179.7 15.4 11.7

Formation

Burguita Fm. 17 3? 5.7

Navay Fm. 186 22.2 8.4

Guayacan Fm. 87 (ls)= 1.6= 54.4 (ls)=43 (ss-sh) 3.8 11.3 (ss-sh)

Aguardiente Fm. 10 4.4? 22.7?

Well 22 (50 km N of Locality 8)

Age

Maastrichtian 577 6.3 92.6


Con.–Sant. 44 5.5 8

Turonian 20 4.5 4.4



Santonian–Ceno. 90 15.4 5.8




Thickness Time Rate

(m) (m.y.) (mm=1000 yr)

Well 22 (50 km N of Locality 8) (continued)

FormationLa Luna* Fm. 90 15.4 5.8

Well 54 (Locality 9)

Age



Con.–Sant. 49 5.5 8.9

Turonian 6 4.5 1.3




Formation

La Luna* Fm. 164 15.4 10.6Well 55 (100 km NE of Locality 8)

Age



Con.–Sant. 40 (est.) 5.5 7.3

Turonian 32 4.5 7.1

Cenomanian 11 5.4 2



Formation

La Luna* Fm. 83 15.4 5.4

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