Birth and death of the Late Cretaceous “La Luna Sea”, and origin of the Tres Esquinas phosphorites

Journal of South American Earth Sciences 13 (2000) 21±45

Birth and death of the Late Cretaceous ``La Luna Sea'', and origin of the Tres Esquinas phosphorites R.N. Erlich a,*, O. Macsotay I. b, A.J. Nederbragt c, 1, M. Antonieta Lorente d a BP Amoco Corporation, PO Box 3092, Houston, TX 77253, USA UrbanizacioÂn El Trygal Norte, Avenida Atlantico 155-61B, Valencia, Edo. Carabobo, Venezuela c Vrije Universiteit, Faculteit der Aardwetenschappen, Amsterdam, The Netherlands d PDV ExploracioÂn y ProduccioÂn, Apartado 829, Caracas 1010A, Venezuela

b

Received 30 April 1999; accepted 30 November 1999

Abstract Deposition of organic carbon-rich intervals of the La Luna and Navay formations of northwestern Venezuela was governed by the development of key paleobathymetric barriers (Santa Marta and Santander massifs, Paraguana Block, and ancestral MeÂrida Andes). These enhanced the development of anoxia in the ``La Luna Sea'' by causing poor circulation and limited ventilation. Anoxia was also promoted by high evaporation and low precipitation rates (high salinity bottom water), and high levels of marine algal productivity (high organic matter ¯ux). Nutrient supply was augmented by infrequent ¯uvial sources. Bottom water oxygen levels increased from the Late Santonian through the end of the Cretaceous. Ventilation of anoxic bottom waters may have been enhanced by more frequent or intense seasonal upwelling (caused by higher wind stress) and catastrophic overturn, as well as the removal of a key paleobathymetric barrier. Common byproducts of overturn events were massive phytoplankton blooms, which produced red tides. Fish and marine reptile bone beds within the Tres Esquinas Member (La Luna Formation) are attributed to massive mortality during these events, and are correlative with similar Campanian units in eastern Colombia. During the Maastrichtian, increasing ventilation, combined with siliciclastic dilution, ultimately produced sediments with lower total organic carbon (TOC) content. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Upper Cretaceous; La Luna Formation; Paleo-oceanography; Phosphate rocks; Organic carbon; Maracaibo Basin

Resumen La deposicioÂn de intervalos ricos de carboÂn orgaÂnico de las Formaciones La Luna y Navay de Venezuela noroccidental fue controlado por el desarollo de barreras paleobatimeÂtricas importantes (los macizos Santa Marta y Santander, el Bloque Paraguana, y los Andes de MeÂrida antõ guo). Estos mejoraron el desarollo de anoxia en el ``Mar La Luna'' causando pobre circulacioÂn y limitada ventilacioÂn. Anoxia fue atribuida tambieÂn a la alta evaporacioÂn y al baja medidas de precipitacioÂn (produciendo aguas profundas de alta salinidad), y altos niveles de productividad de algas marinas (alto ``¯ux'' de materia orgaÂnica). El suministro de nutrimentos fue aumentado por fuentes ¯uviales infrequentes. El nivel de oxõ geno de los aguas profundas aumento desde el Santoniense Tardõ o al ®nal del CretaÂceo. VentilacioÂn de los aguas profundas anoÂxõ cas podria haber sido aumentada por ``upwelling'' maÂs frequente y temporal (causado por mas ``stress de vientos'') y vuelco catastro®co, asõ como el traslado de una barrera paleobatimeÂtrica importante. Subproductos comunes de los eventos de volcar fueron pelusas masivas de ®toplankton, los cuales producieron ``Mareas Rojas''. Capas de huesos de pescados y reptiles marinos dentro del Miembro Tres Esquinas (FormacioÂn La Luna) son atributidos a mortalidad masiva durante estos

* Corresponding author. Present address: Burlington Resources International, 400 North Sam Houston Parkway East, Suite 1200, Houston, TX 77060. Tel: +1-281-878-3875; fax: 1-281-878-3830. E-mail address: [email protected] (R.N. Erlich). 1 Present address: Department of Geological Sciences, University College London, Gower Street, London, WC1E 6BT, UK 0895-9811/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 5 - 9 8 1 1 ( 0 0 ) 0 0 0 1 6 - X

22

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

eventos, y son correlativos con unidades Campaniense similares encontradas en Colombia oriental. Durante el Maastrichtiense, el aumento de ventilacioÂn, combinada con dilucioÂn siliciclaÂstica, ®nalmente produjo sedimentos con contenido de carboÂn orgaÂnico total (COT) maÂs bajo. 7 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction Cretaceous organic carbon-rich sediments have been the focus of much study, due at least in part to the large hydrocarbon resources generated from them (Klemme and Ulmishek, 1991). Over 40 billion barrels of oil sourced from the La Luna and Navay formations have been produced in western Venezuela alone (see Talukdar and Marcano, 1994). Units such as the La Luna and Navay have also yielded important data on the paleoecology of organic carbon-rich rocks, as well as the factors that governed their deposition (Erlich et al., 1999a,b). However, it is their relationship to the formation of the vast hydrocarbon resources of western Venezuela and eastern Colombia that has made the study of these units so critical. In this paper, we identify the in¯uence of paleoceanography and paleoclimate on the deposition of the organic carbon-rich sediments that generated these resources. This study also focuses on the formation and regional signi®cance of phosphorites in western Venezuela and eastern Colombia, and their links to paleoceanographic and paleoclimatic variables. 1.1. Study area and summary of the structural evolution of western Venezuela The Maracaibo and Barinas/Apure basins of northwestern Venezuela are separated by the northeast± southwest trending MeÂrida Andes Mountains (Fig. 1). The Maracaibo Basin is bordered on the north by the Gulf of Venezuela and on the west by the Sierra Perija Mountains. The Barinas/Apure Basin is bordered on the east by the Precambrian Guayana Shield, on the north by the Paleozoic El Baul arch, and in the south continues into Colombia as the Llanos Basin. The Mesozoic structural evolution of these areas began during the Late Jurassic with formation of large rift grabens (Machiques, Uribante, and Barquisimeto Troughs), oriented roughly parallel to and underlying the present day MeÂrida Andes and Sierra Perija (Fig. 2). The grabens were ®lled by lacustrine and ¯uvial red beds and volcanics and later by Neocomian alluvial fans and braided stream deposits (Feo-Codecido et al., 1984; Maze, 1984; Lugo and Mann, 1995). Lower Aptian to Cenomanian marine rocks accumulated along a passive continental margin (Zambrano et al., 1971; Gonzalez de Juana et al., 1980; Bartok et al.,

1981; Parnaud et al., 1995). Granitic and metamorphic basement blocks that bordered the Sierra Perija (Santa Marta and Santander massifs) and the MeÂrida Andes (Paraguana Block, and Arauca, MeÂrida, and El Baul arches) acted as paleotopographic and paleobathymetric features, with signi®cant thinning of Cretaceous sediments (Renz, 1959; 1982; Lugo, 1999; Lugo and Mann, 1995). Reactivation of Jurassic normal faults during the Cenomanian and Turonian produced di€erential subsidence in the central and southern Maracaibo Basin, with a relatively stable to slowly subsiding eastern margin (proto-MeÂrida Andes; Macellari, 1988; Cooper et al., 1995; Vergara, 1997a). The deposition of marine rocks continued through the end of the Cretaceous (Gonzalez De Juana et al. 1980; Parnaud et al. 1995). However, tectonic uplift in northern and eastern Colombia during the Campanian through Maastrichtian provided a sediment source for upper deltaic and non-marine siliciclastics, which eventually ®lled the Maracaibo and Barinas/Apure basins (Kellogg, 1984; Shagam et al., 1984; Macellari, 1988; Pindell and Barrett, 1990; Audemard, 1991; SaÂnchez NunÄez et al., 1994; Cooper et al., 1995; Parnaud et al., 1995; Villamil, 1998; 1999). Southward-directed compression and transpression between the Caribbean and South American plates caused initial uplift (inversion the Machiques Trough) of the Sierra Perija in the Middle Eocene, and subsequent development of the northeastern Maracaibo foreland basin (also see Villamil, 1999). Northward-directed compression and transpression between the Paci®c, Caribbean, and South American plates in the Late Miocene to Early Pliocene caused uplift and inversion of the northeastern Maracaibo foreland basin and MeÂrida Andes (Barquisimeto Trough), and segmentation of the Maracaibo and Barinas/Apure basins (Macellari, 1984; Giegengack, 1984; Stephan, 1985; James 1990; Audemard, 1991; SaÂnchez NunÄez et al., 1994; Colletta et al., 1997). 1.2. Previous work Early studies of the La Luna and Navay formations have emphasized aspects of their stratigraphy and sedimentology (Hedberg and Sass, 1937; Sutton, 1946; Rod and Maync, 1954; Renz, 1959; 1982; Renz, 1961; Stainforth, 1962; Gonzalez De Juana et al., 1980; Macellari, 1988; Lugo and Mann, 1995; Parnaud et al., 1995). Similar studies in the Barinas/Apure Basin

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

23

Fig. 1. Location map of western Venezuela showing all wells (circles) and outcrops (triangles) in the database (see Appendix), and important geological features. Numbers indicate outcrops and wells used to calculate sedimentation rates cited in the text and Figs. 7 and 8. Roman numerals I, II, III, IV indicate general locations of stratigraphic columns shown on Figs. 3 and 4.

(Rod and Maync, 1954; Renz, 1959; Kiser, 1961; 1989; Renz, 1961; Gaenslen, 1962; Chigne, 1985; Helenes et al., 1994, 1998), and in eastern Colombia (BuÈrgl, 1961; Ward et al., 1973; Macellari and De Vries, 1987; Martõ nez, 1989; Barrio and Coeld, 1992; FoÈllmi et al., 1992; Vergara, 1991a,b; Villamil, 1998) have established the regional framework of equivalent units over northwestern South America. Work by Ford and Houbolt (1963), Ghosh (1984), and Martõ nez and Hernandez (1992) dealt primarily with detailed facies relationships within the La Luna and Navay formations, and Macellari and De Vries (1987), Barrio and Coeld (1992), FoÈllmi et al. (1992), Villamil (1996; 1998), and Erlich (1999a, b) emphasized their paleoclimatic and paleoceanographic signi®cance.

Geochemical studies of the La Luna and Navay formations concentrated primarily on characterizing key organic components and biomarkers (Hedberg, 1931; Dufour, 1957; Bockmeulen et al., 1983; Blaser and White, 1984; Zumberge, 1984; Chigne, 1985; Vierma, 1985; Talukdar and De Toni, 1988; Escobar et al., 1989; Tribovillard and Stephan, 1989; Talukdar and Marcano, 1994; Yurewicz et al., 1998) The characterization of depositional environment using organic geochemical data has also been a focus of much study (Talukdar et al., 1986; Krause and James, 1989; Mendez, 1989; Tribovillard et al., 1991; EscandoÂn et al., 1993; Sweeney et al., 1995; Baptista and Scherer, 1996; Davis et al., 1999). However, the studies of Tribovillard et al. (1991), Mann and Stein (1997), Vergara (1997a,b), Villamil and Arango (1998), and Erlich et

24

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

al. (1999b) best demonstrate the link between geochemical data and regional paleoclimate. 1.3. Summary of ®eld methods, analyses, and data sources Outcrops examined in this study were sampled extensively for planktic foraminiferal biostratigraphy, nannofossils, palynology, organic and inorganic geochemistry, and thin section petrography. Samples were taken at intervals ranging from 1 to 5 m, or as needed in order to characterize rapid lithologic changes. Each outcrop was measured in one-meter increments with corresponding gamma log measurements (via handheld scintillometer); lithologic changes or evidence of condensed sections or unconformities were measured in 10 cm increments where appropriate. Outcrop lithologies were determined by bulk x-ray di€raction (XRD) and x-ray ¯uorescence (XRF) analyses. A detailed discussion of inorganic and organic

geochemical data can be found in Erlich et al. (1999b). Thin section descriptions of rock fabrics and constituent grains were made for 290 samples in order to check bulk mineralogical results and provide additional information on depositional environments. A total of 158 wells and 62 outcrops (Fig. 1 and Appendix) were used to construct the lithostratigraphic framework presented in this study. Well data were obtained from proprietary (Maraven S.A. and Amoco Exploration and Production ®les) and published sources, and consist of wireline logs, biostratigraphy (planktic and benthic foraminiferal, nannofossil, and palynological analyses), and lithology (as indicated by conventional and sidewall cores and cuttings) (Appendix). Correlations between wells and between wells and outcrops were done using standard sequence stratigraphic techniques, and were veri®ed by biostratigraphically-determined ages and interpretations of depositional environments (e.g., data of Fuenmayor, 1989). The reader is referred to Erlich et al. (1999a)

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

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

for greater detail on the depositional history of western Venezuelan Cretaceous rocks. 1.4. Calculation of sedimentation rates Biostratigraphic data from three key wells and 14 outcrop sections from this study (Fig. 1) and well documented published studies (Rod and Maync, 1954; Renz, 1959; Boesi et al., 1988; Tribovillard et al., 1991; Helenes et al., 1998) were used to establish average, minimum, and maximum sedimentation rates for the La Luna and Navay formations. Sedimentation rates (expressed in mm 1000 yrÿ1) were calculated by dividing the thickness of speci®c units by the amount of time represented (time scale from Gradstein et al., 1995); unit thicknesses were not corrected for compaction or erosion and therefore represent minimum estimates. In addition, we recognize that the amount of time represented for incomplete lithostratigraphic sections may not have been determined correctly. Nevertheless, the sedimentation rates determined in this study represent reasonable estimates of sedimentation during deposition of the La Luna and Navay. 2. Paleoecology of the La Luna and Navay formations 2.1. Benthic foraminifera Paleoecological interpretations in this study are based on quantitative analysis of foraminiferal faunas in thin section, as well as benthic foraminiferal and macrofossil (primarily mollusc and gastropod) assemblages. The number of foraminifera per unit surface area was estimated, as well as ratios (expressed as percentages) between benthic and planktic foraminifera; keeled and globular planktic foraminifera; and between spiral and biserial planktic foraminifera. A more detailed quantitative analysis of planktic foraminiferal faunal composition was not feasible, as too few specimens could be positively identi®ed. Therefore, species and genus distribution patterns were compiled from qualitative presence/absence data. A more complete discussion of the Late Cretaceous biostratigraphy of western Venezuela can be found in Erlich et al. (1999a). Analysis of benthic foraminiferal faunas in thin section was limited to recognition of the morphotypes that are abundant in some of the samples. These include representatives of genus Orthokarstenia, Siphogenerinoides, Praebulimina, and Neobulimina; the last two groups may include representatives of other genera with similar morphologies. ``Low trochospiral forms'' are common in some samples and cover a number of morphotypes (e.g., Gavelinella; but others cannot be determined reliably at the genus or even

25

family level). Ford and Houbolt (1963) also described Lenticulina sp. and Bolivina sp. as abundant in some levels. All morphotypes listed above have been classi®ed as low-oxygen opportunists (e.g., Bernhard, 1986; Ly and Kuhnt, 1994). Under ``normal'' open marine conditions, benthic faunal composition is a useful indicator of paleobathymetry, as in the ratio of benthic/planktic foraminifera (Wright, 1977; Van Marle et al., 1987). In the Venezuelan units, however, variation in the percentage of benthic foraminifera is also a function of variation in surface water productivity and extent of bottom oxygenation. Shifts in benthic/planktic ratios within a section therefore re¯ect changing conditions in the water column as well as changes in water depth. Geographic variations between age-equivalent samples are interpreted to represent a paleobathymetric signal, in that lower percentages of benthic foraminifera would occur deeper in an oxygen minimum zone (OMZ). Benthic foraminiferal faunas have very low diversity or are monospeci®c. In certain parts of the La Luna Formation, Orthokarstenia sp. or Praebulimina sp. may form the bulk of all foraminiferal faunas. Neobulimina sp. appears to be more abundant in sections that appear to transition into deeper (more pelagic) environments dominated by planktic foraminifera. Species composition, as well as the low diversity in general, is indicative of intermittent low-oxygen conditions in the water column. Planktic foraminifera are generally rare in upper parts (Late Santonian to Early Maastrichtian) of the La Luna Formation and usually comprise less than 30% of the total 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 (in numbers of individuals per surface area in thin section) in more ®ne-grained samples are also much lower than in the older units. Paleobathymetric shallowing, resulting in lower numbers of planktic foraminifera may explain the shift from the older planktic foraminiferal faunas to faunas dominated by benthic foraminifera. This decrease in planktic foraminifera may also be due to environmental factors such as changing productivity and oxygenation of the water column. A decrease in planktic foraminiferal productivity could occur if nutrient levels became too high or too variable. 2.2. Molluscan fauna and trace fossils The molluscan fauna of pre-La Luna Formation rocks are dominated by several species of Exogyra spp., which are indicative of well-oxygenated benthic conditions with good circulation. The dominantly benthic fauna also consist of bivalves such as Ceratos-

26

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

treon, Pterotrigonia, Acila, Icanotia, Toucasia, Neithea, and Cucullaea. The abrupt replacement of Exogyra spp. fauna by bivalves such as Inoceramus spp., Mytilodes, and Didymotis, along with the gastropods Helcion and Scurria within the Late Albian to Early Cenomanian parts of the La Luna (Figs. 3±4), is indicative of the rapid onset of low bottom water oxygen levels in the Cenomanian (Macsotay, 1980). In the southern Maracaibo and Barinas/Apure basins, a shallow shelf depositional environment for the Cenomanian Aguardiente and Escandalosa formations is indicated by the presence of abundant gastropod and bivalve macrofossils (Cassiope conoidea (Sowerby) gastropods, and Plicatula fourneli (Coquand), Amphidonte africana (Lamarck), Exogyra olisiponensis (Coquand), and Pycnodonta vesicularis (Sow) bivalves), as well as the common occurrence of the trace fossils Thalassinoides, Teichichnus, Skolithos,

and Arenicolites (Aguasuelos, 1991). Stratigraphically equivalent carbonate rocks are highly bioturbated (Skolithos, Glossifungites ) and contain more gastropods (Nerinea, Anchura, Mesalia ), again indicating deposition on a shallow marine shelf (Macsotay, 1975; Garcia Jarpa et al., 1980; Frey et al., 1990). Low-oxygen molluscan genuses such as Inoceramus, Mytilopsis, and Didymotis (Sutton, 1946; Renz, 1959), along with juvenile ammonites and terebratulid brachiopods that occur in shallower-water depositional environments of the La Luna Formation, show that the basin margins were at least intermittently oxic during the Cenomanian and Turonian (Aguasuelos, 1991). Ammonites are extremely common within the Cenomanian to Santonian parts of the La Luna Formation in the northeastern MeÂrida Andes (Fig. 3); detailed descriptions can be found in Renz (1982). Post-Turonian bivalves such as Pseudocucullae, Plica-

Fig. 3. Stratigraphic columns for the northwestern and central Maracaibo Basin (I) and northeastern MeÂrida Andes (II) (approximate locations shown on Fig. 1). Depositional environments are indicated by I/S, inner shelf; M/S, middle shelf; O/S, outer shelf; SL, slope; B, basin.

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

tula, Trigonarca, and Veniella of the MeÂrida Andes occurred in middle to outer shelf environments and are indicative of low-oxygen conditions (Macsotay, 1980). 3. Lithostratigraphy of the La Luna and Navay formations The La Luna Formation of the Maracaibo Basin is composed predominantly of thin-bedded black limestones and shales containing variable amounts of planktic foraminifera. Shallow water fauna are rare or non-existent in most of the La Luna Formation (see Erlich et al., 1999a, for a recent summary of the paleoecology of the La Luna). The La Luna is siliceous and phosphatic in the southwestern MeÂrida Andes, where the upper part is divided into two mem-

27

bers, the Ftanita de TaÂchira (TaÂchira Chert) and the Tres Esquinas (Hedberg and Sass, 1937; Renz, 1959; Stainforth, 1962; Ford and Houbolt, 1963; Trump and Salvador, 1964; Gonzalez de Juana et al., 1980; Ghosh, 1984; De Romero and Galea-Alvarez, 1995; Figs. 3±4). The La Luna is subdivided into three members in the northeastern MeÂrida Andes, the La Aguada, the ChejendeÂ, and the Timbetes (Renz, 1959, 1982; Renz, 1961; Tribovillard et al., 1991). The Socuy Member marks the uppermost La Luna in the central and northwestern parts of the Maracaibo Basin (Sellier De Civrieux, 1952; MartõÂ nez, 1989; Canache et al., 1994; Parnaud et al., 1995). The Navay Formation of the Barinas/Apure Basin is composed of interbedded black shales and limestones with variable phosphate and silica content; it can be generally divided into two members, the La Morita

Fig. 4. Stratigraphic columns for the southwestern Maracaibo Basin and southern MeÂrida Andes (III), and west central Barinas/Apure Basin (IV) (approximate locations shown on Fig. 1). Depositional environments are indicated by I/S, inner shelf; M/S, middle shelf; O/S, outer shelf; SL, slope; B, basin.

28

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

(mostly shale) and Quevedo (mixed limestones and shales; Fig. 4). Shallow water faunas are also rare in the Navay, although broken and abraded mollusc fragments do occur in the upper parts of the formation (Erlich et al., 1999a). The La Luna Formation in the southern and central MeÂrida Andes is laterally equivalent to prodelta shales of the Seboruco Member (Capacho Formation) and shallow shelf sandstones of the Escandalosa Formation (Gaenslen, 1962; Kiser, 1989; Galea-Alvarez et al., 1994; Parnaud et al., 1995; Fig. 4). In the northern MeÂrida Andes, the La Luna is composed of outer shelf to slope black limestones and shales in the northern part of the basin (Hedberg, 1931), grading laterally into the Seboruco towards the south (Notestein et al., 1944). In the northern part of the Maracaibo Basin, the La Luna unconformably overlies shallow water carbonate platform sediments of the Maraca Formation (Canache et al., 1994; Parnaud et al., 1995; Truskowski et al., 1995; 1996; Fig. 3). Middle to outer shelf shales of the La Morita Member unconformably overlie shallow marine Guayacan Member (Escandalosa Formation) limestones (locally known as the ``O'' Limestone; Kiser, 1989; Gamarra et al., 1994; Aquino et al., 1995) and shallow marine Escandalosa sandstones over most of the Barinas/ Apure Basin (Fig. 4). The La Luna Formation is laterally equivalent to the La Morita in the Barinas/Apure Basin (Bejarano and Zorrilla, 1994). In the northwestern and southwestern Maracaibo Basin, the Socuy Member of the La Luna Formation is overlain by the homogeneous lower Maastrichtian outer shelf to slope shales of the ColoÂn Formation (Sutton, 1946; MartõÂ nez, 1989; Lorente and Duran, 1995; Parnaud et al., 1995). ColoÂn shales change abruptly to the south into inner to middle shelf siliceous limestones and cherts of the Ftanita de TaÂchira Member and the cherts and shales of the Quevedo Member (Navay Formation, Fig. 4; Bejarano and Zorrilla, 1994). In the northern Barinas Basin, where the Quevedo is not recognized in the subsurface, ColoÂn shales grade into Navay shales, siltstones, and sandstones (Figs. 3±4). In the central and northern Barinas/Apure Basin, the ColoÂn Formation is laterally continuous with the inner shelf to deltaic sandstones, siltstones, and shales of the Maastrichtian BurguÈita Formation (Fig. 4). The BurguÈita unconformably overlies the Navay Formation in the central and northern Barinas/Apure Basin (De Monroy and Van Erve, 1988; Helenes et al., 1994; 1998). 4. Anoxia and biological productivity: origin of the ``La Luna Sea'' The initiation, development, maintenance, and ter-

mination of anoxia in the Maracaibo and Barinas/ Apure basins involved a complex interplay between paleobathymetry, paleoclimate, and paleoceanography. Important regional marine transgressions in the Albian±Cenomanian and Cenomanian±Turonian provided an opportunity for the development of pelagic marine facies in western Venezuela (Parnaud et al., 1995; Truskowski et al., 1996; Erlich et al., 1999a,b). However, marine transgression alone is insucient to explain the occurrence of such a continuous and uniform sequence of organic carbon-rich deposits (Van Cappellen and Ingall, 1994). The in¯uence of pre-existing basement topography on Cretaceous depositional patterns was shown by Renz (1959; 1982), Macellari and De Vries (1987), Macellari (1988), and Erlich et al. (1999a). Speci®cally, Cretaceous strata thin towards the Santa Marta and Santander massifs. In addition, reactivation of Jurassic normal faults in the southeastern part of the Maracaibo Basin produced di€erential subsidence along the trend of the present MeÂrida Andes (Macellari and De Vries, 1987; Macellari, 1988; Lugo and Mann, 1995). These earlier studies indicate that the ``ancestral'' MeÂrida Andes, along with the Santa Marta and Santander massifs and Paraguana Block, acted as stable, bathymetrically ``shallow'' platforms during deposition of the La Luna and Navay formations. These features produced a restricted basin with connections to open/ surface-water marine conditions only in the north and northeast (Fig. 5). Interpretations of paleobathymetry based on foraminiferal biostratigraphic data (Fig. 6) indicate that inner to middle shelf environments initially developed during the Late Cenomanian and Turonian in the MeÂrida Andes, with water depths increasing towards the north and east (Erlich et al., 1999a). These interpretations are supported by similar results from subsurface sections reported by Lugo and Mann (1995) and from outcrop sections in central and eastern Colombia (Vergara, 1994; Villamil, 1998). The presence of bathymetric barriers caused poor circulation and low rates of water column ventilation. High evaporation and low precipitation rates (Bush and Philander 1997; Erlich et al. 1999b) also enhanced stagnant bottom water conditions. Palynological data (De Monroy and Van Erve, 1988; Herngreen and Jimenez, 1990; Helenes et al., 1998; Dino et al., 1998) con®rm that the early Late Cretaceous climate of western Venezuela was arid to semi-arid, and later, alternately wet and dry; wet intervals were apparently brief while dry intervals were more prolonged (Erlich et al., 1999b). Palynological data, the regional abundance of eolian quartz silt (Vergara, 1994), and lack of ¯uvio±deltaic sediments suggest that climatic conditions (intensity of winds and rains) during the Cenomanian through Early Santonian were much di€erent from the present.

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

One reason for this di€erence may be that the intensity of easterly Trade Winds and the Inter-Tropical Convergence Zone (ITCZ) was signi®cantly reduced from the present patterns of strong, seasonally modulated winds and high rainfall rates. A second reason may be that an as yet unidenti®ed topographic feature east of the Barinas/Apure Basin acted as a precipitation shadow over the area. Regardless of the mechanism, these climatic conditions promoted formation of a high salinity (>35-), low-oxygen water mass in shallow shelf areas of the Maracaibo and Barinas/Apure basins. Subsequent entrapment of this water mass in restricted basinal areas produced conditions similar to those proposed by Wignall (1991) for the Kimmeridgian of the North Sea. Bottom water anoxia can be inferred from the microlaminated, non-bioturbated character of the Cenomanian to Santonian part of the La Luna Formation

29

(see criteria of Savrda and Bottjer, 1991) and from organic geochemical data (Talukdar et al. 1986; Mendez 1989; Erlich et al., 1999b). Further evidence for at least low bottom water oxygen content can be found in the rare molluscan and benthic foraminiferal faunal assemblages which, as stated previously, are known to have been associated with low oxygen levels. While bottom waters within the Maracaibo and Barinas/Apure basins appear to have been anoxic, exchange of surface waters did occur with the Atlantic Ocean north and northeast around the Paraguana Block (Fig. 5), as shown by the cosmopolitan ammonite genera of the La Luna Formation (Renz, 1982; Johnson, 1999). Limited surface water exchange also occurred with the Paci®c Ocean to the south through Colombia (Vergara, 1997a; Johnson, 1999). In this regard, the Maracaibo and Barinas/Apure basins may have responded to ¯uctuations of sea level and dis-

Fig. 5. Tectonic reconstruction of the major tectonic features shown in Fig. 2 (see Erlich et al., 1999a, for a detailed description of the displacements used in the reconstruction). Syndepositional bathymetric or topographic highs are shown in stippled pattern.

30

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

solved oxygen content in a manner more similar to epicontinental seaways such as the Cretaceous Interior Seaway (Slingerland et al., 1996) rather than as a deep oceanic basin. As in other areas, anoxia was not necessarily a requirement for the accumulation of organic carbonrich sediments in western Venezuela (Pedersen and Calvert, 1990). Considering an average sedimentation rate of 7.2 mm 1000 yrÿ1 (derived from locations shown on Fig. 1), normal marine productivity levels apparently characterized the La Luna and Navay formations during their deposition. However, this value is only 60% of the rate calculated by Martõ nez and Hernandez (1992) for Colombian La Luna rocks (12 mm 1000 yrÿ1), and only 51% of the rate calculated by Helenes et al. (1998) for the Navay in the west central Barinas/Apure Basin (14 mm 1000 yrÿ1). Nevertheless, this rate represents a basin-wide average for all locations. Areas closer to the Colombian border (Sierra Perija mountain front; Fig. 1) have sedimentation rates much closer to the value published by Martõ nez and Hernandez (1992) (10.4 mm 1000 yrÿ1; outcrop locations 51±54, Appendix). Intervals of high productivity are none-the-less well documented from the La Luna and Navay formations and their equivalents. For example, Tribovillard et al. (1991) described the common occurrence of dino¯agellate cysts in the sections from the central and northern MeÂrida Andes. Helenes et al. (1998) also described

very abundant dino¯agellate cysts from wells in the west central Barinas/Apure Basin, and FoÈllmi et al. (1992) and Guerrero and Sarmiento (1996) reported dino¯agellate cysts in upper La Luna sections from Colombia. Recently, Vergara (1994, 1997b) documented algal and planktic foraminiferal productivity from La Luna equivalents in Colombia. Based on fossil evidence, it is clear that planktic foraminifera and marine algae (dino¯agellates) dominated surface water productivity. Productivity, when enhanced or elevated by episodic supply of nutrients via ¯uvial runo€ (Erbacher et al., 1996), would have helped to further lower bottom water oxygen levels and would have prevented oxic degradation of rapidly deposited organic matter. Nutrient supply to the productivity system during the Santonian also occurred through episodic (seasonal?) upwelling along the eastern margin of the Maracaibo Basin (``ancestral'' MeÂrida Andes). Intensi®cation of the easterly Trade Winds acted in combination with the well established Eastern Paci®c Equatorial Upwelling Zone (Erlich et al., 1996) and local paleobathymetry to drive seasonal upwelling (Kruijs and Barron, 1990; Mikolajewicz et al., 1993; Barron et al., 1995; Bush and Philander, 1997). Although silica±secreting organisms (such as radiolaria and sponges) probably utilized the nutrients provided during upwelling, they may have been only a minor part of the overall productivity system during the Late Cretaceous. Nevertheless, the dissolution of

Fig. 6. Regional paleobathymetric interpretation of Upper Cretaceous rocks from outcrops in the Sierra Perija and MeÂrida Andes Mountains. Each circled location re¯ects the average paleoenvironment for multiple nearby outcrop locations and also represents an average for the Late Albian through Early Maastrichtian.

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

siliceous organisms during burial may account for much of the silica found in Andean La Luna Formation rocks (Erlich et al., 1999b; also see examples in Coniglio, 1987; Maliva and Siever, 1989; Murray et al., 1993). Burial diagenesis has made direct veri®cation of silica-secreting organisms within the La Luna dicult. However, radiolaria (along with siliceous sponge spicules) have been reported from many areas of the MeÂrida Andes and in central and eastern Colombia (Ford and Houbolt, 1963; Vergara, 1994; Villamil and Arango, 1998; and this study). The increased deposition of siliceous sediments and decrease in organic matter accumulation and preservation during the Santonian (especially Late Santonian) and Campanian can be linked to an increase in the frequency or intensity of upwelling, and it signaled the end of anoxia and stagnation within the Maracaibo and Barinas/Apure basins. 5. End of the ``La Luna Sea'': implications for Late Creataceous paleoclimate and paleoceanography We believe that the end of anoxia and salinity strati®cation of the ``La Luna Sea'' was caused primarily by changes in local/regional wind and rainfall patterns (due to changes in paleotopography and global paleoclimate) and by the loss of a key paleobathymetric barrier (and in¯ux of a new source of cooler, oxygenated water into the Maracaibo Basin). Late Cretaceous rainfall patterns have been inferred from numerical climate models (Kruijs and Barron, 1990; Mikolajewicz et al., 1993; Barron et al., 1995; Bush and Philander, 1997). In these models, high ¯uvial runo€ rates are predicted for northern South America during the Late Santonian to Maastrichtian. Con®rmation of model results rests primarily within the lithostratigraphic record, which shows that siliciclastic input (and delta progradation) into the Maracaibo and Barinas/Apure basins increased from the Late Santonian to the Early Campanian (Notestein et al., 1944; Gonzalez de Juana et al., 1980; Macellari, 1988; Kiser, 1989; Martõ nez, 1989; Lugo and Mann, 1995; Guerrero and Sarmiento, 1996; Vergara, 1997a; Erlich et al., 1999a,b). Consequently, the overall organic richness of sediments within the Maracaibo and Barinas/Apure basins decreased due to siliciclastic dilution. Delta progradation also coincided with uplift of the Central Cordillera of Colombia (Vergara, 1997a; Villamil, 1998). It could be argued that the rapid uplift of a large paleotopographic barrier to the west of the Maracaibo and Barinas/Apure basins, coupled with intensi®ed easterly Trade Winds (see paleoclimate models described previously), dramatically altered local precipitation patterns (high precipitation on the eastern

31

side of the mountain range, a rain-shadow on the western side). Further investigation of this possibility is certainly warranted; however, de®nition of the link is beyond the scope of this study. Disruption of anoxia and the accumulation of organic carbon-rich sediments would have also been promoted by the removal of key paleobathymetric barriers. This would have facilitated venting and exchange of water masses between the oxygenated Atlantic and stagnant Maracaibo and Barinas/Apure basins. Unfortunately, such an exchange can only be inferred. However, strong support for such an exchange exists in the paleoecological record of western Venezuela. Interpretations of paleoecological data suggest that conditions typical of open marine Atlantic basins were in place in western Venezuela by at least the end of the Cretaceous (Lorente and Duran, 1995; Lorente et al., 1996; Truskowski et al., 1996). Therefore, some exchange between the western Venezuelan basins and the Atlantic must have occurred. This exchange could have been the indirect result of foreland basin subsidence in western Venezuela, due to uplift of the Central Cordillera of Colombia (Cooper et al., 1995; Vergara, 1997a; Villamil, 1999). Subsidence that began during the Santonian and accelerated in the Maastrichtian may have lowered the Paraguana Block suciently to allow highly saline anoxic water to be vented north into the Atlantic. While only speculation, this mechanism would have eventually replaced stagnant Maracaibo and Barinas/Apure basin water with oxygenated water from the Atlantic. Emplacement of oxygenated water into the Maracaibo and Barinas/Apure basins beginning in the Late Santonian to earliest Campanian and continuing through the end of the Cretaceous would also have provided a means for re-establishing infaunal activity, thereby decreasing the preservation of organic carbonrich units. However, the MeÂrida Andes and numerous basement arches (Fig. 5) prevented complete ventilation of the western Barinas/Apure Basin (upper Navay Formation) until the Late Maastrichtian. 6. Origin and signi®cance of the Tres Esquinas Member phosphorites The origin of phosphatic and glauconitic facies of the upper Tres Esquinas Member (upper La Luna Formation) and equivalents is somewhat problematic. Deposition of the Tres Esquinas is inferred to have occurred during the third major transgression to a€ect western Venezuela, in the Late Santonian through Early Maastrichtian; however, interpretations of its depositional environment vary. Phosphorites in western Venezuela have been

32

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

described by Sutton (1946), Renz (1959), Stainforth (1962), Ford and Houbolt (1963), Tribovillard et al. (1991), De Romero and Galea-Alvarez (1995), Lorente and Duran (1995), Lorente et al. (1996), and Truskowski et al. (1996). However, FoÈllmi et al. (1992) were the ®rst to suggest a classi®cation for mid- to Upper Cretaceous phosphorites in eastern Colombia as a means to determine the genesis of these units. FoÈllmi et al. (1992) presented three possible origins for the eastern Colombian phosphorites. First, as pristine beds, formed due to microbial mediation. Second, as condensed beds, composed of phosphatized fossil

debris and lithoclasts and formed by winnowing of shelf units over long periods of time. Third, as allochthonous beds, composed of phosphatized and non-phosphatized particles transported as gravity ¯ows or as admixtures within winnowed units. FoÈllmi et al. (1992) associated allochthonous and condensed beds with transgressive systems tracts (TST) and the basal parts of high-stand systems tracts (HST) in eastern Colombia, where they may represent erosional lag deposits of inner to outer shelf depositional environments. Nearly all of the phosphorites of the MeÂrida Andes described in this study and in the literature closely

Fig. 7. A) Thin section photomicrograph of coarse-grained phosphatic Tres Esquinas Member, Upper Santonian, Las Delicias outcrop (location 32). F=phosphatic ®sh bone, Q=quartz silt. Scale bar in all photomicrographs=250 mm. B) Thin section photomicrograph of quartz-rich Navay Formation phosphatic bed, Upper Campanian, RõÂ o Sto. Domingo outcrop (location 21). C) Thin section photomicrograph of shaly, phosphatic, quartz-rich Tres Esquinas, Lower Maastrichtian, Las Vegas outcrop (35). BF=benthic foraminifera, G=glauconite. D) Thin section photomicrograph of glauconitic, phosphatic, quartz-rich Tres Esquinas, Lower Maastrichtian, RõÂ o Guaruries outcrop (location 44). Note rare occurrence of oolitically coated phosphatic grains (O).

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

match the description of condensed and allochthonous phosphorites by FoÈllmi et al. (1992) and FoÈllmi (1996). For example, Tres Esquinas Member phosphorites from the northern and southern MeÂrida Andes (Figs. 7 and 8) appear to be allochthonous, according to the criteria of FoÈllmi et al. (1992) and FoÈllmi (1996). However, phosphorites from the central MeÂrida Andes show characteristics (such as down cutting into underlying microlaminated La Luna Formation sediments) typical of condensed phosphorites (Fig. 8), indicating that multiple origins may be possible for certain deposits. Other Tres Esquinas units more closely resemble the condensed phosphorites of FoÈllmi et al. (1992) and

33

FoÈllmi (1996), although no clear determination is possible. As noted previously, phosphorites found in the MeÂrida Andes contain phosphatic ®sh bones, scales and teeth, lithoclasts, crustacean pellets (such as Favreina sp.), large benthic foraminifera (especially Siphogenerinoides sp., Lenticulina sp., Bolivina sp., and Praebulimina sp.), encrusting polychaetes (Hamulus onyx Morton, H. squamosus Gabb, Sclerostyla macropus (Sowerby)), quartz silt, and marine reptile bones and teeth (J. Horner, 1993, pers. comm.; Figs. 7 and 8). The units are poorly sorted and often graded, and they sometimes contain erosional bases. Components found

Fig. 8. A) Tres Esquinas Member bone bed (phosphatic conglomerate), Lower Santonian, RõÂ o Chama outcrop (location 47). Conglomerate truncates (arrows) underlying laminated to burrowed phosphatic La Luna Formation siliceous limestone. Scale=15 cm. B) Thin section photomicrograph of burrowed, foraminifera-rich Tres Esquinas, Upper Santonian, RõÂ o Guaruries outcrop. BF=benthic foraminifera. C) Thin section photomicrograph of phosphatic lime mudstone, Turonian La Luna, RõÂ o Higuerones outcrop (location 18). MO=mollusc, F=phosphatic ®sh bone. D) Thin section photomicrograph of phosphatic lime mudstone, Turonian La Luna, RõÂ o Higuerones outcrop. PF=planktic foraminifera, Q=quartz silt.

34

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

in the coarse grained phosphorites are frequently microbored and have micritic rims, indicating extended periods of exposure on the sea ¯oor. This suggests that the units underwent periods of slow or no deposition, interrupted by rapid or catastrophic reworking and redistribution, followed later by normal marine shelf sedimentation. Regardless of their classi®cation, severe condensation, erosion, and re-distribution can certainly be inferred from the very low sedimentation rates for Tres Esquinas rocks in the central and southern MeÂrida Andes (average 1 mm 1000 yrÿ1). Bioturbation of phosphatic deposits was also very common following redeposition events. The trace fossil assemblage of the Cruziana ichnocoenose suggests the intervening periods were characterized by relatively low energy conditions and intermittent bottom water oxygenation (Ekdale et al., 1984), while the common occurrence of low-oxygen benthic foraminifera (Siphogenerinoides sp., Lenticulina sp., Bolivina sp., and Praebulimina sp.) suggest ¯uctuating oxygen levels. Coincidentally, Tres Esquinas Member phosphate±rich rocks (>5% ¯uorapatite; Erlich et al., 1999b) generally have lower average TOC content than equivalent, phosphate±poor rocks (Table 1), implying formation and (possibly) deposition under episodically agitated, oxic or low-oxygen conditions. BuÈrgl and Botero (1967), Cathcart and Zambrano (1967), FoÈllmi et al. (1992), Mann and Stein (1997), and Vergara (1997b) also observed that phosphorites of the Central and Table 1 Average corrected TOC content (see Erlich et al., 1999b) of selected Upper Cretaceous formations Unit

Avg TOC corr No. of Analyses (wt.%)

All La Luna/Navaya La Luna/Navay (onlyb) La Luna Transitional Faciesc Socuy La Luna/Navay (Campanian onlyd) All Tres Esquinas Tres Esquinas >5% Phosphate Age Equivalent to Tres Esquinase

2.26 2.59 1.43 1.2 0.94 0.92 0.87 1.79

159 122 11 2 36 34 11 85

a Includes all La Luna/Navay Formation samples (analyses include all Tres Esquinas, Ftanita de TaÂchira, and Socuy Member samples; no other formations). b Includes only La Luna and Navay Formation samples (a subset of all samples, without Tres Esquinas, Ftanita de TaÂchira, and Socuy members). c Includes only samples within the La Luna Ð Maraca or La Luna Ð Guayacan facies transition. d Includes only Campanian La Luna/Navay Formation samples (without Tres Esquinas, Ftanita de TaÂchira, and Socuy members). e Includes only analyses of Santonian to Early Maastrichtian formations equivalent to Tres Esquinas Member (includes all La Luna/ Navay, ColoÂn, BurguÈita Formation, and Ftanita de TaÂchira and Socuy Member samples; excludes Tres Esquinas Member samples).

Eastern Cordillera of Colombia often have low TOC content. The occurrence of low TOC content in phosphorites is a common characteristic of other ancient and modern examples outside northern South America (FoÈllmi, 1990; Glenn and Arthur, 1990; Heggie et al., 1990; Guthrie and Pratt, 1994; Kupecz, 1995). This does not mean, however, that the occurrence of phosphorites was not linked to age-equivalent organic carbon-rich La Luna and Navay Formation rocks deposited to the northwest and southeast. On the contrary, large amounts of organic carbon continued to accumulate in some parts of the basin during the Early and Late Santonian. This much is obvious from TOC values (up to 15.62%) of laminated shelf and slope La Luna/Navay rocks that contain abundant transported phosphatic grains. However, the overall La Luna/ Navay succession within the Maracaibo and Barinas/ Apure basins clearly shows decreasing TOC content towards the end of the Santonian (Erlich et. al., 1999b). An intermittently oxygenated middle shelf depositional environment for the Tres Esquinas Member conforms well with occurrences of reworked glauconite and authigenic illite/smectite precursors common in the uppermost part of the unit (e.g., Bromley, 1967; Porrenga, 1967; Ganz et al., 1990; Glenn and Arthur, 1990; Carson and Crowley, 1993; Amorosi, 1995). In Santonian rocks, rare pristine phosphorites were found only as authigenic cements in®lling the tests of Siphogenerinoides sp. in one outcrop section in the southern MeÂrida Andes. These may have formed in the presence of ¯uctuating redox conditions below the sedimentwater interface. Pristine phosphorites also occur within thin (less than 1 m) Navay Formation shale beds in the southwestern Barinas/Apure Basin (Cardenas, 1985; Baptista and Scherer, 1996) and as diagenetic nodules within pre-Santonian La Luna Formation rocks in the northwestern Maracaibo Basin (observed in this study). Paleoenvironmental interpretations based on biostratigraphic data indicate that Tres Esquinas Member phosphatic conglomerates were ultimately deposited within inner to middle shelf and outer shelf to (possibly) upper slope environments (also see De Romero and Galea-Alvarez, 1995). Interpretations of the Santonian to Early Maastrichtian paleobathymetry of the southeastern Maracaibo Basin re¯ect a similar inner to outer shelf depositional environment (Lugo and Mann, 1995). With this in mind, two types of phosphorites are interpreted in this study: the ®rst as coarse-grained shelf-derived turbidites (condensed and allochthonous types), and the second as condensed/winnowed o€shore bars (as in Filippelli, 1997). Both types occur throughout the central and southern MeÂrida Andes. These interpretations are consistent with the origin of similar phosphorites from the subsurface of the southwestern

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

Barinas/Apure Basin (SaÂnchez and Lorente, 1977; Bejarano and Zorrilla, 1994). The preceding observations contrast somewhat with the model of FoÈllmi et al. (1994); in their study, FoÈllmi et al. (1994) directly linked the occurrence of phosphorites to high productivity and concomitant deposition of TOC-rich sediments. Certainly, allochthonous phosphorites that occur within the Turonian in the northern MeÂrida Andes (Fig. 8) are probably linked to upwelling-related productivity. However, the variations noted by Erlich et al. (1999b) for western Venezuelan phosphorites suggest a more complex temporal and spatial relationship. In that study, Erlich et al. (1999b) showed that Upper Santonian through Maastrichtian phosphorites were derived from multiple sources (including re-working of older deposits), whereas earlier phosphorites are linked directly to age-equivalent high-TOC intervals (as in FoÈllmi et al., 1994).

7. Colombian phosphorites and the origin of bone beds A more complete understanding of the formation and accumulation of the Tres Esquinas Member phosphorites can be derived from an examination of Colombian phosphorites. Upper Cretaceous phosphorites of Colombia are concentrated primarily on the eastern side of the Cordillera Central (within the backarc basin) and are mostly age equivalent with the Tres Esquinas±Ftanita de TaÂchira Member deposits of the MeÂrida Andes. Phosphorites range throughout the Upper Cretaceous section but are concentrated mainly in Upper Santonian to Upper Campanian parts of the La Luna/Galembo/Chipaque formations, Guadalupe Group, and lower Umir Formation (BuÈrgl and Botero, 1967; Cathcart and Zambrano, 1967; Ward et al., 1973; Barrio and Coeld, 1992; FoÈllmi et al., 1992; Guerrero and Sarmiento, 1996; Vergara, 1997a; Villamil, 1998; 1999). The high concentration of ®sh bones and teeth within the phosphorites is of particular signi®cance. Fish and marine reptile bone beds are prevalent throughout the La Luna Formation and equivalents from the northeastern Venezuelan Andes to the Upper Magdalena Valley of Colombia. These beds are most common within the Upper Santonian to Upper Campanian part of the section (BuÈrgl and Dumit ToboÂn, 1954; BuÈrgl and Botero, 1967; Cathcart and Zambrano, 1967; Ward et al., 1973; Tribovillard et al., 1991; Barrio and Coeld, 1992; FoÈllmi et al., 1992; Guerrero and Sarmiento, 1996; Mann and Stein, 1997; Vergara, 1997a; Villamil, 1998; 1999). One possible explanation for the sudden appearance of bone beds within the upper parts of the La Luna may be changing environmental or oceanographic conditions at the

35

end of the Santonian, along with high levels of productivity. The accumulation of large amounts of ®sh bones in modern marine environments has been linked to largescale ®sh kills caused by sudden episodes of marine algal growth in surface or mid-water depths. These ®sh kills are directly attributable to the toxins released during dino¯agellate blooms (Adams et al., 1968; Anderson, 1994), which occur as a result of elevated nutrient levels and sea surface temperatures. Recent studies of dino¯agellate blooms on modern continental shelves and slopes have shown that ¯uvially-derived nutrients strongly in¯uenced each event. These algae blooms are believed responsible for recent episodes of water column strati®cation and anoxia (Malone, 1991; Rabalais et al., 1991; Tyson and Pearson, 1991; Van Der Zwaan and Jorissen, 1991; Sen Gupta et al., 1996). When upwelling events or excessive runo€ overload the surface water layer with nutrients, the resulting algal bloom (``red tide'') and accompanying oversupply of organic matter promote the uptake of dissolved oxygen faster than the system can be recharged. The resulting loss of oxygen contributes to ®sh mortality; their subsequent decay supplies more nutrients to the system, continuing the cycle. In such instances, even large marine mammals succumb to the toxins released by the algae, ostensibly via the consumption of ®sh that contain lethal concentrations of the poison (Anderson, 1994; Culotta, 1996). A direct link between ¯uvially-induced nutrient ¯ux and algal productivity within the La Luna Formation was made by Tribovillard et al. (1991) and is corroborated by data presented by Guerrero and Sarmiento (1996), who observed high concentrations of dino¯agellate cysts immediately following episodes of siliciclastic sedimentation in age-equivalent Colombian rocks. Supporting this interpretation is the observation by Macsotay (1980) that heliciform gastropods, thought to have been sensitive to red tide algae, are absent from La Luna rocks in the MeÂrida Andes. It is therefore reasonable to conclude that the regionally common phosphatic bone beds and high siliciclastic input (increasing nutrient ¯ux to the system) are in some way related to dino¯agellate productivity (also see Erbacher et al., 1996). Analogous bone beds and high concentrations of bone material, including marine reptile bones, within the upper La Luna Formation and equivalents may therefore represent the redeposited remnants of red tide ®sh kills. Ghosh (1984) and Tribovillard et al. (1991) interpreted phosphatic bone beds within the upper La Luna as having been caused by ``phytoplankton blooms''. Similar inferences have been made for dino¯agellate- and TOC-rich rocks from the Precambrian to the Pleistocene (Rossignol-Strick, 1987a,b;

36

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

Leckie et al., 1990; Spero, 1990; Piasecki and Stemmerik, 1991; Littke et al., 1991; Prauss et al., 1991). Within the presumably warm, strati®ed ``La Luna Sea,'' nutrient overload could have been accomplished via more frequent catastrophic (seasonal?) upwelling and overturn of anoxic water, in combination with massive fresh water out¯ow due to ¯oods (paleoclimate models of Kruijs and Barron, 1990; Mikolajewicz et al., 1993; Barron et al., 1995; Bush and Philander, 1997). In this regard, the bone beds could represent event markers (spanning as little as one week to more than 100,000 years) and may be correlative over fairly large areas. Reworking of the shallow shelf bone deposits as condensed and allochthonous phosphorites in deeper shelf and slope settings could have been accomplished during episodic or seasonal storms (Glenn and Arthur, 1990), thus accounting for their higher residence time prior to ®nal deposition. 8. Conclusions Uplift of the Central Cordillera of Colombia during the Turonian through Campanian caused the development of the ``ancestral'' MeÂrida Andes paleobathymetric high. The rapid uplift of this feature, combined with the presence of the positive Santa Marta and Santander massifs, caused poor circulation and limited ventilation within the Maracaibo and Barinas/Apure basins. Evidence for low-oxygen or anoxic conditions during deposition of the La Luna and Navay formations includes a lack of infaunal activity and the predominance of low-oxygen-tolerant bivalves, ammonites, and benthic and planktic foraminifera. Anoxia was also enhanced by high evaporation and low precipitation rates (high salinity bottom water) and high levels of marine algal productivity (high organic ¯ux). Nutrient supply was augmented by infrequent ¯uvial sources (Erlich et al., 1999b). Deposition of the Socuy and Tres Esquinas members of the La Luna Formation re¯ects a change in paleoclimate and paleoceanography in western Venezuela. Although upper La Luna and Socuy depositional environments were similar, an increase in bottom water

oxygenation facilitated infaunal activity, which ultimately greatly reduced the preservation of organic matter. Increased tectonic activity and global climate change contributed to higher rainfall rates and intensi®ed wind-driven upwelling, which caused frequent overturn of anoxic water in western Venezuela. Ventilation of the Maracaibo and Barinas/Apure basins may also have been facilitated by the removal of an important paleobathymetric barrier and subsequent exchange with oxygenated water from the Atlantic. Paleoclimatic changes such as increased rainfall rates were expressed through the rapid progradation of Campanian and Maastrichtian deltaic clastics into the Maracaibo and Barinas/Apure basins. Phosphatic ®sh and reptile bone beds found within the Tres Esquinas Member are the result of explosive dino¯agellate productivity (red tides ), induced by intensi®ed upwelling and episodic ¯uvial supply of nutrients; these are correlative with similar Campanian phosphorites of eastern Colombia.

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 assistance in the ®eld in 1990±1996. D. Pocknall and J. Bergen contributed important paleontological analyses to earlier studies, and C. Yeilding contributed photographs and thin sections for analysis. We also thank 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 and H. Krause, who supported the idea for this work and the formation of this diverse study team. W. Schlager, J. Van Hinte, K. FoÈllmi, and D. Leythaeuser made signi®cant improvements to the manuscript. R. Squyres and E. Kreger drafted the ®gures for this report. Finally, we wish to thank PDV Exploration and Production and BP Amoco Exploration and Production for permission to publish this report.

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

37

Appendix A1. List of all wells used in this study and the types of data available from each location

Data Point

Well

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

La Paz P-90 La Paz P-106 Totumos-1 Totumos-2 Totumos-3 Totumos-4 Totumos-5 La Concepcion-156 Ensenada-2 Ambrosio-161 Urdaneta-6 Urdaneta-7 Urdaneta-144 Urdaneta-171 Garcia-15 Garcia-16 Garcia-17 Garcia-18 Garcia-19 Alpuf-1 Alpuf-2 Alpuf-3 Alpuf-4 Alpuf-5 Alpuf-6 Alpuf-7 Alpuf-8 Alpuf-9 Alpuf-10 Zulia-14G-1 Zulia-17G-1 Zulia-36E-2 San Jose-1 San Jose-2 San Julian-1 San Julian-2 Machiques-1 Machiques-2 Machiques-3 Alturitas-1 Alturitas-2 Alturitas-3 Alturitas-4 Alturitas-5 Alturitas-6 Alturitas-7

E-Logs X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

Seismic

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

Lithology X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

Paleo

X X X X X

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

Fm Tops X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

Geochem

X

X X

X

38

47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

Alturitas-8 Alturitas-9 Alturitas-10 Alturitas-11 Alturitas-12 Alturitas-13 Alturitas-14 Alturitas-15 Alturitas-16 Alturitas-17 Alturitas-18 Alturitas-19 Sol-6 VLA-722 VLA-810 VLA-976 VLB-693 VLE-400 VLE-686 VLE-733 VLE-747 VLG-3693 SLA-6-2X SLA-8-2X SLE-1-2X Rio de Oro-1 Rio de Oro-18 Campo Rosario-4 Campo Rosario-5 Campo Rosario-9 Campo Rosario-10 Campo Rosario-11 Campo Rosario-12 Tarra-99 Tarra-230X Tarra-231X West Tarra-2 West Tarra-3 West Tarra-5 West Tarra-6 West Tarra-8 West Tarra-11 West Tarra-35 West Tarra-51 West Tarra-55 West Tarra-60 West Tarra-61 West Tarra-62 West Tarra-63 West Tarra-65 Rio Zulia-2 Mucurera-3 Cerrito-1 Guaruries-1 S Milagro Sur-1X La Ceiba-1X (Barinas)

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

X X X X X X X X X X X X X X

X X X X X X X X X X X X X X X X X X X X X X X

X X X

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

X X X X X X X X X X X X X X X X X X X X X

X X

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

X X

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158

Burgua-3 Burgua-4 Jordan-1X Cutu®to-1 La Victoria-1 La Ceiba-1X (Maracaibo) Poco-2X Las Duaras-1 S Conquistado-1 Butaque-1 S Uzcategui-2X Gavilancito-1 Lechozote-1 Lechozote-2 Lechozote-3 Lechozote-4 Bumbum-1 Socopo-1 Socopo-2 Capitanejo-1 Capitanejo-2 Chorro-1 Chorro-2 Pedraza-1 Yaure-2 Rosalia-1 Agua Linda-1 Cano Munoz-1X Amparo-1 Guasdalito-1X Apure-2 Apure-3 Ichazu-1 Paguey-1 Zamoa-34-1 North San Vincente-1 Aceituno-1 Conso-3 Conso-5 Torunos-1X Sinco-1 Sinco-2 Sinco-4 Sinco-5 Sinco-6 Paez-1 Paez-2 Paez-3 Paez-4 Paez-5 Paez-6 Gua®ta-2X Guanarito 15-107 Guanarito 15-201 Guanarito 15-501 Guanarito 15-507

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

X X X X X X X X

X X X X X X X X X X X X

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

39

X X X X X X X X X X X

X

X

X X X X X X X X X X X X X

X X X X X

40

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

A2. List of all outcrop locations used in this study and the types of data availble from each location

Data Point

Outcrops

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Quebrada San Pedro Quebrada Porras Humucaro Humucaro Bajo Barbacoas La Aguada Carora Curarigua Cabudare Atarigua Isu Quebrada Sonadora Chejende Arbol Redondo El Bano Rio Mimbos Torondoy Rio Higuerones Rio Calderas Quebrada Bellaca Rio Santo Domingo Rio Quiu Santa Barbara La Vueltosa La Fundacion Rio Caparo Quebrada Zorca Rio Navay La Libertad Quebrada Escandalosa Rio Camburito Las Delicias Paso Andino Independencia Las Vegas Urena San Pedro Del Rio Quebrada La Sucia Mucujun Quebrada La Molina La Fria Omuquena Rio Lora Rio Guaruries Zea Rio Guayabones Rio Chama Chiguara

E-Logs

X

X

X

X X X X X

X X

Seismic

Lithology

Paleo

Fm Tops

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

Geochem

X

X X

X X X

X X X X X

X X

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

49 50 51 52 53 54 55 56 57 58 59 60 61 62

La Azulita Rio Sucumo Rio Yasa Rio Negro Rio Aponcito Quebrada La Luna Quebrada Maraca Quebrada Sta. Rosita Quebrada El Mene La Luna Quarry Quebrada La Ge Cano Cusare Rio Socuy Quebrada Cerrejon

References Adams, J.A., Seaton, D.D., Buchanan, J.B., Longbottom, M.R., 1968. Biological observations associated with the toxic phytoplankton bloom o€ the east coast. Nature 220, 24±25. Aguasuelos Engineers, 1991. ModernizacioÂn de Datos GeoloÂgicos en el Frente de MontanÄa. In: Internal Report, p. 200. Amorosi, A., 1995. Glaucony and sequence stratigraphy: A conceptual framework of distribution in siliciclastic sequences. Journal of Sedimentary Research B65, 419±425. Anderson, D.M., 1994. Red tides. Scienti®c American 271, 62±68. Aquino, R., de Guerra, C., Boujana, Y.M., 1995. Facies costera de una plataforma carbonaÂtica CretaÂcica: el Miembro ``O'' de la Formacion Escandalosa. Sedimentologõ a, diageÂnesis, bioestratigrafõ a y anaÂlisis secuencial en nuÂcleos de la Cuenca de Barinas. Boletõ n de Geologõ a, Special Publication 11, 76±79. Audemard, F. 1991. Tectonics of Western Venezuela. Unpublished PhD thesis, Rice University, Houston, TX USA, 245 p. Baptista, N., Scherer, W., 1996. Geology and geochemistry of the La Luna Formation type sections in the Maracaibo Basin, Venezuela. Abstracts of the Second AAPG/SVG International Congress and Exhibition (Caracas, September 8±11). Bulletin of the American Association of Petroleum Geologists 80, 1271± 1272. Barrio, C.A., Coeld, D.Q., 1992. Late Cretaceous stratigraphy of the Upper Magdalena Basin in the Payande±Chaparral segment (western Girardot Sub-basin), Colombia. Journal of South American Earth Sciences 5, 123±139. Barron, E.J., Fawcett, P.J., Peterson, W.H., Pollard, D., Thompson, S.L., 1995. A ``simulation'' of mid-Cretaceous climate. Paleoceanography 10, 953±962. Bartok, P., Reijers, T.J.A., Juhasz, I., 1981. Lower Cretaceous Cogollo Group, Maracaibo Basin, Venezuela: Sedimentology, diagenesis, and petrophysics. American Association of Petroleum Geologists Bulletin 65, 1110±1134. Bejarano, C., Zorrilla, O., 1994. Paleoenvironmental signi®cance of the Cretaceous phosphatic facies in the Barinas-Apure Basin: A contribution to the stratigraphy of the area. In: Proceedings of the 5th Simposio Bolivariano: ExploracioÂn Petrolera en las Cuencas Subandinas, Puerto La Cruz, Venezuela (March 13±16), pp. 213±215. Bernhard, J.M., 1986. Characteristic assemblages and morphologies of benthic foraminifera from anoxic organic carbon-rich deposits: Jurassic through Holocene. Journal of Foraminiferal Research 16, 207±215. Blaser, R., White, C., 1984. Source-rock and carbonization study,

41

X X X X X X X X

X X X X X X X X

X X X X X X X X

X X X X X

X X X X X

X X X X X

X

X

X

Maracaibo Basin, Venezuela. In: Demaison, G.J., Murris, R.J. (Eds.), Petroleum Geochemistry and Basin Evaluation, American Association of Petroleum Geologists Memoir, 35, pp. 229±252. Bockmeulen, H., Barker, C., Dickey, P.A., 1983. Geology and geochemistry of crude oils, Bolivar Coastal Fields, Venezuela. American Association of Petroleum Geologists Bulletin 67, 242± 270. Boesi, T., Galea, F., Rojas, G., Lorente, M.A., DuraÂn, I., Velasquez, M., 1988. Estudio estratigra®co del Flanco Norandino en el sector Lobatera-El Vigia. In: Proceedings of the 3rd Simposio Bolivariano: ExploracioÂn Petrolera de las Cuencas Subandinas, Caracas, Venezuela (March 13±16), pp. 2±41. Bromley, R.G., 1967. Marine phosphorites as depth indicators. Marine Geology 5, 503±509. BuÈrgl, H., 1961. SedimentacioÂn cõ clica en el geosinclinal CretaÂceo de la Cordillera Oriental de Colombia. Boletõ n GeoloÂgico 7, 85±118. BuÈrgl, H., Botero, G.D., 1967. Las capas fosfaÂticas de la Cordillera Oriental. Boletõ n GeoloÂgico 15, 7±44. BuÈrgl, H., Dumit ToboÂn, Y., 1954. El CretaÂceo Superior en la region de Girardot. Boletõ n GeoloÂgico 2, 23±48. Bush, A.B.G., Philander, S.G.H., 1997. The late Cretaceous: Simulation with a coupled atmosphere-ocean general circulation model. Paleoceanography 12, 495±516. Canache, M., Pilloud, A., Truskowski, I., Crux, J.A., Gamarra, S., 1994. Stratigraphic revision of the Cretaceous section of the Maraca River, Sierra De PerijaÂ, Venezuela. In: Proceedings of the 5th Simposio Bolivariano: ExploracioÂn Petrolera en las Cuencas Subandinas, Puerto La Cruz, Venezuela (March 13±16), pp. 240± 241. Cardenas, H., 1985. Areniscas fosfaÂticas de la Formacion Navay, TaÂchira suroriental. In: Proceedings of the 6th Congreso GeoloÂgico Venezolano, Caracas, Venezuela (September 29± October 6), pp. 3854±3891. Carson, G.A., Crowley, S.F., 1993. The glauconite±phosphate association in hardgrounds: examples from the Cenomanian of Devon, southwest England. Cretaceous Research 14, 69±89. Cathcart, J.B., Zambrano, F., 1967. Roca fosfaÂtica en Colombia. Boletõ n GeoloÂgico 15, 65±162. Chigne, N., 1985. Aspectos relevantes en la exploracioÂn de Apure. In: Proceedings of the 2nd Simposio Bolivariano: ExploracioÂn Petrolera en las Cuencas Subandinas, BogotaÂ, Colombia (August), p. 52. Colletta, B., Roure, F., De Toni, B., Loureiro, D., Passalacqua, H., 1997. Tectonic inheritance, crustal architecture, and contrasting structural styles in the Venezuelan Andes. Tectonics 16, 777±794. Coniglio, M., 1987. Biogenic chert in the Cow Head Group

42

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

(Cambro±Ordovician), western Newfoundland. Sedimentology 34, 813±823. Cooper, M.A., Addison, F.T., Alvarez, R., Hayward, A.B., Howe, S., Pulham, A.J., Taborda, A., 1995. Basin development and tectonic history of the Llanos Basin, Colombia. In: Tankard, A.J., SuaÂrez Soruco, R., Welsink, H.J. (Eds.), Petroleum Basins of South America, American Association of Petroleum Geologists Memoir, 62, pp. 659±665. Culotta, E., 1996. Red menace in the world's oceans. In: Pirie, R.G. (Ed.), Oceanography: Contemporary Readings in the Ocean Sciences, Third ed. Oxford University Press, New York, NY USA, pp. 269±271. De Monroy, Z., Van Erve, A., 1988. Revision Palinoestratigra®ca del CretaÂcico y Terciario de Apure (Venezuela suroccidental). In: Proceedings of the 3rd Simposio Bolivariano: ExploracioÂn Petrolera de las Cuencas Subandinas, Caracas, Venezuela (March 13±16), pp. 143±167. De Romero, L.M., Galea-Alvarez, F., 1995. Campanian Bolivinoides and microfacies from the La Luna Formation, western Venezuela. Marine Micropaleontology 26, 385±404. Dino, R., Pocknall, D.T., Dettmann, M.E., 1998. Elater-bearing palynomorphs from the late Albian to Cenomanian of Brazil and Ecuador: Morphology, structure, and implications for anity. Revue of Paleobotony and Palynology 105, 201±235. Ekdale, A.A., Bromley, R.G., Pemberton, S.G., 1984. Ichnology: The Use of Trace Fossils in Sedimentology and Stratigraphy. Society of Economic Paleontologists and Mineralogists Short Course 15, 3±17. Erbacher, J., Thurow, J., Littke, R., 1996. Evolution patterns of radiolaria and organic matter variations: A new approach to identify sea-level changes in mid-Cretaceous pelagic environments. Geology 24, 499±502. Erlich, R.N., Astorga, A., Sofer, Z., Pratt, L.M., Palmer, S.E., 1996. Palaeoceanography of organic carbon-rich rocks of the Loma Chumico Formation of Costa Rica, Late Cretaceous, eastern Paci®c. Sedimentology 43, 691±718. Erlich, R.N., Macsotay I., O., Nederbragt, A.J., Lorente, M.A., 1999a. Palaeoceanography, palaeoecology, and depositional environments of Upper Cretaceous rocks of western Venezuela. Palaeogeography, Palaeoclimatology, Palaeoecology 153. 203± 238. Erlich, R.N., Palmer-Koleman, S.E., Antonieta Lorente, M., 1999b. Geochemical characterization of oceanographic and climatic changes recorded in upper Albian to lower Maastrichtian strata, western Venezuela. Cretaceous Research 20, 547±581. EscandoÂn, M., Murat, B., Gambino, F., PeÂrez, P., 1993. Geochemical study of Cretaceous source rocks from the Perij front, Venezuela. In: Mello, M.R., Trindale, L.A.F. (Eds.), Third Latin American Congress on Organic Geochemistry, Latin American Assoc. of Organic Geochemistry, pp. 45±47. Escobar, M., Pasquali, J., Tocco, R., 1989. Estudio geoquõ mico de bitumenes de la Formacion La Luna, en el pozo 33f-1x, Campo Alpuf, Estado Zulia: Evidencias de migracion primaria. In: Proceedings of the 7th Congreso GeoloÂgico Venezolano, Barquisimeto, Venezuela (November 12±18), pp. 1498±1519. Feo-Codecido, G., Smith Jr, F.D., Aboud, N., de Di Giacomo, E., 1984. Basement and Paleozoic rocks of the Venezuelan Llanos Basin. In: Bonini, W.E., Hargraves, R.B., Shagam, R. (Eds.), The Caribbean-South American Plate Boundary and Regional Tectonics, Geological Society of America Memoir, 162, pp. 175± 187. Filippelli, G.M., 1997. Controls on phosphorous concentration and accumulation in oceanic sediments. Marine Geology 139, 231± 240. FoÈllmi, K.B., 1996. The phosphorus cycle, phosphogenesis and marine phosphate-rich deposits. Earth-Science Revues 40, 55±124. FoÈllmi, K.B., 1990. Condensation and phosphogenesis: Example of

the Helvetic mid-Cretaceous (northern Tethyan margin). In: Notholt, A.J.G., Jarvis, I. (Eds.), Phosphorite Research and Development, Geological Society of London Special Publication, 52, pp. 237±252. FoÈllmi, K.B., Garrison, R.E., Ramirez, P.C., Zambrano-Ortiz, F., Kennedy, W.J., Lehner, B.L., 1992. Cyclic phosphate-rich successions in the Upper Cretaceous of Colombia. Palaeogeography, Palaeoclimatology, Palaeoecology 93, 151±182. FoÈllmi, K.B., Weissert, H., Bisping, M., Funk, H., 1994. Phosphogenesis, carbon-isotope stratigraphy, and carbonate platform evolution along the Lower Cretaceous northern Tethyan margin. Geological Society of America Bulletin 106, 729±746. Ford, A., Houbolt, J.J.H.C., 1963. The Microfacies of the Cretaceous of Western Venezuela. In: International Sedimentary Petrographical Series 6. Brill, Leiden, The Netherlands, p. 109. Frey, R.W., Pemberton, S.G., Saunders, T.D.A., 1990. Ichnofacies and bathymetry: A passive relationship. Journal of Paleontology 64, 155±158. Fuenmayor, A.N., 1989. Manual de Foraminiferos de la Cuenca Maracaibo. In: Maraven Internal Report, p. 191 Maracaibo. Gaenslen, G., 1962. A discussion of the Cretaceous stratigraphy of the southwest Barinas mountain front. BoletõÂ n Informativo 5, 65± 74. Galea-Alvarez, F.A., Morales, M., CallejoÂn, G.A., 1994. GeologõÂ a y GeoquõÂ mica del occidente de Apure zona fronteriza ColomboVenezolana: Resultados preliminares. In: Proceedings of the 7th Congreso Venezolano de GeofõÂ sica, Caracas, Venezuela (September 4±8), pp. 299±306. Gamarra, S., Boujana, M., Pilloud, A., Suarez, C., Truskowski, I., 1994. Nuevos aportes sobre la estratigrafõÂ a del CretaÂcico el Flanco Surandino, Estado MeÂrida, Venezuela. In: Proceedings of the 5th Simposio Bolivariano: ExploracioÂn Petrolera en las Cuencas Subandinas, Puerto La Cruz, Venezuela (March 13±16), pp. 171±173. Ganz, H.H., Luger, P., Schrank, E., Brooks, P., Fowler, M.G., 1990. Facies evolution of Late Cretaceous black shales form southeast Egypt. In: Huc, Y.-Y. (Ed.), Deposition of Organic Facies, American Association of Petroleum Geologists Studies in Geology, 30, pp. 217±229. Garcia Jarpa, R., Ghosh, S., Rondon, F., Fierro, I., Sampol, M., Benedetto, G., Medina, C., Odreman, O., Sanchez, T., Useche, A., 1980. CorrelacioÂn estratigrõÂ ®ca y sõÂ ntesis paleoambiental del CretaÂceo de los Andes Venezolanos. BoletõÂ n de GeologõÂ a 26, 3± 88. Ghosh, S.K., 1984. Late Cretaceous condensed sequence, Venezuelan Andes. In: Bonini, W.E., Hargraves, R.B., Shagam, R. (Eds.), The Caribbean-South American Plate Boundary and Regional Tectonics, Geological Society of America Memoir, 162, pp. 317± 324. Giegengack, R., 1984. Late Cenozoic tectonic environments of the Central Venezuelan Andes. In: Bonini, W.E., Hargraves, R.B., Shagam, R. (Eds.), The Caribbean-South American Plate Boundary and Regional Tectonics, Geological Society of America Memoir, 162, pp. 343±364. Glenn, C.R., Arthur, M.A., 1990. Anatomy and origin of a Cretaceous phosphorite±greensand giant, Egypt. Sedimentology 37, 123±154. Gonzalez de Juana, C., Arozena, J.A., Picard Cadillat, X., 1980. GeologõÂ a de Venezuela y sus cuencas petrolõÂ feras. Ediciones Fonives, Caracas, p. 1031. Gradstein, F.M., Agterberg, F.P., Ogg, J.G., Hardenbol, J., Van Veen, P., Thierry, J., Huang, Z., 1995. A Triassic, Jurassic, and Cretaceous time scale. In: Berggren, W.A., Kent, D.V., Aubry, M.-P., Hardenbol, J. (Eds.), Geochronology Time Scales and Global Stratigraphic Correlation, Society of Economic Paleontologists and Mineralogists Special Publication, 54, pp. 95± 106.

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45 Guerrero, J., Sarmiento, G., 1996. Estratigrafõ a fõ sica, palinoloÂgica, sedimentoloÂgica y secuencial del CretaÂcico Superior y Paleoceno del Piedemonte Llanero. Implicaciones en exploracioÂn petrolera. Geologõ a Colombiana 20, 3±66. Guthrie, J.M., Pratt, L.M., 1994. Geochemical indicators of depositional environment and source-rock potential for the Upper Ordovician Maquoketa Group, Illinois Basin. American Association of Petroleum Geologists Bulletin 78, 744±757. Hedberg, H.D., 1931. Cretaceous limestone as petroleum source rock in northwestern Venezuela. American Association of Petroleum Geologists Bulletin 15, 229±246. Hedberg, H.D., Sass, L.C., 1937. Sinopsis de las formaciones geoloÂgicas de la parte occidental de la Cuenca de Maracaibo, Venezuela. Boletõ n de Geologõ a y Mineralogõ a 1, 77±120. Heggie, D.T., Skyring, G.W., O'Brien, G.W., Reimers, C., Herczeg, A., Moriarity, D.J.W., Burnett, W.C., Milnes, A.R., 1990. Organic carbon cycling and modern phosphate formation on the East Australian continental margin: An overview. In: Notholt, A.J.G., Jarvis, I. (Eds.), Phosphorite Research and Development, Geological Society of London Special Publication, 52, pp. 87± 117. Helenes, J., De Guerra, C., VaÂsquez, J., 1998. Palynology and chronostratigraphy of the Upper Cretaceous in the subsurface of the Barinas area, western Venezuela. American Association of Petroleum Geologists Bulletin 82, 1308±1328. Helenes, J., De Guerra, C., VaÂsquez, J., 1994. Estratigrafõ a por secuencias del CretaÂcico Superior en el subsuelo del area de Barinas. In: Proceedings of the 5th Simposio Bolivariano: ExploracioÂn Petrolera en las Cuencas Subandinas, Puerto La Cruz, Venezuela (March 13±16), pp. 29±39. Herngreen, G.F.W., Jimenez, H.D., 1990. Dating of the Cretaceous Une Formation, Colombia and the relationship with the Albian± Cenomanian African-South American micro¯oral province. Revue of Palaeobiology and Palynology 66, 345±359. James, K., 1990. The Venezuelan hydrocarbon habitat. In: Brooks, J. (Ed.), Classic Petroleum Provinces, Geological Society of London Special Publication, 50, pp. 9±35. Johnson, C.C., 1999. Evolution of Cretaceous surface current circulation patterns, Caribbean and Gulf of Mexico. In: Barrera, E., Johnson, C.C. (Eds.), Evolution of the Cretaceous Ocean-Climate System, Geological Society of America Special Paper, 332, pp. 329±343. Kellogg, J.N., 1984. Cenozoic tectonic history of the Sierra de PerijaÂ, Venezuela-Colombia, and adjacent basins. In: Bonini, W.E., Hargraves, R.B., Shagam, R. (Eds.), The Caribbean-South American Plate Boundary and Regional Tectonics, Geological Society of America Memoir, 162, pp. 239±261. Kiser, G.D., 1961. Review of the Cretaceous stratigraphy of the southwest Barinas mountain front. Boletõ n Informativo 4, 335± 359. Kiser, G.D., 1989. Relaciones Estratigra®cas de la Cuenca Apure/ Llanos con Areas Adyacentes Venezuela Suroeste y Colombia Oriental. Sociedad Venezolana de GeoÂlogos Monograph 1, 77. Klemme, H.D., Ulmishek, G.F., 1991. E€ective petroleum source rocks of the world Ð Stratigraphic distribution and controlling depositional factors. American Association of Petroleum Geologists Bulletin 75, 1809±1851. Krause, H.H., James, K.H., 1989. Hydrocarbon resources of Venezuela Ð Their source rocks and structural habitat. In: Ericksen, M.T., CanÄas, Pinochet, Reinernund, J.A. (Eds.), Geology of the Andes and its Relation to Hydrocarbon and Mineral Resources, Circum±Paci®c Council for Energy and Mineral Resources, Earth Science Series, 11, pp. 405±414. Kruijs, E., Barron, E., 1990. Climate model prediction of paleoproductivity and potential source-rock distribution. In: Huc, A.-Y. (Ed.), Deposition of Organic Facies, American Association of Petroleum Geologists Studies in Geology, 30, pp. 195±216.

43

Kupecz, J.A., 1995. Depositional setting, sequence stratigraphy, diagenesis, and reservoir potential of a mixed-lithology, upwelling deposit: Upper Triassic Shublik Formation, Prudhoe Bay, Alaska. American Association of Petroleum Geologists Bulletin 79, 1301±1319. Leckie, D.A., Singh, C., Goodarzi, F., Wall, J.H., 1990. Organic carbon-rich, radioactive marine shale: A case study of a shallowwater condensed section, Cretaceous Shaftsbury Formation, Alberta, Canada. Journal of Sedimentary Petrology 60, 101±117. Littke, R., Baker, D.R., Leythaeuser, D., RullkoÈtter, J., 1991. Keys to the depositional history of the Posidonia Shale (Toarcian) in the Hils Syncline, northern Germany. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Shelf Anoxia, Geological Society of London Special Publication, 58, pp. 311±333. Lorente, M.A., Duran, I., 1995. Late Cretaceous in Western Venezuela: A review and new biostratigraphical data [abstract]. In: Abstracts of the IGCP 362 Annual Meeting, Maastricht. Lorente, M.A., Duran, I., Ruiz, M., 1996. Late Cretaceous in western Venezuela Ð A new biostratigraphical approach [abstract]. Abstracts of the Second AAPG/SVG International Congress and Exhibition, Caracas, Venezuela (September 8±11). American Association of Petroleum Geologists Bulletin 80, 1309±1310. Lugo, J., 1999. The MeÂrida Arch: Tectonic control on deposition from Late Mesozoic to Early Cenozoic in Western Venezuela. In: Proceedings of the 5th Simposio Bolivariano: ExploracioÂn Petrolera en las Cuencas Subandinas, Puerto La Cruz, Venezuela (March 13±16), pp. 291±310. Lugo, J., Mann, P., 1995. Jurassic±Eocene tectonic evolution of Maracaibo Basin, Venezuela. In: Tankard, A.J., SuaÂrez Soruco, R., Welsink, H.J. (Eds.), Petroleum Basins of South America, American Association of Petroleum Geologists Memoir, 62, pp. 699±725. Ly, A., Kuhnt, W., 1994. Late Cretaceous benthic foraminiferal assemblages of the Casamance shelf (Senegal, NW Africa) Ð Indication of a Late Cretaceous oxygen minimum zone. Revue of MicropaleÂontologie 37, 49±74. Macellari, C.E., 1984. Late Tertiary tectonic history of the Tachira Depression, southwestern Venezuelan Andes. In: Bonini, W.E., Hargraves, R.B., Shagam, R. (Eds.), The Caribbean-South American Plate Boundary and Regional Tectonics, Geological Society of America Memoir, 162, pp. 333±341. Macellari, C.E., 1988. Cretaceous paleogeography and depositional cycles of western South America. Journal of South American Earth Sciences 1, 373±418. Macellari, C.E., De Vries, T.J., 1987. Late Cretaceous upwelling and anoxic sedimentation in northwestern South America. Palaeogeography, Palaeoclimatology, Palaeoecology 59, 279±292. Macsotay, O., 1975. Ambiente deposicional de la porcion basal de la Formacion Taguarumo de®nido por el icnofoÂsil Skolithos. Boletõ n Informativo 18, 107±133. Macsotay, O. 1980. Mollusques benthiques du CreÂtace Inferieur: Une meÂthode de correÂlation entre la Tethys mesogeÂenne et le domaine paleÂo-Caraibe (VeÂneÂzueÂla). Unpublished PhD thesis, Claude Bernard University, Lyon, France, 186 p. Maliva, R.G., Siever, R., 1989. Nodular chert formation in carbonate rocks. Journal of Geology 97, 421±433. Malone, T.C., 1991. River ¯ow, phytoplankton production and oxygen depletion in Chesapeake Bay. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Shelf Anoxia, Geological Society of London Special Publication, 58, pp. 83±93. Mann, U., Stein, R., 1997. Organic facies variations, source rock potential, and sea level changes in Cretaceous black shales of the Quebrada Ocal, Upper Magdalena Valley, Colombia. American Association of Petroleum Geologists Bulletin 81, 556±576. Martõ nez, R.J.I., 1989. Foraminiferal biostratigraphy and paleoenvironments of the Maastrichtian ColoÂn mudstones of northern South America. Micropaleontology 35, 97±113.

44

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45

Martõ nez, R.J.I., Hernandez, R., 1992. Evolution and drowning of the Late Cretaceous Venezuelan carbonate platform. Journal of South American Earth Sciences 5, 197±210. Maze, W.B., 1984. Jurassic La Quinta Formation in the Sierra de PerijaÂ, northwestern Venezuela: Geology and tectonic environment of red beds and volcanic rocks. In: Bonini, W.E., Hargraves, R.B., Shagam, R. (Eds.), The Caribbean-South American Plate Boundary and Regional Tectonics, Geological Society of America Memoir, 162, pp. 263±282. Mendez, B.J., 1989. La Formacion La Luna. Caracteristica de una cuenca anoxica en una plataforma de aguas someras. In: Proceedings of the 7th Congreso GeoloÂgico Venezolano, Barquisimeto, Venezuela (November 12±18), pp. 852±866. Mikolajewicz, U., Maier-Reimer, E., Crowley, T.J., Kim, K.-Y., 1993. E€ect of Drake and Panamanian gateways on the circulation of an ocean model. Paleoceanography 8, 409±426. Murray, R.W., Leinen, M., Isern, A.R., 1993. Biogenic ¯ux of Al to sediment in the central equatorial Paci®c Ocean: Evidence for increased productivity during glacial periods. Paleoceanography 8, 651±670. Notestein, F.B., Hubman, C.W., Bowler, J.W., 1944. Geology of the Barco concession, Republic of Colombia, South America. Geological Society of America Bulletin 55, 1165±1216. Parnaud, F., Gou, Y., Pascual, J.-C., Capello, M.A., Truskowski, I., Passalaqua, H., 1995. Stratigraphic synthesis of Western Venezuela. In: Tankard, A.J., SuaÂrez Soruco, R., Welsink, H.J. (Eds.), Petroleum Basins of South America, American Association of Petroleum Geologists Memoir, 62, pp. 681±698. Pedersen, T.F., Calvert, S.E., 1990. Anoxia vs. productivity: What controls the formation of organic±carbon±rich sediments and sedimentary rocks? American Association of Petroleum Geologists Bulletin 74, 454±466. Piasecki, S., Stemmerik, L., 1991. Late Permian anoxia in central East Greenland. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Shelf Anoxia, Geological Society of London Special Publication, 58, pp. 275±290. Pindell, J.L., Barrett, S.F., 1990. Geological evolution of the Caribbean region: A plate-tectonic perspective. In: Dengo, G., Case, J.E. (Eds.), The Caribbean Region, The Geology of North America, Geological Society of America H, chapter 16, pp. 405± 432. Porrenga, D.H., 1967. Glauconite and chamosite as depth indicators in the marine environment. Marine Geology 5, 495±501. Prauss, M., Ligouis, B., Luterbacher, H., 1991. Organic matter and palynomorphs in the `Posidonienschiefer' (Toarcian, Lower Jurassic) of southern Germany. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Shelf Anoxia, Geological Society of London Special Publication, 58, pp. 335±351. Rabalais, N.N., Turner, R.E., Wiseman Jr, W.J., Boesch, D.F., 1991. A brief summary of hypoxia on the northern Gulf of Mexico continental shelf: 1985±1988. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Shelf Anoxia, Geological Society of London Special Publication, 58, pp. 35±47. Renz, H.H., 1961. Correlation of geologic formations in Venezuela. Boletõ n Informativo 4, 199±203. Renz, O., 1959. Estratigrafõ a del CretaÂceo en Venezuela occidental. Boletõ n de Geologõ a 5, 3±48. Renz, O., 1982. The Cretaceous Ammonites of Venezuela. BirkhaÈuser Verlag, Basel, Switzerland, p. 132. Rod, E., Maync, W., 1954. Revision of Lower Cretaceous stratigraphy of Venezuela. American Association of Petroleum Geologists Bulletin 38, 193±283. Rossignol-Strick, M., 1987a. Rainy periods and bottom water stagnation initiating brine accumulation and metal concentrations: 1. The Late Quaternary. Paleoceanography 2, 333±360. Rossignol-Strick, M., 1987b. Rainy periods and bottom water stagnation initiating brine accumulation and metal concentrations: 2.

Precambrian gold-uranium ore beds and banded iron formations. Paleoceanography 2, 379±394. SaÂnchez, T.U., Lorente, M.A., 1977. PaleoecologõÂ a del Miembro Quevedo (Formacion Navay) en las proximidades de Santa Barbara. In: Proceedings of the 5th Venezuelan Geological Congress, Caracas, Venezuela, vol. 1, pp. 107±133. SaÂnchez NunÄez, M.A., Audemard, F., Giraldo, C., Ruiz, F., 1994. InterpretacioÂn sõÂ smica y gravimeÂtrica de un per®l a traveÂs de los Andes Venezolanos. In: Proceedings of the 7th Congreso Venezolano de GeofõÂ sica, Recursos y Oportunidades, Caracas, Venezuela (September 4±8), pp. 251±258. Savrda, C.E., Bottjer, D.J., 1991. Oxygen-related biofacies in marine strata: An overview and update. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Shelf Anoxia, Geological Society of London Special Publication, 58, pp. 201±220. Sellier De Civrieux, J.M., 1952. Estudio de la microfauna de la seccioÂn tipo del Miembro Socuy de la Formacion ColoÂn, Distrito Mara, Estado Zulia. BoletõÂ n de GeologõÂ a 2, 231±330. Sen Gupta, B., Turner, R.E., Rabalais, N.N., 1996. Seasonal oxygen depletion in continental-shelf waters of Louisiana: Historical record of benthic foraminifers. Geology 24, 227±230. Shagam, R., Kohn, B.P., Banks, P.O., Dasch, L.E., Vargas, R., Rodriguez, G.I., Pimental, N., 1984. Tectonic implications of Cretaceous-Pliocene ®ssion-track ages from rocks of the circum± Maracaibo Basin region of western Venezuela and eastern Colombia. In: Bonini, W.E., Hargraves, R.B., Shagam, R. (Eds.), The Caribbean-South American Plate Boundary and Regional Tectonics, Geological Society of America Memoir, 162, pp. 385± 412. Slingerland, R., Kump, L., Arthur, M., Fawcett, P., Sageman, B., Barron, E., 1996. Estuarine circulation in the Turonian western Interior Seaway of North America. Geological Society of America Bulletin 108, 941±952. Spero, H.J., 1990. Evidence for seasonal low-salinity surface waters in the Gulf of Mexico over the last 16,000 years. Paleoceanography 5, 963±975. Stainforth, R.M., 1962. De®nition of some new stratigraphic units in Western Venezuela: Las Pilas, Cocuiza, Vergel, El Jebe, Tres Esquinas and Nazaret. BoletõÂ n Informativo 5, 279±282. Stephan, J.-F., 1985. Andes et Chaine Caribe sur la transversale de Barquisimeto (Venezuela): Evolution geÂodynamique. In: Mascle, A. (Ed.), Geodynamique des Caribes. Editions Technip, Paris, France, pp. 505±529. Sutton, F.A., 1946. Geology of Maracaibo Basin, Venezuela. American Association of Petroleum Geologists Bulletin 30, 1621± 1741. Sweeney, J.J., Braun, R.L., Burnham, A.K., Talukdar, S., Vallejos, C., 1995. Chemical kinetic model of hydrocarbon generation, expulsion, and destruction applied to the Maracaibo Basin, Venezuela. American Association of Petroleum Geologists Bulletin 79, 1515±1532. Talukdar, S., De Toni, B., 1988. Geochemical modeling in the unexplored Guarumen Sub-basin, Venezuela. In: Proceedings of the 3rd Simposio Bolivariano: ExploracioÂn Petrolera de las Cuencas Subandinas, Caracas, Venezuela (March 13±16), pp. 756±782. Talukdar, S., Gallango, O., Chin-A-Lien, M., 1986. Generation and migration of hydrocarbons in the Maracaibo Basin, Venezuela: An integrated basin study. Organic Geochemistry 10, 261±279. Talukdar, S.C., Marcano, F., 1994. Petroleum systems of the Maracaibo Basin, Venezuela. In: Magoon, L.B., Dow, W.G. (Eds.), The Petroleum System Ð From Source To Trap, American Association of Petroleum Geologists Memoir, 60, pp. 463±481. Tribovillard, N.-P., Stephan, J.-F., 1989. Surmaturation des black shales CreÂtaceÂs des Andes de MeÂrida (VeÂneÂzueÂla): RoÃles respectifs de l'enfouissement seÂdimentaire et des charriages d'aÃge eÂoceÁne. C. R. Acadamie Science, Paris 309, 2085±2091.

R.N. Erlich et al. / Journal of South American Earth Sciences 13 (2000) 21±45 Tribovillard, N.-P., Stephan, J.-F., Manivit, H., Reyre, Y., Cotillon, P., JauteÂe, E., 1991. Cretaceous black shales of Venezuelan Andes: Preliminary results on stratigraphy and paleoenvironmental interpretations. Palaeogeography, Palaeoclimatology, Palaeoecology 81, 313±321. Trump, G.W., Salvador, A., 1964. Guidebook to the Geology of Western TaÂchira, AsociacioÂn Venezolano GeologõÂ a, MineralogõÂ a y PetroÂleo 25. Truskowski, I., Galea-Alvarez, F., Sliter, W., 1995. Cenomanian hiatus in Venezuela [abstract], Abstracts of the Geological Society of America Annual Meeting, New Orleans, Louisiana, USA (November 6±9) A±303. Truskowski, I., Galea-Alvarez, F., Sliter, W., 1996. Late Cretaceous biostratigraphy of the La Luna Formation, Maracaibo Basin [abstract]. Abstracts of the Second AAPG/SVG International Congress and Exhibition, Caracas, Venezuela (September 8±11). American Association of Petroleum Geologists Bulletin 80, 1341. Tyson, R.V., Pearson, T.H., 1991. Modern and ancient shelf anoxia: An overview. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Shelf Anoxia, Geological Society of London Special Publication, 58, pp. 1±24. Van Cappellen, P., Ingall, E.D., 1994. Benthic phosphorous regeneration, net primary production, and ocean anoxia: A model of the coupled marine biogeochemical cycles of carbon and phosphorous. Paleoceanography 9, 677±692. Van Der Zwaan, G.J., Jorissen, F.J., 1991. Biofacial patterns in river-induced shelf anoxia. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Shelf Anoxia, Geological Society of London Special Publication, 58, pp. 65±82. Van Marle, L.J., Van Hinte, J.E., Nederbragt, A.J., 1987. Plankton percentage of the foraminiferal fauna in sea¯oor samples from the Australian±Irian Jaya continental margin, eastern Indonesia. Marine Geology 77, 151±156. Vergara, S.L. 1994. Stratigraphic, Micropaleontologic and Organic Geochemical Relations in the Cretaceous of the Upper Magdalena Valley, Colombia. Unpublished PhD Thesis, University of Giessen, Germany, 179 p. Vergara, S.L., 1997a. Stratigraphy, foraminiferal assemblages and paleoenvironments in the Late Cretaceous of the Upper Magdalena Valley, Colombia (Part I). Journal of South American Earth Sciences 10, 111±132. Vergara, S.L., 1997b. Cretaceous black shales in the Upper

45

Magdalena Valley, Colombia. New organic geochemical results (Part II). Journal of South American Earth Sciences 10, 133±145. Vierma, L., 1985. Caracterizacion geoquõ mica de kerogeno en rocas carbonaticas de la FormacioÂn Luna. In: Proceedings of the 6th Congreso GeoloÂgico Venezolano, Caracas, Venezuela (September 29±October 6), pp. 2248±2262. Villamil, T., 1996. Depositional and geochemical cyclicity in the Cretaceous ®ne-grained strata of Colombia. A model for organic matter content. Ciencia, Tecnologõ a y Futuro 1, 5±23. Villamil, T., 1998. Chronology, relative sea-level history and a new sequence stratigraphic model for basinal Cretaceous facies of Colombia. In: Pindell, J.L., Drake, C. (Eds.), Paleogeographic Evolution and Non-Glacial Eustacy, Northern South America, Society of Economic Paleontologists and Mineralogists Special Publication, 58, pp. 161±216. Villamil, T., 1999. Campanian±Miocene tectonostratigraphy, depocenter evolution and basin development of Colombia and western Venezuela. Palaeogeography, Palaeoclimatology, Palaeoecology, 153, pp. 239±275. Villamil, T., Arango, C., 1998. Integrated stratigraphy of latest Cenomanian and early Turonian facies of Colombia. In: Pindell, J.L., Drake, C. (Eds.), Paleogeographic Evolution and NonGlacial Eustacy, Northern South America, Society of Economic Paleontologists and Mineralogists Special Publication, 58, pp. 129±159. Ward, D.E., Goldsmith, R., Cruz, B.J., Restrepo, A.H., 1973. Geologõ a de los Cuadrangulos H-12 Bucaramanga y H-13 Pamplona, Departamento De Santander, Boletõ n GeoloÂgico 132. Wignall, P.B., 1991. Model for transgressive black shales? Geology 19, 167±170. Wright, R.G., 1977. Planktonic±benthonic ratio in foraminifera as paleobathymetric tool. Quantitative evaluation [abstract]. In: Proceedings of the Society of Economic Paleontologists and Mineralogists Annual Meeting, Washington, p. 65. Yurewicz, D.A., Advocate, D.M., Lo, H.B., HernaÂndez, E.A., 1998. Source rocks and oil families southwest Maracaibo Basin (Catatumbo Subbasin), Colombia. American Association of Petroleum Geologists Bulletin 82, 1329±1352. Zambrano, E., Vasquez, E., Duval, B., Latreille, M., Conieres, B., 1971. Sõ ntesis paleogeogra®ca y petrolera del occidente de Venezuela. In: Proceedings of the 4th Congreso GeoloÂgico Venezolano, Special Publication, 5, pp. 483±552.