RESEARCH REPORTS 305

Copyright Q 2003, SEPM (Society for Sedimentary Geology) 0883-1351/03/0018-0305/$3.00

The Paleoecology of Late Cretaceous Upwelling Eventsfrom the Upper Magdalena Basin, Colombia

J. IGNACIO MARTINEZUniversidad EAFIT, Dept. Geologa, Grupo de Ciencias del Mar, A.A. 3300 Medelln, Colombia,

Email: jimartin@eafit.edu.co

PALAIOS, 2003, V. 18, p. 305320

Ten planktonic foraminiferal zones are identified and sixstratigraphic sequences are interpreted in the TuronianMaastrichtian interval of the Michu-1 well in the UpperMagdalena Basin. Anoxic bottom-water conditions pre-vailed during the Turonian and Coniacian followed by dys-oxic conditions in the Santonian and Campanian. Oxicconditions were established during the Maastrichtian age.Upwelling occurred for most of the Late Cretaceous. Cocco-lithophorids, planktonic foraminifera, radiolaria, ammo-nites, clupeoid fish, and mosasaurs dominated the food-webstructure of the TuronianConiacian interval. Following amajor turnover during the ConiacianSantonian boundaryinterval, dinoflagellates were the primary producers thatsustained radiolarians, a sparse population of planktonicforaminifera, clupeoid fish, and ammonites. Dinoflagellateblooms (peridinoids) were frequent during the Campanianinterval and sustained a sparse population of planktonic fo-raminifera and abundant clupeoid fish whose feces (phos-phatic pellets) were grazed extensively by a specialized pop-ulation of buliminids dominated by the genus Siphogener-inoides.

INTRODUCTION

Sedimentation of Upper Cretaceous organic carbon(Corg)-rich facies in northern South America occurred intwo different oceanographic settings: a carbonate platformin the Maracaibo region of Venezuela where Corg-rich unitscorrespond to oceanic anoxic events (OAEs), and a troughin the Magdalena Basin and the Eastern Cordillera of Co-lombia where upwelling persisted through the Late Cre-taceous (Macellari and deVries, 1987). Sedimentation ofCorg-rich shales on the Venezuelan carbonate platform hasbeen investigated by a number of researchers, includingFord and Houbolt (1963), Ghosh (1984), Tribovillard et al.(1990), Martnez and Hernandez (1992), and Erlich et al.(1999). Sedimentation of Corg -rich shales in the trough hasbeen studied by Burgl (1961), Follmi et al. (1992), Vergara(1994), Villamil (1996), Martnez and Vergara (1999),Guerrero et al. (2001), and Tchegliakova and Mojica(2001), among others. Increased sea-surface productivitywas first proposed by Hedberg (1931) to account for the ac-cumulation of Corg in the TuronianCampanian La LunaFormation. Cathcart and Zambrano (1967) proposed theKazakov (1937) upwelling model to explain the enrich-ment of phosphates in the Senonian interval in the troughregion. The thick accumulations of Corg-rich shales in Cen-tral Colombia are also likely products of upwelling (e.g.,

Macellari and deVries, 1987; Villamil, 1996; Mann andStein, 1997; Vergara, 1997a). Cyclicity in the AlbianSan-tonian interval of the Upper Magdalena Basin corre-sponds to variations in upwelling and sediment supply(Villamil, 1996). Cycle frequencies have been correlated toorbital rhythms with considerable uncertainty arisingfrom biostratigraphy (Villamil, 1996).

Previous attempts to understand the deposition of Corg-rich shales in the Upper Magdalena Basin have not inter-preted them in a regional paleogeographic or global paleo-ceanographic context. The paleontology of these depositsalso has not been interpreted in terms of food-chain struc-ture and its relationship to Corg accumulation. Therefore,the objectives of this investigation were to: (1) describe themicropaleontology of the Michu-1 well; (2) compare thedifferent pelagic/hemipelagic intervals (or condensed sec-tions); and (3) determine the food-chain structures, mech-anisms of organic-matter deposition and preservation,and their implications for the formation of petroleumsource rocks.

The Michu-1 well is located close to the western marginof the Upper Magdalena Basin and represents offshore,pelagic to hemipelagic deposition during the Late Creta-ceous. Sediments from the well thus contain a clear recordof paleoceanographic events. The Michu-1 well also pro-vides precise biochronology for Upper Cretaceous se-quences in the basin (cf., Loutit et al., 1988), building ondetailed studies by Villamil (1996) for the CenomanianTuronian boundary interval. This paper integrates se-quence stratigraphic, biostratigraphic, and paleooceano-graphic interpretations to construct the temporal and spa-tial setting for deposition of Corg-rich units in the UpperMagdalena Basin during the Late Cretaceous.

Geologic Setting

The Upper Magdalena Basin is an intermontane basinlocated in the upper reaches of the Magdalena River be-tween the Central and Eastern Cordillera of Colombia.The Basin has been divided into the northern, Girardotsubbasin and the southern, Neiva subbasin (Fig. 1), sepa-rated by the Natagaima high. The Basin is filled with Me-sozoic and Cenozoic rocks that include: (1) Triassic and Ju-rassic sedimentary and volcanic sequences deposited in anarrow rift valley, (2) Cretaceous marine to coastal sedi-ments deposited in back-arc and/or marginal-sag basins,and (3) Cenozoic terrestrial sediments deposited in a col-lision-related foreland basin.

The beginning of marine Cretaceous sedimentation inthe Upper Magdalena Basin has been dated as late Aptianto early Albian. Maximum flooding surfaces have been

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FIGURE 1Location map of the Upper Magdalena Basin. Note thelocation of the Michu-1 well in the Girardot subbasin.

recognized in the middle Albian, lower Turonian, and San-tonian (Etayo et al., 1976; Etayo, 1994; Vergara, 1994). Pa-leogeographic reconstructions suggest that by the late Al-bian the proto-Caribbean Plate was being subducted west-ward under the Caribbean Plate located west of the UpperMagdalena Basin (Pindell, 1993; Acton et al., 2000). Bythis time, sedimentation in northern South America oc-curred on the passive, southern margin of the proto-Carib-bean Seaway. The eastward motion of the Caribbean Platecontinued throughout the Late Cretaceous, culminating incollision along the Romeral Suture during the Campani-an. By this time, the Central Cordillera of Colombia wasuplifted, and the Cretaceous Colombian seaway was near-ly closed (Pindell, 1993).

The geographic distribution and stratigraphic thick-nesses of clastic sediments suggest a restricted connectionbetween the Colombian seaway and the open PacificOcean during much of the Cretaceous (Etayo et al., 1976;Fabre, 1985; Diaz, 1994). A sill at the mouth of the seawayaffected circulation and contributed to the accumulation ofCorg-rich shales (Fabre, 1985). The Corg-rich CenomanianCampanian interval has been correlated to the La LunaFormation of northern Colombia and western Venezuela,and to the Villeta Group of central Colombia (Perez, 1995;for a discussion on stratigraphic nomenclature in the ba-sin see Patarroyo, 1993; Vergara et al., 1995; and Vergara,1997b). Stratigraphic thickness for the so-called La Lunalithosome (Perez, 1995) reaches a maximum of ;21009(;630 m) along the northeast-southwest depocenter in the

eastern part of the Girardot subbasin; the lithosome isthinner towards the eastern margins of the Central andEastern Cordilleras as a result of erosional truncation(Perez, 1995).

Cretaceous Corg -rich shales of northern South Americahave long been recognized as excellent petroleum sourcerocks (e.g., Hedberg, 1931). The generation capacity of theLa Luna lithosome in Colombia recently has been estimat-ed to be ;25.5 billion equivalent barrels of oil (BEBO;Perez, 1995). The Upper Magdalena Basin currently pro-duces more than 60,000 barrels of petroleum per day(BOPD), derived from Upper Cretaceous source rocks(Vergara, 1997a).

MATERIAL AND METHODS

The Michu-1 well is located west of Valle de San Juanvillage (Fig. 1). The well was selected for micropaleonto-logical analyses because of its location close to the center ofthe Upper Magdalena Basin, its stratigraphic complete-ness, and the apparent lack of tectonic disturbance.

Cuttings from the uppermost 35009 (;1050 m) were col-lected every 309 (;9.1 m). Fifteen to twenty grams of sam-ple were broken down to fragments ;0.5 cm in diameter.Samples were heated, varsol and EXTRAN detergentwere added, then washed through 63 mm and 150 mmsieves. About 300 specimens of benthonic and planktonicforaminifera were identified and counted in the .150 mmfraction, and the number of specimens per gram of sedi-ment was calculated. The number of dinoflagellate specieswas obtained after processing samples using standard pal-ynological procedures.

Micropaleontological analyses, gamma ray logs (GR),and the content of authigenic minerals were used to builda sequence stratigraphic model. These data were com-pared with published palynological and micropaleontolog-ical information.

RESULTS

Generalized sedimentation patterns based on the litho-logic logs of a number of exploratory wells are illustratedin Figure 2 for the CenomanianTuronian and Santoni-anCampanian intervals. In both intervals, carbonate(hemipelagic) units follow a northeast-southwest trend,whereas shale sedimentation predominates in the north-west and eastern regions. This sedimentation pattern sup-ports the silled-basin model (sensu Fabre, 1985), assumingthat clastic material was derived from the ancestral Cen-tral Cordillera in the west and the Guyana Shield in theeast.

Michu-1 Well

Lithologic Succession: The Upper Cretaceous of the Up-per Magdalena Basin is composed of: (1) the Hondita andLomagorda Formations (also called the Villeta Formation;Patarroyo, 1993; Vergara et al., 1995); and (2) the OliniGroup, which consists of the Lidita Inferior Formation, theNivel de Lutitas Formation, the Lidita Superior Forma-tion, and the Lutitas and Arenas Formations. Overlyingthese units is the La Tabla Formation. The uppermost33009 (1005.8 m) of the Michu-1 well are represented by

PALEOECOLOGY OF LATE CRETACEOUS UPWELLING EVENTS 307

FIGURE 2Upper Magdalena Basin lithofacies maps for the CenomanianTuronian and the SantonianCampanian. Exploratory wells areindicated by stars; lms5limestone, ss5sandstone, sh5shale. Note that the pelagic limestone facies is distributed along a northeastsouthwesttrend. The location of the Michu-1 well also is indicated.

the Hondita and Lomagorda Formations, and the OliniGroup (Fig. 3). The boundary between the first two units isdefined by the lowermost occurrence of cherts (Vergara,1994). This definition is not applicable using ditch-cuttingsamples; consequently, the two formations are not sepa-rated in this investigation.

In the Michu-1 well, the Hondita and Lomagorda For-mations consist of dark-gray to black calcareous mud-stones and hemipelagic foraminiferal limestones; pyrite isa common authigenic mineral, except in the uppermostpart where phosphatic pellets and glauconite are common.Lamination, micritic concretions, fish debris (clupeoidfish), inoceramids, and ammonites are observed in out-crops (Barrio and Coffield, 1992; Vergara, 1997b).

The Lidita Inferior Formation consists of dark-gray to

black cherts, porcellanites, and laminated shales in whichplanktonic foraminifera are abundant. Framboidal pyriteis common, whereas glauconite and phosphatic pellets arefound only at the base of the unit. In outcrops, the unitconsists of laminated brown siliceous shales and chertswith ammonites, fish debris, and inoceramids (Barrio andCoffield, 1992; Vergara, 1997b).

The Nivel de Lutitas Formation consists of dark-gray,laminated calcareous mudstones with abundant forami-nifera and variable amounts of phosphatic pellets, pyritecrystals, and glauconite; sandstone interbeds also are pre-sent. Barrio and Coffield (1992) referred to the sandstoneunit as El Cobre Sandstone, described as a 1520-m thick,fine- to medium-grained, pale-yellow to gray sandstonewith lenticular-, flaser-, and cross-bedding.

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FIGURE 3The Michu-1 well: stratigraphy, lithology, informal foraminiferal zones, selected datums (first and last appearances) of planktonicand benthonic foraminifera, and characteristic species. L.S.5Lidita Superior, L.I.5Lidita Inferior. Characteristic species (all foraminifera species,except for 12, 13, and 21, which are radiolarians): 15S. bramlettei; 25G. sandidgei; 35R. macrocephala; 45H. striata; 55R. rugosa; 65S.bermudezi; 75S. horridens; 85B. explicata; 95S. colonensis; 105R. fornicata; 115P. kickapooensis; 125P. floresenses?; 135Crucella sp.?;145Ammobaculites sp.; 155A. blowi, 165M. sinuosa, 175H. globulosa, 185W. archaeocretacea, 195H. delrioensis, 205W. baltica, 215ra-diolarian.

The Lidita Superior Formation is composed of dark-gray siliceous mudstones, calcareous porcellanites, cherts,and phosphorites; pyrite and phosphatic pellets are alsocommon. Wavy bedding is commonly observed in outcrops(Barrio and Coffield, 1992).

The Lutitas y Arenas Formation is composed of gray cal-careous siltstones and mudstones, and fine-grained cal-careous sandstone. Fish fragments and phosphatic pelletsare abundant at the base of this unit, whereas pyrite andglauconite are present near the top. Symmetrical to slight-ly asymmetrical ripples are observed in the sandstones inoutcrops (Barrio and Coffield, 1992).

La Tabla Formation (or Monserrate Formation of oth-ers) consists of quartz sandstones, siltstones, and a lime-stone breccia at the base of the unit. Barrio and Coffield(1992) described this breccia as a coarse-grained fossilif-erous (bivalve) packstone with a coarse-grained quartzosematrix.

Biostratigraphy: The application of global planktonic fo-raminifera schemes (Caron, 1985) in the Upper Magdale-na Basin is hampered by the paleoecological exclusion ofzonal marker species (i.e., many keeled forms) and the rar-ity of planktonics due to diagenetic alteration (Tcheglia-

kova and Mojica, 2001). Until recently, the only time con-trol available for the Cretaceous of the Upper MagdalenaBasin was the benthonic foraminiferal zonation of Petters(1955). However, this scheme was of limited applicabilitybecause of the rarity of benthonic foraminifera in Turoni-an to Santonian sedimentary rocks. Here, an informalplanktonic-benthonic zonation scheme is presented for theuppermost 32009 (960 m) of the Michu-1 well. This local,informal planktonic-benthonic zonation scheme proposedwill serve as a starting point on which to establish a zona-tion scheme for the Upper Magdalena Basin. This zona-tion scheme compliments recent efforts to summarize themicropaleontology of Cretaceous sedimentary rocks in theUpper Magdalena Basin (Vergara, 1994, 1997b; Tcheglia-kova and Mojica, 2001).

The TuronianMaastrichtian interval is divided intoten zones (Fig. 3), with ages derived from Caron (1985),based on planktonic foraminiferal biostratigraphy, andJaramillo and Yepes (1994), based on dinoflagellate bio-stratigraphy of the Coyaima section located 44 km to thesouth.

The Whiteinella archaeocretacea Zone is recognized bythe common occurrence of W. archaeocretacea, W. baltica,

PALEOECOLOGY OF LATE CRETACEOUS UPWELLING EVENTS 309

Hedbergella cf. delrioensis, and H. simplex, and the rare oc-currence of Heterohelix moremani and other species. Thetop of the zone is defined by the first occurrence (FO) ofHeterohelix globulosa. Spumellarian radiolarians alsowere found. This zone is Turonian in age.

The Heterohelix globulosa Zone is recognized by thecommon occurrence of H. globulosa, W. archaeocretacea,and H. cf. delrioensis, and the occasional occurrence ofHebdergella planispira, H. flandrini, and other species.The top of the zone is defined by the last occurrence (LO) ofW. archaeocretacea. This zone is late Turonian to earlyConiacian in age.

The Whiteinella balticaMarginotruncana sinuosa Zoneis recognized by the common occurrence of H. globulosaand W. baltica, and the occasional occurrence of Margino-truncana sinuosa, M. renzi, Whiteinella aprica, and otherspecies. The top of the zone is defined by the LO of W. bal-tica and the abrupt decrease in abundance of H. globulosa.This zone is late Coniacian in age.

An undetermined planktonics interval is distinguishedby abundant internal casts of unidentified planktonic fo-raminifera found in the rocks from this interval. This zoneis late Coniacian in age.

Agglutinated foraminiferaArchaeoglobigerina blowiZone is recognized by the common occurrence of Ammo-baculites spp. and Trochammina sp., and the occasionaloccurrence of Lagena sp., Reophax sp., Praebulimina kick-apooensis, and A. blowi. The base of this zone is defined bythe FO of P. kickapooensis and its top by the LO of Ammo-baculites spp. and Trochammina sp. The zone is thoughtto be early Santonian in age.

The Rosita fornicataPraebulimina kickapooensis Zoneis recognized by the common occurrence of P. kickapooen-sis and the rare occurrence of Dicarinella asymetrica, D.primitiva, A. blowi, R. fornicata, and other species. The topof this zone is recognized by the decrease in abundance ofP. kickapooensis; the LOs of D. asymetrica, D. primitiva,and A. blowi occur within the zone. Elsewhere in the Up-per Magdalena Basin, these events lie close to the top ofthe zone, whereas the LO of R. fornicata sometimes liesabove it. Spumellarian radiolarians were also found. Thezone is thought to be late Santonian in age.

The Sitella colonensis Zone is recognized by the commonoccurrence of Siphogenerinoides bermudezi and the rareoccurrence of S. colonensis. The zone is thought to be Cam-panian in age.

The Bolivina explicata Zone is recognized by the com-mon occurrence of P. kickapooensis, S. bermudezi, S. uhli,S. revoluta, Bolivina explicata (a junior synonym of Af-robolivina afra?, Reyment, 1959; Fayose, 1969), Gyroidi-noides sp., Hoeglundina supracretacea, Gavelinella san-didgei, Nodosaria spp., Dentalina spp., and other taxa.The zone is thought to be Campanian in age.

The Siphogenerinoides bermudezi Zone is recognized bythe common occurrence of S. bermudezi, P. kickapooensis,and P. cushmani, and the sparse occurrence of Gavelinellasandidgei, Globigerinelloides sp., and other species. Thiszone is late Campanian in age.

The Siphogenerinoides bramletteiRugoglobigerina ru-gosa Zone is recognized by the common occurrence of Siph-ogenerinoides bramlettei, S. clarki, P. kickapooensis, G.sandidgei, G. nacatochensis, R. rugosa, R. macrocephala,and Heterohelix striata; and the sparse occurrence of Gy-

roidina depressa, Bolivina incrassata, Globotruncana ae-gyptiaca, Globotruncanita pattersi, and other species. Thiszone is thought to be Maastrichtian in age.

Even though this biostratigraphic scheme broadly ap-plies to the whole basin, noticeable differences are found.For example, the distribution of agglutinated foraminif-era, which are abundant either immediately below theLidita Superior Formation in the Girardot-Narino section(Burgl and Dumit, 1954) located ;75 km northeast of theMichu-1 well, or immediately above the Lidita InferiorFormation in the Michu-1 well, suggests a strong environ-mental control. Therefore, the LOs shown in Figure 3 arelikely controlled by regional ecological factors and do nothave global significance (Emery and Myers, 1996).

Quantitative Microfossil Content: Results of counts (%)of planktonic and benthonic foraminifera are treated sep-arately. Planktonic foraminifera are separated into threegroups: heterohelicids, keeled forms, and unkeeled forms(Fig. 4). Unkeeled planktonic foraminifera dominate mostof the sampled interval of the Michu-1 well. Significantpercentages (.20%) of heterohelicids are found only in theupper part of the Nivel de Lutitas y Arenas Formation(30095009, 91.4 m152.5 m), in the lower part of the Nivelde Lutitas Formation (1600918009, 487.7 m548.6 m)),and the Lomagorda Formation (2200928009, 670.6 m853.4 m; H. globulosa Zone), whereas keeled planktonic fo-raminifera occur in the lower part of the Nivel de LutitasFormation (1600919009, 487.7 m579.1 m) and below theLidita Inferior Formation (below 22009, or below 670.6 m).

Benthonic foraminifera are divided into superfamilygroups: buliminids, nodosarids, rotaliids, and agglutinat-ed taxa (Fig. 4). Calcareous benthonic foraminifera (domi-nated by buliminids) are found almost exclusively in theOlini Group (240921009, 73.2 m640.1 m). Nodosarids arefrequent only in the upper part of the Nivel de Lutitas For-mation (at ;10009, or 304.8 m), whereas rotaliids exceed20% in the lower part of La Tabla Formation (30095009,91.4 m152.4 m), the upper part of the Nivel de LutitasFormation (;12009, or 365.8 m), and the middle part ofthe Villeta Formation (;29009, or 883.9 m). Agglutinatedforaminifera dominate the lower part of the Nivel de Lu-titas Formation (1700921009, 518.2 m640.1 m).

The abundance of planktonic and benthonic foraminif-era in the Michu-1 well, expressed as the number of spec-imens/gram of sediment, is shown in Figure 5. The num-ber of radiolarians/g and the diversity (the number of spe-cies) of planktonic and benthonic foraminifera and dino-flagellates also are shown. A significant change in themicrofossil content occurred at the Coniacian/Santonianboundary at ;21009 (640.1 m). Below 21009 (640.1 m),planktonic foraminifera are abundant and display a pat-tern of cyclical fluctuations with four peaks in abundanceexceeding 100 specimens/gram. The lowermost peak(31009, or 944.9 m) corresponds to a peak in radiolarianabundance that reaches 50 specimens/gram.

Above 21009 (640.1 m), the abundance of benthonic fo-raminifera remains below 110 specimens/gram, except forthe Lidita Superior Formation, where there are two peaksin abundance in excess of 200 specimens/g (Fig. 5). By con-trast, the highest diversity of benthonic foraminifera ispresent in the upper and lower parts of the Olini Group(between ;3009 and 21009, ;91.4 m640.1 m). The num-ber of species in a sample, however, never exceeds 12. It is

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FIGURE 4Vertical distribution of planktonic and benthonic foraminifera from the Michu-1 well (percentage values). Planktonic foraminiferaare distributed according to their morphology as: heterohelicids (a), keeled (b), and unkeeled (c). Benthonic foraminifera are distributed in theirdominant superfamilies as: buliminids (d), nodosarids (e), rotaliids (f), and agglutinated (g).

FIGURE 5Abundance and diversity of microfossils in the Michu-1 well. Planktonic foraminifera/gram of sediment (a), benthonic foraminifera/gram of sediment (b), planktonic foraminiferal species (c), benthonic foraminiferal species (d), dinoflagellate species (e), and radiolarians/gramof sediment (f). Note the major change in composition at the Coniacian/Santonian boundary (;21009, ;640.1 m).

important to note that the benthonic foraminifera diver-sity pattern is analogous to the diversity pattern in the Gi-rardot-Narino section (Burgl and Dumit, 1954; Tcheglia-kova and Mojica, 2001).

The number of dinoflagellate species shows similar pat-terns to the Coyaima section (Fig. 6, redrawn from Jar-amillo and Yepes, 1994). In both sections, the maximumdiversity of dinoflagellate species occurs in the Nivel de

PALEOECOLOGY OF LATE CRETACEOUS UPWELLING EVENTS 311

FIGURE 6Dinoflagellate content of the Olini Group in the Coyaima section (redrawn from Jaramillo and Yepes, 1994). Note that the maximumdiversity occurs in the Nivel de Lutitas Formation where the Gonyaulacysta group is dominant and the gonyaulacoid/peridinoid ratio (G/P spp.)is maximum.

Lutitas Formation where the Gonyaulacysta group isdominant. The gonyaulacoid/peridinoid ratio (G/P), whichis a productivity proxy (Harland, 1973; Eshet et al., 1994),shows a maximum value in the upper part of the Nivel deLutitas Formation. The significance of this pattern is dis-cussed below.

Paleoxygenation Indices at the Seafloor: Comparisons ofCretaceous benthonic foraminifera with their extant hom-eomorphs are used to infer oxygen levels at the sediment-water interface and organic-matter rain rates (Bernhard,1986; Koutsoukos and Hart, 1990; Sen Gupta and Ma-chain-Castillo, 1993; Kaiho and Hasegawa, 1994; Bern-hard and Sen Gupta, 1999; van der Zwaan et al., 1999). Ingeneral, benthonic species that inhabit the upper few cen-timeters of the sediment (infaunal species) show a highsurface to volume ratio and have a flattened or cylindricaland highly porous morphology, considered to be morpho-logical adaptations to low oxygen levels at the sediment-water interface (Bernhard, 1986; Koutsoukos and Hart,1990). Benthonic foraminifera withstand lower oxygenlevels than macrofaunas, making them ideal proxies of an-oxic conditions.

In this study, the abundance of authigenic minerals (py-rite, glauconite, and phosphatic pellets) is used as a proxyof early-diagenetic redox conditions (Berner, 1981), andbenthonic species are grouped into three categories ac-cording to their resistance to low oxygenation levels (oxic,dysoxic 2, and dysoxic 1; Kaiho and Hasegawa, 1994; seeTable 1 and Fig. 7). These terms imply oxygen levels rang-ing from .1.0 ml/l for the oxic group to ;0.5 ml/l for thedysoxic 2 group (see Allison et al., 1995, for a discussion onpaleoxygenation and terminology). The benthonic forami-niferal oxygen index (Fig. 7; Kaiho and Hasegawa, 1994)also has been calculated. Benthonic foraminiferal assem-blages classified according to their tolerance to low oxy-genation levels display high values for the dysoxic 1 cate-gory between ;4809 and ;18009 (;146.3 m ;548.6 m);whereas the dysoxic 2 and oxic categories dominate be-tween ;3309 and ;4809 (;100.5 m ;146.3 m).

Pyrite (mainly framboidal) is common in the Honditaand Lomagorda Formations up to ;22409 (;682.8 m).Glauconite and phosphatic pellets occur in varying pro-portions. Glauconite is common at discrete levels (i.e..;22009(;670.6 m), ;18009 (; 548.6 m), ;15009 (;457.2

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TABLE 1Benthonic foraminifera paleoxygenation indices for the Late Cretaceous of the Upper Magdalena Basin. Adapted from Kaiho andHasegawa (1994).

Dysoxic 1 indices Siphogenerinoides bermudezi, S. revoluta, S. reticulata, S. aff. cretacea, S. uhli, Praebuliminia kickapooensis,Praebulimina spp., Bolivina explicata, Sitella colonensis.

Dysoxic 2 indices Siphogenerinoides clarki, S. bramlettei, Bolivina incrassata, Gyroidinoides spp., Hoeglundina supracretacea, No-dosarrids.

Oxic indices Gavelinella spp., Eponides sp.?

FIGURE 7Authigenic minerals and oxygenation indices of benthonicforaminifera in the Michu-1 well. Note the conspicuous dominance ofpyrite in the Hondita and Lomagorda Formations and the increase inthe Oxic and Dysoxic 2 assemblages and oxygen index above ;9009(;274.3 m).

FIGURE 8Gamma ray (GR) log and sequence stratigraphy modelfor the Michu-1 well. System tracts: HST5highstand, LST5low stand,TST5transgressive. SB5sequence boundary, ts5transgressive sur-face, mfs5maximum flooding surface. Note the high GR values forthe TuronianConiacian interval and the Campanian/Maastrichtianboundary. Also indicated are the total organic carbon (TOC), and theintervals of hemipelagic sedimentation and phosphogenesis.

m), ;12009 (;365.8 m). ;10709 (;326.1 m), ;9009(;274.3 m), ;4809 (;146.3 m), and ;3009 (;91.4 m).Phosphatic pellets occur throughout the sequence, butpredominantly occur between 4809 and 17009 (146.3 m518.2 m), and are the only authigenic mineral between;5509 and ;7009 (167.6 m213.4 m).

High benthonic foraminiferal oxygen index values (Fig.7) imply high oxygenation levels and show a close corre-spondence to the presence of species belonging to the oxicgroup (i.e., in the upper part of the Nivel de Lutitas andNivel de Lutitas y Arenas Formations). However, the in-dex of Kaiho and Hasegawa (1994) is only applicable in theuppermost 18009 of the Michu-1 well where benthonic fo-raminifera are present in sufficient quantities.

Sequence Stratigraphic Interpretation: A sequence strati-graphic interpretation of the Upper Cretaceous of the Mi-

PALEOECOLOGY OF LATE CRETACEOUS UPWELLING EVENTS 313

FIGURE 9Average sedimentation rates for the Michu-1 well. Stageboundaries and the Harland et al. (1989) time scale were used toconstruct the graph. Note the higher sedimentation rates for the Hon-dita and Lomagorda Formations compared to the underlying units.

chu-1 well is shown in Figure 8. This interpretation isbased on gamma ray (GR) logs, microfossil abundance anddiversity, rates of faunal turnover, total organic carbon(TOC) values, and authigenic mineral occurrence, usingconcepts developed by Loutit et al. (1988), Vail and Wor-nardt (1990), Powell (1992), Parsley et al. (1993), Holland(1996), and Emery and Myers (1996). The sequence strati-graphic interpretation needs to be tested with additionaldata from wells and outcrops in the Upper Magdalena Ba-sin.

Microfaunal composition and GR logs show a conspicu-ous change at the level of the Lidita Inferior Formation(the Coniacian/Santonian boundary). Below this unit,maximum flooding surfaces (mfs) are characterized bypeaks in GR (sometimes corresponding to high TOC val-ues), and high numbers of planktonic foraminifera/gram,whereas transgressive surfaces (tss) are characterized bypeaks in GR, turnover in the number of planktonic spe-cies, and relatively low TOC values (Fig. 8). Above the Lid-ita Inferior Formation, excluding the exceptionally highpeaks of the Lidita Superior Formation, mfss are charac-terized by relatively low GR values, but marked increasesin the number of benthonic foraminifera, dinoflagellatespecies, and abundance of glauconite grains. Phosphaticpellets are more abundant in transgressive systems tract(TST) intervals (i.e., in the upper part of the Nivel de Lu-titas Formation coinciding with the FO of the genus Siph-ogerinoides at ;10209 (310.9 m)). However, the phospho-genetic interval appears to extend from the middle part ofthe Nivel de Lutitas Formation to the Nivel de Lutitas yArenas Formation (Fig. 8).

Five sequences were distinguished between the Turoni-an and the Campanian stages (second- to third-order cy-cles in Haq et al., 1988). Below the Lidita Inferior Forma-tion, lowstand systems tracts (LSTs) and TSTs tend to bethicker than highstand systems tracts (HSTs), comparedto those above this formation where the HSTs appear to bethicker (Fig. 8). Intervals of pelagic/hemipelagic sedimen-tation include parts of the Hondita and Lomagorda For-mations and the Lidita Inferior and Superior Formations(Fig. 8) that presumably were deposited offshore duringtimes of intensive upwelling and TSTs. The hemipelagicintervals also correlate to mfss, except for the Lidita Infe-rior Formation (Fig. 8). The discussion focuses on these in-tervals as they are of major interest for their presumed or-igin in upwelling regions and as possible hydrocarbonsource rocks. However, it should be noted that sedimen-tation under anoxic conditions (Corg-rich horizons) is notrestricted to TSTs.

Sedimentation Rates: Average sedimentation rates forthe Michu-1 well were estimated using stage boundariesfrom the time scale of Harland et al. (1989; Fig. 9). How-ever, the position of the stage boundaries is preliminaryand the succession is not corrected for compaction. Alsonote that due to low chronostratigraphic resolution, thecondensed sections (i.e., the Lidita Inferior and SuperiorFormations) are not separated. Therefore, average sedi-mentation rates are highly preliminary and may repre-sent minimum estimates. Sedimentation rates are higherfor the Hondita-Lomagorda Formations than for the over-lying units. This pattern has important implications forthe preservation of organic matter. Calculated sedimen-tation rates are significantly higher than those estimated

by Vergara (1997a) for the Ataco section, located approxi-mately 70 km south, but display similar stratigraphictrends.

Relative Sea-level Changes: Relative sea-level changesfor the entire sequence are displayed in Figure 10. Paleo-bathymetric reconstructions in the TuronianConiacianinterval are hampered by the lack of benthonic microfau-nas and the adverse influence of the oxygen minimumzone on intermediate-dwelling (keeled) planktonic fora-minifera. Because the TuronianConiacian was a time ofmaximum coastal onlap (Haq et al., 1988), the Honditaand Lomagorda Formations were interpreted as being de-posited in an outer-shelf environment (Vergara, 1997b).Slightly shallower conditions (up to the inner shelf) aresuggested for the mfss of the SantonianMaastrichtian in-terval (Vergara, 1997b). Paleobathymetric distribution ofbenthonic foraminifera in the TuronianConiacian inter-val appears to have been influenced more by the supply oforganic matter than by bathymetry. The paleobathyme-tric reconstruction thus is loosely constrained, but the res-olution is greater than the scheme used by Vergara(1997b).

DISCUSSION

TuronianConiacian Upwelling System

The absence of benthonic microfossils, the presence ofinoceramids, the high TOC and pyrite contents, the low di-versity of planktonic foraminifera, the rare occurrence ofdeep-dwelling (keeled) taxa, and the high GR values forthe TuronianConiacian interval indicate sedimentation

314 MARTINEZ

FIGURE 10Late Cretaceous paleoceanography of the Upper Magdalena Basin. Note that paleowater depths varied between the inner andouter shelves. Anoxia was intensive during the Turonian and Coniacian ages, whereas phosphogenesis predominantly occurred during theCampanian age. Upwelling (bold curve arrow) occurred during most of the TuronianCampanian time span, but food-web structures variedsignificantly. SB5sequence boundary, ts5transgressive surface, mfs5maximum flooding surface.

during an oceanic anoxic event (OAE) (e.g., Villamil, 1996;Vergara, 1997b). As elsewhere in northern South Americaand northern Africa, anoxic conditions in the Upper Mag-dalena Basin persisted through the TuronianConiacianages, suggesting an extensive upwelling system (Schlan-ger et al., 1987).

Recent planktonic foraminifera are known to dwell fromthe sea surface down to depths of .600 m (Be, 1977; Hem-leben et al., 1989). Similar depth stratification is known tohave occurred in the Cretaceous, based on stable isotopicstudies of homeomorphic taxa (Hart, 1980; Caron andHomewood, 1983; Corfield et al., 1990). Heterohelicidsand unkeeled planktonic foraminifera are assumed to beshallow-dwelling and some may have been symbiont-bear-ing, whereas keeled planktonic foraminifera are assumed

to be deep-dwelling and very probably asymbiotic, liketheir modern counterparts (Caron and Homewood, 1983;Leckie, 1987). However, in epicontinental seas, the rela-tive proportion of the two groups is affected by oxygen lev-els in the water column as well as by paleobathymetry(Leckie, 1987; Arthur et al., 1987; Martnez and Hernan-dez, 1992). In contrast with the modern ocean, no keeledtaxa have been identified related to upwelling. The biser-ial to multiserial heterohelicid group (a morphology ex-tremely rare in todays oceans) is thought to have behavedopportunistically by dwelling preferentially in shallowepicontinental seas under upwelling regimes (Leckie,1987; Kroon and Nederbragt, 1990).

As noted, unkeeled planktonic foraminifera are thedominant group throughout the TuronianMaastrichtian

PALEOECOLOGY OF LATE CRETACEOUS UPWELLING EVENTS 315

interval in the Michu-1 well, except for discrete depths(;32009 (;975.4 m), 2200927009 (670.6 m823 m) and;17009 (;518.2 m); Fig. 4) where heterohelicids are.20%. The TuronianConiacian assemblage has been re-ported elsewhere in northern South America (Hedberg,1931; Petters, 1955; Ford and Houbolt, 1963; Renz, 1981;Martnez and Hernandez, 1992; Vergara, 1994, 1997b)and is thought to reflect sedimentation under anoxic,deep-water conditions.

The sparse occurrence of radiolarians may result fromthe remobilization of opal during early burial. Turonianradiolarian cherts have been found elsewhere in northernSouth America (Burgl and Dumit, 1954; Etayo, 1968; Mar-tnez and Hernandez, 1992). However, their absence inthe Michu-1 well may be due to regional variations in thesupply of silica to the sea-surface as a result of the locationof the upwelling plume and/or dilution by terrigenous ma-terial.

TuronianConiacian upwelling conditions in northernSouth America have been reconstructed by general circu-lation models (Parrish and Curtis, 1982; Barron, 1985;Parrish, 1995). However, the major upwelling plumes arepredicted to straddle the equator (present Maracaibo re-gion) and the west coast of South America south of ;108S(modern location of Ecuador and Peru). All models indi-cate that upwelling was not significant in the Colombiatrough region. The discrepancy in the location of the pre-dicted and observed upwelling plumes may be due to thelow resolution of the general circulation models. However,major variations in the preservation of organic matter areexpected in the trough region resulting from varying loca-tion of upwelling plumes, basin physiography (i.e., the sil-led basin), subsidence (Fig. 10), and sea level.

The paucity of silica in the Turonian of the Michu-1 wellsuggests that upwelling plumes were not present in thisinterval. This may imply that the plumes were locatedalong the present Eastern Cordillera and towards the Ma-racaibo region. Unkeeled planktonic foraminifera (Hed-bergella spp.) in the Turonian, and heterohelicids in theConiacian, would proliferate along the margins of the up-welling plumes. Both foraminiferal groups behaved oppor-tunistically under paleoenvironmental conditions that areconsidered adverse to other forms (Leckie, 1987; Kroonand Nederbragt, 1990), and support the interpretation ofpeak upwelling conditions for the Lomagorda and HonditaFormations elsewhere in the Upper Magdalena Basin(Vergara, 1997b). However, this microfaunal assemblageindicates a shift of the upwelling center over the Michu-1well site only during the Coniacian age. Coniacian upwell-ing conditions also are supported by the presence of chertbeds and Ba contents in excess of 200 ppm in the Yaguaraand Olini reference sections in the Upper Magdalena Ba-sin (Villamil, 1996). In the former section, Ba increasesmarkedly from the Turonian to the Coniacian. Bariumcontent in deep-sea sediments is a reliable paleoproductiv-ity proxy, except in suboxic sediments where diageneticremobilization occurs (Dymond et al., 1992). Thus, bariumcontent is of limited use compared to planktonic foraminif-era for reconstructing the evolution of the TuronianCon-iacian upwelling system in the Upper Magdalena Basin.

Even though planktonic algae are not preserved as mi-crofossils, geochemical evidence (high hydrogen indices,HI . 500 mg HC/gC, and low oxygen indices, OI ,40 mg

HC/gC; Vergara, 1994, 1997a; Mann and Stein, 1997) sug-gests an algal origin for the organic matter of the HonditaFormation. Seasonal diatom and, to a lesser degree, coc-colithophorid blooms, whose preservation potential is low,may have been the main phytoplankton that sustained arelatively simple food web typical of eutrophic regions.Blooms of Pithonella (a calcareous dinoflagellate) havebeen reported elsewhere as typical of the Cenomanian/Tu-ronian boundary (Fischer and Arthur, 1977). Consequent-ly, planktonic foraminifera may have grazed on diatomblooms. Nektonic crustaceans (i.e. copepods, unpreservedin the fossil record) may have grazed on planktonic fora-minifera and sustained other nektonic groups, such as clu-peoid fish (Macellari and de Vries, 1987), ammonites, andmosasaurs (Paramo, 1994). At the seafloor, inoceramidswere the only invertebrates that tolerated low oxygen lev-els, and exploited the rain of organic matter (Fig. 10).

The four peaks in abundance of planktonic foraminiferain the TuronianConiacian interval (Fig. 5a) may suggestdeeper-water conditions, increases in productivity, or re-ductions in the supply of clastic material to the UpperMagdalena Basin. In the case of the lowermost peak, anincrease in productivity is supported by an increase in theradiolarian content (Fig. 5f), whereas deeper-water condi-tions are supported by high GR values, which are inter-preted as maximum flooding surfaces (Fig. 8). Thesepeaks in planktonic abundance may be used as biostrati-graphic markers in a region where traditional foraminif-eral biostratigraphic schemes are of limited resolution.

ConiacianSantonian Upwelling System (Lidita Inferior)

The Coniacian/Santonian boundary is a time of risingrelative sea level, relatively low sea-surface temperatures,widespread oceanic anoxia, and rare volcanic activity (fora review see Frakes et al., 1992). As at the Cenomanian/Turonian boundary, a positive excursion in the carbon iso-tope record is observed, suggesting extensive burial of or-ganic matter in the world ocean (Arthur et al., 1985).

As in other localities in the Upper Magdalena Basin,Upper Cretaceous phosphorites can reach up to ;28%P2O5 (Cathcart and Zambrano, 1967; Mojica, 1978). Thehigher concentrations occur in the ConiacianSantonianLidita Inferior Formation (Burgl and Botero, 1967) andcorrespond to an oceanwide anoxic event (OAE3) that ledto higher burial rates of Corg (Arthur et al., 1990; Follmi etal., 1993).

A major oceanic turnover is recorded in the Lidita Infe-rior Formation. Plankton consist of unkeeled planktonicforaminifera, radiolarians (Vergara, 1994) and/or diatoms,and a few dinoflagellate species (Fig. 6). Benthonic fora-minifera are absent in the Michu-1 well (Fig. 4), in con-trast to the sparse occurrence of buliminids in the Atacosection located ;80 km south (Vergara, 1994). TOC is,2%. The Lidita Inferior Formation likely was depositedduring the early stages of a TST (Fig. 8). Deposition of sil-ica is interpreted as the product of upwelling that sus-tained radiolarians (and/or diatoms) and a sparse popula-tion of planktonic foraminifera and dinoflagellates. In con-trast to the underlying interval, heterohelicid foraminiferswere not found; although this group is considered to be op-portunists, they likely could not compete with radiolariansand dinoflagellates at the center of the upwelling plume.

316 MARTINEZ

Ammonites and clupeoid fish appear to be the primaryconsumers in an apparently simple food web. Anoxic con-ditions at the seafloor favored inoceramids over other mol-luscan groups.

Immediately after deposition of the Lidita Inferior For-mation, the abundance of buliminids, rotaliids, and agglu-tinated foraminifers increased along with the diversity ofbenthonic foraminifera, dinoflagellates, and the content ofphosphatic pellets. This dramatic change represents a ma-jor reorganization of the food web and a return to mesotro-phic marine conditions that peaked during the late San-tonian, when dinoflagellate diversity and the G/P ratiowere at a maximum, and the Gonyaulacysta group wasdominant (Fig. 6).

Dinoflagellate species reported by Jaramillo and Yepes(1994) are typical of the Malloy Suite that dominated theTethyan region during the Late Cretaceous (Lentin andWilliams, 1980). The geographic distribution of the MalloySuite correlates closely with the distribution of Late Cre-taceous phosphorites and benthonic foraminiferal assem-blages dominated by buliminids of the genera Siphogener-inoides, Praebulimina, and Afrobolivina reported fromwest Africa and northern South America (Petters, 1955;Fayose, 1969; Ly and Kuhnt, 1994). This extensive arealikely was characterized by upwelling-induced dinoflagel-late blooms that sustained a large fish (and zooplankton?)population whose fecal pellets sustained a benthonic fora-miniferal population dominated by buliminids on the sea-floor.

Dinoflagellate blooms, or red tides (Purdie, 1996), havebeen proposed to explain phosphatic accumulations innorthern South America (Erlich et al., 1999). At present,dinoflagellates, planktonic diatoms, prymnesiophyte fla-gellates, and ciliate blooms that have deleterious effectson the pelagic food web in coastal areas are referred to asharmful algal blooms (HAB; e.g., Smayda, 1997; Horner etal., 1997; Turner and Tester, 1997). HABs occur when: (1)nutrients (nitrate and/or ammonium, phosphate, and sili-ca) are elevated in concentration; (2) irradiance levels inthe mixed layer are high; and/or (3) there is an imbalancebetween production of micro-algae and grazing by herbi-vores, advection, or sedimentation out of the photic zone(Purdie, 1996).

Campanian Upwelling System (Lidita Superior)

Continuing a trend established at the Coniacian/San-tonian boundary, the Lidita Superior Formation is domi-nated by benthonic foraminifera, peridinoid dinoflagel-lates (Fig. 6), and radiolarians (Vergara, 1994). The lattertwo groups dominated the euphotic zone, outcompetingplanktonic foraminifera. Diversity of benthonic foraminif-era peaked in the Lidita Superior Formation, but absoluteabundance (number of specimens/gram) is only significantin the upper part (Fig. 5). As indicated above, benthonic fo-raminifera oxygenation indices and the presence of fram-boidal pyrite suggest dysoxic (,1.0 ml/l) levels at the sed-iment-water interface (Fig. 7). In the Trinidad and theSergipe Basins (northeastern Brazil), planktonic and ben-thonic foraminifera show a conspicuous increase in diver-sity and abundance at the Coniacian/Santonianboundary,indicating increasing sea-surface temperatures and only

periodically euxinic conditions on the sea floor (Koutsou-kos and Merrick, 1985; Koutsoukos et al., 1991).

Phytoplankton populations in the Campanian upwell-ing system were dominated by peridinoid dinoflagellates.Spumellarian radiolarian species and a sparse populationof unkeeled planktonic foraminifera represent primaryconsumers. This plankton population sustained abundantclupeoid fish and, apparently, was responsible for their ep-isodic demise by the production of toxins. Such paleoenvi-ronmental reconstructions are limited by a preservationalbias against soft-bodied organisms. This is particularlytrue in that nektonic crustaceans than may have been im-portant primary consumers were not preserved. Theabundant phosphatic pellets found throughout the San-tonianCampanian interval in northern South Americaare interpreted here as deriving from fish feces. The size,structureless character, and similarity to modern clupeoidfish fecal pellets supports this interpretation (see Stare-sinic et al., 1983). In modern environments, fecal pelletsare a key means of transporting organic matter to the sea-floor (Urrere and Knauer, 1981), particularly in upwellingsystems where clupeoid fish replace copepods in the pro-duction of fecal pellets (Staresinic et al., 1983). Therefore,in the Campanian of the Upper Magdalena Basin, fish fe-cal pellets may have sustained an important population ofbuliminids dominated by the genus Siphogenerinoides.

Geographical differences in the species composition ofthe genus Siphogenerinoides (Petters, 1955) may be usedto reconstruct the Campanian upwelling system. Work inprogress (i.e., a taxonomic review of the genus Siphogener-inoides) has revealed morphological differences controlledby environmental factors (Martnez, 1995; Gonzalez andMartnez, 1998). This may be analogous to interspecificvariations in modern buliminids that are related to oxy-genation and organic-matter content at the seafloor.

Follmi et al. (1992) suggested that phosphatic particles(including fecal pellets) and pristine phosphate in the Up-per Cretaceous of the Eastern Cordillera accumulated me-chanically at the middle and the end of fourth-order rela-tive sea level shallowing-upward sequences, respectively.The same mechanism also may have operated in the Up-per Magdalena Basin; however, increased precipitation ofpristine phosphorites would be expected in this offshorelocation. Compared to the Eastern Cordillera, there is alow content of fecal pellets and a high P2O5 content in theLidita Inferior Formation (Burgl and Botero, 1967), whichsupport the idea of enhanced precipitation of pristinephosphorites in the Upper Magdalena Basin.

Plate Tectonics, Upwelling, and Source Rocks

Villamil et al. (1999) proposed that the northward mo-tion of the South American plate across the equator at;86 Ma caused Ekman pumping and upwelling of subsur-face nutrient-rich water away from the continental mar-gin; before 86 Ma, the Ekman mechanism would havecaused downwelling on the margin. Villamil et al. (1999)also proposed that upwelling was caused by the position ofthe Intertropical Convergence Zone (ITCZ) at 58N. Thereare a number of problems with these arguments: (1) Ac-cording to the maps of Villamil et al. (1999), the Berriasianto lower Albian succession was isolated from the proto-Pa-cific Ocean, whereas the western limit of the middle Albi-

PALEOECOLOGY OF LATE CRETACEOUS UPWELLING EVENTS 317

an to Turonian succession is unknown, but presumablyrepresents the flooding of a sill barrier during a high sea-level stand. Chert deposition during the AlbianTuronian,therefore, took place during a period of major sea-level risewhen the Pacific upwelling plumes reached the area westof Bogota (Martinez and Vergara, 1999). Other areasalong the Magdalena Basin might have received similarsiliceous supply, but this is undetected in the stratigraphicrecord due to dilution with terrigenous material. (2) Theincrease in phosphate deposition during the Late Creta-ceous in Colombia is more than a regional phenomenon;therefore any explanation must apply to most of the Teth-yan region. (3) The present ITCZ is a climatic feature thatis not linked to upwelling (Wyrtki, 1981).

Preservation of organic matter in the Late Cretaceous ofColombia is thought to be a result of a high accumulationrate of clastic material, which limited oxidation of organicmatter at the sea floor (Villamil, 1996). However, the con-trast between the high sedimentation rates of the hemi-pelagic, organic-rich TuronianConiacian interval and thelower sedimentation rates of the more clastic and relative-ly organic-lean Campanian interval in the Michu-1 well(Fig. 9) suggests that anoxia was the critical factor con-trolling preservation of organic matter.

The food-webs proposed are still schematic and in needof further testing. For example, uncertainties regardingplankton composition due to a preservational bias (i.e., theapparent absence of diatom fossils) potentially can besolved by organic geochemical (biomarker) analysis (e.g.,De Leeuw et al., 1995).

CONCLUSIONS

(1) Ten planktonic foraminiferal zones that are poten-tially applicable on a regional basis are recognized in theTuronianMaastrichtian interval of the Michu-1 well. TheTuronianConiacian zones are dominated by planktonicforaminifera, whereas the SantonianMaastrichtian onesare dominated by benthonics (buliminids).

(2) Anoxic conditions prevailed during the TuronianConiacian, followed by dyxosic conditions from the Conia-cian/Santonian boundary interval until the Campanian/Maastrichtian boundary. Oxic conditions followed duringthe Maastrichtian age.

(3) Six stratigraphic sequences are interpreted in theTuronianMaastrichtian interval. Below the Coniacian/Santonian boundary, maximum flooding surfaces (mfs)are characterized by peaks in the GR signal, high TOC,and high numbers of planktonic foraminifera/gram,whereas transgressive surfaces (TST) are characterizedby brief peaks in the GR signal, turnover in the number ofplanktonic species, and low TOC. Above the Coniacian/Santonian boundary, mfss are characterized by relativelylow GR values, but a marked increase in the number ofbenthonic foraminifera and dinoflagellate species, andglauconite.

(4) Upwelling conditions were present in the UpperMagdalena Basin for most of the Late Cretaceous.

(5) The food-web structure for the TuronianConiacianwas dominated by coccolithophorids that sustained radio-larians, planktonic foraminifera, ammonites, clupeoidfish, and mosasaurs.

(6) Following a major turnover during the Coniacian

Santonian boundary, dinoflagellates were the primaryproducers that sustained radiolarians, a sparse popula-tion of planktonic foraminifera, clupeoid fish, and ammo-nites. Peridinoid dinoflagellates dominated over gonyau-lacoids during siliceous intervals (Lidita Inferior and Lid-ita Inferior Formations).

(7) Dinoflagellate blooms (peridinoids) dominated dur-ing the Campanian interval and sustained a sparse popu-lation of planktonic foraminifera and abundant clupeoidfish. Feces (phosphatic pellets) produced by these fishwere grazed extensively by a specialized population of bu-liminids, dominated by the genus Siphogenerinoides.

ACKNOWLEDGEMENTS

This study was initiated while I was working at ICP-Ecopetrol. I thank Kurt Baker, Fernando Munoz, and LuisSarmiento for their support at this early stage. Luis Sar-miento kindly provided Figure 2. Special thanks to PatrickDe Deckker, Tom Loutit, Robert Erlich, Tim Bralower,and two anonymous colleagues for careful reviews of themanuscript.

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