assemblages of potential source rocks from the western African margin,. Central Atlantic ...... (MacNeil & Caldwell 1981) and New Jersey slope. (Nyong & Olsson ...
Upper Cretaceous palaeoenvironments and benthonic foraminiferal assemblages of potential source rocks from the western African margin, Central Atlantic A N N H O L B O U R N 1, W O L F G A N G K U H N T 2, A B D E R R A Z Z A K E L A L B A N I 2, T H O M A S P L E T S C H 2, F L O R I A N L U D E R E R 2 & T H O M A S W A G N E R 3
1Department o f Palaeontology, Natural History Museum, Cromwell Road, London S W 7 5BD, UK 2Geologisch-Paliiontologisches Institut, Christian-Albrechts- Universitiit zu Kiel, Olshausenstr. 40, D-24118 Kiel, Germany 3Fachbereich 5, Geowissenschaften, Universitiit Bremen, Klagenfurter Str. 5, 28359 Bremen, Germany Abstract: Upper Cretaceous benthonic foraminiferal assemblages, clay minerals, kerogen
types and carbonate microfacies are examined in organic-rich sediments from western African coastal basins along a latitudinal transect extending from southern Morocco to Nigeria. Samples from onshore sections of the Tarfaya Basin, southern Morocco, from DSDP Site 369, from offshore commercial wells on the Casamance Shelf, Senegal, from DSDP Site 367, from ODP Site 959 and from onshore sections of the Benue Trough in Nigeria, are analysed. Organic-rich sediments from these sites are characterized by smectiterich clay mineral assemblages, high amounts of marine organic matter and a typical benthonic foraminiferal biofacies dominated by buliminids and bolivinids. Benthonic foraminifera provide robust proxy indicators of palaeoproductivity and organic matter flux to the seafloor. The palaeobiogeographic distribution of benthonic foraminiferal biofacies can be used to identify areas and stratigraphic intervals of hydrocarbon accumulation. Lower Turonian benthonic foraminiferal assemblages exhibit extremely low diversity and are strongly dominated by a Gabonita biofacies, in contrast to Coniacian to Maastrichtian assemblages, which display higher diversity and contain numerous species of Buliminella, Praebulimina, Afrobolivina, Bolivina and Orthokarstenia. The palaeobathymetric distribution of these high productivity benthonic foraminiferal biofacies reflects the establishment of a productivity driven oxygen minimum zone along the western African shelf, associated with an upwelling system active off coastal Africa during most of the Late Cretaceous. The extent of the high productivity equatorial belt appears to have shifted latitudinally during the Late Cretaceous as sea level and circulation patterns changed. The zone of highest productivity was broadest in the Early Turonian, when it coincided with the maximum sea-level rise and highest atmospheric temperature in the Late Cretaceous. The Late Cretaceous was generally characterized by high atmospheric CO 2 concentrations, warm climates without major ice-caps, flatter equator to pole temperature gradients than today's and elevated sea levels (Barron & Washington 1985; Berner 1991; Cerling 1991; Frakes et al. 1992; Freeman & Hayes 1992; Barron et al. 1995; Huber et al. 1995; Bush & Philander 1997). The focus of deepwater formation switched from the poles to the low latitudes as extensive, shallow epicontinental seas, w i t h high evaporation rates, became the sources of warm, oxygen-depleted bottom waters (Brass et aL 1982; Hay 1988; Barron & Peterson 1990; Spicer & Corfield 1992; Barron et al. 1993; Hay 1995a, b). At times of highest sea level, in the Early Turonian and Early Campanian, the widespread flooding of continental blocks triggered major changes in palaeocirculation and palaeoproductivity, which led to
the global accumulation of organic-rich sediments (Eicher & Diner 1991). These hydrographic changes had a major impact on the nutrient and oxygen budgets of coastal basins along the western African margin, where general circulation models predict that a major cell of upwelling was active during most of the Late Cretaceous (Kruijs & Barron 1990; Bush & Philander 1997), which led to increased surface water productivity and to the establishment of intensified oxygen minimum zones on the outer shelf. Environmental conditions along the western African margin were, therefore, exceptionally favourable for the accumulation of thick sequences of organic-rich sediments and coastal basins became the prime targets for the trapping of potential hydrocarbon resources during the Late Cretaceous. As the composition of benthonic foraminiferal
HOLBOURN, A., KUHNT, W., EL ALBANI, A., PLETSCH, T., LUDERERE 8r WAGNER, T. 1999. Upper Cretaceous palaeoenvironments and benthonic foraminiferal assemblages of potential source rocks from the western African margin, Central Atlantic. ln. CAMERON,N. R., BATE, R. H. & CLURE,V. S. (eds) The Oil and Gas Habitats of the
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assemblages is strongly dependent on the productivity of surface waters, and on the oxygenation of bottom water masses, high productivity benthonic foraminifera in fossil assemblages may serve as proxies of original phytodetritus flux rates to the seafloor and bottom water oxygenation. Both these factors are also influential in the formation of source rocks and benthonic foraminiferal palaeobiogeographic distribution patterns may, therefore, be used to trace the distribution of potential source rocks in sedimentary basins. Benthonic foraminifera have radiated to occupy most niches in surficial sediments of marine ecosystems, where they represent an essential link in the oceanic food chain (Gooday 1994). Recent studies by Lutze & Colbourn (1984), Altenbach & Sarnthein (1989) and Gooday & Turley (1990) have indicated that food and oxygen are the most influential factors regulating the distribution of benthonic foraminifera. Tyson & Pearson (1991), Barmawidjaja et al. (1992) and Jorissen et al. (1992) also suggested that the depth of the redox potential discontinuity below the sediment surface has a profound influence on the distribution of benthonic foraminifera within sediments, while Linke & Lutze (1993) considered distribution to be a dynamic process reflecting the need to optimize food acquisition. Loubere (1994, 1996) also showed that benthonic foraminiferal assemblage composition can be quantitatively related to surface ocean productivity and to bottom water oxygen content. Modern benthonic foraminiferal communities in areas of increased organic matter export flux rates, which result in an enhanced food supply and dysoxic conditions at the seafloor, are characterized by high numbers of small buliminids and bolivinids regarded as opportunistic infaunal detritus feeders (Lutze & Colbourn 1984; Lutze et al. 1986; Corliss & Chen 1988; Hermelin 1992; Jian et al. 1999; Kuhnt et al. 1999). Sedimentological data, generated from X-ray diffractometry of the clay fraction, carbonate microfacies analysis and Rock-Eval pyrolysis, are used as independent indicators of siliciclastic and carbon flux to the seafloor, bottom water oxygenation and sea-level changes. The composition of clay mineral assemblages provide information on the proximality or distality from the sediment source, as detrital minerals such as illite, chlorite and kaolinite are preferentially deposited in the most proximal part of sedimentary basins, whereas smectite is selectively deposited in more distal environments on account of its higher buoyancy (Chamley 1989). Variations in the degree of sorting, maturity and grain size of sediments reflect fluctuations in the intensity of the
terrigenous flux, which may also be related to sea-level highstands or lowstands. Observations of fine laminations in sediments with a high organic matter content indicate strong dysoxia, or even anoxia, in the original environment of deposition. Measurements of the 'total organic content' (TOC) in sediments provide evaluation of organic carbon accumulation at the seafloor and the degree of oxicity/anoxicity at the sediment-water interface at the time of deposition. Finally, kerogen analyses allow the distinction of lacustrine (Type I) and marine (Type II) organic matter, which are both derived from planktonic organisms, and organic matter originating from land plants (Type III). The palaeobathymetric, biostratigraphic and palaeobiogeographic distribution of Upper Cretaceous benthonic foraminifera in organicrich sediments from coastal basins along the western African margin from southern Morocco to Nigeria are investigated. The study is concentrated on two main time slices: the Early Turonian and the Early Campanian, which correspond to the two most prominent highstands of the Late Cretaceous. Samples from onshore sections of the Tarfaya Basin in southern Morocco, from DSDP Site 369 on the continental slope off southern Morocco, from offshore commercial wells on the Casamance Shelf off Senegal, from DSDP Site 367 in the Cape Verde Basin, from ODP Site 959 on the C6te d'Ivoire-Ghana Marginal Ridge and from onshore sections of the Benue Trough in Nigeria are examined. Samples from these locations represent a latitudinal transect along the east Atlantic margin, which may be used to trace the position of the main upwelling systems during the Late Cretaceous.
Location and geological setting Tarfaya Basin
The Tarfaya Basin is a Mesozoic basin extending along the coast of southern Morocco between latitudes 28~ and 2 4 ~ (Fig. 1), which has remained tectonically stable since the Cretaceous. A palaeoenvironmental evolution of the basin during the Late Cretaceous is presented in E1 Albani et al. (1999). Bituminous marls were deposited in this basin during the Turonian in an open shelf setting, in water depths of the order of 200-300 m (Kuhnt et al. 1990). At the more distal site of Well $75 the Turonian sediment (40 m thick) consists almost exclusively of organic matter and biogenic carbonate. The carbonate content can reach up to 90% and is principally composed of planktonic foraminifera and
UPPER CRETACEOUS PALAEOENVIRONMENTS
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