The Demerara Rise is a prominent submarine plateau located at ~5°N off the coasts of Surinam and French Guyana (Figs. F1, F3). The rise stretches ~380 km along the coast and is ~220 km wide from the shelf break to the northeastern escarpment, where water depths increase rapidly from 1000 to >4500 m. The plateau lies in shallow water (~700 m), but the northwestern margin is a gentle ramp that reaches depths of 3000 to 4000 m. Much of the plateau is covered by 2 to 3 km of sediment. This sedimentary cover thins near the northeastern escarpment and exposes the lower stratigraphy of the sediment column and underlying basement at water depths of 3000 m to >4500 m. In contrast, the gentle ramp on the northwest margin is covered by a nearly uniform drape of pelagic sediment down to water depths >4000 m.
The Demerara Rise is built on rifted continental crust of Paleozoic and early Mesozoic age. Tectonic reconstructions of the equatorial Atlantic place the Demerara Rise just south of Dakar, Senegal, prior to rifting of Africa from South America (Fig. F4). The South American margin in the vicinity of the Demerara Rise was one of the last areas in contact with West Africa during opening of the equatorial Atlantic (Fig. F4). Barremian basaltic volcanics have been recovered in industry wells from the Demerara Rise, suggesting that rifting began in the early Cretaceous. A piston core taken by Koninklijke/Shell penetrated BarremianAptian shales 2 meters below seafloor (mbsf) on the northeast escarpment, and Jurassic sandstone was dredged in the same area (Fox et al., 1972).
The first known marine sediments on the Demerara Rise are Neocomian in age, and the northern edge of the plateau is thought to have subsided rapidly and reached water depths of nearly 2 km by late Cenomanian time (Arthur and Natland, 1979). Upper Albian sediments are mostly green claystones with interbedded black shales. The Cenomanian to Santonian sequence consists almost exclusively of laminated black shale with occasional stringers of limestone. The black shale is a principal source rock for oil production in coastal French Guyana and Surinam and has total organic carbon contents of up to 68 wt% in industry wells near the middle of the plateau. Apparently, upwelling conditions persisted over the Demerara Rise well into the Late Cretaceous. Campanian to Paleogene sediments are calcareous oozes and chalks. Sediment drifts formed along the top of the northeastern escarpment in the Eocene and Oligocene, but all of the Cenozoic cover thins or pinches out at the escarpment, exposing Cretaceous deposits. Pronounced thickening of all of these units inboard indicates that a relatively complete Paleogene sequence is preserved on Demerara Rise. A prominent submarine channel system developed in the early Miocene. The channeled surface is unique in the seismic stratigraphy of the Demerara Rise (Fig. F5) and can be traced over the northwestern surface of the plateau. The channels carried sediment east to west over the flank of the plateau and into feeder channels for a submarine fan that formed northwest of the Demerara Rise. The channel system was short-lived, and most of the Neogene sediments (hemipelagic and pelagic deposits) are thin or absent from the distal portions of the plateau.
DSDP Site 144 was drilled in 1970 during Leg 14 on the outermost edge of Demerara Rise (water depth = 2957 meters below sea level [mbsl]; maximum depth of penetration = 327 mbsf). Results from this site confirm the complete absence of Neogene sediments on the Demerara Rise escarpment. Instead, Oligocene foraminiferal nannofossil oozes crop out at the seafloor and are underlain by a shallowly buried succession of mid-Cretaceous through Eocene sediments (Fig. F5). Basal sediments are of late Aptian age, and these are overlain by Albian through Maastrichtian strata (Fig. F5). The Cretaceous sequence is unconformably overlain by lower Paleocene through middle Eocene sediments, and these are, in turn, unconformably overlain by lower Oligocene sediments (Fig. F5). No chert is present anywhere in the section.
In the three decades that have elapsed since DSDP Leg 14, the paleoceanographic significance of the sedimentary record at Site 144 has not gone unnoticed (e.g., Berger and von Rad, 1974; Barron, 1983; Arthur et al., 1990; Thurow et al., 1992). The Cretaceous sediments at Site 144 are particularly notable in this regard because they consist of an expanded sequence of laminated black shales (now thought to be correlative to OAEs-1b, -2, and -3) and homogenous olive-green pelagic oozes to carbonaceous claystones that are similar to Aptian and Albian lithologies from Blake Nose (ODP Leg 171B) and the Tethyan Umbria Marche Basin (Norris and Wilson, 1998; Erbacher et al., 2001). These dominantly clay-rich Albian through Santonian sediments, like the overlying Upper Cretaceous and Paleogene oozes and marls, yield well-preserved carbonate microfossils when subjected to the same gentle desegregation procedures that are routinely applied to Neogene samples. Examination under binocular microscope shows that the tests of these foraminifers are glassy in appearance and that their chambers are free of calcite infilling. Scanning electron microscopy reveals the preservation of detailed wall and aperture structures (Fig. F6).
It has long been appreciated that ancient calcareous fossils from shallowly buried clay-rich lithologies can display remarkable primary textural structure. Recent work on Cretaceous foraminifers from Blake Nose (ODP Leg 171B) has shown that such fossils are also remarkably well preserved with respect to their stable isotope geochemistry. This fact significantly increases the potential for developing reliable paleoecological, -oceanographic, and -climatological records for the Cretaceous (Norris and Wilson, 1998; Erbacher et al., 2001; Wilson and Norris, 2001; Huber et al., 2002). Therefore, we have generated new stable isotope data from multiple planktonic species in the upper Cenomanian sediments from Site 144 (Fig. F7). Results indicate that these foraminifers are also geochemically well preserved. Downcore records are noise-free, showing small but consistent interspecies offsets in 13C and 18O values (~3.5 to 4.0 PDB) that are consistent with predictions for tropical sea-surface temperatures during the so-called mid-Cretaceous greenhouse.
Next Section | Table of Contents