Tectono-sedimentary controls on Cretaceous black shale deposition along the opening Equatorial Atlantic Gateway (ODP Leg 159)

Abstract

The Cretaceous Equatorial Atlantic Gateway between the Central and South Atlantic basins is of interest not only for palaeoceanographic and palaeoclimatic studies, but also because it provided particularly favourable conditions for the accumulation and preservation of organic-rich sediments. Deposition of carbonaceous sediments along the Côte d’Ivoire-Ghana Transform Margin (Ocean Drilling Program Leg 159) was intimately linked to the plate tectonic and palaeoceanographic evolution of this gateway. Notably, the formation of a marginal basement ridge on the southeastern border of the transform margin provided an efficient shelter of the landward Deep Ivorian Basin against erosive and potentially oxidizing currents. Different subsidence histories across the transform margin were responsible for the development of distinct depositional settings on the crest and on both sides of the basement ridge. Whereas the southern, oceanward flank of the basement ridge was characterized by rapid, continuous deepening since the latest Albian-early Cenomanian, marine sedimentation on the northern, landward flank was interrupted by a period of uplift and erosion in the late Albian, and rapid subsidence started after the early Coniacian. Organic-rich sediments occur throughout almost the entire Cretaceous section, but hydrogen-rich marine black shales were exclusively recovered from core sections above an uplift-related unconformity. These black shales formed when separation of Africa and South America was sufficient to allow permanent oceanic midwater exchange after the late Albian. Four periods of black shale accumulation are recorded, some of them are correlated with the global oceanic anoxic events: in the latest Albian-earliest Cenomanian, at the Cenomanian-Turonian boundary, during the middle Coniacian-early Campanian, and in the mid-Maastrichtian. These periods were characterized by increased carbon fluxes to the seafloor, induced by enhanced palaeoproductivity and intensified supply of terrestrial organic matter. Black shale deposition appears to be intimately linked to periods of rising or maximum eustatic sea level and to the expansion of the oxygen minimum zone, as indicated by foraminiferal biofacies. Intervals between black shale units, in contrast, indicate a shrinking oxygen minimum zone and enhanced detrital flux rates, probably related to lowering sea level. Upper Cretaceous detrital limestones with high porosities may provide excellent hydrocarbon reservoirs, although their areal extent appears to be limited. Palaeogene porcellanites, capped by Neogene pelagic marls and clays, extend over a wider area and may provide another target for hydrocarbon exploration.

Redox sensitivity of P cycling during marine black shale formation: Dynamics of sulfidic and anoxic, non-sulfidic bottom waters

“…A similar conclusion was drawn by Friedrich and Erbacher (2006), based on benthic foraminiferal assemblages at ODP Sites 1257 and 1259. They argue that during nannofossil biozone CC15, a fluctuation of the oxygen minimum zone at the more shallow parts of Demerara Rise occurred, due to establishment of a shallow-water connection to the South Atlantic via the Equatorial Atlantic Gateway (Wagner and Pletsch, 1999). During CC17, the first deep-water connection was established via this gateway, leading to sea floor ventilation at Site 1259 (Friedrich and Erbacher, 2006) and successively terminating the semi-restricted nature of the Proto-North Atlantic.…”

“…However, given very high rates of TOC delivery to the sea floor, sulfidic bottom water conditons over time intervals of thousands of years, a still rather restricted Equatorial Atlantic Gateway in the Late Cretaceous (Wagner and Pletsch, 1999;Friedrich and Erbacher, 2006), and lower sulfate concentrations in the Cretaceous oceans relative to today (Hardie, 1996;Horita et al, 2002;Paytan et al, 2004;Wortmann and Chernyavsky, 2007), the consumption of sea water sulfate by microbial reduction may have exceeded its supply via continental erosion. We note that each of the observed P peaks (Fig.…”

Significance of ODP results on deepwater hydrocarbon exploration – Eastern equatorial Atlantic region

“…The Marginal Ridge is a fossil ridge, which connects laterally with the Romanche Fracture Zone, and separates the Deep Ivorian Basin from the oceanic Gulf of Guinea Abyssal Plain. This ridge isolated the Deep Ivorian Basin and was probably at least partially responsible for the accumulation of black shale facies within the basin by acting as a circulation barrier (Wagner and Pletsch, 1999).…”

“…A review of scientific drilling locations suggests that sites relevant to the hydrocarbon exploration potential of the Equatorial portion of the West African margin were included in ODP Leg 159 (Wagner and Pletsch, 1999), Fig. 1.…”

Seismostratigraphy of the Ceará Plateau: Clues to Decipher the Cenozoic Evolution of Brazilian Equatorial Margin

“…(1) occupies a stable low-latitudinal setting, (2) is characterized by oceanographic conditions that have not changed significantly (Wagner and Pletsch, 1999;Müller et al, 2008;Torsvik et al, 2009;Heine et al, 2013).…”