Distribution of deep-sea black shales of Cretaceous age in the eastern Equatorial Atlantic from seismic profiling

Abstract

The presence of thick (∼ 200 m) carbonaceous shales of Cretaceous age in the deep waters of the eastern Equatorial Atlantic was first revealed by drilling at DSDP sites 367 and 368 near the Cape Verde Islands. More recent seismic investigations show that black shales are widely distributed in this region. In the Gambia Basin they are associated with a group of seismic reflectors lying at depths of 400–600 m below the sea floor. These can be traced southwards from the drill sites into the Sierra Leone Basin and eastwards beneath the African continental rise to depths at which ambient temperatures fall within the oil generation window. In the Gambia Basin the reflectors continue westwards from the Cape Verde Archipelago before terminating on Turonian oceanic basement (∼ 92 Ma). South of the Guinea Fracture Zone near 10° N extension of the same seismic sequence on to younger oceanic crust indicates that deep-sea carbonaceous deposits formed close to the present equator as late as the early Campanian (∼ 80 Ma), long after a series of oceanic anoxic events in the Aptian–Albian (OAE1) and over the Cenomanian–Turonian boundary (OAE2) led to widespread accumulation of black shales. This late period of deposition may correspond to the more locally developed OAE3. High surface productivity in equatorial upwelling zones during the late Cretaceous probably played an important role in delivering large amounts of organic carbon to the sea floor when meridional movement of deep water between the South and North Atlantic was restricted by closely-spaced, east–west fracture zones. In some areas seismic data show that the black shales level out rather than drape pre-existing topography, implying significant downslope transport of organic-rich material from upwelling cells near the African margin.