Conspicuous seismic reflections in Upper Cretaceous sediments as evidence for black shales off South Africa

Abstract

The Upper Cretaceous is commonly associated with greenhouse climate, Oceanic Anoxic Events (OAE), and the ongoing break up of Gondwana, which resulted in strong variations of the ocean's currents flow paths and climate change. Little is known about these changing conditions, in particular south of South Africa. A high-resolution seismic reflection data set from the submarine Transkei Basin off South Africa revealed various depositional stages for this region for the past ∼90 Ma. In these seismic sections, a recurrently appearing high-amplitude horizon within weak to homogeneous Upper Cretaceous reflections was observed. Due to the inaccessibility of any drill hole data from the Transkei Basin and adjacent regions, the origin of this reflector is difficult to specify. It is roughly dated at ∼80–∼85 Ma, which falls within the last big OAE in Upper Cretaceous (OAE 3). A seismostratigraphic analysis led to an exclusion of several features as the origin for this reflector, such as gas hydrates, opal bearing horizons or biogenic oil. Other possible features, such as lithologic variations, intrusions or black shales, are discussed in more detail. According to its appearance and reflection characteristics it could possibly be the first report of black shales in this region.