Hybrid turbidite-contourite system on the upper-slope continental margin of the offshore southern Tanzania

Abstract

Cretaceous-Cenozoic hybrid turbidite-contourite (HTCs) systems on the upper-slope region of the northern Rovuma Basin, southern offshore Tanzania have been poorly studied. HTCs have been widely studied from several places due to their petroleum prospectivity, link to geohazards, and potential to help understand deep marine circulation processes and past climates. We have studied 3D seismic data into the northern Rovuma Basin to establish variation and orientation of seismic facies, architectural elements and geomorphologies (based on RMS amplitude maps) characterizing the Cretaceous-Cenozoic HTCs linked to channelized sediment transport and deposition. Two different migration patterns for channelized systems are recognized. They include Cretaceous-Middle Miocene and Middle Miocene-Holocene systems migrating toward SE and NW, respectively. Timing of change in migration trends coincides with change in traverse directions of major submarine channels that allowed sediment transportation by gravity flows. Trajectory of the submarine channels shifted from S-N to SW-NE and approximately W-E during the Middle Miocene, agreeing with the change in channel fills migration trends. The trajectory shift, for the revealed submarine channels was caused by tectonic activity linked to East African Rift System, so does the distribution and migration trends of the HTCs. The study area has hydrocarbon accumulations in the Oligo-Miocene sandstones containing HTCs characterized by a clearly defined depositional system with high RMS amplitudes. The Middle Miocene-Early Pliocene interval has a clear depositional configuration as well, but its low RMS amplitude indicates dominance of fine-grained HTCs with limited reservoir potential. The Late Pliocene-Holocene interval has very high RMS amplitude, but its chaotic configuration implies the presence of poorly sorted sedimentary fills having limited reservoir potential as well. This may be one of the reasons for hydrocarbon discoveries in the study area being limited within the Oligo-Miocene sandstones, and not the younger stratigraphic levels.