SEQUENCE STRATIGRAPHY OF THE DEEPWATER MIOCENE IN THE LOWER CONGO BASIN AND ITS CONTROLLING FACTORS

The Lower Congo Basin is a basin with a wide continental shelf. In such a basin, when the relative sea-level falls, depocenters on the inner shelf will shift to the outer to form shelf-margin deltas. It will result in the acceleration of sediment supply to the deep-water area, and then lowstand systems tract, such as debris-flow deposits, turbidites and frontal fan deposits will develop. By contrast, when the relative sea-level rises, depocenters will shift towards the middle and inner shelf, then transgressive systems tract and highstand systems tract characterized by hemipelagic and pelagic sediments and muddy debris-flow deposits will form. The deepwater sequence is bounded by condensed layers. There are muddy debris-flow deposits or turbidites above, and hemipelagic and pelagic sediments below the sequence boundary. The sequence boundaries in the well logs usually are lithological discontinuities. In GR logs,they turn from "micro-tooth-like" to retrogradated "bell-like" or aggradated "box-like" in succession. According to the depositional cycles mentioned above, we divided the Miocene in the basin into seven third-order sequences and two second-order sequences. The cyclic change in sea level decided the location of the depocenter as well as the sediment supply to the deep water area, in addition to the tectonic and climatic influences. Humid climate(ice house climate) plus tectonic uplifting will accelerate the erosion of land and increase the sediment supply and therefore influence the development of deep water sequences and their distribution.