MCGINNIS, JOHN P., GARRY D. KARNER, and NEAL W. DRISCOLL, Lamont-Doherty Geological Observatory, Palisades, NY; WILLIAM D. BRUMBAUGH, Conoco, Worldwide Exploration Services, Houston, TX; and NICK CAMERON, Conoco (UK) Ltd., London, England

ABSTRACT: Tectonic Significance of Synrift Sediment Packages Across the Congo Continental Margin

The tectonic and stratigraphic development of the Congo continental margin reflects the timing, magnitude, and distribution of lithospheric extension responsible for its formation. Details of the lithospheric extension process are recorded in the stratigraphic successions preserved along and across the margin. By using the stratal relationships (e.g., onlap, downlap, and truncation) and lithofacies determined from seismic reflection and exploratory well data as input into our basin-modeling strategy, we have developed an integrated approach to determine the relationship between the timing, magnitude, and distribution of lithospheric extension across the margin.

Two hinge zones, an eastern and Atlantic hinge, formed along the Congo margin in response to discrete extensional events occurring from the Berriasian to the Aptian. The eastern hinge zone demarcates the eastern limit of the broadly distributed Berriasian extension. This extension resulted in the formation of deep anoxic, lacustrine systems as evidenced by the silts and shales of the Sialivakou and lower Djeno formations and turbidites of the upper Djeno Formation. Our modeling predicts approximately 1.5 to 2 km of asymmetric footwall uplift across the eastern hinge zone in response to the mechanical unloading of the lithosphere during the first phase of rifting. In contrast, the Atlantic hinge, located (approx.) 90 km west of the eastern hinge, marks the eastern limit of a second pha e of extension, which began in the Hauterivian. Consequent footwall uplift and rotation exposed the earlier synrift and prerift stratigraphy to at least wave base causing varying amounts of erosional truncation across the Atlantic hinge zone along much of the Gabon, Congo, and Angola margins. We interpret the thickness variations of reworked clastic sediment (e.g., the Melania Formation) between the hinge zones as indicative of variations in the degree of uplift and erosional truncation of the Atlantic hinge. Therefore, the absence of the Melania Formation across the Congo margin implies that uplift of the Atlantic hinge was relatively minor compared to that across the Angola and Gabon margins. In addition, material eroded from the adjacent and topographically higher hinge zones may in p rt account for the thick wedge of sediment deposited seaward of the Congo Atlantic hinge. Thus, we propose that this thick wedge of reworked clastics represents deposition by along-axis gravity flows within a deep-water environment ((approx.) 2 km).

A third phase of extension reactivated both the eastern and Atlantic hinge zones and was responsible for creating the accommodation space for Marnes Noires source rock deposition. Two possible scenarios exist for the lateral distribution of the Marnes Noires Formation. If the reactivated rift flank topography across the Atlantic hinge was significant, then sedimentation would be restricted between the hinge zones within discrete lacustrine settings. Alternatively, if hinge zone uplift was relatively minor, then an archipelago may have developed parallel to the margin with open communication across the Atlantic hinge zone. In this latter scenario, the Marnes Noires Formation would be laterally extensive across the entire continental margin. Finally, by Aptian, the remaining accommodati n space between the hinge zones was filled with deltaic sands and shales of the "Tchibota"/Pointe Indienne (Argilles Vertes) formations until bypass transported sediment into the outer basin to form the "Chela" Formation west of the Atlantic hinge.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.