The tectonic development of the west African continental margin is recorded in the stratigraphic successions preserved along and across the margin in terms of stratal relationships (e.g., onlap, downlap, truncation), lithofacies, biostratigraphy, and paleo- water depths. Two hinge zones, an Eastern and Atlantic, formed along the Gabon-Cabinda margin in response to three discrete extensional events occurring from Berriasian to Aptian time. The Eastern hinge zone demarcates the eastern limit of a broadly-distributed Berriasian extension that resulted in the formation of deep anoxic, lacustrine systems as evidenced by the silts and shales of the Sialivakou and lower Djeno formations and the regressive packages of the upper Djeno formation. Approximately 1.5 to 2 km of asymmetric footwall uplift was induced across the Eastern hinge zone in response to the mechanical unloading of the lithosphere during this 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 phase of extension that began in the Hauterivian. Footwall uplift and rotation exposed earlier syn-rift and pre-rift sediments to at least wavebase causing varying amounts of erosional truncation across the Atlantic hinge zone along much of the Gabon-Cabinda margins. We interpret the thickness variations of reworked clastic sediment of this age (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. For example, the absence of Melania Formation across the Congo margin implies that uplift of the Atlantic hinge was relatively minor compared to that across the Cabinda and Gabon margins, the latter being characterized by significant thicknesses of Melania Formation (or equivalent). Material eroded from the Cabinda and Gabon Atlantic hinge zone may in part account for the thick wedge of sediment deposited seaward of the Gabon-Cabinda Atlantic hinge (the Erva Formation). Our modeling suggests that this wedge of reworked clastics represents deposition by along-axis gravity flows within a deep water (approx 2 km) environment.
A third and final phase of extension in the late Barremian-early Aptian was responsible for breaching the continental lithosphere to form the ocean/continent boundary and seafloor spreading between Africa and South America. This third rift phase reactivated both the Eastern and Atlantic hinge zones thereby creating accommodation for the Marnes Noires Formation (and equivalent) source rock deposition across the margin. 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 (e.g., Congo margin). Alternatively, if hinge zone uplift was relatively minor, then a coral-rimmed archipelago may have developed parallel to the margin with open communication across the Atlantic hinge zone (e.g., Cabinda margin). In this latter scenario, the distribution of Marnes Noires source rocks would be more laterally extensive across the continental margin.
By late Aptian time, the remaining accommodation between the hinge zones was partially filled by across- and along-axis prograding deltaic systems of the Argilles Vertes and Tchibota formations. The progradation and interaction of the Argilles Vertes depositional lobes resulted in the formation of residual paleo-relief. Subsequent marine incursions and flooding of this paleo-relief led to the development of basal conglomerates (the Chela "lag" unconformity) grading upward into fine- grained sands and evaporites. Consequently, an inverse relationship should exist between evaporite thickness and the thickness of the underlying Argilles Vertes and Tchibota formations. Variations in Loeme evaporite thickness is a consequence of stratigraphic and structural control with salt instability influencing local variability.
