TECTONIC SIGNIFICANCE OF SYN-RIFT SEDIMENT PACKAGES ACROSS THE GABON-CABINDA CONTINENTAL-MARGIN

The tectonic development of a continental margin is recorded in the st ratigraphic successions preserved along and across the margin in terms of stratal relationships (e.g., onlap, downlap, truncation), lithofac ies, biostratigraphy, and paleo-water depths. By using these observati ons coupled to a kinematic and flexural model for the deformation of t he lithosphere, we have elucidated the tectonic significance of the pr eserved stratigraphy that comprises the Gabon-Cabinda margin of west A frica. Two hinge zones, an Eastern and Atlantic, formed along the Gaba n-Cabinda margin in response to three discrete extensional events occu rring from Berriasian to Aptian time. The Eastern hinge zone demarcate s the eastern limit of a broadly distributed Berriasian extension that resulted in the formation of deep anoxic, lacustrine systems as evide nced by the silts and shales of the Sialivakou and lower Djeno Formati ons and the regressive packages of the upper Djeno Formation. Approxim ately 1.5 to 2 km of asymmetric footwall uplift was induced across the Eastern hinge zone in response to the mechanical unloading of the lit hosphere during this first phase of rifting. In contrast, the Atlantic hinge, located approximately 90 km west of the Eastern hinge, marks t he eastern limit of a second phase of extension that began in the Haut erivian. Footwall uplift and rotation exposed earlier syn-rift and pre -rift sediments to at least wavebase causing varying amounts of erosio nal truncation across the Atlantic hinge zone along much of the Gabon- Cabinda margins. We interpret the thickness variations of reworked ela stic sediment of this age (e.g. the Melania Formation) between the hin ge zones as indicative of variations in the degree of uplift and erosi onal truncation of the Atlantic hinge. For example, the absence of Mel ania Formation across the Congo margin implies that uplift of the Atla ntic hinge was relatively minor compared to that across the Cabinda an d Gabon margins, the latter being characterized by significant thickne sses of Melania Formation (or equivalent). Material eroded from the Ca binda and Gabon Atlantic hinge zone may in part account for the thick wedge of sediment deposited seaward of the Gabon-Cabinda Atlantic hing e (the Erva Formation). Our modelling suggests that this wedge of rewo rked elastics represents deposition by along-axis gravity flows within a deep water (approximate to 2 km) environment. A third and final pha se of extension in the late Barremian-early Aptian was responsible for breaching the continental lithosphere to farm the ocean/continent bou ndary and thus the installation of open marine conditions. Elsewhere, the environments will tend to be marginal marine to brackish, dependin g on the efficiency of the Atlantic hinge zone to act as a barrier to marine enchroachment. This third rift phase reactivated both the Easte rn and Atlantic hinge zones thereby creating accommodation for the Mar nes Noires Formation (and equivalent) source rock deposition between t he hinges and the Falcao source rock equivalent seaward of the Atlanti c hinge. 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 setting s (e.g., Congo margin), Alternatively, if hinge zone uplift was relati vely minor, then a coral-rimmed archipelago may have developed paralle l to the margin with restricted communication across the Atlantic hing e zone (e.g. , Cabinda margin), In this latter scenario, dilution of t he Marnes Noires source rocks by terrigenous input from the eroding At lantic hinge zone should be relatively minor thereby enhancing source rock quality, Furthermore, potential marine upwelling outboard of the Atlantic hinge zone is likely the cause for the production and accumul ation of organic-rich material associated with the Falcao source rock of the Kwanza basin. 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 Formati ons. The progradation and interaction of the Argilles Vertes depositio nal lobes resulted in the formation of residual paleo-relief. Subseque nt marine incursions and flooding of this paleo-relief led to the deve lopment of basal conglomerates (the Chela 'lag' unconformity) grading upward into fine-grained sands and evaporites, Consequently, an invers e relationship should exist between evaporite thickness (in particular , the lower members) and the thickness of the underlying Argilles Vert es and Tchibota Formations. Variations in Loeme evaporite thickness is a consequence of stratigraphic and structural control with salt insta bility influencing local variability, Our modeling suggests the occurr ence of two distinct evaporite sequences on the Congo margin, an earli er evaporite deposited seaward (west) of the Atlantic hinge during the second and third rift phases and the late Aptian Loeme Formation depo sited between the hinge zones. An evaporite sequence seaward of the At lantic hinge is inferred on the basis of extensive diapirs and salt te ctonic structures observed in seismic data. In order to match the dist ribution and thickness of the observed post-salt stratigraphy across t he basin, however, we require large paleowater depths west of the Atla ntic hinge during the later Aptian, The existence of large paleowater depths precludes the formation of thick evaporite sequences within the outer basin, Consequently, we propose that the evaporites seaward of the Atlantic hinge were formed during the syn-rift development of the margin and are not contemporaneous with the post-rift Loeme salts depo sited between the hinge zones. This double salt hypothesis is consiste nt with observations from the conjugate Brazilian margin. (C) 1998 Els evier Science Ltd, All rights reserved.