Tectono-Stratigraphic Evolution of the Onshore Namibe-Benguela-Kwanza Basins, Angola - Implications for Margin Evolution Models

Rifts III: Catching the Wave 22-24 March 2016 The Geological Society, Burlington House, LondonOnshore Angolan outcrops have been studied as part of a Brazil-West African conjugate margin project. Focus has been on rift margin to break-up axis tectono-stratigraphic evolution and associated strike variability of oblique and orthogonal rifted segments (Higgins et al., this conference). In this contribution we highlight key observations from Pre- and Post-Salt outcrops that occur as an elongate coastal strip extending from Namibe in the south to Kwanza in the north. Rift initiation deposits include interbedded basalts, rhyolites and aeolian-alluvial clastics that sit with low angle discordance on Pre-Cambrian basement. Volcanics (dated 132Ma) have geochemical affinity to the Parana-Etendeka LIP. Pre-rift fluvial-lacustrine deposits are locally evident beneath the volcanics, preserved as incised antecedent valleys. Rift climax and early sag sediments are absent onshore, with deposition focused outboard of the hinge line (present day offshore), whilst areas inboard of the hinge underwent uplift, erosion and sediment by-pass. By Early Aptian times a progressively onlapping-overstepping fluvial to marine Pre-Salt succession was deposited along the entire West African margin, cumulating in deposition of Loeme Fm evaporites. This succession includes spectacular fluvial-tidal incised palaeovalleys overlain by algal laminate-sabkha facies with halite pseudomorphs that pass transitionally into the overlying evaporite. Importantly for geodynamic models, this latest sag megasequence transgresses the subaerial rift margin, and effectively ¿seals¿ rift-related faults. The presence of evaporites within all studied palaeovalleys indicates extreme aridity and rift margin drainage shut-off. Sag outcrops also include lacustrine and ¿dam and cascade¿ carbonates (travertines, Sharp et al., 2013). Secondary diagenesis is complex (karstification, dolomitisation, silicification). The onshore observation of rift-initiation clastics/volcanics overlain by latest sag transgressive sediments, separated by a significant unconformity, can be linked to offshore seismic stratigraphic megasequences and distal margin evolution. Original onshore depositional thickness of evaporite, and percentage halite, is hard to estimate - 200-500m is postulated based on halokinetic structures within the overlying Pinda Gp, the basal part of which is pre-halokinetic (Binga Mbr), whilst the overlying mixed carbonate-clastic succession (Dondo-Catumbela) is syn-halokinetic, with spectacular growth geometries. Significant hinge line rotation is also recorded by the Albian succession. In all 3 basins Pinda Gp clastics show a close spatial relationship to Pre-Salt Aptian and modern valleys, indicating W. African drainage is antecedent in nature. Late Albian platform collapse follows, associated with extensional faulting, establishment of U. Albian slope successions and widespread emplacement of shallow sub-volcanic intrusives, subaerial to subaqeous lava flows and major volcanic centres (Sumbe Volcanics). Volcanism is diachronous along strike (Kwanza - 100-97Ma, Benguela - 91-86Ma, Namibe - 88-79Ma). In the Namibe Basin volcanism is proceeded by thick (>1500m) Albian-Cenomanian alluvial clastics with divergent hinge line growth geometries. Clastics are point sourced, associated with palaeovalley and palaeofan morphologies. Disconformably overstepping the Albian and older units is a Cenomanian-Coniacian transgressive sequence (Salinas Fm), itself intruded and overlain by Sumbe Volcanics. Volcanism in Namibe was locally associated with a return to subareal to lacustrine conditions (including travertines). Finally, flat lying successive transgressive-regressive passive margin wedges of Santonian-Campanian, Maastrichtian, Eocene, Miocene, Plio-Pleistocene and Holocene age were deposited, locally associated with spectacular seismic scale clinoforms.Rifts III