Controls on turbidite sand deposition during gravity-driven extension of apassive margin: examples from Miocene sediments in Block 4, Angola

In recent years, exploration of the Lower Congo Basin in Angola has focused on the Neogene turbidite sand play of the Malembo Formation. Gravity tecto nics has played an important role during deposition of the Malembo Formatio n and has imparted a well-documented structural style to the post-rift sedi ments. An oceanward transition from thin-skinned extension through mobile s alt and eventually to thin-skinned compressional structures characterises t he post-rift sediments. There has been little discussion, however, regardin g the influence of these structures on the deposition of the Malembo Format ion turbidite sands. Block 4 lies at the southern margin of the Lower Congo Basin and is dominated by the thin-skinned extensional structural style. U sing a multidisciplinary approach we trace the post-rift structural and str atigraphic evolution of this block to study the structural controls on Neog ene turbidite sand deposition. In the Lower Congo Basin the transition from terrestrial rift basin to full y marine passive margin is recorded by late Aptian evaporites of the Loeme Formation. Extension of the overlying post-rift sequences has occurred wher e the Loeme Formation has been utilised as a detachment surface for extensi onal faults. Since the late Cretaceous, the passive margin sediments have m oved down-slope on the Loeme detachment. This history of gravity-driven ext ension is recorded in the post-rift sediments of Block 4. Extension commenc ed in the Albian in the east of the block and migrated westwards with time. In the west, the extension occurred mainly in the Miocene and generated al lochthonous fault blocks or "rafts", separated by deep grabens. The Miocene extension occurred in two main phases with contrasting slip vectors; in th e early Miocene the extension vector was to the west, switching to southwes t-directed extension in the late Miocene. Early Miocene faults and half-gra bens trend north-south whereas late Miocene structures trend northwest-sout heast. The contrast in slip vectors between these two phases emphasises the differences in driving mechanisms: the early Miocene faulting was driven b y basinward tilting of the passive margin, but gravity loading due to sedim entary progradation is considered the main driver for the late Miocene exte nsion. The geological evolution of the late Miocene grabens is consistent w ith southwest-directed extension due to southwest progradation of the Congo fan. High-resolution biostratigraphic data identifies the turbidite sands in Blo ck 4 as early Miocene (17.5-15.5 Ma) and late Miacene (10.5-5.5 Ma) in age. Deposition of these sands occurred during the two main phases of gravity-d riven extension. Conditions of low sedimentation rates relative to high fau lt displacement rates were prevalent in the early Miocene. Seafloor depress ions were generated in the hangingwalls of the main extensional faults, ult imately leading to capture of the turbidity currents. Lower Miocene turbidi te sand bodies therefore trend north-south, parallel to the active faults. Cross-faults and relay ramps created local topographic highs capable of def lecting turbidite flows within the half grabens. Flow-stripping of turbidit y currents across these features caused preferential deposition of sands ac ross, and adjacent to, the highs. Turbidite sands deposited in the early pa rt of the late Miocene were influenced by both the old north-south fault tr ends and by the new northwest-southeast fault trends. By latest Miocene tim es turbidite channels crosscut the active northwest-southeast-trending faul ts. These latest Miocene faults had limited potential to capture turbidity currents because the associated hangingwall grabens were rapidly filled as pro-delta sediments of the Congo fan prograded across the area from the nor theast. (C) 2001 Elsevier Science Ltd. All rights reserved.