Regionally extensive 3D seismic data from the Lower Congo Basin, offshore Angola, have been used to investigate the influence of salt‐related structures on the location, geometry and evolution of Miocene deep‐water depositional systems. Isochron variations and cross‐sectional lap‐out relationships have then been used to qualitatively reconstruct the syn‐depositional morphology of salt‐cored structures. Coherence and Red‐green‐blue‐blended spectral decomposition volumes, tied to cross‐sectional seismic facies, allow imaging of the main sediment transport pathways and the distribution of their component seismic facies. Major sediment transport pathways developed in an area of complex salt‐related structures comprising normal faults, isolated diapirs and elongate salt walls with intervening intraslope basins. Key structural controls on the location of the main sediment transport pathways and the local interaction between lobe‐channel‐levee systems and individual structures were the length and height of structures, the location and geometry of segment boundaries, the growth and linkage of individual structures, and the incidence angle between structural strike and flow direction. Where the regional flow direction was at a high angle to structural strike, transport pathways passed progressively through multiple intraslope basins in a fill and spill manner. Segment boundaries and structural lows between diapirs acted as spill points, focusing sediment transport between intraslope basins. Channel–lobe transitions are commonly associated with these spill points, where flows expanded and entered depocentres. Deflection of channel‐levee complexes around individual structures was mainly controlled by the length of structures and incidence angle. Where regional flow direction was at a low angle to structural strike, sediment transport pathways ran parallel to structure and were confined to individual intraslope basins for many tens of kilometres. Spill between intraslope basins was rare. The relative position of structures and their segment boundaries was fixed during the Miocene, which effectively pinned the locations where sediment spilled from one intraslope basin to the next. As a result, major sediment transport pathways were used repeatedly, giving rise to vertically stacked lobe‐channel‐levee complexes along the pathways. Shadow zones devoid of coarse clastics developed in areas that were either structurally isolated from the sediment transport pathways or bypassed as a result of channel diversion.

No abstract

Overthe last decade comprehensiveexploration for turbiditeplays hasbeenrewarded byasignificant numberofhydrocarbon discoveries. However,the industry isstill critically challenged withrespecttoappropriate andrefined prediction ofsand/shaled istributions withint hesed epositionals ystems. Alongwithastronga nd sustained commercialinterest intheseplays,extensivesubsurface datasets,includinganewgeneration ofcoreandwell/log-calibrated seismic faciessuites,havebecomeavailable.Based on thisv aluablei nformation,systematic 2Da nd3Dseismic sequence stratigraphicalanalysisw ith particularfocus on seismic faciesandseismic geomorphologywasperformed infivedifferent basins alongNorth andSouthAtlantic margins. Each subsurface examplewascharacterized withreference to its relativesequence stratigraphic orderw ithinah ierarchy. Evolvingg eologicalp arameters like shelfa ccommodation,shelf-break position,slopegradient,slopeandbasinbathymetry/topographyandthird-, fourth-andoccasionally fifth-order stackingpatterns wereassessed withinsecond-ordercyclesineach case.Based on theseobservations,recognition criteria for subsurface lithologyp rediction wered erived.Conceptualknowledgef rom outcrop analysisw asan essentialandintegralpart ofthe interpretation process throughout the study.The subsurface recognition criteria, highlighted inthisstudy,emphasizesimilarity andvariability betweentwo end-memberbasins tyles:h igh shelf-to-basinr elief/sediment under-filled basins wheresand-pronenearshore systems don ot prograde overdeep-waterfans ystems inasecond-ordercyclec ontext; low shelf-to-basin relief/sediment over-filled basins wheresand-pronenearshoresystems doprograde overdeep-waterfansystems inasecond-ordercyclecontext.The most strikingc ontrast withrespectt odeep-waterr eservoirs andstoned istribution int he two basin categoriesrelatestothe fourth-andfifth-orderstackingpatterns offanbuilding-block cycleswithinsecond-and third-ordercycles,which tendto show low progradation/aggradation ratioint he high relief basins andh igh progradation/aggradation ratioint he low relief basins. Givenanu nderstandingofg eologicalcontext,these seismically identifiablepatterns provide morea ccurateprediction ofsanda ndshaled istribution withindeepwaterclastic systems.

The West African continental margin evolution is preserved in a small source-distant setting (20 • 30 km area) by changes in lobe-channel-levee seismic geomorphological elements within a threefold seismic stratigraphic hierarchy. The c. 32 Ma depositional record of rift, drift and depositional outbuilding of the margin by gravity-driven adjustment, deformation and deposition produced a hierarchy of second-through fourth-order stratigraphic cycles bounded by laterally continuous fine-grained drapes inferred to record prolonged periods of sediment starvation. The margin outbuilding phase, the focus of this contribution, consists of three second-order adjustment bounded cycles (ABCs) that record major adjustment and/or modification of the deepmarine depositional system. Seven third-order cycles also show changes in depositional trend and seismic facies architecture. Ten fourth-order cycles, best resolved within the upper part of the succession, consist of multiple, wedge-shaped and compensating, lobe-channel-levee complexes up to 20 km wide. These complexes show an upward increase in channel-levee and decrease in lobe proportion. They also show an upward change from lobes incised by sinuous channels to channels deflected to lobe flanks. Outcrop and shallow core-calibrated analogues from the Permian Brushy Canyon Formation, and modern Amazon and Zaire Fans help constrain these patterns. Changes in the sediment composition and volume of subaqueous flows at their point of origin, and subsequent gravitational deformation, syn-sedimentary mass-wasting and large-scale fan avulsion punctuating deep-marine sedimentation, adjust deep-marine depositional pattern during basin margin outbuilding. Lobe-channel-levee distributions in this sediment source-distant setting record a progressive increase in local topographic relief and gradient related to the basinward migration of deformation during depositional outbuilding of the continental margin. Two important conclusions derived from this record include (1) the importance of local seabed topography and gradient on producing changes in depositional pattern, and (2) that repeated and cyclic changes in these patterns reflect adjustment/deformation within, and probably restricted to, the deep-marine record. Integrated seismic stratigraphic and geomorphic analysis delineates multiple scales of these adjustment-bounded cycles. The evolving map patterns record adjustment by shifts in geomorphic pattern and orientation. These local geomorphic changes can be used to predict longerterm and larger-scale changes in the depositional record of the continental margin evolution. This analytical approach should have general utility along high shelf-to-basin relief margins with similar gravity-driven deformation. Integrated seismic stratigraphic/ geomorphic analysis Chrono strati g raphic f r a m e w o r kThe north-south oriented seismic type-section displays the geology of a c. 3 s (two-way travel time)

No abstract