Abstract A phenomenon unique to fine-grained sediment is its ability to alter the physical characteristics of the overlying water column. Although the present state of research recognizes many aspects of fine-grained seabed and water column interactions, this study documents how an energetic sandy, shallow marine system can autogenically transition to a system capable of accumulating fine-grained bedforms to clinoforms. To understand these transitional processes this study examines the lithostratigraphy and depositional history of the Suriname portion of the Guiana Coast (French Guiana, Suriname, and Guyana). Four major lithologic facies (Pre-Holocene silty clay; peat-rich silty clay; sandy mud; silty clay with cheniers) were derived from the Late Holocene sea level rise and influx of sediments emitted from the Amazon River. Since approximately 6000 BP, ~10 to 20% of Amazon-derived sediments bypass the Amazon shelf and are transported northwestward toward the study area. Along the Suriname coast (~900 km from the Amazon), however, significant mud accumulation did not commence until 3000 to 3500 BP. Suspended sediments can travel this distance in less than 1 month. A migrating (1.5 km/yr) mud bank could travel the 900 km from the Amazon mouth in 600 years or, after formation of a 400-km-wide subaqueous Amazon delta (by 1200 BP), migrate the remaining 500 km by 4500 BP, still 1000 to 1500 years prior to the time period during which radiocarbon dates indicate significant mud accumulation began. Consequently, either there was a major hiatus in sediment transport and accumulation between 6000 BP to 3000 BP or some other transport process other than suspension or mud-bank migration-controlled initial mud accumulation. Assuming steady-state conditions, lateral accretion rates from 6000 BP to 3000 BP equate to 0.3 to 0.4 km/yr. These rates, which are similar to migration rates cited by previous studies for the trailing edges of mud banks in French Guiana, may reflect postmigration erosion of the initial mud banks. Whether there is an erosional overprint or some other process, this lateral accretion rate is an indicator of the amount of fluid mud necessary for ‘mud to beget mud.’ More general prerequisites necessary for a shallow marine setting to autogenically form fine-grained clinoform-scale accumulations are, first, a single large source of muddy sediments (a major river) and, secondly, unidirectional transport processes to concentrate and continuously supply mud sediments to the system.

Abstract The recent surge of exploration activities over distal margins, with the acquisition of more and more high-quality and deep seismic data, has led to enhance concepts of the deformation and subsidence history of passive margins in general and sheared margins in particular. The French Guiana sheared margin is very narrow. The thinning of the upper crust is accommodated by few major faults relayed by well-expressed transfer zones, giving a general oblique trend to the margin. Another possible effect of the shear component during the rifting is the presence in the distal domain of a Moho high. Its exhumation is coeval with the emplacement of a deltaic system coming from the Demerara Plateau, evidencing a probably important early subsidence of the margin. This early subsidence in the late-rifting stage is increased during the early drifting, when the thinned crust reached its isostatic/thermal equilibrium in the Cenomano-Turonian before suffering an important Late Cretaceous sedimentation load. In the Palaeogene, starving of the margin and significant uplifts in the Guiana Craton are observed, possibly resulting from the rise of the Purus Arch (Andes fore-bulge?). Finally, the Amazon deposition by the Late Miocene–Pliocene provoked a large subsidence in the distal domain.

Abstract The morphology of a 1250 km 2 portion of the middle slope off the western Niger Delta shows that gradients on the Pleistocene slope vary both spatially and at different stratigraphic levels. In the deeper section, three lower-gradient steps are connected by three higher-gradient ramps, generating a stepped-slope morphology. Through time, preferential accumulation of slope aprons, composed of mass-transport deposits, compensationally stacked lobes, and overbank deposits (wedge-shaped outer levees), helped fill slope accommodation, smoothing over the gradient change across ramps and steps, and vice versa. Consequently at the local scale, the stepped slope evolved into a smoother slope that is nearly graded at the modern seafloor. As in other studies, preferential accumulation of sediment on the slope is believed to reflect in part the deceleration of sediment gravity flows (both turbidity currents and debris flows) as they encountered lower-gradient steps. Down-slope changes in slope morphology also caused variations in the amount, and presumably rate, of erosion along the axes of canyons in the study area—with increased incision depth where knickpoints cut through positive-relief bathymetric structures in an attempt to establish a graded profile. Along the Benin-major Canyon there is an inverse linear relationship between the thickness of deposits that accumulate on the slope adjacent to the canyon and the amount of vertical erosion along its axis. The thickest outer levee deposits coincide with canyon segments that have the shallowest incision, in turn corresponding to slope segments showing a sharp decrease in pre-incision gradient. This implies that the increase in sediment flux to outer levees on some parts of the stepped slope results from a combination of increased overspill from flows passing through shallower canyon reaches, and increased sedimentation caused as mud-dominated flows decelerated on lower-gradient slope segments immediately adjacent to the canyon. Thus there appears to be an intimate relationship between slope morphology, canyon incision depth, and the thickness of overbank deposits adjacent to canyons.