High resolution sedimentologic and stratigraphic investigation of a storm-dominated carbonate ramp, Hoogland Platform (ca. 549 Ma), Nama Group, Namibia

Abstract

A high resolution stratigraphic study of the Terminal Proterozoic (ca 549 Ma) Hoogland Member of the Kuibis Subgroup, Nama Group, Namibia, revealed the controls on overall carbonate ramp platform morphology in a storm-dominated foreland basin setting. It was found that a generalized suite or class of physical processes that act to restore the seafloor to a relatively flat state by sweeping sediment from the highs and deposit in the lows does not allow for the continued inheritance and propagation of relief between sedimentary "elements" (e.g. laminae, bed, bedset) or larger scale "entities" (e.g. parasequences, systems tracts, depositional sequences). This damping of topographic elements on the seafloor was found to act at a variety of temporal and spatial scales from the sub-annual and sub-meter laminae and facies scale, through the Milankovitch band and meter-scale of parasequences up to the millions of years and scores to hundreds of meters of a depositional sequence. At the meter-scale, bioherms at the base of the studied stratigraphic interval are shown to have been covered and smothered by heterolithic interbeds of shale and fine grained carbonate mud-dominated facies. Facies-scale microbial laminites are shown to posses paleogeographic dip position-dependent centimeter-scale roughness elements that increase in roughness downdip. Relief produced along the tops of laminae is damped out by an overlying sediment rich layer suspended by storms or produced as whitings in the water collumn. Lateral extents of carbonate capped parasequences were found to vary as a function of position within a systems tract or genetic (accomodation) cycle. Late transgressive parasequences are found to be more extensive than those deposited during early transgressive and late highstand conditions due to the presence of basinward thickening shale wedges at their bases. This accommodation space-filling shale acted to decrease the slopes on the platform and allowed the influence of storm wave and current induced seabottom shear stresses to act over a greater area. The presence of a basinward accommodation-filling sediment source evidenced in the basal shale wedges of this foreland basin setting precluded the development of a steep sided rimmed shelf edge. A self-reinforcing ramp profile was maintained despite the fact that thrombolitic and stromatolitic reef forming organisms and processes were present.