ABSTRACT

A terminal Proterozoic (ca. 550 Ma) isolated carbonate platform developed downdip on a carbonate ramp of the Nama Group of southern Namibia. The stratigraphic evolution of the platform was reconstructed from an extensive dataset that was compiled by using digital surveying technologies and traditional outcrop observations. Digital field geology techniques enabled rapid and accurate mapping over the 10 km outcrop belt, with spatial and geological information captured at scales ranging from centimeters to kilometers. Furthermore, these digital geology techniques made the reconstruction and interpretation of the development of the carbonate platform considerably more efficient and quantitative.

The carbonate platform deposits encompass three accommodation cycles in which each subsequent cycle experienced progressively greater influence of a long-term accommodation increase. Aggradation and progradation during the first cycle produced a flat-topped, sheet-like platform. Deposits of the first cycle show a coarsening-upward motif grading from dominantly columnar stromatolitic thrombolites and interbedded mudstone-grainstone at the base, to dominantly massive and cross-bedded dolostones toward the top. The second cycle features aggradation and is typified by the formation of a distinct margin and development of a bucket-shaped geometry. The margin contained thrombolite mounds and domes, whereas columnar stromatolitic thrombolites dominated the platform interior. The third and last cycle records a gradual deepening trend with initial aggradation and formation of well-bedded, thin deposits in the interior and microbial mounds at the margins. While the interior drowned, the platform margins kept up with rising sea level and complex pinnacle reefs formed, which contained fused and coalesced thrombolite mounds flanked by bioclastic grainstones and collapse breccias. The top of the unit consists of a set of isolated large thrombolite mounds flanked by shale, and is interpreted to indicate the final stage of the carbonate platform before drowning.

The overall geometric evolution and spatial scale of this platform, from flat-topped to bucket-shaped with elevated margins, is recorded in many Proterozoic and Phanerozoic isolated carbonate platforms. This study clearly illustrates that accommodation changes are a fundamental control on the development of carbonate platforms, regardless of whether they are formed from microbial or metazoan components, or are of Precambrian or Phanerozoic age. Significantly, the architectural elements and spatial dimensions are comparable throughout earth history.