DIGITAL MAPPING OF THE GEOMETRY AND STRATAL PATTERNS OF MICROBIAL REEFS, TERMINAL NEOPROTEROZOIC NAMA GROUP, NAMIBIA

We have used digital surveying technology to quantitatively characterize the geometry and stratal patterns of well-exposed, seismic-scale, Terminal Neoproterozoic microbial reefs of the Nama Group of central Namibia. Combining 3D digital data obtained from differential GPS mapping with digital elevation models (DEM) derived from quantitative photogrammetry facilitates the visualization and understanding of important stratal and geometrical relationships. The ca. 5 cm resolution of the differential GPS allows one to map dm-scale structures such as individual reef mounds, as well as large-scale stratal patterns and facies distributions. The studied reefs are an analogue to subsurface hydrocarbon reservoirs in the South Oman Salt Basin. The digital mapping of the hierarchy and length-scale variations of the Namibian outcrop analogues will help in the construction of quantitative 3D reservoir models in Oman.

Two study areas have been mapped. The first area along the Zebra River canyon cuts through several stratigraphic levels of the Omkyk Member of the Nama Group. Continuous biostromes are developed in an updip ramp position, but break up into patch reefs in a downdip direction. Reefs are built by stromatolitic and thrombolitic columns. Individual reefs interfinger with and are onlapped by both grainstones and shales. The second area lies approximately 80 km down dip paleogeographically. In this locale, a large isolated pinnacle reef outcrops on the Slangpoortrand Mountain. This reef has a width of 8 km and a thickness of approximately 300 m. Several stages of reef development are recognized and the general growth, stratal patterns, and spatial distribution of facies have been mapped. The platform interior and margin facies consist of thrombolites and stromatolites forming mounds and columns that alternate with grainstones, breccias and irregularly laminated carbonates. Detailed mapping has also revealed numerous facies in the platform-to-basin transition, including irregularly laminated carbonates, turbiditic grainstones, breccias, as well as toe-of-slope thrombolite mounds.