FABRICS AND GENESIS OF MICROBIAL REEFS IN THE TERMINAL NEOPROTEROZOIC NAMA GROUP (CENTRAL NAMIBIA)

Microbial reefs occur at several stratigraphic levels in the Terminal Neoproterozoic Nama Group of central Namibia. Reefs commonly developed during flooding periods on a high-energy carbonate ramp. The location on the ramp, available accommodation space, and variations in sediment flux controlled the geometry of reefs and possibly also the internal fabrics. Field data were collected with digital surveying techniques to enable 3-dimensional visualization of reef geometry, stratal patterns and facies distribution at a scale of 5 cm or better.

Reefal facies are dominantly microbial, including simple stromatolite columns, stromatolite rinds, mounds of stromatolitic thrombolites and thrombolites, thrombolite columns, and dendrolites. Additional facies include talus breccias, irregularly laminated carbonates and Cloudina-Namacalathus skeletal grainstones.

Field observations suggest that the microbial organisms constructed the main reef framework. The associated fossils apparently preferred the stable substrate in the reef, but they did not play an active role in reef construction. In addition, skeletal grainstones may have provided firm sites for initial reef nucleation.

Reef structures on different scales (individual microbial column to entire reef) show a succession from massive fabrics in the center to laminated fabrics around the margins. This feature is attributed to (1) a change in the microbial community, or (2) higher sediment fluxes around the margins of these structures.

Fabrics have a clear relationship to reef geometry. Biostromes and small patch reefs are usually composed of stromatolite columns, smaller thrombolite columns, and grainstones. In contrast, pinnacle reefs contain larger thrombolite mounds, talus breccias and neptunian dikes that are evidence for significant synoptic relief.