NEOPROTEROZOIC MICROBIAL-METAZOAN REEFS, NAMA BASIN, NAMIBIA

The terminal Proterozoic–early Cambrian Nama Group (> 3000m) outcrops extensively in southern Namibia, as well-exposed, fossiliferous shallow marine carbonate and siliciclastic rocks. In the Zaris Subbasin, reef complexes are found in the Omkyk Member, within the Zaris Formation of the Kuibis Subgroup. These include the Driedoornvlagte pinnacle reef complex, over 300 m thick and at least 7 km long, which developed within a stable carbonate ramp succession. Sequence stratigraphic and chemostratigraphic records, calibrated by high-precision U-Pb zircon dates from volcanic ash-beds, provide an exceptionally well-constrained chronostratigraphy. An ash-bed that immediately overlies the Omkyk Member has been dated as 548.8 ± 1 Myr BP. Reefs are dominated by thrombolitic and stromatolitic fabrics, and may be surrounded by fossiliferous wackestones, packstones or grainstones. Reefs have yielded three taxa of skeletal metazoans: the weakly skeletal, solitary Cloudina and Namacalathus, and the large, robustly skeletal, and modular Namapoikia. Cloudina may be found apparently attached to thrombolitic heads as well as providing abundant bioclastic debris. Namacalathus cups are often enclosed within the microbial reef fabrics, although no direct attachment of the supposed stalk to a microbial substrate has been observed. Namapoikia is noteworthy for its unusual paleoecology, as this organism encrusts the walls of vertical, open fracture systems that formed through early lithification, now expressed as neptunian dyke complexes. Individuals may completely fill the dyke void or be encrusted by stromatolites. The preferred habit of Namapoikia within of microbial mounds is noteworthy in that it adds a novel ecological dimension to early reef communities. Terminal Proterozoic reefs have been thought to be ecologically simple and of low biodiversity, but the presence of three skeletal taxa confirms both a significant metazoan component to these communities, and that differentiation of reef biota into distinct open surface and cryptic inhabitants - so characteristic of Phanerozoic reefs - had taken place by the end of terminal Proterozoic time.