Grotzinger 9628257 This project will attempt to improve our understanding of the first-order, long-term evolution of carbonate platforms by investigating the transition from Proterozoic platforms formed of microbial and abiotic carbonates to Phanerozoic platforms formed of calcified metazoan and metaphyte skeletons. The proposed research will attempt to document the detailed paleoenvironments of several carbonate platforms and reefs in the Nama and Witvlei groups (Namibia) that span terminal Neoproterozoic time, and the first influences of calcified invertebrates and metaphytes on sediment production, platform paleoecology, and facies development. We will specifically study: 1) the composition and distribution of various facies, 2) the relationships between microbialite morphology and microfabric, 3) the relationships between microbialities and calcified metazoans and metaphyytes, and 4) primary mineralogy of skeletal grains, microbial components, and cements. We have selected three representative units for investigation based on their diversity of facies, microbialite and skeletal fossil assemblages, and progressively younger stratigraphic positions and age. In ascending stratigraphic order these include: 1) the Bushmanskippe Formation (Witvlei Group), whose basal unit (Bildah Member) is considered to be a "cap carbonate" (peculiar carbonates possibly associated with deglaciation of Neoproterozoic ice sheets; 2) the Omkyk Member of the basal Nama Group which has 3-D stratigraphic exposures and contains spectacular microbialite-Cloudina pinnacle reefs; and, 3) the Huns Member of the middle Nama Group which contains calcified metaphyte-microbialite patch reefs and microbialite-shelly fossil pinnacle reefs. The proposed research will provide important information that will be useful in understanding interactions within the earliest calcified metazoan/metaphyte and microbial ecosystems, the sources of terminal Neoproterozoic carbonate sediment, and impact of skeletal sediment on the f acies and long-term evolution of carbonate depositional systems.
