Environmental and Biologic Controls on Carbonate Platform and Reef Development, Terminal Neoproterozoic Nama Group, Namibia

本项目将通过研究由微生物和非生物碳酸盐形成的元古代台地到由钙化的后生动物和变质动物骨骼形成的显生宙台地的转变，试图提高我们对碳酸盐台地一级长期演化的理解。本研究将尝试记录纳米比亚Nama和Witvlei群跨越新元古代晚期的几个碳酸盐台地和礁体的详细古环境，以及钙化无脊椎动物和变质动物对沉积物产生、台地古生态和相发育的初步影响。我们将具体研究：1)各种相的组成和分布，2)微生物岩形态与微组构之间的关系，3)微生物性与钙化后生动物和变质动物之间的关系，以及4)骨骼颗粒，微生物成分和胶结物的初级矿物学。根据其相、微生物岩和骨骼化石组合的多样性，以及逐渐年轻的地层位置和年龄，我们选择了三个具有代表性的单元进行研究。在上升的地层顺序中，包括：1)Bushmanskippe组（Witvlei组），其基底单元（Bildah组）被认为是一种“帽状碳酸盐岩”（可能与新元古代冰盖脱冰有关的特殊碳酸盐岩）；2)Nama组基底的Omkyk组，具有三维地层暴露，包含壮观的微生物岩- cloudina尖顶礁；3)纳玛群中匈奴成员，含钙化变质微生物岩斑块礁和微生物岩壳壳化石尖顶礁。该研究将为了解最早钙化后生/变质动物与微生物生态系统之间的相互作用、新元古代晚期碳酸盐沉积物的来源以及骨骼沉积物对碳酸盐沉积体系的相和长期演化的影响提供重要信息。

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.