MULTIPLE ORIGINS OF DIVERSE MICROBIAL FABRICS IN PRECAMBRIAN THROMBOLITES

Constraining the processes influencing microbialite morphological and textural variability is important for interpreting temporal and environmental distributions of stromatolites and thrombolites throughout the geologic record. In the Neoproterozoic Beck Spring Dolomite, the co-occurrence of diverse microbialites provides an exceptionally interesting opportunity to evaluate the environmental, biological, and taphonomic controls on microbialite morphogenesis, especially microfabric development. Beck Spring thrombolitic microbialites are of particular interest given the paucity of Precambrian thrombolites; they provide further evidence that thrombolites are not biostratigraphically limited to Phanerozoic strata, and their excellent textural preservation provides insights into the origins of Precambrian thrombolitic microfabrics.

Beck Spring stratigraphic and petrographic relationships demonstrate that stromatolitic, thrombolitic, and composite microbialites formed in close spatial and temporal association in similar shallow water environments. Although environmental conditions influenced large-scale microbialite distributions, internal fabrics reflect diverse community compositions within a single depositional environment. Filmy laminated fabrics can alternate with primary clotted fabrics on a sub-mm scale. This co-occurrence of laminated and clotted fabrics demonstrates that morphologically distinct microbial communities grew simultaneously in the same environment. Thus, the morphology and organization of the constructing communities influenced growth microfabric. However, other microfabrics were significantly influenced by taphonomic processes including biological degradation and physical disruption during very early diagenesis. These fabrics consist of diffuse patches of inclusion-rich micrite that resemble crude clots and define a diffuse lamination. Sub-mm scale textural intergradation between taphonomic and filmy microfabrics suggests both were derived from a laminated growth fabric. Thus, diverse microfabrics can develop from distinct microbial communities or from variable preservation of a single community. In particular, clotted fabrics originate from both taphonomic and primary biological processes.