CLOTTEDNESS IN CAMBRIAN MICROBIALITES: EXAMINING TEXTURAL VARIATIONS IN THE DEFINING CHARACTERISTIC OF THROMBOLITES

Thrombolites reflect the interplay between microbial communities, sedimentation, and lithification, and therefore provide a record of sediment-organism interactions through much of Earth history. Although thrombolites are defined as internally clotted microbialites, the character of clottedness is difficult to define and describe. Thus, ‘thrombolites’ include many dissimilar structures, including both those with a distinct clotted framework and those with irregular, patchy textures, which makes it difficult to interpret their significance. This study describes microbialites in Cambrian carbonates of the southern Great Basin, CA and NV. Specifically, we categorized internal textures of microbial buildups typically described as ‘thrombolites’ and ‘cryptic microbial boundstones’ by examining the features that impart a clotted, or patchy, character. Microbialites were described in the field in a stratigraphic context, and further studied in polished slabs. We examined the presence and distribution of different components (e.g. calcimicrobial clusters, dense micrite, coarse spar, skeletal material), their organization and distribution, and the relationships between them. Some mounds are composed of distinct micritic mesoclots with peloidal microfabrics in a framework structure. The clotted appearance of these mounds likely reflects a primary growth structure. Other mounds lack a framework structure, and a patchy texture is defined by intermixed light and dark micrite in irregularly shaped patches with smooth margins. In this case, the clotted appearance likely reflects intermixing of two sediment types following deposition, possibly from bioturbation. Other densely micritic mounds lack a distinct internal structure, and appear patchy from discontinuous zones of coarse spar. These mounds all share a ‘clotted’ or patchy appearance, yet are made up of different components with different geometries. This indicates that the character of clottedness is produced from a wide range of processes. Recognizing variations in clotted and patchy textures will allow identification of different types of thrombolites and will improve interpretations of ancient thrombolite environments and ecosystems.