Microstructural analysis of the Lower Ordovician Cool Creek Formation stromatolites, Arbuckle Mountains, Southern Oklahoma
The Cool Creek Formation is a Lower Ordovician shallow marine carbonate consisting primarily of mudstones and wackestones with abundant stromatolites. Digitates (branching and non-branching) are the dominant morphotype, although mats intergrading with laterally linked hemispheroids (LLH), and dendrites are common throughout the formation. The microstructure of all morphotypes studied consists primarily of clotted (fenestrate) fabrics growing from basal surfaces of zones within the stromatolites. Complete laminae are rare and zones of varying clot density with significant vertical and horizontal heterogeneity are common. The clotted zones may represent the extracellular polymeric substances (EPS) of the microbial biofilms that formed the stromatolites by initiating the precipitation of carbonate that caused the death or migration of the microbial community confined within the EPS. This process preserved the topography of the biofilms, formed a hard surface for the growth of a new biofilm, and was the primary factor responsible for the vertical growth of the stromatolites. The voids within the clots may represent the original pores in the biofilms used for nutrient uptake and excretion. The larger voids between the clots represent either channels separating different portions of the biofilm or folds and invaginations inherent within the three dimensional architecture of the biofilms. All of the voids have been filled and modified by calcite spar. Physical erosion and biotic sloughing of microbial biofilms contributed significantly to the horizontal heterogeneity in the microstructure of the stromatolites.