Microstructural Analysis of Lower Ordovician Cool Creek Formation Stromatolites, Arbuckle Mountains, Oklahoma

Headd, Brendan, Department of Geosciences, Texas Tech University, Lubbock, Texas, 79409-1053

The Cool Creek Formation is a Lower Ordovician shallow marine carbonate consisting primarily of mudstones and wackestones with abundant stromatolites and peloids. Stromatolites were collected from the southbound lanes of Interstate 35 in Murray County and examined for morphotype and microstructure. Digitates (branching and non-branching) are the dominant morphotype, although mats intergrading with laterally linked hemispheroids (LLH), thrombolites (with incomplete laminae), 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 represent the original pores in the biofilms used for nutrient uptake and excretion and have been modified by calcite spar. Erosion and biotic sloughing of microbial biofilms contributed to much of the horizontal heterogeneity in the microstructure of the stromatolites.