Mill Springs Limestone Submember (new) of the Point Peak Member, Wilberns Formation, Moore Hollow Group, Texas

This study proposes a new mappable unit, the Mill Springs Limestone Submember, within the Upper Cambrian Point Peak Member of the Wilberns Formation of the Moore Hollow Group in the Llano Uplift, central Texas. The submember is characterized by thick microbial boundstones that distinguish it from the thinly bedded heterolithic mixed clastic and carbonate strata of the Point Peak Member. The submember contains discontinuous individual bioherms up to 4.5 meters (m) thick, and cluster up to 15 m thick. The Mill Springs Limestone generally occurs in the upper and middle portions of the Point Peak, from west to east in the study area, and when present is mappable at the quadrangle (1:24,000) scale. In the western part of the study area the Peak Member contains facies that reflect a shift from tidal-flat deposition in the lower part of the Point Peak, to more open, outer-shelf conditions in the upper part of the Point Peak and development of thick bioherms characteristic of the Mill Springs Limestone Submember. The new designation aids geologic mapping and future paleoenvironmental studies.