Depositional history of a Mississippian crinoidal mound on the east flank of the Illinois basin

Abstract

The middle Mississippian Stobo buildup, Edwardsville Formation (Borden Group), Monroe County, Indiana, was deposited as a broad mound with low relief in an interdistributary area near the Borden delta platform edge. Echinoderm skeletal debris comprises most of the mound, which is similar to several other mounds in the Edwardsville Formation of southern Indiana. Six distinct facies occur in rocks comprising and surrounding the Stobo mound. Echinoderm rudstone and echinoderm packstone were deposited under relatively low-energy conditions with the locally derived skeletal grains. Echinoderm grainstone was deposited under higher energy conditions, perhaps during storms. Dolomitic echinoderm wackestone and mudstone were deposited in low-energy areas on the mound surface. Intertonguing siltstone was deposited when silt deposition temporarily overtook carbonate deposition near the mound margin. Echinoderm-bearing silstone was deposited when silt buried the uppermost echinoderm-bryozoan meadow. Progradation of the Borden delta eventually covered the mound with silt. Stobo mound is part of a spectrum of Mississippian buildups which range from the large, relatively deep-water, mud-rich, classical Waulsortian mounds to smaller, grain-dominated, platform-depth mounds such as Stobo and other Borden mounds.