Fossil Communities of the Borden (Mississippian) Delta in Indiana and Northern Kentucky

The Borden siltstone delta in Indiana and northern Kentucky contains shelly communities that are distinctive with respect to their occurrence in delta platform, delta slope, and prodeltaic environments. Isopach maps of the New Providence Shale and total Borden Group indicate that two distinct lobes of the delta existed in Indiana. The delta platform exhibits the greatest variety of habitats, including carbonate banks, distributary channel sandstones, and interdistributary mudstones and siltstones. Each of these environments was occupied by a distinctive tiered shelly community dominated by crinoids and bryozoans. The carbonate bank was dominated by large brachiopods (Spirifer, Imbrexia) and camerate (Alloprosallocrinus) and disparid (Halysiocrinus) crinoids. The distributary channel was occupied by a variety of small, short-stemmed advanced inadunates (Abrotocrinus), and the interdistributary mudstones by a great variety of fenestrate (Fenestella)and cystoporate (Cheilotrypa) bryozoans, as well as crinoids and brachiopods. The delta slope community was generally not tiered and consisted primarily of large brachiopods (Orthotetes, Syringothyris) that could remain fixed in position on unstable slopes. The prodeltaic community included a variety of bryozoans (Rhombopora, Cystodictya) and crinoids (Halysiocrinus, Synbathocrinus) as well as gastropods and pelecypods (Bembexia, Phestia, Ctenodonta). The slope community originated by migration down from the platform by some species and up from the prodelta by others. Species diversity a t individual sites on the delta platform range from 47 to 155, on the slope from 2 to 23, and on the prodelta from 35 to 103. INTRODUCTION Kentucky and adjacent Indiana. In order to DURING DEVONIAN and Mississippian time a integrate these various studies and provide a series of deltas gradually filled in large areas regional picture of the Borden fossil commuof shallow marine basins of the eastern interior nities, all three of us conducted field investiof the United States. These deltas began with gations of fossils in the Carwood and Locust the Devonian Catskill delta to the east, folPoint formations in the fall of 1977, thereby lowed by the Devonian-Mississippian Bedproviding needed information on delta slope ford-Berea delta in Ohio, and terminated with communities. In order to provide a regional the Middle Mississippian Borden delta of Inframework within which to set our paleontodiana, Illinois, and Kentucky (Text-fig. 1). logical efforts we undertook construction of Only seaward portions of the Borden delta are iospach maps of total Borden and New Provstill preserved, mainly in the subsurface of idence thicknesses in southern Indiana. Those southern Illinois and southwestern Indiana, maps were constructed using information debut also in a narrow outcrop belt in southern rived from well logs housed in the Petroleum Indiana and Kentucky. The non-marine part Section of the Indiana Geological Survey. of the delta subsequently has been eroded to This is not a taxonomic study. Most of the the north and east, the presumed source difossils reported here are identified only to the rections. The fossil communities of the Borden generic level, especially those from the Cardelta, as exhibited in outcrop in southern and wood and Locust Point. Specific identificacentral Indiana, are the subject of this report. tions of most of the delta platform and proOne of us began study of Borden deltaic deltaic and basinal species are available but communities 15 years ago (Lane, 1963). Subnot reported here because they add little to the sequently, Ausich has studied delta platform overall paleoecological interpretation based on communities in Monroe County and Kammer genera. is conducting research on the prodeltaic and contiguous basinal communities in northern PHYSICAL FRAMEWORK O F THE