Print this pageAbstract: Lithofacies Relations in Alexandrian and Early Niagaran Rocks (Silurian) in Indiana and Parts of Illinois, Kentucky, and Ohio
Early Silurian depositional patterns indicate a structural setting different from that of the Ordovician Period and not yet defining patterns recognized later in the Silurian Period. The oldest Silurian rocks have a limited distribution. Shale and argillaceous carbonate rocks (Wilhelmi) in northern Indiana and adjacent Illinois contain much reworked Ordovician material and in Indiana pinch out southward. Their distribution appears to be related to subsidence in the Michigan basin. Contemporaneous impure carbonate rocks (Belfast) in central Kentucky and more argillaceous sedimentary rocks in Ohio suggest a depositional encroachment from the Appalachian basin.
The Brassfield-Sexton Creek complex represents deposition in extensive seas lacking strong tectonic control. In Indiana distribution of impure cherty carbonate rock corresponds closely to the present Illinois basin, but these deposits are continuous north into the present Michigan basin. Two north-south-trending linear positives in eastern Indiana bound this facies on the east. East of the northern positive the lithology changes to relatively pure platform carbonate rocks, whereas a distinct facies is present on the Ripley Island positive on the south and gives way east and south mainly to a cherty carbonate rock. Intercalations of shale in the Noland Formation above this cherty carbonate rock reflect a southeastern or eastern clastic source.
After cessation of deposition in much of the area, dominantly carbonate deposition resumed except that a tongue of shale (Estill-Osgood) from the east spread into central Kentucky and southeastern Indiana where there is a facies change north and west into carbonate materials. The carbonate rocks (Salamonie-St. Clair) that are contemporaneous with and directly overlying this shale represent an extensive thin blanket deposit lacking terrigenous material but thickening northward.
AAPG Search and Discovery Article #90975©1976 GCAGS- GC Section SEPM Annual Meeting Shreveport, Louisiana