Print this pageThe study of the Lead Creek Limestone Member (of the Mansfield Formation), herein recognized as Atokan, was designed to test for geographic continuity of deposition, possible time transgression, and paleoenvironmental variation as well as for age determination. Compilation of a conodont zonation for the Pennsylvanian rocks of the Illinois Basin will be an important reference for comparison with other areas of the world.
The Lead Creek is readily recognizable on outcrop in Hancock County, Kentucky, and in Spencer, Perry, and Dubois Counties in southern Indiana and in the immediately adjacent subsurface area to the west and north. It consists of two named dominantly limestones beds, the Fulda Bed below and the Ferdinand Bed above, and a shaley interval between, except that to the north generally only a single ledge of limestone is present in the Lead Creek position. The Fulda commonly is a dark, dense, argillaceous and shaley limestone, and the Ferdinand a light colored, fine-grained, cherty limestone. The shale and siltstone unit between them contains thin coal and paleosol in places. We interpret the environment of deposition as shallow, but open sea for the limestone and shoal to nonmarine conditions for the middle unit.
The Lead Creek conodont fauna is of low diversity and is dominated by Idiognathodus and Idiognathoides. Hindeodus, Neognathodus, and Diplognathodus are minor elements. The presence of Idiognathoides and Neognathodus atokaensis distinguishes the fauna from that of the next overlying named carbonate unit, the Desmoinesian Perth Limestone Member of the Staunton Formation. At present it is not possible to correlate the Lead Creek with any of the minor marine intervals in its approximate position in Illinois.
AAPG Search and Discovery Article #90926©1999 AAPG Eastern Section Meeting, Indianapolis, Indiana