Book Description

Excerpt from The Mississippian Pennsylvanian Unconformity in Southern Illinois A systematic study of the mississippian-pennsylvanian unconformity in the southern part of the Illinois basin, using both surface and subsurface data, indicates that many factors were involved in the geologic history of the erosional surface. Criteria are presented for the recognition of the position of the unconformity in the subsurface and the rocks above and below the contact are compared and contrasted. An areal geologic map of the pre-pennsylvanian formations shows three major features: beveling of younger formations at the edges of the Illinois basin; beveling of small structures in the basin; and long, deep, narrow channels as features of the erosional surface. Several isopach maps indicate the configuration of the channel systems and the upland surface between channels. The geologic history of the unconformity started with uplift and withdrawal of the sea in late Chester time followed by a long period of subaerial erosion. A peneplain was formed across beveled minor structures and older Chester and Lower Mississippian formations at the structurally higher basin borders. After peneplanation and a subsequent second uplift a new cycle of erosion began that resulted in the incision of deep channels in the peneplain surface. In early Pennsylvanian time the topography was buried by detritus laid down by aggrading streams which later merged with coarse sediment advancing westward from the Appalachian geosyncline. During and after Pennsylvanian time the unconformity was warped, folded, and faulted to its present attitude. The unconformity between the Mississippian and Pennsylvanian systems is well below ground level throughout the area covered by this paper (fig. Thus only subsurface geologic methods could be used in mapping the unconformity. There are many published descriptions of the unconformity for areas where the mississippian-pennsylvanian contact is exposed; in fact, almost every areal geology report covering such areas in southern Illinois, southern Indiana, and western Kentucky includes some discussion Of the erosional surface, but there has been no integrated regional study Of the outcrops and no detailed study of the unconformity in the subsurface. Field work in the southern part Of the Eastern Interior basin has been helpful to the writer in interpreting the subsurface data. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.