Regional Correlation and Zonation of Lower Pennsylvanian Mansfield Formation of Southwestern Indiana

Lloyd C. Furer

Several zones within the Mansfield Formation produce hydrocarbons, and many names have been applied to individual reservoirs. The time-stratigraphic relationships among these productive zones and the seven Mansfield members designated in the outcrop are poorly understood.

A format method of correlation of the Mansfield from the type section in Parke County, Indiana, to the thickest Mansfield in Vanderburgh County permits subdivision into lower, middle, and upper zones. At the type section, only 30 ft (9 m) of Mansfield (all upper zone) is present; south in Vanderburgh County the upper zone is 120 ft (37 m) thick. All three zones, totaling about 800 ft (244 m), are present in the Evansville paleovalley in Vanderburgh County. About 250 ft (76 m) of the thickening is related to local fill of the paleovalley.

The northward and eastward thinning of the Mansfield can be explained by any of three fundamental mechanisms: (1) an intraformational unconformity could be present with deeper erosion to the east and the north, (2) lower sedimentation rates might have existed in the east and the north, or (3) latest Mississippian and/or Early Pennsylvanian erosion or nondeposition related to mild tectonic uplift on the LaSalle, Kankakee, and Cincinnati arches could have created a channelized erosion surface. Detailed paleontologic subdivision of the Mansfield is needed, if possible, to select the correct hypothesis. However, preliminary correlations by the format method favor the third thinning mechanism. If so, the lower Mansfield most likely represents nonmarine fill on the post-Mississippian topogr phy. At the top of the middle Mansfield an extensive shale that is possibly marine and a discontinuous marine limestone indicate a regional transgression.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.