ABSTRACT: Recognition of Shoaling Cycles in the Maquoketa Group (Upper Ordovician) of Southern Indiana

SWENSON, ALAN, and BRIAN D. KEITH, Indiana Geological Survey, Bloomington, IN

The Maquoketa Group in Indiana is predominantly shale with lesser amounts of limestone. Marker beds within the Maquoketa were defined on the basis of geophysical-log characteristics and were correlated across a 30-county area in south-central and southeastern Indiana on a series of regional stratigraphic cross sections. In the study area, the Maquoketa thins from more than 850 ft in the east in Union County to less than 400 ft in the west in Greene County. From east to west, progressively younger beds of the Maquoketa disconformably onlap the surface of the underlying Trenton Limestone (Ordovician). From east to west, progressively older beds at the top of the Maquoketa lie in disconformable contact with the overlying Silurian due to erosion.

Two major shoaling cycles can be identified in the subsurface. The boundary between these cycles occurs approximately at the midpoint of the Maquoketa section and is traceable across the entire study area. Both cycles contain a lower, predominantly shale interval that changes upwards to a dominantly limestone interval.

Over most of the study area, the thickness between individual log markers within the Maquoketa is remarkably constant. The major exception occurs along the Mt. CarmeI fault, a northwest-trending high-angle normal fault in Monroe, Lawrence, and Washington counties. The limestone interval of the lower cycle thins by approximately 40 ft in less than 12 mi, from east to west across the fault. The coincidence of this stratigraphic change and the Mt. Carmel fault suggests fault movement during Maquoketa deposition.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)