Variability of Facies and Depositional Environments Represented in Basal Pennsylvanian Rocks in Wallace Quadrangle of West-Central Indiana

David A. Fishbaugh

Field mapping has graphically demonstrated that a great variety of facies and depositional environments is represented in the basal Pennsylvanian rocks of Indiana. Initial sedimentation took place on an erosion surface developed on rocks of Mississippian age. This surface consisted of upland areas as well as valleys as deep as 100 m. The subsequent infilling and covering of this irregular surface with the basal Pennsylvanian sediments have added to the complexity of the area. Relief on this surface is reflected in the Pennsylvanian rocks in the form of debris flows that contain Mississippian fossils. Lithofacies change laterally within a few hundreds of meters. Fluvial channel-fill sandstones grade laterally into rhythmically laminated gray shales and lenticular to wavy-b dded sandstones that represent subtidal to intertidal environments.

Primary sedimentary structures include large (1-3 m), medium (0.4-1 m) and small (5-40 cm) planar and trough cross-beds. Ripple marks, burrows, mud-chip pebble conglomerates, desiccation cracks, peat rip-up, dewatering structures, and convolute bedding are some of the common sedimentary features found in these basal Pennsylvanian rocks. The paleocurrent direction of any one sandstone body is unimodal. Paleocurrent indicators in the fluvial-dominated sandstones indicate flow to the west, southwest, or northwest. The tide-dominated sandstones have paleocurrents indicating a predominant northeasterly or easterly trend.

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