DENNISON, JOHN M., Geology Department, University of North Carolina at Chapel Hill, Chapel Hill, NC

ABSTRACT: Fragment Shape Used for Detailed Stratigraphic Mapping in Devonian Shales of Central Appalachians

Sections of Devonian shale measured over 40,000 square kilometers of Pennsylvania, Maryland, West Virginia, and Virginia in several fold belts reveal a persistent stratigraphic position for different shapes of weathering fragments. These fragments can often be used for very detailed mapping even within members where bedrock is not clearly visible. Detailed time lines in the Needmore, Marcellus, Mahantango, Harrell, Millboro, and Brallier Formations can be traced using silty tongues, black shales, five limestone concretionary and bedded zones, and several layers of Tioga Ash Beds. The sequence ranges 70-300 m thick, with stratigraphic convergence westward. Weathering fragment shapes are reduced to dimensionless ratios of length, width, and thickness. Qualitative fieldterms can be expre sed as domains on scattergram plots from measured samples of the materials used for field terminology. Fragment shapes are designated as sheety, platy, chippy, pencily, and equant. Equant fragments are divided into blocky and lumpy, dependent on degree of preservation of planar edges.Individual zones a few meters thick with specific fragment shapes sometimes can be traced over thousands of square kilometers and for more than 100 kilometers distance.

Distribution of weathering fragment shapes is plotted in cross section panels for comparison with conventional stratigraphic cross sections based on measurements of continuous outcrops of the beds, using lithology and color as the classic stratigraphic criteria. Weathering shapes seem related to siliciclastic grain size, organic matter, calcite cement, bioturbation, turbidites, and the distribution of joints and fracture cleavage.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.