GROUND PENETRATING RADAR IMAGERY OF A PENNSYLVANIAN SANDSTONE AT GRAND LEDGE, MICHIGAN

Pennsylvania strata exposed at Grand Ledge, Michigan have been widely studied. Despite numerous datasets from the exposures and cores from the formation, vertical and lateral heterogeneity over short distances makes the sedimentology difficult to interpret. This has inspired continued debate about the depositional environment(s), age, and tectonic setting. Related facies are exposed at several locations around the Grand Ledge area, but this project focuses on a massive channelized and cross bedded sandstone at the Oak Park locality (42.758297, -84.754648). Paleoenvironmental interpretations of this facies include beach, fluvial, or possibly both, as part of a complex marginal marine environment. Uneven weathering of the outcrops and the adjacent Grand River prevents observation of the big picture sedimentological features, so the goal of this project is to use ground penetrating radar (GPR) to investigate the structure of the sandstone at Oak Park in continuity. GPR data was collected using 100 and 200 MHz antennas mounted on sleds pulled along the flat hiking trail atop the exposed strata. This resulted in a 200 meter subsurface profile of the sandstone. The prominent features observed in the GPR profile include: the cross section of an asymmetrical channel feature with dimensions of about 4 meters deep and about 50 meters wide near the “bankfull” top; a prominent, wavy, and discontinuous reflector corresponding to the bottom of the sandstone and top of the siltstone and coal beds; recurring sub-vertical joints in the sandstone facies. The shape of the channel feature gives the appearance of a point-bar and cutbank in cross section and could be interpreted as a meandering fluvial channel. This 200 meter stretch of data supports previous sedimentology-based studies that suggest a sinuous meandering channel was cutting through this section of a dynamic marginal marine environment.