STRATIGRAPHIC ANALYSIS OF AREAL DISCONTINUITIES OF LATE WISCONSINAN TILL SHEETS NEAR PYMATUNING RESERVOIR, NORTHWESTERN PENNSYLVANIA

State geological survey workers from Pennsylvania and Ohio mapped the extent of late Wisconsinan glaciations in northeastern Pennsylvania and northwestern Ohio during the late 1950’s and 1960’s. Workers in 1959 proposed that a fine-grained till exists in lobes that extend into lowlands beyond the generally mapped limits of the till sheets. Workers in 1969 suggested that a medium-grained till extended beyond its original mapped extent and questioned the existence of fine-grained lobes of till mapped by the early workers.

Samples collected during the summer of 2011 from 21 borings around Pymatuning Reservoir in northeastern Ohio and northwestern Pennsylvania were described and analyzed to help resolve discrepancies in mapping of tills. A brown diamict averages 42% sand, 41% silt, and 17% clay, and a light olive diamict overlies that and averages 26% sand, 59% silt, and 15% clay. A clayey diamict weathers yellowish brown and averages 13% sand, 41% silt, and 46% clay. The sandy diamicts, where at the surface are leached to an average depth of 2 m but only 1 m where overlain by some other unit. Silty diamicts are leached to a depth of 1.5 meters on average; whereas the clayey diamict is leached throughout when encountered. Unweathered samples of the sandy diamict are bluish gray having a diffraction intensity ratio (DI) of about 1 and contain 4-5% carbonate. Silty and clayey diamicts are usually weathered and devoid of carbonates, and DI’s are generally below 1. The occurrence of the clayey diamict stratigraphically atop the silty diamict in a borehole south of the reservoir suggests that they may be different units. In addition to finer-grained diamicts being found together south of the reservoir, separate occurrences of the silty diamict flanking the east- and west-facing edges of the reservoir and the clayey diamict bordering the southern tip of the reservoir suggests that both previous studies may have elements of accuracy.