STRATIGRAPHIC ANALYSIS OF AREAL DISCONTINUITES OF LATE WISCONSINAN TILL SHEETS NEAR CONNEAUT LAKE, NORTHWESTERN PENNSYLVANIA

Previous workers have mapped boundaries of late Wisconsinan glaciations in northwestern Pennsylvania in two contrasting ways. Early geologists suggested that tongues of Hiram ice extended southward in valleys beyond the terminus of the ice on the uplands to deposit fine-grained till, whereas later mappers extended the boundary of the Lavery advance to account for isolated patches of fine-grained till beyond the previously mapped terminus on the uplands. Samples from 15 borings and a large gravel pit in the vicinity of Conneaut Lake in northwestern Pennsylvania were described and analyzed to determine which mapped boundary is correct. Fine-grained diamicts weather yellowish brown and average 15% sand, 49% silt, and 36% clay, whereas although sandy diamicts weather yellowish brown also, their texture averages 33% sand, 44% silt, and 23% clay. Generally, fine-grained diamicts are leached of carbonates to a depth of about 1 m; in some borings sandy diamicts are devoid of carbonates to a depth of 3 m. Fine-carbonate contents of diamicts are somewhat similar averaging about 1% calcite and 2.5% dolomite, but gray, unweathered coarse diamicts contain mean, total fine-carbonate contents of 5.5%. Ratios of illite to kaolinite and chlorite suggest development of weathering profiles at locations of most boreholes. Fine-grained diamicts have a ratio similar to those of Paleozoic rocks exposed upice; unweathered coarse-grained diamicts have a ratio reflecting the local shales. An anomalous fine-grained diamict overlies older glacial sediments in a large gravel pit in the highest part of the study area on the east side of the lake. This diamict is silt rich and has a fine-carbonate content averaging 17.9% similar to that of the Lavery Till. The distribution of fine-grained diamicts in a lobate pattern and their lack of occurrence on topographic highs within an area mapped as Kent Till west of the lake suggest that the original hypothesis of a lowland lobe is correct.