ANATOMY OF THE TYPE AREA OF THE LAVERY TILL OF NORTHWEST PENNSYLVANIA: A SEDIMENTOLOGICAL MOSAIC OF TILLS

It has been nearly fifty years since an in-depth, detailed study of the late Wisconsinan glacial deposits of northwest Pennsylvania has been conducted. The tills in this region were deposited by the Grand River lobe which also extended into northeast Ohio. Large databases for similar tills in Ohio have been constructed that permit separation of units based not only on texture, but also mineralogic and lithologic parameters. We attempt to apply this database to the type area of the Lavery Till in northwest Pennsylvania. Whereas, previous studies relied on small road cuts and ditches, we examined samples from eleven borings and three road cuts in the type area of the Lavery Till. The area contains surface deposits currently mapped as till units from three separate late Wisconsinan advances within an area of less than five square miles.

Our laboratory analyses show the presence of three separate till sheets in this region. Unit A is a silty till that contains more sand than clay, contains approximately 4% total carbonate, and has a calcite to dolomite ratio of 0.5. Unit A occupies the lowest stratigraphic position of all the interpreted late Wisconsinan tills. Unit B, a silty till, contains similar amounts of sand and clay, and has a total carbonate amount between 7 and 8 %. The calcite to dolomite ratio of this unit is 0.8. Unit C has carbonate contents similar to unit B, but may be distinguished from unit B by its more clayey texture and smaller sand content. The three units have similar diffraction intensity ratios typical of lower Paleozoic shales. The identification of three separate units is congruent with previously published data; however, large spatial and vertical variability exists both within and among the textures and compositions of the tills. The reasons for the variations may be a result of the complex nature of moraine deposits, the positions in which the sediment was carried in the ice, or the trusting of sheets near the glacier's margin. Further investigation of soil maps, surface expression, and geomorphology may help differentiate among these deposits.