SEDIMENTOLOGY OF THE JACKSVILLE ESKER-DELTA COMPLEX IN WESTERN PENNSYLVANIA

The internal composition and structure of the late Wisconsinan Jacksville esker-kame delta complex is exposed at the Glacial Sand and Gravel Company quarry near Slippery Rock, Pennsylvania. The sediment was deposited approximately 23,000 years ago during the Kent glaciation and is composed of esker, prograding kame delta, and pro-glacial lake deposits. Both proximal and distal locations (relative to the retreating glacier) within the quarry were examined. Samples were collected from the two locations and sieved for sediment size distribution, and the lithology of the gravel fraction was identified. Delta foreset bed orientations were measured to determine paleo-flow direction, and stratigrahic columns were generated.

The proximal location is primarily composed of coarse-grained sand with two gravel dominated beds. Gravel lithology is mostly sandstone, and there are three beds with distinct layers composed solely of pebble-sized coal clasts, some of which are imbricated. Crystalline (igneous and metamorphic) clasts do not exceed 5% and generally decrease up-section. The distal location is mostly composed of medium-grained sand, with crystalline clast percentages up to 20%. Both sample locations contain fining-upward successions and have predominantly well-sorted delta foreset beds overlain by a gravel topset bed. Flow direction, as recorded by foreset bed orientation, was to the southwest at the proximal location and to the southeast in the distal location.

The presence of coal pebble layers, abundant sandstone clasts, and general lack of crystalline clasts suggest that glacial drainage was locally organized around the ice margin, and erratics did not significantly contribute to the sediment load deposited during glacial retreat. Both locations show that erratic deposition decreased through time as the glacier retreated, and the presence of coal-clast layers suggests that the coal gravel and sand are hydraulically equivalent. The younger, distal location is composed of finer grained sediment, as expected in a prograding kame-delta complex. The varying foreset bed orientations support either or a combination of the following hypotheses: 1) at least two primary outwash sources entered the delta complex, 2) these sources likely changed with time, and 3) several lobes may have composed the delta complex