ABSTRACT

Laminated silt- and clay-size sediments exposed in cutbanks along Woodcock Creek in northwestern Pennsylvania are of glaciolacustrine origin. The presence of convolutions and graded laminae indicate that deposition of these sediments was by turbidity currents. Small-scale cross-bedding in some of the coarser sediments within the laminae suggests that deposition of an individual graded lamination required no more than a few days' time. Thus, although these sediments superficially appear to be annual varves, they are in fact rhythmites.

The turbidity currents primarily responsible for the deposition and deformation of the laminated sediments probably originated from an outwash stream which flowed into an ice-dammed lake which temporarily occupied Woodcock. Creek Valley sometime during Pleistocene time. As the sediment-laden stream entered the lake, sand- and gravel-size sediments were deposited, forming a delta-kame which presently exists. along the valley wall about km from the exposed lake sediments. When the stream currents which flowed across the delta reached the delta front, they had a higher density (suspended-sediment load) than the lake water. Thus, the stream currents flowed down the delta face onto the prodelta slope as turbidity currents. These turbidity c rrents are believed to have deposited the graded laminae now exposed along the valley bottom.