STRATIGRAPHY OF VALLEY FILL DEPOSITS UPSTREAM OF A SMALL COLONIAL-AGE MILL DAM, WHITE CLAY CREEK, PENNSYLVANIA

We measured stratigraphic sections 1.91 km upstream of a 1-m high extant colonial mill dam on the W. Br. White Clay Creek in southeastern Pennsylvania. Exposed deposits can be divided into 4 characteristic facies: 1) matrix-supported (define roundness) cobbles and boulders; 2) clast-supported (define roundness) pebbles and cobbles; 3) Sand and mud with organic fragments (wood, leaves, stems); and 4) massive muddy sand and sandy mud. Matrix-supported gravels are interpreted as colluvium; these deposits are extensive, often overlie bedrock, and may be exhibit deformed strata suggesting transport by mass wasting. Clast-supported gravels are basal deposits exposed as thin (< 0.5 m) layers, often immediately overlying matrix-supported gravels (colluvium), occasionally preserved as triangular-shaped lenses ~10m long and < 1 m thick. These deposits are interpreted as channel gravels, while the triangular lenses represent buried bars. At two sections, basal gravels are overlain by decimeter-thick, laterally discontinuous layers of sand and mud with abundant organic fragments. Leaves and wood from these deposits yield radiocarbon dates of 308-473 and 918-1000 years B.P., indicating that these deposits pre-date European settlement and its associated watershed disturbance. Massive muddy sand and sandy mud represent the uppermost 0.5 - 2 m of nearly all the sections. Excavations of exposed tree roots and dendrochronology demonstrate that the upper decimeters are contemporary overbank deposits. Deeper layers have similar characteristics and are therefore also interpreted as overbank deposits. A decimeter-thick, dark-colored layer is exposed at a few locations at depths of ~ 1m within this unit; this is interpreted as a buried A horizon representing the elevation of a slowly aggrading floodplain before European settlement. Overbank deposits increase in thickness immediately upstream of the mill dam, but laminated deposits typical of subaqueous deposition in mill ponds are absent. Valley-fill deposits of our study area are a complex mosaic of colluvium (likely pre-Holocene in age), pre-Settlement alluvium consisting of channel and bar gravels, low-lying floodplain deposits, and small localized wetland deposits, and post-Settlement overbank and lateral accretion deposits.