SEDIMENTOLOGY OF QUATERNARY SEDIMENTS WITHIN AND ADJACENT WESTERNMOST FALL CREEK VALLEY, TOMPKINS COUNTY, NEW YORK

Late Quaternary glaciation in south-central New York State generated sediment accumulations now mostly hidden beneath forests, fields, and towns. Elevations of the contact between unconsolidated sediment and Devonian bedrock imply that the ancestral position of Fall Creek Valley (FCV) west of 76.43° longitude trended west-southwest across what is now the plain between FCV and Cascadilla Creek valley; two new deep boreholes traverse that valley fill. Outcrop and pit details within the 2.5 km (8200 ft) long sector of modern FCV studied and the boreholes ~1.25 km south of the westernmost valley wall exposure reveal a complex mosaic of time-variable and spatially variable conditions in the ancestral FCV fill. Along the north wall of modern FCV and at the two boreholes, sediments 24-63 m (80-200 ft) thick show an upper part contemporaneous with the last widespread glacial cover of this area, the Valley Heads (VH) readvance, and substantial sediment thickness predating the VH readvance. At the westernmost valley-wall section, ~30 m of sediment include a basal till and an uppermost VH till, between which sand deposits dominate with several meters of interbedded gray muds. The sands and muds were deposited from water with high degrees of suspended sediment load, potentially in a set of small proglacial lakes or pools at the front of a retreating glacier. Eastward, ~39 m of sediment at Varna High Bank contain three gravel-dominated units and VH till. The gravels reveal distinct depositional conditions: stream flow, followed upward by foreset beds formed where a stream merged into a deep pool, followed again by stream flow. At the easternmost >35-m-thick section also occur three major units below VH till, different than to the west. The base is clay with floating cobbles reflecting, first, lacustrine deposition and, second, glacial overriding. Next above are well stratified silty sands, representing deposition from rapidly flowing water with a high load of suspended sediment. Above those, the well sorted gravels are likely glacial outwash. The two boreholes, with 24 and 63 m of Quaternary fill, each penetrates a surficial till inferred to be VH, an interval ~5-m-thick of very well laminated silty clay interpreted to be lacustrine, and then a matrix-rich till that persists to bedrock in one and to at least 30 m subsurface in the other.