INTERPRETATION OF ICE-MARGINAL HYDROLOGIC CONDITIONS BASED ON SEDIMENT PRESERVATION IN CLARKSVILLE CAVE; EAST-CENTRAL NEW YORK STATE

Quaternary sediments in Clarksville Cave, in Albany County, NY (228m to 203m msl), permit interpretation of ice marginal mechanisms of flow transport and deposition. Sediments faithfully record Wisconsinan subglacial, proglacial and climatic conditions. Major cave development occurred beneath warm-based ice, an analogue to alpine Castleguard Cave in Canada. Subglacial meltwaters dissolutionally-enlarged joints and steeply-dipping fault planes (~14°SE to 32°SE) within Devonian Onondaga limestone. Within 60m of water inlet points and shafts, vadose flow was forced southward along a strike-oriented 14m² master conduit largely situated within a thrust zone, down thrown against impermeable Schoharie sandstone. During glacial retreat, sediment and erratic cobble influx occurred through now relict conduits (at least 17) either under subglacial or, more likely, proglacial conditions. The short distance between sediment influx points and the master conduit allow interpretation of hydrologic conditions during deposition because little sediment redistribution occurred. Sediment “marker beds” are traceable throughout the cave, indicating contemporaneous sediment and cobble inputs via braided streams, alternating between glacier ice-proximal and distal zones. Initial deposition of finely laminated lake clays and silts occurred in a clean, nearly sediment-free, cave situated near and back-flooded behind a proglacial lake (within 60m; 218m msl). Above basal glaciolacustrine sediments, a sequence of subaqueous, pebble-cobble, debris flow diamictons are punctuated by stratified and graded gravels, sands, silts, and clays (11 cycle minimum with quiescent periods) that once filled the cave to the ceiling. An upper subaqueous turbulent underflow deposit (yellow silts and clays) with climbing ripples provides an important marker throughout the cave. Above this, a calcite-coated, pebble-rich, debris flow that truncates a glacier-fed delta deposit in McNab Hall and massive ceiling collapses atop stratified sediments, provide evidence of a short-lived glacial re-advance before retreat and later sediment excavation by a cave stream. Clarksville Cave provides a unique and exceptional setting for examination of episodic and seasonal glacial outflows close to the source.