A periglacial stratified slope deposit in the valley and ridge province of central Pennsylvania, USA: Sedimentology, stratigraphy, and geomorphic evolution

The sedimentology and stratigraphy of a stratified slope deposit in central Pennsylvania together with existing pollen data and global climate simulations are used to develop a model for stratified slope deposition within the context of a waxing and waning Late Wisconsinan periglacial climate. The deposit consists of well-stratified beds of platy, subangular shale chips ranging in size from very fine to very coarse gravel and is characterized by two facies: a matrix-supported facies and a clast-supported facies. Mechanical breakdown of bare, highly fractured shale bedrock by frost shattering and freeze-thaw activity in a periglacial environment in front of the Late Wisconsinan glacial boundary provided an abundant and continuous sediment supply. Gelifracts were transported from bedrock source areas to depositional sites along valley side-slopes by sheetwash and debris flow processes, creating an inclined (10o), aggradational surface of low relief. Surface runoff was derived from snow melt and rainfall onto impermeable frozen ground and bedrock surfaces. The cyclic nature and timing of deposition indicate that sediment was produced and transported immediately prior to and after the Late Wisconsinan glacial maximum. The Late Wisconsinan glacial maximum was characterized by permafrost conditions with reduced sediment supply and erosional truncation of the stratified slope deposit. Frost cracks, ice wedges and soil wedges developed on an unconformity within the deposit during the glacial maximum. La sedimentologie et la stratigraphie d'un depot de pente stratifie du centre de la Pennsylvanie, ainsi que des donnees palynologiques et des simulations globales du climat ont ete considerees pour developper un modele de formation de depots de pente stratifies dans le contexte du climat periglaciaire de la fin de la derniere glaciation. Le depot consiste dans des lits bien stratifies de debris plats subanguleux de schiste variant en taille, de tres fins a tres grossiers, et caracterises par deux facies: un facies a matrice et un facies sans matrice. La desagregation mecanique du bedrock schisteux denude par les actions de gel/degel dans un environnement periglaciaire situe non loin du front glaciaire wisconsinien, a fourni une alimentation abondante et continue en sediments. Les debris fractures ont ete transportes depuis la roche en place vers les sites de depots le long des pentes de la vallee par le ruissellement en nappe et des laves torrentielles, en creant une surface faiblement inclinee (10o) d'accumulation. L'ecoulement en surface provenait de la fonte des neiges et des precipitations sur un sol gele et un bedrock impermeables. La nature cyclique et l'tpoque de depbt indiquent quc les sediments ont ete produits et transportes immediatement avant et apres le maximum glaciaire wisconsinien. Ce maximum glaciaire a ete caracterise par des conditions de pergelisol avec une fourniture reduite en sediments et une periode d'erosion tronquant le depbt de pente stratifie. Des fissures de gel, des coins de glace et des coins de sol sont apparus sur cette discordance pendant le maximum glaciaire.