ENTRAINMENT MECHANISMS PRODUCING DEBRIS TAILS ASSOCIATED WITH TRANSLATING SUPRAGLACIAL CLASTS, MATANUSKA GLACIER, ALASKA

This Research builds upon two previous studies that were conducted at the Matanuska Glacier, south-central Alaska, to investigate translation of supraglacial clasts. In these earlier studies significant insights were developed regarding clast rotation and translation phenomena; however, the development of fine-to-medium grained debris tails behind translating clasts remained enigmatic. In this study, translation of clasts across the ice surface occurred in a southwesterly direction near the Matunuska’s terminus at a rate of 5cm/d over a study period of three weeks during the months of July and August. Clast movement resulted from ice pedestals melting preferentially on their sunny sides (SW), tilting of the pedestal, and clast slippage when the top of the pedestal was oversteepened beyond the angle of repose. These translating clasts typically are associated with a concentrated, linear pile of sediment, referred to as debris tails, which trace past locations. Multiple working hypotheses were designed to evaluate the source of sediment within these debris tails. Three potential mechanisms to generate the debris tails are (1) incorporation of up-ice sediment by sheet wash, (2) in-site production of debris by spallation of the cobble or boulder on the pedestal, or (3) debris incorporation by clast over-ride. Experimental plots were designed to test each hypothesis. Three weeks of observations and measurements were completed; the genesis of debris tails by incorporation from sheet wash and clast over-ride was demonstrated. Nine debris tails revealed over-ride, while three tails displayed debris entrainment by sheet wash. No evidence for spallation was observed during the study period. These results provide evidence that the debris-tail development may be a factor of multiple sedimentary processes.