BASIN MORPHOLOGY, SEDIMENTOLOGY, AND HISTORY OF A SMALL PROGLACIAL LAKE, MATANUSKA GLACIER, ALASKA

This study examines the basin morphology, sedimentology, geologic evolution of a small proglacial lake, which is now almost completely drained, located less than a kilometer north of the terminus of the Matanuska Glacier, Alaska. The methods of analysis include pits dug into drained lake deposits, Livingston cores taken from two, small lake remnants, and ground-penetrating-radar data collected during the summer of 2003. The proglacial lake was approximately 120m long by 85m wide and had a maximum depth of 8m as determined by a topographic survey of the modern lake bed. The lake developed in a topographic low as the Matanuska Glacier receded from a recessional moraine that probably formed 150-200 years ago; analysis of aerial photography indicates the lake drained most of its water sometime after 1949. The basin morphology was complicated by the development of a push moraine during a small re-advance early in lake development. Due to proximity of the lake to the glacier, it is assumed that the majority of water input was derived from glacial melt. Analysis of GPR transects, basin topography, and stratigraphy reveals three distinct facies: (1) a deltaic facies consisting of prograding forsets and interlayered laminated silt and gravelly-sand deformed by loading structures; (2) a basinal facies consisting of finely-laminated silt with minor amounts of clay and fine sand; and (3) a push-moraine facies consisting of diamicton found in a subtle ridge. Horizontal laminations, graded beds, and leveled topography, along with some inclined laminations and draping of the basinal facies onto topographic highs, indicate basinal sedimentation was controlled by a combination of density underflows and suspension settling from interflows/overflows. The influence of underflows and lack of noticeable clay partings make annual varves undetectable. Since the draining of the lake basin, small-scale channel development and polygonal freezing features have appeared on the surface.