High-Resolution Sedimentary Record from temperate Alaskan Glaciers deposited in Bays and Fjords

Ellen A. Cowan, Department of Geology, Appalachian State University, Boone, NC 28608 and Ross D. Powell, Department of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL 60115

The glacimarine sedimentary record near Hubbard Glacier, in Southern Alaska contains distinctive annual deposits recognized as varves. Each varve consists of a clast-rich diamicton layer deposited in winter by intense iceberg rafting and a summer meltwater deposit of thinnly laminated mud and turbidite sand strata. Although iceberg rafting occurs year-round, clast concentrations are only high enough in winter/early spring to form diamicton because of the longer residence time of icebergs in the basin and the reduction of fine-grained sediment delivered by meltwater. The summer record consists of fine-grained rhythmites with a coarse silt or very fine sand lamina that grades upward into mud, formed by suspension settling from meltwater plumes interacting with tidal currents. High sediment discharge can produce daily silt and mud couplets of measurable thickness (>2 mm) over the meltwater season from May to September. Rapid deposition prevents bioturbation thus preserving laminae. Short gravity cores (<3 m) show that these season-specific processes occur regularly and produce a decade-long varve record verified with ²¹⁰Pb dating. Glacimarine varves occur in other bays with tidewater glaciers along the Gulf of Alaska coast from Prince William Sound south to Glacier Bay.

Hubbard Glacier continues to slowly advance into the sea and twice in the last 17 years, the glacier and its morainal bank dammed the narrow mouth of Russell Fiord. This created a lake that filled to 25 m.a.s.l. in 1986 and 18.6 m.a.s.l. in 2002. The catastrophic outburst of this lake completely eroded the bank and transported a large flux of sediment into the proglacial basin. The sediment record of the 1986 flood proximal to the outburst includes channelized debris flow and overbank deposits of thick sandy diamicton with mud clasts. In the distal basin, suspension settling from the turbid plume and non-channelized turbidity currents were dominant. Although these processes are also active in the proglacial basin during the summer, the flood deposits are on the order of 1-2 m thick and can be identified by anomalously low magnetic susceptibility. This is attributed to a change in source area for the flood deposits to the Chugach terrane, which lacks magnetite and thus is responsible for lower magnetic susceptibility than sediments deposited in the basin under ‘normal’ conditions originating from the Alexander and Yakutat terranes.