Retreat Sequences from the Last Glacial Maximum and Little Ice Age Preserved in Muir Inlet, Glacier Bay National Park, Southeastern Alaska

The marine sedimentary deposits that accumulated within the basins of Muir Inlet, a deep (150-300 m), narrow (<2.5 km), 48 km-long silled-fjord have been interpreted based on known history of glacial advance and retreat. Ten basins were imaged by two sets of high-resolution seismic reflection profiles. Interpretations are based on sequence stratigraphic analysis of the reflection profiles, the environmental setting of seismic facies, and depositional models developed from extent temperate, tidewater glaciers within Glacier Bay. The USGS collected seismic data between 1978 and 1980. These high-resolution profiles imaged sediment in the basins >100 m thick deposited during Little Ice Age (LIA) retreat over the last 100 years. In 2004, we collected a dual GI-gun profile down the axis of Muir Inlet that imaged up to 300 m of previously unknown sediment below the acoustic basement on the USGS profiles. Although these lower deposits have not been dated, they are interpreted to be sediments laid down during retreat from the Last Glacial Maximum (LGM), the largest known glacial advance prior to the LIA. The LGM ice sheet estimated to be 1700 m thick appears to have eroded to bedrock, while the thinner LIA glacier did not. Another Neoglacial advance can be identified on the fjord entrance sill.

Seismic facies comprising basin fill includes a chaotic transparent facies likely deposited proximal to the retreating glacier and overlying medium- to high-amplitude, horizontal continuous reflections suggesting strata that are horizontal and continuous across each basin. Piston cores through this upper facies have recovered laminated mud and cm thick sand beds, which are interpreted as deposits from turbid meltwater plumes and distal sediment gravity flows, respectively. Modern sediment accumulation rates indicate that deposition decreased exponentially once the glacier retreated from the basin although failure of ice-cored sediment from the fjord walls may supply sediment later.