INFERENCES INTO TEMPERATE FJORD DEGLACIATION: EVIDENCE FROM GLACIAL AND PARAGLACIAL SUBMARINE LANDFORMS IN SOUTHEAST ALASKA

Characterization of submarine landforms has been used for reconstructing glacial dynamics in polar, polythermal and temperate glacial regimes. Unlike polar regimes where subglacial bedforms are often preserved as seafloor morphologic features from LGM retreat, constructional landforms deposited at grounding lines during glacial retreat dominate the temperate glacimarine setting of Southeast Alaska. Fjord seafloor morphologies therefore, provide excellent records of both grounding line processes and inferred glacial dynamics. In this study, we infer processes at former glacier grounding lines in Disenchantment Bay and Muir Inlet, from high-resolution multibeam sonar. Additional seismic reflection and multibeam backscatter data are used to evaluate the spatial extent and composition of these deposits. Submarine glacial landforms adjacent to the modern advancing terminus of Hubbard Glacier in Disenchantment Bay are considered analogous to morainal bank deposits imaged down fjord at Blizhni Point and at the entrance sill of Muir Inlet. Therefore, these morphologies comprising grounding-line fans, and morainal bank landforms, including push forms, provide evidence of periods of slowed retreat or stagnation with intervening readvances and significant meltwater associated sedimentation during longer-term post-LIA glacial retreat. In areas of more rapid retreat, such extensive grounding-line deposits are either not present or are buried due to high sedimentation. Significant post-glacial marine reworking of glacial deposits is observed in several areas in the form of gullies and channelized erosion. Similar paraglacial diatomaceous mound morphologies are observed in Lynn Canal and Lisianski Inlet. In our study areas, seafloor exposure times after glacial retreat range from <5 to 1000s of years, which allow us to investigate settings that have been under the influence of glacial, paraglacial, and fully oceanic conditions for varying lengths of time, yielding insight into the complex spatial and temporal variability of these landforms. In addition to providing information about the nature of tidewater glacier retreat, this work elucidates interpretation of the temperate glacimarine sedimentary record by distinguishing glacial, paraglacial and marine landforms.