DEFINING THE TIMING AND EXTENT OF THE MARINE TRANSGRESSION FOLLOWING THE LAST GLACIAL MAXIMUM IN SOUTHEASTERN ALASKA

During the Last Glacial Maximum (LGM), the Cordilleran Ice Sheet (CIS) flowed south and westward from the crest of the coastal mountains to the coastal plain exposed by lower sea levels and crustal response to ice loading. The exact extent of the LGM ice sheet in southeastern Alaska is poorly defined because much of the evidence is now submerged due to postglacial sea level rise and the land’s response to deglaciation. New data is expanding our understanding of the timing and complexity of the marine transgression following the LGM. This data also defines the timing of the retreat of the CIS from the Archipelago. An extensive literature search and years of field reconnaissance and sample collection have resulted in a dataset of over 600 shell-bearing raised marine deposits throughout Southeast Alaska. It includes site location, elevation, and description when available, and over 300 radiocarbon dates beginning at ~48,000 Cal BP. Interpretation of this data gives insight on the timing and complexity of isostatic crustal adjustments that resulted from glaciation and deglaciation, eustatic sea level change, and subsequent tectonic uplift. From this data, preliminary relative sea level curves have been developed for much of Southeast Alaska allowing for modeling of the paleoshorelines through time. The modeling suggests a peripheral forebulge developed west of the ice front along the whole of the coast of Southeast Alaska expanding the area of previously modeled coastal refugia. The above mentioned data set shows most of Southeast Alaska deglaciated by a median age of 13,708 Cal BP. This age corresponds closely to the published dates for meltwater pulse 1a and a peak rate of sea level rise at ~13,800 Cal BP. During meltwater pulse 1A rising sea-level outpaced isostatic rebound. By ~13,700 Cal BP ice retreated from all of Southeast’s fjords, channels, and passages to the elevation of the highest shell occurrences. The large coastal rivers like the Taku, Stikine, Iskut, Unuk, and Nass River Valleys would have been fiords with tidewater glaciers in their valleys. The largest islands such as Admiralty, Kupreanof, Kuiu, and Mitkof would have consisted of several smaller islands at that time. A forebulge persisted along the coast from 16,700 to 11,100 Cal BP implying that glacial ice was present on the mainland until its collapse.