Two diamictons in a landslide scarp on Admiralty Island, Alaska, and the tectonic insignificance of an intervening peat bed
Two till-like diamictons, 700 feet above present sea level on Admiralty Island, Alaska, are separated by peat near the top of a landslide scarp. The lower diamicton is glaciomarine; the upper diamicton is probably a mudflow. The lower diamicton contains the foraminifer Elphidium clavatum Cushman, a species typical of fiords. Similar diamicton crops out along Gastineau Channel near Juneau, 15 miles eastward. Diamicton in both areas reflects deposition in glacier-free fiords during land depression and sea transgression. Consequently, till-like deposits less than 700 feet above sea level elsewhere in southeastern Alaska should be considered as possibly glaciomarine. Sliding occurred after late 1962, but before July 1964; the March 27, 1964, Alaska earthquake might have caused sliding of diamicton saturated by melting snow and spring rains. The peat in the landslide scarp has a published radiocarbon date of 3,400±250 years B.P. (W-1955). The peat was expected to be older, and to relate to Holocene tectonic movement of Admiralty Island. The unexpectedly young date led to additional field study that showed the material over the peat to be a probable mudflow derived from glaciomarine diamicton upvalley; the date has no tectonic significance.