Anders E. Carlson Abstract

Anders E. Carlson ('00) (anders@geology.wisc.edu)

This study determined the composition, genesis and internal architecture of a large Little Ice Age end-moraine of the Matanuska Glacier, Alaska 0.6 km from the glacier terminus. The moraine age is based on radiocarbon dating and foliage cover. The exposure studied is located at the southern most extent of the moraine and was formed, and is maintained by undercutting of an active outwash stream. Exposed is a steep 100m x 16m cross section of the core of the moraine at a 45° angle to the axial trend. The base is covered in 3 to 4 meters of scree and slump, leaving 625 m² exposed (~78%). Detailed sedimentological, fabric and grain size analyses demonstrate that the moraine consists almost entirely of proglacial sediment flow diamicton, with small silty beds and gravel and sand lenses. The deposited sediment was all reworked sediment derived from the glacier. No melt out or lodgment tills were observed. Type I flows occurred once (4.3%) depositing 44.4 m² (7.9%) of sediment. Type I transition flows occurred once (4.3%) depositing 32 m² of sediment. Type II flows occurred seven times (30.4%) depositing 231.1 m² (40.9%) of sediment. Type III flows occurred 10 times (43.5%) depositing 123.1 m² (21.8%) of sediment. Type IV flows occurred ~4 times (17.4%) providing 133.8 m² (23.7%) of sediment. The sediment flows (types I-IV) differ in viscosity (water content), bedding and lamination and pebble fabric, varying from type I, having the highest viscosity, little to no lamination and the lowest S1 value, to a type III, having a lower viscosity, complex bedding and lamination and the highest S1 value, to a type IV having the lowest viscosity, laminations and no pebble fabric. (Lawson, 1979) A companion study of the volume, frequency and deposition rates of modern type I, II, III and IV sediment flows at the current Matanuska Glacier terminus suggests that the glacier deposited this moraine in a relatively brief period (10's to ~100 years), depending on the amount of water available during deposition to saturate the sediment. The process of sediment flow initiation on basal ice suggests that the glacier was in retreat during sedimentation. Sediment flows deposited the moraine from west to east, flowing southwest, west or northwest on low angle slopes (0°-7°) producing thick sequences of cyclic layers (reoccurrence of the same flow type over a previous flow moving in the same direction).