MULTIPLE GLACIAL EROSION ON THE CHUKCHI BORDERLAND, ARCTIC OCEAN, AS INDICATED BY PRELIMINARY DATA ON SEDIMENT STRATIGRAPHY AND SEA-FLOOR MORPHOLOGY

Bedforms indicative of sea-floor sculpting by coherent ice masses (ice sheets/shelves) have been identified on SCICEX-2000 sidescan records from the Chukchi Borderland at water depths reaching 700+ m (Polyak et al., 2001). We present initial data on the stratigraphy of these ice-scoured areas based on sediment cores collected by USGS in 1991 and SCICEX subbottom chirp-sonar records.

A distinct feature on chirp records is an erosional unconformity of bedrock strata similar to that on the Lomonosov Ridge (Jakobsson, 1999; Polyak et al., 2001). Unconformity occurs at several sites on the Chukchi Plateau and Rise with water depths reaching 700 m. However, other records bear no evidence of erosion at depths as shallow as 350 m. This picture indicates a complex pattern of ice grounding that was probably controlled by sea-floor topography. At shallow depths, above 350 m, sea floor is extensively reworked by chaotically-oriented iceberg scours.

Cores collected along the transect from the Chukchi shelf to Northwind Ridge from water depths above 700 m recover stiff sediments that differ from typical Quaternary sediments of the Amerasian Arctic basin and possibly have pre-Quaternary age. They are separated by a hiatus from overlying Late/Middle Pleistocene sediments. We correlate this hiatus to the erosional unconformity depicted by chirp records from the Chukchi Rise. Although we cannot yet conclusively establish the age control for the overlying Pleistocene sediments, it is obvious that in cores from shallower water depths of <450 m the corresponding sedimentary record is stratigraphically shorter than in cores from larger depths. This indicates two episodes of sea-floor erosion, with the older one reaching larger water depths. This conclusion is consistent with the distribution of glacigenic bedforms that show two sets of lineations (flutes) with different orientation at water depths down to 450 and 700+ m, respectively (Polyak et al., 2001). The development of age stratigraphy for Pleistocene sediments above the erosional unconformity will constrain the ages of glacial grounding on the Chukchi Borderland. Together with reconstructing ice flow trajectories, this result will elucidate the largely unknown history of Pleistocene ice-sheet/shelf expansions into the Arctic Ocean.