ABSTRACT: Late Quaternary shorelines and deformation of the Old Crow Basin, Yukon Territory.

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Abstract

The Old Crow basin is an elliptical depression about 130 km long by 70 km wide with its long axis oriented northwest-southeast. An elliptical bedrock-cored ridge on the floor of the basin near the mouth of Timber Creek parallels the basin's long axis, indicating that local bedrock structure is concordant with basin configuration. The basin probably is structurally controlled, and deformation may be continuing to the present day. Oriented thaw lakes on the forested basin floor have rectangular outlines and are elongated parallel to the basin's long axis. These rectangular lakes differ in both form and setting from the elliptical thaw lakes on the windswept Alaskan Arctic Coastal Plain, which are shaped by prevailing winds. Modern warping of the basin floor is indicated by tilting of the lakes, which typically have elevated shorelines along margins toward the basin edge and marshy submergent shores along margins toward the basin center. The rectangular lake forms might reflect fracture patterns created by deformation of the permanently frozen sediments that underlie the basin floor. Ongoing subsidence in the Old Crow basin also is suggested by the present-day course of Old Crow River, which flows southeast down the basin's axis into its center, then turns 90 degrees to breach the southwest flank of the basin. Late Pleistocene shorelines around the margins of the Old Crow basin are most common at about 300-380 m asl, with some evidence for higher stands up to about 450 m asl. Shoreline configurations reflect generally smooth lake margins except to the southwest, where Mt. Schaeffer and bedrock outliers of the Old Crow Range projected into the lake. Recognizable shoreline features around the basin include beach ridges, wave-cut scarps, spits, deltas, and cuspate forelands. During the 1970 field season, we traversed shorelines at the north and south ends of the basin as well as along the basin's northeastern flank. We found lithic flakes and sparse artifacts at sites we interpreted as overlooks or hunting stations of late to possibly middle Holocene age, but no evidence for habitation structures or for human occupation contemporaneous with the late Pleistocene lake stands. Altimeters available to us in 1970 lacked the precision necessary to determine whether any of the shorelines had been tilted or broadly warped by tectonism. Modern GPS surveys, however, would allow such determinations to be easily carried out. More recent field studies in the Noatak basin of northwest Alaska have documented additional relict shoreline features that may have analogues in the Old Crow and other basins of the northern Yukon. Ice shove in the Noatak basin formed prominent gravel embankments on lee shores that may indicate dominant paleowinds during spring breakup. Horseshoe-shaped iceberg scars mark where bergs grounded on sloping flanks of the Noatak basin, indicating paleowind direction as well as the former presence of calving glaciers. Lagoons or marshlands are present in reentrants and other areas of unusually gentle slopes. Where studied in the Noatak basin, their sediments commonly contain organic remains suitable for radiocarbon dating and paleoenvironmental reconstruction. Deltas and extensive sand and gravel embankments formed where clastic sediments were abundant in both the Noatak and Old Crow basins. These deposits generally accumulated at the mouths of principal inlet streams, and may be useful indicators of basin paleohydrology. Both lagoonal areas and deltas may have supported productive wetlands that attracted wildlife and perhaps early humans

Bibliography of Canadian Geomorphology