Paper No. 4
Presentation Time: 2:30 PM
LINKAGES AND LAGTIMES BETWEEN THE DESERT HILLSLOPES AND STREAMS OF GRAND CANYON DURING RESPONSES TO GLACIAL-INTERGLACIAL CLIMATE CHANGES
Eastern Grand Canyon contains a well exposed stratigraphic record of the responses of hillslopes, ephemeral streams, and the Colorado River to Quaternary climate change. The results of field and geochronologic study of Pleistocene deposits in the arid Chuar Valley-Furnace Flats region suggest both relatively direct responses of, as well as disconnections between, these three landscape components through climate changes. The famous weathering-limited bedrock escarpments of eastern Grand Canyon have a preserved stratigraphy of remnant colluvial mantles at their toes, which grade downslope in local catchments to thick tributary stream deposits. Combined OSL, U-series, and cosmogenic-nuclide dating indicate tributary deposition was occurring >108±23 ka (S4), 50±8 to 30±6 ka (S3), 12±3 to 7±2 ka (S2). The relatively minor S2 deposit is distinct in its field relations, in that it is derived from the incision and remobilization of older alluvium and colluvium rather than being continuous with hillslope sediment sources. Further downslope along the mainstem Colorado River, at least seven inset fluvial gravel deposits have been identified, which are consistently overlain by prograding tributary and hillslope stratigraphic equivalents. Existing dates indicate mainstem aggradation occurred from 385±14 to 322±10 ka (M5), 151±5 and 118±3 ka (M4), and 71±11 to 55±12 ka (M3). The best-constrained last 70 ky of this record thus far suggests a 10-20 ky lagtime between deposition by the Colorado River versus the trunk streams of local catchments, and that the Colorado River had actually begun incising before the main pulse of tributary aggradation after ~50 ka. We hypothesize that this delay in the deposition/incision response of local catchments may reflect both: a) progressive downslope-directed sedimentation from bedrock hillslopes, which first must become sediment-mantled themselves, driven by high weathering rates and possibly combined with decreased effective discharge during OIS 4-3; and then b) incision driven by earlier baselevel fall along the mainstem, but delayed upslope in local catchments possibly due to continued sediment loading of streams. These hypotheses are being tested with continued field and geochronologic work.