QUATERNARY INCISION HISTORY AND DETRITAL SANIDINE PROVENANCE OF THE UPPER COLORADO RIVER, GLENWOOD CANYON AND RIFLE, COLORADO

The effects of bedrock resistance must be incorporated into interpretations of temporal and spatial patterns of fluvial incision. Glenwood Canyon, located in the upper Colorado River basin, represents a knickzone comprised of Precambrian crystalline rocks overlain by resistant Paleozoic sandstone and limestone units. Downstream of the knickzone (Rifle, Colorado), incision rates ranged from ~290-260 m/m.y. over the past ~2 Ma, based on cosmogenic radionuclide burial ages of ancient Colorado River deposits. Upstream of the knickzone (Dotsero, Colorado), incision rates were 320-200 m/m.y. from ~3.0-0.6 Ma and then slowed to ~100 m/m.y. from ~0.6 Ma to the present. This deceleration could reflect bedrock incision through the resistant Paleozoic and Pecambrian units that comprise the lower levels of the canyon. Additional luminescence dates on Colorado River terraces upstream and downstream of the knickzone will further evaluate the effects of bedrock type on rates of fluvial incision.

Detrital sanidine (DS) ⁴⁰Ar/³⁹Ar geochronology was also used to try and date probable Plio-Pleistocene Colorado River deposits. Four Colorado River sand samples using the 125-250 um fraction yielded 355 DS grains <38 Ma, but no Plio-Pleistocene grains were found; the youngest grain is ~10 Ma. However, the DS data provide insight on ancient Colorado River sediment provenance. Based on comparisons between published ages of volcanic tuffs and the high-precision DS ages, it is likely that some of the ancient Colorado River grains were eroded from volcanic rocks that crop out within the Never Summer Range of the upper Colorado River basin. However, other DS ages correlate well with eruption ages from the Southern Rocky Mountain volcanic field and, possibly, the Basin and Range. These grains probably originated as air-fall deposits that accumulated within the Colorado River watershed, and were subsequently reworked by the river. The recognition of far-traveled sanidine grains derived from areas outside the Colorado River watershed is important for interpreting sediment provenance of fluvial deposits elsewhere.