TIMING AND RATES OF LATE CENOZOIC INCISION BY THE COLORADO RIVER IN GLENWOOD CANYON, COLORADO

New geologic mapping and ⁴⁰Ar³⁹Ar dating of basaltic rocks refine conclusions concerning rates of incision of the Colorado River presented in the pioneering study by Larson et al (1975). On Spruce Ridge, 4.6 km south of and 847 m above the Colorado River in Glenwood Canyon, a basaltic flow (7.80±0.04 Ma) overlies Colorado River gravel and is cut by a channel filled with gravel. On Gobblers Knob 1.9 km to the east of the Spruce Ridge locality, a flat-lying, thick basaltic flow (3.03±0.02 Ma), probably deposited on a flood plain or other flat surface near river level, is about 732 m above the river. A basaltic flow (1.36±0.04 Ma) at the base of a sequence of flows on Triangle Peak, 15 km NNW from Aspen adjacent to and 396 m above the Roaring Fork River, overlies stream gravels. Based on these data points and assuming that the flows on Spruce Ridge and Gobblers Knob were emplaced at or near the level of the Colorado River, the river incised 115 m in late Miocene and early Pliocene between 7.8 and 3.03 Ma at an average rate of 24 mm/k.y. In the late Pliocene and Quaternary an additional 732 m of incision occurred at an average rate of 241 mm/k.y., an order of magnitude greater than in the previous 4.77 m.y. The rate of incision by the Roaring Fork at Triangle Peak is 291 mm/k.y. for the past 1.36 m.y., which is comparable with the rate for the Colorado River, although in Glenwood Canyon the Colorado River cuts more resistant rocks than does the Roaring Fork downstream from Triangle Peak.

In the Carbondale collapse center, an area which collapsed at least 800 m due to dissolution of evaporite, one basaltic flow previously dated by K/Ar as 7.8±0.4 Ma (9.89±0.06 Ma in this study) was used in the earlier calculations of incision rates. In the collapse center, 10 and 3 Ma basaltic flows are about equally deformed, indicating that the greatly accelerated incision in late Pliocene and Quaternary time led to greatly increased rates of evaporite dissolution and concomitant collapse of overlying volcanic and sedimentary rocks.