PRELIMINARY RESULTS OF DEEP DRILLING IN WESTERN UNAWEEP CANYON (COLORADO): A PALEOZOIC TO CENOZOIC HISTORY REVEALED

Unaweep Canyon is enigmatic. The Precambrian inner gorge carves a wide, deep, flat path through the Uncompahgre Plateau, and houses two underfit creeks emanating from a divide within it. The canyon's genesis has been attributed to Cenozoic fluvial incision or glaciation, or late Paleozoic glaciation and Cenozoic exhumation. To assess genesis, we drilled and cored a deep well in western Unaweep Canyon.

Preliminary analyses distinguish three units within the 320.3 m core. The upper ~150+ m comprises reddish-brown, poorly indurated conglomerate intercalated with moderately sorted sand. Both Mesozoic sandstone and Precambrian basement clasts occur, ranging from 1 cm to 1.2 m in diameter. Sand composition (Gazzi-Dickinson method) is approximately Q (~25), F (~70), A (<3) and Lv (<3), and the pollen is Cenozoic. The middle unit comprises ~150 m of greenish-gray, well sorted, poorly indurated fine sand, silt, and clay containing locally abundant carbonaceous debris. Sand composition is highly variable, characterized by increasing Q (<80) and Lv (<10) with depth, and the pollen is Cenozoic. The well terminated within a brownish-gray, moderately indurated diamictite containing late Paleozoic-aged pollen, and an approximate sand composition of Q (15-20), F (65-75), A (11) and Lv (0).

The provenance and pollen data indicate a Cenozoic age for the upper two units, but a late Paleozoic age for the lowest. These results support the interpretation that Unaweep's inner gorge originated glacially, in the late Paleozoic and was buried. Significant Lv and Q sand within the middle interval reflect the ancestral Gunnison River, which fluvially exhumed (Neogene) the buried gorge. A lacustrine origin for this unit suggests that the ancestral Gunnison became blocked, leading to aggradation within, and ultimate abandonment of the canyon. This lacustrine fill might correlate to young lakebeds reported by others near Cactus Park. The youngest unit records passive filling of the inner gorge by mass wasting. We suggest Unaweep Divide originated as the West Creek and East Creek drainages evolved in recent time, and bears no relation to the Gunnison River, nor Uncompahgre neotectonics.

[Q=quartz, F=feldspar, Lv=volcanic lithics, A=amphibole. We thank Himes Drilling, Inc for their generosity in absorbing partial costs of drilling.]