INCISION HISTORY OF THE BLACK CANYON OF THE GUNNISON: UPDATE AND SUMMARY OF AVAILABLE CONSTRAINTS AND PROBLEMS

The Black Canyon of the Gunnison is one of the deepest (~ 900 m) and narrowest (~350 m) bedrock canyons in the western U.S. The Gunnison is fed by several stranded and underfit tributaries, including the abandoned Shinn-Bostwick "river" on the south and Grizzley Creek on the north. The incision histories of all these drainages are linked to more regional puzzles about the relationship between surface uplift and climate in the creation of present topography and relief. Fill gravels are preserved intermittently from the confluence of the North Fork Gunnison River to the highest part of the rim of the Black Canyon. At the confluence of the north rim and Grizzley Creek, gravels are a mix of local Elk Mountain volcanics and Precambrian clasts, indicating that incision into basement was initiated at the time Grizzley Creek was graded to this level (900m).

New river profiles incorporate all terrace heights and available timing constraints on gravels (from interbedded ashes). The Shinn-Bostwick "river" was graded to the paleo Gunnison River at 2140 m at Red Canyon. The presence of Mesa Falls (1.2 Ma) and Lava Creek B Ash (640 ka) from just below these gravels may indicate relative stability of the Shinn-Bostwick river at this elevation for a period of ~600 Ka. Using the younger age, ~half of the incision of the Black Canyon has occurred in the past 640 Ka, that the Canyon had begun incising well before 1.2 Ma, and that the entire canyon may have been cut in the last 2 Ma (Hanson 1965). Longitudinal profiles of the Gunnison River display a knickpoint near the Painted Wall, where the river drops 400 m in elevation over ~4 km (100 m /km) through Vernal Mesa granite, a massive 1.42 Ga, relatively unfoliated and unjointed megacrystic granite. The rock strength of this unit may be inhibiting knickpoint migration, but we hypothesize a transient knickpoint generated by drainage reorganization and/or neotectonics. Different incision rates above (90-100 m/Ma) and below (170-250 m/Ma) the knickpoint suggest continuing upstream knickpoint migration. Ongoing studies plan for cosmogenic burial dating for terraces/gravels at numerous locations within the Black Canyon (at Shinn-Bostwick Park, Grizzley Creek, and the North Fork Gunnison River) in an effort to more fully understand the timing and evolution of the carving of the Black Canyon of the Gunnison.