LONG PROFILE ANALYSIS OF THE PLEISTOCENE BOSTWICK RIVER, WITH IMPLICATIONS FOR THE INCISION OF THE BLACK CANYON OF THE GUNNISON

Lava Creek B ash localities were studied to the north of the San Juan Mountains in western Colorado. These deposits provide a datum for a late Pleistocene Bostwick paleovalley. Through the examination of this Quaternary paleovalley, geometries can be compared between current and past fluvial systems to deduce basinal changes, resulting from fluvial incision that has occurred since the deposition of the Yellowstone Lava Creek B ash at 640 ka This study also provides valuable insight into the incision of the Black Canyon of the Gunnison, a part of the fluvial systems included in this study.

The paleovalley is interpreted from the presence of 20-m-thick lenticular gravel deposits, the gravel deposits are then in turn overlain by fine-grained valley-fill material reaching up to 55 m in thickness. The fluvial gravels are temporally constrained by the presence of the Lava Creek B ash, which overlies above the gravels. The fluvial gravels unconformably overlie the Cretaceous age Mancos shale, the bedrock in the area. GPS measurements of various gravel-bedrock contacts as well as imbrication measurements show the valley was roughly 1 mile wide with a northward paleoflow, originating from the San Juan Mountains. This is further supported by the dominance (>70%) of volcanic clasts comprising the gravel.

Long profile comparisons between the modern Uncompaghre and Gunnison Rivers and the Bostwick River (long profile constructed from ash localities) yield interesting results. First, there is a distinct divergence in slope between the paleoriver and modern rivers which is likely due to the upstream migration of a knickpoint in the modern Uncompaghre and Gunnison Rivers over the past 640 ka. Secondly, there is a clear elevation difference between the current and paleoriver profiles, suggesting that local base level for the fluvial system has lowered significantly in the last 640 ka. This fluvial study has important implications for the evolution of the nearby Black Canyon of the Gunnison. By using the Lava Creek B Ash as a datum, a maximum incision rate was calculated to be 191 m/Ma for areas near to the Black Canyon of the Gunnison. This high incision rate maybe a local phenomena rather than the result of neo-tectonics or large-scale climate change.