EVOLUTION OF AN ANCIENT BRAIDED DRAINAGE: ANALYSIS OF FLUVIAL GRAVELS NEAR GUNNISON, COLORADO

The study of gravel deposits located in McIntosh Mountain Quadrangle near Gunnison, Colorado offer insights into the evolution of a landscape surrounding an ancient braided drainage. The gravels are associated with mid-Oligocene volcanics that were erupted between 30 and 27 Ma from the San Juan Volcanic Field to the south and the West Elk Volcano to the north. The deposits consist of eroded piles of angular to well rounded clasts of granite, amphibolite, gneiss, schist, and hornfels derived from Proterozoic strata; quartzite, mudstone, siltstone, quartz arenite and feldspathic arenite derived from Paleozoic sources; and Tertiary igneous rocks. The correlation of gravel clast lithologies to possible source areas suggest multiple periods of channel development and erosion from the Sawatch Range to the east and northeast of the study area. The deposits also include local contributions of intermediate volcanics from the West Elk Mountains and silicic ash-flow tuffs from the San Juan Mountains. The gravel deposits are considered to be Mid to Late Oligocene in age. This is based on the fact that at several locations, the deposits are found interbedded with Oligocene ash-flow tuffs. In other places, erosion has obscured the stratigraphic relationship between the gravel and volcanic deposits and thus the relative age is unknown. If these gravels post-date Oligocene volcanism, then the gravels may represent remnant fluvial terraces created by the ancestral Gunnison River as it downcut through the volcanic stratigraphy.