WHY THE COLORADO NEVER FLOWED THROUGH UNAWEEP: SIGNIFICANCE OF GRAND MESA IN GEOMORPHIC EVOLUTION OF THE UNCOMPAHGRE PLATEAU

Miocene basalts of Grand Mesa comprise the oldest regional landscape marker from which relative timing and placement of later erosional and tectonic events may be hypothetically determined. Before eruption of the 9.8 Ma flows, integrated drainage was eastward from the low relief of the Uncompahgre uplift, and northward from the Gunnison uplift, eventually joining the Colorado R. Blocked by the eruptions, and partially reversed in slope by laccolithic doming, this now-ponded and tectonically disturbed upland drainage of the ancestral Gunnison R. would have spilled west into the adjacent Dolores R. drainage, across the low divide formed by the Uncompahgre uplift. For the Colorado R., this diversion of Gunnison R. discharge would have caused entrenchment in the segment above the Dolores, and, farther upstream, valley widening resulting from reduced incision near the former Gunnison R. confluence. Later, as the Miocene closed, the resulting canyon of the Colorado above the Dolores fixed the course of the Colorado across the nose of the Uncompahgre, while topography associated with Grand Mesa and the stranded drainage to the south indicates the Colorado had been effectively separated from the then-present upland Gunnison R. drainage. By mid-Pliocene, the Gunnison R. had a well-established course across the tectonically quiescent Uncompahgre and was cutting Unaweep Canyon in the crystalline core of the uplift. With passage of the Farallon Plate, regional extension associated with opening of the no-slab window allowed isostatic adjustment and consequent renewed differential uplift to sweep abruptly northeastward across the Uncompahgre uplift beginning ~2 Ma to the south, and evident by 1.8 Ma northward in the vicinity of Unaweep Canyon, as proposed previously.