PALEOGENE TOPOGRAPHY OF THE GUNNISON AREA, GUNNISON COUNTY, COLORADO

New mapping of five quadrangles at 1:24000 has allowed reinterpretation of the Paleogene topography of the southern portion upper Gunnison Basin. The Paleogene surface was initially formed on the NE flank of the Laramide Gunnsion uplift and SW flank of the Sawatch uplift and formed a SE extension of the Piceance Basin. The Paleogene surface was buried by intermediate volcanic and volcaniclastic rocks from the Lake Fork (~32Ma), Sawtooth (~32 Ma) and West Elk (~30 Ma) volcanic centers and a series of large ash-flow sheet including Wall Mountain (36.7 Ma), Saguache Creek (32.2 Ma), Blue Mesa (28.5 Ma), Dillon Mesa (28.4 Ma), Sapinero Mesa (28.35 Ma), Fish Canyon (28.0 Ma) and Carpenter Ridge (27.5 Ma) tuffs. Partial exhumation of this surface allows production of a detailed topographic map covering ~1500 km². The map was produced with a 20’ contour interval with good topographic control over ~75% of the area. The main stem of the drainage system closely follows the current path of Tomichi Creek and drained a region with up to 2500’ of buried topographic relief. A coherent drainage network can be produced between the Cimmaron Fault to the SW and the Wood Gulch Fault to the east, both of which have possible Neogene movement. This paleo-drainage disappears below volcaniclastic rocks from the West Elk volcanic center just west of the town of Gunnison at a modern elevation of ~7700‘. A paleo- tributary to the main stem of the drainage disappears below the volcanics at a modern elevation~ 7600’ 13 km to the west of Gunnison. The paleo-surface shows multiple examples of inverted topography, paleo-stream channels that cut modern drainages at high angles, and places where modern drainages closely follow paleo-drainages.