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Show Scott C. Lundstrom U. S. Geological Survey MS 913, Box 25046 Denver, CO LATE CENOZOIC HISTORY OF FORTYMILE WASH, SOUTHERN GREAT BASIN, NEVADA: LATE MIOCENE TO PLIOCENE DRAINAGE BASIN EVOLUTION AND LATE QUATERNARY HYDROLOGIC AND GEOMORPHIC RESPONSE TO CLIMATE CHANGE Fortymile Wash ( FW), presently an ephemeral desert wash, is hypsometrically the largest and highest tributary of the upper Amargosa River in the southern Great Basin, and has built a large alluvial fan into the central Amargosa Desert. The establishment of the watershed of FW and its subsequent highly varied hydrologic response to climate change probably contributed to ( and perhaps dominated) a variable history of streamflow in the Amargosa River which controlled a varying habitat for fish and other aquatic biota of the basin. Two major aspects of FW history considered here are: ( 1) the late Miocene to Pliocene evolution and establishment of the FW drainage basin, and ( 2) the late Quaternary hydrologic and geomorphic response of the FW alluvial fan to climate change during the past 100 k. y. A major change in surface drainage pattern during late Miocene to Pliocene time is constrained by geologic mapping, stratigraphy, and provenance analysis of alluvial basin fill interbedded with dated lava flows, tuffs, and distal ash beds. FW now provides southward surface drainage to 60 percent of the physiographically well- expressed 11.5 Ma Timber Mountain Caldera through Fortymile Canyon, a major breach in the southern caldera margin. The northern half of Fortymile Canyon is incised up to 300 m through the basaltic Dome Mountain shield volcano (- 10.5 Ma) and the overlying rhyolite flows of the 10.3 Ma Shoshone Mountain volcanic center. These large constructional volcanoes formed a drainage divide in the area now occupied by the canyon. This is evidenced by flows from both centers that slope away from each center, but with especially extensive flows northward into the east caldera moat. In the moat, the flows are overlain by gravelly alluvium that also grades and fines northward and contains clasts derived from each of the centers, indicating a northward direction of sediment transport opposite that of southward sloping FW. This late Miocene alluvium in the east moat is overlain by tuffs of the 9.3 Ma Thirsty Canyon Group, erupted from a location that would have dammed an outlet to the above drainage. Transformation to a closed basin is evidenced by laminated sands which include a 40- cm- thick basaltic ash over a thin white ash. The ash couplet is not dated but its thickness and position suggest correlation with the 8 Ma Thirsty Canyon basalts or the voluminous early Pliocene basalts of Crater Flat to the southwest or the Thirsty Mesa Shield to the west. It is likely that southward drainage from the east moat was initiated when the closed basin filled and spilled southward through the saddle between Dome and Shoshone Mountains into a minor drainage in the location of lower Fortymile Canyon. Integration of the east moat with the lower base level of the extensional Jackass Flat basin resulted in incision of Fortymile Canyon, and a partly superposed drainage pattern in the east moat. This event resulted in at least a fourfold increase in the area of the FW watershed, adding areas that became the highest and wettest part of the Amargosa River basin. A lithologically unusual gravel along FW south of the Canyon is interpreted to be a record of |
