OCR Text |
Show lower Fortymile Canyon before the upper basin overflow, and includes a slightly reworked ash with chemistry suggesting it is airfall of the 7.5 Ma Spearhead Tuff. Well- integrated southward drainage of the upper FW basin existed by the time the 2.8 Ma basalt flows of Buckboard Mesa were extruded, because the flows slope southward above FW on a grade projecting about 200 m below the rim of Fortymile Canyon. Therefore, establishment of the near- present drainage basin area occurred between 7.5 and 2.8 Ma. During the Quaternary, episodic hydroclimatically driven erosion of the upper basin, Fortymile Canyon, and tributary uplands was accompanied by both aggradation and fanhead incision south of Fortymile Canyon. In this area of the central Amargosa Desert, FW has built one of the largest alluvial fans ( about 200 km2) of the Southern Great Basin . This fan crowds the Amargosa River against the Funeral Range, and has extensive areas of fine- grained light- toned deposits associated with past groundwater discharge in the area of its confluence with the Amargosa River. We have studied the late Quaternary history of Fortymile Wash through geologic mapping, stratigraphic and sedimentologic studies, thermoluminesence dating of the eolian component, and U- series dating of the authigenic opal and carbonate components of surflcial and buried soils and groundwater discharge areas. A major episode of aggradation occurred during 120 to 50 ka. Subsequent incision of up to 20 m of the uppermost part of the fan occurred by 24- 36 ka, during climates known to be significantly cooler and effectively wetter than during the preceding earlier part of the late Pleistocene. After colder and drier, but effectively moist climates of the last glacial maximum, several meters of deposition of bouldery alluvium occurred on the lower part of the fan by 8 ka during flood events embedded within wanner and wetter conditions. These relationships, data on varying alluvial provenance during the late Quaternary, and consideration of the significant effect of downwash infiltration and loss of streamflow competence to transport sediment, suggest the following model. Fortymile Wash aggrades during interglacial and transitional climates which produce high intensity rainfall that erodes hillslopes and supplies sediment to the fluvial system. Incision of the upper part of the fan of FW occurs during relatively cool and moist climates when hillslopes are relatively stable and supply relatively little sediment to competent flows generated by concentrated spring snowmelt from the upper basin of FW. Records of groundwater discharge at the lower end of the FW fan indicate groundwater recharge to and perennial discharge from the fan into the Amargosa River during both aggradation and incision, but with greater groundwater flux and discharge during the most effectively moist climates associated with fanhead incision during 40 to 25 ka. The adjoining major tributary basins to the upper Amargosa River - Oasis Valley, Crater Flat, Topopah Wash, Rock Valley, and Mercury Valley - differ from FW in both integration history and in response to late Quaternary climate change. Though the detailed histories for these basins have yet to be worked out, each have evidence for past drainage geometries within the last 10 M. y. that were substantially different than present. On reconnaissance, each basin appears to have had late Quaternary alluvial aggradational events similar to that of FW, but had substantially less fanhead incision, consistent with these lower basins producing less snowmelt than FW during periods of greatest effective moisture. |