BASIN ANALYSIS OF THE CENTRAL GRAND WASH TROUGH, NW ARIZONA: NEW INSIGHTS INTO A MIOCENE EXTENSIONAL BASIN

The Grand Wash Trough (GWT), NW Arizona, is the first extensional basin transected by the Colorado River west of the Colorado Plateau. The GWT formed as a hangingwall basin above the Grand Wash fault and is associated with detachment faulting in the south Virgin Mountains. Basin fill deposits of the GWT document the tectonic history of the eastern Lake Mead region during the Miocene. Newly measured stratigraphic sections and geologic mapping along an E-W transect across the basin near Pearce Ferry allow for an accurate cross sectional view of the basin. The oldest rocks of the GWT, dated by previous workers at 15 Ma, record the formation of an internally drained, half graben basin created by movement along the Grand Wash fault. Older deposits on the western edge of the basin are pebble-boulder conglomerates deposited as debris flows derived from the south Virgin Mountains. These conglomerates, interpreted as large alluvial fan deposits, dominated the western side of the basin until ~9 Ma. The conglomerates grade eastward into planar laminated sandstones and siltstones deposited in an axial playa setting. At 10 Ma the playa deposits transitioned to fresh water limestones deposited in a lacustrine environment that persisted until 6 Ma. Along the eastern edge of the basin a Paleozoic clast conglomerate overlies the limestone and overlaps the basin bounding Grand Wash fault. Generally speaking our interpretations are consistent with previous workers results with a few exceptions. A previously undocumented normal fault, antithetic to the Grand Wash fault, was found ~2 km west of the eastern edge of the basin. This fault was active during the final stages of limestone deposition in the GWT and contradicts previous interpretations that the GWT was tectonically inactive from 11-6 Ma. The GWT is a unique basin because of the aerial distribution and coarseness of the western alluvial fan conglomerates relative to the size of the basin. The source area for the conglomerates may have been isostatically uplifted as the result of detachment faulting in the south Virgin Mountains.