Oligocene to middle Miocene fluvial, lacustrine and volcanic strata are well exposed in southcentral Nevada and southeastern California and record an early phase of extensional deformation in the Basin and Range province. These strata are approximately 2,000 m thick and predominantly mapped as the Horse Spring Formation and Rocks of Pavits Spring. Stratigraphic and sedimentologic data show that the stratigraphy of these combined deposits consists of three main units. The lower unit consists of a few hundred meters of gastropod and ostracod-rich limestones. This lower unit is overlain by several hundred meters (possibly up to 1000 m) of pebble-cobble conglomerate, coarse sandstone, and red siltstone. Individual conglomerate beds can be greater than 30 m thick. Above the conglomerate-rich unit is a thick section (over 1000 m) of volcaniclastic sandstone, bedded tuff and airfall tuff. This three-unit stratigraphy can be correlated in outcrop from the Funeral Mountains of eastern California east to the Frenchman Flat area of the Nevada Test Site. Analysis of cuttings from several of the wells drilled by Nye County south of the Yucca Mountain area also show a similar three-unit subsurface stratigraphy. We interpret the lower limestone-rich unit to represent initiation of extension that resulted in the ponding of existing fluvial drainages along the axes of the newly developed extensional basins. The middle conglomerate-rich unit is interpreted to represent active extension and the development of regional through-going fluvial drainage systems. The upper volcanic-rich unit is interpreted to represent the late phase of extension when attenuation of the crust resulted in widespread regional volcanism to the north of this area.

Our stratigraphic analyses coupled with those of regional structure and geophysics can be used to test various hypotheses of early Basin and Range extension, including the proposed 90 degree rotation of extension (north-south to east-west), detachment faulting, and Walker Lane strike-slip displacements.

Work supported by the US NRC (Contract NRC-02-97-009). The work is an independent product of the CNWRA and does not necessarily reflect the views or regulatory position of the NRC.