STRATIGRAPHY AND TECTONIC IMPLICATION OF UPPER OLIGOCENE AND LOWER MIOCENE STRATA IN THE YUCCA MOUNTAIN AREA

The Oligocene-Miocene Horse Spring Formation and Rocks of Pavits Spring consist of over 2000 m of strata in the Yucca Mountain area. Outcrops in the Funeral Mountains and across the Nevada Test Site reveal that these units commonly dip toward the Amargosa Desert. Our stratigraphic and sedimentologic data show that the stratigraphy of these combined deposits can be divided into three main lithologic units. The lower unit is dominated by 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 30m thick. Above the conglomerate-rich unit is a thick section, over 1000m, of volcaniclastic sandstone, flows, and airfall tuff. This three-unit stratigraphy can be correlated from the Funeral Mountains east to the Frenchman Flat area of the Nevada Test Site. Analysis of cuttings from several wells, located adjacent to 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 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. The strata constrain interpretations of nascent extension of the Basin and Range and tectonic reconstructions that restore the region to its pre-extensional configuration. The presence of basinal strata across the region suggests that Basin and Range extension was underway by the beginning of the late Oligocene or early Miocene. Moreover, the regional distribution of Horse Spring Formation and Rocks of Pavits Spring suggests that late Miocene and Pliocene low-angle detachment faulting did not substantially disrupt the depositional alignment of these basinal strata.

[This abstract is an independent product of the CNWRA and does not necessarily reflect the views or regulatory positions of the U.S. Nuclear Regulatory Commission]