EXAMINATION OF THE RESTING SPRING FORMATION AND THE POSSIBILITY OF LATE OLIGOCENE TO EARLY MIOCENE DEFORMATION IN THE SOUTHWESTERN BASIN AND RANGE

Basinal deposits of Late Oligocene to Middle Miocene age crop out discontinuously in the southwestern Basin and Range province from Lake Mead in the east to the Funeral and Cottonwood Mountains around Death Valley in the west. Previous studies have used these deposits to examine the character of Middle to Late Miocene extension in the region. Correlative Tertiary deposits crop out in the central Nopah Range and north of the Kingston Range in easternmost California. These rocks are called the Resting Spring Formation, but fieldwork suggests that they can be divided into three temporally, lithologically and environmentally distinct packages of rock. The lower formation consists of alluvial fan, lacustrine limestone and braided stream deposits. It rests unconformably upon Early Paleozoic bedrock, dips moderately east, and is correlative with deposits that crop out near Death Valley and Lake Mead. The middle formation consists of poorly indurated fine-grained sandstones, siltstones and shales, which crop out in low relief. It is deposited unconformably against paleotopographic features formed by the Paleozoic and lower Tertiary formations, and is shallowly dipping. The uppermost portion of the middle formation is disrupted; the top is marked by a tuff deposit that is up to about 1 m thick, is highly variable in attitude and which contains spherical accretionary bombs up to 5cm. The upper formation is more or less comformable atop the middle formation and consists of volcanic breccia, with variable meter-scaled bed thicknesses, and very rare interbedded banded tuffs. Clasts are pebble-size on average and compositions range from dacitic to rhyolitic. Bedding surfaces are indistinct at the outcrop, but banded tuffs and broader views suggest the deposits have low dips. The lower formation is related to regional Oligocene and Early Miocene deposits; the middle formation records local lacustrine and perhaps distal volcanic deposition; the upper formation is contemporaneous with crustal extension and volcanism related to the emplacement of the Kingston Peak granite at about 12 Ma. This implies that the Middle to Late Miocene extension was preceded by a Late Oligocene to Early Miocene tectonic event that affected the Early Paleocene and lower formation rocks prior to the deposition of the middle and upper formations.