THE EARLY MIOCENE HORSE CAMP BASIN, NAVADA: ARCHITECTURE, DEPOSITIONAL HISTORY, AND TECTONIC EVOLUTION OF A CLASSIC EXTENSIONAL HALF GRABEN

The Horse Camp basin area in the White Pine/Grant Ranges of eastern Nevada exposes a spectacular cross sectional view of an early Miocene half-graben basin fill. This sequence is >2500 m thick and includes mud matrix boulder debrites deposited on a subaerial alluvial fan, rhythmic graded hyperconcentrated flows deposited on a fan delta, near shore lacustrine limestone, delta front turbidites, and deep lacustrine shale. New detailed geologic mapping, stratigraphic analysis, and U-Pb LA-ICPMS and 40Ar/39Ar dating of interbedded ash layers within the Horse Camp Formation place important constraints on the age, architecture, and evolution of this extensional basin. The Horse Camp Formation is entirely Miocene as it overlies a 22.96 ± 0.12 Ma ignimbrite. The basin fill is divided into 3 major units, including: (I) a basal 100-350 m-thick fluvial and alluvial sandstone and pebble conglomerate succession that displays little lateral or vertical variation in facies, (II) a “syntectonic” sequence that grades abruptly from >1200 m of subaerial boulder conglomerate to <300 m of deep lacustrine shale, and (III) a >1200 m-thick “late- or post-tectonic” sequence of near shore and offshore lacustrine limestone, shale, sandstone and minor pebble conglomerate that fill in the previously generated basin. Syndepositional seismic activity during deposition of II is suggested by interstratified megabreccia bodies derived from adjacent ranges and spectacular olistostrome deposits produced by subaqueous avalanches off the upper edge of the fan delta. Cross sectional reconstructions also indicate that the lake rapidly deepened during deposition of II, from <100 m to ~900 m. Interbedded ash layers tightly constrain the deposition of most of unit II and part of III between 16.2 ± 0.1 and 15.4 ±0.1, at average sediment accumulation rates of 1.2 ± 0.1 mm/year.