MIOCENE EXTENSIONAL TECTONICS AS RECORDED BY THE HORSE SPRING FORMATION, LONGWELL RIDGES, LAKE MEAD, NEVADA: POTENTIAL FOR 100-KY TIME-SCALE BASIN ANALYSIS USING A MULTIDISCIPLINARY APPROACH

Detailed mapping and stratigraphic analysis in the Longwell Ridges area of the Lake Mead Miocene extensional domain has allowed us to document the development of the NW margin of an extensional basin and correlate to faulted sub-basins farther east. 40Ar/39Ar geochronology on 10 tuffs provides chronostratigraphic control between faulted sub-basins and preliminary sedimentation rates. The geochronology, and geochemical, sedimentological, and structural data, shows potential to understand the stratigraphy at a 100-ky time scale resolution. Our study suggests that the lower Horse Spring Formation spans from ~17 to ~14.5 Ma at Longwell Ridges. The lower facies show vertical (~180 – 200 m) and lateral changes from alluvial fan conglomerate and fluvial sandstone, to stream, lake and marsh siltstone, gypsum, and limestone, to lacustrine limestone. A basal conglomerate together with tuff ages indicates a buttress unconformity that climbed to the NW up a NE trending range front with modest relief. Lacustrine limestones are commonly developed near the ancient mountain front. Lacustrine facies change to a thick upper section (~1 km) of fluvial sandstone and siltstone; the uppermost section comprises 200-300 m of gypsum that may be correlative with the Bitter Ridge Limestone. The tuff ages, taken at face value, indicate a relatively slow sedimentation rate for the lower section (~110 m/my) and a poorly constrained rapid (~1000 m/my) rate for the upper section. The change in sedimentation rate probably occurred at ~15.3 Ma if it is coincident with the abrupt change from limestone to sandstone. Four additional, undated tuffs at key levels show the potential for 100-ky time scale analysis and more refined sedimentation rates. Three of the current tuff ages are from biotite that are consistent with the sanidine ages; however, sanidine – biotite pairs from 2 samples showed sanidine ages to be more precise and younger, so reanalysis of the samples will further refine the results. More 40Ar/39Ar ages and an ongoing geochemistry-based fingerprinting study should refine correlations to critical sub-basins locally and farther west and east. Finally, a detailed stable isotope study of the lacustrine facies of the Bitter Ridge Limestone, combined with 40Ar/39Ar, should allow analysis of cyclicity at the 100-ky and greater scale.