Abstract
East-central Basin and Range extension is recorded in the late-Oligo-Miocene Horse Spring Formation (HSF) in the Lake Mead Area, NV. Reconstructing the evolution of the HSF is key to understanding tectonic processes and the paleogeography of the region. The lowest member of the HSF, the Rainbow Gardens Member (RG), is thought to predate the main period of extension at approximately 16.5 to 13 Ma. Beard (1996) interpreted the RG to represent deposition in a sag basin from >24 Ma to approximately 17.5 Ma. New provenance data presented here, combined with a detailed stratigraphic study recognizing a new unconformity (Lamb et al., this volume), point to uplift at approximately 19 Ma or earlier. As part of a larger study of the RG, we examined over 60 thin sections and demonstrate an abrupt change in composition at this unconformity. The RG is comprised of three submembers. Rice (1987) showed that the basal conglomerate, the lowest of the three submembers, is sourced from Paleozoic and Mesozoic sedimentary sequences to the west and north, except at southernmost exposures where less than 3% of clasts are granite or granitic gneiss. In this study, we focus on the middle RG submember, comprised of sandstones, limestones and minor gypsum. Most middle RG sandstones are sourced from the Paleozoic and Mesozoic sequences to the west and north or volcanic fields to the northeast. In contrast, we have identified a stratigraphic interval that contains pebble to sand-size plutonic and metamorphic grains whose source we interpret as Proterozoic exposures to the south. Point counts and photomicrographs record this Proterozoic input at 6 localities across the southern portion of the palinspastically-reconstructed basin. Sandstones deposited closest to this uplift contain over 30% Proterozoic grains. Toward the north, sandstones contain fewer Proterozoic grains but the signal persists halfway across the basin. The only source for these grains is to the south. In addition, the grain size of Proterozoic clasts fines northward from pebble to medium sand and the stratigraphic sections thin southward above the unconformity, further supporting a southern source area. We interpret that this change in source area is the result of gentle uplift in the Gold Butte to White Hills and Lost Basin range area starting at 19 Ma or earlier. This suggests that extension may start earlier than previously thought.