PUSHING BACK THE ONSET OF EXTENSION IN THE EAST-CENTRAL BASIN AND RANGE: A PREVIOUSLY UNRECOGNIZED UNCONFORMITY IN THE LOWER HORSE SPRING FORMATION, LAKE MEAD AREA, SOUTHERN NEVADA

The Horse Spring Formation ranges in age from >24 to 12 Ma and records Basin and Range extension in the Lake Mead area. Neither the details of extension nor the paleogeography of this area are fully understood yet figure prominently in tectonic reconstructions of the southwest and understanding Colorado River drainage evolution. The lowest member of the formation, the Rainbow Gardens member (RG), was interpreted as a pre-extensional, sag basin (Beard 1996). The end of the RG member and the beginning of extension was estimated at ~17 Ma by previous workers. Using detailed stratigraphic, tephrochronologic and provenance analyses and new 40Ar/39Ar dates, we reconstruct the RG basin and interpret that uplift to the south began ~19-23 Ma. We measured 13 detailed sections, collected data from 8 additional sections of Beard (1996), and mapped to correlate stratigraphic layers. In the southern RG basin, we document a previously unrecognized unconformity within the middle submember. Over 80 m of section deposited at one locality is missing due to erosion and/or non-deposition within 4 km to the south. Below this unconformity, the two localities have almost identical thickness and facies. At the unconformity there is an abrupt increase in grain size, from fine- and medium-sized sandstones to pebbly, coarse sandstones, and an influx of granitic and metamorphic clasts. This same change is recorded at two other localities 6 and 20 km apart in a palinspastic reconstruction thus suggesting a regional event. A provenance study (Foster et al., this volume) documents the dramatic change in the source area and further supports this interpretation. The only known source area for these grains is Proterozoic basement exposed in the northern part of the Kingman Uplift to the south. To the north and west, Sevier uplifts expose Paleozoic and Mesozoic sedimentary sequences. To the east, these same strata were at the surface. We suggest this regional event is due to uplift to the south of the RG basin. The cause of this uplift is not fully known but strongly suggests that extension begins by 19 Ma and possibly earlier. This is supported by Fitzgerald et al. (2009) who found rapid cooling initiated at 18 Ma to the south in the White Hills - Lost Basin Range, and that the onset of rapid cooling must predate the age of the oldest dated sample at 18 Ma.