Abstract
This study focuses on the lower Horse Spring Formation in the Frenchman Mountain block located in the western Lake Mead domain. The lower part of the formation contains the Rainbow Gardens Member ( approximately 25- approximately 16 Ma) and the Thumb Member ( approximately 16-14.5 Ma). We constructed a 1:10,000 scale geologic map of the Rainbow Gardens area on the Frenchman Mountain block and analyzed the facies and stratigraphy of the Rainbow Gardens and Thumb members in detail to produce a south-north, stratigraphic panel of the lower Horse Spring Formation in this area. The Rainbow Gardens Member has a conglomerate at its base, with red sandstones and interbedded limestones in the middle, and palustrine limestone beds on top. The Thumb Member includes the following sequence in the Frenchman Mountain area: in the lower part lacustrine limestones and calcarenites, gypsum, and conglomerate all interbedded to the north with sandstone; in the upper part interbedded red sandstone, gypsum, and megabreccia in a thick section with common tuffs. The contact between the two members is discrete, and is similar in lithology to the South Virgin Mountains, north of the Gold Butte Block. In this study and in other areas, the upper part, or the top, of the Rainbow Gardens Member records the onset of extensional faulting, while the Thumb Member records widespread faulting. Facies analysis shows a basinal transition in the following order upward: (Rainbow Gardens Member) gravel alluvial fans and plain derived from Kingman Arch to south; sandy fluvial plain; carbonate lake and lake margins; (Thumb Member) playa and sandy margins, fluvial plain with alluvial fans derived from southeast; fluvial plain with playas and landslide breccias derived from Gold Butte; a mix of shallow lake and fluvial plain with landslide breccias continuing from Gold Butte. The study area records the center to northern edge of the basin during lower Horse Spring deposition. The megabreccia sequence, lithologies, and new tuff ages indicate a systematic derivation of landslides from the NE to NW part of Gold Butte from 15.5 to 14.6 Ma. Preliminary paleogeographic maps are compatible with all known data if the Frenchman Mountain block is placed adjacent to the north to NW side of Gold Butte from 19-14.5 Ma.