PALEOSOLS OF THE RAINBOW GARDENS MEMBER OF THE MIOCENE HORSE SPRING FORMATION, LAKE MEAD REGION, NEVADA: PALEOENVIRONMENTS, TECTONICS, AND LANDSCAPE EVOLUTION

Within the Basin and Range region, the Horse Spring Formation (HSF) north of Lake Mead, Nevada gives insight into the evolution of the upper plate of a low-angle detachment fault system. The HSF contains significant paleosols, both in carbonate and siliciclastic sequences, that can shed light on Miocene paleogeography and the tectonic evolution of the area. In particular, we can use the physical and chemical properties of these paleosols and their lateral variation to test the validity of retrodeformed basin models. Additionally, stratigraphic (vertical) changes in these paleosols constrain models of Miocene landscape evolution and can provide insight into paleoclimate. The oldest of the four members of the HSF is the Rainbow Gardens member (RBG; ~18-24 Ma) that records the pre-extensional paleogeography of the region. We sampled paleosols from eight localities in this member. New isotopic, petrographic, and outcrop data from these eight sections has allowed us to examine the paleogeography of a retrodeformed Rainbow Gardens basin in considerably more detail, characterizing the evolution of a unique palustrine system through time. Preliminary isotopic results suggest that isotopes trend according to the approximate maturity of the paleosols, with oxygen isotopes becoming heavier with more mature soils. This proxy can be used to understand both vertical and lateral variation in paleosol maturity and, thus, the changing paleogeography of the Rainbow Gardens basin. Preliminary results show an evolution from swampy, fluvio-palustrine settings to carbonate-dominated palustrine/lacustrine paleoenvironments. This trend terminates with a fully lacustrine carbonate unit at the base of the overlying Thumb member. Furthermore, lateral variation of RBG paleosols illuminates the paleogeography of a hypothesized retrodeformed Rainbow Gardens basin.