OLIGO-MIOCENE PALEOGEOGRAPHY OF THE LAKE MEAD REGION: THE RAINBOW GARDENS MEMBER OF THE HORSE SPRING FORMATION RECORDS PRE-EXTENSIONAL TECTONICS OF THE CENTRAL BASIN AND RANGE

The Oligo-Miocene Horse Spring Formation (HSF), found throughout the Lake Mead region, records the onset of and main phase of extension in the central Basin and Range and reflects a response to tectonic activity on extensional, strike-slip and contractional features. Its lowest member, the Rainbow Gardens (RG, 24-18 Ma), is crucial to understanding the paleogeography at the start of extension and inherited structures and prior tectonic events that affected extensional processes. There are few other basins of this age in the region and so it provides key data on the tectonic and climatic evolution of the southwest. Most sedimentary basins are created by faulting but most or all of RG deposition predates extension by 6-8 Ma: 1) It is relatively thin (75-225m) and sedimentation rates are low for typical fault-bounded basins, and 2) exhumation data indicate faulting began at 17 Ma.

The RG records deposition within a high-energy, alluvial fan dominated system that progressed to lower-energy systems and eventually became a marshy, palustrine, carbonate-dominated lowland. Paleocurrent data from the RG, along with detailed stratigraphic work, suggest deposition in a north-south elongate basin created by surrounding topographic highs, each with its own timing and uplift mechanism. To the south, Beard (2010) presented evidence for a Laramide reverse fault related to the Kingman Arch. The transition from the northern edge of this arch to the RG lowland immediately to the north coincides regionally with the northern boundary of Saleeby’s (2003) shallow slab segment and resultant corridor of Laramide uplifts. To the west and north of the RB basin were modest highlands from the eastern limit of Sevier thrusting that locally included a portion of the Muddy and South Virgin Mountains. To the east, our paleocurrent data suggest a basin edge but not a major uplift. This supports an emerging hypothesis of Flowers et al. (2008) and others that the southwestern interior of the Colorado Plateau may have experienced uplift in the Mid-Tertiary. Finally, to the north and northeast, the early Miocene eruption of the Caliente Volcanic Field and associated thermal uplift likely formed a shoulder at the north edge of the basin (Beard, 1996). These last two events likely created the final edges of the basin and triggered deposition.