ABSTRACT Tule Springs Fossil Beds National Monument (TUSK) preserves 22,650 acres of the upper Las Vegas Wash in the northern Las Vegas Valley, Nevada, USA. TUSK is home to extensive and stratigraphically complex groundwater discharge (GWD) deposits, called the Las Vegas Formation, which represent springs and desert wetlands that covered much of the valley during the late Quaternary. The GWD deposits record hydrologic changes that occurred here in a dynamic and temporally congruent response to abrupt climatic oscillations over the last ~300 ka (thousands of years). The deposits also entomb the Tule Springs Local Fauna (TSLF), one of the most significant late Pleistocene (Rancholabrean) vertebrate assemblages in the American Southwest. The TSLF is both prolific and diverse, and includes a large mammal assemblage dominated by Mammuthus columbi and Camelops hesternus. Two and possibly three distinct species of Equus, two species of Bison, Panthera atrox, Smilodon fatalis, Canis dirus, Megalonyx jeffersonii, and Nothrotheriops shastensis are also present, and newly recognized faunal components include micromammals, amphibians, snakes, and birds. Invertebrates, plant macrofossils, and pollen also occur in the deposits and provide important and complementary paleoenvironmental information. This field compendium highlights some of the classic stratigraphic sequences of the Las Vegas Formation within TUSK, emphasizes the significant hydrologic changes that occurred in the area during the recent geologic past, and examines the subsequent and repeated effect of rapid climate change on the local desert wetland ecosystem.

ABSTRACT This field guide synthesizes more than a half century of research by many geologists and paleontologists on Frenchman Mountain and Rainbow Gardens, southern Nevada, USA. The field-trip consists of seven stops to be visited in one day. The guide was written not only for field-trip participants on the occasion of the 2022 ­Cordilleran/Rocky Mountain Geological Society of America Joint Section Meeting in Las Vegas, but also with future users in mind. The Frenchman Mountain/Rainbow Gardens block of crust exposes an extraordinary sample of Earth history. The geologic features include (1) Proterozoic crystalline rocks, (2) the Great Unconformity, (2) a Paleozoic interval that is essentially a ­tilted section of the western Grand Canyon, (3) a Mesozoic interval that preserves strata that were eroded off the southern Colorado Plateau during the Miocene “Great Denudation” episode, and (4) a Cenozoic section that records a wealth of paleo­climatic, paleontological, and tectonic data. Among the many stories that are recorded in the rocks of the Frenchman Mountain/Rainbow Gardens block, I have chosen three to emphasize in this field guide: (1) the history recorded in the Proterozoic Vishnu Basement Complex and the Great Unconformity; (2) the stratigraphy, sedimentology, paleontology, and geochronology of the Cambrian Tonto Group [with a focus on (a) trilobite biostratigraphy in the Bright Angel Formation, (b) the significance of the abundance of glauconite in the Tonto Group, and (c) the Frenchman Mountain Dolostone]; and (3) the tectonic story recorded in the Thumb Member of the Horse Spring Formation. The basement rocks record events and processes associated with the assembly of supercontinents Nuna and Rodinia. The Great Unconformity records the breakup of Rodinia and the associated denudation interval that played a role in triggering the Cambrian explosion. The Tonto Group, which was recently expanded to include the Frenchman Mountain Dolostone, records the Sauk Transgression. And the Thumb Member of the Horse Spring Formation contains rock avalanche deposits that have played a key role in sorting out the tectonic history of the southern Nevada region and the translational history of the Frenchman Mountain/Rainbow Gardens block.

Abstract There is a long history of debate over glacial versus non-glacial interpretations of both Quaternary and pre-Quaternary diamicts in various places around the world, and the Spring Mountains in southern Nevada are the site of one such debate. Here the debate focuses not only on Quaternary diamicts, but also on landforms and erosional features. The deposits and geomorphic features in question will be examined on this field trip. The Spring Mountains are developed in a fault block in the southern Basin and Range Province; elevations range from ~4000 ft (1220 m) at the eastern base to 11,918 ft (3634 m) at Charleston Peak. The range is farther south than any other glaciated range in Nevada; however, the glaciated San Gorgonio Mountains on the border of the Basin and Range in California are farther south, though in a much more maritime position. The Spring Mountains lie in the rain shadow of the Sierra Nevada; rainfall increases from ~4 in (10 cm) per year in the Las Vegas Valley to ~20 in (50 cm) per year at the crest of the range. A published interpretation of sedimentary and geomorphic features at the head of Kyle Canyon claims that steep valley heads of Kyle Canyon and Big Falls wash are degraded cirques, that a ridge at the mouth of Big Falls wash is a lateral moraine, and that diamicts exposed in the ridge include glacial till. An alternative view is that the “cirques” are normal valley heads as are found in high-relief desert ranges, that the “lateral moraine” owes some of its ridge character to erosion along Big Falls wash and may originally have been a debris flow levee or a protalus rampart, and that the “till” is actually colluvium. Abundant clast striations constitute a key element of the glacial interpretation, and much rests on whether glacial striations can be distinguished from mass movement striations.