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 The Grand Canyon is perhaps our planet’s most widely recognized and single most important geologic landform. The goals for this trip are to give participants an understanding of the canyon’s formation and its dynamic hydrologic system. While our destination is clear, the journey will also provide opportunities to discuss Arizona’s larger geologic setting within the Basin and Range, Transition Zone, and Colorado Plateau physiographic provinces. Stops and discussions will include: (1) geologic setting and groundwater environment of the Phoenix basin; (2) Cenozoic landscape development of the Transition Zone; (3) Montezuma Well, a unique arid-land spring contained within a travertine mound; (4) ascent of the Mogollon Rim, the state’s second largest landform and entryway to the Colorado Plateau; (5) the San Francisco Volcanic Field and surrounding volcanic features, including Sunset Crater, a late Holocene scoria cone; and (6) multiple stops in Grand Canyon National Park to discuss its varied geology. The principal focus here will be on evolving concepts of the canyon’s formation since the time of John Wesley Powell, including the flurry of research results proffered in the past 20 years. Participants will walk the Trail of Time, Earth’s largest man-made geologic exhibit at over 2 km. Another equally important discussion will cover the modern hydrologic system of the canyon, which yields a tenuous supply of potable water from a single inner-canyon spring for over six million annual visitors and 2,500 full-time residents. The National Park Service has prioritized the replacement of the Trans-Canyon Waterline due to climate change concerns.