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 This field trip will visit the southern Black Mountains volcanic center (SBMVC) and its associated products in northwestern Arizona. Post-volcanic extension and erosion of this semi-arid area afford the opportunity to examine a Miocene-aged (ca. 19–17 Ma) volcanic terrain in detail, using an approach that integrates fieldwork, petrography and geochemistry, and remotely sensed data. This approach was recently used in a three-year National Science Foundation Research Experience for Undergraduate (NSF REU) program, from which some results will be highlighted. This integrated approach can provide insight into the amounts and types of information that can be gleaned from various types of remotely sensed data of a volcanic terrain. Over three days, we will work our way through the volcanic section associated with the SBMVC as exposed near Oatman and Kingman, Arizona, to observe: (1) felsic to mafic shallow intrusions; (2) deposits from felsic to intermediate explosive and effusive eruptions; (3) deposits from sedimentary processes; and (4) the results of subsequent extensional faulting. The petrologic, physical volcanic, and morphologic characteristics of observed outcrops will be discussed at each field stop, and will be used as points of discussion while in the field, for observations of presumably volcanic terrains on other planetary bodies. An appended road log serves as a geologic guide to this volcanic center and associated rocks.

Abstract Little is known about the effect of thrusting on lithological and petrophysical properties of reservoir sandstone. Here we use field observations, probe permeability measurements and thin-section analysis along ten transects from the Muddy Mountain thrust contact downwards into the underlying Jurassic Aztec Sandstone to evaluate the nature and extent of petrophysical and microstructural changes caused by the thrusting. The results reveal a decimetre- to metre-thick low-permeable (≤50 mD) and indurated (0–3% porosity) zone immediately beneath the thrust contact in which dominant microscale processes, in decreasing order of importance, are (1) cataclasis with local fault gouge formation; (2) pressure solution; and (3) very limited cementation. From this narrow zone the petrophysical and microstructural effect of the thrusting decreases gradually downwards into a friable, highly porous ( c. 25%) and permeable (≤2 D) sandstone some 50–150 m below the thrust, in which strain is localized into deformation band populations. In general, the petrophysical properties of the sandstone as a result of overthrusting reveal little impact in overall primary reservoir quality below some tens of metres into the footwall, except for the relatively minor baffling effect of deformation bands.