Abstract An ~90-m-thick interval of mixed siliciclastic-carbonate strata, including a cap dolostone unit, overlies diamictite of the upper Scout Mountain Member of the Pocatello Formation in the Fort Hall Mine area south of Portneuf Narrows, southeastern Idaho, and is ≤ ca. 665 Ma. Six facies comprise this interval: silty sandstone (reworked diamictite matrix), laminated dolomite, dolomite and sandstone, sandstone, dolomite-chip breccia, and argillite and limestone. Sedimentary structures and bedding geometries of facies indicate paleoenvironments ranging from below storm wave base to upper shoreface. The edgewise, mounded, and parallel-bedded dolomite-chip breccia indicates slope failure and reworking of the lower shoreface during large storms. Facies relationships allow generalized division of these strata into three units. The lowermost unit, Unit A, comprises intimately interbedded laminated dolomite (“cap dolostone”), dolomite-chip breccia, and sandstone facies and is 17 m thick. Unit A apparently grades upward into Unit B, a 45-m-thick interval of the sandstone facies. Unit C, 28 m thick, rests sharply on Unit B, and comprises a basal laminated dolomite facies and the limestone and argillite facies. Units A and B may indicate a regressive wave-dominated coast that was influenced by large storms (highstand systems tract). Unit C indicates near storm wave base deposition and an overall deepening, as shown by dark argillite beds of the overlying upper member of the Pocatello Formation (transgressive systems tract). δ13C and δ18O values from dolomite and limestone samples of Units A and C are similar to values from local, regional, and transglobal cap carbonate intervals. δ13C values range from −1.9 to −5.6‰ and δ18O values range from −10.2 to −17.4‰, with no systematic correlation between C- and O-isotope values. δ13C values are consistent with previously reported values from the Pocatello Formation and are similar to values from the alleged Marinoan Noonday Formation in Death Valley, California, and the Marinoan Maieberg Formation in Namibia. Collective data from the cap dolostone and associated strata of the Pocatello Formation suggest protracted mixed siliciclastic-carbonate deposition on a stormdominated shelf at ca. 665 Ma.

Abstract Our studies of the Neoproterozoic Uinta Mountain Group focus on the Red Pine Shale in the western Uinta Mountains and the undivided clastic strata in the eastern Uinta Mountains, which record deltaic-marine and braided-fluvial to shoreline deposition, respectively. We conclude that the Red Pine Shale postdates the < 770 Ma eastern clastic strata, and the Uinta Mountain Group represents deposition in a rift basin predating the rift episode recorded at ~ 700 Ma in western Laurentia. Measured sections and stratigraphic mapping of the Red Pine Shale show that it is ~ 550-1200 m thick in the western part of the range, thins to < 300 m in the east-central range, and is missing in the eastern range. Measured sections show organic-rich shale interbedded with medium- to coarse-grained sandstone. Sedimentary structures include graded bedding, hummocky cross stratification, parallel to ripple lamination, tabular crossbeds, ripple marks, and slump folds. Fossils include Bavlinella faveolata, filaments, leiosphaerid acritarchs, and, more rarely, vase-shaped microfossils and ornamented acritarchs. Preliminary whole-rock δ 13 Corg analysis of organic-rich shales reveal 13.9%o (PDB) variability (values range from -16.9 to -30.8%o PDB) and TOC values range from 0.07 to 5.9%. Combined data suggest deposition below and near fair-weather wave base in a marine deltaic system, and correlation with the ~ 770 to > 742 Ma Chuar Group, Grand Canyon. The undivided clastic strata of the Uinta Mountain Group, eastern Uinta Mountains, are dominated by trough- and tabular-cross- bedded and massive sandstone showing south-southwestern paleocurrent flow. At least three laterally continuous (kilometer-scale) ~ 50-m-thick intervals of gray-green, organic-bearing shale have been mapped amongst these sandstone intervals and contain ripple marks, mud-crack casts, ripple cross laminae, and gypsum casts and molds. The lowermost shale interval allows subdivision of the clastic strata into three informal units. Fossils from shale in the middle-upper (?) interval of the clastic strata include acritarchs and possible vase-shaped microfossils. Simple sphaeromorphs and carbonaceous filaments have also been found in black to green shale near the base of the section. The clastic strata represent a sandy braid system with possible marine drowning events from the west. In addition to an alluvial-fan setting, the Jesse Ewing Canyon Formation, the basal unit of the UMG below the clastic strata, represents high-energy shoreline and fan-delta deposition.

Abstract Basaltic volcanism in the Snake River Plain of southern Idaho has long been associated with the concept of a mantle plume that was overridden by North America during the Neogene and now resides beneath the Yellowstone plateau. This concept is consistent with the time-transgressive nature of rhyolite volcanism in the plain, but the history of basaltic volcanism is more complex. In the eastern Snake River Plain, basalts erupted after the end of major silicic volcanism. The basalts typically erupt from small shield volcanoes that cover up to 680 km 2 and may form elongate flows that extend 50–60 km from the central vent. The shields coalesce to form extensive plains of basalt that mantle the entire width of the plain, with the thickest accumulations of basalt forming an axial high along the length of the plain. In contrast, basaltic volcanism in the western Snake River Plain formed in two episodes: the first (ca. 7–9 Ma) immediately following the eruption of rhyolites lavas now exposed along the margins of the plain, and the second forming in the Pleistocene (≤2 Ma), long after active volcanism ceased in the adjacent eastern Snake River Plain. Pleistocene basalts of the western Snake River Plain are intercalated with, or overlie, lacustrine sediments of Pliocene-Pleistocene Lake Idaho, which filled the western Snake River Plain graben after the end of the first episode of basaltic volcanism. The contrast in occurrence and chemistry of basalt in the eastern and western plains suggest the interpretation of volcanism in the Snake River Plain is more nuanced than simple models proposed to date.

Abstract The Quaternary record of the Uinta Mountains of northeastern Utah has been studied extensively over the past decade, improving our understanding of the Pleistocene glacial record and fluvial system evolution in a previously understudied part of the Rocky Mountains. Glacial geomorphology throughout the Uintas has been mapped in detail and interpreted with reference to other well-studied localities in the region. In addition, studies in Browns Park and Little Hole in the northeastern part of the range have provided information about paleoflooding, canyon cutting, and integration of the Green River over the Uinta Mountain uplift. Notable contributions of these studies include (1) constraints on the timing of the local last glacial maximum in the southwestern Uintas based on cosmogenic surface exposure dating, (2) insight into the relationship between ice dynamics and bedrock structure on the northern side of the range, and (3) quantification of Quaternary incision rates along the Green River. This guide describes a circumnavigation of the Uintas, visiting particularly well-documented sites on the north and south flanks of the range and along the Green River at the eastern end.

Abstract The Fremont River drainage basin has a variety of geologic and geomorphic features that provide insight into the long-term landscape development of the catchment. Volcanic rocks that are ca. 26 to 4 Ma and are offset by Basin-and-Range style normal faulting underlie the western third of the drainage basin. Fish Lake Hightop and Boulder Mountain show evidence of Pleistocene glaciation. Recent mapping and surface exposure dating suggests that the glacial deposits around these two mountains were deposited during the last glacial maximum (LGM). Mass movement and fluvial deposits in the catchment are predominantly composed of volcanic boulders derived from the volcanic rocks atop Boulder and Thousand Lakes Mountains. Fremont River and tributary incision rates estimated from surface exposure dating of these deposits range from 0.20 to 0.43 m/k.y. Longer-term estimates of exhumation rates in the drainage basin based on emplacement depths of igneous rocks range from 0.10 to 0.38 km/m.y.

Abstract The Kaiparowits Basin, located mostly within Grand Staircase–Escalante National Monument, preserves an outstanding record of Late Cretaceous sedimentation in a foreland basin setting. Hosted in these rocks is one of the most continuous and complete records of this period’s ecosystems known from any one geographic area in the world. Recent work in the basin has emphasized macrovertebrate remains and documented many new sites of high scientific value. Recent stratigraphic studies have further refined our knowledge of the depositional systems and chronostratigraphic relationships. Provided is an overview of some of these recent advances, along with the necessary background to provide context .

Abstract Within the western Great Basin, a system of dextral strike-slip faults accommodates a significant fraction of the North American–Pacific plate motion. The northern Walker Lane in northwest Nevada and northeast California occupies the northern terminus of this fault system and is one of the youngest and least developed parts of the North American–Pacific transform plate boundary. Accordingly, the northern Walker Lane affords an opportunity to analyze the incipient development of a major strike-slip fault system. In northwest Nevada, the northern Walker Lane consists of a discrete ~50-km-wide belt of overlapping, curiously left-stepping dextral faults, whereas a much broader zone of disconnected, widely-spaced northwest-striking faults characterizes northeast California. The left steps accommodate little shortening and are not typical restraining bends. The left-stepping dextral faults may represent macroscopic Riedel shears developing above a nascent lithospheric-scale transform fault. Strands of the northern Walker Lane terminate in arrays of northerly striking normal faults in the northwestern Great Basin and along the eastern front of the Sierra Nevada. These relations suggest that dextral shear in the northern Walker Lane is transferred to ~NW-SE extension in the Great Basin. Offset segments of a west-trending Oligocene paleovalley suggest ~20–30 km of cumulative dextral slip across the northern Walker Lane. Strike-slip faulting began between 3 and 9 Ma, indicating a long-term slip rate of ~2–10 mm/yr, which is compatible with GPS geodetic observations of the current strain field .

Abstract The interaction among brittle deformation, fluid flow, and diagenesis is displayed at Valley of Fire, southern Nevada, where diagenetic iron oxide and hydroxide stains provided a visible record of paleofluid flow in Jurassic Aztec Sandstone. Deformation features include deformation bands, joints, and faults composed of deformation bands and sheared joints. Faults formed by shear along joints, formation of splay fractures, and linkage of fault segments. Measurements of fault permeability, combined with numerical permeability upscal-ing, indicate that these faults impede cross-fault fluid flow, with cross-fault permeability reduced by two orders of magnitude relative to the host sandstone, whereas fault-parallel permeability is enhanced by nearly one order of magnitude. A reconstruction of paleofluid flow in the Aztec Sandstone is based on detailed mapping of multicolored alteration patterns and their cross-cutting relations with brittle structures. These patterns resulted from syndepositional reddening of the eolian sandstone and repeated episodes of dissolution, mobilization, and reprecipita-tion of iron oxide and hydroxide. The distribution of alteration patterns indicates that regional-scale fluid migration pathways were controlled by stratigraphic contacts, thrust faults, and high-angle oblique-slip faults. Outcrop-scale focusing of fluid flow was controlled by structural heterogeneities such as joints, joint-based faults, and deformation bands as well as the sedimentary architecture. The complex interaction of structural heterogeneities with alteration in this exhumed analog of a fractured and faulted sandstone aquifer is consistent with their measured hydraulic properties demonstrating the significance of structural heterogeneities for focused fluid flow in porous sandstone aquifers.

Abstract The commercial development of Utah’s state rock, coal, by the Union Pacific and Denver and Rio Grande Western railroads began in the late 1800s. Ninety-five percent of the coal produced in Utah today is used to generate electricity, and recent developments in coal-bed methane characterization and extraction are promising for the state’s economy . The Emery coalfield in central Utah consists of sediments deposited along the western margin of the Cretaceous Western Interior Seaway and subsequently deformed during the Laramide orogeny and Basin and Range deformation. The coal-field contains important methane and bituminous deposits in the Ferron Sandstone Member of the Cretaceous Mancos Shale . Ferron coal beds exposed on the northwest-dipping western flank of the San Rafael Swell have been burning for decades. The collapse of strata overlying the Ferron as a consequence of burning provides conduits for the circulation of oxygen, thereby promoting the smoldering that can be observed today. Dating clinker in the Emery coalfield would provide useful information about the timing of certain geologic events in Utah on a local and perhaps regional scale . Federal agencies and laws help safeguard coal mining in the United States today. The environmental effects of Utah’s natural burning and mine-related coal fires are unknown. However, such fires elsewhere are responsible for pollution, including acid rain, and they are responsible for a variety of human diseases .

Abstract On this field trip, you will visit two important archaeological cave sites that provide the most compelling evidence for latest Pleistocene and earliest Holocene human occupation in the Bonneville Basin. Danger Cave, located near Wendover, Utah/Nevada, is famed for its deeply stratified archaeological deposits dating as old as 10,300 radiocarbon yr B.P., when the remnant of Lake Bonneville stood at the Gilbert shoreline. Bonneville Estates Rockshelter, located south of Danger Cave at the Lake Bonneville highstand shoreline, also contains well-preserved stratified deposits, including artifacts and cultural features dated to at least 11,000 radiocarbon yr B.P., making it one of the oldest known archaeological occupations in the Great Basin. We describe results of our recent research at these sites and show the stratigraphic evidence for these earliest human occupations. We also review recent work at the Old River Bed Delta, on Dugway Proving Ground, that has documented hundreds of Paleoarchaic occupation sites dating 11,000–8500 radiocarbon yr B.P. Together these localities give us an unparalleled picture of human occupation during the first few thousand years of known human occupation in the region, during a time of dramatic environmental change. Packrat middens, pollen sampling localities, and geomorphic features that illustrate the history of Pleistocene Lake Bonneville and the environmental history of the western Bonneville Basin will also be observed on this trip .

Abstract The Wasatch fault is a 370-km-long zone of normal faulting that forms the eastern edge of the Basin and Range Province in Utah and southeastern Idaho. The fault zone is subdivided into ten segments that range from 30 to 60 km in length and are each capable of generating earthquakes of M ~7. For the five central segments, multiple surface-faulting earthquakes have occurred during the Holocene, and vertical slip rates are ~1 mm/yr. Recurrence intervals for the individual central segments range from ~1300 to 2500 yr. The fault poses a significant seismic hazard to the highly urbanized Wasatch Front in north-central Utah. The field localities described in this guide provide an overview of the surface and subsurface character of the Wasatch fault zone. Five field trip stops are located along the Nephi, Provo, and Salt Lake City segments. We will observe fault scarps on Quaternary deposits, which record tectonic displacements associated with Holocene earthquakes, and fault-zone rocks exhumed from depths in excess of 10 km that are hydrothermally altered and have evidence of brittle and ductile deformation. Specific topics of discussion include the nature of piedmont fault scarps; the use of paleoseismic trenching and fault-scarp geomorphology to infer earthquake timing, recurrence intervals, and fault slip rates; and the subsurface structure and rheology of the fault . Keywords : faulting, paleoseismology, neotectonics, earthquake geology.

Abstract This one-day field trip examines two Neoproterozoic sections in the Portneuf Narrows area, SE of Pocatello, Idaho. Rocks to be examined on the first traverse belong to the Scout Mountain Member, Pocatello Formation, and include <710 Ma glacial diamictites, dolomite cap, 667 Ma tuff, and upper caplike carbonate. The second traverse, in Blackrock Canyon, exposes the cyclic Blackrock Canyon Limestone, with upward-shallowing siliciclastic to carbonate cycles and microbial mounds.

Abstract Analysis of fault zone structure and composition of two intermediate-displacement faults in the Colorado Plateau reveal how fault structure varies as a function of lithology, and how faults impact fluid flow. The Little Grand Wash fault cuts Jurassic Summerville through Cretaceous Mancos Shale rocks, and consists of a complex set of interweaving fault strands. Fault relays are developed where sandstone and shale are cut by the fault in roughly equal amounts. Ancient and modern fluid flow is documented by the presence of travertine and tufa deposits, an oil seep, and CO 2 gas seeps. Analysis of the water, travertine, and gas composition indicate that the CO 2 emanates from a reservoir 1.5–2 km deep, and charges a shallow aquifer. Cross-fault flow is inhibited, and the gas and water flows in the footwall damage zone of the fault. Analysis of the Bighole fault in the San Rafael Swell shows how fault structure varies with displacement. The fault is exposed entirely in the aeolian Jurassic Navajo Sandstone, and consists of a dense fault core interpreted to be a densely packed set of deformation bands bounded by a narrow slip surface. The fault zone consists of conjugate deformation band sets in the hanging wall and footwall of the fault, and the fault core thickness does not vary significantly with net slip.

Abstract Small intrusions (< km 2 ) on the margins of the Henry Mountains intrusive complex of southern Utah are exceptionally well exposed in three dimensions and have a variety of shapes. Our examination of the geometry, structures, and fabric of the Maiden Creek sill, Trachyte Mesa laccolith, and the Black Mesa bysmalith (cylindrical intrusion bounded by vertical faults) suggests that this range of intrusion geometry may reflect a continuum of igneous emplacement as volume increases through magma sheeting. Intrusions begin as thin sills and through incremental injection of additional sheets, inflate into laccoliths. Marginal wall rocks are strained and rotated upward. Further sheet emplacement leads to the formation of a fault at the margin of the inflating intrusion. This fault accommodates piston-like uplift of the intrusion’s roof and results in the formation of a bysmalith. All three of these intrusions exhibit evidence for sheeting, although the evidence is weakest on the margins of the Black Mesa bysmalith. Solid-state shear zones exist between sheets in the Maiden Creek sill and on the margins of the Trachyte Mesa laccolith. Cataclastic zones also separate sheets within the Trachyte Mesa laccolith. Evidence for sheeting in the interior of the Trachyte Mesa laccolith is solely based on differences in weathering and jointing patterns. Evidence for sheeting on the margins of the Black Mesa bysmalith is based on the differences in lineation patterns and also on the distribution of cataclastic zones.

Abstract Recent structural analysis of the Grey’s Landing ignimbrite offers new insights into the emplacement of rheomorphic ignimbrites. We present several key localities, where volcanological and structural features reveal the emplacement history of a lavalike ignimbrite and the evolution of ductile deformation structures during and after deposition across complex topography. Excellent three-dimensional exposure allows us to interpret structural features of the Grey’s Landing ignimbrite in the context of diverse emplacement models for rheomorphic ignimbrites elsewhere and to consider field criteria to distinguish between lava-like ignimbrites and extensive silicic lavas .