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.

A critical factor required to unravel processes that have shaped other planets is a solid understanding of geologic processes as they operate on Earth, and a logical way to understand those processes is to go into the field and view them. We provide a field guide to three locations: (1) Cima volcanic field, south of Baker, California; (2) Rainbow Basin, north of Barstow, California; and (3) Red Rock Canyon and vicinity in Nevada and California, all within the Mojave Desert of the southwestern United States. These locations highlight three processes that have affected Earth and other planets: volcanism, sedimentation, and tectonism. Volcanism is explored by looking at the basaltic cinder cones, lava flows, lava tube, and xenoliths of the later Tertiary and Quaternary Cima volcanic field. Felsic ash and volcaniclastic material interbedded with lacustrine, siliciclastic sedimentary rocks are examined in Rainbow Basin, a Tertiary strike-slip basin. The interplay between volcanic and sedimentary processes is examined at this locality, while deformation of the basin makes it ideal for examining structural and tectonic aspects. Broader-scale tectonism is observed in the hanging wall (Ordovician carbonates) and footwall (Jurassic sandstone) rocks to the Keystone thrust fault. The fault is visible given the color contrast between the lower (white and red) and upper (gray) plates. In Red Rock Canyon, Nevada, exposures of the Jurassic Aztec Sandstone display excellent examples of large-scale cross-stratification from eolian dune deposition. Each locale holds lessons pertinent for the study of processes that have operated on other planets in the solar system.

Abstract This field trip covers sedimentological and paleontological research being conducted on the Jurassic Aztec Sandstone and Lower Cretaceous Willow Tank Formation in Valley of Fire State Park. Valley of Fire State Park is located in southern Nevada, just outside of the town of Overton. The Jurassic Aztec Sandstone is equivalent to the Navajo and Nugget Sandstones; these formations together record an aerially large erg complex along the western margin of North America during the time of deposition. Invertebrate and vertebrate ichnofossils are not uncommon in portions of these Jurassic formations. The Willow Tank Formation is composed of the deposits of both a braided and anastomosed fluvial system. This system drained off the paleohigh of the Sevier fold and thrust front to the west, during Early Cretaceous time. Recently a diverse vertebrate assemblage has been discovered from this formation. The fauna of the Willow Tank Formation are similar to other Early Cretaceous faunas from western North America. The vertebrate remains recovered include three taxa of fish, three to four taxa of turtle, crocodilian, iguanodontian, thyreophoran, dromaeosaur, tyrannosauroid, two theropod ootaxa, and a titanosauriform. In addition to the vertebrate elements, two fern morphotypes have been found. Through the course of this field trip participants will see extensive exposures of Aztec Sandstone, including vertebrate ichnofossils. Participants will also hike to vertebrate bearing-beds of the Willow Tank Formation.

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 purpose of this field trip is to provide an overview of Miocene basin development in the Lake Mead region, demonstrate how basin-fill deposits reflect tectonic activity on a variety of structures, and highlight the work of Ernie Anderson in this region. The Basin and Range province is superb for the study of major normal and strike-slip fault systems that accommodate large-magnitude extension. Within this province, the Lake Mead region provides exceptional exposures of synextensional Miocene basins and faults and is a transition zone between predominantly half-graben–style basins and ranges to the north and the highly extended Colorado River Extensional Corridor to the south. The region also embraces a change from thick Phanerozoic sedimentary rocks in the north to Precambrian crystalline basement rocks overlain by late Tertiary volcanic rocks in the south. The early Paleozoic “Cordilleran hingeline” and the southeast margin of east-directed Mesozoic thrusting are also within this transition zone, but the area contains a strong overprint of late Tertiary tectonism. This overprint is strongest near the intersection between two major strike-slip fault systems: the right-lateral Las Vegas Valley shear zone and the left-lateral Lake Mead fault system. Miocene sedimentary rocks record the onset of major extension and the development of numerous, complex structures. Details of the extension and the resulting complexity are not fully understood and many issues remain unresolved. The relative importance of normal versus strike-slip faults is debated as are the details of how and why faults develop through time.

The Cowhole Mountains, near Baker, California, are an east-tilted remnant of the Jurassic magmatic arc that extends from western Nevada through southeastern California into Arizona and Sonora, Mexico. In Middle Jurassic time, two northeast-erly trending grahens bounded by syndepositional faults accumulated 550 m and 700 m of predominantly eolian sandstone (Aztec Sandstone). Because east dip in one of the grabens increases from 30° at the base of the sandstone section to 65° at its top, we infer that structure to have developed during tilting to the west on east-dipping, listric normal faults. Westward tilting continued to accumulate during extrusion of an overlying Middle Jurassic volcanic series (Cowhole volcanics). Felsic volcanic rocks that buried the grabens comprise ignimbrites, volcaniclastic rocks, and flow breccias, emplaced in a proximal but extra-caldera setting. Plutons and sills were intruded at shallow levels, possibly during both graben filling and extrusion of volcanics. Local magmatism of significant volume ended with intrusion of a dike swarm that may be correlative with the Independence dike swarm (148 Ma). After the range tilted 90° toward the east, small glide blocks and landslides detached from high portions of the eroding range and descended into the southern graben. Our model suggests that two sets of high-angle normal faults, oriented nearly at right angles to each other, were active during the same Middle Jurassic interval. A modern analogue may be provided by the Central American arc graben, in which similar transtensional structures have developed perpendicular to the regional graben where it is intersected obliquely by the Chiapas shear zone.

The Upper Cretaceous Iron Springs Formation in southwest Utah consists of 1,000 m of fluvial sandstone, conglomerate, mudstone, and minor lacustrine carbonate rock. The formation was deposited in the proximal reaches of the southern sector of the Sevier foreland basin. These strata record uplift and erosion of thrust sheets in the fold/thrust belt to the west. Facies associations indicate deposition in alluvial fan and fluvial braidplain environments. Laterally extensive sheets of clast-supported cobble conglomerate were deposited during episodes of unconfined sheetflood flow. Mud matrix-supported boulder conglomerate represents locally derived muddy debris flows. Lenticular bodies of sandstone and conglomerate were deposited in channels of a sandy braided stream system. Sheets of mudstone and siltstone represent floodplain sedimentation during flood events. To the north, an upward-coarsening trend in conglomerates of the lower part of the section is attributed to eastward advancement of the thrust belt by ramping and erosion of three thrust sheets. A shift in sediment dispersal patterns from east to north-northeast, shown by paleocurrent data, indicates evolution from an earlier lateral drainage pattern to development of a trunk stream system that paralleled the orogenic front. Abrupt fining of sediment concurrent with this shift suggests an increase in the rate of subsidence, which inhibited progradation of coarser material basinward. This trunk system was likely fed by tributary alluvial fans emerging from the fold/thrust belt to the west. Variances in sandstone and conglomerate composition among different localities reflect deformation of different sedimentary lithologies ranging in age from Precambrian to lower Mesozoic along strike of the mountain belt. At the northernmost studied locality, locally derived basal mudflow deposits show north-directed transport and indicate intraforeland deformation and development of a mudflow-dominated fan. Overlying quartzitic conglomerate and sandstone show east-directed flow and were derived from erosion of Precambrian-Cambrian quartzite in the upper plate of the Wah Wah thrust. An upsection increase in carbonate detritus indicates emplacement of the carbonate-dominated Blue Mountain and Escalante thrusts. In contrast, the Iron Springs Formation at the southernmost studied locality is dominated by fine-grained quartz arenite derived from erosion of the Jurassic Aztec Sandstone during movement on the Weiser syncline and Summit-Willow Tank thrust, located in the Muddy Mountains of southern Nevada. An increase in lithic fragments upsection indicates uplift and erosion of the Muddy Mountain and Glendale thrust system.