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.

The eastern Lake Mead region, central Basin and Range Province, contains an abrupt boundary between the Colorado Plateau and Basin and Range, west of which decades of tectonic studies have documented extreme (at least 60 km) westerly translation of the Frenchman Mountain structural block away from the boundary, currently at a distance of ~95 km. Detachment-style faulting and large lengthening in the eastern Lake Mead region are generally accepted (eight of ten papers in a 2010 compilation of recent research) as integral to the large province-wide lengthening at this latitude. Presented here is field geologic evidence of the contribution of karsting and tilting on multiple fixed axes to the Miocene strain history. Together with newly recognized strike slip on northerly striking faults, associated steep-axis bending, and evidence against fault listricity, the region probably contributes little to Province-wide lengthening, but estimates of westerly translation of the Frenchman Mountain block remain unchanged.

The Lake Mead region contains major Miocene disruptions of structures formed during Mesozoic tectonic shortening. Erosion by the Colorado River and its tributaries has produced exceptional exposures of diverse structures and basin deposits recording the disruptions. Here we provide an overview of the results of studies of these features that started in earnest in 1934 when Chester Longwell began assessing the geology of the reservoir floor prior to impoundment of Lake Mead. The analysis was reinvigorated in the 1970s and early 1980s with geological mapping and structural and stratigraphic studies by Ernie Anderson and Bob Bohannon, as well as geochemical and volcanological studies by Gene Smith and his students, and has culminated in numerous subsequent studies.

The Lake Mead region of northwest Arizona and southeast Nevada contains exceptional exposures of extensional basins and associated normal and strike-slip faults of mainly Miocene age. The Salt Spring Wash Basin is located within the hanging wall of a major detachment fault in the northern White Hills in northwest Arizona, the South Virgin–White Hills detachment fault. The basin is the focus of a detailed basin analysis designed to investigate its three-dimensional structural and stratigraphic evolution in order to determine how a major reentrant in the detachment fault formed. Geochronology and apatite fission-track thermochronology from other studies constrain movement on this detachment fault system to ca. 18–11 Ma, while our study suggests faulting from ca. 16.5 to 11 Ma. Salt Spring Wash Basin consists of variably tilted proximal rock avalanche and alluvial-fan deposits shed from uplifting hanging-wall and predominantly footwall blocks. The basinal strata were deformed during early to middle Miocene faulting on the detachment fault, normal faults, and a faulted rollover fold within the basin. New and existing 40 Ar/ 39 Ar ages on tilted volcanic tuffs and basalt lava flows within the basin strata constrain deposition of these deposits from 15.19 to 10.8 Ma. An apparent lag between the initiation of footwall uplift at 18–17 Ma (based on thermochronology) and basin subsidence at 16.5–16 Ma in the eastern Lake Mead region may be explained by the influences of preexisting paleotopography, or it may be an artifact of lack of exposure of the base of the basin. An early phase of faulting and basin sedimentation from 16.5–16 to 14.6 Ma generated the relief to produce a 500+-m-thick lower section of megabreccia (landslide) and conglomerate (debris flows). Salt Spring Wash Basin experienced relatively high sedimentation rates of 200–600 m/m.y. during its early history. A 14.64 Ma basalt lies at a facies change to 650 m of conglomerate of the middle sequence that was deposited in an alluvial-fan to braid-plain setting. Changes in basin geometry included the development of the reentrant in the northern Salt Spring Wash Basin with the rollover fold at its southern margin. The middle sequence records a significant decrease in sedimentation rates from hundreds of meters per million years to ~60–30 m/m.y., major facies changes, and decreased rate of uplift of footwall rocks. The upper sequence of the basin includes ca. 11–8 Ma basalts interbedded with conglomerate. The ca. 6 Ma lacustrine Hualapai Limestone caps the section and indicates a profound change in sedimentation. The history of the Salt Spring Wash Basin indicates that there was a step-over geometry in the detachment fault that was linked across the southern margin of the reentrant in the basin during deposition of the middle sequence.

The Oligocene-Miocene Horse Spring Formation consists of sedimentary strata that record the onset and evolution of Miocene extensional tectonics in the Lake Mead region. The sedimentary basins of this formation hold critical clues for evaluating and testing competing models that attempt to explain the tectonic evolution of this important part of the Basin and Range. Detailed sedimentology, stratigraphy, isotope geochem istry, and new geochronology of carbonates of the Horse Spring Formation shed light on the details of middle Miocene depositional systems and provide important paleoclimatic and paleotopographic data that further our understanding of the geological evolution of this area. We investigated four carbonate sections in detail, two from the Bitter Ridge Limestone Member (Slot Canyon section, near the Gale Hills, and the West Longwell section, at the Bitter Ridge), one from the Thumb Member (East Longwell section, near the Bitter Ridge), and one from the Rainbow Gardens Member (Rainbow Gardens Recreation Area section), to understand the evolution of carbonate lake systems, to extricate paleoclimatic from tectonic signals in the sedimentary record, and to develop a more clear picture of the evolution of Horse Spring sedimentary basins. New 40 Ar/ 39 Ar dates from the Bitter Ridge Limestone, combined with dates in the published literature, suggest that the Bitter Ridge Lake may have evolved time-transgressively from the White Basin area in the east to the Rainbow Gardens area in the west. Possibly contemporaneous with this evolution, the lake gradually shifted from an open to a closed lake system, most likely due to tectonic partitioning of the basin or the creation of a tectonic sill that cut off the overflow for the lake. Stable isotope and lithofacies analyses provide one of the first detailed proxy records of paleoclimate for the Miocene of the Basin and Range and show strong evidence for an orbitally forced climate signal that represents changes in the precipitation/evaporation ratio for the Bitter Ridge Lake system. Because we can effectively show a climatic signal in the Bitter Ridge Limestone units over 100 k.y. and, likely, 40 k.y. time cycles, longer time-scale shifts in isotopic ratios are more likely due to tectonic processes. Based on a strong negative shift in oxygen isotopic ratios, previous researchers have suggested that the Lake Mead region experienced an increase in paleoelevation during Horse Spring time, while the remainder of the central Basin and Range to the north experienced a decrease in elevation for the same time period. Our data, when compared with data from the Pliocene Hualapai Limestone and those presented by previous researchers, appear to constrain the timing of this isotopic shift to between 15 and 13 Ma, coincident with the timing of the onset of rapid extension in this part of the Basin and Range. We hypothesize that this isotopic shift was due not to a change in paleoelevation due to magmatic activity alone, but was influenced by either (1) longer travel distances of air masses and the development of increased topographic corrugation as the Lake Mead region experienced accelerated rates of extension or (2) drainage reorganization of the early Colorado Plateau and the infusion of isotopically lighter waters from this emergent source.

The central Basin and Range of the southwestern United States is known for large-magnitude Cenozoic extension and a unique combination of normal and major strike-slip faults. The Lake Mead region constitutes the eastern portion of this domain and has been the site of numerous mapping and detailed structural studies, which have led to several models explaining the complex faulting and folding of the region, as well as the tectonic drivers of this deformation. The syntectonic basin fill of the Oligocene-Miocene Horse Spring Formation records a considerable portion of this deformation. A more detailed understanding of the Horse Spring Formation is important to determining the deformation history of the area and to constraining regional tectonic reconstructions. In this study, we present results of detailed mapping and stratigraphic analyses of the Lower Horse Spring Formation in the Longwell Ridges area, Nevada. Detailed measured sections combined with 1:5,000 scale mapping allow us to recognize and document lithofacies and their detailed architecture within the Lower Horse Spring Formation and highlight the extreme lateral and vertical facies changes within this portion of the formation. New 40 Ar/ 39 Ar ages and volcanic ash geochemical data support these analyses. These data record deposition within a range of environments, including alluvial-fan, lacustrine, and fluvial settings. Deposition occurred within an asymmetric basin with a main bounding fault lying east of the modern Overton Arm of Lake Mead. Activity on this fault began around 17 Ma and increased significantly at ca. 15.5 Ma.

Three major low-angle normal faults in the eastern Lake Mead area, Nevada and Arizona, are segments of a regional, 55-km-long, detachment fault. This fault, the South Virgin–White Hills detachment, consists of the Lakeside Mine, Salt Spring, and Cyclopic Mine fault segments. All three segments dip gently west and record top-to-the-west displacement. Based on apatite fission-track and apatite and titanite (U-Th)/He thermochronology of footwall rocks, tilt relations, and 40 Ar/ 39 Ar dates on tuffs and basalts within hanging-wall synextensional sedimentary sequences, significant extension along the South Virgin–White Hills detachment occurred between 16.5 and 14 Ma. Minor extension continued until ca. 8 Ma. Displacement on the South Virgin–White Hills detachment decreases from a maximum of ~17 km at the Gold Butte block in the north to 5–6 km at the Cyclopic Mine in the south. The along-strike, southward decrease in displacement is accompanied by a change in type of fault rock from mylonite along the Lakeside Mine fault (northern segment), to chloritic cataclasite along the Salt Spring fault (central segment), to unconsolidated fault breccia along the Cyclopic Mine fault (southern segment). Differences in fault rock may reflect decreasing exhumation of footwall rocks as a result of decreased displacement to the south. About 40% of the displacement gradient can be accommodated along a series of left-slip faults in the upper plate of the detachment. The Golden Rule Peak lineament, an east-trending alignment of structural and topographic features, may be a transverse structure that accommodates differential displacement between the Salt Spring and Cyclopic Mine faults. The trace of the South Virgin–White Hills detachment is highly sinuous in map view and is marked by three prominent salients that define west-plunging antiformal warps in the detachment surface. We interpret the corrugations in the South Virgin–White Hills detachment to have formed by a process of linkage of originally separate en echelon fault segments followed by eastward tilting of the footwall. Depositional patterns, particularly between the Lakeside Mine and Salt Spring segments, support this interpretation. The Grand Wash fault forms the present-day physiographic boundary between the Colorado Plateau and the Basin and Range Provinces; however, based on greater amount of displacement and exhumation, we suggest that the South Virgin–White Hills detachment is the principal structure accommodating regional extension in the eastern Lake Mead extensional domain.

The hypothesized presence of a detachment underlying the Lake Mead region has created a dichotomy in the interpretations of the roles of strike-slip faults of the Lake Mead fault system in accommodating regional deformation. Our detailed field mapping reveals a previously unnamed left-lateral strike-slip segment of the Lake Mead fault system and a dense cluster of dominantly west-dipping and related normal faults located near Pinto Ridge. We suggest that the strike-slip fault that we refer to as the Pinto Ridge fault: (1) was kinematically related to the Bitter Spring Valley fault; (2) was responsible for the creation of the normal fault cluster at Pinto Ridge; and (3) utilized these normal faults as linking structures between separate strike-slip fault segments to create a longer, through-going fault. Results from numerical models demonstrate that the observed location and curving strike patterns of the normal fault cluster are consistent with the faults having formed as secondary structures as the result of the perturbed stress field around the slipping Pinto Ridge fault, regardless of whether or not the Pinto Ridge fault merges into a regional detachment at depth. Calculations of mechanical efficiency of various normal fault geometries within extending terranes suggest that a preferred west dip of normal faults likely reflects a west-dipping anisotropy at depth, such as a detachment. The apparent terminations of numerous strike-slip faults of the Lake Mead fault system into west-dipping normal faults suggest that a west-dipping detachment may be regionally coherent.

Late Miocene and early Pliocene sediments exposed along the lower Colorado River near Laughlin, Nevada, contain evidence that establishment of this reach of the river after 5.6 Ma involved flooding from lake spillover through a bedrock divide between Cottonwood Valley to the north and Mohave Valley to the south. Lacustrine marls interfingered with and conformably overlying a sequence of post–5.6 Ma fine-grained valley-fill deposits record an early phase of intermittent lacustrine inundation restricted to Cottonwood Valley. Limestone, mud, sand, and minor gravel of the Bouse Formation were subsequently deposited above an unconformity. At the north end of Mohave Valley, a coarse-grained, lithologically distinct fluvial conglomerate separates subaerial, locally derived fan deposits from subaqueous deposits of the Bouse Formation. We interpret this key unit as evidence for overtopping and catastrophic breaching of the paleodivide immediately before deep lacustrine inundation of both valleys. Exposures in both valleys reveal a substantial erosional unconformity that records drainage of the lake and predates the arrival of sediment of the through-going Colorado River. Subsequent river aggradation culminated in the Pliocene between 4.1 and 3.3 Ma. The stratigraphic associations and timing of this drainage transition are consistent with geochemical evidence linking lacustrine conditions to the early Colorado River, the timings of drainage integration and canyon incision on the Colorado Plateau, the arrival of Colorado River sand at its terminus in the Salton Trough, and a downstream-directed mode of river integration common in areas of crustal extension.

Abstract This field trip will visit the River Mountains volcanic section (12.17 ± 0.02 to 13.45 ± 0.02 Ma) and Wilson Ridge pluton (13.10 ± 0.11 Ma) in southern Nevada and northwestern Arizona. Although this volcanic-plutonic system was disrupted by the Saddle Island detachment fault during Miocene crustal extension, there are convincing lithological, mineralogical, geochemical and geochronological indicators that suggest a cogenetic relationship. The trip consists of 17 stops that emphasize evidence that links the volcanic and plutonic sections. In addition we will visit the Saddle Island detachment fault at its type locality on Saddle Island.