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.

New studies of selected basins in the Miocene extensional belt of the northern Lake Mead domain, southern Nevada, suggest refinements on previous models for the early extensional history of the region. Critical data come from (1) the Longwell Ridges area, west of Overton Arm and within the Lake Mead fault system; (2) the Salt Spring Wash Basin, in the hanging wall of the South Virgin Mountains–White Hills detachment fault; and (3) previously studied subbasins of the South Virgin Mountains in the Gold Butte step-over region. Our model focuses on the early history of extension and involves analysis of the lower Horse Spring Formation and correlative strata. The basins and fault patterns suggest two stages of basin development related to two distinct faulting episodes, an early period of detachment faulting, followed by a switch to faulting mainly along the Lake Mead transtensional fault system while detachment faulting waned. Apatite fission-track ages suggest that the footwall block of the detachment fault began cooling at 18–17 Ma. The 18–17 Ma time period appears to be the age of the upper limestone of the Rainbow Gardens Member of the Horse Spring Formation, which is interpreted to be a pre-extensional unit deposited only north of Gold Butte block in the Gold Butte step-over basin, where facies patterns and slow rates of sedimentation make faulting uncertain. The first definite basin stage occurred ca. 16.5–15.5 Ma, during which there was slow to moderate faulting and basin subsidence in a contiguous basin along the South Virgin Mountains–White Hills detachment fault and in the Gold Butte step-over basin; the step-over basin had complex fluvial and lacustrine facies and was synchronous with landslides and debris flows in the basin in the hanging wall of the detachment fault. At ca. 15.5–14.5 Ma, there was a dramatic increase in sedimentation rate related to formation or increased activity on the Gold Butte fault, a change from lacustrine to widespread fluvial, playa, and local landslide facies in the step-over basin, and the peak of exhumation and faulting rates on the detachment fault. The simple early Gold Butte step-over basin broke up into numerous subbasins at ca. 15.5–14.5 Ma as initial faults of the Lake Mead fault system formed. From 14.5 to 14.0 Ma, a major change occurred from dominantly detachment faulting to dominantly transtensional (strike-slip + normal) faulting in the Lake Mead fault system as detachment faulting waned. At this time, the Lake Mead fault system began to propagate to the west, and activity on faults and in subbasins north of Gold Butte slowed or ceased, accompanied by major progradation of alluvial conglomerates over the step-over basin. The geometry of the South Virgin Mountains–White Hills detachment fault that dominated the early Lake Mead extension history fundamentally controlled patterns of faulting and magmatism throughout the rest of the extensional history, even as the detachment faulting itself slowed from 14 to 11 Ma, when it ceased to be active. In a regional view, the detachment faulting in eastern Lake Mead is linked to and forms the northern end of the ca. 20–11 Ma northern Colorado River extension corridor. Similar to the rest of the corridor, faulting and exhumation peaked at 15 Ma, but at the north end of the corridor in eastern Lake Mead, detachment faulting changed rapidly to dominantly transtensional left-lateral faulting of the Lake Mead fault system. Eastern Lake Mead shows evidence for a spatial boundary between the southern and central Basin and Range that is best thought of as a northeast-southwest–trending feature located on numerous older tectonic boundaries. The area also records a temporal change from detachment to transtensional faulting characteristic of the central Basin and Range after 15 Ma.

The eastern Lake Mead region, to the north of the belt of metamorphic core complexes that define the Colorado River extensional corridor, underwent large-magnitude extension in the middle to late Miocene. We present two speculative new models for extension in this area that resolve several puzzling and paradoxical relations. These models are based on new field mapping and structural, geochronologic, and thermochronologic data from the northern White Hills, Lost Basin Range, and south Wheeler Ridge. The Meadview fault, a previously underappreciated structure, is an east-side-down normal fault that separates the northern Lost Basin Range to the west from south Wheeler Ridge to the east. Proterozoic crystalline rocks of the northern Lost Basin Range yielded an apatite fission-track (AFT) age of 15 Ma, whereas 2 km to the east, across the Meadview fault, crystalline rocks of south Wheeler Ridge yielded a 127 Ma AFT age. Similarly, at the south end of the Lost Basin Range, crystalline rocks with ca. 15 Ma AFT ages lie within 5 km of crystalline rocks of Garnet Mountain that yielded a 68 Ma AFT age across the Grand Wash fault. Neither of these relations can be explained by existing tilted crustal section or tilt-block models. In our “classic” metamorphic core complex model, the Grand Wash fault (breakaway), the Meadview fault, and the South Virgin–White Hills detachment represent different structural levels of a single, regional detachment that was active between ca. 16 and 11 Ma. The hanging wall of the detachment consists of rocks at south Wheeler Ridge, the Paleozoic ridges, and possibly part of the crystalline basement of the Gold Butte block, sedimentary and volcanic rocks in the hanging walls of the Salt Spring and Cyclopic Mine faults, and possibly stranded tilt blocks beneath the Grand Wash Trough supradetachment basin. The footwall, exhumed by subvertical simple shear and characterized by middle Miocene AFT ages, includes the central and western Gold Butte block, Hiller Mountains, and crystalline rocks of the White Hills and the Lost Basin Range. The east-dipping Meadview fault bounds the crystalline core on the east; the west-dipping South Virgin–White Hills detachment bounds the core on the west. Therefore, the Grand Wash fault represents the structurally highest part of the detachment, and the South Virgin–White Hills detachment represents the structurally deepest exposed part of the detachment. In the modified core complex model, the Grand Wash, Meadview, and South Virgin–White Hills detachment faults are separate structures, and the Grand Wash Trough is a “trailing-edge” basin bound on the east by the Grand Wash fault and on the west by the Meadview fault. The South Virgin–White Hills detachment is the main detachment along which extension was accommodated, and the Meadview fault is a major antithetic normal fault that facilitated exhumation of the core at the trailing edge of the detachment system.

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 purpose of this study is to reconstruct the geologic history of a sinkhole collapse basin that formed during late Pleistocene time in Meade County, Kansas; to determine the nature of the vertebrate faunas that lived in and around this basin during Illinoian glacial, Sangamon interglacial, and Wisconsin glacial times; and to reconstruct, on the basis of the fossils, some of the local environments and climatic conditions that existed in the southern High Plains during late Pleistocene time. Small fossils were recovered from the sedimentary deposits by washing and screening large quantities of matrix. Most of these fossils were identified by comparing them with Recent skeletal remains of the same species. Remains of larger, extinct mammals also were found. Mapping of the geology was facilitated by the use of aerial photographs, topographic maps, and Brunton compass. The basin under study is called the Butler Spring Basin and is located in southern Meade County, Kansas, about 1 mile north of the Cimarron River. It is approximately 1 mile in diameter and is believed to have formed when subsurface solution of salt and anhydrite from Permian beds caused the collapse of overlying beds of Pleistocene age. Studies of fossils preserved in the sediments filling the basin have demonstrated the presence of four superimposed vertebrate faunas. Associated with some of these faunas are mollusks, freshwater ostracods, and pollen. The oldest fauna occurs in stream deposits of the ancestral Cimarron River which lie topographically below the High Plains surface and which represent a period of downcutting during late Pleistocene time. Younger faunas occur in a series of overlying silts and sands of sinkhole origin. On the basis of stratigraphic position, faunal composition, and associated floral (pollen) evidence, the faunas are assigned to the third (Illinoian) and fourth (Wisconsin) glacials and the intervening third (Sangamon) interglacial. The Adams local fauna (Illinoian) contains fragmentary remains of large mammals taken from sands and gravels of the ancestral Cimarron River. In the Butler Spring local fauna (late Illinoian), the occurrence of Sorex cinereus , masked shrew; Citellus richardsoni , Richardson ground squirrel; and Microtus pennsylvanicus , meadow vole, whose present ranges do not extend as far south as Meade County, Kansas, suggests summers that were cooler and more moist than those in the area today. The Cragin Quarry local fauna (Sangamon) occurs in sediments which overlie the Butler Spring local fauna. It includes a large and diverse mammalian assemblage; many of the species are upland prairie forms. The remains of the tortoises Gopherus and Geochelone indicate a warm, interglacial climate. A massive caliche immediately above the faunal zone suggests a period of prolonged weathering and aridity—perhaps during middle Sangamon time. The Robert local fauna (late Wisconsin) consists of microvertebrates collected from a 1-foot, dark-gray to black silt or soil zone just below the surface in the immediate vicinity of Butler Spring. Most of the mammals are characteristic of marshy conditions or moist, low meadows. A radiocarbon date of 11,000 ± 390 years B.P. was obtained by dating shells of Succinea ovalis Say. Several of the mammalian species represented in the fauna are not found as far south as Meade County, Kansas, at the present time. Eleven of the extant species may be found living today in northeastern South Dakota. From a study of temperature and effective precipitation in both areas, it appears that the Robert local fauna lived in Meade County, Kansas, during the late Wisconsin glacial in a climate which had cooler summers and more effective moisture than occur in the area today, and that this climate is probably similar to that of northeastern South Dakota today.

Abstract The Middle Jurassic San Rafael Group and Upper Jurassic Morrison Formation of the San Juan basin consist of complexly interrelated conglomerate, sandstone, silt- stone, mudstone, limestone, and gypsum. The San Rafael Group, consisting of the Entrada Sandstone, Wanakah Formation, Cow Springs Sandstone, and sandstone at Mesita, was deposited in eolian, sabkha, minor fluvial, and marine or possibly lacustrine environments. The Morrison Formation, consisting of the Salt Wash, Recapture, Westwater Canyon, and Brushy Basin Members, was deposited in widespread fluvial, lacustrine, and eolian environments. The presence of eolian rocks in both the San Rafael Group and Morrison Formation led to mapping and correlation problems in the southern half of the basin. The chief problem was that a widespread eolian facies of the Recapture Member of the Morrison was considered a part of the eolian Cow Springs Sandstone by earlier workers. Recent work in the southern and western parts of the basin shows that eolian beds of the two units can be distinguished by lateral relationships and sedimentologic features including sorting, sedimentary structures, and crossbed dip-vector resultants. Separation of the two units yields an improved understanding of depositional processes and paleoenvironmental distributions. Problems in this interval still exist in the southeastern part of the basin where the relationship of the San Rafael Group to the Recapture Member of the Morrison remains unclear.

Barren mud and salt flats comprise a low-lying coastal plain at the north-western end on the Gulf of California, between the mouth of the Colorado River and the town of San Felipe, Baja California. The region is arid, and characterized by a maximum spring tide range of 8 to 10 m. Three morphologic or environmental units are distinguished in the coastal mud flats: (1) the high flats, approximating the level of extreme spring tides; (2) the intertidal flats, dipping seaward at gradients of 0.1–0.2 degree from spring higher-high to spring lower-low tide level; and (3) the subtidal mud flats, extending 11–12 m below mean sea level. Deposits of the coastal mud flats cover an area of 2000 km 2 , and are about 16 m thick. Examination of surface sediments in terms of color, minor structures, texture, and composition of the coarse fraction reveals a zonation of sediment types across the mud flats related to variable exposure to subaerial drying and evaporite crystallization, wave action, tidal currents, and the activity of burrowing organisms. In a seaward direction, i.e ., with decreasing elevation, the sequence of sediment types includes: (1) chaotic muds and evaporites; (2) moderate brown, well-laminated clayey silts; (3) brown to gray, mottled silty clays; and (4) gray, poorly laminated silty clays and clayey silts. Mineralogy, grain size, and the areal distribution indicate that the silts and clays are derived from suspended load of the Colorado River, and are carried to the site of deposition by Gulf tidal currents. Borings encounter a similar sequence of muds beneath the high flats, indicating development of the mud flats through depositional regression. Seaward growth was initiated during the final stages of late Wisconsin sea-level rise when accretion of tidally supplied muds exceeded the ability of small Gulf waves to rework and disperse. Subsequent growth resulted in onlap of the tidal mud flats across the Pleistocene piedmont plain to the west, and progradation over sandy tidal current ridges of the deeper Gulf to the east. Mud supply diminishes toward the south, and wave effects are accentuated accordingly. Waves truncated the Pleistocene piedmont plain causing rejuvenation and entrenchment of the piedmont washes. Coarse sand supplied thereby is carried northward to form prominent longshore spits which finger out into the intertidal muds. Restriction to tidal flooding imposed by the spits initiated erosion of prominent tidal channels which dissect the southern mud flats. Depositional regression in the southern area has been limited in extent, and has occurred by strand plain development in response to mudflat encroachment from the north. Sediment supply has been much reduced for the past 50–60 years due to diversion of the Colorado River into the Salton Sea and the subsequent construction of Hoover Dam. Consequently, waves have winnowed the poorly segregated mud-flat deposits, piled coarse mollusk remains into beach ridges fringing the northern high flats, and developed a fine sand and shell veneer over the intertidal zone. Older beach ridges, now largely encased by intertidal muds, record an earlier period of low mud supply and reworking which was probably initiated 1000 to 1500 years B.P. by diversion of the Colorado River into the Salton Basin to the north.

Abstract The numerous CO 2 reservoirs in the Colorado Plateau region of the United States are natural analogues for potential geological CO 2 sequestration repositories. To understand better the risk of leakage from reservoirs used for long-term underground CO 2 storage, we examine evidence for CO 2 migration along two normal faults that cut a reservoir in east-central Utah. CO 2 -charged springs, geysers, and a hydrocarbon seep are localized along these faults. These include natural springs that have been active for long periods of time, and springs that were induced by recent drilling. The CO 2 -charged spring waters have deposited travertine mounds and carbonate veins. The faults cut siltstones, shales, and sandstones and the fault rocks are fine-grained, clay-rich gouge, generally thought to be barriers to fluid flow. The geological and geochemical data are consistent with these faults being conduits for CO 2 moving to the surface. Consequently, the injection of CO 2 into faulted geological reservoirs, including faults with clay gouge, must be carefully designed and monitored to avoid slow seepage or fast rupture to the biosphere.

Abstract In northwest Arizona, the relatively unextended Colorado Plateau gives way abruptly to the highly extended Colorado River extensional corridor within the Basin and Range province along a system of major west-dipping normal faults, including the Grand Wash fault zone and South Virgin–White Hills detachment fault. Large growth-fault basins developed in the hanging walls of these faults. Lowering of base level in the corridor facilitated development of the Colorado River and Grand Canyon. This trip explores stratigraphic constraints on the timing of deformation and paleogeographic evolution of the region. Highlights include growth-fault relations that constrain the timing of structural demarcation between the Colorado Plateau and Basin and Range, major fault zones, synextensional megabreccia deposits, nonmarine carbonate and halite deposits that immediately predate arrival of the Colorado River, and a basalt flow interbedded with Colorado River sediments. Structural and stratigraphic relations indicate that the current physiography of the Colorado Plateau–Basin and Range boundary in northwest Arizona began developing ca. 16 Ma, was essentially established by 13 Ma, and has changed little since ca. 8 Ma. The antiquity and abruptness of this boundary, as well as the stratigraphic record, suggest significant headward erosion into the high-standing plateau in middle Miocene time. Thick late Miocene evaporite and lacustrine deposits indicate that a long period of internal drainage followed the onset of extension. The widespread distribution of such deposits may signify, however, a large influx of surface waters and/or groundwater from the Colorado Plateau possibly from a precursor to the Colorado River. Stratigraphic relations bracket arrival of a through-flowing Colorado River between 5.6 and 4.4 Ma.