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At least 109 records · Page 6Linked to original sources
USGS1971

Basin and range structure: A system of horsts and grabens produced by deep-seated extension

Basin and Range structure can be interpreted as a system of horsts and grabens produced by the fragmentation of a crustal slab above a plastically extending substratum. According to this view, the extension of the substratum causes the basal part of the slab to be pulled apart along narrow, systematically spaced zones which in turn cause the downdropping of complex horizontal prisms (grabens) in the brittle upper crust. The grabens form valleys at the surface; the intervening areas are horsts, or tilted horsts. Not all geologists have agreed, however, that Basin and Range structure consists of a system of horsts and grabens. Instead, the structure is commonly considered to consist of tilted blocks in which the upslope part of an individual block forms a mountain and the downslope part a valley. Recent detailed studies, including geophysical work, suggest that the horst and graben model may be more generally applicable. Many of the valleys in the Great Basin are bounded on both sides by faults that drop the valley block down; these faults are exposed at the surface or can be inferred from steep gravity gradients indicative of steep faulted subsurface bedrock slopes. Some areas that were thought to represent a typical series of tilted blocks may be a series of highly asymmetrical grabens in which one side of a valley is marked by a master fault and the other side by valleyward tilt. With present knowledge, most, or perhaps all, of the major valleys in the Great Basin can plausibly be considered to be grabens, and most or all of the mountains can be considered to be horsts or tilted horsts. The grabens, and the underlying inferred deep zones of extension that cause them, are systematically distributed in the Great Basin. They are generally north-trending features spaced 15 to 20 mi apart. Locally, the pattern is more complex, and individual grabens divide and trend away from each other at acute or high angles. In a few places, the pattern may even be roughly polygonal. The distribution pattern of the grabens and the related deep zones of extension resemble crack patterns in small-scale tensional systems, and both patterns may be mechanically related. By analogy with the small-scale systems, the areas of generally north-trending and parallel grabens require east-west extension, whereas the areas with a possible polygonal pattern of grabens must extend radially. The geometry of block faulting related to Basin and Range structure requires sizable east-west extension, estimated at about 1.5 mi on the average for each major valley and at about 30 to 60 mi across the entire Great Basin. Most of this extension has taken place in the last 17 m.y., or perhaps even in the last 7 to 11 m.y., indicating a rate of extension in the range of 0.3 to 1.5 cm/yr.

Arizona, California, Idaho, Nevada, Oregon, Utah↗
USGS1947

Flysch and molasse

By definition European geologists consider a sequence of limestones, sandstones, and shales, the beds of which are thin, regular, and alternating, and which are deposited in a geosyncline or foredeep shortly before a major orogeny, as the flysch . The waste products that accumulate as a deposit flanking mountains and built in part of the deformed flysch make up the molasse . In field practice the groups of sediments called flysch and molasse, or facies of them, are formations in the American sense. However, the Europeans would not recognize all groups of beds deposited in a geosyncline just before an orogeny as flysch; the beds must possess the proper lithologic and bedding characteristics. Europeans have tacitly tied the variable of lithology and stratification to the variable of orogeny, with attendant difficulties. Originally only the first was denoted, but later the second was emphasized in definition if not in field use. Americans have considered the terms chiefly in their orogenic sense and thereby have called certain sequences flysch that do not fully meet the requirements of European usage. The writers are of the opinion that little is gained by the use of the words; by their application no new fact is told or discovery made. They simply elaborate a conclusion by way of analogy. On the other hand, a possibility of confusion is introduced by the use of the terms, and in America it is best to avoid them.

GSA Bulletin↗
USGS2017

Active tectonics of the northern Mojave Desert: The 2017 Desert Symposium field trip road log

The 2017 Desert Symposium field trip will highlight recent work by the U.S. Geological Survey geologists and geophysicists, who have been mapping young sediment and geomorphology associated with active tectonic features in the least well-known part of the eastern California Shear Zone (ECSZ). This area, stretching from Barstow eastward in a giant arc to end near the Granite Mountains on the south and the Avawatz Mountains on the north (Fig. 1-1), encompasses the two major structural components of the ECSZ—east-striking sinistral faults and northwest-striking dextral faults—as well as reverseoblique and normal-oblique faults that are associated with topographic highs and sags, respectively. In addition, folds and stepovers (both restraining stepovers that form pop-up structures and releasing stepovers that create narrow basins) have been identified. The ECSZ is a segment in the ‘soft’ distributed deformation of the North American plate east of the San Andreas fault (Fig. 1-1), where it takes up approximately 20-25% of plate motion in a broad zone of right-lateral shear (Sauber et al., 1994) The ECSZ (sensu strictu) begins in the Joshua Tree area and passes north through the Mojave Desert, past the Owens Valley-to-Death Valley swath and northward, where it is termed the Walker Lane. It has been defined as the locus of active faulting (Dokka and Travis, 1990), but when the full history from about 10 Ma forward is considered, it lies in a broader zone of right shear that passes westward in the Mojave Desert to the San Andreas fault (Mojave strike-slip province of Miller and Yount, 2002) and passes eastward to the Nevada state line or beyond (Miller, this volume). We will visit several accessible highlights for newly studied faults, signs of young deformation, and packages of syntectonic sediments. These pieces of a complex active tectonic puzzle have yielded some answers to longstanding questions such as: How is fault slip transfer in this area accommodated between northwest-striking dextral faults and eaststriking sinistral faults? How is active deformation on the Ludlow fault transferred northward, presumably to connect to the southern Death Valley fault zone? When were faults in this area of the central Mojave Desert initiated? Are faults in this area more or less active than faults in the ECSZ to the west? What is the role of NNW-striking faults and when did they form? How has fault slip changed over time? Locations and fault names are provided in figure 1-2. Important turns and locations are identified with locations in the projection: UTM, zone 11; datum NAD 83: (578530 3917335).

California↗
USGS2023

Detrending Great Basin elevation to identify structural patterns for identifying geothermal favorability

Topography provides information about the structural controls of the Great Basin and therefore information that may be used to identify favorable structural settings for geothermal systems. The Nevada Machine Learning Project (NVML) tested the use of a digital elevation map (DEM) of topography as an input feature to predict geothermal system favorability. A recent study re-examines the NVML data, identifying the DEM as the most important feature, showing a broad uniform pattern of high-favorability in the lower-elevation west and low-favorability in the higher elevation east of their study area in north-central Nevada. This regional elevation trend conflicts with the geologic notion that local relative topography should be used to identify geologic structures associated with favorable structural settings for hydrothermal upflow. Specifically, local relative topography gives information about position in the mountains, in the valleys, or at the transitions between, aiding in identification of faults and fault intersections. As part of U.S. Geological Survey efforts to engineer features that are useful for predicting geothermal resources, we construct a detrended elevation map that emphasizes local relative topography and highlights features that geologists use for identifying geothermal systems (i.e., providing machine learning algorithms with features that may improve predictive skill by emphasizing the information used by geologists). Herein, we describe the removal of the regional trend in elevation to emphasize the basin-and-range scale structural features, creating detrended elevation maps. Regional elevation trends were estimated using a local linear regression and subtracted from the actual elevation using a 30-m DEM. In an effort to optimize the detrended surface, alternate versions were produced with different rates of smoothness resulting in three detrended elevation maps. The resulting elevation trend surfaces (a proxy for crustal thickness) are compared with conductive heat flow maps, and a general pattern was observed of a negative correlation between heat flow and regional elevation in many areas, indicating that thinner crust may be causing elevated heat flow in some areas and thicker crust may cause the observed heat flow lows. Because these detrended elevation maps emphasize geologic structure and relative displacement, these products may also be useful for other geologic research including mineral exploration, hydrologic research, and defining geologic provinces.

Geothermal Resources Council Transactions↗
USGS1915

Contributions to the stratigraphy of southwestern Colorado

In the course of field work of the United States Geological Survey in the San Juan region of Colorado observations have been made in the last three seasons that considerably extend our knowledge of the great stratigraphic break below the La Plata sandstone, which is currently assumed to be of Jurassic age. The new data pertain partly to the relations existing in the Gunnison Valley, north of the San Juan Mountains, where the unconformity marking this break was already known at certain places, and partly to the conditions in the Piedra Valley, on the south side of the mountains, where the unconformity had not before been noted. The Piedra Valley is of special interest, and it seems well to call attention to the relations observed even though they were examined only in a reconnaissance. The first part of this paper is devoted to the evidence of the overlap of the La Plata sandstone; the second to the stratigraphic relations in the Piedra Valley. The section of sedimentary formations in Piedra Canyon is of much interest because none of the pre-La Plata formations are known east of this locality on the south side of the San Juan Mountains. Most of these formations exhibit a notably different facies where they reappear from beneath the overlying beds at their nearest exposures in New Mexico, southeast of the Piedra Valley. It is believed that the character of the formations in the Piedra section should be recorded for the benefit of geologists who may be studying the Paleozoic and Mesozoic rocks of New Mexico, and accordingly the second part of the paper presents details of the structure and the stratigraphic section of Piedra Valley.

Colorado↗
USGS2017

Investigating the landscape of Arroyo Seco—Decoding the past—A teaching guide to climate-controlled landscape evolution in a tectonically active region

Introduction Arroyo Seco is a river that flows eastward out of the Santa Lucia Range in Monterey County, California. The Santa Lucia Range is considered part of the central California Coast Range. Arroyo Seco flows out of the Santa Lucia Range into the Salinas River valley, near the town of Greenfield, where it joins the Salinas River. The Salinas River flows north into Monterey Bay about 40 miles from where it merges with Arroyo Seco. In the mountain range, Arroyo Seco has cut or eroded a broad and deep valley. This valley preserves a geologic story in the landscape that is influenced by both fault-controlled mountain building (tectonics) and sea level fluctuations (regional climate). Broad flat surfaces called river terraces, once eroded by Arroyo Seco, can be observed along the modern drainage. In the valley, terraces are also preserved like climbing stairs up to 1,800 feet above Arroyo Seco today. These terraces mark where Arroyo Seco once flowed.The terraces were formed by the river because no matter how high they are, the terraces are covered by gravel deposits exactly like those that can be observed in the river today. The Santa Lucia Range, Arroyo Seco, and the Salinas River valley must have looked very different when the highest and oldest terraces were forming. The Santa Lucia Range may have been lower, the Arroyo Seco may have been steeper and wider, and the Salinas River valley may have been much smaller. Arroyo Seco, like all rivers, is always changing. Some-times rivers flow very straight, and sometimes they are curvy. Sometimes rivers are cutting down or eroding the landscape, and sometimes they are not eroding but depositing material. Sometimes rivers are neither eroding nor transporting material. The influences that change the behavior of Arroyo Seco are mountain uplift caused by fault moment and sea level changes driven by regional climate change. When a stream is affected by one or both of these influences, the stream accommodates the change by eroding, depositing, and (or) changing its shape. In the vicinity of Arroyo Seco, the geologically young faulting history is relatively well understood. Geologists have some sense of the most recent faulting event and of the faulting in the recent geologic past. The timing of regional climate changes is also well accepted. In this area, warm climate cycles tend to cause the sea level to rise, and cool climate cycles tend to cause the sea level to fall. If we understand the way the terraces form and their ages in Arroyo Seco, we can draw conclusions about whether faulting and (or) climate contributed to their formation. This publication serves as a descriptive companion to the formal geologic map of Arroyo Seco (Taylor and Sweetkind, 2014) and is intended for use by nonscientists and students. Included is a discussion of the processes that controlled the evolution of the drainage and the formation of the terraces in Arroyo Seco. The reader is guided to well-exposed landscape features in an easily accessible environment that will help nonscientists gain an understanding of how features on a geologic map are interpreted in terms of earth processes.

California↗
USGS2006

Aeromagnetic and gravity data over the Central Transantarctic Mountains (CTAM), Antarctica: a website for the distribution of data and maps

Near complete coverage of the East Antarctic Shield by ice hampers geological study of crustal architecture important for understanding global tectonic and climate history. Limited exposures in the central Transantarctic Mountains (CTAM), however, show that Archean and Proterozoic rocks of the shield as well as Neoproterozoic-lower Paleozoic sedimentary successions were involved in oblique convergence associated with Gondwana amalgamation. Subsequently, the area was overprinted by Jurassic magmatism and Cenozoic uplift. To extend the known geology of the region to ice-covered areas, we conducted an aeromagnetic survey flown in draped mode by helicopters over the Central Transantarctic Mountains and by fixed-wing aircraft over the adjacent polar plateau. We flew more than 32,000 line km covering an area of nearly 60,000 km2 at an average altitude of 600 m, with average line spacing 2.5 km over most areas and 1.25 km over basement rocks exposed in the Miller and Geologists ranges. Additional lines flown to the north, south, and west extended preliminary coverage and tied with existing surveys. Gravity data was collected on the ground along a central transect of the helicopter survey area.

Open-File Report↗
USGS1966

Geology of the Arabian Peninsula: Sedimentary geology of Saudi Arabia

Systematic mapping of the sedimentary geology of Saudi Arabia by Arabian-American Oil Co. (Aramco) began in 1933. By 1959, exploration parties of one type or another had surveyed more than 1,300,000 square kilometers (500,000 square miles) of sedimentary outcrop. The foundation for sedimentary deposition is the Arabian Shield a vast Precambrian complex of igneous and metamorphic rocks that occupies roughly one-third of the Arabian Peninsula in the west and crops out sporadically along the southern coast. Since the outset of the Paleozoic Era the shield has been amazingly stable, subject only to gentle, epeirogenic movement. On this rigid land mass was deposited an aggregate total of nearly 5,500 meters (18,000 feet) of sedimentary rocks ranging in age from presumed Cambrian to Pliocene(?). Paleozoic, Mesozoic, and lower Tertiary strata are magnificently exposed in central Arabia where they crop out in a great curved belt bordering the shield. Here the landscape is dominated by a series of essentially parallel west-facing escarpments, each supported by a resistant limestone cap. Exposures are unusually good, and many rock units can be traced without significant interruption for 500 to nearly 1,000 km. Beds reflecting buried basement configuration dip gently and uniformly away from the escarpment region into the Persian Gulf and Rub' al Khali basins. East of the escarpment belt is a broad expanse of relatively low-relief terrain in which Tertiary and younger deposits effectively mask older units. Clues to the character of pre-Tertiary rocks in this large area, which includes the Rub' al Khali desert and most of northeastern Arabia, are afforded only by widely scattered bore holes and oil wells. In extreme northwestern Arabia, largely lower Paleozoic sedimentary rocks are exposed, although a basinal area bordering the Paleozoic rocks on the north is characterized by thick Upper Cretaceous to Tertiary strata. Tertiary to Recent volcanic rocks cover substantial parts of the area. In general the older sedimentary rocks are exposed in north-central and northwestern Arabia near the Precambrian basement where as much as 2,000 m of lower Paleozoic rocks are present. Although unfossiliferous, the lower 600 m can be equated, at least in part, to rocks of certain Cambrian age in Jordan. Higher beds contain intervals confidently dated as Lower Ordovician, Silurian, and Lower Devonian. Lower Paleozoic rocks are chiefly coarse-grained sandstone of terrestrial origin, although marine shale occurs at several levels and the upper 300 m is mainly shale with thin beds of limestone. Lower Paleozoic strata are succeeded in the central escarpment region by a thick sequence (about 1,000 m) of Upper Permian and Triassic sedimentary rocks. The initial deposit, the Khuff Formation, is mostly shallow-water limestone; overlying beds are nonmarine elastics except for thick carbonate units in the middle part of the section. Above the Triassic System is some 200 to 500 m of Lower and Middle Jurassic rocks which, near the middle of the escarpment region, are interbedded marine shale and shelf limestone. These grade to sandstone, in part continental, in the northern and southern areas of outcrop. The Middle Jurassic is overlain by a great sequence-of nearly pure carbonate rocks, highly fossiliferous and accurately dated as Upper Jurassic and early Lower Cretaceous. The Jurassic System is spectacularly displayed in central Arabia where it forms the backbone of the escarpment region the Tuwayq Mountains. Carbonate sedimentation was interrupted several times in the closing stages of the Jurassic b^ the onset of evaporite conditions which gave rise to cyclic deposits of anhydrite and calcarenite. The resulting sequence the Arab Formation is of prime importance for its porous carbonate members contain billions of barrels of proved oil reserves The carbonate sequence is succeeded by a thick body of late Lower and Middle Cretaceous sandstone. (The Middle Cretaceous Series and Epoch, as defined by European geologist-, are used in this report.) Late Lower Cretaceous rocks are nonmarine and appear only in the middle and southern parts of the escarpment region. Middle Cretaceous rocks, nonmarine in the south, become progressively more marine in the north where they follow a transgressive path northwest across older beds as far as Jordan. Upper Cretaceous and Eocene rocks, almost exclusively in limestone and dolomite facies, are extensively exposed alorc the eastern edge of the escarpment belt and continue northwest into Iraq. The sequence, with an average thickness of about 5^0 m, includes rocks of Upper Cretaceous, Paleocene, lower Eocene, and middle Eocene ages. The stratigraphic sequence above the Eocene consists of 200 to 600 m of Miocene and Pliocene rocks, mostly of nonmarine origin. These deposits a heterogeneous assemblage of marly sandstone, sandy marl, and sandy limestone blanket the Rub' al Khali and northeastern Arabia. Above the Miocene and Pliocene rocks are unconsolidated Quaternary deposits which comprise great sand deserts and widespread gravel sheets. Sand of the Rub' al Khali Desert alone covers about 600,000 sq km (230,000 sq mi) or most of southern Arabia. Two major structural provinces are recognized within the Arabian Peninsula and adjacent areas. One is the comparatively stable interior region whose rigidity is controlled by the Precambrian basement. The other is the great mobile belt of Taurus, Zagros, and Oman Mountains, bordering the stable region on the north and east. Saudi Arabia falls entirely within the stable region. The interior stable region contains the Arabian shield as well as the Arabian Shelf an extension of the basement thinly veneered with little-disturbed sedimentary rocks. Widespread structural events, presumably related to epeirogenic movement within the basement, have divided the Arabian Shelf into several distinct and significant structural elements the Interior Homocline, the Interior Platform, and several basinal areas. Bordering the shield is a great belt of sedimentary rocks whose dip basinward is so slight and uniform as to be impreceptible to the eye. This Interior Homocline has an average width of about 400 km and a persistent dip varying from slightly more than 1°00' in older units to less than 0°30' in the youngest. One structural feature of the homocline the central Arabian arch has greatly influenced the present surface distribution of sedimentary rocks in the interior escarpment region. The arch, which affects all rocks from the basement up, marks the area of maximum curvature of the homocline in central Arabia. Although the arch has a varied history, it is apparently a residual high between the periodically sinking Persian Gulf and Rub' al Khali basins rather than a true independent positive feature. Support for this concept conies from the presence of a great arc of tensional structures the central Arabian graben and trough system near the crest of the arch and along the hinge line between the Persian Gulf basin and the stable western block. Bordering the homocline is the Interior Platform, a remarkably flat area of varying width in which systematic dip off the crystalline core no longer prevails. Superimposed on the platform are several major north-south anticlinal trends which include the great oil fields of Arabia Adjacent to the platform are several basinal areas that have from time to time received thick sedimentary deposits. Such basinal sags have developed on the shelf in the northeastern Rub' al Khali, northern Persian Gulf, Dibdibah and Sirhan-Turayf areas.

Professional Paper↗
USGS1954

Uranium in the Mayoworth area, Johnson County, Wyoming - a preliminary report

The uranium mineral, metatyuyamunite, occurs in the basal limestone of the Sundance formation of late Jurassic age along the east flank of the Bighorn Mountains, about 2 miles southwest of the abandoned Mayoworth post office. This occurrence is of particular interest because it is the first uranium mineralization reported from a marine limestone in Wyoming. The discovery uranium claims were filed in July 1953, by J.S. Masek, Dan Oglesby, and Jack Emery of Casper, Wyo. Subsequent reconnaissance investigations have been made by private individuals and geologists of the U.S. Geological Survey and Atomic Energy Commission. The metatyuyamunite is concentrated in a hard gray oolitic limestone that forms the basal bed of the Sundance formation. A selected sample of limestone from a fresh face in the northernmost deposit known at the time of the field examination contained 0.70 percent equivalent uranium and 0.71 percent uranium. Eight samples of the limestone taken at the sample place by the Atomic Energy Commission contained from 0.007 to 0.22 percent uranium. A chip sample from the weathered outcrop at the top of this limestone half a mile to the southeast contained 0.17 percent equivalent uranium and 0.030 percent uranium. A dinosaur bone from the middle part of the Morrison formation contained 0.044 percent equivalent uranium and 0.004 percent uranium. metatyuyamunite forms a conspicuous yellow coating along fracture planes cutting the oolitic limestone and has also replaced many of the oolites within the solid limestone and has also replaced many of the oolites within the solid limestone even where fractures are not present. Many radioactive spots in the basal limestone of the Sundance formation were examined in a reconnaissance fashion along the outcrop for a distance of half a mile south of the initial discovery. Samples were taken for analysis only at the northern and southern margins of this interval. Outcrops farther north and south were not studied. There are not sufficient data to make even rough estimates of tonnage and grade of the occurrences. The extent of the limestone, the approximate boundaries of the area of above-normal radioactivity, and the possibilities of other radioactive zones have not been thoroughly investigated. Although dinosaur bones in the Morrison formation were radioactive wherever they were tested, no significant amount of radioactivity was observed in rocks adjacent to the bones.

Wyoming↗
USGS1948

Geology of the Southern Guadalupe Mountains, Texas

This report deals with an area of 425 square miles in the western part of Texas, immediately south of the New Mexico line. The area comprises the south end of the Guadalupe Mountains and the adjacent part of the Delaware Mountains; it includes the highest peaks in the State of Texas. The area is a segment of a large mountain mass that extends 50 miles or more northward and southward. The report describes the geology of the area, that is, the nature of its rocks, tectonics, and surface features, and the evidence that they give as to the evolution of the area through geologic time. Incidental reference is made to the geology of surrounding regions in order to place the area in its environment. The Permian system of the southwestern United States has been until recently one of the intriguing but little known subjects of American stratigraphy. In the latter half of the nineteenth century after the western. United States was settled, the "red bed" sections of the Permian were studied and reported on by many geologists, but up to 1920 the existence of a contemporaneous marine sequence in western Texas and southeastern New Mexico was little appreciated. Since that year the discovery of extensive oil fields and potash beds in this region gave an impetus to the study of the Permian rocks, and furnished the geologist with records of hundreds of drill holes from which to deduce the nature of the strata not exposed at the surface. At the same time geologists have studied the rocks in the outcropping areas, and have compared them with the strata encountered by drilling. Much remains to be done in order to understand the history of Permian time in the region. The physical and chemical conditions that caused. the deposition of .the various and often complexly related deposits need to be better understood. More of the fossils of the rich and interesting marine faunas should be described, and the relations of the fossils to their environments should be determined. Further, a satisfactory scheme of correlation is needed, and also a subdivision into series that will express the contemporaneity of strata in different areas. One useful contribution to the solution of these problems is the detailed study of sequences of rocks exposed at the surface in the different mountain ranges of Texas and New Mexico. This report deals with one such sequence of rocks in western Texas, the one exposed in the southern Guadalupe Mountains (fig. 1). Here, the Permian rocks are magnificently exposed, to a thickness of about 4,000 feet (pl. 1). They are all of marine origin, and belong to the middle part of the system, with the base concealed and the top absent. Overlying and underlying beds, however, are found in nearby areas.

Texas↗
USGS2023

Hydrogeology, karst, and groundwater availability of Monroe County, West Virginia

Monroe County is in southeastern West Virginia, encompassing an area of 474 square miles. The area consists of karst and siliciclastic aquifers of Ordovician, Silurian, Devonian, and Mississippian age and is in parts of two physiographic provinces: the Valley and Ridge Province to the east of Peters Mountain, and the Appalachian Plateau Province to the west of Peters Mountain. This study was developed in response to inquiries from the Monroe County Commission requesting assessment of the water resources of the county to better understand the quantity of the county’s groundwater resources, for both current [2023] and future demand, and to provide information to support protection and management of the county’s valuable groundwater resources. Various products were developed for this study that provide knowledge with respect to water availability and contamination susceptibility of the karst aquifers within the county. U.S. Geological Survey (USGS) geologists conducted extensive geologic mapping in support of the project, producing (1) a countywide bedrock geologic map, (2) a countywide hydrogeologic map, and (3) a light detection and ranging (lidar)-derived countywide digital elevation model and associated sinkhole map. A significant part of this work was to map in detail the Greenbrier Group at the formation level, which prior to this study had only partially been completed. The report also includes (4) a description of the lithologic units identified as part of the geologic mapping process. U.S. Geological Survey hydrologists completed several additional products for the hydrology part of the effort, including development of (1) a countywide potentiometric surface (water-table) map, (2) a countywide base-flow stream assessment, (3) countywide water-budget estimates, (4) well log surveys for 15 wells to better understand subsurface controls on groundwater flow within the study area, (5) two groundwater tracer tests to better refine the groundwater divide from the northern and southern parts of the karst aquifer in Monroe County; and finally, based on all available data collected for the study including the potentiometric surface map, geologic map, current [2023] and legacy fluorometric groundwater tracer tests, and base-flow stream assessments, (6) groundwater-basin delineations were reassessed for principal groundwater basins within the Greenbrier aquifer. In Monroe County, four principal hydrogeologic settings produce large yields of water for residential, agricultural, and other uses. The most relied upon water-bearing zone with respect to current [2023] public water supply is from springs along Peters Mountain. These springs are derived from intervals of fractured sandstone and resultant alluvial deposits. Groundwater flows downslope through these permeable alluvial deposits and discharges at the contact with less permeable strata, such as the Reedsville Shale. The second most relied upon water-bearing zone in Monroe County is within the karstic Greenbrier Group aquifer, in which the basal Hillsdale Limestone overlies the less permeable Maccrady Shale. This geologic contact between the Hillsdale Limestone and Maccrady Shale is not only targeted as a source of water for agricultural supply but also is targeted as a source of water for residential supply. The third most relied upon water-bearing zone is composed of shallow perched aquifers within the Greenbrier Group. The discontinuous nature of these perched aquifers makes mapping their extent impossible, but they are related to permeable geologic strata, such as karstified limestones with solutionally enhanced permeability that overlies less permeable shale or chert bedrock. During geologic mapping of the county, several of these perched aquifers were documented in the Pickaway, Union, and Alderson Limestones. A fourth zone consists of springs from Ordovician carbonates at the base of Peters Mountain, which are influenced by sinking streams as well as upwelling along faults. In terms of water quantity, the most sustainable springs are those having deeper-sourced flows. Public supplies are a principal source of water used for residential and commercial supply in the region, accounting for 0.49 million gallons per day (Mgal/d) of fresh-water withdrawals (0.14 Mgal/d of groundwater and 0.35 Mgal/d of surface water) for residential and commercial use and serving 6,645 individuals (49.2 percent of the population). An estimated 6,861 people, (50.8 percent of the population) primarily rely on private wells or other unregulated sources, such as springs, and withdraw 0.55 Mgal/d of groundwater for their residential use. Public water supply in the region is primarily (71.4 percent) derived from springs and augmented by stream withdrawals (backup sources mainly during low-flow periods), with the remaining portion (28.6 percent) derived from groundwater withdrawals from wells. For rural residents, however, 100 percent of their withdrawals are derived from groundwater (wells or springs).

West Virginia↗
USGS2009

Gold and trace element zonation in pyrite using a laser imaging technique: Implications for the timing of gold in orogenic and carlin-style sediment-hosted deposits

Laser ablation ICP-MS imaging of gold and other trace elements in pyrite from four different sediment- hosted gold-arsenic deposits has revealed two distinct episodes of gold enrichment in each deposit: an early synsedimentary stage where invisible gold is concentrated in arsenian diagenetic pyrite along with other trace elements, in particular, As, Ni, Pb, Zn, Ag, Mo, Te, V, and Se; and a later hydrothermal stage where gold forms as either free gold grains in cracks in overgrowth metamorphic and/or hydrothermal pyrite or as narrow gold- arsenic rims on the outermost parts of the overgrowth hydrothermal pyrite. Compared to the diagenetic pyrites, the hydrothermal pyrites are commonly depleted in Ni, V, Zn, Pb, and Ag with cyclic zones of Co, Ni, and As concentration. The outermost hydrothermal pyrite rims are either As-Au rich, as in moderate- to high- grade deposits such as Carlin and Bendigo, or Co-Ni rich and As-Au poor as in moderate- to low-grade deposits such as Sukhoi Log and Spanish Mountain. The early enrichment of gold in arsenic-bearing syngenetic to diagenetic pyrite, within black shale facies of sedimentary basins, is proposed as a critical requirement for the later development of Carlin-style and orogenic gold deposits in sedimentary environments. The best grade sediment-hosted deposits appear to have the gold climax event, toward the final stages of deformation-related hydrothermal pyrite growth and fluid flow. ?? 2009 Society of Economic Geologists, Inc.

Economic Geology↗
USGS1983

Map showing outcrops of ash-flow tuffs, Basin and Range Province and vicinity, Trans-Pecos Texas

This map report is one of a series of geologic and hydrologic maps covering all or parts of the States within the Basin and Range province of the western United States, resulting from work under the U.S. Geological Survey's program for geologic and hydrologic evaluation of the Basin and Range province to identify potentially suitable regions for future study relative to isolation of high-level nuclear waste (Bedinger, Sargent, and Reed, 1984) . The map report on the ash-flow tuffs of Trans-Pecos Texas was prepared from published maps and reports and from recent work in progress by geologists of the Texas Bureau of Economic Geology, and was compiled utilizing the project guidelines of Sargent and Bedinger (1984). The map shows the outcrops and localities of measured thicknesses of the ash-flow tuffs that resulted from silicic volcanism 38 to 28 million years ago (Henry and McDowell, 1982). Locally, however, some thin, inadequately studied tuffs of limited distribution have been excluded from the map. Such tuffs are reported in the "lower rhyolite" in the Eagle Mountains (Underwood, 1963), in the southern Davis Mountains (Parker, 1977), within parts of the Shely Group and Morita Ranch Formation in the Chinati Mountains (Cepeda and Henry, 1983) , and in the Rawls Formation in the Bofecillos Mountains (McKnight, 1969). The distribution of most ash-flow tuffs in the northern Davis and Wylie Mountains is not well known. The Chambers Tuff, largely tuffaceous sedimentary rocks, is reported to contain several ash-flow tuffs (Walton, 1972), but their distribution is poorly documented. Metamorphosed ash-flow tuffs of Precambrian age in the Thunderbird Group in the Franklin Mountains (Thomann, 1981) and in the Carrizo Mountain Group near Van Horn (Rudnick , 1983) were not included on the map. In some areas studied in reconnaissance, the definitive lithology of some volcanic units described as lava flows or tuffs has not been determined. In the Description of Map Units, the general location, caldera source and volume of tuff, isotopic age, if available, lithologic data, and the reference sources for each geologic unit are described. The nomenclature of the geologic units is from published reports and does not necessarily conform to U.S. Geological Survey usage.

Texas↗
USGS2010

Structural analysis of three extensional detachment faults with data from the 2000 Space-Shuttle Radar Topography Mission

The Space-Shuttle Radar Topography Mission provided geologists with a detailed digital elevation model of most of Earth's land surface. This new database is used here for structural analysis of grooved surfaces interpreted to be the exhumed footwalls of three active or recently active extensional detachment faults. Exhumed fault footwalls, each with an areal extent of one hundred to several hundred square kilometers, make up much of Dayman dome in eastern Papua New Guinea, the western Gurla Mandhata massif in the central Himalaya, and the northern Tokorondo Mountains in central Sulawesi, Indonesia. Footwall curvature in profile varies from planar to slightly convex upward at Gurla Mandhata to strongly convex upward at northwestern Dayman dome. Fault curvature decreases away from the trace of the bounding detachment fault in western Dayman dome and in the Tokorondo massif, suggesting footwall flattening (reduction in curvature) following exhumation. Grooves of highly variable wavelength and amplitude reveal extension direction, although structural processes of groove genesis may be diverse.

GSA Today↗
USGS2009

Evaluation of hazardous faults in the intermountain west region: Summary and recommendations of a workshop

The U.S. Geological Survey’s (USGS) Earthquake Hazards Program (EHP) has the responsibility to provide nationwide information and knowledge about earthquakes and earthquake hazards as a step to mitigating earthquake-related losses. As part of this mission, USGS geologists and geophysicists continue to study faults and structures that have the potential to generate large and damaging earthquakes. In addition, the EHP, through its External Grants Program (hereinafter called Program), supports similar studies by scientists employed by state agencies, academic institutions, and independent employers. For the purposes of earthquake hazard investigations, the Nation is geographically subdivided into tectonic regions. One such region is the Intermountain West (IMW), which here is broadly defined as starting at the eastern margin of the Rocky Mountains in New Mexico, Colorado, Wyoming, and Montana and extending westward to the east side of the Sierra Nevada mountains in eastern California and into the Basin and Range-High Plateaus of eastern Oregon and Washington. The IMW contains thousands of faults that have moved in Cenozoic time, hundreds of which have evidence of Quaternary movement, and thus are considered to be potential seismic sources. Ideally, each Quaternary fault should be studied in detail to evaluate its rate of activity in order to model the hazard it poses. The study of a single fault requires a major commitment of time and resources, and given the large number of IMW faults that ideally should be studied, it is impractical to expect that all IMW Quaternary faults can be fully evaluated in detail. A more realistic approach is to prioritize a list of IMW structures that potentially pose a significant hazard and to focus future studies on those structures. Accordingly, in June 2008, a two-day workshop was convened at the USGS offices in Golden, Colorado, to seek information from representatives of selected State Geological Surveys in the IMW and with knowledgeable regional experts to identify the important structures for future studies. Such a priority list allows Program managers to guide the limited resources toward studies of features that are deemed to potentially pose the most serious hazards in the IMW. It also provides the scientific community with a list of structures to investigate because they are deemed to pose a substantial hazard to population centers or critical structures. The IMW encompasses all or large parts of 12 states, including Arizona, New Mexico, extreme west Texas, Colorado, Utah, Nevada, eastern California, eastern Oregon, eastern Washington, Idaho, western Wyoming, and western Montana. In Utah, and more recently in Nevada, geoscientists have taken steps to evaluate geologic data related to well-studied faults and to develop a statewide priority list of hazardous structures. In contrast to Utah and Nevada, the other IMW states contain substantially fewer Quaternary faults, so there have not been any previous efforts to develop similar priority lists. This workshop was organized to address this matter and create a more balanced perspective of priorities throughout the entire IMW region. Because working groups and workshops had already been convened to specifically deal with Quaternary fault priorities in Utah and Nevada, this workshop specifically emphasized structures outside of these two states.

Arizona, California, Colorado, Idaho, Montana, Nev↗
USGS2016

Geology, selected geophysics, and hydrogeology of the White River and parts of the Great Salt Lake Desert regional groundwater flow systems, Utah and Nevada

The east-central Great Basin near the Utah-Nevada border contains two great groundwater flow systems. The first, the White River regional groundwater flow system, consists of a string of hydraulically connected hydrographic basins in Nevada spanning about 270 miles from north to south. The northernmost basin is Long Valley and the southernmost basin is the Black Mountain area, a valley bordering the Colorado River. The general regional groundwater flow direction is north to south. The second flow system, the Great Salt Lake Desert regional groundwater flow system, consists of hydrographic basins that straddle the Utah-Nevada border, with a length of about 150 miles from north to south. The general regional groundwater flow direction is from south to north towards the Great Salt Lake Desert. For 15 years with support from the Southern Nevada Water Authority (SNWA), hydrologists, geologists, and geophysicists studied the basin connections and the groundwater resources in these and adjacent flow systems over an area of about 25,000 square miles. A major first part of the SNWA study was constructing a 3-dimensional digital hydrogeologic framework based on geologic maps and cross sections at 1:250,000 scale. This framework documents the presence of three major aquifers: (1) Paleozoic carbonate rocks, (2) Eocene to Miocene volcanic rocks, and (3) Miocene to Holocene basin-fill sediments, as well as confining units that constrain flow. We interpret that movement of most groundwater through and across basins is by fracture-dominated flow along faults/fractures, yet in most places flow is prevented or retarded across faults, so mapping structures gives a first approximation to conduits and barriers to flow. The most important structures by far are high-angle normal faults of the basin-range episode of east-west extensional deformation. This event began at about 20 Ma, although most deformation and the formation of the present topography took place between 10 Ma and present. This topography consists of north-trending basins (mostly grabens) that alternate with north- trending ranges (mostly horsts); erosion of the ranges filled the basins with clastic alluvial basin-fill deposits. Geophysics provides data on the third dimension (cross sections) of the hydrogeologic framework. Audiomagnetotelluric profiles and gravity inversion located faults and enabled us to estimate thicknesses of basin-fill deposits. To this framework, hydrologic studies addressed precipitation, surface water, and springs, as well as groundwater levels, volumes, geochemistry, water budgets, and monitoring. At nearly the same time as our study, the Utah Geological Survey (UGS) and U.S. Geological Survey (USGS) addressed the same issues in many of the same areas, and publication of the efforts by all three agencies reveals a surprising similarity of conclusions, with some critical exceptions, which therefore demonstrates the great value of many scientists independently studying the same complex scientific problem. The differences in conclusions include directions and volumes of some ground- water flow paths, such as one proposed by the USGS of unlikely groundwater flow from Steptoe Valley to southern Snake Valley, and another proposed by the UGS of unlikely significant groundwater recharge flow from the Snake Range to the Fish Springs complex.

Utah↗
USGS2010

Teachers guide to geologic trails in Delaware Water Gap National Recreation Area, Pennsylvania–New Jersey

T he Delaware Water Gap National Recreation Area (DEWA) contains a rich geologic and cultural history within its 68,714 acre boundary. Following the border between New Jersey and Pennsylvania, the Delaware River has cut a magnificent gorge through Kittatinny Mountain, the Delaware Water Gap, to which all other gaps in the Appalachian Mountains have been compared. Proximity to many institutions of learning in this densely populated area of the northeastern United States (Fig. 1 ) makes DEWA an ideal locality to study the geology of this part of the Appalachian Mountains. This one-day field trip comprises an overview discussion of structure, stratigraphy, geomorphology, and glacial geology within the gap. It will be highlighted by hiking a choice of several trails with geologic guides, ranging from gentle to difficult. It is hoped that the “professional” discussions at the stops, loaded with typical geologic jargon, can be translated into simple language that can be understood and assimilated by earth science students along the trails. This trip is mainly targeted for earth science educators and for Pennsylvania geologists needing to meet state-mandated education requirements for licensing professional geologists. The National Park Service, the U.S. Geological Survey, the New Jersey Geological Survey, and local schoolteachers had prepared “The Many Faces of Delaware Water Gap: A Curriculum Guide for Grades 3–6” ( Ferrence et al., 2003 ). Copies of this guide will be given to trip participants and can be downloaded from the GSA Data Repository 1 . The trip will also be useful for instruction at the graduate level. Much of the information presented in this guidebook is modified from Epstein (2006) .

New Jersey, Pennsylvania↗
USGS1950

Preliminary report on the stratigraphy and structure of the upper Utukok and Kokolik Rivers area

U. S. Geological Survey Navy Oil Unit Party I examined the surface geology of the area of the upper Utukok River and east fork of the Kokolik River from June 28 to September 3, 1950. The party consisted of six men: E. G. Sable and M. D. Mangus, geologists; C. L. Hummel and F. H. Shannon, field assistants; R. D. Gerard, cook; and L. E. Hall, weasel mechanic. Three weasels were used for transportation in the field. The area examined approximates 1,000 square miles and is in the extreme southwest corner of Naval Petroleum Reserve No. 4. It is bounded on the west by Iligluruk Creek (east fork of the Kokolik) and on the east by Driftwood Creek. With the exception of Driftwood Creek, major streams were ascended to their headwaters in the De Long Mountains. A traverse southward over the Utukok-Kugururok Rivers divide was successfully completed, providing a "tie-in" with the work of Navy Oil Unit Party 7. Several traverses were made 8 miles north of the Driftwood anticline and to the west fork of the Colville River. The objectives of this part of the summer's work were (1) the geologic mapping and stratigraphic study of the rocks exposed in this area, and (2) measurement of stratigraphic thicknesses of sediments older than those exposed in the Driftwood anticline. Geology was plotted on vertical and trimetrogon oblique aerial photographs. Altitudes were established by altimeter traverses. The northern part of this area was previously investigated by members of the Navy Oil Unit. In 1947 R. M. Thompson and W. L. Barksdale1/ visited outcrops along the Utukok River, and in 1949 R. M. Chapman and E. G. Sable2/ examined exposures along the Kokolik River. (See fig. 1, index map.)

Alaska↗