Geology topics
V. J. S. Grauch
Publications and source records attributed to V. J. S. Grauch.
The U.S. Geological Survey earth mapping resources initiative’s efforts to map the Rocky Mountain region
No abstract available.
Revisiting geophysical interpretations of the Midcontinent Rift below Lake Superior— Insights from GLIMPCE seismic-reflection line C
The 1.1 Ga Midcontinent Rift System (MRS) has been investigated in the Lake Superior region for more than a century. The most influential geophysical data for modern paradigms has come from seismic-reflection profiles collected by the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) in the late 1980s. We are revisiting seismic-reflection GLIMPCE line C by constructing a detailed velocity model for conversion of the seismic data measured in two-way travel time to a section plotted versus depth. Preliminary results from the velocity modeling, depth conversion, and ties to onshore geology have led to a significantly different view of Line C as primarily a sag basin rather than a half-graben. Narrow intervals of high-velocity material that emanate upwards along both sides of the sag basin are interpreted as mafic intrusions connected to the mantle. This new interpretation suggests that basin subsidence was the dominant process in the development of rift stage troughs rather than major half-graben structures.
Compressional-wave seismic velocity, bulk density, and their empirical relations for geophysical modeling of the Midcontinent Rift System in the Lake Superior region
Compressional-wave seismic velocity (velocity) and bulk density (density) data were compiled from published sources for rock suites and earth materials that are significant for geophysical modeling of the Mesoproterozoic Midcontinent Rift System in the Lake Superior region. The data include laboratory measurements of outcrop and drill core samples, seismic refraction studies, and a sonic log from a 1.5-kilometer-deep exploration well. Rock suites of the Midcontinent Rift System include basalts of the Mesoproterozoic Keweenawan Supergroup, Oronto Group sedimentary rocks (divided into arenaceous versus argillaceous units), and several sedimentary formations overlying the Oronto Group that have been correlated across the area. Intrusive units include diabase, gabbro, and felsic igneous rocks. Other geologic units important for geophysical modeling in the Lake Superior region include Archean crystalline crust, Paleoproterozoic metasedimentary and crystalline rocks, lower Mesoproterozoic sedimentary rocks, and Holocene to Pleistocene surficial deposits. Empirical velocity-density relations for each rock suite were determined by comparing the compiled data to published relations, such as the Nafe-Drake curve, Gardner’s relation, and best-fit equations developed for different rock types from laboratory studies. Graphical representations of these velocity-density relations provide a way to easily understand how velocity and density differ between tectonic settings and by rock type. Overlaps in velocity and density ranges for different geologic units are significant and have especially important implications for geologic interpretation of seismic data. Important examples include similar velocities but differing densities for argillaceous Oronto Group versus units overlying the Oronto Group and arenaceous Oronto Group versus basalt of the Keweenawan Supergroup. Similar densities but differing velocities were found for diabase versus gabbro. In addition, expected velocity ranges by rock type show that high-velocity intervals (6.9–7.1 kilometers per second) interpreted as basalt in previous seismic-reflection studies more likely indicate diabase or gabbro, suggesting that these interpretations may warrant additional consideration.
Exploring the geology of the Midcontinent Rift under western Lake Superior using a preliminary velocity model of seismic line GLIMPCE C
Seismic-reflection data were collected in the 1980s as part of the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) to investigate the 1.1 Ga Midcontinent Rift System (MRS). GLIMPCE Line C crosses western Lake Superior from north to south shores (Fig. 1 inset). Many previous workers have interpreted the MRS in Line C as an asymmetric central graben filled with 10–20 km of subaerial basalt flows, overlain by 7-10 km of sedimentary section, and underlain by magmatic underplating. The central graben was interpreted to have formed from extensional normal faults, later reactivated as high-angle reverse faults. The northern part of Line C crosses over a prominent gravity low called the Grand Marais Ridge (GMR; Fig. 1 inset), previously interpreted as an Archean granitic basement high.
Using high-resolution aeromagnetic surveys to map subsurface hydrogeology in sediment-filled basins: A case study over the Rio Grande Rift, Central New Mexico, USA
High-resolution aeromagnetic surveys were acquired for the Albuquerque basin in the central Rio Grande rift, a basin filled with poorly consolidated sediments. The surveys proved successful in efficiently and economically mapping previously unknown hydrogeologic features of the shallow subsurface. This success suggests that aeromagnetic methods may be useful in hydrogeologic studies of other sediment-filled basins. The aeromagnetic surveys were used primarily to delineate buried igneous rocks and to locate faults within the basin fill, both important for understanding the subsurface hydrogeology. Buried igneous rocks were recognized from their high-frequency, high-amplitude magnetic responses and characteristic map patterns. The horizontal-gradient and local wavenumber methods were used to obtain estimates of their source depths. The aeromagnetic surveys were also successfully used to locate faults within the basin fill. Magnetic signatures associated with faults are produced by the juxtaposition of sediments having differing magnetic properties rather than the products of secondary processes. Expression of faults is abundant throughout the basin, revealing patterns that cannot be mapped at the surface due to widespread cover. A fault signature recognized in the high-resolution data that has multiple inflection points is best explained by a fault with a thin magnetic layer on the upthrown block and thick magnetic layer on the downthrown block, called the thin-thick layers model. Geologically, this signature indicates erosion of the upthrown block or a growth-faulting scenario: fault-controlled sedimentation for faults that offset sediments, and successive accumulation of basalt on the downthrown block for faults that offset volcanic rocks.
A comprehensive survey of faults, breccias, and fractures in and flanking the eastern Española Basin, Rio Grande rift, New Mexico
A comprehensive survey of geologic structures formed in the Earth’s brittle regime in the eastern Española Basin and flank of the Rio Grande rift, New Mexico, reveals a complex and protracted record of multiple tectonic events. Data and analyses from this representative rift flank-basin pair include measurements from 53 individual fault zones and 22 other brittle structures, such as breccia zones, joints, and veins, investigated at a total of just over 100 sites. Structures were examined and compared in poorly lithified Tertiary sediments, as well as in Paleozoic sedimentary and Proterozoic crystalline rocks. Data and analyses include geologic maps; field observations and measurements; orientation, kinematic, and paleostress analyses; statistical examination of fault trace lengths derived from aeromagnetic data; mineralogy and chemistry of host and fault rocks; and investigation of fault versus bolide-impact hypotheses for the origin of enigmatic breccias found in the Proterozoic basement rocks. Fault kinematic and paleostress analyses suggest a record of transitional, and perhaps partitioned, strains from the Laramide orogeny through Rio Grande rifting. Normal faults within Tertiary basin-fill sediments are consistent with more typical WNW-ESE Rio Grande rift extension, perhaps decoupled from bedrock structures due to strength contrasts favoring the formation of new faults in the relatively weak sediments. Analyses of the fault-length data indicate power-law length distributions similar to those reported from many geologic settings globally. Mineralogy and chemistry in Proterozoic fault-related rocks reveal geochemical changes tied to hydrothermal alteration and nearly isochemical transformation of feldspars to clay minerals. In sediments, faulted minerals are characterized by mechanical entrainment with minor secondary chemical changes. Enigmatic breccias in rift-flanking Proterozoic rocks are autoclastic and isochemical with respect to their protoliths and exist near shatter cones believed to be related to a previously reported pre-Pennsylvanian impact event. A weak iridium anomaly is associated with the breccias as well as adjacent protoliths, thus an impact shock wave cannot be ruled out for their origin. Major fault zones along the eastern rift-flank mountain front are discontinuous and unlikely to impede regional groundwater flow into Española Basin aquifers. The breccia bodies are not large enough to constitute aquifers, and no fault- or breccia-related geochemical anomalies were identified as potential contamination sources for ground or surface waters. The results of this work provide a broad picture of structural diversity and tectonic evolution along the eastern flank of the central Rio Grande rift and the adjacent Española Basin representative of the rift as a whole and many rifts worldwide.
A shifting rift—Geophysical insights into the evolution of Rio Grande rift margins and the Embudo transfer zone near Taos, New Mexico
We present a detailed example of how a subbasin develops adjacent to a transfer zone in the Rio Grande rift. The Embudo transfer zone in the Rio Grande rift is considered one of the classic examples and has been used as the inspiration for several theoretical models. Despite this attention, the history of its development into a major rift structure is poorly known along its northern extent near Taos, New Mexico. Geologic evidence for all but its young rift history is concealed under Quaternary cover. We focus on understanding the pre-Quaternary evidence that is in the subsurface by integrating diverse pieces of geologic and geophysical information. As a result, we present a substantively new understanding of the tectonic configuration and evolution of the northern extent of the Embudo fault and its adjacent subbasin. We integrate geophysical, borehole, and geologic information to interpret the subsurface configuration of the rift margins formed by the Embudo and Sangre de Cristo faults and the geometry of the subbasin within the Taos embayment. Key features interpreted include (1) an imperfect D-shaped subbasin that slopes to the east and southeast, with the deepest point ∼2 km below the valley floor located northwest of Taos at ∼36° 26′N latitude and 105° 37′W longitude; (2) a concealed Embudo fault system that extends as much as 7 km wider than is mapped at the surface, wherein fault strands disrupt or truncate flows of Pliocene Servilleta Basalt and step down into the subbasin with a minimum of 1.8 km of vertical displacement; and (3) a similar, wider than expected (5–7 km) zone of stepped, west-down normal faults associated with the Sangre de Cristo range front fault. From the geophysical interpretations and subsurface models, we infer relations between faulting and flows of Pliocene Servilleta Basalt and older, buried basaltic rocks that, combined with geologic mapping, suggest a revised rift history involving shifts in the locus of fault activity as the Taos subbasin developed. We speculate that faults related to north-striking grabens at the end of Laramide time formed the first west-down master faults. The Embudo fault may have initiated in early Miocene southwest of the Taos region. Normal-oblique slip on these early fault strands likely transitioned in space and time to dominantly left-lateral slip as the Embudo fault propagated to the northeast. During and shortly after eruption of Servilleta Basalt, proto-Embudo fault strands were active along and parallel to the modern, NE-aligned Rio Pueblo de Taos, ∼4–7 km basinward of the modern, mapped Embudo fault zone. Faults along the northeastern subbasin margin had northwest strikes for most of the period of subbasin formation and were located ∼5–7 km basinward of the modern Sangre de Cristo fault. The locus of fault activity shifted to more northerly striking faults within 2 km of the modern range front sometime after Servilleta volcanism had ceased. The northerly faults may have linked with the northeasterly proto-Embudo faults at this time, concurrent with the development of N-striking Los Cordovas normal faults within the interior of the subbasin. By middle Pleistocene(?) time, the Los Cordovas faults had become inactive, and the linked Embudo–Sangre de Cristo fault system migrated to the south, to the modern range front.
A paleomagnetic age estimate for the draining of ancient Lake Alamosa, San Luis Valley, south-central Colorado, U.S.A.
In September 2009, a 99.4-m (326-ft) deep well was drilled proximal to Great Sand Dunes National Park and Preserve to explore the history and subsurface geology of the San Luis Valley, Colorado. Of particular interest was deciphering the evolution of ancient Lake Alamosa, which filled much of the San Luis Valley in the Pliocene and Pleistocene. Thick intervals of massive clay, recovered at the well (BP-3-USGS [U.S. Geological Survey]), represent deposition as part of this extensive lacustrine environment. Oriented paleomagnetic samples from the BP-3-USGS core allow for estimation of sediment deposition ages. Paleomagnetic analyses show magnetic reversals correlated to the Brunhes-Matuyama boundary (0.78 Ma) at about the 75.2-m (247-ft) depth and the start of the Jaramillo subchron (1.07 Ma) at the 90.9-m (298-ft) depth. These paleomagnetic age ties imply an average sediment accumulation rate of 0.10 mm/yr for the sediments above the Brunhes-Matuyama reversal and 0.05 mm/ yr for the sediments below the reversal. The relatively low average sedimentation rate (0.05 mm/yr) from below the reversal roughly corresponds with an observed lithological change at 70.7 m (232 ft) from deep-water lacustrine clays below the reversal, to increasing littoral and basin-margin deposits above. Magnetic reversal ages provide estimates for when the sedimentary environment at the well site shifted away from a deep-water lacustrine-dominated system to an alluvial system. This shift in depositional environment may correspond to the draining of Lake Alamosa and indicate when the San Luis Valley was incorporated into the downstream Rio Grande drainage network. The last extensive deposit of lacustrine clay at the well site is encountered at the 36.3–40.8 m (119–134 ft) depth, which corresponds to 423–376 ka. A lack of broadly distributed lacustrine clays above this layer suggests that Rio Grande incorporation and onset of Lake Alamosa draining occurred about 376 ka, similar to lake drainage estimates from 3 He cosmogenic nuclide dating of Lake Alamosa and Rio Grande Gorge landforms. As such, our analyses suggest that the San Luis Valley likely became incorporated into the Rio Grande approximately 376 ka. Local, shallower lake systems appear to have existed at the well site until about 250 ka. This may corroborate the results of previous works that propose the drainage of Lake Alamosa began about 400 ka and was ultimately finished by 200 ka.
Geologic map and cross sections of the Embudo Fault Zone in the Southern Taos Valley, Taos County, New Mexico
The southern Taos Valley encompasses the physiographic and geologic transition zone between the Picuris Mountains and the San Luis Basin of the Rio Grande rift. The Embudo fault zone is the rift transfer structure that has accommodated the kinematic disparities between the San Luis Basin and the Española Basin during Neogene rift extension. The eastern terminus of the transfer zone coincides with the intersection of four major fault zones (Embudo, Sangre de Cristo, Los Cordovas, and Picuris-Pecos), resulting in an area of extreme geologic and hydrogeologic complexities in both the basin-fill deposits and the bedrock. Although sections of the Embudo fault zone are locally exposed in the bedrock of the Picuris Mountains and in the late Cenozoic sedimentary units along the top of the Picuris piedmont, the full proportions of the fault zone have remained elusive due to a pervasive cover of Quaternary surficial deposits. We combined insights derived from the latest geologic mapping of the area with deep borehole data and high-resolution aeromagnetic and gravity models to develop a detailed stratigraphic/structural model of the rift basin in the southern Taos Valley area. The four fault systems in the study area overlap in various ways in time and space. Our geologic model states that the Picuris-Pecos fault system exists in the basement rocks (Picuris formation and older units) of the rift, where it is progressively down dropped and offset to the west by each Embudo fault strand between the Picuris Mountains and the Rio Pueblo de Taos. In this model, the Miranda graben exists in the subsurface as a series of offset basement blocks between the Ponce de Leon neighborhood and the Rio Pueblo de Taos. In the study area, the Embudo faults are pervasive structures between the Picuris Mountains and the Rio Pueblo de Taos, affecting all geologic units that are older than the Quaternary surficial deposits. The Los Cordovas faults are thought to represent the late Tertiary to Quaternary reactivation of the old and deeply buried Picuris-Pecos faults. If so, then the Los Cordovas structures may extend southward under the Picuris piedmont, where they form growth faults as they merge downward into the Picuris-Pecos bedrock faults. The exceptionally high density of cross-cutting faults in the study area has severely disrupted the stratigraphy of the Picuris formation and the Santa Fe Group. The Picuris formation exists at the surface in the Miranda and Rio Grande del Rancho grabens, and locally along the top of the Picuris piedmont. In the subsurface, it deepens rapidly from the mountain front into the rift basin. In a similar manner, the Tesuque and Chamita Formations are shallowly exposed close to the mountain front, but are down dropped into the basin along the Embudo faults. The Ojo Caliente Sandstone Member of the Tesuque Formation appears to be thickest in the northwestern study area, and thins toward the south and the east. In the study area, the Lama formation thins westward and southward. The Servilleta Basalt is generally thickest to the north and northwest, thins under the Picuris piedmont, and terminates along a major, linear, buried strand of the Embudo fault zone, demonstrating that the Servilleta flows were spatially and temporally related to Embudo fault activity.
Preliminary geophysical interpretations of regional subsurface geology near the Questa Mine Tailing Facility and Guadalupe Mountain, Taos County, New Mexico
This report presents geophysical interpretations of regional subsurface geology in the vicinity of the Tailing Facility of the Questa Mine near Guadalupe Mountain, Taos County, New Mexico, in cooperation with the New Mexico Environment Department. The interpretations were developed from aeromagnetic data, regional gravity data, data from four ground magnetic traverses, geologic mapping, a digital elevation model, and information from a few shallow wells. The resolution of the geophysical data is only appropriate for a broad assessment of the regional setting. Aeromagnetic data provided the most comprehensive information for interpretation. Qualitative and semiquantitative interpretations indicate the nature and extent of volcanic rocks, their relative depths, and inferred contacts between them, as well as conjectured locations of faults. In particular, the aeromagnetic data indicate places where volcanic rocks extend at shallow depths under sedimentary cover. Trachydacites of Guadalupe Mountain are magnetic, but their associated aeromagnetic anomalies are opposite in sign over the northern versus the southern parts of the mountain. The difference indicates that lavas erupted during different magnetic-polarity events in the north (reverse polarity) versus the south (normal polarity) and therefore have different ages. We postulate a buried volcano with reverse-polarity magnetization lies under the northeast side of Guadalupe Mountain, which likely predated the exposed trachydacites. Faults interpreted for the study area generally align with known fault zones. We interpret a northern extension to one of these faults that crosses northwesterly underneath the Tailing Facility. Gravity data indicate that Guadalupe Mountain straddles the western margin of a subbasin of the Rio Grande rift and that significant (>400 meters) thicknesses of both volcanic and sedimentary rocks underlie the mountain.
Sample descriptions and geophysical logs for cored well BP-3-USGS, Great Sand Dunes National Park and Preserve, Alamosa County, Colorado
The BP-3-USGS well was drilled at the southwestern corner of Great Sand Dunes National Park in the San Luis Valley, south-central Colorado, 68 feet (ft, 20.7 meters [m]) southwest of the National Park Service’s boundary-piezometer (BP) well 3. BP-3-USGS is located at latitude 37°43ʹ18.06ʺN. and longitude 105°43ʹ39.30ʺW., at an elevation of 7,549 ft (2,301 m). The well was drilled through poorly consolidated sediments to a depth of 326 ft (99.4 m) in September 2009. Water began flowing from the well after penetrating a clay-rich layer that was first intercepted at a depth of 119 ft (36.3 m). The base of this layer, at an elevation of 7,415 ft (2,260 m) above sea level, likely marks the top of a regional confined aquifer recognized throughout much of the San Luis Valley. Approximately 69 ft (21 m) of core was recovered (about 21 percent), almost exclusively from clay-rich zones. Coarser grained fractions were collected from mud extruded from the core barrel or captured from upwelling drilling fluids. Natural gamma-ray, full waveform sonic, density, neutron, resistivity, spontaneous potential, and induction logs were acquired. The well is now plugged and abandoned. This report presents lithologic descriptions from the well samples and core, along with a compilation and basic data processing of the geophysical logs. The succession of sediments in the well can be generalized into three lithologic packages: (1) mostly sand from the surface to about 77 ft (23.5 m) depth; (2) interbedded sand, silt, and clay, decreasing in overall grain size downward, from 77 to 232 ft (23.5 to 70.7 m) depth; and (3) layers of massive clay alternating with layers of fine sand to silt from 232 to 326 ft (70.7 to 99.4 m), the total depth of the well. The topmost clay layers of the deepest package have a blue tint, prompting a correlation with the “blue clay” of the San Luis Valley that is commonly considered as the top of the confined aquifer. However, a confining clay was intercepted 113 ft (34.4 m) higher than the blue clay in BP-3-USGS. Most of the geophysical logs have good correspondence to the lithologic variations in the well. Exceptions are the gamma-ray log, which is likely affected by naturally occurring radiation from abundant volcanic detritus, and one interval within the deepest lithologic package, which appears to be abnormally electrically conductive. Resistivity logs and variations in sand versus clay content within the well are consistent with electrical resistivity models derived from time-domain electromagnetic geophysical surveys for the area. In particular, the topmost blue clay corresponds to a strong electrical conductor that is prominent in the electromagnetic geophysical data throughout the park and vicinity. BP-3-USGS was sited to test hypotheses developed from geophysical studies and to answer questions about the history and evolution of Pliocene and Pleistocene Lake Alamosa, which is represented by lacustrine deposits sampled by the well. The findings reported here represent a basis from which future studies can answer these questions and address other important scientific questions in the San Luis Valley regarding geologic history and climate change, groundwater hydrology, and geophysical interpretation.
Airborne electromagnetic and magnetic survey data of the Paradox and San Luis Valleys, Colorado
In October 2011, the U.S. Geological Survey (USGS) contracted airborne magnetic and electromagnetic surveys of the Paradox and San Luis Valleys in southern Colorado, United States. These airborne geophysical surveys provide high-resolution and spatially comprehensive datasets characterizing the resistivity structure of the shallow subsurface of each survey region, accompanied by magnetic-field information over matching areas. These data were collected to provide insight into the distribution of groundwater brine in the Paradox Valley, the extent of clay aquitards in the San Luis Valley, and to improve our understanding of the geologic framework for both regions. This report describes these contracted surveys and releases digital data supplied under contract to the USGS.
Geologic cross sections and preliminary geologic map of the Questa Area, Taos County, New Mexico
In 2011, the senior authors were contacted by Ron Gardiner of Questa, and Village of Questa Mayor Esther Garcia, to discuss the existing and future groundwater supply for the Village of Questa. This meeting led to the development of a plan in 2013 to perform an integrated geologic, geophysical, and hydrogeologic investigation of the Questa area by the New Mexico Bureau of Geology & Mineral Resources (NMBG), the U.S. Geological Survey (USGS), and New Mexico Tech (NMT). The NMBG was responsible for the geologic map and geologic cross sections. The USGS was responsible for a detailed geophysical model to be incorporated into the NMBG products. NMT was responsible for providing a graduate student to develop a geochemical and groundwater flow model. This report represents the final products of the geologic and geophysical investigations conducted by the NMBG and USGS. The USGS final products have been incorporated directly into the geologic cross sections. The objective of the study was to characterize and interpret the shallow (to a depth of approximately 5,000 ft) three-dimensional geology and preliminary hydrogeology of the Questa area. The focus of this report is to compile existing geologic and geophysical data, integrate new geophysical data, and interpret these data to construct three, detailed geologic cross sections across the Questa area. These cross sections can be used by the Village of Questa to make decisions about municipal water-well development, and can be used in the future to help in the development of a conceptual model of groundwater flow for the Questa area. Attached to this report are a location map, a preliminary geologic map and unit descriptions, tables of water wells and springs used in the study, and three detailed hydrogeologic cross sections shown at two different vertical scales. The locations of the cross sections are shown on the index map of the cross section sheet.
Identifying buried segments of active faults in the northern Rio Grande Rift using aeromagnetic, LiDAR,and gravity data, south-central Colorado, USA
Combined interpretation of aeromagnetic and LiDAR data builds on the strength of the aeromagnetic method to locate normal faults with significant offset under cover and the strength of LiDAR interpretation to identify the age and sense of motion of faults. Each data set helps resolve ambiguities in interpreting the other. In addition, gravity data can be used to infer the sense of motion for totally buried faults inferred solely from aeromagnetic data. Combined interpretation to identify active faults at the northern end of the San Luis Basin of the northern Rio Grande rift has confirmed general aspects of previous geologic mapping but has also provided significant improvements. The interpretation revises and extends mapped fault traces, confirms tectonic versus fluvial origins of steep stream banks, and gains additional information on the nature of active and potentially active partially and totally buried faults. Detailed morphology of surfaces mapped from the LiDAR data helps constrain ages of the faults that displace the deposits. The aeromagnetic data provide additional information about their extents in between discontinuous scarps and suggest that several totally buried, potentially active faults are present on both sides of the valley.
Digital data from the Great Sand Dunes airborne gravity gradient survey, south-central Colorado
This report contains digital data and supporting explanatory files describing data types, data formats, and survey procedures for a high-resolution airborne gravity gradient (AGG) survey at Great Sand Dunes National Park, Alamosa and Saguache Counties, south-central Colorado. In the San Luis Valley, the Great Sand Dunes survey covers a large part of Great Sand Dunes National Park and Preserve. The data described were collected from a high-resolution AGG survey flown in February 2012, by Fugro Airborne Surveys Corp., on contract to the U.S. Geological Survey. Scientific objectives of the AGG survey are to investigate the subsurface structural framework that may influence groundwater hydrology and seismic hazards, and to investigate AGG methods and resolution using different flight specifications. Funding was provided by an airborne geophysics training program of the U.S. Department of Defense's Task Force for Business & Stability Operations.
Introduction in New perspectives on Rio Grande rift basins: from tectonics to groundwater
Basins of the Rio Grande rift have long been studied both for their record of rift development and for their potential as host of natural resources. Early workers described the basin geomorphology and the character of infilling sediments (e.g. Siebenthal, 1910; Bryan, 1938; Speigel and Baldwin, 1963), and subsequent research compilations provided general stratigraphic and tectonic overviews of rift basins and described their geophysical characteristics within the crust (Hawley, 1978; Riecker, 1979; Baldridge et al., 1984; Keller, 1986). Subsurface knowledge gained from hydrocarbon exploration activities coupled with detailed surface studies of basins and their flanking uplifts were presented in Geological Society of America (GSA) Special Paper 291, edited by Keller and Cather (1994a).
The utility of gravity and magnetic methods for understanding subsurface hydrogeology in large alluvial watersheds: Examples from urbanized basins of the Western United States
Population continues to grow rapidly within the large alluvial watersheds associated with structural basins of the Basin and Range Province and the Rio Grande rift of the western United States. Increasing demands on ground‐water resources in these basins, combined with water‐rights disputes, have amplified the need for improved understanding of subsurface hydrogeology. Gravity and magnetic methods provide cost‐effective information critical to the understanding of the subsurface geology that controls hydrology at watershed scales. Gravity models are used to estimate the variations in the overall thickness of basin‐fill aquifers and to define major subbasin boundaries that partition flow systems. High‐resolution aeromagnetic surveys can be used to map the distribution of volcanic and other crystalline rocks in the shallow subsurface that impede flow. In certain geologic settings, the aeromagnetic data can be used to infer the base of basin aquifers or reveal buried, shallow paleotopography. In addition, the utility of high‐resolution aeromagnetic data to locate partially or wholly concealed faults within basin sediments is a non‐conventional application that has gained prominence in recent years. Examples of these uses of gravity and magnetic methods come from studies of basins within the Albuquerque‐Santa Fe, NM, urban corridor, the Virgin Valley in the tristate area of NV, AZ, and UT, the upper Verde River watershed near Prescott, AZ, and the San Luis Valley surrounding Alamosa, CO.