Geology topics

Benjamin J. Drenth

Publications and source records attributed to Benjamin J. Drenth.

36 records · Page 2Linked to original sources

USGS2015

What lies beneath: geophysical mapping of a concealed Precambrian intrusive complex along the Iowa–Minnesota border

Large-amplitude gravity and magnetic highs over northeast Iowa are interpreted to reflect a buried intrusive complex composed of mafic–ultramafic rocks, the northeast Iowa intrusive complex (NEIIC), intruding Yavapai province (1.8–1.72 Ga) rocks. The age of the complex is unproven, although it has been considered to be Keweenawan (∼1.1 Ga). Because only four boreholes reach the complex, which is covered by 200–700 m of Paleozoic sedimentary rocks, geophysical methods are critical to developing a better understanding of the nature and mineral resource potential of the NEIIC. Lithologic and cross-cutting relations interpreted from high-resolution aeromagnetic and airborne gravity gradient data are presented in the form of a preliminary geologic map of the basement Precambrian rocks. Numerous magnetic anomalies are coincident with airborne gravity gradient (AGG) highs, indicating widespread strongly magnetized and dense rocks of likely mafic–ultramafic composition. A Yavapai-age metagabbro unit is interpreted to be part of a layered intrusion with subvertical dip. Another presumed Yavapai unit has low density and weak magnetization, observations consistent with felsic plutons. Northeast-trending, linear magnetic lows are interpreted to reflect reversely magnetized diabase dikes and have properties consistent with Keweenawan rocks. The interpreted dikes are cut in places by normally magnetized mafic–ultramafic rocks, suggesting that the latter represent younger Keweenawan rocks. Distinctive horseshoe-shaped magnetic and AGG highs correspond with a known gabbro, and surround rocks with weaker magnetization and lower density. Here, informally called the Decorah complex, the source body has notable geophysical similarities to Keweenawan alkaline ring complexes, such as the Coldwell and Killala Lake complexes, and Mesoproterozoic anorogenic complexes, such as the Kiglapait, Hettasch, and Voisey’s Bay intrusions in Labrador. Results presented here suggest that much of the NEIIC is composed of such complexes, and broadly speaking, may be a discontinuous group of several intrusive bodies. Most units are cut by suspected northwest-trending faults imaged as magnetic lineaments, and one produces apparent sinistral fault separation of a dike in the eastern part of the survey area. The location, trend, and apparent sinistral sense of motion are consistent with the suspected faults being part of the Belle Plaine fault zone, a complex transform fault zone within the Midcontinent rift system that is here proposed to correspond with a major structural discontinuity.

Iowa, Minnesota↗

USGS2015

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.

Open-File Report↗

USGS2014

Geophysical expression of a buried niobium and rare earth element deposit: the Elk Creek carbonatite, Nebraska, USA

The lower Paleozoic Elk Creek carbonatite is a 6–8-km-diameter intrusive complex buried under 200 m of sedimentary rocks in southeastern Nebraska. It hosts the largest known niobium deposit in the U.S. and a rare earth element (REE) deposit. The carbonatite is composed of several lithologies, the relations of which are poorly understood. Niobium mineralization is most enriched within a magnetite beforsite (MB) unit, and REE oxides are most concentrated in a barite beforsite unit. The carbonatite intrudes Proterozoic country rocks. Efforts to explore the carbonatite have used geophysical data and drilling. A high-resolution airborne gravity gradient and magnetic survey was flown over the carbonatite in 2012. The carbonatite is associated with a roughly annular vertical gravity gradient high and a subdued central low and a central magnetic high surrounded by magnetic field values lower than those over the country rocks. Geophysical, borehole, and physical property data are combined for an interpretation of these signatures. The carbonatite is denser than the country rocks, explaining the gravity gradient high. Most carbonatite lithologies have weaker magnetic susceptibilities than those of the country rocks, explaining why the carbonatite does not produce a magnetic high at its margin. The primary source of the central magnetic high is interpreted to be mafic rocks that are strongly magnetized and are present in large volumes. MB is very dense (mean density 3200 kg/m 3 ) and strongly magnetized (median 0.073 magnetic susceptibility), producing a gravity gradient high and contributing to the aeromagnetic high. Barite beforsite has physical properties similar to most of the carbonatite volume, making it a poor geophysical target. Geophysical anomalies indicate the presence of dense and strongly magnetized rocks at depths below existing boreholes, either a large volume of MB or another unknown lithology.

Nebraska↗

USGS2014

Porphyry copper assessment of eastern Australia

The U.S. Geological Survey (USGS) conducts national and global assessments of resources (mineral, energy, water, and biologic) to provide science in support of decision making. Mineral resource assessments provide syntheses of available information about where mineral deposits are known and suspected to occur in the Earth’s crust and which commodities may be present, together with estimates of amounts of resources that may be present in undiscovered deposits. The USGS collaborated with geologists of the Geological Survey of New South Wales and Geoscience Australia (formerly the Australian Geological Survey Organisation) on an assessment of Phanerozoic-age porphyry copper resources in Australia. Porphyry copper deposits contain about 11 percent of the identified copper resources in Australia. This study addresses resources of known porphyry copper deposits and expected resources of undiscovered porphyry copper deposits in eastern Australia. A three-part form of assessment was used for estimation of undiscovered resources. Using this method, four tracts were delineated that are permissive for porphyry copper deposits. A probabilistic estimate of the expected number of deposits in each tract was prepared on the basis of existing information about geology, geochemistry, geophysics, exploration history, and mineral occurrences. Monte Carlo simulation was used to combine the estimated number of deposits with an appropriate model of grade and tonnage for porphyry copper deposits to provide a probabilistic estimate of metal content and total tonnage for undiscovered deposits. The Delamerian permissive tract comprises igneous rocks of Cambrian age in the Delamerian Orogen, which borders the western margin of the Tasmanides. The Delamerian tract contains no known porphyry copper deposits, but the Adelaide sub-tract, one of three sub-tracts that compose the Delamerian tract, contains four porphyry copper prospects. The Adelaide sub-tract is estimated to contain 2.5±2.2 undiscovered deposits in an area of about 50,700 square kilometers. The Macquarie permissive tract comprises volcanic, volcaniclastic, and minor exposed intrusive igneous rocks of the Macquarie Arc. The nine known deposits in this tract are now estimated to contain a total of about 13.5 million metric tons of copper and 1,700 metric tons of gold. This tract is estimated to contain 6.9±3.5 undiscovered deposits for a total of about 16 deposits in an area of about 41,500 square kilometers. The Yeoval permissive tract includes subequal areas of permissive volcanic and intrusive rocks of Silurian to Devonian age exposed in and around the Cowra-Buchan Rift System, which overlaps the previously accreted Macquarie Arc. The Yeoval tract contains one porphyry copper deposit and several porphyry copper prospects. This tract is estimated to contain 1.3±0.75 undiscovered porphyry copper deposits, for a total of about 2 expected deposits in an area of about 53,200 square kilometers. The East Tasmanide permissive tract includes a semi-continuous belt of plutonic and subordinate volcanic rocks along the eastern margins of Queensland and northeastern New South Wales. The East Tasmanide tract contains 14 known porphyry copper deposits and many porphyry copper prospects, which are all in the Central sub-tract. This sub-tract is expected to contain 4.8±3.3 undiscovered porphyry copper deposits, for a total of about 19 deposits in an area of about 291,000 square kilometers. This assessment estimates that 15 undiscovered deposits contain an arithmetic mean of ~21 million metric tons or more of copper in four tracts, in addition to the 24 known porphyry copper deposits that contain identified resources of ~16 million metric tons of copper. In addition to copper, the mean expected amount of undiscovered byproduct gold predicted by the simulation is ~1,500 metric tons. The probability associated with these arithmetic means is on the order of 30 percent. Median expected amounts of metals predicted by the simulations may be ~50 percent lower than mean estimates.

Scientific Investigations Report↗

USGS2013

Geophysical constraints on Rio Grande rift structure in the central San Luis Basin, Colorado and New Mexico

Interpretation of gravity, aeromagnetic, and magnetotelluric (MT) data reveals patterns of rifting, rift-sediment thicknesses, distribution of pre-rift volcanic and sedimentary rocks, and distribution of syn-rift volcanic rocks in the central San Luis Basin, one of the northernmost major basins that make up the Rio Grande rift. Rift-sediment thicknesses for the central San Luis Basin determined from a three-dimensional gravity inversion indicate that syn-rift Santa Fe Group sediments have a maximum thickness of ~2 km in the Sanchez graben near the eastern margin of the basin along the central Sangre de Cristo fault zone, and reach nearly 1 km within the Monte Vista graben near the western basin margin along the San Juan Mountains. In between, Santa Fe Group thickness is negligible under the San Luis Hills and estimated to reach ~1.1 km under the Costilla Plains (although no independent thickness constraints exist, and a range of thicknesses of 600 m to 2 km is geophysically reasonable). From combined geophysical and geologic considerations, pre-rift, dominantly sedimentary rocks appear to increase in thickness from none in the Sanchez graben on the east to perhaps 800 m under the San Luis Hills on the west. The pre-rift rocks are most likely early Tertiary in age, but the presence of Mesozoic and Paleozoic sedimentary rocks cannot be ruled out. Geophysical data provide new evidence that an isolated exposure of Proterozoic rocks on San Pedro Mesa is rooted in the Precambrian basement. This narrow, north-south–trending basement high has ~2 km of positive relief with respect to the base of the Sanchez graben, and separates the graben from the structural depression beneath the Costilla Plains. A structural high composed of pre-rift rocks, long inferred to extend from under the San Luis Hills to the Taos Plateau, is confirmed and found to be denser than previously believed, with little or no overlying Santa Fe Group sediments. Major faults in the study area are delineated by geophysical data and models; these faults include significant vertical offsets (≥1 km) of Precambrian rocks along the central and southern zones of the Sangre de Cristo fault system. Other faults with similarly large offsets of the Santa Fe Group include a fault bounding the western margin of San Pedro Mesa, and other faults that bound the Monte Vista graben in an area previously assumed to be a simple hinge zone at the western edge of the San Luis Basin. A major north-south–trending structure with expression in gravity and MT data occurs at the boundary between the Costilla Plains and the San Luis Hills structural high. Although it has been interpreted as a down-to-the-east normal fault or fault zone, our modeling suggests that it also is likely related to pre-rift tectonics. Aeromagnetic anomalies over much of the area are interpreted to mainly reflect variations of remanent magnetic polarity and burial depth of the 5.3–3.7 Ma Servilleta Basalt of the Taos Plateau volcanic field. Magnetic-source depth estimates are interpreted to indicate patterns of subsidence following eruption of the basalt, with maximum subsidence in the Sanchez graben.

Colorado, New Mexico↗

USGS2013

Advancements in understanding the aeromagnetic expressions of basin-margin faults—An example from San Luis Basin, Colorado

Advancements in aeromagnetic acquisition technology over the past few decades have led to greater resolution of shallow geologic sources with low magnetization, such as intrasedimentary faults and paleochannels. Detection and mapping of intrasedimentary faults in particular can be important for understanding the overall structural setting of an area, even if exploration targets are much deeper. Aeromagnetic methods are especially useful for mapping structures in mountain-piedmont areas at the margins of structural basins, where mineral exploration and seismic-hazard studies may be focused, and where logistical or data-quality issues encumber seismic methods. Understanding if the sources of aeromagnetic anomalies in this context originate from sedimentary units or bedrock is important for evaluating basin structure and/or depth to shallow exploration targets. Advancements in aeromagnetic acquisition technology over the past few decades have led to greater resolution of shallow geologic sources with low magnetization, such as intrasedimentary faults and paleochannels. Detection and mapping of intrasedimentary faults in particular can be important for understanding the overall structural setting of an area, even if exploration targets are much deeper. Aeromagnetic methods are especially useful for mapping structures in mountain-piedmont areas at the margins of structural basins, where mineral exploration and seismic-hazard studies may be focused, and where logistical or data-quality issues encumber seismic methods. Understanding if the sources of aeromagnetic anomalies in this context originate from sedimentary units or bedrock is important for evaluating basin structure and/or depth to shallow exploration targets. Although explorationists have surmised that subtle, narrow, linear aeromagnetic anomalies or gradients are caused by intrasedimentary faults, the nature of the magnetic sources has been debated. A common and intuitive explanation for the linear anomalies considers that the magnetic properties of the fault zone have been altered by secondary chemical processes, either through the growth or destruction of magnetic minerals. However, comprehensive, multidisciplinary studies of partially exposed intrasedimentary faults in basins within the central Rio Grande Rift, New Mexico, have shown that the anomalies can be completely explained by the tectonic juxtaposition of strata of differing magnetic properties at the fault (summarized in Grauch and Hudson, 2007 , 2011 ). Whereas a reduction in magnetic susceptibility was detected at some fault zones in the laboratory, the slight reduction and small volume of material affected were insufficient to produce aeromagnetic anomalies ( Hudson et al., 2008 ). A key finding of the studies summarized by Grauch and Hudson (2007 , 2011 ) is that multiple magnetic contrasts (sources) can be vertically stacked at one fault. This situation requires rethinking of common assumptions when modeling faults as simple steps or when interpreting depth-estimation results. The multiple contrasts in these case studies arise from the tectonic juxtaposition of stratified sediments with differing magnetic properties. Multiple, vertically stacked magnetic sources also can occur where volcanic layers are interbedded with the sedimentary section or where faults offset both shallow bedrock and its overlying sedimentary cover. The latter situation is common at basin margins. Herein, we summarize and expand on an investigation of the sources of aeromagnetic anomalies related to faults along the eastern margin of the San Luis Basin, northern Rio Grande Rift, Colorado ( Grauch et al., 2010 ). Similar to the faults examined in the central Rio Grande Rift, magnetic sources can be completely explained by tectonic juxtaposition and produce multiple, vertically stacked magnetic contrasts at individual faults. However, the geologic sources are different. They arise from both the sedimentary cover and the underlying bedrock rather than from stratified sediments. In addition, geologic evidence for secondary growth or destruction of magnetic minerals at the fault zone is lacking.

Colorado↗

USGS2013

Expression of terrain and surface geology in high-resolution helicopter-borne gravity gradient (AGG) data: examples from Great Sand Dunes National Park, Rio Grande Rift, Colorado

Airborne gravity gradient (AGG) data are rapidly becoming standard components of geophysical mapping programs, due to their advantages in cost, access, and resolution advantages over measurements of the gravity field on the ground. Unlike conventional techniques that measure the gravity field, AGG methods measure derivatives of the gravity field. This means that effects of terrain and near-surface geology are amplified in AGG data, and that proper terrain corrections are critically important for AGG data processing. However, terrain corrections require reasonable estimates of density for the rocks and sediments that make up the terrain. A recommended philosophical approach is to use the terrain and surface geology, with their strong expression in AGG data, to the interpreter’s advantage. An example of such an approach is presented here for an area with very difficult ground access and little ground gravity data. Nettleton-style profiling is used with AGG data to estimate the densities of the sand dunefield and adjacent Precambrian rocks from the area of Great Sand Dunes National Park in southern Colorado. Processing of the AGG data using the density estimate for the dunefield allows buried structures, including a hypothesized buried basement bench, to be mapped beneath the sand dunes.

Colorado↗

USGS2013

Porphyry copper assessment of Southeast Asia and Melanesia: Chapter D in Global mineral resource assessment

The U.S. Geological Survey collaborated with member countries of the Coordinating Committee for Geoscience Programmes in East and Southeast Asia (CCOP) on an assessment of the porphyry copper resources of Southeast Asia and Melanesia as part of a global mineral resource assessment. The region hosts world-class porphyry copper deposits and underexplored areas that are likely to contain undiscovered deposits. Examples of known porphyry copper deposits include Batu Hijau and Grasberg in Indonesia; Panguna, Frieda River, and Ok Tedi in Papua New Guinea; and Namosi in Fiji. This assessment covers the countries of Cambodia, Indonesia, Lao People’s Democratic Republic, Malaysia, Myanmar, Papua New Guinea, Singapore, Thailand, and parts of southeastern China, India, the Solomon Islands, Vanuatu, and Fiji. Twenty-two geographic areas were delineated as tracts that are permissive for porphyry copper deposits in Southeast Asia. Permissive tracts are grouped into four broadly defined geographic/geologic areas, as follows: (1) the Indochina Peninsula area, (2) Indonesian and Malaysian Islands, (3) New Guinea Island and Papuan New Guinea islands, and (4) Melanesia. Individual tracts range from less than 1,000 to more than 350,000 square kilometers in area. Permissive tracts are based on mapped and inferred subsurface (<1 kilometer depth) distributions of igneous rocks of specific age ranges that define magmatic arcs and magmatic belts that are likely to contain porphyry copper deposits. Most of these magmatic arcs are subduction-related, although some have porphyry-style deposits occurring in postcollisional and (or) poorly understood tectonic settings. Although maps at a variety of different scales were used in the compilation, the final tract boundaries are intended for use at a scale of 1:1,000,000. Global grade and tonnage models for porphyry copper deposits were evaluated. Most of the known deposits are best described as fitting the copper-gold (Cu-Au) subtype of porphyry copper deposit. For some permissive tracts, a general porphyry copper-gold-molybdenum (Cu-Au-Mo) model was used. Assessment participants estimated numbers of undiscovered deposits at different levels of confidence for most of the permissive tracts. These estimates were combined with grade and tonnage models using a Monte Carlo simulation to estimate undiscovered resources. Additional resources in extensions of deposits with identified resources were not evaluated. Assessment results, presented in tables and graphs, show mean amounts of metal and mineralized rock in undiscovered deposits at different quantile levels, as well as the arithmetic mean for each tract. This assessment estimated a mean of 89 undiscovered porphyry copper deposits for the assessed permissive tracts in Southeast Asia and Melanesia. About 288 million metric tons (Mt) of copper and 18,000 metric tons (t) of gold, as well as byproduct molybdenum and silver, could be associated with undiscovered deposits. This represents about four times the number of deposits with identified resources (23) already discovered in Southeast Asia; reliable reported identified resources for those 23 deposits total 84 Mt of copper and 6,000 t of gold. Eleven permissive tracts have no known porphyry copper deposits with reported resources. Three of those 11 tracts lacked sufficient information for a probabilistic assessment and are discussed in qualitative terms. On a regional basis, both the Indochina Peninsula area and the Indonesian-Malaysian Islands area are estimated to contain about 10 times as much in place copper in undiscovered porphyry copper deposits as has been identified to date. For the New Guinea Island areas, the ratio of undiscovered to identified copper resources is about 2. Some parts of the region have a long history of porphyry exploration cycles and mine development, interrupted at times by political and social unrest, environmental concerns, and natural disasters. Changes in mining laws within the region and the recent high price of gold on the world market have prompted renewed interest in porphyry copper deposits in Southeast Asia and Melanesia. However, predicted undiscovered deposits may not be found, and if found, may not be developed. This assessment includes an overview of the assessment results with summary tables. Detailed descriptions of each tract are included in appendixes, with estimates of numbers of undiscovered deposits, and probabilistic estimates of amounts of copper, molybdenum, gold, and silver that could be contained in undiscovered deposits for each permissive tract. A geographic information system (GIS) that accompanies the report includes tract boundaries and a database of known porphyry copper deposits and significant prospects.

Scientific Investigations Report↗

USGS2012

Geophysical study of the San Juan Mountains batholith complex, southwestern Colorado

One of the largest and most pronounced gravity lows over North America is over the rugged San Juan Mountains of southwestern Colorado (USA). The mountain range is coincident with the San Juan volcanic field (SJVF), the largest erosional remnant of a widespread mid-Cenozoic volcanic field that spanned much of the southern Rocky Mountains. A buried, low-density silicic batholith complex related to the volcanic field has been the accepted interpretation of the source of the gravity low since the 1970s. However, this interpretation was based on gravity data processed with standard techniques that are problematic in the SJVF region. The combination of high-relief topography, topography with low densities, and the use of a common reduction density of 2670 kg/m 3 produces spurious large-amplitude gravity lows that may distort the geophysical signature of deeper features such as a batholith complex. We applied an unconventional processing procedure that uses geologically appropriate densities for the uppermost crust and digital topography to mostly remove the effect of the low-density units that underlie the topography associated with the SJVF. This approach resulted in a gravity map that provides an improved representation of deeper sources, including reducing the amplitude of the anomaly attributed to a batholith complex. We also reinterpreted vintage seismic refraction data that indicate the presence of low-velocity zones under the SJVF. Assuming that the source of the gravity low on the improved gravity anomaly map is the same as the source of the low seismic velocities, integrated modeling corroborates the interpretation of a batholith complex and then defines the dimensions and overall density contrast of the complex. Models show that the thickness of the batholith complex varies laterally to a significant degree, with the greatest thickness (∼20 km) under the western SJVF, and lesser thicknesses (<10 km) under the eastern SJVF. The largest group of nested calderas on the surface of the SJVF, the central caldera cluster, is not correlated with the thickest part of the batholith complex. This result is consistent with petrologic interpretations from recent studies that the batholith complex continued to be modified after cessation of volcanism and therefore is not necessarily representative of synvolcanic magma chambers. The total volume of the batholith complex is estimated to be 82,000–130,000 km 3 . The formation of such a large felsic batholith complex would inevitably involve production of a considerably greater volume of residuum, which could be present in the lower crust or uppermost mantle. The interpreted vertically averaged density contrast (–60 to –110 kg/m 3 ), density (2590–2640 kg/m 3 ), and seismic expression of the batholith complex are consistent with results of geophysical studies of other large batholiths in the western United States.

Colorado↗

USGS2011

Notes on interpretation of geophysical data over areas of mineralization in Afghanistan

Afghanistan has the potential to contain substantial metallic mineral resources. Although valuable mineral deposits have been identified, much of the country’s potential remains unknown. Geophysical surveys, particularly those conducted from airborne platforms, are a well-accepted and cost-effective method for obtaining information on the geological setting of a given area. This report summarizes interpretive findings from various geophysical surveys over selected mineral targets in Afghanistan, highlighting what existing data tell us. These interpretations are mainly qualitative in nature, because of the low resolution of available geophysical data. Geophysical data and simple interpretations are included for these six areas and deposit types: (1) Aynak: Sedimentary-hosted copper; (2) Zarkashan: Porphyry copper; (3) Kundalan: Porphyry copper; (4) Dusar Shaida: Volcanic-hosted massive sulphide; (5) Khanneshin: Carbonatite-hosted rare earth element; and (6) Chagai Hills: Porphyry copper.

Open-File Report↗

USGS2011

Recovery and reprocessing of legacy geophysical data from the archives of the State Company of Geology and Mining (GEOSURV) of Iraq and Iraq Petroleum Company (IPC)

Aeromagnetic data belonging to the State Company of Geology and Mining of Iraq (GEOSURV) have been recovered from magnetic tapes and early paper maps. In 1974 a national airborne survey was flown by the French firm Compagnie General de Geophysique (CGG). Following the survey the magnetic data were stored on magnetic tapes within an air conditioned archive run by GEOSURV. In 1990, the power supply to the archive was cut resulting in the present‐day poor condition of the tapes. Frontier Processing Company and the U.S. Geological Survey (USGS) have been able to recover over 99 percent of the original digital data from the CGG tapes. Preliminary reprocessing of the data yielded a total magnetic field anomaly map that reveals fine structures not evident in available published maps. Successful restoration of these comprehensive, high quality digital datasets obviates the need to resurvey the entire country, thereby saving considerable time and money. These data were delivered to GEOSURV in a standard format for further analysis and interpretation. A parallel effort by GETECH concentrated on recovering the legacy gravity data from the original field data sheets archived by IPC (Iraq Petroleum Company). These data have been compiled with more recent GEOSURV sponsored surveys thus allowing for the first time a comprehensive digital and unified national gravity database to be constructed with full principal facts. Figure 1 shows the final aeromagnetic and gravity data coverage of Iraq. The only part of Iraq lacking gravity and aeromagnetic data coverage is the mountainous areas of the Kurdish region of northeastern Iraq. Joint interpretation of the magnetic and gravity data will help guide future geophysical investigations by GEOSURV, whose ultimate aim is to discover economical mineral and energy resources.

SEG Technical Program Expanded Abstracts↗

USGS2011

Digitized data from ground geophysical surveys in Afghanistan: A website for distribution of data

This document describes the process of digitization of a 1974 report on geophysical work undertaken by Soviet geophysicists in southern and eastern Afghanistan. These data, uncovered in Afghanistan, represent magnetic and electrical ground surveys for which locations are not well defined. Due to lack of location information, these surveys were georeferenced using the cities, rivers, and surrounding geology found on the maps used to plot survey locations. A geologic map found in the Soviet report contains profile lines that correspond to the geophysical maps, allowing these data to be georeferenced. The profiles correspond to sets of resistivity, chargeabiliy, and magnetic data. Some datasets were presented as graphs and needed to be gridded into a useable image. Only the vertical component of the magnetic field was collected, so conversion to total field anomaly was necessary. The magnetic data were collected in either gammas or milliorstead, both of which required conversion to standard SI units. To be useful to modern studies, the datasets and images contained in this report have been digitized, georeferenced, and in some cases converted into computer-ready formats.

Open-File Report↗

USGS2011

Geologic map of Big Bend National Park, Texas

The purpose of this map is to provide the National Park Service and the public with an updated digital geologic map of Big Bend National Park (BBNP). The geologic map report of Maxwell and others (1967) provides a fully comprehensive account of the important volcanic, structural, geomorphological, and paleontological features that define BBNP. However, the map is on a geographically distorted planimetric base and lacks topography, which has caused difficulty in conducting GIS-based data analyses and georeferencing the many geologic features investigated and depicted on the map. In addition, the map is outdated, excluding significant data from numerous studies that have been carried out since its publication more than 40 years ago. This report includes a modern digital geologic map that can be utilized with standard GIS applications to aid BBNP researchers in geologic data analysis, natural resource and ecosystem management, monitoring, assessment, inventory activities, and educational and recreational uses. The digital map incorporates new data, many revisions, and greater detail than the original map. Although some geologic issues remain unresolved for BBNP, the updated map serves as a foundation for addressing those issues. Funding for the Big Bend National Park geologic map was provided by the United States Geological Survey (USGS) National Cooperative Geologic Mapping Program and the National Park Service. The Big Bend mapping project was administered by staff in the USGS Geology and Environmental Change Science Center, Denver, Colo. Members of the USGS Mineral and Environmental Resources Science Center completed investigations in parallel with the geologic mapping project. Results of these investigations addressed some significant current issues in BBNP and the U.S.-Mexico border region, including contaminants and human health, ecosystems, and water resources. Funding for the high-resolution aeromagnetic survey in BBNP, and associated data analyses and interpretation, was from the USGS Crustal Geophysics and Geochemistry Science Center. Mapping contributed from university professors and students was mostly funded by independent sources, including academic institutions, private industry, and other agencies.

Texas↗

USGS2011

Geophysical expression of elements of the Rio Grande rift in the northeast Tusas Mountains - Preliminary interpretations

New interpretations of the nature of the Rio Grande rift and pre-existing rocks in the northeast Tusas Mountains region are derived from new and existing gravity and aeromagnetic data. 12-15 mGal amplitude gravity lows are interpreted to mainly reflect large thicknesses of the upper Oligocene to upper Miocene, syn-rift Los Pinos Formation and possibly significant amounts of the Eocene El Rito Formation. The Broke Off Mountain sub basin, named after the location of its greatest inferred depth, is interpreted to be a ~40 km long and ~13 km wide structure elongated in a northwest trend at the western margin of the San Luis Basin. The sub basin is interpreted to contain a maximum combined thickness of 900-2300 m of the Los Pinos Formation and El Rito Formation, with the Los Pinos Formation constituting the majority of the section. Sub basin age is constrained to be older than 21.6 ± 1.4 Ma, the age of a Hinsdale Formation basalt flow that caps the Los Pinos Formation section at Broke Off Mountain. This age constraint and surface geology indicate a pre- and early-rift age. The structural fabric of the northeast Tusas Mountains region is dominated by northwest-trending normal faults, as indicated by geologic mapping and interpretation of aeromagnetic data. Preliminary analysis of the aeromagnetic data suggests that lineaments, possibly reflecting faulting, trend through volcanic rocks as young as Pliocene in age. If correct, these interpretations challenge commonly held beliefs regarding two stages in the structural style of rifting, where early (Oligocene-Miocene) rifting was characterized by broad, shallow basins bounded by northwest-trending faults and later (Miocene-Pliocene) rifting was characterized by deep, narrow basins bounded by north-trending faults. The Broke Off Mountain sub basin is a counter example of a pre- and early-rift, deep and narrow basin. We hypothesize that the Broke Off Mountain sub basin may represent a southward extension of the Monte Vista graben in Colorado, based on similarities in geophysical expression, stratigraphy, and its position at the western portion of the San Luis Basin

Colorado, New Mexico↗

USGS2010

Global mineral resource assessment: porphyry copper assessment of Mexico: Chapter A in Global mineral resource assessment

Mineral resource assessments provide a synthesis of available information about distributions of mineral deposits in the Earth’s crust. A probabilistic mineral resource assessment of undiscovered resources in porphyry copper deposits in Mexico was done as part of a global mineral resource assessment. The purpose of the study was to (1) delineate permissive areas (tracts) for undiscovered porphyry copper deposits within 1 km of the surface at a scale of 1:1,000,000; (2) provide a database of known porphyry copper deposits and significant prospects; (3) estimate numbers of undiscovered deposits within those permissive tracts; and (4) provide probabilistic estimates of amounts of copper (Cu), molybdenum (Mo), gold (Au), and silver (Ag) that could be contained in undiscovered deposits for each permissive tract. The assessment was conducted using a three-part form of mineral resource assessment based on mineral deposit models (Singer, 1993). Delineation of permissive tracts primarily was based on distributions of mapped igneous rocks related to magmatic arcs that formed in tectonic settings associated with subduction boundary zones. Using a GIS, map units were selected from digital geologic maps based on lithology and age to delineate twelve permissive tracts associated with Jurassic, Laramide (~90 to 34 Ma), and younger Tertiary magmatic arcs. Stream-sediment geochemistry, mapped alteration, regional aeromagnetic data, and exploration history were considered in conjunction with descriptive deposit models and grade and tonnage models to guide estimates. The 12 permissive tracts delineated for this assessment are grouped by age (Jurassic-Early Cretaceous, Laramide (~90 to 34 Ma), Tertiary) and range in size from ~3,000 to 184,000 km 2 . Probabilistic estimates of numbers of undiscovered deposits were made for 10 tracts. A qualitative discussion is included for one Jurassic tract that extends into the U.S. from northern Mexico, and a preliminary outline of the northernmost part of a Tertiary tract that continues well to the south of Mexico is included in summary figures for reference. This assessment estimates that 39 undiscovered deposits contain an arithmetic mean estimate of ~144 million metric tons of copper or more in ten tracts for which probabilistic estimates were made, in addition to 21 porphyry copper deposits that contain identified resources of ~52 million metric tons of copper. Approximately 70 percent of the estimated mean undiscovered copper resources are associated with permissive tracts that contain identified resources; the remaining estimated resources are associated with permissive tracts with no reported porphyry copper resources. In addition to copper, the mean expected values of undiscovered byproduct resources predicted by the simulation are ~4 million metric tons of molybdenum, ~48 thousand metric tons of silver, and 4 thousand metric tons of gold. The probability associated with these arithmetic means is on the order of 30 percent. Median expected amounts of metals predicted by the simulations may be ~50 percent lower than mean estimates, and in some cases, zero. For tracts that contain identified resources, the ratios of undiscovered to identified copper resources indicate that: Jurassic-Early Cretaceous tracts MX-J1 and MX-J2 contain fewer estimated undiscovered copper resources (ratios <1) than identified resources; Laramide tract MX-L1 may contain significantly more copper than has been identified; Laramide tract MX-L2, the tract that contains most of the known deposits in Mexico, including the world-class deposits at Cananea and La Caridad, may contain about as much copper, molybdenum, and silver in undiscovered deposits and more gold than has been identified; Tertiary tracts MX-T1 and MX-T2, where no porphyry copper deposits are known, may contain undiscovered deposits; Tertiary tract MX-T3 may contain about three times more copper than has been identified; and Permissive tracts with no known deposits (MX-J3, MX-J5, MX-L3, MX-T1, MX-T2) contain approximately 30 percent of the total estimated undiscovered copper resources. Most porphyry copper exploration in Mexico focused on the exposed northern parts of the Laramide arc. This assessment suggests that the exposed and shallowly buried (<1 km) parts of the Laramide and Tertiary arcs delineated as permissive tracts are more likely to contain undiscovered deposits than are older (Jurassic-Early Cretaceous) arc segments. Interest in gold has prompted exploration of historical precious metal prospects and small mines in Mexico, some of which may represent high-sulfidation epithermal systems overlying or adjacent to porphyry copper systems. This report includes a brief overview of porphyry copper deposits in Mexico, a description of the assessment process used, a summary of results, and appendixes. Appendixes A through K contain summary information for each tract, as follows: location, the geologic feature assessed, the rationale for tract delineation, tables and descriptions of known deposits and significant prospects, exploration history, model selection, rationale for the estimates, assessment results, and references. The accompanying digital map files (shapefiles) provide permissive tract outlines, assessment results, and data for deposits and prospects in a GIS format (appendix L).

Scientific Investigations Report↗

USGS2009

High-Resolution Aeromagnetic Survey To Image Shallow Faults, Poncha Springs and Vicinity, Chaffee County, Colorado

High-resolution aeromagnetic data were acquired over the town of Poncha Springs and areas to the northwest to image faults, especially where they are concealed. Because this area has known hot springs, faults or fault intersections at depth can provide pathways for upward migration of geothermal fluids or concentrate fracturing that enhances permeability. Thus, mapping concealed faults provides a focus for follow-up geothermal studies. Fault interpretation was accomplished by synthesizing interpretative maps derived from several different analytical methods, along with preliminary depth estimates. Faults were interpreted along linear aeromagnetic anomalies and breaks in anomaly patterns. Many linear features correspond to topographic features, such as drainages. A few of these are inferred to be fault-related. The interpreted faults show an overall pattern of criss-crossing fault zones, some of which appear to step over where they cross. Faults mapped by geologists suggest similar crossing patterns in exposed rocks along the mountain front. In low-lying areas, interpreted faults show zones of west-northwest-, north-, and northwest-striking faults that cross ~3 km (~2 mi) west-northwest of the town of Poncha Springs. More easterly striking faults extend east from this juncture. The associated aeromagnetic anomalies are likely caused by magnetic contrasts associated with faulted sediments that are concealed less than 200 m (656 ft) below the valley floor. The faults may involve basement rocks at greater depth as well. A relatively shallow (<300 m or <984 ft), faulted basement block is indicated under basin-fill sediments just north of the hot springs and south of the town of Poncha Springs.

Open-File Report↗

USGS2008

Geologic Map of the San Luis Quadrangle, Costilla County, Colorado

The map area includes San Luis and the primarily rural surrounding area. San Luis, the county seat of Costilla County, is the oldest surviving settlement in Colorado (1851). West of the town are San Pedro and San Luis mesas (basalt-covered tablelands), which are horsts with the San Luis fault zone to the east and the southern Sangre de Cristo fault zone to the west. The map also includes the Sanchez graben (part of the larger Culebra graben), a deep structural basin that lies between the San Luis fault zone (on the west) and the central Sangre de Cristo fault zone (on the east). The oldest rocks exposed in the map area are the Pliocene to upper Oligocene basin-fill sediments of the Santa Fe Group, and Pliocene Servilleta Basalt, a regional series of 3.7?4.8 Ma old flood basalts. Landslide deposits and colluvium that rest on sediments of the Santa Fe Group cover the steep margins of the mesas. Rare exposures of the sediment are comprised of siltstones, sandstones, and minor fluvial conglomerates. Most of the low ground surrounding the mesas and in the graben is covered by surficial deposits of Quaternary age. The alluvial deposits are subdivided into three Pleistocene-age units and three Holocene-age units. The oldest Pleistocene gravel (unit Qao) forms extensive coalesced alluvial fan and piedmont surfaces, the largest of which is known as the Costilla Plain. This surface extends west from San Pedro Mesa to the Rio Grande. The primary geologic hazards in the map area are from earthquakes, landslides, and localized flooding. There are three major fault zones in the area (as discussed above), and they all show evidence for late Pleistocene to possible Holocene movement. The landslides may have seismogenic origins; that is, they may be stimulated by strong ground shaking during large earthquakes. Machette and Thompson based this geologic map entirely on new mapping, whereas Drenth supplied geophysical data and interpretations.

Scientific Investigations Map↗

USGS2007

Preliminary Geologic Map of the Sanchez Reservoir Quadrangle and Eastern Part of the Garcia Quadrangle, Costilla County, Colorado

This geologic map is based entirely on new mapping by Thompson and Machette, whereas the geophysical data and interpretations were supplied by Drenth. The map area includes most of San Pedro Mesa, a basalt covered mesa that is uplifted as a horst between the Southern Sangre de Cristo fault zone (on the west) and the San Luis fault zone on the east. The map also includes most of the Sanchez graben, a deep structural basin that lies between the San Luis fault zone (on the west) and the Central Sangre de Cristo fault zone on the east. The oldest rocks in the map area are Proterozoic granites and Paleozoic sedimentary rocks, which are only exposed in a small hill on the west-central part of the mesa. The low hills that rise above San Pedro mesa are comprised of middle(?) Miocene volcanic rocks that are undated, but possibly correlative with mapped rocks to the east of Sanchez Reservoir. The bulk of the map area is comprised of the Servilleta Basalt, a regional series of flood basalts of Pliocene age. The west, north, and northeast margins of the mesa are covered by extensive landslide deposits that rest on poorly exposed sediment of the Santa Fe Group. Rare exposures of the sediment are comprised of siltstones, sandstones, and minor fluvial conglomerates. Most of the low ground surrounding the mesa is covered by surficial deposits of Quaternary age. The piedmont alluvium is subdivided into three Pleistocene units, and three Holocene units. The oldest Pleistocene gravel (unit Qao) forms an extensive coalesced alluvial fan and piedmont surface that is known as the Costilla Plains. This surface extends west from San Pedro Mesa to the Rio Grande. The primary geologic hazards in the map are are from earthquakes and landslides. There are three major fault zones in the area (as discussed above), and they all show evidence for late Pleistocene to possible Holocene movement. Two generations of landslides are mapped (younger and older), and both may have seismogenic origins.

Open-File Report↗