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V. J. S. Grauch

Publications and source records attributed to V. J. S. Grauch.

At least 55 records · Page 3Linked to original sources
USGS2004

Digital aeromagnetic data and derivative products from a helicopter survey over the town of Blanca and surrounding areas, Alamosa and Costilla Counties, Colorado

This CD-ROM contains digital data, image files, and text files describing data formats and survey procedures for aeromagnetic data collected during a helicopter geophysical survey in southern Colorado during October 2003. The survey covers the town of Blanca and surrounding communities in Alamosa and Costilla Counties. Several derivative products from these data are also presented, including reduced-to-pole, horizontal gradient magnitude, and downward continued grids and images.

Colorado↗
USGS2004

Thickness of Santa Fe Group sediments in the Espanola Basin south of Santa Fe, New Mexico, as estimated from aeromagnetic data

In the southern Espa?ola basin south of Santa Fe, New Mexico, weakly magnetic Santa Fe Group sediments of Oligocene to Pleistocene age, which represent the primary aquifers for the region, are locally underlain by moderately to strongly magnetic igneous and volcaniclastic rocks of Oligocene age. Where this relationship exists, the thickness of Santa Fe Group sediments, and thus the maximum thickness of the aquifers, can be estimated from quantitative analysis of high-resolution aeromagnetic data. These thickness estimates provide guidance for characterizing the ground-water resources in between scattered water wells in this area of rapid urban development and declining water supplies. This report presents one such analysis based on the two-step extended Euler method for estimating depth to magnetic sources. The results show the general form of a north-trending synclinal basin located between the Cerrillos Hills and Eldorado with northward thickening of Santa Fe Group sediments. The increase in thickness is gradual from the erosional edge on the south to a U-shaped Santa Fe embayment hinge line, north of which sediments thicken much more dramatically. Along the north-south basin axis, Santa Fe Group sediments thicken from 300 feet (91 meters) at the hinge line near latitude 35o32'30'N to 2,000 feet (610 meters) at the Cerrillos Road interchange at Interstate 25, north of latitude 35o36'N. The depth analysis indicates that, superimposed on this general synclinal form, there are many local areas where the Santa Fe Group sediments may be thickened by a few hundred feet, presumably due to erosional relief on the underlying Oligocene volcanic and volcaniclastic rocks. Some larger areas of greater apparent thickening occur where the presence of magnetic rocks directly underlying the Santa Fe Group is uncertain. Where magnetic rocks are absent beneath the Santa Fe Group, the thickness cannot be estimated from the aeromagnetic data.

Open-File Report↗
USGS2003

Geophysical and isotopic constraints on crustal structure related to mineral trends in north-central Nevada and implications for tectonic history

We combined information from Sr and Pb isotope data and magnetotelluric models to develop a new magnetic and gravity interpretation of the crustal structure of north - central Nevada to better understand the origin of mineral trends . The new interpretation suggests a crustal structure that is composed of Precambrian continental crust, transitional crust, and primarily oceanic crust that are separated by northwest- and northeast-striking fault zones. The magnetic expression of the buried Precambrian continental crust is recognized for the first time. Low magnetic values primarily reflect magnetite-poor crystalline crust rather than elevated temperatures at depth. Northwest- and northeast-striking crustal boundaries are defined by isotopic data and abrupt gradients in gravity and magnetic data. The Carlin and Battle Mountain-Eureka mineral trends are associated with two of three northwest-striking boundaries. The Carlin boundary is primarily defined by a change in density and isotopic character of the lower to middle crust. The Battle Mountain-Eureka boundary coincides with a density contrast in the upper crust and a change in isotopic character in the lower to middle crust. Magnetotelluric models suggest that the Battle Mountain-Eureka boundary represents a crustal fault zone for most of its extent, but that deep-rooted faulting is more complex near and northwest of Battle Mountain. Crustal fault zones inferred from the magnetotelluric models near the Carlin trend are oblique to it, suggesting that they may not have been controlled by the deep boundary seen in the gravity and isotopic data. The third northwest-trending boundary is related to the western edge of the buried Precambrian continent in west- central Nevada , but lacks an associated mineral trend . A northeast-striking boundary forms the northern limit of Precambrian continental and transitional crust. The boundaries may have originated as rift or transform faults during Precambrian breakup of Rodinia or as faults accommodating lateral movements or accretion during later Paleozoic tectonic events. Comparing the crustal structure to tectonic elements produced by successively younger events shows that it had a profound influence on subsequent sedimentation, deformation, magmatism, extension, and most important, mineralization.

Nevada↗
USGS2003

Paleomagnetic evidence for a Tertiary not Triassic age for rocks in the lower part of the Grober-Fuqua #1 well, southeastern Albuquerque Basin, New Mexico

A sedimentary sequence penetrated in the lower part of the Grober-Fuqua #1 well in the southeastern Albuquerque Basin has previously been interpreted as either Triassic or Eocene in age. Paleomagnetic study of three specimens from two core fragments yielded a 54.5?? mean inclination of remanent magnetization relative to bedding. This inclination is like that expected in Tertiary time and is distinct from an expected low-angle Triassic inclination. Although the data are very few, when considered in combination with stratigraphic relations and the presence of a gravity low in this southeastern part of the basin, the paleomagnetic evidence favors a Tertiary age for strata in the lower part of the Grober-Fuqua #1 well.

New Mexico Geology↗
USGS2003

Evidence for a Battle Mountain-Eureka crustal fault zone, north-central Nevada, and its relation to Neoproterozoic-Early Paleozoic continental breakup

Combined evidence from gravity, radiogenic isotope, and magnetotelluric (MT) data indicates a crustal fault zone that coincides with the northwest-trending Battle Mountain-Eureka (BME) mineral trend in north-central Nevada, USA. The BME crustal fault zone likely originated during Neoproterozoic-Early Paleozoic rifting of the continent and had a large influence on subsequent tectonic events, such as emplacement of allochthons and episodic deformation, magmatism, and mineralization throughout the Phanerozoic. MT models show the fault zone is about 10 km wide, 130-km long, and extends from 1 to 5 km below the surface to deep crustal levels. Isotope data and gravity models imply the fault zone separates crust of fundamentally different character. Geophysical evidence for such a long-lived structure, likely inherited from continental breakup, defies conventional wisdom that structures this old have been destroyed by Cenozoic extensional processes. Moreover, the coincidence with the alignment of mineral deposits supports the assertion by many economic geologists that these alignments are indicators of buried regional structures.

Journal of Geophysical Research B: Solid Earth↗
USGS2002

Geologic and geophysical evidence for the influence of deep crustal structures on Paleozoic tectonics and the alignment of world-class gold deposits, north-central Nevada, USA

Geologic data concur with geophysical and isotopic data that suggest the presence of deep crustal fault zones along the Battle Mountain-Eureka (BME) trend and elsewhere in Nevada. The fault zones may have originated during Proterozoic rifting of the continent and were likely substantially reactivated and modified during Paleozoic tectonism. Five distinct Paleozoic structural and stratigraphic domains are defined that demonstrate the complexity of Paleozoic tectonic events and also lead to hypotheses about ways in which the margin could have been modified. The current locations of these domains adjacent to the geophysically and isotopically defined indicators of the buried continent edge corroborate their interactions with the continental margin. During the Tertiary, preexisting crustal fault zones were intersected and reopened during episodes of extension and served as the conduits for deep-sourced, gold-rich fluids, which were disseminated into Paleozoic slope facies sedimentary rocks, forming sediment-hosted Carlin-type and other deposits. Multiple factors including the locations of these deep-seated structures, the original configuration of the lower Paleozoic continental margin of Nevada, and its subsequent reactivation during the Paleozoic all were fundamental controls on the location of younger mineral deposits. A clearer understanding of the original configuration of the margin and of the effects of subsequent Paleozoic and Mesozoic tectonic events on the margin would provide insight into the locations of these and other prospective mineral belts. ?? 2002 Elsevier Science B.V. All rights reserved.

Ore Geology Reviews↗
USGS2002

Merged digital aeromagnetic data for the middle Rio Grande and southern Espanola basins, New Mexico

The U. S. Geological Survey (USGS) recently conducted a multi-disciplinary study of the Middle Rio Grande basin (Bartolino and Cole, 2002; Fig. 1). The main purpose of this study was to gain a better multi-dimensional understanding of the basin's hydrogeologic framework and use this new understanding to construct an improved regional ground-water flow model. The Middle Rio Grande basin fill serves as the primary water resource for Albuquerque and surrounding communities (Thorn and others, 1993). It is composed of poorly consolidated, Tertiary to Quaternary sediments, collectively called the Santa Fe Group. These sediments were deposited during the Tertiary to Quaternary development of the Rio Grande rift (Fig. 1, inset). The strata vary in thickness from 1,000 to more than 4,000 m and range from mudstone to conglomerate (Kelley, 1977; May and Russell, 1994).

New Mexico↗
USGS2002

Preliminary Results of a High-Resolution Aeromagnetic Survey to Identify Buried Faults at Dixie Valley, Nevada

Preliminary results from a high-resolution aeromagnetic survey (200 m line spacing) acquired in Dixie Valley early in 2002 provide confirmation of intra-basin faulting based on subtle surface indications. In addition the data allow identification of the locations and trends of many faults that have not been recognized at the surface, and provide a picture of intrabasin faulting patterns not possible using other techniques. The data reveal a suite of northeasterly-trending curving and branching faults that surround a relatively coherent block in the area of Humboldt Salt Marsh, the deepest part of the basin. The producing reservoir occurs at the north end of this coherent block, where rampart faults from the northwest side of the valley merge with anthithetic faults from the central and southeast parts of the valley.

Conference Paper↗
USGS2001

High-resolution aeromagnetic data, a new tool for mapping intrabasinal faults: Example from the Albuquerque basin, New Mexico

High-resolution aeromagnetic surveys flown over the Albuquerque basin, New Mexico, demonstrate that aeromagnetic methods can successfully map concealed and poorly exposed faults in sediment-filled basins. This is the first known use of aeromagnetic data as an aid to surficial mapping and hydrogeologic studies in a basin. Aeromagnetic maps show detailed fault patterns within the basin fill that revise the structural view of the basin. Concealed faults are more numerous and more closely spaced than expected. The Hubbell Springs fault is the central splay of three generally north-striking fault splays that can be traced for nearly 50 km. The splays converge on the north and may represent the southern extension of the Tijeras fault, contradicting the proposed southwest extension of the Tijeras fault across the basin. In profile view, the linear aeromagnetic anomalies associated with faults show a variety of signatures. One signature has potential for mapping fault-controlled sedimentation in the subsurface because it identifies increases in magnetic, likely coarse-grained, material in the hanging walls of faults.

New Mexico↗
USGS2001

Aeromagnetic expression of faults that offset basin fill, Albuquerque basin, New Mexico

High-resolution aeromagnetic data acquired over the Albuquerque basin show widespread expression of faults that offset basin fill and demonstrate that the aeromagnetic method can be an important hydrogeologic and surficial mapping tool in sediment-filled basins. Aeromagnetic expression of faults is recognized by the common correspondence of linear anomalies to surficial evidence of faulting across the area. In map view, linear anomalies show patterns typical of extensional faulting, such as anastomosing and en echelon segments. Depths to the tops of faulted magnetic layers showing the most prominent aeromagnetic expression range from 0 to 100 m. Sources related to subtler fault expressions range in depths from 200 to 500 m. We estimate that sources of the magnetic expressions of the near-surface faults likely reside within the upper 500-600 m of the subsurface. The linear anomalies in profile form show a range of shapes, but all of them can be explained by the juxta-position of layers having different magnetic properties. One typical anomaly differs from the expected symmetric fault anomaly by exhibiting an apparent low over the fault zone and more than one inflection point. Although the apparent low could easily be misinterpreted as representing multiple faults or an anomalous fault zone, geophysical analysis, magnetic-property measurements, and geologic considerations lead instead to a "thin-thick model" in which magnetic layers of different thickness are juxtaposed. The general geometry of this model is a thin magnetic layer on the upthrown block and a thick magnetic layer on the downthrown block. The thin-thick model can be represented geologically by growth faulting and syntectonic sedimentation, where relatively coarse-grained sediment (which is more magnetic than fine-grained material) has accumulated in the hanging wall. This implies that the aeromagnetic data have potential for mapping growth faults and locating concentrations of coarse-grained material that may have high hydraulic transmissivity. Although cementation along fault zones is common in portions of the area, we found no evidence that this secondary process results in measurable aeromagnetic anomalies. This observation differs from the findings in other sedimentary basins suggesting that magnetic anomalies arise from secondary magnetization along fault planes.

New Mexico↗
USGS2000

Principal facts for gravity data collected in the southern Albuquerque Basin area and a regional compilation, central New Mexico

Principal facts for 156 new gravity stations in the southern Albuquerque basin are presented. These data fill a gap in existing data coverage. The compilation of the new data and two existing data sets into a regional data set of 5562 stations that cover the Albuquerque basin and vicinity is also described. Bouguer anomaly and isostatic residual gravity data for this regional compilation are available in digital form from ftp://greenwood.cr.usgs.gov/pub/openfile- reports/ofr-00-490.

Open-File Report↗
USGS1990

Magnetic forward models of Cement oil field, Oklahoma, based on rock magnetic, geochemical, and petrologic constraints

Magnetic forward models of the Cement oil field, Oklahoma, were generated to assess the possibility that ferrimagnetic pyrrhotite related to hydrocarbon seepage in the upper 1 km of Permian strata contributes to aeromagnetic anomalies at Cement. Six bodies having different magnetizations were constructed for the magnetic models, based on geology and on petrologic and geochemical results, supplemented by rock magnetic measurements of shallow-core and outcrop samples. The column of rock through which hydrocarbons have passed is divided into three sulfide zones on the basis of pyrrhotite content, and the column is capped by a 30 m thick zone that contains ferric oxide minerals formed mainly from oxidized pyrite. Red beds unaffected by sulfidization, as well as a zone of rock depleted in hematite but lacking sulfide, surround sulfidic zones.The synthetic magnetic profiles are controlled mainly by pyrrhotite-bearing strata at depths of 200-500 m. The magnetizations of these bodies are calculated from: (1) petrographic estimates of pyrrhotite content relative to pyrite; (2) content of sulfide sulfur determined from chemical analysis; and (3) values for the magnetic susceptibility of monoclinic pyrrhotite. Total magnetizations of the bodies of highest pyrrhotite content range from about 3 X 10 (super -3) to 56 X 10 (super -3) A/m in the present field direction and yield magnetic anomalies (at 120 m altitude) having amplitudes of less than 1 nT to approximately 6 to 7 nT, respectively. Such amplitudes are much lower than those (as high as 60 nT) reported from the original total-field survey over the Cement field.Numerous assumptions were made in the generation of the models, and thus the results neither prove nor disprove the existence of aeromagnetic anomalies related to hydrocarbon seepage at Cement. Nevertheless, the results suggest that pyrrhotite, formed via hydrocarbon reactions and within a range of concentrations estimated at Cement, is capable of causing magnetic anomalies.

Geophysics↗