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At least 937 records · Page 52Linked to original sources

USGS2017

Population dynamics of brown trout (Salmo trutta) in Spruce Creek Pennsylvania: A quarter-century perspective

We examined the relationship between density-independent and density-dependent factors on the demography of a dense, relatively unexploited population of brown trout in Spruce Creek Pennsylvania between 1985 and 2011. Individual PCAs of flow and temperature data elucidated groups of years with multiple high flow versus multiple low flow characteristics and high versus low temperature years, although subtler patterns of variation also were observed. Density and biomass displayed similar temporal patterns, ranging from 710 to 1,803 trout/ha and 76–263 kg/ha. We detected a significantly negative linear stock-recruitment relationship ( R 2 = .39) and there was no evidence that flow or water temperature affected recruitment. Both annual survival and the per-capita rate of increase ( r ) for the population varied over the study, and density-dependent mechanisms possessed the greatest explanatory power for annual survival data. Temporal trends in population r suggested it displayed a bounded equilibrium with increases observed in 12 years and decreases detected in 13 years. Model selection analysis of per-capita rate of increase data for age 1, and adults ( N = eight interpretable models) indicated that both density-dependent (five of eight) and negative density-independent processes (five of eight, i.e. high flows or temperatures), affected r . Recruitment limitation also was identified in three of eight models. Variation in the per-capita rate of increase for the population was most strongly affected by positive density independence in the form of increasing spring–summer temperatures and recruitment limitation. Model selection analyses describing annual variation in both mean length and mass data yielded similar results, although maximum w i values were low ranging from 0.09 to 0.23 (length) and 0.13 to 0.22 (mass). Density-dependence was included in 15 of 15 interpretable models for length and all ten interpretable models for mass. Similarly, positive density-independent effects in the form of increasing autumn–winter flow were present in seven of 15 interpretable models for length and five of ten interpretable models for mass. Negative density independent effects also were observed in the form of high spring–summer flows or temperatures ( N = 4), or high autumn–winter temperatures ( N = 1). Our analyses of the factors affecting population regulation in an introduced population of brown trout demonstrate that density-dependent forces affected every important demographic characteristic (recruitment, survivorship, the rate of increase, and size) within this population. However, density-independent forces in the form of seasonal variations in flow and temperature also helped explain annual variation in the per-capita rate of increase, and mean length and mass data. Consequently, population regulation within this population is driven by a complex of biotic and environmental factors, although it seems clear that density-dependent factors play a dominant role.

Pennsylvania↗

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↗

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↗

USGS2026

Geophysical architecture and geochronology of the Neoarchean Mentor anorthosite intrusive complex, northwestern Minnesota: Largest anorthosite complex of the Superior Province?

The Neoarchean Mentor anorthosite intrusive complex (MAIC) lies within the Wawa subprovince in northwestern Minnesota, in a region where the Wawa, Quetico, and Wabigoon subprovinces are juxtaposed in close proximity. Archean rocks are entirely concealed, and interpretations are developed from aeromagnetic, gravity, and borehole samples. The MAIC includes both anorthosite (dense, weakly magnetized) and oxide-rich gabbro (strongly magnetized) lithologies. Anorthosite is coarse-grained to megacrystic, intensely altered, and locally brecciated. Pervasive epidote alteration enhanced the density of the anorthosite via introduction of higher-density mineral assemblages, explaining why the MAIC produces a significant gravity high. Oxide-rich gabbro forms a border phase of the MAIC and has potential for vanadium, chromium, and titanium mineralization, and produces a strong aeromagnetic high. The MAIC is interpreted to extend over an area of 640 km 2 , making it the largest known anorthosite complex of the Superior Province, as measured by preserved areal extent. Modeling indicates the MAIC extends more than 6 km into the subsurface. A new Pb–Pb zircon age of 2737.2 ± 4.5 Ma is interpretated as the crystallization age of anorthosite within the MAIC, showing that the MAIC formed well before the ca. 2690 Ma Shebandowanian orogeny, and raising new questions about correlations with other parts of the Wawa subprovince. A low-density batholith, here informally called the Fertile batholith, is interpreted to intrude the southern part of the MAIC. A new Pb–Pb zircon age of 2701.1 ± 6 Ma is interpreted as the magmatic age of the Fertile batholith.

Minnesota↗

USGS1994

Seismic images of a tectonic subdivision of the Greenville Orogen beneath lakes Ontario and Erie

New seismic data from marine air-gun and Vibroseis profiles in Lake Ontario and Lake Erie provide images of subhorizontal Phanerozoic sediments underlain by a remarkable series of easterly dipping reflections that extends from the crystalline basement to the lower crust. These reflections are interpreted as structural features of crustal-scale subdivisions within the Grenville Orogen. Broadly deformed, imbricated, and overlapping thrust sheets within the western Central Metasedimentary Belt are succeeded to the west by a complex zone of easterly dipping, apparent thrust faults that are interpreted as a southwest subsurface extension of the boundary zone between the Central Metasedimentary Belt and the Central Gneiss Belt. The interpreted Central Metasedimentary Belt boundary zone has a characteristic magnetic anomaly that provides a link from the adjacent ends of lakes Ontario and Erie to structures exposed 150&ensp;km to the north. Less reflective, west-dipping events are interpreted as structures within the eastern Central Gneiss Belt. The seismic interpretation augments current tectonic models that suggest the exposed ductile structures formed at depth as a result of crustal shortening along northwest-verging thrust faults. Relatively shallow reflections across the boundary region suggest local, Late Proterozoic extensional troughs containing post-Grenville sediments, preserved possibly as a result of pre-Paleozoic reactivation of basement structures.

Lake Erie, Lake Ontario↗

USGS2013

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.

Colorado↗

USGS1997

Geophysical setting of the Wabash Valley fault system

Interpretation of existing regional magnetic and gravity data and new local high-resolution aeromagnetic data provides new insights on the tectonic history and structural development of the Wabash Valley Fault System in Illinois and Indiana. Enhancement of short-wavelength magnetic anomalies reveal numerous NW- to NNE-trending ultramafic dikes and six intrusive complexes (including those at Hicks Dome and Omaha Dome). Inversion models indicate that the interpreted dikes are narrow (≤3 m), lie at shallow depths (<200 m) and are steeply dipping. Some of the interpreted dikes closely follow mapped faults; their abundance suggests that the Wabash Valley Fault System contains many more faults than those mapped. Both the interpreted dike pattern and mapped Wabash Valley Fault System terminate near the Reelfoot-Rough Creek-Rome rift system. Based on the interpretation of both the regional magnetic and gravity data and the high-resolution magnetic data, we propose that the shallow faults and deep-seated rift structures in the Wabash Valley terminate at or near the Rough Creek-Shawneetown Fault System. The Grayville Graben (∼20 km wide, ∼700 m maximum basement relief, and <40 km long [Bear et al. , this volume]) underlying the Wabash Valley developed during rifting, perhaps in response to stress concentrations generated by a bend in the Reelfoot-Rough Creek-Rome rift system. We therefore hypothesize that although the Reelfoot Rift and Rough Creek Graben represent tectonic intraplate structures of large areal extent (>500 km long and generally >50 km wide) and with deep basins (locally >3 km thick), the ancestral Wabash Valley faults express, in comparison, minor tectonic structures and probably do not represent a failed rift arm. There is a lack of any obvious relation between the Wabash Valley Fault System and the epicenters of historic and prehistoric earthquakes. Five prehistoric earthquakes lie conspicuously near structures associated with the Commerce geophysical lineament, a NE-trending magnetic and gravity lineament lying oblique to the Wabash Valley Fault System and possibly extending over 600 km from NE Arkansas to central Indiana.

Arkansas, Illinois, Indiana, Kentucky, Missouri, T↗

USGS2000

SHRIMP U-Pb zircon ages for Big Creek gneiss, Wyoming and Boulder Creek batholith, Colorado: Implications for timing of Paleoproterozoic accretion of the northern Colorado province

Sensitive, high-resolution, ion microprobe (SHRIMP) U-Pb zircon ages from a sample of the high-grade, hornblende-feldspathic Big Creek gneiss of the southeastern Sierra Madre, along with samples of a quartz monzonitic phase of the Boulder Creek batholith, help define timing of three major Paleoproterozoic thermo-tectonic events within the northern Colorado province at approximately 1810, 1710, and 1610 Ma. Previous ages determined for these key rock units were problematic; they hindered regional tectonic interpretations of the Paleoproterozoic crustal accretion history of the Colorado province that extends from the Cheyenne belt of southern Wyoming to north-central New Mexico. The Colorado province has been popularly modelled as a series of accreted oceanic volcano-plutonic arc systems and associated sediments, although alternative interpretations suggest that the series represents continental-margin arc rocks. The Big Creek gneiss has been interpreted as a high-grade basement equivalent of the oldest arc volcanic rocks exposed within the Green Mountain arc terrane, but it also has been suspected of being either an older block of pre-arc basement or perhaps an allochthonous piece of crust from slightly older orogens to the east and north. Previous ID-TIMS work on mg-size zircon fractions yielded U-Pb concordia upper-intercept ages of 1618 ± 22 and 1684 ± 5 Ma as well as negative lower-intercept ages, indicating complex U-Pb isotopic systematics involving at least two ages of zircon growth overprinted by at least one episode of Pb-loss. Zircons from this gneiss were analyzed using the SHRIMP, and a total of 32 spot analyses on both centers and rims produced a range of different 207 Pb/ 206 Pb ages between ∼1840 and ∼1560 Ma. The weighted mean of the oldest 207 Pb/ 206 Pb ages is 1812 ± 12 Ma and is interpreted to estimate the age of the protolith that appears to be slightly older than lower-grade metabasalts and associated plutons at ∼1790–1775 Ma. This protolith age of 1812 Ma further implies that significantly older crust (> 1820 Ma; e.g., Penokean orogeny) is not found in the Green Mountain magmatic arc. The youngest 207 Pb/ 206 Pb ages of ∼1610 Ma are interpreted to represent a time of new zircon growth during highly localized high-grade metamorphism—an event that also produced local granitic magmatism at ∼1625 Ma. The Boulder Creek batholith had been dated previously using the ID-TIMS, U-Pb zircon technique that yielded ages at ∼1670 and ∼1714 Ma, a 45-m.y. discrepancy that left the true age of the batholith in doubt. Zircons from two samples, previously dated using the ID-TIMS method, were analyzed using SHRIMP, and yielded concordia upper-intercept ages of 1713 ± 10 and 1721 ± 15 Ma. These results, combined with two earlier U-Pb zircon determinations, help to establish the age of the Boulder Creek batholith at 1714.4 ± 4.6 Ma (weighted mean), an age more compatible with those for the other large, tonalitic to quartz monzonitic, syntectonic plutons within the northern Colorado province. The new Boulder Creek age helps to establish a discrete period of plutonism (∼1735–1705 Ma) that is syn- to post-tectonic with respect to major regional structures of deformation and metamorphism in the northern Colorado province. Assuming the multiple oceanic arc accretion model, the new age for the mid-crustal emplacement of this batholith into a deforming composite back-arc basin may date the closure of that basin during crustal shortening.

Colorado, Wyoming↗

USGS1990

Hydrologic framework of Long Island, New York

Long Island, N.Y., is underlain by a mass of unconsolidated geologic deposits of clay, silt, sand, and gravel that overlie southward-sloping consolidated bedrock. These deposits are thinnest in northern Queens County (northwestern Long Island), where bedrock crops out, and increase to a maximum thickness of 2,000 ft in southeastern Long Island. This sequence of unconsolidated deposits consists of several distinct geologic units ranging in age from late Cretaceous through Pleistocene, with some recent deposits near shores and streams. These units are differentiated by age, depositional environment, and lithology in table 1. Investigations of ground-water availability and flow patterns may require information on the internal geometry of the hydrologic system that geologic correlations and interpretation alone cannot provide; hydrologic interpretations in which deposits are differentiated on the basis of water-transmitting properties are generally needed also. This set of maps and vertical sections depicts the hydrogeologic framework of the unconsolidated deposits that form Long Island's ground-water system. These deposits can be classified into eight major hydrogeologic units (table 1). The hydrogeologic interpretations presented herein are not everywhere consistent with strict geologic interpretation owing to facies changes and local variations in the water-transmitting properties within geologic units. These maps depict the upper-surface altitude of seven of the eight hydrogeologic units, which, in ascending order, are: consolidated bedrock, Lloyd aquifer, Raritan confining unit, Magothy aquifer, Monmouth greensand, Jameco aquifer, and Gardiners Clay. The upper glacial aquifer—the uppermost unit—is at land surface over most of Long Island and is, therefore, not included. The nine north-south hydrogeologic sections shown below depict the entire sequence of unconsolidated deposits and, together with the maps, provide a detailed three-dimensional interpretation of Long Island's hydrogeologic framework. The structure-contour map that shows the upper-surface altitude of the Cretaceous deposits is included to illustrate the erosional unconformity between the Cretaceous and overlying Pleistocene deposits. Pleistocene erosion played a major role in determining the shape and extent of the Lloyd aquifer, the Raritan confining unit, and the Magothy aquifer, and thus partly determined their hydrogeologic relation with subsequent (post-Cretaceous) deposits.

Hydrologic Atlas↗

USGS1985

Aeromagnetic maps of the Latir Peak and Wheeler Peak Wildernesses and the Columbine-Hondo Wilderness Study Area, Taos County, New Mexico

This report interprets data from a helicopter-borne magnetic survey to help delineate subsurface geology. Such data, acquired near the ground, reflect greater magnetic detail than do data acquired at higher altitude. Data from a high-altitude reconnaissance aeromagnetic survey (U.S. Geological Survey, 1976) are used selectively to assist in the interpretation of the helicopter data. Interpretations are chiefly empirical, wherein magnetic anomalies characterizing known geologic features in the region are used as interpretive criteria for identifying such features where they are concealed. Identifying geologic environments favorable for mineral deposits is a primary objective of this report. Geophysical interpretations of a broader and more general scope are made from other geophysical data by Cordell and Jones (in press).

New Mexico↗

USGS2006

Micropaleontology of selected wells and seismic shot holes, northern Alaska

This report provides micropaleontologic data (foraminifera, pollen, spores, and microplankton) and interpretations of the rocks penetrated by 49 wells and 3,134 seismic shot holes distributed among 73 seismic lines (figs. 1, 2; table 1). All shot holes and 30 wells are located within the National Petroleum Reserve in Alaska (NPRA); the remaining 19 wells are located adjacent to the NPRA. The biostratigraphic zonation scheme, stratigraphy, and geologic ages followed in this study are summarized in figure 3. This update brings paleontologic analyses performed at various times over several decades to a current, unified set of interpretations that benefit from the evolution of northern Alaska biostratigraphic understanding developed during the past 33-years by Mickey and Haga. For each well, paleontologic information includes microfossil distribution charts, data spreadsheets, diversity graphs, and interpretive reports describing age and environments of deposition. Three biostratigraphic well-correlation sections that relate Chukchi Sea wells to onshore northwestern NPRA wells are also included. For all analyzed seismic shot hole samples, foraminiferal age and environmental interpretations are provided; palynological interpretations are provided only for those shot hole samples collected and analyzed after 1976, a little less than half of the total number of samples.

Open-File Report↗

USGS2008

Using the U.S. Geological Survey National Water Quality Laboratory LT-MDL to Evaluate and Analyze Data

A long-term method detection level (LT-MDL) and laboratory reporting level (LRL) are used by the U.S. Geological Survey?s National Water Quality Laboratory (NWQL) when reporting results from most chemical analyses of water samples. Changing to this method provided data users with additional information about their data and often resulted in more reported values in the low concentration range. Before this method was implemented, many of these values would have been censored. The use of the LT-MDL and LRL presents some challenges for the data user. Interpreting data in the low concentration range increases the need for adequate quality assurance because even small contamination or recovery problems can be relatively large compared to concentrations near the LT-MDL and LRL. In addition, the definition of the LT-MDL, as well as the inclusion of low values, can result in complex data sets with multiple censoring levels and reported values that are less than a censoring level. Improper interpretation or statistical manipulation of low-range results in these data sets can result in bias and incorrect conclusions. This document is designed to help data users use and interpret data reported with the LTMDL/ LRL method. The calculation and application of the LT-MDL and LRL are described. This document shows how to extract statistical information from the LT-MDL and LRL and how to use that information in USGS investigations, such as assessing the quality of field data, interpreting field data, and planning data collection for new projects. A set of 19 detailed examples are included in this document to help data users think about their data and properly interpret lowrange data without introducing bias. Although this document is not meant to be a comprehensive resource of statistical methods, several useful methods of analyzing censored data are demonstrated, including Regression on Order Statistics and Kaplan-Meier Estimation. These two statistical methods handle complex censored data sets without resorting to substitution, thereby avoiding a common source of bias and inaccuracy.

Open-File Report↗

USGS2009

Investigation into the Origin and Character of Surficial Sedimentary Deposits at the Midshore Regional Solid Waste Facility near Easton, Maryland

A temporary exposure at the Midshore Regional Solid Waste Facility near Easton, MD, provided an opportunity to document the characteristics of the complex assemblage of surficial facies in that area. This unusually large cross section allowed interpretation of the changing processes that shaped the landscape in response to climate change through the late Pleistocene. Eight stratigraphic units were recognized: (1) gray, fossiliferous, muddy silt of the marine Miocene Choptank Formation; (2) coarse, crossbedded conglomerate of the late Miocene to Pliocene fluvial Pensauken Formation; (3) bioturbated muddy conglomerate interpreted as deposits of small colluvial fans; (4) pebbly, quartzose sand overlying a planar erosional surface reflecting a marine transgression; (5) irregular pods and lenses of sand and gravel deformed into bowl-shaped folds and faulted, which are interpreted as wind deposits over a semipermanent snow cover (niveo-aeolian deposits); (6) crossbedded sand and conglomerate with abundant mud partings indicating tidal influences on sinuous stream channels; (7) heavily bioturbated silt and sand with abundant root casts and flattened vesicles interpreted as aeolian loess deposits in marshy fens; and (8) pebbly sand and mud with scattered boulders and cobbles that reflect modern infill of the excavation by the operators. Soils formed on units 3, 4, and 7. Superimposed on units 4, 5, and 7 is evidence of deep freezing and permafrost development and subsequent thermokarst development after thawing, which includes large, complexly filled wedge-shaped cracks, deformed bedding and faults, fluid-injection structures, and spherical blobs of sand and mud. Each of the stratigraphic units has irregular distributions and lateral changes. The results of this study provide a unique insight into the geometry of surficial deposits that will help facilitate mapping of units, interpretation of cored intervals, and understanding of ground-penetrating radar profiles. The study also documents the widespread effects of permafrost during the last glacial episode well south of the maximum advance of ice sheets.

Open-File 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↗

USGS2009

Land-Cover Change in the East Central Texas Plains, 1973-2000

Project Background: The Geographic Analysis and Monitoring (GAM) Program of the U.S. Geological Survey (USGS) Land Cover Trends project is focused on understanding the rates, trends, causes, and consequences of contemporary U.S. land-use and land-cover change. The objectives of the study are to: (1) develop a comprehensive methodology for using sampling and change analysis techniques and Landsat Multispectral Scanner (MSS) and Thematic Mapper (TM) data for measuring regional land-cover change across the United States, (2) characterize the types, rates and temporal variability of change for a 30-year period, (3) document regional driving forces and consequences of change, and (4) prepare a national synthesis of land-cover change (Loveland and others, 1999). Using the 1999 Environmental Protection Agency (EPA) Level III ecoregions derived from Omernik (1987) as the geographic framework, geospatial data collected between 1973 and 2000 were processed and analyzed to characterize ecosystem responses to land-use changes. The 27-year study period was divided into five temporal periods: 1973-1980, 1980-1986, 1986-1992, 1992-2000, and 1973-2000. General land-cover classes such as water, developed, grassland/shrubland, and agriculture for these periods were interpreted from Landsat MSS, TM, and Enhanced Thematic Mapper Plus imagery to categorize land-cover change and evaluate using a modified Anderson Land-Use Land-Cover Classification System for image interpretation. The interpretation of these land-cover classes complement the program objective of looking at land-use change with cover serving as a surrogate for land use. The land-cover change rates are estimated using a stratified, random sampling of 10-kilometer (km) by 10-km blocks allocated within each ecoregion. For each sample block, satellite images are used to interpret land-cover change for the five time periods previously mentioned. Additionally, historical aerial photographs from similar timeframes and other ancillary data such as census statistics and published literature are used. The sample block data are then incorporated into statistical analyses to generate an overall change matrix for the ecoregion. For example, the scalar statistics can show the spatial extent of change per cover type with time, as well as the land-cover transformations from one land-cover type to another type occurring with time. Field data of the sample blocks include direct measurements of land cover, particularly ground-survey data collected for training and validation of image classifications (Loveland and others, 2002). The field experience allows for additional observations of the character and condition of the landscape, assistance in sample block interpretation, ground truthing of Landsat imagery, and helps determine the driving forces of change identified in an ecoregion. Management and maintenance of field data, beyond initial use for training and validation of image classifications, is important as improved methods for image classification are developed, and as present-day data become part of the historical legacy for which studies of land-cover change in the future will depend (Loveland and others, 2002). The results illustrate that there is no single profile of land-cover change; instead, there is significant geographic variability that results from land uses within ecoregions continuously adapting to the resource potential created by various environmental, technological, and socioeconomic factors.

Open-File Report↗

USGS2013

Characterization of major lithologic units underlying the lower American River using water-borne continuous resistivity profiling, Sacramento, California, June 2008

The levee system of the lower American River in Sacramento, California, is situated above a mixed lithology of alluvial deposits that range from clay to gravel. In addition, sand deposits related to hydraulic mining activities underlie the floodplain and are preferentially prone to scour during high-flow events. In contrast, sections of the American River channel have been observed to be scour resistant. In this study, the U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers, explores the resistivity structure of the American River channel to characterize the extent and thickness of lithologic units that may impact the scour potential of the area. Likely lithologic structures are interpreted, but these interpretations are non-unique and cannot be directly related to scour potential. Additional geotechnical data would provide insightful data on the scour potential of certain lithologic units. Additional interpretation of the resistivity data with respect to these results may improve interpretations of lithology and scour potential throughout the American River channel and floodplain. Resistivity data were collected in three profiles along the American River using a water-borne continuous resistivity profiling technique. After processing and modeling these data, inverted resistivity profiles were used to make interpretations about the extent and thickness of possible lithologic units. In general, an intermittent high-resistivity layer likely indicative of sand or gravel deposits extends to a depth of around 30 feet (9 meters) and is underlain by a consistent low-resistivity layer that likely indicates a high-clay content unit that extends below the depth of investigation (60 feet or 18 meters). Immediately upstream of the Watt Avenue Bridge, the high-resistivity layer is absent, and the low-resistivity layer extends to the surface where a scour-resistant layer has been previously observed in the river bed.

California↗

USGS2015

Second Projet de Renforcement Institutionnel du Secteur Minier de la République Islamique de Mauritanie (PRISM-II) Phase V

In 1996, at the request of the Government of the Islamic Republic of Mauritania, a team of U.S. Geological Survey (USGS) scientists produced a strategic plan for the acquisition, improvement and modernization of multidisciplinary sets of data to support the growth of the Mauritanian minerals sector and to highlight the geological and mineral exploration potential of the country. In 1999, the Ministry of Petroleum, Energy, and Mines of the Islamic Republic of Mauritania implemented a program for the acquisition of the recommended basic geoscientific information, termed the first Projet de Renforcement Institutionnel du Secteur Minier (Project for Institutional Capacity Building in the Mining Sector, PRISM-I). As a result of the PRISM-I efforts, a great deal of new geological, geophysical, geochemical, remote sensing, and hydrological data became available for evaluation and synthesis. However, the Ministry of Petroleum, Energy, and Mines recognized that additional work was required to extract the full benefit of the data before it could be of greatest use to the international community and of benefit to the Mauritanian minerals and development sector. To achieve this benefit, the Ministry of Petroleum, Energy, and Mines implemented a second Projet de Renforcement Institutionnel du Secteur Minier (PRISM-II) in 2006 to consolidate, synthesize, and interpret all of the existing data, create a new 1:1,000,000 scale geologic map, and define the mineral resource potential of the country. A consortium in which the USGS was the lead scientific agency carried out the majority of the PRISM-II work. In 2008, the USGS Mauritania Minerals Project was interrupted due to political changes in Mauritania. PRISM-II work resumed in 2011, and was completed in 2013 with the delivery of over 40 separate written reports and plates, an access file containing the Mauritanian National Mineral Deposits Database, and an interactive GIS containing all of the multi-disciplinary data and interpretive areas of mineral resource potential in Mauritania. This report contains the USGS results of the PRISM-II Mauritania Minerals Project and is presented in cooperation with the Ministry of Petroleum, Energy, and Mines of the Islamic Republic of Mauritania. The Report is composed of separate chapters consisting of multidisciplinary interpretive reports with accompanying plates on the geology, structure, geochronology, geophysics, hydrogeology, geochemistry, remote sensing (Landsat TM and ASTER), and SRTM and ASTER digital elevation models of Mauritania. The syntheses of these multidisciplinary data formed the basis for additional chapters containing interpretive reports on 12 different commodities and deposit types known to occur in Mauritania, accompanied by countrywide mineral resource potential maps of each commodity/deposit type. The commodities and deposit types represented include: (1) Ni, Cu, PGE, and Cr deposits hosted in ultramafic rocks; (2) orogenic, Carlin-like, and epithermal gold deposits; (3) polymetallic Pb-Zn-Cu vein deposits; (4) sediment-hosted Pb-Zn-Ag deposits of the SEDEX and Mississippi Valley-type; (5) sediment-hosted copper deposits; ( 6) volcanogenic massive sulfide deposits; (7) iron oxide copper-gold deposits; (8) uranium deposits; (9) Algoma-, Superior-, and oolitic-type iron deposits; (10) shoreline Ti-Zr placer deposits; (11) incompatible element deposits hosted in pegmatites, alkaline rocks, and carbonatites, and; (12) industrial mineral deposits. Additional chapters include the Mauritanian National Mineral Deposits Database are accompanied by an explanatory text and the Mauritania Minerals Project GIS that contains all of the interpretive layers created by USGS scientists. Raw data not in the public domain may be obtained from the Ministry of Petroleum, Energy, and Mines in Nouakchott, Mauritania.

Open-File Report↗

USGS2015

Geologic map of Mauritania (phase V, deliverables 51a, 51b, and 51c)

Open-File Report↗