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At least 901 records · Page 50Linked to original sources
USGS1987

The Alaska Mineral Resource Assessment Program: Background information to accompany folio of geologic and mineral resource maps of the Circle quadrangle, Alaska

The geology, geochemistry, geophysics, and Landsat imagery of the Circle quadrangle were investigated by an interdisciplinary research team for the purpose of assessing the mineral potential of the area. The quadrangle covers approximately 15,765 km 2 in east-central Alaska; most of it is included in the mountainous Yukon-Tanana Upland physiographic division, but the northernmost part is in the low-lying Yukon Flats section. The Circle mining district, in the east-central part of the quadrangle, has been a major producing area of placer gold since its discovery in 1893. For descriptive purposes, the Circle quadrangle is divided into three areas: the northwest Circle quadrangle, the area north of the Tintina fault zone, and the area south of the Tintina fault zone. The Tintina fault zone extends northwesterly through the northern part of the quadrangle. The northwest Circle quadrangle contains mostly folded and faulted, slightly metamorphosed sedimentary rocks that are intruded by Tertiary granitic plutons. In the northern part of the area north of the Tintina fault zone (Little Crazy Mountains and northern east Crazy Mountains), the rocks consist primarily of the gabbro and basalt of the Circle Volcanics and minor associated chert, graywacke, and limestone. Elsewhere in this area (south of the Circle Volcanics and in the western Crazy Mountains), the rocks are mostly slightly metamorphosed Paleozoic sedimentary rocks that have been folded and faulted. Rocks in the largest part of the quadrangle, the area south of the Tintina fault zone, consist largely of pelitic rocks that are regionally metamorphosed to greenschist and amphibolite facies. Felsic plutons, mostly Tertiary in age, occur throughout the area. The metamorphic rocks are separated from sedimentary rocks on the northwest by thrust faulting. The aeromagnetic and gravity data show clear differences between the areas north and south of the Tintina fault zone. The metamorphic terrane to the south has low overall gravity and local gravity lows over exposed granitic plutons. It is hypothesized that magnetic chlorite schist infolded with nonmagnetic quartzite and schist account for east-northeast-trending magnetic highs that approximately parallel the regional strike of the most prominent foliation in the metamorphic rocks. North of the Tintina fault zone, the Circle Volcanics are characterized by high gravity and east-west-trending magnetic highs. The Tintina fault zone has an intense magnetic high near the western margin of the Circle quadrangle overlying the magnetic granodiorite of the Victoria Mountain pluton. A magnetic high near Circle Hot Springs is less intense, but broader, and could reflect a buried magnetic pluton similar to that of the Victoria Mountain pluton. Computer-enhanced Landsat images of the Circle quadrangle show trends and patterns of concentrations of linear features. Features trending northeast-southwest predominate throughout the quadrangle; northwest-southeast-trending linear features are found mostly south of the Tintina fault zone. High concentrations of linear features were not found to correspond to areas of known mineralization in any consistent or significant way that could presently be used in locating areas of mineralization. Geochemical and mineralogical studies of stream sediment and heavy-mineral concentrates from the Circle quadrangle identify areas of anomalous concentrations of metallic elements, including gold, silver, tin, tungsten, lead, antimony, zinc, thorium, uranium, and beryllium. The data delineate areas of known mineral occurrences and areas that may contain undiscovered mineral resources. To date, placer gold has been the only significant metallic mineral resource from the Circle quadrangle, but the general geologic setting, especially the presence of post-orogenic plutons, is similar to that of regions that contain tin greisen deposits, tungsten skarn deposits, lode gold deposits in metasedimentary rocks, and uranium vein deposits. Six areas or tracts were identified in which such deposits might occur, and two more tracts were delineated as possible for the occurrence of shale-hosted Lead-zinc deposits. The discovery of two diamonds in the gravels of Cooked Creek point to the slight possibility of finding placer or lode diamond deposits. Although most of the past and present gold mining has taken place in four areas in the quadrangle, a sedimentary basin near the town of Central was identified as possibly containing buried placer gold deposits or sedimentary uranium deposits.

Alaska↗
USGS2007

Sedimentary properties of shallow marine cores collected in June and September 2006, Hanalei Bay, Kaua'i, Hawai'i

Sedimentary facies, short-lived isotopes 7 Be, 137 Cs, and 210 Pb, and magnetic properties of sediment cores in Hanalei Bay, Kaua‘i, Hawai‘i, were used to assess sediment sources and patterns of deposition associated with seasonal flooding of the Hanalei River. Sediment cores were collected from the seafloor in June and September of 2006 to supplement similar data collected during the summer of 2005. The youngest and thickest terrigenous sediment was observed on the east side of the bay: near the Hanalei River mouth and in a bathymetric depression, known locally as the Black Hole, that acts as a temporary sediment sink. Deposits from floods that occurred between February and April 2006 left flood deposits in the eastern bay that, by June of 2006, were on the order of 10 cm thick. A flood occurred on August 7, 2006, that was smaller than floods that occurred the previous winter but was a substantial discharge event for the summer season. Deposits from the winter 2006 floods continued to dominate the sedimentary record in the eastern bay through early fall, even after the addition of newer sediment during the August 7 flood; this is consistent with the much higher sediment input of the winter floods compared with the August 7 flood. Broad variations in magnetic grain size and relative magnetite-hematite abundance in several sediment cores indicate many sources of upland terrigenous sediment. As a group, recent flood deposits show much less variation in these properties compared with older deposits, implying either that the 2006 winter–spring flood sediment originated from one or more distinct upland settings, or that substantial mixing of sediment from multiple sources occurred during transport. Sediment is most readily remobilized and advected out of the bay during winter, when oceanic conditions are energetic. In summer, wave and current measurements made concurrently with this study showed weak currents and little wave energy, indicating that sediment delivered during summer floods most likely remains in the bay until winter storms can remove it. Increased turbidity and sedimentation on corals resulting from floods of the Hanalei River could affect the sustainability of coral reefs and their many associated species. This possibility is of particular concern during summer months when wave energy is low and sediment is not readily remobilized and transported out of the bay. The timing (seasonality) and magnitude of sediment input to the coastal ocean relative to seasonal variations in wave and current energy could have significant ecological consequences for coral-reef communities in the Hawaiian Islands.

Hawai'i↗
USGS2008

Multibeam Sonar Mapping and Modeling of a Submerged Bryophyte Mat in Crater Lake, Oregon

Traditionally, multibeam data have been used to map sea floor or lake floor morphology as well as the distribution of surficial facies in order to characterize the geologic component of benthic habitats. In addition to using multibeam data for geologic studies, we want to determine if these data can also be used directly to map the distribution of biota. Multibeam bathymetry and acoustic backscatter data collected in Crater Lake, Oregon, in 2000 are used to map the distribution of a deep-water bryophyte mat, which will be extremely useful for understanding the overall ecology of the lake. To map the bryophyte's distribution, depth range, acoustic backscatter intensity, and derived bathymetric index grids are used as inputs into a hierarchical decision-tree classification model. Observations of the bryophyte mat from over 23 line kilometers of lake-floor video collected in the summer of 2006 are used as controls for the model. The resulting map matches well with ground-truth information and shows that the bryophyte mat covers most of the platform surrounding Wizard Island as well as on outcrops around the caldera wall.

Data Series↗
USGS2013

Geology, sequence stratigraphy, and oil and gas assessment of the Lewis Shale Total Petroleum System, San Juan Basin, New Mexico and Colorado: Chapter 5 in Total petroleum systems and geologic assessment of undiscovered oil and gas resources in the San Juan Basin Province, exclusive of Paleozoic rocks, New Mexico and Colorado

The Lewis Shale Total Petroleum System (TPS) in the San Juan Basin Province contains a continuous gas accumulation in three distinct stratigraphic units deposited in genetically related depositional environments: offshore-marine shales, mudstones, siltstones, and sandstones of the Lewis Shale, and marginal-marine shoreface sandstones and siltstones of both the La Ventana Tongue and the Chacra Tongue of the Cliff House Sandstone. The Lewis Shale was not a completion target in the San Juan Basin (SJB) in early drilling from about the 1950s through 1990. During that time, only 16 wells were completed in the Lewis from natural fracture systems encountered while drilling for deeper reservoir objectives. In 1991, existing wells that penetrated the Lewis Shale were re-entered by petroleum industry operators in order to fracture-stimulate the Lewis and to add Lewis gas production onto preexisting, and presumably often declining, Mesaverde Group production stratigraphically lower in the section. By 1997, approximately 101 Lewis completions had been made, both as re-entries into existing wells and as add-ons to Mesaverde production in new wells. Based on recent industry drilling and completion practices leading to successful gas production from the Lewis and because new geologic models indicate that the Lewis Shale contains both source rocks and reservoir rocks, the Lewis Shale TPS was defined and evaluated as part of this U.S. Geological Survey oil and gas assessment of the San Juan Basin. Gas in the Lewis Shale Total Petroleum System is produced from shoreface sandstones and siltstones in the La Ventana and Chacra Tongues and from distal facies of these prograding clastic units that extend into marine rocks of the Lewis Shale in the central part of the San Juan Basin. Reservoirs are in shoreface sandstone parasequences of the La Ventana and Chacra and their correlative distal parasequences in the Lewis Shale where both natural and artificially enhanced fractures produce gas. The Lewis Continuous Gas Assessment Unit (AU 50220261) is thought to be self-sourced from and self-sealed by marine shales and mudstones deposited within the Lewis Shale that enclose clastic parasequences in the La Ventana and Chacra Tongues. The gas resource is thought to be a continuous accumulation sourced from the Lewis Shale throughout the depositional basin. In the Lewis Continuous Gas Assessment Unit (AU 50220261), for continuous gas resources, there is an F95 of 8,315.22 billion cubic feet of gas (BCFG) and an F5 of 12,282.31 BCFG, with a mean value of 10,177.24 BCFG. There is an F95 of 18.08 million barrels of natural gas liquids (MMBNGL) and an F5 of 47.32 MMBNGL, with a mean of 30.53 MMBNGL.

Colorado;New Mexico↗
USGS2007

Subsurface stratigraphic cross sections of cretaceous and lower tertiary rocks in the Wind River Basin, central Wyoming: Chapter 9 in Petroleum systems and geologic assessment of oil and gas resources in the Wind River Basin Province, Wyoming

The stratigraphic cross sections presented in this report were constructed as part of a project conducted by the U.S. Geological Survey to characterize and evaluate the undiscovered oil and gas resources of the Wind River Basin (WRB) in central Wyoming. The primary purpose of the cross sections is to show the stratigraphic framework and facies relations of Cretaceous and lower Tertiary rocks in this large, intermontane structural and sedimentary basin, which formed in the Rocky Mountain foreland during the Laramide orogeny (Late Cretaceous through early Eocene time). The WRB is nearly 200 miles (mi) long, 70 mi wide, and encompasses about 7,400 square miles (mi 2 ) (fig. 1). The basin is structurally bounded by the Owl Creek and Bighorn Mountains on the north, the Casper arch on the east, the Granite Mountains on the south, and the Wind River Range on the west.

Wyoming↗
USGS2014

Maps and geospatial data for the Shorty’s Island and Myrtle Bend substrate enhancement pilot projects, Kootenai River near Bonners Ferry, Idaho, 2014

The U.S. Geological Survey, in cooperation with the Idaho Department of Fish and Game, conducted a study to characterize the physical habitat occupied by Kootenai River white sturgeon during spawning and early-life phases. The objective was to gain a better understanding of spawning behavior, site selection, and type of habitat used during egg incubation in two sub-reaches of the Kootenai River. Habitat characterizations generated by this study will assist in the design of a substrate enhancement pilot project. This report presents the methods used to develop georeferenced portable document format maps and geospatial data that describe spawning locations and physical habitat characteristics (including egg mat locations, bathymetry, surficial sediment facies, and streamflow velocity) within the substrate enhancement pilot project study area. The results are presented as two maps illustrating the physical habitat characteristics along with proposed habitat enhancement areas, aerial imagery, and hydrography. The results of this study will assist researchers, policy makers, and management agencies in deciding the spatial location and extent of the substrate enhancement pilot project.

Idaho↗
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↗
USGS1997

Hydrogeologic framework of western Cape Cod, Massachusetts

The aquifer of western Cape Cod consists of several hydrogeologic units composed of sand, gravel, silt, and clay (fig. 1) that were deposited during the late Wisconsinan glaciation of New England. The aquifer is a shallow, unconfined hydrologic system in which ground-water flows radially outward from the apex of the ground-water mound near the center of the peninsula toward the coast (fig.2). The aquifer is the sole source of water supply for the towns of Bourne, Sandwich, Falmouth, and Mashpee, and the Massachusetts Military Reservation (MMR). Previous geologic studies summarized the characteristics and relative ages of the glacial moraines and meltwater deposits and the relation of these sediments to the extent of the ice-sheet lobes during the last glaciation of southern New England (Oldale and Barlow, 1986; Hartshorn and others, 1991). Hydrogeologic studies in western Cape Cod characterized the shallow regional ground-water-flow system (LeBlanc and others, 1986) and analyzed simulated responses of the aquifer to changes in hydrologic stresses (Guswa and LeBlanc, 1985; Barlow and Hess, 1993; Masterson and Barlow, 1994; and Masterson and others, 1996). Recent concerns about widespread ground-water contamination, especially from sources on the MMR, have resulted in extensive investigations to characterize the local hydrogeology of the aquifer near the MMR (ABB Environmental Services, 1992). Masterson and others (1996) illustrated the strong influence of geology on ground-water flow and the importance of characterizing the hydrogeology to predict the migration of the contaminant plumes beneath the MMR. This report, a product of a cooperative study between the National Guard Bureau and the U.S. Geological Survey (USGS), characterizes the regional hydrogeology of the western Cape Cod aquifer on the basis of surficial glacial geology previously described by Mather and others (1940) and Oldale and Barlow (1986), and presents a new analysis of the subsurface hydrogeology. The characterization of the regional hydrogeologic framework includes a detailed analysis of the glacial sediments, including deltaic and lacustrine deposits and their sedimentary facies; a structure-contour analysis of the transition between the shallow sand and gravel deposits and the underlying fine sand, silt, and clay deposits; and a summary of the relation between lithologic characteristics (grain size and stratigraphy) and empirically determined hydraulic-conductivity values.

Massachusetts↗
USGS1989

Geologic map of the Latir Volcanic Field and adjacent areas, northern New Mexico

This map was first published as a printed edition in 1989. The geologic data have now been captured digitally and are presented here along with images of the printed map sheet and component parts as PDF files. This map encompasses all or parts of ten 7.5 minute quadrangles in the Taos Range of the Sangre de Cristo Mountains in northern New Mexico. Geologic mapping was initiated in this area by the U.S. Geological Survey in response to its mandate under the Wilderness Act of 1964 to evaluate the mineral resource potential of the Latir and Wheeler Peak Wildernesses and the Columbine-Hondo Wilderness Study Area in the Carson National Forest. The mapping was later extended to adjacent areas in order to better understand the regional geology and geologic history of the range. The present map focuses on the early Tertiary (largely Oligocene) Latir Volcanic Field and plutonic rocks associated with it. The basement rocks in the map area are Paleoproterozic amphibolite-facies metasedimentary and metavolcanic rocks intruded by large bodies of quartz monzonite, granodiorite and gabbro dated at between 1,750 and 1,690 Ma. The basement rocks are locally overlain by Mississippian limestone or by Pennsylvanian and Permian redbeds. The basement rocks were thrust eastward across a thick section of similar late Paleozoic rocks along low-angle faults during the Laramide Orogeny. Post-Laramide erosion largely removed the sedimentary cover from the basement rocks and reduced the topography to a low-relief surface locally overlain by scattered lenses of Eocene to Oligocene shale, sandstone, and conglomerate. It was on this surface that the earliest rocks of the Latir Volcanic Field were deposited, beginning in the Oligocene, about 30 Ma. The early volcanic rocks comprise a thick sequence of andesite, dacite, and minor rhyolite, deposited as flows, breccia, and volcaniclastic sediments from numerous local volcanic centers; thin layers of rhyolite tuff are from distant eruptions, some probably in the San Juan Mountains. These volcanic rocks probably record early growth of an upper-crustal batholith. At 25 Ma enormous eruptions of peralkaline rhyolite ash-flow tuff were accompanied by collapse of the roof of the growing batholith to form the Questa Caldera, a volcanic depression at least 14 km across. The ash flows traveled for tens of kilometers from the caldera rims to form a widespread sheet of densely welded tuff; ash also fell back to fill the subsiding caldera depression. Concurrent extension along northwest-trending faults segmented the volcanic edifice and rotated the originally sub-horizontal layers, so that some are nearly vertical. Formation of the caldera was accompanied by further batholith growth, involving intrusion of large plutons of granite, granodiorite, and related rocks into both caldera fill and the adjacent rocks, including both the earlier volcanic rocks and the basement rocks. The large molybdenum deposit along the Red River east of Questa is related to the emplacement of one of these late granitic plutons. Intrusive activity continued into the Miocene. Development of the Rio Grande Rift beginning at about 15 Ma was accompanied by development of the north-south trending normal faults along the western foot of the Taos Range. The rift is filled with thousands of meters of clastic sediments interleaved with basalt flows, some as young as 3.6 Ma. The bounding faults of the rift cut the Questa Caldera so that the western part of the original structure is now deeply buried beneath the rift fill.

New Mexico↗
USGS1995

Geologic map of Arsia Mons Volcano, Mars

The geologic map shows the flanks and summit caldera of Arsia Mons, the southernmost of three large shield volcanoes ( Arsia , Pavonis, and Ascrae us Montes) that form the Tharsis Montes volcanic chain (fig. 1). The volcanoes lie along the cre st of a regional northeast-trending rise that extends more than 3,000 km along or near the planet-wide boundary that separates th e southern highlands from the northern lowlands. Previous geologic mapping of this region (Scott and others, 1981; Scott and Tanaka, 1986) shows six major lava flows that were extruded from the Tharsis volcanoes during the Early Hespe rian to Late Amazonian Epochs; the flows cover and area exceeding 7x10 6 km 2 around the volcanic centers. All of these flows – mapped as members of the Tharsis Montes Formation – are pre sent in the map area. Broad, fan-shaped surficial deposits that resemble landslides extend down the northwest sides of each volcano and of Olympus Mons, about 1,500 km northwest of Arsia Mons (fig. 1). The largest of these gently sloping, lobate deposits is on Arsia Mons, where ambiguous relations bet wee n one facies of the fan-shaped accumulations and underlying topographic features have provoked questions as to the origin of the deposits.

IMAP↗
USGS1998

Geologic map of Pavonis Mons Volcano, Mars

The geologic map shows lava flows and fan-shaped deposits on Pavonis Mons, the central of three large shield volcanoes ( Arsia , Pavonis, and Ascr aeus Montes) that form the Tharsis Montes volcanic chain. The volcanoes lie along the crest of a regional northeast-trending rise that extends more than 3,000 km across the western equatorial region of Mars (fig. 1). The volcanic history of Pavonis Mons is similar to that of other volcanoes in the western equatorial region of Mars (Scott and others, 1981 a-c; Scott and Tanaka, 1981, 1986; Zimbelman and Edgett , 1992). Previous geologic mapping of this region (Scott and others, 1981a-c; Scott and Tanaka, 1981, 1986) shows six major lava flows that were extruded from the Tharsis volcanoes during the Early Hes perian to Late Amazonian Epochs; four of these lava flow members are present in the map area. On the northwest flank of Pavonis Mons, broad, lobate, fan-shaped deposits form a surficial cover similar to other fan-shaped deposits on the northwest flanks of Arsia , Arcaeus , and (to a lesser degree) Olympus Mons. Similar to those of Arsia Mons , the fan-shaped deposits of Pavonis Mons consists of several facies whose origins are attributed to glaciation, mass wasting, and pyroclastic volcanism origins.

IMAP↗
USGS1979

Maps showing late Pleistocene and Holocene evolution of the South Texas continental shelf

Interpretation of acoustical profiles has provided insight into the late Quaternary geologic history of the Continental Shelf off South Texas. (See the geographic index map on sheet 1 for location of the area studied.) The profiles reveal the interplay of tectonism, sedimentation, and cyclic fluctuations of sea level in the building and geologic evolution of the continental terrace. The sequence of sediments studied extends to about 200 meters (m) beneath the sea-floor surface. Four seismic-stratigraphic units underlain by four prominent sound reflectors were identified and mapped. This geologic synthesis, for which the research was funded by the U.S. Bureau of Land Management (BLM), is but one aspect of a coordinated, multidisciplinary environmental study of the South Texas Outer Continental Shelf sponsored by BLM (Berryhill, 1976, 1977). The environmental studies are keyed to the leasing of Federal Outer Continental Shelf (OCS) lands for petroleum exploration and production. Their purpose is to provide the data development of petroleum resources on the OCS, as well as to provide the basis for predicting the impact of oil and gas exploration and production on the marine environment. Of primary concern is the recognition of geologic conditions that might be hazardous to structures placed on the sea floor. Geologic hazards relate directly to the potential for significant movement of the sea floor in the future. Judging sea-floor stability and recognizing geologic features that are potentially hazardous require an understanding of the recent geologic history of the area, which, in turn, entails determining the relative rates and interactions of sedimentation and tectonism through time. In addition to the primary objective, the synthesis provides knowledge about the magnitude and extent of sea-level fluctuations in the western Gulf of Mexico, and it provides a depositional facies model of possible use in appraising the resource potential of the more deeply buried sediments.

Texas↗
USGS1984

Geochronometric and lead isotope data on samples from the Wallace 1 degree by 2 degrees Quadrangle, Montana and Idaho

Most of the bedrock in the Wallace quadrangle belongs to the Belt Supergroup, a thick (about 18,000 m) sequence of generally fine-grained clastic and carbonate rocks of Middle Proterozoic age. Regional metamorphism prior to Cambrian time prograded the Belt rocks to greenschist facies, and some metal-bearing veins were emplaced in fractures. The Belt rocks were intruded in Late Proterozoic time by basic dikes and sills.

Idaho, Montana↗
USGS1982

Stratigraphic framework and coal correlation of the Upper Cretaceous Fruitland Formation, Bisti-Ah-Shi-Sle-Pah area, San Juan Basin, New Mexico

This report illustrates and describes the detailed stratigraphic framework and coal correlation of the Upper Cretaceous Fruitland Formation exposed in isolated badlands and along washes within a 20-mile outcrop belt in the Bisti-Ah-Shi-Sle-Pah area, southwestern San Juan Basin, Nex Mexico (see index). The stratigraphic framework showing the vertical and lateral distributions of rock types and the lateral continuity of coal beds is illustrated in cross sections. The cross sections were constructed from 112 stratigraphic sections measured at an average distance of 0.4 mi apart. Each section contained key marker beds (sandstone, coal, and tonstein) that were physically traced to adjacent sections. Each measured section was "hung" on multiple marker beds arranged in a geometric best-fit method that accounts for the differential compaction and facies associations of the deposits.

New Mexico↗
USGS1986

Median-permeability contour maps of the J sandstone, Dakota Group, in the Denver Basin, Colorado, Nebraska, and Wyoming

The Lower Cretaceous J sandstone of the Dakota Group (MacKenzie, 1965) is present in the Denver basin in eastern Colorado, southeastern Wyoming, and southwestern Nebraska. This informally named unit deposited during a regression of the Cretaceous epi continental sea and is composed primarily of sandstone and shale of deItaic and near-shore marine origin. The J sandstone can be divided into an upper transgressive sand, a middle marginal-marine and deltaic facies, and a lower prodelta sequence (Clark, 1978). The depth from the surface to the top of the J sandstone increases from about 4,000 ft on the gently-dipping eastern flank of the bass n to more than 8,000 ft near the steeply-dipping flank at the western boundary. Permeability values compiled in this J sandstone study were determined from cores from 134 widely scattered boreholes. Median permeabilities, rather than average permeabilities, were used in order to minimize the effect of anomalous samples. Thirty-five oil companies and independent operators supplied core data. Core Laboratories of Denver, Colorado analyzed the core, which was submitted over a period of 25 years.

Colorado, Nebraska, Wyoming↗
USGS1992

Metallogenic map of volcanogenic massive sulfide occurrences in Wyoming

Volcanogenic massive sulfide occurrences of Late Archean age and Early Proterozoic age occur in metavolcanic and associated metasedimentary rocks in southeastern, eastern, and central Wyoming. Vein deposits in Archean volcanic terrane have accounted for a large part of the gold production in the State (Miners Delight, locality 8). Past production from all massive sulfide occurrences in the State is poorly known, but it probably did not exceed $13 million at current metal prices. The bulk of the production was from gold mines in the southern Wind River Range (localities 8 and 17). For the most part, the host rocks of the massive sulfide occurrences have been metamorphosed to amphibolite facies. As a result, the depositional environments are, in many cases, difficult to determine. For purposes of discussion, the occurrences are classified by geologic age.

Wyoming↗
USGS1987

Median-porosity contour maps of the J Sandstone, Dakota Group, in the Denver Basin, Colorado, Nebraska, and Wyoming

The Lower Cretaceous J sandstone of the Dakota Group is present in the Denver basin in eastern Colorado, southeastern Wyoming, and southwestern Nebraska. Deposited during a regression of the Cretaceous epicontinental sea, this informally named unit is composed primarily of sandstone and shale of deltaic and near shore-marine origin. The J sandstone can be divided into an upper transgressive sand, a middle marginal-marine and deltaic facies, and a lower prodelta sequence (Clark, 1978). The depth from the surface to the top of the J sandstone increases from about 4,000 ft on the gently-dipping eastern flank of the basin to more than 8,000 ft at the basin ax is near the steeply-dipping western flank. Porosity data compiled in this study were determined from J sandstone cores from 134 widely spaced boreholes. Porosity in areas of poor core coverage was determined from neutron density logs from an additional 20 boreholes ( corrected to core average grain density). Median, rather than average, porosity was used in order to minimize the statistical effect of anomalously high and low porosity values. Thirty-five oil companies and independent operators supplied core porosity data. Core porosities were determined by means of helium porosimetry, primarily by Core Laboratories of Denver, Colo.

Colorado, Nebraska, Wyoming↗
USGS1995

Photogeologic map showing distribution of sinkholes south of Fairplay, Park County, Colorado — A possible geologic hazard

A large group of at least 50, and perhaps significantly more, sinkholes partially surrounds Black Mountain 6-10 mi south Fairplay in South Park, Park County, Colorado. The sinkholes occur in bedrock in the evaporite facies of the Middle Pennsylvania Minturn Formation, and in Quaternary soil, alluvium, and glacial outwash gravels that overlie the evaporite beds. Sinkholes range in size from small depressions a few feet across to large holes several hundred feet across. Measured sinkholes range in size from about 25 ft in diameter and 2 ft deep to about 235 ft in diameter and 25 ft deep. In places, several sinkholes have coalesced to form depressions as much as 750 ft long and 400 ft wide. One large cluster of small craters is about 1,8000 ft long and 600 ft wide. As reported to us by a resident rancher, one small sinkhole collapsed about 10 years ago. The area of sinkholes extends into land now under development for residences, and the sinkholes thus pose a potential hazard that needs to be considered in future development. Also, they might jeopardize existing farmland, structures, ans roads (including U.S. Highway 285), as well as projected roads and airstrips. This report is not a comprehensive evaluation of the distribution and origin of the sinkholes; its intent is to call attention to their presence and to encourage further study. Many by not all of the sinkholes were visited; the geologic map is based mainly on the interpretation of aerial photographs by D.R. Shawe.

Colorado↗