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At least 289 records · Page 16Linked to original sources

USGS2015

Underpressure in Mesozoic and Paleozoic rock units in the Midcontinent of the United States

Potentiometric surfaces for Paleozoic strata, based on water well levels and selected drill-stem tests, reveal the control on hydraulic head exerted by outcrops in eastern Kansas and Oklahoma. From outcrop in the east, the westward climb of hydraulic head is much less than that of the land surface, with heads falling so far below land surface that the pressure:depth ratio in eastern Colorado is less than 5.7 kPa/m (0.25 psi/ft). Permian evaporites separate the Paleozoic hydrogeologic units from a Lower Cretaceous (Dakota Group) aquifer, and a highly saline brine plume pervading Paleozoic units in central Kansas and Oklahoma is attributed to dissolution of Permian halite. Underpressure also exists in the Lower Cretaceous hydrogeologic unit in the Denver Basin, which is hydrologically separate from the Paleozoic units. The data used to construct the seven potentiometric surfaces were also used to construct seven maps of pressure:depth ratio. These latter maps are a function of the differences among hydraulic head, land-surface elevation, and formation elevation. As a consequence, maps of pressure:depth ratio reflect the interplay of three topologies that evolved independently with time. As underpressure developed, gas migrated in response to the changing pressure regime, most notably filling the Hugoton gas field in southwestern Kansas. The timing of underpressure development was determined by the timing of outcrop exposure and tilting of the Great Plains. Explorationists in western Kansas and eastern Colorado should not be surprised if a reservoir is underpressured; rather, they should be surprised if it is not.

Arkansas, Colorado, Kansas, Iowa, Missouri, Nebras↗

USGS1988

Hydrodynamics of Denver basin: Explanation of subnormal fluid pressures

Anomalously low fluid potential (and hence subnormal fluid pressure) is found in Mesozoic and Paleozoic rocks of the Denver basin. The potentiometric surface for the Dakota and basal Cretaceous sandstones is 2,000-3,000 ft (600-900 m) beneath the land surface in parts of the Denver basin in Colorado and Nebraska. The potentiometric surface for pre-Pennsylvanian carbonate rocks is 1,500 ft (450 m) lower than the potentiometric surface for the Dakota Sandstone in southeastern Colorado and western Kansas. The low fluid potential seems especially anomalous considering the high elevation of the outcrops along the Laramie and Front Ranges and the Black Hills. A quasi-three-dimensional numerical flow model is used to investigate the regional flow system in the Denver basin and adjacent Mid-Continent. The model simulates flow through the entire Phanerozoic sedimentary column and indicates that subnormal pressures are a consequence of hydraulic insulation of the strata within the basin from their recharge zones as compared to their discharge zones. The Dakota Sandstone and underlying hydrostratigraphic units are insulated from the overlying water table by low-permeability shales of Cretaceous age, and from their own high-elevation outcrops by a zone of low permeability coincident with the basin deep. Subnormal pressures in the area of Denver, Colorado, and southward are further enhanced by faulting along the Front Range that isolates the stra a within the basin from their outcrops. The results of this study show that (1) subnormal fluid pressures can be explained as a consequence of steady-state regional ground-water flow, (2) shale is an important factor in the regional flow system, and (3) depth is an important control on the distribution of hydraulic conductivity.

Colorado, Iowa, Kansas, Nebraska, South Dakota, Wy↗

USGS2006

Regional and local species richness in an insular environment: Serpentine plants in California

We asked how the richness of the specialized (endemic) flora of serpentine rock outcrops in California varies at both the regional and local scales. Our study had two goals: first, to test whether endemic richness is affected by spatial habitat structure (e.g., regional serpentine area, local serpentine outcrop area, regional and local measures of outcrop isolation), and second, to conduct this test in the context of a broader assessment of environmental influences (e.g., climate, soils, vegetation, disturbance) and historical influences (e.g., geologic age, geographic province) on local and regional species richness. We measured endemic and total richness and environmental variables in 109 serpentine sites (1000-m2 paired plots) in 78 serpentine-containing regions of the state. We used structural equation modeling (SEM) to simultaneously relate regional richness to regionalscale predictors, and local richness to both local-scale and regional-scale predictors. Our model for serpentine endemics explained 66% of the variation in local endemic richness based on local environment (vegetation, soils, rock cover) and on regional endemic richness. It explained 73% of the variation in regional endemic richness based on regional environment (climate and productivity), historical factors (geologic age and geographic province), and spatial structure (regional total area of serpentine, the only significant spatial variable in our analysis). We did not find a strong influence of spatial structure on species richness. However, we were able to distinguish local vs. regional influences on species richness to a novel extent, despite the existence of correlations between local and regional conditions. ?? 2006 by the Ecological Society of America.

California↗

USGS2014

Layered hydrothermal barite-sulfide mound field, East Diamante Caldera, Mariana volcanic arc

East Diamante is a submarine volcano in the southern Mariana arc that is host to a complex caldera ~5 × 10 km (elongated ENE-WSW) that is breached along its northern and southwestern sectors. A large field of barite-sulfide mounds was discovered in June 2009 and revisited in July 2010 with the R/V Natsushima , using the ROV Hyper-Dolphin . The mound field occurs on the northeast flank of a cluster of resurgent dacite domes in the central caldera, near an active black smoker vent field. A 40 Ar/ 39 Ar age of 20,000 ± 4000 years was obtained from a dacite sample. The mound field is aligned along a series of fractures and extends for more than 180 m east-west and >120 m north-south. Individual mounds are typically 1 to 3 m tall and 0.5 to 2 m wide, with lengths from about 3 to 8 m. The mounds are dominated by barite + sphalerite layers with the margins of each layer composed of barite with disseminated sulfides. Rare, inactive spires and chimneys sit atop some mounds and also occur as clusters away from the mounds. Iron and Mn oxides are currently forming small (<1-m diam, ~0.5-m tall) knolls on the top surface of some of the barite-sulfide mounds and may also drape their flanks. Both diffusely and focused fluids emanate from the small oxide knolls. Radiometric ages of the layered barite-sulfide mounds and chimneys vary from ~3,920 to 3,350 years. One layer, from an outcrop of 10- to 100-cm-thick Cu-rich layers, is notably younger with an age of 2,180 years. The Fe-Mn oxides were <5 years old at the time of collection in 2009. Most mound, chimney, and layered outcrop samples are dominated by barite, silica, and sphalerite; other sulfides, in decreasing order of abundance, are galena, chalcopyrite, and rare pyrite. Anglesite, cerussite, and unidentified Pb oxychloride and Pb phosphate minerals occur as late-stage interstitial phases. The samples contain high Zn (up to 23 wt %), Pb (to 16 wt %), Ag (to 487 ppm), and Au (to 19 ppm) contents. Some layered outcrop samples are dominated by chalcopyrite resulting in ≤4.78 wt % Cu in a bulk sample (28 wt % for a single lens), with a mean of 0.28 wt % for other samples. Other significant metal enrichments are Sb (to 1,320 ppm), Cd (to 1,150 ppm), and Hg (to 55 ppm). The East Diamante mound field has a unique set of characteristics compared to other hydrothermal sites in the Mariana arc and elsewhere. The geochemical differences may predominantly reflect the distribution of fractures and faults and consequently the rock/water ratio, temperature of the fluid in the upper parts of the circulation system, and extensive and prolonged mixing with seawater. The location of mineralization is controlled by fractures. Following resurgent doming within the caldera, mineralization resulted from focused flow along small segments of linear fractures rather than from a point source, typical of hydrothermal chimney fields. Based on the mineral assemblage, the maximum fluid temperatures were ~260°C, near the boiling point for the water depths of the mound field (367–406 m). Lateral fluid flow within the mounds precipitated interstitial sphalerite, silica, and Pb minerals within a network of barite with disseminated sulfides; silica was the final phase to precipitate. The current low-temperature precipitation of Fe and Mn oxides and silica may represent rejuvenation of the system.

Economic Geology↗

USGS1950

Dispersion of copper from the San Manuel copper deposit, Pinal County, Arizona

At San Manuel, near Tucson, Arizona, recent churn drilling has blocked out large reserves of low-grade "porphyry copper" ore. This virgin deposit has a small outcrop and seems ideally suited for a geochemical study of the dispersion pattern produced by weathering in a desert climate. Samples of soils, alluvium, ground water, and vegetation were analyzed for copper. To avoid grinding, to accentuate differences in copper concentration, and to decrease sampling error, sampling was confined to the silt and clay fraction of the soil and alluvium. A sensitive field test for copper, using hydrochloric acid for a digestant and dithizone for copper determination, proved both quick and reliable. The results of the study show that at present little copper from the ore body dissolves in the ground water or runoff, and also that very little is taken up by plants growing on the ore outcrop. For this reason, prospecting for similar deposits by the sampling and analysis of ground water or vegetation is unlikely to be fruitful. A study of plant ecology, however, shows that certain plant species grow preferentially on outcrops of copper ore and may be useful as indicators of ore. Chrysocolla along joints carries most of the copper in the oxidized zone. The chrysocolla is slowly disintegrated mechanically as the rock weathers and enters the fine fraction of the soil. Slope wash and soil creep carry the copper along with the other soil materials into the nearby washes, where they are incorporated in the alluvium and swept downstream during floods. Abrasion probably continues to concentrate the copper in the fines, but the net change downstream is a decrease in copper content caused by dilution. As copper can now be readily determined by chemical analysis in the field, analysis of soil and alluvium seems to be one of the best geochemical methods of prospecting for copper in a desert environment. Wherever appreciable copper is found in alluvium or soils, upstream or upslope sampling can be used to trace the copper back to its source.

Arizona↗

USGS2023

Designation of a composite-stratotype section for the lower Paleocene (Danian) Brightseat Formation in Prince George’s County, Maryland, U.S.A.

The lower Paleocene (Danian) Brightseat Formation consists of fine-grained, dark-gray, micaceous sand and silty clay, with glauconite and abundant, but generally poorly preserved, fossils. The Brightseat Formation represents deposition of lower, but not lowermost, Paleocene sediments that were deposited on the middle to outer shelf of what is now the Atlantic Coastal Plain. A basal disconformity separates the Brightseat from the underlying Upper Cretaceous Severn Formation, and it is disconformably overlain by the middle to upper Paleocene Aquia Formation. The type locality of the Brightseat Formation, previously exposed approximately 1 mile (1.6 km) west-southwest of the former village of Brightseat in Prince George’s County, Maryland, just outside ofWashington, D.C., has been destroyed by urban development, prompting the establishment of a composite-stratotype section from nearby outcrops approximately a half mile to the south-southwest of the original location. The outcrop at Cabin Branch is hereby designated as the principal reference section for the Brightseat Formation and the “Cabin Creek” outcrop (an informally designated tributary of Cabin Branch) is designated as a supplementary reference section that illustrates the boundary of the Brightseat with the overlying Aquia Formation. Paleontological analyses, grain-size analyses, and petrographic techniques are used to document this new stratotype section and to correlate and contrast it with underlying and overlying sediments. This composite-stratotype section is designated using the guidelines outlined in Article 8, remarks (d) and (e) of the North American Code of Stratigraphic Nomenclature.

Maryland↗

USGS1952

Cyclic fluctuations of water level as a basis for determining aquifer transmissibility

In coastal areas, wells near bodies of tidal water frequently exhibit sinusoidal fluctuations of water level, in response to periodic changes of tidewater stage. Inland, the regulation of a surface reservoir often produces correlative changes of ground-water stage in wells adjacent either to the reservoir or to its attendant stream. As the stage of the surface water rises, the head upon the subaqueous outcrop of the aquifer increases and thereby either increases the rate of inflow to the aquifer or reduces the rate of outflow therefrom. The increase in recharge or reduction in discharge results in a general recovery of water level in the aquifer. On the subsequent falling stage this pattern is reversed. When the stage of the surface body fluctuates as a simple harmonic motion a train of sinusoidal waves is propagated shoreward through the sub-outcrop of the aquifer. With increasing distance from the sub-outcrop, the amplitude of the transmitted wave decreases and the time lag of a given maximum or minimum increases.

Report↗

USGS1950

Stratigraphy and structure of the Okpikruak and Kiruktagiak Rivers area, Alaska

Navy Oil Unit Party 4, during the summer field season of 1949, examined the surface geology of the area of the Okpikruak and Kiruktagiak Rivers. The party consisted of six men: two geologists, two field assistants, a weasel mechanic, and a cook. Three weasels were used for transportation in the field. The area covered is bounded on the west by the Okpikruak River, on the east by the Chandler River, and on the south by the north front of the Brooks Range. Tuktu Bluff on the Chandler River and its westward extension forms the northern boundary. The area is drained by the Chandler, Kiruktagiak, Ayiyak, Okokmilaga, and Okpikruak Rivers. The objectives of this summer's work was the geologic mapping and stratigraphic study of the rocks that crop out in this area. All outcrops were visited and the geology was plotted on vertical and trimetrogon oblique aerial photographs. Altitudes were established by an altimeter traverse. Parts of this area had been investigated previously by members of the Navy Oil Unit. During the summer of 1945 George Gryc, E. J. Webber, and Karl Stefansson visited outcrops along the. Chandler and Kiruktagiak Rivers and in the vicinity of Castle Mountain. In the same year L. A. Warner and C. E. Kirschner examined outcrops along the Okpikruak River in conjunction with their survey of the Klink and Colville Rivers. R. L. Detterman included detailed stratigraphic studies of cutbanks near the confluence of the Kiruktagiak and Chandler Rivers in his geological mapping of the lower Chandler during the summer of 1948.

Alaska↗

USGS1963

Geology of the Jewel Cave SW Quadrangle, Custer County, South Dakota

The Jewel Cave SW quadrangle is in the southwestern part of the Black Hills in Custer County, S. Dak., about midway between Edgemont, S. Dak., and Newcastle, Wyo. All the rocks that crop out within the quadrangle are of sedimentary origin and range in age from Pennsylvanian to Early Cretaceous. The Minnesota Formation of Pennsylvania and Permian age, which is about 1,000 feet thick, was studied in outcrop and from two diamond-drill cores. In the subsurface the upper part of the formation consists of gray sandstone, very fine grained dolomite, and anhydrite. The anhydrite has been leached from the formation near the outcrop, perhaps in the early part of the Cenozoic Era, and the resulting subsidence has produced collapse breccias in the Minnelusa and milder deformation in the overlying units. In the collapse breccias the rocks have been oxidized and are red, whereas in the subsurface they are gray. The anhydrite cement of the subsurface sandstone has been replaced by calcite, and the dolomite beds have been partially converted to limestone. The Opeche Formation of Permian age consists of 75 to 115 feet of red siltstone and shale and two thin gypsum beds. The Minnekahta Limestone of Permian age is about 40 feet thick. The Spearfish Formation of Permian and Triassic age is about 550 feet thick and consists of red siltstone red sandstone, dolomite, and gypsum. The dolomite and gypsum beds are restricted to the lower half of the formation. In the northeast corner of the quadrangle the gypsum beds have been dissolved by ground water. The Sundance Formation of Late Jurassic age is divided into five members that have a total thickness of about 360 feet. The Morrison Formation of Late Jurassic age ranges in thickness from 60 to 120 feet. It consists of blocky weathering noncarbonaceous mudstone and subordinate beds of limestone and sandstone. The Inyan Kara Group of Early Cretaceous age has been subdivided into the Lakota Formation and the Fall River Formation. The Lakota Formation consists of 200 to 300 feet of carbonaceous siltstone blocky-weathering claystone, and fine-grained to conglomeratic sandstone. These rocks were deposited in stream channels, flood plains, and ponds. The Fall River Formation is about 110 to 130 feet thick. Along the northeast side of the outcrop the formation consists of fine- to medium-grained sandstone, which forms an elongate body at least 1-1/2 miles wide and more than 25 miles long. To the southwest the formation consists of thinly stratified interbedded sandstone, carbonaceous siltstone, and varicolored mudstone. The Skull Creek and Mowry Shales of Early Cretaceous age consist of black fissile shale. The Mowry contains abundant fish scales and weathers to a silver gray. Alluvium fills the bottom of many intermittent streams, and small gravel-covered terraces mark the former high levels of these streams. Gravel, which caps hills at altitudes of 4,460 to 4,620 feet, is believed to have been deposited by a Pleistocene stream that drained southeastward toward the town of Minnekahta. Many landslides are present along the northward- and eastward-facing scarp of the Inyan Kara hogback. The Dewey fault, trending N. 75 deg E., crosses the quadrangle. It is probably a vertical dip-slip fault, and has an apparent displacement of 250 to 440 feet. Two northwest-trending anticlines are in the quadrangle - one extends from the Edgemont NE quadrangle to near the center of the Jewel Cave SW quadrangle, and the other is limited to the center of the Jewel Cave SW quadrangle. Collapse structures, which were produced by the solution of anhydrite, are (a) breccias in the Minnelusa Formation, (b) limestone-dolomite breccias in the Spearfish Formation, (c) undulations and normal faults in the formations overlying the Minnelusa and (d) breccia pipes that extend upward from the Minnelusa to at least as high as the Lakota Formation. The leaching probably occurred in early Cenozoic time. Minor deformationa

Bulletin↗

USGS2002

Composition of the Rex Chert and associated rocks of the Permian Phosphoria Formation: Soda Springs area, SE Idaho

This study, one in a series, reports bulk chemical and mineralogical compositions, as well as petrographic and outcrop descriptions of rocks collected from three measured outcrop sections of the Rex Chert member of the Phosphoria Formation in SE Idaho. The three measured sections were chosen from ten outcrops of Rex Chert that were described in the field. The Rex Chert overlies the Meade Peak Phosphatic Shale Member of the Phosphoria Formation, the source of phosphate ore in the region. Rex Chert removed as overburden comprises part of the material disposed in waste-rock piles during phosphate mining. It has been proposed that the chert be used to cap and isolate waste piles, thereby inhibiting the leaching of potentially toxic elements into the environment. It is also used to surface roads in the mining district. The rock samples studied here constitute a set of individual chert beds that are representative of each stratigraphic section sampled. The informally named cherty shale member that overlies the Rex Chert in measured section 1 was also described and sampled. The upper Meade Peak and the transition zone to the Rex Chert were described and sampled in section 7. The cherts are predominantly spicularite composed of granular and mosaic quartz, and sponge spicules, with various but minor amounts of other fossils and detrital grains. The cherty shale member and transition rocks between the Meade Peak and Rex Chert are siliceous siltstones and argillaceous cherts with ghosts of sponge spicules and somewhat more detrital grains than the chert. The overwhelmingly dominant mineral is quartz, although carbonate beds are rare in each section and are composed predominantly of calcite and dolomite in addition to quartz. Feldspar, mica, clay minerals, calcite, dolomite, and carbonate fluorapatite are minor to trace minerals in the chert. The mean concentrations of oxides and elements in the Rex Chert and the cherty shale member are dominated by SiO2, which averages 94.6%. Organic-carbon contents are generally very low in the chert, but are up to 1.8 wt. % in cherty shale member samples and up to 3.3% in samples from the transition between the Meade Peak and Rex Chert. Likewise, phosphate (P2O5) is generally low in the chert, but can be up to 3.1% in individual beds. Selenium concentrations in Rex Chert and cherty shale member samples vary from <0.2 to 138 ppm, with a mean concentration of 7.0 ppm. This mean Se content is heavily dependent on two values of 101 and 138 ppm for rocks from the transition zone between the Meade Peak and Rex Chert. Without those two samples, the mean Se concentration would be <1.0 ppm. Other elements of environmental interest, As, Cr, V, Zn, Hg, and Cd, are generally near or below their content in average continental shale. Stratigraphic changes (equivalent to temporal changes in the depositional basin) in chemical composition of rocks are notable either as uniform changes through the sections or as distinct differences in the mean composition of rocks that compose the upper and lower halves of the sections. Q-mode factors are interpreted to represent the following rock and mineral components: chert-silica component consisting of Si (± Ba); phosphorite-phosphate component composed of P, Ca, As, Y, V, Cr, Sr, and La (± Fe, Zn, Cu, Ni, Li, Se, Nd, Hg); shale component composed of Al, Na, Zr, K, Ba, Li, and organic C (± Ti, Mg, Se, Ni, Fe, Sr, V, Mn, Zn); carbonate component (dolomite, calcite, silicified carbonates) composed of carbonate C, Mg, Ca, and Si (± Mn); tentatively organic matter-hosted elements (and/or sulfide-sulfate phases) composed of Cu (± organic C, Zn, Mn Si, Ni, Hg, and Li). Selenium shows a dominant association with the shale component, but correlations and Qmode factors also indicate that organic matter (within the shale component) and carbonate fluorapatite may host a portion of the Se. Consideration of larger numbers of factors in Qmode analysis indicates that native Se (a factor containing Se (± Ba)) may also comprise a minor component of the Se compliment.

Idaho↗

USGS2006

Gravity investigations of the Chickasaw National Recreation Area, south-central Oklahoma

The geological configuration of the Arbuckle Uplift in the vicinity of Chickasaw National Recreation Area in south-central Oklahoma plays a governing role in the distribution of fresh and mineral springs within the park and in the existence of artesian wells in and around the park. A confining layer of well-cemented conglomerate lies immediately below the surface of the recreation area, and groundwater migrates from an area of meteoric recharge where rocks of the Arbuckle-Simpson Aquifer crop out as close as two kilometers to the east of the park. Prominent, Pennsylvanian-aged faults are exposed in the aquifer outcrop, and two of the fault traces project beneath the conglomerate cover toward two groups of springs within the northern section of the park. We conducted gravity fieldwork and analysis to investigate the subsurface extensions of these major faults beneath Chickasaw National Recreation Area. By defining gravity signatures of the faults where they are exposed, we infer that the Sulphur and Mill Creek Faults bend to the south-west where they are buried. The South Sulphur Fault may project westward linearly if it juxtaposes rocks that have a density contrast opposite that of that fault's density configuration in the Sulphur Syncline area. The Sulphur Syncline, whose eastern extent is exposed in the outcrop area of the Arbuckle-Simpson Aquifer, does not appear to extend beneath Chickasaw National Recreation Area nor the adjacent City of Sulphur. The South Sulphur Fault dips steeply northward, and its normal sense of offset suggests that the Sulphur Syncline is part of a graben. The Mill Creek Fault dips vertically, and the Reagan Fault dips southward, consistent with its being mapped as a thrust fault. The Sulphur and Mill Creek Synclines may have formed as pull-apart basins in a left-lateral, left-stepping strike-slip environment. The character of the gravity field of Chickasaw National Recreation Area is different from the lineated gravity field in the area of Arbuckle-Simpson Aquifer outcrop. This change in character is not due to the presence of the overlying conglomerate layer, which is quite thin (<100 m) in the area of the park with the springs. The presence of relatively high-density Precambrian basement rocks in a broader region suggests that significant gravity anomalies may arise from variations in basement topography. Understanding of the geological configuration of Chickasaw National Recreation Area can be improved by expanding the study area and by investigating complementary geophysical and borehole constraints of the subsurface.

Open-File Report↗

USGS2012

Geology of the Devonian Marcellus Shale--Valley and Ridge province, Virginia and West Virginia--a field trip guidebook for the American Association of Petroleum Geologists Eastern Section Meeting, September 28-29, 2011

Detailed and reconnaissance field mapping and the results of geochemical and mineralogical analyses of outcrop samples indicate that the Devonian shales of the Broadtop Synclinorium from central Virginia to southern Pennsylvania have an organic content sufficiently high and a thermal maturity sufficiently moderate to be considered for a shale gas play. The organically rich Middle Devonian Marcellus Shale is present throughout most of the synclinorium, being absent only where it has been eroded from the crests of anticlines. Geochemical analyses of outcrop and well samples indicate that hydrocarbons have been generated and expelled from the kerogen originally in place in the shale. The mineralogical characteristics of the Marcellus Shale samples from the Broadtop Synclinorium are slightly different from the averages of samples from New York, Pennsylvania, northeast Ohio, and northern West Virginia. The Middle Devonian shale interval is moderately to heavily fractured in all areas, but in some areas substantial fault shearing has removed a regular "cleat" system of fractures. Conventional anticlinal gas fields in the study area that are productive from the Lower Devonian Oriskany Sandstone suggest that a continuous shale gas system may be in place within the Marcellus Shale interval at least in a portion of the synclinorium. Third-order intraformational deformation is evident within the Marcellus shale exposures. Correlations between outcrops and geophysical logs from exploration wells nearby will be examined by field trip attendees.

Virginia;West Virginia↗

USGS2015

Geochemical and mineralogical sampling of the Devonian shales in the Broadtop synclinorium, Appalachian basin, in Virginia, West Virginia, Maryland, and Pennsylvania

Reconnaissance field mapping and outcrop sampling for geochemical and mineralogical analyses indicate that the Middle Devonian Marcellus Shale in the Broadtop synclinorium and nearby areas from southeastern West Virginia to south-central Pennsylvania has an organic content sufficiently high and a thermal maturity sufficiently moderate to be considered for a shale gas play. The organic matter-rich Marcellus Shale is present throughout most of the synclinorium, being absent only where it has been eroded from the crest of anticlines. Geochemical analyses of outcrop and well drill-cuttings samples indicate that variable levels of hydrocarbons have been generated and expelled from the kerogen originally in place in the mudstone. The mineralogical characteristics of the Marcellus Shale samples from the study area are conducive to a continuous resource play, but the Middle Devonian strata are folded, faulted, and moderately to heavily sheared in the Broadtop synclinorium. The presence of conventional anticlinal gas fields in the study area that are productive from the underlying Lower Devonian Oriskany Sandstone suggests that an unconventional (or continuous) shale gas system may be in place within the Marcellus Shale in the study area. Results of this study indicate that the Marcellus Shale in the Broadtop synclinorium generally is similar in organic geochemical nature throughout its extent, and based on the sample analyses, there are no clearly identifiable high potential areas (or “sweet spots”) in the study area. This report contains analyses of 132 outcrop and well drill-cuttings samples.

Maryland, Pennsylvania, Virginia, West Virginia↗

USGS2018

Digital representation of exposures of Precambrian bedrock in parts of Dickinson and Iron Counties, Michigan, and Florence and Marinette Counties, Wisconsin

The U.S. Geological Survey (USGS) conducted a program of bedrock geologic mapping in much of the central and western Upper Peninsula of Michigan from the 1940s until the late 1990s. Geologic studies in this region are hampered by a scarcity of bedrock exposures because of a nearly continuous blanket of unconsolidated sediments resulting from glaciation of the region during the Pleistocene ice ages. The USGS mapping, done largely at a scale of 1:24,000, routinely recorded the location and extent of exposed bedrock to provide both an indication of where direct observations were made and a guide for future investigations to expedite location of observable rock exposures. The locations of outcrops were generally shown as colored or patterned overlays on printed geologic maps. Although those maps have been scanned and are available as Portable Document Format (PDF) files, no further digital portrayal of the outcrops had been done. We have conducted a prototype study of digitizing and improving locational accuracy of the outcrop locations in parts of Dickinson County, Michigan, to form a data layer that can be used with other data layers in geographic information system applications.

Michigan, Wisconsin↗

USGS1975

Palynological and stratigraphic investigations of four deep wells in the Salisbury Embayment of the Atlantic Coastal Plain

Use of palynological zones defined by Brenner and Doyle from the Cretaceous outcrop belt in Maryland, Delaware, and New Jersey permit correlation of the Cretaceous section in four wells drilled to basement in the Salisbury Embayment in the Eastern Shore of Maryland and in Accomack Co., Virginia. These correlations, supplemented by electric log correlations between wells, clarify the Mesozoic depositional history of the outer margin of the Atlantic Coastal Plain. The bulk of the Cretaceous section consists of continental to marginal is marine sediments correlative with and probably older than the outcrop Potomac Group (Aptian-lower Cenomanian?). Lower Cretaceous palynlogical assemblages also occur in at least the top of basal "red beds" considered Triassic or Jurassic by earlier authors. Dinoflagellates and lithological criteria indicate several transgressive-regressive cycles within the generally transgressive Potomac sequence, with maximum marine incursions in the Late Albian and the Early Cenomanian. Equivalents of the lower Raritan Formation of New Jersey (middle-upper Cenomanian?), absent at the Maryland outcrop, are recognized for the first time in the Taylor and Bethards wells. Potomac and lower Raritan equivalents are separated from overlying Magothy equivalents (upper Santonian-lower Campanian) by a major regional unconformity. Well data suggest that further downdip, under the continental shelf, most of the formations should be marine and contain abundant organic matter. Given sufficient heat and trapping structures, oil and gas may have accumulated.

Open-File Report↗

USGS1976

Textural and mineralogical study of sandstones from the onshore Gulf of Alaska Tertiary Province, southern Alaska

Petrographic examination of 74 outcrop samples of Paleocene through Pliocene age from the onshore Gulf of Alaska Tertiary Province indicates that sandstones of the province characteristically are texturally immature and mineralogically unstable. Diagenetic alteration of framework grains throughout the stratigraphic sequence has produced widespread zeolite cement or phyllosilicate grain coatings and pseudomatrix. Multiple deformation and deep burial of the older Tertiary sequence--the Orca Group, the shale of Haydon Peak, and the Kulthieth and Tokun Formations--caused extensive alteration and grain interpenetration, resulting in low porosity values. Less intense deformation and intermediate depth of burial of the younger Tertiary sequence--the Katalla, Poul Creek, Redwood, and Yakataga Formations--has resulted in a greater range in textural properties. Most sandstone samples in the younger Tertiary sequence are poorly sorted, tightly packed, and have strongly appressed framework grains, but some are less tightly packed and contain less matrix. Soft and mineralogically unstable framework grains have undergone considerable alteration, reducing pore space even in the youngest rocks. Measurements of porosity, permeability, grain density, and sonic velocity of outcrop samples of the younger Tertiary sequence indicate a modest up-section improvement in sandstone reservoir characteristics. Nonetheless porosity and permeability values typically are below 16 percent and 15 millidarcies respectively and grain densities are consistently high, about 2.7 gm/cc. Low permeability and porosity values, and high grain densities and sonic velocities appear to be typical of most outcrop areas throughout the onshore Gulf of Alaska Tertiary Province.

Open-File Report↗

USGS1980

Potentiometric map of the Ripley Aquifer in northeastern Mississippi, October and November 1978

This potentiometric map of the Ripley aquifer in northeastern Mississippi is the fifth in a series of maps, prepared by the U.S. Geological Survey in cooperation with the Mississippi Department of Natural Resources, Bureau of Land and Water Resources, delineating the potentiometric surfaces of the major aquifers in Mississippi. In the outcrop area of the Ripley aquifer the potentiometric surface is strongly affected by recharge from precipitation, by topography, and by drainage of the aquifer by streams. The potentiometric surface slopes generally to the west away from the area of outcrop and is mildly affected by pumpage from wells and by discharge of flowing wells in the valleys. Historically, water levels in or near the outcrop have shown little or no long-term changes. (USGS)

Open-File Report↗

USGS1980

Potentiometric map of the Coffee Sand Aquifer in northeastern Mississippi, October and November 1978

This potentiometric map of the Coffee Sand aquifer in northeastern Mississippi is the fourth in a series of maps, prepared by the U.S. Geological Survey in cooperation with the Mississippi Department of Natural Resources, Bureau of Land and Water Resources, delineating the potentiometric surfaces of the major aquifers in Mississippi. In the outcrop areas the potentiometric surface is strongly affected by recharge from precipitation, topography, and drainage of the aquifer by streams. The potentiometric surface slopes generally to the west away from the area of outcrop and is mildly affected by moderate ground-water withdrawals by wells in Tippah and Union County. Historically, water levels in or near the outcrop of the Coffee Sand have shown little or no long-term changes as shown by a hydrograph of one well in Alcorn County. In the downdip part of the aquifer water-level declines of 2 feet per year are common. (USGS)

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