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At least 595 records · Page 33Linked to original sources
USGS2015

Geology and hydrocarbon potential of the Hartford-Deerfield Basin, Connecticut and Massachusetts

The Hartford-Deerfield basin, a Late Triassic to Early Jurassic rift basin located in central Connecticut and Massachusetts, is the northernmost basin of the onshore Mesozoic rift basins in the eastern United States. The presence of asphaltic petroleum in outcrops indicates that at least one active petroleum system has existed within the basin. However, to-date oil and gas wells have not been drilled in the basin to test any type of petroleum trap. There are good to excellent quality source rocks (up to 3.8% present day total organic carbon) within the Jurassic East Berlin and Portland formations. While these source rock intervals are fairly extensive and at peak oil to peak gas stages of maturity, individual source rock beds are relatively thin (typically less than 1 m) based solely on outcrop observations. Potential reservoir rocks within the Hartford-Deerfield basin are arkosic conglomerates, pebbly sandstones, and finer grained sandstones, shales, siltstones, and fractured igneous rocks of the Triassic New Haven and Jurassic East Berlin and Portland formations (and possibly other units). Sandstone porosity data from 75 samples range from less than 1% to 21%, with a mean of 5%. Permeability is equally low, except around joints, fractures, and faults. Seals are likely to be unfractured intra-formational shales and tight igneous bodies. Maturation, generation, and expulsion likely occurred during the late synrift period (Early Jurassic) accentuated by an increase in local geothermal gradient, igneous intrusions, and hydrothermal fluid circulation. Migration pathways were likely along syn- and postrift faults and fracture zones. Petroleum resources, if present, are probably unconventional (continuous) accumulations as conventionally accumulated petroleum is likely not present in significant volumes.

Connecticut, Massachusetts↗
USGS2006

Assessing recreation impacts to cliffs in Shenandoah National Park: Integrating visitor observation with trail and recreation site measurements

The rock outcrops and cliffs of Shenandoah National Park provide habitat for several rare and endangered plant and animal species, including the federally endangered Shenandoah Salamander (Plethodon shenandoah; Ludwig et al., 1993). The location of the well-known park tour road, Skyline Drive, along the ridgeline provides exceptional access to many outcrops and cliffs throughout the park for a large number of the park?s 1.2 million annual visitors. Consequently, visitor use of cliff areas has led to natural resource impacts, including marked decreases in size and vigor of known rare plant populations. Despite the clear ecological value and potential threats to the natural resources at cliff areas, managers possess little information on visitor use of cliff sites and presently have no formal planning document to guide management. Thus, a park wide study of cliff sites was initiated during the 2005 visitor use season. As part of this research effort, our study used an integrative approach to study recreational use and visitor-caused resource impacts at one of the more heavily visited cliff sites in the park: Little Stony Man Cliffs (LSMC). In particular, this study integrated data from resource impact measurements and visitor use observation to help assess the effects of recreational use on the natural resources of LSMC. Procedures derived from campsite and trail impact studies were used to measure and characterize the amount of visitor-caused resource impacts on LSMC (Marion & Leung, 2001; Marion, 1995). Visitor use observations were conducted on top of LSMC to document and characterize the type and amount of recreational use the cliffs receive and the behaviors of recreationists that may contribute to cliff-top resource impacts. Resource impact measurement data show trampling disturbance present at LSMC, characterized by vegetation loss, exposed soil, and root exposure. Documentation of informal trails, soil erosion, tree damage, and tree stumps provide further indicators of resource damage at LSMC. Results of visitor use observation offer several insights into contributory factors of cliff-top resource damage by showing differences in use and behavior between visitor types. The findings from this study suggest that a management approach characterized by visitor education, some site hardening, and concentration of visitor use on durable surfaces, along with the installation of fixed anchors at the top of popular climbing routes is likely to have the greatest success at balancing visitor enjoyment with resource protection at LSMC.

Journal of Park and Recreation Administration↗
USGS1993

40Ar-39Ar ages of bentonite beds in the upper part of the Yazoo Formation (Upper Eocene), west-central Mississippi

Bentonite beds recorded from both outcrops and cores in the upper Eocene Yazoo Formation offer opportunities to date the uppermost Eocene of this region and to provide information on the age of the Eocene/Oligocene boundary. This report gives radiometric age dates for three bentonites sampled from the upper Yazoo Formation. Two bentonites are from outcrops near Satartia in western Mississippi and one is from a core hole at Society Ridge in west-central Mississippi. The upper bentonite at Satartia was studied independently at two laboratories using different techniques but with the same results, an age of 34.3 Ma (million years). Results from the Society Ridge bentonite gave the same age. -from Authors

Mississippi Geology↗
USGS1997

Bedrock geology of snyderville basin: Structural geology techniques applied to understanding the hydrogeology of a rapidly developing region, Summit County, Utah

The availability of ground water is a problem for many communities throughout the west. As these communities continue to experience growth, the initial allocation of ground water supplies proves inadequate and may force restrictions on existing, and future, development plans. Much of this new growth relies on ground water supplies extracted from fractured bedrock aquifers. An example of a community faced with this problem is western Summit County, near Park City, Utah, This area has experienced significant water shortages coupled with a 50% growth rate in the past 10-15 years. Recent housing development rests directly on complexly deformed Triassic to Jurassic sedimentary rocks in the hanging wall of the Mount Raymond-Absaroka thrust system. The primary fractured bedrock aquifers are the Nugget Sandstone, and limestones in the Thaynes and Twin Creek Formations. Ground water production and management strategies can be improved if the geometry of the structures and the flow properties of the fractured and folded bedrock can be established. We characterize the structures that may influence ground water flow at two sites: the Pinebrook and Summit Park subdivisions, which demonstrate abrupt changes (less than 1 mi/1.6 km) within the hydrogeologic systems. Geologic mapping at scales of 1:4500 (Pinebrook) and 1:9600 (Summit Park), scanline fracture mapping at the outcrop scale, geologic cross sections, water well data, and structural analysis, provides a clearer picture of the hydrogeologic setting of the aquifers in this region, and has been used to successfully site wells. In the Pinebrook area, the dominate map-scale structures of the area is the Twomile Canyon anticline, a faulted box-like to conical anticline. Widely variable bedding orientations suggest that the fold is segmented and is non-cylindrical and conical on the western limb with a fold axis that plunges to the northwest and also to the southeast, and forms a box-type fold between the middle and eastern limbs with a fold axis that plunges to the northeast. The fold is cut by several faults including the Toll Canyon fault, which we interpret as a west-directed folded hanging-wall splay off the east-directed Mt. Raymond thrust. These complex geometries may be due to at least two phases of deformation. Results from outcrop analyses show that the fractured bedrock aquifers are lithologically heterogeneous, anisotropic, and compartmentalized. Two exposures of the Toll Canyon fault show that even though the fault cores may be thin, extensive damage zones develop in the Nugget Sandstone and Thaynes Limestone, and shale smears form in the Triassic shales. The damaged zones may be regions of enhanced fracture permeability, whereas the shale smears act as flow barriers. The orientation, density, and hydrogeologic characteristics for predominate fracture sets vary within meters. In the Summit Park area, chronic water shortages required new wells to be sited in the northeast-plunging Summit Park anticline. The anticline experienced two phases of folding and at least one episode of faulting. Structural analysis of the fold defined the geometry of the structure, and a down plunge projection along the fold hinge was used to estimate the location of the Nugget Sandstone at a depth of 700 ft (213 m). The crestal region of the anticline was drilled in order to intercept regions of higher fracture density in the fold. The test well penetrated the Nugget Sandstone at 698 ft depth, and two production wells with long-term yields of 120 and 180 gpm completed. One well in the Sliderock Member (Twin Creek Formation) experiences seasonal fluctuations whereas production in the Nugget sandstone has only subdued seasonal variations, suggesting the Nugget may have great storage. Complex structures work against the typical basin yield approach for water budgets, therefore, water supply estimates may benefit from detailed studies within local areas. The results of this study demonstrate how tradition

Brigham Young University Geology Studies↗
USGS2003

The chrono- and lithostratigraphic significance of the type section of the Middendorf Formation, Chesterfield County, South Carolina

The name Middendorf Formation has been widely used in the Coastal Plain of South Carolina, eastern Georgia, and southern North Carolina since 1904, despite conflicting interpretations of the age and stratigraphic relations of the unit at its type locality. Between 1995 and 1998, the U.S. Geological Survey, in cooperation with the South Carolina Department of Natural Resources, drilled three continuously cored holes to the south and to the east of the type section of the Middendorf Formation, which is located in Chesterfield County, South Carolina. In addition, two outcrops to the northeast of the type section were sampled for biostratigraphic control. The litho- and biostratigraphic relations of the units in these cores and outcrops provide significant insights into the age of the Middendorf Formation at its type locality, and how this age impacts regional correlations of the formation. A projection of formational contacts and thicknesses from downdip areas into the type locality of the Middendorf Formation indicates that the type section is most likely a facies of either the uppermost Bladen Formation (of the Black Creek Group), or the uppermost Bladen and the lowermost part of the Peedee Formations. This report documents the evidence that support this interpretation. The implication of this interpretation is that the name "Middendorf" has been applied to a variety of units throughout the southeastern United States, all of which display a similar lithology, but differ significantly in stratigraphic position and age. For these reasons, we recommend that the name Middendorf be restricted for use with strata that occur only in the vicinity of the type locality, and use of the name "Middendorf" for units elsewhere in the Coastal Plain be reconsidered.

Southeastern Geology↗
USGS2008

Nearshore substrate and morphology offshore of the Elwha River, Washington

The planned removal of two dams on the Elwha River, Washington, will likely increase river sediment flux to the coast, which may alter coastal habitats through sedimentation and turbidity. It is therefore important to characterize the current habitat conditions near the river mouth, so that future changes can be identified. Here we provide combined sonar and video mapping results of approximately 20 km2 of seafloor offshore of the Elwha River collected with the purpose to characterize nearshore substrate type and distribution prior to dam removal. These combined data suggest that the nearshore of the western delta and Freshwater Bay are dominated by coarse sediment (sand, gravel, cobble, and boulders) and bedrock outcrops; no fine-grained sediment (mud or silt) was identified within the survey limits. The substrate is generally coarser in Freshwater Bay and on the western flank of the delta, where boulders and bedrock outcrops occur, than directly offshore and east of the river mouth. High variation in substrate was observed within much of the study area, however, and distinct boulder fields, gravel beds and sand waves were observed with spatial scales of 10-100 m. Gravel beds and sand waves suggest that sediment transport is active in the study area, presumably in response to tidal currents and waves. Both historic (1912) and recent (1989-2004) distributions of Bull Kelp (Nereocystis sp.) beds were preferentially located along the boulder and bedrock substrates of Freshwater Bay. Although kelp has also been mapped in areas dominated by gravel and sand substrate, it typically has smaller canopy areas and lower temporal persistence in these regions.

Northwest Science↗
USGS2011

Survey of invasive ants at Hakalau Forest National Wildlife Refuge

We conducted a survey for invasive ants at Hakalau Forest National Wildlife Refuge, Hawai‘i Island, during 2009–2010 to evaluate potential threats to native arthropod communities and food webs. The focal area of the survey was the upper portion of the Hakalau Unit of the refuge, where native forest was being restored in abandoned cattle pastures. This area, between 1575 and 1940 m elevations, contained much alien kikuyu grass (Pennisetum clandestinum), but koa (Acacia koa) trees and other native species that were planted in the past 20 years were rapidly filling in the pasture. We surveyed for ants at predetermined points along roads, fences, and corridors of planted koa. Sampling methods primarily consisted of hand searching and pitfall traps, but bait cards were used additionally in some instances. Our results indicated that a single species, Cardiocondyla kagutsuchi, was widespread across the upper portion of the refuge. Cardiocondyla kagutsuchi seemed absent, or at least rare, in areas of tall, dense grass. Due to the undulating topography of the area, however, the dense grass cover was interspersed with outcroppings of exposed, gravelly soil. Presumably due to warming by the sun, many of the outcropped habitats supported colonies of C. kagutsuchi. We did not detect ants in the old-growth forest below the abandoned pastures, presumably because microhabitat conditions under the forest canopy were unsuitable. Although ecological impacts of C. kagutsuchi have not been reported, they may be limited by the small size of the ant, the relatively small size of colonies, and the apparent preference of the ant for disturbed areas that are dominated by alien species. Notably, our survey of Keanakolu-Mana Road between the Observatory Road (John A. Burns Way) and the town of Waimea detected a population of Argentine ants (Linepithema humile) approximately 5.1 km north of the Maulua Section of the refuge. We also surveyed for ants on the Kona Forest Unit of the refuge. This small survey focused on approximately 14 km of roads located below about 1600 m elevation. We found two species, Solenopsis papuana and Nylanderia bourbonica. Solenopsis papuana was more widespread, being found along the southern, northern, and western boundaries, while N. bourbonica was detected only at 790 m elevation on the southern boundary. Of the two species, S. papuana seemed more likely to affect native arthropod communities due to its tendency to form relatively large, aggressive colonies and its ability to inhabit intact mesic and wet forests below 1100 m elevation. In contrast, the restriction of N. bourbonica to disturbed habitats indicated a reduced threat to native arthropod communities. Our results on the Kona Forest Unit corroborated those of a study conducted during 1999–2000, although the earlier study was more intensive over time and yielded small numbers of two additional species, Cardiocondyla wroughtonii and Tetramorium bicarinatum, both of which were detected below 792 m elevation along the southern boundary.

Hawaii↗
USGS1976

Glaciomarine deposits of Miocene through Holocene age in the Yakataga Formation along the Gulf of Alaska margin, Alaska

Perhaps the world's longest and most complete onshore sedimentary record of late Cenozoic glaciation is preserved in the Gulf of Alaska Tertiary province that extends 800 km along the coast of southern Alaska. The Yakataga Formation, with an aggregate outcrop thickness of about 5,000 m, is characterized by variable amounts of distinctive neritic marine tillite-like diamictite and laminated siltstone containing dropstones interpreted as ice-rafted glacial debris. The lithology, sedimentary structures and molluscan fauna of the formation suggest that active tidal glaciers or an ice shelf were present along the landward margin of the basin possibly beginning in early or early middle Miocene time. Dropstone distribution in outcrop sections indicates that glaciers reached tidewater intermittently during the Miocene and were almost continually present throughout the Pliocene and much of the Pleistocene. Paleomagnetic and nannoplankton dating of the upper 1,181 m of the Yakataga Formation at Middleton Island indicate that this part of the sequence probably was entirely deposited during the Matuyama reversed polarity epoch of the Pleistocene during which the sedimentation rate was of the order of 1 m/1,000 years. Lithologically similar deposits of poorly consolidated sandy mud and pebbly mud continue to accumulate locally near tidal glaciers in the same area, and older late Quaternary deposits are undoubtedly present offshore. The anomalous late Cenozoic glaciation recorded in the sediments along the Gulf of Alaska margin reflects a fortuitous combination of a subsiding shelf basin adjacent to an area of extremely high elevations and exceptionally heavy precipitation, an environment that persists to the present.

Alaska↗
USGS2013

Field guide to the geology of the Denali National Park Road and the Parks Highway from Cantwell to Healy

The Denali National Park & Preserve area provides one of the few opportunities in Alaska for road-side access to good rock outcrops. The rocks and surficial deposits exposed in the Denali area span from the Paleozoic to the Quaternary. It is a structurally complex area that contains a history of rifting, accretion, and orogeny. There is evidence of multiple metamorphic events in the Mesozoic, mountain building in the Tertiary, and faulting in the present day. The region is the site of active faulting along one of the largest intra-continental fault systems, the Denali Fault system, which was the locus of a 7.9 M earthquake in 2002. This guidebook describes the key outcrops viewable along the Denali Park Road from the entrance to the Eielson Visitor Center, and along the Parks Highway from Healy to Cantwell.

Alaska↗
USGS1980

Airborne Fraunhofer line discriminator (FLD) luminescence imaging systems and its application to exploration problems

Experiments with an imaging airborne Fraunhofer line discriminator (FLD) are being conducted to establish the feasibility of delineating the areal extent of luminescent materials on the earth's surface from aircraft and spacecraft. All luminescence measurements are related to a standard set of conditions with rhodamine wt dye used as a reference standard. The FLD has a minimum detectable rhodamine wt concentration of 0.1 parts per billion (ppb) at a signal-to-noise ratio of 5.0. Luminescence, when expressed in a signal-to-noise ratio (R) is related to equivalent ppb rhodamine wt through the relationship ppb=(0.1R-0.4). Luminescent materials imaged from an aircraft altitude of approximately 2400 m above terrain include fluorite in association with molybdenum, Pinenut Mountains, Nevada (R=62.0); mineralized playas, Claunch, New Mexico (R=960.0); uranium and vanadium-bearing outcrops, Big Indian Valley, Utah (R=105.0); uranophane sandstones, Sandia Mountains, New Mexico (R=60.0); phosphate outcrops, Pine Mountain, California (R=76.0); and marine oil slicks, Santa Barbara Channel, California (R=24.0). Correlation between the amount of fluorite in the rocks and soils of the Pinenut Mountains and luminescence, measured by the FLD, is as high as 0.88 at the 95 percent confidence level.

Sixth Annual Pecora Symposium and Exposition↗
USGS1987

Field guide to sedimentary structures in the Navajo and Entrada sandstones in southern Utah and northern Arizona

This field-trip guide describes the common sedimentary structures that occur in eolian sands. The outcrops that are described occur in the Navajo and Entrada Sandstones between the areas of Page, Arizona and St. George, Utah (figure I), but the sedimentary structures of these two sandstones are typical of most eolian deposits. The main part of the guide discusses the geologic setting and the origin of the various structures, and the road log discusses which structures are best displayed at selected outcrops.

Arizona, Utah↗
USGS2015

Large river bed sediment characterization with low-cost sidecan sonar: Case studies from two setting in the Colorado (Arizona) and Penobscot (Maine) Rivers

Mapping subaqueous riverbed sediment grain size across channels and in nearshore areas typically used by fish and benthic invertebrates is difficult where and when the water flow is too swift or deep to wade yet impractical to access with large boats and instruments. Fluvial characteristics can further constrain sampling options, particularly where flow depth, water column turbidity or channel bottom structure prohibit use of aerial or bottom deployed imaging platforms. Here we discuss considerations in the use of sidescan sonar for riverbed sediment classification using examples from two large rivers, the Colorado River below Glen Canyon Dam in Arizona and the Upper Penobscot River in northern Maine (Figure 3). These case studies represent two fluvial systems that differ in recent history, physiography, sediment transport, and fluvial morphologies. The bed of the Colorado River in Glen Canyon National Recreation Area is predominantly graveled with extensive mats of submerged vegetation, and ephemeral surficial sand deposits exist below major tributaries. The bed is imaged periodically to assess the importance of substrate type and variability on rainbow trout spawning and juvenile rearing habitats and controls on aquatic invertebrate population dynamics. The Colorado River bed further below the dam in Grand Canyon National Park is highly dynamic. Tributary inputs of sand, gravel and boulders are spatially variable, and hydraulics of individual pools and eddies vary considerably in space and in response to varying dam operations, including experimental controlled flood releases to rebuild eroding sandbars. The bed encompasses the full range of noncohesive sediments, deposited in complicated spatial patterns. The mobile portion of the Penobscot River is generally more uniform, and consists predominantly of embedded gravels interspersed between bedrock outcrops with small isolated sand patches in sections with modest or low gradients. Patches of large cobbles, boulders and bedrock outcrops are present in the lower reaches of the river near locations of two recent dam removal projects but are of limited extent below the "head of tide" on the river. Aggregations of coarse materials often correspond to locations with abrupt bed elevation drops in the Upper Penobscot River.

Arizona, Maine↗
USGS1976

Sedimentary depositional environments of uranium and petroleum host rocks of the Jackson Group, south Texas

Determination of depositional environments in rocks of the Jackson Group of late Eocene age aids in the exploration for uranium and petroleum in south Texas. During deposition of the Jackson Group, conditions were similar to those existing along the modern Gulf Coast of Texas except for a less arid climate and a greater supply of volcanic sediment. Sedimentary depositional environments recognized in both the modern coast and the Jackson rocks, in order from sea toward land, are: shoreface, beach, lagoon and bay, and paludal. Sediment deposited in these environments in places is interbedded with or transected by fluvial sediment. Both uranium and petroleum are found in the shoreface, beach, and fluvial facies. These facies are characterized by permeable well-sorted sandstone, which is commonly overlain and underlain by less permeable mudstone. A typical vertical sequence in ascending order through a unit containing a beach sandstone facies is (1) lagoonal or paludal clay stone and siltstone, (2) crossbedded fine-grained beach sandstone which contains Ophiomorpha burrows and, which, in places, is locally interbedded with lacustrine or lagoonal mudstone, (3) weakly laminated beach sandstone which contains root impressions at the top and Ophiomorpha , and (4) lagoonal or paludal claystone and siltstone. Gulfward, the beach sandstone apparently grades into highly burrowed, generally finer grained, more poorly sorted, less porous shoreface rocks. The beach facies may include barrier island beaches and mainland beaches, some of which were probably cheniers. Beach facies sandstone units extend many miles along the depositional strike, which nearly parallels the present outcrop strike, but extend only a mile or two (2-3 km) normal to the depositional strike into the subsurface. Recognition of the various facies in outcrops and drill holes is necessary for determination of true stratigraphic relations and for aid in exploration for beach and fluvial sandstone favorable for uranium and petroleum accumulations.

Texas↗
USGS1973

Accessory apatite from hybrid granitoid rocks of the southern Snake Range, Nevada

Analytical data, optical properties, and unit-cell parameters are presented for 24 samples of accessory apatites recovered from hybrid granitoid rocks of the southern Snake Range, Nev. A complete chemical analysis is given for one. In the Snake Creek-Williams Canyon outcrop area, where the hybrid rocks grade from granodiorite with 63 percent SiO 2 to a quartz monzonite with 76 percent SiO 2 within a horizontal distance of about 3 miles, abundance and crystal habit of the apatite change with rock chemistry. The apatite probably is slightly more than 90 mole percent fluorapatite in the most felsic rocks, slightly less in the most mafic. The range of F->OH substitution in apatite from this outcrop area is much smaller than in the coexisting biotites. Except for manganese, strontium, sodium, and the rare earths, there is very limited substitution for calcium in the crystal structure of these apatites. Apatites from the more mafic rocks tend to contain a lighter, more basic assemblage of rare earths, in agreement with results obtained for coexisting allanites, monazites, and zircons.

Nevada↗
USGS2015

Architecture and evolution of an Early Permian carbonate complex on a tectonically active island in east-central California

The newly named Upland Valley Limestone represents a carbonate complex that developed on and adjacent to a tectonically active island in east-central California during a brief interval of Early Permian (late Artinskian) time. This lithologically unique, relatively thin limestone unit lies within a thick sequence of predominantly siliciclastic rocks and is characterized by its high concentration of crinoidal debris, pronounced lateral changes in thickness and lithofacies, and a largely endemic fusulinid fauna. Most outcrops represent a carbonate platform and debris derived from it and shed downslope, but another group of outcrops represents one or possibly more isolated carbonate buildups that developed offshore from the platform. Tectonic activity in the area occurred before, probably during, and after deposition of this short-lived carbonate complex.

California↗
USGS1973

Use of machine-processable field notes in a wilderness mapping project (Granite Fiords area), southeastern Alaska

For reconnaissance geologic mapping and mineral resource evaluation of the Granite Fiords wilderness study area, we developed and used a system of machine-processable field notes. Preprinted field forms standardize notes and serve as checklists that insure collection of all available data. The use of this system cut in half the time required to record data at an outcrop. The system consists of three related but different types of preprinted field sheets, a key to abbreviations and codes, and a set of written instructions. The field sheets include a station sheet for basic outcrop data, a specimen sheet for rock samples, and a geochemical sheet for materials to be chemically analyzed. Data on the field sheets are keypunched on standard IBM cards, then arranged in subfiles and retrieved by using a card sorter. Our system is designed specifically for a region of granitic and metamorphic rocks but is easily modified for use in different geologic terranes. We offer four guidelines for developing a system of machine-processable field notes: (1) Time and money spent to develop the system must be worth its anticipated benefits. (2) The system should be as flexible as possible. (3) It is necessary to tailor the system to a particular geologic terrane or project objective. (4) The system should be as simple and self-explanatory as possible.

Alaska↗
USGS2016

Late Neogene deformation of the Chocolate Mountains Anticlinorium: Implications for deposition of the Bouse Formation and early evolution of the Lower Colorado River

Deformation related to late Neogene dextral shear can explain a shift from an estuarine to lacustrine depositional environment in the southern Bouse Formation north of Yuma, Arizona. We infer that late Neogene deformation in the Chocolate Mountain Anticlinorium (CMA) created a barrier that blocked an estuary inlet, and that pre-existing and possibly active structures subsequently controlled the local course of the lower Colorado River. Structural patterns summarized below suggest that the CMA absorbed transpressional strain caused by left-stepping segments of dextral faults of the San Andreas fault system and/or the eastern California shear zone and Gulf of California shear zone. For this hypothesis to be correct, about 200-250 m of post-6 Ma, pre- ~5.3 Ma uplift along the CMA crest would be required to cut off a marine inlet. The 220-km-long CMA, cored by the early Paleogene Orocopia Schist subduction complex, extends from the Orocopia Mountains (Calif.) southeastward through the Chocolate Mountains (parallel to the southern San Andreas fault). Where Highway 78 crosses the Chocolate Mountains (Fig. 1), the CMA turns eastward through the Black Mountain-Picacho area (Calif.) and Trigo Mountains (Ariz.) into southwest Arizona. It separates southernmost Bouse Formation outcrops of the Blythe basin from subsurface Bouse outcrops to the south in the Yuma area. South of Blythe basin the CMA is transected by the lower Colorado River along a circuitous path. Here we focus on the geology of an area between the central Chocolate Mountains and the Yuma Proving Grounds in Arizona. Specific landmarks include the southeast Chocolate Mountains, Midway Mountains, Peter Kane Mountain, Black Mountain, Picacho Peak, and Gavilan Hills. For simplicity, we refer to this as the eastern Chocolate Mountains.

Conference Paper↗
USGS1993

Coalbed methane potential of the Upper Cretaceous Mesaverde and Meeteetse formations, Wind River Reservation, Wyoming

The environments of deposition of the uppermost part of the Cody Shale and the Mesaverde and Meeteetse Formations of Late Cretaceous age were studied on outcrop in the Shotgun Butte area in the north-central part of the Wind River Reservation. A shoreface sandstone occurs in the lower part of the Mesaverde Formation at all localities studied, and is directly overlain by a coaly interval. Repetitive coarsening-upward cycles of mudstone, siltstone, and sandstone occur in the 200 ft interval of the upper part of the Cody Shale below the shoreface sandstone. These Cody sandstones are typically hummocky cross stratified with symmetrical ripples near the top, indicating that they are largely storm surge deposits that were later reworked. Channel-form sandstones from 10 to 20 ft thick, with abundant locally derived clayey clasts, occur in a 75 ft thick interval below the shoreface at one locality. These unusual sandstones are largely confined to a narrow area of the outcrop and grade laterally into more typical storm surge deposits. They may be unusually large storm surge channels created when high-energy flow conditions were localized to a limited area of the shelf. The Mesaverde Formation above the shoreface sandstone is divided into a middle member and the Teapot Sandstone Member. The lower part of the middle member is everywhere coaly. Erosional-based sandstones in this coaly interval are highly variable in thickness and architecture. Thin, single channel sandstone bodies were deposited by moderate to high sinuosity streams, and thick, multistory channel sandstone bodies were deposited by rapidly switching fluvial channel systems that remained relatively stationary for extended periods of time. The architecture of the fluvial channel sandstones in the overlying noncoaly interval appears to be highly variable as well, with complex multistory sandstones occurring at different stratigraphic levels at different localities. This distribution may be explained by long term stability of fluvial channel systems followed by major avulsion events. The Teapot Sandstone Member consists of fairly persistent to lenticular white multistory sandstone units that are as much as 85 ft thick and contain trough cross beds as much as 5 ft high. These sandstone units are interbedded with gray mudstones and carbonaceous shales. Paleosols are preserved at the tops of individual sandstones in the multistory units in some places. It is suggested that these sandstones were deposited largely by low-sinuosity to braided streams. The Meeteetse Formation consists of alternating coal and sandstone-rich intervals. The coal-rich intervals have relatively thin fluvial channel sandstones probably deposited by medium to high sinuosity streams whereas the sand-rich intervals have thick (to 105 ft) multistory fluvial channel sandstones possibly deposited by low-sinousity to braided streams.

Conference Paper↗