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At least 109 records · Page 6Linked to original sources
USGS1978

The structure and stratigraphy of the Pen Argyl Member of the Martinsburg Formation in Lehigh and Berks counties, Pennsylvania

The Pen Argyl Member, the upper claystone slate member of the Martinsburg Formation, was studied in three quadrangles in Lehigh and Berks Counties, Pennsylvania. Graptolites collected from the Pen Argyl Member at Lehigh Gap indicate a lower Upper Ordovician (Edenian-Maysvillian) age for the Pen Argyl Member. The Pen Argyl Member in this area is located on the normal limb and in the brow of the large, recumbent Musconetcong nappe. It is a deep water flysch deposit emplaced by turbidity currents from a southeasterly source. Sedimentologic and structural evidence show that the Pen Argyl member overlies the sandy middle Ramseyburg Member, thus supporting the tripartite subdivision of the Martinsburg Formation. Field and thin section study indicates that the penetrative slaty cleavage formed in an indurated rock probably by pressure solution and neocrystallization under lower greenschist facies metamorphism. Strain-slip cleavage formed as a result of a stress couple operating parallel to the slaty cleavage that transposed the slaty cleavage into a more spaced cleavage. Both cleavages are believed to have formed within the same stress continuum and in close succession. Analysis of the folds in the Pen Argyl Member indicate six phases of major and minor folding. The earliest folding, F1, resulted in the development of the recumbent nappe. F2 folds can only be determined statistically; these axes plunge either northeast or southwest Asymmetric folds, F3, and associated F4 crenulations formed within the same stress continuum. F5 folds are large open folds and are exemplified by the Mosservi!le anticline. Kink folds, F6 and associated crenulations are fault related and were the last folds to form. Faults in the Pen Argyl Member range from small displacements along slaty cleavage to large reverse faults. The largest of these, the Eckville fault, is recognized throughout the three quadrangle area. It is a high angle reverse fault that separates the Shochary sequence from the Pen Argyl member to the north. Detailed fabric analysis of the Pen Argyl Member indicates that (1) the strike of the slaty cleavage is consistent throughout the study area, (2) bedding strikes are undulose indicating that the rocks were folded prior to slaty cleavage development, (3) slaty cleavage-bedding intersections indicate an early northeast-southwest fold set and a later east-west trend of fold axes, and (4) slaty cleavage-strain-slip cleavage intersections indicate two periods of strain-slip cleavage development, the later period being fault related. Synthesis of field work and fabric data suggest that the Pen Argyl Member was deposited in the waning stages of flysch deposition during the Taconic orogeny. The nappe, F1, was formed at this time as a result of stress generated by plate convergence to the southeast. Further Taconian deformation of the normal limb of the nappe resulted in the northeast-southwest plunging F2 folds. Initial Alleghenian deformation resulted in the F3 asymmetric folds and slaty cleavage, S1. Later in the same stress continuum the F4 crenulations and strain-slip cleavage, S2, formed. Subsequently, F5 open folding occurred. Kink folds and crenulations, F6, and strain-slip cleavage, S3, formed in conjunction with late Alleghenian reverse faults such as the Eckville fault.

Open-File Report↗
USGS1976

Preliminary survey of modern glaciolacustrine sediments for earthquake-induced deformational structures, south-central Alaska

Bottom sediments of four lakes in South-Central Alaska were studied in the summer of 1975 for evidence of earthquake-induced deformation. The lakes are: Summit, Upper Trail, and Skilak, all on the Kenai Peninsula, and Eklutna, northeast of Anchorage (fig. 1). Interest in these lakes was stimulated by hypotheses developed from a study of sediments in Van Norman Reservoir, California, after the 1971 San Fernando earthquake (Sims, 1973). During that study three zones of deformational structures were found and correlated with moderate earthquakes that shook the San Fernando area in 1930, 1952, and 1971. Results of that study, coupled with the experimental formation of deformational structures similar to those from Van Norman Reservoir, led to a search for similar structures in Pleistocene and Holocene lakes and lake sediments in other seismically active areas. The lakes for this study were chosen specifically because of their location within the area affected by the 1964 Prince William Sound earthquake, and the probability of obtaining varved sediments which would allow counting of years between sedimentologic events.

Alaska↗
USGS2009

The Shawangunk and Martinsburg Formations revisited; sedimentology, stratigraphy, mineralogy, geochemistry, structure and paleontology

In southeastern New York Middle Silurian Shawangunk Formation (Figure 1), containing gray conglomerate, sandstone and shale, lies unconformably above the Ordovician Martinsburg Formation, consisting of shales and graywackes. In southwestern New York, near the Port Jervis area, The Shawangunk Formation is overlain by the Bloomsburg Red Beds, the same stratigraphic sequence that occurs in Pennsylvania and New Jersey to the southwest. The Shawangunk Formation thins gradually from Port Jervis to its pinchout near Hidden Valley and Binnewater, New York. Two tongues of the upper part of the Shawangunk are: the Ellenville Tongue that extends from the Ellenville-Accord area to its feather edge just southwest of the New York-New Jersey border and, the High View Tongue that is restricted to the Wurtsboro area (Epstein and Lyttle, 1987; Epstein, 1993). Early in the Paleozoic carbonate banks lay along the east coast of the ancient North American continent. During the Ordovician plate convergence commenced in the closing of the Iapetus Sea and a deep basin developed into which thick muds and dirty sands were deposited. These were later lithified into the Martinsburg Formation. Eventually, with continued compression, these sediments were folded and faulted during the complex deformation of the Taconic Orogeny. The trend of these folds in southeastern New York is approximately N20E. As one proceeds westward across the Wallkill Valley these structures become less intense. Subsequent to the Taconic Orogeny mountains rose to the east and coarse sediments were transported westward and deposited as the conglomerates and sandstones of the Shawangunk Formation across the beveled folds of the Martinsburg. Deposition occurred on a plain of alluviation and in a marine basin to the northwest. Erosion of the source area was intense, and the climate, based on the mineralogy of the rocks, was warm and at least semiarid. The source was composed predominately of sedimentary and low-grade metamorphic rocks with exceptionally abundant quartz veins and small local areas of gneiss and granite. As the source highlands were eroded, the steep braided streams of the Shawangunk gave way to more gentle-gradient streams of the Bloomsburg Red Beds.

New York, Pennsylvania↗
USGS2007

A wireline piston core barrel for sampling cohesionless sand and gravel below the water table

A coring device has been developed to obtain long and minimally disturbed samples of saturated cohesionless sand and gravel. The coring device, which includes a wireline and piston, was developed specifically for use during hollow-stem auger drilling but it also offers possibilities for cable tool and rotary drilling. The core barrel consists of an inner liner made of inexpensive aluminum or plastic tubing, a piston for core recovery, and an exterior steel housing that protects the liner when the core barrel is driven into the aquifer. The core barrel, which is approximately 1.6m (5.6 feet) long, is advanced ahead of the lead auger by hammering at the surface on drill rods that are attached to the core barrel. After the sampler has been driven 1.5m (5 feet), the drill rods are detached and a wireline is used to hoist the core barrel, with the sample contained in the aluminum or plastic liner, to the surface. A vacuum developed by the piston during the coring operation provides good recovery of both the sediment and aquifer fluids contained in the sediment. In the field the sample tubes can be easily split along their length for on-site inspection or they can be capped with the pore water fluids inside and transported to the laboratory. The cores are 5cm (2 inches) in diameter by 1.5m (5 feet) long. Core acquisition to depths of 35m (115 feet), with a recovery greater than 90 percent, has become routine in University of Waterloo aquifer studies. A large diameter (12.7cm [5 inch]) version has also been used successfully. Nearly continuous sample sequences from sand and gravel aquifers have been obtained for studies of sedimentology, hydraulic conductivity, hydrogeochemistry and microbiology.

Groundwater Monitoring & Remediation↗
USGS2006

Flume experiments on the alignment of transverse, oblique, and longitudinal dunes in directionally varying flows

For more than a century geologists have wondered why some bedforms are orientated roughly transverse to flow, whereas others are parallel or oblique to flow. This problem of bedform alignment was studied experimentally using subaqueous dunes on a 3–6-m-diameter sand-covered turntable on the floor of a 4-m-wide flume. In each experiment, two flow directions (relative to the bed) were produced by alternating the turntable between two orientations. The turntable was held in each orientation for a short time relative to the reconstitution time of the bedforms; the resulting bedforms were in equilibrium with the time-averaged conditions of the bimodal flows. Dune alignment was studied for five divergence angles (the angle between the two flow directions): 45°, 67–5°, 90°, 112–5° and 135°. The flow depth during all experiments was approximately 30 cm; mean velocity was approximately 50 cm s -1 and mean grain diameter was 0–6 mm. Each experiment continued for 30–75 min, during which time the flume flow was steady and the turntable position changed every 2 min. At the end of each experiment, water was slowly drained from the flume and dune alignment was measured. Transverse dunes (defined relative to the resultant transport direction) were created when the divergence angle was 45° and 67–5°, and longitudinal dunes were created when the divergence angle was 135°. At intermediate divergence angles, dunes with both orientations were produced, but transverse dunes were dominant at 90°, and longitudinal dunes were dominant at 112–5°. One experiment was conducted with a divergence angle of 135° and with unequal amounts of transport in the two flow directions. This was achieved by changing the orientation of the turntable at unequal time intervals, thereby causing the amount of transport to be unequal in the two directions. The dunes formed during this experiment were oblique to the resultant transport direction. These experimental dunes follow the same rule of alignment as wind ripples studied in previous turntable experiments. In both sets of experiments, the bedforms developed with the orientation having the maximum gross bedform-normal transport (the orientation at which the sum of the bedform-normal components of the two transport vectors reaches its maximum value). In other words, the bedforms develop with an orientation that is as transverse as possible to the two flows. In those cases where the two flows diverge by more than 90° and transport equal amounts of sand, bedforms that are as transverse as possible to the two separate flows will be parallel to the resultant of the two flow vectors. Although such bedforms have been defined by previous work as longitudinal bedforms, they are intrinsically the same kind of bedform as transverse bedforms.

Sedimentology↗
USGS2011

Stratigraphic, sedimentologic, and dendrogeomorphic analyses of rapid floodplain formation along the Rio Grande in Big Bend National Park, Texas

The channel of the lower Rio Grande in the Big Bend region rapidly narrows during years of low mean and peak flow. We conducted stratigraphic, sedimentologic, and dendrogeomorphic analyses within two long floodplain trenches to precisely reconstruct the timing and processes of recent floodplain formation. We show that the channel of the Rio Grande narrowed through the oblique and vertical accretion of inset floodplains following channel-widening floods in 1978 and 1990–1991. Vertical accretion occurred at high rates, ranging from 16 to 35 cm/yr. Dendrogeomorphic analyses show that the onset of channel narrowing occurred during low-flow years when channel bars obliquely and vertically accreted fine sediment. This initial stage of accretion occurred by both bed-load and suspended-load deposition within the active channel. Vegetation became established on top of these fine-grained deposits during years of low peak flow and stabilized these developing surfaces. Subsequent deposition by moderate floods (between 1.5 and 7 yr recurrence intervals) caused additional accretion at rapid rates. Suspended-sediment deposition was dominant in the upper deposits, resulting in the formation of natural levees at the channel margins and the deposition of horizontally bedded, fining-upward deposits in the floodplain trough. Overall, channel narrowing and floodplain formation occurred through an evolution from active-channel to floodplain depositional processes. High-resolution dendrogeomorphic analyses provide the ability to specifically correlate the flow record to the onset of narrowing, the establishment of riparian vegetation, the formation of natural levees, and ultimately, the conversion of portions of the active channel to floodplains.

Texas↗
USGS2008

Geologic assessment of undiscovered gas resources of the Eastern Oregon and Washington Province

The purpose of the U.S. Geological Survey's (USGS) National Oil and Gas Assessment is to develop geology-based hypotheses regarding the potential for additions to oil and gas reserves in priority areas of the United States, focusing on the distribution, quantity, and availability of oil and natural gas resources. The USGS has completed an assessment of the undiscovered oil and gas potential of the Eastern Oregon and Washington Province of Oregon and Washington (USGS Province 5005). The province is a priority Energy Policy and Conservation Act (EPCA) province for the National Assessment because of its potential for oil and gas resources. The assessment of this province is based on geologic principles and uses the total petroleum system concept. The geologic elements of a total petroleum system include hydrocarbon source rocks (source rock maturation, hydrocarbon generation and migration), reservoir rocks (stratigraphy, sedimentology, petrophysical properties), and hydrocarbon traps (trap formation and timing). In the Eastern Oregon and Washington Province, the USGS used this geologic framework to define one total petroleum system and two assessment units within the total petroleum system, and quantitatively estimated the undiscovered gas resources within each assessment unit.

Oregon, Washington↗
USGS2021

Major reorganization of the Snake River modulated by passage of the Yellowstone Hotspot

The details and mechanisms for Neogene river reorganization in the U.S. Pacific Northwest and northern Rocky Mountains have been debated for over a century with key implications for how tectonic and volcanic systems modulate topographic development. To evaluate paleo-drainage networks, we produced an expansive data set and provenance analysis of detrital zircon U-Pb ages from Miocene to Pleistocene fluvial strata along proposed proto-Snake and Columbia River pathways. Statistical comparisons of Miocene-Pliocene detrital zircon spectra do not support previously hypothesized drainage routes of the Snake River. We use detrital zircon unmixing models to test prior Snake River routes against a newly hypothesized route, in which the Snake River circumnavigated the northern Rocky Mountains and entered the Columbia Basin from the northeast prior to incision of Hells Canyon. Our proposed ancestral Snake River route best matches detrital zircon age spectra throughout the region. Furthermore, this northerly Snake River route satisfies and provides context for shifts in the sedimentology and fish faunal assemblages of the western Snake River Plain and Columbia Basin through Miocene−Pliocene time. We posit that eastward migration of the Yellowstone Hotspot and its effect on thermally induced buoyancy and topographic uplift, coupled with volcanic densification of the eastern Snake River Plain lithosphere, are the primary mechanisms for drainage reorganization and that the eastern and western Snake River Plain were isolated from one another until the early Pliocene. Following this basin integration, the substantial increase in drainage area to the western Snake River Plain likely overtopped a bedrock threshold that previously contained Lake Idaho, which led to incision of Hells Canyon and establishment of the modern Snake and Columbia River drainage network.

Alberta, British Columbia, Idaho, Montana, Oregon,↗
USGS2007

Stratigraphy and facies of Cretaceous Schrader Bluff and Prince Creek Formations in Colville River Bluffs, North Slope, Alaska

Stratigraphic and sedimentologic studies of facies of the Upper Cretaceous rocks along the Colville River Bluffs in the west-central North Slope of Alaska identified barrier shoreface deposits consisting of vertically stacked, coarsening-upward parasequences in the Schrader Bluff Formation. This vertical stack of parasequence deposits represents progradational sequences that were affected by shoaling and deepening cycles caused by fluctuations of sea level. Further, the vertical stack may have served to stabilize accumulation of voluminous coal deposits in the Prince Creek Formation, which formed braided, high-sinuosity meandering, anastomosed, and low-sinuosity meandering fluvial channels and related flood plain deposits. The erosional contact at the top of the uppermost coarsening-upward sequence, however, suggests a significant drop of base level (relative sea level) that permitted a semiregional subaerial unconformity to develop at the contact between the Schrader Bluff and Prince Creek Formations. This drop of relative sea level may have been followed by a relative sea-level rise to accommodate coal deposition directly above the unconformity. This rise was followed by a second drop of relative sea level, with formation of incised valley topography as much as 75 ft deep and an equivalent surface of a major marine erosion or mass wasting, or both, either of which can be traced from the Colville River Bluffs basinward to the subsurface in the west-central North Slope. The Prince Creek fluvial deposits represent late Campanian to late Maastrichtian depositional environments that were affected by these base level changes influenced by tectonism, basin subsidence, and sea-level fluctuations.

Alaska↗
USGS2008

Geologic assessment of undiscovered, technically recoverable coalbed-gas resources in Cretaceous and Tertiary rocks, North Slope, and adjacent state waters, Alaska

The purpose of the U.S. Geological Survey's (USGS) National Oil and Gas Assessment is to develop geology-based hypotheses regarding the potential for additions to oil and gas reserves in priority areas of the United States, focusing on the distribution, quantity, and availability of oil and natural gas resources. The USGS has completed an assessment of the undiscovered, technically recoverable coalbed-gas resources in Cretaceous and Tertiary rocks underlying the North Slope and adjacent State waters of Alaska (USGS Northern Alaska Province 5001). The province is a priority Energy Policy and Conservation Act (EPCA) province for the National Assessment because of its potential for oil and gas resources. The assessment of this province is based on geologic principles and uses the total petroleum system concept. The geologic elements of a total petroleum system include hydrocarbon source rocks (source rock maturation, hydrocarbon generation and migration), reservoir rocks (stratigraphy, sedimentology, petrophysical properties), and hydrocarbon traps (trap formation and timing). In the Northern Alaska Province, the USGS used this geologic framework to define one composite coalbed gas total petroleum system and three coalbed gas assessment units within the petroleum system, and quantitatively estimated the undiscovered coalbed-gas resources within each assessment unit.

Alaska↗
USGS2004

Stratigraphy and sedimentology of the Upper Cretaceous (Campanian) Anacacho Limestone, Texas, USA

The Upper Cretaceous Anacacho Limestone is exposed in outcrops between the cities of San Antonio and Del Rio, Texas. A detailed study of four outcrops (Blanco Creek section, Sabinal River section, Seco Creek section, Hondo Creek section) shows that the Anacacho Limestone rests on the Upson Clay (which contains fauna of early Campanian age) and is overlain by the Corsicana Marl (which contains fauna of early Maastrichtian age). An unconformity within the Anacacho Limestone is used herein to separate the limestone into a lower member and an upper member. The lower Anacacho member contains fauna of early Campanian age, whereas the upper Anacacho member contains fauna of middle Campanian age. The lower Anacacho member consists predominantly of wackestones to packstones, which are overlain by packstones to grainstones capped by the unconformity. This unconformity is interpreted as a marine flooding surface, delineating a transition from carbonate grainstones deposited in shallow water (<30 m depth) to a chalk deposited in deeper water. Above the unconformity, the upper Anacacho member is characterized by a chalk, overlain by wackestones and packstones. The uppermost section of the Anacacho Limestone consists of packstones and grainstones with abundant and diverse fossils. Most of the Anacacho Limestone developed in relatively shallow water (<50 m depth) leeward of a large carbonate build-up (possibly a rudistid reef) that now comprises the Anacacho Mountains. The environment, however, was open to marine water throughout deposition of the Anacacho Limestone. ?? 2004 Elsevier Ltd. All rights reserved.

Cretaceous Research↗
USGS2011

A reevaluation of the Pallett Creek earthquake chronology based on new AMS radiocarbon dates, San Andreas fault, California

The Pallett Creek paleoseismic record occupies a keystone position in most attempts to develop rupture histories for the southern San Andreas fault. Previous estimates of earthquake ages at Pallett Creek were determined by decay counting radiocarbon methods. That method requires large samples which can lead to unaccounted sources of uncertainty in radiocarbon ages because of the heterogeneous composition of organic layers. In contrast, accelerator mass spectrometry (AMS) radiocarbon dates may be obtained from small samples that have known carbon sources and also allow for a more complete sampling of the section. We present 65 new AMS radiocarbon dates that span nine ground‐rupturing earthquakes at Pallett Creek. Overall, the AMS dates are similar to and reveal no dramatic bias in the conventional dates. For many layers, however, individual charcoal samples were younger than the conventional dates, leading to earthquake ages that are overall slightly younger than previously reported. New earthquake ages are determined by Bayesian refinement of the layer ages based on stratigraphic ordering and sedimentological constraints. The new chronology is more regular than previously published records in large part due to new samples constraining the age of event R. The closed interval from event C to 1857 has a mean recurrence of 135 years ( σ = 83.2 years) and a quasiperiodic coefficient of variation (COV) of 0.61. We show that the new dates and resultant earthquake chronology have a stronger effect on COV than the specific membership of this long series and dating precision improvements from sedimentation rates.

California↗
USGS2010

The sedimentology and dynamics of crater-affiliated wind streaks in western Arabia Terra, Mars and Patagonia, Argentina

Wind streaks comprise recent aeolian deposits that have been extensively documented on Venus, Earth and Mars. Martian wind streaks are among the most abundant surface features on the planet and commonly extend from the downwind margins of impact craters. Previous studies of wind streaks emerging from crater interior deposits suggested that the mode of emplacement was primarily related to the deposition of silt-sized particles as these settled from plumes. We have performed geologic investigations of two wind streaks clusters; one situated in western Arabia Terra, a region in the northern hemisphere of Mars, and another in an analogous terrestrial site located in southern Patagonia, Argentina, where occurrences of wind streaks emanate from playas within maar craters. In both these regions we have identified bedforms in sedimentary deposits on crater floors, along wind-facing interior crater margins, and along wind streaks. These observations indicate that these deposits contain sand-sized particles and that sediment migration has occurred via saltation from crater interior deposits to wind streaks. In Arabia Terra and in Patagonia wind streaks initiate from crater floors that contain lithic and evaporitic sedimentary deposits, suggesting that the composition of wind streak source materials has played an important role in development. Spatial and topographic analyses suggest that regional clustering of wind streaks in the studied regions directly correlates to the areal density of craters with interior deposits, the degree of proximity of these deposits, and the craters' rim-to-floor depths. In addition, some (but not all) wind streaks within the studied clusters have propagated at comparable yearly (Earth years) rates. Extensive saltation is inferred to have been involved in its propagation based on the studied terrestrial wind streak that shows ripples and dunes on its surface and the Martian counterpart changes orientation toward the downslope direction where it extends into an impact crater. ?? 2009 Elsevier B.V.

Geomorphology↗
USGS1987

Marine and nonmarine gas-bearing rocks in Upper Cretaceous Blackhawk and Neslen Formations, eastern Uinta Basin, Utah: Sedimentology, diagenesis, and source rock potential

The Upper Cretaceous Blackhawk and Neslen Formations in the eastern Uinta basin contain large amounts of thermogenic gas that was generated from interbedded humic-rich source rocks. The geometry and distribution of hydrocarbon source and reservoir rocks are controlled by depositional environment. The Blackhawk, composed of laterally extensive sandstone and locally interbedded carbonaceous siltstone and minor coal, reflects deposition in nearshore marine and backshore environments. The Neslen contains organic-rich siltstone and mudstone with lesser amounts of carbonaceous shale, coal, and lenticular sandstone that formed in coastal and lower alluvial-plain depositional settings. Potential reservoir sandstones are composed dominantly of monocrystalline quartz grains and sedimentary lithic fragments. Mechanical compaction during early burial was followed by the precipitation of quartz, carbonate, and barite later in the burial history. Variations in porosity and permeability (2-10%; < 1 md) reflects the presence of authigenic clay, mineral cements, and dissolved lithic grains. Natural fractures, cemented with carbonate, barite, and kaolinite, occur locally. Active hydrocarbon generation occurred in the Neslen and Blackhawk during the Oligocene and Miocene when these units were near their maximum burial depth and temperature. The rate of hydrocarbon generation decreased from the late Miocene to the present, owing to widespread cooling that occurred in response to regional uplift and erosion associated with the development of the Colorado Plateau. Temporally equivalent rocks in other areas of the basin may have experienced similar diagenetic and hydrocarbon generation histories.

Colorado, Utah↗
USGS1998

Significance of nearshore trace-fossil assemblages of the cambro-ordovician deadwood formation and Aladdin Sandstone, South Dakota

The Cambro-Ordovician Deadwood Formation and Aladdin Sandstone represent intertidal and subtidal, nearshore deposystems that contain few well-preserved body fossils, but contain abundant trace fossils. The present study uses the much neglected trace-fossil fauna to describe the diverse paleoenvironments represented in the Deadwood-Aladdin deposystems, and to better understand the environmental conditions that controlled benthic life in the Early Paleozoic. The Deadwood-Aladdin ichnotaxa can be separated into three distinct assemblages based on the changing sedimentologic and hydrodynamic conditions that existed across the Cambro-Ordovician shelf. Trace-fossil assemblages and corresponding lithofacies characteristics indicate that the Deadwood-Aladdin deposystems formed within an intertidal-flat and subtidal-shelf environment. Based on the distribution and numbers of preserved ichnotaxa, the intertidal flat can be subdivided further into an ecologically stressful inner sand-flat environment, and a more normal marine outer sand-flat environment, both of which belong to a mixed, Skolithos-Cruziana softground ichnofacies. The inner sand flat is characterized by low diversity, low numbers, and a general lack of complexly constructed ichnotaxa. Trace fossils common to both assemblages tend to be smaller in the inner flat compared to the outer sand flat. Taphonomic effects, such as substrate type and sediment heterogeneity, also aid in differentiating between the inner and outer sand-flat assemblages. The subtidal shelf environment is categorized in the Cruziana Ichnofacies. Ichnological evidence of periodic tempestite deposition and hardground development within this subtidal regime is manifested by high diversity and low abundance of ichnogenera.

Annals of Carnegie Museum↗
USGS2011

Depositional setting and geochemistry of phosphorites and metalliferous black shales in the Carboniferous-Permian Lisburne Group, Northern Alaska

Phosphatic rocks are distributed widely in the Lisburne Group, a mainly Carboniferous carbonate succession that occurs throughout northern Alaska. New sedimentologic, paleontologic, and geochemical data presented here constrain the geographic and stratigraphic extent of these strata and their depositional and paleogeographic settings. Our findings support models that propose very high oxygen contents of the Permo-Carboniferous atmosphere and oceans, and those that suggest enhanced phosphogenesis in iron-limited sediments; our data also have implications for Carboniferous paleogeography of the Arctic. Lisburne Group phosphorites range from granular to nodular, are interbedded with black shale and lime mudstone rich in radiolarians and sponge spicules, and accumulated primarily in suboxic outer- to middle-ramp environments. Age constraints from conodonts, foraminifers, and goniatite cephalopods indicate that most are middle Late Mississippian (early Chesterian; early late Visean). Phosphorites form 2- to 40-cm-thick beds of sand- to pebble-sized phosphatic peloids, coated grains, and (or) bioclasts cemented by carbonate, silica, or phosphate that occur through an interval =12 m thick. High gamma-ray response through this interval suggests strongly condensed facies related to sediment starvation and development of phosphatic hardgrounds. Phosphorite textures, such as unconformity-bounded coated grains, record multiple episodes of phosphogenesis and sedimentary reworking. Sharp bed bases and local grading indicate considerable redeposition of phosphatic material into deeper water by storms and (or) gravity flows. Lisburne Group phosphorites contain up to 37 weight percent P2O5, 7.6 weight percent F, 1,030 ppm Y, 517 ppm La, and 166 ppm U. Shale-normalized rare earth element (REE) plots show uniformly large negative Ce anomalies Ce/Ce*=0.11 + or - 0.03) that are interpreted to reflect phosphate deposition in seawater that was greatly depleted in Ce due to increased oxygenation of the atmosphere and oceans during the Carboniferous evolution of large vascular land plants. Black shales within the phosphorite sections have up to 20.2 weight percent Corg and are potential petroleum source rocks. Locally, these strata also are metalliferous, with up to 1,690 ppm Cr, 2,831 ppm V, 551 ppm Ni, 4,670 ppm Zn, 312 ppm Cu, 43.5 ppm Ag, and 12.3 ppm Tl; concentrations of these metals covary broadly with Corg, suggesting coupled redox variations. Calculated marine fractions (MF) of Cr, V, and Mo, used to evaluate the paleoredox state of the bottom waters, show generally high CrMF/MoMF and VMF/MoMF ratios that indicate deposition of the black shales under suboxic denitrifying conditions; Re/Mo ratios also plot mainly within the suboxic field and support this interpretation. Predominantly seawater and biogenic sources are indicated for Cr, V, Mo, Zn, Cd, Ni, and Cu in the black shales, with an additional hydrothermal contribution inferred for Zn, Cd, Ag, and Tl in some samples. Lisburne Group phosphorites formed in the Ikpikpuk Basin and along both sides of the mud- and chert-rich Kuna Basin, which hosts giant massive sulfide and barite deposits of the Red Dog district. Lisburne Group phosphatic strata are coeval with these deposits and formed in response to a nutrient-rich upwelling regime. Phosphate deposition occurred mainly in suboxic bottom waters based on data for paleoredox proxies (Cr, V, Mo, Re) within contemporaneous black shales. Recent global reconstructions are consistent with Carboniferous upwelling in northern Alaska, but differ in the type of upwelling expected (zonal versus meridional). Paleoenvironmental data suggest that meridional upwelling may better explain phosphorite deposition in the Lisburne Group.

Alaska↗
USGS2014

Geologic implications of gas hydrates in the offshore of India: results of the National Gas Hydrate Program Expedition 01

The Indian National Gas Hydrate Program Expedition 01 (NGHP-01) is designed to study the occurrence of gas hydrate along the passive continental margin of the Indian Peninsula and in the Andaman convergent margin, with special emphasis on understanding the geologic and geochemical controls on the occurrence of gas hydrate in these two diverse settings. The NGHP-01 expedition established the presence of gas hydrates in the Krishna-Godavari and Mahanadi Basins, and the Andaman Sea. The expedition discovered in the Krishna-Godavari Basin one of the thickest gas hydrate accumulations ever documented, in the Andaman Sea one of the thickest and deepest gas hydrate stability zones in the world, and established the existence of a fully developed gas hydrate petroleum system in all three basins. The primary goal of NGHP-01 was to conduct scientific ocean drilling/coring, logging, and analytical activities to assess the geologic occurrence, regional context, and characteristics of gas hydrate deposits along the continental margins of India. This was done in order to meet the long-term goal of exploiting gas hydrate as a potential energy resource in a cost effective and safe manner. During its 113.5-day voyage, the D/V JOIDES Resolution cored and/or drilled 39 holes at 21 sites (1 site in Kerala-Konkan, 15 sites in Krishna-Godavari, 4 sites in Mahanadi, and 1 site in the Andaman deep offshore area), penetrated more than 9250 m of sedimentary section, and recovered nearly 2850 m of core. Twelve holes were logged with logging-while-drilling (LWD) tools and an additional 13 holes were wireline logged. The science team utilized extensive on-board laboratory facilities to examine and prepare preliminary reports on the physical properties, geochemistry, and sedimentology of all the data collected prior to the end of the expedition. Samples were also analyzed in additional post-expedition shore-based studies conducted in leading laboratories around the world. One of the specific objectives of this expedition was to test gas hydrate formation models and constrain model parameters, especially those that account for the formation of concentrated gas hydrate accumulations. The necessary data for characterizing the occurrence of in situ gas hydrate, such as interstitial water chlorinities, core-derived gas chemistry, physical and sedimentological properties, thermal images of the recovered cores, and downhole measured logging data (LWD and/or conventional wireline log data), were obtained from most of the drill sites established during NGHP-01. Almost all of the drill sites yielded evidence for the occurrence of gas hydrate; however, the inferred in situ concentration of gas hydrate varied substantially from site to site. For the most part, the interpretation of downhole logging data, core thermal images, interstitial water analyses, and pressure core images from the sites drilled during NGHP-01 indicate that the occurrence of concentrated gas hydrate is mostly associated with the presence of fractures in the sediments, and in some limited cases, by coarser grained (mostly sand-rich) sediments.

Marine and Petroleum Geology↗
USGS2001

A lone biodetrital mound in the Chesterian (Carboniferous) of Alabama?

A carbonate mound in the Chesterian Bangor Limestone of Lawrence County, Alabama, consists chiefly of packstone and grainstone dominated by echinoderm ossicles and fragments of fenestrate bryozoans. In-situ colonies of the rugose coral Caninia flaccida comprise about 8% of the mound by volume. The exposed portion of the mound is approximately 25 m wide, 1.6 m thick at the thickest point and roughly circular in plan. The mound developed on top of a shallow ooid shoal that had been cemented and stabilised during an earlier episode of sub-aerial exposure. Subsequent flooding of the exposed shoal surface permitted establishment of the mound biota. Lateral and vertical facies relationships suggest that the mound possessed about 45 cm of synoptic relief when fully developed. Rugose corals, fenestrate and ramose bryozoans, stalked echinoderms, and sessile soft-bodied organisms encrusted by foraminifera colonised the shoal, forming a mound. Baffling resulted in deposition of mixed-fossil packstone containing locally derived debris and coated grains from the surrounding sea floor. Strong currents within the mound are indicated by preferred orientation of corals and by coarse, commonly cross-stratified grainstone in channels between neighboring coral colonies. Corals are most abundant on the windward side of the mound, where they account for about 13% of the mound compared to 6- 10% in the central part of the mound, and 2-4% on the leeward flank. Biodetrital mounds such as the one described here are uncommon in upper Paleozoic strata and previously unknown in the Bangor Limestone. Of 10 carbonate buildups we examined in the Bangor in Alabama and Tennessee, only one is a biodetrital mound. Two are rugose coral-microbial reefs, one is a coral biostrome, and six are dominated by microbialite. The Bangor shelf, previously interpreted as sedimentologically simple, appears to contain many small mounds of quite varied characteristics. Also, the discovery of a biodetrital mound in the Chesterian of Alabama suggests that there may be more kinds of upper Paleozoic mounds than commonly acknowledged. ?? 2001 Elsevier Science B.V. All rights reserved.

Sedimentary Geology↗