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At least 973 records · Page 54Linked to original sources
USGS2017

The role of deep-water sedimentary processes in shaping a continental margin: The Northwest Atlantic

The tectonic history of a margin dictates its general shape; however, its geomorphology is generally transformed by deep-sea sedimentary processes. The objective of this study is to show the influences of turbidity currents, contour currents and sediment mass failures on the geomorphology of the deep-water northwestern Atlantic margin (NWAM) between Blake Ridge and Hudson Trough, spanning about 32° of latitude and the shelf edge to the abyssal plain. This assessment is based on new multibeam echosounder data, global bathymetric models and sub-surface geophysical information. The deep-water NWAM is divided into four broad geomorphologic classifications based on their bathymetric shape: graded, above-grade, stepped and out-of-grade. These shapes were created as a function of the balance between sediment accumulation and removal that in turn were related to sedimentary processes and slope-accommodation. This descriptive method of classifying continental margins, while being non-interpretative, is more informative than the conventional continental shelf, slope and rise classification, and better facilitates interpretation concerning dominant sedimentary processes. Areas of the margin dominated by turbidity currents and slope by-pass developed graded slopes. If sediments did not by-pass the slope due to accommodation then an above grade or stepped slope resulted. Geostrophic currents created sedimentary bodies of a variety of forms and positions along the NWAM. Detached drifts form linear, above-grade slopes along their crests from the shelf edge to the deep basin. Plastered drifts formed stepped slope profiles. Sediment mass failure has had a variety of consequences on the margin morphology; large mass-failures created out-of-grade profiles, whereas smaller mass failures tended to remain on the slope and formed above-grade profiles at trough-mouth fans, or nearly graded profiles, such as offshore Cape Fear.

Marine Geology↗
USGS1992

Mid-Mesozoic (Mid-Jurassic to Early Cretaceous) evolution of the Georges Bank Basin, U.S. North Atlantic outer continental shelf: Sedimentology of the Conoco 145-1 well

The Conoco 145-1 exploratory well, located in the southeastern portion of the Georges Bank Basin, was drilled to a total depth of 4303 m below the sea floor. The oldest sedimentary rocks sampled are of Middle Jurassic age (Late Bathonian-Callovian). A dolomite-limestone-evaporite sequence dominates the section below 3917 m; limestone is the predominant lithology in the intervals of 3271-3774 m, 2274-3158 m, and 1548-1981 m. Siliciclastics dominate the remainder of the drilled section. Calcite tightly cements most of the rocks below 1548 m; dolomite, silica, siderite, and diagenetic clay cements are locally important. Restricted inner marine environments, representing lagoonal and tidal flat conditions, prevailed at the wellsite during much of the deposition recorded by the Callovian-Bathonian age Iroquois Formation. These environments gave way to a carbonate platform, which formed part of the > 5,000 km long Bahama-Grand Banks gigaplatform that lasted through the end of the Late Jurassic (encompassing the uppermost portion of the Iroquois Formation and the Scatarie Limestone and Bacarro Limestone Members of the Abenaki Formation). The absence of a skeletal-reef association and the dominance of muddy limestone fabrics are evidence that the 145-1 wellsite was located on the platform interior. Major periods of siticiclastic deposition interrupted carbonate deposition, and they are recorded by stratigraphic equivalents of the Mohican Formation, Misaine Shale Member of the Abenaki Formation, and the Mohawk and Mic Mac Formations. A series of sustained prograding delta systems, the earliest of which is preserved as the Missisauga Formation, buried the carbonate platform following its drowning in the Early Cretaceous (Berriasian-Valanginian). The sparser, primarily allochthonous lignite content and better-sorted, glauconite-bearing sands of the Missisauga strata at the 145-1 wellsite suggest that shallow marine or barrier-bar environments were more prevalent than the low delta-plain facies recorded farther shoreward at the COST G-1 wellsite.

Sedimentary Geology↗
USGS2006

Contaminant exposure and potential effects on terrestrial vertebrates residing in the National Capital Region network and Mid-Atlantic network

Part of the mission of the National Park Service is to preserve the natural resources, processes, systems, and associated values of its units in an unimpaired condition. Environmental contamination and pollution processes are well recognized stressors addressed by its management policies and plans. A recent study indicates that contemporary terrestrial vertebrate ecotoxicological data are lacking for 59 of 126 Park Service units located in coastal watersheds exhibiting serious water quality problems or high vulnerability to pollution. Based upon these findings, a more in-depth evaluation of contaminant threats and ecotoxicological data gaps related to terrestrial vertebrates was undertaken at 23 Inventory and Monitoring National Park units in National Capital Region and Mid-Atlantic Networks. Ecotoxicological data were compiled for each park unit through literature searches and meetings with Park Service personnel. Information on contemporary and on persistent legacy pollutants in air, water, soil, and terrestrial vertebrates (amphibians, reptiles, birds and mammals) were evaluated. To identify contaminant threats in proximity to the 23 Park Service study units, data was gathered on National Priority List Superfund sites, Section 303(d) Impaired Waterbodies, the number and relative toxicity of current use pesticides and herbicides, Toxic Release Inventory sites and discharge of priority pollutants, and Fish Consumption Advisories. A metric was derived that described the quality and quantity of existing data for each park, and in combination with known contaminant threats, park units in need of additional study were identified. Results demonstrated that over half of the Park Service study units are near Toxic Release Inventory sites discharging dioxins, polychlorinated biphenyls, lead or mercury into air or water, and fish consumption advisories are in effect at or near 22 of the 23 study units. Pesticide and herbicide use at the park units is minimal, with the exception of those units with significant agricultural leases. Despite highly regulated use, many of the pesticides and herbicides applied are believed to be highly toxic to amphibians, and some of the compounds are also highly toxic to birds. Only 70 reports were found that describe terrestrial vertebrate ecotoxicology data on or near the study units. Of the greater than 75,000 compounds in commerce in the United States, existing terrestrial vertebrate exposure and effects data in the pr esent study were limited to 58 legacy organochlorine pesticides, polychlorinated biphenyls and individual congeners, insecticides and rodenticides, metals, and some contemporary compounds (e.g., polybrominated diphenyl ether flame retardants, and alkylphenol and ethoxylate surfactants). Based upon these and other findings, ecotoxicological monitoring and research investigations of terrestrial vertebrates are warranted at several National Parks. These include Shenandoah National Park, Richmond National Battlefield, Chesapeake & Ohio Canal National Historic Park, Valley Forge National Historic Park, Hopewell Furnace National Historic Site, Monocacy National Battlefield, and Harpers Ferry National Historic Park. The types of investigations vary according to the species present at these parks and potential contaminant threats, but should focus on contemporary use pesticides and herbicides, polychlorinated biphenyls, mercury, lead, and perhaps, emerging contaminants including antibiotics, flame retardants, pharmaceuticals, and surfactants. Other management recommendations include additional training for natural resource staff members in the area of ecotoxicology, inclusion of terrestrial vertebrate contaminant monitoring and the Contaminant Assessment Process (U.S. Geological Survey Biomonitoring of Environmental Status and Trends Project) into the National Park Service Vital Signs Program, development of protocols for hand ling and toxicological analysis of dead or seemingly affected wildlife, consideration of some alternative methods and compounds for pest management and weed control, and use of non-toxic fishing tackle by visitors.

Maryland, Pennsylvania, Virginia↗
USGS2018

Paleoclimate Records: Providing context and understanding of current Arctic change

At present, the Arctic Ocean is experiencing changes in ocean surface temperature and sea ice extent that are unprecedented in the era of satellite observations, which extend from the 1980s to the present (see sections 5c,d). To provide context for current changes, scientists turn to paleoclimate records to document and study anthropogenic influence and natural decadal and multidecadal climate variability in the Arctic system. Paleoceanographic records extend limited Arctic instrumental measurements back in time and are central to improving our understanding of climate dynamics and the predictive capability of climate models. By comparing paleoceanographic records with modern observations, scientists can place the rates and magnitudes of modern Arctic change in the context of those inferred from the geological record. Over geological time, paleoceanographic reconstructions using, for instance, marine sediment cores indicate that the Arctic has experienced huge sea ice fluctuations. These fluctuations range from nearly completely ice-free to totally ice-covered conditions. The appearance of ice-rafted debris and sea ice-dependent diatoms in Arctic marine sediments indicate that the first Arctic sea ice formed approxi-mately 47 million years ago (St. John 2008; Stickley et al. 2009; Fig. SB5.1), coincident with an interval of declining atmospheric carbon dioxide (CO2) concentration, global climate cooling, and expansion of Earths cryosphere during the middle Eocene. The development of year-round (i.e., perennial) sea ice in the central Arctic Ocean, similar to conditions that exist today, is evident in sediment records as early as 1418 million years ago (Darby 2008). These records suggest that transitions in sea ice cover occur over many millennia and often vary in concert with the waxing and waning of circum-Arctic land ice sheets, ice shelves, and long-term fluctuations in ocean and atmospheric temperature and atmospheric CO2 concentrations (Stein et al. 2012; Jakobsson et al. 2014). Over shorter time scales, shallow sediment records from Arctic Ocean continental shelves allow more detailed, higher-resolution (hundreds of years resolution) reconstructions of sea ice history extending through the Holocene (11 700 years ago to present), the most recent interglacial period. A notable feature of these records is an early Holocene sea ice minimum, corresponding to a thermal maximum (warm) period from 11 000 to 5000 years ago, when the Arctic may have been warmer and had less summertime sea ice than today (Kaufman et al. 2004). However, it is not clear that the Arctic was ice-free at any point during the Holocene (Polyak et al. 2010). High-resolution paleosea ice records from the western Arctic in the Chukchi and East Siberian Seas indicate that sea ice concentrations increased through the Holocene in concert with decreasing summer solar insolation (sunlight). Sea ice extent in this region also varied in response to the volume of Pacific water delivered via the Bering Strait into the Arctic Basin (Stein et al. 2017; Polyak et al. 2016). Records from the Fram Strait (Mller et al. 2012), Laptev Sea (Hrner et al. 2016), and Canadian Arctic Archipelago (Vare et al. 2009) also indicate a similar long-term expansion of sea ice and suggest sea ice extent in these regions is modulated by the varying influx of warm Atlantic water into the Arctic Basin (e.g., Werner et al. 2013). Taken together, available records support a circum-Arctic sea ice expansion during the late Holocene. A notably high-resolution summer sea ice history (<5-year resolution) has been established for the last 1450 years using a network of terrestrial records (tree ring , lake sediment, and ice core records) located around the margins of the Arctic Ocean (Kinnard et al. 2011). Results summarized in Fig. SB5.2 indicate a pronounced decline in summer sea ice extent beginning in the 20th century, with exceptionally low ice extent recorded since the mid-1990s, consistent with the satellite record (see section 5d). While several episodes of reduced and expanded sea ice extent occur in association with climate anomalies such as the Medieval Climate Warm Period (AD 8001300) and the Little Ice Age (AD 14501850), the magnitude and pace of the modern decline in sea ice is outside of the range of natural variability and unprecedented in the 1450-year reconstruction (Kinnard et al. 2011). A radiocarbon-dated driftwood record of the Ellesmere ice shelf in the Canadian High Arctic, the oldest landfast ice in the Northern Hemisphere, also demonstrates a substantial reduction in ice extents over the 20th century (England et al. 2017). A supporting sediment record indicates that inflowing Atlantic water in Fram Strait has warmed by 2C since 1900, driving break up and melt of sea ice (Spielhagen et al. 2011). Complementary mooring and satellite observations show the Atlantification of the eastern Arctic due to enhanced inflow of warm saline water through Fram Strait (Nilsen et al. 2016) and nutrient-rich Pacific water via the Bering has increased by more than 50% (Woodgate et al. 2012), further driving sea ice melt and warming seas. Similar high-resolution proxy records from Arctic regions also indicate that the modern rate of increasing annual surface air temperatures has not been observed over at least the last 2000 years (McKay and Kaufman 2014). Scientists conclude that broad-scale sea ice variations recorded in the paleo record were dominantly driven by changes in basin-scale changes in atmospheric circulation patterns, fluctuations in air temperature, strength of incoming solar radiation, and changes in the inflow of warm water via Pacific and Atlantic inflows (Polyak et al. 2010). There is general consensus that ice-free Arctic summers are likely before the end of the 21st century (e.g., Stroeve et al. 2007; Massonnet et al. 2012), while some climate model projections suggest ice-free Arctic summers as early as 2030 (Wang and Overland 2009). Paleoclimate studies and observational time series attribute the decline in sea ice extent and thickness over the last decade to both enhanced greenhouse warming and natural climate variability. While understanding the interplay of these factors is critical for future projections of Arctic sea ice and ecosystems, most observational time series records cover only a few decades. This highlights the need for additional paleoceanographic reconstructions across multiple spatial and temporal domains to better understand the drivers and implications of present and future Arctic Ocean change.

Bulletin American Meteorological Society↗
USGS1966

Hydrology of the Upper Capibaribe Basin, Pernambuco, Brazil - A reconnaissance in an Area of Crystalline Rocks

The upper Capibaribe basin is the western three-fourths, approximately, of the valley of the river that empties into the Atlantic Ocean at Recife, the capital of the State of Pernambuco, Brazil. It is the part of the drainage basin that is within the Drought Polygon of northeast Brazil, and it totals about 5,400 square kilometers. It receives relatively abundant precipitation in terms of the annual average, yet is regarded as hot subhumid to semiarid because the precipitation is uneven from year to year and place to place. The dependable water supply, therefore, is small. The basin has water, which could be put to better use than at present, but the opportunities for augmenting the usable supply are not great. The streams are intermittent and therefore cannot be expected to fill surface reservoirs and to keep them filled. The ground-water reservoirs have small capacity--quickly filled and quickly drained. A rough estimate based on the records for 1964 suggests that, of 4,700 million cubic meters of precipitation in the upper Capibaribe basin, 2,700 million cubic meters (57 percent) left the basin as runoff and 2,000 million cubic meters {43 percent) went into underground storage or was evaporated or transpired. The bedrock of the upper Capibaribe basin is composed of granite, gneiss, schist, and other varieties of crystalline rocks, which have only insignificant primary permeability. They are permeable mainly where fractured. The principal fracture zones, fortunately, are in the valleys, where water accumulates and can feed into them, but the volume of fractured rock is small in relation to the basin as a whole. A well in a large water-filled fracture zone may yield up to 20,000 liters per hour, but the average well yields less than one-fourth this amount, and some wells yield none. The saprolite, or weathered rock, is many meters thick at some places especially in the eastern half of the upper Capibaribe basin. It contains water locally, but ordinarily will yield only small quantities to wells. The alluvium probably is the most productive aquifer in the basin, but is limited to narrow bands along the rivers that generally are no more than a few hundred meters wide and 5 meters thick. The alluvium contains variable amounts of silty sand capable of yielding small to moderate quantities of water to wells. Wells driven or dug into the alluvium could solve many small water problems. The chemical quality of the water in the upper Capibaribe basin ranges from good to bad and generally presents a major problem that cannot be solved solely by applying geological criteria. Mineralized water is widespread in the area, both in streams and underground, and .the choice of aquifers is small. All known aquifers contain, at one place or another, water that is mineralized, leaving no alternative for a natural supply of good-quality water. Although much of the available water is unsatisfactory for human consumption, it is generally acceptable for animals and therefore meets one of the principal water needs. Some of the ground water could be made potable by diluting it with rainwater, which could be collected during rainy seasons and temporarily stored in cisterns or reservoirs.

Water Supply Paper↗
USGS2007

Origin and emplacement of impactites in the Chesapeake Bay impact structure, Virginia, USA

The late Eocene Chesapeake Bay impact structure, located on the Atlantic margin of Virginia, may be Earth's best-preserved large impact structure formed in a shallow marine, siliciclastic, continental-shelf environment. It has the form of an inverted sombrero in which a central crater ∼40 km in diameter is surrounded by a shallower brim, the annular trough, that extends the diameter to ∼85 km. The annular trough is interpreted to have formed largely by the collapse and mobilization of weak sediments. Crystalline-clast suevite, found only in the central crater, contains clasts and blocks of shocked gneiss that likely were derived from the fragmentation of the central-uplift basement. The suevite and entrained megablocks are interpreted to have formed from impact-melt particles and crystalline-rock debris that never left the central crater, rather than as a fallback deposit. Impact-modified sediments in the annular trough include megablocks of Cretaceous nonmarine sediment disrupted by faults, fluidized sands, fractured clays, and mixed-sediment intercalations. These impact-modified sediments could have formed by a combination of processes, including ejection into and mixing of sediments in the water column, rarefaction-induced fragmentation and clastic injection, liquefaction and fluidization of sand in response to acousticwave vibrations, gravitational collapse, and inward lateral spreading. The Exmore beds, which blanket the entire crater and nearby areas, consist of a lower diamicton member overlain by an upper stratified member. They are interpreted as unstratified ocean-resurge deposits, having depositional cycles that may represent stages of inward resurge or outward anti-resurge flow, overlain by stratified fallout of suspended sediment from the water column.

Chesapeake Bay↗
USGS2022

Mapping areas of groundwater susceptible to transient contamination events from rapid infiltration into shallow fractured-rock aquifers in agricultural regions of the conterminous United States

Current time-invariant groundwater vulnerability assessments may not capture intermittent contamination events in landscape areas that experience rapid infiltration following precipitation or snowmelt. Occurrences of rapid infiltration and intermittent degradation of groundwater quality are frequently reported in fractured-rock aquifers. This investigation identifies landscape areas underlain by fractured rock within the conterminous United States (CONUS) that may be susceptible to rapid infiltration and where groundwater is a principal source of water supply to the population. Our analysis shows that approximately 27 percent of the CONUS, corresponding to a population of approximately 150 million people, is both underlain by fractured rock and denoted as an area of significant groundwater use. The results of this survey identified shallow fractured-rock aquifers underlying glacial sediments in the upper Midwest and northeastern United States as areas that may be subject to rapid infiltration events. Additionally, aquifers associated with the early Mesozoic basins located in the northeastern and mid-Atlantic United States and bands of carbonate aquifers in the southeastern United States show high susceptibility to rapid infiltration. Index values used in this investigation indicate isolated areas in the western half of the United States also show high susceptibility to rapid infiltration. The isolated areas in Oklahoma, Texas, Arkansas, and southwestern Missouri correspond to karst regions of carbonate aquifers. The isolated areas showing high susceptibility to rapid infiltration and contamination from agricultural sources are locations where more detailed investigations of transient contamination events are warranted. This survey also addresses the potential for contaminant longevity in fractured-rock aquifers stemming from intermittent contamination events. Contaminants that can dissolve into the groundwater following infiltration may be introduced into fractures, and the dissolved constituents can diffuse from fractures into the porosity of the adjacent rock matrix. These constituents can then diffuse back into permeable fractures and adversely affect groundwater quality at downgradient locations over an extended time frame. Rock types with larger matrix porosities have the capacity to retain and then release larger quantities of dissolved constituents, resulting in longer residence times for dissolved groundwater contaminants. The magnitude of the dissolved contaminant concentration infiltrating to the water table will also dictate whether the contaminant concentration in the groundwater exceeds limits for human consumption over the duration of a contamination event. In general, sedimentary- and carbonate-rock aquifers have larger matrix porosities in comparison to igneous- and metamorphic-rock aquifers, and thus, they are more susceptible to longer contaminant residence times. Aquifers composed of sedimentary or carbonate rock constitute approximately 51 percent of the CONUS, and 19 percent of the CONUS is associated with sedimentary- or carbonate-rock aquifers that are of significance for groundwater use. Depending on the contaminants of concern and the concentration of the contaminants introduced into the groundwater from infiltrating water, it would be beneficial for investigations of susceptibility to rapid infiltration to also consider the potential for contaminant longevity. This investigation identifies areas of rapid infiltration into fractured rock using index values applied to the attributes (1) depth to the water table, (2) depth to bedrock, and (3) percentage of sand in soil, where larger index values indicate a greater susceptibility to rapid infiltration. These attributes are selected as the most likely factors that affect rapid infiltration to the water table. The combination of depth to water table and depth to bedrock highlight those aquifer settings that are characterized as shallow fractured-rock aquifers, where the water table may reside either in the bedrock or in overlying unconsolidated geologic materials. In addition, we consider the percentage of agricultural use as a land-use attribute when formulating an index of susceptibility to rapid infiltration and contamination. Agricultural areas are well recognized as nonpoint sources of contaminants that can affect groundwater quality because of seasonal amendments applied to the land surface. Rural agricultural areas are also characterized by septic tanks and leach fields for onsite treatment of wastewater, which may also be a source of contamination that may be introduced into the groundwater following precipitation or snowmelt events.

Conterminous United States↗
USGS2000

Hydrologic budget of the late Oligocene Lake Creede and the evolution of the upper Rio Grande drainage system

The filling history, hydrologic budget, and geomorphic development of ancient Lake Creede and its tributary basin are evaluated to determine the factors that controlled its character. The lake filled the Creede caldera that formed in the late Oligocene as a consequence of the eruption of the Snowshoe Mountain Tuff. The caldera's sedimentary fill accumlated to a depth of about 1.26 km and had a volume of about 89 km 3 . The highest lake level was ~3300 m (10,800 ft) present altitude before it drained eastward across a broad volcanic plateau as the ancestral Rio Grande. A tributary canyon several hundred meters deep was cut into hard rhyolite in the north wall of the caldera before the lake was more than half full; its presence demonstrates that ancient Lake Creede filled slowly and thus occupied a long-lived, closed basin. The slow filling rate is incompatible with the present water flux through the Creede caldera basin, because such a flow would fill the basin geologically instantaneously. This mismatch, together with the recognition that the Oligocene climate was similar to that of today, forces the reexamination of the hydrologic and geomorphic history of the caldera. That appraisal shows that the caldera cannot have resurged rapidly immediately after caldera collapse, and that ancient watershed must have been lass than half as large as the present upper Rio Grande basin. The ancient lake had a more or less constant surface area of about 200 km 2 that approximated a steady-state condition between inflow and evaporation. Although the lake level fluctuated with climatic variations, its surface elevation steadily climbed as sediment accumulated, accelerating as resurgance and dome growth usurped spacewithin the basin. It could have had one playa stage early in its development and another after the basin had nearly filled with sediment, but there is no direct evidence for either. At least the lower half of the sedimentary column (the part sampled by the scientific drilling) formed in an euxinic environment. This argues against a persistent early playa, although evaporative accumulation of brine was inevitable. When the rate of resurgance was rapid relative to sedimentary infilling, the lake would have been deep (i.e., bordered by bedrock rather than sedimentary fans). The geomorphic evolution of the Creede caldera and its watershed tracks a two-phase topographic history, the first the Oligocene through Miocene, and the second for Pliocene to the recent. In Oligocene time, the San Juan volcanic field was a hydrologically immature, gently undulating, and outward sloping, constructional volcanic plateau straddling the ancient Continental Divide. West of the Creede caldera, a dendritic drainage discharged northeastward into ancestral Cebolla Creek (a tributary of the ancestral Gunnison River) through an early stage of the Clear Creek graben in the vicinity of Spring Creek Pass. Miocene basalt choked, but did not reconstruct, the drainage. By the end of Miocene time a mature topography of moderate relief developed, exposing some of the higher ores in the Creede district to weathering. In the late Miocene-early Pliocene time the San Juan Mountains were uplifted and titled eastward; the ancestral Rio Grande was revitalized and cut deeply into the older terrain, excavating much of the accessible sediment from the moat of the Creede caldera and exposing successively lowe levels in the Creede district to oxidation. Simultaneously, the southeast end of the Clear Creek graben was reactivated and breached the southwest wall of the Creede caldera. The rejuvenated Rio Grande captured the formerly northeast-directed headwaters of ancestral Cebolla Creek, shifting more than 1000 km 2 from the Pacific-directed drainage to the Atlantic. The water budget for ancient Lake Creede was strictly limited by the early stages of the fist geomorphic cycle; the modern water budget is the product of the second cycle.

Colorado↗
USGS2022

Geochemical insights into formation of enigmatic ironstones from Rio Grande rise, South Atlantic Ocean

Rio Grande Rise (RGR) is an intraplate oceanic elevation in the South Atlantic Ocean that formed at a hotspot on the Mid-Atlantic Ridge during the Cretaceous. In spreading center and hotspot environments, ironstones form mainly by biomineralization of reduced Fe from hydrothermal fluids or oxidation of sulfide deposits. However, RGR has been considered aseismic and volcanically inactive for the past 46 Ma. Here, we investigate the origin of ironstones collected from the summit of RGR using multiple techniques: petrographic observations, X-ray diffraction, U-Th/He geochronology, and chemical composition. The ironstones from RGR consist of finely laminated goethite containing igneous rock fragments, carbonate fluorapatite, and calcite. Our results suggest that Fe oxyhydroxides were precipitated by Fe-oxidizing bacteria forming bacterial mats. The bacterial Fe mats underwent compaction, dewatering, goethite crystallization, and cementation that created the ironstone deposits. U-Th/He geochronology reveals protracted goethite minimum ages extending from the late Miocene to the Quaternary, probably due to multiple generations of mats, slow mineralization rates, and Fe-oxide dissolution-reprecipitation cycles. Flame-like goethite structures underneath FeMn crusts and a chimney-shaped goethite sample with a central channel indicate that the dewatering fluid flowed upward through the deposits, or a thermal fluid source may have been introduced from below the ironstone deposits. High Fe/Mn ratios, low trace metals contents (Ni + Co + Cu), and very low Fe/REY ratios suggest ironstone precipitation from a hydrothermal fluid; however, REY SN plots and bivariate Ce SN /Ce SN ⁎ versus Y SN /Ho SN and Ce SN /Ce SN ⁎ versus Nd plots are inconclusive, and a proximal source of magma was unlikely during the period of mat formation. Given this evidence, we hypothesize that a geothermal circulation system may have facilitated ironstone mineralization at RGR.

Marine Geology↗
USGS1960

Position of the salt-water body in the magothy(?) formation in the Cedarhurst-Woodmere area of southwestern Nassau county, Long Island, N.Y.

The position and chloride concentration of a sizable body of salt water , moving slowly landsyard from the south-shore bays of Long Island and the Atlantic Ocean were defined by recent test drilling in the Cedarhurst-Woodmere area of southwestern Nassau County, Long Island, N. Most of the salt-water body is in the lower part of a permeable artesian aquifer; the lowermost part of the salt-water body is in clay deposits underlying the permeable aquifer. The upper limit of the salt-water body in this area was found at depths increasing progressively in a landward direction. It was 318 feet below sea level at a well in Cedarhurst and 541 feet below sea level at a well in Woodmere. The lower limit of the salt-water body was determined at depths between 578 and 630 feet below sea level in the Cedarhurst-Woodmere area. The salt-water body is more than 300 feet thick at a well in Cedarhurst, and it thins out to zero in the vicinity of a pumping center about 1- miles northeast of the Cedarhurst well. Chloride concentration in the salt-water body in the Cedarhurst-Woodmere area ranged from about 40 to 16,000 ppm (parts per million). Isochlors define a zone of diffusion about a mile wide in the Cedarhurst-Woodmere area. They indicate a thickness of diffused water ranging from a few tens to more than 150 feet vertically. Electrical-log data show that the upper boundary of the salt-water body moved upward 21 feet between 1952 and 1958 at a site in Woodmere about half a mile southwest of the pumping center. ' From this information it is inferred that between 1952 and 1958 the leading edge of the saltwater front moved landward about 2,000 feet toward the pumping center.

New York↗
USGS2010

Nitrogen Loads in Groundwater Entering Back Bays and Ocean from Fire Island National Seashore, Long Island, New York

Fire Island is a barrier island that lies south of central Long Island, N.Y. It is about 60 km (37 mi) long and 0.5 km (1/4 mi) wide and is bounded by the Great South Bay, Narrow Bay, and Moriches Bay estuaries to the north; by the Atlantic Ocean to the south; by Fire Island Inlet to the west; and by Moriches Inlet to the east (fig. 1). Fire Island National Seashore (FIIS) encompasses a 42-km (26-mi) length of Fire Island that is bordered by Robert Moses State Park to the west and Smith Point County Park to the east (fig. 2). Interspersed throughout FIIS are 17 residential beach communities that together contain about 4,100 homes. The barrier island's summer population increases 50-fold through the arrival of summer residents and vacationers. The National Park Service (NPS) has established several facilities on the island to accommodate visitors to FIIS. About 2.2 million people visit at least one of the 17 communities and (or) Smith Point County Park, the waterways surrounding Fire Island, or a FIIS facility annually (National Park Service, 2007). Combined visitation on a peak-season weekend day can be as high as 100,000 (National Park Service, 2002). Most homes and businesses in the 17 barrier-island communities discharge untreated wastewater directly to the shallow (water-table) aquifer through private septic systems and cesspools; the NPS facilities discharge wastewater to this aquifer through leach fields and cesspools. (The community of Ocean Beach (fig. 2) has a treatment plant that discharges to tidewater.) Contaminants in sewage entering the shallow groundwater move through the flow system and are ultimately discharged to adjacent marine surface waters, where they can pose a threat to coastal habitats. A contaminant of major concern is nitrogen, which is derived from fertilizers and human waste. The continuous inflow of nitrogen to surface-water bodies can lead to increased production of phytoplankton and macroalgae, which in turn can cause oxygen depletion, decreases in size of estuarine fish and shellfish communities, and loss of submerged seagrass habitat through light limitation (Valiela and others, 1992). The FIIS boundary extends roughly 1.2 km (0.8 mi) into the back-barrier estuaries of Great South Bay, Narrow Bay, and Moriches Bay (fig. 1). Within this estuarine zone are extensive areas of seagrass, shellfish, and finfish habitat, as well as intense recreational activity (Bokuniewicz and others, 1993). Management strategies for protection of these habitats require data on (1) concentrations and movement of nutrients and other human-derived contaminants that enter the groundwater system from on-site septic systems, and (2) aquifer characteristics and groundwater flow patterns. These data can then be used in three-dimensional flow models of the shallow aquifer system to predict the rates of groundwater discharge to the marine surface waters that bound Fire Island and the concentrations of nitrogen entering these water bodies from the aquifer's discharge zones. In 2004, the U.S. Geological Survey (USGS), in cooperation with the NPS, began a 3-year investigation to (1) measure groundwater levels within four local study areas at FIIS, (2) collect groundwater samples from these areas for nutrient (nitrogen) analysis, (3) develop a three-dimensional model of the hydrologic system and adjacent saltwater bodies for groundwater-flow delineation and particle tracking, and (4) apply the results of groundwater-discharge simulations to calculate the annual nitrogen loads in these discharges, particularly those entering Great South Bay, which together with the other back bays receives an estimated 80 percent of the total groundwater discharge from Fire Island. The four areas on which the investigation focused were the communities of Kismet and Robbins Rest, the NPS Visitor Center at Watch Hill, and the undeveloped Otis Pike Fire Island High Dune Wilderness (shown in panels A, B, C, and D in fig. 2); these were

Open-File Report↗
USGS2009

Size distribution of submarine landslides along the U.S. Atlantic margin

Assessment of the probability for destructive landslide-generated tsunamis depends on the knowledge of the number, size, and frequency of large submarine landslides. This paper investigates the size distribution of submarine landslides along the U.S. Atlantic continental slope and rise using the size of the landslide source regions (landslide failure scars). Landslide scars along the margin identified in a detailed bathymetric Digital Elevation Model (DEM) have areas that range between 0.89??km 2 and 2410??km 2 and volumes between 0.002??km 3 and 179??km 3 . The area to volume relationship of these failure scars is almost linear (inverse power-law exponent close to 1), suggesting a fairly uniform failure thickness of a few 10s of meters in each event, with only rare, deep excavating landslides. The cumulative volume distribution of the failure scars is very well described by a log-normal distribution rather than by an inverse power-law, the most commonly used distribution for both subaerial and submarine landslides. A log-normal distribution centered on a volume of 0.86??km 3 may indicate that landslides preferentially mobilize a moderate amount of material (on the order of 1??km 3 ), rather than large landslides or very small ones. Alternatively, the log-normal distribution may reflect an inverse power law distribution modified by a size-dependent probability of observing landslide scars in the bathymetry data. If the latter is the case, an inverse power-law distribution with an exponent of 1.3 ?? 0.3, modified by a size-dependent conditional probability of identifying more failure scars with increasing landslide size, fits the observed size distribution. This exponent value is similar to the predicted exponent of 1.2 ?? 0.3 for subaerial landslides in unconsolidated material. Both the log-normal and modified inverse power-law distributions of the observed failure scar volumes suggest that large landslides, which have the greatest potential to generate damaging tsunamis, occur infrequently along the margin. ?? 2008 Elsevier B.V.

Marine Geology↗
USGS1988

Relic magma chamber structures preserved within the Mesozoic North Atlantic crust?

The North Atlantic Transect seismic reflection data, collected southwest of Bermuda, have been reinterpreted following post-stack migration and reveal two major intracrustal reflections. The shallower of these two events, located ∼1 s below the igneous basement, is a subhorizontal, undulating surface that in some places is continuous for as much as 10 km. On the basis of its position within the section and its laterally discontinuous nature, we believe that this upper crustal reflection corresponds to the intermittently sharp contact between the sheeted dikes and the underlying isotropic gabbro. A second set of lower crustal reflections, dipping ∼20°-40° eastward, is also prominent on the migrated profile and terminates downdip against the subhorizontal reflection Moho. Several lines of evidence argue against these features being either artifacts or out-of-the-plane events. Instead, their presence may be ascribed either to crustal-penetrating fault zones or to mafic-ultramafic cumulate layers frozen into the oceanic crust at the time of formation at the paleo-spreading center. Because of the laminated character of these events and their typical occurrence within 1.0 to 1.5 s of the reflection Moho, we prefer a compositional versus a structural interpretation for their origin. The gradual thinning in the crust approaching the fracture zones is shown to be more complex than was originally inferred; although the interpretation that the crust gradually thins toward fracture zones may still apply in a few localities, significant departures are recognized elsewhere. Similarly, the improved image on the migrated profile documents an increase in complexity across the localized region directly surrounding the Blake Spur fracture zone. An interpretation advocating crustal thickening in this narrow zone is proposed as an alternative to the crustal-thinning model of Mutter and others.

Geological Society of America Bulletin↗
USGS1984

Models for the deposition of Mesozoic-Cenozoic fine-grained organic-carbon-rich sediment in the deep sea

The widespread occurrence of organic-carbon-rich strata (‘black shales’) in certain portions of Jurassic, Cretaceous and Cenozoic sequences has been well-documented from Deep Sea Drilling Project sites in the Atlantic and Pacific Oceans and from sequences, now exposed on land, originally deposited in the Tethyan ocean. These ancient black shales usually have been explained by analogy with examples of modern deep-sea sediments in which organic matter locally is preserved by (1) increasing the supply of organic matter, (2) increasing the rate of sedimentation, and/or (3) decreasing the oxygen content of the bottom water. However, detailed examination of many black shales reveals characteristics that cannot be explained by simple local models, including: their approximate coincidence in time globally; their occurrence in a variety of different environments, including open oxygenated oceans, restricted basins, deep and shallow water; their interbedding with organic-carbonpoor strata which often dominate a so-called black shale sequence; their deposition by pelagic, hemipelagic, turbiditic and other processes; and the variations in type and amount of organic matter that occur even within the same sequence. A more complex model for the origin of black shales therefore appears most appropriate, in which the cyclic preservation of organic matter depends on the interplay of the three main variables, namely supply of organic matter, sedimentation rate, and deep-water oxygenation, each of which varies independently to some extent. The variation and relative importance of these parameters in individual basins and widespread black shale deposition in general are linked globally and temporally by changes in global sea-level, climate and related changes in oceanic circulation. An important and often overlooked factor for the supply of organic matter to deep-basin sediments is the frequency and magnitude of redepositional processes. The interplay of these variables is discussed in relation to the middle Cretaceous and Cenozoic organic-carbon-rich strata, in particular, which show marked differences in the relative importance of the different variables.

Geological Society of London Special Publications↗
USGS1997

A geologic history of the north-central Appalachians. Part 1. Orogenesis from the Mesoproterozoic through the Taconic orogeny

The north-central Appalachians lie just southwest of the boundary between the central/southern and northern Appalachians, occupying a critical position within the 3000+ km-long Appalachian orogen. The history of tectonic activity in the north-central Appalachians spans more than one billion years, from the assembly and breakup of a Neoproterozoic supercontinent, through active orogenesis during Laurentia's Paleozoic northward journey off the western margin of West Gondwana, to the Mesozoic transformation of the active orogen into a passive margin during Pangea's disassembly. The major tectonic events include five compressional orogenies and two extensional episodes: 1. The late Mesoproterozoic Grenville orogeny assembled several continental masses into the Neoproterozoic supercontinent, Rodinia. It was the most widespread but now least exposed tectonism, and rocks involved in this event underlie as basement most of the exposed north-central Appalachians. Large fragments of Laurentian Grenville rock were subsequently broken off and incorporated in later Appalachian orogenesis. 2. Crustal extension and rifting late in the Neoproterozoic and into the earliest Cambrian separated Laurentia from West Gondwana, thereby forming the intervening Theic ocean and two continental rifts on Laurentia's eastern margin, the Catoctin rift and, later in the Middle? to Late Cambrian, the so-called "Rome" trough. The initial siliciclastic sedimentation on the margin migrated westward through time onto the craton, supplanted by a thick and increasingly wide carbonate shelf on Laurentia's eastern margin. Two microcontinents of Grenville-age, non-Laurentian(?) continental rock became positioned east of Laurentia, thereby creating the Octoraro sea as an arm of Theia. 3. In Theia, east of the two microcontinents, magmatic arcs developed over a subduction zone late in the Cambrian. Convergence within Theia caused the Potomac orogeny, which obduced the arcs (Wilmington Complex, Cecil Amalgamate) over the microcontinents and associated Theic deposits (including accretionary wedge sediments). 4. Continued westward convergence collapsed the Octoraro sea, producing the Middle to Late Ordovician Taconic orogeny in which the Potomac-deformed magmatic arcs and associated Theic elements were obduced onto the Laurentian continental margin. This obduction: drowned the carbonate shelf with siliciclastic sediments (Martinsburg Formation); drove continental rise and basinal deposits over the carbonate shelf on the Martic thrust; slid the Hamburg klippe onto the shelf; and accreted the Potomac package of microcontinent/arc/basinal-sediments onto the Laurentian margin. This orogeny transformed the broad Early Paleozoic carbonate shelf into the Appalachian basin that persisted throughout the Middle and Late Paleozoic. 5. The Middle Devonian Acadian orogeny ended the largely paralic environment that dominated the Appalachian basin during the Late Silurian. Active orogenesis in New England probably extended southward to the north-central Appalachians, because a vast amount of terrigenous sediment was introduced into the Appalachian basin to form the Catskill delta; however, evidence of actual Acadian deformation and metamorphism is lacking at this latitude. These presumed internal Acadides have yet to be found. 6. The convergence of West Gondwana and Laurentia during the Late Carboniferous and earliest Permian produced the Permian Alleghany orogeny in the north-central Appalachians. This widespread decollement tectonism directly affected a larger area of the presently exposed central and southern Appalachians than any earlier Paleozoic tectonic event. An early layer-parallel shortening phase gave way to a fold-thrust development above a basal decollement. This Alleghany fold-and-thrust tectonism created long, arcuate folds in the Appalachian basin. Late in the Alleghany orogeny, rock thrust northward over the Carboniferous rocks in the Anthracite region of northeastern Pennsylvania caused anthracitization of the underlying coals. The internal Alleghanides are not presently exposed. 7. Crustal extension in the Late Triassic and Early Jurassic produced numerous local, closed basins along eastern North America. Igneous intrusions and effusions marked the beginning of the Jurassic. By the end of the Early Jurassic, horizontal crustal rebound in response to opening of the Atlantic Ocean rotated the basins by crustal inversion, which folded some within-basin rocks and produced a prominent topographic ridge along the Piedmont, up-dip of the basins. Subsequent erosion of this Piedmont ridge and other parts of the Appalachian orogen fed large volumes of sediment to offshore basins during the remainder of the Mesozoic and throughout the Cenozoic. Each of these orogenies affected most of the Appalachian orogen. The tectonic expression of each orogeny varied along and across the orogen. The elements and structural bodies involved in each also varied along strike. However, many common elements persist from one part of the orogen to another; only a few features are singular in time and space. The tectonic boundary between the central and northern Appalachians is one of these singular features-it is solely an Alleghanian artifact.

American Journal of Science↗
USGS2003

The early Mesozoic Birdsboro central Atlantic margin basin in the Mid-Atlantic region, eastern United States

The early Mesozoic Birdsboro basin (new name) was a single, elongate depositional trough in the present Mid-Atlantic area of the eastern United States, extending north-eastward from central Virginia across Maryland, Pennsylvania, and New Jersey into southern New York. What now remains after erosion comprises the Barboursville, Culpeper, Gettysburg, and Newark remnants. Some 7?? km of late Triassic and early Jurassic continental sediments of varying provenances entered and spread across in the Birdsboro basin in several depositional environments. The five resulting sedimentary lithosomes include feldspathic sandstone, quartzose sandstone, red silty mudstone, gray shale, and fanglomerate. The extensive interbedding, intertonguing, and lateral gradation among these lithosomes suggest that they were contemporary and closely interrelated. The feldspathic sandstone lithosome contains sediment with a southeastern provenance that accumulated in a bajada environment extending the length of the southeastern side of the basin. Sediment in the quartzose sandstone lithosome had a northwestern provenance-the coarse-grained fraction formed regional alluvial fans at the mouths of four major input centers. The finer-grained fraction was deposited in the distal reaches of these fans and in the playa environments in the interfan areas; this fraction formed the red silty mudstone lithosome. Gray/black shales and argillites of the gray shale lithosome accumulated in lacustrine environments in the interfan areas. The fanglomerate lithosome comprises numerous small, lobate deposits of poorly sorted sediment along both basin margins. The location and time of activity of the northwest input centers largely determined the distribution and areal extent of the various depositional environments and consequent lithosome along the length and across the width of the basin. The Birdsboro basin was deformed (tilted, faulted, and folded) sometime after the deposition of the youngest preserved rocks (early Sinemurian). The deformation varied along the length of the basin, producing differences in the amount of tilting, structural elevation, and subsequent erosion. The present erosional remnants create the illusion of four originally separate depositional basins.

Geological Society of America Bulletin↗
USGS2000

Hydrogeology and simulation of ground-water flow at Dover Air Force Base, Delaware

Dover Air Force Base in Kent County, Delaware, has many contaminated sites that are in active remediation. To assist in this remediation, a steady-state model of ground-water flow was developed to aid in understanding the hydrology of the system, and for use as a ground-watermanagement tool. This report describes the hydrology on which the model is based, a description of the model itself, and some applications of the model.Dover Air Force Base is underlain by unconsolidated sediments of the Atlantic Coastal Plain. The primary units that were investigated include the upper Calvert Formation and the overlying Columbia Formation. The uppermost sand unit in the Calvert Formation at Dover Air Force Base is the Frederica aquifer, which is the deepest unit investigated in this report. A confining unit of clayey silt in the upper Calvert Formation separates the Frederica aquifer from the lower surficial aquifer, which is the basal Columbia Formation. North and northwest of Dover Air Force Base, the Frederica aquifer subcrops beneath the Columbia Formation and the upper Calvert Formation confining unit is absent. The Calvert Formation dips to the southeast. The Columbia Formation consists predominately of sands, silts, and gravels, although in places there are clay layers that separate the surficial aquifer into an upper and lower surficial aquifer. The areal extent of these clay layers has been mapped by use of gamma logs. Long-term hydrographs reveal substantial changes in both seasonal and annual ground-water recharge. These variations in recharge are related to temporal changes in evaporation, transpiration, and precipitation. The hydrographs show areas where extensive silts and clays are present in the surficial aquifer. In these areas, the vertical gradient between water levels in wells screened above and below the clays can be as large as several feet, and local ground-water highs typically form during normal recharge conditions. When drought conditions persist, water drains off these highs and the vertical gradients decrease. At the south end of Dover Air Force Base, hydrographs of water levels in the Frederica aquifer show that off-Base pumping can cause the water levels to decline below sea level during part of the year.A 4-layer, steady-state numerical model of ground-water flow was developed for Dover Air Force Base and the surrounding area. The upper two layers represent the upper and lower surficial aquifers, which are in the Columbia Formation. In some areas of the model, a semi-confining unit is used to represent an intermittent clay layer between the upper and lower surficial aquifer. This semi-confining unit causes the local groundwater highs in the surficial aquifer. The third model layer represents the upper part of the Calvert Formation, a confining unit. The fourth model layer represents the Frederica aquifer. The model was calibrated to hydraulic heads and to ground-water discharge in Pipe Elm Branch, both of which were measured in September 1997. For the calibrated model, the root-mean-squared errors for the hydraulic heads and the ground-water discharge in the Pipe Elm Branch were 9 percent of the range of head and 3 percent of discharge, respectively. Heads simulated by use of the model were consistent with a map showing average water levels in the region. The U.S. Geological Survey?s MODPATH program was used to simulate ground-water-flow directions for several areas on the Base. This analysis showed the effects of the local groundwater highs. In these areas, ground water can flow from the highs and then dramatically change flow direction as it enters the lower surficial aquifer. The steady-state model has several limitations. The entire ground-water system is under transient hydraulic conditions, due mainly to seasonal and yearly changes in recharge and to withdrawal from irrigation wells. Yet this steady-state model is still considered to be an effective tool for understanding the ground-water-flow system u

Water-Resources Investigations Report↗
USGS2001

Deposition of sedimentary organic matter in black shale facies indicated by the geochemistry and petrography of high-resolution samples, blake nose, western North Atlantic

A transect of three holes drilled across the Blake Nose, western North Atlantic Ocean, retrieved cores of black shale facies related to the Albian Oceanic Anoxic Events (OAE) lb and ld. Sedimentary organic matter (SOM) recovered from Ocean Drilling Program Hole 1049A from the eastern end of the transect showed that before black shale facies deposition organic matter preservation was a Type III-IV SOM. Petrography reveals that this SOM is composed mostly of degraded algal debris, amorphous SOM and a minor component of Type III-IV terrestrial SOM, mostly detroinertinite. When black shale facies deposition commenced, the geochemical character of the SOM changed from a relatively oxygen-rich Type III-IV to relatively hydrogen-rich Type II. Petrography, biomarker and organic carbon isotopic data indicate marine and terrestrial SOM sources that do not appear to change during the transition from light-grey calcareous ooze to the black shale facies. Black shale subfacies layers alternate from laminated to homogeneous. Some of the laminated and the poorly laminated to homogeneous layers are organic carbon and hydrogen rich as well, suggesting that at least two SOM depositional processes are influencing the black shale facies. The laminated beds reflect deposition in a low sedimentation rate (6m Ma-1) environment with SOM derived mostly from gravity settling from the overlying water into sometimes dysoxic bottom water. The source of this high hydrogen content SOM is problematic because before black shale deposition, the marine SOM supplied to the site is geochemically a Type III-IV. A clue to the source of the H-rich SOM may be the interlayering of relatively homogeneous ooze layers that have a widely variable SOM content and quality. These relatively thick, sometimes subtly graded, sediment layers are thought to be deposited from a Type II SOM-enriched sediment suspension generated by turbidities or direct turbidite deposition.

Special Papers of the Geological Society of London↗