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At least 595 records · Page 33Linked to original sources
USGS2021

Deepwater debrites and linked megaturbidites in confined basins: An example from the Onnuri Basin, East Sea of Korea

We analyzed data from seven piston cores, multi-channel seismic-reflection (MCS) and chirp profiles, and multibeam echosounder (MBES) data to study the distribution, emplacement time, sedimentary facies, and depositional processes of sediment-gravity-flow deposits in the Onnuri Basin, a confined basin in the East Sea. These data reveal that debris flows have traveled ca. 30 km downslope, forming a seismic facies consisting of stacked, wedge-shaped, transparent units separated by high-amplitude continuous reflectors. Analysis of piston cores shows three distinct sedimentary units, throughout the basin. The lowest unit, I, is a debrite containing numerous mud clasts of varying size and color distributed in a mud-rich matrix; it is absent over elevated basinal highs or ridges, such as the Onnuri Ridge, suggesting that local topography controls its distribution. The debrite forms a recognizable acoustically transparent layer on subbottom chirp profiles (av. 7 m thick), covers approximately 500 km 2 , and has an estimated volume of ∼ 3.5 km 3 . The overlying unit, II, contains normally graded beds composed of massive sand, laminated and cross-laminated sand and silt, and a thick cap of structureless mud. This unit is interpreted to be a megaturbidite deposited from turbidity currents that originated from the flow transformation of debris flows on the upper continental slope. The megaturbidite covers the entire basin (at least 650 km 2 ), and has an average thickness of 2.8 m (maximum thickness of 4.35 m), and comprises a volume of 1.8 km 3 . Variations in grain size and sedimentary structures suggest that the megaturbidite was deposited by progressively waning flows that reflected off basin flanks and ridges. The thick (up to 3.65 m) structureless mud cap further indicates deposition in a confined basin. The sharp basal contact, together with the lack of hemipelagic sediments between debrite and overlying megaturbidite, suggest that both were deposited during the same flow event, likely to have originated from a single catastrophic slope failure. Collapsing slide material evolved into a debris flow, from which a turbidite formed by dilution of the debris flow. Radiocarbon dates suggest that the slope failure occurred about 13–11 ka, a time when sea level was ca. 50 m lower than at the present day. Hemipelagic sediments in the topmost unit, III-2, above the megaturbidite indicate that the basin has been stable since ca. 11 ka. We provide robust evidence that submarine slope failures evolve downslope into slides, debris flows, and finally, thick megaturbidites. This contribution highlights the importance of seafloor morphology on the distribution and stratigraphy of submarine flows in confined basins.

Onnuri Basin↗
USGS2022

Sedimentary geochemistry of deepwater slope deposits in southern Lake Tanganyika (East Africa): Effects of upwelling and minor lake level oscillations

Lake Tanganyika ranks among the most valuable modern analogs for understanding depositional processes of carbonaceous sediments in ancient tropical rifts. Prior research on Lake Tanganyika has emphasized the importance of bottom-water anoxia, depositional processes (hemipelagic settling versus gravity flows), and large-scale (100s of meters) lake level change on the quality of sedimentary organic matter content. Here, facies analysis and numerous organic geochemical tools (elemental, carbon isotope, and programmed pyrolysis) were applied to a radiocarbon-dated core from southern Lake Tanganyika to investigate the accumulation of carbonaceous sediments in a deepwater slope environment influenced by high-frequency climatic fluctuations accompanied by only minor (10s of meters) lake level changes. Considerable variability in lithofacies and geochemistry characterizes the ∼ 1030-year-long core record, chiefly driven by climate-mediated changes to the lake's upwelling system. Laminated diatom oozes and sapropels with mean total organic carbon (TOC) concentrations and hydrogen indices of 6.9 wt.% and 385 mg hydrocarbon/g TOC, respectively, characterize sediments deposited during periods of strong upwelling and variable water levels. Silty sediments deposited via gravity-flow processes were likewise rich in organic matter, likely due to preservation-enhancing bottom-water anoxia. Dilution by reworked tephra was the chief constraint on organic enrichment at the study site. Data from this study reveal that oscillations in atmospheric and limnological processes in the absence of major shoreline movements can result in geochemically diverse deepwater slope sediments, which have implications for improving depositional models of petroliferous continental rift basins.

Journal of Sedimentary Research↗
USGS2013

The role of vermetid gastropods in the development of the Florida Middle Ground, northeast Gulf of Mexico

The Florida Middle Ground is a complex of north to northwest trending ridges that lie approximately 180 km northwest of Tampa Bay, Florida. The irregular ridges appear on the otherwise gently sloping West Florida shelf and exhibit between 10-15 m of relief. Modern studies interpret the ridges as remnants of a Holocene coral-reef buildup that today provide a hard substrate for growth of a variety of benthic organisms including hydrocorals, scleractinians, alcyonarians, and algae. Recent rotary coring reveals that the core of the eastern ridge of the Florida Middle Ground complex consists of unconsolidated marine calcareous muddy sand that is capped by a boundstone composed primarily of the sessile vermetid gastropod Petaloconchus sp., and overlays a weathered, fossiliferous limestone. Accelerator Mass Spectrometry radiocarbon ages (uncalibrated) on the 3.6-m thick vermetid worm rock indicate that it developed during a sea-level stillstand in the early Holocene (8,225 ±30-8,910 ± 25 yr B.P.). Our observations suggest that the Florida Middle Ground is a remnant of a series of shore parallel bars that formed in the early Holocene and were capped by a 3.6-m thick unit of vermetid gastropods. During a rapid sea-level rise that began ~8,000 yr B.P. the vermetids growth ceased and the worm rock preserved the ridges structure. Diver observations document that the edges of the ridges are currently being eroded and undermined by biological activity and current action, leading to calving of large capstone blocks.

Florida↗
USGS2018

Unsaturated zone CO2, CH4, and δ13C-CO2 at an arid region low-level radioactive waste disposal site

Elevated tritium, radiocarbon, Hg, and volatile organic compounds associated with low-level radioactive waste (LLRW) at the USGS Amargosa Desert Research Site (ADRS) have stimulated research on factors and processes that affect contaminant gas distribution and transport. Consequently, we examined the sources, mixing, and biogeochemistry of CO 2 and CH 4 , two additional important species in the unsaturated zone at ADRS. In spring 2015 and 2016, shallow unsaturated zone gas samples were collected from the 1.5-m depth both inside and outside the LLRW disposal area. Samples also were collected from two 110-m-deep multilevel gas-sampling boreholes and a distant background site. These samples were analyzed for CO 2 mole fraction ( x CO 2 ) and C isotopic composition (δ 13 C-CO 2 ) and CH 4 mole fraction ( x CH 4 ). Graphical analysis of the results indicates mixing of CO 2 characteristic of the root zone (δ 13 C −18 to −19‰), deep soil gas of the capillary fringe (−13 to −15‰), and CO 2 produced by microbial respiration of organic matter disposed in the LLRW trenches (−22 to −25‰). Distribution of CH 4 overall reflects atmospheric sources and production in anaerobic microzones in the LLRW area and methanotrophy in the undisturbed shallow subsurface outside the LLRW area. Although x CH 4 reflecting lateral transport from the LLRW area is decreasing with time in the deep profiles, deep unsaturated zone x CO 2 has changed little in recent decades. The results imply that CH 4 and δ 13 C-CO 2 may serve as good tracers of anthropogenic effects in the unsaturated zone even when CO 2 primarily reflects natural processes.

Nevada↗
USGS2014

Loess as a Quaternary paleoenvironmental indicator

Loess is aeolian sediment that is dominated by silt-sized particles. Unlike either coarser dune sand or finer-grained, long-range-transported dust, loess is relatively poorly sorted, reflecting a combination of transport processes, including saltation, low suspension, and high suspension. Loess can be readily identified in the field; deposits range in thickness from a few centimeters to many tens of meters, and are found over large areas of Eurasia, South and North America (Fig. 1), and smaller areas of New Zealand, Australia, Africa and the Middle East. Loess covers approximately 10% of the Earth’s land surface and is therefore one of the most important terrestrial archives of paleoenvironmental change during the Quaternary. In many regions, loess sections consist of deposits of mostly unaltered sediment with intercalated paleosols. Paleosols represent periods of landscape stability when loess deposition ceased altogether, or at least slowed significantly. Loess can be dated directly using luminescence, radiocarbon, and amino acid geochronology methods.

Past Global Changes↗
USGS1983

Late Holocene ice wedges near Fairbanks, Alaska, USA: Environmental setting and history of growth

Test trenches excavated into muskeg near Fairbanks in 1969 exposed a polygonal network of active ice wedges. The wedges occur in peat that has accumulated since about 3500 yr BP and have grown episodically as the permafrost table fluctuated in response to fires, other local site conditions and perhaps regional climatic changes. Radiocarbon dates suggest one or two episodes of ice-wedge growth between about 3500 and 2000 yr BP as woody peat accumulated at the site. Subsequent wedge truncation evidently followed a fire that charred the peat. Younger peat exhibits facies changes between sedge-rich components that filled troughs over the ice wedges and woody bryophytic deposits that formed beyond the troughs. A final episode of wedge development took place within the past few hundred years. Pollen data from the site indicate that boreal forest was present throughout the past 6000 yr, but that it underwent a gradual transition from a predominantly deciduous to a spruce-dominated assemblage. This change may reflect either local site conditions or a more general climatic shift to cooler, moister summers in late Holocene time. The history of ice-wedge growth shows that wedges can form and grow to more than 1 m apparent width under mean annual temperatures that probably are close to those of the Fairbanks area today (-3.5°C) and under vegetation cover similar to that of the interior Alaskan boreal forest. The commonly held belief that ice wedges develop only below mean annual air temperatures of -6 to -8°C in the zone of continuous permafrost is invalid.

Alaska↗
USGS2017

Thorne Cave, northeastern Utah: Geology

Geologic interest in Thorne Cave stems from its link with valley alluvium along Cliff Creek, which accumulated to a height of 48 ft., continued to build up another 13 ft. while men lived here, and then reached 30 ft. higher-sealing in the signs of man. Mineralogic study shows that ground water then circulated through the cave deposits for a considerable time. The alluvium is correlated with the lower part of the Tsegi Formation of the Navajo country. Cutting of a terrace at mid-depth in the valley alluvium reopened Thorne Cave , probably before the Christian era, and desert varnish then began to form on the cave brow. Radiocarbon dates from presumably correlative deposits suggest that the cave debris is about 4000 years old-a conclusion consistent with dates of 4230 and 4170 years from Thorne Cave .

Utah↗
USGS2024

Neotectonic mapping of Puerto Rico

Puerto Rico is part of the Puerto Rico-Virgin Islands microplate, along the Caribbean–North American plate boundary between the Puerto Rico trench subduction zone and the Muertos Trough incipient subduction zone. Despite recent seismicity and geodetically constrained deformation of ~3 mm/yr of left-lateral shear across the island, Quaternary fault locations remain largely uncertain. Preservation of recent faulting in the landscape is masked by distributed faulting, high weathering rates of the tropical climate, steep topography, frequent landsliding, and extensive agriculture and urbanization along coastlines and river valleys. We present remote neotectonic mapping of historical aerial imagery from the 1930s-60s and <1-m lidar topography, integrated with field observations, to create an updated active fault map of Puerto Rico. We focus on faults that offset younger geomorphic surfaces, with ages estimated by geologic mapping, OSL and radiocarbon dating, and morphologic interpretations. We present new evidence for Quaternary activity on seven faults, including the South Lajas, Salinas, Punta Montalva, Great Southern Puerto Rico, Cerro Goden, Parguera, and San Marcos faults. We find that active faulting occurs preferentially along the southern and western coasts and does not always spatially coincide with preexisting bedrock faults, possibly reflecting partial reactivation of older faults in the modern strain field.

Puerto Rico↗
USGS1992

A postulated new source for the White River Ash, Alaska: A section in Geologic studies in Alaska by the US. Geological Survey, 1990

The White River Ash (Lerbekmo and others, 1968), product of two of the most voluminous pyroclastic eruptions in North America in the past 2,000 yr, blankets much of the Yukon Terrtory, Canada, and a small part of adjoining eastern Alaska. Lerbekmo and Campbell (1969) narrowed the source of the ash to an area northeast of the Mt. Bona-Mt. Churchill massif in the St. Elias Mountains of southern Alaska. Based on indirect evidence, Lerbekmo and Campbell (1969) further suggested that the vent was beneath the Klutlan Glacier, adjacent to a mound of coarse pumice, 16 km northeast of Mt. Bona. Recently discovered pumice and ash deposits and a possible vent structure near the summit of Mt. Churchill suggest an alternate source area. The White River Ash is a bilobate plinian fallout deposit covering more than 340,000 km 2 and containing an estimated 25-50 km 3 of tephra (Bostock, 1952; Berger, 1960; fig. 1). Radiocarbon ages indicate that the northern lobe was deposited about 1,887 yr B.P. and the eastern, and larger, lobe about 1,250 yr B.P. (Lerbekmo and others, 1975). The axes of the two lobes converge near Mt. Bona (16,420 ft (5,005 m)) and Mt. Churchill [15,638 ft (4,766 m)], which together form a prominent massif in the St. Elias Mountains. The Klutlan Glacier, a large valley glacier that flows eastward into Canada, has its principal source on the eastern flank of the massif.

Alaska↗
USGS2003

Tufts submarine fan: turbidity-current gateway to Escanaba Trough

Turbidity-current overflow from Cascadia Channel near its western exit from the Blanco Fracture Zone has formed the Tufts submarine fan, which extends more than 350 km south on the Pacific Plate to the Mendocino Fracture Zone. For this study, available 3.5-kHz high-resolution and airgun seismic-reflection data, long-range side-scan sonar images, and sediment core data are used to define the growth pattern of the fan. Tufts fan deposits have smoothed and filled in the linear ridge-and-valley relief over an area exceeding 23,000 km2 on the west flank of the Gorda Ridge. The southernmost part of the fan is represented by a thick (as much as 500 m) sequence of turbidite deposits ponded along more than 100 km of the northern flank of the Mendocino Fracture Zone. Growth of the Tufts fan now permits turbidity-current overflow from Cascadia Channel to reach the Escanaba Trough, a deep rift valley along the southern axis of the Gorda Ridge. Scientific drilling during both the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP) provided evidence that the 500-m-thick sediment fill of Escanaba Trough is dominantly sandy turbidites. Radiocarbon dating of the sediment at ODP Site 1037 showed that deposition of most of the upper 120 m of fill was coincident with Lake Missoula floods and that the provenance of the fill is from the eastern Columbia River drainage basin. The Lake Missoula flood discharge with its entrained sediment continued flowing downslope upon reaching the ocean as hyperpycnally generated turbidity currents. These huge turbidity currents followed the Cascadia Channel to reach the Pacific Plate, where overbank flow provided a significant volume of sediment on Tufts fan and in Escanaba Trough. Tufts fan and Tufts Abyssal Plain to the west probably received turbidite sediment from the Cascadia margin during much of the Pleistocene.

Escanaba Trough↗
USGS2019

Digital database of the geologic map of the middle east rift geothermal subzone, Kīlauea Volcano, Hawai‘i

This database release contains all the information used to produce Geologic Investigations Series I-2614 ( https://pubs.usgs.gov/imap/2614/ ). The main component of this digital release is a geodatabase prepared using ArcGIS, but Esri shapefiles are included as well. Kīlauea is an active shield volcano in the southeastern part of the Island of Hawai‘i. The middle East Rift Zone (MERZ) map includes about 27 square kilometers of the MERZ and shows the distribution of the products of 34 separate eruptions during late Holocene time. Lava flows erupted during 1983–86 have reached the mapped area. The subaerial part of the MERZ is 3–4 km wide and about 18 km long. It is a constructional ridge, 50–150 m above the adjoining terrain, marked by low spatter ramparts and cones as high as 60 m. Lava typically flowed either northeast or southeast, depending on vent location relative to the topographic crest of the rift zone. The MERZ receives more than 100 inches of rainfall annually and is covered by tropical rain forest. Vegetation begins to grow on lava a few months after its eruption. Relative heights of trees can be a guide to relative ages of underlying lava flows, but proximity to faults, presence of easily weathered cinders, and human activity also affect the rate of growth. The rocks have been grouped into five basic age groups. The framework for the ages assigned is provided by eight radiocarbon ages from nearby mapping by the authors and a single date from within this investigation area. The numerical ages are supplemented by observations of stratigraphic relations, degree of weathering, soil development, and vegetative cover.

Hawaii↗
USGS2006

Geologic map of the middle east rift geothermal subzone, Kīlauea Volcano, Hawaiʻi

Kīlauea is an active shield volcano in the southeastern part of the Island of Hawai'i. The middle east rift zone (MERZ) map includes about 27 square kilometers of the MERZ and shows the distribution of the products of 37 separate eruptions during late Holocene time. Lava flows erupted during 1983-96 have reached the mapped area. The subaerial part of the MERZ is 3-4 km wide and about 18 km long. It is a constructional ridge, 50-150 m above the adjoining terrain, marked by low spatter ramparts and cones as high as 60 m. Lava typically flowed either northeast or southeast, depending on vent location relative to the topographic crest of the rift zone. The MERZ receives more than 100 in. of rainfall annually and is covered by tropical rain forest. Vegetation begins to grow on lava a few months after its eruption. Relative heights of trees can be a guide to relative ages of underlying lava flows, but proximity to faults, presence of easily weathered cinders, and human activity also affect the rate of growth. The rocks have been grouped into five basic age groups. The framework for the ages assigned is provided by eight radiocarbon ages from previous mapping by the authors and a single date from the current mapping effort. The numerical ages are supplemented by observations of stratigraphic relations, degree of weathering, soil development, and vegetative cover.

Hawaii↗
USGS2003

Ferricrete, manganocrete, and bog iron occurrences with selected sedge bogs and active iron bogs and springs in the upper Animas River watershed, San Juan County, Colorado

During 1996 to 2000, the Bureau of Land Management, National Park Service, Environmental Protection Agency, United States Department of Agriculture (USDA) Forest Service, and the U.S. Geological Survey (USGS) developed a coordinated strategy to (1) study the environmental effects of historical mining on Federal lands, and (2) remediate contaminated sites that have the greatest impact on water quality and ecosystem health. This dataset provides information that contributes to these overall objectives and is part of the USGS Abandoned Mine Lands Initiative. Data presented here represent ferricrete occurrences and selected iron bogs and springs in the upper Animas River watershed in San Juan County near Silverton, Colorado. Ferricretes (stratified iron and manganese oxyhydroxide-cemented sedimentary deposits) are one indicator of the geochemical baseline conditions as well as the effect that weathering of mineralized rocks had on water quality in the Animas River watershed prior to mining. Logs and wood fragments preserved in several ferricretes in the upper Animas River watershed, collected primarily along streams, yield radiocarbon ages of modern to 9,580 years B.P. (P.L. Verplanck, D.B. Yager, and S.E. Church, work in progress). The presence of ferricrete deposits along the current stream courses indicates that climate and physiography of the Animas River watershed have been relatively constant throughout the Holocene and that weathering processes have been ongoing for thousands of years prior to historical mining activities. Thus, by knowing where ferricrete is preserved in the watershed today, land-management agencies have an indication of (1) where metal precipitation from weathering of altered rocks has occurred in the past, and (2) where this process is ongoing and may confound remediation efforts. These data are included as two coverages-a ferricrete coverage and a bogs and springs coverage. The coverages are included in ArcInfo shapefile and ArcInfo interchange file format.

Colorado↗
USGS2004

Photomosaics and logs of trenches on the San Andreas Fault, Thousand Palms Oasis, California

We present photomosaics and logs of the walls of trenches excavated for a paleoseismic study at Thousand Palms Oasis (Fig. 1). The site is located on the Mission Creek strand of the San Andreas fault zone, one of two major active strands of the fault in the Indio Hills along the northeast margin of the Coachella Valley (Fig. 2). The Coachella Valley section is the most poorly understood major part of the San Andreas fault with regard to slip rate and timing of past large-magnitude earthquakes, and therefore earthquake hazard. No large earthquakes have occurred for more than three centuries, the longest elapsed time for any part of the southern San Andreas fault. In spite of this, the Working Group on California Earthquake Probabilities (1995) assigned the lowest 30-year conditional probability on the southern San Andreas fault to the Coachella Valley. Models of the behavior of this part of the fault, however, have been based on very limited geologic data. The Thousand Palms Oasis is an attractive location for paleoseismic study primarily because of the well-bedded late Holocene sedimentary deposits with abundant layers of organic matter for radiocarbon dating necessary to constrain the timing of large prehistoric earthquakes. Previous attempts to develop a chronology of paleoearthquakes for the region have been hindered by the scarcity of in-situ 14C-dateable material for age control in this desert environment. Also, the fault in the vicinity of Thousand Palms Oasis consists of a single trace that is well expressed, both geomorphically and as a vegetation lineament (Figs. 2, 3). Results of our investigations are discussed in Fumal et al. (2002) and indicate that four and probably five surface-rupturing earthquakes occurred along this part of the fault during the past 1200 years. The average recurrence time for these earthquakes is 215 ± 25 years, although interevent times may have been as short as a few decades or as long as 400 years. Thus, although the elapsed time since the most recent earthquake, about 320 years, is about 50% longer than the average recurrence time, it is not necessarily unprecedented.

California↗
USGS2004

The Catfish Lake Scarp, Allyn, Washington: Preliminary field data and implications for earthquake hazards posed by the Tacoma fault

The Tacoma fault bounds gravity and aeromagnetic anomalies for 50 km across central Puget lowland from Tacoma to western Kitsap County. Tomography implies at least 6 km of post-Eocene uplift to the north of the fault relative to basinal sedimentary rocks to the south. Coastlines north of the Tacoma fault rose about 1100 years ago during a large earthquake. Abrupt uplift up to several meters caused tidal flats at Lynch Cove, North Bay, and Burley Lagoon to turn into forested wetlands and freshwater marshes. South of the fault at Wollochet Bay, Douglas-fir forests sank into the intertidal zone and changed into saltmarsh. Liquefaction features found beneath the marsh at Burley Lagoon point to strong ground shaking at the time of uplift. Recent lidar maps of the area southwest of Allyn, Washington revealed a 4 km long scarp, or two closely spaced en-echelon scarps, which correspond closely to the Tacoma fault gravity and aeromagnetic anomalies. The scarp, named the Catfish Lake scarp, is north-side-up, trends east-west, and clearly displace striae left by a Vashon-age glacier. A trench across the scarp exposed evidence for postglacial folding and reverse slip. No organic material for radiocarbon dating was recovered from the trench. However, relationships in the trench suggest that the folding and faulting is postglacial in age.

Washington↗
USGS2004

Pocomoke Sound Sedimentary and Ecosystem History

Summary of Results: Pocomoke Sound Sediment and Sediment Processes Transport of sediment from coastal marshes. Analyses of pollen and foraminifera from surface sediments in Pocomoke Sound suggest that neither the upstream forested wetlands nor coastal marshes bordering the sound have contributed appreciably to particulate matter in the 10- to 1000-micron size range that is currently being deposited in the sound. Sediment processes derived from short-lived isotope. Analyses of beryllium-7, cesium-137 and lead-210 and redox sensitive elements from Pocomoke sediments showed that there has been a significant increase in anthropogenic elements since the late 1940's when the Delmarva Peninsula became more accessible from the Baltimore-Washington region. Cesium-137 was found to be a useful tool to determine changes in sedimentation within the system. Three major stages of sedimentation occurred. Before 1950, the system was equilibrium with the agriculture activity in the watershed, whereas urbanization and agricultural activity changes during and immediately preceding World War II resulted in increased sediment flux. Around 1970, the sediment flux diminished and there was an apparent increase in bank erosion sediment to the deeper parts of the system. Rates of sediment deposition. Radiocarbon, lead-210, and pollen dating of sediment cores from Pocomoke Sound indicate relatively continuous deposition of fine-grained sediments in the main Pocomoke channel at > ~7 m water depths. Mean sediment accumulation rates during the past few centuries were relatively high (>1 cm yr -1 ). The ages of coarser-grained sediments (sands) blanketing the shallow (< ~ 7 m water depth) flanks of Pocomoke Sound are not well constrained but were probably deposited discontinuously. Impacts of land-use on benthic biota. The Pocomoke Sound paleoecological record shows that in the 1940-50s and again in the 1970-80s, the sound experienced unprecedented changes in the benthic assemblages of both ostracodes and foraminifera that can be attributed to degradation in water quality. These changes represent perturbations to the natural variability in faunal assemblages, which are normally driven by climatically influenced changes in salinity regimes. Changes in 20th century benthic communities were characterized by the rise to dominance of facultative anaerobic taxa tolerant of hypoxia and detrital-feeding species, reflecting increased influx of organic matter, and perhaps greater turbidity. Results support the hypothesis of Orth et al. (2002) and Orth and Moore (1983) that unprecedented changes to the bay ecosystem affected submerged aquatic vegetation in the Tangiers-Pocomoke region prior to large-scale monitoring began in the 1970s and 80s. Comparison of Pocomoke paleoecological record with those from the mainstem bay indicate that environmental degradation during the 20th century was nearly synchronous bay-wide within the limits of sediment core chronology (10-20 years). Stable isotopic evidence for decadal water quality changes. Stable isotopic records from benthic foraminifera in Pocomoke Sound sediment cores, especially oxygen isotopes, document regional decadal and centennial climate processes which influence salinity and water quality over the past few centuries. These results provide indirect evidence for discharge-driven changes in freshwater and presumably river-borne sediment from the watershed to the sound. They are consistent with studies in the mainstem indicating the important influence of climatic and hydrological processes on water quality. Pollen evidence for high sedimentation and vegetation change during colonial land clearance. Pollen assemblages from sediment cores in Pocomoke Sound document high sedimentation rates (0.7->4.0 cm yr -1 ) at most sites throughout the Sound in post-Colonial time. These results confirm those from other regions of the bay that land-clearance increased the flux of river-borne sediment to certain r

Open-File Report↗
USGS2010

Biological, Physical, And Chemical Data From Gulf of Mexico Core PE0305-GC1

This paper presents benthic foraminiferal census data, and magnetic susceptibility, 210Pb , radiocarbon, and geochemical measurements from gravity core PE0305-GC1 (=GC1). Core GC1 was collected from the Louisiana continental shelf as part of an initiative to investigate the geographic and temporal extent of hypoxia, low-oxygen water, in the Gulf of Mexico. Hypoxia (<1.4 ml/l or <2 ppm oxygen concentration) in Gulf of Mexico waters can eventually lead to death of marine species. The development of hypoxia off the Mississippi delta has increased steadily since routine and systematic measurements were begun in 1985 and has been linked to the use of fertilizer in the Mississippi basin. Benthic foraminifers provide a proxy to track the development of hypoxia prior to 1985. Previous work determined that the relative occurrence of three low-oxygen-tolerant species is highest in the hypoxia zone. The cumulative percentage of these three species (% Pseudononion atlanticum + % Epistominella vitrea, + % Buliminella morgani = PEB index of hypoxia) was used to investigate fluctuation in paleohypoxia in four cores, including the upper 60 cm of GC1. In this report, we compile all available data from GC1 as the basis for further publications.

Open-File Report↗
USGS2006

Preliminary surficial geologic map of a Calico Mountains piedmont and part of Coyote Lake, Mojave desert, San Bernardino County, California

This 1:24,000 scale detailed surficial geologic map and digital database of a Calico Mountains piedmont and part of Coyote Lake in south-central California depicts surficial deposits and generalized bedrock units. The mapping is part of a USGS project to investigate the spatial distribution of deposits linked to changes in climate, to provide framework geology for land use management (http://deserts.wr.usgs.gov), to understand the Quaternary tectonic history of the Mojave Desert, and to provide additional information on the history of Lake Manix, of which Coyote Lake is a sub-basin. Mapping is displayed on parts of four USGS 7.5 minute series topographic maps. The map area lies in the central Mojave Desert of California, northeast of Barstow, Calif. and south of Fort Irwin, Calif. and covers 258 sq.km. (99.5 sq.mi.). Geologic deposits in the area consist of Paleozoic metamorphic rocks, Mesozoic plutonic rocks, Miocene volcanic rocks, Pliocene-Pleistocene basin fill, and Quaternary surficial deposits. McCulloh (1960, 1965) conducted bedrock mapping and a generalized version of his maps are compiled into this map. McCulloh's maps contain many bedrock structures within the Calico Mountains that are not shown on the present map. This study resulted in several new findings, including the discovery of previously unrecognized faults, one of which is the Tin Can Alley fault. The north-striking Tin Can Alley fault is part of the Paradise fault zone (Miller and others, 2005), a potentially important feature for studying neo-tectonic strain in the Mojave Desert. Additionally, many Anodonta shells were collected in Coyote Lake lacustrine sediments for radiocarbon dating. Preliminary results support some of Meek's (1999) conclusions on the timing of Mojave River inflow into the Coyote Basin. The database includes information on geologic deposits, samples, and geochronology. The database is distributed in three parts: spatial map-based data, documentation, and printable map graphics of the database. Spatial data are distributed as an ArcInfo personal geodatabase, or as tabular data in the form of Microsoft Access Database (MDB) or dBase Format (DBF) file formats. Documentation includes this file, which provides a discussion of the surficial geology and describes the format and content of the map data, and Federal Geographic Data Committee (FGDC) metadata for the spatial map information. Map graphics files are distributed as Postscript and Adobe Acrobat Portable Document Format (PDF) files, and are appropriate for representing a view of the spatial database at the mapped scale.

Open-File Report↗