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33 records · Page 2Linked to original sources
USGS2001

A geostatistical approach to predicting sulfur content in the Pittsburgh coal bed

The US Geological Survey (USGS) is completing a national assessment of coal resources in the five top coal-producing regions in the US. Point-located data provide measurements on coal thickness and sulfur content. The sample data and their geologic interpretation represent the most regionally complete and up-to-date assessment of what is known about top-producing US coal beds. The sample data are analyzed using a combination of geologic and Geographic Information System (GIS) models to estimate tonnages and qualities of the coal beds. Traditionally, GIS practitioners use contouring to represent geographical patterns of "similar" data values. The tonnage and grade of coal resources are then assessed by using the contour lines as references for interpolation. An assessment taken to this point is only indicative of resource quantity and quality. Data users may benefit from a statistical approach that would allow them to better understand the uncertainty and limitations of the sample data. To develop a quantitative approach, geostatistics were applied to the data on coal sulfur content from samples taken in the Pittsburgh coal bed (located in the eastern US, in the southwestern part of the state of Pennsylvania, and in adjoining areas in the states of Ohio and West Virginia). Geostatistical methods that account for regional and local trends were applied to blocks 2.7 mi (4.3 km) on a side. The data and geostatistics support conclusions concerning the average sulfur content and its degree of reliability at regional- and economic-block scale over the large, contiguous part of the Pittsburgh outcrop, but not to a mine scale. To validate the method, a comparison was made with the sulfur contents in sample data taken from 53 coal mines located in the study area. The comparison showed a high degree of similarity between the sulfur content in the mine samples and the sulfur content represented by the geostatistically derived contours. Published by Elsevier Science B.V.

International Journal of Coal Geology↗
USGS2007

Multibeam observations of mine burial near Clearwater, FL, including comparisons to predictions of wave-induced burial

A Kongsberg Simrad EM 3000 multibeam sonar (Kongsberg Simrad, Kongsberg, Norway) was used to conduct a set of six repeat high-resolution bathymetric surveys west of Indian Rocks Beach (IRB), just to the south of Clearwater, FL, between January and March 2003, to observe in situ scour and burial of instrumented inert mines and mine-like cylinders. Three closely located study sites were chosen: two fine-sand sites, a shallow one located in ??? 13 m of water depth and a deep site located in ???14 m of water depth; and a coarse-sand site in ???13 m. Results from these surveys indicate that mines deployed in fine sand are nearly buried within two months of deployment (i.e., they sunk 74.5% or more below the ambient seafloor depth). Mines deployed in coarse sand showed a lesser amount of scour, burying until they present roughly the same hydrodynamic roughness as the surrounding rippled bedforms. These data were also used to test the validity of the Virginia Institute of Marine Science (VIMS, Gloucester Point, VA) 2-D burial model. The model worked well in areas of fine sand, sufficiently predicting burial over the course of the experiment. In the area of coarse sand, the model greatly overpredicted the amount of burial. This is believed to be due to the presence of rippled bedforms around the mines, which affect local bottom morphodynamics and are not accounted for in the model, an issue currently being addressed by the modelers. This paper focuses specifically on two instrumented mines: an acoustic mine located in fine sand and an optical instrumented mine located in coarse sand. ?? 2007 IEEE.

IEEE Journal of Oceanic Engineering↗
USGS1996

Petrology and U-PB geochronology of the Robertson River Igneous Suite, Blue Ridge province, Virginia - Evidence for multistage magmatism associated witn an early episode of Laurentian rifting

The Late Neoproterozoic (735-702 Ma) Robertson River Igneous Suite includes at least eight plutons ranging in composition from syenogranite to alkali feldspar granite to alkali feldspar syenite. These plutons intruded Mesoproterozoic (1.2-1.0 Ga) gneissic basement of the Blue Ridge anticlinorium in northern and central Virginia during an early episode of Laurentian rifting. Robertson River plutons range in composition from metaluminous to peralkaline and, relative to other granite types, exhibit compositional characteristics of A-type granitoids including (1) marked enrichment in Nb, Zr, Y, REE (except Eu), and Ga, (2) high Ga/Al and FeO (total) /MgO, and (3) depletion of Ba and Sr. High Ga/Al ratios are particularly diagnostic of the suite and serve as an effective discriminant between originally metaluminous and peralkaline bulk compositions, providing a useful proxy for widely used indicators based on major elements that are prone to remobilization. U-Pb isotopic analyses of zircons indicate that the suite was emplaced in two pulses, occurring at 735 to 722 and 706 to 702 Ma. Metaluminous magmas were emplaced during both pulses, formed most of the main batholith, and fractionated as independent, time-correlative groups. Peralkaline magmas were emplaced only during the final pulse, formed a volcanic center that erupted unknown quantities of rhyolite, and experienced a style of fractionation similar to the metaluminous types. Differences in Ce/Nb, Y/Nb, and Yb/Ta ratios suggest that the metaluminous and peralkaline magmas were derived from different sources. The Robertson River Igneous Suite is part of a regional group of Late Neoproterozoic (760-700 Ma) plutons including at least 20 other A-type granitoid bodies exposed throughout the Laurentian terrane of Virginia and northwestern North Carolina. Like the Robertson River, most of the other granitoids are metaluminous in composition, typically form multi-intrusive, elongate plutons, and are not geographically constrained to the vicinity of major Neoproterozoic volcanic centers. The Late Neoproterozoic plutons of the Laurentian Appalachians are similar in lithology, composition, and style of emplacement to Miocene granitoids that occur on the southern flanks of the present Red Sea rift and that are considered a younger analog of the Appalachian plutons. However, the Red Sea granitoids were emplaced within a few million years of the initial development of oceanic crust whereas the Appalachian plutons appear largely to have pre-dated development of the Iapetus Ocean by about 130 my.

Virginia↗
USGS2014

Model behavior and sensitivity in an application of the cohesive bed component of the community sediment transport modeling system for the York River estuary, VA, USA

The Community Sediment Transport Modeling System (CSTMS) cohesive bed sub-model that accounts for erosion, deposition, consolidation, and swelling was implemented in a three-dimensional domain to represent the York River estuary, Virginia. The objectives of this paper are to (1) describe the application of the three-dimensional hydrodynamic York Cohesive Bed Model, (2) compare calculations to observations, and (3) investigate sensitivities of the cohesive bed sub-model to user-defined parameters. Model results for summer 2007 showed good agreement with tidal-phase averaged estimates of sediment concentration, bed stress, and current velocity derived from Acoustic Doppler Velocimeter (ADV) field measurements. An important step in implementing the cohesive bed model was specification of both the initial and equilibrium critical shear stress profiles, in addition to choosing other parameters like the consolidation and swelling timescales. This model promises to be a useful tool for investigating the fundamental controls on bed erodibility and settling velocity in the York River, a classical muddy estuary, provided that appropriate data exists to inform the choice of model parameters.

Virginia↗
USGS2004

Ecological correlates of fish movement in a network of Virginia streams

Identifying factors that influence fish movement is a key step in predicting how populations respond to environmental change. Using mark-recapture (four species) and trap capture (eight species) data, we examined relationships between three attributes of movement and 15 ecological variables. The probability of emigrating from a reach was positively related to intermittency (one species) and body size (one species) and negatively related to distance from the mainstem creek (two species) and habitat complexity (one species). The number of fish moving upstream through traps was positively related to increases in flow (five species), day length (three species), and water temperature (two species); the number moving through downstream traps was positively associated with increases in flow (three species). Distance moved was greater for fish moving through unsuitable reaches (one species). Floods have a pervasive effect on fish movement, and human activities that affect flows will have widespread implications. The importance of other factors varies interspecifically, which may translate into variation in persistence and colonization rates. For example, species that exhibit reach fidelity in complex habitats may increase movement if habitats are homogenized. These species may suffer population declines because of the cost of increased movement and may ultimately be replaced by ecological generalists.

Canadian Journal of Fisheries and Aquatic Sciences↗
USGS2000

Depletion of Appalachian coal reserves - how soon?

Much of the coal consumed in the US since the end of the last century has been produced from the Pennsylvanian strata of the Appalachian basin. Even though quantities mined in the past are less than they are today, this basin yielded from 70% to 80% of the nation's annual coal production from the end of the last century until the early 1970s. During the last 25 years, the proportion of the nation's coal that was produced annually from the Appalachian basin has declined markedly, and today it is only about 40% of the total. The amount of coal produced annually in the Appalachian basin, however, has been rising slowly over the last several decades, and has ranged generally from 400 to 500 million tons (Mt) per year. A large proportion of Appalachian historical production has come from relatively few counties in southwestern Pennsylvania, northern and southern West Virginia, eastern Kentucky, Virginia and Alabama. Many of these counties are decades past their years of peak production and several are almost depleted of economic deposits of coal. Because the current major consumer of Appalachian coal is the electric power industry, coal quality, especially sulfur content, has a great impact on its marketability. High-sulfur coal deposits in western Pennsylvania and Ohio are in low demand when compared with the lower sulfur coals of Virginia and southern West Virginia. Only five counties in the basin that have produced 500 Mt or more exhibit increasing rates of production at relatively high levels. Of these, six are in the central part of the basin and only one, Greene County, Pennsylvania, is in the northern part of the basin. Decline rate models, based on production decline rates and the decline rate of the estimated, 'potential' reserve, indicate that Appalachian basin annual coal production will be 200 Mt or less by the middle of the next century. Published by Elsevier Science B.V.Much of the coal consumed in the US since the end of the last century has been produced from the Pennsylvanian strata of the Appalachian basin. Even though quantities mined in the past are less than they are today, this basin yielded from 70% to 80% of the nation's annual coal production from the end of the last century until the early 1970s. During the last 25 years, the proportion of the nation's coal that was produced annually from the Appalachian basin has declined markedly, and today it is only about 40% of the total. The amount of coal produced annually in the Appalachian basin, however, has been rising slowly over the last several decades, and has ranged generally from 400 to 500 million tons (Mt) per year. A large proportion of Appalachian historical production has come from relatively few counties in southwestern Pennsylvania, northern and southern West Virginia, eastern Kentucky, Virginia and Alabama. Many of these counties are decades past their years of peak production and several are almost depleted of economic deposits of coal. Because the current major consumer of Appalachian coal is the electric power industry, coal quality, especially sulfur content, has a great impact on its marketability. High-sulfur coal deposits in western Pennsylvania and Ohio are in low demand when compared with the lower sulfur coals of Virginia and southern West Virginia. Only five counties in the basin that have produced 500 Mt or more exhibit increasing rates of production at relatively high levels. Of these, six are in the central part of the basin and only one, Greene County, Pennsylvania, is in the northern part of the basin. Decline rate models, based on production decline rates and the decline rate of the estimated, `potential' reserve, indicate that Appalachian basin annual coal production will be 200 Mt or less by the middle of the next century.

International Journal of Coal Geology↗
USGS2012

Assessment of salinity intrusion in the James and Chickahominy Rivers as a result of simulated sea-level rise in Chesapeake Bay, East Coast, USA

Global sea level is rising, and the relative rate in the Chesapeake Bay region of the East Coast of the United States is greater than the worldwide rate. Sea-level rise can cause saline water to migrate upstream in estuaries and rivers, threatening freshwater habitat and drinking-water supplies. The effects of future sea-level rise on two tributaries of Chesapeake Bay, the James and Chickahominy (CHK) Rivers, were evaluated in order to quantify the salinity change with respect to the magnitude of sea-level rise. Such changes are critical to: 1) local floral and faunal habitats that have limited tolerance ranges to salinity; and 2) a drinking-water supply for the City of Newport News, Virginia. By using the three-dimensional Hydrodynamic-Eutrophication Model (HEM-3D), sea-level rise scenarios of 30, 50, and 100 cm, based on the U.S. Climate Change Science Program for the mid-Atlantic region for the 21st century, were evaluated. The model results indicate that salinity increases in the entire river as sea level rises and that the salinity increase in a dry year is greater than that in a typical year. In the James River, the salinity increase in the middle-to-upper river (from 25 to 50 km upstream of the mouth) is larger than that in the lower and upper parts of the river. The maximum mean salinity increase would be 2 and 4 ppt for a sea-level rise of 50 and 100 cm, respectively. The upstream movement of the 10 ppt isohaline is much larger than the 5 and 20 ppt isohalines. The volume of water with salinity between 10 and 20 ppt would increase greatly if sea level rises 100 cm. In the CHK River, with a sea-level rise of 100 cm, the mean salinity at the drinking-water intake 34 km upstream of the mouth would be about 3 ppt in a typical year and greater than 5 ppt in a dry year, both far in excess of the U.S. Environmental Protection Agency's secondary standard for total dissolved solids for drinking water. At the drinking-water intake, the number of days of salinity greater than 0.1 ppt increases with increasing sea-level rise; during a dry year, 0.1 ppt would be exceeded for more than 100 days with as small a rise as 30 cm.

Newport News↗
USGS2007

Mars reconnaissance orbiter's high resolution imaging science experiment (HiRISE)

The HiRISE camera features a 0.5 m diameter primary mirror, 12 m effective focal length, and a focal plane system that can acquire images containing up to 28 Gb (gigabits) of data in as little as 6 seconds. HiRISE will provide detailed images (0.25 to 1.3 m/pixel) covering ∼1% of the Martian surface during the 2‐year Primary Science Phase (PSP) beginning November 2006. Most images will include color data covering 20% of the potential field of view. A top priority is to acquire ∼1000 stereo pairs and apply precision geometric corrections to enable topographic measurements to better than 25 cm vertical precision. We expect to return more than 12 Tb of HiRISE data during the 2‐year PSP, and use pixel binning, conversion from 14 to 8 bit values, and a lossless compression system to increase coverage. HiRISE images are acquired via 14 CCD detectors, each with 2 output channels, and with multiple choices for pixel binning and number of Time Delay and Integration lines. HiRISE will support Mars exploration by locating and characterizing past, present, and future landing sites, unsuccessful landing sites, and past and potentially future rover traverses. We will investigate cratering, volcanism, tectonism, hydrology, sedimentary processes, stratigraphy, aeolian processes, mass wasting, landscape evolution, seasonal processes, climate change, spectrophotometry, glacial and periglacial processes, polar geology, and regolith properties. An Internet Web site (HiWeb) will enable anyone in the world to suggest HiRISE targets on Mars and to easily locate, view, and download HiRISE data products.

Journal of Geophysical Research E: Planets↗
USGS2005

Phosphate reactivity in long-term poultry litter-amended southern Delaware sandy soils

Eutrophication caused by dissolved P from poultry litter (PL)-amended agricultural soils has been a serious environmental concern in the Delaware-Maryland-Virginia Peninsula (Delmarva), USA. To evaluate state and federal nutrient management strategies for reducing the environmental impact of soluble P from long-term PL-amended Delaware (DE) soils, we investigated (i) inorganic P speciation; (ii) P adsorption capacity; and (iii) the extent of P desorption. Although the electron microprobe (EMP) analyses showed a strong correlation between P and Al/Fe, crystalline Al/Fe-P precipitates were not detected by x-ray diffraction (XRD). Instead, the inorganic P fractionation analyses showed high levels of oxalate extractable P, Al, and Fe fractions (615-858, 1215-1478, and 337-752 mg kg-1, respectively), which were susceptible to slow release during the long-term (30-d) P desorption experiments at a moderately acidic soil pHwater. The labile P in the short-term (24-h) desorption studies was significantly associated with oxalate and F extractable Fe and Al, respectively. This was evident in an 80% reduction maximum in total desorbable P from NH4 oxalate/F pretreated soils. In the adsorption experiments, P was strongly retained in soils at near targeted pH of lime (???6.0), but P adsorption gradually decreased with decreasing pH near the soil pHwater (???5.0). The overall findings suggest that P losses from the can be suppressed by an increase in the P retention capacity of soils via (i) an increase in the number of lime applications to maintain soil pHwater at near targeted pH values, and/or (ii) alum/iron sulfate amendments to provide additional Al- and Fe-based adsorbents. ?? Soil Science Society of America.

Soil Science Society of America Journal↗
USGS2008

High Resolution Imaging Science Experiment (HiRISE) observations of glacial and periglacial morphologies in the circum-Argyre Planitia highlands, Mars

The landscape of the Argyre Planitia and adjoining Charitum and Nereidum Montes in the southern hemisphere of Mars has been heavily modified since formation of the Argyre impact basin. This study examines morphologies in the Argyre region revealed in images acquired by the High Resolution Imaging Science Experiment (HiRISE) camera and discusses the implications for glacial and periglacial processes. Distinctive features such as large grooves, semicircular embayments in high topography, and streamlined hills are interpreted as glacially eroded grooves, cirques, and whalebacks or roche moutonnée, respectively. Large boulders scattered across the floor of a valley may be ground moraine deposited by ice ablation. Glacial interpretations are supported by the association of these features with other landforms typical of glaciated landscapes such as broad valleys with parabolic cross sections and stepped longitudinal profiles, lobate debris aprons interpreted as remnant debris covered glaciers or rock glaciers, and possible hanging valleys. Aligned boulders observed on slopes may also indicate glacial processes such as fluting. Alternatively, boulders aligned on slopes and organized in clumps and polygonal patterns on flatter surfaces may indicate periglacial processes, perhaps postglaciation, that form patterned ground. At least portions of the Argyre region appear to have been modified by processes of ice accumulation, glacial flow, erosion, sediment deposition, ice stagnation and ablation, and perhaps subsequent periglacial processes. The type of bedrock erosion apparent in images suggests that glaciers were, at times, wet based. The number of superposed craters is consistent with geologically recent glacial activity, but may be due to subsequent modification.

Journal of Geophysical Research E: Planets↗
USGS2005

Habitat models to assist plant protection efforts in Shenandoah National Park, Virginia, USA

During 2002, the National Park Service initiated a demonstration project to develop science-based law enforcement strategies for the protection of at-risk natural resources, including American ginseng (Panax quinquefolius L.), bloodroot (Sanguinaria canadensis L.), and black cohosh (Cimicifuga racemosa (L.) Nutt. [syn. Actaea racemosa L.]). Harvest pressure on these species is increasing because of the growing herbal remedy market. We developed habitat models for Shenandoah National Park and the northern portion of the Blue Ridge Parkway to determine the distribution of favorable habitats of these three plant species and to demonstrate the use of that information to support plant protection activities. We compiled locations for the three plant species to delineate favorable habitats with a geographic information system (GIS). We mapped potential habitat quality for each species by calculating a multivariate statistic, Mahalanobis distance, based on GIS layers that characterized the topography, land cover, and geology of the plant locations (10-m resolution). We tested model performance with an independent dataset of plant locations, which indicated a significant relationship between Mahalanobis distance values and species occurrence. We also generated null models by examining the distribution of the Mahalanobis distance values had plants been distributed randomly. For all species, the habitat models performed markedly better than their respective null models. We used our models to direct field searches to the most favorable habitats, resulting in a sizeable number of new plant locations (82 ginseng, 73 bloodroot, and 139 black cohosh locations). The odds of finding new plant locations based on the habitat models were 4.5 (black cohosh) to 12.3 (American ginseng) times greater than random searches; thus, the habitat models can be used to improve the efficiency of plant protection efforts, (e.g., marking of plants, law enforcement activities). The field searches also indicated that the level of occupancy of the most favorable habitats ranged from 49.4% for ginseng to 84.8% for black cohosh. Given the potential threats to these species from illegal harvesting, that information may serve as an important benchmark for future habitat and population assessments.

Virginia↗
USGS2002

SHRIMP and Conventional UPb ages of Ordovician granites and tonalites in the central Appalachian Piedmont: Implications for Paleozoic tectonic events

Discordant Cambrian U-Pb ages of questionable reliability for metamor-phosed plutonic rocks in the central Appalachian Piedmont of northern Virginia, Maryland, and the District of Columbia have led to different regional geologic interpretations and controversies. In this study, we use the sensitive high resolution ion microprobe (SHRIMP) for dating 25-m diameter areas on individual zircons from Piedmont granitoids in an effort to resolve previous controversial ages. Cathodoluminescence images of zoning within the zircons are used to select pristine areas of cores and overgrowths for dating and to help interpret the results. For comparison, hand-picked fractions of zircons from most of the samples were also dated by the conventional isotope dilution thermal ionization mass spectrometry (TIMS) U-Pb method. The combined SHRIMP and TIMS results of this study demonstrate that the Dale City Quartz Monzonite, Norbeck Intrusive Suite, Kensington Tonalite, Goldvein pluton, Lake Jackson pluton, Falls Church Intrusive Suite, Occoquan Granite (main batholith and satellite Bull Run Marina pluton), Georgetown Intrusive Suite, Dalecarlia Intrusive Suite, and related granitoids are parts of an extensive Early to Middle Ordovician magmatic arc. Emplacement of these plutons from about 485 to 450 Ma generally was contemporaneous with major regional deformation and metamorphism, called the Taconic orogeny in rocks of the northern Appalachians. The Ordovician ages supersede most of the discordant Cambrian(?) to Neoproterozoic dates that previously suggested a Potomac or Penobscot orogeny in Piedmont rocks of the Potomac Valley, raising questions about the nature and timing of that event. The Devonian Guilford Granite crosscuts all structures, providing a minimum age for these Paleozoic events.

Virginia, Maryland↗
USGS2007

Northward extension of Carolina slate belt stratigraphy and structure, South-Central Virginia: Results from geologic mapping

Geologic mapping in south-central Virginia demonstrates that the stratigraphy and structure of the Carolina slate belt extend northward across a steep thermal gradient into upper amphibolite-facies correlative gneiss and schist. The Neoproterozoic greenschist-facies Hyco, Aaron, and Virgilina Formations were traced northward from their type localities near Virgilina, Virginia, along a simple, upright, northeast-trending isoclinal syncline. This syncline is called the Dryburg syncline and is a northern extension of the more complex Virgilina synclinorium. Progressively higher-grade equivalents of the Hyco and Aaron Formations were mapped northward along the axial trace of the refolded and westwardly-overturned Dryburg syncline through the Keysville and Green Bay 7.5-minute quadrangles, and across the northern end of the Carolina slate belt as interpreted on previous geologic maps. Hyco rocks, including felsic metatuff, metawacke, and amphibolite, become gneisses upgrade with areas of local anatexis and the segregation of granitic melt into leucosomes with biotite selvages. Phyllite of the Aaron Formation becomes garnet-bearing mica schist. Aaron Formation rocks disconformably overlie the primarily felsic volcanic and volcaniclastic rocks of the Hyco Formation as evidenced by repeated truncation of internal contacts within the Hyco on both limbs of the Dryburg syncline at the Aaron-Hyco contact. East-northeast-trending isograds, defined successively by the first appearance of garnet, then kyanite ?? staurolite in sufficiently aluminous rocks, are superposed on the stratigraphic units and synclinal structure at moderate to high angles to strike. The textural distinction between gneisses and identifiable sedimentary structures occurs near the kyanite ?? staurolite-in isograd. Development of the steep thermal gradient and regional penetrative fabric is interpreted to result from emplacement of the Goochland terrane adjacent to the northern end of the slate belt during Alleghanian orogenesis. This mapping study indicates that the Carolina slate belt does not terminate on the north against through-going faults or rest on higher-grade basement as previously suggested.

American Journal of Science↗
USGS2016

Erosional and depositional history of the Atlantic passive margin as recorded in detrital zircon fission-track ages and lithic detritus in Atlantic Coastal plain sediments

Comparison of fission-track (FT) ages of detrital zircons recovered from Atlantic Coastal Plain sediments to FT ages of zircons from bedrock in source terranes in the Appalachians provides a key to understanding the provenance of the sediments and, in turn, the erosional and depositional history of the Atlantic passive margin. In Appalachian source terranes, the oldest zircon fission-track (ZFT) ages from bedrock in the western Appalachians (defined for this paper as the Appalachian Plateau, Valley and Ridge, and far western Blue Ridge) are notably older than the oldest ages from bedrock in the eastern Appalachians (Piedmont and main part of the Blue Ridge). The age difference is seen both in ZFT sample ages and in individual zircon grain ages and reflects differences in the thermotectonic history of the rocks. In the east, ZFT data indicate that the rocks cooled from temperatures high enough to partially or totally reset ZFT ages during the Paleozoic and (or) Mesozoic. The majority of the rocks are interpreted to have cooled through the ZFT closure temperature (∼235 °C) at various times during the late Paleozoic Alleghanian orogeny. In contrast, most of the rocks sampled in the western Appalachians have never been heated to temperatures high enough to totally reset their ZFT ages. Reflecting their contrasting thermotectonic histories, nearly 80 percent of the sampled western rocks yield one or more zircon grains with very old FT ages, in excess of 800 Ma; zircon grains yielding FT ages this old have not been found in rocks in the Piedmont and main part of the Blue Ridge. The ZFT data suggest that the asymmetry of zircon ages of exposed bedrock in the eastern and western Appalachians was in evidence by no later than the Early Cretaceous and probably by the Late Triassic. Detrital zircon suites from sands collected in the Atlantic Coastal Plain provide a record of detritus eroded from source terranes in the Appalachians during the Mesozoic and Cenozoic. In Virginia and Maryland, sands of Early Cretaceous through late early Oligocene age do not yield any old zircons comparable in age to the old zircons found in bedrock in the western Appalachians. Very old zircons yielding FT ages >800 Ma are only encountered in Coastal Plain sands of middle early Miocene and younger age. Miocene and younger fluvial-deltaic deposits associated with the major mid-Atlantic Coastal Plain rivers that now head in the western Appalachians (the Hudson, Delaware, Susquehanna, Potomac, James, and Roanoke) contain abundant clasts of fossiliferous chert and quartzite and other distinctive rock types derived from Paleozoic rocks of the western Appalachians. These distinctive clasts have not been reported in older Coastal Plain sediments. The ZFT and lithic detritus data indicate that the drainage divide for one or more east-flowing mid-Atlantic rivers migrated west into the western Appalachians, and the river(s) began transporting western Appalachian detritus to the Atlantic Coastal Plain, sometime between the late early Oligocene and middle early Miocene. By no later than late middle Miocene most if not all of the major rivers that now head west of the Blue Ridge were transporting western Appalachian detritus to the Coastal Plain. Prior to the drainage divide migrating into the western Appalachians, the ZFT data are consistent with the dominant source of Atlantic Coastal Plain sediments being detritus from the Piedmont and main part of the Blue Ridge, with possible input from distant volcanic sources. The ZFT data suggest that the rapid increase in the rate of siliciclastic sediment accumulation in middle Atlantic margin offshore basins that peaked in the middle Miocene and produced almost 30 percent of the total volume of post-rift siliciclastic sediments in the offshore basins began in the early Miocene when Atlantic river(s) gained access to the relatively easily eroded Paleozoic sedimentary rocks of the western Appalachians.

American Journal of Science↗
USGS2017

Petrology and geochronology of Mesoproterozoic basement of the Mount Rogers area of southwestern Virginia and northwestern North Carolina: Implications for the Precambrian tectonic evolution of the southern Blue Ridge province

Results from new geologic mapping, SHRIMP U-Pb geochronology, and petrologic studies indicate that Mesoproterozoic basement in the northern French Broad massif near Mount Rogers consists of multiple, mostly granitic plutons, map- and outcrop-scale xenoliths of pre-existing crustal rocks, and remnants of formerly overlying meta-sedimentary lithologies. Zircon and titanite ages demonstrate that these rocks collectively record nearly 350 m.y. of tectonic evolution including periods of igneous intrusion at ca . 1190 to 1130 Ma (Early Magmatic Suite) and ca . 1075 to 1030 Ma (Late Magmatic Suite) and three episodes of regional metamorphism at ca . 1170 to 1140, 1070 to 1020, and 1000 to 970 Ma. The existence of ca . 1.3 Ga age crust is indicated by (1) orthogranofels of ca . 1.32 Ga age in a map-scale xenolith, (2) inherited zircons of ca . 1.33 to 1.29 Ga age in Early Magmatic Suite plutons, and (3) ca . 1.36 to 1.30 Ga age detrital zircons in meta-sedimentary lithologies. Mineral assemblages developed in amphibolites and granofelses indicate that metamorphism during both Mesoproterozoic episodes occurred at upper amphibolite- to lower granulite-facies conditions. Syn-orogenic Early Magmatic Suite plutons emplaced at ca . 1190 to 1145 Ma are characterized by high-K, variably magnesian, dominantly calc-alkalic compositions, and have trace-element characteristics indicative of continental-arc magmatic origin involving melting of thick continental crust. In contrast, ca . 1140 Ma age quartz syenite displays A-type features indicating derivation from depleted crustal sources with increased mantle input during waning stages of regional contraction. Plutons of the compositionally bimodal Late Magmatic Suite include (1) ca . 1060 Ma meta-granite with geochemical characteristics transitional between silicic rocks of arc systems and post-collisional granites of A-type lineage, and (2) ca . 1055 Ma monzodioritic rocks with A-type compositional characteristics that likely reflect derivation from fertile, mafic sources in the lower crust. Collectively, these data suggest that Mesoproterozoic rocks of the study area preserve evidence of multiple orogenic episodes that likely involved continental-arc development and deformation at ca . 1150 Ma followed by crustal thickening at ca . 1060 Ma. Field relations and geochronologic data indicate that regional uplift and sedimentation occurred at ca . 1100 Ma between the two episodes of overlapping magmatism and orogenesis. The nature and timing of Mesoproterozoic events recorded in basement rocks of the study area illustrate significant differences in the lithologic assemblages and geologic history preserved by Mesoproterozoic basement of the adjacent Shenandoah and French Broad massifs, suggesting that the Blue Ridge massifs occupied different locations within the regional Grenville-age orogen until about 1070 Ma when the effects of Ottawan-age tectonics began to affect both areas. The near ubiquity of Ottawan-age orogenic activity recorded in Mesoproterozoic rocks of the Blue Ridge, other Appalachian inliers such as the New Jersey Highlands, and the Grenville province of Canada, including the Adirondacks, suggests that these formerly disparate terranes were amalgamated to form a common, regional orogen by this time.

North Carolina, Virginia↗