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The Ratcliff Site in southwestern Virginia lies in a small second-order stream valley filled with approximately 3.5 m of organic-rich deposits that contain bones of mammoth, mastodon, deer (or antelope), logs, and plant macrofossils. Radiocarbon analyses indicate the age of the organic-rich sediment ranges from > 44,000 to 29,100 14C yr BP, a time period with no fossil remains reported in this region of the Appalachians. Analyses of field observations, textural data, organic carbon content, and plant macrofossils indicate that the organic-rich sediments contain interbedded standing-water and debris-flow deposits. Up to 6 m of oxidized debris-flow sediments bury the organic-rich sediments. The presence of Rubus parviflorus (Thimble Berry) throughout the deposit indicates the site had a boreal environment from > 44,000 to 29,100 14C yr BP. Plant macrofossil evidence indicates the uplands had stands of spruce/jack-pine forests while the valley contained ponds and associated wetlands. Three debris flows occurred at the site between approximately 38,000 and 29,000 14C yr BP, suggesting a recurrence interval for major storms of approximately 3000 yr, even though the apparent stability of the boreal environment implies a climate not conducive to catastrophic rainstorms. This conflicting combination of features suggests that during the middle Wisconsinan this area experienced generally cool climates, dominated by polar air masses, but was punctuated by relatively brief warm periods marked by incursions of tropical air masses. 相似文献
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Cactus Hill is located in the Virginia Coastal Plain on a terrace above the Nottoway River. The site has a record of occupation that spans the Holocene and also offers evidence of humans late in the Pleistocene before Clovis time. Soil investigations identified several deposit types, and demonstrated that multisequal eolian sands forming the site's primary core are arrayed in spatially and temporally discrete horizons. Resting atop an ancient paleosol, the earliest sand stratum (19,540 ± 70 14C yr B.P.) is marked by a conspicuous but culturally sterile buried surface horizon. Eolian sand above this surface supports another sequum in which Clovis and underlying “Blade” artifacts are associated with a fainter surface horizon and pronounced subsoil lamellae. Early Archaic and successively younger artifacts occur above the Clovis level in a more weakly developed uppermost sequum. This soil and cultural stratigraphy, together with considerations of regional topography, demonstrate that the landscape has evolved incrementally since about the last glacial maximum. © 2004 Wiley Periodicals, Inc. 相似文献
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W. Cullen Sherwood 《Environmental Geology》1989,14(2):99-106
Loading trends and sources of CI– in the South Fork of the Shenandoah River, Virginia were analyzed for the period 1929–1982. CI– has increased from approximately 2 mg/L (2,776 tons/yr) to over 10 mg/L (14,256 tons/yr). Natural CI– is estimated to be 1.01 mg/L (1,388 tons/yr) with precipitation providing 0.99 mg/L and rocks 0.02 mg/L. From 1929 to 1949 CI– concentrations were relatively constant and independent of discharge, conforming to the Type II curve of Davis and Zobrist (1978), indicative of natural or relatively uncontaminated streams. Since 1952 CI– concentrations increased exponentially as river discharge decreases conforming to the Type I curve of Davis and Zobrist for polluted streams. Since 1965 anthropogenic CI– loading at 12,868 tons/yr has remained relatively constant. Four major sources contribute 92.2 percent (11,871 tons/yr) of the anthropogenic CI–: (1) deicing salts—4,149 tons/yr, (2) domestic sewage—3,015 tons/yr, (3) livestock and poultry wastes—2,458 tons/yr, and (4) commercial fertilizers—2,249 tons/yr. 相似文献
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D.L. Cremeens 《Geoarchaeology》2005,20(6):581-597
Micromorphology samples from Cotiga Mound (ca. 2100 yr B.P.) reveal distinct microstructures associated with mound construction and post‐construction pedogenesis of the mound materials. The interface between layers of basketloads displays sharp, irregular contacts with little to no evidence of pedoturbation, suggesting rapid construction. Evidence of clay and iron translocation, along with well‐preserved charcoal fragments containing identifiable cell structure, were observed in samples collected from hearth features. Both the prepared surface and associated central burial feature at the base of the mound have a complex microstructure, characterized by charcoal layers and zones of clay accumulation, along with microstratification in the prepared surface. Clay translocation, formation of a complex void network, and ironmanganese accumulation were the dominant pedogenic processes that modified the completed mound. © 2005 Wiley Periodicals, Inc. 相似文献
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More than 3,200 indoor radon-222 (222Rn) measurements were made seasonally in an area of about 1,000 square kilometers of the Coastal Plain and Piedmont physiographic provinces in Virginia, U.S.A. Results of these measurements indicate that some geological units are associated, on the average, with twice as much indoor222Rn as other geological units, and that indoor222Rn varies seasonally. The Kruskal-Wallis test was used to test whether indoor222Rn concentrations for data gathered over the winter and summer seasons differ significantly by rock unit. The tests concluded that indoor222Rn concentrations for different rock units were not equal at the 5-percent significance level. The rocks associated with the highest median indoor222Rn concentration are specific rocks in the Mesozoic Culpeper basin, including shale and siltstone units with Jurassic diabase intrusives, and mica schists in the Piedmont physiographic province. The pre-Triassic Peters Creek Schist has the highest ranking in terms of indoor222Rn concentration. The rocks associated with the lowest indoor222Rn concentrations include coastal plain sediments, the Occoquan Granite, Falls Church Tonalite, Piney Branch Mafic and Ultramafic complex, and unnamed mafic and ultramafic inclusions, respectively. The rocks have been ranked according to observed222Rn concentration by transforming the average rank of indoor222Rn concentrations to z scores. 相似文献
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Acrylic panels were employed to follow fouling community development at Channel Points, Lynnhaven Bay, Virginia, from April 1977 through March 1979. Panel treatments were devised to study the effects of predator exclusion and panel area on community development. An additional panel was introduced in April 1978 to test the repeatability of initial community development. Early development was similar both years with hydroids, barnacles, and the polychaetePolydora ligni the initial macroinvertebrate colonists. Early community development in both years was dominated by the solitary tunicateMolgula manhattensis. Molgula was able to invade and rapidly overgrow all resident speices except largeBalanus eburneus. Panel area was shown to affect the community structure and development. The community on caged panels (predator exclusion) was significantly different from controls, although the differences could not be unequivocally attributed to predation. 相似文献
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Summary Ferroan-gahnite, (Zn,Fe)Al2O4, is a common accessory mineral found in association with the metamorphosed volcanogenic massive sulfide ore bodies of the Mineral District, Virginia. Gahnite is present throughout the mineralized zones that contain pyrite, sphalerite, chalcopyrite, and sulfosalts. Although constituting only 1–2 modal percent of most samples and displaying not well-defined distribution pattern, gahnite is especially abundant at the margins of the mineralized zones and locally constitutes as much as 35 modal percent. The compositions vary slightly from one deposit to another but all lie within the range of 68 to 88 mol.% gahnite (ZnAl2O4), 8 to 25 mol.% hercynite (FeAl2O4), and 1–17 mol.% spinel (MgAl2O4). Gahnite occurs within a wide variety of mineral assemblages, with or without coexisting sphalerite.The reflectance and unit cell dimension are sensitive to composition and decrease with increasing gahnite component. The approximate 8% reflectance at 540 nm is intermediate between most silicates and ore minerals such as sphalerite and magnetite and thus provides a ready method for recognition in reflected light. The gahnite is believed to have formed during amphibolite grade regional metamorphism as a result of either sphalerite desulfidation or the reaction of original zinc oxide phases.
With 7 Figures 相似文献
Gahnit in metamorphen schichtgebundenen Sulfid-Lagerstätten des Mineral-Distriktes, Virginia, U.S.A.
Zusammenfassung Eisenhaltiger Gahnite (Zn,Fe)Al2O4 ist in den metamorphen vulkanogenen massiven Sulfiderzkörpern Virginias als akzessorisches Mineral weit verbreitet. Er kommt durchwegs in den Pyrit, Zinkblende, Kupferkies und Sulfosalze enthaltenden mineralisierten Zonen vor. Obwohl die meisten Proben nur 1–2 Modal % Gahnit enthalten, und keine klar definierte Verteilung zu erkennen ist, tritt er besonders häufig an den Rändern der mineralisierten Zonen auf und erreicht lokal 35 Modal %. Die Zusammensetzung schwankt leicht von Vorkommen zu Vorkommen, liegt aber stets zwischen 68 und 88 Mol.% Gahnit (ZnAl2O4) 8 und 25 Mol.% Hercynit (FeAl2O4) und 1 und 17 Mol.% Spinell (MgAl2O4) Gahnit ist Bestandteil sehr verschiedener Mineralparagenesen mit und ohne Zinkblende.
With 7 Figures 相似文献
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Suspended and equivalent active sediments were collected from streams at 84 sites at the eastern and western sides of a ridge with stratabound Zn mineralization, Rockingham County, Virginia (USA). The purpose was to evaluate the relative effectiveness of suspended and bottom sediment in the geochemical exploration for Mississippi-Valley type Zn ores. The orebody is composed of sphalerite and minor amounts of galena with some chalcopyrite, strikes parallel to the enclosing dolomite beds at N34°E, and cuts vertically across the 30°SE dip. Oxidation of the ore extends to about 100 m in depth.Zinc was effective in detecting the mineralized zone. Zinc in the suspensates provided a denser anomaly system (7 strong and 2 moderate samples) than Zn in the <150 μm size of the bottom sediment, either totally dissolved (4 strong and 1 moderate anomalies) or extracted with cold 3% HCl (5 strong and 3 moderate anomalies). The magnitude of the high values for suspensate Zn was considerably higher than those for the other sample types studied. Suspensate Zn was complemented by 3 strong suspensate Cu anomalies. The suspensate Zn anomaly dispersion was about 850 m, 400 m of which was upstream from the mineralization. Lead was detected only in the cold acid extraction and in combination with Cu and Zn gave an extensive overlapping multielement anomaly string that targeted the ore zone and had a dispersion downstream from the mineralization of more than 1000 m. The Pb anomalies extended about 900 m upstream and 400 m downstream from the ore zone. Together with suspensate Zn, the Pb anomalies suggest a continuation of mineralization upstrike from the known orebody.The major anomaly systems are along the eastern side of the ridge with comparatively few anomalies present in the drainage along the western side. The concentration of anomalies in the eastern drainage and anomaly disposition with respect to known mineralization result from structural control on the hydromorphic dispersion of metals from the orebody. 相似文献
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Chemical partitioning data are of fundamental interest to exploration geochemists. This paper is one of the few studies which has investigated the relative proportions of the rare elements in various soil extracts. The dispersion of trace elements from weathering pegmatites in Powhatan Country, Virginia, was found to be restricted to the immediate vicinity of the pegmatites. A sequential extraction procedure was used to measure the distribution of Be, La, Nb, Sn, U, Li, and Ni,among the following fractions of the B soil horizons: exchangeable, Fe and Mn oxyhydroxide, residual, and aqua regia digestion. The elements Sn, Be, Li, and U were found to be associated with soils over the complex Herbb No. 2 pegmatite, whereas La and Ni were generally associated with the background soils.A geochemical exploration model was developed using stepwise discriminant function analysis to determine the combination of elements and soil extracts that best differentiates between complex pegmatitic, simple pegmatitic, and background soils. Log-transformed aqua regia extract concentrations of Sn, La, U, and Li were the most effective variables when used to separate complex pegmatitic from simple pegmatitic soils. 相似文献
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Soil and stream sediment sampling have been the primary geochemical exploration tools in the Appalachian piedmont to date. However, the great thicknesses of soil and saprolite found in the region coupled with the dense vegetation frequently encountered favor biogeochemistry as an alternative or supplemental method since deep-rooted plants sample closer to bedrock. To evaluate this method, an orientation survey was performed in which soils and vegetation at 17 sites north of Mineral, Virginia, were sampled and analyzed for Ag, Cd, Cu, Pb and Zn. The traverse included stations over the host rocks of massive sulfide mineralization, as well as over apparent “barren” country rock. Samples were analyzed by atomic absorption spectrophotometry using standard digestion and analytical techniques.Both A- and B-horizon soil metals generally appear to be reliable indicators of mineralization, with soils developed over sulfides showing up to three-fold enrichment in metal content relative to the average soils developed on the country rock. Correlation of metal concentrations in vegetation to soil metal concentrations reveal plant concentrations expressed on a dry-weight basis correlate stronger and more frequently to soil metals than do ash-weight concentrations. Copper shows some promise in selected organs and species, Ag appears fair but data are limited to one organ of one species, and plant Pb seems totally unresponsive to soil metal concentrations perhaps because foliar absorption is an important plant uptake mechanism here. However, Zn and Cd in organs of the oak group, especially mature leaves and twigs of the current year's growth show the greatest promise as prospecting tools. They correlate well with soil metals and when compared directly to the geology they reliably reflect mineralization. Although results using White oak were slightly less profound than those obtained from the Black-Red oak group, White oak may be preferred as it is a single, more widespread, easily-identifiable species. Copper and especially Zn although essential elements to plants, do not appear to be “difficult” elements for biogeochemical prospecting in the Appalachian piedmont. 相似文献
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A variety of processes and sources account for the total accumulation of sediment on a fringe marsh. The rates of accretion across Brockenberry fringe marsh at the south end of the Delmarva Peninsula, Virginia, were determined by Pb-210 radiogeochronology. Rates are governed by the surface elevation with respect to midtide elevation, the rate of sea-level rise, and outwash from the mainland. Only some portions of the fringe marsh are able to keep pace with sea-level rise and thus migrate up the mainland slope during transgression. 相似文献
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David R. Gray 《Journal of Structural Geology》1981,3(3):265-277
A non-coaxial deformation involving pre-folding initiation of cleavage perpendicular to bedding is proposed to explain non-axial planar cleavage associated with mesoscopic folds in part of the Appalachian foreland thrust-belt of southwest Virginia. Folds are gently plunging, asymmetric, upright to slightly inclined, sinusoidal forms with non-axial fanning cleavage. They show extreme local variations in type and degree of transection and the consistency of transection direction. These relations are further complicated by hinge migration.Cleavage-fan angles, bedding-cleavage angles and δ transection values appear influenced by fold tightness, and in part by fold flattening strain. Fold flattening increments are considered simultaneous with folding. Axial surface traces, and not cleavage traces, coincide with the principal extension direction in fold profiles. Geometric modelling of cleavage fanning and bedding-cleavage angle variations for various theoretical folding modes suggest that folding in limestone and sandstone layers was by tangential longitudinal strain. Significant shape modification and change in bedding-cleavage relations occurred after limb dips of 40 and 50° were attained in limestone and sandstone respectively. Mud-rock class 1C folds with convergent cleavage fans show features transitional between buckling and flexural flow. Initiation of ‘cleavage’ fabrics during layer-parallel shortening prior to significant folding may be important for cleavage evolution in some deformed rocks. 相似文献
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Low grade submarine exhalative gold mineralization occurs at the London-Virginia deposit in central Virginia Piedmont. The deposit consists of a series of localized but conformable units — basal garnetiferous-chlorite schist, magnetite schist, quartz-muscovite schist, ferruginous quartzite, and chlorite-biotite schist — which represent a mixture of submarine epiclastic volcanic debris and exhalative chemical sediments. Finely disseminated gold occurs dispersed with minor amounts of pyrite, sphalerite, chalcopyrite, galena, and tennantite in the ferruginous quartzite and quartz-muscovite schist. The deposit is believed to have formed by processes analogous to those currently active in the Atlantis II Deep of the Red Sea. Silicarich, hypersaline brines discharged through fractures in the sea floor and ponded in a local basin. Epiosodic influx of clastic debris and extensive deposition of hydrothermal silica diluted the concentration of sulfides and gold to produce a low-grade, siliceous mineralized zone. Emanation from the exhalative vent was terminated when the basin was capped by a lava flow. Subsequent regional greenschist grade metamorphism has recrystallized the silica into a granular quartzite and produced minor remobilization of the gold and sulfides. 相似文献
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Cotiga Mound, a 2100-year-old Woodland burial mound, was constructed on a mid-Holocene terrace along the Tug Fork of the Big Sandy River. The terrace is the only stable, level landscape in the area. The intact nature of the terrace soil horizons indicated that little if any of the mound materials were taken from the terrace surface in the immediate vicinity of the mound. The mound was constructed of silty material and contained basket-loading macrofabric and a rock ring of sandstone fragments. Seismic fan refraction indirectly indicated the center of the rock ring, while ground-penetrating radar (GPR) did not detect the rock ring. A stratigraphic cross section, based on particle-size analysis, indicates that the mound consisted of 10 lithologic layers. The uppermost layer resulted from faunal disturbance. The remaining nine layers represented construction episodes; the upper four extending across large expanses of the mound in caplike fashion, and the lower five representing more specific inner-mound localized construction. Particle-size analysis quantitatively substantiates field observations of mound stratigraphy and highlights subtle changes in materials used for construction. This approach provides a basis for quantitative comparisons with other mounds and earthworks by delineating stratigraphic details associated with episodes of construction and the ceremonial implications of the episodes. © 1997 John Wiley & Sons, Inc. 相似文献
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Physical and geophysical data collected at a fractured-rock research site in the Blue Ridge Province of Virginia, USA indicate that recharge rates to a fractured-rock aquifer are controlled by a small-scale thrust fault associated with regional thrust faulting. Recharge rates appear to be correlated to spatial variation in the hydraulic conductivity of the regolith, which has been influenced by weathering rates and the metamorphic and structural history of the underlying parent material. The angle of dip of the thrust-fault shear zone and the fracturing within the crystalline rock adjacent to the fault plane appear to serve as geologic controls that preferentially direct infiltrated meteoric water to a deeper confined fractured-rock aquifer. In-situ analysis of electrical resistivity, matric potential and moisture content shows two distinctly different recharge processes which are spatially correlated with the structure of the shallow subsurface (regolith overlying the vertically oriented shear zone and regolith overlying the thrust-fault hanging wall), and which have strong temporal correlations with the dynamics of the underlying saturated conditions. Recharge flux within the regolith overlying the vertically oriented shear zone is strongly controlled by the orientation and areal extent of the thrust-fault shear zone, highlighting the importance of accurate delineation of recharge areas in crystalline rock aquifer systems. 相似文献
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Ben K. Odhiambo Virginia Brown Gayle Armentrout Leanna C. Giancarlo Chelsea Wegner 《Environmental Earth Sciences》2013,69(6):2103-2117
Damming of the North Anna River in 1972 created Lake Anna, a cooling water source for the Dominion nuclear power plant as well as a popular recreation site in Spotsylvania and Orange counties, Virginia, USA. Previously dated (210-Pb) sediment cores from seven locations within the lake and three locations in the adjoining Waste Heat Treatment Facilities (WHTF) were analyzed for trace metals (Al, Ba, Zn, Cd, Cu, Fe, Mn and Pb) and polychlorinated biphenyls (PCBs) to examine the environmental evolution of the reservoir system. The reservoir has a history of mining activities in its watershed and unusually elevated concentrations of PCBs were found in fish tissues from previous studies. Therefore, dated sediment cores provided the framework for both the temporal and spatial analysis of possible sources and flux histories for both trace metals and PCBs. The trace metals results suggest that, though the upper reaches are relatively less impacted, the old mine tailings from the now ceased mining activities in the watershed of Contrary Creek tributary continue to dominate the sediment chemistry of the lower portion of the lake basin, signified by sediment enrichment of Pb, Cd, Cu, and Zn. Lagoon-2 of the WHTF also seems to be receiving unusually high loadings of Cd (12.5 ± 1.07 μg/g) that is probably associated with waste materials from the nuclear power plant that maintains the lagoons. PCB sediment concentrations were relatively low in the lower sections of the basins with values typically being <3.5 ng/g. The upper reaches of the basin had several PCB hotspots, with the surface sediments of Terry’s Run tributary having values as high as 53.13 ng/g. The spatial distribution of PCBs seems to suggest the upper reaches of the basin as the probable source, with the unusually high concentrations near bridges suggesting a possible link between the PCBs and old bridge fill materials. The oldest lacustrine sediments also had relatively high trace metals and PCB values signifying a probable role of soil disruption and sediment reconcentration during reservoir construction. 相似文献