Sedimentary environments in Long Island Sound: A guide to sea-floor management in a large urbanized estuary
No abstract available.
Connecticut, New York↗
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Map showing geochemical characteristics of rock samples from the Kalmiopsis Wilderness, southwestern Oregon
This report summarizes 1,307 semiquantitative chemical analyses of rocks collected to aid int he evaluation of the mineral resources of the Kalmiopsis Wilderness, Oreg. Chemical analyses and sample location maps are presented by Grimes, Leinz, and Speckman (1981). The area is located in the Kilamath Mountains geoporphic province of southwestern Oregon, and the area's location and boundaries are shown on the accompanying map. The geology of the study area is described by Page and others (1981), Gray (1980), and Ramp (1975). The rocks of the area cosist of dismembered ophiolite, island-arc, and sedimentary rock sequesnces that lie within the Western Jurassic belt of the Klamath Mountains. All units normally found within these assemblages are present; however, thrusting ans subsequent normal faulting have disrupted the geometric relations, producing a complexly faulted series of imbricate thrust slices.
Oregon↗
A New Occurrence Model for National Assessment of Undiscovered Volcanogenic Massive Sulfide Deposits
Volcanogenic massive sulfide (VMS) deposits are very significant current and historical resources of Cu-Pb-Zn-Au-Ag, are active exploration targets in several areas of the United States and potentially have significant environmental effects. This new USGS VMS deposit model provides a comprehensive review of deposit occurrence and ore genesis, and fully integrates recent advances in the understanding of active seafloor VMS-forming environments, and integrates consideration of geoenvironmental consequences of mining VMS deposits. Because VMS deposits exhibit a broad range of geological and geochemical characteristics, a suitable classification system is required to incorporate these variations into the mineral deposit model. We classify VMS deposits based on compositional variations in volcanic and sedimentary host rocks. The advantage of the classification method is that it provides a closer linkage between tectonic setting and lithostratigraphic assemblages, and an increased predictive capability during field-based studies.
Open-File Report↗
Potash—A vital agricultural nutrient sourced from geologic deposits
This report summarizes the primary sources of potash in the United States. Potash is an essential nutrient that, along with phosphorus and nitrogen, is used as fertilizer for growing crops. Plants require sufficient potash to activate enzymes, which in turn catalyze chemical reactions important for water uptake and photosynthesis. When potassium is available in quantities necessary for healthy plant growth, disease resistance and physical quality are improved and crop yield and shelf life are increased. Potash is a water-soluble compound of potassium formed by geologic and hydrologic processes. The principal potash sources discussed are the large, stratiform deposits that formed during retreat and evaporation of intracontinental seas. The Paradox, Delaware, Holbrook, Michigan, and Williston sedimentary basins in the United States are examples where extensive potash beds were deposited. Ancient marine-type potash deposits that are close to the surface can be mined using conventional underground mining methods. In situ solution mining can be used where beds are too deep, making underground mining cost-prohibitive, or where underground mines are converted to in situ solution mines. Quaternary brine is another source of potash that is recovered by solar evaporation in manmade ponds. Groundwater from Pleistocene Lake Bonneville (Wendover, Utah) and the present-day Great Salt Lake in Utah are sources of potashbearing brine. Brine from these sources pumped to solar ponds is evaporated and potash concentrated for harvesting, processing, and refinement. Although there is sufficient potash to meet near-term demand, the large marine-type deposits are either geographically restricted to a few areas or are too deep to easily mine. Other regions lack sources of potash brine from groundwater or surface water. Thus, some areas of the world rely heavily on potash imports. Political, economic, and global population pressures may limit the ability of some countries from securing potash resources in the future. In this context, a historical perspective on U.S. potash production in a global framework is discussed.
Open-File Report↗
High-Flow Experimental Outcomes to Inform Everglades Restoration, 2010–22
The Decompartmentalization Physical Model (DPM) was an experimental facility in the central Everglades operated between 2010 and 2022 to release high flows through a levee-enclosed area of degraded ridge and slough wetland that had been isolated from flow for sixty years. The purpose of DPM experimental program was to make measurements before, during, and after seasonal high-flow releases that could help guide the Congressionally authorized Everglades restoration project known as the Decompartmentalization and Sheet Flow Enhancement Project. The DPM facility was operated by the South Florida Water Management District, with the U.S. Geological Survey (USGS) and several universities participating in experimental design and leading aspects of the research. The USGS research at DPM focused on measuring high-flow hydraulics and its sedimentary and ecological responses in downstream wetlands. USGS investigated interactions between flow and vegetation and microtopography that influenced flow velocity and water depth, bed shear stress, sediment entrainment, and the resulting downstream transport of suspended sediment and fate of particle-associated phosphorus. USGS also investigated high-flow changes in water-column mixing and gas exchange and resulting effects on metabolism of the aquatic ecosystem (primary productivity and respiration). USGS also investigated effects of built structures such as levee gaps that were constructed to reconnect levee-enclosed basins. This report describes the methods and results of the USGS-led data collection at DPM. The USGS studies at DPM have identified factors that influence effectiveness of restoration, specifically how high-flow releases maximize sheet flow and affect sediment and nutrient dynamics while minimizing undesirable outcomes caused by past management that bypassed wetlands by conveying polluted water through canals to ecologically sensitive downstream areas. The DPM high-flow experiments reconnected the Water Conservation Area 3A and Water Conservation Area 3B basins, and it therefore has become a central feature of the restoration’s Decompartmentalization and Sheet Flow Enhancement Project. DPM’s scientific findings have already influenced the adaptive management of Everglades restoration in guiding elements of the final design and implementation of the Central Everglades Planning Project-South. In addition to serving Everglades restoration, the DPM has the potential to influence similar adaptive management programs throughout the nation’s network of federal and state-managed river corridors, floodplains, and riparian ecosystems.
Florida↗
Mineral deposits in western Saudi Arabia; a preliminary report
Mineral deposits in Saudi Arabia include a variety of deposits which were formed in many geologic environments. These include magmatic and late magmatic deposits in igneous masses, contact metamorphic deposits along the margins of igneous bodies, and stratiform sulfide deposits and veins. Notable deposits of sedimentary origin include deposits of iron oxides and phosphate. Strata-bound massive sulfide deposits containing copper, zinc, and nickel sulfides associated with pyrite and pyrrhotite in complexly deformed volcanic rocks in Saudi Arabia have been the subject of controversy. They are considered by some geologists to be of syngenetic or volcanogenic origin, and by others to be epigenetic. The principal mineral deposits in western Saudi Arabia are localized in mineral belts within tectonic zones that trend northerly, northeasterly, and northwesterly. These belts contain the exploration target areas most promising for future exploration. Noteworthy among these are: 1) the northerly-trending Bidah copper-zinc belt; 2) the Sayid copper-zinc zone; 3) the northwest-trending Nuqrah copper zinc-silver belt; and 4) the north-northwesterly-trending Al Amar belt. Gold and silver deposits are mainly localized in north-trending structural features of the Hijaz geotectonic cycle and northwest Najd trends. Saudi Arabian metal deposits known thus far contain reserves in the 2 billion dollar range. Further exploration should result in discoveries that could significantly increase this amount; Saudi Arabia may well have workable metallic and nonmetallic mineral resources for industrial developments in many parts of the Kingdom. In order to effectively carry on a search for new mineral deposits, the belts should be mapped in detail, with emphasis on the delineation of stratigraphic and structural features that control metallization. In addition, geochemical and geophysical studies should be made of promising areas to outline exploration targets. These targets could then be systematically explored.
Open-File Report↗
Reconnaissance geology of the Jabal Hashahish Quadrangle, sheet 17/41 B, Kingdom of Saudi Arabia
The Jabal Hashahish quadrangle (sheet 17/41 B) lies between lat 17?30' and 18?00' N. and long 41?30' and 42?00' E. and encompasses an area of 2,950 km2, of which only about 600 km2 is land; the remainder is covered by the Red Sea. The geologic formations exposed in the quadrangle include Precambrian layered and intrusive rocks, Tertiary gabbro dikes, Quaternary basaltic lavas and pyroclastic rocks, and Quaternary surficial deposits. The Precambrian rocks include layered sedimentary and volcanic rocks that have been assigned to the Baish, Bahah, and Ablah groups. These rocks have been folded, metamorphosed, and invaded by intrusions. They are cut by Miocene gabbro dikes that were intruded during the initial stages of the opening of the Red Sea rift. The Quaternary rocks also include basalt that was extruded during a continuation of that opening, after the uplift that formed the escarpment that parallels the eastern shore of the Red Sea, but before the Holocene erosional cycle. Coastal, pediment, and alluvial, and eolian deposits of various kinds are also of Quaternary age. The economic potential of the quadrangle lies essentially in the agricultural value of its flood-plain deposits, though these are not so widely used as those in Wadi Hali and Wadi Yiba, which are located in the Manjamah quadrangle. The coral reefs possibly could provide raw materials for use in a cement industry, if any such industry were ever required in this area.
Open-File Report↗
Geomorphology, active tectonics, and landscape evolution in the Mid-Atlantic region
In 2014, the geomorphology community marked the 125th birthday of one of its most influential papers, “The Rivers and Valleys of Pennsylvania” by William Morris Davis. Inspired by Davis’s work, the Appalachian landscape rapidly became fertile ground for the development and testing of several grand landscape evolution paradigms, culminating with John Hack’s dynamic equilibrium in 1960. As part of the 2015 GSA Annual Meeting, the Geomorphology, Active Tectonics, and Landscape Evolution field trip offers an excellent venue for exploring Appalachian geomorphology through the lens of the Appalachian landscape, leveraging exciting research by a new generation of process-oriented geomorphologists and geologic field mapping. Important geomorphologic scholarship has recently used the Appalachian landscape as the testing ground for ideas on long- and short-term erosion, dynamic topography, glacial-isostatic adjustments, active tectonics in an intraplate setting, river incision, periglacial processes, and soil-saprolite formation. This field trip explores a geologic and geomorphic transect of the mid-Atlantic margin, starting in the Blue Ridge of Virginia and proceeding to the east across the Piedmont to the Coastal Plain. The emphasis here will not only be on the geomorphology, but also the underlying geology that establishes the template and foundation upon which surface processes have etched out the familiar Appalachian landscape. The first day focuses on new and published work that highlights Cenozoic sedimentary deposits, soils, paleosols, and geomorphic markers (terraces and knickpoints) that are being used to reconstruct a late Cenozoic history of erosion, deposition, climate change, and active tectonics. The second day is similarly devoted to new and published work documenting the fluvial geomorphic response to active tectonics in the Central Virginia seismic zone (CVSZ), site of the 2011 M 5.8 Mineral earthquake and the integrated record of Appalachian erosion preserved on the Coastal Plain. The trip concludes on Day 3, joining the Kirk Bryan Field Trip at Great Falls, Virginia/ Maryland, to explore and discuss the dramatic processes of base-level fall, fluvial incision, and knickpoint retreat.
Virginia↗
Geology and hydrology for environmental planning in Marquette County, Michigan
Marquette County, in the glaciated area of the Upper Peninsula of Michigan, includes 1,878 square miles. Precipitation averages 32 inches per year. Bedrock and glacial deposits contain materials that are good aquifers. Sedimentary bedrock units generally yield sufficient water for domestic supply and, in places, may yield more than 100 gallons per minute to large-diameter wells. In the glacial deposits, sand and gravel beds are the principal aquifers; yields to wells range from less than 10 to 200 gallons per minute. Igneous and metamorphic rocks yield little or no water to wells. Suitable sewage and refuse disposal sites are not readily available because of the abundance of wetlands, streams, and lakes susceptible to infiltrating leachate.
Michigan↗
Geology and ground water in the central part of Apache County, Arizona
The central part of Apache County, Ariz., includes an area of about 3,300 square miles between the Navajo Indian Reservation to the north and U.S. Highway 60 to the south. Sedimentary rocks in the area range from Pennsylvanian to Quaternary in age and from 2,000 to more than 6,000 feet in thickness. The strata were tilted to the northeast, and part of the Upper Triassic and all the Jurassic and Lower Cretaceous rocks were eroded away before strata of Late Cretaceous age were deposited. Basaltic lava flows and cinder cones, representing four general periods of eruption in late Miocene to Quaternary time, are widespread in the southern part of the area. Pennsylvanian and Permian rocks overlie basement rocks of granite and diorite and include the Supai Formation, the Coconino Sandstone, and the Kaibab Limestone. The Supai Formation is 1,000 to 2,000 feet thick and consists of interbedded red and brown mudstone, siltstone, sandstone, limestone, and evaporites. It contains water of very poor quality outside Apache County. The Coconino Sandstone is 200 to 250 feet thick and consists of light-gray fine- to medium-grained sandstone. It contains water suitable for domestic use in the south and water unsuitable for most purposes in the north. The Coconino Sandstone underlies all Central Apache County in the subsurface. The yellowish-gray to dark-gray Kaibab Limestone is present in the southern two-thirds of the area and is 0 to 350 feet thick. It contains water where it is fractured and combines with the Coconino Sandstone to form a single hydrologic unit that yields from 6 to 74 gpm (gallons per minute) of water per foot of drawdown. An unconformity Heparates the Permian rocks from the overlying Triassic rocks, which comprise the Moenkopi and Chinle Formations and the Wingate Sandstone. The Moenkopi Formation is 35 to 250 feet thick and consists of intercalated brownish-red siltstone, sandstone, and conglomerate. It contains salty water in some areas but is dry in most. The Chinle Formation is 0 to 1,550 feet thick and unconformably overlies the Moenkopi. The Chiule consists of multicolored claystone, mudstone, siltstone, sandstone, and conglomerate. Some of the sandstone units yield small amounts of water, usually of a quality unsuitable for domestic use. The Wingate Sandstone is about 250 feet thick and is present only in the extreme northeastern corner of the area. It consists of intercalated, reddish-brown sandstone and siltstone and does not contain water. The Upper Cretaceous rocks comprise the Dakota Sandstone, from 50 to 115 feet thick; the Mancos Shale, about 150 feet thick; and the Mesaverde Group, as much as 200 feet thick. These rocks consist of yellowish-gray, light-green, and reddish-brown sandstone and carbonaceous siltstone. Some of the sandstone units contain water of suitable quality for domestic use, and wells in these units yield from 10 to 1,000 gpm. Sedimentary rocks of Eocene(?) age are about 800 feet thick and unconformably overlie Cretaceous rocks. They consist of light-brown and medium-red conglomerate, sandstone, and siltstone. These sedimentary rocks contain small amounts of water suitable for domestic use and yield from 10 to 25 gpm in the Springerville area. The Datil Formation of Miocene(?) Tertiary age consists of more than 800 feet of sedimentary rocks, which are composed largely of volcanic fragments. The Datil Formation does not contain water in the one small area where it crops out. The Bidahochi Formation of Pliocene age consists of 0 to 800 feet of white, green, and brown claystone, mudstone, and sandstone. Locally it yields from 10 to 50 gpm of water suitable for domestic use. Quaternary rocks consist of as much as 500 feet of alluvium, sand, gravel, travertine, cinders, and lava. The alluvium along the large drainages contains water that differs in quality from place to place. In most areas where it occurs, the lava
Water Supply Paper↗
Geologic and geophysical investigations of Climax Stock intrusive, Nevada
The Climax stock is a composite granitic intrusive of Cretaceous age, composed of quartz monzonite and granodiorite, which intrudes rocks of Paleozoic and Precambrian age. Tertiary volcanic rocks, consisting of ashflow and ash-fall tuffs, and tuffaceous sedimentary rocks overlie the sedimentary rocks and the stock. Erosion has removed much of the Tertiary volcanic rocks. Hydrothermal alteration of quartz monzonite and granodiorite is found mainly along joints and faults and varies from location to location. The Paleozoic carbonate rocks have been thermally and metasomatically altered to marble and tactite as much as 457 m (1,500 ft) from the contact with the stock, although minor discontinuous metasomatic effects are noted in all rocks out to 914 m (3,000 ft). Three major faults which define the Climax area structurally are the Tippinip, Boundary and Yucca faults. North of the junction of the Boundary and Yucca faults, the faults are collectively referred to as the Butte fault. The dominant joint sets and their average attitudes are N. 32? W., 22? NE; N. 60? W., vertical and N. 35? E., vertical. Joints in outcrop are weathered and generally open, but in subsurface, the joints are commonly filled and healed with secondary mineral s. The location of the water table and the degree of saturation of the granitic rocks are presently unknown. Measurement from drill holes indicated that depth to perched water levels ranges from 30 to 244 m (100-800 ft). Recent field investigations have shown the contact between the Pogonip marble and the granodiorite is a contact rather than a fault as previously mapped. The thickness of the weathered granodiorite is estimated to be 8 to 46 m (25 to 150 ft).
Open-File Report↗
A multidisciplinary-based conceptual model of a fractured sedimentary bedrock aquitard: improved prediction of aquitard integrity
A hydrogeologic conceptual model that improves understanding of variability in aquitard integrity is presented for a fractured sedimentary bedrock unit in the Cambrian-Ordovician aquifer system of midcontinent North America. The model is derived from multiple studies on the siliciclastic St. Lawrence Formation and adjacent strata across a range of scales and geologic conditions. These studies employed multidisciplinary techniques including borehole flowmeter logging, high-resolution depth-discrete multilevel well monitoring, fracture stratigraphy, fluorescent dye tracing, and three-dimensional (3D) distribution of anthropogenic tracers regionally. The paper documents a bulk aquitard that is highly anisotropic because of poor connectivity of vertical fractures across matrix with low permeability, but with ubiquitous bed parallel partings. The partings provide high bulk horizontal hydraulic conductivity, analogous to aquifers in the system, while multiple preferential termination horizons of vertical fractures serve as discrete low vertical hydraulic conductivity intervals inhibiting vertical flow. The aquitard has substantial variability in its ability to protect underlying groundwater from contamination. Across widespread areas where the aquitard is deeply buried by younger bedrock, preferential termination horizons provide for high aquitard integrity (i.e. protection). Protection is diminished close to incised valleys where stress release and weathering has enhanced secondary pore development, including better connection of fractures across these horizons. These conditions, along with higher hydraulic head gradients in the same areas and more complex 3D flow where the aquitard is variably incised, allow for more substantial transport to deeper aquifers. The conceptual model likely applies to other fractured sedimentary bedrock aquitards within and outside of this region.
Minnesota↗
Spatial correlation of shear-wave velocity in the San Francisco Bay Area sediments
Ground motions recorded within sedimentary basins are variable over short distances. One important cause of the variability is that local soil properties are variable at all scales. Regional hazard maps developed for predicting site effects are generally derived from maps of surficial geology; however, recent studies have shown that mapped geologic units do not correlate well with the average shear-wave velocity of the upper 30 m, V s (30). We model the horizontal variability of near-surface soil shear-wave velocity in the San Francisco Bay Area to estimate values in unsampled locations in order to account for site effects in a continuous manner. Previous geostatistical studies of soil properties have shown horizontal correlations at the scale of meters to tens of meters while the vertical correlations are on the order of centimeters. In this paper we analyze shear-wave velocity data over regional distances and find that surface shear-wave velocity is correlated at horizontal distances up to 4 km based on data from seismic cone penetration tests and the spectral analysis of surface waves. We propose a method to map site effects by using geostatistical methods based on the shear-wave velocity correlation structure within a sedimentary basin. If used in conjunction with densely spaced shear-wave velocity profiles in regions of high seismic risk, geostatistical methods can produce reliable continuous maps of site effects.
California↗
Geology of Porcupine Mountains in Carp River and White Pine Quadrangles, Michigan
The Porcupine Mountains are the eroded remnants of the upper limb of an anticline that is faulted along most of its southern, overturned, limb. The exposed rocks include about 5,000 ft (1,525 m) of middle Keweenawan intermediate to mafic lava flows interbedded with subordinate lithic sedimentary rocks and also include about 3,000 ft (915 m) of overlying middle and upper Keweenawan sedimentary rocks. The sediments are derived from lower Keweenawan and older rocks. The structural relief is at least 8,000 ft (2,440 m) between the crest of the Porcupine Mountains anticline and the trough of the Iron River syncline about 5 mi (8 km) to the south. The reverse fault along the southern, overturned, limb of the anticline has a displacement of about 5,000 ft (1,525 m).
Michigan↗
Preliminary geologic map of the Al Jawf Quadrangle, sheet 29D, Kingdom of Saudi Arabia
The Al Jawf quadrangle (Sheet 29 D) lies in the northwestern part of the Kingdom of Saudi Arabia about 900 km north of Jeddah. The quadrangle is located between lat 29°00'-30°00' N. and long 39°00'-40°30' E. It includes the southeastern rim of the Sirhan-Turayf basin, and is underlain by sedimentary rocks of Paleozoic to Cenozoic age. More than half of the quadrangle is covered by surficial deposits. The Phanerozoic sedimentary rocks of the map area were deposited under both marine and continental conditions. Marine deposits, produced by transgressions and regressions of the sea, are of outershelf to near-shore and coastal-lagoon origin, and the continental deposits are largely of fluvial and deltaic origin. Part of the Miocene-Pliocene rocks are believed to have been deposited in a lacustrine environment. Tensional structures, such as linear macro joints, grabens, block-faulting, and some folding are characteristic of the geology of the map area, and reflect a combination of Red Sea rifting and movement of the Hail Arch. This report introduces a new structural concept that extends the Wadi as Sirhan graben complex southeastward in the Al Jawf quadrangle into the An Nafud (Great Desert). This concept greatly increases the area of interest in the potentially oil-and-gas-bearing rocks of the Wadi as Sirhan region to include those of the An Nafud basin. Discovery of the largest specimen of Prototaxites sp. in the world, in the Al Jubah area of the northeastern part of the quadrangle, has helped solve a long-standing stratigraphic problem involving Devonian and Cretaceous rocks of the southeastern part of the Sirhan-Turayf basin. Phosphorite, a commodity of major economic interest in the Sirhan-Turayf basin, is exposed in the map area along escarpments that form the southeastern rim of the basin. The grade of the phosphorite is as much as 21 percent P 2 O 5 , but the beds are thin and lenticular.
Open-File Report↗
Geologic map and profiles of the north wall of the Snake River Canyon, Pasadena Valley and Ticeska quadrangles, Idaho
The Snake River Plain is a broad, arcuate region of low relief that extends more than 300 mi across southern Idaho. The Snake River enters the plain near Idaho Falls and flows westward along the southern margin of the eastern Snake River Plain (fig. 1), a position mainly determined by the basaltic lava flows that erupted near the axis of the plain. The highly productive Snake River Plain aquifer north of the Snaked River underlies the most of the eastern plain. The aquifer is composed of basaltic ricks that are interbedded with fluvial and lacustrine sedimentary rocks. The top of the aquifer (water table) is typically less than 500 ft below the land surface, but is deeper than 1,000 ft in few areas. The Snake River had excavated a canyon into the nearly flat-lying basaltic and sedimentary rocks of the eastern Snake River Plain between Milner Dam and King Hill (fig. 2), a distance of almost 90 mi. For much of its length the canyon intersects the Snake River Plain aquifer, which discharges from the north canyon wall as springs of variable size, spacing, and altitude. Geologic controls on springs are of importance because nearly 60 percent of the aquifer's discharge occurs as spring flow along this reach of the canyon. This report is one of several that describes the geologic occurrence of springs along the northern wall of the Snake River canyon from Milner Dam to King Hill. To understand the local geologic controls on springs, the Water Resources Division of the U.S. Geological Survey initiated a geologic mapping project as part of their Snake River Plain Regional Aquifer System-Analysis Program. Objectives of the project were (1) to prepare a geologic map of a strip of land immediately north of the Snake River canyon, (2) to map the geology of the north canyon wall in profile, (3) to locate spring occurrences along the north side of the Snake River between Milner Dam and King Hill, and (4) to estimate spring discharge from the north wall of the canyon.
Idaho↗
The Giles County, Virginia, seismogenic zone-- Seismological results and geological interpretations
This paper describes and interprets a newly-recognized 40-km-long seismogenic zone, which is inferred to have been the locus of a damaging earthquake in 1897. That shock was the second largest known to have occurred in the southeastern United States (MMI VIII, m b estimated at 5.8, felt over 725,000 km 2 ). It struck Giles County in southwestern Virginia, and a recurrence would affect populous regions on and near the central Atlantic seaboard. This paper attempts to aid in evaluating that hazard by presenting and synthesizing new seismological data with geological inferences and deductions. A five-station, 60-km aperture seismic network has been in operation in the Giles County locale since early 1978. For the subsequent 3-year monitoring period, 10 microearthquakes (M < 2) have been detected. Eight of those 10 events, plus an additional 4 relocated felt earthquakes (3.2 < M < 4.1; 1959-1976) have a tabular distribution centered at Pearisburg, Virginia. That distribution is about 40 km long, 10 km wide, strikes N. 43° E., and has a nearly vertical extent of from 5 km to 25 km in depth. Thus, a Giles County seismogenic zone is defined presently by 12 earthquakes that span 4 orders of magnitude (0 < M < 4) and 2 decades of time (1959-1980). We conclude that the 1897 earthquake occurred on that seismogenic zone. From the orientation of the tabular zone, from evidence that greatest horizontal compressive stress trends east-northeasterly at seismogenic depths in and near Giles County and from sparse P-wave first-motion data, we infer that the monitored microseismicity probably occurs by right-reverse motion on the seismogenic zone, with the southeast side dropping down with respect to the northwest side. In the Giles County locale, the upper 3-6 km of the crust are Paleozoic sedimentary rocks that have moved some tens of kilometers northwest on nearly horizontal detachment faults. The above-mentioned reliable hypocenters for the region lie below the deepest likely detachment, indicating that Giles County seismicity probably has no simple relationship to surface geology. Since Precambrian time, three deformational episodes could have formed steep faults under today's surface structures, at the observed hypocentral depths. These episodes were as follows: (1) As the Iapetus Ocean (Atlantic's predecessor) opened in late Precambrian or early Paleozoic time, northeast-striking normal faults formed, probably at the inferred Iapetan continental edge in central Virginia and at least as far northwest of that locus as Giles County. (2) In late Paleozoic time, detachment faults loaded the crust with several kilometers of overthrust sedimentary rocks, perhaps forming northeast-striking thrust-load faults in a brittle analogue of isostatic depression caused by thrust masses and much lighter continental glaciers. (3) As the Atlantic Ocean opened in Mesozoic time, other northeast-striking normal faults formed on the present continental margin and inland of it. The N. 43° E.-striking seismogenic zone seems most likely to have resulted from compressional reactivation of an Iapetan normal fault, which also may have been reactivated by late Paleozoic compression and Mesozoic extension. First, the seismogenic zone probably does not occur on a thrust-load fault. The zone underlies detached structures of southern Appalachian orientations (east-northeast), but those structures are not known to be displaced where they cross the zone. Thus, if the zone occurs on a thrust-load fault, the fault and its coeval causative central Appalachian detachments would pre-date the southern Appalachian structures. That deduction contradicts stratigraphic and structural estimates of relative ages of southern and central Appalachian detachments. Second, the zone probably does not result from a Mesozoic normal fault, because known locations of Mesozoic normal faults and grabens are well to the southeast of Giles County. Not yet known is where else in the East reactivated Iapetan normal faults might generate shocks similar to that of 1897. However, our analysis enables us to suggest specific geological and geophysical investigations that may produce results useful in answering that question. Such investigations can concentrate on defining the area of probable occurrence of other Iapetan normal faults, and on determining whether the one inferred to underlie Giles County is uniquely active or is typical of others that might exist elsewhere.
Virginia↗
Geologic, geochemical, and isotopic studies of a carbonate- and siliciclastic-hosted Pb-Zn deposit at Lion Hill, Vermont
Zn-, Pb-, Cu-, and Fe-bearing rocks of the Lion Hill area in western Vermont formed during the Early Cambrian by syngenetic sedimentary-exhalative and diagenetic replacement processes. Sphalerite, galena, chalcopyrite, pyrite, and, locally, magnetite form stratabound and broadly stratiform lenticular zones, -300 meters long and 25-50 meters thick, which are uneconomic at the present time. The lenses are structurally disrupted and metamorphosed to greenschist facies, probably due to the Taconic orogeny. Textural evidence suggests that mineralizing fluids permeated the sediments prior to lithification and that a dilatant fracture zone, possibly a feeder zone, contains some of the discordant veins at Lion Hill. The veins may have formed when the sediments were in a plastic, semiconsolidated state. The association of layered iron formation containing base-metal sulfide minerals provides possible lithologic evidence for syngenetic mineralization by submarine exhalative activity. Sand bars and tidal channels present in the sedimentary section could have acted as permeable pathways for movement of mineralizing fluids. The complex interlayering in the sedimentary sequence of carbonate and siliciclastic rock types having widely varying permeabilities created numerous fluid traps. Homogenization temperatures of primary and secondary inclusions in vein sphalerite range from 152°C to 196°C; salinities range from 11.5 to 14.0 equivalent weight percent NaCl. δ 34 S values of sulfides from Lion Hill vary from -25.9 to +10.0 per mil, and fall within the expected range for sulfide produced from bacteriogenic reduction of sulfate with δ 34 S values of 25 to 30 per mil. In addition, some pyrite probably formed from sulfate in trapped pore fluid that resulted in heavier isotopic values characteristic of more closed-system behavior. Three sphalerite samples that have heavier sulfur isotopic values may reflect a change in the source of sulfur during a later episode of mineralization, perhaps a change to a deep-seated source. Lead isotopic compositions of galenas from mineralized zones at Lion Hill range from 18.351 to 18.632 for 206 Pb/ 204 Pb, from 15.546 to 15.618 for 207 Pb/ 204 Pb, and from 38.126 to 38.496 for 208 Pb/ 204 Pb. The lead isotopic compositions of galena from Lion Hill and fluid inclusion and sulfur isotopic values for the Lion Hill sulfides are more like those of Pb-Zn-Ag deposits of Ireland than those of MVT or Appalachian-type Zn deposits. The prospect of an Irish-type sedimentary-exhalative origin for stratabound Pb-Zn deposits of the Paleozoic shelf of North America is of considerable importance to understanding the timing of mineralization relative to platform evolution and for evaluating the mineral resource potential of the region. Our study of the Lion Hill deposit indicates a potential for Irish-type Pb-Zn deposits in platform rocks of western Vermont; however, at Lion Hill they contain enrichments of Pb, Zn, and Cu rather than a Pb, Zn, and Ag association.
Vermont↗