Geology Reports⌕ Search

Geology topics

William C. Burton

Publications and source records attributed to William C. Burton.

At least 19 recordsLinked to original sources
USGS2024

Bedrock geologic map of the Woodstock quadrangle, Grafton County, New Hampshire

The bedrock geology of the Woodstock 7.5-minute quadrangle consists of highly deformed metasedimentary rocks of the Central Maine trough, including the Silurian Rangeley and Perry Mountain Formations and the Devonian Littleton Formation. The central, northern, and eastern parts of the quadrangle are underlain by the oldest rocks in the area, the Rangeley Formation. In the southwest and south-central part of the quadrangle, metaturbidites of the Perry Mountain Formation and subsequent Littleton Formation overly the Rangeley Formation in a deformed F 1 synform, herein informally called the Bagley Brook basin. The metasedimentary rocks were intruded by widespread syn- to post-tectonic granitoids of the Devonian New Hampshire Plutonic Suite and minor post-metamorphic Jurassic-Cretaceous mafic dikes of the White Mountain Plutonic-Volcanic Suite. The metasedimentary rocks were affected by at least two episodes of deformation in the Devonian Acadian orogeny. The dominant regional foliation is second-generation (S 2 /D 2 ) and formed during the development of sillimanite-muscovite mineral assemblages. Large bodies of the Early Devonian Kinsman Granodiorite intruded the metasedimentary rocks semi-concordantly during D 2 deformation. Dikes of the Late Devonian Concord Granite cut the Kinsman Granodiorite and the metasedimentary rocks and were emplaced either syn- or post-D 2 . The map pattern in the Rangeley Formation is dominated by northeast to northwest trending, moderately to steeply north-dipping F 2 and F 3 folds. Map-scale F 1 folds are defined by the Bagley Brook basin. Previous division of Rangeley Formation stratigraphy in this region into “upper” and “lower” parts was not corroborated by 1:24,000-scale mapping of lithodemic units, and rocks previously mapped as part of the Smalls Falls and Madrid Formations are here reassigned to the Rangeley Formation. Some rocks previously mapped as the lower part of the Littleton Formation are now assigned to the Perry Mountain Formation. The Littleton Formation on this map is approximately equivalent to rocks previously mapped as the upper part of the same formation. Steeply dipping fractures in the quadrangle show a preferred northeast orientation, consistent with subsurface fracture orientations in the well fields near Mirror Lake. Jurassic-Cretaceous mafic dikes and normal faults show preferred northeast orientations, similar to the fractures, suggesting that the extensional stress field that controlled dike orientation during the Mesozoic also produced the dominant brittle fabrics in the area.

New Hampshire↗
USGS2022

Geologic map of the South Boston 30' × 60' quadrangle, Virginia and North Carolina

This 1:100,000-scale geologic map of the South Boston 30 ’ × 60 ’ quadrangle, Virginia and North Carolina, provides geologic information for the Piedmont along the I–85 and U.S. Route 58 corridors and in the Roanoke River watershed, which includes the John H. Kerr Reservoir and Lake Gaston. The Raleigh terrane (located on the eastern side of the map) contains Neoproterozoic to early Paleozoic(?) polydeformed, amphibolite-facies gneisses and schists. The Carolina slate belt of the Carolina terrane (located in the central part of the map) contains Neoproterozoic metavolcanic and metasedimentary rocks at greenschist facies. Although locally complicated, the slate-belt structure mapped across the South Boston map area is generally a broad, complex anticlinorium of the Hyco Formation (here called the Chase City anticlinorium) and is flanked to the west and east by synclinoria, which are cored by the overlying Aaron and Virgilina Formations. The western flank of the Carolina terrane (located in the western-central part of the map) contains similar rocks at higher metamorphic grade. This terrane includes epidote-amphibolite-facies to amphibolite-facies gneisses of the Neoproterozoic Country Line complex, which extends north-northeastward across the map. The Milton terrane (located on the western side of the map) contains Ordovician amphibolite-facies metavolcanic and metasedimentary gneisses of the Cunningham complex. Crosscutting relations and fabrics in mafic to felsic plutonic rocks constrain the timing of Neoproterozoic to late Paleozoic deformations across the Piedmont. In the eastern part of the map, a 5- to 9-kilometer-wide band of tectonic elements that contains two late Paleozoic mylonite zones (Nutbush Creek and Lake Gordon) and syntectonic granite (Buggs Island pluton) separates the Raleigh and Carolina terranes. Amphibolite-facies, infrastructural metaigneous and metasedimentary rocks east of the Lake Gordon mylonite zone are generally assigned to the Raleigh terrane. In the western part of the map area, a 5- to 8-kilometer-wide band of late Paleozoic tectonic elements includes the Hyco and Clover shear zones, syntectonic granitic sheets, and amphibolite-facies gneisses along the western margin of the Carolina terrane at its boundary with the Milton terrane. This band of tectonic elements is also the locus for early Mesozoic extensional faults associated with the early Mesozoic Scottsburg, Randolph, and Roanoke Creek rift basins. The map shows fluvial terrace deposits of sand and gravel on hills and slopes near the Roanoke and Dan Rivers. The terrace deposits that are highest in altitude are the oldest. Saprolite regolith is spatially associated with geologic source units and is not shown separately on the map. Mineral resources in the area include gneiss and granite quarried for crushed stone, tungsten-bearing vein deposits of the Hamme district, and copper and gold deposits of the Virgilina district. Surface-water resources are abundant and include rivers, tributaries, the John H. Kerr Reservoir, and Lake Gaston. Groundwater flow is concentrated in saprolite regolith, along fractures in the crystalline bedrock, and along fractures and bedding-plane partings in the Mesozoic rift basins.

North Carolina, Virginia↗
USGS2021

Paleozoic and Mesozoic tectonic events west of the Waterbury Dome: Results of new mapping in the western Connecticut Highlands

This field trip highlights the results of recent U.S. Geological Survey (USGS) bedrock geologic mapping in four 7.5 min quadrangles in the western Connecticut highlands near Southbury, Connecticut, USA. The rocks are broadly within what Rodgers (1985) called the Hartland and Gneiss Dome belts of the Connecticut Valley Synclinorium ( Rodgers, 1985 ; Fig. 1 ), the latter of which is now known as the Connecticut Valley–Gaspe Trough (Hibbard et al., 2006). The mapping occurred over two intervals: 2003–2005 and 2016–present. In the first, the goal was a detailed map of the early Mesozoic Pomperaug basin, which overlaps the four quadrangles. Portions of the basin had been separately mapped during the statewide 7.5 min quadrangle mapping campaign spanning the 1950s–1970s, resulting in an inaccurate depiction of the basin on the 1985 state geologic map ( Rodgers, 1985 ). A new map of the basin was proposed in part to benefit an ongoing project by the USGS Connecticut Water Science Center to determine the contributions of natural and artificial contaminants to a public water-supply well in Woodbury, Connecticut, under the National Water-Quality Assessment (NAWQA) Program ( Starn and Brown, 2007) . The NAWQA project was partially funded by the Pomperaug River Watershed Coalition, which also provided logistical support to the geologic mapping project. Mapping of the basin was also a high priority for the Connecticut State Geologist at the time (Ralph Lewis, 2002, pers. comm.). The mapping resulted in an NEIGC (New England Intercollegiate Geologic Conference) field guide ( Burton et al., 2005) and a 1:12,000-scale USGS open-file map ( Burton, 2006) and was funded by the USGS National Cooperative Geologic Mapping Program (NCGMP). The second phase of the mapping began in 2016 after the discovery of elevated levels of uranium and arsenic in domestic water wells in the igneous and metamorphic rocks that surround the sedimentary and volcanic rocks of the Pomperaug basin ( Flanagan and Brown, 2017) . Structural measurements in the surrounding crystalline rocks were made during Pomperaug basin mapping to better understand the tectonic setting, but a revision of the crystalline map units on Rodgers’ 1985 geologic map was not attempted. Nonetheless, discrepancies were noted between the new mapping and the 1985 map, particularly within the two northern quadrangles of Woodbury and Roxbury, which were originally mapped by Gates (1954) and Gates (1959) , respectively. Based on these discrepancies, a new NCGMP project was proposed to remap the Woodbury and Roxbury 7.5 min quadrangles, commencing in the fall of 2016. The expected USGS product will be a two-quadrangle, 1:24,000-scale Scientific Investigations Map (SIM). This field guide is not meant as a comprehensive review of all of the geologic research done in this area of Connecticut; rather, it looks at previous bedrock geologic mapping from the perspective of new and recent mapping in the four-quadrangle area and discusses the structural, stratigraphic, and nomenclatural revisions necessary for the next revision of the state geologic map.

Connecticut↗
USGS2020

Photoluminescence imaging of whole zircon grains on a petrographic microscope—An underused aide for geochronologic studies

The refractory nature of zircon to temperature and pressure allows even a single zircon grain to preserve a rich history of magmatic, metamorphic, and hydrothermal processes. Isotopic dating of micro-domains exposed in cross-sections of zircon grains allows us to interrogate this history. Unfortunately, our ability to select the zircon grains in a heavy mineral concentrate that records the most geochronologic information is limited by our inability to predict internal zonation from observations of whole grains. Here we document the use of a petrographic microscope to observe and image the photoluminescence (PL) response of whole zircon grains excited under ultraviolet (UV) light, and the utility of this PL response in selecting grains for geochronology. While zircon fluorescence has long been known, there is limited documentation of its utility for and application to geochronologic studies. Our observations of zircon from an un-metamorphosed igneous rock, two meta-igneous rocks, and a placer deposit show that variations in the PL color are readily observable in real-time, both among grains in a population of zircons and within single grains. Analyses of cross-sections of the same grains demonstrate that the changes in PL correlate with zoning in backscattered electron (BSE) and cathodoluminescence (CL) images as well as with changes in U + Th concentration and spectroscopic proxies for radiation damage. In other words, the whole grain PL provides a low-resolution preview of the U + Th zoning expected in a cross-sectioned grain. We demonstrate the usefulness of this “preview” in identifying and selecting the subset of zircon grains in a heavy mineral separate that has metamorphic rims of sufficient width to date by secondary ionization mass spectrometry (SIMS). The data are also used to place preliminary constraints on the age and U + Th concentrations at which a yellow PL response is observed in natural samples. The PL response of zircon is well-known among spectroscopists, and these simple applications demonstrate several ways in which the response might be more effectively used by geochronologists.

Minerals↗
USGS2020

Geochronologic age constraints on tectonostratigraphic units of the central Virginia Piedmont, USA

New geologic mapping coupled with uranium-lead (U-Pb) zircon geochronology (sensitive high-resolution ion microprobe-reverse geometry [SHRIMP-RG] and laser ablation-inductively coupled plasma-mass spectrometry [LA-ICP-MS]) analyses of 10 samples, provides new constraints on the tectonostratigraphic framework of the central Virginia Piedmont. Detrital zircon analysis confirms that the Silurian-Devonian Quantico Formation is a postorogenic successor basin, with zircons derived primarily from Ordovician Chopawamsic Formation volcanic rocks. Detrital zircons from strata of the Long Island syncline, previously mapped as a separate successor basin, have a peri-Gondwanan component distinct from Laurentian-sourced rocks of the Potomac terrane to the west. Volcanism of the Chopawamsic Formation spanned at least 14 million years during the Ordovician. The Chopawamsic Formation contains sheet-like Late Ordovician-Silurian granodioritic and tonalitic intrusions that were once mapped as Carboniferous. Biotite-muscovite migmatitic paragneiss, which borders the Chopawamsic Formation on its southeast side and also occurs east of the Lakeside fault, preserves evidence of Silurian deformation and metamorphism, with a Carboniferous (Alleghanian) overprint. Limited SHRIMP-RG analysis of detrital zircons from this paragneiss yields a Laurentian (Mesoproterozoic) signature, which suggests that the structurally concordant contact between volcanic rocks of the Chopawamsic Formation and paragneiss is either a pre-Alleghanian fault or an unconformity.

Virginia↗
USGS2020

Preliminary bedrock geologic map of the Lahore 7.5-minute quadrangle, Orange, Spotsylvania, and Louisa Counties, Virginia

Introduction Bedrock geologic mapping of the Lahore, Va., 7.5-minute quadrangle was completed as part of a broader project, undertaken jointly between the U.S. Geological Survey, the Virginia Division of Geology and Mineral Resources, and other Federal and State agencies to better understand the causative mechanisms of the magnitude-5.8 (M5.8) earthquake that occurred near Mineral, Va., on August 23, 2011. This project involved detailed mapping of eight quadrangles in the epicentral region of the Mineral, Va., earthquake in order to improve our understanding of the geologic framework of the central Virginia seismic zone, which has a long record of historical and prehistoric seismicity. The Lahore 7.5-minute quadrangle contains the contact between Ordovician to Silurian, dioritic and granodioritic rocks of the Lahore and Ellisville plutons and older metasedimentary and metavolcanic rocks. The Lahore quadrangle is northeast of the Ferncliff and Louisa, Va., quadrangles, where the Shores complex is intruded by the Ellisville pluton along the pluton’s southwestern margin. The new mapping in the Lahore quadrangle shows that the Shores complex continues northeast of the Ellisville pluton. A northeast-trending mafic- and ultramafic-bearing belt within the Shores complex is a fault-bounded accretionary zone (accretionary wedge) between rocks of the Chopawamsic Formation and Laurentian slope-and-rise deposits. In the Lahore quadrangle, this belt contains several mappable, northeast- to southwest-trending mafic bodies and also includes small exposures of gabbro and talc schist. The Lahore quadrangle contains structures of both early- and late-Paleozoic age that correspond to the Taconic and Alleghanian orogenies. Taconic (Late Ordovician) S 1 schistosity in layered rocks is typically fine-grained and parallel to compositional layering, when present. Alleghanian (Pennsylvanian) S 2 schistosity is coarser and more micaceous than S 1 and is locally accompanied by a lineation that is represented by mineral lineations, micro-crenulations, or mullion fabric, and represents the hinges of F 2 folds. A foliation in the plutonic rocks is represented by an equilibrium assemblage of aligned mafic minerals and is early Paleozoic in age. Metamorphic grade in the non-plutonic rocks of the Lahore quadrangle ranges from lower-greenschist to the northwest to upper-greenschist to the southeast, as represented by mineral assemblages in non-plutonic rocks. The biotite isograd may be, in part, lithologically controlled by the contact between the informally-named Hardware and Byrd Mill formations, and locally affected by contact metamorphism by the Lahore pluton (western part of map). The Taconic garnet isograd is defined by the sparse presence of small (less than 1 millimeter), euhedral garnet crystals. Both the biotite and garnet isograds continue along strike to the southwest into the Ferncliff and Louisa quadrangles, where the isograds have been identified as Ordovician age (Taconic orogeny) based on muscovite, biotite, and amphibole 40 Ar/ 39 Ar cooling ages. Regionally, the most common trend and plunge of joints is northwest and subvertical, respectively, and orthogonal to the regional strike of foliation. Early Mesozoic extension may have reactivated the Harris Creek fault (southeast corner of map), a late Paleozoic (Alleghanian orogeny) transpressional fault that marks the contact between granodiorite of the Ellisville pluton and the Chopawamsic Formation.

Virginia↗
USGS2019

Geologic map of the Ferncliff and Louisa quadrangles, Louisa, Fluvanna, and Goochland Counties, Virginia

The area encompassed by the geologic map of the Ferncliff and Louisa, Va., 7.5-minute quadrangles includes the hypothetical surface projection of the Quail fault, which is the subsurface fault that was responsible for the 2011 magnitude 5.8 (M5.8) Mineral, Va., earthquake. The mapping shows that the Quail fault appears to have reactivated the Harris Creek fault, a Paleozoic fault that has been mapped and named in the study area and marks the boundary between the Ellisville pluton neck and Chopawamsic Formation. The Harris Creek fault was also reactivated in the early Mesozoic. Another result of the mapping is a well-defined, southwest to northeast, narrow zone of metagraywacke and ultramafic rocks (both part of the informal Shores complex) that marks the closure of a small ocean basin and the accretion of the 468- to 460-Ma (mega-annum) Ordovician Chopawamsic volcanic arc (part of the Carolina terrane) onto Laurentia. The accretion zone is truncated by the Ordovician-Silurian (444 Ma) Ellisville pluton; the 444-Ma age of the pluton therefore represents the minimum age of the accretion zone and indicates likely closure of the ocean basin during the Taconic orogeny. Across the map area, the metamorphic grade ranges from lower-greenschist facies in the northwest to amphibolite facies in the southeast. 40 Ar/ 39 Ar age-dating across this metamorphic gradient indicates that Taconic metamorphism was overprinted by late-Paleozoic Alleghanian metamorphism that was accompanied by refolding and faulting of Taconic structures. Quaternary terraces mapped along the South Anna River record a long history of incision and downcutting. A continuing question is how much of this downcutting was a result of neotectonic uplift in the central Virginia seismic zone. This report consists of a single geologic map sheet and an online geographic information systems database that includes bedrock geologic unit contacts and polygons, surficial geologic polygons, faults, and structural geologic information.

Virginia↗
USGS2019

Geology of the Mineral and Lake Anna West Quadrangles, Virginia

This map product is a cooperator series publication and, as such, does not have a specific abstract. Geologic mapping for this map product was completed between 2014 and 2017, with most of the field work occurring between January 2016 and May 2017. Numerous foot traverses were completed along creeks and roads throughout the field area; the shore of Lake Anna was accessed by kayak to provide additional data. Distributions of soil units were considered when assigning bedrock type in areas where outcrop was lacking and helped to distinguish fluvial terrace deposits. Hill-shade raster images created from LiDAR datasets and geophysical data sets proved useful in the field to trace bedrock and surficial units. GPS location control and field data were collected and recorded in digital databases using a variety of geologic mapping applications for an iPad 3rd Gen Model A1403 and Motion C5v tablet using Fieldmove 2013.1. Structural measurements were also plotted on field maps. Data collected included lithology and the orientation of foliations, folds, lineations, joints, and faults. Representative rock samples of significant formations were thin-sectioned for petrographic analysis; a few samples from these and surrounding quadrangles were analyzed for zircon U-Pb geochronology and geochemistry.

Virginia↗
USGS2018

Geologic map of the Washington West 30’ × 60’ quadrangle, Maryland, Virginia, and Washington D.C.

The Washington West 30’ × 60’ quadrangle covers an area of approximately 4,884 square kilometers (1,343 square miles) in and west of the Washington, D.C., metropolitan area. The eastern part of the area is highly urbanized, and more rural areas to the west are rapidly being developed. The area lies entirely within the Chesapeake Bay drainage basin and mostly within the Potomac River watershed. It contains part of the Nation's main north-south transportation corridor east of the Blue Ridge Mountains, consisting of Interstate Highway 95, U.S. Highway 1, and railroads, as well as parts of the Capital Beltway and Interstate Highway 66. Extensive Federal land holdings in addition to those in Washington, D.C., include the Marine Corps Development and Education Command at Quantico, Fort Belvoir, Vint Hill Farms Station, the Naval Ordnance Station at Indian Head, the Chesapeake and Ohio Canal National Historic Park, Great Falls Park, and Manassas National Battlefield Park. The quadrangle contains most of Washington, D.C.; part or all of Arlington, Culpeper, Fairfax, Fauquier, Loudoun, Prince William, Rappahannock, and Stafford Counties in northern Virginia; and parts of Charles, Montgomery, and Prince Georges Counties in Maryland. The Washington West quadrangle spans four geologic provinces. From west to east these provinces are the Blue Ridge province, the early Mesozoic Culpeper basin, the Piedmont province, and the Coastal Plain province. There is some overlap in ages of rocks in the Blue Ridge and Piedmont provinces. The Blue Ridge province, which occupies the western part of the quadrangle, contains metamorphic and igneous rocks of Mesoproterozoic to Early Cambrian age. Mesoproterozoic (Grenville-age) rocks are mostly granitic gneisses, although older metaigneous rocks are found as xenoliths. Small areas of Neoproterozoic metasedimentary rocks nonconformably overlie Mesoproterozoic rocks. Neoproterozoic granitic rocks of the Robertson River Igneous Suite intruded the Mesoproterozoic rocks. The Mesoproterozoic rocks are nonconformably overlain by Neoproterozoic metasedimentary rocks of the Fauquier and Lynchburg Groups, which in turn are overlain by metabasalt of the Catoctin Formation. The Catoctin Formation is overlain by Lower Cambrian clastic metasedimentary rocks of the Chilhowee Group. The Piedmont province is exposed in the east-central part of the map area, between overlapping sedimentary units of the Culpeper basin on the west and those of the Coastal Plain province on the east. In this area, the Piedmont province contains Neoproterozoic and lower Paleozoic metamorphosed sedimentary, volcanic, and plutonic rocks. Allochthonous mélange complexes on the western side of the Piedmont are bordered on the east by metavolcanic and metasedimentary rocks of the Chopawamsic Formation, which has been interpreted as part of volcanic arc. The mélange complexes are unconformably overlain by metasedimentary rocks of the Popes Head Formation. The Silurian and Ordovician Quantico Formation is the youngest metasedimentary unit in this part of the Piedmont. Igneous rocks include the Garrisonville Mafic Complex, transported ultramafic and mafic inclusions in mélanges, monzogranite of the Dale City pluton, and Ordovician tonalitic and granitic plutons. Jurassic diabase dikes are the youngest intrusions. The fault boundary between rocks of the Blue Ridge and Piedmont provinces is concealed beneath the Culpeper basin in this area but is exposed farther south. Early Mesozoic rocks of the Culpeper basin unconformably overlie those of the Piedmont and Blue Ridge provinces in the central part of the quadrangle. The north-northeast-trending extensional basin contains Upper Triassic to Lower Jurassic nonmarine sedimentary rocks. Lower Jurassic sedimentary strata are interbedded with basalt flows, and both Upper Triassic and Lower Jurassic strata are intruded by diabase of Early Jurassic age. The Bull Run Mountain fault, a major Mesozoic normal fault characterized by down-to-the-east displacement, separates rocks of the Culpeper basin from those of the Blue Ridge province on the west. On the east, the contact between rocks of the Culpeper basin and those of the Piedmont province is an unconformity, which has been locally disrupted by normal faults. Sediments of the Coastal Plain province unconformably overlie rocks of the Piedmont province along the Fall Zone and occupy the eastern part of the quadrangle. Lower Cretaceous deposits of the Potomac Formation consist of fluvial-deltaic gravels, sands, silts, and clays. Discontinuous fluvial and estuarine terrace deposits of Pleistocene and middle- to late-Tertiary age flank the modern Potomac River valley unconformable capping these Cretaceous strata and the crystalline basement where the Cretaceous has been removed by erosion. East of the Potomac River, the Potomac Formation is onlapped and unconformably overlain by a westward thinning wedge of marine sedimentary deposits of Late Cretaceous and early- and late-Tertiary age. Basement rooted Coastal Plain faults of Tertiary to Quaternary age occur along the Fall Zone and this part of the inner Coastal Plain. These Coastal Plain faults have geomorphic expression that appear to influence river drainage patterns. The geologic map of the Washington West quadrangle is intended to serve as a foundation for applying geologic information to problems involving land use decisions, groundwater availability and quality, earth resources such as natural aggregate for construction, assessment of natural hazards, and engineering and environmental studies for waste disposal sites and construction projects. This 1:100,000-scale map is mainly based on more detailed geologic mapping at a scale of 1:24,000.

Maryland, Virginia↗
USGS2015

Geologic framework and evidence for neotectonism in the epicentral area of the 2011 Mineral, Virginia, earthquake

The epicenters of the main shock and associated aftershocks of the 2011 moment magnitude, M w 5.8 Mineral, Virginia (USA), earthquake, and the updip projection of the possible fault plane that triggered the quakes, are contained in the areas of 2 adjoining 7.5′ quadrangles in the central Virginia Piedmont. These quadrangles have therefore been the focus of concentrated geologic study in the form of bedrock and surficial mapping and near-surface trenching in order to identify potential seismogenic structures. Bedrock mapping has outlined a series of northeast-southwest–trending lithologic belts that include the Ordovician Chopawamsic and Quantico Formations, the narrow neck of the Late Ordovician Ellisville pluton, and mélange zone III of the Mine Run Complex. The region was affected by at least two ductile deformational events, one in the early Paleozoic that was broadly synchronous with the intrusion of the pluton, and one later in the Paleozoic. The earlier deformation produced the Quantico synclinorium and other regional folds, and the later deformation produced faults with associated high-strain zones. Two of these faults have been trenched at their intersection along the east-dipping eastern contact of the Ellisville neck, near where the causative fault for the earthquake projects to the surface. The trenches have exposed abundant evidence of post-Paleozoic fracturing and faulting, including brecciated quartz-tourmaline veins, slickensided thrust and strike-slip faults, and clay-filled fractures. Fluvial and colluvial gravels that overlie these brittle structures have yielded optically stimulated luminescence ages ranging from ca. 27 to 10 ka. These structures are likely representative of surface features associated with Quaternary earthquakes in the Central Virginia seismic zone.

GSA Special Papers↗
USGS2014

Geology and neotectonism in the epicentral area of the 2011 M5.8 Mineral, Virginia, earthquake

This fi eld guide covers a two-day west-to-east transect across the epicentral region of the 2011 M5.8 Mineral, Virginia, earthquake, the largest ever recorded in the Central Virginia seismic zone. The fi eld trip highlights results of recent bedrock and surficial geologic mapping in two adjoining 7.5-min quadrangles, the Ferncliff and the Pendleton, which together encompass the epicenter and most of the 2011–2012 aftershocks. Tectonic history of the region includes early Paleozoic accretion of an island arc (Ordovician Chopawamsic Formation) to Laurentia, intrusion of a granodiorite pluton (Ordovician Ellisville pluton), and formation of a post-Chopawamsic successor basin (Ordovician Quantico Formation), all accompanied by early Paleozoic regional deformation and metamorphism. Local transpressional faulting and retrograde metamorphism occurred in the late Paleozoic, followed by diabase dike intrusion and possible local normal faulting in the early Mesozoic. The overall goal of the bedrock mapping is to determine what existing geologic structures might have been reactivated during the 2011 seismic event, and surfi cial deposits along the South Anna River are being mapped in order to determine possible neotectonic uplift. In addition to bedrock and surfi cial studies, we have excavated trenches in an area that contains two late Paleozoic faults and represents the updip projection of the causative fault for the 2011 quake. The trenches reveal faulting that has offset surfi cial deposits dated as Quaternary in age, as well as numerous other brittle structures that suggest a geologically recent history of neotectonic activity.

Virginia↗
USGS2014

Subsurface geologic features of the 2011 central Virginia earthquakes revealed by airborne geophysics

Characterizing geologic features associated with major earthquakes provides insights into mechanisms contributing to fault slip and assists evaluation of seismic hazard. We use high-resolution airborne geophysical data combined with ground sample measurements to image subsurface geologic features associated with the 2011 moment magnitude (M w ) 5.8 central Virginia (USA) intraplate earthquake and its aftershocks. Geologic mapping and magnetic data analyses suggest that the earthquake occurred near a complex juncture of geologic contacts. These contacts also intersect a >60-km-long linear gravity gradient. Distal aftershocks occurred in tight, ~1-km-wide clusters near other obliquely oriented contacts that intersect gravity gradients, in contrast to more linearly distributed seismicity observed at other seismic zones. These data and corresponding models suggest that local density contrasts (manifested as gravity gradients) modified the nearby stress regime in a manner favoring failure. However, along those gradients seismic activity is localized near structural complexities, suggesting a significant contribution from variations in associated rock characteristics such as rheological weakness and/or rock permeability, which may be enhanced in those areas. Regional magnetic data show a broader bend in geologic structures within the Central Virginia seismic zone, suggesting that seismic activity may also be enhanced in other nearby areas with locally increased rheological weaknesses and/or rock permeability. In contrast, away from the M w 5.8 epicenter, geophysical lineaments are nearly continuous for tens of kilometers, especially toward the northeast. Continuity of associated geologic structures probably contributed to efficient propagation of seismic energy in that direction, consistent with moderate to high levels of damage from Louisa County to Washington, D.C., and neighboring communities.

Virginia↗
USGS2014

Stitching the western Piedmont of Virginia: Early Paleozoic tectonic history of the Ellisville Pluton and the Potomac and Chopawamsic Terranes

The theme of the 2014 Virginia Geological Field Conference is the tectonic development, economic geology, and seismicity of the western Piedmont of Louisa County, Virginia. It is timely for the conference to turn its attention here, for during the past decade these aspects of western Piedmont geology have garnered the renewed attention of researchers. In terms of regional tectonics, it has been hypothesized that the major structure in the region, the Chopawamsic fault system, represents the most significant boundary in the Appalachian orogen, the main Iapetan suture (Hibbard et al., 2014). Economically, recent elevated market values of metals— particularly that of gold—has spurred reconsideration of the economic geology of the western Piedmont. Finally, the August 23, 2011, M5.8 earthquake, with its epicenter in our field area, startled the North American east coast and has revived awareness of the seismic potential of the region. This renewed interest in the geology of the western Piedmont of north-central Virginia has led to new detailed bedrock mapping, detailed surficial mapping, high-resolution UPb TIMS zircon geochronology, U-Pb LA-ICPMS detrital zircon geochronology, radiogenic isotope geochemistry, major/minor/REE geochemistry, and geophysical studies (e.g. Bailey et al., 2005, 2008; Bailey and Owens, 2012: Berti et al., 2012; Burton et al., 2014; Burton, in progress; Harrison, 2012; Horton et al., 2010, in press; Hughes, 2010, 2014; Hughes et al., 2013a, 2013b, 2014, in press a, in press b; Malenda, in progress; Owens et al., 2013; Spears and Gilmer 2012; Spears et al. 2013, Terblanche, 2013; Terblanche and Nance, 2012). A host of institutions have taken part in the research, including North Carolina State University, the Virginia Department of Mines, Minerals, and Energy, the U.S. Geological Survey, Virginia Tech, Lehigh University, and the College of William and Mary. Many of these investigations remain active. The majority of the data presented herein is the product of research conducted from 2010 to 2014 by geologists at North Carolina State University. This field trip guide is intended to complement a Geological Society of America field guide (Hughes et al., 2014) that covers the western Piedmont geology along strike to the northeast in the vicinity of Fredericksburg. Geologic mapping and geochronologic and geochemical sampling were coordinated between these two areas as part of a study funded in part by the National Science Foundation and the USGS EDMAP program. Some of the stops in this guide have previously been written up in past field guides (Hughes, 2010; Burton et al., 2014) and are reused here because of their ease of access for large groups and because of new data that update the context and our understanding of the outcrops.

Virginia↗
USGS2013

The Chaitén rhyolite lava dome: Eruption sequence, lava dome volumes, rapid effusion rates and source of the rhyolite magma

We use geologic field mapping and sampling, photogrammetric analysis of oblique aerial photographs, and digital elevation models to document the 2008-2009 eruptive sequence at Chaitén Volcano and to estimate volumes and effusion rates for the lava dome. We also present geochemical and petrologic data that contribute to understanding the source of the rhyolite and its unusually rapid effusion rates. The eruption consisted of five major phases: 1. An explosive phase (1-11 May 2008); 2. A transitional phase (11-31 May 2008) in which low-altitude tephra columns and simultaneous lava extrusion took place; 3. An exogenous lava flow phase (June-September 2008); 4. A spine extrusion and endogenous growth phase (October 2008-February 2009); and 5. A mainly endogenous growth phase that began after the collapse of a prominent Peléean spine on 19 February 2009 and continued until the end of the eruption (late 2009 or possibly earliest 2010). The 2008-2009 rhyolite lava dome has a total volume of approximately 0.8 km3. The effusion rate averaged 66 m3s-1 during the first two weeks and averaged 45 m3s-1 for the first four months of the eruption, during which 0.5 km3 of rhyolite lava was erupted. These are among the highest rates measured world-wide for historical eruptions of silicic lava. Chaitén’s 2008-2009 lava is phenocryst-poor obsidian and microcrystalline rhyolite with 75.3±0.3% SiO2. The lava was erupted at relatively high temperature and is remarkably similar in composition and petrography to Chaitén’s pre-historic rhyolite. The rhyolite’s normative composition plots close to that of low pressure (100-200 MPa) minimum melts in the granite system, consistent with estimates of approximately 5 to 10 km source depths based on phase equilibria and geodetic studies. Calcic plagioclase, magnesian orthopyroxene and aluminous amphibole among the sparse phenocrysts suggest derivation of the rhyolite by melt extraction from a more mafic magmatic mush. High temperature and relatively low viscosity enabled rapid magma ascent and high effusion rates during the dome-forming phases of the 2008-2009 eruption.

Chaitén Volcano↗
USGS2013

Merapi 2010 eruption—Chronology and extrusion rates monitored with satellite radar and used in eruption forecasting

Despite dense cloud cover, satellite-borne commercial Synthetic Aperture Radar (SAR) enabled frequent monitoring of Merapi volcano's 2010 eruption. Near-real-time interpretation of images derived from the amplitude of the SAR signals and timely delivery of these interpretations to those responsible for warnings, allowed satellite remote sensing for the first time to play an equal role with in situ seismic, geodetic and gas monitoring in guiding life-saving decisions during a major volcanic crisis. Our remotely sensed data provide an observational chronology for the main phase of the 2010 eruption, which lasted 12 days (26 October–7 November, 2010). Unlike the prolonged low-rate and relatively low explosivity dome-forming and collapse eruptions of recent decades at Merapi, the eruption began with an explosive eruption that produced a new summit crater on 26 October and was accompanied by an ash column and pyroclastic flows that extended 8 km down the flanks. This initial explosive event was followed by smaller explosive eruptions on 29 October–1 November, then by a period of rapid dome growth on 1–4 November, which produced a summit lava dome with a volume of ~ 5 × 10 6 m 3 . A paroxysmal VEI 4 magmatic eruption (with ash column to 17 km altitude) destroyed this dome, greatly enlarged the new summit crater and produced extensive pyroclastic flows (to ~ 16 km radial distance in the Gendol drainage) and surges during the night of 4–5 November. The paroxysmal eruption was followed by a period of jetting of gas and tephra and by a second short period (12 h) of rapid dome growth on 6 November. The eruption ended with low-level ash and steam emissions that buried the 6 November dome with tephra and continued at low levels until seismicity decreased to background levels by about 23 November. Our near-real-time commercial SAR documented the explosive events on 26 October and 4–5 November and high rates of dome growth (> 25 m 3 s − 1 ). An event tree analysis for the previous 2006 Merapi eruption indicated that for lava dome extrusion rates > 1.2 m 3 s − 1 , the probability of a large (1872-scale) eruption was ~ 10%. Consequently, the order-of-magnitude greater rates in 2010, along with the explosive start of the eruption on 26 October, the large volume of lava accumulating at the summit by 4 November, and the rapid and large increases in seismic energy release, deformation and gas emissions were the basis for warnings of an unusually large eruption by the Indonesian Geological Agency's Center for Volcanology and Geologic Hazard Mitigation (CVGHM) and their Volcano Research and Technology Development Center (BPPTK) in Yogyakarta — warnings that saved thousands of lives.

Merapi Volcano↗
USGS2013

Bedrock geologic and joint trend map of the Pinardville quadrangle, Hillsborough County, New Hampshire

The bedrock geology of the Pinardville quadrangle includes the Massabesic Gneiss Complex, exposed in the core of a regional northeast-trending anticlinorium, and highly deformed metasedimentary rocks of the Rangeley Formation, exposed along the northwest limb of the anticlinorium. Both formations were subjected to high-grade metamorphism and partial melting: the Rangeley during the middle Paleozoic Acadian orogeny, and the Massabesic Gneiss Complex during both the Acadian and the late Paleozoic Alleghanian orogeny. Granitoids produced during these orogenies range in age from Devonian (Spaulding Tonalite) to Permian (granite at Damon Pond), each with associated pegmatite. In the latest Paleozoic the Massabesic Gneiss Complex was uplifted with respect to the Rangeley Formation along the ductile Powder Hill fault, which also had a left-lateral component. Uplift continued into the early Mesozoic, producing the 2-kilometer-wide Campbell Hill fault zone, which is marked by northwest-dipping normal faults and dilational map-scale quartz bodies. Rare, undeformed Jurassic diabase dikes cut all older lithologies and structures. A second map is a compilation of joint orientations measured at all outcrops in the quadrangle. There is a great diversity of strike trends, with northeast perhaps being the most predominant.

New Hampshire↗
USGS2013

Bedrock geology and outcrop fracture trends in the vicinity of the Savage Municipal Well Superfund site, Milford, New Hampshire

The Savage Municipal Well Superfund site consists of an eastward-directed plume of volatile organic compounds, principally tetrachloroethylene (PCE), in alluvium and glacial sand and gravel in the Souhegan River valley, just south of the river and about 4 kilometers west of the town of Milford, New Hampshire. Sampling of monitoring wells at the site has helped delineate the extent of the plume and has determined that some contaminant has migrated into the underlying crystalline bedrock, including bedrock north of the river within 200 meters of a nearby residential development that was constructed in 1999. Borehole geophysical logging has identified a northeast preferential trend for bedrock fractures, which may provide a pathway for the migration of contaminant under and north of the Souhegan River. The current study investigates the bedrock geologic setting for the site, including its position relative to known regional geologic structures, and compiles new strike and dip measurements of joints in exposed bedrock to determine if there are dominant trends in orientation similar to what was found in the boreholes. The site is located on the northwestern limb of a northeast-trending regional anticlinorium that is southeast of the Campbell Hill fault zone. The Campbell Hill fault zone defines the contact between granite and gneiss of the anticlinorium and granite and schist to the northwest and is locally marked by lenses of massive vein quartz, minor faults, and fracture zones that could potentially affect plume migration. The fault zone was apparently not intercepted by any of the boreholes that were drilled to delineate the contaminant plume and therefore passes to the north of the northernmost borehole in the vicinity of the new residential area. Joints measured in surface exposures indicate a strong preferred direction of strike to the north-northeast corroborating the borehole data and previous outcrop and geophysical studies. The north-northeast preferred direction matches the direction of elongation of the cone of depression formed during a pump test of the bedrock wells and could explain a potential pathway for the migration of contaminant north of the river.

New Hampshire↗