Vermont Geological Survey

Montpelier, VT, United States

Vermont Geological Survey

Montpelier, VT, United States
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Becker L.R.,Vermont Geological Survey | Patriarco S.P.,Northeast States Emergency Consortium | Marvinney R.G.,Maine Geological Survey | Thomas M.A.,Connecticut Geological Survey | And 2 more authors.
Special Paper of the Geological Society of America | Year: 2013

In New England, earthquakes pose a risk to the built environment. Emergency preparedness and mitigation planning are prudent in this region as older unreinforced masonry buildings and numerous critical facilities are common. New England state geological surveys cooperate with the Northeast States Emergency Consortium (NESEC) to improve risk communication with emergency managers. To that end, Connecticut, Maine, Massachusetts, and Vermont employed surficial geologic maps, deglaciation history, knowledge of the glacial stratigraphy, and professional judgment to reclassify surficial geologic material units into one of the five National Earthquake Hazards Reduction Program (NEHRP) site classifications (A, B, C, D, and E). These new classifications were used as a substitute for the HAZards U.S. Multi- Hazard (HAZUS-MH) site class value of "D," which is used throughout New England as a default value. In addition, coding of surficial geologic materials for the five NEHRP site classifications was compared with classifications using the Wald methodology, a method that uses a slope analysis as a proxy for shear-wave velocity estimates. Comparisons show that coding to site classes using the Wald methodology underestimates categories A (high-velocity shear-wave materials, least relative hazard) and E (lowest-velocity shear-wave materials, greatest relative hazard) when evaluated side by side with coding done with the aid of surficial geologic maps. North of the glacial limit, derangement of drainage resulted in extensive ponding of meltwaters and the subsequent deposition of thick sequences of lacustrine mud. Inundation by the sea immediately following deglaciation in New England resulted in the deposition of spatially extensive and locally thick sequences of glacial marine mud. Surficial geologic maps better capture this circumstance when compared with the Wald topographic slope analysis. Without the use of surficial geologic maps, significant areas of New England will be incorrectly classified as being more stable than the site conditions that actually exist. By employing surficial geologic information, we project an improved accuracy for HAZUS-MH earthquake loss estimations, providing local and regional emergency managers with more accurate information for locating and prioritizing earthquake planning, preparedness, and mitigation projects to reduce future losses. © 2012 The Geological Society of America. All rights reserved.

Coish R.,Middlebury College | Kim J.,Vermont Geological Survey | Morris N.,Middlebury College | Johnson D.,Middlebury College
Canadian Journal of Earth Sciences | Year: 2012

Metamorphosed mafic rocks from west-central Vermont crop out in tectonic slices of the Stowe Formation within the Rowe-Hawley Belt of New England. The rocks include greenstone and amphibolite, which are interpreted to have been basaltic flows and gabbroic intrusions, respectively. Even though the rocks have been metamorphosed to greenschist or amphibolite facies, their igneous origins can be deciphered through careful use of geochemistry. Three geochemical types have been identified. Type 1 and 2 samples have geochemical characteristics similar to those found in mid-ocean ridge basalts (MORB), except that they have slightly elevated light rare-earth element (LREE) concentrations and are higher in Nb/Y ratios. Their Nb/Y ratios are similar to basalts found in Iceland and parts of the Afar region of the East African Rift. Types 1 and 2 are similar to metabasalts of the Caldwell and Maquereau formations in southern Quebec. The less-common type 3 samples have highly enriched LREE and are high in Nb/Y and Zr/Y ratios, similar to some alkali basalts from Afar and Iceland. Detailed analysis of the geochemistry suggests that greenstones and amphibolite from the Stowe Formation formed as basaltic eruptions during very late stages in rifting of the Rodinian continent that eventually led to formation of the Iapetus Ocean. This interpretation is consistent with tectonic models of the Vermont and Quebec Appalachians.

Ryan P.C.,Middlebury College | Kim J.,Vermont Geological Survey | Wall A.J.,Pennsylvania State University | Moen J.C.,Middlebury College | And 4 more authors.
Applied Geochemistry | Year: 2011

In the fractured bedrock aquifer of northern Vermont, USA, As concentrations in groundwater range from <1 to 327μg/L (<13-4360nm/L) and these elevated occurrences have a general spatial association with ultramafic rock bodies. The ultramafic rocks in this region are comprised mainly of serpentinites and talc-magnesite rocks with average As concentration of 93ppm and a range from 1 to 1105ppm. By comparison, the other main lithologies in the study area are depleted in As relative to the ultramafics: the average As concentration in metabasaltic rocks is 4.1ppm with a range of <1-69ppm, and mean As concentration in meta-sedimentary phyllites and schists is 22ppm with a range of <1-190ppm. In the ultramafic rocks, As is correlated with Sb and light rare earth elements, indicating that As was introduced to the ultramafic rocks during metasomatism by fluids derived from the subducting slab. Evidence from sequential chemical extraction, X-ray diffraction (XRD) and stoichiometric analysis indicates that the majority of the As is located in antigorite and magnesite (MgCO3) with lesser amounts in magnetite (Fe3O4). Hydrochemistry of monitoring wells drilled into fractured ultramafic rock in a groundwater recharge area with no anthropogenic As source reveals above background As (2-9μg/L) and an Mg-HCO3 hydrochemical signature that reflects dissolution of antigorite and magnesite, confirming that As in groundwater can be derived from ultramafic rock dissolution. Arsenic mobility in groundwater affected by ultramafic rock dissolution may be enhanced by alkaline pH values and relatively high HCO3- concentrations. © 2011 Elsevier Ltd.

Coish R.,Middlebury College | Kim J.,Vermont Geological Survey | Twelker E.,Middlebury College | Zolkos S.,Middlebury College | Walsh G.,U.S. Geological Survey
American Journal of Science | Year: 2015

The Moretown Formation, exposed as a north-trending unit that extends from northern Vermont to Connecticut, is located along a critical Appalachian litho-tectonic zone between the paleomargin of Laurentia and accreted oceanic terranes. Remnants of magmatic activity, in part preserved as metamorphosed mafic rocks in the Moretown Formation and the overlying Cram Hill Formation, are a key to further understanding the tectonic history of the northern Appalachians. Field relationships suggest that the metamorphosed mafic rocks might have formed during and after Taconian deformation, which occurred at ca. 470 to 460 Ma. Geochemistry indicates that the sampled metamorphosed mafic rocks were mostly basalts or basaltic andesites. The rocks have moderate TiO2 contents (1-2.5 wt %), are slightly enriched in the light-rare earth elements relative to the heavy rare earths, and have negative Nb-Ta anomalies in MORB-normalized extended rare earth element diagrams. Their chemistry is similar to compositions of basalts from western Pacific extensional basins near volcanic arcs. The metamorphosed mafic rocks of this study are similar in chemistry to both the pre-Silurian Mount Norris Intrusive Suite of northern Vermont, and also to some of Late Silurian rocks within the Lake Memphremagog Intrusive Suite, particularly the Comerford Intrusive Complex of Vermont and New Hampshire. Both suites may be represented among the samples of this study. The geochemistry of all samples indicates that parental magmas were generated in supra-subduction extensional environments during lithospheric delamination.

Kim J.J.,Vermont Geological Survey | Comstock J.,Vermont Agency of Agriculture | Ryan P.,Middlebury College | Heindel C.,Waite Heindel Environmental Management | Koenigsberger S.,Middlebury College
Science of the Total Environment | Year: 2016

In 2000, elevated nitrate concentrations ranging from 12 to 34 mg/L NO3[Formula presented] were discovered in groundwater from numerous domestic bedrock wells adjacent to a large dairy farm in central Vermont. Long-term plots and contours of nitrate vs. time for bedrock wells showed “little/no”, “moderate”, and “large” change patterns that were spatially separable. The metasedimentary bedrock aquifer is strongly anisotropic and groundwater flow is controlled by fractures, bedding/foliation, and basins and ridges in the bedrock surface. Integration of the nitrate concentration vs. time data and the physical and chemical aquifer characterization suggest two nitrate sources: a point source emanating from a waste ravine and a non-point source that encompasses the surrounding fields. Once removed, the point source of NO3 (manure deposited in a ravine) was exhausted and NO3 dropped from 34 mg/L to < 10 mg/L after ~ 10 years; however, persistence of NO3 in the 3 to 8 mg/L range (background) reflects the long term flux of nitrates from nutrients applied to the farm fields surrounding the ravine over the years predating and including this study. Inferred groundwater flow rates from the waste ravine to either moderate change wells in basin 2 or to the shallow bedrock zone beneath the large change wells are 0.05 m/day, well within published bedrock aquifer flow rates. Enrichment of 15N and 18O in nitrate is consistent with lithotrophic denitrification of NO3 in the presence of dissolved Mn and Fe. Once the ravine point-source was removed, denitrification and dilution collectively were responsible for the down-gradient decrease of nitrate in this bedrock aquifer. Denitrification was most influential when NO3[Formula presented] was > 10 mg/L. Our multidisciplinary methods of aquifer characterization are applicable to groundwater contamination in any complexly-deformed and metamorphosed bedrock aquifer. © 2016 Elsevier B.V.

Ryan P.C.,Middlebury College | Kim J.J.,Vermont Geological Survey | Mango H.,Castleton State College | Hattori K.,University of Ottawa | Thompson A.,Middlebury College
Applied Geochemistry | Year: 2013

Elevated As levels have been reported by the Vermont Geological Survey in groundwater from public and domestic bedrock wells in northwestern New England (USA). The study area in southwestern Vermont is underlain by pyrite-rich, organic-rich slates that were thrusted over carbonate and clastic sedimentary rocks of the continental shelf during the Ordovician Taconian Orogeny, and the distribution of wells with elevated As shows that they were completed in slates. Hydrochemical and bedrock geochemical analysis indicates that elevated As in the aquifer system is controlled by the following: (1) the presence of black slates that are rich in arsenian pyrite (200-2000. ppm As); (2) release of As via the dissolution of As-rich pyrite; (3) geochemically-reducing and slightly alkaline conditions, where high As values occur at Eh. <. 200. mV and pH. >. 7; and (4) physical hydrogeological parameters that foster low Eh and high pH, particularly long groundwater flow paths and low well yields (i.e. high residence time) which provides high rock to water ratios. Where all four factors affect As contents in groundwater, 72% of wells in a zone of distal groundwater flow/low-relief topography exceed 10. μg/L (ppb) and 60% of wells in this zone exceed 25. ppb As. Where flow paths are shorter in slates and groundwater has higher Eh and lower pH (i.e. in regions of higher-relief topography closer to recharge zones), only 3% of wells contain >10. ppb As and none contain >25. ppb.Overall, 28% (50/176) of low-elevation wells (<245meters above sea level [masl]) exceed 10ppb As; only 3% (2/60) of higher-elevation wells (245-600masl) exceed 10ppb As. Over the entire aquifer system, 22% of bedrock wells (52/236) exceed 10ppb and the mean As concentration is 12.4ppb. Strong positive correlations among Fe, SO4 and As in groundwater confirm that dissolution of pyrite is the dominant As source. Positive correlations among SO4, Na and As indicate that, in reducing (Eh<200mV) groundwater, Fe(II) is exchanged for Na on mineral surfaces following pyrite dissolution and As remains in solution; conversely, in oxidizing groundwater (recharge zones), Fe(II) is oxidized to Fe(III) and the subsequent formation of ferrihydrite removes As (V) from solution. © 2013 Elsevier Ltd.

Kim J.,Vermont Geological Survey | Ryan P.,Middlebury College | Klepeis K.,University of Vermont | Gleeson T.,McGill University | And 4 more authors.
Geofluids | Year: 2014

In polyorogenic regions, the superposition of structures during a protracted tectonic history produces complex fractured bedrock aquifers. Thrust-faulted regions, in particular, have complicated permeability patterns that affect groundwater flow paths, quantity, and quality. In the Appalachian foreland of northwestern Vermont, numerous bedrock wells that are spatially related to the Paleozoic Hinesburg thrust have elevated naturally occurring radioactivity and/or low yields. The association of groundwater quality and quantity issues with this thrust was a unique opportunity to investigate its structural and hydrogeologic framework. The Hinesburg thrust juxtaposed metamorphic rocks of the hanging wall with sedimentary rocks of the footwall during the Ordovician. It was then deformed by two orthogonal Devonian fold sets and was fractured during the Cretaceous. Median well yields in the hanging wall aquifer are significantly lower than those of the footwall aquifer, consistent with the respective permeability contrast between metamorphic and carbonate rocks. For wells drilled through the Hinesburg thrust, those completed closest (vertically) to the thrust have the highest median yields, whereas others completed farther below have yields in the footwall range. The geochemical signature of the hanging wall and footwall aquifers correlates with their whole-rock geochemistry. The hanging wall aquifer is enriched in alpha radiation, Na+K-Cl, Ba, and Sr, whereas the footwall aquifer is enriched in Ca-Mg-HCO3 and alkalinity. Wells that penetrated the Hinesburg thrust generally have hanging wall geochemical signatures. A simple hydrogeologic model for the permeability evolution of the Hinesburg thrust involves the ductile emplacement of a low-K hanging wall onto a high-K footwall, with subsequent modification by fractures. The Hinesburg thrust juxtaposed hanging wall metamorphic rocks with footwall sedimentary rocks along a ductile fault zone during the Ordovician and was subsequently deformed by three additional tectonic events from the Devonian to the Cretaceous. Aquifers from the hanging wall and footwall have separable groundwater geochemical signatures and low and high well yields, respectively; however, wells drilled through the thrust have groundwater with hanging-wall-dominant or weakly mixed geochemical affinities. A hydrogeologic model involving a low-K hanging wall overlying a high-K footwall, which was modified by later folds and fractures, explains these observations. © 2014 John Wiley & Sons Ltd.

Castonguay S.,Geological Survey of Canada | Kim J.,Vermont Geological Survey | Thompson P.J.,University of New Hampshire | Gale M.H.,Vermont Geological Survey | And 3 more authors.
Bulletin of the Geological Society of America | Year: 2012

In the pre-Silurian lithotectonic units of the northern Vermont Appalachians, the timing of orogenesis and tectonometamorphism has traditionally been ascribed to the combined effects of the Middle Ordovician Taconian orogeny and Middle to Late Devonian Acadian orogeny. However, numerous geo chronological studies throughout the Northern Appalachians, including neighboring southern Quebec, have obtained Silurian and Early Devonian age data that document more or less continuous tectonometamorphic activity throughout the Ordovician-Devonian. The structural and metamorphic evolution of northern Vermont can be separated into three regional phases, which are characterized by distinct structures, fabrics, and metamorphic parageneses. The first phase (D1), associated with westward emplacement of various thrust slices leading to crustal thickening and regional metamorphism, and the second phase (D2), characterized by bivergent structures and metamorphic overprint, have both been considered to be Taconian. The third phase, the structure and fabric of which are also observed in the Silurian- Devonian rocks to the east, is considered to be Acadian. We present new step-heating and spot fusion 40Ar/39Ar geochronological data on amphibole and fabric-forming muscovite from samples taken across the Green Mountain anticlinorium, which, coupled with published data, provide improved age constraints on tectonometamorphism of D1 (latest Cambrian to Middle Ordovician), D2 (Silurian-Early Devonian), and D3 (Middle Devonian) events. By comparing structural and metamorphic characteristics, and now timing, these phases are interpreted to be correlative to the tripartite tectonometamorphic evolution documented in southern Quebec, and they further exemplify the along-strike diachronism of tectonism induced by the inherited irregular geometry of the Laurentian margin. © 2012 Geological Society of America.

Dorais M.J.,Brigham Young University | Atkinson M.,Clemson University | Kim J.,Vermont Geological Survey | West D.P.,Middlebury College | Kirby G.A.,United Road Services
Canadian Journal of Earth Sciences | Year: 2012

The ~470 Ma Ammonoosuc Volcanics of the Bronson Hill terrane of New Hampshire have back-arc basin basalt compositions. Major and trace element compositions compare favorably to coeval volcanic rocks in the Miramichi Highlands of New Brunswick and the Munsangan and Casco Bay volcanics of Maine, back-arc basin basalts of known peri-Gondwanan origins. Additionally, the Ammonoosuc Volcanics have Nd and Pb isotopic compositions indicative of peri-Gondwanan provenance. Thus, the Ammonoosuc Volcanics correlate with Middle Ordovician, peri-Gondwanan, Tetagouche- Exploits back-arc rocks of eastern New England and Maritime Canada. This correlation indicates that the Red Indian Line, the principle Iapetus suture, lies along the western margin of the Bronson Hill terrane. However, the younger (~450 Ma) Oliverian Plutonic Suite rocks that intruded the Ammonoosuc Volcanics, forming domes along the core of the Bronson Hill anticlinorium, have Laurentian isotopic signatures. This suggests that the Ammonoosuc Volcanics were thrust westwardly over the Laurentian margin, and that Laurentian basement rocks are present under the Bronson Hill terrane. A plausible explanation for these relationships is that an easterly dipping subduction zone formed the Ammonoosuc Volcanics in the Tetagoughe-Exploits oceanic tract, just east of the coeval Popelogan arc. With the closure of the Iapetus Ocean, this terrane was thrust over the Laurentian margin. Subsequent to obduction of the Ammonoosuc Volcanics, subduction polarity flipped to the west, with the Oliverian arc resulting from a westerly dipping subduction zone that formed under the Taconic Orogeny- modified Laurentian margin.

Ryan P.C.,Middlebury College | West D.P.,Middlebury College | Hattori K.,University of Ottawa | Studwell S.,Middlebury College | And 2 more authors.
Science of the Total Environment | Year: 2015

Elevated As occurs in many meta-sedimentary bedrock aquifers where elevated bulk-rock As content is one of the primary controls on the concentration of As in groundwater. This study was designed to determine As concentrations in a black shale, black slate and black phyllite sequence that comprises the bedrock aquifer system of the Taconic Mountain region of southwestern Vermont and adjacent New York State. Variability in groundwater As concentrations provides the impetus for this study: 25% of wells in weakly metamorphosed shales and slates (

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