A3:CURRENT RESEARCH IN STRUCTURE, STRATIGRAPHY, AND HYDROGEOLOGY IN THE CHAMPLAIN VALLEY BELT OF WEST- CENTRAL VERMONT
Jonathan Kim
Vermont Geological Survey
Montpelier, VT 05620
Keith Klepeis
Dept. of Geology
University of Vermont
Burlington, VT 05405
Peter Ryan
Middlebury College
Middlebury, VT 05753
Edwin Romanowicz
Center for Earth and Environmental Science
SUNY at Plattsburgh
Plattsburgh, NY 12901
INTRODUCTION
Over the past four years, the Vermont Geological Survey and professors and undergraduate students from the University of Vermont, Middlebury College, and SUNY at Plattsburgh geology departments have formed a multidisciplinary fractured bedrock consortium. This consortium integrates varying expertise and resources to comprehensively address applied geologic issues in Vermont, such as groundwater quality (i.e. radionuclides, arsenic, nitrates, fluoride, and manganese), groundwater quantity of domestic and public wells, groundwater-surface water interaction, and shallow geothermal energy. The purpose of this trip is to visit field sites in the Champlain Valley Belt of west-central Vermont that illustrate our group’s current research efforts in fractured bedrock hydrogeology. At each site, we will discuss how structural geology, stratigraphy, and hydrogeology (including geophysical well logging) bear on a specific environmental issue. This trip will not only visit classic sites such as the Champlain Thrust at Lone Rock Point and the Hinesburg Thrust at Mechanicsville, where we will discuss refined structural chronologies, but also locations that exhibit a strike-slip fault zone in the Winooski River Spillway (Williston), a well-described wrench fault site in Shelburne, phosphorite layers that explain elevated radioactivity in the bedrock aquifer (Milton), and a site in Hinesburg where field mapping of fractures has been correlated with those in geophysical logs. The following Bedrock Geology of Vermont, Field Area Geology, Structural Geology, Metamorphism, and Geochronology sections are modified from Kim et al. (2011).

A3:CURRENT RESEARCH IN STRUCTURE, STRATIGRAPHY, AND HYDROGEOLOGY IN THE CHAMPLAIN VALLEY BELT OF WEST- CENTRAL VERMONT

Meeting Point - Colchester Park and Ride Lot

STOP 1. Elevated Radioactivity in Groundwater from the Clarendon Springs Formation, Milton, Vermont
Introduction Elevated radionuclide levels were reported in groundwater from ∼30% of bedrock wells tested in the Clarendon Springs Formation in the towns of Colcheser Quadrangle.
Lithology Massive gray dolostone of the Late Cambrian Clarendon Springs Formation.
Structure The Muddy Brook Thrust (MBT), which is described below, is located in the valley to the east. In this area, the dominant fracture set strikes NE, dips steeply, and is associated with cross faults that offset the MBT and lithologies on both sides (Kim and Thompson, 2001).
Tectonic/Stratigraphic Context This site is at the north end of the Hinesburg Synclinorium. The shallowly east- dipping MBT carried black phyllites with thin dolostone interlayers over the Clarendon Springs Formation dolostones during the Taconian Orogeny. The MBT is west of and probably synchronous with the Hinesburg Thrust (Kim and Thompson, 2001).
Hydrogeology and Groundwater Geochemistry Groundwater produced from the Clarendon Springs Formation in Milton and Colchester is known to contain elevated uranium, radium, and alpha radiation. Radioactivity is high enough that the average alpha radiation from 131 domestic bedrock wells tested in a 12 km2 area is 32 picocuries per liter (pCi/L), and 22 % of these wells produce concentrations of alpha radiation above the EPA’s maximum contaminant level (MCL) of 15 pCi/L; furthermore, the three most contaminated wells, which occur within a kilometer of each other, average 841 pCi/L. Dark grey to black phosphorites (with 7 to 37 % P2O5) occur throughout the region that contains elevated radionuclides in well water; in outcrop, these phosphorites occur in two main forms: (1) subangular, pebble-sized clasts in a dolostone matrix, sometimes with imbricated clasts that suggest deposition from a current, and (2) wispy beds of dark grey phosphorite (possibly “hardground”). Breccias are more common than wispy sedimentary layers, and both indicate that the concentrated phosphate is syndepositional in origin. McDonald (2012) cited a model involving upwelling sea water and reducing biochemical conditions as factors responsible for deposition of phosphorite with elevated U.
Rock samples collected from outcrops located upgradient of the most highly contaminated wells exhibited the following: (1) the phosphorites contain 80 to 430 mg/kg uranium while the dolostone matrix contains less than 10 mg/kg U; (2) the phosphorite mineral — as determined by powder XRD — is fluoroapatite, and trace amounts of autunite also occur in some phosphorites; (3) a gamma ray survey of a 160 m deep bedrock well documents the interbedding of U-rich phosphorite beds and dolostone beds throughout the Ccs in Milton-Colchester, perhaps alternating cyclically. U-rich phosphorites produce a gamma ray signal of 500 to 3400 cps whereas U-poor carbonates have a signal < 50 cps gamma radiation; and (4) SEM-EDS element maps (Bachman, 2015) show that phosphorite clasts and layers lacking significant post-depositional deformation contain the highest levels of uranium. SEM-EDS also indicates that U occurs in two broadly-defined mineralogical forms: (1) diffusely distributed U in cryptocrystalline fluoroapatite, and (2) perhaps more importantly, concentrated U in microcrystalline minerals (e.g. autunite, coffinite, brannerite) scattered throughout the phosphorite. This may suggest that uranium was initially substituted for Ca in fluoroapatite — during crystallization on the seafloor (consistent with literature reports)—, but then was incorporated into anhedral secondary minerals when (at least some of the) fluoroapatite dissolved. This could have occurred any time from very early diagenesis to dolomitization. Weathering of pyrite may trigger U release by locally lowering pH — evidence for this is the occurrence of autunite [Ca(UO2)2(PO4).nH2O] in rusty Fe hydroxide matrix at the boundary of weathered pyrite and adjacent, unweathered fluorapatite.

STOP 2. Champlain Thrust at Lone Rock Point
Introduction This stop description was modified from West et al. (2011). The Champlain thrust fault at Lone Rock Point is arguably the iconic geologic feature in Vermont and perhaps the finest thrust fault exposure in eastern North America. The “older-on-top-of-younger” relationship exposed here is a fundamental indicator of thrust faulting. Hitchcock et al. (1861) was the first to recognize that the contact relationships exposed at Lone Rock Point are the result of major regional faulting.
Lithology Early Cambrian massive dolostone of the Dunham Formation structurally overlies Late Ordovician Iberville Formation black shales (Figure 5).
Structure An interesting feature at Lone Rock Point is the interplay between brittle deformation and ductile flow mechanisms. Brittle deformation involves the breaking of material along discrete surfaces, which can be fractures, veins or, if they accommodate slip, faults. These two styles are not completely independent of one another, and commonly occur together to accommodate shortening. The material type and the conditions under which deformation occurs typically dictates the types of deformation processes. As such, the type of parent lithology (what type of rock was present prior to deformation) is a critical influence on style of deformation. At Lone Rock Point two significantly different rock types are juxtaposed with the strong Dunham dolostone thrust above the weak Iberville shale (Fig. 5A). Deformation is not restricted to slip along the fault plane, and can be dived into two domains as described by Stanley (1987). These are composed of an inner fault-zone including the fault surface at the base of the Dunham dolostone and a proximal region consisting of broken limestone and highly contorted shale. The outer fault-zone occurs in the Iberville Shale and consists of a high concentration of veins, subordinate faults, and tightly folded compositional layering. By recognizing various features within both the Iberville Shale and the Dunham dolostone we can establish various mechanisms of deformation and compare the way these two units accommodate shortening. For this reason, the exposure of the Champlain Thrust fault at Lone Rock point is an excellent location to teach fault-zone processes and the influence of material properties on deformation.
The following sections briefly describe key features that document deformation styles and the motion history of Lone Rock Point.
Structural Slickenlines. The basal surface of the Dunham Formation is the slip surface on which a significant amount of displacement has occurred. This surface contains corrugations referred to as fault mullions with wavelengths on the order of half a meter. These features, as depicted in Figure 8 can form with crest lines parallel to the motion of the fault and, therefore, can help constrain motion direction. In addition to fault mullions, striations can occur from the scraping and gouging of the fault surface by resistant objects. These also help constrain the motion of the Champlain Thrust fault (Figure 5B).
Original Bedding. Despite the highly deformed nature of the Iberville Formation, compositional layering is visible as resistant quartz rich layers. These layers are frequently folded and faulted into small isolated pods surrounded by soft clay rich rock. Although dolostones of the Dunham Formation are massively bedded, depositional surfaces are intact and relatively undisturbed (Figure 5A). Veins and Mineral Slickenlines. Veins form from the deposition of material from solutions that fill voids in rocks. The calcite veins at Lone Rock Point display complex geometries, including folding, faulting, and shearing. The shear veins, in particular, can be good indicators of the motion history of deformation and frequently form lineated surfaces known as mineral slickenlines. The formation of mineral slickenlines generally involves the infilling of void space with material created by offsets on the fault surface (Figure 5C).
Cleavage. Cleavage planes are formed from the preferential alignment of mineral grains due to flattening, which is accommodated by dissolution and the removal of soluble material. Cleavage is common in shales of the Iberville Formation, and are generally is oriented at low angles to the fault surface. However, as the distance increases away from the fault, the orientation of cleavage planes tends to steepen. This rotation of cleavage planes can be used to infer the sense of motion on the thrust surface.
Fractures. Unlike the intensely deformed Iberville Formation, massive dolostones of the Dunham Formation are relatively intact and show little evidence of internal deformation (i.e., no visible cleavage). However, fractures (breaking of the rock along discrete planes without differential motion), are common in this more competent unit. New fracture data will be presented for the hanging wall.
Subordinate Faults. Many small scale faults can be observed within the footwall rocks of the Iberville Formation that offset folds, cleavage, and veins. These faults do not continue into structurally overlying dolostones of the Dunham Formation. These small scale faults in the Iberville tend to rotate into the direction of the fault motion with proximity to the fault (Stanley, 1987). Because the Champlain thrust fault at Lone Rock Point contains numerous features that help constrain the transport direction along the fault plane, an exercise can be constructed to identify as many of the features that contain motion information (i.e. fault mullions, gouges, and mineral slickenlines). These features generally indicate displacement and tectonic transport in a west-north-west direction. Teaching considerations include the following questions: How does the geometry of folded compositional layering change with proximity to the fault? How does the orientation of the dominant cleavage change with proximity to the fault surface? Is displacement constrained to slip along the fault surface only? What is the general temporal progression of deformational style using cross-cutting relations? What fundamental influence does the type of bedrock have on deformation style? These questions address the fundamental aspects of thrust faulting and the inherent relationship between initial lithology and deformation mechanisms.
New Work on Foot Wall Structures
The following has been modified from Strathearn et al. (2015). Although the Champlain Thrust has been studied previously at Lone Rock Point, the multiple generations of ductile and brittle structure in shales of the footwall have never been systematically defined. We present the following relative chronology of structures:
1) Formation of bedding planes (S0), characterized by thin layers of carbonate within black shale.
2) Formation of rootless isoclinal folds (F1) of brittle carbonate layers and the development of an spaced pressure solution cleavage (S1) that parallels the axial planes of the folds.
3) The S1 cleavage is deformed into asymmetric S-C shear bands that merge into parallelism with, and are cut by intraformational thrusts. The thrusts form oblate, eye-shaped structures that are stacked on top of one another forming thrust duplexes. A second cleavage (S2) defines a part of the S-C fabric and is intensified in thrust zones. Calcite slickenlines on fault surfaces plunge to the SE and NW and slip directions fan up to 40 degrees with respect to one another in different thrust horses
4) Formation of sets of upright, north (F3) and east-striking (F4) folds of S2 warping the CT.
5) Formation of conjugate sets of normal faults that record top-down-to-the–north and -south kinematics.
6) Formation of the steeply-dipping fracture sets (N-S and E-W striking) that cut across competent lithologies.
Hydrogeology and Groundwater Geochemistry In a case study examined on this field trip (Hinesburg Thrust; Kim et al., 2014a), the hanging wall is sometimes responsible for producing groundwater with elevated radionuclides. In the case of the Champlain Thrust, the culprit is the footwall. Of 35 tests for fluoride (note: F can cause bone disease), 37 % (13/35) exceeded the Vermont recommended F level in public water systems (0.7 mg/L). [Relative to the EPA MCL of 4 mg/L, however, only 3/35 wells were above the F threshold]. Sodium is elevated in ∼ 45 % of footwall wells (13/29) relative to EPA's 20 mg/L Drinking Water Equivalency Level (guidance level), and the average Na concentration in footwall wells (82 mg/L) is four times greater than the DWEL. Both issues are related to the behavior of illite in this black shale-influenced bedrock aquifer system. Regarding fluoride, diagenetic illites typically contain 0.2 to 0.5 % F in isomorphous substitution for OH (Thomas et al., 1977), so dissolution of illite or exchange of Cl- or OH- for F- are potential F sources in groundwater.
Dissolution of apatite (4 % F) could also be a source, although it is far less abundant and likely less reactive than illite. Other potential F minerals (e.g. fluorite, titanite) are not likely F sources in this system. Regarding elevated Na, we observe a weak but positive correlation of Na and Ca in solution, an occurrence that likely relates to Na-Ca ion exchange. When Ca is released to solution upon weathering of calcite (nearly ubiquitous in these Ordovician black shales), the higher-charge, less-hydrated Ca+2 cation is more strongly attracted to cation exchange sites (e.g. on illite) than is Na+1, and the exchange reaction releases Na+1 into solution. This also is cited as the cause of high Na in black slate-influenced wells of the Taconics of southwestern Vermont (Ryan et al., 2013). Another element worth noting in footwall wells of the Champlain thrust is arsenic, which exceeds 10 ppm in 3 % of wells tested for As and exceeds 5 ppm in 10 % of wells (3/29); by comparison, in Taconic slates, 22 % of bedrock wells (52/236) exceed 10 ppb and 24 % exceed 5 ppb. Deeper anoxia in the Taconic seaway relative to open-shelf Iberville and Stony Point shale depositional environments is likely the cause of greater amounts of pyrite and arsenic in Taconic black slates.

STOP 3. Strike Slip Fault Zone in the Winooski River Gorge, Williston/Essex, Vermont
Introduction The Winooski River flows through a bedrock gorge downstream of a hydroelectric dam at the Essex/Williston border. During bedrock mapping in the Town of Williston (Kim et al., 2007), it was observed that most of the bedrock channels were northeast-striking fracture intensification domains (FIDs). Further investigation revealed a more complicated scenario.
Lithology Massive gray dolostone of the Late Cambrian Clarendon Springs Formation.
Structure Preliminary mapping and analysis of the brittle structures in the Winooski River Gorge reveals that the river channels are northeast- striking and steeply-dipping fracture intensification domains (FIDs) that are also strike-slip fault zones. Throughout the river gorge, these faults are intersected by steeply-dipping, north-northeast striking Riedel shear zones. Riedel shears usually form at a 10-20 degree angle to the main strike slip fault, in clockwise and counterclockwise directions, respectively, to right lateral and left lateral displacements (e.g Twiss and Moores, 2006). The Riedel shears in this gorge are consistent with left lateral displacement on the main strike-slip faults.
Tectonic/Stratigraphic Context This body of Clarendon Springs Formation is bounded to the east by the Ordovician Muddy Brook Thrust Fault and to the west by the presumed Mesozoic down-to-the-east St. George normal fault. Hydrogeology and Groundwater Geochemistry Bedrock geochemical analysis indicates some interesting differences in Ccs composition here compared to the phosphorite-bearing localities in Milton-Colchester and Highgate (McDonald, 2012). P2O5 here in the Winooski gorge is < 0.5 % whereas P2O5 ranges from to 7.4 to 36.9 % in phosphorite-rich dolostone and phosphorite layers or clasts. Uranium is also low in the Ccs in the Winooski River gorge (1.3 to 8.3 ppm) relative to Milton-Colchester (where it reaches 432 ppm). No phosphorite clasts or layers are observed in the Winooski River gorge, explaining the low amounts of P and U at this locality. Geochemically-notable are two samples of black wispy clasts coated with Fe hydroxide that contain 120 and 283 ppm As (for comparison, As < 16 ppm at Milton-Colchester outcrops). These black wispy clasts occur in a dolomitic breccia, and yet while this facies appears similar in outcrop to some of the phosphorite occurrences at Milton-Colchester, the black wispy clasts in Winooski gorge contain < 0.5 % P2O5 and < 10 ppm U. In terms of major elements, the Winooski gorge Ccs black clasts are dominantly Si, Mg and Ca; otherwise, only elevated iron (5.7 and 10.1 % Fe2O3, respectively) and arsenic (120 and 283 ppm, respectively) are anomalous relative to other dolomitic Ccs rocks. One possible interpretation: if the black color is from mature organic matter, these wispy black clasts may represent an organic-rich shallow marine or subaerial layer that was incorporated into a cave breccia. The As-bearing Fe hydroxide is likely remnants of weathered disseminated pyrite.
The municipalities in the vicinity of Winooski gorge are part of the Champlain Water District and their drinking water is supplied from Lake Champlain. We know of no chemical data for bedrock wells in this area, so it is unknown how the composition of this belt of Clarendon Springs Formation may affect groundwater.

STOP 4. Wrench Faults and Fractures at the Shelburne Boat Access, Shelburne, Vermont
Introduction This site was used by Rolfe Stanley of the University of Vermont as a teaching outcrop for his structural geology classes for decades and is currently used by Keith Klepeis. Stanley (1974) wrote a Geological Society of America Bulletin article that details the structural history of this site.
Lithology Early Cambrian ferruginous quartzite of the Monkton Formation. Stanley (1974) reported that the quartzite at this site is composed of 93% quartz, 5% hematite, 1% microcline, and traces of zircon, plagioclase, detrital staurolite, and dolomite.
Structure As outlined by Stanley (1974), there are three major structural features at this site, which are:
1) High – Angle Wrench Faults:
A) Generation 1 Northeast to west striking, moderately-steeply dipping faults (Figure 7).
B) Generation 2 North-northeast to north-northwest striking, steeply dipping faults that offset generation 1 faults (Figure 7).
2) En Echelon Fractures: Dextral and sinistral en echelon fracture arrays that are filled with quartz. The trend of the en chelon arrays corresponds to the strike of a particular fault. These arrays are only associated with the first generation of high angle faults (see fracture arrays on Figure 7).
3) Fractures: The four major fractures sets (contoured maxima) at this field site are:
A) East-west striking, steeply north-dipping,
B) North-south striking, steeply west-dipping,
C) Northwest striking, steeply southwest-dipping, and
D) Northeast striking, steeply northwest dipping.
Set A is associated with the first generation wrench faults whereas sets B, C, and D overprint set A and are associated with the second generation wrench fault generation.
Tectonic/Stratigraphic Context Stanley (1974) proposed that the east-west striking and steeply dipping wrench faults were related to the Shelburne Bay cross-fault, which offsets the Champlain Thrust. Both sets of wrench faults were presumed to be related to the Devonian Acadian Orogeny. Kim et al. (2011; 2014) described dome and basin fold patterns in the Champlain Valley Belt that were the result of the superposition of fold sets with steeply dipping ∼north-south and ∼east-west striking axial surfaces. Could the two sets of wrench faults and their associated fractures be related to these folding events?
Hydrogeology and Groundwater Geochemistry We are using this study as a detailed context for fracture analysis throughout the Champlain Valley Belt. These brittle structures may influence groundwater flow in the bedrock aquifer.

STOP 5. Mount Philo State Park in Charlotte, Vermont
Introduction This stop description, which was modified from Kim et al. (2011), is designed to give an overview of the four major tectonic zones in the Lake Champlain area of west-central Vermont and eastern New York, which are, from west to east, and from structurally lowest to highest: 1) Autochthon, 2) Parautochthon, 3) Hanging Wall of the Champlain Thrust, and 4) Hanging Wall of the Hinesburg Thrust. A major thrust fault separates the Autochthon from the Parautochthon on the west side of Lake Champlain.
Lithology At the top of Mt. Philo, you are standing on the upper member of the Monkton Formation, which is a ferruginous quartzite. The lower dolomitic sandstone member of the Monkton Formation is exposed near the Champlain Thrust below.
The autochthonous rocks on the west side of Lake Champlain include Mesoproterozoic metamorphic rocks of the Adirondacks (see Yu in Figure 3) that are uncomformably overlain by sedimentary rocks of the Beekmantown Group (Isachsen and Fisher, 1970). The Autochthon is structurally overlain by by: 1) Late Cambrian - Middle Ordovician weakly metamorphosed sedimentary rocks of the Parautochthon, 2) weakly metamorphosed sedimentary rocks of the Hanging Wall of the Champlain Thrust; and 3) low grade (chlorite-sericite to biotite) rift clastic metasedimentary rocks of the Hanging Wall of the Hinesburg Thrust.
Structure At this lookout, the Champlain Thrust is ∼160 vertical feet (50 m.) below your feet, where the dolomitic lower member of the Monkton Quartzite is in tectonic contact with the Middle Ordovician Stony Point Shale. From Mt. Philo, one can see the geomorphic expression of this thrust to the south as the slope breaks (steep = Monkton and gentler = Stony Point) on Buck and Snake mountains, and to the north on Pease Mountain.
Tectonic/Stratigraphic Context At the Lone Rock Point exposure of the Champlain Thrust in Burlington (e.g Stanley, 1987)(Stop 2), this thrust juxtaposes the Lower Cambrian Dunham Dolostone with the Iberville Shale; this suggests the presence of an along-strike ramp that climbs ∼2000’ (610 m.) up section between Burlington and Mt. Philo. The stratigraphic throw of the Champlain Thrust is ∼9000’ (2743 m.) at Lone Rock Point and ∼6000’ (1830 m.) at Mt. Philo (Stanley, 1987). Total displacement along the Champlain Thrust ranges from 34-62 miles (55-100 km.) (Stanley, 1987; Rowley, 1982).
Apatite fission track work by Roden-Tice (2000) indicates that the exhumation of the Adirondacks was complete by the Cretaceous. This uplift may have reactivated faults between the Champlain Valley and Adirondacks.
Hydrogeology and Groundwater Geochemistry Based on domestic well logs, average yields from the Monkton Formation in the Town of Charlotte are very favorable (Springston et al., 2010). Although we do not know of any bedrock groundwater wells that penetrate through the Monkton Formation into the underlying shales, Charles Welby (pers. Comm., 2009) said that numerous wells do. Such wells are often characterized by unpleasant amounts of hydrogen sulfide (rotten egg smell) gas. A Middlebury College student will be investigating the groundwater chemistry of the hanging and foot wall aquifers of the Champlain Thrust during 2015-2016.

STOP 6. Hinesburg Thrust at Mechanicsville, Vermont
Introduction Elevated radionuclide levels have been reported in groundwater from numerous bedrock wells completed in the Hanging Wall of the Hinesburg Thrust and drilled through this thrust (Kim et al., 2014a). This stop description was modified from Kim et al. (2011).
Lithology The Hinesburg Thrust at Mechanicsville places overturned light green and gray metapsammitic schists and quartzites of the Early Cambrian Cheshire Quartzite (argillaceous member) on top of a slice of deformed dolomites, limestone and marble of the Ordovician Bascom Formation (Thompson et. al., 2004). The hanging wall rocks display a lower greenschist facies metamorphic mineral assemblage that includes chlorite, quartz, sericite, and biotite. The footwall is dominated by a chloritic and graphitic carbonate mineral assemblage, indicating that the Hinesburg Thrust is marked by an abrupt change in metamorphic grade as well as a change in lithology.
Structure The thrust fault itself is defined by a narrow zone (<10 cm thick) of cataclastic dolomite at the top of the Bascom Formation (Figure 8). The dolomites below the fault are highly fractured and sheared. Ductile shear bands (C’ type) deform a penetrative phyllitic cleavage (S1) up to at least a meter beneath the fault. The shear bands are variable in orientation, but most dip gently to the northeast (298 31 NE). Quartz rods define a mineral stretching lineation on C’ shear planes. This lineation plunges gently to the east and southeast (093 02). The asymmetry of the shear bands yields a consistent top-to-the-northwest sense of displacement on the Hinesburg Thrust.
Above the Hinesburg Thrust at Stop 6 the metapsammite schists and quartzite layers of the Cheshire Fm are mylonitic (Strehle, 1985; Strehle and Stanley, 1986). The oldest structures preserved include a compositional layering (S0) defined by alternating bands of micaceous and quartz-rich laminae and >3 cm thick quartzite beds (Figure 8). This layering, which most likely represents sheared, stretched, and recrystallized bedding planes, is deformed into a series of tight-isoclinal, reclined-recumbent folds (F1). A fine grained mylonitic foliation (S1) defined by the alignment of graphite, mica, and recrystallized quartz parallels the axial planes of the F1 folds (Figure 8). On S1 surfaces, a penetrative mica and quartz mineral lineation (L1) plunges gently to the east and southeast. Locally, and especially within thin quartzite bands, the L1-S1 fabric is deformed by a series of shear bands (C’ type) similar to those in the foot wall rocks (339 13 NE). Both sets yield a similar top-to-the-northwest sense of shear parallel to the L1 mineral lineation (106 12). The S1 foliation also parallels the surface of the Hinesburg Thrust and is interpreted here to reflect early ductile thrusting at depth prior to the final emplacement of the Cheshire Fm onto the Bascom Fm along the semibrittle Hinesburg thrust fault.
The structural features observed in the mylonitic rocks above the Hinesburg Thrust display an elegant interplay between ductile deformation, in the form of folds and cleavages, and brittle deformation, in the form of veins. Throughout the outcrop the mylonitic S1 foliation locally is cross cut by a set of quartz veins (V1) that are tightly folded within the F1 folds, indicating that they formed during folding, probably as a result of pressure solution and fluid transfer processes. Cross cutting both the S1 cleavage and the V1 veins is a second set of asymmetric quartz tension gashes (V2) that localized within thick (>30 cm) quartzite layers (Figures 8, 9a). The tips of the asymmetric veins penetrate into the mylonitic schist surrounding the quartzite layers. A close inspection of the V2 veins (both on the outcrop and in thin section) indicates that they are sheared and protomylonitic (look for the milky white, recrystallized appearance and the presence of quartz ribbons). These characteristics contrast with a younger set of quartz tension gashes (V3) that cross cut the V2 set in the same quartzite layers (Figure 8). The V3 vein set is only weakly deformed, less recrystallized than the V2 veins, and are mostly symmetric to slightly asymmetric. A black, quartz-poor pressure solution selvage surrounds the veined quartzite layers (Figure 9a) strongly suggesting that the vein material was locally derived and that dissolution and fluid migration depleted these zones of silica during progressive deformation. These relationships indicate that crystal-plastic deformation alternated with brittle deformation as the superposed sets of tension gashes formed.
Both the V3 and V2 vein sets, as well as the F1 folds, S1 cleavage, and quartzite layers, are all deformed into a series of northwest-vergent asymmetric folds (F2) of variable tightness (Figure 18). The fanning of the V2 vein sets around fold hinges is a good indicator that they are folded (Figure 19c, 19d). The tightest folds are recumbent and tend to occur nearest the Hinesburg Thrust close to the base of the mylonitic section. Farther above the thrust, the F2 folds tend to be more open and upright to gently inclined. This increase in fold tightness and orientation suggests that the folds record an increase in finite strain downward toward the Hinesburg Thrust.
A spaced crenulation cleavage (S2) parallels the axial planes of the F2 folds and also displays variable dips (Figures 8, 9d). In addition to recording a strain gradient, the variability in axial plane and cleavage orientation with increasing fold tightness provides kinematic information. The rotation of fold axial planes and S2 to the northwest as fold tightness (and finite strain) increases, yields a top-to-the-northwest sense of shear identical to that indicated by the shear bands (Figure 9d). This relationship indicates that the F2 folds reflect progessive deformation during the same ductile thrusting event that produced the S1 mylonites and F1 folds.
The following model, which is based on sketches of features at Mechanicsville, explains the evolution of the veins and the F2 fold structures. See if you can find features on the outcrop that record each of these stages:
Stage 1 (Figure 9a).
En echelon arrays of quartz veins (V2) open perpendicular to the direction of maximum stretch (X) of the instantaneous strain ellipse (ISE) in quartzite layers.
Stage 2 (Figure 9b)
After the V2 veins finish forming, noncoaxial shear zones localized by the rheological contrast between the shale and the quartzite causes the parts of the vein tips that extend into shale to deform and rotate to the left. This process causes the veins to become asymmetric. A new set of veins (V3) open perpendicular to X-direction of instantaneous strain ellipse (ISE). A comparison of instantaneous and finite strain ellipses and the asymmetry of the two vein sets yield a top-to-the-NW sense of shear, identical to that recorded by shear bands in the mylonitic matrix. Note that this process differs than that which forms sigmoidal veins in brittle shear zones where the veins continue to open during shearing (Figure 10). In this latter model, a comparison of instantaneous and finite strain ellipses yields a top-to- the-SE (normal) sense of shear. This is because, in this latter case, the tips of V2 veins are younger than their centers and so the former record instantaneous strains (ISE, Figure 10) and the latter record finite strains (FSE, Figure 10). We ruled out this model because, given the top-to-the-NW sense of shear at Mechanicsville, it would produce V2 and V3 vein asymmetries opposite to those observed (Figure 19). In the Mechanicsville model, the vein is required to form quickly and finish opening before ductile shear begins, yielding an asymmetry similar to that of a shear band.
Stage 3 (Figure 9c)
As the rotation of the veins during noncoaxial shear continues, the F2 folds begin to form along with an axial planar crenulation cleavage (S2). The F2 axial planes and S2 cleavage initially form at 45° to the quartzite layers and then rotate to the northwest toward the shear plane (defined by S1). The V3 vein sets also begin to rotate toward the northwest as noncoaxial shear continues.
Stage 4 (Figure 9d)
As noncoaxial shear continues the F2 folds continue to rotate and tighten, recording a progressive increase in finite strain durign ductile thrusting. The S2 crenulation cleavage rotates to the northwest along with the folds. The V2 veins exhibit a characteristic fanning geometry around fold hinges, indicating that they also rotated during folding. The F1 and S1 structures are transposed parallel to S2. The rotation of F2 axial planes to the left with increasing fold tightness yields a top-to-the-northwest sense of shear, which is the same as that indicated by the asymmetric veins and shear bands.
The Hinesburg Thrust surface, and all other structures above and below it, are corrugated by two orthogonal sets of gentle, upright folds, forming a dome and basin pattern with a wavelength on the order of a few meters (Figure 10). This fold geometry mimics a kilometer scale dome and basin interference pattern formed by north-plunging (F3) and east and west plunging (F4) folds across the field trip area. These orthogonal fold sets are among the youngest ductile structures preserved at Stop 6. On the thrust surface itself two orthogonal crenulation lineations (L3, L4) mark the presence of the corrugation folds (Figure 11). A regional correlation of similar structures across the field area indicates that the orthogonal fold sets everywhere postdate thrust sheet emplacement and imbrication on the Champlain, Hinesburg and Iroquois thrusts. Earle et al. (2010) suggested that the two folds sets formed together as a result of a constrictional style of deformation during the Acadian orogeny, possible reflecting the reactivation of inherited basement faults or lateral thrust ramps. However, it is also possible that the fold sets formed in sequence as separate events.
Hydrogeology and Groundwater Geochemistry Elevated levels of alpha radiation (> 15 pCi/L as gross alpha) were observed in 38 % (12/31) of bedrock wells drilled into the hanging wall, including wells that penetrated the thrust into carbonates below; for comparison, no wells [0/21] in the carbonate-dominated footwall west of the thrust front exceeded 15 pCi/L (the EPA MCL). The source of the elevated radioactivity was evaluated by testing groundwater from hanging wall and footwall bedrock wells and by analyzing compositions of bedrock from local outcrops. The chemical composition of groundwater in hanging wall and footwall aquifers is mainly controlled by whole-rock geochemistry. Hanging wall groundwater is enriched in alpha radiation, K, Cl, Ba, Sr and U, whereas footwall groundwater is enriched in Ca, Mg, and HCO3. These signatures reflect the distinct compositions of phyllite-dominated bedrock in the hanging wall compared to Ca-Mg-CO3-rich limestones and dolostones of the footwall. Elevated alpha radiation and U in wells that produce from footwall carbonates below the thrust recorded compositions that are intermediate to hanging wall and footwall end-members, indicating that alpha and U are transported in groundwater downward through the thrust via fractures into the footwall below (Kim et al., 2014a).

STOP 7. Multiple Structural Generations and Hydrogeology at Geprags Park, Hinesburg, Vermont
Introduction The Town of Hinesburg drilled three bedrock wells in Geprags Park in 1996. Although these wells were completed as future public water supplies, surface water contamination in one of these wells precluded them from ever being used. The Vermont Geological Survey and SUNY at Plattsburgh conducted comprehensive geophysical logging of these wells during 2012-2014 using temperature, conductivity, gamma, caliper, and accoustic televiewer tools. During 2014, Hinesburg drilled two new wells at a nearby site, which is also located on Shelburne Falls Road, but closer to the intersection with Route 116. These wells were drilled to increase water production for the town. One of these new wells (#48477) was logged with the previously mentioned tools. One well in Geprags Park (#7797) and another on the Wainer property were logged using a heat pulse flowmeter. Structures and lithologies were also mapped in the field to compare with those in the wells.
Lithology In order of decreasing age, Early Ordovician Shelburne Formation (marble and dolostone), Late Cambrian Clarendon Springs Formation (dolostone) and Danby Formation (dolomitic sandstone), and Middle Cambrian Winooski Formation (dolostone).
Structure All lithologies are folded by the north-plunging Hinesburg Synclinorium, which is thought to have formed during the Ordovician Taconian Orogeny. In the western half of Geprags Park, this synclinorium is truncated by the north striking and steeply east-dipping St. George Fault, a down to the east normal fault of presumed Cretaceous age (Figure 2). The dominant fracture set in this area is axial planar to parasitic folds in the Hinesburg Synclinorium (Figure 12). Isolated east-west trending fracture zones also occur (Figure 13). The integrated geophysical logs for Geprags Park well #7797 showed that this well was completed through the St. George Fault.
Tectonic/Stratigraphic Context This site sits in a similar tectonic context to Stop 3, in the hanging wall of the Champlain Thrust just east of the St. George Fault.
Hydrogeology and Groundwater Geochemistry Using temperature, conductivity, caliper, and acoustic televiewer logs for the four wells, we were able to delineate water producing zones. A summary cross section for Geprags Park only was drawn by Kim et al. (2014b) In addition, the attitude of structures intersecting each well were calculated from the acoustic televiewer logs. Using structural data from bedding and fractures acquired in the field as a template, we were able to accurately categorize the borehole structures. Finally, we were able to determine which structures were producing groundwater in each well (Kim et al, (2015). All of our data has been presented to the Consulting Hydrogeologist for the Town of Hinesburg.