A3: STRUCTURAL AND STRATIGRAPHIC FEATURES OF THE TACONIC FORELAND, NW VERMONT
Adam Schoonmaker
Department of Geosciences
Utica College
Utica, NY 13502
adschoon@utica.edu
William S.F. Kidd
formerly of Geological Sciences
University at Albany
Albany, NY 12222
wkidd@albany.edu
INTRODUCTION
This trip will focus on some of the spectacular structural and stratigraphic features exposed in NW Vermont at the Highgate Gorge, and at Lessor’s Quarry, and the outcrop known as “The Beam”, on South Hero Island. The last two are Rolfe Stanley Memorial outcrops. The trip will cover the nearly continuously exposed upper Cambrian to Ordovician section in the Highgate Gorge. This section exposes the shelf to basin transition including bedded limestone and massive dolomite, a spectacular shelf slope limestone breccia, as well as basinal slates containing likely turbidite-transported carbonate shelf beds. The out-of-sequence cataclastic Highgate Falls Thrust is well-exposed, as well as a number of other Taconic-aged structures including folds, axial plane cleavage, and rotated en echelon fractures. The “Beam” is a superb meso-scale illustration of duplex faulting in thin-skinned foreland belts, while Lessors Quarry exposes fault-bend fold structures as well as a number of fault zone features, and an excellent example of an out-of-sequence thrust. This trip is highly recommended for students to see a number of well-exposed and instructive stratigraphic and structural features. Part of this material has appeared in a previous NEIGC field trip (Schoonmaker and Kidd, 2009).
Walking in the gorge can be treacherous and difficult both on the boulder field in the channel and the outcrop; both may be very slippery if wet. Care should be observed, and adequate footwear is advised.

A3: STRUCTURAL AND STRATIGRAPHIC FEATURES OF THE TACONIC FORELAND, NW VERMONT

Assembly Point - Price Chopper in Warrensburg

STOP 1. Upper Highgate Falls Gorge.
Continue straight and immediately bear right onto Mill Hill Road. Park at bottom of hill at terminus of pedestrian bridge and hydroelectric dam. A nice view of the gorge can be seen from the pedestrian bridge including the Highgate Falls Thrust. Proceed by foot along canoe portage westward through the woods along the north side of the gorge for about 50 feet. Find trail on the left that leads down to the gorge floor below the dam.
CAUTION!
ACCESS TO GORGE FLOOR WILL REQUIRE SCRAMBLING DOWN STEEP ROCKS AND ONCE ON THE GORGE FLOOR, WALKING ON THE BOULDER FIELD CAN BE TREACHEROUS, ESPECIALLY WHEN WET.
PLEASE ALSO NOTE THAT IF HORN SOUNDS, EXIT GORGE AS QUICKLY AS POSSIBLE IN THE EVENT OF AN UNLIKELY, BUT POSSIBLE DAM RELEASE.
The Mississquoi River laps against the walls in part of the gorge, so it is cut into upper (Stop 1) and lower (Stop 2) sections with different access points.
The entire (both upper and lower) east-west oriented Highgate Falls Gorge on the Mississquoi River continuously exposes a cross-section (Figure 3) of upper Cambrian to lower Ordovician rocks that include the upper section of the Gorge Formation (dolomitic arenites and quartz sandstones), the entire Highgate Formation (a variable unit containing bedded micrite, limestone clast breccias, calcareous slate, and argillaceous micrite), and the lower part of the Morses Line Formation (argillaceous slates with dolomitized, boudinaged micrite beds, bedded micrite, and limestone breccias;). The Highgate Falls Thrust repeats part of this section (Gorge Fm.) in the part of the gorge nearest the dam. In the upper gorge we can see the upper part of the Highgate Formation, lower section of the Morses Line Formation, and part of the Gorge Formation above the Highgate Falls Thrust.
Once in the gorge, the Highgate Falls Thrust is clearly visible in the north wall, approximately 200’ downstream from the point where the path descends (Figure 6E). A dolomite clast breccia with dolomitic matrix of the Gorge Formation is thrust over dark slates and a bedded argillaceous micrite and carbonate breccia interval within the Morse Line slates along a shallowly east-dipping surface characterized by phyllonitic fault zone cleavage (cataclastic breccia- to gouge-sized material); beds and cleavage in the lower block are bent in the direction of transport of the upper block. In the lower block, directly beneath the thrust, regional cleavage in the slates and bedded micrite can be seen to be bent as it approaches the fault zone indicating a westward sense of motion of the upper block. Because the regional cleavage is deformed by the motion of the upper block, the Highgate Falls Thrust is inferred to be an out-of-sequence thrust fault.
To the west and slightly downstream from the highway bridge that can be seen high above the gorge and upstream from the powerhouse and tailrace visible on the south bank, the contact between calcareous slates of the uppermost Highgate Formation are overlain by black slates of the Morses Line Formation (Loc. A, Figure 5). The contact on the north bank is mesoscopically folded and marked by a minor thrust fault containing a slickenlined and stepped calcite vein on the north limb (Figure 6C). On the south bank, the south limb is an observable depositional contact. Landing (1983) interpreted the micrite clast breccia (unit 29, Figure 5) that occurs beneath the calcareous slate as a tectonic breccia and the base of the black slates as an unnamed thrust. In contrast, Haschke (1994) suggested the slickensided surface on the north limb was a detachment that cut out a major thrust between the Highgate and Morses Line Formations, similar to relationships described by Hayman and Kidd (2002) for the northern part of the Taconic Allochthon. The observable conformable contact relationships on the south bank in the Highgate Falls Gorge do not support either of these hypotheses. Further, minor thrust faults are observed in the Morse Line slates (Figure 6D) and cleavage can also be seen in the Highgate Formation limestones, suggesting that a significant structural boundary is not present within this section, which contrasts to the Quebec and Taconic allochthons where deformation and cleavage deformation occurred prior to final emplacement (Rowley and Kidd, 1981; St. Julien, 1977; Lebel and Kirkwood, 1998).
Within the Morses Line Formation, dismembered dolomitized micrite beds can be observed, wrapped by the regional cleavage. Some of these beds are cut by small pre-cleavage faults with a normal sense of displacement. These normal faults are interpreted to be from passive continental margin downslope slumping. Similar dolomite beds in slate are observed further north in Vermont and in Quebec, well east of the strike of these beds, where they form part of the upper block of the Highgate Falls thrust and are mapped as Stanbridge Slates (Figure 3).
Structurally just below the Highgate Falls Thrust, there is a small fold of bedding in slate and thin limestone beds whose axial plane is parallel to the regional cleavage in the slates; this can be seen in the outcrop close to the river channel.

STOP 2. Lower Highgate Falls Gorge.
The lower gorge displays the contact between the Gorge and Highgate Formations, slightly below the Cambrian-Ordovician boundary (Figures 3 and 4). Here, the depositional contact the Gorge Formation and the overlying Highgate Formation can be observed (Figure 3, Loc. D). In contrast, Landing (1983) placed the contact at Loc. B (Figure 3). Very similar argillaceous limestones and limestone breccias are present both below and above this level. We prefer to define the contact at the lower horizon (Figure 3, Loc. D; Figure 6B), which separates dolomitic sandstones below (Gorge Formation) from the argillaceous limestones and limestone breccias above (Highgate Formation), as a much more readily identifiable contact in the field and likely representing a significant change in depositional environment from sand-dominated carbonate deposition to shale-dominated carbonate deposition. Regardless of the placement of the Gorge- Highgate contact (Figure 3. Loc. D vs. Loc. B) within the Highgate Falls Gorge, no major structural boundary is observed in this section, up to the position of the out-of sequence Highgate Falls Thrust.
The contact is readily identifiable in the gorge by the abrupt change from thick-bedded dolomitic arenites of the Gorge Formation below, to thinner bedded argillaceous micrites and limestone breccias of the Highgate Formation above (Figure 6b). In Quebec, this contact is inferred to be a major fault boundary between the autochthonous shelf platform rocks (Milton Dolomite) and allochthonous, deep marine rocks (Stanbridge Complex).

STOP 3. “The Beam”, a Rolfe Stanley Memorial Outcrop.
NO HAMMERS, PLEASE. “The Beam” is one of two Rolfe Stanley Memorial Outcrops visited on this trip. Over the years, many thousands of introductory geology students, as well as undergraduate and graduate students in Rolfe’s structural geology courses have visited these sites. Both beautifully expose important features of foreland deformation, including thrust faulting, fault duplexes, fault bend folds, ramps and ramp faults, en echelon fractures and rotation, and pressure solution cleavage. Detailed descriptions and interpretation for both of these outcrops can be found in Stanley (1988).
“The Beam” occurs in the Cumberland Head Formation, and is a ∼1 foot thick micrite bed sandwiched between calcareous shale, and is bounded by bedding parallel thrust faults that display slickensided and stepped calcite veins, and fault zone cleavage (Figure 7). The micrite bed is relatively uncleaved while the surrounding shale displays a strong, steeply east-dipping spaced pressure solution cleavage, similar to that seen throughout the region, containing clay selvedges. Along the thin fault zones, a fault zone cleavage dips more shallowly to the east. Near the fault zone, the spaced cleavage in the shale is rotated towards parallelism with the fault zone cleavage indicating westward transport along the thrust faults. This is consistent with fiber orientation and slickenside fracture steps developed within veins in the fault zones.
Several ramps have been cut through the micrite bed, merging with the bounding bedding-parallel floor and roof faults, creating several horsts. Ramp faults show progressive development from initial slight buckle folding, en echelon fracture formation along the west-dipping limb, and subsequent rotation and failure through the trace of the fractures. Some fractures show diachronous development where older fractures are rotated and cut by younger, unrotated ones. Fracture arrays dominantly dip to the east (west-climbing), parallel and are cut by ramps, but a few dip to the west (east-climbing) and are not cut by through-going faults.
Deformation in “the Beam” contrasts with that in the surrounding calcareous shale. “The Beam” has shortened through rigid body displacement along thrust faults with little cleavage development within the bed, and minor folding over the ramps (∼13%), while shortening in the shale is accommodated by pressure solution development (∼11-16%), with some minor bedding parallel faults present in the shale. Stanley (1988) interpreted the development of ramp faults to be time transgressive, from east to west across the outcrop. This diachronous shortening in “the Beam” was accompanied by a concurrent progressive pressure solution cleavage development from east to west in the surrounding shale (Figure 8).

STOP 4. Lessor’s Quarry, a Rolfe Stanley Memorial Outcrop.
NO HAMMMERS PLEASE. There is plenty of loose material on the quarry floor to collect. Abundant fossils can be seen both in the walls of the quarry and in the loose debris. This is the second of two Rolfe Stanley Memorial Outcrops we will be visiting.
Lessor’s Quarry exposes several bedding parallel and bedding cutting faults in the Glens Falls Limestone. The north wall displays several interesting features (Figure 9) including an out-of-sequence thrust that is both bedding parallel and bedding-cutting indicating that is preserves parts of its initial ramp and flat sections (labeled “Thrust Fault” in figure 9). It cuts an earlier folded thrust with fault splay and tip (labeled “Synformal Thrust Fault” in figure 9). The folding of the earlier thrust is thought to be the result of an unseen ramp beneath the quarry (Stanley, 1988); e.g. a fault bend fold similar to those seen in the roof thrusts at the “Beam”. Since the younger through-going thrust cuts the earlier folded thrust, the younger thrust must be an out-of-sequence fault.
Also related to the younger out-of-sequence fault is the orientation of bedding in the block above the thrust. In the eastern part of the wall, bedding parallels the thrust, while in the middle parts of the north wall, bedding is truncated by the thrust. Then, in the western part of the wall, bedding returns to a parallel position (Figure 9). This suggests that the currently flat thrust in the middle of the wall, initially developed as a ramp, cutting bedding in the upper block. This part of the upper block was then transported off the ramp and onto a nearby flat section of fault, resulting in counterclockwise rotation of the upper block in this section. Note also, the spaced pressure solution cleavage in the upper block is rotated in a counterclockwise manner in the middle part of the wall, over the “ramp” section. Because the cleavage was rotated during movement, the fault must have evolved subsequent to cleavage development, consistent with its interpretation as an out-of-sequence fault (recall that the Highgate Falls Thrust also rotates cleavage).
Along the western wall, next to the entrance, is a steeply dipping fault surface decorated with smeared calcite where the upper block has been removed by quarry operations. It is abundantly lineated with slickenfibers; well-developed steps indicate an east-over-west sense of transport for the removed upper block.