The geology of the Vermont Valley and the western flank of the Green Mountains between Dorset Mountain and Wallingford, Vermont
Date of Award
Master of Science (MS)
The carbonate sequence of the Vermont Valley has been generally believed to lie on the east limb of a major unfaulted syncline ("Middlebury Synclinorium"), and the exposed Precambrian basement and overlying cover rocks of the Green Mountain massif on the west limb of the adjacent major unfaulted anticline ("Green Mountain Anticlinorium"). Except for the Basal Thrust of the Taconic Allochthon, all faults shown on previous maps die out north of Dorset Mountain.
In contrast, detailed mapping for this thesis in the Vermont Valley and the western flank of the Green Mountain massif has revealed several major north/south trending thrust faults, which can be traced through the field area, and document the progressive evolution of the Vermont Valley as a complicated thrust-and-fold belt in four different stages.
A tectonically derived, highly-strained rock unit, previously mapped as the Baker Brook "volcanics", which separates Ordovician black phyllites of the Hortonville Formation from the shelf carbonates to the east, shows a pervasive transposed and differentiated layering. The fault zone fabrics in these rocks display an anastomosing mylonitic foliation, locally containing coarser fragments derived from intermediate-silicic plutonic rocks, presumed to originate from the Grenville basement. Asymmetrical feldspar porphyroclasts and recrystallized quartz grain shape fabrics give a clear and consistent east-over-west sense of shear. Adjacent Ordovician marbles are mylonites too, with a steeply plunging stretching lineation. This contact between the Middle Ordovician black phyllites and the Cambrian to Early Ordovician shelf units has previously been interpreted as an angular unconformity ("Tinmouth unconformity"). In contrast, I suggest there is a major thrust fault here, which has transported the carbonates in the east over the black phyllites of the Hortonville Formation in the west, named the Baker Brook Thrust (T1).
The carbonate sequence is interpreted as a duplex thrust system (T2), that was progressively developed and is necessary to explain the complex structural relationships of the Vermont Valley carbonate shelf units. While overriding a footwall ramp, or alternatively by footwall plucking, underlying basement slices were detached and brought up by the Pine Hill Thrust. The Pine Hill Thrust, which extends through the field area, is believed to be entirely a T2- imbricate thrust of the shelf duplex. Detailed correlation of units of the Vermont marble belt suggest a single, however internally imbricated, large marble slice, attached to the sole of the moving Basal Taconic Thrust and emplaced with the Taconic Allochthon to the west. The overthrust, named the Dorset Mountain Thrust (T2r), truncates all earlier structures north of the Dorset Mountain massif and follows the Basal Taconic Thrust throughout the Vermont Valley.
Further progressive shortening during the last stage of deformation culminated in foreland directed thrust faults (Ts), and folding of the shelf duplex; these are presumed to belong to the Taconic Frontal Thrust System. This stage is probably responsible for cross-cutting relationships along the complexly folded western flank of. the Green Mountains (Green Mountain Thrust), and reactivation of some earlier T2-thrust faults.
The regional interpretation strongly suggests that the entire Vermont Valley, the eastern part of the Taconic Allochthon to the west, and probably the Green Mountain massif to the east, are underlain by a complicated deformed shelf duplex, which has been cut by major north/south trending, eastward dipping late thrust faults.
Herrmann, Rolf, "The geology of the Vermont Valley and the western flank of the Green Mountains between Dorset Mountain and Wallingford, Vermont" (1992). Geology Theses and Dissertations. 75.
Plate 1 - Geological Map of the Vermont Valley and the western flank of the Green Mountains between Dorset Mountain and Wallingford, Vermont (uncoloured outcrop map, scale 1: 31,250)
HerrmannMSpl2.pdf (126 kB)
Plate 2 - Cross-section through Dorset Mountain (coloured geological cross-section, scale 1: 31,250)
HerrmannMSpl3.pdf (150 kB)
Plate 3 - Cross-section through South Wallingford (coloured geological cross-section, scale 1: 31,250)
HerrmannMSpl4.pdf (68 kB)
Plate 4 - Generalized regional geological Map of the Vermont Valley, western New England (coloured geological map)
herrmannclmsmap.pdf (4183 kB)
Geological Map of the Vermont Valley and the western flank of the Green Mountains (coloured computer-drawn version of plate 1 geological map, scale ~1: 37,700)
Herrmann, R., 1992. The geology of the Vermont Valley and the western flank of the Green Mountains between Dorset Mountain and Wallingford, Vermont.
Unpublished MSc. thesis, State University of New York at Albany. 127pp., +xiii; 4 folded plates (maps)
University at Albany Science Library call number: SCIENCE Oversize (*) QE 40 Z899 1992 H47