A STRUCTURAL INVESTIGATION OF FOLDED BASCOM FM., US HIGHWAY 279 IN NORTH BENNINGTON, VT

A new road cut exposed north of Bennington, VT provides an excellent opportunity for collaborative teaching and research. During the fall of 2014, the metamorphic structural geology class from the University of Massachusetts, Amherst spent several days investigating the new cut, which exposes the Ordovician Bascom Fm. along the southbound side of US Highway 7. The Bascom Fm. here consists of well-bedded dolostone and limestone with finer-grained, phyllitic beds. Carbonate-dominated layers host stylolites, whereas phyllites preserve bedding surfaces, some with ripples and rip-up clasts. The structure consists of a mesoscopic (>100 m), upright fold train with a central zone of vertical bedding. We subdivided the outcrop into 6 structural domains, with domain 1 beginning on the westernmost edge of the outcrop and domain 6 ending before a chaotic section at the east end of the outcrop. Domains 1, 2, & 6 contain open synforms and antiforms, whereas domain 4 contains steeply dipping to vertical beds. Domains 3 & 5 contain a transition from the open folds to the vertical beds, and these domains contain parasitic and tight, asymmetric folds. Phyllite layers host subtle S-C fabrics, with bedding defining the C-surfaces. S-C fabrics in domains 1, 2 & 6 consistently suggest west-directed, bedding-parallel thrusting. Some S-C fabrics in “transitional” domains 3 & 5 show top-east vergence, suggesting flexural shearing during folding. Sub-horizontal tensions gashes are present in domain 4, suggesting vertical extension. Bedding orientations were obtained in all six domains (D). From west to east, fold axis orientations are: D1 & D2: 24/143; D3: 28/167; D4: 29/166; D5: 20/163; and D6: 09/165. The shallower plunge of the eastern domains may reflect some rotation across the vertical bedding in domains 4 & 5. We interpret the structure to indicate early west-directed thrusting and folding followed by nearly pure-shear shorting with strain accumulating on the steep central fold limb. We propose this outcrop could be an excellent collaborative research and teaching location, where classes from New England colleges and universities might study aspects of the structural geology and contribute to a common database and discussion website. In this spirit, our measurements will be made available to serve as a starting point for future research.