Date of Award
Master of Science (MS)
The geology of the eastern limb of the Green Mountain Anticlinorium consists of a series of Paleozoic metasedimentary rocks with lithologic boundaries arranged in a remarkably straight trend approximately parallel to the axis of the Green Mountains. Published reports of the area, consisting largely of reconnaissance mapping, have treated this complex series of polyphase deformed rocks as an essentially upright autocthonous sedimentary sequence. Boundaries between rock units have, for the most part, been assumed to be primary in origin, as have various structural elements within the rock units. More recent work in selected areas within the Ludlow Quadrangle has revealed problems in the application of stratigraphic techniques toward field mapping in these medium grade metamorphic rocks.
One of the main problems is the consistent misidentification of demonstrably secondary mesostructures as primary sedimentary features by early workers. Many rocks in the area have been shown to contain secondary structures which mimic such primary features as bedding and conglomeratic pebbles (Gregg and Nisbet, in preparation) and which have in fact been mistaken for these structures in the past. In addition to problems arising from incorrect interpretation of mesostructures, other problems have resulted from the failure of some workers to distinguish various fold groups on the basis of overprinting relationships,
This investigation deals with a subarea within the Ludlow quadrangle where detailed structural mapping was performed by the author from 1971 to 1974. The central feature of the area is the ultramafic zone consisting dominantly of serpentinized ultramafic rock masses up to 1 km long with minor zones of talc carbonate rocks around the boundaries, This ultramafic zone is situated along the boundary of two rock units, the Moretown member and: the Cram Hill member of the Missisquoi Formation. Although these units have been called members of the same formation by early workers, the author has found a number of striking contrasts in the deformational features of the units. For example, the Cram Hill phyllites have been involved in only two phases of deformation, while the Moretown gneisses contain structures from at least one earlier deformational phase. The Cram Hill phyllites contain layering which may be of sedimentary origin and which is moderately deformed in most cases. The Moretown member has been severely deformed, and all trace of initial layering is obliterated. In addition to these contrasts the author has shown a discordancy between early layering and the contact between the units which is probably of tectonic origin. Because. of these contrasts the author considers that there is little basis for classifying the various rock types by the "Formational" and "Member" designations, and that future structural work in the Central Vermont area will result in the abandonment of the stratigraphic nomenclature now being applied.
During field work and thin section examination the author observed a number of tabular garnets in the rocks of the Moretown member. In most cases the garnets were formed during the earliest deformational event and were deformed into tabular shape by later deformation. The deformation, however, was not the typical flattening assumed by most workers, but a slicing process in which segments of garnets from an initially equidimensional crystal are sheared parallel to rock layering. A number of examples of partially sliced crystal sections were observed on mesoscopic and microscopic scales.
Gregg, William J., "Structural Studies in the Moretown and Cram Hill Units near Ludlow, Vermont" (1975). Geology Theses and Dissertations. 29.