Stratigraphy and Structure at the North End of the Taconic Range in West-Central Vermont

Abstract

The stratigraphy of the Taconic sequence at the north end of the Taconic Range in west-central Vermont has been revised as follows: Pawlet formation: interbedded black slate and greywacke; Middle Ordovician Mount Hamilton group: undivided black, gray, green, and red argillite, with minor limestone, limestone conglomerate, and ankeritic quartzite; Upper Cambrian to Middle Ordovician West Castleton formation: black slate and phyllite, with a limestone unit near the base; Lower Cambrian Bull formation: Lower Cambrian (1) Mettawee facies. Purple and green slate, mudstone, and phyllite; this facies constitutes the bulk of the formation. (2) North Brittain conglomerate member. An intraformational limestone-pebble, slate-matrix conglomerate (3) Mudd Pond member. A thin but persistent orthoquartzite (4) Zion Hill member. A discontinuous graywacke or sub-graywacke unit ranging from a pebble conglomerate to a mudstone (5) Bomoseen member. A massive, olive-drab, medium-grained graywacke Biddie Knob formation: a chloritoid-bearing purple and green slate and phyllite with minor beds of limestone and quartzite. Lower Cambrian (?) The sense of the Lower Cambrian succession is given by load casting and graded bedding in the Zion Hill, and in rare arkoses mapped with the Bomoseen. The sense of succession in the Mount Hamilton group is given by cross-bedding, graded bedding, and by channel filling. Structurally, the Taconic sequence consists of nested thrust slices subsequently folded together. Although each slice has its own characteristic stratigraphy and structure, correlation of the units, down to the formational level, presents little difficulty. The structure is typified by the Giddings Brook recumbent bottoming fold in the area between the Taconic Range and Lake Bomoseen. The fold is at least 5 miles in amplitude, the movement was cast to west, and the axial plane is nearly horizontal. Digitations on this major structure explain the map pattern. Key units of the Taconic sequence have been recognized east of the Taconic Range; the east flank of the Range is now underlain by an inverted sequence of Lower Cambrian rocks. The Sudbury nappe at the north end of the map area is composed of the Lower Ordovician Chip-man formation. The structure does not root in the east limb of the Middlebury synclinorium as reported by Cady, but is a thrust slice, whose southeast edge tucks under the Taconic sequence. Structurally, this unit is part of the latter. The Whipple marble at the north end of Whipple Hollow is part of the Lower Ordovician Bascom formation, which in this area has been moved westward over the younger black phyllite through a recumbent fold involuted into the Taconic sequence. This structure indicates the persistence of intense deformation after the development of the Taconic structure. Topologically, the Taconic sequence is in the southward continuation of the Middlebury synclinorium. Coupled with the discovery of the inverted Lower Cambrian rocks east of the Taconic skyline, the evidence supports an allochthonous origin of the Taconic sequence. The gradational contact between the green and black phyllites east of the Taconic Range, and the presence in the black phyllite of beds characteristic of the Taconic sequence show that part of the black phyllite here is allochthonous. The thrusting is dated as Late Trenton on the basis of exotic blocks of Taconic rocks in an autochthonous black slate at Forbes Hill. Similar rocks have also been found elsewhere at the periphery of the Taconic sequence. The thrusting may have occurred as submarine gravity slides of soft rocks; slump structure is indeed abundant in the Taconic sequence. The intimate mixing of the black slates of the Taconic sequence and of the Trenton mud may make the mapping of the Taconic fault unfeasible even in principle. Continued deposition of black mud after the emplacement of the Taconic nappes could account for the reported Trenton unconformity at the margin of the Taconic sequence south of the Castleton area. The Taconic sequence probably was deposited in the area of the present Green Mountains. The upper part of the Mount Hamilton group correlates by fossils with the Moretown-Cram Hill formations of eastern Vermont. The Mount Hamilton group is free of volcanic rocks and is thin, in contrast with the Moretown-Cram Hill formations; this change is consistent with paleogeographical requirements, as the Cambro-Ordovician, typically eugeosynclinal deposits of eastern Vermont, must somehow change into the synchronous, but typically miogeosynclinal deposits of western Vermont and eastern New York.