ABSTRACT
The Champlain Thrust System, responsible for the final emplacement of the Taconic Allochthon, and which represents the surface trace of the main detachment under the Taconic suite, can be traced throughout the study area in northwestern Vermont, northward into southern Quebec. In the Highgate and St. Albans area this imbricated assemblage consists of 3 main carbonate thrust slices (Highgate Springs-, Philipsburg-, Rosenberg slice), each of which contain rocks deposited during some part of early Cambrian to medial Ordovician time. The Highgate Springs sequence comprises a mildly deformed early to medial Ordovician carbonate-shale succession. Late Cambrian to early Ordovician carbonates of the Philipsburg sequence, some quartz-arenaceous to argillaceous, form a northeast-plunging syncline structure that appears as an intercalated wedge within the thrust system. The Rosenberg sequence consists of early Cambrian to medial Ordovician shallow water siliciclastics and carbonates of the continental shelf and shelf edge that has been believed thoughout the last decades to represent a largely unfaulted regional synclinal structure ("St. Albans Synclinorium"). In particular, the eastern contact of the Rosenberg carbonates with the overlying Morses Line Formation has been interpreted as a stratigraphically intact, rapid facies change from shallow water siliciclastics and carbonates into deeper water sediments of the continental slope and/or rise. In contrast, my detailed lithostructural study of the Highgate and St. Albans region favors a substantially faulted nature of this contact. The significant contrast of the dip angle of the average cleavage foliation (~15°) across the Highgate-Morses Line Formation contact suggests a substantial structural discontinuity between the Rosenberg and the Morses Line sequence. This structural break is interpreted as a continuous detachment fault ("St. Albans Detachment") that caused a counter-clockwise rotation of the average cleavage fabric within the Morses Line Formation and juxtaposes mildly deformed Cambro-Ordovician siliciclastics, carbonates and siltstones/shales of the Rosenberg sequence with intensely strained slates of the medial Ordovician Allochthon. The extension of the St. Albans Detachment can be extrapolated between Burlington, Vermont, and Drummondville, Quebec, where it causes a strongly varying thickness of the shelf carbonate strata [Dunham Dolomite, Monkton Quartzite, Winooski Dolomite, Danby Formation] that are exposed adjacent to intensely strained slates, and, in Canada, juxtaposes melange with Taconic sequences [Stanbridge Nappe, Granby Nappe]. In addition, a closely spaced suite of northeast-southwest trending normal/tear faults cross-cuts the main structural north-south trend in the study area and can be extrapolated across the international border.

Haschke, M.R., 1994. The Champlain Thrust System in Northwestern Vermont - structure and lithology of the Taconic foreland sequence in the Highgate Center quadrangle.  Unpublished MSc. thesis, State University of New York at Albany. 124 pp., +x; 1 folded plate (map)
University at Albany Science Library call number:  SCIENCE Oversize (*) QE 40 Z899 1994 H38

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