A reappraisal of the allochthonous nature of the Rosenberg slice and Stanbridge Group of southern Quebec and northwestern Vermont

Abstract

The Ordovician Stanbridge Group of Quebec has long been considered an allochthonous nappe. It is an internally coherent unit that consists of lower slaty limestone overlain by slate, which is correlated with the Highgate and Morses Line formations, respectively, in Vermont. In Quebec, the basal limestones have been inferred to be thrust over Cambrian dolomites (Gorge Formation in Vermont) of the Rosenberg slice, part of the parauthochthonous shelf, although this contact is not exposed there. In the Missisquoi River gorge of Vermont, a conformable sequence of upper Gorge–Highgate–Morses Line formations is exposed. The map distribution of rock units indicates that this conformable relationship probably extends up to at least the middle unit of the Stanbridge Group. Therefore, the relationships in Vermont require that the Stanbridge Group must be part of the parauthochthonous Taconic foreland rather than a far-traveled nappe. The Rosenberg slice in Quebec forms a large anticline (Highgate anticline) whose western limb is truncated by the Rosenberg thrust. In Vermont, the anticline is internally cut by the Highgate Falls Thrust, which is an out-of-sequence thrust that decreases in displacement northwards to the International Border.