Northeastern Section - 37th Annual Meeting (March 25-27, 2002)

Recent mapping in the Colchester quadrangle, Vermont, confirms the presence of the Muddy Brook thrust fault between the gently eastward-dipping Cambrian-Ordovician carbonate bank sequence and the overlying, in part coeval, deeper-water calcareous shales (Skeels Corners Formation), in a highly deformed, overturned fold limb below the Hinesburg thrust. The Muddy Brook thrust cuts across successively younger units toward the south, where relationships are complicated by a north-striking normal fault, the St. George fault, and presumably Mesozoic cross-faults. The Skeels Corners continues south across the Winooski River to new exposures near Taft Corner in Williston. From there south it correlates with the Brownell Mountain Phyllite, which occupies a similar position below the Hinesburg thrust and above carbonates at the north end of the Middlebury synclinorium (original type locality for the Muddy Brook thrust). Our interpretation thus revives earlier models for the Brownell Mountain as allochthonous above Bascom Formation. The incompetent shales were squeezed westward as the advancing Hinesburg thrust rode up over the shelf edge. Strongly cleaved, discontinuous limestones along the thrust may be fault slivers picked up from younger units on the shelf, or faulted depositional carbonate blocks, which are common in the Skeels Formation away from any faults.