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Outcrops of Trenton and younger limestones and shales between Plattsburgh, New York, and Malletts Bay, Vermont, show eastwardly increasing progressive deformation fabric in the foreland fold and thrust belt of northwestern Vermont.
The deformation sequence within the calcareous Stony Point and Ibervillle shales is: (1) bed-parallel slip, marked by grooved calcite-covered surfaces; (2) folding accompanied by pressure solution cleavage; (3) overturned folds with frequent faulting along overturned limbs; fault surfaces, marked by calcite slickensides, at high angles to calcite-filled extension fractures; cleavage frequency increases and is rotated into a lower angle with bedding; (4) late-stage features 2-3 km (1.2-1.9 mi) from the Champlain thrust include high-angle faults and pervasive shearing of early fabric. Less than 1 km (0.6 mi) from the Champlain thrust (Malletts Bay area), fold hinges are sheared out and closely spaced cleavage is folded.
The Cumberland Head Argillite (transitional between the Trenton Glens Falls Limestones and the younger Stony Point Shale) contains medium to thick micrite beams in a calcareous shale matrix. Deformation in the beams occurs by brittle failure and minor thrusting up ramps in the beams. The surrounding shale deforms by bed-plane slip and pressure-solution cleavage.
Glens Falls Limestone in the study area consists of medium to thick beds of fossiliferous micrite. A widely spaced pressure solution cleavage is intensified and rotated in ramp-and-fold zones. The cleavage is crosscut by late high-angle faults.
A progressive increase in deformation fabric from west to east across the Ordovician limestones and shales of the northern Champlain Valley allows the fold and thrust chronology to be determined for the different lithologies.
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