STRUCTURE OF GRAYWACKE AREAS AND TACONIC RANGE, EAST OF TROY, NEW YORK

Abstract

The report deals with the structure of graywacke areas, east of the Hudson River near the latitude of Troy, New York, and the structure of a part of the Taconic Range in the same latitudes. It has been claimed that much of this area is underlain by allochthonous far-travelled masses of rocks, resting on flat thrusts. The contact zones of the Rensselaer graywacke, and of smaller graywacke areas south of it, show that the graywacke layers are conformably interbedded with greenish and purple slates and siltstones which underlie large portions of the Hudson lowland. If thrusts are present, they must lie below the levels previously assumed, to take cognizance of the uninterrupted sequences of strata below the present edges of the gray-wacke terranes. The coarsest phases of graywacke crop out near the western edges of the area. Here are also found the thickest sections of graywacke and several lenses of coarse, bouldery conglomerate. Eastward, fine-grained graywacke forms individual lenses in phyllitic slates and schists of the Taconic Range. The Taconic Range consists of a thick mass of quartz-chlorite schist, with subordinate zones of gray siltstone, quartzitic and graphitic lenses, and a tuffaceous series of rocks along its western border. The rocks have been folded, overturned to the west, but isoclinal folds appear to be rare. Slip cleavage dips east, and east of the graywacke areas a well-developed lineation plunges east-southeast, parallel to the direction of movement along many thrust faults. About 8 miles south of Bennington, Vermont, the rocks of the Taconic Range approach the Green Mountain front. This tract, referred to as the Pownal upland, is structurally complex, and unfortunately poor exposures leave many of its problems open. Flat thrusts may occur here, but the magnitude of their displacements is uncertain. The structural relations between the Pownal upland and the Green Mountain front are discussed, and the writer attempts to reconstruct the tectonic history of the area in the light of those features of deformation that are exposed and believed to allow a fairly reliable interpretation. Folding, flowage, and thrust faulting of an autochthonous series of argillaceous Lower Cambrian and Ordovician rocks are thought to explain satisfactorily most, if not all, observable features of the region.