GEOLOGY OF THE WOODSVILLE QUADRANGLE, VERMONT-NEW HAMPSHIRE

Abstract

The Woodsville quadrangle, in east-central Vermont and west-central New Hampshire, was studied primarily to determine the stratigraphic and structural relations between two contrasting sedimentary sequences, one in western and central New Hampshire, the other in eastern and central Vermont. The Vermont rocks in this quadrangle consist of three formations, probably Middle Ordovician, with a total thickness between 15,000 and 20,000 feet. A fault separates these rocks from the New Hampshire rocks, which are pre-Silurian, Silurian, and Lower Devonian and total approximately 20,000 feet thick. Volcanic rocks, mostly pyroclastic, constitute about 10 per cent of the stratified rocks in the Woodsville quadrangle. Plutonic and hypabyssal rocks, probably late Devonian, occurs as dikes, sills, large intrusive sheets, and small stocks. The quadrangle is divided into four major tectonic units by three faults, which trend north-northeast. The Ammonoosuc fault dips about 36°W. and is younger than the folding and regional metamorphism. The Northey Hill and Monroe faults are steep and are older than most of the metamorphism. Because of the faults the major folds are fragmentary, preserved only as single limbs or as parts of synclines. Countless minor folds are present throughout the region. Progressive metamorphism is well displayed in the quadrangle. Whereas the rocks in a central north-south belt belong in the chlorite zone, the metamorphism increases toward the east and west margins of the quadrangle, where the rocks lie in the staurolite zone. The sillimanite zone has been attained locally around plutonic rocks. Retrograde metamorphism is especially strong along the Ammonoosuc thrust. The folding, thrusting, metamorphism, and igneous intrusions were all phases of the late Devonian Acadian orogeny. Special attention was given to the origin and chronology of the cleavage and minor folds in the Vermont part of the quadrangle, where there is evidence of two stages of deformation. During the earlier stage, schistosity formed parallel to the axial planes of contemporaneous minor folds. Because of the later deformation, these earlier folds are inconspicuous. During the later stage, slip cleavage formed parallel to the axial planes of contemporaneous minor folds. In many places these later folds are very conspicuous and if interpreted as ordinary drag folds may lead to an incorrect interpretation of the stratigraphy and major structure. Toward the west the slip cleavage becomes schistosity. The concept of two stages of deformation has important applications to the geology of New England and other metamorphic terranes.