Revision of Grenville stratigraphy and structure in the Balmat-Edwards district, northwest Adirondacks, New York

Abstract

Integration of much new surface and subsurface data suggests major reinterpretations of stratigraphy and structure of the Grenville series of the Precambrian in the Balmat-Edwards district, New York. Only two major metasedimentary units appear to exist, (1) marble and (2) migmatitic paragneiss. The paragneiss seems to be about 3000 feet thick and is the oldest formation. The overlying marble may be as much as 2000 feet thick and is differentiated into 15 units. Most units are silicated dolomite; one contains anhydrite and traces of halite, another hydrogen sulfide and graphite. A potassic migmatite (injected sandy shale), about 400 feet thick, and minor marble overlie this sequence. Major variations in composition and form of metasedimentary units are common, but most variations are metamorphic in origin. The metasedimentary sequence is complexly sheared and intricately plicated into a series of northeast-trending, gently plunging folds. These merge into and are blurred by northwest-plunging cross folds. Both types of folds range from crumples to folds with either an amplitude or a radius of curvature of a mile or more. The northeast-trending folds are accordant elements of a regional system. Cross folds are confined largely to zones on opposite sides of the Grenville Lowlands where the lowlands merge into igneous massifs. Parts of these zones are major shears, involving displacements of at least 3 miles, along which stratigraphic units are obliterated. The two fold sets and shear zones may be ascribed to either (1) closely related stages in a gigantic, upward diapiric flow of metasediments and intrusives of the Grenville Lowlands in response to southeast-northwest crustal shortening or (2) more distinctly separated periods of deformation, in which the cross folds formed through refolding of the preexisting structures in response to a northeastward crustal couple. The second interpretation is preferred largely because of (1) the large size of the cross folds and of the belts formed by them, (2) the common direction sense of cross folds, and (3) evidence that the axes of many cross folds intersect axes of the northeast-trending, gently plunging folds at large angles.