Abstract
Analysis of the Paleozoic stratigraphy in the northern Allegheny synclinorium indicates that (1) Gross deformation was downward in early and middle Paleozoic. (2) The greatest stratigraphic thicknesses are in extensive semilenses of detrital sediment laid in depressions complementary to uplifts of the Taconian and Acadian orogenies, in deltageosynclines. (3) The Cincinnati and Adirondack axes geographically define the Allegheny belt; they had arches for limited times, were hinges of oscillation, and at other times were more depressed than the central part of the belt. (4) The axes were less depressed than the elliptical Michigan geosyncline to the west and the linear Champlain geosyncline on the east. (5) There may have been warping (epeirogenic) and sea-level (eustatic) movements synchronous with and related to the orogenies, as well as depression produced by loading (isostatic adjustment); the causes and interrelations are not proven, however.
Later Paleozoic structures trend more southwesterly than the Adirondack axis; the rectilinear margin of the late Paleozoic (Virginian) geosynclinal belt crosses the axis in the central Appalachians. The eastern margin of the southern part of the Allegheny belt is concealed beneath the first extensive Appalachian thrust sheet. Inasmuch as this sheet bears middle Ordovician rocks stratigraphically similar to those in the Champlain belt in the north, it forms an allochthone, and the thrust developed along the Adirondack axis. Appalachian folds that extend into the northern Allegheny synclinorium parallel the Appalachian Structural Front, the western principal fold in the mountains. The Front has the arcuate trend of deltageosynclinal isopachs in the north and of the rectilinear trend of the margin of the Virginian belt in the south. The northern margin of the synclinorium was block-faulted in the early Tertiary.
