Abstract

The stratigraphy and geologic structure of Precambrian to Lower Cambrian rocks of the Blue Ridge province of northwestern Virginia was established by mapping. Major rock divisions are: Precambrian gneiss, Precambrian and Lower Cambrian metasedimentary and metavolcanic rocks, and Lower Cambrian quartzites, phyllites, and carbonate rocks. The sedimentary-volcanic sequence consists of local sediments and sub-aerial basalts, deposited unconformably upon the gneiss. Thinning of this sequence toward the north may be due either to original variations in thickness or to erosion prior to deposition of overlying rocks. Widespread Lower Cambrian sediments of the platform facies were deposited in a shallow sea that covered the older rocks.

Blue Ridge and Short Hill are parts of the west limb of the Catoctin Mountain-Blue Ridge anticlinorium, formed by Paleozoic deformation. Each contains an overturned anticline and syncline. During deformation, flexure folding graded into shear folding at different times in rocks of different competency. Near the close of deformation, flow cleavage, formed earlier during the shear folding of incompetent rocks, was folded, and slip cleavage developed. No major thrust faults are recognized in the area; but the west limb of the Catoctin Mountain-Blue Ridge anticlinorium is repeated by a fault, here interpreted as a normal fault of Triassic age. The rocks, including the gneiss, which underwent retrogressive changes during Paleozoic folding, are in the greenschist facies of metamorphism.

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