Geology of the Humpback Mountain area of the Blue Ridge in Nelson and Augusta counties, Virginia

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Virginia Polytechnic Institute and State University

Mapping of a 190 square km area along the western flank of the Blue Ridge anticlinorium in central Virginia has defined three major rock groups: (1) the earlier Precambrian Virginia Blue Ridge Complex; (2) the Late Precambrian Catoctin Group; and (3) the Early Cambrian Chilhowee Group. The Virginia Blue Ridge Complex is subdivided from oldest to youngest into the Pedlar, Marshall and Levingston formations. The Pedlar and Marshall formations, partially of metasedimentary origin, were metamorphosed to the granulite facies, retrograded to as low as the greenschist facies, and deformed into a series of east-trending folds prior to Late Precambrian. The relationships between the two metamorphic events and structural deformation was not determined. An angular unconformity separates the Catoctin Group from the Virginia Blue Ridge Complex upon which 300 m of topographic relief was developed.

The Catoctin Group is subdivided from oldest to youngest into the Swift Run, Catoctin and Loudoun formations which are subdivided into phyllitic units of both sedimentary and pyroclastic origin separated by sequences of greenstone flows. Early Catoctin Group volcanism, originating in situ from northeast-trending dike complexes, was accompanied by normal faulting along the northwest-trending Stony Creek fault. Swift Run and lower Catoctin Formation sediments were transported principally from the southeast quadrant or were derived in situ from saprolite and colluvial deposits. Loudoun and upper Catoctin Formation arkosic sediments were derived from localized sources, west of the Blue Ridge, and transported in a southwesterly direction parallel to the Appalachian structural trend. Pyroclastics of the Catoctin Group probably were derived from a northwest source. The upper Catoctin Formation is older than 700 my.

Paleozoic metamorphism of the Catoctin Group altered the lava flows toward the following greenschist assemblages of high oxidation state: quartz and albite plus (1) epidote, penninite and magnetite; or (2) penninite, magnetite and calcite. Subsequently, metamorphism indicative of a lower oxidation state altered the lavas toward the greenschist assemblage of: quartz, albite, epidote and actinolite. Actinolite content increases with depth at the expense of relict pyroxene, penninite and magnetite. Metamorphosed pyroclastics are characterized by sericite, quartz, magnetite and/or hematite.

The Chilhowee Group is subdivided, from oldest to youngest, into the Weverton, Harpers and Antietam formations. A thrust fault zone separates metamorphosed Weverton and Harpers elastics from unmetamorphosed Antietam elastics. Metamorphosed Chilhowee and Catoctin sediments are characterized by formation of the following metamorphic minerals: epidote, penninite, sericite, and microcrystalline quartz. Chilhowee detritus was derived from the west-northwest and transported eastward and/or parallel to the Appalachian structural grain.

Post-Precambrian deformation includes folding about northeast and north-trending axes followed by development of southeast-dipping cleavage during Paleozoic metamorphism. Cleavage development preceded northwestward thrusting of metamorphosed rocks of the Blue Ridge over unmetamorphosed Paleozoic rocks of the Valley and Ridge. Subsequently the principal joint trends were established followed by normal and strike-slip movement along northeast-trending high-angle faults. Intrusion of Mesozoic dikes post-dates high-angle faulting.