Abstract

Detailed mapping of bedding, cleavage, and lineation in the Lower Cambrian clastic rocks and underlying metavolcanics of Catoctin Mountain in Maryland and northern Virginia indicates that the area is not a syncline as previously interpreted, but a tightly folded, eastward-dipping sequence which forms the upper and eastern limb of the South Mountain anticlinorium. This interpretation necessitates a revision of the stratigraphy. Strata mapped as Loudoun along the eastern border of Catoctin Mountain constitute a zone including the upper Weverton quartzite and the basal Harpers phyllite.

Evidence suggests that the Catoctin metabasalt and associated rhyolite tuffs are closely related in time to the overlying Chilhowee group and that volcanic activity continued during deposition of the lower portion of the Chilhowee group.

A statistical study of the megascopic structures, supplemented by petrofabric analysis, contributes to an understanding of the deformation plan. Folds are asymmetrically overturned to the west; flow cleavage dips eastward and fans gently toward the west; the principal movement has been along flow-cleavage planes so that shear folds result with thickened crests and troughs and thinned limbs; fold axes are subhorizontal; lineation in a results from a crenulation of the flow cleavage and a stretching of minerals down the dip of the flow-cleavage plane; and the intersection of flow cleavage and bedding produces a lineation in b.

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