PRELIMINARY STRAIN ANALYSIS OF THE ANTIETAM FM., SOUTHERN CENTRAL APPALACHIANS, VIRGINIA

The Antietam Fm., a lower Cambrian, quartz arenite, that crops out as hogbacks along the length of the western limb of the Blue Ridge anticlinorium in the southern Central Appalachians, Virginia, displays a wide variety of deformational features. The general strike of the Antietam Fm. on the western limb of the anticlinorium is N30E, with a variable dip to the NW of 30-overturned, however local deviations from this geometry are present within local folds and thrust sheets. The major brittle folding and thrusting that formed the anticlinorium is thought to be associated with Alleghanian orogenesis. We have begun a study of widespread plastic deformation of these rocks as recorded in the shape of deformed Skolithos tubes present in the Antietam Fm. We have calculated the magnitude and orientation of strain on the bedding plane of over 1000 tubes at 21 different sites along 140 km of the western limb of the anticlinorium. Site-composite Rf ratio values vary from 1.15 to 1.61, but mostly converge around 1.3, with a general trend of N30E for S1. With few exceptions, this trend is consistent, even as beds locally deviate from this strike, such as in the plunges of local folds. Although not quantified, the long axis of the tubes, where seen, appears to be orthogonal to bedding. Thus the Skolithos are recording a penetrative flattening, parallel to bedding that differs in structural style from the larger-scale brittle strain commonly observed in the Antietam Fm. In addition, although S1 is nominally oriented parallel to the trend of the Blue Ridge anticlinorium, the strain is not correlated with respect to local structural position or dip. We interpret this strain to be a result of earlier shortening that had an overall similar orientation to Alleghanian shortening. This penetrative flattening is probably associated with modest deformation of a Taconic foreland.