STRUCTURAL IMPLICATIONS OF CONICAL FOLDS IN THE ORDOVICIAN WINDSOR TOWNSHIP FORMATION NEAR KUTZTOWN, PA

Outcrop-scale conical folds recently exposed near Kutztown, PA, reveal the local-scale kinematic complexity of Taconic deformation. The 30 meter long outcrop of the Ordovician Windsor Township Formation is divided into three structural domains separated by fault zones. Domains 1 and 3 contain inclined, SW-plunging folds in generally W- to SW-dipping sandstone and shale. SW-dipping bedding in Domain 2 is not folded. Fault zones between the domains contain veins with crystal fiber lineations, phyllitic lenses with stretching lineations, and slickenlined surfaces. Fault-related lineations plunge gently NW, perpendicular to the fold hinges in Domains 1 and 3.

Although the conical geometry of folds in Domains 1 and 3 is apparent in outcrop, the conical characteristics are ambiguous when plotted on stereonets. To help verify and analyze the conical geometry of the folds, we compared stereonets of field data with stereonets generated by measuring ideal cylindrical and conical objects. Specifically, measurements were taken on a traffic cone (representing a narrow cone), a funnel (wide cone), and a bucket (nearly perfect cylinder). When we rotated the measurements from ideal conical folds into plunging and reclined positions, the resulting stereonets closely resembled plots of field data from plunging cylindrical folds. These results highlight that plunging conical folds and plunging cylindrical folds may be difficult to distinguish in the field, such that conical folds are under-recognized and under-reported in the literature.

Previously published studies have attributed conical folds to drag folding along adjacent strike-slip faults or superimposed folding events with oblique stress directions. The proximity of the conical folds to fault zones containing lineations oblique to fold hinges favors the strike-slip drag mechanism. However, superimposed folding during Taconic, Acadian and Alleghanian deformation events is equally likely in Ordovician rocks of the Appalachians. Because small-scale folds usually mimic the regional pattern, the presence of outcrop-scale conical folds has broad implications for Taconic deformation in the Appalachian Great Valley. Additional work focuses on multiple sets of cleavage within the outcrop to unravel the kinematic significance of the conical folds.