STRESS-CONTROLLED CLEAVAGE IN CONICAL FOLDS

Outcrop-scale conical folds in inter-bedded lithic sandstone and shale in the Ordovician Windsor Tonwship Formation in Berks County, Pennsylvania show two distinct cleavage orientations on opposite fold limbs and form a ‘V’ shape towards the apex of the fold. The similarity of the geometry of conical folds to periclinal folds indicates that the local stress field is controlling the cleavage orientation. Based on the proposed passive drag folding model, in which a layer is subjected to oblique slip to the strike or dip of the bedding, the stress field of a conical fold is neither intuitively understood, nor documented in the literature.

In this study, 3D visco-elastic finite element analysis is used to investigate the mechanism of conical fold initiation and to document the resulting stress field. The models are based on a setup of a competent layer embedded in a less competent matrix subjected to oblique slip to the strike of the bedding. Several sensitivity analyses are conducted to investigate the role of geometrical input parameters, material properties and model boundary conditions on the resulting geometrical shape and stress distribution of conical folds. Based on these results the conical fold geometry in the outcrop of the Windsor Township Formation is simulated and the local stress orientations compared to the cleavage orientation.