Timing of regional deformation of the hill end trough: A reassessment

Abstract

Recent mapping around the margins of the former Hill End Trough shows that the Upper Devonian to ?Lower Carboniferous Lambie and Catombal Groups are deformed into overturned, tight to isoclinal folds with an axial‐surface slaty or fracture cleavage. This primary cleavage is parallel to slaty cleavage in underlying rocks both within, and on the margins of, the former Hill End Trough. On the basis of this parallelism, and the gross congruence of major dimensions, shape, symmetry, and orientation of folds in the Lambie‐Catombal Group and subjacent rocks, we conclude that the regional deformation of the Hill End Trough occurred after deposition of the Lambie and Catombal Groups. A consideration of both structural‐strati‐graphic and radiometric time constraints shows that regional deformation occurred in the latest Devonian to Early Carboniferous, possibly around 360 m.y. ago. The discrepancy between our latest Devonian to Early Carboniferous timing of the regional deformation of the Hill End Trough and the commonly accepted mid‐Devonian timing arises from the erroneous assumption that local unconformities reflect deformations that affected the entire Lachlan Fold Belt.