Intraformational Contorted Rocks in the Upper Carboniferous of the Southern Pennines

Abstract

Summary Beds of contorted shale, both overlain and underlain by normally bedded shale, occur in the Millstone Grit and Lower Coal Measures of the Goyt Trough to the west of Buxton. A brief synopsis of the geology of this area is given. The beds of contorted shale, usually from 6 inches to 2 feet 6 inches in thickness, are remarkably constant in horizon throughout the area; the majority directly overlie marine bands, but at least two are unassociated with these fosailiferous shales. Structures discernible within the contorted beds comprise sheer-planes and small folds. On each limb of the Goyt Syncline both the axial planes of the small folds, and the shear-planes, clip towards the major axis at an angle greater than the stratal dip. Wherever the pitch of the small folds can be observed it is northerly, in the same direction as the pitch of the major fold. Contacts of the contorted beds are almost invariably plane surfaces, often polished or bearing slickensides running along the line of dip. The contorted shsles are considered to be of tectonic origin and to be an expression of adjustment through bedding-plane slip during the folding of a succession of rocks of varying competency. It is thought that shearing, resulting from bedding-plane slip, affected the least competent beds in the succession. Shear-planes are interpreted as fracture cleavage and the small folds as congruous drag-folds. Similar beds of contorted shale occur at some of the same horizona on the borders of the North Staffordshire Coalfield and in Lancashire.