The structure of the Taff Gorge area,Glamorgan, and the stratigraphy of the Old Red Sandstone—Carboniferous Limestone transition

Lithostratigraphical and structural mapping of the Taff Gorge area, Glamorgan, has revealed a number of E-W trending folds and thrusts. These were developed during the early stages of the Hercynian deformation as a result of an approximately N-S principal compressive strain. Coeval with the folding, a conjugate set of sub-vertical wrench faults, symmetrical about the principal compressive strain, divided the area into tectonic blocks and to some extent controlled the progressive development of folds and thrusts within the individual blocks. Later Hercynian deformation gave rise firstly to oblique-slip and finally to dip-slip movement on the faults. The lithological transition between the Old Red Sandstone and Carboniferous Limestone is described in detail, including the diagenetic history of the limestones. Marine horizons are shown to be present within the dominantly fluviatile Old Red Sandstone, and the base of the Carboniferous Limestone is defined at the base of the lowest limestone of the continuous marine sequence. Studies of spores, macroplants, conodonts and brachiopods indicate that the whole of the Upper Old Red Sandstone and Lower Limestone Shale in the Taff Gorge are of Tournaisian age.