Slumping in the Carboniferous rocks of Pembrokeshire

Abstract

The coastal cliffs between Tenby and Telpyn Point afford a fine section through part of the Carboniferous strata of southern Wales. Here the writer came across some thirty localities showing evidence of slumping. The evidence takes several forms : (1) contortion and slip-bedding; (2) slump-ball structures of a few inches to 24 feet across, generally composed of sandstone and embedded in mudstone. Lumps of clay-ironstone, resembling concretions, appear to have a similar origin; (3) crumpled balls, resembling the slump balls, but of a less compact structure, of only a few inches across and occurring embedded in sandy shale; (4) slipped blocks, giving the appearance of cross-bedding. These occur amongst other slump structures, as separate masses, generally a few feet across, which have slid without much internal distortion; (5) pseudo-ripple marks, which are regular slight folds measuring a few inches from crest to crest, each crest persisting only a short distance along the strike. They resemble ripple marks, but through a thickness of a few feet all laminae are bent in “similar folds” with the crests directly above each other. The thickness of the slump sheets varies from a foot or less to about 15 feet. For each sheet the thickness remains almost constant over large areas. The most extensive sheet is exposed for a mile along the coast around Telpyn Point and for 200 or 300 yards at right angles to this direction. At Waterwynch Bay four sheets are superposed. These larger sheets are composed of slump balls embedded in mudstone. They alternate with undisturbed beds. The slopes on the sea-floor during the slumping must have been exceedingly slight, since there is plentiful evidence of shallow water. Moreover, most individual, undisturbed beds can be traced for considerable distances, each showing a constant thickness. The slump balls are explained by quasi-turbulent motion of the sliding mud. An experimental investigation of the mechanics is being conducted in conjunction with Dr. van Straaten. Difficulty is sometimes found in distinguishing between slump structures and buckling in anticlinal cores, the two features frequently being closely associated in the field. Although it is not deemed probable, the possibility must be admitted that the so-called buckling is due, at least in part, to slumping. Anticlines, also, may have tended to form where slumping had previously caused irregularity in the bedding.