Microstructures in deformed sediments, Denbigh Moors, North Wales

Abstract

In the Denbigh Moors area of North Wales some of the Silurian sedimentary rocks are intensely disrupted. Recently the British Geological Survey has confirmed that this is largely the result of extensive slumping of the sediments, which was part of a long sequence of deformation events ranging from what they called ‘penecontemporaneous movements’ and related ‘additional processes’ to at least three periods of regional tectonism. New knowledge of the microstructural behaviour of deforming sediments obtained from laboratory studies allows the prospect of assigning field structures to a position within this long scenario and a better understanding of the complex pre-lithification deformation processes. Experimental deformation of argillaceous sediments has shown that they deform, over a wide range of water content, by intense slip within narrow shear zones rather than by pervasive grain slip. Shear zones of strikingly similar appearance are common in parts of the Denbigh Moors succession, and, being demonstrably pre-diagenetic, can be used as indicators of pre-lithification disturbance. The shear zones give a polished, finely striated, and crenulated appearance to many of the exposure surfaces in the area. The diverse orientations of the zones reflect to some extent different locations within the sliding sedimentary masses. The style of the zones gives an indication of the water content of the sediment and hence the burial depth at the time of deformation. Some beds show features such as truncated upper contacts and are therefore thought to have been disturbed before lithification, yet the rocks lack shear zones. These are interpreted as having been deformed while very near surface, with a water content too high for the shear zone mechanism to have operated. The possibility remains to be explored that some of the shear zones in the area are the result of regional tectonism acting on unlithified sediments.