Lower Silurian distal shelf storm-induced turbidites in the Welsh Borders: sediments, tool marks and trace fossils

The Upper Llandovery Hughley Shales of Shropshire are interpreted as storminduced distal shelf turbidites. They comprise a sequence of calcareous sandstones interbedded with purple mudstones. The trace fossils, body fossils, and palaeogeography indicate a deep shelf position for their deposition. The thin sandstone units are interpreted as having been deposited from density current bedload and suspension. Storm surge ebb traction currents, arising from intermittent major offshore storms, are considered to have initiated the density currents. A deepened storm wave base allowed transportation of fine sand in suspension out onto the distal shelf. Internally, the resultant sands are differentiated from distal (fan) turbidites by multidirectional bedload currents, which are shown most clearly by spectacular tool marks on sandstone bases. The tools are clearly identifiable in many cases as indigenous benthonic organisms uch as horn corals, coral colonies, crinoids, brachiopods and trilobites. The sandstone beds were later colonized by burrowers and other track-producing organisms. The trace fossils include Diplocraterion (or Corophioides), Palaeophycus, Skolithos, Chondrites, Scolicia, Rusophycus, Cruziana and Diplichnites (Cruziana facies assemblage). A new ichnospecies, Walcottia deuilsdingli is established for small burrows with regular lateral appendage marks at certain points.