Short Paper: Hummocky cross-stratification in the Downton of the Welsh borderland

Abstract

Hummocky cross-stratification is recorded from the classic Upper Silurian Whitcliffe Group (Ludlow Series)/Downton Group (Pridoli Series) boundary section at Ludlow, Shropshire. This necessitates a revision of previous interpretations of palaeoenvironments which infer either a tidal mudflat depositional setting. Evidence for tidal influence is lacking. Instead the importance of storm activity, probably on a shoreface, is emphasized. The absence of articulate brachiopods and other open marine faunal elements in the Downton strata is attributed to changes in salinity in the restricted Downton sea shortly before its elimination by Acadian uplift. The boundary between the Upper Whitcliffe Formation (Ludlow) and the overlying Downton Castle Sandstone Formation (Pridoli), formerly proposed as the stratotype section for the base of the fourth Series of the Silurian System, is designated at Ludford Corner, Ludlow, Shropshire [SO 51237413] (Fig. 1, Bassett et al. 1982 and references therein). It has received considerable attention, with particular emphasis on faunal changes through the section. In addition to its stratigraphic significance, the section occurs at a crucial position in the transition from marine to non-marine environments associated with the Acadian (late Caledonian) inversion of the Welsh Basin ( Woodcock et al. 1988 ). The Downton Castle Sandstone Formation in the Ludlow area overlies shelly calcareous siltstones of the top Ludlow, Upper Whitcliffe Formation and comprises a coarsening and cleaning-upward sequence, about 15 to 20 m thick ( Bassett et al. 1982 ). Coastal mudflat deposits of the Temeside Shale Formation lie directly above but are not seen at Ludford corner. The basal part of the