More gap than record? Qualitative and quantitative assessment of stratigraphic gaps in a field based study, with examples from the Lower Silurian Pentamerus Beds of Shropshire, England and the Lower Ordovician Ribband Gp of County Wexford, Ireland

Abstract

To assess the extent to which it is possible to identify, characterize and quantify gaps in the stratigraphic record, two contrasting sections, the Silurian Pentamerus Beds of Shropshire, England, and the Ordovician Ribband Gp of County Wexford, Ireland, have been examined in the field. In the Pentamerus Beds the life assemblages contained in silty mudstones suggest slow to little/no deposition during fair weather periods. Interbedded lenticular calcirudites characteristically feature death assemblages of thick-shelled brachiopods and are interpreted as storm deposits, implying erosional breaks in accumulation. In contrast, the Ribband Gp's regular ribbon banding of fine sandstones, siltstones and mudstones/silty mudstones suggests a more distal shelf environment. Fodinichnial trace fossils are witness to periods of more or less continuous sedimentation. The succession may have formed during seasonal alternations of stormy (thin cross-laminated sandstone beds) and fair weather periods (laminated mudstones); though longer-term climatic variations cannot be ruled out. There is minimal evidence for hiatuses in the accumulation.