Quaternary deposits and glacial stratigraphy in Ireland

Abstract

Quaternary stratigraphic studies in Ireland have been based largely on lithostratigraphy. The division of complex sedimentary sequences into the traditional framework of Munsterian and Midlandian events has created a series of stratigraphic scenarios which cannot be fully substantiated either by biostratigraphy or by facies basin analysis. Many of the current problems of glacial stratigraphy in Ireland therefore arise directly from the field procedures used and the frameworks used to subdivide sedimentary sequences. These problems are centred on the selection of type-sites, the subdivision of sedimentary sequences into unnatural divisions, the concepts of ‘direct’ and ‘indirect’ stratigraphy, classical models of climatically determined ice advance/retreat cycles and the genetic inferences of the term till. The conceptual basis of various stratigraphic schemes has changed little since the work of Farrington (1934–1966). It is argued that a more realistic approach to description and assessment of glacigenic sediments should be based on basinal facies analysis and must include a documentation of the composition and geometry of three dimensional facies variations. Palaeoenvironmental interpretations of complex glacigenic sequences, especially in coastal locations, should take account of the influences of marine ice sheets on the sedimentary record. In the past most sequences were interpreted in terms of glacioterrestrial models. A large body of sedimentary evidence now indicates that shallow, glaciomarine environments occurred along the margins of the last ice sheet during its maximum and subsequent retreat phases. The locations and patterns of associated icemarginal systems indicate that the magnitude of, and patterns of crustal deformation were greater and more complex than formerly realised. Thus the primary control on sea-level variation at successive ice sheet margins was largely a function of glacioisostatic disequilibrium. Although complex glacial and glacially-related sequences are known it is argued here that most of the multiple glacigenic sequences in Ireland can be accommodated within the Midlandian Cold Stage. The basal units from certain sequences and erratic fans have been attributed to events earlier than the Midlandian. However, it is difficult to compare these traces of earlier events from different sectors of the island due to poor dating correlation and the fact that ‘direct’ and ‘indirect’ stratigraphy is not adequately defined or separated. The age of the Gortian-style interglacial sequences, the implications of their floristic composition and their relationships to drift stratigraphy remains a major problem in stratigraphic constructions of the Irish Quaternary. The stratigraphic significance of many till units must remain largely unknown until these matters are resolved. At present it is not clear if the Gortian sequences represent more than one interglacial. A series of exposures in Ulster provide evidence for major environmental changes during the Midlandian Cold Stage. These include: an early stadial, an early forest biozone (>48.1 ka BP), an early herb biozone (>41.5 ka BP), a middle ‘interstadial’ phase of cold but non-glacial aspect (30.5 ka BP), a late stadial (max. 24-22 ka BP) and a late glacial climatic oscillation. The Late Midlandian stadial comprises a major glaciation that almost covered the island and, a later, more restricted phase when drumlins formed in the northern and western parts of the island (max. 17 ka BP).