Gauging the impact of glacioeustasy on a mid-latitude early Silurian basin margin, mid Wales, UK

The early Silurian (Llandovery) Gondwanan South Polar ice sheet experienced episodes of ice retreat and re-advance. Marine base level curves constructed for the interval are widely assumed to provide a record of the associated glacioeustasy. In revealing a series of progradational sequences (progrades) bounded by flooding surfaces, recent work on the Type Llandovery succession in mid Wales (UK) has provided an opportunity to test this hypothesis. The grouping of these progrades into three composite sequences underpins the construction of both low order (small amplitude, high frequency) and high order (large amplitude, low frequency) base level movement curves. Revised biostratigraphical datasets for the type succession permit the accurate dating of base level events. The composite sequences record progradational acmes in the acinaces, lower convolutus and upper sedgwickii-halli graptolite biozones. A series of transgressions that postdate the Hirnantian glacial maximum culminated in an upper persculptus Biozone high-stand. Maximum flooding events also occurred during the revolutus and lower sedgwickii biozones, and the base of the early Telychian guerichi Biozone also marked the onset of a pronounced deepening. A review of 62 published datasets, including global and other regional base level curves, records of glacial activity, isotope data, patterns of facies and faunal flux and putative climate models, permits an evaluation of the origins of these local base level events. The concept of a Eustasy Index is introduced and shows that the impacts of global sea level movements can only be demonstrated within narrow ‘eustatic windows’ coincident with times of ice sheet collapse. At other times, the geometry of Llandovery area progrades reflects their accumulation across a faulted basin margin where, during periods of slow ice sheet advance, epeirogenic processes outstripped sea level movements as the dominant forcing factors. Increased levels of Telychian subsidence at first enhanced and then overwhelmed the influence of glacioeustasy as part of the region's response to the north European Scandian deformation.