Sedimentological reconstruction of Glan-y-môr Isaf, North Wales: A model for the formation of stratified subglacial till assemblages from glaciolacustrine deposits

Abstract

Reconstruction of an understudied region of North Wales, north of the Llŷn Peninsula, redefines the dynamics and interaction of the Welsh Ice Cap and Irish Sea Ice Stream during the deglaciation of the last British-Irish Ice Sheet. Sedimentological research highlights the role of topography in creating distinct glacial environments as the Irish Sea Ice Stream retreated northwards from North Wales. Insights are gained into the production of stratified tills from previously deposited glaciolacustrine sediments and outwash sandur in a region where an ice-margin advanced over the site. Sedimentary analyses, including data on clast form, fabric, and diamicton particle size, are used to produce lithofacies profiles from which interpretations can be made into the styles of deposition and environmental change. Multiphase deposition, initially from Welsh ice flowing northwest, details a transition from a subglacial traction till to a distal, ice-marginal glaciolacustrine setting with laminated varves marking retreat. Above, sands and gravels formed by gravity flow, turbidity currents, and outwash document ice advance, then are capped by another subglacial traction till composed of Irish Sea deposits.

Sediments capture one of many oscillations of the Irish Sea Ice Stream along the North Wales coast while the ice remained anchored on Anglesey. Sedimentological analysis fortifies evidence that the last ice sheet, especially its ice stream outlets, was highly dynamic and oscillatory, responded heavily to topography, and created various ice-marginal environments during deglaciation.

This study presents a four-stage model for the formation of stratified tills at Glan-y-môr Isaf, beginning with initial glaciolacustrine sedimentation, ice then advanced towards the site, and finally overrode the pre-existing sediments. Processes forming stratified tills began with the contemporaneous deposition of outwash and deformation via glaciotectonism. Till stratification occurred from the cannibalisation of pre-existing laminated material into a subglacially deforming till layer, which laterally homogenised sediments over short transport distances and continued to occur well after ice had readvanced into the area. Crucially, sediments portray the complex behaviour of ice sheets in forming glacial sediments and provide knowledge that must be taken into consideration when modelling contemporary ice sheets.