Stratigraphic model of the Quaternary sediments of the Western Irish Sea Mud Belt from core, geotechnical and acoustic data

A new geotechno-stratigraphic model for Quaternary deposits in the Western Irish Sea Mud Belt area is presented. This area, located in the North Irish Sea, has been heavily influenced by the advance and retreat of the British and Irish Ice Sheet during the last glaciation and subsequent Holocene marine transgression. This study uses a synthesis of lithostratigraphy, geotechnical data (from in situ cone penetration testing) and seismic profiles to generate a stratigraphic framework describing the geometry, distribution and characteristics of Quaternary deposits in an area of the Western Irish Sea Mud Belt at increased resolution. Within a regional context, this stratigraphy is compared with other established offshore stratigraphies in the Irish Sea and terrestrial successions. Four stratigraphic units are identified consisting of abasal subglacial (lodgement) till (Unit 4) emplaced by the Irish Sea Ice Stream. Unit 4 overlies irregular bedrock and is overlain in turn by glaciomarine to glaciolacustrine, ice-proximal outwash and glaciomarine ice-proximal sand and sandy muds (Units 3 and 2 respectively; previously undifferentiated). A thick succession (up to 27m) of Holocene marine muds (Unit 1) caps the sequence up to the contemporary seabed. Results show that these deposits, and their geotechnical properties, have significant implications for the anthropogenic use of the area, such as the development of offshore renewable energy infrastructure. Revealed are relatively thick (up to 30m) weak, under-consolidated sediments at the surface and highly heterogeneous, often over-consolidated, sediments that have limited groundtruthing at depth. Furthermore, localised shallow gas is imaged on seismic profiles. The influence on the geotechnical properties of the sediments by this gas revealed no significant effect on shear strength values from cone penetration testing data, although it may have implications for the long-term behaviour of the sediments.