New insights into the Quaternary evolution of the Bristol Channel, UK

Abstract

A synthesis of new publically available borehole and bathymetric data, combined with a wealth of other existing disparate data sources, reveals new insights into the Quaternary history of the Bristol Channel area. Sediment boreholes throughout the Bristol Channel confirm the area was glaciated in the Pleistocene. Till is present below marine deposits and, in some areas, is visible morphologically as submerged moraines. In the central and eastern Bristol Channel the submerged valley course of the palaeo‐Severn is very clear in new high‐resolution bathymetric surveys. This former river course and associated tributaries cross‐cut through glacial sediments in the Bristol Channel. At least three phases of glaciation are recorded in the Bristol Channel, one related to the southern limits of a Late Devensian Substage (∼Marine Isotope Stage (MIS) 2) Welsh Ice Cap which reached into Swansea Bay, an earlier Devensian (MIS 4–3) glaciation associated with Irish Sea ice, and another older glaciation that is associated with ice that filled the entire outer and central Bristol Channel. The age of the older Bristol Channel glaciation is still open, although it pre‐dates the Devensian (Late Pleistocene) and must date to the Middle Pleistocene. It is therefore evident that Pleistocene glacial and fluvial activity, combined with subsequent post‐glacial sea transgression, directly account for current morphometries of the Bristol Channel and Severn Estuary, and the current geography of the SW British Isles.

Re‐assessment of the age and depositional origin of the Paviland Moraine, Gower, south Wales, UK

“…Third, recent bathymetric data (Gibbard et al . ) reveal the probable LGM end moraine to an ice lobe filling Swansea Bay, supporting previous sea‐floor sediment investigations (Blackley ; Culver & Bull ), recent views about LGM ice encroachment onto southeast Gower (Hiemstra et al . ) and icecap modelling (Patton et al .…”

“…Confinement of the southern limit of the ice lobe to the peninsula is supported by: (i) glacigenic sediment in dry valley fans between Eastern Slade and Oxwich Point being relatively small in calibre, sparse and mostly mixed with limestone head, contrasting with fans immediately adjacent to the moraine where it occurs as thick discrete lenses that include some coarse clasts and a mixed glacigenic–limestone diamict; and (ii) recent bathymetric data showing no evidence of moraine development offshore from west Gower (Gibbard et al . ). We attribute the subdued morphology of the W–E section of the moraine in large part to the influence of an extensive network of sinks and passages in the underlying limestone (J. Cooper, pers.…”

Maximum extent and readvance dynamics of the Irish Sea Ice Stream and Irish Sea Glacier since the Last Glacial Maximum

“…1; cf. Rolfe et al ., 2012) although the nature of any direct evidence of glaciation and associated timings in the outer Bristol Channel remain equivocal (Carr et al ., 2017; Gibbard et al ., 2017; Hiemstra et al ., 2019). However, the distribution of glacigenic sediments and landforms on Scilly (Scourse, 1991; Hiemstra et al ., 2006; Smedley et al ., 2017a) is difficult to explain without a major portion of the ISIS east flank overriding at least the western part of the Lundy Platform (Lockhart, 2019).…”

Dynamics and stratigraphy of a tidal sand ridge in the Bristol Channel (Nash Sands banner bank) from repeated high‐resolution multibeam echo‐sounder surveys

“…Gibbard et al . (2017) showed outer end moraine underlying westerly Nash Sands, which correspond with the undulating morphology marked ‘glacial till’ in Fig. 2.…”

“…During the Flandrian transgression (3 to 5 ka BP), sea level reached its present level in the Bristol Channel (Jennings et al, 1998) and the modern tidal regime became established (Stride & Belderson, 1990). Gibbard et al (2017) showed outer end moraine underlying westerly Nash Sands, which correspond with the undulating morphology marked 'glacial till' in Fig. 2.…”