The late devensian and holocene evolution of Barmouth Bay, Wales

A marine seismic and sedimentological investigation of the Late-Devensian and Holocene sediments of Barmouth Bay, North Wales, was undertaken to reconstruct late-glacial and Holocene sedimentary environments, and to evaluate post-glacial sedimentation rates. In the last glacial, the Devensian, many Welsh estuaries were the sites of valley glaciers flowing from the Welsh Ice cap onto the modern inner-shelf. At Barmouth, the Mawddach valley glacier flowed into the present inner-shelf of Cardigan Bay. The modern courses of the Mawddach Estuary and the river Gwril were overdeepened by erosive sub-glacial flows, and are later infilled by a coarse-grained sub-glacial and pro-glacial infill sequence. With late-glacial relative sea-level rise and retreat of the Welsh Ice, the Mawddach valley tidewater glacier became grounded and confined to the valley. Re-equilibration of the ice profile occurred, and a morainal complex was formed at its toe. In the early Holocene, the morainal complex formed the seaward boundary of a sheltered nearshore basin where fine-grained, partly organic, sediments were deposited in a lagoonal or estuarine environment. Holocene reworking of the sediments by shallow-marine processes has formed a lag surface to the morainal complex, and has introduced gravel of Irish Sea Ice character from offshore. Wave action has formed a shingle coastal barrier, narrowing the mouth of the modern estuary. A shoreface-attached sand wedge, which attains a maximum thickness of 4 m beneath the modern ebb-tidal delta, overlies the lag surface and onlaps the coastal barrier. Since 8–9 ky BP, the mean sediment accumulation rate of estuarine and shallow-marine sediments has been 2.3-3.1 mm/yr, roughly equivalent to the long-term mean rate of sea-level rise for Cardigan Bay. This mean rate masks rapidly increasing accumulation rates in the Mawddach Estuary in the last 5–6 ky, which now may be 82 mm/yr, and which are due to reworking of shoreface and barrier sands into the estuary under a dynamic tidal regime which has a marked time-velocity asymmetry.