Post-glacial depositional environments in a mid-high latitude glacially-overdeepened sea loch, inner Loch Etive, western Scotland

Abstract

A high-resolution seismic and gravity coring survey has been conducted on inner, and part of outer Loch Etive, a 30 km long, 150 m deep sea loch on the west coast of Scotland. The seismic reflection profiles reveal a 30–60 m thick sediment pile that has accumulated on the floor of the loch since the last (Younger Dryas stadial) glacial re-advance 11–10 ka BP. Four seismic facies have been identified grouped into two seismic sequences, A and B, which are separated by a distinct reflector termed ‘E1’. A highly reflective facies is interpreted as direct river input, an acoustically well-laminated, draped facies interpreted as fine-grained river sediments and a well-laminated to transparent facies interpreted as glaciomarine and glaciolacustrine. Throughout the loch, methane gas-rich sediments occur producing a chaotic seismic facies. The transition from glacially-influenced to river-dominated deposition is displayed by the seismic reflector E1. Core analysis identified eight lithofacies. The oldest sediments are grey muddy sands and olive sandy silts, overlain by younger laminated greeny-grey muddy sand and greeny-grey and greeny-black homogeneous watery sandy mud. Abundant shell material is present throughout, notably the bivalve Arctica islandica, from which a 14C age of 9490±90 yr BP was obtained from the laminated greeny-grey muddy sands. The sediments are interpreted as being of glaciomarine and glaciolacustrine (Younger Dryas to early-mid Holocene) and more modern riverine-derived origin (mid-latest Holocene).