Various styles of deposition of Pleistocene glacigenic deposits in Essex, England

Abstract

Late Ordovician siliciclastic glacial and related deposits on the North Gondwana continental shelf comprise striking incised features referred to as large-scale channel-fill structures. These are described from exposures and aerial photographs in two areas (Adrar and Hodh) in Mauritania. The channel-fill structures are up to several kilometres in length and several hundred metres wide. They are slightly sinuous narrow sandstone bodies deeply incised into the Late Ordovician glacial drift and the Cambro–Ordovician bedrock. Palaeogeographical reconstructions indicate that these structures were located in the ice-marginal zone. They are preferentially oriented parallel to palaeo-ice-flow directions. Sedimentary facies analysis reveals three vertically stacked architectural units infilling a U-shaped erosional basal unconformity. Unit 1 is thin and made up of conglomeratic sandstones of various origins comprising debris-flows, deltaic-like progradational foresets and trough cross-stratified sands. In Adrar, architectural unit 2 shows sheet-like, evenly laminated, fine- to medium-grained, well sorted sandstones characteristic of a high-energy marine environment. In contrast, in the Hodh area, unit 2 displays vertically stacked successions of coarse-grained sandstones that indicate high-discharge sediment-laden flows; these are interpreted as meltwater sediments. In both Adrar and Hodh, architectural unit 3 is made up of gravelly coarse-grained, trough cross-stratified sandstone emplaced in a braided, low sinuosity fluvial environment. The three-stage infilling history and the palaeogeographic location combined with size and shape criteria make it possible to compare the channel-fill structures with Pleistocene tunnel-valleys found in areas of low-relief in cool temperate and low Arctic zones. Such features are incised by high-pressure subglacial meltwater and later infilled by proglacial to postglacial deposits. Based on this comparison, architectural unit 1 represents subglacial or proglacial outwash, high sediment discharge in unit 2 took place in a proglacial environment in the Hodh area while an isostatically downwarped shelf was responsible for a marine incursion in the Adrar channels, and deposits in unit 3 are fluvial postglacial sediments. Identification of preserved channel-fill structures may be useful in reconstructing Late Ordovician ice-sheet dynamics.