Sedimentology a Grenvillian Neoproterozoic foreland basin succession in northern Scotland: a new combined interpretation

Bonsor, H.; Krabbendam, M.; Strachan, R.; Prave, T.. 2018 Sedimentology a Grenvillian Neoproterozoic foreland basin succession in northern Scotland: a new combined interpretation. [Lecture] In: 57th BSRG Annual General Meeting, Edinburgh, UK, 17-20 Dec 2018. British Geological Survey. (Unpublished)

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Abstract/Summary

The early Neoproterozoic Moine and Torridonian rocks, which occupy much of the Northern Highlands in Scotland, have traditionally been interpreted as separate entities, deposited in distinct early Neoproterozoic rift basins. The rocks are now divided across the later Caledonian Moine thrust structure: the Torridonian rocks (>6 km thick) occur in the Caledonian foreland, and the Moine rocks (>3km thick) in the hanging wall of the Moine thrust. New work, based on structural geology, sedimentology and detrital mineral dating presents evidence for these successions to be now correlated, and to have formed in a single foreland basin in front of the Grenville Orogen. As such the deposits provide arguably the largest preserved and most accessible record of the Grenvillian foreland basin. This paper presents the sedimentological data collected over the last 10 years across the Neoproterozoic succession in northern Scotland, to present a new combined stratigraphic interpretation of these rocks. Taken collectively, the early Neoproterozoic sequence is ca. 9km thick and displays alternating fluvial, and tidally-influenced shoreface lithofacies, within several cycles of progradation and retrogradation. Five main phases of sedimentological evolution are interpreted: (1) A progradational sucession from tidally-influenced shoreface facies to fluvial facies at the base of the succession (Sleat Group). (2) An abrupt but (near|)conformable switch to large-scale progradation with rapid and sustained high-energy braided river deposition (Applecross Fm and lower Morar group). (3) These high-energy fluvial lithofacies gradually are replaced upwards by lower-energy braidplain fluvial facies, or tidally-influenced braidplan and shoreface facies further east within a gradual large-scale (ca. 3km thick) retrogradational succession (Aultbea Fm and middle Morar Group). (4) This retrogradational phase is indicated to have culminated in a relatively rapid transgression and marine deposition (upper Morar Group). (5) A further progradation and retrogradation succession sequence is preserved in the uppermost Morar Group. The lithofacies suggest overall lower-energy depositional conditions in this phase - from with shallow-marine to tidally-influenced distal braidplain depositional settings. The final retrograding succession is followed by a second transgression to shallow marine conditions resulting in deposition of pelite. This represents the last phase of deposition preserved in upper most parts of the early Neoproterozoic rocks in Northern Scotland. These five main phases of sedimentological evolution provide insight to the development of the depositional environments within the Grenville foreland basin, resulting from changes in the basin form, accommodation space and sediment flux.