Evidence, or not, for late Tonian break-up of Rodinia? The Dalradian Supergroup, Scotland

• Published in: Journal of the Geological Society Vol. 180; no. 2; p. 1
• Main Authors: Prave, A. R, Fallick, A. E, Kirsimae, K
• Format: Journal Article
• Language: English
• Published: Oxford Geological Society of London 03-03-2023; The Geological Society of London; Geological Society Publishing House
• Subjects: Appin Group; Argyll Group; Basins; Cambrian; Carbonate rocks; Carbonates; Cryogenian; Dalradian; Deformation; Europe; Flysch; Frameworks; Geochemistry; Geophysics; Grampian Group; Grampian Highlands; Great Britain; Isotopes; Knoydartian Orogeny; Neoproterozoic; Ordovician; Orogeny; Oxygen; Oxygen isotopes; plate tectonics; Precambrian; Precambrian studies; Proterozoic; Rodinia; Scotland; Scottish Highlands; sedimentary rocks; solid Earth (tectonophysics); Stratigraphy; Tonian; Turbidites; United Kingdom; upper Precambrian; Western Europe; United Kingdom > UK; Scotland
• ISSN: 0016-7649; 2041-479X
• DOI: 10.1144/jgs2022-134

The Tonian-Cambrian Dalradian Supergroup, Scotland, is a siliciclastic-carbonate succession as much as 10 km thick. The consensus view is that its lower part, the mid-late Tonian Grampian and Appin groups, formed in rift basins: deep-marine turbidites of the former infilled rift depocentres and shallow-marine strata of the latter marked basin-bounding palaeohighs. That scenario is used as a key line of evidence to infer the onset of break-up of Rodinia between Laurentia and Baltica. However, deformation during the mid-Ordovician Caledonian Orogeny obscured original depositional frameworks. Reconstructing those frameworks (and hypothesised rift basins) has relied on trace- and major-element log-ratio geochemistry of minor carbonate rocks to assign units to either the Grampian or Appin groups, i.e. to rift depocentres or basin-bounding palaeohighs, respectively. We report new carbon and oxygen isotope and geochemical data to create a revised stratigraphic framework for the Grampian and Appin groups. Our findings show that the previous geochemical-based correlations are unreliable and that there is no evidence for palaeohighs or rift basins. Instead, the Grampian-Appin groups are a deeper-marine flysch to a shallower-marine molasse succession formed in response to the mid-Tonian Knoydartian Orogeny. From a Scottish perspective, evidence for the break-up of Rodinia is recorded higher in the Dalradian succession during deposition of the early Cryogenian Argyll Group. Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wils on-cycle Supplementary material: https://doi.org/10.6084/m9.figshare.c.6317830