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

Abstract

The Tonian–Cambrian Dalradian Supergroup in Scotland is a siliciclastic–carbonate succession that can be up to 10 km thick. The consensus view is that its lower part, the mid- to late Tonian Grampian and Appin groups, formed in rift basins: the deep marine turbidites of the Grampian Group infilled rift depocentres, whereas the shallow marine strata of the Appin Group mark basin-bounding palaeohighs. This scenario is used as a key line of evidence to infer the onset of the break-up of Rodinia between Laurentia and Baltica. However, deformation during the mid-Ordovician Caledonian Orogeny obscured the original depositional frameworks. Reconstructing these frameworks (and hypothesized rift basins) has relied on the trace and major element log-ratio geochemistry of minor carbonate rocks to assign the units to either the Grampian or Appin group – that is, to rift depocentres or basin-bounding palaeohighs, respectively. We report new carbon and oxygen isotope and geochemical data and use these 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 the deposition of the early Cryogenian Argyll Group. Supplementary material: Geochemical analysis of samples is available at https://doi.org/10.6084/m9.figshare.c.6317830 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle