Evidence for a major Neoproterozoic orogenic unconformity within the Dalradian Supergroup of NW Ireland

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doi: 10.1144/0016-764903-094
Authors:Hutton, D. H. W.; Alsop, G. I.
Author Affiliations:Primary:
University of Birmingham, Earth and Environmental Sciences, Birmingham, United Kingdom
Other:
University of St. Andrews, United Kingdom
Volume Title:Journal of the Geological Society of London
Source:Journal of the Geological Society of London, 161(4), p.629-640. Publisher: Geological Society of London, London, United Kingdom. ISSN: 0016-7649
Publication Date:2004
Note:In English. 59 refs.; illus., incl. 3 plates, geol. sketch maps
Summary:An intriguing and contradictory scenario has recently developed in the Caledonides of the British Isles concerning the ages of deposition and deformation of the Neoproterozoic Dalradian Supergroup. Isotopic evidence, although limited, suggests that the lower parts of the sequence may have been deposited and undergone Precambrian deformation (i.e. pre-600 Ma) prior to deposition of the upper parts of the sequence (i.e. post-600 Ma). Given these existing constraints, it is clear that a major break (or breaks) would be required in the intervening sequence to maintain a coherent tectonostratigraphy. We present evidence for such an unconformity preserved at the base of the Easdale Subgroup in NW Ireland. Reworked clasts that contain a pre-existing tectonometamophic history are identified within a conglomerate that lies along this regionally recognized boundary. The underlying sequence also exhibits pre-existing deformational fabrics that display erosional truncation at the base of the conglomerates. These relationships, together with significant erosion and excision of the footwall sequence, and extensive thickness and facies variations in the hanging-wall units, imply that a major tectonic unconformity exists within this succession. As the conglomerate lies stratigraphically below Precambrian (c. 600 Ma) lavas, the structural fabrics contained within the clasts and the underlying sequence must also be of Precambrian age and totally unrelated to the well-established Early Palaeozoic (Caledonian) orogenic deformation observed in the younger parts of the sequence. The Dalradian Supergroup, as defined, may actually comprise (at least) two distinct tectonostratigraphic sequences.
Sections:Petrology
Subsections:General
Subjects:Dalradian; Deformation; Depositional environment; Fabric; Facies; Faults; Folds; Geochemistry; Isotopes; Lithostratigraphy; Mapping; Metamorphic rocks; Mineral composition; Neoproterozoic; Orogeny; Paleoenvironment; Petrography; Precambrian; Proterozoic; Sedimentary rocks; Sedimentation; Structural analysis; Tectonics; Tectonostratigraphic units; Unconformities; Upper Precambrian; Caledonides; Europe; Great Britain; Ireland; United Kingdom; Western Europe; Appin Group; Argyll Group; Colonsay Group; Easdale Subgroup; Grampian Group; Rhinns Complex; South Highland Group; Termon Formation
Coordinates:N540000 N553000 W0060000 W0080000
Abstract Numbers:04M/2848
Record ID:2004061367
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from The Geological Society, London, London, United Kingdom
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