Identification and implications of an orogenic unconformity in the Neoproterozoic Dalradian of Ireland
D.H.W. Hutton1, & G.I. Alsop2
1School of Earth Sciences, University of Birmingham, Birmingham, B15 2TT UK
2
CGG, School of Geography & Geosciences, University of St. Andrews, KY16 9AL UKThe identification of unconformities in ancient orogenic belts containing tectonised Precambrian rocks is severely hindered by the lack of age constraints across units, which reflects the difficul-ties in applying precise isotopic dating techniques to the depositional ages of deformed rocks. In addition, deformation may modify original stratigraphic cut offs across unconformities, whilst structures generated during different periods of orogenesis may display similar orientations and geometries reflecting the inherited tectonic control of the orogenic template. Although the Caledonides of the British Isles represents one of the most intensively studied portions of an orogenic belt, an intriguing and contradictory scenario is now developing whereby the lower parts of the Dalradian were deposited and underwent Precambrian (ie. pre-600 Ma.) deforma-tion prior to deposition of the upper parts of the sequence at post-600 Ma. Given these existing constraints, it is increasingly clear that a major break (or breaks) are required within the inter-vening sequence to maintain a coherent tectono-stratigraphy. Evidence for such an unconformity is preserved at the base of the Easdale subgroup within the Dalradian succession of NW Ireland where reworked clasts that contain a pre-existing tectonometamophic history are identified within conglomerates which lie along this regionally recognised boundary. The sequence under-lying this boundary also contains pre-existing deformational fabrics which display erosional trun-cation at the base of the conglomerates. These relationships imply that a major tectonic uncon-formity exists within this succession which traditionally is regarded as a continuous sequence. As the conglomerate lies stratigraphically below lavas which have consistently given Precambrian dates, the deformation contained within the clasts and underlying sequence must also be of Precambrian age and totally unrelated to that observed in the younger parts of the sequence. The recognition of a tectonic unconformity has a number of implications. a) It allows for a more geologically realistic interpretation of Dalradian stratigraphy in which, far from recording a continuous history of deposition over 300 Ma., the succession actually incorporates significant depositional hiatuses. b) It allows for a more geologically realistic interpretation of evidence for a earlier (pre-600 Ma.) Precambrian orogeny in the lower parts of the Dalradian succession. Thus the lower part of the sequence was deposited, deformed and eroded prior to deposition of the overlying succession. c) The Dalradian Supergroup is actually punctuated by Precambrian orogenesis and in reality, it therefore comprises at least two tectono-stratigraphic sequences.
Caption: Conglomerate containing m-scale sub-angular quar-tzite boulders developed along the unconformity in the Dalradian of NW Ireland. A4 notebook for scale.