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Dalradian

Chronostratigraphy The Dalradian Supergroup of Scotland and Ireland is a metasedimentary and igneous rock succession that was deposited on the eastern margin of Laurentia between the late Neoproterozoic (~800 Ma) and Early Cambrian (~510 Ma).

Dating the succession, using lithostratigraphic, radiometric and chemostratigraphic has improved our knowledge of the Proterozoic ocean chemistry and the environmental conditions of deposition within the Dalradian sedimentary basin. Most models for Dalradian deposition invoke a long, shallow-marine, ensialic basin which underwent prolonged extension during the late Neoproterozoic, resulting in the eventual separation of Laurentia from western Gondwana at ca. 550 Ma. Extensional tectonics played a major role in the genesis of the upper portions of the Dalradian basin during the latest Neoproterozoic to Early Cambrian. Correlation of the Dalradian Supergroup is hampered by the paucity of volcanic horizons suitable for U-Pb geochronology and the lack of biostratigraphically diagnostic fossils. Also, many portions of the Dalradian exhibit extreme facies variability along strike, and have experienced complex polyphase deformation and metamorphism. Despite these issues, a coherent lithostratigraphy has been established from western Ireland to the Shetland Islands.

The Dalradian Supergroup consists of four groups which are from oldest to youngest; the Grampian, Appin, Argyll and Southern Highland groups. The Grampian Group consists of up to 7 km of predominantly marine, quartzo-feldspathic psammites and semi-pelites, displaying sharp lateral variations typical of a syn-rift origin. The Appin Group consists of up to 4 km of quartzite, semipelites and phyllites deposited as a post-rift, thermal subsidence sequence. The overlying Argyll Group records rapid deepening of the basin following the shallow marine conditions of the Appin Group, and consists of a heterogeneous succession of shelf sediments which passes upwards into deep water turbidite and basinal facies and associated mafic volcanics. The marked change from a shelf setting to deep-water sedimentation is widely attributed to the onset of syn-depositional rifting. The basal subgroup (Islay Subgroup) of the Argyll Group is marked by a distinctive and persistent tillite horizon; the Port Askaig Formation (probably related to the Sturtian glaciations at ~650 Ma), correlatives of which are traceable from Connemara in western Ireland to Banffshire in NE Scotland. The upper part of the Argyll Group is marked by the Tayvallich Volcanic Formation, a sequence of basaltic pillow lavas and hyaloclastite and airfall tuffs. An eruption of of 601 ± 4 Ma has been obtained from the U-Pb dating of zircons from a felsic tuff. The Southern Highland Group marks the top of the Dalradian succession and consists of ca. 4 km of coarse-grained turbiditic clastics and volcaniclastic strata, and represents the change from a period of continental rifting and rupture to that of a thermally subsiding margin (extracts part from Rooney al. 2011).

Further Information
Prave, A.R., Fallick, A.E., Thomas, C.W. & Graham, C.M. (2009). A composite C-isotope profile for the Neoproterozoic Dalradian Supergroup of Scotland and Ireland. Journal of the Geological Society, London, 166, 845-857.

Rooney, A.D., Chew, D.M., Selby, D., (2011). Re-Os geochronology of the Neoproterozoic – Cambrian Dalradian Supergroup of Scotland and Ireland: Implications for Neoproterozoic stratigraphy, glaciations and Re-Os systematics, Precambrian Research, doi:10.1016/j.precamres.2011.01.009.

Strachan, R.A., Smith, M., Harris, A.L. & Fettes, D.J. (2002). The Northern Highland and Grampian terranes. In: Trewin, N. (ed) Geology of Scotland  (4th edition). Geological Society, London, 81-147.
(CS, MWH, RS)