Crawford Group, Ordovician, Southern Uplands

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Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.

Crawford Group

Representative structural-stratigraphical profiles across the Ordovician–Silurian Southern Uplands terrane. P912326. Part 1.
Representative structural-stratigraphical profiles across the Ordovician–Silurian Southern Uplands terrane. Part 2. P912327.

The oldest rocks found in the Southern Uplands belong to the Crawford Group and are of late Arenig to Llanvirn age. They comprise the dominantly volcanic and chert successions of the Raven Gill and Kirkton formations, which underlie the Moffat Shale Group in the Northern and Central belts (P912326 and P912327).

The Raven Gill Formation (55–70 m) consists of an interbedded succession of basaltic pillow lavas, blue-grey radiolarian cherts and fossiliferous brown mudstones, with sheet-like dolerite intrusions or lava flows. The stratigraphical limits of the formation are unclear since its southern margin (presumed base) is faulted and no overlying unit is seen, though it is possible that it may be enclosed within the Kirkton Formation (see below) as an olistostrome unit. The brown mudstones at the Raven Gill type locality (NS 9204 1989) have yielded fragmentary graptolites and a conodont assemblage of late Arenig age. Only three proven outcrops are known, all within a strike length of about 2 km in the Abington area. There they form part of an extensive linear belt of black shale and chert (mostly Moffat Shale Group) inliers, up to 1.8 km wide across strike, which crop out within the imbricate Leadhills Fault Zone. These inliers are interpreted as the thrust-faulted repetition of a single, thin basal succession that may have originally been in conformable contact with the (overlying) Kirkcolm Formation.

Pillow lavas and chert are exposed at several other localities within the Leadhills Fault Zone, as well as in association with the analogous Fardingmullach Fault farther south, and have previously been assumed to correlate with those at Raven Gill. However, recent studies have shown that there is considerable variation in the geochemical characteristics of the lavas. For example, samples from the Leadhills Fault Zone at Abington (NS 9295 2141) exhibit the characteristics of mid-ocean-ridge basalts (MORB), whereas those from the Fardingmullach Fault Zone at Gabsnout Burn (NX 2020 6116) near Glenluce are chemically more like island-arc basalts. It seems likely that the lavas underlying the Moffat Shale Group were erupted during several distinct and unrelated volcanic episodes.

The Kirkton Formation (>120 m) is a sequence of red and grey cherts and red and green siliceous mudstones with chert nodules that appears to lie between the Raven Gill Formation and the Glenkiln Shale (Moffat Shale Group) within the Leadhills Fault Zone. Similar cherts crop out at other localities throughout the Northern Belt along the tract-bounding faults including, for example, at Normangill Burn (NS 971 241) north-east of Abington, at numerous localities in the Leadhills area, and at Morroch Bay (NX 017 524) south of Portpatrick on the Rhins of Galloway. North of Portpatrick, adjacent to the Glaik Fault (NW 976 588) the chert is associated with hyaloclastite containing broken lava pillows. At many of these localities the cherts yield conodont faunas of late Llanvirn and early Caradoc age. Farther south, the Kirkton Formation is represented by volcaniclastic rocks and chert, underlying apiculatus-ziczac Subzone mudstone, that crop out adjacent to the Laurieston Fault in a linear zone extending for about 7 km north-eastward from Crossmichael (NX 735 670).

Curiously, there is a complete absence from the Crawford Group of any evidence for the well-established conodont zones intervening between the upper Arenig and the upper Llanvirn. This absence, together with the restricted occurrence of the Raven Gill Formation and its uncertain relationship with the Kirkton Formation, suggests that the Raven Gill Formation may simply be one or more large olistostromes within the Kirkton Formation.


Bibliography

Clarkson, E N K, Harper, D A T, Owen, A W, and Taylor, C M. 1992. Ordovician faunas in mass-flow deposits, Southern Scotland. Terra Nova, Vol. 4, 245–253.

Duller, P R, and Floyd, J D. 1995. Turbidite geochemistry and provenance studies in the Southern Uplands of Scotland. Geological Magazine, Vol. 132, 557–569.

Elders, C F. 1987. The provenance of granite boulders in conglomerates of the Northern and Central Belts of the Southern Uplands of Scotland. Journal of the Geological Society of London, Vol. 144, 853–863.

Floyd, J D. 2001. The Southern Uplands Terrane: a stratigraphical review. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 91, 349–362.

Floyd, J D, and Kimbell, G S. 1995. Magnetic and tectonostratigraphic correlation at a terrane boundary: the Tappins Group of the Southern Uplands. Geological Magazine, Vol. 132, 515–521.

Floyd, J D, and Rushton, A W A. 1993. Ashgill greywackes in the Southern Uplands of Scotland: an extension of the Ordovician succession in the Northern Belt. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 84, 79–85.

Floyd, J D, and Trench, A. 1989. Magnetic susceptibility contrasts in Ordovician greywackes of the Southern Uplands of Scotland. Journal of the Geological Society of London, Vol. 146, 77–83.

Kemp, A E S. 1986. Tectonostratigraphy of the Southern Belt of the Southern Uplands. Scottish Journal of Geology, Vol. 22, 241–256.

Leggett, J K. 1987. The Southern Uplands as an accretionary prism: the importance of analogues in reconstructing palaeogeography. Journal of the Geological Society of London, Vol. 144, 737–752.

Merriman, R J, and Roberts, B. 1990. Metabentonites in the Moffat Shale Group, Southern Uplands of Scotland: Geochemical evidence of ensialic marginal basin volcanism. Geological Magazine, Vol. 127, 259–271.

Morris, J H. 1987. The Northern Belt of the Longford–Down Inlier, Ireland and Southern Uplands, Scotland: an Ordovician backarc basin. Journal of the Geological Society of London, Vol. 144, 773–786.

Phillips, E R, Smith, R A, and Floyd, J D. 1999. The Bail Hill Volcanic Group: alkaline withinplate volcanism during Ordovician sedimentation in the Southern Uplands, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 89, 233–247.

Phillips, E R, Evans, J A, Stone, P, Horstwood, M S A, Floyd, J D, Smith, R A, Akhurst, M C,and Barron, H F. 2003. Detrital Avalonian zircons in the Laurentian Southern Uplands terrane, Scotland. Geology, Vol. 31, 625–628.

Stone, P, and Evans, J A. 2001. Silurian provenance variation in the Southern Uplands terrane, Scotland, assessed using neodymium isotopes and linked with regional tectonic development. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 91, 447–455.

Stone, P, Breward, N, Merriman, R J, and Plant, J A. 2004. Regional geochemistry of cryptic geology: variations in trace element distribution across the Southern Uplands terrane, Scotland. Applied Earth Science (Transactions of the Institution of Mining and Metallurgy B), Vol. 113, B43–B57.

Stone, P, Breward, N, Merriman, R J, and Barnes, R P. 2006. The interpretation and application of regional geochemistry: lessons from the Paratectonic Caledonides. Scottish Journal of Geology, Vol. 42, 65–76.

Tucker, R D, Krogh, T E, Ross, R J, and Williams, S H. 1990. Time-scale calibration by high-precision U-Pb zircon dating of interstratified volcanic ashes in the Ordovician and Lower Silurian stratotypes of Britain. Earth and Planetary Science Letters, Vol. 100, 51–58.

Waldron, J W F, Floyd, J D, Simonetti, A, and Heaman, L M. 2008. Ancient Laurentian detrital zircon in the closing Iapetus Ocean, Southern Uplands Terrane, Scotland. Geology, Vol. 36, 527–530.

Warren, P T. 1964. The stratigraphy and structure of the Silurian rocks south-east of Hawick, Roxburghshire. Quarterly Journal of the Geological Society of London, Vol. 120, 193–218.