Inverclyde Group, Carboniferous of the Tweed and Northumberland–Solway basins
|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.|
Along the southern margin of the Southern Uplands massif the Upper Devonian Stratheden Group passes conformably up into the Inverclyde Group. The base of the latter is taken at Kinnesswood the first appearance of calcrete palaeosol and so is likely to be diachronous; in many places Formation it remains ill defined. The oldest strata of the Inverclyde Group belong to the Kinnesswood Formation of Late Devonian (Famennian) to early Tournaisian (Courceyan) age. The best biostratigraphical resolution is provided by a Courceyan miospore microflora from the Tweed Basin near Eyemouth, but fish macrofaunas from the Eyemouth and Langholm areas could be either Famennian or Courceyan in age.
The Kinnesswood Formation comprises red and yellow-brown sandstones and siltstones with locally developed conglomerate and thin beds of palaeosol containing concretionary carbonate nodules (calcrete or ‘cornstone’); thicker beds of calcrete appear near the top of the formation. The clastic rocks were deposited by ephemeral streams that drained the Southern Uplands landmass under semi-arid, seasonally wet, weathering conditions. In the Tweed Basin, the Cheviot volcanic massif was the source of the conglomerates, whilst farther west their provenance lay in the Caledonian igneous and Lower Palaeozoic sedimentary rocks of Galloway.
Birrenswark and Kelso volcanic formations
The initially rapid, extensional fault-controlled subsidence of the Northumberland–Solway Basin was accompanied by extrusion of basaltic lavas along its northern margin. In the west, around Langholm, the Birrenswark Volcanic Formation comprises thin flows of microphyric, feldspar-rich basalt totalling 100 m in thickness at Burnswark but reducing to some 15 m at Kirkbean. An olivine basalt flow at Watch Hill (NY 435 908), north-east Langholm, has given a K-Ar whole-rock age of 361 ± 12 Ma. Several small volcanic vents containing agglomerate and/or intrusive material occur in the vicinity of Langholm, and whilst no direct genetic link has been established, it is likely that they acted as feeders for the lavas of the Birrenswark Volcanic Formation, or possibly for the slightly younger Glencartholm Volcanic Member of the Tyne Limestone Formation (see below). Sedimentary debris in the agglomerate-filled vents includes Lower Palaeozoic lithologies and dolostone and chert of probable Late Devonian age; the principal igneous component is olivine-basalt.
Farther east, in Roxburghshire, the Kelso Volcanic Formation comprises at least six flows, and possibly as many as twelve, of alkaline olivine basalt together with subordinate tuff beds. The formation is about 120 m thick. A scattering of small basaltic intrusions is spatially associated with the lavas and one, from Mellerstain Hill (NT 641 397) has given a K-Ar whole-rock age of 361 ± 7 Ma. Like the date from the Birrenswark lavas, this suggests volcanism at around the Devonian–Carboniferous boundary. The outcrop of the Kelso Volcanic Formation extends southward across the English border near Carham (NT 799 384), whilst a very similar volcanic assemblage, the Cottonshope Volcanic Formation, crops out just to the south of the border on the south-west flank of the Cheviot Hills.
The lavas of the Kelso and Birrenswark volcanic formations interfinger with, and are overlain conformably by the Ballagan Formation; where it does not overlie the lavas the Ballagan Formation conformably follows the underlying Kinnesswood Formation. The Ballagan Formation is of late Tournaisian to early Visean (Courceyan–Chadian) age and crops out in the Tweed Basin, the Langholm area, and on the Solway Coast. In the Tweed Basin, underlying the Merse of Berwickshire, the Ballagan Formation comprises about 430 m of mudstone with thin interbeds of argillaceous dolostone (‘cementstone’) and sandstone (P667218), with sporadic lenticular bodies of cross-bedded, channel-fill sandstone up to 30 m thick. Locally, the sequence succeeding the Kelso Volcanic Formation also contains concretionary calcrete, the most notable example being the Carham Limestone Bed (‘Carham Stone’), a cherty magnesian limestone up to 7.5 m thick, thought to have accumulated by chemical precipitation from waters enriched in lime by showers of volcanic dust.
In several parts of the Tweed Basin the Ballagan Formation contains miospores and a fossil fauna that includes environmentally tolerant bivalves and ostracods, and at Foulden Burn in Berwickshire (NT 921 553) has yielded twelve or more species of fresh- to brackish-water fish from the mudstone of the Foulden Fish Bed. Farther west, at the margin of the Solway Basin around Langholm, the Ballagan Formation contains plant fossils and a sparse faunal assemblage that includes gastropods, ostracods, modiolid bivalves and fish remains. The fossils occur in discrete associations of alluvial, fluvial and fluviodeltaic sedimentary rocks, intercalated with lacustrine, coastal plain and sporadic marine deposits (P912351).
In its outcrop from the Langholm area, westward along the Solway coast, the Ballagan Formation is in the order of 100 to 150 m thick (thinning westward) and is made up of thinly interbedded sandstone, mudstone and lagoonal dolostone (‘cementstone’) with some anhydrite (P579357). It is a terrestrial/fluvial to peritidal assemblage known in its Solway coastal outlier as the Kirkbean Cementstone Member. It was deposited in shallow channels within mudflats in a low-lying, coastal environment but with only a limited and intermittent marine influence (P912351). The Kirkbean Cementstone Member contains plant fragments and a quasi- to nonmarine fauna of bivalves, gastropods and ostracods, although fully marine bivalves are present locally. Elsewhere, in Annandale around Dalton (NY 104 740), boreholes prove the Kirkbean Cementstone Member to be at least 100 m thick and to contain abundant fibrous gypsum (satin spar) bands up to 4 cm thick. The ‘cementstones’ are underlain by dark red, fine-grained to conglomeratic sandstone, the Annandale Sandstone Beds. The anhydrite (or its precursor, gypsum) formed either subaqueously in shallow water or subaerially in coastal sabkhas, but following burial to depths greater than a few hundred metres, CaSO4 is usually preserved as anhydrite. Both gypsum and anhydrite are readily removed in the surface weathering zone and so are rarely seen at outcrop.
Another ‘cementstone’ sequence is seen in the Langholm–Newcastleton area, where it is up to 200 m thick and passes downwards into fine-grained, massive, fluviatile and deltaic sandstones, derived from the north, which are collectively referred to the Whita Sandstone Beds. These ‘cementstones’ were formerly regarded as part of a ‘Lower Border Group’ (P912354) and demonstrate the uncertainties that bedevil correlation between the various cementstone–sandstone sequences. The ‘cementstones’ above the Whita Sandstone could equally well be regarded as part of the Ballagan Formation or the Lyne Formation (Border Group — see below), whilst the Whita Sandstone is assigned to the Ballagan Formation.
The most westerly outcrop of the Ballagan Formation forms part of the Rerrick Outlier on the north coast of the Solway Firth. There, the Wall Hill Sandstone Member rests with angular unconformity on Wenlock strata of the Raeberry Castle Formation. It has a maximum thickness of about 360 m and comprises upward-fining sequences of conglomeratic sandstones, siltstones and mudstones. The fine-grained rocks were laid down on a floodplain by a braided fluvial system of low to moderate sinuosity, with the coarse-grained rocks deposited in channels. Palaeocurrent data indicate a predominately north and north-westerly source from the Southern Uplands terrane, and the conglomerates include clasts derived from the roof of the Criffel–Dalbeattie pluton. Local but intense sediment fluidisation developed at a number of localities, perhaps influenced by synsedimentary activity on the nearby North Solway Fault.
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