Carboniferous of the Oldhamstocks Basin, Dunbar
|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.|
South-east of Dunbar, as far as Cockburnspath, the Midland Valley lithostratigraphy (P912386) can be recognised in a coastal outlier of Carboniferous rocks. The coastal section exposes a succession of strata, the Inverclyde, Strathclyde and Clackmannan groups, of Courceyan to Brigantian age (P912352), which accumulated at the south-western margin of the Oldhamstocks Basin, an area of independent subsidence peripheral to the south-east margin of the much larger Midland Valley Basin (P912346 and P912349). The Dunbar–Cockburnspath outlier is bounded to the south-west by the Innerwick Fault, but inland exposure is very limited. To the south, the outlier is separated from the Tweed Basin by the Lammermuir Hills and Coldingham Moor outcrops of Lower Palaeozoic strata, which probably formed a contemporary, terrestrial barrier between the two areas of subsidence. Many of the Strathclyde and Clackmannan groups’ limestones have been worked locally for agricultural lime, with some kilns still preserved. Clackmannan Group limestones are now worked on a large scale near Dunbar for cement production.
At the base of the group, the Kinnesswood Formation, here probably of Tournaisian (Courceyan) age, comprises red and green sandstones and pedogenic limestones (‘cornstones’). Fragmentary fish fossils have been found at several levels, and of particular importance is the occurrence at Hawk’s Heugh (NT 790 714) of Remigolepis (the only British example of this fish), Holoptychius and Bothriolepis. Generally, this combination would be taken to indicate a Famennian (latest Devonian) age, only possibly ranging up into the lower part of the Tournaisian. However, from Hawk’s Heugh, a miospore assemblage confirms a Courceyan (earliest Tournaisian) age. Though in general the formation conformably overlies Upper Devonian strata of the Greenheugh Sandstone Formation (Stratheden Group), in the coastal section at Hawk’s Heugh the boundary is faulted. Close to Oldhamstocks, probable Kinnesswood Formation strata are associated with a thin succession of red volcanic breccia and basaltic lava that may correlate with the Kelso Volcanic Formation, described earlier in this account from its outcrop farther south where it is thought to be of Courceyan age and to succeed the Kinnesswood Formation. As discussed in Chapter 5, there is considerable uncertainty over the precise position of the Devonian–Carboniferous boundary within the lithostratigraphy.
The sandstones of the Kinnesswood Formation pass up into the Ballagan Formation, characterised by the appearance of grey mudstones and ferroan dolostones (‘cementstones’). The palynology of the Ballagan Formation shows it to be Courceyan to Chadian in age, and it has a macrofossil fauna that includes quasimarine bivalves and ostracods. The lower part of the Ballagan Formation comprises about 60 m of silty, ripple cross-laminated sandstone, grey mudstone, and dolostone. These are alluvial floodplain deposits and culminate in the distinctive Eastern Hole Conglomerate, with its red, coarse-grained sandstone matrix and fossil content of fish scales and spines, and the overlying Sanguinolites band with coalified wood (Lepidodendron) fragments, the eponymous bivalve and rare ostracods and fish scales. Higher in the sequence are thin beds of calcareous sandstone, siltstone and dolostone, the sandstone containing bivalves, ostracods and fish fragments. About 56 m of thicker sandstone beds, the Horse Road Sandstone, succeed the highest dolostone and are predominantly cross-bedded with common slump structures. Some of the sandstone beds contain rip-up clasts of mudstone, and some feature distinctive, spherical, calcareous concretions up to 1.5 m in diameter.
The Horse Road Sandstone is unfossiliferous and is interpreted as the infill of a major fluvial channel. The succeeding sequence, about 90 m thick, sees the recurrence of fine-grained sandstone, dolostone and mudstone of floodplain origin. Finally, at the top of the Ballagan Formation, thick sandstone beds reappear in a sequence that includes the Kip Carle Sandstone at Cockburnspath, which is 21 m thick and has been interpreted as a fluvial channel-fill deposit.
In the Dunbar–Cockburnspath outlier, the Strathclyde Group is divided into the Gullane and Aberlady formations (P912347, P912348 and P912352). About 125 m of Gullane Formation strata are present. The lower part of the formation comprises mostly mudstone interbedded with thin coals (<0.3 m), succeeded by 56 m of cross-bedded, channel-fill sandstone, the Heathery Heugh Sandstone, which has a median parting of soft mudstone, 3.4 m thick. The upper part of the formation is made up of argillaceous sandstone and soft carbonaceous mud-stone. Miospores of early Visean age are present in places, as well as plant fragments and nonmarine bivalves; poorly developed ‘marine bands’ contain the brachiopod Lingula and various molluscs.
The Aberlady Formation is about 140 m thick at Cove Harbour and includes red-orange weathering sandstone (including the prominent Cove Harbour and Bilsdean sandstones), bioclastic limestone with a varied shelly fauna, and the two Cove Marine bands, the lower of which is a 3.5 m-thick sandstone, and the upper a somewhat thinner, blue-grey silty mud-stone. The marine bands contain a shelly fauna (including goniatites) and miospores of Asbian age. The Cove Marine Bands have been correlated with the Macgregor Marine Bands of the Lothians and East Fife, which represent the first fully marine incursions to affect east central Scotland in Carboniferous times. There, the lowermost of the multiple Macgregor Marine Bands is taken as the formal base of the Aberlady Formation so, if the correlation is secure, the lower of the Cove Marine Bands would mark that level in the Oldhamstocks Basin. Above the upper Cove Marine Band lies the Cove Harbour Sandstone, about 25 m thick, and then a sequence of grey silty mudstone with nodular siderite horizons that is capped by the probably lacustrine Cove Oil Shale containing algal traces and pyritised plant stems. Together with the succeeding Bilsdean Sandstone, the sequence is interpreted as mostly crevasse splay and crevasse channel floodplain deposits, with the thicker, cross-bedded, red-stained sandstones originating as fluvial channel fills.
The Longcraig to Barns Ness section of the Aberlady Formation is composed of about 19 m of interbedded coals, mudstones and limestones, the latter including the Hollybush and Blackbyre limestones, formerly known respectively as the Lower and Middle Longcraig limestones. The formation was deposited in fluviodeltaic and marine transgressive environments, and most of the limestones contain a rich and varied fauna that includes corals, brachiopods, bivalves and crinoids. The Blackbyre Limestone is nodular and contains abundant Siphonodendron corals, brachiopods and bryozoan debris. It overlies bioturbated sandstone and is overlain by a sequence of mudstones, seatclays with Stigmaria, and thin coals. The upper surface of the Blackbyre Limestone, best seen at Catcraig (NT 715 774) is remarkable in that it is pitted with seatearth-filled hollows that are thought to mark the sites of trees growing on what was probably a palaeokarst surface (P006000). The top of the Aberlady Formation is taken at the base of the Hurlet Limestone, formerly known as the Upper Longcraig Limestone.
The Hurlet Limestone marks the base of the Clackmannan Group and of its lowest division, the Lower Limestone Formation. The latter is characterised by strongly cyclical, upward-coarsening sequences of limestone, mudstone, siltstone and sandstone, which may be capped by seatearth and coal. The limestones are laterally extensive and have a rich and diverse fauna of marine fossils that includes foraminifera, corals, bryozoa, brachiopods, gastropods, bivalves, nautiloids, crinoids and echinoids. The formation was deposited in progradational lobate deltas during marine transgressive–regressive cycles.
The Lower Limestone Formation is Visean (Brigantian) in age on the basis of its shelly fauna and miospore flora. The Hurlet Limestone notably includes Koninckophyllum corals whilst the higher Craigenhill (formerly the Lower Skateraw) Limestone is characterised by the brachiopod Gigantoproductus. Farther up the sequence the Blackhall (formerly the Middle Skateraw) Limestone is crinoidal and contains a 25 cm-thick band packed with foraminifera, whilst the Neilson Shell Bed immediately above it contains brachiopods, trilobites and crinoids. The succeeding Chapel Point and Barns Ness limestones (local names applied to two of the more widely identified Hosie limestones) are only sparsely fossiliferous but bellerophontid gastropods, coiled nautiloids and crinoids are present locally, whilst trace fossils are abundant.
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