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.|
In the Scottish Borders, along the northern margin of the Northumberland–Solway Basin and underlying the Tweed Basin, the oldest Carboniferous rocks, of Tournaisian age, belong to the Inverclyde Group. They include the basaltic lavas of the Kelso Volcanic Formation (‘Kelso Traps’) and the Birrenswark Volcanic Formation which overlie red-bed strata assigned to the Kinnesswood Formation. The latter rests either conformably on red-bed strata of Devonian age (Stratheden Group) or with marked angular unconformity upon an irregular surface eroded across the steeply dipping, Lower Palaeozoic turbidite sequence. Stratigraphically above the lavas lies the Ballagan Formation, which forms the upper part of the Inverclyde Group. This earliest Carboniferous sedimentary succession of the Tweed and Solway basins is distinct from the rest of the Carboniferous succession in being lithologically similar to coeval successions in the Midland Valley of Scotland. Accordingly, the same lithostratigraphical formational terminology is applied (P912386 and P912348).
Higher parts of the Carboniferous succession are not present in the Scottish sector of the Tweed Basin, with the exception of a structurally isolated coastal outcrop north of Berwick. Elsewhere, along the north-west margin of the Nothumberland–Solway Basin, there is a marked departure from the Midland Valley lithofacies, reflected in the application of different lithostratigraphical terminology. There (and across northern England) the Inverclyde Group is succeed by the Border Group, which includes the Lyne Formation, partly coeval with the Ballagan Formation and largely restricted to the Solway area, and the Fell Sandstone Formation.
The Border Group is largely of early Visean age but marine lithofacies are rare within it and there are few stratigraphically useful fossils. Consequently there are difficulties in making a detailed correlation of the succession across the region, particularly toward the bottom of the sequence. For much of this early Carboniferous interval the basins formed narrow gulf-like extensions of the open sea, widening to the south-west, and their sedimentary rock successions reflect the interplay of fluviodelatic and paralic depositional systems (P912351). The northern emergent margins of the Northumberland and Solway basins were sources of clastic sediment during the early period of deposition, but for much of Dinantian time axial drainage systems were dominant, building from the north and east towards a shallow sea in the west. However, marginal clastic deposition was relatively persistent in the Solway Basin adjacent to the active North Solway Fault system. The variations in local lithostratigraphy for this ‘Dinantian’ interval are summarised and correlated in P912352.
The Border Group is succeeded by the Yoredale Group, a Visean to Namurian succession characterised by repeated upward-coarsening sedimentary cycles from limestone through mudstone to sandstone, capped by seatearth and coal. The ‘Yoredale’ cycles range in thickness up to about 35 m with an average of about 20 m; some are incomplete, and the proportions of the different lithologies vary in response to subtle changes in depositional environment (P912353). The Yoredale Group is divided, in upward sequence, into the Tyne Limestone, Alston and Stainmore formations based largely on the relative abundance of the different rock types within cycles. The base of the group (and of the Tyne Limestone Formation) is of early Asbian age on the northern margin of the Northumberland–Solway Basin, but becomes diachronously younger southwards. The Alston Formation is mostly of Brigantian age but its uppermost strata are Pendleian so that the formation straddles the Visean–Namurian chronostratigraphical boundary. The Stainmore Formation (which is only sparsely represented in southern Scotland) extends upwards to the top of the Namurian.
The Yoredale Group is succeeded in the Langholm–Canonbie area of the Scottish Borders by Westphalian fluviodeltaic strata of the Pennine Coal Measures Group. The group includes the Pennine Lower, Pennine Middle and Pennine Upper Coal Measures formations and comprises repeated mudstone–sandstone–coal cyclothems that individually range up to about 15 m in thickness (P912353). Late in the Westphalian, the Pennine Coal Measures Group was succeeded in the Solway Basin by the red-bed succession of the Warwickshire Group.
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