Carboniferous of the Southern Uplands - an introduction

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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.

Introduction

Palaeogeographical reconstructions from the Ordovician to the Cretaceous. P912314.
Structural setting for Carboniferous sedimentation across the south of Scotland. P912346.
Stratigraphical classification of Carboniferous rocks in the south of Scotland. P912386.

At the end of the Devonian Period, Scotland lay in low latitudes just south of the Equator, within the northern part (Laurussia) of the huge Pangaea ‘supercontinent’. During Carboniferous times, Pangaea drifted slowly northward, thus moving Scotland from arid, through humid tropical and back to arid climatic zones (P912314d). Throughout this time, the Southern Uplands was a terrestrial area of low to moderate relief forming an elongate but discontinuous barrier between depositional basins in the Midland Valley to the north and the Northumberland–Solway Basin to the south; a more open connection between these two basins probably existed to the north-east of the present North Sea coastline and extended onshore as the Tweed Basin (P912346). The Northumberland–Solway Basin was itself the northernmost depositional centre within the larger Pennine Basin, which covered much of northern and central England and had sedimentary links westwards into North Wales and Ireland, and eastwards into mainland Europe.

Across the south of Scotland, Carboniferous strata are now preserved along the eastern and southern margins of the Southern Uplands: the Dunbar district, much of Berwickshire and Roxburghshire, the south of the Langholm district (wherein lies the Canonbie Coalfield) and the Solway coastal fringe of Dumfriesshire. Other sequences form outliers that originated as the fill of palaeovalleys within the massif: examples include the outliers at Whitecleuch, Sanquhar, Thornhill and Stranraer. A range of lithofacies and depositional environments are represented.

As described in Chapter 5, the youngest Devonian and oldest Carboniferous rocks form a continuous terrestrial (fluvial and aeolian) sequence containing sporadic miospore floras and rare fish faunas. Accordingly, there has been persistent difficulty in defining a reliable and consistent base for the Carboniferous throughout the region. In south and central Scotland the base of the Kinnesswood Formation is commonly correlated with the base of the Carboniferous, and is taken at the incoming of a pedogenic nodular limestone (calcrete or cornstone)-bearing sequence that probably represents a general and widespread change to a seasonally wetter climate. However, though assigned to the Carboniferous and included within the Inverclyde Group (P912386), the lower part of the Kinnesswood Formation displays ‘Upper Old Red Sandstone’ lithofacies and is similar in character to the underlying Stratheden Group. In places, like the higher formations of that group, the Kinnesswood Formation in the south of Scotland contains a probably Famennian (Late Devonian) fish fauna as described in Chapter 5 of this account; elsewhere in the region, Carboniferous miospores have been recorded close to the base of the Kinnesswood Formation. To compound the problem, both Carboniferous and Devonian miospores have been recovered from low in the Kinnesswood Formation in different parts of the Midland Valley. On a regional scale the base of the formation therefore seems likely to be diachronous. In some parts of southern Scotland the Kinnesswood Formation is succeeded by basaltic lavas of earliest Carboniferous age, but the error range on the available radiometric dates precludes a definitive chronostratigraphical allocation.

During the Tournaisian the sea gradually encroached onto the Southern Uplands massif, giving rise to a conformable transition in south-east Scotland, within the Inverclyde Group, from the fluvial, calcrete and red-bed facies of the Kinnesswood Formation up into the lagoon-al mudstone with dolostone (cementstone) facies of the Ballagan Formation. The sedimentary transition was interrupted locally by the subaerial eruption of basalt lava around Langholm and Kelso. Some of the larger valleys such as Nithsdale experienced partial marine inundation and became narrow arms of the sea connected to the more open seaways along the Solway Firth and the Midland Valley, but there was never an open marine connection across the massif through these palaeovalleys. Through the late Visean and Namurian, marginal and shallow marine conditions prevailed with repeated flooding cycles creating a characteristic succession of ‘Yoredale-type’ cyclothems. When fully developed, each individual cyclothem has a limestone at the base, which is overlain sequentially by mudstone, sandstone, seatearth and coal. The cycles commonly attain a thickness of around 20 m but may be much thicker. Each one records a marine transgression followed by a progressive shallowing and change to fluvial conditions with subsequent emergence and the growth of delta-top swamp vegetation. The deltaic conditions then became dominant during the Westphalian, with extensive development of the characteristic Coal Measures cyclic sequences in which limestone is generally absent, replaced by a thicker mudstone development followed by sandstone, seatearth and coal; individual cyclothems mostly range up to about 10 m in thickness, some attain 15 m.

As might be expected in narrow basins within a generally stable, upland area, the Carboniferous outliers within the region have thin attenuated successions, with numerous non-sequences and unconformities. However, along the southern and eastern margins of the massif deposition was more continuous and successions can generally be correlated with those in the contemporaneous basins developing in the Northumberland–Solway Basin, where relatively thick and complete marine and deltaic sequences were laid down.


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Related articles

Stratigraphical principles
Regional structure
Coal-bearing sequences
Tweed and Northumberland–Solway basins
Oldhamstocks Basin, Dunbar
Central outliers (Whitecleugh, Sanquhar and Thornhill)
Magmatism