Regional structure of the Carboniferous, 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.

Regional Structure

Structural setting for Carboniferous sedimentation across the south of Scotland and the surrounding regions. P912346.
Main structures associated with Carboniferous deposition and subsequent late Carboniferous to early Permian deformation. P912349.
Thirlstane Sandstone Beds, Powillimount Sandstone Member, showing liquefaction structures. P001161.
Palaeogeographical reconstructions from the Ordovician to the Cretaceous. P912314.
Anticline in the Penton Limestone exposed in the Liddel Water at Penton Linns. P221694.

The Southern Uplands and the Northumberland–Solway Basin form the northern components of a regional arrangement of tectonic blocks and basins that extends southwards, as detailed in the companion volume — Northern England — in the British Regional Geology series. Only the northern edge of the Northumberland–Solway Basin extends across the border into the south of Scotland. In general, the Tournaisian lithofacies characteristic of the Midland Valley succession — the Inverclyde Group — extends southwards across the Scottish Border region into the northern margin of the Northumberland–Solway Basin. During the Visean, the Cheviot Block separated the basin from the Tweed Basin in Berwickshire (P912346) but ceased to be an effective barrier by the beginning of the Namurian.

Early to mid Carboniferous extension

Lithospheric extension under the influence of broadly north–south tension affected much of Laurussia from late in the Devonian; it was particularly active until the end of the Visean but continued intermittently through much of Carboniferous time. The interaction of the north– south extensional regime with the pre-existing Caledonian fault pattern led to reactivation of Caledonian faults as the marginal structures of transtensional basins. One such major basin complex opened in the Midland Valley of Scotland, with a variable, mostly dextral, strike-slip component. Thence the transtensional regime was propagated across the Southern Uplands massif where several small basins opened with their margins controlled both by the major north-east-trending strike faults and by the north-west- and north-north-east-trending Caledonian cross faults. To the south, the Solway and Northumberland basins opened in response to north–south extension across the Iapetus Suture Zone, the pre-existing orientation of which determined the broadly east-north-east trend of the basin boundary faults. Interpretation of cross-basin geophysical profiles indicates that the faults defining the southern margin of the basin (Maryport–Stublick–Ninety Fathom) were rooted into the suture zone, where extension and growth faulting was facilitated by the pre-existing intracrustal detachment.

Throughout much of its length the northern margin of the Northumberland–Solway basin is also formed by a system of en echelon, synsedimentary dislocations downthrowing predominantly to the south-east; they include the North Solway, Waterbeck, Gilnockie and Alwinton faults (P912349). North-westerly trending cross faults acted to transfer downthrow between different fault blocks. The extensional tectonics active at the basin margin early in the Carboniferous facilitated the eruption of lavas and emplacement of a variety of minor igneous intrusions. So, for example, the deposition of the Kinnesswood Formation alluvial-plain sediments was followed by eruption of the lavas of the coeval Birrenswark Volcanic and Kelso Volcanic formations. These volcanic formations are considered below in their lithostratigraphical context, but the broader manifestations of Carboniferous magmatism are considered separately in the final section of this chapter.

Syndepositional faulting

The northern margin of the Solway Basin saw rapid extensional subsidence during the Tournaisian deposition of the Inverclyde Group, with normal fault movement continuing through the Visean to influence deposition of the Border Group. An example of a major north-easterly fault with such a syndepositional history is the Waterbeck Fault, north of Annan (P912349). It throws Yoredale Group strata down to the south-east and cuts out parts of the Border Group formations to the north. There, and in general within the Border and Yoredale groups, abrupt transitions are observed from relatively flat lying beds to zones of tightly folded strata. This pattern is particularly well observed in the Fell Sandstone Formation as exposed in the River Annan north of Brydekirk (NY 186 705). Elsewhere in the region, an analagous structural pattern is observed in the Tournaisian and Visean strata of Berwickshire, where zones of steeply dipping and folded strata are believed to have formed during periods of synsedimentary deformation.

Close to the north-east to south-west trending North Solway Fault, an inherited Caledonoid structure, there is much evidence within the Tournaisian to Visean, basin-margin, coarse clastic sedimentary rocks of the Rerrick and Colvend Outliers of periodic synsedimentary, dip-slip, normal movement. Synsedimentary fault deformation of hanging-wall strata increases towards the North Solway Fault, whilst cyclicity within alluvial fan deposits of the Rerrick Outlier may be attributed to variation in rate of fault-controlled subsidence. Further away from the basin margin, strata of the Kirkbean Outlier exhibit characteristics of a more stable environment marginal to shallow marine deposition, but rates of subsidence may nevertheless have been controlled tectonically. Evidence of synsedimentary seismic activity is particularly convincing within the coastal outcrop of the Thirlstane Sandstone Beds (Powillimount Sandstone Member, Fell Sandstone Formation) (P001161).

Late Carboniferous (Variscan) compression

Basin shortening and inversion took place in late Westphalian times as a peripheral effect of the Variscan Orogeny farther south; many of the basin margin faults were reactivated with a reverse sense of displacement. By this time, the thermal subsidence that had controlled the Namurian and Westphalian patterns of sedimentation had all but ceased whilst, to the south, the Gondwanan continent had collided with Laurussia to create the supercontinent of Pan gaea (P912314d). The Laurussia–Gondwana collision generated the Variscan Orogeny with a mountain fold belt produced as its culmination. The approximate northern limit of the fold belt (the Variscan Front) extends from the mainland of Europe across southern England and Wales and into Ireland. The south of Scotland lay in a foreland setting well to the north of the deformation front. Although tectonic activity in the foreland region was much weaker than in the fold belt to the south, some folds were formed, both independently and as adjuncts to fault reactivation. An anticline affecting the Penton Limestone (Alston Formation), as exposed in the Liddel Water near Langholm, is illustrated in P221694.

Seismic profiles across the Solway Basin show Carboniferous strata folded into the north-north-east-trending Solway Syncline and, to the south-east, the complementary Carlisle Anticline (P912349). To the north-west of the syncline, and along the northern margin of the Solway Basin, are north-east orientated and plunging anticlinal structures, well seen in the Kirkbean Outlier.Farther north-east, along the northern margin of the Northumberland Basin, smaller scale, generally north-east-trending structures predominate, but the Cheviot Block produces much local complication. Around the eastern side of the block the structural trend swings toward a north–south orientation, as shown in north-east Northumberland by the Holborn Anticline and its associated Hetton Fault (P912349). North of the border, in eastern Berwickshire, the north-north-west-trending Berwick Monocline faces east, with the steep limb coinciding with a high angle reverse fault that juxtaposes Carboniferous rocks against Silurian and Devonian strata. The complexity of these relationships is inferred to arise from the reverse reactivation of original syndepositional faults.


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