Carlisle Basin, Permian-Triassic
|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 Permian to Triassic strata in southern Scotland form the marginal sequence of the Carlisle Basin, most of which lies to the south of the border (P912359) and is described in the companion volume for Northern England. The basin has an extensive onshore outcrop across northern Cumbria, where the top of the sequence is Early Jurassic in age, and extends offshore under the innermost part of the Solway Firth. Thence, towards the south-west, it is linked over a low ridge to the somewhat larger, offshore Solway Firth Basin that also contains Permian, Triassic and Lower Jurassic rocks.
The lowermost Permian strata of the Carlisle Basin, though of probable mid Permian (Guadalupian) age, rest with strong unconformity on Carboniferous rocks. The latter were folded and uplifted during Variscan basin inversion in late Carboniferous to early Permian times and rapidly eroded to a relatively smooth peneplain prior to Permian sedimentation. In places, for example where Tournaisian or Visean beds lie beneath the sub-Permian unconformity, several thousand metres of strata have been removed. Conversely, in the down-folded areas such as the Solway syncline, late Westphalian beds lie beneath the unconformity and the thickness of eroded strata is probably no more than a few hundred metres. There is ubiquitous reddening of the subjacent Carboniferous strata for several metres beneath the unconformity.
The Scottish outcrop of Carlisle Basin strata extends across the Canonbie–Gretna–Annan area. Exposure is very limited and it is only relatively recently that boreholes and geophysical surveys have enabled a proper assessment of the sequence. Around the Canonbie Coalfield, early workers had observed basal breccias and ‘brick red’ sandstones above the ‘Barren Red Coal Measures’ (now, in part, the Pennine Upper Coal Measures Formation and Warwickshire Group). Locally, the breccias were shown to be overlain by an argillaceous unit, the ‘Robgill Marls’ or ‘St Bees Shales’, which passed up into an upper sandstone division, variously named and divided as part of the ‘Annan Series’. Data from recently drilled boreholes and geophysical surveys have radically improved the interpretation of the sequence, age, lithology and stratigraphy of these Permian to Triassic strata, and allowed their correlation with the established Carlisle Basin succession. Late Permian evaporites (gypsum–anhydrite) and microfloras have also recently been recognised and described for the first time from onshore Scotland.
Three groups of strata are now recognised in the Scottish outcrop of the Carlisle Basin sequence (P912360). The locally derived, basal sedimentary breccias and sandstones are referred, respectively to the Brockram and Penrith Sandstone Formation of the Permian Appleby Group. Above the Appleby Group, the argillaceous and evaporite-bearing strata form the Eden Shales Formation of the Cumbrian Coast Group. The Eden Shales grade upwards into the St Bees Sandstone Formation, which is the lowermost division of the Sherwood Sandstone Group. The lowest beds of the St Bees Sandstone are late Permian in age, but most of the Sherwood Sandstone Group is Triassic.
Along the northern margin of the Carlisle Basin, the basal few metres of the Permian sequence comprise sedimentary breccia and sandstone of local origin, which rest unconformably on Carboniferous or older strata. Most of these strata can be referred to as ‘Brockram’, a general term for the coarse clastic, basal Permian breccias of north-west England. These are alluvial fan and flash-flood deposits that interfinger with red aeolian sandstones, the characteristic lithology of the Penrith Sandstone Formation. Its local derivation means that Brockram is highly heterolithic and can vary considerably in its clast assemblage. In the Scottish outcrop, around Annan, two units have been differentiated from small, isolated outcrops. The Kelhead Breccia is up to 15 m thick and comprises angular clasts of Visean limestone and sandstone. The Kettleholm Breccia is some 20 m thick and is composed mostly of Lower Palaeozoic wacke sandstone and mudstone clasts. Both units rest unconformably on Lower Palaeozoic strata and can be regarded as part of the wider Brockram lithofacies. These locally derived, basal clastic strata accumulated as small alluvial fans shed either from low hills that rose above the general peneplain or from the higher ground of the Southern Uplands massif to the north (P912363a).
The red, aeolian sandstone of the Penrith Sandstone Formation has only a very restricted presence in the Scottish outcrop of the Carlisle Basin succession. A small outcrop of red sandstone with ‘millet seed’ grains is seen on the south bank of the River Esk at Canonbie (NY 392 761), and there forms the base of the Permian sequence. This sequence is only about 35 m thick, but to the south the formation has an extensive outcrop in the Vale of Eden, on the south side of the Carlisle Basin, where it attains a maximum thickness of over 500 m. There, much of the formation is aeolian sandstone (with wind direction from the east or south-east) with interbedded fluvial sandstones and alluvial fan breccias (Brockram). There is little or no biostratigraphical evidence for the age of the Penrith Sandstone Formation but it is likely to be mid Permian from its conformable relationship with the overlying, and demonstrably middle to upper Permian, Eden Shales Formation of the Cumbrian Coast Group.
Cumbrian Coast Group
Strata of the Cumbrian Coast Group succeed the Appleby Group in the Annan to Gretna sector of the Carlisle Basin’s northern margin (P912359 and P912360). Only one of the group’s divisions, the Eden Shales Formation, is present in the Scottish outcrop, where it is about 100 m thick. Exposures in the Kirtle Water (NY 248 745) show the formation to consist principally of siltstone, commonly micaceous and sandy, fine-grained sandstone and silty mudstone, all mostly purplish-red or reddish-brown. Interbeds of more coarsely grained red sandstone, conglomerate and breccia also occur. Grains of gypsum and anhydrite are widely disseminated throughout these rocks and cross-cutting fibrous gypsum veins are common. In the lower part of the formation, beds and nodules of gypsum–anhydrite have been identified in a series of boreholes sunk at the site the former Chapelcross Power Station (around NY 220 700). These were the first recorded occurrences of Permo-Triassic evaporites in onshore Scotland. The boreholes showed that there is an increase in the proportion of sandstone towards the top of the formation, which is gradational into the overlying St Bees Sandstone Formation.
Throughout much of the formation, interbedded siltstone and fine-grained sandstone are either irregularly or regularly laminated, whereas the less common, finer-grained siltstone and mudstone appear structureless. Similar rock types are typical of this formation in its more extensive outcrop in Cumbria and have also been proved by boreholes in the Solway Firth Basin (P912359) and farther south in the East Irish Sea Basin. The Eden Shales are thought to have been deposited by accretion of fine wind-blown and sheet-flood detritus across mudflats, with the intermittent establishment of evaporitic conditions in sabkhas and shallow lakes (P912363b). An arid climate is indicated by the presence of the evaporite beds, by the evidence for periodic desiccation such as mud cracks, and by the local reworking of mud flakes into sandstones and conglomerates.
With the exception of the lower, evaporitic (gypsum–anhydrite) part of the formation, the sequence and lithology of the Eden Shales in the Chaplecross boreholes broadly correspond to what is seen at outcrop, albeit the transition into the St Bees Sandstone Formation is not well exposed. In addition, the boreholes allow, from a combination of geophysical log signatures and core lithology, three informal lithostratigraphical divisions to be recognised in the ‘Scottish’ Eden Shales. The lower and middle divisions fine upwards overall, but the highest of the divisions is a coarsening-upwards sequence passing into the overlying St Bees Sandstone.
The Chapelcross boreholes have provided the first record of late Permian palynomorphs from Scotland. Miospores are the sole or dominant component of the assemblages and comprise associations dominated by saccate pollen, principally of conifer origin. The miospores Sherwood Sandstone Group originated from the contemporary land flora and allow correlation with rocks in the Kazanian and Tatarian stages of the classical type-Permian succession in Russia. Of particular note is the bisaccate pollen Lueckisporites virkkiae, a distinctive species that appears first in the mid Guadalupian in its type region and ranges to the top of the Permian sequence there and elsewhere. Hence a Guadalupian (mid Permian) to Lopingian (late Permian) age range is likely for the Eden Shales Formation.
Sherwood Sandstone Group
The Eden Shales Formation is succeeded by the sandstone-dominated St Bees Sandstone Formation, the lowest division of the Sherwood Sandstone Group. The sandstone is typically reddish brown and fine grained, and in the restricted Scottish outcrop it was extensively worked for building stone near Annan, at Corsehill (NY 205 701) and nearby at several quarries close to the Kirtle Water; one of these, Cove Quarry (NY 254 710), was still being worked early in the 21st century. The sandstone is sporadically micaceous and/or silty, some beds grade up to medium-grained, and intraformational mud clasts are common at some levels. Many of the sandstone beds are tabular and composite, with flat or gently undulating erosion surfaces at their base. A wide range of sedimentary structures is displayed, including parallel and low-angle cross-lamination, planar tabular and trough cross-bedding and convolute bedding. There are numerous, generally thin partings of siltstone, silty mudstone and mudstone, particularly towards the basal transition into the underlying Eden Shales Formation. The maximum thickness of the St Bees Sandstone in its Scottish outcrop is about 170 m, proved in a borehole at Staffler (NY 3398 7226), but it is much thicker to the south where its outcrop extends over much of onshore west Cumbria and the Carlisle and Vale of Eden basins.
The lower part of the formation marks a transition from the mudflat deposition of the Eden Shales to the fluvial environment of the St Bees Sandstone. By comparison with the similar strata in west Cumbria, the lower part of the formation is thought to record sedimentation by unconfined flood events fed by a large fluvial system that regional palaeocurrent data suggest probably flowed towards the north. This fluvial system — the Budleighensis River — was the dominant depositional influence for the St Bees Sandstone across much of north-west England. Higher in the formation, but proved only in boreholes at Staffler, Cranberry (NY 3072 6949) and Becklees (NY 3517 7158), the fluvial regime progressively evolved into a channel sandstone facies association. This produced a thick, sand-filled channel complex (P912363c), the sandstone bodies still largely derived from the south and interbedded with less common overbank siltstone and mudstone.
Around Gretna, the St Bees Sandstone contains a scattering of wind-rounded quartz grains and relatively little interbedded mudstone. This might indicate a transition into the overlying Ormskirk Sandstone Formation (the Kirklinton Sandstone in some older literature), a higher division of the Sherwood Sandstone Group that has a mixed fluvial and aeolian origin, as recognised in the Carlisle Basin to the south of the Border.
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