Gala Group, Silurian, Southern Uplands
|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.|
The Gala Group is composed of the sandstone-dominated strata that crop out between the Orlock Bridge Fault and, to the south, the faulted boundary with the Ettrick Group. The outcrop is divided by strike-parallel faults, marked by discontinuous outcrops of the underlying Moffat Shale Group, into tracts that range from a few hundreds of metres to a few kilometres in outcrop width. In the Southern Uplands of south-west Scotland the age of the sandstone becomes progressively younger in successive tracts southwards (P912326 and P912327), with an overall range spanning the lower to middle Llandovery acuminatus to guerichi biozones. In the central and north-eastern parts of the Southern Uplands the earlier and later parts of the group are repeated by faulting but the middle part is missing. The Orlock Bridge Fault has traditionally been seen as one of the more important Southern Uplands strike faults since it was thought to separate sandstone of Ordovician and Silurian age. This view is partly an artifact of the long-established ‘Northern Belt versus Central Belt’ terminology and, at best, the biostratigraphical break across the Orlock Bridge Fault appears to be no greater than that across most of the other tract boundary faults (P912326 and P912327).
The orientation of palaeocurrent indicators suggests that sediment transport during deposition of Gala Group sandstones was predominantly towards the south-west. Sandstone composition is typically quartzo-feldspathic, with quartz forming up to 55 per cent and feldspar (plagioclase and K-feldspar in varying proportions) 20–30 per cent of the rock. Mica generally forms a minor component but is more abundant in fine-grained sandstone. Pyroxene and amphibole occur sporadically in the older formations where, together with andesitic and basaltic lithic debris, they may very rarely comprise as much as 20 per cent of the sandstone. Other lithic grains comprise clastic sedimentary, felsic igneous, and spilitic volcanic rocks, the latter becoming more abundant in the younger part of the group. A particular feature brought out by the regional geochemical distribution of chromium in stream sediment (P912328b) is the abundance of detrital chrome-spinel in the sandstones that make up the older Gala Group tracts. Grains of garnet, tourmaline, zircon and epidote are present in the sandstones forming all of the tracts.
Mudstone–siltstone units are commonly interbedded with the Gala Group sandstones and represent fine-grained elements of the submarine fan succession deposited in areas sheltered from sandstone deposition. The mudstone and siltstone may dominate the succession locally. In the older tracts, dark grey graptolite-bearing mudstone commonly forms units up to 10 m thick, and locally up to 80 m, which are effectively interbedded, lateral equivalents of the pelagic Birkhill Shale Formation. In younger tracts, massive or laminated siltstone dominates units that are tens, and locally hundreds, of metres thick and which contain only a small proportion of medium- to coarse-grained sandstone in thin beds.
As in the Leadhills Supergroup, the variation in age of adjacent Gala Group tracts (albeit based on weak biostratigraphical control in some cases), coupled with subtle variations in the character of their component sandstone successions, has encouraged treatment of each fault-bounded tract as a separate stratigraphical unit. Formation names have been applied in some parts of the outcrop where an adequate degree of lithological distinction can be recognised, but these are difficult to extend with any confidence through the large areas with little exposure. The tops of units thus defined are taken at tract-bounding faults, their formal bases at their contact with the underlying Moffat Shale Group, though at outcrop both top and bottom of ‘formations’ are commonly faulted with no Moffat Shale Group strata preserved. Consequently, a ‘tectonostratigraphical’ numbering scheme (Gala 1–8) was adopted in the south-west of the Southern Uplands (P912384) and extrapolated north-eastward into the area east of the Cairnsmore of Fleet pluton and the Thornhill Basin. Farther north-east, in the central and north-eastern Southern Uplands, repetition of lithologically and biostratigraphically similar rocks across a number of tracts led to the use there of a different suite of formation names, but again the top boundaries are invariably faulted. Overall then, the difficulties of correlation are compounded by the loss of some tracts as faults merge, whilst other parts of the succession expand across several, relatively narrow tracts as faults bifurcate. Hence, though eight units with formational status are recognised on the Rhins of Galloway (P912327), in central to north-eastern parts of the Southern Uplands the Gala Group is divided into only two formations (P912326); correlation is shown in P912384.
The older parts of the Gala Group are best preserved in south-west Scotland (P912327) where much of the succession is exceptionally well exposed on the west coast of the Rhins of Galloway and to the east of Luce Bay. The Gala 1 tract is composed of massive and bedded sandstone facies locally named the Kilfillan Formation (up to 2000 m). Sparse units of dark grey to black mudstone interbedded with the sandstone facies range up to 10 m thick. Slumped units up to 80 m thick, comprising blocks of wacke and mudstone in a mudstone or siltstone matrix, are exposed in the north of Luce Bay (NX 199 153) and Penkiln Burn (NX 413 669). A restricted graptolite fauna suggests a level in the acuminatus Biozone. Farther east, the Gala 1 tract is lost as its boundary faults merge.
The outcrop of the Mindork Formation (up to 1000 m) extends from Luce Bay eastwards across the full length of the Southern Uplands into the north of the Lammermuir Hills. In Wigtownshire it forms the Gala 2 tract, but east of the Cairnsmore of Fleet pluton it is the northernmost of the Gala Group units preserved and locally spans at least two structural tracts. The component sandstone is generally well bedded and is petrographically distinguished by pyroxene and amphibole crystal debris together with andesitic and basaltic lithic clasts. Locally the mafic volcanic material forms up to 20 per cent of the rock (e.g. at Mindork Fell (NX 321 583)), but its proportion varies widely and the sandstone commonly contains no discernible volcanic material. There is an increase in lithological variation north-eastward with mudstone, and locally conglomerate, units appearing laterally along strike. The conglomerates, for example the Raeshaw Conglomerate near New Channelkirk (NT 484 555), contain clasts of igneous rock and Ordovician Moffat Shale Group mudstone. Graptolite faunas from the interbedded mudstones are mostly long ranging and of low diversity but are broadly indicative of the acuminatus Biozone, with one fauna indicative of the middle part of that zone.
The Mindork Formation also forms the northernmost preserved tract on the Rhins of Galloway, although there, andesitic detritus is additionally seen in the lower part of the Money Head Formation in the Gala 3 tract (about 900 m). This latter division is restricted to the western side of the Rhins and generally comprises thickly bedded (up to 5 m) sandstone with intervals of laminated siltstone up to 8 m and several beds up to 3 m thick of intraclast slump breccia. A sedimentary contact with the underlying Moffat Shale Group is exposed at Cairnweil Burn (NX 085 495) and Strandfoot (NX 052 482) where graptolites suggestive of the acinaces Biozone provide a maximum age for the Money Head Formation.
The Gala 4 (up to 1500 m) tract, preserved only to the west of the Cairnsmore of Fleet pluton, is composed of interbedded sandstone and siltstone, with intervals of silty mudstone up to 30 m thick occurring in a few places. The tract is locally termed the Sinniness Formation in Wigtownshire and the Float Bay Formation in the Rhins of Galloway. The uppermost 200 m of the Float Bay Formation is dominated by laminated siltstone. Soft sediment deformation is evident in a number of places ranging from slump folding of thin wacke interbeds in silty mud-stone to complete disruption of bedding, locally forming mélange units from a few metres up to 70 m thick. The age of the succession in the Gala 4 tract is well constrained by graptolite faunas. The basal sedimentary transition from the Moffat Shale Group exposed at Culroy (NX 255 540) yields revolutus Biozone faunas, and mudstone beds within the turbidite sequence contain faunas that indicate a range from the revolutus Biozone into the triangulatus Biozone.
The Gala 5 tract (up to 900 m) is well exposed in the Rhins of Galloway and east of Luce Bay in Wigtownshire where it is locally named the Stinking Bight beds and the Garheugh Formation respectively. It is composed of alternating thinly bedded and more massive turbidite sandstone units (P008401), locally with interbedded laminated siltstone and mudstone. In Wigtownshire, one such interbedded unit near Rocks of Garheugh (NX 268 501) is 10 m thick and includes very thin beds of red mudstone and bentonite. Sedimentary breccia, composed of sandstone clasts in a coarse-grained sandstone matrix, occurs sporadically in the Garheugh Formation in units up to a few metres thick, and in Wigtownshire is associated with a unit of matrix- and clast-supported conglomerate up to 30 m thick, intermittently exposed for 6 km along strike. Individual clasts are up to 30 cm in diameter and are mostly well-rounded cobbles of crystalline quartz or quartz-arenite. Other clast lithologies are black silty mudstone (one example yielding Ordovician brachiopod fragments), laminated sandstone, rare granodiorite and a highly ferruginous rock. These accessory clasts are up to a few centimetres in size and vary from angular to well rounded. Deposition of the conglomerate in a system of anastomosing channels seems most likely.
Between the Cairnsmore of Fleet granite and the Permian Thornhill Basin, a medium- to very thick-bedded sandstone sequence assigned to the Gala 5 tract includes units of matrix- and clast-supported, cobble to boulder conglomerate. The conglomerate beds are up to 300 m thick and are lenticular over 1–2 km of strike length, e.g. at Castramon Hill (NX 780 835). Composed of generally well-rounded clasts of quartz, quartz arenite, andesite, limestone, chert and mudstone, these conglomerate units are probably equivalent to that in the Garheugh Formation in Wigtownshire.
The Gala 5 turbidite succession in the south-west of the Southern Uplands is largely unfossiliferous, though one fauna from an interbedded mudstone in the Dalmacallan Forest (NX 816 885) indicates the triangulatus Biozone or younger. However, farther west, the maximum age is constrained by a magnus Biozone fauna from underlying mudstone of the Moffat Shale Group at The Hooies (NX 068 446) on the Rhins of Galloway. In the north-east of its outcrop, the Garheugh Formation includes an exceptional conglomeratic unit that crops out north-east of Peebles near Fountainhall (NT 423 501). There, a sequence of thickly bedded sandstones includes a laterally discontinuous body, up to 500 m thick, of intraformational pebbly sandstone known locally as the Dyker Law Conglomerate Member; a triangulatus Biozone age has again been determined from graptolites in a rare mudstone interbed. It may be that the Garheugh Formation becomes diachronously younger to the south-west.
In the Rhins of Galloway, the Grennan Point Formation (Gala 6) (300–600 m) is well- exposed above several tectonic repetitions of the basal part of the succession and the top of the underlying Moffat Shale Group at Drumbreddan Bay (NX 075 437). A few metres of grey mudstone and laminated siltstone occur at the transition, overlain by a succession that mostly comprises sandstone but includes thick units of laminated siltstone, the latter sporadically including thin beds of pale grey and red mudstone. A convolutus Biozone fauna has been obtained from the base of the formation at Grennan Point (NX 075 439) and in Drumbreddan Bay. In Wigtownshire, a lenticular sandstone tract in an analogous structural position extends south of the Cairnsmore of Fleet granite. Farther east the stratigraphical relationships become more complicated, with anastomosing tract-boundary faults resulting in multiple tracts of similar composition and overlapping, mid Llandovery age; this has previously led to interpretations involving an apparent structural reversal of the Gala 5 and 6 tracts. All of these mid Llandovery tracts are probably equivalent to the Queensberry Formation as exposed to the east of Thornhill (see below and P912384).
In the south-west of the Southern Uplands, the Gala Group outcrop extends southward with the Gala 7 tract (which locally bifurcates) comprising the Mull of Logan Formation (about 1800 m). The southern part of the outcrop is largely composed of well-bedded wacke sandstone with mudstone intervals varying from thin laminae to units 40 cm thick. Very thick (<2 m) sandstone beds occur dispersed throughout the succession and locally, for example north-east of Elrig (NX 331 484), even thicker units of massive sandstone are evident. The northern part of the outcrop, well exposed north from the Mull of Logan on the west coast of the Rhins of Galloway and inland on the western side of the Wigtown peninsula, is composed of well-bedded sandstone with interbedded units of laminated siltstone, generally less than 15 m thick but rarely up to 100 m. Sparse graptolite faunas from the mudstone range across the guerichi, turriculatus and crispus biozones. Beds of laminated red mudstone up to 3 m thick occur locally towards Dumfries, but only thin laminae of red mudstone are present in the south-western part of the tract.
The northern part of the Gala 7 tract includes sporadic and laterally discontinuous units of massive sandstone up to several hundred metres thick, with individual beds recognisable up to 8 m thick, and spectacular intraformational breccia deposits most probably emplaced as debris flows. The breccia is composed of angular to subrounded clasts of siltstone and sandstone, variably matrix- or clast-supported in a sandstone matrix. In the Rhins of Galloway, breccia clasts range up to 10 m across in a 550 m-thick breccia-dominated succession. East of Luce Bay, breccia with clasts from a few centimetres up to 50 cm in diameter forms two units, 150 m and 200 m thick, separated by about 350 m of thickly bedded sandstone. In both areas the breccia units die out laterally within a few kilometres.
As originally conceived, the tract sequence of the Gala Group extended to a Gala 8 tract, or Port Logan Formation (about 800 m), with an outcrop restricted to the Rhins of Galloway and western Wigtownshire (P912325). It is largely composed of wacke-type sandstone beds with mudstone intervals ranging from thin laminae to beds 40 cm thick. On the Rhins of Galloway, several thicker mudstone units, up to 135 m thick, include laminated siltstone and thin-bedded sandstone and, locally, chert layers with carbonate laminae. The transition from the Moffat Shale Group into the sandstone succession is exposed at Clanyard Bay (NX 101 380) with faunas from the mudstone ranging up to the sedgwickii Biozone. Graptolites from mudstone interbeds within the sandstone succession range from the guerichi to the crispus biozones.
The regional relationships of the southern part of the Mull of Logan Formation in the Gala 7 tract, and also of the Port Logan Formation in the Gala 8 tract, both became ambiguous following the establishment of the Ettrick Group in the central Southern Uplands. There is an overlap in age between the Gala 7 and 8 tracts and the Ettrick Group, whilst all three divisions share a distinctive sandstone composition, shown most clearly by the whole rock geochemistry. Likely correlation between the Ettrick Group and the previously defined Gala Group tracts is summarised in Table 3 and discussed further, below, as part of the Ettrick Group account.
Eastward from the Cairnsmore of Fleet pluton, an increasingly complex array of anastomosing faults is indicated by discontinuous outcrops of Moffat Shale Group until, east of Moffat, ten or more relatively narrow, laterally discontinuous tracts can be recognised, all of similar lithological character and overlapping in age. The succession repeated in these tracts (uncertain but probably around 1000 m) comprises the Queensberry Formation (P912331). At its base, a few metres of unfossiliferous grey siltstone overlies the Moffat Shale Group (e.g. at Dob’s Linn (NT 196 159)) and is succeeded by alternations of thickly and thinly bedded sandstone sequences; interbedded units of siltstone range upward from a few metres in thickness, commonly to 50 m and more rarely to 200 m. Units of conglomerate and intraformational breccia occur locally. These are generally up to a few metres thick but rarely 100 m of breccia is seen, associated with massive sandstone. The breccia is variably matrix- to clast-supported and is mostly composed of angular to rounded, fine-grained sandstone and siltstone clasts, generally a few centimetres in diameter but locally up to 50 cm, in a coarse sandstone matrix. In the north-east of the Southern Uplands, the Queensberry Formation continues through the Lammermuir Hills from Oxton to the limit of the Silurian outcrop south-west of Dunbar.
The maximum age of the Queensberry Formation is constrained by the youngest faunas in the numerous outcrops of the underlying Moffat Shale Group. Faunas indicative of the convolutus Biozone and, in more southerly tracts, the sedgwickii Biozone are common; at Dob’s Linn, at the southern edge of the outcrop, the youngest fauna proves the halli Biozone. Graptolite faunas from interbeds within the formation itself range through the sedgwickii, halli and guerichi biozones.
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