Aufsiitze

Revision of the Stratigraphy of Permian and Supposed Permian Rocks of Southern Scotland By M. E. BROOKFIELD, Guelph *) With 19 figures

Zusammenfassung Erstmals wird hier eine stratigraphisehe Gliederung der permischen Beckensedimente im siidlichen Schottland gegeben. Diese Gesteine bestehen aus Piedmont-Brekzien und Laven, die yon Fanglomeraten und Dfinenkomplexen iiberlagert werden. Alluviale F~icher und Schwernmfl~iehen-Sedimente gehSren hierzu. Diese Serien sollen ein Alter von Stephan bis Unter-Perm aufweisen und vergleichbar mit 5hnlichen kontinentalen Sedimenffolgen in England und im Unter-Botliegenden von Mitteleuropa sein.

Abstract The formal stratigraphy of the Permian basins of southern Scotland is described for the first time. These rocks consist of piedmont breccias and lavas, overlain by wadi-fan and dune complexes, and alluvial fan and floodplain sediments. The sequences are inferred to be Stephanian to Lower Permian in age and correlatable with similar continental sediments in England and with the Lower Rotliegendes of Germany and the North Sea.

R6sum6 La stratigraphie des formations des bassins permiens du Sud de l'Ecosse est ici d6crite pour la premi6re lois. Ces roehes sont des br~ches de piedmont et des laves, surmont6es par un complexe de d6p6ts d'6pandage et de dunes, et par des cones et des d6p6ts alluviaux. Les couches seraient h rapporter au St6phanien et au Permien inf6rieur, et ~ correler avec des s6diments continentaux semblables en Angleterre et avec le Rothliegend inf6rieur d'Allemagne et de la Mer du Nord.

I~paTKoe eo~ep~i~aHHe Bnepm,ie npoBe~eHo cTpaTHI'padpHqecKoe noffpa3~e~eHHe oca~oqHI=IX nopo~ 5acce~iHa HepMCKOPO Bo3pacTa tO--Oft IilOTJIaH~HH. 9TH IIopo~I,I eOCTOHT H3 5pe~qn~ Piedmont H JiaB, nepeKpl~iTmX dpaHrJloraepaTa~a n I~OMIIJIeKcaMa~mH. Cm~a me npHna~J~e~aT a~J~mBHaYrbHB~e OTJ~O~eHHa paaJm~Horo rpaHy~oMewpn~iec~oro cocTa~a. ~TH cepHrI HMeIOT no-BrI~HraoMy Bo3pacT OT CTe~0aHCKOFO BeHa ~o HHX~HeI~ nepMH; HX MO~-I~HO eOIIOeTa,BHTI~ e IIO~D~O~HBIMHMaTepHI~OBBIMH o c a ~ o q HBIMH CBHTaMtI ~A_HrJIHH rI HH>KHefI rlepm~ epe~Hel~ EBpOHBI.

1. Introduction Rocks of supposed Permian age outcrop in isolated basins in southern Scotland and in some cases are continuous with more extensive offshore basins in the Irish Sea and Firth of Clyde (fig. 1). The Permian basins of southern Scotland present special stratigraphic problems. The sediments consist of laterally variable continental elastics, generally lack fossils usable for correlation, are poorly ex*) Author address: M. E. BnOOKFIELD,Department of Land Resource Science, University of Guelph, Guelph, Ontario, Canada. 110

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Aufsiitze have not been dated radiometrieally, partly because of unsuitable lithology and alteration (SMITrt et al., 1974). The rocks were first assigned to the Lower Permian by MtIRCHISON and HARKNESS (1864) on lithologieal grounds, though they were earlier and later considered to be Triassic in age (HARKNESS, 1850; SHERLOCK,1926). Recently they have been again placed in the Lower Permian (SMITH et al., 1974). The basins discussed in detail here are the Dumfries, Lochmaben, Thornhill, Moffat and Stranraer basins, together with some smaller outliers. The Dumfries, Lochmaben, Stranraer and Moffat basins have not been studied since the work of HARKNESS (1850, 1856) and GEmlE et al. (1871, 1878, 1877). Though the Thornhill basin formed part of an excellent study by SIMPSONand RICHEY (1986) no detailed sedimentologieal study has ever been done on any of the Permian basins. Since various shades of red are the prevailing colour, the formation definitions do not include colour. Fluviatile palaeoeurrents given are usually based on very few measurements (less than 5) per locality, and are considered to give only a rough approximation to the general current direction. Palaeowind directions are based on numerous readings of cross-lamination (often more than 10O per locality), corrected for tectonic disturbance on a stereonet according to SHOTTON'S (1956) method. Borehole data is from LAWRIE and CRAIG (1945), CARTER and MCADAM (1968). The purpose of this paper is to revise and formalize the stratigraphy of the various basin and discuss its implications for the Permian history of southern Scotland. The detailed sedimentology will be discussed elsewhere. Stratigraphy The formal stratigraphy follows the recommendations of HARLANDet al, (1972) which closely follow the recommendations of the International Subcommittee on Stratigraphie Classification (HEDBERC, 1972), differing mainly in the use of a trinomial rather than a binomial naming system for formations, and is entirely lithostratigraphic. To some the plethora of names proposed will seem excessive, but I adhere to the concept of a formation as a well-defined lithologieal mapping unit showing demonstrable stratigraphic continuity. Since the Permian basins are isolated teetonically, some of the formations named here are probably equivalent, but until this can be proved separate names are justified. In the course of this study I have visited all presently accessible exposures of Permian sediments in the basins. Dumfries Basin This enclosed semi-elliptical basin contains up to 1600 metres of sediment, probably mostly of Permian age (CRAIG, 1965). On the east, thick aeolian sandstones rest unconformably on an irregular basement of Ordovician and Silurian greywackes, argillites, and cherts. These aeolian sandstones underlie, and are interbedded with, breccias and fluviatile sandstone towards the western margin of the basin. In places the western margin is entirely composed of massive debrisflow and sheet-flood breceias with thin impersistent fluviatile and aeolian sandstone partings, which are downfaulted against Ordovician sediments and the Criffel granadiorite (fig. 2). 112

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The Permian sediments were first thoroughly studied by HArdiNESS (1850, 1856) who, in the neighbourhood of Dumfries, divided them up into three units: thickbedded sandstones, about 180 metres thick; overlain by about 100 metres of conglomerate; in turn overlain by soft, fine-grained sandstones about 100 metres thick (HARKNESS, 1850). But this sequence (fig. 8) was measured in only part of the basin and gives a misleading impression of the succession which has been perpetuated ever since (SMITH et al., 1974). After GEIKIE et al. (1877) mapped part of the area, only a few intermittent observations have been made on the Permian rocks, e.g. by CAMEaON-SMITH (1925), WATT (1902), SHOTTON (1956) and GLENNm (1970). The structure of the basin is simple, consisting of a synclinal depression centred near the western margin of the basin. The structure is readily explained as due to coarse sediment input from the west being accommodated by subsidence along western boundary faults. Closely analogous modern basins exist among the intermontane arid basins of the western U.S.A. (SPENCXa, 1969). Stratigraphic sections, 114

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Sandstone

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This formation takes its name from the town of Loeharbriggs, near Dumfries, where three large quarries expose up to 25 metres of aeolian sandstone (CAM~aON SMirn, 1925)9 The formation consists of medium to fine grained quartz sandstone, showing aeolian cross-bedding. The sandstones were first described by HARDNESS 116

M. E. BROOKFIELD- - Revision of the Stratigraphy of Permian Rocks of Scotland

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117

Aufs~itze used the name for their Dumfries Series, It can hence not be used for a formation (H~,DB~.~G, 1972). The type section of the Locharbriggs Sandstone Formation is designated as Locharbriggs North Quarry (NX 810 990) which is still actively worked. There, 20 metres of medium to fine grained quartz sandstones are arranged in wedgeshaped and planar tabular sets between 0.5 and 2.0 metres thick, dipping between 10 ~ and 80 ~ south-west. The bounding surfaces of the sets are frequently covered with aeolian ripples striking down the dip. The aeolian foresets show a consistent south-westerly dip defining a resultant wind vector from the north-east (fig. 2). The tabular sets, uniformly dipping foresets and other structures most closely approximate the transverse dune type of McKE~ (1966); but I have elsewhere noted that much larger bedforms, draas, were probably responsible for the structures observed (BRooKFI~.LD, 1977). The sandstones consist of subrounded to very well-rounded quartz, with less than 5 percent of very wellrounded basalt, felspar and rock fragments (fig. 6 D), weakly cemented by silica, and with interstitial iron oxide. Individual laminae are less than 1 millimetre thick and show typical excellent aeolian sorting of individual laminae. Towards the bases of the foresets, thin silt laminae separate the sand laminae and probably represent sedimentdropped out of suspension in the lee of the 'dunes' (cf. McKEE and MOIOLA, 1975). Neither base nor top of the sandstones are exposed at Locharbriggs. At Quarrelwood (loc, 88, fig. 2), the sandstones lie unconformably on vertical Ordovician greywacke and argillite, but show no change in grain-size at the base. On the western side of the basin, e. g. at Craigs (loe. 16),and in the Cargen Water (loc.7), reworking of contemporary fluviatile sediment by the wind is shown by the greater percentage of rock and basalt fragments; the latter probably derived from the Thornhill basin to the north. The Craigs sample (fig. 6 c) illustrates a typical bimodal reg deposit (cf. FOLK, 1968). o n the north-western side of the basin, thick aeolian sandstones underlie fluviatile sandstones and finegrained breccias at MiUigantown (locs. 1, 2). Computation indicates a minimum thickness at outcrop of 700 metres. Geophysical evidence suggests up to 1000 metres at depth (CRAIC, 1965). The Locharbriggs Sandstone Formation is distinguished by its large scale aeolian cross-bedding and its fine to medium-grained very well sorted, unimodal or polymodal sandstones. It covers the whole of the eastern and northern parts of the Dumfries basin, and has a relatively uniform lithology and mineralogy. It lies unconformably on Lower Palaeozoic sediments on the east, and is overlain and interbedded with breccias and fluviatile sandstones on the west. Doweel

Breccia

Formation

The name is taken from Doweel Farm, west of Dumfries. The formation consists of coarse to fine-grained, moderately to well-sorted dominantly sandy breccias, interbedded in places with fluviatile sandstone, breccia sandstone and aeolian sandstone. No finer grained units are exposed. Towards the base of the I.C.I. borehole section, very thin and rare siltstones and mudstones are interbedded with fluviatile sandstones (loc. 9, fig, 4), On the west the breccias consist of thick-bedded coarse tabular breccias with thin impersistent sandstone partings. The breceias tend to become finer grained and better sorted towards the east, 118

M. E. BROOKFIELD

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Revision of the Stratigraphy of Permian Rocks of Scotland

where small t o large-scale scour and fill sometimes occurs. Tongues o f aeolian sandstone are frequently intercalated between the breccias' The t y p e section is designated as the old railway cutting south of Doweel Farm, west of Dumfries (NX 985 789 - - NX 941 740: loc. 8, figs. 2, 4). Twenty metres of tabular medium to fine-grained red breccias, in units of 1--2 metres are well exposed though smoke-blackened in the 600 metre long, 6 metre high cutting. The breceias dip gently north-east and consist of angular to sub-angular clasts up to 20 cm. in diameter of intrusive porphyrite (85Yo), grey and purple greywacke (80~o), grey argillite (80~o) and grey schist and vein quartz (5~o). Visual roundness of the clasts range between 0.2 and 0.5, based on clasts with average diameter of 8 4 cm. (KauxIBEIN, 1941). The matrix consists of subrounded to sub-angular silty sand with only traces of 'millet seed' aeolian sand grains (fig. 6 A, B). Lenticular sand and pebble lenses occur within and between the breccia units. The tabular clasts are often imbricated, defining a consistent easterly current flow. Rough vertical size grading of individual breccia units is common, but there is no sign ohf channelling of one unit by another. Average size of the breccia clasts is between 1 and 5 cm. The breccias of this type section resemble fluid debris-flow and sheet.flood deposits of alluvial fans. Sections north and east of the type section, and stratigraphically lower show similar features, but contain greater proportions of aeolian sand grains, are often finer-grained and sometimes show large-scale cross-bedding. They also have thin aeolian sandstone interbeds, and the thick breccia units are frequently separated by fluviatile sandstones. Occasional unsorted breccias with a sand-silt-clay matrix occur. As well as debris-flow and sheet-flood deposits, these include braided stream deposits and were probably deposited on the lower parts of alluvial fans. The unsorted breccias are probably viscous debris-flows. Each of the ridges south of Dumfries represent breccia tongues overlying fluviatile and aeolian sandstones and mark periods of advance of the alluvial fan deposits out into the basin. A second group of ridges occurs to the south around Bankend, separated from the Dumfries ones by lower lying ground. This more southerly group differs from the northern group in the presence of abundant, sometimes dominant, pink granadiorite clasts and rare Carboniferous limestones clasts in a sometimes arkosic matrix. These indicate a source including the main Criffel granadiorite and the Carboniferous limestone sediments south of it. Thus two alluvial fan complexes were probably present along the western margin of the basin. It is possible to give each of these breccia groups a formal name. Thus the southern group could be called the Caerlaverock Member of the Doweel Breccia Formation; while the group around Dumfries could be called the Cargen Member. Further subdivision of each member into tongues is possible, e.g. the Maidenbower Craigs Tongue. Such subdivisions are used informally in the stratigraphic columns, but I am not sympathetic to formalizing such refined terminology in thick sequences of laterally variable elastics. The Doweel Breccia is characterized by interbedded breccias, fluviatile sandBasin, forming a small syncline closing north (fig. 2). The breccias may once have extended across the basin, but the increase in interbedded fluviatile and aeolian sandstone towards the east and north suggests that the breccias form wedges thinning away from the western margin of the basin, The presence of aeolian 119

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Fig. 7. Outline geological map of the Loehmaben basin, with localities cited. Symbols as for fig. 2, with addition. Partly from NAmN (1956), PALLISTER(1952). 120

M. E. BROOKFIELD

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Revision of the Stratigraphy of Permian Rocks of Scotland

sand in the lower part of the formation and its absence from the topmost units suggests that the Doweel Breccia Formaion is partly contemporaneous and partly younger than the Locharbriggs Sandstone Formation. The Doweel Breccia is chracterized by interbedded breccias, fluviatile sandstone and minor aeolian sandstone, with clasts of westerly provenance derived from local rocks. As defined here it includes the Craigs Breccia and Castledykes Sandstone of SMITH et al. (1974) which they placed above both the Doweel Breccia Formation and Locharbriggs Sandstone Formation (fig. 8). The Craigs Breccia simply marks the first breccia tongue at outcrop, though others occur at depth (borehole 18, fig. 4). The Castledykes Sandstone consists dominantly of tabular fine-grained breccias and fluviatile sandstones, and is a minor facies variant of the Maidenbower Breccia Tongue. It is identical to other fine-grained breccias elsewhere in the Dumfries area. Lochmaben

Basin

The Lochmaben basin lies to the east of the Dumfries basin and is separated from it by a ridge of Lower Palaeozoic sediments. To the northeast, the Lochmaben basin is separated from the narrow elongate Moffat basin by a thin transverse strip of Ordovician sedimentary rocks. On the north-west, west and east, the basin is bounded by Lower Palaeozoic greywaeke and mudstones, and on the south by Devonian and Carboniferous lavas, carbonates and various clastics. The known sediments in the basin are dominantly aeolian sandstones (fig. 7). Breccias occur along the north, east and south margins; but are surprisingly absent on the west. Together with the absence of breccias along the eastern margin of the Dumfries basin this indicates that the Lower Palaeozoic ridge, at present dividing the two basins, may not have formed a markedly positive area during Lower Permian times. On both sides of this ridge aeolian sandstones rest directly on the Lower Palaeozoic, with only sporadic thin basal breccias between. On the north, the breccias are sometimes faulted against (lot. 42, fig. 7) and sometimes unconformably on the Ordovician (locs. 38, 41, fig. 7). Sections showing breecias and aeolian sandstones unconformable on the Lower Palaeozoic are also found along the eastern margin (locs. 50, 51, figs. 7, 8). The southern margins are complicated by numerous faults (fig. 7), some of which were undoubtedly active during Permian times. Small outliers of lava and associated breccia to the north of the Lochmaben basin indicate a former connection with the Thornhill basin to the north-west. Geophysical evidence provides some indication of the form of the basin (fig. 9) and indicates a maximum thickness of sediment of about 900 metres (BoTT and MASSON-SMITH,1960). The surface exposures, and two boreholes (fig. 8) suggest that most of the southern part of the basin is dominated by aeolian sandstone, grading north into interbedded breccias, fluviatile sandstones and aeolian sandstones. The basin has few good exposures, except along the northern margins. Partly because of this the lithostratigraphy has of necessity to deal with small areas, and is perhaps unduly complex. Four formations are distinguished: the Corncockle Sandstone Formation, the Hartfield Breccia Formation, the Kettleholm Breccia Formation, and the Locker121

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~SANDSTON~ NEW RED I~BRECCIA JSANDSTONE ~CARBONIFEROUS N I 2Km!

Fig. 9, Map of the Dumfries and Lochmaben basins showing sediment thickness. After BOTT and MASSoN-SMITn(1960), CaAIG (1965) and personal observations. bie Breccia Formation: the last two being based on very small, isolated exposures. Corncockle

Sandstone

Formation

The sandstone quarries at Corncockle (NY 085 870, loc. 47) were early famous for the abundance of reptilian footprints found in them (see SAnaEANT, 1974), but little has been published on tile sediments. The sandstones, like those at Locharbriggs consist of fine to medium grained, well sorted, red quartz sandstone, with minor amounts of felspar (fig. 10 D). At the type section of Corncockle Quarry the sandstones are arranged in tabular sets dipping up to 40 ~ south-west, and show large scale aeolian cross-bedding (ef. SHOTTON, 1956). Up to 80 metres are exposed in the vertical and now inaccessible faces of the quarry. The easterly

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F M. CORNCOCKLE LOC. 47 ENV. AEOLIAN FORESET 30

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Fig. 10. Representative grain-size analyses of sediments from the Lochmaben basin. Symbols as for fig. 6. parts of the quarry show very uniform steeply dipping cross-laminated tabular sets. These pass to the southwest into less steeply dipping more irregular sets, often bounded by concave downwards curving bounding surfaces. The sections exposed c a n be briefly interpreted as dome-shaped 'dunes' overlain by large barchanoid 'dunes' (McK~E, 1966). An usually coarse-grained facies can be examined at Corsua (NY 075 880; loc. 45, fig. 10 c). At Aebriggend (loc. 27, fig. 7) on the western side of the basin, the aeolian sandstones can be seen resting on the 124

M. E. BROOKFIELD -

-

Revision of the Stratigraphy of Permian Rocks of Scotland

Lower Palaeozoic with only a thin breccia sandstone intervening. To the north the aeolian sandstones overlie and are interbedded with the fine-grained breeeias of the Hartfield Breccia Formation. At Nethercleugh (loe. 50, fig. 7) on the eastern side of the basin, basalt fragments occur in the aeolian sandstones which rest either on the Lower Palaeozoic, or on thin breeeias and fluviatile sandstones of the Hartfield Breccia Formation. The Corneoekle Sandstone is a typical aeolian sand deposit, as shown by the large-scale cross bedding, the excellenee of sorting within individual laminae, the absence of clay, and the grain-size eharaeteristies (fig. 10 C, D). As at Locharbriggs, the uniform dip of the cross-lamination indicates winds blowing dominantly from the east-north-east (fig. 7). Hartfield

Breccia

Formation

This formation takes its name from the farm of Hartfield on the banks of the Kinnel Water (loe. 42, fig. 7) where interbedded fine-grained breccia and fluviatile sandstone are intermittently exposed upstream and downstream in the banks of the Kinnel Water (NY 068 925 to NY 068 918). The type section lies along the Kinnel Water and extends for 0.5 Kin. north and south of the farm. In three major sections, the important lithologies of the formation can be examined, In the northermost locality (NY 068 925, loe. 42) 15 metres of well-sorted tabular, fine-grained breceias are interbedded with tabular fine-grained breccia sandstones and fluviatile sandstones in units between 0.2 and 1.0 metre thick. The breccias consist of up to 25~o of angular clasts in a matrix of silty sand. To the south (NY 068 922) about 5 metres of interbedded fine to medium grained fluviatile and aeolian sandstones dip 15 ~ south. Just south of Hartfield farm (NY 070 920 to NY 068 918) fine-grained sandy breccias and breccia sandstones are exposed for a considerable distance along both banks of the Kinnel Water. They consist of tabular, medium to fine-grained sandstone, with variable proportions of highly angular breccia clasts. In contrast to the northernmost locality, these breeeias show frequent irregular channelled bases, cut and fill crossbedding, clast imbrication and low-angle cross-bedding in the sandstones. The dominant clasts of all these breceias are a distinctive greenish-buff angular to sub-angular fine-grained tuffaceous and mieaeeous sub-greywaeke of local derivation (visual roundness 0.4--0.6), with up to 10~o (combined total) of angular amygdaloidal basalt, purple greywaeke and red argillite. The basalt elasts show aeolian fluting and polishing, and the matrix contains variable proportions of 'millet seed' aeolian sand (fig. 10 B). A source between north-west and north is indicated by clast imbrieation and by the relative abundance of basalt clasts, which could only have come from the basalt flows to the north and north-west, in and to the south-east of the Thornhill basin (figs. 7, I2). There are. no indications of basalt feeder dikes or flows east of this. The breccias and sandstones indieate fluid debris flow, sheetflood and braided stream deposits of wadi-fans. The abundance of aeolian sand and wind-polished elasts indicate extensive reworking of aeolian deposits and areas of deflation, possibly on the old fan surfaces. To the south in the Kinnel Water, the Hartfield Breccia Formation is sharply overlain by the aeolian sandstones of the Corneockle Formation. In the Mollin 125

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F M. KETTLEHOLM LOC. 53 ENV. FLUVIATILE

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30 F M. THoRNHILL LOC. 71 ENV. AEOLIAN ACCRETION

F M. DURISDEER LOC. 57 ENV. FLUVIATILE @

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Fig. 11. Representative grain-size analyses of sediments from the Lochmaben and Thornhill basins. Symbols as for fig. 6. Burn to the west (loc. 41, fig. 8), the Hart.field Formation is thinner and interbedded with aeolian sandstone. Here, the basal breccias rest sharply on an irregular surface of reddened Ordovieian greywacke and argillite. On the eastern side of the basin, the Harffield Formation is represented by fine-grained tabular greywaeke and basalt-bearing breccias and fluviatile sandstones, with aeolian 'millet seed' grains, resting unconformably on Ordovician greywacke (loc. 50, figs. 7, 8). 126

M. E. BROOKFIELD -

-

Revision of the Stratigraphy of Permian Rocks of Scotland

A fine-grained facies of the formation is exposed on the western side of the basin at Trailfleet (loc. 56, fig. 7), and also in the centre of the basin in the Lochmaben water borehole (bore 16, fig. 7). At Trailfleet, 40 metres of medium to fine-grained, poorly sorted fluviatile sandstone, with rare greywacke and basalt clasts, are interbedded with fine-grained micaceous sandy siltstones. The Hartfield Breccia Formation is distinguished by its generally fine-grained nature, the presence of aeolian sand and wind-facetted basalt clasts, and the dominance of greenish-buff subgreywacke clasts. It extends in a wide belt around the northern and eastern sides of the basins, and probably extends as thin tongues at depth into the centre of the basin. The clasts are derived from the north-west. The breccias are identical in lithology and stratigraphic position to those of the Locherben Breccia Formation of the Thornhill basin. One isolated exposure south of Locherben (loc. 85, fig. 7) shows a possible relic of a once continuous sheet between the two areas. Kettleholm

Breccia

Formation

This formation takes its name from the village of Kettleholm where about 20 metres of breccia are exposed in the Water of Milk (loc. 58, fig. 7, 8, NY 148 768). This exposure is designated as the type section. The breccias consist of massive and poorly bedded coarse to medium-grained breccia, with relatively well-rounded clasts of Silurian greywacke and argillite (visual roundness 0.7), up to 20 cm. in diameter though averaging only 5 cm., in matrix of granular rock fragments and silt. In contrast to most of the other breccia formations, coarse to medium-grained quartz sand is virtually absent (fig. 11 A). Bedding is poorly developed, but irregular tabular units about 1 metre thick dip 5 ~ south-west. Imbrication is poorly developed but some measurements indicate a westerly current flow. The breccias are moderately well-sorted within individual units. There are a few granular sandy silt-stones interbedded in places, but no sign of any chanelling or cross-bedding in the breccias. The breecias are however usually packstones, with the breccia clasts in contact. These features are suggestive of fluid debris flow and possibly sheetflood deposits of alluvial fans. The distinctive features of the Kettleholm Breccia Formation are the overwhelming dominance of red micaceous greywacke clasts of local provenance, their subrounded nature, the absence of basalt clasts and rarity of quartz sand, and the poor sorting of the matrix. The formation resembles some units of the Doweel Breccia Formation, but is closest in character to the Snar Breccia Formation of the Snar valley far to the north (fig. 19). Lockerbie

Breccia

Formation

Around Lockerbie, coarse-medium grained sandy breccias do not resembly any of the formations of the Lochmaben basin described above. The breccias consist of angular clasts of greywacke (60~o), argillite (25~o), green to red micaceous siltstone (10~o), with white Carboniferous limestone, basalt and porphyrite (5~o). The porphyrite forms very large angular clasts up to 0.5 metres in diameter; but the average clast size ranges between 1 and 8 em. The silty sand matrix consists of medium to fine grained quartz sand with up to 10~o of 'millet seed' aeolian 127

Aufs~itze sand (fig. 11 B). The percentage of clasts in the breccias varies from 10 to 50~o. Though dominantly in tabular units between 0.2 and 0.5 metres thick, the breccias show abundant imbrication, small-scale cut and fill, low angle crossbedding and occasional channelling. The type section is designated as the road cutting on the main Carlisle to Glasgow road, just north of Lockerbie (loc. 52, fig. 7, NY 181 828) where about 4 metres are exposed dipping gently east. The sediments represent sheet-flood and braided stream deposits of alluvial fans. A carbonate rich facies of the Lockerbie Breccia Formation occurs in the southern extremity of the Lochmaben basin (loc. 54, fig. 7, NY 110769). It consists of coarse tabular breccias, with thin impersistent lenses of fine-grained calcareous sandstone. Here about 5 metres of breccia consist of tabular units between 0.2 and 1.0 metre thick, containing large angular blocks of bioclastic limestone up to 0.5 metres in diameter, together with smaller clasts of limestone (80~o), greywacke (20~o), argillite (20~o), vein quartz (20~o) and basalt (10~o) in a matrix of silty sand. Ctast size is very variable but tends to average between 2--8 cm, The limestone clasts are derived from outcrops of the Lower Limestone Group of the Carboniferous to the south. The basalt clasts are identical with the probably Lower Carboniferous Birrenswark Basalt lavas which are extensively exposed to south and east of Daltonhook (fig. 7). The sandstones are very thin, impersistent, and consist of very fine-grained quartz sand and silt, with traces of basalt and 'millet seed' aeolian sand grains. Both breccia and sandstone units are very poorly sorted, with elasts 'floating' in the silty sand. All these features, together with the large size of some limestone elasts, suggests deposition by debris-flows on an alluvial fan. The stratigraphic relationship of the Lockerbie Breccia Formation to other Permian units in the Lochmaben basin is not directly determinable. However, the presence of 'millet seed' sand grains and the debris-flow nature of the sedimentation places the formation firmly within the environmental framework of the known Permian sediments. It is likely that the formation dips below the aeolian sandstones in the centre of the basin and is thus approximately equal in age to the Hartfield Formation. Where indurated, the Lockerbie Breccia Formation is silica-cemented. It closely resembles some units of the Doweel Breccia Formation in the Dumfries basin. Thornhill

Basin

The Thornhill basin occupies a narrow faulted belt north of the Dumfries basin (figs. 1, 12). Unlike the other basins it contains extensive exposures of Carboniferous sediment below the Permian. The Permian rocks are relatively thin and much has been removed by later erosion. One major Permian outlier at Locherben on the east, and other minor outliers to the south-east and south (figs. 7, 12) show probable former connections with the Dumfries and Lochmaben basins. These, for convenience, are treated with the Thornhill strata. In the Thornhill basin, the Permian rocks are better exposed than in the other basins, though the stratigraphy is complicated by the numerous faults. In the north, basal breccias and thin olivine basalt lava flows are overlain by breccias, fluviatile sandstones and siltstones, with interbedded and overlying 128

M. E. BaOOKFIELD- - Revision of the Stratigraphy of Permian Rocks of Scotland

Fig. 12. Outline geological map of the Thornhill basin, with cited localities. Symbols as for fig. 2 with additions. aeolian sandstones. In the south, the aeolian sandstones overstep the lower units and rest directly on the pre-Permian rocks (fig. 12). In the available exposures along the eastern side of the basin, palaeocurrents indicate Permian drainage south, along the basin axis, with sediment input from the eastern side. Aeolian sandstones show winds dominantly from the east-north-east, as in the other 9 Geologische Rundschau, Bd. 67

19 9

Aufs~itze Localities 57

59.-60

61-63

64

66

71

72

73-74

76

H THORNHILL FORMATION

.-~.! '

0, '

!

Fig. 13. Stratigraphic sections in the Thornhill basin, with cited localities. Symbols as for fig. 4. basins. The last comprehensive study of the whole basin was by SIMPSON and RiCnEV (1986), and their excellent survey has been used as the basis for more recent summaries (SMITH et al., 1974; CRAm, 1965; MYKURA,1967). However, no detailed environmental studies have been published and no formalized names proposed. The Permian sequence can be divided up into four formations. The Carton Basalt Formation, consists of a thin sequence of amygdaloidal basalt flows and associated thin breccias and sandstones. The Durisdeer Formation consists of breccia, breccia sandstones, fluviatile 130

M. E. BROOKFIELD -

-

Revision of the Stratigraphy of Permian Rocks of Scotland

sandstones, siltstones and rare mudstones, with abundant basalt detritus and aeolian sand grains. The Thomhill Sandstone Formation consists entirely of coarse to fine-grained aeolian sandstone, with very rare thin sfltstone laminae. The Locherben Breccia Formation consists of coarse to medium grained breccias and sandy breccias with only traces of basalt detritus, but abundant aeolian sand in the matrix. Carron

Basalt

Formation

The name is taken from the Carron Water, which drains the northern part of the Thornhill basin (fig. 12). The type section is designated as the stream section exposed in a gorge of the Carron Water, north-east of Carronbridge station (loc. 60, fig. 12, 18, NS 885 017 to NS 888 022), where about 20 metres of olivine basalt flows, with breccias and sandstones consisting of basalt, greywacke and ed. At the base of the section, a sandy breccia with small angular clasts of greywacke and basalt apparently rests directly on the Upper Carboniferous, though the contact is covered. Above this comes three olivine basalt flows, each consisting of a lower aphanatie and upper amygdaloidal layer. The tops of the basalt flows are fissured, infilled and overlain by medium to fine-grained laminated sandstone and fine-grained sandy breccias. The breccias and sandstones consist predominantly of angular basalt, greywacke and argillite clasts with only a low percentage of quartz sand and no 'millet seed' aeolian sand grains (fig. 14 B). In the southern part of the basin, at Sandrum (loc. 67) the lavas rest directly on Carboniferous sediments, with no breccia intervening, and are overlapped southwards by aeolian sandstones. Far to the south, in the Windyhill Burn (loc. 85, fig. 7) a small patch of breccia, with angular basalt and greywacke clasts is overlain by amygdaloidal olivine basalt (cf. GEIKIE, 1897, p. 61). The Carron Basalt Formation is characterized by the dominance of olivine basalt flow, with breccias and sandstones consisting of basalt, greywacke and argillite fragments, and by the scarcity of aeolian grains within the sediment interbedded with the lava flows. Durisdeer

Formation

This formation takes its name from the village of Durisdeer, to the south and west of which are extensive exposures of the formation. The formation consists of tabular sandy breccias, breccia sandstones, medium to fine-grained sandstones, with rare siltstones and mudstones. Sporadic lenses of aeolian sandstone occur, dominantly towards the top of the formation where it gradually passes up into the Thornhill Sandstone Formation. The type section is designated as the almost continuous exposures in the banks of the Hapland Burn (loc. 59, figs. 12, 18, NS 888 028 to NS 889 025). Here the formation directly overlies, and passes down into, the Carron Basalt Formation. The type section consists of about 70 metres of tabular sandy breccias, with clasts of amygdaloidal basalt up to 20 cm. in diameter, in a matrix of silty sand which contains up to 10~o of 'millet seed' aeolian sand grains (fig. 11 C). Most of the larger basalt clasts are wind-facetted and polished with very low roundness (visual roundness 0.4). Fine to medium grained laminated tabular sandstones, 131

Aufs~itze m

v

50

50

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F M. THORNHILL LOC. 71 ENV. AEOLIAN FORESET

F M. CARRON LOCo 60 ENV. FLUVIATILE

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F M, LOCHERSEN LOC, 75 ENV. FLUV!ATILE

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20

(g)

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Fig: 14. Representative grain-size analyses of sediments from the Thornhill and Locherben basins. Symbols as for fig. 6. breccia sandstones and, rarely, aeolian sandstones are interbedded. The breccias are graded, but only rarely show imbrication of clasts or cut and fill. The sand132

M. E. BROOKFIELD -

-

Revision of the Stratigraphy of Permian Rocks of Scotland

stones are dominantly evenly laminated with occasional low-angle cross-lamination. The sediments are pre-dominantly sheet-flood and braided stream deposits of pediments. At Durisdeermill (loc. 57) a highly irregular and weathered basalt surface with up to 5 metres of relief is overlain by coarse to fine-grained basalt breccias and occasional aeolian sands. Higher in the section, fining upward breccias and sandstone, with large basalt clasts at the base and abundant cut and fill and lowangle cross-bedding indicate braided stream channels; though the rarity of imbrication suggests rapid deposition during flood surges, and therefore only intermittent stream flow. The rare imbrication recorded indicates southerly flowing streams. A finer grained facies is exposed at East Morton (loc. 64), where thinly laminated fine-grained well sorted sandstone, siltstones and purple micaceous mudstones overlie fine-grained tabular basalt breccias. The section is disturbed, possibly by faulting, but more likely by solution of evaporite minerals, e.g. gypsum, which may once have cemented the sediments (cf. GLENNIE, 1970). Mudcracks and coarse well-sorted laminae of 'milled seed' sand are present. The sediments most closely resemble inland sabkha or ephemeral desert lake deposits. The Durisdeer Formation lies directly on the Carron Basalt Formation and passes gradually upwards into the aeolian Thornhill Sandstone Formation. The Durisdeer Formation is characterized by the abundance of wind-faeetted basalt clasts, abundant aeolian sand grains and wind-rounded basalt grains, and the rarity of greywacke and argillite clasts. Many of the sediments are finer-grained than in any other formation, e.g. the siltstone-mudstone units. The formation cannot be traced outside the Thornhill basin, but has affinities with the Locherben Breccia Formation and the Hartfield Breccia Formation in the presence of wind-facetted basalt elasts and abundant aeolian sand. The general depositional environment of the Durisdeer Formation is that of the lower parts of a wadi fan complex, where ephemeral braided streams debouch into a desert environment. In places, as at Durisdeermill, these streams flowed across a partially dissected pediment. Thornhill

Sandstone

Formation

The name is taken from the town of Thornhill. The type section is in Crichope Linn in the southern part of the Thornhill basin. In the type section (lot. 71, figs. 12, 18), the Criehope Burn has cut a deep gorge exposing about 80 metres of coarse to fine-grained well sorted laminated quartz sandstone in massive tabular and wedge shaped cross-stratified sets. The base of the formation is not exposed, but only 1 metre of covered ground separates grey-purple Carboniferous sandstones and siltstones from the lowest coarse-grained sandstones of the Thornhill Sandstone Formation. At the base the formation consists of very coarse, redgreen, silica-cemented evenly laminated coarse-grained quartz sandstone, passing up into large-scale cross-stratified fine-grained, well-sorted quartz sandstone (fig. 11 D, 14 A). Coarser grained bimodal sandstones and finer-grained, better sorted and cross-laminated sandstones alternate throughout the sequence. The sandstone consist of rounded to very well rounded quartz sand, with up to 5~ of very well rounded felspar and basalt grains. 'Millet seed' sand and basalt 133

Aufs~tze granules are relatively abundant in the coarser sandstone, but rare in the finergrained ones. In the southern part of the basin, the Thornhill Sandstone Formation rests directly on pre-Permian rocks, and the Carton Basalt and Durisdeer Formations are absent. From Sandrum north (loc. 67) the Thornhill Sandstone Formation overlies and is interbedded with the Durisdeer Formation. The Thornhill Sandstone Formation is characterized by well-sorted, wellrounded coarse to fine-grained quartz sandstones in tabular and wedge-shaped cross-stratified sets. It is a typical 'dune' deposit. Unfortunately exposures are not comprehensive enough to determine the type of dunes present. Locherben

Breccia

Formation

This formation is named from the farm of Locherben in the valley of the Garrel Water. The type section (loc. 75, fig. 12, 18; NY 960 967) exposes about 10 metres of red, well-sorted sandy breccia sandstone in tabular units between 0.2 and 0.5 metres thick, normally massive and ungraded, but occasionally showing clast imbrication and cut-and-fill. Clasts range in size from 0.2 to 10 era., with an average of 1 cm. The clasts consist dominantly of angular tabular and bladed greywacke and argillite of local derivation (visual roundness 0.4 0.5), with occasional clasts, especially the larger ones, of purple amygdaloidal basalt. The matrix consists of sub-angular to well-rounded quartz sand with up to 20~o of 'millet seed' sand, but only traces of basalt grains (fig. 14 C). In the higher beds (loc. 76) the breccias are finer-grained, contain no basalt detritus and interfinger with the basal beds of the Thornhill Sandstone Formation (loc. 76, fig. 18). These upper beds also show minor cut-and-fill and low angle cross-bedding in places. The Locherben Breccia Formation is similar to the Durisdeer Formation, but lacks abundant basalt detritus and finer-grained siltstone and mudstone. It is, however, identical in lithology to the Harffield Breccia Formation of the Lochmaben basin. The Locherben Breccia Formation represents fluid debris flow and sheetflood deposits of wadi-fans. Some low-sinuosity braided stream deposits are present in the upper beds. Moffat

Basin

This narrow elongate trough is entirely surrounded b y Lower Palaeozoic uplands (fig. 15). Breccias and aeolian sandstones are poorly exposed along its eastern margin, but are obscured by drift and scarcely exposed at all along the western margin. The breccias and aeolian sandstones are partly unconformable on a highly irregular Lower Palaeozoic basement, and partly faulted against them on the north and east. A major fault probably runs down the centre of the trough, controlling the course of the Annan Water. The sediments are possible thinner than their equivalents in the other basins, but no boreholes have penetrated the Permian. They may have been deposited in part within a basin scooped out of Carboniferous sediments. At one locality (loc. 85, fig. 15) a thin sequence of granular red mudstone and white, well-sorted medium-grained sandstone lies unconformably on the Ordovician and are themselves unconformably overlain by Permian breccias. This thin sequence is lithologically akin to 134

Fig. 15. Outline geological map of the Moffat basin. Symbols as for figs. 2, 7. 135

Aufs~.~e Localities

80

81

82

83-84

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10metres Fig. 16. Stratigraphic sections in the Moffat basin. Symbols as for fig, 4. Upper Carboniferous sequences in the Thornhill basin, but totally unlike any of the exposed Permian sequences. In the northern part of the basin, a basal breccia and sandstone sequence is succeeded by aeolian sandstones; and these, probably, by a fine-grained breccia and breccia sandstone sequence. To the south, the entire succession consists of coarse to fine-grained sandy-silty breccias with occasional interbeds of fine-grained sandstone. No fossils have ever been recorded in the Moffat basin, and the Permian age of the sediments is inferred on purely lithological grounds. Four formations are distinguished: basal breccias and sandstones, the Corehead Sandstone Formation, the Auchencat Breccia Formation, the Bellcraig Breccia Formation. Basal

Breccias

and Sandstones

These are not given a formal name, and are exposed at only two localities; in the Auchencat Burn (loc. 81) and north-east of Corehead farm (loc. 80) where they underlie the aeolian sandstones of the Corehead Sandstone Formation. The most complete section occurs at locality 81, where about 5 metres massive unsorted breccias, consisting of large, completely unrounded blocks of greywacke and argillite (visual roundness 0.1--0.2) lie in an unsorted granular, silty, sandy, clayey matrix. These breccias are probably scree deposits and are directly over136

M. E. BROOKFIELD

-

-

Revision of the Stratigraphy of Permian Rocks of Scotland

lain by fine-grained sandstones and breccias, consisting of rounded greywacke and argillite clasts in a matrix of sub-angular to angular sand, with lenses of intraformational clay clasts, and traces of 'millet-seed' sand. Thin silty lenses are common in the sandstones, and cut-and-fill cross-lamination occurs. These basal sediments are sharply overlain by aeolian sandstones of the Corehead Sandstone Formation. The relatively sudden transition to fine-grained, well-sorted breccias and sandstones, probably deposited by sinuous-braided streams, indicates that no alluvial fan development succeeded the screes. The fine-grained breccias and sandstones were apparently deposited directly on the scree breccias by streams flowing longitudinally along the trough, Unfortunately, the current directions obtained from the cut-and-fill cross-lamination are widely scattered, but tend to indicate a southerly flowing current. Corehead

Sandstone

Formation

The formation takes its name from the farm of Corehead in the north of the Moffat basin. The type section is designated as the exposures along an unnamed burn to the east of Corehead farm (loc. 80, fig. 15, 16, NY 078 125) where about 80 metres of aeolian sandstone can be measured. The sediments consist of alternations of finely laminated, fine-grained cross-stratified sandstone, with evenly laminated coarser sandstone. The grains consist entirely of sub-angular to subrounded quartz sand, with no 'millet seed' grains (fig. 17 c). No consistent dip could be measured, partly because of tectonic disturbance, partly because of the cross-bedding. However, the aeolian foresets are dominantly asymptotic towards the west, indicating a wind generally from the east. The Corehead Sandstone Formation directly overlies the basal sandstones and breccias. It may be overlain by the Auchencat Breccia Formation, but the junction is obscured and probably faulted. As a working hypothesis, the Corehead Formation is considered to be mainly older than the Auchencat Breccia Formation. Auchencat

Breccia

Formation

The type section of this formation is designated as the lower reaches of the Auchencat Burn, a tributary of the river Annan (loc. 81, figs. 15, 16); NT 076 104 to NT 079 104), where about 50 metres of sandy breccia, breccia sandstone and fine-grained sandstone are exposed. Neither base nor top to the formation is exposed. The breccias are moderately well-sorted in tabular units between 0.2 and 0.8 metres thick. They consist of angular to sub-angular tabular greywacke and argillite clasts up to 10 cm. in diameter, though averaging 1--3 cm. (visual roundness 0.4), in a matrix of fine to medium-grained quartz sand and rock fragments, with up to 50~o of 'millet seed' quartz sand. The tabular breccia sandstones average about 0.2 metres thick. Imbrication of clasts, grading and channelling are rarely developed. The few imbricated clasts noted define a south-westerly current flow. These sediments are separated by thin, less than 5 cm. thick layers of evenly laminated quartz sandstone in alternating laminae of fine-grained quartz sand and coarser 'millet seed' sand with small windrounded quartz sand and coarser 'millet seed' sand with small wind-rounded greywacke and argillite granules. 137

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Fig. 17. Representative grain-size analyses of sediments from the Moffat and Snar Valley basins. Symbols as for fig. 6. The stratigraphie position of the Auchencat Breccia Formation is debatable9 It contains vast quantities of aeolian sand and represents sheet-flood and rarely, braided stream deposits of a wadi-fan. Since the Corehead Sandstone Formation directly overlies the basal sandstones and breccias, and contains one thin breccia bed at its top, and since the Auchencat Formation apparently overlies the Corehead Sandstone Formation at locality 81, the Auchencat Formation is tentatively placed stratigraphically above the Corehead Formation9 The Auchencat Breccia Formation is characterized by highly angular local greywacke and argillite clasts, abundance of aeolian sand grains, and the absence 138

M. E. BROOKFIELD -

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Revision of the Stratigraphy of Permian Rocks of Scotland

of basalt clasts, and fine-grained siltstone and mudstone units. It represents sheetflood and rare braided stream deposits of wadi-fans. Bellcraig

Breccia

Formation

From just north of Moffat southwards, thick breccias occupy the whole of the Moffat basin and differ significantly from the other units of the Moffat basin. The name is taken from Bellcraig Lynn, a deep gorge on the eastern side of the basin, where over 80 metres of breccia are exposed in extensive vertical facies (loc. 84, fig. 12, 18, NT 106 013 to NT 113 014). Exposures further down the stream indicate a probably thickness in excess of 100 metres. The sediments consist of coarse to fine-grained well-sorted tabular sandy breccias, with thin interebedded fine-grained cross-stratified breccias and sandstones. In many cases, the breccias and associated sandstones are lenticular and show channelling, synsedimentary slumping and low angle cross-bedding. Imbrication is often well-developed. The breccia clasts consist of angular to subangular greywacke and argillite (visual roundness 0.4 to 0.5) with some chert clasts. Only traces of 'millet seed' sand occur in the lower and upper breccias (fig. 17 B); but some of the sandier breccias towards the centre of the sequence contain up to 50~o of 'millet seed' sand in their sand fractions. The whole sequence lacks fine-grained siltstone and mudstone units, and the sediments can be interpreted as fluid debris flow, sheet flood and braided stream deposits of alluvial fans. Clast imbrication shows a consistent west-south-west current flow. There is no regular variation in sedimentary character within the sequence. Identical, but thinner sections are exposed at locality 82 to the north at Moffat, and sporadically on the hillsides north and south of Bellcraig Lynn. At the head of Bellcraig Lynn, the breccias can be seen lying unconformably on a thin sequence of granular red mudstone and white sandstone, which, as mentioned earlier, is probably of Carboniferous age. Elsewhere the breccias presumably lie unconformably on Ordovician rocks. The Bellcraig Breccia Formation is characterized by the coarseness and angularity of the greywacke and argillite clasts, the presence of breccia channelling and marked imbrication in some beds, the sandy nature of the breccia matrix, the absence of fine-grained siltstone and mudstone units, and the absence of basalt clasts. Its stratigraphic position is unknown, as it can not be stratigraphically related to the formations in the northern part of the Moffat basin. Since, however, it contains abundant aeolian sand in its middle, it may be equivalent to the whole sequence of the northern Moffat basin. SnarValley

Basin

This isolated basin occupies a tectonic depression entirely within Ordovician rocks (fig. 1). The sediments consist of a monotonous sequence, at least 500 metres thick, of moderately well-sorted, coarse to fine-grained, dominantly tabular breccias, containing rounded greywacke, argillite and chert clasts (visual roundness 0.6 to 0.7) in a matrix of granular silty sand. Clasts range up to 30 cm. in diameter, but average between 1 and 5 cm. Some breccias are graded, show marked imbrication of clasts, and are sometimes overlain by thin, irregular gran139

Aufs~itze ular silty sandstones; while others are tabular, ungraded, very poorly sorted and between 0.2 and 1 meter thick. Marked variations in grain size between adjacent breccia units is common. A coarser unsorted facies is well-exposed in a tributary of the Glendorch Burn (NS 877 205 - - NS 879 191) where the breccias can be seen resting unconformably on the Ordovician. The breccias show very variable dips, and are probably faulted in places; though the uniformity of the sediments makes detection of faults impossible. The sediments were briefly described by SMITH (1900), but have not been referred to since except by SMITH (1972). They are here called the S n a r B r e c c i a Formation, and the type section is designated as the exposures in the Glendowran burn (NS 870 210 to NS 881 200) where the breccias dip in various directions but predominantly north-east. The Snar Breccia Formation is characterized by relatively well-rounded clasts, a rarity of quartz sand, and its poor sorting (fig. 17 A). It represents deposits of debris flows, sheet-floods and occasionally braided streams in an alluvial fan environment. It closely resembles the Kettleholm Breccia Formation of the Lochmaben basin. Its stratigraphic position is doubtful, but it is probably of Permian age. Ballantrae

Basin

The few exposures on the Ayshire coast at Ballantrae mark the south-eastern extremity of a cuvette extending into the Firth of Clyde (fig. 1). It is convenient to divide the rocks into two formations. Ballantrae

Breccia

Formation

In the type section at Ballantrae (NX 088 882), about 50 metres of well-sorted, graded breccias are exposed just east of the harbour, dipping about 5 ~ north. Rare thin silty sandstone lenses occur between the breccias and become commoner upwards. The breccias consist of relatively well-rounded clasts (visual roundness 0.6--0.7) of serpentine, greywacke, argillite and chert (90~o) with minor amounts of gabbro, and other of the diverse Lower Palaeozoic rock types in the Ballantrae area. They are thus locally derived. The breccia matrix consists of granular silty sand, with no well-rounded 'millet seed' aeolian grains. Imbrication, channelling and grading is well-developed. Large-scale, low-angle crossbedding, pebble lenses and marked grain size variation among adjacent units is characteristic. Some of the pebble lenses are carbonate-cemented and have no clastic matrix. These deposits closely resemble braided stream deposits of permanent or semi-permanent streams. Corseclays

Sandstone

Formation

In the central part of the bay, isolated exposures of orange, tabular, finegrained laminated micaceous sandstones and fine-grained sandy breccias rest unconformably on Lower Palaeozoic rocks and contain traces of 'millet seed' aeolian sand, as well as thin orange siltstone laminae. GEIKm et al. (1869) recorded rippled surfaces and raindrop imprints. SMITH (1972) estimated the thickness of these sandstones to be more than 1,000 metres. As far as can be seen, these sandstones overlie the Ballantrae Breccia Formation, and represent 140

M. E. BROOKFIELD- - Revision of the Stratigraphy of Permian Rocks of Scotland channel and overbank deposits of predominantly sand-bed ephemeral streams as described by WILLIAMS (1971). The type section is designated as the exposures west of Corseclays Farm (NX 098 848 to NX 095 858) where about 15 metres of bimodal finely laminated medium to fine-grained orange micaceous sandstones with rare breccia sandstones are intermittently exposed in the stream, where they dip between 5 ~ and 20 ~ west. Stranraer

Basin

Thick massive breccias are exposed along the Loch Ryan coast north of Stranraer. Elsewhere, the basin is covered by surficial deposits. Geophysics indicate that the basin closes northwards, and is asymmetric, with up to 1200 metres of sediment at its south-eastern margin (fig. 18) (MANSFIELD and KENNET, 1963). On the west, the Permian breccias are unconformable on Carboniferous and Lower Palaeozoic greywackes and argillites (FULLER, 1958). The eastern boundary is faulted. KELLING and WELSH (1966) estimated a down-throw of this fault of about 1500 metres westward, from offsets of structural belts in the

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Aufs[itze Lower Palaeozoic on each side of Loch Ryan. This closely corresponds with the estimate of the maximum sedimentary fill, and suggests that vertical movements of the Loch Ryan fault were contemporaneous with deposition of the Permian sediments (MANSFIELDand KENNETT,1968). One borehole shows that the thick breccia succession is Underlain by sandstone at Stranraer, but as the sandstone has not been described, only the breccias will we given a formal name. Loch

Ryan

Breccia

Formation

This formation is named from Loch Ryan. The formation consists of coarse to fine-grained, moderately to well-sorted breccia, with rare thin interbeds of silty sandstone and fine-grained sandstone. No finer grained units are exposed. The type section is designated as the shore and cliff exposures between Clachan Heugh and Corsewall farm (Grid ref. NX 086 695 - - 035 704, loc. 90, fig. 18), east of the village of Kirkcolm on the western side of Loch Ryan. Here up to 50 metres of breccia are well-exposed, dip gently south-east, and consist of tabular breccia units between 20 cm. and 1 metre thick. The clasts consist almost entirely of angular to sub-rounded greywacke, argillite and chert, with less than 5~o of reworked, but cemented, intraformational fine-grained breccia and sandstone clasts, and only traces of Carboniferous sedimentary clasts (cf. FVLLEa, 1958). The matrix of the breccias consists of granular sub-angular silty sand without 'millet-seed' aeolian sand grains. Occasionally thin siltstone and finegrained sandstone lenses occur, especially towards the top of the breccia units. In the lower part of the type section, the breccias are tabular, consisting of angular clasts in a silty sand matrix (visual roundness 0.4--0.5). Clasts vary in size from 1 to 80 cm., with average 5 cm. The breccias range from moderately to well-sorted, are ungraded, generally packstoneS and only rarely show imbrication of clasts. The few directions obtained indicate currents flowing between east and south-west (fig. 18). These can be interpreted as dominantly fluid debris flow or sheet-flood deposits of alluvial fans. In the upper part of the section, as well as the above breccias, about 50~o of the section consists of well-sorted, graded breccias, showing large-scale low angle cross-bedding, minor cut and fill, and thin sandstone or siltstone tops, often partly eroded prior to deposition of the overlying breccias. Imbrication is still, however, rare. These breccias may be considered a s dominantly sheet-flood deposits of alluvial fans. Apart from the type section the breeeias are only exposed in small stream sections, and intermittently in the raised beach cliff southwards to Stranraer. The Loch Ryan Breccia Formation closely resembles the upper units of the Doweel Breccia Formation, but differs from the latter in the total absence of aeolian sand grains. 8. Evidence of age

The rocks of all the basins are remarkably unfossiliferous. In the Mauchline basin, plant remains in sediments between the basal lava flows indicate an Upper Carboniferous to Lower Permian age (MYKvRA, 1965; WAGNER, 1966; 142

M. E. BROOKFIELD

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Revision of the Stratigraphy of Permian Rocks of Scotland

S~ITrt et al., 1974). Reptilian footprints in the aeolian sandstones of the Lochmaben and Dumfries basins were thoroughly studied in the 19th century (for history see SARGEANT,1974) and more recently by DELAIa (1966, 1967, 1969). HAUBOLD (1971) considered that some of the footprints were made by caseasaurs (edaphosaurs), which reached their acme in the Upper Carboniferous to Lower Permian. In the upper breccias of the Vale of Eden, to the south of the areas considered, pebbles of igneous rock are thought to have been derived from the Whin Sill. This sill gives radiometric dates of 295 -4- 6 million years, i. e. Upper Carboniferous (Stephanian) (FITCH and MILLER, 1967). The breeeias are overlain by the Hilton Plant Beds, which are of Upper Permian age (SMITH et al., 1974). A latest Carboniferous to Lower Permian age for the Vale of Eden breccias and sandstones can reasonably be inferred (SMITH et al., 1974). Since these are lithologically identical to sediments in the Scottish Basins and to the Lower Rotleigendes of the North Sea and Germany, a latest Carboniferous to Lower Permian age is reasonable for the Scottish rocks. 4. Correlation among the Scottish Basins

Many of the conclusions here are similar to those of SMITH et al. (1974). The few footprints and plants found in some of the sediments show only an Upper Carboniferous - - Lower Permian age, and cannot be used for correlation between the basins. The relative ages of the formations may, however, be roughly worked out using the constituents of the sediments and their depositional environments if four main assumptions arc made. Firstly, that basalt eruption was confined to one period in all basins, and that these eruptions were contemporaneous with basalt eruptions in the Mauchline basin. Secondly, that most of the detritus in the breccias is first-cycle. Thirdly, that extensive movement of aeolian sand was confined to one period during the Permian (SMITH et al., 1974). Fourthly, that major changes in breccia characteristics are climatically controlled; hence the breccias with well-rounded clasts and low percentage of quartz sand are younger than the wadi-fan, sandy breccias, and were deposited when the Permian climate had changed from arid to semi-arid or even humid (SMITH et al.,

1974). The first assumption appears reasonable, since in all basins the lavas, where present, are at the base of the sequence, predate and are partly contemporaneous with sediments with aeolian sand grains, and basalt detritus decreases upwards in the overlying sediments (cf. MYKURA, 1967; LtrraSDEN et al. 1967). The second assumption is justified in most cases by the angularity of the clasts, which from comparison with recent alluvial fan environments, show similar roundness to first-cycle fan deposits. The third assumption is less certain. In more complete Permian sequences in northern England, aeolian sandstones occur in the Lower Permian, are overlain by fluviatile sediments, and only at a much younger period were minor aeolian deposits again laid down, e. g. in some units of the St. Bees Sandstone of Triassic age. The fourth assumption is justified by the roundness of clasts in the Snar and Kettleholm Breccia formations, which, despite their coarseness and unsorted nature have much more rounded clasts than even the finer-grained units of the other breccias. The roundness of the clasts in the Ballantrae Breccias is possible a function of distance of transport, since they 143

Aufs~itze are of a more distal facies than the above two formations. In contrast to D. B. SMITa (1976, pers. comm.), I prefer to consider the clast roundness to be mainly a function of type of weathering and distance of transport, rather than of differences in lithology. Highly angular clasts of identical lithologies occur in breccia formations in the other basins. Constituents B a s a I t. Basalt lava flows occur in two main areas: within and to the southeast of the Thornhill basin where they are post-Westphalian age; to the south of the Lochmaben basin, where they are of Lower Carboniferous age. The Thornhill lavas were apparently erupted over a relatively short time span, as they are thin and occur at a constant stratigraphic level. They must once have extended much further east and south-east, as isolated patches of lava occur at Locherben and in the Windyhill burn (locs. 85, 78, figs. 7, 12). The Hart_field Formation of the Lochmaben basin has abundant basalt clasts, indicating a source to the north-west, and a drainage from the Thornhill basin to the southeast during deposition. Small grains of basalt in the Dumfries Formation and Locharbriggs Formation may have been carried west by the wind. The presence of abundant basalt detritus indicates a low stratigraphic level, since basalt detritus decreases upwards in the Locherben Breccia Formation, indicating gradual stripping of the basalt from the hills and covering by sediment in the lowlands. In the south of the Lochmaben basin, the Lockerbie Formation has basalt clasts derived from the Birrenswark lavas; whereas the Kettleholm Formation does not, even though it is closer to the lavas. This suggests that the Kettleholm Formation is younger and was deposited after the Birrenswark lava scarp had retreated to almost its present position. This suggestion is also supported by a similar relationships for limestone clasts, derived from the Carboniferous to the south. 'Millet seed' quartz s a n d . The presence of aeolian frosting on quartz grains is not a good indication of environment, since these can be extensively reworked and frosting imparted simply by chemical action; furthermore frosting can quickly be removed during only a short transportation (KtrENEN and PERDOK, 1962). However, almost spherical 'millet seed' sand indicates extensive aeolian transport and can survive reworking intact. The percentage of 'millet seed' sand in a sample is thus a good reflection of the content of far-travelled aeolian sand in a sample; though confined to the coarser grain sizes (figs. 10, 11, 14, 17). If large-scale transport of sand by the wind was confined to one period, the presence of significant amounts of 'millet seed' sand indicates either that the sediment was laid down during this period, or that it was partly derived from aeolian sands of that period being reworked. Since aeolian sands accumulate at low levels within a basin, it seems likely that most sediments with 'millet seed' sand were laid down contemporaneously with or only shortly after the major period of aeolian activity. There is no sign in the sections that the aeolian sediments ever underwent an intra-Permian phase of erosion by through-flowing streams, 'Millet seed' sand is present in most of the wadi-fan environments described; but it notably decreases in the upper levels of the Dumfries Breccia Formation, 144

M, E. Baoo~sm~D - - Revision of the Stratigraphy of Permian Rocks of Scotland

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Aufs~itze the youngest stratigraphically located sediment of the basins, where interbedded aeolian sandstones are not present. Since the prevailing wind would have carried sand onto this area ff aeolian action had been continuing, its absence indicates cessation of aeolian activity (SMITa et al., 1974). C l a s t s. The clasts in all the breeeias associated with aeolian sands or deposited in wadi-fan environments are highly angular, even in finer-grained sandstone units. However, the Kettleholm, Ballantrae and Snar Breccia Formations, despite their coarseness have relatively well-rounded elasts. These formations thus represent either reworked gravels or more intense chemical weathering in the source area. In view of the thickness of the Snar b:eccias, reworking of pre-existing gravels seems unlikely, and the roundness of the clasts must be attributable to more intense chemical weathering at source and hence more humid climate. Since the upper units of the Dumfries breccia apparently accumulated after the main phase of aeolian sand movement, the Snar, Ballantrae and Kettleholm Breccias probably are younger and represent a culmination of the Dumfries breccia trend. However, this also means that the three breccia formations may be Triassic and not Permian in age. An alternative view is that the Snar and Ballantrae breceias belong at the base of the Permian sequences; and were laid down prior to the onset of desert conditions and before extensive reddening of the pre-Permian land surface (D. B. SMITa, 1972, and pers. comm., 1976). The correlation shown by these methods is illustrated in fig. 19. It has some implications for the sedimentary and tectonic history of the basins during Permian times. Development

of the Basins

The first event was the eruption of basalt lavas in the Thornhfll basin and areas to the east, predominantly in basins scooped out of Carboniferous sediment. Minor piedmont breccias accumulated between and on top of the lavas and were then followed by the development of extensive wadi-fan networks, with associated aeolian sandstones in the Thornhill and Lochmaben basins and in the Moffat basin. Drainage was from the Thornhfll basin south-eastwards into the Loehmaben basin, and also southwards through the Moffat basin. The Dumfries basin does not appear to have existed as a basin of deposition at this time. As the uplands were worn down, aeolian sands spread over the Thornhill, Lochmaben and Moffat basins, but at the same time, faulting along its present western margin initiated sedimentation in the Dumfries basin, where breccias and aeolian sandstones interfinger. Minor breccias may also have been deposited on the southern and eastern sides of the Lochmaben basin (Lockerbie Formation). Discontinuous fault movement may be reflected in the building out of breccia tongues into the Dumfries basin at various times, separated by static periods when dune sandstone advanced onto the old fans or this could be due to climatic change (cf. WILLIAMS, 1978). Sometime during this ,period, rejuvenation of the uplands, perhaps by faulting, occurred along the eastern margin of the Moffat basin, and thick breecias were built out. The relationships of this breccia unit (the Bellcraig Breccia Formation) to other units is not known. Since its middle portion contains abundant aeolian sand, it could have been laid down during 146

M. E. BROOKFIELD

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Revision of the Stratigraphy of Permian Rocks of Scotland

aeolian sand accumulation, since the prevailing winds were off the breccia fans. However, since the source of the aeolian sand must have been Carboniferous sediments to the east of the present basins (cf. WAUGrI, 1970, on the Permian aeolian sandstones of northern England), this supply m a y have been exhausted b y the time the u p p e r beds of the Belleraig Formation were deposited. Deposition of the Belleraig Breccia m a y thus have spanned most of the lower Permian. If the sandstone below the thick breccias of the Stranraer basin are aeolian, then the stratigraphic sequence can almost directly be c o m p a r e d with than in the Dumfries basin and they are tentatively considered to be of the same age. The last or first known event was the deposition of the Kettleholm and Snar breccias on true alluvial fans. The braided stream conglomerates and overlying fluviatile sandstones with aeolian grains of the Ballantrae Basin m a y also belong here; but they could equally well be equated with the minor Lower Triassic phase of aeolian transport represented by parts of the St. Bees Sandstone in north-western England (SMITHet al., 1974). Acknowledgements Mr. B. S. TURNER of I.C.I. (Plastics), Drungans, gave permission for, and greatly assisted in, an examination of the borehole sections in his charge. The Institute of Geological Sciences (Edinburgh) gave permission to cite their detailed well and borehole logs. Mr. A. E. TRUCKELL(Dumfries Museum) pointed out many obscure localities and helped with the tracking down of borehole records. The manuscript was critically read and materially improved by Dr. D. B. SMITH, though be does not agree with everything in it. References

BOTT, M. H. P. and MASSON-SMITn,D.: A gravity survey of the Criffell Granadiorite and the New Red Sandstone deposits near Dumfries. Proc. Yorks. Geol. Soc., 32: 317--882. 1960. BROOKFIELD, M. E.: The origin of bounding surfaces in ancient aeolian sandstones. Sedimentology, 24, 808~882, 1977. CAMERON-SMITH, A.: The Locharbriggs Sandstone. Trans. J. Dumfries Galloway nat. Hist. Antiq. Soc., 12: 281--286. 1925. CARTER, A. V. F. and MCADAM,A. D.: Record of wells in the areas of Scottish one-inch geological sheets Girvan (7), Carrick (8), Maxwelltown (9), Dumfries (10), Langholm (11), Campbeltown (12), South Arran (18), Ayr (14), Sanquhar (15), Moffat (16), Jedburgh (17), and Morebattle (18). Gt. Brit. Geol. Surv. Water Supply Well Catalogue Series, 75 pp. 1968. COLTER, V. S. and BARE, K. W.: Recent developments in the Geology of the Irish Sea. In: WOOOLAND,W. A. (Editor), Petroleum and the Continental Shelf of North-West Europe. Vol. 1, Geology. Applied Sci. Pub. Ltd., Barking. 501 p., pp. 61--73. 1975. CRAIC, G. Y.: Permian and Triassic. In: CRAIG, G.Y. (Editor), The Geology of Scotland. Oliver and Boyd, Edinburgh. 556 p., pp. 383~400. 1965. DELAm, J.B.: Fossil footprints from Dumfriesshire, with descriptions of new forms Annandale. Trans. J. Dumfries Galloway nat. Hist. Antiq. Soc., 43" 14--30. 1966. DELAIR, J.B.: Additional records of British Permian footprints. Trans J. Dumfries Galloway nat. Hist. Antiq. Soc., 44" 1--5. 1967. DELAXR, J.B.: Preliminary notice of vertebrate footprints from the Trias of Dumfriesshire. Trans. J. Dumfries Galloway nat. Hist. Antiq. Soc., 46: 178--179. 1969. FITCH, F.J. and MILLER, J.A.: The age of the Whin. Sill. Geol. J., 5: 288---250. 1967. 147

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Die Gr6dener Schichten (Perm, Siidalpen). Sedimentologische und geochemische Untersuchungen zur Unterseheidung mariner und kontinentaler Sedimente Von Werner BUGGISCH,Darmstadt *) Mit 11 Abbildungen und 8 Tabellen Zusammen{assung

Die mittelpermischen Gr(idener Schichten kSnnen in eine kontinentale Fazies mit Kaolinit und SiiBwasseralgen und in eine marine Fazies mit Chlorit und Dolomit als Sedimentkomponenten und vereinzelten Foraminiferen gegliedert werden. Im t~bergangsbereich finden sich die Charakteristika einer Kiistenregion mit marginalen Lagu:hen: Gipse, Kohlen, Tetrapodenf~ihrten, eingedriftete Cephalopoden und Foraminiferen. *) Anschrift des Verfassers: Dr. W. BUGGISCH, Geologisch-Paliiontologisches Institut, Technischen Hochschule Darmstadt, Schnittspahnstr. 9, D-6100 Darmstadt Band 67, Heft 1, 1978, Seite 149--180

149