Ordovician sedimentary succession, Girvan–Ballantrae district
|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 Arenig rocks of the Ballantrae Complex, which form the irregular ‘basement’ to the younger, Ordovician and Silurian sedimentary sequence in the Girvan area, had been obducted onto the southern margin of the Midland Valley terrane by Llanvirn times. These ‘basement’ rocks were then deeply eroded prior to deposition of a mostly shallow- to moderate-depth marine ‘cover sequence’ of conglomerates, sandstones, siltstones, mudstones and minor reefal limestones. The ‘cover sequence’ shows distinctive variations across its principal areas of outcrop (P912323): a Main Outcrop inland from Girvan and along the coast to the south of the town, and the Craighead Inlier to the north-east of Girvan (P912319). There is evidence of tectonic instability and significant fault control of sedimentation, with major changes in thickness of (particularly) Ordovician conglomerate units across contemporaneous, broadly east–west faults. The faults developed sequentially northwards, with downthrow to the south, and so produced a northward transgressive onlap of the sedimentary sequence (P912324). The counterpart of this is a southward transition to more distal, deeper-water lithofacies at any given stratigraphical level. An example of this effect is preserved on the south side of the River Stinchar, south-westward from its confluence with the Water of Gregg.
Water of Gregg
In general, the southern margin of the Ordovician–Silurian ‘cover sequence’ is formed by the Stinchar Valley Fault, but there is one important exception. On the south side of the River Stinchar, and cropping out intermittently for about 10 km south-west from the river’s confluence with the Water of Gregg, Ordovician sedimentary rocks appear on the south side of the Stinchar Valley Fault, in the footwall of the Pyet Thrust (P912319). This thrust dips to the south and has transported Tappins Group strata northwards; as described in Chapter 3, the Tappins Group forms the northernmost division of the Southern Uplands terrane. Along strike to both the north-east and the south-west, the Pyet Thrust is cut out by the Stinchar Valley Fault.
Part of the succession between the Stinchar Valley Fault and the Pyet Thrust can be assigned to the Barr Group, which is more fully developed to the north and is accordingly described below from the Main Outcrop. However, to the south of the River Stinchar, the relatively shallow-water deposits of the Barr Group appear to be overlain by more than 500 m of deep-water deposits: wacke sandstone, mudstone and conglomerate. These form the Albany Group (P912319 and P912323), much of which comprises the Craigmalloch Formation, a clastic unit of conglomerate, sandstone and siltstone. Towards the top of the succession, the mudstones of the Doularg Formation, about 25 m thick, interfinger with the Craigmalloch Formation conglomerates, and contain a significant deep-water fauna, mainly of trilobites. This shelly fauna, together with graptolites from the Craigmalloch Formation, establishes the gracilis Biozone and allows biostratigraphical correlation with the upper part of the Barr Group in the Main Outcrop (P912323 and P912324).
Main Outcrop and coastal section
The oldest part of the succession in the Main Outcrop, the Barr Group, crops out in the south along and on the north flank of the Stinchar Valley, where the Kirkland Conglomerate Formation (thicknesses for this and all of the other units subsequently described are given in P912323) forms the core of an anticline plunging gently north-eastwards. Though the base is nowhere seen, the conglomerate is assumed to rest on Ballantrae Complex rocks and contains a large proportion of ophiolitic and igneous detritus, both within the matrix and as clasts. This rudite is overlain by the Confinis Formation, a flaggy sandstone unit with minor calcareous members (e.g. the Auchensoul algal limestone and calcareous mudstone). The calcareous aspect increases significantly upwards into the succeeding Stinchar Limestone Formation, a massive reef unit containing both shelly and conodont faunas. The conodonts define the local base of the Llandeilian Stage within the reef limestone sequence. A return to fine-grained clastic sedimentation is seen in the overlying graptolitic Superstes Mudstone Formation, which is in turn overlain, and channelled into, by the thick Benan Conglomerate Formation. An important feature of this conglomerate is its dramatic change in thickness (640 m to 55 m) when traced north-west across strike, a feature thought to have been caused by its deposition against steep submarine fault scarps (P912324), the faults having downthrow to the south-east and most probably forming a listric set stepping sequentially northward. As one result of this transgression, the Benan Conglomerate is the only part of the Barr Group seen in the Girvan coastal section, where it is presumed to rest unconformably on Ballantrae Complex lavas (P912323), the exposed contacts being faulted.
Although many of the clasts in the Benan Conglomerate can be matched with the subjacent Barr Group sedimentary rocks and Ballantrae Complex lithologies, there are many boulders of igneous rock (e.g red granite) for which no obvious source is known at outcrop. Radiometric age-dating of some of these granitic boulders has demonstrated a relatively short time span between their intrusion (cooling dates cluster around 560 Ma and 470 Ma) and their subsequent erosion into the conglomerate (depositional age ca 460 Ma). The most likely source of the granitic boulders is a magmatic arc to the north, within the Laurentian, Midland Valley terrane, that was initiated by the establishment (or rejuvenation) of northward subduction following the arc–continent collision that emplaced the Ballantrae Complex. This most probably involved a reversal of subduction direction (cf. P912321) to establish oceanic subduction northward beneath the Midland Valley terrane. This would have initiated supra-subduction magmatism there and led, ultimately, to the development of the Southern Uplands accretionary complex as described in Chapter 3
The Ardmillan Group succeeds the Barr Group and although it contains numerous conglomerate lenses, it is generally a more sandstone- and mudstone-dominated succession than its predecessor; turbidity currents dominated the depositional regime. The Ardwell, Whitehouse and Drummuck subgroups are recognised, though the latter is restricted to the Craighead Inlier in the north (P912319 and P912323).
At the base of the Ardmillan Group (and the Ardwell Subgroup), the Balclatchie Formation is a thick mudstone unit with important faunas, both shelly and graptolitic, at several levels; the graptolitic strata include the palaeontologically renowned Laggan Member, which has yielded remarkable 3D graptolites dissolved out from calcareous nodules. The presence of both shelly and graptolitic faunas has been of particular importance for biostratigraphical correlations. Some substantial conglomerate units also occur within the Balclatchie Formation, including the Kilranny Conglomerate Member, which has formerly been much confused with the older Benan Conglomerate. The succeeding Ardwell Farm Formation is well exposed on the coast northward from Kennedy’s Pass (NX 150 933) to Ardwell Bay and consists mainly of fissile turbidite sandstone beds, some micaceous, some calcareous, with sporadic lenticular conglomerate members interspersed; the conglomerates are mainly confined to the inland part of the outcrop. In its coastal outcrop the formation’s strata are folded into a spectacular series of angular ‘box’ folds (P005823) for which a soft-sediment slumping origin has been proposed, though not universally accepted.
The shore section between Ardwell Bay and Girvan displays an excellent strike section through the middle part of the Ardmillan Group, the Whitehouse Subgroup and its constituent formations. At the base of the subgroup (P912323), the South Shore Formation is a calcareous turbidite succession of wacke-type sandstone and mudstone, with coarse shelly debris in places. The overlying Three Mile Formation comprises a fine-grained and laminated, sandstone–siltstone–mudstone sequence characteristic of deposition from low-density turbidite flows. Black graptolitic mudstone characterises the succeeding Penwhapple Formation with a linearis Biozone fauna indicating equivalence to the upper part of the Lower Hartfell Shales in the Southern Uplands terrane (see Chapter 3). This lithology is succeeded by the distinctive red and green silty mudstones of the Myoch Formation, a deep water deposit with an extensive shelly fauna that includes unusual trilobite species which were either blind or had very large eyes. Above this, the Mill Formation is lithologically diverse with mottled and dark mudstones, fissile, calcareous sandstone, and mud-clast conglomerate; it is formally subdivided into several members. One of the calcareous sandstone members at the top of the formation (Stacks Member) has yielded a diverse and abundant brachiopod fauna that establishes a Pusgillian age. The Shalloch Formation, at the top of the Whitehouse Subgroup, is a fairly uniform succession of thinly bedded turbidite sandstone with a few interbedded green mudstone beds and some turbidite units composed largely of limestone detritus. The important zonal graptolite Dicellograptus anceps occurs within this formation, indicating correlation with the Upper Hartfell Shales of the Southern Uplands (see Chapter 3). These are the youngest Ordovician strata seen in the Main Outcrop at Girvan, being cut off by the unconformity beneath the lowermost Silurian rocks, as seen at Woodland Point (NX 170 953). The Shalloch Formation provides a correlative overlap with the youngest part of the Girvan Ordovician succession, which is only preserved in the Craighead Inlier (P912323).
Located some 7 km north of the Main Outcrop, the Craighead Inlier contains a sequence that spans the Ordovician–Silurian boundary. The local ‘basement’ is a volcanic assemblage of basaltic pillow lavas and associated (perhaps infolded) bedded chert, the Craighead Volcanic Formation, which has commonly been assumed to form the northern outpost of the Arenig Ballantrae Complex. However, the cherts have yielded conodonts of Llanvirn (late Abereiddian or Llandeilian) age and, depending on the exact relationship of the cherts with the lavas, it is possible that the latter are younger than those of the Ballantrae Complex. In the disused Craighead Quarry (NS 235 014), the Craighead Limestone Formation (here the basal unit of the Ardwell Subgroup) rests with marked unconformity on an irregular surface of lava. It includes algal and pelmatozoan limestones and limestone breccias with an interesting, mid Caradoc shelly and ostracod fauna of North American affinity. Some interfingering mudstone members (e.g. the Kiln and Sericoidea members) are also richly fossiliferous and provide useful correlation between the graptolitic (basal clingani Biozone) and shelly biozonal schemes. The overlying Ardwell Farm Formation is locally in faulted contact with the Craighead Limestone and, as in the more extensive outcrops farther south, consists of fissile calcareous sandstones and siltstones (the latter dominating the Plantinhead Member) with a graptolite fauna indicative of the clingani Biozone.
The component formations of the Ardwell Subgroup, described above, crop out in the southern part of the Craighead inlier and are in faulted contact with a succession of younger Ordovician and Silurian strata that form the northern part of the inlier (P912319). The outcrop pattern of this younger, northern succession is controlled by its distribution around a large anticlinal structure plunging gently towards the north-east. The lowest stratigraphical unit at outcrop, the Shalloch Formation, is the only visible representative of the Whitehouse Subgroup and consists of a cyclic sequence of fissile, wacke-type sandstone and siltstone similar to that occurring to the south in the Main Outcrop. The succeeding Drummuck Subgroup, known only from the Craighead Inlier, includes five conformable formations extending up to the local base of the Silurian (P912323).
The oldest division of the Drummuck Subgroup, the Auldthorns Formation, comprises coarse green sandstone, conglomerate and minor mudstone. Rich faunas have been obtained from thin shelly horizons throughout the formation. The succeeding green and purple mud-stone with conglomerates and mass-flow deposits constitute the Quarrel Hill Formation, with well-preserved brachiopod faunas at various horizons throughout. The grey, silty mudstone of the overlying Lady Burn Formation has yielded only a few brachiopods and trilobites. However, the South Threave Formation, comprising mudstone, sandstone and siltstone, has several highly fossiliferous members including, at its base, the Farden Member with the well-known Starfish Beds. The latter are thought to be mass-flow deposits which have transported and redeposited a rich shelf faunal assemblage (Plate 11) into deeper water. The youngest unit seen in the Ordovician succession of the Craighead Inlier is the High Mains Formation, a fine- to medium-grained grey-green sandstone body cropping out only in the core of the northeast-plunging anticline. It has a scattered shelly fauna of Hirnantian age that includes trilobites and brachiopods. The Ordovician succession is here truncated by the gently unconformable base of the Silurian, with, in chronostratigraphical terms, relatively little Ordovician strata missing (P912323). Although the principal structure in the northern part of the Craighead Inlier is anticlinal, the sub-unconformity distribution of the Ordovician strata is broadly synclinal (cf. P912324); hence the restriction of the High Mains Formation to the anticlinal hinge zone.
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