Campsie Lava Member

From Earthwise
Jump to navigation Jump to search

Campsie Lava Member (CMLA), Campsie Block, Carboniferous, Midland Valley of Scotland[edit]

Campsie Lava Member is part of the Clyde Plateau Volcanic Formation.

Name[edit]

Previously referred to as four separate successions named the Lower South Campsie Lavas, Lower North Campsie Lavas, Upper South Campsie Lavas and Upper North Campsie Lavas (Craig, 1980[1]; Forsyth et al., 1996[2]; Hall et al., 1998[3]).

Lithology[edit]

The Campsie Lava Member has been divided stratigraphically into lower and upper sequences and geographically into northern and southern sequences that reflect petrographically their source vent systems. Although the various sequences are distinctive in some areas, enabling their boundaries to be identified, elsewhere they interdigitate laterally and have no distinctive stratigraphical markers to separate them vertically. Hence it is not possible to draw continuous boundaries between them on a map and therefore they have all been assigned to a single formal member. The four sequences have been given informal names that have been widely used on maps and in literature (Craig, 1980[1]; Forsyth et al., 1996[2]; Hall et al., 1998[3]). The informal sequences are the lower south Campsie lavas, the lower north Campsie lavas, the upper south Campsie lavas and the upper north Campsie lavas.

The distinctive characteristic of the whole member is that it is composed almost entirely of plagioclase-microphyric basalt and trachybasalt (basaltic hawaiite and hawaiite) lavas (i.e. of ‘Jedburgh’ type). Within this restricted range there is considerable petrographical variation and there are systematic variations between the south and north Campsie lavas. The south Campsie lavas contain significant amounts of clinopyroxene, whereas in the north Campsie lavas it is present only as small granules in the groundmass. The plagioclase microphenocrysts also differ, being labradorite to calcic andesine in the south but sodic andesine in the north. These variations reflect the whole-rock compositions, which are dominantly basalt to basaltic hawaiite in the south and hawaiite to mugearite in the north. The lower and upper Campsie lava sequences can be identified where they are separated locally by one of several other lava members of distinctly different petrography. These are the Craigentimpin Lava Member, the Loup of Fintry Lava Member, the Laird’s Hill Lava Member and the Overton Lava Member. Since it is not certain if those four members are contemporaneous, the boundaries between the lower and upper Campsie lavas, where drawn, do not necessarily involve lavas of exactly the same age everywhere.

The lower south Campsie lavas are microporphyritic trachybasalts (basaltic hawaiite and hawaiite of ‘Jedburgh type’) with microphenocrysts of sodic labradorite and a generally quite high content of mafic minerals which have been extensively replaced, especially in the case of olivine and augite, by carbonate, as has some plagioclase. Basalt lavas also occur. In the western Campsie Fells, in the Forking Burn (NS 653 790), there are ten proximal-facies lavas, totalling 80 m in thickness, showing toe structures and other features typical of pahoehoe lavas. These are separated by ‘slaggy’ agglomerates which make up about 25 per cent of the total thickness of the section. The individual lavas are generally thin (3 to 8 m) and highly amygdaloidal. Viriditic material is prominent in the groundmass. In the Garrel Burn (NS 697 807), the lavas are thicker, more massive, less amygdaloidal, and more distal in character, and the total thickness is reduced to approximately 50 m.

The lower north Campsie lavas consist mainly of olivine-microphyric basalt and trachybasalt (basaltic hawaiite and hawaiite of ‘Jedburgh’ type). There is considerable modal variation but clinopyroxene is generally lacking except as very small grains in the groundmass. Plagioclase microphenocrysts tend to be in the sodic andesine range. The best section is in the western escarpment overlooking the Blane Water valley at Black Craig (NS 558 812) where nine lavas are present, although the lowest of these is interpreted as part of the lower south Campsie lavas (Craig, 1980[1]; Hall et al., 1998[3]). These form a near-vertical face with well-developed tuff and scoria, up to 2 m thick, at the top of each lava. The lowest three lavas are each approximately 15 m thick, while the remainder are noteably thinner, averaging about 10 m each. Crude columnar jointing is evident in most of the lavas, except the second which locally has irregular subvertical platy jointing and comprises several ‘flow units separated by rather impersistent slaggy layers’ (Craig, 1980)[1]. Despite petrographical differences between the north and south Campsie lavas, it is not readily possible to distinguish between the two in all areas, and the two sequences are interpreted as interdigitating in many sections between the northern and southern Campsie Fells and Kilsyth Hills (Craig, 1980)[1].

The upper south Campsie lavas consist of microporphyritic trachybasalt (hawaiites of ‘Jedburgh’ type), transitional to basaltic-trachyandesite (mugearite) with a very low content of augite. These lavas are well exposed in the Goat Burn (NS 637 793), above the southern escarpment in the eastern Campsie Fells, where a sequence of lavas, 40 m thick, is developed. The sequence comprises four lavas, the lower three averaging 12 m in thickness, and the fourth 5 m. The third lava is compound, as may also be the lowest, which is poorly exposed. The uppermost lava, and one of the flow units of the compound third lava, die out eastwards, as does the sequence as a whole.

The upper north Campsie lavas consists of plagioclase-microphyric trachybasalt and some basalt (basalt to hawaiite of ‘Jedburgh’ type) with scoriaceous agglomerates and tuffs. Basaltic trachyandesite (mugearite) also occurs. The lavas are generally poorly exposed and there are few sections through them. In the Muir Toll Burn (NS 628 828) the total thickness is about 40 m, part of which is repeated by the South Campsie Muir Fault. Farther east, on Cairnoch Hill (NS 690 852), Haugh Hill (NS 682 842) and Little Bin (NS 674 830), thick distal facies sequences occur, but are poorly exposed. In the Gonachan (NS 602 838) and Clachie (NS 612 840) burns, faulted sections, partly obscured by superficial deposits, occur. These are of proximal-facies lavas which are thin and carbonated, with considerable scoriaceous agglomerates and tuffs between the lavas. Interdigitation with lavas derived from the south, which are more mafic and more basaltic than those of the upper north Campsie lavas, is characteristic. This appears likely to be the case in the central and western Campsie Fells, including Ballagan Burn Glen (north of NS 5726 8043), Finglen (north of NS 5956 8043) and Campsie Glen (north of NS 6239 8080). Similar interdigitation is seen in the River Carron (between NS 766 848 and NS 771 846) in the easternmost Kilsyth Hills, where a sequence, dipping to the east, of five or six relatively thin trachybasalts, transitional to basaltic trachyandesite, is exposed.

Stratotype[edit]

The Campsie Fells and Kilsyth Hills, between Strathblane (NS 555 797) and the Carron Valley, east to (NS 785 833), north and north-east of Glasgow.

Lower and upper boundaries[edit]

In the northern Campsie Fells, the lower boundary of the Campsie Lava Member is taken as the first appearance of basalt or trachybasalt lava on the underlying North Campsie Pyroclastic Member of the Clyde Plateau Volcanic Formation, and the contact appears to be conformable or disconformable. However, in the western and south-western Campsie Fells, the North Campsie Pyroclastic Member is absent and the Campsie Lava Member overlies the Clyde Sandstone Formation of the Inverclyde Group. Farther east in the southern Campsie Fells, at three localities between the Finglen Burn (NS 599 800) and the Forking Burn (NS 653 790), it rests directly, and unconformably, on argillaceous rock, dolostone and sandstone of the older Ballagan Formation of the Inverclyde Group, suggesting that the lower boundary is an unconformity, both there and maybe also in the northern Campsie Fells. To the east of the East Bachille Fault (NS 6915 7974) and west of the Chapmen’s Graves Fault (NS 7063 8046), the lower boundary is taken as the first appearance of basalt or trachybasalt on the plagioclase-macrophyric trachybasalt (hawaiite) lavas of the Laird’s Loup Lava Member.

In the central and western Campsie Fells, the Campsie Lava Member is overlain with apparent unconformity by the Fin Glen Lava Member. The lithological change is to trachyte, with basaltic trachyandesite (mugearite), trachybasalt (hawaiite) and basalt. In the eastern Campsie Fells and western Kilsyth Hills, the member is overlain variably disconformably to unconformably by the Lower Lecket Hill Lava Member, and in some places the member was probably entirely removed by erosion prior to emplacement of the Lower Lecket Hill Lava Member; for example south-east of Cort-ma Law (NS 6515 7995), south and east of Brown Hill (NS 6645 7897) and around Laird’s Hill (NS 6957 8018). In the north-eastern Kilsyth Hills, the Campsie Lava Member is overlain, probably unconformably by the lavas of the Gargunnock Hills Lava Member, or in the south-east Kilsyth Hills, by the Kilsyth Hills Lava Member.

Thickness[edit]

Between 80 and 265 m.

Distribution and regional correlation[edit]

The Campsie Lava Member accounts for over 30 per cent of the outcrop of volcanic rocks in the structural Campsie Block, which comprises the Campsie Fells and Kilsyth Hills, north and east of Glasgow. It is present throughout the block with the exception of the southern escarpment between Garrel Hill (NS 704 805) and Tappetknowe (NS 761 819).

The lower south Campsie lavas are generally restricted to the southern part of the block and are developed along most of the length of the southern Campsie Fells and southern Kilsyth Hills. They also crop out from the western Campsie Fells (NS 5540 7977) eastwards, immediately to the north of the Campsie Fault, to the south sides of Lairs (NS 640 791) and Brown Hill (NS 665 785), to the south side of Garrel Hill (NS 7068 8053). They are also interdigitated with and undivided from, the lower north Campsie lavas, eastwards across the Kilsyth Hills, from the north-east of Little Bin (NS 6726 8339), to the south of the Carron Valley Reservoir (NS 700 825) to the lower western, southern and eastern flanks of Dundaff Hill (NS 7273 8486) to (NS 7467 8470), north of Denny Muir (NS 7505 8254) and along the River Carron, north of Tarduff Hill (NS 7486 8387) to (NS 7580 8463), and to the west and east of Northshields (NS 762 838) to (NS 7783 8396). The extent to which they extend farther northwards across the Campsie Fells and Kilsyth Hills is uncertain, but they are likely to be interdigitated with the lower north Campsie lavas across most of that area.

The lower north Campsie lavas are generally restricted to the northern part of the Campsie Block and are developed along most of the length of the northern Campsie Fells and northern Kilsyth Hills. They crop out from the western Campsie Fells (NS 5520 8180) eastwards, across Craigbarnet Muir (NS 575 835) to the Endrick Water (NS 6617 8619). They are also interdigitated with and undivided from, the lower south Campsie lavas southwards across the Campsie Fells and Kilsyth Hills as detailed above.

The upper south Campsie lavas are restricted to the southern part of the Campsie Block and are developed along most of the length of the southern Campsie Fells and southern Kilsyth Hills. They crop out in the escarpment north of the Campsie Fault, and extend westwards across the southern Campsie Fells to the Ballagan Burn (NS 5726 8043). They also extend eastwards across the Kilsyth Hills to the River Carron (NS 766 848) and (NS 771 846), where they are interdigitated with the upper north Campsie lavas. The extent to which they extend northwards across the Campsie Fells and Kilsyth Hills is uncertain, but they might be interdigitated with the upper north Campsie lavas across most of the area.

The upper north Campsie lavas are generally restricted to the northern part of the Campsie Block and are developed along most of the length of the northern Campsie Fells and northern Kilsyth Hills. These rocks crop out from the western Campsie Fells (NS 562 820) eastwards, across Craigbarnet Muir (NS 575 830) to the Gonachan (NS 602 830) and Clachie (NS 612 840) burns. They also extend eastwards across the Kilsyth Hills, where they are interdigitated with the upper south Campsie lavas as detailed above.

Age[edit]

Mid Visean (Arundian to Asbian).

References[edit]

  1. 1.0 1.1 1.2 1.3 1.4 Craig, P M. 1980. The volcanic geology of the Campsie Fells area, Stirlingshire. Unpublished PhD Thesis, University of Lancaster
  2. 2.0 2.1 Forsyth, I H, Hall, I H S, and McMillan, A A. 1996. Geology of the Airdrie district. Memoir of the British Geological Survey, Sheet 31W (Scotland)
  3. 3.0 3.1 3.2 Hall, I H S, Browne, M A E, and Forsyth, I H. 1998. Geology of the Glasgow district. Memoir of the British Geological Survey, Sheet 30E (Scotland)