Geology of the Aberfoyle district: Strathmore Group (SEG)

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This topic provides a summary of the geology of the Aberfoyle district – covered by the British Geological Survey. 1:50k geological map sheet 38E (Scotland).
Authors: C W Thomas, A M Aitken, E A Pickett, J R Mendum, E K Hyslop, M G Petterson, D Ball, E Burt, B Chacksfield, N Golledge and G Tanner (BGS).

The rocks of the Strathmore Group are generally much finer grained than those of the Arbuthnot–Garvock Group, and are dominated by mudstones, siltstones and sandstones. Conglomeratic rocks still occur locally, but are much less common. The rocks assigned to the Strathmore Group are Emsian (400–391 Ma) in age (Browne et al., 2001[1]).

Cromlix Mudstone Formation (CXF)[edit]

The Cromlix Mudstone Formation comprises poorly sorted mudstones, siltstones and sandstones. These strata have previously been interpreted as distal parts of alluvial fans that remained unreworked by fluvial processes (Armstrong et al., 1985[2]). However, lithologically similar modern analogues from Australia have been interpreted as aeolian mud pellet deposits, where the pellets lose their identity soon after deposition (Dare-Edwards, 1984[3]; Yang, 1997[4]). The formation crops out within the district between Auchmar [NS 442 913] and the valley north of Cock Hill [NN 618 057], but exposure is poor north-east of Ben Gullipen [NN 598 043]. The rocks within the formation are monotonously uniform between Gartartan [NS 533 982] and Auchmar, but north-east of Shannochill [NS 543 993] massive siltstone gives way to sandstones assigned to the Shannochill Sandstone Member, and conglomeratic beds (Malling and Tom Dubh Conglomerate members) gradually increase in thickness.

SHANNOCHILL SANDSTONE MEMBER (SHAS)
In the ground north-west of Lake of Menteith, the Shannochill Sandstone Member comprises well-bedded, fine-grained sandstones, interdigitated with sandy siltstones typical of the lower part of the Cromlix Mudstone Formation. Also present are subordinate, thin beds of conglomerate and coarse sandstone, that provide useful marker horizons. The interdigitation of sandstone and siltstone is readily observed in ground almost free of superficial deposits around Shannochill [NS 543 993]. Between Shannochill and Malling [NN 562 006], siltstone is progressively replaced by sandstone in a north-easterly direction, in the interval between the underlying Gartartan Conglomerate and the overlying Malling Conglomerate members. These conglomeratic members gradually thicken at the expense of sandstone north-eastwards from Lake of Menteith. The Shannochill Sandstone Member reaches a maximum thickness of 300 m near Arntamie [NN 565 010].

Volcanic and metamorphic clasts dominate the clast assemblage in the conglomerates and associated coarse sandstones. The latter may be polycyclic, but the state of the volcanic material indicates relatively limited transport for this component. Once again, sand-grade material is mineralogically diverse, reflecting a variety of sources dominated by volcanic and metamorphic rocks.

MALLING CONGLOMERATE MEMBER (MALC)
The Malling Conglomerate Member forms a pronounced topographical feature, ranging, over ten kilometres north-eastwards from the River Forth [NN 538 985] to Ben Gullipen [NN 598 043] (P001225). It also crops out intermittently between the Forth and the Kelty Water east of Corrie [NS 493 952], and varies in thickness from 1 m to more than 180 m. The maximum conglomerate clast size increases north-eastwards in direct proportion to the thickness of the unit. At Shannochill [NS 543 993], the maximum clast size is 120 mm, whereas small boulders occur on Ben Gullipen. The conglomerate is poorly sorted and matrix supported in the low ground, and clast supported in the Menteith Hills, with common planar bedding. Clasts consist of vein quartz with quartzite and acid to intermediate igneous lithologies.

TOM DUBH CONGLOMERATE MEMBER (TDCO)
The Tom Dubh Conglomerate Member forms a pronounced feature on the north-west limb of the Strathmore Syncline, between the Lake of Menteith and Meall Leathan Dhail [NN 667 117] to the east of the district. The conglomerate consists of poorly sorted, coarse and cobble sized clasts of vein quartz and quartzite, in a lithic sand matrix. The thickness varies between 110 m and 300 m north of Port of Menteith [NN 583 015], before decreasing rapidly around Nether Glenny [NN 5719 0183]. The base of the Tom Dubh Conglomerate is not seen, but topographical expression indicates a sharp contact. The underlying lithology is invariably poorly sorted siltstone or sandstone, typical of the Cromlix Mudstone Formation. The upper contact is transitional into and diachronous with the sandstone and pebbly sandstone of the Teith Sandstone Formation.

Teith Sandstone Formation (THF)[edit]

The Teith Sandstone Formation crops out in the broad axial zone of the Strathmore Syncline, with an outcrop width of up to ten kilometres (Figure 6). It is relatively monotonous lithologically, consisting for the most part of locally pebbly lithic sandstones with interbedded sequences of siltstone and mudstone, the latter representing fluvial overbank deposits that thicken towards the top of the formation. The formation underlies much of the carselands and the villages of Arnprior, Buchlyvie, Drymen and Port of Menteith, and up to three members are recognised.

Although the base of the Teith Sandstone Formation is generally defined by a transition from poorly sorted, poorly bedded siltstone or sandstone, to well-sorted, well-bedded sandstone (Dalmary Sandstone Member), conglomerate is developed in ground extending north-east from the northern shores of Lake of Menteith; this coarse lithology is assigned to the Bracklinn Falls Conglomerate Member. To the south-east, the formation is represented by a generally finer-grained sequence (Buchlyvie Sandstone Member).

BRACKLINN FALLS CONGLOMERATE MEMBER (BFCO)
The Bracklinn Falls Conglomerate Member (BFCO) represents the last major episode of conglomerate deposition in the Devonian of the Aberfoyle district. It crops out only in the ground to the north-east of Lake of Menteith, forming the steep ground marking the south-eastern edge of the Hills of Menteith (P219918).

Total thickness varies between 150 m and 350 m. The base is poorly exposed, but appears to be diachronous and transitional to sandstones typical of the Teith Sandstone Formation. The arenaceous sequence separating the Tom Dubh Conglomerate from the Bracklinn Falls Conglomerate thins north-eastwards, and the two members are separated by only a thin sequence of sandstones, assigned to the Dalmary Sandstone Member in the Cock Hill area [NN 618 057]. The top of the Bracklinn Falls Conglomerate is defined by a sharp, diachronous contact with the sandstones and mudstones of the Dalmary Sandstone.

The Bracklinn Falls Conglomerate is composed of bedded but poorly sorted, clast-supported conglomerate, with clasts of andesite, poly- and monocrystalline quartz, quartzite, psammite and semipelite up to cobble size. Also present are metamorphic clasts and epidotised metabasalt. Rare carbonate clasts may represent reworked intraclasts of diagenetic concretions, or very locally developed calcretes. The matrix is fine- to medium-grained sandstone with variable clay content. The matrix is dominated by quartz, andesitic volcaniclastic fragments and plagioclase, with minor phyllosilicates, oxides, epidote and metamorphic lithic clasts, and is fine to medium grained.

DALMARY SANDSTONE MEMBER (DASA)
The Dalmary Sandstone Member (DASA) crops out on the north-west limb of the Strathmore Syncline, between Loch Lomond and Loch Rusky [NN 615 035]. The outcrop of the member is up to 3.5 km wide, and the total thickness is estimated to be about 1500 m. The base of the member is reasonably sharp, resting on either the Bracklinn Falls Conglomerate or the Tom Dubh Conglomerate (Cromlix Mudstone Formation). The top of the Dalmary Sandstone is transitional into the red sandstone, siltstone and mudstone lithologies of the overlying Buchlyvie Sandstone Member (see below).

The principal lithology within the Dalmary Sandstone is fine- to coarse-grained sandstone, commonly in upward fining cycles, with subsidiary siltstone and pebbly sandstone, and rare beds of mudstone and conglomerate. Pebbles include vein quartz with subsidiary quartzite, and andesite; intraformational rip-up clasts of mudstone and siltstone are locally abundant, especially near the bases of sedimentary cycles, and may be plastically deformed. The larger clasts are dominated by andesitic material, polycrystalline quartz and metamorphic rocks, whilst the sand-grade material is mineralogically diverse and relatively immature. Carbonate is observed replacing original matrix and/or volcanic clasts (Phillips and Aitken, 1998[5]). Haematitic and chloritic clayey cements are typical.

The characteristic colour is greenish grey, but a pinkish grey colour prevails towards the base. The bedding planes are locally rich in fragmentary plant remains. Jack and Etheridge (1877)[6] described lepidodendroid flora from four localities in the Aberfoyle district. During the course of the recent survey, fragmentary plant material was found at Fir Hill Caravan Park [NS 5394 9675], 750 m east-north-east of Gartur at [NS 5797 9853], 350  m east-north-east of Gartrenich at [NS 5592 9855], at three localities in the Ward Burn [NS 5070 9345; 5146 9429; 5345 9481] and on the West Highland Way in the Garadhban Forest at [NS 4687 9075].

BUCHLYVIE SANDSTONE MEMBER (BUCS)
The Buchlyvie Sandstone Member crops out in the axial zone of the Strathmore Syncline, centred on Buchlyvie and Ballat. Although this unit is estimated to be only about 400 m thick, the outcrop is some 5–7 km wide and underlies the villages of Arnprior, Dykehead, Buchlyvie and Drymen.

The base of the member is transitional from the predominantly greenish lithologies of the Dalmary Sandstone Member, to generally red sandstone. The top is the faulted or unconformable contact with Upper Devonian sediments of the Stockiemuir Sandstone Formation (see below). An angular unconformity with the Upper Devonian units is seen in a small exposure in the headwaters of the Arngibbon Burn, 300 m south-west of Badenkep at [NS 5926 9181]. Brick red quartz sandstone of the Formation, with rare vein quartz pebbles, fills a washout cut in very soft, pinkish grey, feldspathic sandstone of the Buchlyvrie Sandstone Member. Intense Devonian weathering of the Lower Devonian substrate, leading to reddening and softening of the rock, indicates close proximity to the unconformity, especially to the south-west and south-east of Badenkep Farm [NS 595 920].

The principal components of this part of the succession are reddish grey, grey and reddish brown, lithic and sublithic feldspathic sandstones, with locally significant developments of red siltstone and mudstone. Pebbly sandstone beds are much rarer than in underlying units and conglomerates are absent. Pebble composition is variable, but clasts include vein quartz, quartzite, andesite and mudstone. The lithic sandstones have an ‘ashy’ appearance; they can be soft and thus easily eroded by streams, as in the Arngibbon Burn at [NS 5989 9427]. Sequences of siltstone and mudstone several metres thick are common and often result in the development of waterfalls. Silty mudstone crops out extensively in the Altquhur Burn below Craigievern [NS 493 910].

References[edit]

  1. Browne, M A E, Smith, R A, and Aitken, A M. 2001. A lithostratigraphical framework for the Devonian (Old Red Sandstone) rocks of Scotland south of a line from Fort William to Aberdeen. British Geological Survey Research Report, RR/01/04.
  2. Armstrong, M, Paterson, I B, and Browne, M A E. 1985. Geology of the Perth and Dundee district. Memoir of the British Geological Survey, Sheets 48W, 48E and 49 (Scotland).
  3. Dare-Edwards, A J. 1984. Aeolian clay deposits of south-eastern Australia; parna or loessic clay? Transactions of the Institute of British Geographers, Vol. 9, 337–344.
  4. Yang, C X. 1997. Quaternary sedimentation, parna, landforms, and soil landscapes of the Wagga Wagga 1:100 000 map sheet, south-eastern Australia. Australian Journal of Soil Research, Vol. 35, 643–668.
  5. Phillips, E R, and Aitken, A M. 1998. The petrology and composition of the Lower Old Red Sandstone exposed in the Aberfoyle area (Sheet 38E), central Scotland. British Geological Survey Technical Report, WG/98/10.
  6. Jack, R L, and Etheridge, R. 1877. On the discovery of plants in the Lower Old Red Sandstone of the neighbourhood of Callander. Quarterly Journal of the Geological Society of London, Vol. 33, 212–222.

Geology of the Aberfoyle district - contents[edit]