Geology of the Aberfoyle district: Stratheden Group
|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).
Stockiemuir Sandstone Formation (SCK)
All the Upper Devonian strata within the Aberfoyle District are assigned to the Stockiemuir Sandstone Formation within the Stratheden Group (Paterson and Hall, 1986). The lithologies are mainly red brown, commonly pebbly, cross bedded sandstones with some conglomerates. The formation crops out in the south-east of the district in the Endrick valley around Balfron and on Ballindalloch, Buchlyvie and Kippen muirs. No fossils have been found in these rocks in the Aberfoyle district, but assemblages of fossil fish found in similar strata elsewhere in the Midland Valley indicate a Late Devonian (Famennian) age.
The base of the Stockiemuir Sandstone Formation is exposed in the headwaters of the Arngibbon Burn, 300 m south-west of Badenkep [NS 597 920] (it is better exposed in adjacent areas, for example at Arrochymore Point on Loch Lomond-side [NS 410 917]). Although an unconformable relationship with Lower Devonian strata occurs on Buchlyvie Muir [NS 595 920], over most of the area the boundary with older rocks is faulted. Intense reddening of the Lower Devonian strata near the boundary indicates proximity to the unconformity. Over most of the district, the top of the formation is marked by a transition into the overlying Kinnesswood Formation except north-east of Balmaha [NS 435 925] in ground to the north-west of the Highland Boundary Fault. Here, the Stratheden Group appears to be absent and the Kinnesswood Formation lies directly on Dalradian rocks.
The lowest strata within the formation are seen near Badenkep [NS 597 918] and Lintmiln [NS 612 926], where about 20 m of strata are exposed. In these sections the rocks comprise red, coarse and medium grained, cross bedded sandstones. They are pebbly in places and interbedded with some thin conglomerates. The clasts are angular, up to 80 mm across and mostly of quartz, though mica schist and red mudstone clasts are also present. The clasts are contained within a medium to coarse grained matrix composed mostly of detrital quartz grains. Other constituents include lithic fragments similar to the clasts, feldspar, greenish and yellowish patches of clay and rare flakes of mica. Detrital material appears to have been derived partly from Lower Devonian and partly from Highland source rocks. The pebbly and conglomeratic rocks are overlain by fine-grained red sandstones, with some red siltstones, as seen near Little Drumtie at [NS 573 906], the latter overlain by more pebbly sandstones interbedded with conglomerates. The cement in these rocks is mainly iron oxide, though calcareous cement occurs locally.
The overlying 100 m of strata dominantly comprise cross bedded red sandstone with locally occurring pebble or mudstone clast beds. In places, the clast-rich beds occur in upward fining channel-fill sequences, up to 8 m thick. Thin pebble beds or isolated pebbles occur at the base of the channels. Aeolian dune bedding occurs locally, but most sedimentary features indicate fluvial sedimentation. Mudstone clasts are more common and extrabasinal clasts less common in higher parts of this sequence.
The uppermost 100 m of the formation consists of thin, parallel or cross bedded red sandstones, with rare siltstones. Interbedded with these rocks are soft, brick red sandstones that are partly cross bedded (with downward wedging foresets) and partly flat bedded. Evidence that these sandstones are at least partly aeolian in origin comes from ‘pin stripe’ lamination (Fryberger and Schenk, 1988) and well-rounded ‘millet-seed’ quartz grains. However, features characteristic of water lain deposition include mica covered bedding planes, parting lineations, rare mudstone clasts, and small quartz pebbles.
- Paterson, I B, and Hall, I H S. 1986. Lithostratigraphy of the late Devonian and early Carboniferous rocks in the Midland Valley of Scotland. Report of the British Geological Survey, No. 18/3. (London: HMSO for the British Geological Survey.)
- Fryberger, S G, and Schenk, C J. 1988. Pin stripe lamination; a distinctive feature of modern and ancient eolian sediments. 1–15 in Eolian sediments. Hesp, P, and Fryberger S G (editors). Sedimentary Geology, Vol. 55 (1–2).