Geology of the Aberfoyle district: Magnetic data

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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).
File:AberfoyleSD fig11.jpg
Figure 11    Map of the regional variation in magnetic anomaly across the Aberfoyle district. Anomalies range from about -270 (blue) to +10 nT (red). Note the generally more magnetic character over ground underlain by the Loch Katrine Volcaniclastic Formation in the north-west, and along the Highland Boundary Fault Zone (HBFZ), particularly in the north-east. Faults shown in red and geological boundaries to lithostratigraphical units shown in blue for reference. LTFZ: Loch Tay Fault Zone. DPF: Dukes Pass Fault. Shaded relief underlay derived from NextMap digital elevation data.

The map of magnetic variation across the Aberfoyle district reveals a series of concentric magnetic anomalies with values higher than adjacent areas, dispersed along the Highland Boundary Fault (Figure 11). Most of these, including a prominent magnetic high in the adjacent Ben Lomond district, are possibly due to the presence mafic units within the Highland Border Complex, notably the Lime Hill Serpentinite and the Corrie Burn Hornblende Schist, and the Balleich Lava Member (Craig of Monievreckie Conglomerate Formation). The elongate magnetic high north-east of Loch Venachar, immediately south-east of the Highland Boundary Fault, may also be due to the Balleich Lava Member. However, this particular anomaly may also reflect in part the more abundant volcanic detritus within the Craig of Monievreckie Conglomerate in this area.

A second, broader, north-easterly trending belt of magnetic highs, up to 60 nT in amplitude, occurs within the ‘Flat Belt’ in the extreme north-west of the Aberfoyle district and the adjacent Ben Lomond district. These anomalies probably reflect in part the presence of quartz dolerite and microdiorite dykes that crop out in this ground. However, the generally less negative to positive magnetic character of the north-western part of the district (Figure 11) coincides broadly with the outcrop of the Loch Katrine Volcaniclastic Formation. Thus, the lithologies in this unit, with their dominantly volcaniclastic character, are somewhat more magnetic than the lithologies in the other Dalradian units to the south-east.

Other more subtle local anomalies, forming short ridges and strings of local circular highs, correlate approximately with quartz dolerite dykes that crop out within the metasedimentary rocks of the Southern Highland Group (see Minor intrusions).

South-east of the Highland Boundary Fault, the magnetic signature is generally more subdued with a progressive decrease in the magnetic field towards the south-east over the Devonian and Carboniferous succession.

Geology of the Aberfoyle district - contents[edit]