Boyne Bay – Whitehills – Inverboyndie – Macduff – Tarlair. Day 5. Excursion to the Banffshire Coast
Unpublished report prepared for the Edinburgh Geological Society Excursion to the Banffshire Coast. 14th – 21st May, 2005. Leaders: John Mendum, Douglas Fettes, David Stephenson and David Gould (British Geological Survey)
Introduction
The excursion will examine a section from Tarlair Bay, near Macduff, westwards to Whitehills. All the rock lie within the Macduff slate Formation (Southern Highland Group). The rocks at Tarlair lie just west of the regional Turriff Syncline, which extends from Whitehills to Fraserburgh. The section from Tarlair westwards to Whitehills starts at the highest stratigraphical and structural level and traverses into older and structurally more complex rocks. The metamorphic grade rises from greenschist to amphibolite facies from Tarlair westwards.
Tarlair [NJ 720 647]
(Figures 32, 33 and 34; Plate 16)
Drive to swimming pool at Tarlair and park in large parking area.
The exposures are in the Macduff Slate Formation. This is the highest part of the Dalradian succession in this area and represents a time when depositional conditions were highly unstable. The rocks are mainly turbiditic metasandstones and metamudstones. Complex rip-up clasts, flame structures, etc may be examined on the low tidal outcrops below the parking area. The sedimentary structures were first described by Sutton and Watson (1955). The structure here is relatively simple. Only one phase of deformation may be seen. This forms large upright folds; a synclinal-anticlinal pair may be seen on the cliffs behind the parking area. The folds carry a simple cleavage and examination of the cleavage bedding relationships clearly demonstrates that the folds are right-way-up. These relationships are best examined immediately east of the crag adjacent to the parking place. The metamorphic grade is greenschist facies, with the development of porphyroblastic biotite.
The Sclates, Macduff [NJ 714 649]
(Figures 35 and 36; Plate 17)
On return from Tarlair, stop at playing field above swimming pool. Access cannot now be safely obtained from the path seaward of the fence. From the edge of the playing field overlooking the rocky shore a large pink and white quartzofeldspathic boulder may be seen in one of the faces. This is one of a small number of exotic clasts, which are thought to be dropstones from floating ice (Sutton and Watson, 1954). Stoker et al, (1999) have published a section characterizing the rocks as marine sediments deposited close to grounded ice.
Old Banff station [NJ 687 647]
(Figure 37; Plate18)
Drive round past the harbour and park in the parking bay. The metamorphic grade has increased from Macduff and this locality lies immediately west of the cordierite-in isograd (Fig. 37). On the shore immediately below, metasiltstone beds have knots representing retrograded cordierite. Excellent examples of coarse metasandstones with rip-up clasts may also be seen.
Inverboyndie [NJ 664650]
(Figure 37; Plates 19 to 24)
Leave Banff and drive towards Whitehills. Just before entering town take the narrow road to the left down to Red Well and the shore and park at the bottom. Examine the shore section to the east.
At this point the rocks lie within the Macduff Slate Formation close to the top of the Knock Head Grit Member. The andalusite and staurolite-in isograds have been crossed and large porphyroblasts of andalusite are the characteristic feature of the area. Dark grey round cordierites are locally abundant and small brown staurolites are also seen, but tend to be confined to specific rather narrow pelitic bands. These assemblages form part of the metamorphic progression west from Tarlair and are typical of the Buchan-type metamorphism.
The increase in grade is accompanied by the appearance of secondary fold structures. These may be clearly seen on the foreshore. In thin section crenulations associated with this deformational event can be clearly seen to bend and envelope the andalusite porphyroblasts, demonstrating that the peak of metamorphism in this area splits the two deformational events.
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