Banffshire coast – an excursion – introduction to geology

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Regional setting

During late Neoproterozoic times the Grampian Terrane of the Scottish Highlands lay adjacent to the outboard edge of the Laurentian Continental mass (Figures 1, 2). Harris et al. (1994) and Soper (1994) have interpreted the currently exposed Dalradian sequence to have formed in several rift‑thermal subsidence cycles in the late Proterozoic and early Palaeozoic at between c. 750 Ma and 510 Ma. Dewey and Mange (1999) postulated that this succession was subject to a brief Grampian orogenic event during Early Ordovician times at around 470 Ma, with subsequent uplift and further deformation in the Late Ordovician and in the Silurian. In contrast, evidence of complex and multiple subduction zones in the Iapetus Ocean to the southeast of Newfoundland is provided by Van der Pluijm et al. (1995) and Dunning et al. (1991). The tectonic features and timing of events in Ireland and Scotland also suggest a more complex Precambrian to Ordovician history for the Caledonides (see Strachan et al., 2002, for review) and the exposed geology of the Grampian Highlands documents parts of that tectonic history.

The Grampian Terrane is part of the Caledonide Orogen that is bounded to the northwest by the Great Glen Fault and to the southeast by the Highland Boundary Fault (Figure 3). It is composed of a unique southeasterly prograding Neoproterozoic (Late Precambrian to Cambrian) Dalradian sequence of metasedimentary and metavolcanic rocks that cumulatively total c. 15 km in thickness in any one area (Stephenson and Gould, 1995). These rocks show widely varied metamorphic grades ranging from upper amphibolite facies to lower greenschist facies and include the type areas for Buchan and Barrovian metamorphism. Intrusive bodies, ranging in composition from ultramafic to mafic and granitic, are common and are mostly of Ordovician and Silurian age. Northeast Scotland is unique for the abundant occurrence of Ordovician mafic-ultramafic and granitic intrusions, related shear zones, and its relatively low-pressure metamorphic mineralogies. It is interesting that the present sea level broadly reflects the geology with Palaeozoic and Mesozoic sedimentary rocks dominant offshore (Figure 4).

The present day erosion level reveals an oblique section through the orogen with former shallow crustal levels (6 km to 18 km depth) now exposed in the Buchan Block. In contrast, in the Central and Southwest Highlands peak metamorphic conditions imply that the rocks reached crustal depths of 25 km to 35 km. The nature of the basement to the Grampian Terrane and of any Cambrian-Ordovician rocks that once existed to the southeast of the Highland Boundary Fault are largely unknown, with only small fragments preserved in parts of the Highland Border Complex, the Ballantrae Complex and in the Northern Belt of the Southern Uplands. The bulk of such material either lies beneath the Midland Valley or has been excised by the possible Early Ordovician dextral and Late Silurian and Devonian sinistral strike slip movements along the Highland Boundary. By analogy with Connemara and Mayo the missing material may have included arc-related sediments and basic volcanic units, pre-Grenvillean basement rocks, ultramafic ophiolitic material, etc.

The Grampian Highlands can be divided into four main tectonic elements, the Buchan Block, the Central Highlands, the Southern Highlands and the Southwest Highlands. These elements show distinctive structural and metamorphic histories, acquired during the Caledonian orogeny, although their boundaries are gradational in parts. This excursion will address the nature of the Buchan Block, the northeast part of the Central Highlands and their contact zone along the Portsoy Lineament. Although there are some similarities in the geological make-up of the different elements, it will become clear that the Buchan Block is quite distinct from the other parts of the Grampian Highlands.

The Buchan Block is a geophysically discrete area with high Bouguer gravity values (mean 2.76-2.78 gu) reflecting relatively dense material at depths of 5 ‑10 km (Ashcroft et al., 1984; Rollin, 1994). The block is bound in the west by the sub-vertical, north-northeast‑trending Portsoy Lineament and in the south by a complex imbricated zone containing several east‑west trending and gently northerly dipping shear zones, e.g the Glen Mark Slide (Harte, 1979). The Portsoy Lineament, picked out by a steep gravity gradient, is internally characterised by numerous steep shear zones (Fettes et al, 1991; Goodman, 1994), many of which are marginal to the larger basic‑ultrabasic bodies (Ashcroft et al, 1984). Voluminous granites, notably the Mount Battock pluton, subsequently intruded the southern margin. The Buchan Block exposes upper Argyll Group and Southern Highland Group rocks which are affected by ‘Buchan type’ metamorphism and deformed into a series of upright to west‑vergent, north‑trending folds. The sequence is upward facing in the north, but significantly large areas of inverted rocks are recorded farther south in Central Deeside, and in the Stonehaven and Collieston ‑Whinnyfold areas. Metamorphic pressure and temperature (P-T) values range from 2 kb, 500°C on the Banffshire Coast, to c. 7.5 kb and 800°C on‑Mid‑Deeside and in Glen Muick (Goodman, 1991). Layered mafic and ultramafic intrusions of the Northeast Grampian Basic Suite (formerly termed the 'Younger’ and ‘Older Basic' intrusions) are restricted to this area in Scotland and both Ordovician ('Older') and Silurian ('Younger') granites are abundant. Geophysical and well data indicate that the Buchan Block extends offshore northwards to the Halibut‑Orkney Lineament and eastwards to the Peterhead Ridge, Buchan Horst and possibly the Renee Ridge (in line with the Highland Boundary Fault) (Figure 1).

The Central Highlands is formed mainly of Dalradian Grampian Group rocks with representatives of the Appin and Argyll groups in its southwest part. Smith et al. (1999) and Robertson and Smith (1999) recognised the presence of an older largely psammitic, high grade, migmatitic basement, the Dava-Glen Banchor succession, formerly termed the Central Highland Migmatite Complex, which is onlapped by the Dalradian sequence. The Grampian Group units generally exhibit low to moderate Caledonian deformation with upward, west to southwest‑facing folds that show distinctly sinuous axial traces. Note that the original correlation of the Grampian Group (dominantly psammites) with the Moine rocks of the Northwest Highlands is no longer tenable. The Moine Supergroup sensu stricto was deposited between 950 and 873 Ma, whereas the Dalradian succession was laid down between c.750 and 510 Ma. An older (pre-800 Ma) deformational and metamorphic history is evident in the Central Highland ‘basement’ rocks but they are cut by shear zones and have been reworked by the ubiquitous Caledonian deformation and amphibolite grade metamorphism. The overall sequence is affected by Early Ordovician penetrative folding and related pervasive fabrics, particularly well-seen in prominent steep belts (e.g. Kinlochlaggan Steep Belt), and by a later Ordovician phase of widespread folding and pervasive amphibolite grade fabrics. Regionally, metamorphism attained kyanite grade in the southwest and eastern parts of the area, with P-T values of 8 kb and 650°C (Phillips et al, 1999; Beddoe-Stephens, 1990). In its northeast part andalusite prophyroblasts are overprineted by kyanite indicating a pressure increase of around 2.5 kb during Caledonian orogenesis. In the central area local migmatisation of semipelitic and potash‑feldspar‑rich psammitic units occurred. These amphibolite grade conditions, accompanied by some granite intrusion (e.g. Strathspey, Findhorn granites) occurred at around 445-455 Ma (Clayburn, 1981; Highton et al, 1999). Both Ordovician and Silurian age granite plutons are again present.

Dalradian Supergroup

The Dalradian rocks now exposed in Northeast Scotland encompass much of the known succession and range from the older Grampian Group rocks to Appin and Argyll Group, and the younger Southern Highland Group (Figure 5). However, on the ‘Banffshire Coast’ section the older parts of the Grampian Group and younger parts of the Southern Highland Group are absent. Some elements of the Dalradian sequence mapped inland are also absent on the coast, mainly due to the original stratigraphical template or to structural discontinuities. In addition, faulting cuts out some parts of the sequence and the wider sandy bays do not allow continuous exposure, e.g. Sandend Bay, Fraserburgh Bay.

On the coast the Grampian Group rocks are dominantly psammites and quartzites, and these pass conformably up into the Lochaber Subgroup (Appin Group), here composed mainly of micaceous psammites and semipelites, which become dominantly calcareous towards the top. At Sandend Lochaber Subgroup rocks pass with a sharp transition into Ballachulish Subgroup rocks, graphitic pelites, calcareous semipelites and metalimestones. The Corriehabbie Quartzite Formation (º Appin Quartzite) that can be traced from Donegal along almost the whole Dalradian outcrop is absent from the succession. In contrast, the overlying more uniform Blair Atholl Subgroup rocks, mainly metalimestones, semipelites and pelites, are well represented. The ‘Boulder Bed’ at the base of the overlying Argyll Group is also absent from the coastal succession, but large angular boulders and other float can be found inland, and lensoid outcrops are mapped in several parts of the East Grampians. The Islay Subgroup rocks are dominantly psammites and quartzites with subsidiary semipelites. Again their coastal manifestation differs from the quartzite unit seen extensively inland that can be traced to Schiehallion, Islay and Donegal. The Easdale Subgroup rocks are very varied with graphitic pelites, metalimestones, psammites, semipelites and pelites all represented. The rocks lie within the Portsoy Shear Zone and hence are strongly deformed, folded and intruded by abundant mafic and ultramafic bodies. The rocks at Portsoy are not traceable inland, partly due to poor exposure, but partly reflecting the original stratigraphical template. Crinan Subgroup rocks are represented by semipelites, micaceous and feldspathic psammites and occur in areas of high-grade metamorphism. Hence, they are commonly gneissose and exensively veined by quartz and quartzofeldspathic material. The Tayvallich Subgroup rocks are represented by the metalimestones and calcaroues semipelites, which represent former reef and shallow marine, possibly lagoonal environments. The Southern Highland Group succession is marked by the first incoming of turbiditic material, initially dominated by calcareous material but soon dominated by gritty arenites and pelitic material. The metamorphic grade of these turbiditic rocks is lower and sedimentary features are well seen. Evidence of glacial activity at the time of deposition is seen at Macduff.

Structure and metamorphism

The ‘Banffshire Coast’, provides a well-exposed natural across-strike section through the Dalradian succession in Northeast Scotland. The structural and metamorphic patterns that are now exposed are controlled by the late-stage north-trending Turriff Syncline and Buchan Anticline (Figure 6). The youngest Dalradian rocks occur just east of Macduff in the core of the Turriff Syncline and coincide with the lowest metamorphic grades, here in upper greenschist facies. Moving east and west from these greenschist facies rocks metamorphic grade rises as does the degree of structural complexity. Indeed, there is a near-perfect coincidence between the oncoming of andalusite (the andalusite isograd) and a secondary phase of folding (termed F3 here). Although the Portsoy Lineament, which on the coast is expressed as an almost 2 kilometre-wide complex shear zone, the Portsoy Shear Zone, forms a major tectonic disturbance in the Dalradian stratigraphy. It appears to coincide with Easdale and Crinan subgroup units whose original distribution is unclear. There is presently no definable stratigraphical jump across this Portsoy Shear Zone. Other features of the structure are the steep zones, which appear to have formed at both early and late stages in the tectonic history. Although in the literature numbers have been used for the different deformation phases (e.g. D1, D2, D3, etc) and related folds (F1,F2, etc) and fabrics (S1, S2, etc), it is difficult to correlate across the Portsoy Shear Zone and to relate such phases to other parts of the Grampian Highlands. Hence, an overallpicture of tectonic events is not possible without accurate age dating of the deformation and metamorphic phases, and even these are likely to be diachronous. In the Portsoy Shear Zone, numerous individual deformation phases can be recognised but they probably have little regional significance.

Early Ordovician shear zones are not restricted to the Portsoy Lineament but also bound many of the mafic-ultramafic bodies of the Northeast Grampian Basic Suite (Figure 7). The intrusion of these layered bodies was coeval with deformation and shearing, which clearly occurred in a geologically short-lived episode at c. 470 Ma.

The Dalradian succession was folded, thrust and variably metamorphosed during the Early Ordovician Grampian event. However, the earliest penetrative deformation, folding, and related low pressure (2 kb to c.5 kb) metamorphic pattern resulted in a pattern of increasing metamorphism from chlorite grade up to sillimanite grade. This metamorphism predated the intrusion of mafic and ultramafic rocks of the Northeast Grampian Basic Suite as clearly shown by the superimposition of the hornfels on the early S1 fabrics and related regional metamorphic mineralogy. The Early Ordovician thrusting gave rise to a high pressure (c. + 2.5 kb) overprint, with excellent examples of kyanite pseudomorphing andalusite, now seen at Portsoy and in the Keith – Dufftown – Cabrach area. Pressures of 6 kb to 8 kb are implied by the superimposed metamorphic mineralogy. Rapid uplift must have followed this overthrusting to preserve the mineralogy. Hence, it appears that the Buchan Block has had a different early Caledonian tectonic history compared to other parts of the Grampian Highlands. The overall metamorphic pattern is shown in Figure 8.

Intrusive rocks

Precambrian intrusive rocks are found in the Dalradian succession and range from metadolerite and metabasalt sills and lenticular bodies to discrete granites. The earlier mafic rocks intrude rocks from the Islay Subgroup upwards to the lower parts of the Southern Highland Group but whether they have a range of intrusive ages is unclear. In other parts of the East Grampian areas basic igneous activity is recorded at several levels. Foliated and mainly porphyritic granites form series of lenticular granitic bodies that extend from Portsoy southwest to Keith and beyond. U-Pb zircon age dates show that these granites were intruded at 600 Ma, and represent an abortive rifting event signalling the later break up of Rodinia. The lineament along which intrusion was focussed later became the site of WNW-directed thrusting (the Keith Shear Zone).

As noted above the main igneous bodies were intruded in the Early Ordovician (see Stephenson and Gould, 1995). These include the Northeast Grampian Basic Suite, a series of layered mafic and ultramafic rocks whose mineralogy can be assigned to Lower, Middle and Upper zones dependent of the degree of fractionation of the basic magma. However, the form and mode of occurrence is the mafic-ultramafic plutons is still unclear. The Huntly Pluton does contain Lower Zone rocks, but the intrusion lacks a full or even partial zonal sequence and includes contaminated and fine- grained granular gabbroic rocks implying that fractionation processes were interrupted by ingress of fluids (mainly water) during rapid intrusion. The Huntly Pluton appears from magnetic and gravity data to be <1 km thick. The Insch and Boganclogh plutons are thicker and contain Lower, Middle and Upper Zone rocks. The basic igneous activity was followed rapidly by intrusion of diorite, granodiorite and granite bodies (the ‘Older Granites’), but only the Cairns of Ord, Longmanhill, Aberchirder and Strichen granites occur close to the ‘Banffshire Coast’. Intrusion of the ‘Younger Granites’ occurred during the Silurian. Most of these bodies occur on Deeside but the Peterhead Granite forms a major body in the gneissose Dalradain rocks of Buchan. Partial melting, which has giving rise to extensive areas of migmatitic gneisses, has occurred locally, notably adjacent to the Ordovician gabbroic and related dioritic intrusions. The Late Silurian period of granite intrusion was widespread across the Scottish Highlands and was coeval with regional uplift, extension and exhumation of the Caledonian orogenic belt. Erosion of the mountains so-formed provided material for the post-orogenic ‘molasse’ but much of this material has been transported elsewhere.

Devonian

Devonian alluvial and lacustrine rocks are preserved in numerous outliers across the Northeast Grampians, commonly with partly fault-bounded margins, reflecting their typical original half-graben nature. The additional presence of small patches of basal breccia and conglomerate in many places suggest that the present land surface accords closely to that in early Devonian times in many areas. Early Devonian conglomeratic and sandstone outliers occur around Tomintoul and around Cabrach. The Rhynie outlier contains early Devonian breccias, sandstones, siltstones, mudstones and andesitic lavas, and the western marginal faults are the site for the Rhynie Chert. This local unit contains amazingly well-preserved early plants, crustaceans and insects, all silicified in a volcanic hydrothermal spring system about 407 Ma ago. The larger Turriff Outlier contains a sequence of Early Devonian breccias, conglomerates, sandstones and siltstones that constitute the Crovie Sandstone Group and a later Mid Devonian conglomerate and sandstone sequence (Inverness Sandstone Group) that overlies the older sequence unconformably (see Trewin and Thirlwall, 2002).

Mesozoic and Cainozoic rocks and deposits

Only minor small enclaves of Mesozoic rocks are found onshore in Northeast Scotland. The Cainozoic history is significant in that evidence is preserved of Neogene deposits and weathered profiles (Figure 9, 10), as well as fragments of the lengthy post Devonian and pre-Late Devensian history of Scotland. Although Northeast Scotland contains limited evidence of pre-Late Devensian glacial and interglacial deposits, the bulk of glacial and glaciofluvial deposits mainly reflect the last main stadial, the Main Late Devensian, that occurred between c. 28,000 and 15,000 years BP. Figure 11 shows the complex sequence of Pliocene and Quaternary events that occurred in Northeast Scotland or offshore. Evidence relating to events in the earlier parts of the Quaternary is sketchy as late glaciations have effectively obliterated the deposits and features. Figure12 shows the climatic changes and deposits found in Northeast Scotland that relate to the last 140,000 years, whereas the distribution of deposits and notable glacial features is shown in Figure 13. Northeast Scotland is unique in Great Britain in that during the Late Devensian it was covered by ice streams derived from the Cairngorms and Ladder Hills, the Findhorn and Spey valleys, the Moray Firth, and from Strathmore. These separate ice streams have not only differed in their timing, but have also emplaced very different type of glacial till across the area. Figure 14 summarises the stratigraphy across the region and Figure 15 shows the distribution of the main tills that relate to the various ice streams. Towards the end of the last main glacial phase (the Dimlington Stadial) retreat of the inland ice sheets allowed meltwaters to form lakes against the marine-based ice sheets. Hence, from about 18,000 years to around 14,000 years ago a series of proglacial lakes developed depositing sequences of silt and forming a network of overflow channels (Figure 16). The ‘Banffshire Coast’ contains only a few sites where Quaternary deposits are exposed, but ironically much of the immediate hinterland contains thick glacial and glaciofluvial deposits. The detailed description of the Quaternary deposits and events is summarised in Merritt et al, (2003); detailed maps for the Elgin, Portsoy, Banff and Fraserburgh 1:50,000 sheet areas are shown in Figures 17, 18, 19 and 20 respectively.


References

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