OR/15/026 Dalradian Supergroup

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Gunn, A G, Mendum, J R and Thomas, C W. 2015. Geology of the Huntly and Turriff Districts. Sheet description for the 1:50 000 geological sheets 86W (Huntly) and 86E (Turriff) (Scotland). British Geological Survey Internal Report, OR/15/026.

The Dalradian metasedimentary rocks within the Huntly and Turriff districts are dominated by those assigned to the youngest constituent group, the Southern Highland Group. These rocks occupy most of the ground east of the Huntly and Knock mafic-ultramafic plutons and north of the Insch Pluton (Figure 1). The oldest Dalradian rocks belong to the Grampian Group and crop out only in a small area in the west of the Huntly district. Successively younger rocks, assigned to the Appin and Argyll Groups, crop out in the Huntly district, where they are cut by the major shear zones and the mafic-ultramafic intrusions.

The terminology used to describe the Dalradian stratigraphy and lithologies in the literature is somewhat confusing. Some formation names refer to their undeformed protolith rock type, e.g. Fordyce Limestone Formation, but others highlight their metamorphic character, e.g. Mortlach Graphitic Schist Formation, and yet other their parting characteristics, e.g. Findlater Flag Formation. Unfortunately, a single rational scheme based on simple criteria cannot be easily applied to the Dalradian succession and the varied names reflect the field appearance and characteristics of the individual formations. Given that there is no satisfactory alternative scheme, and to maintain continuity with the previous literature, such idiosyncratic lithostratigraphical names are used here, with minor rationalisations.

However, the recently approved BGS lithological schemes for igneous, metamorphic and sedimentary rocks are generally followed throughout this sheet explanation. The lithologies in the Huntly and Turriff districts vary from low grade deformed arenites (sandstones) and argillites (mudstones and siltstones) in the turbiditic Southern Highland Group to their recrystallised and gneissose metamorphic equivalents that retain no trace of their former bedding, typically seen in some Argyll Group rock units. Hence, with increasing metamorphic grade, arenites become psammites, and argillites become pelites or semipelites, dependent on their mica content. The arenites range from quartz arenites (>95% quartz) to the sub-feldspathic (5–25% feldspar) and feldspathic (25–100% feldspar) varieties. Sub-lithic and lithic arenites (>50% rock fragments) are rare. Note that quartz arenites are generally termed quartzites and even in their recrystallised metamorphic state they retain this name. If the matrix percentage rises to over 15 per cent in siliciclastic arenaceous rocks, they are termed wackes and some of the Dalradian turbiditic succession undoubtedly contains feldspathic and possibly locally quartz wackes. As no modes are available for many of the arenites/wackes, in this account they are generally termed arenites or psammites. The pelitic and semipelitic rocks are more susceptible to deformational and metamorphic processes even in the lowest grade rocks. Here they are invariably slaty at low metamorphic grade (greenschist facies) and as the grade rises they are phyllitic and schistose. Hence, in this account they are referred to as pelites and semipelites throughout. The carbonate rocks should be termed metacarbonates, metalimestones, etc but the term limestone is still used here, both in the lithostratigraphical formation names and in places, also in the text.

Grampian Group

Cullen Quartzite Formation

The Grampian Group is represented by a small outcrop on the Hill of Shenwall on the border of sheets 85E (Glenfiddich) and 86W (Huntly) at around [NJ 440444]. There are no exposures and the outcrop is defined by the presence of boulders and white-weathering debris of massive pinkish quartzite. This outcrop, together with a more extensive outcrop on the Hill of Bellyhack on sheet 85E, occurs in the core of the Ardonald Anticline. The rocks are assigned to the Cullen Quartzite Formation on the basis of their stratigraphical position below the Findlater Flag Formation and are termed the Bellyhack Quartzite Member.

Appin Group

Lochaber Subgroup

The Lochaber Subgroup is represented in this district by two formations, both of which comprise dominantly flaggy psammites and semipelites. The noncalcareous Findlater Flag Formation is overlain by the Pitlurg Calcareous Flag Formation, which includes beds of calc-silicate rock and locally impure metalimestones. The junction of the formations is relatively sharp; in the field it is identified by incoming of a pale greenish grey colour, characteristic of most lithologies in the Pitlurg Formation. In thin section, the colour is seen to be due to the presence of abundant fine-grained tremolitic amphibole.

Findlater Flag Formation

There are two outcrops of this formation in the district. The larger one, around the Burn of Aultmore, encompasses almost all of the rocks below and to the north-west of the Keith Shear Zone and is continuous with the type locality around Findlater Castle on the Banffshire coast some 12 km to the north-east. The smaller outcrop lies farther south in the core of the Ardonald Anticline. Lithologies are similar in both outcrops and consist predominantly brown-weathering, grey, thinly banded micaceous psammites and semipelites. Most exposures are distinctly flaggy, but the semipelites are typically schistose, commonly with a marked crenulation cleavage (S3). The semipelites are garnetiferous in places, particularly in the higher parts of the formation.

In the Burn of Aultmore and its tributaries there are several beds of grey to off-white, flaggy, micaceous quartzite and some more massive white quartzites in which cross-bedding is commonly visible. All of the quartzites have thin fissile micaceous partings. The highest exposed part of the formation, immediately beneath the Keith Shear Zone, is more predominantly semipelitic with abundant schistose micaceous beds.

Pitlurg Calcareous Flag Formation/Cairnfield Calcareous Flag Formation

The main outcrop of the Pitlurg Formation is around the eastern closure of the Ardonald Anticline, which includes the type locality of the Den of Pitlurg [NJ 425 452 (in sheet 85E) to NJ 452 453]. Here, there are excellent exposures on the steep sides of a deep, boggy glacial meltwater channel that has been excavated along an E–W strike fault. The formation consists typically of flaggy calcareous psammites and semipelites that show a strong colour banding on a scale of 1 to 2 cm in shades of grey and pale green. Micaceous partings occur throughout, normally muscovite rich but commonly also with biotite. Near the base are persistent beds of schistose garnet-biotite semipelite similar to those in the underlying Findlater Flags. The formation becomes more calcareous towards the top and in places there are quite thick sequences of pale green and spotted calc-silicate rocks and thin beds of impure cream and pale grey metalimestone.

In the area to the south of Keith (mainly in Sheet 85E), the upper part of the Pitlurg Formation is dominantly semipelitic, but with numerous beds of calc-silicate rock and impure metalimestones. Most of the metalimestones are thinly banded, some with pelitic interbeds, but others are pale grey to white, coarsely crystalline and massive. This sequence is termed the Drummuir Calcareous Member. Along strike to the north-east, along the Burn of Drum in the present district, there are grey, flaggy semipelites with phyllitic partings and thin beds of psammite. These have been assigned to the Drummuir Member but here the semipelites are less calcareous and lack metalimestones, making the correlation somewhat uncertain.

Beneath and to the north-west of the Keith Shear Zone, flaggy calcareous rocks above the Findlater Flags are considered to be laterally equivalent to the Pitlurg Formation but, because they have a distinctly different field appearance, they are termed the Cairnfield Calcareous Flag Formation. Both the Pitlurg and the Cairnfield Flag formations contain flaggy, colour-banded, amphibole-bearing rocks, but they differ in the mode of occurrence of the amphibole. Whereas the Pitlurg Flags generally contain disseminated fine-grained amphibole, visible only in thin section, the Cairnfield Flags include beds of coarse-grained amphibole-rich rock, with the amphbole clearly visible in hand specimen and forming crystals up to several centimetres long in places. The Cairnfield Calcareous flags project into the Huntly district in only two small outcrops around [NJ 445 522] and [NJ 489 580], neither of which is exposed.

Apart from the discrete beds of metalimestone, few of the lithologies in the Pitlurg and Cairnfield Calcareous Flag formations contain any appreciable carbonate minerals. Typical thin sections show fine-grained equigranular quartz and colourless prismatic amphibole, varying in proportions from band to band, together with lesser amounts of phlogopite. The amphiboles range from tremolite to tremolitic hornblende to magnesio-hornblende and show hardly any enrichment in either sodium or iron (Stephenson, 1993)[1]. Overall the mineralogy suggests a protolith high in calcium, magnesium and aluminium, with variable amounts of silica, such as would be found in dolomitic marl. Sedimentary bedforms throughout this part of the Dalradian sequence suggest deposition in shallow water, possibly on a marine continental margin (Anderton, 1988)[2]. The magnesium-rich carbonates are most likely to have been precipitated from seawater in restricted lagoons, subject to evaporation in a tropical environment. Stephenson (1993)[1] has argued that the observed colour banding may effectively be primary, reflecting seasonal variations in the amount of silica and aluminium brought into the lagoons as clastic material from a nearby mature landscape with lateritic soils.

Ballachulish Subgroup

Mortlach Graphitic Schist Formation

The base of the Ballachulish Subgroup throughout the north-east Grampian Highlands is marked by a relatively abrupt change to a more pelitic and graphitic background lithology, the Mortlach Graphitic Schist Formation, within which are calc-silicate beds, similar to those of the underlying formation, and beds of metacarbonate rock. At, or close to the base of this formation throughout much of the area is a persistent bed of massive metalimestone, termed the Dufftown Limestone Member, which has been quarried extensively in several localities. In the Huntly district, the Mortlach Graphitic Schist Formation crops out only sparsely near its western margin around the eastern closure of the Ardonald Anticline, in an inferred anticlinal core in the Glen of Coachford, and in small outcrops around Dunnyduff.

The Dufftown Limestone Member is well exposed in the disused Blackhillock Quarry [NJ 440 481] in sheet 85E, and in long-abandoned quarries to the south of Alehousehillock [NJ 459 449 to 461 442]. It can be traced farther south-west around the nose of the Ardonald Anticline and west along its southern limb by small exposures, shallow overgrown pits, and lines of springs. The metalimestone is up to 30 m thick, although tight to isoclinal folding is developed in places, implying at least some local tectonic thickening. Typically it is pale bluish grey and coarsely crystalline with a pronounced banding and locally developed dark pelitic partings.

The metalimestones are overlain directly by black graphitic pelites, which are best exposed at the top of the quarry faces at the Blackhillock and Alehousehillock quarries. Elsewhere its presence is revealed mainly from debris in soil and till. In places thin quartz-rich laminae indicate bedding, but mostly the pelites are schistose with a strong crenulation cleavage (S3), itself deforming an earlier spaced cleavage (S2). Small garnets are common and staurolite locally present. Outcrops assigned to the Mortlach Graphitic Schist in the Burn of Drum, around [NJ 441 497], are atypical lithologically, being phyllitic to slaty, paler grey and more semipelitic. Their correlation with the main Mortlach Graphitic Schist outcrop is uncertain but they seem to be continuous with lithologically similar outcrops to the north-west of the Drummuir Fault in the adjacent Glenfiddich district (Sheet 85E).

Corryhabbie Quartzite Formation

The Corryhabbie Quartzite crops out almost continuously throughout the north-east Grampian Highlands from Deeside to just east of Keith in the Huntly district. It forms two parallel outcrops around the eastern closure of the Ardonald Anticline, where the repetition is interpreted as due to westward thrusting. In its northward extension across the north-west-trending Cairnie Fault, it forms an extensive outcrop underlying most of Balloch Forest defining a complex anticlinal fold closure. A further outcrop, structurally beneath that of the Balloch, extends from the Cairnie Fault, through Dunnyduff to the River Isla and, across the Isla faults, ending in an anticlinal closure on Gallow Hill [NJ 483 526]. Outcrops of highly sheared quartzite within the Keith Shear Zone at Greenbog [NJ 472 545] and between Goukstone [NJ 489 560] and Clochmacreich [NJ 494 580] are lithologically similar to the Corryhabbie Quartzite, but there are no further outcrops to the north. Although this unit can be traced along the Dalradian outcrop from Donegal through the Grampian Highlands, it is not present in Sandend Bay, some 10 km along strike to the north-east. Hence, it must either lens out or have been excised tectonically to the north-east of Keith.

In other areas in the Grampian Highlands, including the adjacent Glenfiddich district (Sheet 85E), there is commonly a striped transitional unit or units of micaceous psammite, which intervene between the underlying graphitic pelite and the quartzite. However, in the Huntly district, the true boundary between the formations is not seen; it is either not exposed, as on the southern limb of the Ardonald Anticline and the Glen of Coachford, or it is inferred to be a tectonic dislocation. There is no sign of striped transitional lithologies as clasts in the subsoil or till of any of the unexposed areas and we infer that the boundary is relatively sharply defined in this district. The tectonised lower boundary of the quartzite is well seen in the east face of Blackhillock Quarry [NJ 4415 4816], where layers of dark green to white clay with shattered quartzite fragments occur. Elsewhere, as in the Burn of Ardonald around [NJ 462 448], there are schistose muscovite-rich quartzites or flaggy quartzites close to the quartzite margin.

Much of the mapping of the Corryhabbie Quartzite is based upon the tracing of ridges of high ground that are commonly littered with angular blocks of white-weathering massive quartzite The typical lithology, well seen for example in Cairdshill Quarry [NJ 443 482] and in a small quarry on the south side of Both Hill [NJ 4438 4050], is a white to fawn, massive, indurated orthoquartzite. Bedding traces are typically well seen due to a pinkish brown weathering of very thin heavy mineral layers. Cross-bedding is common and graded bedding and slump features are seen locally. Way-up criteria are clearly seen at the two quarries mentioned above, but normally are more indistinct. At Cairdshill Quarry there are micaceous interbeds and layers of pale green clay material. Thin, coarse-grained, gritty feldspathic beds occur only rarely, e.g. in the quarry on Gallowhill [NJ 4842 5253].

The upper boundary of the quartzite is exposed in the Burn of Tarnash [NJ 4436 4918], where there is a sharp junction with flaggy to phyllitic, banded semipelites and micaceous psammites of the overlying Tarnash Phyllite and Limestone Formation. Elsewhere the upper boundary is inferred. In places, it seems to cut across the overlying strata and has been interpreted as a tectonic dislocation, as in Mill of Wood Burn [NJ 4555 5050], on the north-west side of Gallow Hill, and on the north-west side of Meikle Balloch Hill.

Tarnash Phyllite and Limestone Formation

This formation is only well exposed in burn sections east and north-east of Keith. The low relief areas that surround The Balloch are mostly underlain by this formation, but most are till-covered and cultivated. The type section lies in the lower part of the Burn of Tarnash around Bridge of Tarnash [NJ 4438 4894] and in its south-eastern tributary, the Birken Burn. Further sections occur in the lower part of the nearby Herrick’s Burn, farther north in limited parts of the Burn of Paithnick, by the Mill of Paithnick [NJ 4817 5383], and the Lime Burn, downstream from Bridge of Bridgend [NJ 5146 5526]. It consists of mid grey to fawn, flaggy to fissile, thin- to medium-bedded, phyllitic, micaceous and highly micaceous psammite and semipelite with prominent metalimestones, and some thin psammite beds. The micaceous psammites are commonly calcareous and locally contain abundant thin bands and lenses of cream to pale green calc-silicate rock, which in parts show well-formed dark green tremolitic amphibole laths. The metalimestones are dominantly calcitic and range from the typical white weathering, pale grey to white and cream, fine-grained, crystalline varieties to white/grey, laminated and thinly banded types, and more rarely to coarsely crystalline, pure white types. They are typically 20 to 30 cm thick but range from 1 cm up to 2 m. A 4 to 8 m-thick, cream to grey, dolomitic metalimestone (in part metadolostone) near the base of the formation forms the prominent waterfall in the type section in the Burn of Tarnash. Rarely thin quartzite beds occur within the sequence.

The formation is internally tightly folded so that estimates of its stratigraphical thickness are problematical. It is probably between 150 and 200 m thick in the Tarnash area, but may be thicker farther north where it is dominantly semipelitic. Generally it passes upwards with rapid transition into dominantly schistose and commonly graphitic, pelites and semipelites that form the lower part of the Fordyce Limestone Formation. Near Balnamoon [NJ 4821 5556], it passes up into directly into the Limehillock Limestone Member, a thick blue-grey metalimestone normally found in the central part of the Fordyce Limestone Formation.

Ailnack Phyllite and Limestone Formation

A poorly exposed outcrop of semipelites, micaceous psammites and thin metalimestones stretches along the eastern side of The Balloch, south-south-west from the valley of the Isla through Auldtown [NJ 494 487] and Botary Mains Farm [NJ 470 450] to reach the western margin of the district at Westfolds [NJ 437 414]. The Burn of Cairnie provides a cross-section though the formation, but even here the exposure is dominated by the numerous metadolerite sheets. A prominent grey metalimestone occurs in the upper part of the sequence. Although these beds occupy a similar stratigraphical position to the Tarnash Phyllite and Limestone Formation, they are attributed here to the laterally equivalent Ailnack Phyllite and Limestone Formation as they form a laterally contiguous outcrop with that unit to the south. A small tight elongate periclinal synform enclosed by the Corryhabbie Quartzite Formation that underlies most of The Balloch contains semipelite, micaceous psammite and a prominent metalimestone unit in its hinge area. This is interpreted as a fold outlier of the Ailnack Phyllite and Limestone Formation.

Blair Atholl Subgroup

Rocks attributed to the Blair Atholl Subgroup are mainly schistose pelites and semipelites, in part graphitic, and with abundant thin beds of grey, crystalline metalimestone and more lenticular calc-silicate rock. A thick unit of blue-tinged, pale to dark grey crystalline metalimestone is present in many places and this distinctive lithology can be correlated readily with other Blair Atholl Subgroup limestone formations farther south-west. The subgroup is characterised by consistent lithologies and represents a period of widespread, stable, shallow marine shelf conditions. The rocks in the Huntly district are termed the Fordyce Limestone Formation, and link north-wards to their type area. However, in some parts of the district lack of exposure prevents the separation of Blair Atholl and Ballachulish subgroup rocks and the metalimestone-bearing pelitic rocks are termed Blair Atholl Subgroup Undivided, as Blair Atholl Subgroup rocks are thought to form the bulk of the outcrop.

Fordyce Limestone Formation

This unit crops out west of the Portsoy Lineament in the Limehillock, Edingight and Burn of Paithnick areas north of the River Isla, where it is folded into a kilometre-scale, east-north-east- plunging syncline-anticline-syncline pattern. South of the River Isla the formation crops out in the core of a recumbent synformal structure on the lower north-west flank of Meikle Balloch Hill. Its type area lies farther north, and good sections through the formation are rare in this district. Coherent exposures are seen in the Mill of Wood Burn [NJ 457 503], in parts of the Burn of Braco around [NJ 498 524] and [NJ 5055 5366], and in the Burn of Paithnick and its tributaries, the Bowie and Balnamoon burns, e.g. around [NJ 4812 5402]. The formation consists of dark to silvery grey, fissile, slaty to schistose, commonly graphitic, muscovite-biotite pelite and semi-pelite. It is locally calcareous and commonly pyritic. Micaceous psammite, commonly finely interbedded with semipelite, and rarely feldspathic psammite, occur in its upper part, notably around Langley [NJ 5160 5674] and Brambleburn [NJ 5101 5639]. Typically, the unit contains abundant metamorphic porphyroblasts of brown to grey staurolite, small red garnets and locally prominent grey kyanite laths up to several centimetres long. Beds of blue-grey, crystalline metalimestone, ranging from a few centimetres to several metres thick, are common within the formation, and a thick central unit is termed the Limehillock Limestone Member. It is a pale to dark grey, typically coarsely crystalline metalimestone that varies from massive to thinly banded with abundant chert and pelitic interbeds. This metalimestone unit correlates with Blair Atholl Dark Limestone formations, such as the Inchrory Limestone that crops out farther south. It forms one of the most persistent lithostratigraphical markers in the Dalradian succession.

The metalimestone is commonly exposed in small and medium sized quarries, from where it has been obtained mainly for local use in liming the agricultural land. Many of these quarries are now disused or infilled but are recorded near to the following farms; Mains of Edingight [NJ 5144 5622], Fortry [NJ 5064 5355], Muiryfold [NJ 4904 5193], Mains of Auchoynanie [NJ 4575 5005], Goukstone [NJ 4915 5616] and Nethertown of Windyhills [NJ 5017 5610]. Note that some of the quarry locations cited here are some distance from the farm itself. At the type locality adjacent to the large, now flooded, Limehillock Quarry [NJ 5155 5190], the unit consists of grey, flaggy to blocky and in parts massive, thin- to medium-bedded, crystalline metalimestone intercalated with thin beds of graphitic pelite and calcareous semipelite. Impure pelitic and psammitic beds and lenticular calc-silicate bands are also abundant. The sequence dips at about 50° to the south-east and totals some 120 m in thickness, but it exhibits tight folding and a notable down-dip extension lineation, testifying to its strongly deformed state. The outcrop is bounded on its south-east side by the Portsoy Lineament/Shear Zone.

The overall thickness of the Fordyce Limestone Formation ranges between about 300 and 400 metres in this district, but the formation thickens northwards. Thicknesses measured from cross-sections are between 330 and 370 m, but such estimates neglect the effects of internally folding and the strong deformation.

The upper contact of the formation with the overlying Durn Hill Quartzite Formation (Islay Subgroup) is not exposed in this district but float and sub-outcrop on the south-east slope of Lurg Hill roughly define its location here. Sparse outcrops on the north-west slope of Sillyearn Hill locally constrain its position and show that the contact must be relatively sharp, marked by a change from graphitic pelite and semipelite to either micaceous psammite or to quartzite with minor interbedded semipelite. The contact on the south-east slope of Sillyearn Hill is indicated by the marked change in slope and the numerous springs.

Blair Atholl Subgroup, undivided

In the valley of the River Deveron, between Aswanley and Cairnborrow, a dominantly semipelitic succession intervenes between the Corryhabbie Quartzite and the quartzite at the base of the Argyll Group (Kymah Quartzite). This succession is truncated to the east by the western elements of the Portsoy Shear Zone. As there is no evidence for tectonic excision of any units within this succession, it has been assigned to the Ailnack Phyllite and Limestone Formation and the Blair Atholl Subgroup (undivided); the boundary between the two units on the map is arbitrary. This succession is intruded by many concordant sheets of metadolerite and metabasalt (see Intrusive igneous rocks section), which account for most of the exposures within its area of outcrop. Exposures of metasedimentary rock mostly consist of schistose micaceous semipelite, banded in places, with some interbedded psammite. At Blairmore School [NJ 4366 3982] a grey impure metalimestone shows signs of baking adjacent to a metadolerite sheet. More extensive exposures of metalimestone and metadolerite, which show complex contact relationships, occur just to the west of the present district, in the River Deveron section around the Haugh of Glass [NJ 424 393].

Appin Group unassigned

Within the Huntly district there are areas of Appin Group rocks that form distinct units, even formations, but then cannot readily be assigned to a particular subgroup. These lithological units mostly have structural boundaries, but their generally poor exposure commonly prevents the determination of their relationships to the known succession. Where the exposure is very poor the rocks can merely be allocated to the Appin Group and comments made as to the possible underlying lithology and its probable affinities. These units are shown on the 1:50 000 map as Appin Group — Unassigned. In this area and in the Glenfiddich district to the west they include the Keith Limestone Formation.

Keith Limestone Formation

The Keith Shear Zone is a zone of deformation up to 1 km wide, marked by a strong foliation and a prominent lineation, locally with associated asymmetrical shear indicators. It dips south-east at between 25° and 40° and cuts across the north-west corner of the Huntly district between the outskirts of Keith [NJ 44 51] and Clochmacreich [NJ 49 58]. The zone is characterised by the presence of several parallel lenticular sheets of foliated metagranite whose outcrops are coincident with the main thrusts. The metasedimentary background lithology is dominantly semipelite, with one thick and several thin metalimestone beds, best exposed in the vicinity of Keith itself in the Glenfiddich district (Sheet 85E). These metasedimentary rocks within the shear zone, collectively termed the Keith Limestone Formation, are lithologically similar to other Appin Group units, but their exact stratigraphical position is uncertain.

The semipelites are typically bluish to purplish grey to dark grey, medium- to coarse-grained and schistose, with both a strong foliation and a penetrative, down-dip stretching lineation. Garnet, muscovite and biotite are obvious in most samples. Close to the sheets of metagranite, porphyroblasts of pink feldspar are developed and the semipelite becomes gneissose, with quartzo-feldspathic augen, similar to those in the metagranite. The boundary between metasedimentary rock and metagranite appears gradational and is difficult to determine precisely in places. Thin beds of flaggy, micaceous, quartzose psammite are present locally.

The Cuthill Limestone Member is a massive, colour-banded, mid grey crystalline metalimestone that has a proven thickness of up to 28 m in Keith itself. It probably extends east-north-east into the present district along the crest of the broad ridge at Drum, but there are no exposures. Thinner metalimestones, also massive and colour-banded, occur structurally beneath the Cuthill Limestone. One of these, a banded impure metalimestone with lenses of dark pelite, was formerly quarried in the present district at [NJ 4438 5135], but the small quarry is now completely infilled.

The Cuthill Limestone is similar in lithology and thickness to the Dufftown Limestone of the Ballachulish Subgroup in its type area, and in the adjacent Glenfiddich district (Sheet 85E) it crops out in a structural position that could be interpreted as supporting such a correlation. However, analyses of the Cuthill Limestone show geochemical characteristics that are unlike metalimestones of the Ballachulish Subgroup, but more typical of those in the Blair Atholl Subgroup (Thomas, 1989)[3]. To emplace Blair Atholl Subgroup rocks in the Keith Shear Zone would involve considerable tectonic transport and complex structural geometry. The structurally adjacent succession, immediately above the Keith Shear Zone, is the Drummuir Calcareous Member of the Pitlurg Formation at the top of the Lochaber Subgroup. This member does comprise a general lithological association of semipelites with thin metalimestones, similar to that of the Keith Limestone Formation. However, it contains no thick developments of metalimestone and is characterised by generally impure metacarbonate rocks that would be expected to show geochemical affinities with Ballachulish rather than Blair Atholl metalimestones. The conflicting evidence and in particular the geochemistry of the metalimestones, which has proved to be a good stratigraphical ‘indicator’ elsewhere in the north-east Grampian Highlands (Thomas, 1989[3], 1999[4]), precludes any detailed correlation of the Keith Limestone Formation with other established Appin Group formations.

The River Isla, between Bridge of Montgrew and Grange, flows through a broad alluvial plain in which there are no rock exposures. Straight banks on either side of the alluvium suggest fault control (the Isla fault system). The ground between the faults is interpreted to be underlain by easily eroded metasedimentary rocks, which, by comparison with surrounding known lithologies, are probably mostly semipelites. Similarly, mainly unexposed ground within the Keith Shear Zone, forms a 200 m to 600 m wide zone north of the River Isla, stretching north-north-east from Mains of Glengarrack [NJ 455 526], across Foggy Moss and into the Portsoy district to the north. This area is also probably underlain by schistose semipelites. Sparse notes by the original surveyors recorded folded and weathered schists, commonly muscovite-rich. A significant metalimestone bed was recorded by the original surveyors adjacent to the B9018 road at [NJ 492580] at the northern margin of the Huntly district. Their structural setting requires that these areas are part of the Appin Group, and they may belong to the upper parts of the Ballachulish Subgroup and/or the lower part of the Blair Atholl Subgroup. However, they cannot be assigned to a particular subgroup or individual formation with any confidence. In part they may also be equivalent to the Keith Limestone Formation.

Argyll Group

Islay Subgroup

Durn Hill Quartzite Formation

This basal formation of the Islay Subgroup underlies Knock Hill, the ridge of Sillyearn Hill, and the south-east flank of Lurg Hill. Exposure is poor, although the nature of the unit is seen in the relatively abundant float blocks. The basal contact of the unit with the Fordyce Limestone Formation is commonly marked by springs, particularly along the Sillyearn and Knock hills, reflecting the fracture-dominated nature of the quartzite and its local role as a permeable aquifer. The formation consists mainly of white to cream, pink-purple and fawn, medium- to coarse-grained, indurated quartzite and feldspathic quartzite. Tabular planar cross-bedding with high foreset-bedding angles is present in boulders on Sillyearn Hill, and is common in the quartzite north and south of the Huntly district. The lowermost units of the formation are exposed in a small quarry by Cairdshill Farm at [NJ 5063 5137], where leached muscovitic semipelite beds are interbedded with grey to purple-tinged, white to fawn, blocky, lineated, fine- to coarse-grained quartzites. More biotite-rich semipelites are seen in a similar situation in minor tributaries of the Lime Burn on the north-west side of Sillyearn Hill. On Lurg Hill there is evidence for grey to green-grey micaceous and highly micaceous psammite at or near the base of the formation, parts of which are unbedded with rare scattered clasts (mostly quartz). This lithology resembles that of the ‘Boulder Bed’, a metadiamictite, which occurs as a lenticular unit either at the base of the quartz- ite or within its lower part. Adjacent to Langley Farm is a large angular boulder of vesicular and possibly pillowed, partly carbonated, amphibolitic metabasic rock. It contains some irregular more mafic patches, a crude banding and some included quartz pebbles. Similar pillowed mafic lavas are associated with the quartzite and the underlying ‘Boulder Bed’ in the Glenlivet district (Sheet 75E) in the Kymah and Muckle Fergie Burn sections respectively.

The Durn Hill Quartzite reaches a thickness of about 55 m on Knock Hill, although its upper contact is not exposed. However, possible structural duplication by thrusting and/or folding may occur here as parts of the Portsoy Shear Zone pass through the central part of the hill. The upper contact of the quartzite with the overlying Castle Point Pelite Formation is either a fault or a slide in the Huntly district. Further north on the Banffshire coast in the Portsoy district (Sheet 96W), this boundary is marked by a rapid transition.

Easdale Subgroup Castle Point Pelite Formation

This formation is composed of dark grey, schistose, biotite-rich, locally graphitic pelite and semipelite. It underlies the almost completely drift covered, low relief area of Glen Barry, where it is intruded by elements of the Knock Pluton and the Portsoy mafic and ultramafic intrusions. It generally lies to the east of the Portsoy Shear Zone and hence its lower contact is likely sheared or faulted. Near the northern margin of the district, north-west of Barnyards of Badenyouchers [NJ 540 571], it also crops out west of the trace of the shear zone, but again its lower contact with the Durn Hill Quartzite is faulted. The bulk of evidence for the nature of the formation comes from shallow boreholes and pits excavated by AMAX in connection with the wider investigations into copper and nickel mineralisation by Exploration Ventures Ltd. in the 1970s (see Metamorphism section). A few of the deeper boreholes drilled by Aberdeen University to investigate the nature of the North-east Grampian Basic Subsuite (‘Younger Basic’) intrusions (Munro, 1970[5]; Munro and Gallagher, 1984[6]) also penetrated the Castle Point Pelite formation. One of these near Drums of Muirake Farm at [NJ 5544 5721] recorded 6 metres of sillimanite-bearing gneiss beneath 2 metres of sandy drift deposits. The semipelite is locally calcareous to the north with thin calc-silicate rock lenses and impure metalimestone interbeds.

The transition upwards from the Castle Point Pelite Formation into the Cowhythe Psammite Formation is not exposed but is obscured by numerous metabasic pods and sheets and probably by dislocations. On the Banffshire coast section in the Portsoy district numerous metacarbonate, semipelite, quartzite and graphitic pelite units constitute the Portsoy Limestone Formation. This unit has been intruded by numerous metabasic and ultramafic intrusions and is strongly deformed and folded. Such rocks appear to be largely missing in the Huntly district. Concealed serpentinised ultramafic pods around Ordiquhill and Gordonstown may signify the presence of a major lineament that marks the eastern edge of the regional Portsoy Shear Zone. This zone also shows evidence of middle to upper amphibolite grade metamorphism, related to the intrusion of the North-east Grampian Basic Subsuite.

Easdale Subgroup unassigned

Partly calcareous semipelitic and subsidiary pelitic lithologies, metalimestones and calc-silicate rock lenses are recorded from two BGS boreholes sited adjacent to the Portsoy Lineament near Drumnagorrach at [NJ 5241 5242] and [NJ 5198 5205]. Similar lithologies are also recorded from other boreholes at the southern and western edges of the Knock intrusion. In parts the rocks appear to be fragmentary on a millimetre to centimetre scale, possibly reflecting an initial sedimentary brecciated limestone-siltstone/mudstone protolith. The metasedimentary rocks are strongly deformed, hornfelsed, and have been metamorphosed mainly at middle amphibolite grade. They are typically interleaved with variably foliated metagabbros, metanorites, serpentinised ultramafic rocks and quartz-feldspar pegmatites. They are attributed to the Easdale Subgroup but their relationship to the Castle Point Pelite Formation is unclear.

Crinan Subgroup

Cowhythe Psammite Formation

This unit, formerly termed the Cowhythe Gneiss (Read, 1923)[7], crops out to the east of the Castle Point Pelite Formation. It is also very poorly exposed in the Huntly district. It underlies a broad swathe that stretches south-south-west from Gordonstown [NJ 564 566] and Finnygaud [NJ 604 545] in the north, to Milltown of Rothiemay [NJ 547 484] and the Huntly Pluton in the south. The formation consists of mid to dark grey, commonly gneissose micaceous psammite, biotite-rich semipelite and pelite with abundant quartz-feldspar segregations. Calc-silicate rock is not common in the sparse exposures but has been intersected in boreholes close to the Knock and Huntly basic-ultrabasic plutons and in the xenolithic gabbroic bodies that punctuate the Cowhythe Psammite outcrop. Quartzose psammite with thin semipelitic partings is recorded both in old quarries [NJ 5882 5637] and in float on Meikle and Little Brown Hill. Coarsely gneissose segregated semipelite was recorded from a now-infilled quarry near Starmires Farm at [NJ 5864 5594].

Crinan/Tayvallich Subgroup

Strichen and Aberdeen Formations

Rocks assigned to the Crinan and/or Tayvallich Subgroup, formerly termed the Old Meldrum Gneisses (Read, 1923)[7], crop out at the eastern end of the Insch Pluton, in the south-east corner of the Turriff district. Again, the succession is generally poorly exposed. Its extent on the north-eastern margins of the Insch Pluton has been clarified by geophysical and pitting work by Aberdeen University workers (e.g. Leslie, 1981)[8]. Small troctolite and gabbroic intrusions lie within the metasedimentary succession and may indicate that mafic rocks occur more widely at shallow depths here.

In the stream section just north of Mill of Easterton [NJ 7775 3085] parts of the Strichen Formation are moderately well exposed. Lithologies range from coarsely schistose semipelite and pelite to migmatitic semipelite with patchy segregations and a gneissose foliation defined by the quartz-feldspar leucosome. Some rocks contain what appear to be ‘xenoliths’ of pelitic material with ghosted margins. The rocks are strongly deformed and sheared in places. In thin section sillimanite, cordierite and andalusite are present and the widespread occurrence of foxy, red-brown biotite suggests that most of the rocks are hornfelsed. Read (1923)[7] reports the occurrence of a garnet-cordierite-biotite-green spinel-quartz-plagioclase rock with scattered magnetite around [NJ 810 293]; this is probably a contaminated igneous rock or a partial melt restite.

Argyll Group unassigned

Lithologies typical of Argyll Group successions elsewhere in the Dalradian occur immediately to the east of the Portsoy Shear Zone and are dominated by locally graphitic pelites and semipelites with thin psammitic beds. Thick turbiditic units of gritty psammite and massive gritty quartzite occur as local lenses, and metalimestones and calc-silicate rocks are generally thin and impersistent. Although formations have been erected and members recognised, they cannot be traced laterally for any distance and do not directly correlate with Argyll Group successions elsewhere in the Grampian Highlands (Fettes et al., 1991)[9]. Hence, it is inappropriate to assign them to subgroups within the Argyll Group.

Beldorney Pelite Formation

The type area for this formation lies in the immediately adjacent part of the Glenfiddich district (Sheet 85E), where good exposures occur in the River Deveron around Beldorney Castle [NJ 421 369]. The dominant lithology is black, phyllitic or slaty, commonly finely laminated pelite that passes into schistose semipelite in parts. Thin psammite beds and quartzose psammite lenses are common. In the Huntly district the formation extends along the western parts of the Portsoy Shear Zone, from Westerpark [NJ 437 382] north-eastwards to the area around Ruthven [NJ 506 470]. The best exposures of the typical pelitic and semipelitic rocks occur in the River Deveron below Bogforth [NJ 472 404], and in the Burn of Cairnie around [NJ 480 448], between Newton and Cairnie. In these exposures coarse-grained schistose muscovite-biotite-garnet semipelites are common, some with conspicuous tourmaline and possible kyanite. At a remarkable exposure in the River Deveron at [NJ 4720 4041] thin (about 5 cm) tightly folded beds of hard, massive, unfoliated quartz-plagioclase-garnet rock occur within schistose semipelite. The beds contain up to 50 per cent, clear, pinkish purple euhedral garnets up to 5 mm in diameter. They may represent a basic metavolcanic or possibly metasomatic protolith.

The Beldorney Pelite Formation is also exposed in the Burn of Aswanley, upstream of [NJ 4467 3920]. Here the north-west margin of the formation, adjacent to a metagabbro at the western limit of the Portsoy Shear Zone, is marked by a grey, gritty feldspathic psammite that can be traced north-eastwards along a ridge, almost to the River Deveron. The psammite contains some pelitic interbeds and quartzose psammite lenses and locally has a strong foliation. Farther to the north a strongly deformed micaceous psammite with some gritty feldspathic beds occupies a similar position, adjacent to the western limit of the shear zone. Good exposures of this psammite, with interesting deformation structures associated with the shear zone (see Concealed geology section), are seen on the south bank of the Burn of Cairnie, below Newton [NJ 478 449].

Blackwater Formation

The Blackwater Formation crops out on the south-eastern side of the Succoth–Brown Hill intrusion and hence is outwith the main zone of deformation in the Portsoy Shear Zone. The lower part of the formation occurs in adjacent areas of the Glenfiddich district. It comprises a mixed sequence of gritty psammites, schistose pelites and abundant metavolcanic rocks; compositonal changes in the latter are used to define three members (Gunn et al., 1990[10]; Fettes et al., 1991[9]). Only the upper part of the formation, comprising two members, occurs in the Huntly district, the lower members having been cut out by the shear zones and the Succoth Fault that bound the Succoth–Brown Hill intrusion.

The Corinacy Pelite Member crops out from the south-eastern slopes of the Deveron valley, on the western edge of the Huntly district at [NJ 436 355], to the valley of the Collonach Burn, near Wellheads [NJ 486 396]. The member is dominantly pelitic, ranging from schistose to phyllitic and slaty. The pelites are graphitic in part and in the Dry Burn [NJ 436 358] they contain a dark grey, ovoid, phosphatic nodule, about 4 cm x 3 cm x 2 cm, with impurities of amorphous iron and manganese. Andalusite is common, either as dark grey blobs or as large, euhedral crystals of white-weathering chiastolite up to 2 cm long. Some rocks also contain garnet and staurolite. In the adjacent Glenfiddich district sheets of metabasalt are abundant in the lower part of the member, but are less abundant higher up. Thin lenticular beds of gritty psammite are present throughout the member, but become thicker and more persistent towards the top, where they locally form distinct low ridge features [NJ 485 380]. In addition to the lithologies already described, the boreholes by Wellheads [NJ 486 395] (see Applied geology section) intersected graphitic quartzite, chert, intraformational breccias and a 5 m-thick metalimestone.

The Grumack Hill Quartzite Member occurs only in the south-western part of the Blackwater Formation outcrop and is cut out to the east by the east-north-east-trending Long Slough Fault. It forms persistent positive features on the north-west flank of the watershed ridge, south-east of the Deveron valley. The buff-grey to off-white massive quartzites are purer than the psammites in the underlying Corinacy Pelite Member, although some gritty beds occur locally. Mixed quartzitic and pelitic debris on top of the features suggests that the member may include significant interbeds of phyllitic pelite.

Formation unassigned

Typical Argyll Group semipelites with minor metalimestone extend to the north and north-east, through poorly exposed ground between Wellheads [NJ 486 395] and Littlemill [NJ 518 474]. This approximately 1 km-wide outcrop lies between sheared metagabbro bodies that may be a drawn-out extension of the Succoth–Brown Hill intrusion, and the western margin of the West Huntly cumulate body. Although the semipelites may possibly be a continuation of the Corinacy Pelite Member (Blackwater Formation) to the south, or the Beldorney Pelite Formation farther west, they lie within the Portsoy Shear Zone. Hence, they are separated from both pelite units by major dislocations, making any such correlations suspect.

Good sections are seen in the road cutting on the north side of the Deveron valley, east of Milton of Cairnborrow between [NJ 478 407] and [NJ 483 406], and in the Burn of Cairnie between [NJ 487 446] and [NJ 495 449]. Here, the rocks are schistose semipelites, dominated by muscovite, biotite and garnet, with some thin beds of gritty psammite. Samples from the eastern end of the Cairnborrow section are spotted and one contains fibrolite mats replacing biotite and white mica, possibly after cordierite. In the part of the Burn of Cairnie adjacent to the Huntly intrusion, the rocks are gneissose and heterogeneous, suggesting partial melting and/or intermixing with basic magma. Blue-grey, fine-grained metalimestone is well exposed in a line of old pits on the south side of the Hill of Cuternach [NJ 479 418].

Southern Highland Group

Rocks assigned to the Southern Highland Group occupy more than half the area of the Huntly and Turriff districts (Sheet 86W/E), forming the major part of the wide outcrop of these rocks which extends northwards into the Banff district (Sheet 96E) and up to the coast. Read (1923)[7] estimated that they covered an area of ‘over 200 square miles’ (about 520 km2) in north-east Scotland. Southern Highland Group rocks are bounded to the west mainly by the Cowhythe Psammite Formation (Argyll Group), but also by the Huntly Pluton around Huntly itself. Near the southern margin of the Huntly and Turriff districts they are transected by the Insch Pluton, and to the east they underlie the Old Red Sandstone of the Turriff basin.

Southern Highland Group rocks consist almost entirely of metamorphosed wacke sandstones, arenites and mud rocks, although very coarse arenites and microconglomerates occur locally in several large lenses. Though rare, thin units of calc-silicate rock and one or two thin metalimestones occur in the western part of the crop.

The Group is divided up into three formations as follows (youngest to oldest):

Macduff Formation
Whitehills Grit Formation/Clashindarroch Formation

Although they differ lithologically, the older two formations are considered, at least in part, to be stratigraphically equivalent, and to interdigitate in the ground south-west of Huntly. Rocks of the carbonate-bearing Whitehills Grit Formation (formerly the ‘Whitehills Group’ of Read, 1923[7]) crop out to the west of the Macduff Formation, north of Huntly, where they overlie the Cowhythe Psammite Formation.

The base of the Clashindarroch Formation is marked in places by the Garnel Burn Pelite Member. This schistose, highly magnetic pelite unit is laterally equivalent to other magnetic pelitic and semipelitic lithologies mapped at this stratigraphical level in other districts, e.g. the Longshank Gneiss Formation in the Ballater district (Sheet 65E) (British Geological Survey, 1995[11]).

The Macduff Formation forms much the greater part of the Southern Highland Group outcrop on Sheets 86E and 86W. Although the formation is a broadly uniform sequence of metaturbidites, in its lower part a major pelite unit, the Hill of Foudland Pelite Member, forms the prominent, east–west trending line of hills (the ‘Slate Hills’) immediately north of the Kennethmont and Insch Intrusions. Also in several places within the formation generally lenticular but moderately thick units of conglomeratic to microconglomeratic arenites can be mapped.

The Southern Highland Group rocks in much of the Turriff district are disposed in the broad open Turriff Syncline (Read, 1955)[12]. The wide complex axial zone of this large-scale structure trends roughly 008°, with its trace lying close to the western edge of the Turriff outlier. However, south of grid line 40, bedding and S1 cleavage swing to strike north-east and then east-north-east to become near parallel the northern margin of the Insch Pluton. Here, the trace of the syncline is unclear and it may well decline in amplitude as it is traced to the west-south-west. The Boyndie Syncline, defined by Sutton and Watson (1956)[13], is effectively a large-scale, east-facing monoform defined by the Knock Head Grit Member on the Banffshire coast section. In the Turriff district its southerly continuation is not well defined and no kilometre-scale monoform structure can be readily identified. The steep limb of the monoform may be represented by the numerous steep and subvertical dips seen in gritty arenites and microconglomerates close to the western boundary of the Macduff Formation and farther south-west in the Whitehills Grit Formation. However, local reversals of dip and changes in younging direction imply that medium-scale F1 folds are present in the succession as seen on the coast section around Banff and Macduff. These generally upright structures commonly have steeply dipping limbs.

The Turriff Syncline is reflected by the metamorphic zonation (Read, 1955)[12]. Chlorite and biotite-grade greenschist facies metasedimentary rocks lying in the core of this structure pass both to the east and west into schistose cordierite- and andalusite-bearing lower amphibolite facies pelites and semipelites within the Macduff and Clashindarroch formations. These schistose, dominantly pelitic and semipelitic rocks were formerly referred to as the ‘Fyvie Schists’ and ‘Boyndie Bay Group’, respectively, and were defined by Read (1923)[7] as separate units from the ‘Macduff Slate’. However, they are lithologically similar and now recognised as a partly metamorphic transition. In the south-east part of the Turriff District, andalusite- and cordierite-bearing rocks of the Macduff Formation overlie gneissose and migmatitic pelites, semipelites and psammites assigned to the Strichen Formation. However, gneissose rocks are absent to the south-west of Huntly where the Clashindarroch Formation overlies metasedimentary rocks assigned to the Argyll Group.

Whitehills Grit Formation

This formation includes andalusite- and tourmaline-bearing semipelite and pelite, gritty quartzites and psammites, and rare metalimestones and calc-silicate rocks. The andalusite-bearing pelitic rocks are highly schistose, with grey commonly elongate porphyroblasts some several millimetres across. The formation crops out in a narrow zone between 0.8 and 4 km wide, stretching from south-west of Huntly, around Brawlandknowes [NJ 507 369], north-north-east to Aberchirder [NJ 625 525], and thence northwards into the Portsoy and Banff districts (Sheet 96W and E) to its type section on the coast immediately west of Whitehills. Exposure inland is generally poor and restricted to isolated outcrops, as noted by Read (1923)[7].

In the area between Marnoch [NJ 597 502] and Drumblade [NJ 585 400] in the Huntly district, a thick lens composed mainly of quartzite occurs within the Whitehills Grit Formation. Its lenticular form is interpreted here to represent original facies variations and may possibly be a large sand channel form. Read (1923, p.53)[7] records that the quartzite ranges from fine-grained to gritty and even pebbly. Near its southern extent it is exposed in a small quarry in Cruchie Wood [NJ 5827 4313] where white, fine- and medium-grained quartzites and psammites with thin schistose pelitic partings locally show cross-bedding and ripple marks. Metalimestones and calc-silicate rocks, though rare, are characteristic features of the Whitehills Grit Formation that help in distinguishing the unit from the Clashindarroch Formation. Read (1923)[7] recorded ‘broken whitish limestones’ near Auchingoul [NJ 610 486]. Thin beds of metalimestone are also exposed in the River Deveron near Marnoch Lodge (NJ 604 496], and in the River Bogie and the railway cutting at around [NJ 525 370] just east of Bucharn [NJ 521 369].

Clashindarroch Formation

The Clashindarroch Formation is almost wholly restricted to the Clashindarroch Forest area in the south-western part of the Huntly district and south-eastern part of the Glenfiddich district (Sheet 85E). The formation is lithologically similar to much of the overlying Macduff Formation, but typically it is more semipelitic. However, its base is marked by the Garnel Burn Pelite Member, a distinctive schistose, biotite-rich, andalusite-bearing pelite that contains several per cent of fine-grained magnetite and hence is highly magnetic. As a result this member can be traced readily from the Daugh of Corinacy [NJ 402 311] north-east over Grumack Hill in the Glenfiddich district and into the Huntly district on the ridge north-west of Red Hill [NJ 437 340]. Its outcrop only appears to extend north-east in the Clashindarroch Forest as far as [NJ 450 356] where it is truncated by the Succoth Fault. Farther along strike to the east-north-east the base of the Clashindarroch Formation is marked by coarse, gritty and locally conglomeratic psammite units that form the crests of Muckle Long Hill [NJ 460365] and Hill of Bogairdy [NJ 485 366]. These psammite units die out laterally and the main semipelitic lithologies of the Clashindarroch Formation lie directly on rocks assigned to the Corinacy Pelite Member (Blackwater Formation, Argyll Group) on the west side of Clashmach Hill [NJ 498 385]. In this area, the Clashindarroch Formation appears to underlie the Whitehills Grit Formation, although at least in part the contact is faulted. Relationships farther to the north-east are obscured by the Huntly Pluton. However, it is possible that the elongate mass of semipelite shown at the base of the Whitehills Grit Formation in the Fourman Hill area, and described above, is an outlier of the Clashindarroch Formation.

Conglomeratic psammites occur in several places, most notably north-west of Whitestones [NJ 480 355] and on the south-east flanks of Craigend Hill at about [NJ 458 342]. In the extreme south-west corner of the Huntly district, parts of the Clashindarroch Formation lie within the contact metamorphic aureole of the Boganclogh sector of the Insch Pluton. They have been metamorphosed into very hard, compact, fine-grained cordierite-and-andalusite-bearing pelitic and semipelitic hornfelses.

Macduff Formation

Read (1923)[7] termed this unit the Macduff Group, but more recently it has generally been called the ‘Macduff Slate Formation’ (e.g. Harris et al., 1994)[14], with the emphasis on its pelitic lithologies. Although slaty pelite and semipelite are locally abundant, there is a range of terrigenous, clastic lithologies in the formation and the term ‘slate formation’ is thus somewhat of a misnomer.

The Macduff Formation is very well exposed along the Banffshire coast section but exposure inland is poor and individual exposures are generally isolated. It is considered to be a turbiditic sequence, exhibiting classic turbidite fan facies (Kneller, 1987[15]; Trewin, 1987[16]). It includes dropstones and debris flow diamictites, which are well exposed at Macduff and have been interpreted as glacigenic in origin (Sutton and Watson, 1954[17]; Hambrey and Waddams, 1981[18]; Stoker et al., 1999[19]). Such lithologies and features have not been recognised within the Huntly and Turriff districts. The formation is considered to have been deposited in deep water in a continental slope setting. Material was derived from an adjacent marine continental margin and shelf which may have undergone periodic active glaciation.

The dominant lithologies are fine- to coarse-grained siltstones and wacke sandstones, with sub-ordinate pelites, arkosic gritty sandstones and rare quartzites. Pebbly zones, microconglomerates and very coarse grained gritty sandstones are locally important.

Coarse-grained gritty feldspathic arenites containing clasts of semipelite up to several tens of centimetres across are seen in Strathbogie at [NJ 5128 3378], to the south on Birch Hill [NJ 51 33] and immediately east of the River Deveron in Logg Wood at [NJ 6437 4736], near Inverkeithny.

Very coarse-grained arenites and microconglomerates were recognised as distinct units by Read and shown on the original (1923) 1: 63 360 scale geological map of Sheet 86. These lithologies consist of interbedded coarsely gritty arenite and pebbly conglomerate with a coarse-grained sand matrix. The pebbles consist mostly of vein-quartz, although Read (1923)[7] also reports the presence of feldspar, quartzite and granitic lithologies. The pebbles are well-rounded and are up to 3–4 cms in diameter. One of these coarse-grained units extends west-south-west from near Culdrain House [NJ 515 338] for some 4 km. At its eastern extent its outcrop possibly terminates against the fault that forms the western side of the Old Red Sandstone Strathbogie Inlier. The best exposures occur in Kirkney Water and on the steep north-facing bank opposite the northern end of Birch Hill [NJ 5096 3293]. To the west, the conglomerate unit lenses out on northern side of Slouch Hill [NJ 4815 3279]. A further major occurrence of gritty arenite and conglomerate outcrops just to the south-east of Bainshole [NJ 611 351]. It is well exposed in a small quarry 400 m east of Bainshole. This unit can be traced intermittently east-north-east across the Glens of Foudland over some seven kilometres towards the Glenmellan–Logie Aulton area to about [NJ 658 381]. Here, it is a massive, coarse-grained gritty arenite and pebble conglomerate, interbedded with strongly cleaved semipelite. As the unit is poorly bedded and generally fractured and weathered, little internal detail can be discerned. The unit may continue along strike to the west-south-west to link with outcrops in the Burn of Stodfold at [NJ 5871 3375]. Blocks of conglomerate are numerous hereabouts. Many are fractured and contain slickensided surfaces, indicative of local faulting. The pebbles, composed predominantly of vein-quartz, are rounded to well-rounded and up to 5 cm in diameter. To the east of Burn of Stodfold, the conglomerate can be traced over the hillside across the fields for about 1 kilometre.

The most extensive continuous development of pelite and semipelite within the Macduff Formation occurs in the ‘Slate Hills’ near the southern margin of the sheet, just north of the Insch Pluton and Kennethmont Intrusion. These distinctive slaty pelitic lithologies, termed the Hill of Foudland Pelite Member, lie mainly within the contact aureole of the Insch Pluton and show varying degrees of contact metamorphism (see Metamorphism section). Their cleavage generally dips steeply southwards, and the due to their massive and partly hornfelsed nature they form generally positive relief features.

Exposure of the pelitic rocks is enhanced considerably by the extensive abandoned slate quarries throughout the ‘Slate Hills’, most particularly on Hill of Foudland [NJ 603 332] itself. The slates were exploited from a narrow zone either just within or just outwith the onset of hornfels textures. Here the pelitic rocks were recrystallised, making them harder and more durable, and yet they were still fissile enough to split along the S1 slaty cleavage to form slates. However, clean bedrock faces are relatively rare in the workings. Much of the abundant loose material is inferior quality slate material, back-fill, and ruins of dwellings, all linked to the uncontrolled nature of slate exploitation in numerous small quarries in the 19th century. The rocks vary from generally poorly to well-cleaved pelites and semipelites with thin siltstone and fine-grained sandstone laminae. They are generally massive in the southern part of the member, where they are more strongly hornfelsed. Way-up is generally difficult to determine and the slates do show complex medium-scale F1 folds, at least locally. Farther northwards the member becomes less pelitic and small-scale sedimentary structures are present in more sandy beds, some of which do indicate a more consistent way-up. The pelitic rocks pass northwards into coarse-grained arenites and wackes with subsidiary pelitic interbeds.

Small, dark green, lath-shaped chloritoid porphyroblasts are abundant in places in the pelites. Greenish-grey, highly cleaved pelites containing chloritoid are found both on the west side of Strathbogie on the summit ridge of the Hill of Kirkney [NJ 505 316], and on the east side on the Hill of Corskie [NJ 547 327]. Chloritoid is also recorded from pelitic rocks on the hills of Foudland and Tillymorgan (Leslie, 1988) and in small exposures at the eastern end of Wishach Hill around [NJ 577 332].

South across Hill of Kirkney towards Glen of Noth, spots and small porphyroblasts of cordierite and andalusite begin to appear in the pelites due to contact metamorphic effects of the Insch Pluton (Boganclogh Sector). At the south-western end of the Hill of Kirkney, the pelitic rocks are thoroughly recrystallised into very hard, weakly to moderately foliated, massive, cordierite-andalusite hornfelses. The foliation is best developed on the outer margins of the hornfelsed zones.

Calc-silicate rocks: Calc-silicate-bearing rocks have been found several localities in the south-west corner of the Huntly district. They occur as thin laminae generally less than 5 cm thick and are interbedded with medium-bedded, dark grey, fine-grained micaceous psammites and semi-pelites. Though conspicuous where present, they are not abundant in the Macduff Formation. The calc-silicate laminae are creamy-white in colour, flecked with green amphibole and pink grossular garnet. In thin section, the general assemblage is plagioclase + quartz + garnet + hornblende ± zoisite ± sphene ± biotite ± white mica. Calcite or other carbonate minerals are absent. The assemblage accords with that recorded by Hudson (1985) for calc-silicate rocks from the coast section west of Banff. Such assemblages are only stable in the presence of very water-rich fluids. This is consistent with their presence in micaceous psammites and semipelites, for which equilibrium fluids have a H2O (water activity) » 1.

The Fyvie Gorge and Foredoun Burn sections

Within the Huntly and Turriff districts, the best-exposed sections through the Macduff Formation occur in the River Ythan section in the Fyvie Gorge and in the lower part of the tributary Fodoun Burn that joins the Ythan at Fyvie. Both sections afford good exposures of the Macduff Formation, here disposed in large, upright folds (see also Concealed geology section). The Fyvie Gorge is an overdeepened glacial meltwater drainage channel within which the diminutive River Ythan is now somewhat of a misfit. The Fyvie Gorge section extends over about 6.5 km from Ardlogie [NJ 780 373] in the west of Braes of Gight [NJ 828 387] in the east. Exposure is also reasonably good in the gorge of the Fordoun Burn, particularly at about 1.5 km south-west of Fyvie, around a prominent bend at about [NJ 755 372].

The rocks vary from pelite and semipelites to dark, greenish-grey, coarse-grained, gritty, wacke sandstones with arenites and rarer pinkish arkosic sandstone beds. Units are generally thinly to thickly bedded, except for some massive, coarse-grained units that attain in excess of 2 m thickness and in part probably comprise amalgamated beds. Graded bedding is common in the thinner and finer grained arenites, allowing elucidation of the major fold structures in these areas (See [[OR/15/026 Structure #Fyvie Gorge [NJ 780 369 to NJ 820 392] |Fyvie Gorge]]. Other sedimentary structures present in these rocks include ripples, load casts, loading structures and trough cross-lamination, all of which can help to determine way-up.

The more aluminous pelitic lithologies commonly contain porphyroblasts of cordierite and andalusite, which are typically weathered out, giving the weathered surface a honeycomb texture. They also show a strongly developed schistosity in places, with the fabric (S2) wrapping the porphyroblasts.

References

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