This report presents a preliminary interpretation of the bedrock geology of Ben Alder, based on
data collected during the 2005 field season. This work contributes to the resurvey of 1:50,000
scale Sheet 54E (Loch Rannoch). The principal aim of this report is to present a
lithostratigraphical framework and structural model for testing during the 2006 field season.
The Ben Alder massif exposes polydeformed psammite and semipelite belonging to the
Neoproterozoic Grampian Group, lower Dalradian Supergroup. It comprises essentially four
formations locally termed, the Lethcois Semipelite (lowest), Ben Alder Psammite, Garbh Choire
Semipelite and Gaick Psammite Formation. The lithological and stratigraphical character of
these formations is compared and contrasted with that of the type-stratigraphies for the
Strathtummel and Corrieyairack Basins. The Ben Alder Grampian Group has an unequivocal
affinity with the Strathtummel Basin-fill.
In the Strathtummel Basin (east of the Geal-charn – Ossian Steep Belt), the resurvey of Sheet
63E (Dalwhinnie) utilised the Corrieyairack Basin’s type-stratigraphy when naming new
lithostratigraphical units. Some of these correlations are questioned and new correlations with
the Strathtummel Basin stratigraphy are suggested.
The stratigraphical pile was progressively deformed by up to four phases of deformation (D1-
D4). Early phase (D1) deformation is only represented by fabric development (S1). By contrast,
the D2 main phase deformation folded the stratigraphy into a number of tight, almost isoclinal,
SE-facing F2-fold structures (e.g. the Ben Alder Anticline) with associated penetrative fabric
development (S2). Subsequently, these F2-structures were refolded by the co-axial and co-planar
F3-folds, which have a more open nature. Late weak and upright folding (F4) served to ‘kink’
the stratigraphical units without altering their distribution, creating alternating steep and more
gently dipping zones.
Displacement across major brittle faults (Ericht-Laidon, Inverpattack-Markie Faults) is relatively
limited, enabling confidence in stratigraphical and structural correlations made between faultbound
blocks. These faults acted as conduits for late- and post-tectonic microdiorite,
lamprophyre and pegmatite intrusions.
ABSTRACT Palaeomagnetic data define a polar wander path for the Dalradian tectonic block of the Scottish Highlands but resolve no relative motion between the Dalradian and Midland Valley blocks from early Ordo vician onwards. The history of structural episodes (D 1 to D 4 ) is traced for the Dalradian rocks of the Southern Highlands, and peak metamorphic temperatures are closely associated with D 3 in the 520–490 Ma (early Ordovician) time period, whilst retrograde events are mostly associated with D 4 . Throughout the structural history, the influence of a lineament or lineaments parallel to the Highland Border is seen and is particularly demonstrated by the formation of the major Highland Border downbend (D 4 ) structure. This monoform is interpreted as lying structurally above a major step in the basement and is associated with uplift of the Dalradian flat belt in the period 460–440 Ma (late Ordovician). The mechanism of emplacement of the slivers of Highland Border Complex (HBC) rocks adjacent to the Dalradian Supergroup along the Highland Boundary fault zone is considered given that structural and palaeontological evidence indicates the emplacement to be post- D 3 in the Dalradian rocks. The steep attitude of rocks along the Highland Boundary fault zone is probably due to vertical movements but either thrust or strike-slip movements are also indicated. Thrust emplacement prior to downbend formation (D 4 in the Dalradian rocks) would also precede uplift and have to take place at considerable depth (10–20 km). Thrust emplacement subsequent to downbend formation is difficult to reconcile with the existing structural relations, whilst strike-slip motion offers a straightforward mechanism of placement.
Synopsis The Strathan Conglomerate lies in the Lower Moinian of Sutherland and has been strongly deformed by two events, f 1 and f 2 . Minimum finite strain values X:Y:Z, calculated from deformed pebbles, range from 51:44.5:1 to 32:25:1 respectively. The size, shape and composition of the pebbles is compared to the nearby undeformed Watch Hill Conglomerate of Old Red Sandstone age. A simple method for estimating the minimum finite strain is outlined. The f 2 deformation has resulted in folding of the pre-existing flattened pebbles in some areas. During this folding and accompanying lower amphibolite grade metamorphism (M 2 ) quartz migration to low stress areas has occurred, resulting in a strong f 2 lineation and quartz rodding, both parallel to the X axis of the f 2 strain ellipsoid. On Ben Hutig a similar event has occurred; pre-f 2 planar quartz veins form a series of rods when folded by f 2 folds. The consequences of these results are discussed.
Abstract The British Geological Survey (BGS) petrology collections contain almost 1500 Scottish rock samples (with thin sections) deposited by Archibald Geikie, including BGSS1, an analcime gabbro from Salisbury Crags, Edinburgh. High-quality thin section images are now available from the BGS's Britrocks online database. The geospatial distribution of these samples is analysed. They reflect the development of geological mapping and igneous petrology in Scotland from the 1850s to the 1890s. Geikie had the opportunity to study Nicol's original thin sections in 1851 and he met both Sorby and Zirkel, early pioneers of petrography. Lacking management support, he cut many of his own thin sections while mapping the Clyde Plateau lavas during the 1860s, leading to publications on Carboniferous and Tertiary volcanism. When appointed Director of the newly formed Geological Survey of Scotland in 1867, he was able to establish a petrological laboratory in Edinburgh. Time pressures resulting from his subsequent promotion to Director-General, and increasing quantities of metamorphic rocks, then necessitated the appointment of Hatch and Teall as petrographers for the Survey. Teall's work was particularly important in the detailed petrography of the gneisses and mylonites associated with the Highlands Controversy.
Summary The oldest rocks of the Outer Hebrides are a supracrustal sequence of metasediments, metavolcanics and associated layered basics which formed at c. 2900 Ma. These rocks were buried or carried to deep levels in the crust where they were intruded by an extensive series of granodioritic and tonalitic magmas, probably during the period 2800–2900 Ma. These rocks were subjected to the polyphasal Scourian event which led to extensive migmatization, high-grade metamorphism and the development of a pervasive gneissic foliation. The culmination of this event occurred in the period 2600–2700 Ma. The pattern of gneiss types resulting from these events defined a NNE-SSW grain in the rocks which is reflected in the present distribution of metamorphic facies. This pattern, which reflects either original compositional variations or the effects of Scourian metamorphism, has controlled much of the later metamorphic and structural evolution of the Lewisian of the Outer Hebrides. The late Scourian period was marked by the emplacement of a suite of dioritic intrusions followed by numerous granitic bodies, the two events being separated by a period of deformation. This deformation died out to the extreme SE and did not affect the eastern part of Barra. These intrusive events probably marked the end of crustal formation and the accompanying deformation is the first evidence of the reworking of a crystalline crust with the development of zones or ‘augen’ of low deformation. The granitic intrusion was followed by a deformational event which, in the now more rigid crust, resulted in the development of localized steep shear zones in the period 2400–2600 Ma. The later stages of this event coincided with the intrusion of a widespread suite of metadoleritic and metanoritic rocks—the Younger Basics of ‘Scourie dyke’ age. The overlap of deformational phases from the late Scourian to the early Laxfordian highlights the difficulty of regarding the Scourian-Laxfordian boundary as one of major importance. Also there may well have been a considerable period ( c. 400 Ma) of relative crustal stability before the main Laxfordian events. There is no evidence that the late Scourian shearing events were associated with substantial uplift, although geochronological evidence from S Harris (Cliff et al. 1983) does suggest a gradual uplift and cooling, at least in that area, at about this time. Certainly the early Laxfordian events appear to have been deep-seated. The intrusion of the late Laxfordian granites at c. 1700 Ma and the associated deformation signalled the first movements which led to substantial uplift, thrusting and unroofing of the complex. The geological history draws attention to the interesting evolution of deformational style and the generation of lineaments with the changes in crustal level. Features such as the Langavat belt of S Harris may result from an early Scourian lineament that has been reactivated many times. In general the Scourian gneiss terrains are broad zones with localized sub-vertical shear zones only becoming established in the late Scourian. The Laxfordian is typified by such zones of reworking, accompanying recrystallization commonly under hydrous conditions. Ductile shear zones are found, although on a local scale the deformation pattern is typically heterogeneous. After the late Laxfordian granitic intrusion the deformation became localized in restricted mylonite zones or along brittle thrusts. The Caledonian reactivation and faulting is largely controlled by the position of this major thrust zone, with mylonization and related low-grade alteration overprinting the late Laxfordian thrust features.
This report summarises the results of detailed geological remapping and related studies under-taken by the Geological Survey in the Huntly and Turriff districts of North-east Scotland. It pro-vides an account of the geology to accompany the published 1:50 000 geological maps sheets 86W (Huntly) and 86E (Turriff). The districts occupy a tract of generally rolling country, which ranges from about 50 to 200 m above sea level in its eastern part, to more elevated hilly country, commonly reaching over 300 m and 400 m above OD, in its western and southern parts. The drainage is dominated by the sinuous incised valley of the River Deveron and its main tributar-ies, the rivers Isla and Bogie, except for the southern part of the Turriff district, which is drained by the headwaters of the River Ythan. Agriculture and forestry are the dominant economic ac-tivities, with tourism and whisky distilling also significant.
The Huntly and Turriff districts are underlain mainly by Dalradian metasedimentary rocks of Neoproterozoic age that have been strongly deformed and metamorphosed during the Grampian event of the Caledonian Orogeny. The Dalradian rocks have been intruded by igneous intrusions that range from large plutons to small pods and dykes. Granite sheets and pods were emplaced at about 600 million years (Ma), followed later by small ultramafic and mafic bodies. However, the main intrusive igneous event occurred in the Early to Mid Ordovician at 474 to 470 Ma, coeval with the Grampian event. It resulted in the emplacement of mafic and ultramafic plutons, com-monly zoned; the Insch Pluton includes monzonites and syenites in its upper zone. Dioritic and granitic bodies are associated with the plutons.
The major structural feature is the north-north-east-trending Portsoy Shear Zone, which traverses the Huntly district. This steeply easterly dipping zone is coincident with a major regional linea-ment, which separates stratigraphically discrete packages of Dalradian rocks with different tec-tonometamorphic histories. The shear zone has also facilitated and in part controlled the emplacement of the Huntly and Knock mafic-ultramafic plutons. The Dalradian metasedimen-tary rocks contain both Buchan and Barrovian metamorphic zonal assemblages. Folding and shearing have caused local repetition of lithological units, but overall the Dalradian sequence be-comes younger eastwards until the broad hinge of the regional Turriff Syncline is reached in the central part of that district. Here, the youngest Dalradian rocks are exposed and metamorphic grade is low (biotite grade). Farther east older Dalradian rocks again crop out and the Buchan metamorphic isograds are re-crossed such that the rocks contain andalusite and sillimanite.
Following the Grampian orogenic event the area was uplifted, and during the Early Devonian fluvial and lacustrine Old Red Sandstone rocks were deposited in the northerly trending fault-bounded basins, namely the Turriff and Rhynie basins. In Strath Bogie a basaltic andesite lava unit linked to the Rhynie chert occurs within the Rhynie Basin succession. Subsequently, a Mid-dle Devonian conglomerate-dominated sequence, linked to the Orcadian Lake farther north, was deposited unconformably on the older succession in the Turriff Basin.
In the Turriff district deep Tertiary weathering profiles and local fluvial sands and gravels are preserved, testifying to only limited erosion by the later Quaternary glaciations. The Devensian glacial and postglacial history of the districts was dominated by an ice sheet derived from the highland areas farther to the south-west. The related glacial and postglacial superficial deposits form a pervasive if generally thin cover over much of the bedrock. Till derived from the Moray Firth is present in the north-eastern part of the Turriff district. Eastward migration of glacial meltwater gave rise to channels that in places have significantly influenced development of the Holocene fluvial drainage pattern.
The Huntly district has been the focus of several periods of exploration since 1970 for platinum group elements, copper and nickel linked mainly to the mafic-ultramafic plutons and their meta-morphic aureoles. The limited drilling and more extensive ground magnetic surveys, in combina-tion with geochemical studies and gravity modelling, have provided detailed information as to the nature, distribution and origin of prospective areas. No economic deposits have been deline-ated to date. However, this BGS report and the geological maps provide a sound foundation for existing and any future commercial and/or conservation-related developments.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
