Structure and stratigraphy of the Morar Group in Knoydart, NW Highlands: implications for the history of the Moine Nappe and stratigraphic links between the Moine and Torridonian successions

The Caledonian Orogen in northern Scotland comprises two major thrust nappes: the Moine and the Sgurr Beag Nappe. The Moine Nappe contains early Neoproterozoic Morar Group rocks (Moine Supergroup) and basement inliers. This paper describes the structure and stratigraphy of the Knoydart peninsula, a key area within the southern Moine Nappe. The geology of Knoydart is dominated by a thick internally coherent sequence of Morar Group rocks. This sequence is shown to be deformed by large-scale, west-vergent and west-facing Caledonian (early Palaeozoic) folds that represent D2 within the southern Moine Nappe. Subsequent D3 deformation led to refolding or tightening of F2 folds, so that the major Morar Antiform is, in essence, a composite F2/F3 fold. F2 and F3 folds are broadly co-axial, but F3 folds have steeper axial planes. The F2/F3 folds refold a regional-scale, originally recumbent, isoclinal F1 fold nappe of probable Knoydartian (mid-Neoproterozoic) age. The F1 fold nappe is cored by a thin sliver of basement gneiss; the lower limb comprises migmatitic Morar Group rocks, exposed in the Morar Window. The upper limb of the F1 fold nappe occupies most of Knoydart and is stratigraphically coherent and right-way-up. Within this sequence, the upper unit of the Lower Morar Psammite is barely deformed, preserving trough-cross-bedding and large-scale channels in thick beds. This suggests braided river deposition, similar to the Torridon Group west of the Moine Thrust and the Morar Group in the northern part of the Moine Nappe. On the basis of lithological similarity and stratigraphic disposition, it is suggested that the lowermost part of the Morar Group in Knoydart correlates with the Neoproterozoic Sleat Group on Skye

Progress report on the geology of 1:50k sheet 64W (Newtonmore)

This report describes the results of solid geology fieldwork in 1:50 000 sheet 64W (Newtonmore) resulting from the 2002 summer mapping season. A full revision of the solid geology at 1:10 000 scale was completed in the north-western part of the sheet (Sheet NN69SE) while rapid mapping/reconnaissance of the solid geology has been carried out in the remaining 85% of the sheet area The superficial geology of the sheet has been completely revised and will be described in a separate report. The north-western part of the sheet contains the transition from the deeper water graded sandy and silty turbidite deposits of the Corrieyairack Subgroup upwards (and south-eastwards) into the shallow water sand-dominated deposits of the Strathtummel Subgroup. East of the A9 trunk road, the Gaick region is confirmed as a single lithostratigraphical package in the Strathtummel Subgroup recording shallow water depositional conditions, greatly thickened by D2 recumbent folding. Axial surfaces of these folds dip gently east overall with gently east plunging to subhorizontal fold axes. Axial traces are generally N-S trending. The main regional (biotite) schistosity is axial planar to these folds and locally, can be seen clearly deforming an earlier bedding near-parallel biotite fabric. The available evidence for stratigraphical younging is limited to a few well-washed river sections but shows that regional facing is always to the south in S2 across the Gaick region. No large-scale F1 folds are recognised with the exception of those at Crubenmore on the A9. Minor undulations of the main regional fabric mean that the sheet dip varies between gently north to gently east across open upright north-east-plunging folds, in marked contrast to the conspicuous pattern of reclined, north-west verging D3 folds deforming the main regional (S2) schistosity in Glen Truim and farther north-west. There appear to be no other major fold sets across this part of the Gaick region The Drummochter Dome thus takes the form of a stack of recumbent D2 folds, modified by steep zones to the north-west (Geal CharnOssian Steep Belt) and south-east (Tummel Steep belt and correlatives). The pattern of early recumbent folds and later steep belts is similar to that seen in the higher structural levels south of the Boundary Slide which include the Tay Nappe

The rocks of Spireslack surface coal mine and its subsurface data : an introduction

Surface coal mining at Spireslack, East Ayrshire, has exposed a one kilometre long, vertical section of over 130 m of Carboniferous strata. This includes a variably exposed complete sequence through the Limestone Coal Formation, one of the main coal producing units in Central Scotland, and also the underlying Lower Limestone Formation. Parts of the Lawmuir and Upper Limestone formations are also exposed. Such laterally continuous exposures of these Carboniferous strata are rarely exposed in nature, and therefore Spireslack allows the opportunity to study their features in detail. These include laterally extensive fluvial sandstone sequences; palaeosol horizons with in situ tree casts; regionally correlateable limestones; fold and fault-related structures and their relation to differing mechanical rock properties; and regionally important marine bands and other fossils within the Carboniferous. Data obtained from abandonment plans of earlier underground coal mining, and from more recent surface coal mining, have been used in combination with 1:10 250 geological fieldslips, to reconstruct the position and structure of seven mined coal seams from Spireslack in a 3D geological model. The model reveals geological folds previously photographed but not mapped before and allows the structure and position of the Muirkirk Syncline to be mapped accurately in this area. This report firstly presents an account of the geology exposed at Spireslack, and secondly the results of the 3D geological model. The 3D model will underpin future geological investigations of Spireslack and act as a foundation for subsequent sub-surface geological modelling (e.g. seismic modelling, fluid flow modelling, etc)

Tectonic synthesis and contextual setting for the Palaeozoic of the Moray Firth region, Orcadian Basin

This report is designed simply to provide a summary tectonic outline and contextual setting against which offshore seismic and well data relating to the Devono-Carboniferous evolution of the Inner Moray Firth region, adjacent areas of the Orcadian Basin, and UK offshore regions can be considered. This summary is intended to help better frame the questions that will arise during interrogation of that data; the findings that result from that analysis are presented elsewhere in the report series (Arsenikos et al., 2016; Kimbell & Williamson, 2016; Monaghan et al., 2016). The pattern of Devonian and Carboniferous tectonics in the Moray Firth region will be strongly influenced by the underlying pattern of (N)NE-(S)SW tending Caledonian basement discontinuities transecting the region, in particular the expression of regional stress patterns along and across the trace of the Great Glen – Walls Boundary Fault Zone (GGFZ). Sinistral motion, on that very large-scale intra-Laurussian structure, is seen throughout most of the Upper Palaeozoic (Dewey and Strachan, 2003) but by the late Carboniferous, interaction of Baltica-Siberia across the Ural Sea foredeep had come into play, promoting dextral shear on the GGFZ (Coward, 1993; cf. Domeier and Torsvik, 2014). In the Devono-Carboniferous, and under the influence of overall sinistral transcurrent motion on the GGFZ, E(SE)-W(NW) directed stretching should be anticipated in the Moray Firth region with N(NE)-S(SW) oriented extensional faults likely. Such structures would be similar in style to the patterns of faulting associated with the Devonian outliers observed onshore in Moray- Buchan, and also as described in the Helmsdale region (Underhill & Brodie, 1993). In addition to a strong ‘basement’ control from inherited Caledonian (N)NE to (S)SW features, there is likely also to be underlying control from any Caledonian plutonic complexes present (Kimbell & Williamson, 2016). The present day pattern of Moray Firth faulting, established depocentres and intra-basinal highs, comprises a strong Mesozoic pattern of tectonic features (e.g. Andrews et al., 1990; Underhill 1991; Thomson and Underhill 1993) superimposed on older (‘post-Caledonian’) tectonic patterns established in the Late Carboniferous, most likely in response to the south-westwards movement of Baltica relative to Laurussia at this time (cf. Coward, 1993). Those regional stresses generated dextral shear in the GGFZ, coincident with strongly partitioned strain in the North Sea basin interior (Leslie et al. 2015). From Late Carboniferous times, N(NW) – S(SE) directed extension in the Inner Moray Firth region generated W(SW)-E(NE) trending extensional faults (e.g. the Banff and Wick faults). This stress régime sets in place the framework of highs and lows preceding any Permian uplift and younger tectonics as the switch to the earliest Atlantic-opening stresses occurred. WNW-ESE directed extension, observed and dated in the Pentland Firth area (267+/-3 Ma, Dichiarante et al. 2015), will likely generate increasingly oblique (sinistral?) wrench on the Wick/Banff Faults from this time. Dip-slip components of movement are likely on the older (N)NE- (S)SW-trending structures inherited from the Caledonian, e.g. the Helmsdale/Strathconnon Faults, and perhaps the GGFZ; the latter may be too steep and fundamental a structure to actively respond in pure extension at this time

Revision of the solid geology shown on the 'Assynt District' special geological map : a report on the 2002 fieldwork

The report provides an overview of the main findings from the first field season in the Assynt District of the Moine Thrust Project. Detailed mapping in the eastern part of the Assynt halfwindow has resulted in a new interpretation of the geometry and behaviour of the Ben More Thrust. This reinterpretation of the thrust satisfactorily resolves the conflicts between the various previous models. The remapping confirmed that the Ben More Thrust can be traced, as shown on the published 1923 Assynt District geological map, along the western flank of Na Tuadhan to Bealach a’ Mhadhaidh. The Ben More Thrust is then traced to [NC 30026 24416] where it is displaced across a steep reverse fault to [NC 30514 23953]. It then continues NNW as a readily traceable feature placing gneisses of the Lewisian Gneiss Complex over quartzite along Leathaid Riabhach [NC 298 252]. Here the Ben More Thrust progressively steepens into a sub-vertical structure that has gneiss to the NE and quartzite to the SW. The thrust follows a prominent gully along Leathaid Riabhach to A’ Chailleach. From here the Ben More Thrust more or less follows the top of a monoclinally folded quartzite that forms the summit of Beinn Uidhe and is exposed in the valley floor NW of A’ Chailleach. It retains thrust geometry with hangingwall gneisses and footwall quartzites and becomes a steep feature that approximately follows ‘Glen Beag’ (the un-named glen south of the Stack of Glencoul). The Ben More Thrust meets, but does not displace the Glencoul Thrust at the head of Loch Glencoul. Therefore it is proposed that there is a branch line here where the two thrusts meet so that all the rocks NE of Loch Glencoul and east of Loch Beag are part of the Ben More Thrust Sheet. Figure 2.7 in the report provides a clear pictorial description of the geometry of the Ben More Thrust in the northern part of the Assynt half-window. A significant new ductile structure has been identified within the Ben More Thrust Sheet, termed the Coire a’ Mhadhaidh Detachment, that mostly follows the Lewisian gneisses/quartzite contact. It has been traced from the northern limits of the Loch Ailsh intrusion across Ben More Assynt, along the eastern slopes of Na Tuadhan, across Cailleach an t-Sniomha to the west of Gorm Loch Mòr and immediately west of the Stack of Glencoul into Glen Coul (Figure 2.1 in the report). The sense of shearing in the detachment is almost always top-to-west. Similar smaller shears have also been recognised within the Lewisian gneisses in the thrust sheet. However, no ductile shearing was noted at the gneiss/quartzite contact below the Ben More Thrust. Several of the complex imbricate structures mapped by previous workers were revisited. The imbricates in the Loch an Eircill–Loch nan Caorach area appear to be simpler than shown on the published Assynt District map. An alternative solution is provided for the southern termination of the Glencoul Thrust south of Inchnadamph although it is noted that more detailed work needs to be done, notably south of Conival. Brief descriptions are given of Moine rocks above the Moine Thrust in the north-eastern part of the Assynt District map. There appears to be a lateral facies change with semipelitic schists dominant in upper Glen Cassley and psammites becoming dominant to the north. Fabrics associated with several deformation phases have largely obliterated sedimentary structures although transposed bedding traces can be seen between a spaced foliation that controls the flaggy character of the psammites. Widely spaced traverses across the major Lewisian outcrop areas, within the Assynt half-window as well as in the western foreland to the thrust belt, largely confirmed the work of the primary surveyors. Thus all of the Lewisian comprises orthogneisses, mostly hornblende-gneisses but with more felsic pyroxene-bearing gneisses in the north, that all contain ultramafic and mafic pods and layers. The traces of the various Scourie dykes are correctly shown on the published Assynt District map. The Canisp Shear Zone has been traced eastwards, south of Canisp, eventually disappears under Cambrian quartzites. A second parallel shear has also been delineated north of Loch Assynt. The polyphase nature of ductile deformation in the Lewisian gneisses elucidated by previous workers is confirmed. However, the deformation state of the gneisses is extremely variable on all scales, with intense deformation confined to specific (shear) zones that vary in thickness from several centimetres up to hundreds of metres. Descriptions of the numerous minor intrusions and the Quaternary deposits studied during the fieldwork are given in separate reports

Tectonic synthesis and contextual setting for the Central North Sea and adjacent onshore areas, 21CXRM Palaeozoic Project

This report is designed simply to provide a summary tectonic outline and contextual setting against which offshore seismic and well data relating to the Devono-Carboniferous evolution of the Central North Sea, Forth Approaches, and adjacent UK onshore region can be considered. This summary is intended to help better frame the questions that will arise during interrogation of that data; the findings that result from that analysis are presented elsewhere in the report series (Arsenikos et al., 2015; Kimbell & Williamson, 2015; Monaghan et al., 2015). Apparently contradictory, wrench- or extension-dominated patterns of Lower Carboniferous basin development are recorded in the Forth Approaches, Quadrant 29, North Dogger and Silverpit basins of the Central North Sea, as well as the Midland Valley of Scotland (MVS) and Northumberland and Solway basins onshore. Partitioning Carboniferous deformation across inherited pre-existing Caledonian or Tornquist structures is likely to be an important control on the tectonic architecture developed in these regions during intervals of the geological record in the Carboniferous. Onshore, spatially separate but contemporaneous domains of extension-dominated tectonics versus wrench-dominated tectonics explain the contrasting tectonic framework of the MVS/Forth Approaches region (wrench-dominated) compared with Northumberland Basin (classic ‘stags head’ structure). NE-SW trending Caledonian inheritance strongly controls the domain boundaries and the patterns of deformation created in each of these domains. Offshore, in the Devono-Carboniferous basins of the Central North Sea, the likelihood that strain is partitioned in a similar way across features inherited from the NW-SE Tornquist trend is proposed and examined. The data currently under consideration suggests that a NW-SE trending wrench-dominated domain is spatially associated with the region underlain by the Dogger Granite pluton; domains affected by extension-dominated tectonics appear to be arranged on either side of that feature, namely the Quadrant 29 and North Dogger basins to the NE, and the Silverpit Basin to the SW. Extension is expressed as a NE-SW directed stretch in both of these domains. Patterns of broadly N-S trending fold axes need to be carefully assessed in terms of their structural setting, as folding cannot implicitly be linked with inversion/compression when partitioned strains are developed. Superficially similar features can develop in the MVS in dextral transpression, in north Northumberland buttressed around the Cheviot Granite in overall dextral wrench, and as superimposed late compressional folds in end-Variscan convergence, for example in the Boldon syncline of County Durham. Offshore, similar inversion effects can be seen in the patterns of transpressive faulting associated with features such as the Murdoch Ridge, and with examples of superimposed NE-SW trending extensional faults active in the latest Carboniferous to early Permian

Geology of the Newtonmore-Ben Macdui district : Bedrock and superficial geology of the Newtonmore-Ben Macdui district : description for sheet 64 (Scotland)

This report provides an account of the geology of the Newtonmore-Ben Macdui district in the Grampian Highlands of Scotland, which extends from the Cairngorm massif in the north-east, west across to the Upper Spey valley and south into the upper parts of Glen Tilt and Glen Fearnach. The district is nearly all remote countryside with steep-sided glens between upland plateaus with relatively few distinct mountain peaks. The entire area lies within the Cairngorm National Park and much of the land is covered by large estates run for game conservation and recreational sports. The bulk of the rocks are metasedimentary and most of these are assigned to the Neoproterozoic Dalradian Supergroup (Figure 1). In the north-west near Newtonmore, a ridge or ‘palaeohigh’ of older metasedimentary rocks, the Glen Banchor Subgroup, is considered to lie below the Dalradian. The Dalradian Supergroup forms a thick succession of originally clastic, carbonate and pelitic rocks. Much of the latter is graphitic and pelagic in origin. The metasedimentary rocks were intruded by relatively minor basic igneous and granitic bodies as the Rodinian palaeocontinent broke up. At about 470 Ma the Laurentian continental margin collided with an island arc causing the Grampian Event of the Caledonian Orogeny. The orogeny is mainly manifest in four deformation phases which included early large nappe-like folds, ductile shear-zones and prograde Barrovian regional metamorphism. Most of the rocks in this district lie within the kyanite zone but, because most of the rocks are siliceous, this index mineral is scarce. Semipelitic rocks are locally migmatitic. The earlier Precambrian metamorphism in the Glen Banchor Subgroup is overprinted by the Grampian metamorphism

Sedimentology, architecture and depositional setting of the fluvial Spireslack Sandstone of the Midland Valley, Scotland: insights from the Spireslack surface coal mine

The Spireslack surface coal mine exposes a section in the Carboniferous Lawmuir Formation (Brigantian) into the Upper Limestone Formation (Arnsbergian). This paper describes the stratigraphy exposed at Spireslack and, in so doing, names for the first time the Spireslack Sandstone, a distinctive erosively based, sandstone-dominated unit in the Upper Limestone Formation. The Spireslack Sandstone consists of two fluvial sandstone channel sets and an upper, possibly fluvio-estuarine, succession. From an analysis of their internal architectural elements, the channel sets are interpreted as a low-sinuosity, sand-dominated, mixed-load fluvial system in which avulsion and variations in sediment load played a significant part. The lower channel set appears to be confined to erosional palaeovalleys of limited lateral extent and significant relief. The upper channel set is much more laterally extensive and shows evidence of a generally lower sediment load with a greater degree of lateral accretion and flooding. Consequently, the Spireslack Sandstone may represent a system responding to base level changes of higher magnitude and longer duration than the glacio-eustatic scale commonly attributed to Carboniferous fluvio-deltaic cycles. The Spireslack Sandstone may represent an important correlative marker in the Carboniferous of the Midland Valley and may provide an alternative analogue for some Carboniferous fluvial sandstone stratigraphic traps