A geological excursion guide to Rum: the Paleocene igneous rocks of the Isle of Rum, Inner Hebrides

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Contents

Acknowledgements

Introduction

Summary of the geology of Rum

Pre-Paleocene Geology

Lewisian Gneiss Complex

Torridon Group

Mesozoic strata

Palaeocene

Pre-Stage 1

The Rum Central Complex

Stage 1

The Main Ring Fault

Minor intrusions: the commencement of Stage 2

Stage 2: The Layered Centre (formerly the ‘Layered Suite’)

The Canna Lava Formation on Rum

Events post-dating the Rum Central Complex

The Pleistocene and later


Excursion 1 Kinloch and surroundings

Excursion 1 Kinloch and surroundings. A. West of Kinloch

Locality 1.1 Kinloch Glen – sandstone intruded by dolerite plug and sheet [NM 3927 9987]

Locality 1.2 Bridge over Allt Bealach Mhic Neill – peridotite plug in sandstone [NM 3803 9983]

Locality 1.3 Allt Bealach Mhic Neill – spectacular spherulithic structures in baked sandstone adjoining gabbro plug [NM 3803 9966]

Excursion 1 Kinloch and surroundings. B. South side of Loch Scresort

Locality 1.4 White House, Kinloch – picrite dyke cutting sandstone [NM 4044 9928]

Locality 1.5 Cro nan Laogh – Torridonian sandstone displaced along the low-angle Mullach Ard Fault [NM 4208 9893]

Excursion 2 The Northern Marginal Zone (NMZ)

Coire Dubh – Meall Breac – Am Màm– Priomh-lochs area

Locality 2.1 Allt Slugan a’Choilich – steeply dipping, indurated mudstone within strands of Main Ring Fault [NM 3935 9835]

Locality 2.2 Allt Slugan a’Choilich – Inner Main Ring Fault [NM 3932 9828]

Locality 2.3 Coire Dubh – basal Coire Dubh Breccia, with incipient fracturing in nearby sandstone [NM 3917 9807]

Locality 2.4 Coire Dubh – tuffisite dyke cutting Coire Dubh Breccia [NM 3927 9794]

Locality 2.5 Coire Dubh – ‘mixed-magma’: rhyodacite plug with mafic enclaves [NM 393 979]

Locality 2.6 Cnapan Breaca – bedded breccias, sandstone and rhyodacite ash flows [NM 396 976]

Locality 2.7 Coire Dubh – Epiclastic sandstone underlying tuffs at base of rhyodacite ash flows [NM 3933 9769]

Locality 2.8 Coire Dubh – rhyodacite fiamme deformed by ‘bombs’ – c. [NM 3915 9775]

Locality 2.9 South end, Meall Breac – intrusion breccia at rhyodacite/gabbro contact [NM 3845 9797]

Locality 2.10 South-west Meall Breac – deformed fiamme in rhyodacite ash flow [NM 3837 9810]

Locality 2.11 Loch Gainmich – Am Màm Breccia underlying gabbro megablock [NM 3825 9874]

Locality 2.12 East of Loch Gainmich – gabbro/gneiss contact, megablock in Am Màm Breccia [NM 3825 9873]

Locality 2.13 Am Màm – Am Màm Breccia with gabbro, gneiss and sandstone clasts [NM 3830 9885]

Locality 2.14 East of Am Mam – gabbro of megablock cut by Am Màm Breccia and rhyodacite [NM3856 9867]

Locality 2.15 North Meall Breac – Am Màm Breccia cut by, and grades into, rhyodacite [NM 3870 9860]

Excursion 2 The Northern Marginal Zone (NMZ). Optional: Traverse west, towards Priomh-lochs

Locality 2.16 Loch Gainmich – small intrusive plug of gabbro inside Main Ring Fault [NM 3805 9895]

Locality 2.17 Ridge east of Priomh-lochs – ‘matrix banding’ in peridotite tongue [NM 3735 9900]

Locality 2.18 Priomh-lochs – faulted and unconformable relationships of gritty sandstone to gneiss [NM 3715 9876]

Locality 2.19 North of Priomh-lochs – explosion breccia injected along line of Main Ring Fault [NM 367 994]


Excursion 3 Hallival and Askival

Locality 3.1 Coire Dubh dam – intrusion breccia on edge of Eastern Layered Intrusion [NM 3892 9782]

Locality 3.2 Coire Dubh – chromite seams and anorthositic troctolite at Unit 7/8 boundary [NM 3896 9742]

Locality 3.3 North of Hallival – ‘wavy contact’ within Unit 9, at troctolite/gabbro boundary [NM 3950 9694]

Locality 3.5 Shelf north-west of Hallival – classic Unit 11/12 boundary with chromite seam [NM 3895 9679]

Locality 3.7 Hallival summit – panoramic views of Eigg, north-west Highlands, Skye and Outer Isles [NM 3953 9626]

Locality 3.8 Coire nan Grunnd – details of troctolite layering displayed in fallen blocks c. [NM 400 961]

Locality 3.9 Allt Mòr na h-Uamha – rafts of hornfelsed basalt in layered troctolite [NM 397 972]

Excursion 4A The Central Intrusion

Locality 4.1 Harris road near Kilmory fork – quarry shows sandstone baked by dolerite plug [NG 3696 0016]

Locality 4.2 Bridge south of Long Loch – evidence for Long Loch Fault in riverside exposures [NM 3639 9944]

Locality 4.3 North of Harris road – layering in troctolite deformed by ‘dropstones’ [NM 3615 9929]

Locality 4.4 ‘Whaleback’ west of Long Loch – depositional and erosional sedimentary structures in troctolite [NM 3620 9908]

Locality 4.5 ‘Whaleback’ – layering in troctolite, transgressed by underlying peridotite [NM 3623 9893]

Locality 4.6 South of Long Loch – layered troctolite/peridotite blocks in deformed layered peridotite [NM 3635 9792]

Locality 4.7 Loch an Dornabac – poikilo-macro-spherulitic structures in peridotite of Central Intrusion [NM 3569 9769]

Locality 4.8 North of Loch an Dornabac – small-scale layering in peridotite of Western Layered Intrusion [NM 3576 9800]

Locality 4.9 Road east of Ard Nev – dense, black picrite dyke intruding peridotite [NM 353 984]

Locality 4.10 Roadside north-east of Ard Nev – baked microgranite and (nearby) altered dolerite, near peridotite [NM 3540 9870]

Locality 4.11 East of Ard Mheall – harrisitic structures in peridotite, some showing erosional features [NM 3502 9772]

Excursion 4B The Central Intrusion

Locality 4.12 North of Harris Bay – roadside exposures of harrisitic structures [NM 341 963]

Locality 4.13 Harris – spectacular layering in gabbros north of bridge [NM 3377 9602]

Locality 4.14 West end, Harris Bay – contact of layered gabbro (with harrisite) and microgranite [NM 3350 9560]

Locality 4.15 Cove west of Harris Bay – contact between gabbro and hybridised microgranite [NM 3348 9565]

Locality 4.16 Harris Bay – shelves eroded in well-layered gabbro [NM 373 958]

Locality 4.16 Harris Bay – shelves eroded in well-layered gabbro [NM 373 958]Locality 4.17 East end Harris Bay – intrusion breccia at gabbro/microgranite contact [NM 3405 9505]

Locality 4.18 Abhainn Rangail – peridotite breccia in Central Intrusion, with chromite seams [NM 3450 9557]

Excursion 5 The Canna Lava Formation in north-west Rum

Locality 5.1 North-west of Malcolm’s Bridge – contact between Central Intrusion and Western Granite [NM 354 998]

Locality 5.2 ‘West Minishal’ – conglomerates containing clasts derived from central complex [NG 3475 0064]

Locality 5.4 East of Fionchra – varied clasts in fluviatile conglomerates; palaeovalley in lavas [NG 342 005

Locality 5.5 North-east side, Fionchra – hyaloclastites (including pillows) exposed in fallen blocks [NG 341 005]

Locality 5.6 North side, Fionchra – stream section through plant-bearing silty sandstones [NG 3365 0068]

Locality 5.7 North side, Fionchra – coarse fluviatile conglomerate underlying feldspar-phyric lava [NG 3346 0088]

Locality 5.8 Coire na Loigh – conglomerates and lavas faulted against Torridonian sandstone [NG 3322 0098]

Locality 5.9 Coire na Loigh – lava with thin, glassy basal selvedge overlying conglomerate [NG 3314 0092]

Locality 5.10 South side, Fionchra – lava and conglomerate ponded against wall of hyaloclastite [NG 3363 0038]

Locality 5.11 Bealach a’Bhràigh Bhig – lava resting on rubbly microgranite surface [NG 3395 0010]

Locality 5.12 Orval – classic locality where lavas were shown to rest on weathered microgranite [NM 335 992]

Locality 5.13 Bloodstone Hill – lavas lying on weathered sandstone, overlain by valley-fill lavas [NG 317 006]

Locality 5.14 North end of Bloodstone Hill – lava on weathered sandstone [NG 3149 0082]

Locality 5.15 West side of Bloodstone Hill – rhyodacite boulder in conglomerate [NG 3107 0035]

Excursion 6 Minishal and north-west Rum

Locality 6.1 Kilmory River – alkaline segregations in peridotite of Central Intrusion [NM 3639 9962]

Locality 6.2 West of Salisbury’s Dam – replacement ‘finger structures’ in peridotite [NG 3583 0018]

Locality 6.3 Minishal – Main Ring Fault, sandstone and microgranite cut by peridotite [NG 3550 0022]

Locality 6.4 Path north of Minishal – peridotite plug with some layering, intruding sandstone [NG 356 002]

Locality 6.5 Sgaorishal – ‘fissure-breccia’ in sandstone, which is baked and bleached [NG 3523 0175]

Locality 6.6 ‘West Sgaorishal’ – contact of sandstone and peridotite exposed in plug [NG 3446 0198]

Locality 6.7 ‘West Sgaorishal’ – matrix banding and strong jointing in peridotite plug [NG 3430 0224]

Locality 6.8 ‘West Sgaorishal’ – Triassic cornstones at peridotite contact containing chalcopyrite and malachite [NG 3427 0229]

Locality 6.9 Monadh Dubh – plant and ostracod remains in Triassic silty sandstones [NG 3404 0300]

Locality 6.10 North of Glen Shellesder – cornstones permeating brecciated Torridonian sandstone [NG 3301 0262]

Locality 6.11 Coast at Glen Shellesder – heavy mineral bands in Torridonian sandstone [NG 3300 0248]

Locality 6.12 Guirdil Bothy – bloodstone and agate pebbles on the foreshore [NG 3196 0134]

Excursions 7, 8 and 9 The Southern Mountains and Dibidil

Excursion 7 Kinloch – Allt nam Bà – Beinn nan Stac – Lower Glen Dibidil

Locality 7.1 Allt na h-Uamha – magmatic sediments [NM 4095 9670]

Locality 7.2 Welshman’s Rock – a doming-related ?slide[NM 417 963]

Locality 7.3 Allt nam Bà – margins of a ultrabasic magma chamber and evidence for subsidence on the Main Ring Fault [NM 4093 9424]

Locality 7.4 Lower south-east flank of Beinn nan Stac – Paleocene basalt lavas and Lewisian gneiss slivers in the Main Ring Fault zone [NM 4033 9343]

Locality 7.5 Beinn nan Stac – folded and tilted Torridonian rocks [NM 4004 9403]

Locality 7.6 Peak of Beinn nan Stac – a volcano-sedimentary succession? [MN 3977 9405]

Locality 7.7 Views from Beinn nan Stac [NM 3693 9410]

Locality 7.8 Lower south-west flank of Beinn nan Stac – subsided slivers of Mesozoic and Paleocene rocks [NM 3988 9341]

Locality 7.9 Dibidil River – ‘intrusive tuffs’ [NM 3934 9307]

Locality 7.10 Dibidil foreshore – a ring fault exposed [NM 3947 9374]

Excursion 8 Lower Glen Dibidil – Nameless and Forgotten corries – Upper Glen Dibidil – Sandy Corrie – Sgurr nan Gillean

Locality 8.1 Lower east side of Sgurr nan Gillean – contact between intrusive breccia and mesobreccia [NM 3899 9294]

Locality 8.2 Lower east side of Sgurr nan Gillean – enigmatic Torridonian rocks above rhyodacite (extrusive?) and mesobreccia [NM 3876 9293]

Locality 8.3 Nameless Corrie – an ignimbrite vent system [NM 3828 9346]

Locality 8.4 Nameless Corrie – a hybrid contact rock (‘needle-rock’) [NM 3863 9349]

Locality 8.5 Forgotten Corrie – tilted Torridonian basement, bedded mesobreccia, and extrusive vs. intrusive rhyodacite [NM 3851 9427]

Locality 8.6 Upper Glen Dibidil – Hughes’ back-veining contact between the ultrabasic and felsic rocks (rhyodacites) [NM 3821 9453]

Locality 8.7 Bealach an Fhuarain – views of Glen Dibidil, Trollaval, Fiachanis and Harris [NM 3790 9487]

Locality 8.8 Sandy Corrie – a rare caldera floor through volcanosedimentary infill succession [NM 3743 9404]

Locality 8.9 Headwall of Nameless Corrie – intrusive rhyodacite meets the extrusive rhyodacite sheet of Sgurr nan Gillean peak [NM 3783 9330]

Locality 8.10 Peak of Sgurr nan Gillean Peak of Sgurr nan Gillean – views of the Hebrides and Western Scottish Highlands [NM 3802 9303]

Excursion 9 Lower Glen Dibidil – shoulder of Sgurr nan Gillean – Papadil

Locality 9.1 Along the Papadil path – evidence for multiple movements on the Main Ring Fault [NM 3916 9263]

Locality 9.2 Southern lower slope of Sgurr nan Gillean – depositional contact between mesobreccia and uplifted gneiss [NM 3845 9248]

Locality 9.3 Southern lower slope of Sgurr nan Gillean – contacts between mesobreccia, rhyodacite, and intrusive Am Màm-type breccia [NM 3826 9245]

Locality 9.4 Shoulder of Sgurr nan Gillean – alternating bedding in the mesobreccia [NM 3810 9246]

Locality 9.5 Shoulder of Sgurr nan Gillean – the Papadil Microgranite [NM 378 923]

Locality 9.6 Sandstone melted by gabbro at Inbhir Gil [NM 3579 9247]

Locality 9.7 Peridotites around Loch Papadil [NM 3620 9228]

Locality 9.8 Melted sandstone around the bay south of Loch Papadil [NM 3660 9173]

Locality 9.9 South-east of Papadil – minor intrusions and heavy-mineral bands in Torridonian sandstone [NM 3671 9164]

References