Northwest Highlands Field Trip
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OPEN UNIVERSITY GEOLOGICAL SOCIETY WEST OF SCOTLAND BRANCH Northwest Highlands Field Trip 11th May to 18th May 2013 LED BY DR IAIN ALLISON, UNIVERSITY OF GLASGOW Achmelvich Beach M Donnelly WRITTEN BY MAGGIE DONNELLY, ANNE MORTON, CHRISTINE HODGSON, LEXY CRUICKSHANK, JIM MARTIN, JOYCE RAMMELL Compiled by M Donnelly Tight folds in “Iain’s surprise quarry’” Cul Mor I would like to thank our Leader, Dr Iain Allison, very much, for sharing with us his knowledge and expertise during the course of this week. Without him this trip would not have been possible. I would also like to thank him for the use of his Excursion Notes in the compilation of this booklet. Maggie Donnelly November 2013 . The field trips were based approximately on the new ‘A Geological Excursion Guide to the Northwest Highlands’, 2011, eds. Kathryn Goodenough, Maarten Krabbendam, published by Edinburgh Geological Society. Day 1 – Excursion 1 Day 2 – Excursion 2 Day 3 – Excursion 10 Day 4 – Excursions 16 & 7 Day 5 – Excursion 6 Day 6 – Excursion 9 Looking towards Ben More Assynt, snow on the hills………… Achmelvich Laxford Bridge Loch Glencoul 4 Achmelvich Loch Assynt 2 1 Inchnadamph Traligill Cam Loch 3 6 5 Knockan Crag 6 Oykel Bridge Ullapool UK North Sheet, BGS Day 1 Loch Assynt and the Achmore Duplex Day 2 Canisp Shear Zone at Achmelvich Day 3 Cam Loch, the Loch Borralan Pluton and the Loch Urigill carbonatite. Day 4 Glencoul Thrust, Laxford Bridge and Traligill Valley Day 5 Knockan Crag Day 6 Glen Oykel & Oykel Bridge Some Key Events………….. Period Ma After Dr Iain Allison Achmelvich Canisp shear zone Northwest Highlands Field Trip 11th May to 18th May 2013 Our trip was planned as an opportunitiy to visit localities, both familiar and unfamiliar, that are described in the new ‘Excursion Guide to the Northwest Highlands’. Our party of sixteen was accommodated in the very comfortable Inchnadamph Lodge, refurbished since our last stay, near the shores of Loch Assynt. Most of the group were from Scotland, but happily we were joined by four ‘further travelled’ from ‘South of the Border’ and abroad. The rocks we would see comprised Late Archaean Lewisian gneisses, reworked during the Laxfordian event ca. 1.7 Ga, the Torridon Group (Applecross Formation) ca. 1000 Ma, the Cambrian/Ordovician of the Caledonian Foreland and Moine Thrust Zone, and Moine metasediments. Our weather was mixed, but fortunately improved as the week went on. So……….on Sat 11th May we gathered in late afternoon, and after dinner our leader gave us an introduction to the geology for the week. We then retired for a ‘sociable refreshment’. Sun 12th May 2013 Loch Assynt and the Achmore Duplex. Margaret Donnelly On a bright but rather cloudy morning we headed northwest for the shores of Loch Assynt beyond Skiag Bridge. Parking in a layby (NC 212 251), we set up a car shuttle, and were told to ‘go and look at the rocks’; so we made our way down the heathery slope towards the loch and came upon cross-bedded crimson sandstones, very fine to medium grained. Unfortunately this was not what our leader wanted us to find…………..he returned and led us down to the shore where there were Lewisian orthogneisses, intruded by a large black undeformed ultramafic Scourie dyke, in places 9 m wide, and trending ESE. The gneiss was composed mainly of pyroxene, quartz and feldspar, with westward dipping foliation, and the dyke was an olivine-rich metaclinopyroxene-norite. We climbed back up to the road, and examined the (crimson) sandstone rocks in the road cut. They were composed of (a) a poorly sorted, tabular-bedded very coarse material with clasts of gneiss and vein quartz supported in a muddy matrix, and (b) a better sorted coarse sand to granule material with matrix-supported pebbles, some with a desert varnish. A number of the beds had erosive bases, there was some parallel lamination, the entire matrix was arkosic, and there were millet seed sand grains and dreikanters. Our leader explained that these deposits had been formed by alluvial fans descending into lacustrine The ‘Double Unconformity’ M Donnelly environments, and that this was the Diabaig Formation, lowest of the Torridon Group, laid down on top of a Lewisian palaeolandscape about 1000 Ma. At this particular location, the unconformity was just out of sight beneath the road but as we walked eastwards it rose up before us (NC 213 251) – reddish conglomerates and pebbly sandstones of the Diabaig Formation lying on top of the highly weathered cream to pale green Lewisian gneisses. Looking across Loch Assynt to the slope of Beinn Gharbh, we could see Torridon rocks on top of Lewisian, and Cambrian Quartz Arenite on top of both, overlapping the first unconformity. This is the ‘double unconformity’ – a line in space perpendicular to the surface of the hill. Further along the road (NC 218 250), the considerable topography of the unconformity was revealed as the Diabaig sandstones infilled huge troughs in the Lewisian palaeolandscape of ‘cnoc and lochan’. We walked on to locality NC 219 247, where massive trough cross-bedded sandstones of the Applecross Formation lay conformably over thinly bedded, laminated granule stones of Diabaig. A little further on, at NC 223 249, well developed glacial striae scored the Applecross which was smoothed and shaped by ice, forming roche moutonnees. After a small ‘debate’, it was decided that the ice had come from the WSW. At locality NC 225 248, the Applecross Formation was set back from the road, and composed of very coarse sand to Massive cross-bedding M Donnelly granule clasts, angular to sub-angular with little silt or clay. It had an abundance of terracotta-coloured feldspar, lithic clasts and vein quartz – an arkosic sandstone – and trough cross-bedding throughout. We climbed a little way uphill to find the top surface of some of the cross-beds and in this 3D view the direction of the palaeocurrent could be observed as towards 100⁰. These rocks were created by deep perennial braided rivers, forming a very large-scale braidplain, and their source was the WNW. The slope of the hill, which was now all Applecross Formation, was broken up into ‘levels’ and ‘rises’ – the ‘rises’ formed the channels of the rivers and the ‘levels’ were eroded mudstones (old channels) where the rivers had changed directions and conditions had become quiet. We walked on towards Skiag Bridge and came to the Cambrian Quartz Arenite (NC 229 247) – this is not quartzite, which is metamorphic, but a sedimentary rock of arkosic sandstone or quartz arenite. The initial outcrops are of light grey weathering, very coarse sandstones and granulestones, full of clasts of quartz grains, pink feldspars, and small scale cross-bedding – these are sub-arkoses. We had a clear view of the Basal Cambrian unconformity (which is not actually seen on the road) extending all the way up the hill, Quinag, above – white well-bedded quartz arenites of the Eriboll Formation, dipping east at 12-15⁰, lay over sub-horizontal Applecross Formation with an angular unconformity. As we progressed along the road the pale sandstone generally became finer, with less feldspar, and it was now a true quartz arenite, with planar tabular cross-bedding, bipolar current directions and herring bone cross-bedding (NC 231 245). There were numerous joint faces, and some slickensides indicating a fault. Then pipes and pimples started to appear in the rock………. a cold wind sprang up and clouds started to gather. We continued to Skiag Bridge and then 20 m up the Kylesku road (NC 235 244) where there are classic outcrops of Pipe Rock. This has large scale planar tabular cross- bedding and is stained red by diagenetic iron oxide, but the pipes stand out as white tubes about 1 cm wide and tens of cms long. They are known as Skolithus and are thought to be the trace fossil of a filter feeder organism. The actual animal has never been found even though similar pipes also occur in Mesozoic rocks. Altogether there are three types of this trace fossil – Skolithus (a single pipe), Monocriterion (a pipe with a funnel), Diplocriterion (a U-shaped tube). Returning to the lochside road, we walked east to where we found the Pipe Rock conformably overlain by orange-brown Fucoid Beds of the An t-Sron Formation (NC 236 242). These have numerous black burrows on the bedding surfaces, and were originally thought to contain fossil seaweeds ………. hence their name. They contain both dolomitic wavy bedded siltstones with the trace fossils Skolithus, Planolites, Cruziana and Rusophycus, and cross- bedded dolomitic grainstones with echinoderm fragments, the latter interpreted as storm events. This is the earliest unit in the Cambrian to contain body fossils – trilobites of the Bonnia-Olenellus Biozone which is of late Early to Middle Cambrian. As we progressed we came to the Salterella Grit, conformably overlying the Fucoid Beds. These are cross-bedded quartz arenites and contain Skolithus as well as the body fossil Salterella, a cone-shaped shell only a few mms long. The organism is unknown but may have been a primitive mollusc. One notable ‘fossil-finder’ in our company found lots of good examples. Finally we came to the grey dolostones of the Ghrudaigh Formation conformably overlying the Salterella Grit, before making our way back to the cars in the layby on the Kylesku road for lunch……..it started to rain heavily! The rain eased……….we walked up the road and through a gate, to a ford over a small stream, and climbed the grassy slope looking for Fucoid Beds, Salterella Grit and the Ghrudaigh Member, in the streambed.