Philip Copestake, Principal Geologist / Stratigrapher
Over the weekend of the 28th June, I was fortunate to join the post IAS (International Association of Sedimentologists) 2024 conference (held the previous week in Aberdeen) field trip to the isles of Skye and Raasay (Figure 1). The trip was led superbly by Stuart Archer (Harbour Energy) and Ron Steel. We visited outcrops of Lower and Middle Jurassic rocks (Pliensbachian to Bathonian). Emphasis was on the depositional environments of the succession, but also the relevance in the wider context of correlation to the North Sea and Mid Norway. The area, which represents the most westerly outcrops of Mesozoic strata in NW Europe, also provides analogues for the subsurface basins to the south west, including the Rockall, Slyne and Porcupine basins, offshore Ireland. This fact was discussed and described in detail by Merlin (2020), in which some of the lithostratigraphic units applied in the Hebridean Basins were extended into the offshore Ireland area. A further aim was to consider the local and wider aspects of Middle Jurassic palaeogeography, taking into account the thermal doming in the Central North Sea at this time.
Unfortunately, as is often the case in this part of the world, the weather was on the damp side, compounded by the predatory midges, however, the camaraderie of the group of international participants, together with Stuart’s humour and encouragement and the superb geology, made for an enjoyable and educational trip. Morale was boosted by supplies of Tunnock’s caramel bars and teacakes, top quality dates (supplied by Hassan from Saudi Arabia), and good choice of playlists in the trip minibuses (ably driven by PhD students from Aberdeen University, in my case by Alice Hall)!
Time was also made to take in some of the additional sites in the area, such as Loch Ness, Eilean Donan Castle, The Old Man of Storr, Kilt Rock and a quick sampling trip to the Whisky and Gin Distillery on Raasay while we waited for the ferry. On a more personal note, the visit to Raasay provided some fond reminiscences of my undergraduate field mapping spent on the island, many years ago.
An important background paper was that of Archer et al. (2019), which described the Bajocian successions that we were visiting on the trip. This paper had also proposed that the base of the Ollach Sandstone, within the Bearreraig Sandstone Formation, correlated with the major depositional sequence boundary (intra J22) at the base of the Brent Group (Broom Formation) in the North Sea. Detailed descriptions of all the sections visited are also provided by Morton & Hudson (1995) and Morton (2004). Of particular interest to me was to examine the successions and to confirm the development of some key regional surfaces (J sequence boundaries) that had been documented in the sections we visited and their potential correlation to the North Sea (as referenced in Copestake & Partington, 2023).
There were three days in the field: at Inverarish and Scrapadal (Raasay), Bearreraig Bay (Trotternish, Skye) and Glasnakille (Strathaird, Skye). The first day we took the ferry to Raasay and the first stop was to view the old Raasay Ironstone mine workings at Inverarish, which are a Historic Environment Scotland scheduled monument. The Raasay Ironstone is a Lower Jurassic (Toarcian) ironstone that is locally developed on the island and on Skye, but which is not known outside these areas. It was worked commercially during the First World War (the ore was sent to Ravenscraig for smelting and use in the British war effort), partly by German prisoners of war. At the mine entrance, we first examined the top of the Lower Jurassic (Pliensbachian) Scalpay Sandstone Formation (a potential analogue for the age-equivalent Cook Formation, Dunlin Group, in the northern North Sea). The bedding plane above which the ironstone was removed in open cast workings (marking the top of the underlying Portree Shale) is still accessible and an exposed face of ironstone visible (Figure 2). The ironstone is a chamositic, sideritic and often oolitic, only a few metres thick, shelly in places and partly cross-bedded (Figure 2). The top of the underlying Portree Shale Formation displays a concentration of belemnites (“belemnite graveyard”) and ammonites (Figure 2).
We next travelled to the north east of the island to see the major cliffs of Bearreraig Sandstone at Screapadal. We parked near the ruined Brochel Castle, former seat of the Lloyd clan of Raasay and Skye. We also saw the beginning of the famous Calum’s Road, evocatively described in a book by Roger Hutchinson.
It was a fairly long hike to the Screapadal cliff and we passed several well exposed sections of PreCambrian Torridonian Sandstone on the way (Figure 3). This unit lies on the upthrown side of the Screapadal Fault today (the faulting timing most likely being Early Cretaceous, according to Archer et al., 2019). We also observed the evocative ruined village of Screapadal, depopulated during the Highland Clearances in the 19th century.
The magnificent cliff sections at Screapadal, described in detail by Archer et al. (2019), exposes over 100m of the Bearreraig Sandstone, at seismic scale. The sandstone (which achieves a total of 485m in gross thickness in the Inner Hebrides Basin, Archer & Steel., 2024) is the thickest Middle Jurassic sandstone in the country (Cox et al., 2002) and represents deposition in a shallow marine, tidal, setting. High resolution drone acquired photographs were obtained by Archer et al. (2019, figs. 11a-e), although much of the succession is inaccessible. One of the attendees on the trip, Sergio Longhitano, flew his drone over this and other outcrops we visited and has since shared the amazing drone footage captured. Figure 4 shows the upper part of the main cliffs at Screapadal, with the channels, erosion surfaces, units of soft sediment deformation and hiatus/draping surfaces (flooding or possibly maximum flooding surfaces) indicated. Figure 5 shows the cross bedding from the formation in a fallen block.
Day 2 included a visit to the famous Bearreraig Bay section, Isle of Skye, which exposes the most typical succession of the Bearreraig Sandstone Formation with all members of the unit clearly exposed. Work on the succession of ammonite faunas across the Aalenian–Bajocian boundary has led to Bearreraig Bay being recognized as an Auxiliary Stratotype Point for the base of the Bajocian Stage (Morton, 2002).
After a challenging descent down to the shore section of the bay, we examined the succession from the Ollach Sandstone, through the Udairn Shale and up to the Holm Sandstone at the top of the waterfall section (Figure 6). The uppermost part of the succession, including the Rigg Sandstone and overlying Garantiana Clay, nor the Dun Caan Shale at the base, were not accessible unfortunately. The Bearreraig Bay succession displays the various members of the Bearreraig Sandstone Formation, which were not discernible on Raasay.
As noted above, the Bearreraig Bay succession shows remarkable correlation with the Aalenian to Bajocian of Mid Norway, where the Ile, Not and Garn formations compare well with the Ollach Sandstone, Udairn Shale and Holm plus Rigg sandstone members respectively. The major flooding surface at the base of the J24 sequence in the Not Formation correlates with the probable MFS within the Udairn Shale Member (Figure 7). Hesselbo & Coe (2000) placed a MFS precisely at the Bajocian/Aalenian boundary, and this is a precise correlation with the position of the base J24 MFS as defined by Copestake & Partington (2023). The formation is capped by the Garantiana Shale, which has been dated by ammonite records as Late Bajocian, Garantiana Zone and therefore ties to the base J26 MFS of Copestake & Partington (2023). The succession in the Hebrides Basin is unlike that of the equivalent section in the North Sea Brent Group at this level where the J26 sequence is developed in a thick succession of non marine sandstones, shales and coals of the upper part of the Ness Formation. In the Hebrides, the J26 sequence is relatively thin and may contain some missing sections on account of this.
To the north of the bay, in the Upper Glen-1 well (Figure 8), several of the sandstone members, have begun to pass laterally into shale, demonstrating the geographically restricted nature of some of the sandstone units. This is analogous to the northern limit of the Brent delta in the Tampen Spur area in Norway, where argillaceous equivalents of the Brent Group are developed, and also associated with significant thinning/wedging out of the J26 sequence (see Copestake & Partington, 2023, Figure 4.5).
Stratigraphic correlation across Sea of the Hebrides Basin half graben; Upper Glen-1 to Screapadal (Raasay) showing key sequence surfaces.
Quaternary landslips extend the whole length of the Tertiary basalt escarpment on the coast from Portree to Flodigarry and are spectacularly displayed at The Quiraing (Figure 9). These landslips have been related in process and scale to the many footwall collapse/degradation aprons associated with some of the major Jurassic tilted fault blocks in the North Sea, such as Brent and Statfjord (Archer & Steel, 2024). Tertiary uplift and tilting, followed by the gravitational instability created by the tilting, together with the density contrast between the basalts and the underlying Upper Jurassic argillaceous sediments were responsible for the landslip (Archer & Steel, 2024). Some surface displacements visible at the top of the cliffs suggesting that landslipping is still active today.
Day 2 finished with a quick visit to view Kilt Rock and the waterfall there, where a thick dolerite sill occurs above Middle Jurassic Great Estuarine Group sediments (Figure 10). The well publicized discoveries of a wide variety of dinosaur fossils (bones plus footprints) and other large reptiles (such as pterosaurs) from the Bathonian Great Estuarine Group were discussed, including the small but excellent Staffin Dinosaur Museum that houses some of these finds. Following the trip, Stuart shared some of his photos of some spectacular dinosaur footprints from the nearby area (Figure 11).
Day 3 comprised a trip to the south western tip of the Strathaird Peninsula, where the important section of Bearreraig Sandstone at Glasnakille was the main focus (Figure 12). This location lies in a separate half graben, of the Inner Hebrides Basin, lying to the south of the Camasunary Fault.
The formation in this section comprises poorly sorted, cross bedded sandstones with a marine fauna (belemnites, bivalves) and ichnofauna; large scale sandwaves up to 4m high can be seen. The bimodal cross stratification is indicative of a tidal setting with sediment transport towards the south. Deposition took place in a narrow seaway generated by rotating fault blocks and emergence of footwall islands (Archer & Steel, 2024).
The trip ended with a stop at Elgol harbour, to quickly take in the view across to the Cuillin Hills and we also discovered a newly cut claystone/shale section behind the new public toilet building at Elgol, which either represents the Garantiana Shale (uppermost Bearreraig Sandstone Formation) or Cullaidh Shale (Great Estuarine Group), most likely the latter. Some reconnaissance samples were taken for potential biostratigraphic analysis.
One of the main conclusions from the trip was that variations in thickness and facies development of the Bearreraig Sandstone across the area, especially in relation to the major faults and half grabens, indicates fault-controlled deposition in the Skye – Raasay region. Upon this are superimposed regional changes, whereby the main transgressive – regressive cycles can be related to regionally developed depositional sequences recognised in the North Sea, Norwegian Sea (mid Norway) and elsewhere at outcrop in the UK; these regional cycles have been recognised by several authors (e.g. Hesselbo & Coe, 1998; Copestake & Partington, 2023) and point to the complex interplay between local and regional geological events. This was one of the most fascinating and intriguing aspects of the trip, which I will take into my ongoing work on the Jurassic both in the subsurface but also elsewhere in the Jurassic outcrops of the United Kingdom. It was also interesting that the Bearreraig Sandstone successions we visited appear more analogous to age equivalent sections in Mid Norway (i.e. the Ile, Not and Garn formations) than the geographically closer Brent Group. I am currently undertaking a project in the Mid Norway area and it was fascinating to see the many similarities between the two areas at outcrop.
Thank you Stuart, Ron and the other participants for a truly memorable and highly enjoyable field trip, ably led by expert geologists. The trip was also an excellent opportunity to meet and learn from other geologists from many institutions, literally worldwide.
References
Archer, S. G., & Steel, R. J. 2024. The Bearreraig Sandstone Formation of the Isles of Skye and Raasay, NW Scotland. Field Guide. IAS 2024 Conference, Aberdeen Field Trip.
Archer, S. G., Steel, R. J., Mellere, D, Blackwood, S. & Cullen, B. 2019. Response of Middle Jurassic shallow-marine environments to syn-depositional block tilting: Isles of Skye and Raasay, NW Scotland. Scottish Journal of Geology, 55, 35-68.
Copestake, P. & Partington, M. A. 2023. Sequence Stratigraphy of the Jurassic–Lowermost Cretaceous (Hettangian–Berriasian) of the North Sea Region. Geological Society, London, Memoirs, 59, https://doi.org/10.1144/M59
Hesselbo, S. & Coe, A. 2000. Jurassic sequences of the Hebrides Basin, Isle of Skye, Scotland. In: Graham, J. R. & Ryan, A (eds) Field Trip Guidebook, International Association of Sedimentologists Meeting, Dublin, 41-58.
Merlin Energy Consortium, 2020. The Standard Stratigraphic Nomenclature of Offshore Ireland: An Integrated Lithostratigraphic, Biostratigraphic and Sequence Stratigraphic Framework. Project Atlas. Petroleum Affairs Division, Department of Communications, Climate Action & Environment, Special Publication 1/21. https://www.gov.ie/en/publication/d4923-the-standard-stratigraphic-nomenclature-of-ireland/
Morton, N. 2002. Bearreraig Bay, Isle of Skye. Chapter 6, The Middle Jurassic Stratigraphy of Scotland, Cox, B. M., Page, K. N. & Morton, N. In; Cox, B.M. & Sumbler, M.G. 2002. British Middle Jurassic Stratigraphy. Geological Conservation Review Series, No. 26, JNCC, Peterborough, ISBN 1 86107 479 4. https://geoguide.scottishgeologytrust.org/p/gcr/gcr26/gcr26_ch6midjurassicstratscotland
Morton, N. 2004. 8, The Hebrides Basin. In; Simms, M. J., Chidlaw, N., Morton, N. & Page, K. N. British Lower Jurassic Stratigraphy. Geological Conservation Review Series, No. 30, Joint Nature Conservation Committee, Peterborough, 458 pp. British Lower Jurassic Stratigraphy: contents | GeoGuide (scottishgeologytrust.org)
Morton, N. & Hudson, J. D. 1995. Field guide to the Jurassic of the Isles of Raasay and Skye, Inner Hebrides, NW Scotland. In; Taylor, P. D. (ed.) Field Geology of the British Jurassic. Geological Society, London, 209 – 280.
