SABRINA TIMES September 2019
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SABRINA TIMES September 2019 Open University Geological Society Severnside Branch Branch Organiser’s Report Hello everyone, Our weekend trip to North Pembrokeshire in July was blessed with good weather, and leader Dave Green led our small but happy group on two days of interesting geology from our base in the coastal village of Newport. There was a variety of things to examine during our travels including a dolerite ‘bluestone’ tor in the Preseli Hills, pillow lavas at Strumble Head, columnar jointing at Fishguard, dramatic folding at Newport Sands, glacial landforms at Cenarth, and so much more. For some of us, the highlight of the weekend was relaxing in the sunshine at the Llangrannog beach café eating cake and ice-cream whilst admiring the turbidite exposures in the cliffs surrounding the small bay – food for the mind and body! August’s highlight was the annual OUGS Symposium, this year held at the Open University in Milton Keynes. With the help of our current president Dr Marianne Schwenzer, the hard-working organising committee put together an impressive programme of talks from OU staff about their research activities at the university. I found the talks about space particularly enjoyable, with Prof. Carole Haswell (Head of Astronomy) talking about her favourite subject of exoplanets; Dr Ben Rozitis describing his role on a NASA mission to bring back a rock sample from an asteroid; Prof. David Rothery explaining our current understanding of Mercury and its geological make-up; and Prof. Nigel Harris applying his vast knowledge of mountain-building on Earth to the evolution of mountains found on other planets in the Solar System. We also had the exciting opportunity of visiting a few OU labs to see a couple of mass spectrometers; a scanning electron microscope used (amongst other things) to analyse both terrestrial rocks and meteorites and a lab used to develop and build instruments for use in space missions. Michelle has been busy organising our Day of Lectures on Saturday 7th December at the National Museum of Wales in Cardiff, and we look forward to a programme of interesting talks from this year’s speakers. Sad to say this will probably be the last time we’ll use the Museum as a venue for our events. In the past we have been fortunate to have had access to the Museum on a free-of-charge basis and of course we’re very grateful to the Museum for this, but a new management policy has now been introduced which means that we will incur charges from now on. We have a discounted rate for our Day of Lectures this year, but charges will increase dramatically in 2020. To avoid these unexpected high charges levied by the Museum, we’ve had to quickly search around for an alternative lower-cost venue for our branch AGM on Sunday 9th February 2020. Janet has now booked the village hall at Langstone, a small village on the outskirts of Newport (South Wales), which is close to the M4 and has free car parking. Please put the AGM date in your diary! Finally, our last field trip of the year takes place on Saturday 28th and Sunday 29th September in the Ludlow area of South Shropshire, led by local expert geologist Andrew Jenkinson. Full details of the weekend trip are on the Severnside website and in this newsletter. Bookings should be made with Michelle, and day visitors are very welcome on either day if you aren’t able to join us for the full weekend. Best wishes, Best wishes, Norman Nimmo-Smith In this issue: Gower 2 Aberystwyth 9 Events 15 Contacts 16 D44 The Gower 8th June 2019 Leader: Gareth George It was a very blustery day but sunny with light cloud. A vast improvement on the stair rods of the previous day and the week that followed. We had a fabulous day with our leader Gareth George whose enthusiasm for geology and sedimentology processes was infectious. The geology of the area tells a story of transgression and regression, of glacial-eustatic rises and falls in sea level. Fig. 1 Geological and Structural map of Gower (George 2015) Figure 1 shows a map of the geology of the Gower and Figure 22 (near the end of the article) is a stratigraphic log of the area and you will find it useful to refer to as you read. We met at Knab Rock car park. Only a short distance away, behind the Mumbles Yacht Club, was an exposure of a palaeokarst surface, sitting at 60o NE slope, potted all over with circular depressions (Fig. 2). This exposure of limestone sits stratigraphically in the middle of the Oxwich Head Limestone Formation and is on the northern flank of the Langland Anticline. These depressions are believed to be where Lepidodendron (club- mosses) and tree ferns had been growing when sea levels were low. Large nodules could be seen (on the left of the photo) which are thought to be the casts of stumps. Fig. 2 Oxwich Head Limestone showing depressions formed by club-mosses We wandered along the road towards the lifeboat station and car park of the Bristol Channel Yacht Club, towards Bracelet Bay. En route we made several stops. We passed the Cut, a quarry dating back to Roman times where iron ore had been mined. This is part of the Cut fault (more of which later). The overlying Triassic beds were the origin of the iron found in the Cut. 2 The Gower 8th June 2019 (contd.) Hiding behind the old lifeboat station was a well-hidden exposure of variegated palaeosol clays and it contains a clast of a club moss (Fig. 3). The palaeosols mark a rapid drop in sea-level resulting in the emergence of a shelf sea. A later rise in sea-level is marked by the eroded upper surface of the palaeosols and the overlying massive limestone beds. Fig. 3 Palaeosol surface (Patrick Moore for scale) and circled Fig. 4 Runnels eroded into Oxwich Head Limestone club-moss cast On the roadside another good exposure of ‘club-moss pits’ was seen and also a runnel formation (solution grooves) (Fig. 4) that was believed to have been formed by marine erosive forces prior to any palaeosol formation. Walking back in time, down the stratigraphic log, we arrived in Bracelet Bay, a great place for good views of Mumbles Head (Fig. 5) with its lighthouse and ‘fort’. Mumbles Head is composed of limestone of the Hunts Bay Oolite Sub-group. This sub-group was laid down in a proximal Inner Carbonate Ramp environment with prograding ooid shoals. There are three Heads and the middle one shows the axis of the eastward plunging Langland Anticline. Fig. 5 View of Mumbles Head - Bracelet Bay Fig. 6 Bedded Shell Lag and Stylolites (marked with red arrow) – Bracelet Bay On the west side of the bay, behind the Castellamare Ristorante, is the type locality for Composita ficoidea (brachiopod) and these are found, along with productids, in bedded shell lags. There are also very good examples of stylolites in a small exposure just behind the restaurant (Fig. 6). Stylolites are crinkled voids in the rock running parallel to and oblique to the bedding plane. They are the result of pressure dissolution that occurs during sediment compaction. 3 The Gower 8th June 2019 (contd.) Just below the restaurant we had a good view of a larger exposure of these rocks showing intertidal deposits with herringbone cross-stratification. As well as the shelly beds, algal oncoliths were present. Fig. 7 Shelly Lag containing brachiopods – Bracelet Bay We wandered back over to the east side of the bay to try to find exposure of fossil demosponges and haematite formation. Unfortunately, the bay was covered in large pebbles obscuring what we were looking for and making it difficult to walk! But the fault at that side of the bay had a lovely slickenside to show us. In our search for haematite we went to Limeslade Bay where the strata are cut by a N-S trending fault zone, the Cut fault. The dolomitised rock is banded with calcite and haematite (Fig. 8) in wonderful shapes and patterns. The haematite has replaced calcite and shows concentric to botryoidal growths. Fig. 8 Haematite – Limeslade Bay This fault and the iron relate to the Roman quarry seen at the beginning of our walk. So ended our morning of great rocks. 4 The Gower 8th June 2019 (contd.) We spent all afternoon in Caswell Bay. As soon as we stepped on to the beach, we could see sedimentation in action and the formation of structures, albeit in miniature. A stream was flowing onto the beach, picking up sand grains, of which some were held in suspension and we could see ripple formation. There were also great examples of standing waves (Fig. 9). Fig. 9 Sedimentation in action – standing waves and ripples Fig. 10 Gareth George standing on Caswell Fig. 11 Caswell Bay Mudstone Bay Oolite We looked at a small exposure which showed Caswell Bay Mudstone (CBM) which sits stratigraphically between Caswell Bay (Gully) Oolite (CBO) and High Tor Limestone (HTL). The CBO was laid down in tropical waters in depths of less than 5m. When sea-level fell palaeokarst surfaces formed due to exposure and subsequent erosion. It became potholed and then infilled with the overlying CBM. Gareth is standing on an outcrop of the CBO in Fig. 10. The Caswell Bay Mudstone was laid down in a tidal lagoon and consists of limestones, dolomites and claystones. The limestone beds show erosional surfaces and fine up in to algal-laminated facies (lime mudstones) (Fig.