D3 July 2011.Pub

SABRINA TIMES JULY 2011 OPEN UNIVERSITY GEOLOGICAL SOCIETY SEVERNSIDE BRANCH In this issue: Branch Organiser’s Report Ireland—Dingle & Portmagee 2 Hello Severnside Cwm Cerrig-gleisiad 11 We have had a good start to the year with some very successful trips. We looked at Brecon Beacons the remnants of glaciers in Cwm Cerrig Gleisiad with Duncan Hawley. It was a steep climb to the rim of the back wall and even steeper on the way back down but well Henllys Vale 12 worth the effort. We also explored Henllys Vale with Adrian Humpage where the day took 2 different styles. In the morning we followed the river in the valley and looked Branch Officer’s 13 at remnants of industrial heritage, all based around the local geology of iron, coal Meeting and lime stone. In the afternoon we climbed up to the hills above and followed the old tram way to look at the old quarries in the limestone. Although the day of our Events 14 visit was fine, the effects of recent rain on the terrain was obvious and made the going very boggy in places. Brilliant views though. Contacts 16 The next day trip is on 11th September when we are visiting the Abberley Hills with Branch Committee 17 Paul Olver. Here he will be showing us a variety of rocks ranging through pre Cambrian basement, Cambrian quartzites, Silurian, Carboniferous and Triassic exposures! Quite a variety. Jan is organising some longer trips as well. In the spring 2012 a trip is going to Fuerteventura and Lanzarote and in September 2012 Brian Williams is leading a trip for us to Canada. More details are available on the website. The Annual Society’s Symposium is taking place in Stirling this year. The theme is ‘North of the Suture’ and as usual there is a interesting programme of lectures plus field trips to look forward to. I think many delegates are planning to extend their visit and travel on to other parts of Scotland. As happens every so often, the traditional pattern of the Symposium is being considered to see if it still offers what members want. Traditionally, all events are charged for as part of the weekend package (except possibly any field trips on the Monday). Should this change? Should the field trip be separated from the lecture programme so that delegates have a choice about which they attend and pay for? Should there not be any field trips at all? If you have any views or ideas on this, I would be pleased to receive them to feed back to the various national committees who guide organisers on these matters. Your views are important because for these events to continue to run successfully they need to offer what members want. I believe we are still looking for a National Information Officer. If you think you could do this, please let me know. We have several new books donated to the library by Philip Clarke. These include a Geol. Soc. publication on James Hutton, lecture notes on ‘Samuel Pepys at the Rhenish wine-house’, and a geological map and explanatory book on the Looking for striations Volcanology of the Chaine des Puys. All of them are available should you wish to at Cwm Cerrig –gleisiad borrow them. (See Page 11) Photo: Philip Clark Best wishes Janet D3 1 Ireland - Dingle and Portmagee April - May 2011 OUGS Severnside Field Trip to South West Ireland 2011. The aim of the trip was to study lower Palaeozoic rocks seen at outcrop on the Dingle Peninsula and on Valentia Island at the west of the Iveragh Peninsula. The group first stayed in Dingle, then moved to Portmagee. The Dingle Peninsula is the northernmost of three peninsulas, which effectively make up County Kerry. The peninsula is a mountainous finger of land about 40Km long with a maximum width of 24Km. The rocks preserve a unique Ordovician to Carboniferous succession including the most complete Old Red Sandstone magnafacies in SW Ireland. The oldest part of the ORS was deposited in Dingle Basin which formed in a transgressive regime during the convergence of Avalonia and Laurentia. Younger ORS sequences were deposited in basins on the northern margin of the Middle to Late Devonian Munster Basin of South Kerry and Cork. Several researchers have studied the geology of the area since the 1970s, but their findings remain controversial. The visit was intended to view a selection of outcrops but not to form definite conclusions about the geological history. 2 Ireland - Dingle and Portmagee April - May 2011 (contd.) Thursday 28th April 2011 The Silurian was visited at the Dunquin Inlier in Ferriter Bay where the group spent time examining the outcrop of sub vertical beds on the beach. The rocks form part of the Dunquin Group which outcrops as a series of inliers along the Dingle Peninsula and in the nearby Blasket Islands. These rocks represent parasequences which are stacked offshore to nearshore regressive phases. 5 repetitions are seen over 250 metres of section. Two members were seen. An older, resistant muddy limestone, with the tabulate coral favosites, trilobites and graptolites suggest formation in an open marine environment, possibly close to shore in a shallow marine shelf. The other different lithology had cross bedded fine sandstone interbedded with coarse grained cross bedded sandstone that View from the north side of Clogher Head towards contained channels and syn-sedimentary deformation. This high energy Clogher Strand and Silurian rocks of the Dunquin environment was free of fossils and some of the coarse beds contained Group mm scale volcanic clasts. The sequence was topped by a 2metre thick bed of pale felsic volcanic deposit. The pumice in coarse sandstone suggests a nearby volcano with intermittent activity which was a precursor for a big eruption that produced the felsic unit The sequence was overlain by a pinstripe laminated siltstone, dominantly parallel bedded, with desiccation cracks but no burrows. This unit is a tidalite and followed by sandstone with coral fossil cast. There are two contrasting theories for the deposition environment for these units: 1. A lagoon with abnormal saline conditions. 2. Open marine with sea level changed riven by a magma chamber inflating and deflating. Our visit to the Silurian Dunquin Group continued in the afternoon at the east side of Clogher Head at the boundary between fossiliferous sandstone of the Ferriter formation and the Lower Clogher Head volcanic formation. We walked south along the clifftop to study several locations. The boundary between the formations is a dextral strike-slip fault with about 300 metres displacement. The volcanics form a unit about 200 m thick, though it is about 900 metres thick at the eruption centre. The Silurian sub vertical beds of the Dunquin inlier northern part of the volcanic deposits consists of ignimbrites. To the south there are lavas. This suggests a Plinian eruption that delivered several layers of material containing welded tuff, fiamme, pumice and ash. These rocks were examined. Trilobite fossil cast in siltstone 3 Ireland - Dingle and Portmagee April - May 2011 (contd.) Thursday 28th April 2011 (contd.) The Group on the Lower Clogher Head volcanic formation This inlet was formed by erosion of a palaeosol after volcan- ism had stopped. Soil formed on top of the volcanic rocks and was itself overlain by sediments. The rocks were turned during mountain building and the soft palaeosol was exposed and eroded. Further along the cliff we saw the Millcove formation which contains distinctive red sandstone about 1.5metres thick. Research has defined this as a groundwater ferricrete with the red colour controlled by the oxidation state of iron. It was possibly formed in an alluvial plane that underwent cyclical drying. The sandstone was deposited during seasonal sheet flow on floodplains. Also seen within the sandstone are signs of volcanic activity in the form of volcanic bombs - mafic material up to 80 cm diameter. The top of the Millcove formation was seen at Drom Point and is a major transgressive surface.This unit was best seen on the beach at Clogher Strand where the marine deposits show hummocky cross stratification indicative of storm conditions, followed upwards by finer sediments with extensive trace fossils Chondrites. Shelly fossils broken in the storms have accumulated in decalcified lenses. The deposition environment was a shore face which is shallow marine typical of the Silurian. Report and Photographs by Phillip Robinson The trace fossil Chondrites seen at Clogher Strand in the Drom Point formation 4 Ireland - Dingle and Portmagee April - May 2011 (contd.) Friday 29 April 2011 Today’s objective was to examine the oldest rocks exposed on the Dingle Peninsula before studying the Lower Devonian rocks of the Dingle Group. First, we drove east of Dingle to Bealacoon Cove in Minard Bay to examine two members of the Annascaul Formation exposed in the Annascaul lnlier. These rocks are the oldest in the Dingle Peninsula and have been dated, using microfossil marine phytoplanktonic cysts called acritarchs, to the early Ordovician Period. The Illaunglass Member is exposed on the western side of the cove and consists of highly deformed, thinly bedded sandstones and laminated grey/green siltstones. Parts of the exposure are red and purple in colour from haematisation but this is secondary in nature as the colour often changes to green shales along the direction of strike. The sandstones are mainly fine-grained and show tight isoclinal folds, often with two cleavages (Fig.1). This deformation occurred early in the Caledonian deformation event. Laboratory analysis of the acritarchs samples taken from the site show they come in all shapes, including square, knobbly and spine, and vary in size between 30 — 40 microns.

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