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Ercall TVGS Field Trip Saturday 18Th October 2014

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Ercall TVGS Field Trip Saturday 18Th October 2014 Ercall TVGS Field Trip Saturday 18th October 2014 Overview of field trip route 1 Met with trip leader Alan Bates at the Forest Glen quarry. Alan asked if we were out of breath as there is only around 7% oxygen in the air – or at least that is what it would be if we were actually back in the Ediacaran (very top of the Precambrian – named after Ediacara Hills in Australia). The 60m rock face in front of us is a pile of volcanic rocks – ash and lava deposits – ejected by very explosive volcanoes associated with an active plate subduction zone. In the late Ediacaran (566Ma +/-1Ma) this place was around 60’ south of the equator and on the northern tectonically active margin of Gondwana. The quarry displays 3 distinct nearly vertical ridges of rock that the quarry operators left. Alan asked why they were left and what they may be made of? Alan explained these are dykes of dolerite intruded into the earlier volcanic rocks. The dykes can be traced into the adjacent Wrekin, which is also formed Ediacaran (Uriconian) volcanic rocks – Rhyolite, high silica content rock that is associated with explosive volcanic activity due to its high viscosity and entrapped gasses. Alan said think of Mount St Helens type eruptions, only on a much larger scale! Forest Glen Quarry – with 2 of the 3 dolerite dykes visible 2 The party then travelled a short distance to a parking area adjacent to the Buckatree Hall Hotel, from where a path leads into the Eracall quarries, all of which are SSSI’s, maintained by the Shropshire Wildlife Trust. The first of these quarries – known simply as ‘Quarry 1’ has a 10m high face of rhyolite, which has light granite like appearance. Examining pieces of scree on the western side of the quarry, you could see a distinct banded appearance. banded rhyolite from Quarry 1 3 Alan explained the banding arose from a stirring action in the very viscous melt – like the banded colour effect you get when you knead two differently coloured pieces of play doe. Moving around to the centre of Quarry 1, the scree is full of small globules or pieces of rock with globules (spherulites) weathering out. Alan explained these globules (some of which can be several centimetres across) are formed from rapidly cooling lava ejected in a nuée ardente type ash flow explosion – nasty stuff! 4 A small dyke at the eastern edge of Quarry 1 was examined, followed by a brief visit to Quarry 4, which although large no longer readily exposes any rock and largely repeats what we had already seen. Walking a short distance east along the valley, the trees suddenly open-out to give a striking view of a large quarry with bright pink/orange coloured rock and a distinct set of beds dipping off the top right edge of the face. This is Quarry 2 and site of the internationally famous Ercall unconformity (strictly a nonconformity) between the Ediacaran igneous rocks and sedimentary rocks at the base of the Cambrian. Quarry 2 with the nonconformity 5 Before climbing to the quarry face, Alan drew out attention to an outcrop, which exposed an almost pure white highly polished quartzite. The surface showed grooves – slickensides – produced along the surface of a strike-slip fault. From feeling the direction of maximum smoothness (towards the west in this case) one can determine the direction of movement on the fault. This fault is a one of a set of strike-slip faults running across the Ercall ridge. Left-lateral strike-slip fault 6 Getting up close to the Quarry 2 exposure, Alan explained the pink-orange rock was a granophyre and was named here the Ercall Granophyre. The unusually bright colour of the rock and its tendency to crumble to gravel size pieces had made it popular as an ornamental surface dressing for the driveways of big estate houses, there is also a reference to the Romans using it to surface the ancient trackway of Watling Street. A granophyre is composed of quartz and alkali feldspar with characteristic angular intergrowths (visible under microscope). The Ercall Granophyre has been intruded into the earlier rhyolites and has been dated at around 560Ma (+/-1Ma), i.e. around 6Ma after the volcanic activity that formed the rhyolites as seen in Forest Glen and Quarry 1. Resting upon the Ercall Granophyre at an angle of around 45’ (dip to SE) are the beds of the Cambrian Wrekin Quartzite Formation, the lowest beds of which are the Wrekin Conglomerate. This major unconformity (nonconformity – as it is sedimentary rocks upon igneous) was once taken as the international boundary of the Precambrian and Cambrian. That honour has now moved to a site at Fortune Head SE Newfoundland. The Ercall Granophyre below the nonconformity shows a white appearance, due to weathering to kaolin. This suggests this surface was exposed to aerial weathering, before burial by the sediments of the Cambrian seas. 7 8 Just around the corner into Quarry 3, your eyes fall upon the delight of a Cambrian beach, as implied by the bedding plane surface of distinct symmetrical ripples. Symmetrical ripples form from the back and forth movement of the water in an inter-tidal environment. Alan explained a good way to determine the profile of ripples is to use the kind of comb like profile gauge you can use when measuring to cut tiles around irregular shapes. Cambrian beach – symmetrical ripples 9 Walking through Quarry 3 parallel to the 45’ SE dipping beds of Wrekin Quartzite Formation, the beds show a change from inter-tidal (beach) conditions through progressively deepening water – a marine transgression over the Ediacaren igneous landmass. The top of the Wrekin Quartzite Formation is marked by a striking change from the massively bedded hard sandstones, to soft dark green (glauconite) silty sediments – the Lower Comley Sandstone Formation – which marks a further deepening of the Cambrian seas. 10 Alan explained that the interface of the Upper and Lower Comley Sandstones includes a thin limestone, which contains a rich internationally renowned phosphatised fossil bed – trilobites. With matching types in Eastern Newfoundland, but not Scotland, which is in keeping with the relative close proximity of Newfoundland at this time as part of Avalonia and Scotland on the far side of the Iapetus Ocean. On leaving Quarry 3 you used to be able to see another slice of Cambrian beach on the far western side of Quarry 2, but this is now obscured by beech trees. The interesting point is why, what is presumably part of the same rippled marked bed, is suddenly displaced some 100m west? The explanation lies with the slip-strike faulting here, as described earlier (see sketch map). Although I have often said ‘Alan explained … ‘ , the explanations came out of us doing our own observing and questioning, then discussing as a group with Alan what might be occurring. No member of the party had visited the Ercall before and had resisted forming any pre-visit view by googling/consulting resources, so all came with a largely ‘blank canvas’ for the geology of the area. That brought to close a very enjoyable field-trip, with a small but enthusiastic group, an excellent leader and exceptionally warm (for mid-October) largely sunny weather. 11 Ercall seen from the Wrekin (Hell Gate) looking north. 12 Sketch map based on data from: BGS W-50 web service and Pringle K & Stimpson I (2013), The Precambrian-Cambrian nonconformity at the Ercall Quarries, The Wrekiin Shropshire UK, Geology Today Vol. 29 No. 5 13 .
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