Field Trip Report the Tortworth Inlier Sunday 26Th April 2015. Dave

Field Trip Report The Tortworth Inlier Sunday 26th April 2015. Dave Green, our group leader, showed us some of the fascinating geology, to the north-east of the city of Bristol. This area is relatively flat but has some undulating topography as it runs into the Vale of Berkeley. It includes the northern part of the once busy Bristol Coalfield. This part of South Gloucestershire is largely rural, dotted with farms, small villages and little townships. It is enclosed by the Mendip Hills to the south, the Cotswold Escarpment to the north and to the east, and the Severn estuary to the west. The area is drained by small streams and rivers like the Frome, whose waters skirt the contours and tumble into the Severn Estuary. Palaeozoic rocks can be found exposed here, while to the east and north east younger Mesozoic rocks overlie them. The geology of South Gloucestershire at first blush appears complex and takes a little unravelling. To begin with; some terminology An inlier is an island of older rock strata, surrounded by younger strata. The Tortworth Inlier comprises Cambrian/Silurian outcrops surrounded by Mesozoic rocks located around Charfield Green, just south of the Little Avon river. It also lies North East of the Carboniferous Coalpit Heath syncline once mined for the bituminous coals it contains. The rock strata we hoped to see Cambrian rocks The oldest strata are the Micklewood beds of upper Cambrian/Lower Ordivician and Tremadoc in age. These are grey micaceous shales with flaggy sandstone or siltstones inbetween. They lie in the southern third of the Tortworth inlier. The underlying and older Breadstone shales make up two thirds of the inlier. These are thin interbedded siltstones a few centimetres thick. Their fauna is typical of that found in the lower Tremadoc series. An Ordovician unconformity There is a notable absence of any later Ordovician rocks in this Palaeozoic succession. Yet to the north in Shropshire and Wales are found the graptolite bearing black shales that characterise much of the Ordovician. Silurian The younger Silurian rock strata are exposed in an orderly north west, south easterly direction. The first of these lies uncomformably upon the Cambrian rocks and are volcanic in origin. These are the so called Lower Trap. (Lower Trap meaning ‘lower step’ from the Sanskrit word for step). These rocks were at one time thought to be intrusive in origin but now considered to be extrusive and comprise of very altered amygdaloidal basalts. Above the Lower Trap are found the Damery Beds made up of thin bedded sandstone, siltstone and mudstone and the occasional layers of impure limestone. They range in colour from greys to greens to reds and purples. These beds contain fossils that are Silurian in age. These in turn are overlain by the Upper Trap a much altered micro-crystalline basalt with quartz xenocrysts visible under a hand lens. These quartz xenoliths distinguish this lava from the lava of the Lower Trap. Again this basalt is much altered. The rock weathers to a distinctive burnt sienna colour again with numerous voids. The voids are distorted gas vesicles indicative of degassing of a cooling, yet mobile magma that flowed and covered the land surface. The Upper Trap is overlain in turn by the Tortworth beds, similar lithologically to the Damery Beds but they are much less fossiliferous. At the base of the Tortworth beds is the Palaeocyclus band, a thin decalcified sandstone containing fossils of button corals. Above these are found the younger Brinkmarsh Beds. Wenlock in age, they are mainly grey mudstone with inter bedded thin bands of siltstone and fine grained sandstone. The Brinkmarsh beds are overlain by the Ludlow series with three distinctive mudstone beds. Devonian The Old Red Sandstone (ORS), the hallmark of the Devonian period, is present but the entire series is incomplete. In this locale, it is divided into the Lower and Upper Sandstone with a marked unconformity between them. They outcrop around the edge of the Coalpit Heath Syncline. Carboniferous The Devonian ORS is inturn overlain by the Lower Carboniferous strata. The carboniferous limestone forms the most extensive outcrop of all Palaeozoic rock in the area. Today it is quarried extensively and is used for road stone and as a construction aggregate. In the past limestone was also heavily quarried for use locally. Carboniferous rocks also include quartzitic sandstones and the coal measures. The Pennant Sandstones were used locally for building and dry stone walling. Coal deposits were mined in several localities and coal mining peaked in the 1870-90s. Mining now has ceased due to the exhaustion of workable seams and increasing difficulties of the geological conditions. Permo-Triassic Rock interface marks a major unconformity. Upper Triassic The Triassic rocks are mostly sandstones, mudstones and conglomerates which lie unconformably on deeply folded Permian beds but in this area they lie directly on top of Carboniferous strata. An enthusiastic group, gathering at the start of the trip. A joyous Heidi, pleased that the sun is shining! The Field Trip 1. Tytherington Hill Quarry, Scarp and Coalpit Heath Syncline. To begin we all met at Tytherington Hill ST672886. We parked at the side of Baden Hill road and as a group we proceeded to examine the exposed surface of the western wall of an abandoned limestone quarry. The angles of dip on the exposed quarry slab face are steep, dipping at some 55 degrees towards the East. This is part of the western arm of the syncline of Carboniferous limestone rocks. Carboniferous fossil remains of broken crinoids, bits of corals and broken shell fragments (brachiopods) were encased in a hard limestone matrix. The shell detritus indicated deposition had taken place within a shallow mobile marine environment subject to the action of waves. An exposure of crinoids in carboniferous limestone We then climbed to the top of the scarp of this old quarry by a steep flight of steps to the left of the quarry. The scarp runs in a NNW direction with the road running below it. Facing East we could see a series of low ridges running parallel to the ridge line we were standing on. These are the more durable outcrops of Carboniferous limestone rock that mark out the Western limb of the Coalpit Heath syncline. The syncline continues over the horizon and we would visit the Eastern part of the syncline later in the day. We were to find that the Eastern parts of the syncline had much shallower angles of dip. The Western limestone ridge on which we were standing runs from Almondsbury though to Thornbury and onwards in a NNE direction. Whilst on the Eastern side the eastern Limestone ridge runs from Chipping Sodbury to Cromhall village in a NNW direction. At the northern end these limestone ridges mark out a horse shoe shaped perimeter of the basin. The limestone in this area has been quarried extensively and continues to be worked today by Hanson, part of the Heidelberg cement group. In the past Hotwells limestone was quarried locally for building the grey limestone walls used to enclose fields and to build houses. It was also used for road making and burnt in lime kilns to make quick lime (Calcium oxide), an ingredient of lime mortar. Quick lime when mixed with water reacts exothermically to make slaked lime. Slaked lime was used to whitewash walls and can also be used to adjust the pH of acidic soils. The divalent Calcium cation in lime has the ability to cross link negatively charged clay particles and to improve the crumb structure of the clinging clays of many local fields. 2. Brinkmarsh Quarry Our next port of call was to Brinkmarsh Quarry. We parked in a lane near the A38 and walked to the quarry at ST675914. The quarry is edged by a line of trees and is hidden from the road by a low hill. In the quarry the Wenlock shales are overlain by limestone beds as part of an small anticline. The limestone has been quarried and removed leaving a muddy reddish brown clay bottom that in wet weather is heavy enough to impede walking. The intrepid group, traversing a muddy field towards the Brinkmarsh quarry. An exposure of crinoids in carboniferous limestone. The Brinkmarsh shale beds are exposed near to the entrance to the quarry. The eagle eyed observers in our party soon began to find fossils. The Silurian rugose coral Pycnastis was abundant and rapidly we had found several samples as well as the fossil remains of several small bivalves. Fossils found in the pycnactis band. A collection of bivalves and Silurian rugose corals! Dave Green pointed out a nodule of Celestine lying above the flaking impervious shales. Celestine is a mineral of Strontium Sulphate found in Yate, Aust and Clifton. The concentration of the element Strontium is low in sea water today but can be much higher in hydrothermal deposits. Locally Celestine is associated with the Mercia Mudstones, which are Triassic in age. Celestine is thought to have been deposited along with other paragenic evaporites like gypsum (Calcium Sulfate) and halite (Sodium Chloride), in land locked seas or lakes. The original concentration of Strontium may have taken place in the aragonite shells of molluscs. Aragonite can contain 8000ppm of Strontium as opposed to calcite which contains only 400ppm. Over time the aragonite changes to its more stable polymorph form, calcite. The change releases Strontium metal ions in the process. The Strontium metal ion is more reactive than the lighter alkaline earth (Group 2) metals Beryllium, Magnesium and Calcium, and which in aqueous conditions Strontium is able to displace them to form Strontium salts.

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