Manchester Building Stones
The North West Geologist
|A Building Stones Guide to Central Manchester|
Third Edition (2014)
Four self-guided walks through the city centre
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Newsletter - September 2017
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Text extracts are given below.
The aim of the trip was to understand the dolomitisation and sedimentation of the limestones on the Great Orme. The lowest unit to be looked at was the Llandudno Pier Dolomite, which is the first unit above the Lower Paleozoic unconformity. Above this are the Tollhouse Mudstones, then the Great Orme Limestone, which are followed by the patchy Craig Croft Sandstones then Bishops Quarry Limestone and finally the Summit Limestone.
These rocks were deposited in a shallow marine setting on the footwall of an extensional fault, within a back-arc extensional regime, to the north of the Wales-Brabant Massif. The shore line of the North Wales platform was approximately where the present east Llandudno shore runs from the Great Orme to the Little Orme; in a Mediterranean type enclosed basin. The depth of the basin was controlled by a series of faults that were periodically reactivated. Ice caps controlled the sea level. Limestone is only deposited in warm (about 20°C), clear water (no clastic inputs) within the photic zone of a relatively calm environment. If sea level rises the limestone beds develop upwards then outwards, but when sea level falls a karst surface will develop. The bed thickness depends on the rate of sea level change; thick beds can form during times of rapid sea level rise when the polar ice is melting quickly and thin beds form when sea level rises slowly. After these rocks were deposited they were buried to a depth of about one kilometre during the Carboniferous period and re-surfaced in the Permian only to sink again to between two and three kilometres in the Jurassic, finally emerging to their present position in the Tertiary (the burial history is recorded in the cements because those precipitated from meteoric water have a different chemical composition to cements from sea water).
Dolomite is a calcium magnesium carbonate CaMg (CO3)2 in which alternate calcium ions are replaced by magnesium ions. It is not fully understood how it forms (see end note), but as long as there is a supply of seawater, or other fluid containing magnesium (may have been scavenged from basin sediments) dolomite can form. Seawater is likely to be the source down to depths of one kilometre; as it flows through faults and sediments. It can form soon after the limestone has been deposited or when deeply buried. Dolomite is about 50% calcite and 50% magnesium, which is part of the continuous solution series calcite-magnesium.
Pier Dolomite is a crinoidal packstone-grainstone, light orange/brown in colour with sparkly dolomite and thin interbedded shale and mud; it was deposited on the shelf edge. The fabric of the original limestone is preserved in places. The dolomitisation only affects some of the limestone before randomly petering out. There is no obvious reason for this, but it could be prevented from forming by a decrease in the supply of Mg, or seawater, or a change in grain size; fine grained beds often dolomitise more easily that coarse grained ones and coarse grained beds allow the fluids to flow more readily, also stylolites and faults can disrupt the flow of the fluids. The Zebra dolomite, which overprint the replacive dolomite, form under high pressure and show that these rocks have been deeply buried. Veins of calcite and lead were abundant.
Tollhouse Mudstones are thinly bedded, crumbly, grey marls that contain very little clay. Sedimentation rates were low and deposition was in a low energy, near shore environment there are also burrows and odd shell fragments. There were several shallowing and deepening episodes as shown by the interbedded limestones. The mudstone is important because there is very little dolomitisation above it. Therefore this layer must have prevented the magnesium rich fluids flowing higher up the succession, except where a later fault has allowed the fluid through. The dolomitised area around these faults is limited to a few tens of millimetres.
The Great Orme Limestone developed during a period when the ice caps were waxing and waning; ten cycles have been identified. When sea level was low a soil or proto-soil horizon surface of each cycle. The proto-soil horizons are orange and the more developed soils tend to be pink to grey, some are lithified; burrows can be seen in the overlaying limestone. The soil horizons can be very small pockets (tens of millimetres thick and half a metre wide) or quite extensive. Above the soil horizons, but not related to them, a fault plain surface could be seen with large chunks of fault breccia to the right, where the fault moved against the stable bedding. There are also some very good examples of syn-sedimentary faulting.
Farther along the road we came to a break in the shelf deposit where wave energy was relatively high as could be seen in the thick beds that have large foresets dipping to the North. Here there are some massive burrows, 50 mm or more in diameter, that are associated with the muddy horizons that were probably made by some type of crustaceans (possibly shrimps). Some of the beds are finely laminated and are of clean fine grains and no burrows so high energy deposition. The fractures here are post compaction because they cross the stylolites.
Craig Croft Sandstone was not seen, but it is a thin, patchy, deltaic, sand deposit that forms the top of the Asbian.
After a very steep climb up a footpath we stopped for a well earned lunch break.
The Bishops Quarry Limestone is immediately above the Craig Croft Sandstone and the first unit of the Brigantian. The environment was beginning to change to deeper water with an influx of clay, which reduced light levels allowing the more tolerant brachiopods and sponges to flourish. The sponge spicules are opaline (hydrated silica) that dissolves and then moves through the sediment to fill the burrows; in the process it changes to chert, which prevents later compression of the beds.
The Summit Limestone in Bishops Quarry has a lot of Gigantoproductids in life positions, concave up. Gigantoproductid brachiopod fauna form death assemblages at the base of the limestone beds. Most were the ordinary none spiny type, but we did find a few with spines. This was the end of the limestone deposition because the water was deepening at the beginning of the first episode of burial; it was also the end of our trip.
I was confused about how well the dolomitisation process was understood; this was Cathy's reply: we know that it can form from seawater under certain conditions and we can precipitate it in the lab at high temperature, but it is not observed in nature forming at the surface, even though geologically we know that this can happen. Sorry if this sounds confusing, but the bottom line is that it is a bit of an enigma!
I would like to thank Dr Cathy Hollis, for a very interesting and informative day and also for checking the facts in my report.
New Natural History Museum Fossil Explorer mobile app
The Natural History Museum in London has just released a new version of the free Fossil Explorer app, a field guide to the common fossils of Britain that helps identify fossils based on where they are found.
Whether seeking ammonites in Lyme Regis, marine reptiles in Whitby or trilobites in Girvan the beginner and more experienced collectors alike can learn about their finds and what else may be beneath their feet.
Through Fossil Explorer users have access to the combined expertise of the Natural History Museum and the British Geological Survey. It is based on information from the Museum's popular British Fossils book series; the app offers details about more than 1,200 fossil taxa as well as local geology.
Thanks to an interactive geological map, Fossil Explorer suggests likely fossil matches based on where they are found, giving a list of fossils known to occur in rocks of the same age. Introductory facts and illustrations help beginner fossil hunters get started while additional information enables more experienced fossil collectors to delve deeper.
New functionality in this release allows users to create and share lists of fossils they have found and set wish lists for future discovery.
We hope the app will encourage budding citizen scientists around the country, inspire a new generation of explorers and get people thinking differently about the natural world.
The app is available for iOS and Android. More information and download links are available on the Museum's website.