St Austell Bay Geology – 'Duporthite' EarthCache
St Austell Bay Geology – 'Duporthite'
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This tide-dependent EarthCache is located on a publically accessible beach at Duporth. Access to the beach is via the South West Coast Path - see additional waypoints. To avoid being cut off by the tide, or not being able to access GZ, you should plan to visit no more than 1 hour either side of Low Tide. Tide times can be found here
The outcrop that you are looking for
Geology
The large outcrop of rock visible at GZ is an altered basic igneous rock, known locally as an elvan or greenstone. Originally intruded into the surrounding rocks as a dolerite sill, it has been chemically altered and weathered into the rock that you see today.
Most of the rock types exposed in St Austell Bay (including the ones that you passed to get to GZ) are metamorphosed sediments known as the Meadfoot Group and are named after Meadfoot Beach in Torquay, where the same rock types were first described in detail.
The Meadfoot Group is further sub-divided into 2 other Formations and the rocks in this bay belong to the Bovisand Formation. Comprising layers of mudstones, siltstones, some volcanics and limestone, these rocks were created under relatively shallow marine conditions around 400 million years ago during the Devonian Period. The layers contain lots of finely divided pyrite (iron sulphide) which gives them their dark-coloured appearance and also suggests that they were laid down under fairly anoxic (low oxygen) conditions. Following their deposition, the sediments were compressed, dewatered and subjected to a process called diagenesis which converted them to hard rock. Subsequent major Earth movements throughout Devonian and Carboniferous times affected much of SW England and caused the rocks in St Austell Bay to become folded, faulted and metamorphosed into rough heavily-fractured slates and sandstones. It is probably during this time that the dolerite intrusion that is the subject of this EarthCache was also emplaced. Later deformation had a big influence on the variety of coastal features that eventually formed and the style of erosion we can see happening today.
The altered intrusion was first described in 1877 by the great Geologist and Mining Engineer Joseph Henry Collins in the inaugural volume of Mineralogical Magazine. Collins thought that he’d discovered a new mineral growing in the veins that dissect the intrusion (see picture below) and proposed the name 'Duporthite' on account of it being first described here at Duporth.
White 'Duporthite' filling a tension gash
However, more recent analysis (using X-Ray Diffraction) has found that the material identified as 'Duporthite' is actually just a weathered mixture of two other minerals - talc and chlorite inter-growing. Collins could not have known this as the two minerals are incredibly fine and difficult to distinguish even with a powerful Scanning Electron Microscope as can be seen below:
'Duporthite' viewed under a Scanning Electron Microscope
Post-emplacement Alteration
As already mentioned the intrusion has been heavily altered by the action of hydrothermal fluids and faulted by major Earth movements. Together, these have significantly changed the nature of the rock and given rise to the following interesting features:
Pseudomorphs
The outline of many original plagioclase feldspar and pyroxene crystals can be seen, but the minerals themselves have been altered and changed. We call these spotty-looking blotches in the rock pseudomorphs. Crystals that were originally pyroxene and feldspar have been changed into tremolite, talc, chlorite and dolomite. The pale-coloured talc is easy to identify as its softness (just 1 on Mohs Scale of Hardness) gives the rock a soapy or greasy feel when you touch it with your fingers.
Mineral Veins
Although the white-coloured veins containing 'Duporthite' (weathered chlorite and talc) are the main reason for interest in this outcrop, there are also lots of other mineral-filled veins too. The green chlorite is a hydrous silicate of aluminium, iron and magnesium. It is relatively soft (1.5-2.5 on Mohs Scale of Hardness), and can be scratched by a finger nail. The brown-coloured veins are composed of siderite (iron carbonate) and other secondary iron oxides. They are easy to identify by their rusty brown colour.
Faulting
The intrusion has a distinctly dissected appearance, having been sheared and faulted by major Earth movements. Many of the fractures created by these Earth movements have been filled with minerals as described above. Geologists describe these mineral-filled veins as tension gashes and if a repeated set of them occur with a similar orientation, they are said to be en échelon. Such en échelon tension gashes filled with quartz are common in the metamorphosed sediments here in St Austell Bay.
In order to claim this EarthCache, go to the published coordinates - You should be standing in front of the large outcrop on the beach. N.B., You do not need to climb on or around the outcrop.
Please message or email me your answers to the following questions through my profile at the top of this page or via this link:-
1) Find a fresh exposure of rock – The rock at the base of the outcrop that is usually covered at high tide is a good place to look. Observe and describe the dominant characteristics - What colour is it, what does it feel like and can you see any crystals – If so, what colour and size are they and what might they be?
2) Find a weathered exposure of rock – There is lots of it above the high water mark. Observe and describe the dominant characteristics – How, if at all, does it differ from the fresh rock? HINT – Suggest what the mineral pseudomorphs look like, how the weathered rock feels and why this might be.
3) Find a mineral-filled vein. Describe the mineral(s) that you can see and estimate the width of the vein. Suggest which mineral(s) the vein is made from and give some reasoning for your choice. HINT – Use the colours and hardness given in the description to try to identify the mineral(s).
Feel free to log your find without having to wait for confirmation from me. However, if your answers are not correct or incomplete etc., I will contact you to verify your find.
Earthcache guidelines now make posting a photograph optional, but feel free to upload pics with your log if you wish.
Thanks for taking the time to visit this Earthcache and I hope you enjoy your visit.
Additional Hints
(No hints available.)