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OGU fieldtrip to Middle Hope

19 August 2013

OGU fieldtrip to Carboniferous volcanics of Middle Hope, north Somerset Coast.

Sunday 18th August saw the OGU decamp from the lab and pop down to the north coast of Somerset for a geology field trip. The countryside surrounding Bristol has some truly excellent geology, and the coastline along the Bristol Channel gives great opportunity for some lovely outcrops by the sea side. This weekend the geologists, biogeochemists and archeologists of the OGU alike took the opportunity of a low tide and a (mostly!) sunny day to explore a particularly special part of the geological history of north Somerset.

The outcrop in question was the cliffs and foreshore of Middle Hope (at ST 324661), a promontory just to the north of Weston-super-Mare, within easy drive of Bristol. Here outcrops the grey Carboniferous limestone which is familiar to many, which also makes up the walls of the Avon gorge, across which spans Bristol’s famous Clifton Suspension bridge.  However, here at Middle Hope the Black Rock Limestone includes something rather special. The Black Rock Limestone represents the remains of a peaceful, coral limestone platform, where colonial syringopora and solitary zaphrentoid and caninioid corals, choenitid brachiopods, and crinoids made their home ~ 350 million years ago in a warm, shallow sea (Faulkner, 1989).

Middle Hope 4

Photo credit: M Badger

However as you follow the rocks from the sea towards the cliffs at Middle Hope evidence of greater drama and turbulence starts to appear. The gentle carbonate platform is interrupted by tuffs – preserved layers of ash which give witness to a nearby volcano raining down ash on the site. This volcanic activity increases up section, with increasing lapilli suggesting increasingly violence until the limestone is ploughed into by massive pillow lavas – evidence of subsea volcanic activity very close by spewing lava into the carbonate platform. The pillow forms as molten rock hits the sea water, abruptly quenching the surface to from a basaltic glass which coats the outside of these lobe like, pillow shaped forms. The lava here forms a bed several metres thick which forms a prominent, upstanding bed jutting into the beach, weathering proud of the tuffs and limestones either side. It has many amygdales – small (here calcite) infills of the bubbles of exsolved gasses from the basalt, and where these outcrop on the beach they have been weathered back to bubble like hollows in the rock. Above the lava, calm returns and the limestones of the cliff itself demonstrate how quickly the peace and calm returned after the cessation of volcanic activity.

Middle Hope 5

Photo credit: M Badger

This sequence is rare in the UK, and the unusual extrustion of lava into a carbonate platform has given this location well deserved Special Site of Scientific Interest (SSSI) classification, so we controlled ourselves and didn’t hammer at all. The group was perfectly content to admire the fossiliferous limestones and amygdaloidal limestone in a lovely summers day before returning to the car park for a well-deserved ice cream

Further reading:

Morgan CL and Reynolds SH (1904)The Ignous Rocks associated with the Carboniferous Limestone of the Bristol District. Quarterly Journal of the Geological Society, 60, p 137-157 doi:10.1144/GSL.JGS.1904.060.01-04.14

Speedyman DL, Volcanic rocks of the Bristol region, in Geological Excursions in the Bristol District. Savage RJG (1977). University of Bristol.  

Faulkner TJ (1989) The early Carboniferous (Courceyan) Middle Hope volcanics of Weston-super-Mare: development and demise of an offshore volcanic high. Proc. Geol. Ass., 100(1), 93-106.

http://avonrigsoutcrop.blogspot.co.uk/2013/03/rigs-of-month-march-middle-hope.html

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Article written by Dr. Marcus Badger, a PDRA in the OGU working with Prof. Rich Pancost.

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