Portishead Old Red Sandstone and Carboniferous

There was actually blue sky this morning, and even though it was a cold wind and clouds kept threatening to the south, on Portishead foreshore it stayed dry even if a bit bracing. 23 takers which was a good number - enough but not too many for comfort - and quite a few people who were on their first field trip, which was the idea of today's outing.


We met up at 10.30 on the Esplanade by the Lake Grounds and after some preliminary explanations about the regional geology set off SW on the path between the cliff and the saltings to have a look at the Old Red Sandstone exposures there.

The series here is at the northern end of a pericline (the Portishead anticline) and on its southern limb which means that we got a good section through a number of beds of Upper ORS age down through to the Lower ORS Nore Sandstone. There's a variety of lithologies ranging from silts through sandstone to conglomerate and the Nore Sandstone has some calcrete horizons at the top. Triassic Dolomitic Conglomerate, an immature sub-aerial fan breccia, has been deposited unconformably on the erosion surface along the front of the cliff , in one place, against a fault scarp and in another showing a classic exposure of an angular unconformity.
The ORS also includes a conglomerate bed - the polymict Woodhill Conglomerate which derives clasts possibly from the Mona complex on Anglesey and much better sorted and rounded clasts. It was interesting to compare this with the Dolomitic Conglomerate - they are very different!

The ORS was deposited on a broad coastal plain on the southern edge of the Old Red Sandstone continent: the beds are fluvial deposits and showed cross stratification, channeling and evidence of overbank deposits.

At the end of the section we went up the steps onto the cliff path and back to the Lake Grounds for lunch.

Afterwards (and after Jan had done some heavy marketing of the July Symposium!) we went down to the NE end of the Esplanade and up onto Portishead Point where we spent some time looking for fossils and working out the folding in the Carboniferous Black Rock Limestone (which also has Dolomitic Conglomerate sitting unconformably on the top). The memorial on the top of the point gave a glimpse of a different limestone - Portland? - an oolitic limestone with numerous Turritella fossils.

Down below the point and lower in the succession the fossils are much more obvious - expecially crinoid ossicles and brachiopods - and also neatly arranged at eye level which is really helpful for those who aren't as bendy as they used to be!
These rocks must have been deposited in a submarine environment, low energy but with occasional currents bringing in the broken shelly material. The Black Rock Limestone is fairly free from siliciclastic sediment so away from any terrestrial sediment source but the lower beds are sandier - so there must have been some input from the land to the north.

Walking back across the beach we came to the lower Avon Group (Lower Limestone Shale) and traced out a series of plunging folds in these calcareous shales which have been stained red by the overlying Trias.
From the oldest ORS in the south to the Black Rock Limestone in the north this area was covered by increasingly deep water, from a coastal plain, through  near offshore, to a carbonate shelf environment in the Lower Carboniferous.
During the Variscan orogeny the rocks were pushed north on a series of thrust sheets which folded the rocks above the tips producing the Portishead anticline as well as the smaller folds in the Avon Group (and of course the bigger folds in the Mendips etc to the south. Uplift and erosion left a land surface on which the Triassic breccia was deposited.
... and very many thanks Severnisde Branch for letting me lead this trip - and for the bottle of 'Castillo de Calatrava' - the header of my blog is actually reddened ash deposits in the Calatrava volcanics in La Mancha, Spain - so very appropriate!