Stop the press: The Jurassic Coast starts in the Permian

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Mervyn Jones (UK)

This Geologists’ Association field meeting followed the publication of Professor John Cope’s Geologists’ Association (GA) Guide No 73, Geology of the South Devon Coast. It is also the companion to GA Guide No 22, Geology of the Dorset Coast. John retired in 2003 after lecturing at Swansea and Cardiff universities. Since then, he has been an Honorary Research Fellow at the National Museum Wales in Cardiff, and has a wide field experience in the UK and Europe, with publications covering many fossil groups over a wide stratigraphical range. Most recently he has been working on redrawing the geological map of South Wales, the subject of an upcoming GA lecture. And, each year, for the past six years, he has provided weekend geological trips to the West Country.

Fig. 1. Prof Cope demonstrates bedding and cleavage.

We met up at Meadfoot Strand to the east of Torquay Harbour. Our mission for the weekend was to examine the complex Devonian succession in the Torbay area and its unconformable relationship to the Permo-Triassic cover. Of great interest was the marine Devonian, first described by Adam Sedgwick, assisted by Roderick Impey Murchison, who finally realised that these facies were contemporaneous with the familiar Old Red Sandstone found north of the Bristol Channel. Since then, the Devonian Stages have been named after rocks in the Czech Republic, Germany and Belgium. The base of the Devonian was the first ‘Global Boundary Stratotype Section and Point’ (GSSP), defined by graptolite zones at Klonk, in the Prague Basin of the Czech Republic – a great geological name.

The area has much to offer enthusiasts of structural geology because the Devonian strata have been tectonised by the closure of the Rheic Ocean during the Variscan orogeny. The story has only been unravelled in the last 50 years. First, sediments filled a series of basins caused by crustal extension; the basement beneath the Devonian rocks may well be a massif of Precambrian mica-schist. Then, from the Early Carboniferous, continental collision caused a series of major thrust structures that progressively moved northward. As a consequence, what Carboniferous rocks were deposited in the Torbay area were rapidly stripped off and the Devonian was covered by sediments from the Permo-Triassic.

Our first exposure was of the basal Devonian Meadfoot Group of Pragian/Emsian age. This gave us the first chance to study the relationship between bedding and cleavage, the nightmare of many a geology undergraduate. The bedding was revealed by compositional differences and the cleavage was refracted at different angles through the beds. The surface of some bedding planes was rucked up into a mullion structure (that is, a structure formed by extension) and beds of sandstone locally squeezed into boudins (Fig. 2). It turned out that we were looking at the overturned lower limb of a major recumbent fold (which has an essentially horizontal axial plane).

Fig. 2. Boudin in the Nordon Formation. Boudins are sausage-shaped structures formed by extension, where a rigid tabular body (such as hornfels) is stretched and deformed inside less competent surroundings, and the competent bed begins to break up.

We walked over to Triangle Point at the southern end of Meadfoot Beach. The promontory is formed of Daddyhole Limestone, the basal Member of the Torquay Limestone Formation. It was deposited on a high point, where the water was shallow enough for reefs to develop. Weathering out on the steeply dipping bedding planes are the closely packed tubes of fossil stromatoporoid colonies (Fig. 3), a type of sponge related to modern sclerosponges (that is, sponges with soft bodies that cover a hard, often massive skeleton made of calcium carbonate). These were the essential builders of Devonian reef structures. They trapped sediment and provided shelter for solitary rugose and colonial tabulate corals, and also bryozoans. At Triangle Point, the cleavage is steeper than the bedding, showing that the beds are the right way up.

Fig. 3. Boulder of Stromatoporoidal Limestone from the Torbay Breccia.

Our next stop was Hope’s Nose, where we slithered and slipped down a steep path crossing the slatey Nordon Formation. This lies above the Meadfoot Group and is intercalated with the Daddyhole Limestone Member, underlying the Walls Hill Member. From the promontory, we had a good view around the bay where we could see faulted blocks of mudstones of the Meadfoot Group, with cleavage and bedding here parallel, and our first igneous rock, a dolerite that had intruded up one of the faults. Out to sea was the Ore Stone, formed of limestone with gently dipping beds divided from a deformed block by a 45o thrust plane, showing that pressure had been exerted from the south.

The quarry in the Daddyhole Limestone posed another structural problem. The best interpretation was thrusting along a disconformity in the succession. To the south of Hope’s Nose, the overlying thin beds showed well-formed boudins, confirming the tectonic activity. And here we found a text-book recumbent fold with well-developed axial cleavage. At the top of the cliff was a section revealing the well-known Pleistocene raised beach.

After lunch, we made our way to Goodrington Sands, starting at Waterside Cove. Here, we found the unconformity between marine Devonian of the Meadfoot Group and Permian Torbay Breccia laid down on land in desert conditions. Both rocks were reddened by the oxidising conditions. Walking south, we crossed a major fault bringing the Upper Devonian Saltern Cove Formation down to beach level. Prof Cope challenged us to discriminate the bedding and cleavage; the former was vertical and the latter horizontal. On the beach, we found beautiful boulders of rubified Torquay limestone full of Stromatoporoids. The cliffs contain goniatites (that is, ammonoid cephalopods), although we found very few, occasional volcanic tuffs and a dolerite sill. The thought of a volcano towering over Torbay was entertaining.

The following morning dawned bright and clear. We started with a group photo at the Geoneedle (Fig. 4) marking the western limit of the so-called “Jurassic Coast” at Orcombe Point Exmouth. Our mission was to explore the Permo-Triassic succession. Mostly terrestrial, it has been very hard to date until magnetostratigraphy became available and established that there are numerous unconformities in the succession. We started on the Aylesbeare Mudstones of late Permian age, laid down in lakes and found some of the mudstone horizons packed with ichnofossils (trace fossils) of burrowing creatures.

Fig. 4. Group photo at the Geoneedle, with the editor resplendent in red. Mervyn (the author of this article) is to his immediate left and Prof Cope is to the extreme left with a blue shirt.

An interesting point to arise from looking at these rocks is that the succession in the Jurassic Coast actually starts in the Permian and ends in the Eocene at Studland Bay, covering over 200 million years – far more geological time than most people realise. And that’s not to mention the Quaternary deposits overlying the bedrock.

We then moved on to Sidmouth where, at the bottom of Jacob’s Ladder (the sandy, western end of Sidmouth’s town beach), we could see the Middle Triassic succession of Otter Sandstone and Sidmouth Mudstone, overlain uncomfortably by the Cretaceous Upper Greensand. The path around the cliff towards Sidmouth centre allows close examination of the cross bedded Otter Sandstone, which exhibits vain attempts by the authorities to stabilise the cliffs and also lovers’ tokens carved into the face. There are vertical rhizoconcretions where calcrete was deposited around plant roots (Fig. 5) and possibly even the burrow of a small creature.

Fig. 5. Rhizoconcretions in the Otter Sandstone.

Finally, we arrived at Budleigh Salterton to view the famous Lower Triassic Pebble Beds by way of the equally famous nudist beach under rapidly eroding cliffs of Otter Sandstone. The unconformable boundary between the two formations is marked by a sandstone leached of its red iron oxide cement by groundwater. This rests on an impermeable mudstone that occasionally produces wind-faceted pebbles. The pebble beds (Fig. 6) include frequent well-rounded “liver-coloured” quartzite pebbles of Ordovician age transported by wadi systems from Brittany and Normandy, 80 to 100 miles to the south.

There are also pebbles of Devonian limestone and metamorphic rocks from Dartmoor. Finally, we passed across another unconformity representing ten million years to the Permian Littleham Mudstone. This incorporates nodules with rare elements, including vanadium and native copper. Some of these are mildly radioactive. The source of the mineralisation is believed to be from within the depositional basin and not from any hydrothermal activity.

Fig. 6. Budleigh Salterton Pebble Bed and Otter Sandstone.

We left well satisfied after another terrific trip with Professor Cope. This was the sixth of the series and we hope we can persuade him to take us again.

A version of this article first appeared in the March 2018 edition of the Magazine of the Geologists’ Association.

About the author

Mervyn Jones is a long-time member of the GA. He studied geology at St Catharine’s College at Cambridge University and is currently studying for an MSc in geoarchaeology at Reading University. Like William Smith, the father of stratigraphy, he is both a surveyor and geologist.

All the photos used in the article were taken by GA member, Martin Brown (UK).

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