Beginning of Mesozoic Marine Overstep of the Mendips: the Rhaetian
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Proceedings of the Geologists’ Association 131 (2020) 578–594 Contents lists available at ScienceDirect Proceedings of the Geologists’ Association journal homepage: www.elsevier.com/locate/pgeola Beginning of Mesozoic marine overstep of the Mendips: The Rhaetian and its fauna at Hapsford Bridge, Vallis Vale, Somerset, UK a a,b,c a a James Ronan , Christopher J. Duffin , Claudia Hildebrandt , Adam Parker , d d,1 a, Deborah Hutchinson , Charles Copp , Michael J. Benton * a School of Earth Sciences, University of Bristol, Life Sciences Building, Tyndall Avenue, Bristol, BS8 1TQ, UK b 146 Church Hill Road, Sutton, Surrey, SM3 8NF, UK c Earth Sciences Department, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK d Bristol Museum and Art Gallery, Queen’s Road, Bristol, BS8 1RL, UK A R T I C L E I N F O A B S T R A C T Article history: One of the most dramatic environmental changes in the Mesozoic history of Europe was the switch from Received 26 November 2019 terrestrial to marine deposition marked by the Rhaetian Transgression, 205 Ma. Beginning with this Received in revised form 24 February 2020 event, the Mendip Hills, composed primarily of uplifted and folded Lower Carboniferous limestones, Accepted 26 February 2020 were flooded in a stepwise manner from the Late Triassic to mid Cretaceous. The basal Rhaetian beds at Available online 29 May 2020 the eastern end of the Mendips (Hapsford Bridge, Vallis Vale) lie directly on Carboniferous limestone, which was bored, indicating it functioned as a hardground. Bored pebbles were then eroded, transported, Keywords: encrusted with bivalves, and deposited in marine muds in the lower parts of the Westbury Formation. At Chondrichthyes certain levels also, suspended mini-conglomerates within finer-grained sediments suggest continuing Osteichthyes Bristol storm activity. The Hapsford Bridge Rhaetian bone bed includes microvertebrate remains of four species Rhaetian of sharks and two species of bony fishes, all of them typical of Rhaetian-aged bone beds. The invertebrate Rhaetian bone bed fauna is especially rich, including bivalves and echinoids, as well as trace fossils. Unusual elements are Penarth Group barnacles and a possible belemnite. Westbury Formation © 2020 The Geologists' Association. Published by Elsevier Ltd. All rights reserved. 1. Introduction Atlantic Magmatic Province and major rifting on the Afro- European and North American sides (Wall and Jenkyns, 2004). The Late Triassic was a time of major environmental change. In South Wales and south-west England, from south Gloucester- The Rhaetian transgression marks the switch from primarily shire to Somerset and Dorset, the Rhaetian and other Mesozoic terrestrial to marine conditions about 205.5 Ma, and it has been sedimentary units accumulated on top of an exposed landscape noted across much of Europe. The event is marked in the UK by the formed from uplifted, folded and subaerially weathered Palaeozoic switch from the Mercia Mudstone Group to the Penarth Group rocks, primarily Lower Carboniferous limestones, but also Devoni- (Fig. 1), starting with the famous Westbury Formation basal bone an sandstones and Upper Carboniferous coal-bearing sediments. bed, which is made up of a rich accumulation of disarticulated These limestones have been quarried for a long time across the vertebrate fossils (Storrs, 1994). The change is evident at many Mendip Hills, south of Bristol, and the quarrying activity often cut localities, especially those such as Aust Cliff (Cross et al., 2018) down through overlying Mesozoic sediments. where the Mercia Mudstone Group largely comprises red beds and Much of this geological story can be seen in Vallis Vale (Fig. 2A), is conformably overlain by the black-coloured shales of the a narrow valley in the south-eastern Mendips, located between the Westbury Formation. villages of Great Elm and Hapsford, just north of Frome. By 1890, The Rhaetian transgression was probably triggered by the the Vale was an industrial site, with lime kilns, iron workings, and break-up of Pangaea, marked by emplacement of the Central woollen mills, all powered by the fast-flowing waters of the tiny Mells River. But, when Buckland and Conybeare (1824, p. 225) visited the steep-sided valleys of Mells and Vallis, they were quiet ‘ * Corresponding author. and secluded, and they noted that [b]eautiful sections may be seen E-mail address: [email protected] (M.J. Benton). in the precipitous sides of these valleys, exhibiting the oolitic strata 1 In honour of Charles Copp (1949–2009) who carried out extensive work on the in an absolutely horizontal position, reposing on the truncated Hapsford Bridge site in the 1980s and from whose unpublished PhD Thesis work we edges of highly inclined strata of mountain limestone [sic].’ Much have drawn upon throughout the paper. https://doi.org/10.1016/j.pgeola.2020.02.005 0016-7878/© 2020 The Geologists' Association. Published by Elsevier Ltd. All rights reserved. J. Ronan et al. / Proceedings of the Geologists’ Association 131 (2020) 578–594 579 Fig. 1. Summary of Late Triassic and Early Jurassic stratigraphy in England (after Swift, 1999; Trueman and Benton, 1997). the same observations were repeated by De la Beche (1846, p. 287), The Rhaetian sections of the Hapsford Bridge end of Vallis Vale when he wrote that Buckland and Conybeare had observed how were mentioned by Reynolds and Richardson (1909); Richardson Vallis Vale and Murdercombe ‘show the inferior oolite [sic] in (1907,1909, 1911), Richardson and Young (1909); Reynolds (1912); nearly horizontal beds upon the upturned edges of the carbonifer- Cox (1941); Savage (1977), and Duff et al. (1985, pp. 135–139). ous limestone [sic], with occasionally an interposed portion of a Detailed studies were carried out in his unpublished thesis by Copp conglomerate referable to the lias [sic], and containing organic (1980), who made extensive collections at Vallis Vale and his remains’. De la Beche (1846, p. 288) illustrated the Carboniferous- sedimentary log was published (Copp in Duffin, 1982), but none of Middle Jurassic unconformity (now commonly referred to as the his other work. Orbell (1973) and then Warrington (1984) reported De la Beche Unconformity) in the old quarry opposite the palynological investigations from the section at Hapsford Bridge, confluence of the Egford Brook and Mells River (ST 7557 4918). comprising miospores and organic-walled microplankton that Charles Moore collected in the area during the early 1860s and indicate Rhaetian age, and equivalence to horizons from Chil- passed some invertebrate fossils to T. Rupert Jones, who named compton. Furthermore, Hapsford Bridge became the type locality Estheria minuta var. brodieana (now Euestheria), based partly on for the penaeid shrimp, Aeger gracilis (Förster and Crane, 1984; this material, and gave a brief sketch of the geological succession at Boomer et al., 1999, pp. 141–144). The Hapsford Bridge sections Vallis Vale (Jones 1862). It was Moore (1867, pp. 488–491), were cleared in 1979–1980 by Charles Copp and colleagues, with however, who provided the first detailed information on the Vallis funding from the Nature Conservancy Council. Vale sections, describing the succession in several quarries, The aim of this paper is to present a report of the Rhaetian at including that with the De la Beche unconformity, and noting Hapsford Bridge in Vallis Vale. We present evidence that the especially that the rocks either side of the Carboniferous-Jurassic Rhaetian Transgression was accompanied by hardground forma- contact could be hard to distinguish in hand specimen, so close was tion and the beginning of overstep of the Mendip islands, as well as the lithological similarity of the two units. He described quarries evidence for several storm events associated with deposition of along the eastern branch of the Vale, towards Hapsford, where he various levels of the Westbury Formation. We also document the observed Liassic-age fissures in the Carboniferous limestone, and Rhaetian bone bed faunas and compare them with examples from made the first mention of bedded Rhaetian-age sediments. He around Bristol and in South Wales. noted (Moore, 1867, p. 489) that ‘A thin bed of a waterworn pebbly Repository abbreviations: BATGM, Bath Royal Literary and conglomerate, which will be found continuous and of greater Scientific Institution, geology collection; BRSMG, Bristol City thickness in succeeding sections, makes its appearance, resting for Museum and Art Gallery, geology collection; BRSUG, University a short distance immediately on the Carboniferous Limestone.’ of Bristol, School of Earth Sciences, geology collection. Moore (1875) provided further description of the geology of Vallis Vale, and later (Moore, 1881, pp. 67–68) commented, ‘At the 2. Geological setting entrance to the Vallis at Hapsford the first sections show irregularly bedded Rhaetic conglomerates resting in depressions 2.1. Quarrying in the Vallis Vale on the edges of the inclined Carboniferous limestone. They are separated by thin blue clays with Avicula contorta and also Discina Hapsford Bridge is part of the Vallis Vale Site of Special Scientific Babiana [sic]. I have no doubt they would yield important Interest (SSSI), scheduled for conservation especially because of vertebrate remains, as I found a very perfect Dinosaurian vertebra; the De la Beche unconformity, where yellow-coloured Middle but, unfortunately, these beds have not been worked for some Jurassic limestones sit horizontally on top of a planed-smooth years.’ Finally, McMurtrie (1885, pp. 103–106) repeated some of palaeotopography of uplifted and steeply dipping Carboniferous Moore’s comments, but especially highlighted how the Carbonif- limestones (De la Beche, 1846, pp. 287–288). Vallis Vale is lined erous limestone along the banks of the Mells River was overlaid with quarry faces which have been abandoned over time and these successively by Rhaetian, Lias and Middle Jurassic rocks, corre- have become overgrown. It is a popular site for walkers, with a sponding to an inferred steeply rising palaeotopography of the stream, car park and footpath (Prudden, 2006).