Westbury Garden Cliff, Gloucestershire

[SO 719 129]

Introduction

Westbury Garden Cliff (simply 'Garden Cliff' in the GCR Unit records) exposes the Mercia Mudstone Group, Penarth, and Lias groups. This site is especially important for the Penarth Group bone bed that preserves fossils of exceptional quality, as well as for the associated trace fossils that are much richer than at other sites, and give detailed evidence on palaeoenvironment.

The first report on Westbury Garden Cliff was by Conybeare and Phillips (1822). Subsequent accounts include Brodie (1845, 1858), Wright (1860), Etheridge (1865, 1872), Lobley (1875), Richardson (1903a), Sykes (1977), and Green (1992). Descriptions of the palaeontology include Richardson (1903a), Benton and Spencer (1995), Storrs et al. (1996) and Dineley and Metcalfe (1999). Trueman and Benton (1997) presented an account of the geochemical taphonomy of the 'Rhaetic Bone Bed' from this section.

Description

Westbury Garden Cliff is on the north bank of the River Severn, some 9 km south-west of Gloucester. The cliff reaches a height of 21 m at the downstream (western) end, and exposes a 1-km-long section of Upper Triassic and Lower Jurassic sediments (Figure 4.10). The beds dip gently south-east and the Mercia Mudstone Group is exposed in the downstream part of the section; the Westbury Formation is seen at the upstream end of the section (Wang, 1993; Storrs et al., 1996). Natural erosion keeps the site free of vegetation, although beds at the top of the cliff are generally inaccessible (Swift, 1995). The base of the cliff is covered by estuarine waters at high tide. Where the Mercia Mudstone Group forms the base of the cliff tidal erosion is generally uniform, and produces a smooth base, which tends to undercut the overlying Westbury Formation, resulting in occasional rockfalls. Where the Penarth Group forms the base of the cliff; exposure is less uniform, as beds such as well-cemented sandstones are more resistant to erosion (Reynolds, 1906; Storrs et al., 1996).

In the literature, the site is referred to as 'Westbury-on-Severn' (Sykes, 1977), 'Garden Cliff' (Etheridge, 1865; Swift, 1995; Storrs et al., 1996), and 'Westbury Garden Cliff' (Trueman and Benton, 1997). The first mention of the 'Westbury Beds' was by Wright (1860) who used it to describe the dark-coloured shales and subordinate sandstones. The unit was formalized as the Westbury Formation by Warrington et al. (1980).

Sedimentology

The sedimentary log given here is modified from those of Richardson (1903a) and Sykes (1977, pp. 206–9):

The Mercia Mudstone Group succession comprises red mudstones and marls of the Twyning Mudstone Formation. These argillaceous lithologies are commonly cut by vertical gypsum veins and in places are laminated or show rippled surfaces. The overlying Blue Anchor Formation is green or grey when freshly exposed but when weathered it acquires a yellowish or bluish tinge (Richardson, 1903a). These beds dip to the south-east at about 2°.

Fossils, especially the remains of fish and coprolites, appear in the Penarth Group. There are two bone-bearing levels, one at the base of the formation, the other some 2 m above the base and 0.4 m above the 'Upper Pullastra Bed' (Richardson, 1903a,b; Reynolds and Vaughan, 1904; Storrs et al., 1996). This discontinuous bone-bearing unit varies in thickness and is approximately 0.03 m thick; it is composed of pyrite-rich, shelly silt or sandstone with mud laminations (Figure 4.4)d. Euhedral unabraded pyrite crystals are commonly scattered across the basal surface of the layer, and the matrix consists of approximately 70% disseminated pyrite. Vertebrate remains, coprolites, and nodules occur throughout the bed and quartz grains occur sporadically (Storrs et al., 1996; Trueman and Benton, 1997). The upper and lower surfaces of the bed often show ripples; on the upper surface, argillaceous sediments may infill small channels. Bones are exquisitely preserved on the lower surface of the bed and may show evidence of poorly developed current alignment (Figure 4.11). The bone beds all contain large quantities of vertebrate remains, as well as quartz pebbles, intraformational mud clasts, and pyrite (Richardson, 1903a; Wang, 1993).

Above the bone beds, the Westbury Formation comprises dark shales with sandstones and limestones, which are often especially fossiliferous. In places, the top surfaces of the sandier units are covered with shells and may also preserve trace fossils, seen for example on the upper surface of the 'Upper Pullastra Bed' (Wang, 1993).

At the top of the cliff section, the Lilstock Formation comprises alternations of pale shales, grey, brown, or blue-grey limestones and occasional sandstones. Many of the beds show primary sedimentary structures such as laminations and ripples. Of note are the 'Insect Limestone' and Tuestheria Bed', both remarkably rich in fossils (Richardson, 1903a).

Palaeontology

Many invertebrate trace and body fossils have been recorded from the Penarth Group in Westbury Garden Cliff Invertebrate body fossils include rare and fragmentary remains of insects and gastropods from beds in the Cotham Member, and several species of bivalve genera such as Ostrea, Protocardium, and Pecten, many of which are preserved as isolated shells and are commonly pyritized (Wang, 1993), and the inarticulate brachiopod Lingula (Richardson, 1903a; Wang, 1993).

Many of the bed boundaries show evidence of bioturbation, and several ichnogenera have been identified. Beds in the lower part of the Westbury Formation show well-defined Diplocraterion burrows that have been infilled with the overlying black argillaceous sediment; also present are Skolithos traces (Wang, 1993).

The locality has received international attention for the large numbers of well-preserved Pachystropheus remains (Storrs and Gower, 1993; Storrs et al., 1996),most of which are isolated bones and bone fragments that weathered out of the cliff (Wickes, 1904) or were found when sections of the cliff collapsed. The well-preserved material from Westbury Garden Cliff has enabled a thorough analysis of Pachystropheus to be completed, indicating that it may be the earliest known choristodere (Storrs et al., 1996), a group of extinct, aquatic diapsid reptiles that were previously first known from Middle Jurassic strata.

Other vertebrate fossils include ichthyosaurs, plesiosaurs, and the fishes Gyrolepis, Ceratodus, and Acrodus (Etheridge, 1865; Richardson, 1903a; Wang, 1993; Benton and Spencer, 1995; Trueman and Benton, 1997; Dineley and Metcalfe, 1999).

Interpretation

The red mudstones of the Twyning Mudstone Formation were deposited under terrestrial conditions, in environments similar to modern sabkha flats. These conditions continued throughout the deposition of the Blue Anchor Formation, although the importance of influxes of water, probably marine, increased.

The Penarth Group marks the end of terrestrial conditions in the area, and the establishment of marginal or marine environments. The Westbury Formation bone beds have been interpreted as fillings of shallow channels within an estuarine environment, a view supported by the limited lateral development of the horizon and the trace fossil assemblage at Westbury Garden Cliff (Wang, 1993; Trueman and Benton, 1997). The presence of large quantities of pyrite and phosphate is consistent with this palaeoenvironmental interpretation. The bones probably accumulated as some form of winnowed channel lag deposit (Trueman and Benton, 1997), or possibly as a shoal or strandline deposit (Storrs et al., 1996).

Analysis of the rare earth elements (REE) in the minerals preserving the bones at this site indicate that chemically there is little to separate them from the bones at Aust Cliff (see below), although the REE signatures of the associated sediments are distinct. This suggests that the two bone accumulations experienced similar early diagenetic processes, and that the Aust Cliff material probably originated in a bone bed similar to the one preserved at Westbury, but was then reworked (Trueman and Benton, 1997).

The remainder of the Westbury Formation is dominated by fine-grained sediments, indicative of shallow marine conditions. At the top of the section, the Cotham Member indicates marine conditions, but with evidence for fresh water and for subaerial exposure at some levels.

Conclusions

Westbury Garden Cliff provides an important sedimentological record of changing environments from terrestrial, hypersaline sabkha plains (Mercia Mudstone Group) to shallow marine (Penarth Group). Fossils, including invertebrate body and trace fossils, as well as vertebrates such as fishes, ichthyosaurs, and plesiosaurs are commonly preserved in the Penarth Group. Of great significance are the bone-bearing horizons, which have gained international fame as rich sources of vertebrate fossils, especially the choristodere Pachystropheus.

References