Bracklesham

[SZ 775 975]–[SZ 844 930]

Introduction

The foreshore between Chichester Harbour and Selsey Bill (Figure 8.21) is one of the classic sites for studying the geology of the Bracklesham Group, the interval of strata overlying the London Clay in the eastern Hampshire Basin, and it has been independently selected for the GCR for Tertiary stratigraphy (under the name Wittering to Selsey Foreshore'; see Daley in Daley and Balson, 1999). Palaeobotanically, this is the best site for studying the flora of the Eocene Bracklesham Group. Sixteen horizons through the upper Ypresian and Lutetian Stages have yielded plant fossils, providing a record of the vegetational changes taking place in Britain during the onset of the post-Ypresian climatic cooling.

The earliest record of plant fossils from here was by Dixon (1850), in his account of the geology of this stretch of coast, while a second edition of the work (Dixon, 1878) included a review of the fossils by Carruthers. Gardner (1884) figured conifer cones from here, and Rendle (1894) and Reid (1897) illustrated examples of Nypa fruits. More recent accounts of the palaeobotany of this site are by Chandler (1961b) and Collinson (1978a, 1996b).

Description

Stratigraphy

The classic account of the geology here was by Fisher (1862) and, although there are more modern accounts (e.g. Curry and Wisden, 1958; Curry et al., 1977), these often still refer to Fisher's numerical classification of the beds. The most recent review is by Daley (in Daley and Balson, 1999). The foreshore sequence (Figure 8.21) consists of mainly arenaceous deposits, the lower few metres belonging to the London Clay Formation, and the rest to the Bracklesham Group (Figure 8.22). Daley (in Daley and Balson, 1999) points out the difficulty of establishing the true thickness of the succession, but it is probably somewhere between 90 m and 120 m.

The Bracklesham Group consists of shallow marine to brackish deposits formed along the coastal margin (Flint, 1983) and are in part coeval with the non-marine Poole and Branskome Sand Formation to the west. There has been some disagreement as to the subdivision of the Group but it is now normally divided into the Wittering, Earnley, Marsh Farm and Selsey Formations. The Wittering Formation belongs to the upper Ypresian Stage, the rest to the Lutetian Stage.

Palaeobotany

Plant fossils have been found here throughout the Bracklesham Group. The commonest are remains of the mangrove vegetation (fruits of Nypa burtinii (Brongniart) Ettingshausen (Figure 8.23) and a possible piece of Nypa bark, together with the enigmatic Wetherellia dixonii (Carruthers) Chandler), especially in the lower Wittering and upper Earnley Formations. Other levels have the remains of brackish aquatic plants (Limnocarpus forbesii (Heer) Chandler, Selseycarpus enormis (Chandler) Collinson) and freshwater plants ('Scirpus' lakensis Chandler (Figure 8.24), Caricoidea obscura Chandler). Sometimes the brackish and freshwater elements are associated with the mangroves, but they are, on the whole, commoner in the upper part of the section (Marsh Farm and Selsey Formations).

Chandler (1961b) described what she interpreted as a rhizome of a pondweed (Potamogetonaceae) from the topmost beds of the Selsey Formation. Collinson (1996b) has reinterpreted this fossil and together with additional material from the same bed, identifies it as a seagrass-like plant. Significantly, also from this bed, Murray and Wright (1974) described forminifera that are very similar to the faunas found today in seagrass communities.

In addition to these aquatic and marginal plants derived from the vegetation near to where these deposits were formed, there are the remains of trees presumably representing the vegetation of the surrounding forests. Most widely found are conifers, including Pinus cones, and leafy shoots known as 'Araucarites' selseyensis Chandler. The latter might in fact have been from the extinct conifer genus Doliostrobus Marion, which belongs to either the redwood or monkey puzzle families (Mai, 1976; Collinson, 1996b). There are also various angiosperm fruits, seeds and foliage, including members of the sedge (Carcoidea), laurel (Laurocalyx sp.), moonseed (Palaeococculus lakensis Chandler, Palaeosinomenium cf. venablesi Chandler), mastic tree (cf. Eomastixia rugosa (Zenker) Chandler), rue and tea families. They occur at several levels within the sequence, but are most common in the upper Wittering Formation (Collinson, 1996b).

Interpretation

The Wittering Formation is contemporaneous with the upper London Clay and Dorset Pipe Clay. However, it complements the palaeobotany of both of these sites, by including both upstream riparian vegetation and mangroves, and is thus intermediate between the coastal mangroves preserved in the London Clay and the inland vegetation seen in the Dorset Pipe Clays.

Bracklesham is also important as the only British site with a good record of plant macrofossils through the late Ypresian-early Lutetian Bracklesham Group. This is a critical part of the palaeobotanical record in northern Europe, reflecting vegetational changes during the onset of global climatic cooling (Collinson et al., 1981). This is especially evident in the record of the aquatic and marginal plants, which are especially well represented here. Of the events given as significant in this vegetational change by Collinson et al. (1981), three can be identified at Bracklesham: (1) the appearance of Limnocarpus forbesii in the Wittering Formation; (2) the appearance of abundant Caricoidea obscura in the Marsh Farm Formation; and (3) the disappearance of 'Scirpus' lakensis at the base of the Selsey Formation. These are fully documented in Collinson (1996b) and the latter is further discussed in Collinson (2000a).

Collinson (1996b) has been able to demonstrate a number of previously unknown features of the 'Scirpus' fruits, including the ultrastructure of the seed envelopes. She had earlier argued (Collinson, 1978a) that these plants may have fulfilled a similar role to Typha, which occurs in the late Eocene floras of Britain. Although the work is still in progress, it is clear that the Bracklesham fossils will play a major role in understanding the affinities of this important Ypresian–lower Lutetian aquatic plant.

The Nypa burtinii from near the top of the Selsey Formation is one of the youngest authenticated examples of that genus known from the fossil record (Collinson, 1996b). The only younger example is a single specimen said to be from the Boscombe Sand Formation and lost since 1894 (Chandler, 1960; see also Collinson, 1996b). The examples of N. burtinii from the Wittering Formation are also of interest, as they were probably deposited nearer where the plant grew than the comparable fossils from the London Clay. They are sufficiently common at one level in the lower Wittering Formation for it to be known as the Nypa Bed and to have been interpreted as representing a strand-line (Collinson, 1996b). Also near this level, Collinson (1996b) reported the discovery of what may be a segment of bark of Nypa, the only such example known from the British Tertiary strata. This all offers one of the best opportunities to study the Eocene Nypa mangroves and to reconstruct this important ecosystem (Collinson, 1993, 1996b).

Bracklesham is the best site for the study of Eocene seagrass communities. Seagrasses are generally very rare in the fossil record and were for a long time only recognized through the presence of the characteristic foraminifera faunas (Brasier, 1975). While macrofossil remains of the plants themselves are known from Herne Bay and Belgium (Stockmans, 1936; Chandler, 1961a; Collinson, 1983b), Bracklesham has yielded the plants with their associated fauna (Collinson, 1996b).

It is evident that Bracklesham is a site of outstanding importance for Tertiary palaeobotany, in understanding both the vegetational changes occurring in the late Ypresian and early Lutetian, and some of the individual plants that made up that vegetation. This was a time of significant climatic and vegetational change (Collinson et al., 1981; Collinson, 2000a,b) and Bracklesham is crucial for understanding them.

Conclusions

Bracklesham is an internationally important site for early and middle Eocene plant fossils, c. 45–50 Ma old. It helps significantly in understanding the changes in vegetation that were taking place at that time, changes that were being triggered by a cooling of the climate. It is one of the best sites for studying the mangrove palms (Nypa) and the 'Scirpus' lakensis plant, both of which were diagnostic elements in the middle Eocene vegetation of Europe. It is also the best site for the study of seagrass communities of this age.

References