Colwell Bay

[SZ 330 884]

Introduction

Colwell Bay is another of the classic lower Tertiary stratigraphical sites in southern Britain, and it is described in detail by Daley (in Daley and Balson, 1999). It is the only site yielding plant fossils from the Linstone Chine Member (upper Headon Beds of earlier authors), of late Eocene age. The flora consists mainly of aquatic plants and includes over 40 species, for some of which this is the only known site.

The plant fossils that occur here in the late Eocene Linstone Chine Member have been described by Chandler (1955, 1963a) and Collinson (1980a,b). Chandler (1961c, p. 101) also described a small flora from the Totland Bay Member here (see (Table 9.2)). Charophytes occur at various levels (Feist-Castell, 1977).

Description

Stratigraphy

The sequence has been described by Keeping and Tawney (1881), Reid and Strahan (1889) and Daley (in Daley and Balson, 1999). There are c. 33 m of the Headon Hill Formation, between the Totland Bay and Osborne Marls Members (Figure 9.20) and (Figure 9.21). The plant fossils mostly occur in carbonaceous bands within a unit of laminated clays and sands, at the base of the Linstone Chine Member. This unit contains abundant fossils of the bivalve Potamomya, indicating brackish conditions.

Palaeobotany

The plant fossils found here are mainly carbonaceous fruits and seeds, often with replacement by amorphous pyrite. Chandler (1963a) states that there are some 38 taxa known from here, 27 of which were named to species. Angiosperms are by far the commonest, and over half of these are aquatic or marsh plants, including rushes, water soldiers, water lilies and pondweeds ((Figure 9.22); a full list of the angiosperm fruits and seeds found here is given in (Table 9.2)). The remains of plants that grew in drier conditions are much scarcer, although fragments of twigs, seeds and cone-scales of the taxodiaceous conifer Sequoia couttsiae Heer (see Footnote 2 to (Table 8.2), this volume) are locally common. In addition, there are rare fruits of elder (Sambucus), fig (Ficus — see comments by Collinson, 1989), bog-myrtle (Myrica) and silkwood (Zanthoxylon).

Ferns are uncommon here, but Chandler (1955) described a single fertile pinnule of the schizaeacean Anemia colwellensis Chandler. Barthel (1976) regarded this species as a synonym of Ruffordia subcretacea (Saporta) Barthel. Ruffordia clearly belongs in a Glade including Anemia and is very similar to Anemia (Collinson, 1996a, in press a). Collinson (1980b) described the megaspore of the water fern Azolla colwellensis Collinson, which is unique to this site, and is the youngest record of the primitive form with many 'floats' in the megaspore (Figure 9.23).

Interpretation

Chandler (1963a) was of the view that the Colwell Bay flora 'is not a very remarkable or informative one'. It is indeed similar in many ways to that from the lower part of the Headon Hill Formation (Totland Bay Member), such as seen at Hordle, consisting predominantly of aquatic and marsh plants. However, there are many features that make the Colwell Bay flora distinctive and important, not least the fact that it is the type locality for nine species: Azolla colwellensis, Anemia colwellensis, Carex colwellensis, Cladiocarya colwellensis, Myrica colwellensis, Decodon vectensis, Sambucus colwellensis (synonymized with S. parvula Chandler by Collinson, 1983a), Epacridicarpum colwellense and Carpolithus colwellensis. The Anemia has been tentatively identified from another locality (Chandler, 1964) and is more widespread if included in Ruffordia subcretacea (Barthel, 1976). Decodon vectensis ranges up into the early Miocene deposits in Europe; the record from Colwell Bay is the oldest (Mai, 2000).

Colwell Bay is the only site to yield plant fossils from the late Eocene Linstone Chine Member (Headon Hill Formation), and as such is important for establishing the pattern of vegetational and climatic change in southern England during the Palaeogene Period. This is most clearly seen in the aquatic part of the flora. It shows the extinction of Stratiotes hantonensis, which becomes replaced by the larger S. headonensis, and the youngest possible occurrences of ?Caricoidea angulata and ?C. obscura. Collinson (1983a) considered these specimens of ?C. obscura to be similar to some from the Bembridge Marls but probably distinct from those in older strata. Also here is the first appearance of Nymphaea in the British fossil record.

The forest component provides less evidence of the environmental changes occurring at this time, but this is probably merely a function of the scarcity of the fossils of this part of the flora. It is nevertheless of note that these beds include the youngest occurrences of Ficus lucidus, Hordwellia crassisperma and Epacridicarpum headonense. Bulk sampling has the potential for yielding plant fossils from other levels in this section and it is worth noting that Stratiotes has been recovered from the Venus Beds (Collinson, pers. obs.). Charophytes, of value in palaeoenvironmental analysis and biostratigraphy, occur throughout the site (Feist-Castell, 1977).

Conclusions

Colwell Bay is the only British site to have yielded plant fossils from the Linstone Chine Member (Headon Hill Formation), of late Eocene age (c. 36 Ma old). It is thus crucial for understanding variations in the geographical and temporal distribution of these floras. The fossils are mainly the fruits and seeds of aquatic and marsh plants, and they help demonstrate the progressive cooling of the climate that was taking part in southern Britain at this time. Although the flora is not as diverse as at some other Lower Tertiary sites in Britain, it has yielded seven species that are unique to this site.

References