Maggot, Kingbarrow and Waycroft quarries

[SY 691 728], [SY 692 731] and [SY 696 729]

Introduction

Sites on Portland (Figure 4.8) afford the best opportunity to examine the trees of the basal Purbeck (latest Jurassic age) 'fossil forest'. This forest was formed primarily of cheirolepidiaceous conifers, whose silicified in-situ boles and fallen trunks and branches can be seen in the fossil soils known as 'dirt beds' and overlying beds. Bennettitaleans up to 1.18 m tall were also present. The plants and the sediments in which they are found are of considerable value for reconstructing latest Jurassic palaeoenvironments in southern England.

The Purbeck Limestone Formation (Purbeck Beds') of southern England have long been known to yield in-situ remains of arborescent plants. Buckland (1828, 1836), Fitton (1836), Carruthers (1870a) and Seward (1897b) described the bennettitalean stems that have been preserved, and more recently Francis (1983, 1984) investigated one of the coniferous species. The Isle of Portland is the best area to study them because of the extensive quarrying of the underlying Portland Stone Formation for building stone, which is still continuing (Figure 4.9).

Description

Stratigraphy

The Upper Jurassic Series in southern Britain ranges from the Kellaways Beds to the Purbeck Beds. The lower Purbeck Beds of Dorset are the uppermost deposits of the Jurassic System, and lie within the Portlandian Stage. The Purbeck Beds are present over much of the Isle of Portland, and extend as a broad band from Swanage in the east to Portesham in the west via Lulworth Cove where the 'fossil forest' level is preserved. They also occur at a few inland sites.

The term 'Portlandian' was first used with reference to the Portland Stone, which is an important building and facing stone. Until fairly recently it was used synonymously with 'the Portland Beds'. However, according to Wimbledon (in Cope et al., 1980a), the Portlandian, as the topmost Jurassic stage, encompasses everything between the top of the Kimmeridgian Stage and the base of the Cretaceous System (base Berriasian Stage). For decades, the boundary between the Jurassic and Cretaceous systems was accepted, somewhat arbitrarily, to be at the base of the mid-Purbeck Cinder Bed, but recent studies (e.g. Wimbledon and Hunt, 1983; Allen and Wimbledon, 1991; Feist et al., 1995) suggest that it is low down in the Lower Purbeck (Lulworth Formation). As noted earlier, the 'fossil forests' of Portland and Lulworth are found in the basal Purbeck (Figure 4.10). They are, therefore, late Portlandian in age.

Palaeobotany

The quarries are famous for the silicified stumps and branches of the cheirolepidiaceous conifer Protocupressinoxylon purbeckensis Eckhold, which have been most recently studied by Francis (1983, 1984; (Figure 4.11)). Most of the stumps are rooted in situ and many are encircled by algal stromatolites (Figure 4.12). The algae grew when the trees were drowned by rising water. In some cases the stromatolites completely covered the stumps, but in others the trunks have almost gone (in some cases removed by collectors) leaving the raised circular stromatolites (burrs) on their own. Small holes in the limestone surrounded by narrow circles of stromatolites mark the original positions of branches. Francis (1984) mapped the positions of stromatolite burrs on Portland and calculated the original density of trees to vary between one tree in 15 m² to one in 54 m², trunks on average being spaced between 3 and 5 m apart. This suggests an open forest, much more open than modern Taxodium swamp forests, although the relative age of the trees needs to be taken into account. Some were dead stumps that became encased by the stromatolites. Large silicified trunks lying close to them support this interpretation. The silicified wood shows fairly well-defined growth rings with very narrow mean-ring widths ranging from 0.05 mm to a maximum of 4.44 mm (Francis, 1984).

The Isle of Portland has also been famous for its cycadeoid stems ever since Buckland (1828, 1836) first described their occurrence, followed by Carruthers (1870a). Seward (1897b) described a particularly well-preserved specimen, 1.18 m tall and 1.7 m in girth, to which he gave the new name Cycadeoidea gigantea. It is elliptical in cross-section, measuring 0.41 m × 0.19 m. Its surface is covered with diamond-shaped ridges, which are the petrified ramenta from around the bases of the leaf petioles (represented by the depressions). Seward saw no flowers but thought that a small bud found on the stem might be an aborted fertile shoot.

Interpretation

This is by far the best locality for studying the 'fossil forests' of the Purbeck 'Dirt Beds'. Stumps are preserved at other sites (e.g. Lulworth) but they are not as accessible as on Portland. The only other Upper Jurassic plant beds in Britain are in Scotland (see Chapter 5), but these are rather older (Kimmeridgian) and yield compression fossils. Late Jurassic floras are generally very rare in Europe and none appears to be Portlandian (Tithonian) in age (Vakhrameev et al., 1978).

Francis (1984) has interpreted these 'fossil forests' as the remains of cheirolepidiaceous gymnosperm trees that grew on the fringes of a shallow, hypersaline lagoon that covered much of southern England during the latest part of the Jurassic Period. The trees appear to have been adapted to growing in a semi-arid environment, as indicated by the growth rings in the wood. These vary in width, but all are narrow. The trees were probably very sensitive to climatic variations. The factor most likely to have affected growth would have been rainfall and, hence, the availability of water. The preservation of the wood was by quartzine, which is chalcedony that formed very slowly. It is often associated with evaporitic environments and high pH values. The source of the silica is not known although it might have come from the dissolution of detrital quartz.

There is also sedimentary and palynological evidence to suggest a seasonal climate. In the neighbourhood of Portesham and Upway, a lagoonal clay, which is the lateral equivalent of the Dirt Bed, contains silicified wood, conifer shoots and seeds and charophytes together with nodules of silicified evaporite pseudomorphs (Barker et al., 1975). Sladen and Batten (1984) and Allen (1998) discussed the climatic implications of combined clay mineral and palaeontological data through both the Purbeck and overlying Wealden successions.

All this information suggests that the marginal and variable climate was probably of Mediterranean-type, with warm, wet winters. The summers would have been hot and arid, stimulating the formation of evaporites. Francis (1984) suggested that intermittent dry spells could have interrupted the growing season for days or even months to produce the 'false' rings seen within the early wood of the growth rings. She further proposed that an analogous modern environment for both the forests and the seasonal lagoonal sediments is to be found on Rottnest Island, near Perth in Western Australia. Here natural stands of the conifer Callitris preissii grow close to small ephemeral lakes.

Conclusions

Maggot, Kingbarrow and Waycroft quarries yield the only conveniently examinable examples of a fossil forest known from the uppermost Jurassic strata of Europe. It is dated at about 145 Ma (WA. Wimbledon, pers. comm.). An association of evaporites and fossil remains in the basal Purbeck Limestone Formation reflect the succession of mature gymnosperm forest of cheirole-pidiaceous conifers growing on the margins of a shallow, variably saline, sometimes hypersaline lagoon. Narrow rings in the wood indicate marginal and highly irregular growth conditions for the trees. The Purbeck climate was probably of Mediterranean-type with hot, arid summers and wet winters.

References