Chapter 9 The Lenham Beds

P Balson

Introduction

In 1854, Harris and Jones (see Prestwich, 1857b) discovered fossiliferous ferruginous sandstones within solution pipes in the surface of the Chalk escarpment between Harrietsham and Charing in Kent. They believed the deposits to be part of the Basement Bed of the London Clay. Prestwich visited the area in 1855 and described a section at Lenham Hill about 1 km north of the village of Lenham. He found the fossiliferous sandstone to be present as 'detached blocks' within the fill of the solution pipes. The sandstones are decalcified with molluscan shells preserved only as moulds. Prestwich soon noticed that certain elements of the fauna contradicted a Palaeogene age for the deposit (Prestwich, 1858). These elements included the bryozoan Lunulites, a large Terebratula, Emarginula and echinoid spines.

Prestwich also noticed a similarity to the fauna of the East Anglian Crags and postulated that the deposits might be equivalent in age with the Coralline Crag or possibly older. Prestwich submitted samples to Searles Wood, the leading authority on Crag molluscs at that time, who cautiously agreed that the fauna was similar to that of the Crag and supported the view that the deposit was older than the Red Crag (Prestwich, 1858).

The process of pipe formation was described by Prestwich (1855) who noted the irregular position of the cemented blocks. Prestwich recognized that the fills had originated by the slow downward warping of an originally horizontally layered sequence into a solution hollow in the surface of the Chalk and gave a reconstruction (Prestwich, 1858, fig. 4) of the former stratigraphy. The formation of the solution pipes and their infill is intimately related to the origin of the 'Clay-with-flints' deposits, which are found over large areas of the Chalk downland of southeast England (Catt and Hodgson, 1976).

By 1872, patches of sand on the surface of the Chalk had been found over a considerable length of the North Downs as far west as Netley Heath, west of Dorking (Whitaker, 1872) (Figure 9.1). Whitaker acknowledged, however, that the deposits were not necessarily all of the same age. Sediments attributed to the Lenham Beds (sensu stricto) are found on the surface of the Chalk escarpment of the North Downs as far east as Folkestone (Reid, 1890; Smart et al., 1966) although the deposits appear to be unfossiliferous east of Charing. Most of these deposits consist of decalcified and unfossiliferous sands and sandstones preserved as discontinuous patches or as the infills of solution pipes. The patches are often only revealed as a result of ploughing on the escarpment dip slope but the solution pipes are exposed as a result of former Chalk quarrying into the steep south-westward slope of the escarpment. Although attempts have been made to use sediment mineralogy to correlate these decalcified sediments with the Lenham Beds, in the absence of faunal evidence, the correlation, and thus age and origin of these unfossiliferous sediments must remain, to some extent, speculative.

The first use of the term 'Lenham Beds' appears to have been by Geikie and Reid (1886). In this paper, Reid drew attention to the presence of the bivalve Arca diluvii (now Scapharca diluvii) (Figure 9.2), a species later believed to be of particular significance in indicating a late Miocene age for the deposit.

Reid (1890) correlated the Lenham Beds with the Diestien Stage, which includes lithologically similar deposits in northern France and Belgium. This observation led Harmer (1898) to reconstruct the palaeogeography of the North Sea Basin that showed the Lenham Beds close to the southern coast of a closed embayment (Figure 9.3). Harmer noted, however (1898, p. 317), that the faunas of the Lenham Beds and Diestien, 'while presenting some resemblance, are, as to individual species, by no means identical'. In fact, Arca diluvii, regarded as the most diagnostic fossil of the Lenham Beds, does not even occur in the Diestien deposits (Newton, 1916). Despite this cautionary note, a correlation with the Belgian Diestien has become accepted more or less up to the present time (e.g. Curry et al., 1978). A new stage, the Lenhamian or 'zone of Arca diluvii', was created for the Lenham Beds (Harmer, 1900a). This stage was later to include the fauna of the Suffolk boxstones (the 'Trimley Sands' of Balson, 1990a) although the two faunas were not regarded as contemporaneous (Harmer, 1910). Newton (1916) regarded the Lenham fauna as the younger, but more recent study of the mollusc faunas suggests that the Lenham Beds fauna is stratigraphically older than that of the 'boxstones' (A.W. Janssen, pers. comm. 1995). Reid (1890) noted that the fauna consisted of a mixture of hard substrate species and those common in sandy substrates but with 'no truly littoral forms'. He believed the fauna to indicate water depths of at least 40 fathoms (73 m).

To the west, the Netley Heath Beds were visited by a field excursion of the Geologists' Association (Stebbing, 1900) and fossils found for the first time. Correlation of these deposits with the Lenham Beds was based on their marine nature and similarity of elevation; the poorly preserved mollusc fauna was not diagnostic enough to correlate with certainty. Subsequent fossil finds showed a correlation of the Netley Heath Beds with the Red Crag of East Anglia to be more likely (Chatwin, 1927). It has recently been shown, however, that the ironstones that contain the fossils are not in situ (John and Fisher, 1984).

Newton (1916, 1917) extended the list of mollusc species from the Lenham Beds to 76 (one scaphopod, 32 gastropods and 43 bivalves) and believed the fauna to be closely related to that of the Coralline Crag, referring both faunas to the Miocene.

Abbott (1916) recorded fossiliferous deposits similar to the Lenham Beds on the South Downs, a discovery later supported by evidence of a Pliocene marine fauna from deposits at Beachy Head (Edmunds, 1927). This discovery casts doubt on Harmer's (1898) reconstruction (Figure 9.3) of the palaeogeography of Lenham Beds times. The similarity of the Beachy Head deposits to the Lenham Beds was confirmed by heavy mineral analysis (Elliot in Worssam, 1963); the Lenham Beds contain abundant andalusite, kyanite, staurolite, anatase, brookite and spinel. Later reconstructions of the palaeogeography show a Wealden island' surrounded by the Pliocene sea (Wooldridge and Linton, 1938) (Figure 9.4). This reconstruction implies a marine connection between the North Sea and the English Channel through the region of the modern Dover Straits which are unlikely to have been open until the middle Pleistocene (Gibbard, 1995). The model also relies on deposits such as the Netley Heath Beds being contemporaneous with the Lenham Beds. Both palaeogeographical reconstructions are therefore unsatisfactory and further work is necessary on the deposits, their relative ages and the tectonic history of the region before the palaeogeographical evolution of south-east England during the Neogene can be reconstructed with any confidence.

Chatwin (in Worssam, 1963) reviewed the Lenham Beds fauna and increased the list of mollusc species to 88. Cardium (Acanthocardia) aff. andreae (now Acanthocardia andreae) and Nucula sulcata were identified as the most common species. Of the mollusc species, 75 are also known from the Coralline Crag, leading Chatwin to the conclusion that the Lenham Beds are 'very little, if at all, older than the Coralline Crag'. Arca diluvii is not known to occur in the Coralline Crag or in the Diestien of Belgium (Chatwin in Worssam, 1963). Lagaaij (1952) correlated the 'Lenhamian' with the 'Lower Diestien' zone a Terebratula perforata of Belgium.

The Lenham Beds have played an important role in the debate over the uplift and denudation history of the Weald (e.g. Wooldridge and Linton, 1938, 1955; Worssam, 1963; Jones, 1980) due to their elevation of up to 680 feet (207 m) above OD (Abbott, 1916). Wooldridge and Linton (1938, 1955) postulated a late Pliocene–early Pleistocene ('Calabrian) transgression which produced a widespread marine planation surface over south-east England up to an elevation of 210 m upon which marine sediments were deposited, including the Netley Heath Beds in the west and the Lenham Beds in the east. However they failed to take into account the age difference between the two deposits, possibly greater than 3 million years, and believed that they were both deposited from a single transgression that spread from east to west, depositing first the Lenham Beds and then the Netley Heath Beds. It is more likely that the Lenham Beds were deposited during a separate transgression that occurred much earlier than the transgression responsible for the deposition of the Netley Heath Beds, and that the role of Plio-Pleistocene marine erosion in forming planation surfaces (Wooldridge and Linton, 1938, 1955) has been overstated (see discussion in Can and Hodgson, 1976).

The magnitude and timing of post-depositional tectonic uplift, particularly of the Wealden axis, and subsidence of the southern North Sea Basin, are key factors in the understanding of the Neogene evolution of south-east England. Reid's (1890) suggestion of a water depth of 40 fathoms (73 m) during deposition of the Lenham Beds would imply a sea level 280 m above present mean sea level. The Coralline Crag of Suffolk was probably deposited in water depths similar to, or less than, those of the Lenham Beds and has a maximum surface elevation of only 18 m above OD. Unless eustatic sea level was greatly different between deposition of the two deposits, this implies that the majority of the difference in elevation is due to tectonic uplift. The relative elevations of the two deposits can be explained by a regional tilt of only 0.1°. Van Voorthuysen (1954) illustrated the magnitude of uplift and subsidence in eastern England and the southern North Sea since Pliocene times with c. 200 m of uplift along the northern limb of the Wealden anticline and over 700 m of subsidence in the Netherlands. A 'hinge' line with no net subsidence or uplift lay in the vicinity of Ipswich and thus in the area of deposition of the Red and Coralline Crags. A more controversial explanation for the Lenham Beds location was offered by Shephard-Thorn (1975) who suggested that the sediments were glacially transported to their present position. If such a model was correct the elevation of the Lenham Beds could not be used to infer past sea levels, nor rates of tectonic uplift.

A reappraisal of the Lenham Beds fauna by Janssen (in Balson, 1990a) suggests that the fauna has closer affinities with the Redonian of Brittany than with the Neogene of the North Sea Basin. This observation may have important implications to the reconstruction of the palaeogeography of Lenham Beds time and suggests that a physical barrier may have been present to the north of the North Downs rather than as a result of Wealden uplift to the south.

The Lenham Beds therefore remain controversial to the present time. Their exact age is still unknown. The environment of deposition and the mechanism by which they came to be preserved within solution pipes has still to be resolved. The resolution of these questions will shed light on the palaeogeography, environment and tectonic history of south-east England during the Neogene.

References