AUTOCHTHONOUS MOLLUSC FAUNAS OF THE RED CRAG

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AUTOCHTHONOUS MOLLUSC FAUNAS OF THE RED CRAG R. DIXON Background W. Whitaker in 1885 said: were it not for the shells, the Red Crag would “probably have been almost unnoticed by geologists”. Quite so.

The Red Crag is a shelly sand deposited in a shallow tidal sea about 2·4 million years ago over much of coastal Suffolk, northern Essex and inland as far as Bishops Stortford. It has long been studied on account of its mollusc fauna – indeed, Crag Mollusca was the subject of the first monograph published by S. V. Wood by the Palaeontographical Society in 1848.

The sediments are characterised by sandwaves, some 3 m or more high, the product of strong marine currents of c. 0·6 m/sec in water depths of up to 30 m, with a general regional flow towards the south-west. The upper part of the Crag, north of the Deben Estuary shows much evidence of channelling, with ebb and flow tidal patterns and shallower water depths of 10 m or less, and including infralittoral and low shore intertidal sediments.

The environment, then, was one where scouring by currents and sandwave migration were dominant, where shells become incorporated into the sediment load and subjected to sedimentary processes, and where comminution and reworking are the norm. At first sight it would thus appear unlikely that truly non-derived faunal assemblages have survived.

However, as long ago as 1927 Boswell (p. 40) commented that “the Crag may display evidence of molluscan communities” and that “the association of certain species does not appear to have been entirely destroyed”. Wood had made comparable comments in his monograph. This was further demonstrated in the mid-1970s by the author in quantitative statistical analyses, where molluscan percentage frequencies of different samples were compared and correlated by cluster analysis, and distinct assemblages recognised in both geographical and temporal distributions. Further, comparisons with the ecologies and environments of present day communities demonstrated that in general, although transport of shells has occurred and sedimentary processes have undoubtedly modified Red Crag communities, fossil assemblages do closely reflect the original communities.

The faunas are dominated by shell gravel assemblages, which change from Glycimeris-Venerupis types in the southern part of the basin to Spisula-Mytilus and Mya-Macoma types further north.

Autochthonous faunas The preservation of truly non-derived assemblages is not common, but nevertheless do occur: 1. Abra alba-rich assemblages with Echinocardium cordatum in muddy bottomsets at Walton-on-the Naze [Abra makes up 22% and mud-tolerant species a further 12%]; and from Spisula-dominated silty fine sands at Ramsholt. In the latter, Spisula ovalis makes up 44% of the assemblage [Abra 16%], and many are articulated and possibly in the life position (both are described in Dixon, 2001). The finding at Walton of fragile but complete E. Cordatum tests, even with spines still attached, indicates that post-death burial must have been prompt and transport minimal.

Trans. Suffolk Nat. Soc. 43 (2007)

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2. At Mason’s Pit at Great Blakenham abundant moulds of Mya arenaria were recently discovered (Dixon, 2005) in a pebble bed assumed to be of Red Crag age overlying the Chalk and underlying the Creeting Sands. It is the first time that complete invertebrate specimens have ever been found at this locality. The moulds were of adult forms and showed that both valves were together and closed upon death, and their upright position indicates they were probably in the life position. Fragmentary moulds of Cardium edule and other species were also present. An inter-tidal or infralittoral environment for the horizon can thus be interpreted.

3. The GeoSuffolk flagship site at Sutton Knoll [Rockhall Wood] has so far yielded three further examples of non-derived faunas.

During Red Crag times the Coralline Crag formed an island (see Dixon, 2006 for background). ‘Fossil’ beaches and cliffs were recognised by Prestwich nearly 150 years ago and described in his classic papers (1871 pt 1, pt 2). The author has long suspected that different habitats existed around the island, and these were colonised by different mollusc communities. English Nature (as then was) cleared much of the site as part of their ‘Facelift’ programme, but also, in order to interpret the site more fully, cleared three faces in the Chicken Pit, which lies outside the SSSI, and dug a total of eight exploratory holes from which the Red Crag could be assessed and samples taken. Analysis of the samples is nearing completion and the results will be published in due course. Figure 1 illustrates the nature of the environment around the island in Red Crag times.

Figure 1. Illustrative sketch showing the Coralline Crag island at Sutton during the time of the Red Crag sea and its environments.

Trans. Suffolk Nat. Soc. 43 (2007)

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A). Part of the Red Crag /London Clay boundary was exposed in the Chicken Pit early in 2006. The entire exposed London Clay outcrop surface, about 2 m × 2 m, was marked by abundant Zirfaea burrows, c. 50 mm deep and c. 40 mm apart. Its occurrence in these numbers is unique for the Red Crag – only isolated examples of Zirfaea burrows have been found before at other localities.

Zirfaea crispata is an infralittoral bivalve, commonly found from ELWS to a depth of c. 10 m. It mechanically bores into soft rock, firm clays, shales, silts, peat and wood. It is evident that the monospecific community thrived in this habitat before being overwhelmed by sandwaves.

Prestwich (1871, pp. 339–341) also comments that Pholas (Zirfaea) can be found boring into Coralline Crag at this site.

B). Colonies of Mytilus edulis are found in abundance at or adjacent to the Red Crag /Coralline Crag boundary wherever it has been exposed at this locality, and indeed were noted by Prestwich. In fact, they can help to identify the boundary itself when the two lithologies may at first sight appear so similar due to the amount of reworked Coralline Crag material in the adjacent Red Crag. The valves are articulated and closed, and the full size range is present, juveniles as well as adults.

Mytilus is typically an intertidal rocky shore species, attaching itself to firm substrates by means of its byssal thread, and the communities discovered at Sutton are undoubtedly in their life positions.

Figure 2. Red Crag overlying Coralline Crag, with boulders of Coralline Crag within the basal Red Crag on the former ‘beach’ surface. Mytilus colonies can be found in the Red Crag above the junction.

Trans. Suffolk Nat. Soc. 43 (2007)

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C). Careful clearing at the western end of the Chicken Pit has revealed another unique and very special fauna. The section has revealed blocks of Coralline Crag, which evidently littered the Red Crag beach/cliff zone, dominated by articulated Mytilus-rich assemblages. Many shells and cobbles of Coralline Crag have Red Crag barnacles attached.

The blocks are interspersed by pockets of relatively clean sand, which reveal many perfectly preserved shells, including articulated Tapes apparently in the life position. Anomia, Buccinum, Colus, Neptunea and the Naticidae family are particularly well represented (Table 1 – see appendix). It is evident that the fallen cliff blocks provided an ideal microenvironment for this unusual assemblage.

Tapes typically occurs in shallow (ELWS and below) sublittoral shell gravels and coarse sand. By contrast, genera dominating assemblages only a short distance away from the ‘island’, such as Glycimeris and Spisula, are very poorly represented and poorly preserved.

Endpiece It is exciting to think that it is only relatively recently that these non-derived faunas have been found in the Red Crag and it is hoped that further close scutiny, particularly at Sutton Knoll, will reveal yet more examples.

References Boswell, P. G. H. (1927). The Geology of the Country around Ipswich. Mem. Geol. Survey of England & Wales. H.M.S.O. Dixon R. G. (2001). A note on the occurrence of Echinocadium cordatum from the Red Crag, Pliocene, East Anglia. Bull. Geol. Soc. Norfolk 51: 105–111.

Dixon R. G. (2005). On the occurrence of a mollusc fauna in the Red Crag pebble bed from Mason’s Pit, Great Blakenham, Suffolk. Proc. Geol. Ass. 116: 139–142.

Dixon R. G. (2006). Sutton Knoll. Trans. Suffolk Nat. Soc. 42: 97–101.

Prestwich, J. (1871). On the structure of the Crag Beds of Norfolk and Suffolk, with some observations on their organic remains. Part 1: The Coralline Crag of Suffolk. Part II: The Red Crag of Essex and Suffolk.

Quart. Journ. Geol. Soc. Lond. 27: 115–146, 325–356.

Wood, S. V. (1848–1882). A Monograph of Crag Mollusca. Palaeontological Society, London. [1848 Vol 1: Univalves. 1851–61Part II: Bivalves. 1872– 1874 1st Supplement. 1879 2nd Supplement. 1882 3rd Supplement.]

Roger Dixon The White House 7 Chapel Street Woodbridge Suffolk IP12 4NF

Trans. Suffolk Nat. Soc. 43 (2007)

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Appendix Table 1. Species list from sand pockets between Coralline Crag boulders in the Chicken Pit Molluscs

*

Molluscs

*

Anomia patelliformis Arctica islandica Astarte obliquata A. omalii A. sp Cardita scalaris C. corbis Cardium parkinsoni Chlamys harmeri Dosinia exoleta Ensis sp Glycimeris glycimeris Hiatella arctica Macoma obliqua Modiolus modiolus

+f +

Calliostoma zizyphinum Capulus obliqua C. ungaricus C. sp Colus sp Emarginula reticulata Gibbula sp Lunatia sp Mangelia sp Nassa granulata N. labiosa N. reticosa Natica multipunctata Neptunea contraria N. ‘despecta’

+f +f +

Nucella tetragona N. lapillus Polinices hemiclausa Potamides tricinctus Searlesia costifer Trophon clathratus T. muricatum Turritella incrassata

+f

Mya arenaria Ostrea edulis Panopaea faujasii Pygocardia rustica Spisula arcuata S. ovalis Tapes sp Zirfaea crispata Admete viridula Buccinum undatum B. dalei Brachiopod Terebratula grandis

f (including 1 articulated pair)

+

+f

+f + +f f f +f +f +f

+f f

+f + +f

*’+’ signifies good preservation; ‘f’ denotes the more frequently occurring species.

Trans. Suffolk Nat. Soc. 43 (2007)