A preliminary account of the stratigraphy of Thorington Park Pit

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Suffolk Natural History,

Vol. 35

A PRELIMINARY ACCOUNT OF THE STRATIGRAPHY OF THORINGTON PARK PIT H. B. MOTTRAM In 1990 a ncw mincral working was started up in north-eastern Suffolk. This was the Thorington Park Pit centred at NGR TM/422727. The northern pari of the pit has now been backfilled and although cxcavation will continuc for several years in the rest of the sile, detailcd examination has become more difficult. It is therefore an appropriate time to set out the findings to date.

Concealed Strata There are no deep boreholes in the immediate vicinity to provide precise details. Extrapolation of regional data suggests that the Chalk is overlain by the London Clay at approximately - 5 0 m OD and that the London Clay is overlain by the Norwich Crag at about - 1 5 m OD.

Norwich Crag up to circa +14 m OD The lowest Norwich Crag seen within the northern part of the site was at about +5 m OD and consisted of fine sand. In some exposures this coarsened upwards into gravel and in other exposures it was abruptly overlain by this gravel. The gravel was moderately well sorted, like that at Covehithe (Mottram, 1989) and the pebbles were dominated by flint (virtually 100% in the 16 to 32 mm size ränge). Rusty bands within the gravel beds suggested that the rusty looking bones which occasionally tumed up in the Screening plant reject heap came from these bands. The site staff confirmed this.

Within the gravel were beds of sand and of mud. The intimate relationship of the mud and the gravel was shown where mud beds split around thin gravel beds and where thin gravel beds coalesced with flat bedded muds. Some rippling and waviness were recorded and even small, vertical burrows (15 mm high x 1-5 mm diam.) were observed. In the northern exposures, the foresets and ripples etc. indicated water flow to the north-east. Exposures near the old farm building indicated water flow to the north-north-west.

The sedimentary struetures encountered were most typical of tidal currcnts.

The dominance of flow in an overall northward direction without any opposing flow showed that this current was extremely persistent and/or generally protected from other influences. This suggests that the deposits could represent tidal flats within a lagoon or an estuary.

Gravel in tidal flat environments is uncommon but it does for example occur in the present day tidal reaches of the Blackwater in north-east Essex (Greensmith & Tucker, 1975; Bristow, 1985). The introduetion of gravel into shallow marine environments can bc from (i) a river discharging nearby, (11) storm reworking of off-shore banks, or, (iii) erosion of the coastline. For a sheltered environment the third mechanism is extremely unlikely.

Norwich Crag above circa +14 m OD The top of the 'good' gravel deposit described in the foregoing generally occurred at a level of between +13 and +15 m OD, but it wasn't always sharply defined. The sueeeeding strata were of pale coloured sand with a few gravel

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horizons and mud horizons. The sand was usually of fine to medium grain si/.e although beds of coarse sand were present. Foresets in the sand and in the gravel beds indicated an averagc of north-eastward flow. Load struetures and flaser like ripples were also present.

Two samples from near the top were analysed for pebble composition. Both confirmed that the gravel was largely composed of flint in the 16 to 32 mm size ränge. One sample had a quartzose content of 9% comparable to the highest Norwich Crag at Covehithe (Mottram, 1989), but the other had 22%, comparable to Bytham (Ingham) or Kesgrave Sand and Gravel. It was noted that the higher quartzose content of the latter sample was mainly due to additional Ordovician quartzite.

A couple of patches of sand in the same face but at different levels yielded fragments of largely deealeified shells. Pale bones and tusks have also been reported by the site staff.

The author has always assumed the Sediment to represent shallow marine Norwich Crag, a basic view also held by Hamblin & Moorlock (1995). The deposits probably represent tidal flats, perhaps in the lagoon behind a barrier island. The sedimentary features are consistent with this interpretation but they are not entirely diagnostic. The occasionally high proportion of quartzose pebbles may therefore imply that the Bytham (Ingham) River was discharging into the sea slightly further north.

Glacial The only Sediment attributable to the Anglian Glaciation was a thin tili. This was not present where the ground level was below approximately +20 m OD. The tili contained marl lenses near its base, but it was otherwise deealeified. In some exposures sand bands were present near the base of the tili.

References Bristow, C. R. (1985). The geology of the country around Chelmsford (sheet 241). Mem. Br. Geol. Surv.

Greensmith, J. T. & Tucker, E. V. (1975). Dynamic strueture in the Holocene chenier plane setting of Essex, England. 252. In Hails, J. & Carr, A. (eds.)

Nearshore sediment, dynamics and Sedimentation. London: John Wiley and Sons.

Hamblin, R. J. O. & Moorlock, B. S. P. (1995). The Kesgrave and Bytham Sands and Gravels of eastern Suffolk.. Quaternary Newsl., 77, 17.

Mottram, H. B. (1989) The Upper Caenozoic sequence around The Long Row, Covehithe. Trans. Suffolk Nat. Soc., 25, 86.

H. B. Mottram The Warren Duckamere Bramford Ipswich Suffolk IP8 4AH

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