Retreat of the Late Pliocene and Lower Pleistocene Crag sea
Tectonics appear to influence the Red and Norwich Crag Formations on more than one scale. The
Formations are found at or below present sea-level on the coast at places such as Easton Bavents and
Sizewell, but at Great Blakenham their base is at 36 m OD and at Elsenham it has risen to 90 m OD,
before reaching 200+ m OD further west. At all these sites, the sediments have nearshore
characteristics and so would have been at, or just below, contemporary sea-level at the time of
deposition. Thus the western area has been uplifted. The uplift represents a major tectonic event that
dramatically altered the palaeogeography of southern England. The uplift is generally associated
with subsidence of the North Sea basin due to the weight of the sediments accumulating there (e.g.
Mathers & Zalasiewiez 1988). However, Boulton (1992) argued that subsidence in the North Sea
Basin was restricted to the Central Grabcn and did not extend as far as the coast of England. The
relative uplift of western Britain he associated with isostatic activity related to sea-floor spreading in
the North Atlantic. The large amount of sediment infilling the southern North Sea was probably
derived from erosion of Tertiary sediments inland. The resulting reduced loading there could have
been a contributory factor in the uplift (Bridgland 2000) The major period of uplift occurred relatively
rapidly late in the deposition of the Norwich Crag (Mathers & Zalasiewicz 1988), immediately
before the Sidestrand Member (Weybourne Crag) was deposited in north Norfolk. Had the North
Sea basin been subsiding continuously during deposition of the Crags, deeper water facies would be
expected to the east.
The distribution of the Red and Norwich Crags in East Anglia (Fig. 1) shows three major depositional
basins, a coastal basin extending from Weybourne to Sizewell, from which two extensions run
northeast-southwest, one through Ipswich (the Ipswich Basin) and the other through central Suffolk
(the Stradbroke-Sudbury Basin). The two reunite in the Sudbury area. There are also small but
significant basins, such as at Great Blakenham, which now do not appear to be linked to the major
basins. The origin of these major basins has not been discussed in the literature, but maybe associated
with the uplift of western Britain outlined above. Bristow (1983) argues that they are fault controlled.
Within the Stradbroke-Sudbury basin there are notable thicknesses of Noiwich Crag, possibly reaching
80 m near Stradbroke, which can be traced as far west as Elsenham. However, in the Ipswich basin
the Norwich Crag is much less important, found only as far west as Ipswich reaching thicknesses of
about 5 m as at Valley Farm. This would seem to imply that the Ipswich basin became less active
after the deposition of the Red Crag while the Stradbroke-Sudbury basin continued to be very active.
Within the basins, there are deeper troughs, the most significant examples being centred on Stradbroke
and on Eye in mid-Suffolk. Similar troughs can be seen in the southern North Sea today, such as the
Silver Pit. The origin of these troughs is debated. Woodland (1946) and Bristow (1983) considered
that they were of tectonic origin. Woodland noted that water is much more difficult to extract from
the Chalk beneath the troughs, suggesting that the joints are closed as would be the ease in a syncline.
However, Funnell (1972) attributed the troughs to tidal scouring and drew parallels with present sea
floor of the English Channel. Zalasiewicz et al. (1988) noted that flie Red and Norwich Crags cut
through the Coralline Crag and topmost London Clay on the southern side of the Sizewell Basin,
suggesting an erosional (= scoured) relationship.
The uplift caused the coastline to retreat to northeast Norfolk (Funnell 1996). The sea was sufficiently
shallow that the area was very sensitive to relative sea-level change (Fig. 2b). Slight oscillations in
sea level tipped the balance between shallow marine and terrestrial freshwater environments while
the Dobbs, How Hill and Mundersley Members of the Wroxham Formation were deposited. The
environment may be likened to the Wadden See of north Germany today. The withdrawal of the sea
to north Norfolk was principally tectonically driven, as discussed above, but the oscillations were
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Essex Naturalist (New Series) 18 (2001)