The Upper Caenozoic sequence around the Long Row, Covehithe

THE UPPER CAENOZOIC SEQUENCES THE LONG ROW,

AROUND

COVEHITHE

H . B . MOTTRAM

T h e Norwich C r a g at Covehithe is regarded as extending down to about - 5 0 m O D which is well below the levels observable in cliff sections or excavations. T h e g r e a t e r part of it is t h e r e f o r e concealed and i n f o r m a t i o n is limited to that inferred f r o m a few wells and b o r e h o l e s in the a r e a . This i n f o r m a t i o n indicates that the concealed strata are mainly c o m p o s e d of sands which w e r e generally deposited u n d e r shallow o f f s h o r e conditions, though not necessarily as a Single s e q u e n c e . T h e exposed Norwich C r a g is largely c o m p o s e d of n e a r s h o r e sands which occur as two sequences. T h e lower s e q u e n c e indicates that in this area the d e v e l o p m e n t of tidal flats was d o m i n a n t , while t h e higher s e q u e n c e indicates that the d e v e l o p m e n t of beach faces was d o m i n a n t . T h e youngest sediments exposed in t h e cliff line reflect a c h a n g e f r o m m a r i n e to fluvial conditions and progressive onset of glacial conditions. It should be n o t e d that the cliffs have been e r o d e d at a fairly rapid rate and so t h e cliff line at the Long R o w in 1987, illustrated by Fig. 1, was about 430m f u r t h e r west than when observed by W h i t a k e r b e t w e e n 1877 and 1880 ( W h i t a k e r , 1887) and s o m e 50m f u r t h e r west than when m e a s u r e d by W e s t in 1974 ( W e s t , 1980).

T h e bases of the cliffs along this Stretch of coast are exposed to best a d v a n t a g e during t h e winter and spring m o n t h s . A t E a s t o n B a v e n t s this has e n a b l e d t h e recording of sands which contain s o m e very shelly b e d s (Larw o o d & M a r t i n , 1953; Funneil & W e s t , 1962). D u e to coastal protection such e x p o s u r e s n o longer occur at this classical location but they can still b e f o u n d against E a s t o n W o o d . M a n y shells a r e w o r n , and although this is not sufficient evidence to prove accumulation u n d e r storm conditions it clearly shows t h e likelihood of significant reworking and mixing of shells f r o m different environments.

A t C o v e h i t h e the present day f o r e s h o r e exposures are d o m i n a t e d by m u d s . G e n e r a l l y , t h e lowest visible m u d s have the richest clay c o n t e n t , the silt and fine sand c o n t e n t increasing towards the t o p of the f o r e s h o r e e x p o s u r e s . A Single black b a n d , smelling of anaerobic products, m a y also be n o t e d . Shells, largely a t t r i b u t e d to subtidal dwellers, sometimes occur in the m u d s but they are prevalent in occasional b a n d s of m e d i u m to coarse sand.

M o r e c o m m o n l y f o u n d in the fine sediments are small cylindrical b u r r o w s , a b o u t 10mm long by 4 m m d i a m e t e r and of variable attitude. Initial examination suggested that the b u r r o w s w e r e f o r m e d by w o r m s or small crust a c e a n s , b u t on two occasions it was f o u n d that they can contain in-situ the bivalve mollusc Macoma calcarea. A s would be expected f r o m the size of the b u r r o w s , t h e in-situ individuals were smaller than 'loose' united or Single valved s p e c i m e n s normally f o u n d at these or other levels. T h e impact of the cold climate of this time (West etal., 1980) may have been responsible for an i m p o v e r i s h e d f a u n a and p o o r d e v e l o p m e n t of individuals in the shallowest e n v i r o n m e n t s . This should suggest that the b u r r o w e d m u d s were deposited

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under intertidal conditions with the larger shells and coarser sand having been thrown up from lower levels by storms. However, M. calcarea is not regarded as having inhabited intertidal levels, being subtidal at the present day. T h e small burrows are very common at certain horizons and the intensity of bioturbation has sometimes totally destroyed any original lamination. The small burrows are usually absent in slumped Sediment but. if present, they are distorted. The slumping referred to appears to have been directed into furrows of somewhat north-south orientation. On several surfaces of slumped and flat beds there are polygonal shrinkage cracks which vvould have formed as drying out occurred at the top of the sediment when it sometimes lay above water level. On some flat surfaces ripples may also be found. Overall, the implication is that the exposed, and possibly some concealed muds, were deposited as flats in a quiet environment around high tide mark. It is therefore likely that the furrows represent shallow drainage features known as rills which fed creeks flowing approximately eastwards.

In the lowest part of the cliff around the Long Row, muds are subordinate to sands and the burrows are larger and subvertical erect to L-shaped.

Sediment instability structures and flasers may be seen and current ripples are more common. The ripples indicate flow to the north-north-east and sometimes to the south-south-west, i.e., reversible and parallel to the coast, which is typical of tidal currents. The deposits are interpreted as representing more active intertidal conditions. At the foot of this section there is a 'loamy' bed containing shells which is considered to be a storm deposit. T h e shells are often bored and are composed almost entirely of M. calcarea with a few Periwinkles, Littorina littorea.

T h e Geological Survey map of 1883 infers that the muds are persistent along the Easton Valley from the cliffs to Frostenden. From this, and the occurrence of muds in boreholes, see Fig. 2, it is suggested that a wedge of muds generally occurs throughout a 5km wide coastal strip, its presence further inland being less clear. T h e wedge continues northwards (West, 1980) and is known to occur around - 1 0 m O D at Pakefield (Brown, 1977).

As the muds are traced to the south of Easton Bavents their development becomes poor and they are absent at Southend Warren. However, the muds probably reappear where development has been favourable. At Southend Warren the muds are replaced by sands, the lowest of which were deposited by north-north-eastward currents. Upwards they alternate with sands deposited by south-south-eastward currents. This alternation often gives the bedding a 'herring bone' pattern, a characteristic feature of nearshore tidal sands. T h e sands are often gravelly and are truncated at an estimated level of + 6 . 5 m O D at Southend Warren. At Covehithe, between this sort of level and the underlying intertidal deposits, the sands contain gravel in sheets and Channels. T h e sand and gravel is moderately sorted and indicates dominantly north-north-eastward currents. T h e gravel fraction often appears to be distinctly quartzose but the 1 6 - 3 2 m m size ränge traditionally used in studying pebbly materials in the region classifies it as a rounded flint deposit.

The top of the intertidal sands and muds occurs at variable levels and throughout much of the cliff line there is a distinct junction with the overlying sediments. This can lead to the supposition that the top of this sequence has

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been significantly eroded. However, erosion can be explained as a contemporaneous event largely due to the scouring and channelling associated with the deposition of the gravelly beds. It is suggested that if a significant erosion plane is present within the exposed Norwich Crag then it is a less distinct plane which lies within the granular strata at about +6.0m O D at the Long Row and that it marks the base of a higher sequence of exposed Norwich Crag. It is also considered that it correlates with the truncation plane at Southend Warren mentioned earlier and that occurring at +7.4m O D in pit C of Hill Farm Pits (Mottram, 1988).

The sands succeeding the truncation plane at Southend Warren were deposited by currents flowing to the south-south-west. Gravel, of rounded flint type, is present as lag planes and in small Channels. Near the top of this Stratum between Southend Warren and Easton Bavents the gravel also occurs in medium-sized Channels up to 30m wide and 3m deep and as low ridges parallel to the Channels. The Channels are less common at Covehithe but exposure here is more important since the Channels can be traced as the cliff recedes and observations have shown that the Channels soon tail out in their upstream direction. The Channels are interpreted as rip Channels cut into beach shoreface sands which were swept by south-south-westward (long-shore dominated) currents. The gravels are moderately sorted and whereas the gravel fraction is not so coarse as that of major Channel infill at Hill Farm Pits the sand fraction is coarser. Thus, although the infill materials may have a common source their modes of deposition were slightly different.

It is envisaged that berms of gravel were thrown up along the beach foreshores by storms and that the berms were reworked by rip currents in the manner described by Leckie and Walker (1982); subsequent to this the gravel was remobilised by longshore currents and spead into major Channels.

Above these depoists at the Long Row is a thin, often iron stained development of sand and gravel that contains 20% quartz and quartzite in the 16-32mm size ränge. It is most likely that it is part of the Kesgrave Formation reported to have been deposited from an early River Thames in a periglacial environment. The base of the overlying beds also contains quartzose gravel but in the main the beds consist of sand with chalk grains and occasional concretionary lenses, the lenses containing plant impressions. In the sands the bedding is frequently irregulär, may show slumping and was deposited from extremely variable directions of flow. These sands are believed to represent the Corton Sands produced as glacial outwash from the Anglian ice sheet. A narrow band of silt separates the sands from the decalcified Lowestoft Till at the top of the cliff.

References Brown, S. (1977). Groundwater quality surveillance near a landfill site comparative study of groundwater sampling techniques. In groundwater quality - measurement, prediction and protection. Water Research Centre, Medmenham. 385.

Funneil, B. M. & West, R. G. (1962). The early Pleistocene of Easton Bavents, Suffolk. Q. J.Geol. Soc. London, 118, 125.

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Larwood, G . P. & Martin, A . J. (1953). Stratigraphy and faunaof the Easton Baventscliffsection, n e a r S o u t h w o l d , Suffolk. Trans. Suffolk Nat. Soc. ,8, 157.

Leckie, D. A . & Walker, R. G . (1982). Storm and tide dominated shorelines in Cretaceous Moosebar - Lower Gates interval, outcrop equivalents of deep basin gas trap in Western Canada. Amer. Ass. Petrol. Geol. Bull.. 66, 138.

Long, P. E. (1974). Norwich Crag at Covehithe, Suffolk. Trans. Suffolk Nat. Soc., 16, 199.

Mottram, H. B. (1988). Norwich Crag at Hill Farm Pits, Wangford. Trans.

Suffolk Nat. Soc., 24, 88.

West, R. G . (1980). The pre-glacial Pleistocene of the Norfolk and Suffolk coasts. Cambridge Univ. Press. Cambridge.

West, R. G . , Funnell, B. M. & Norton, P. E. P. (1980). An Early Pleistocene cold marine episode in the North Sea: pollen and faunal assemblages at Covehithe, Suffolk, England. Boreas, 8. 1.

Whitaker, W. (1887). The geology of Southwold and the Suffolk coast from Dunwich to Covehithe (explanation of quarter-sheet 49, N.). Mem. Geol.

Survey.

H. B. Mottram, 66 Glastonbury Close, Ipswich, IP2 9 E E

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