The Concealed Chalk Surface of Mid-Suffolk

THE CONCEALED CHALK SURF ACE OF MID-SUFFOLK GEOFFREY J . NOTCUTT

Summary T h e concealed Chalk surface of part of Suffolk is examined with information obtained from well records. Certain features, including buried Valleys, depressions and a warp axis, are identified, and used in conjunction with stratigraphic evidence to establish a sequence of events.

Introduction T h e overall form of the concealed Chalk surface of Suffolk is reasonably well knpwn, due to the abundance of water wells sunk to the level of the main aquifer, the Chalk. Particular attention has been paid to narrow, steep sided depressions cut into this surface, and referred to by Whitaker (1904) as 'Drift-filled Channels', by Boswell (1913) as 'buried chanels', and by Woodland (1970) as 'buried tunnel Valleys'. The aim of this paper is to examine the concealed Chalk surface in the area shown on Map 1, which corresponds approximately to G . S . Sheets 175, 176, 190,191.

General Geology T h e Chalk, which underlies the whole of Suffolk, dips gently to the east and south-east. It is rarely exposed in the east of the county, where river erosion has occasionally cut down sufficiently to reveal Upper Chalk (e.g. isolated patches in the Waveney valley, near Diss).

Resting unconformably on the Chalk are Lower Tertiary deposits, predominantly silts and clays, which have in most cases been separated on a lithological basis into two divisions, namely the Woolwich and Reading Beds, and the more argillaceous London Clay. Although limited today to a narrow coastal strip (Map 1), these formations probably once covered a much greater area. Most of the information about them comes from drillers' logs of uncertain reliability, and it is possible that in places sands and silts of Pleistocene and Tertiary age have been confused. In addition, few wells in the area penetrate the Tertiary. Thus, inadequacies in the data permit only broad generalisations to be made about these deposits. The available evidence indicates that the London Clay is missing from the western margin of the subcrop, and

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that b o t h the L o n d o n Clay and the W o o l w i c h and Reading Beds thickert irregularly to the east.

Recent boreholes at H a i e s w o r t h encountered about 9m. of silty clays resting directly on the Chalk and overlain by Pleistocene deposits. Fisher and Williams (1975) studied the f o r a m i n i f e r a o f these clays and showed that they are a shallow marine fauna of Thanet Beds age. Nowhere eise in the area have T h a n e t i a n beds been recognized, although it is possible that elsewhere sediments of this age have been identified incorrectly as W o o l w i c h and Reading Beds.

T h e o v e r l y i n g sands and shelly sands of the various Crags are m o r e extensive in o u t c r o p than the Tertiary deposits, but are again l i m i t e d to the eastern part of the county ( M a p 1).

W i t h a few exceptions (e.g. Stradbroke borehole — T M 233 738), the concealed Crag has not been studied in detail. Beck, F u n n e i l and L o r d (1972) have shown f r o m pollen analysis that 45m. o f L u d h a m Crag at Stradbroke are underlain by 23m. of pre-Ludhamian sediments. T h e pre-Ludhamian faunal assemblages include foraminifera which closely resemble those o f the exposed Red Crag o f Suffolk and Walton. Most of the solid f o r m a t i o n s are concealed by a thick veneer of d r i f t , m a i n l y boulder clay overlying sands and gravels. The boulder clay occasionally exceeds 30m. in thickness, and although near the coast it tends to thin to less than 10m. such attenuation rarely occurs inland. T h e drillers' logs seldom distinguish between glacial and Crag sands and gravels.

T h e Concealed Chalk Surface M a p 2 shows the contours of the concealed Chalk surface, constructed by the author f r o m the logs of over 800 wells.

M o s t o f the logs are the interpretations of the drillers, rather than geologists, but since the prime object of the bores was to reach the m a i n aquifer, the C h a l k , it is probable that the depth to this f o r m a t i o n has been recorded w i t h reasonable accuracy.

T h e concealed surface ranges in height f r o m 46m. above O . D . near Wattisfield ( T M 008 737) to 87m. below O . D . at S o u t h w o l d ( T M 502 762). In the west of the area, the surface f o r m s a p l a t f o r m at about + 2 0 m . O . D . , but east of a line passing approximately N N E - S S W through Haiesworth it slopes d o w n gently to the east. The morphology of the p l a t f o r m and its change o f slope (which for reasons discussed later is considered to be due to warping) are demonstrated on Figl.

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Two broad depressions traverse the surface, both trending NNE-SSW. O n e lies immediately to the west of Eye, and the other to the east: they are referred to respectively as the western Eye depression and the eastern Eye depression. They join about 5km. north of Eye. South of Bungay they merge into a gently undulating surface declining to the north and east.

Adjoining the western Eye depression is a narrow buried valley, several kilometres long, and passing NW/SE beneath Diss. The term 'valley', as opposed to 'depression', connotes a definite morphological distinction. The Valleys are steep sided and 'V' shaped; the depressions, although steep sided, have a wide base.

A n o t h e r buried valley, with a NW/SE trend, passes beneath Haiesworth. It can be traced over a distance of about 4km.

Less positively defined is a shorter buried valley, between 1 and 2 km. long, which joins the eastern Eye depression.

These main features of the concealed Chalk sürface, namely the change of slope, the depressions and the buried valleys, are examined in greater detail in the following sections.

Change of Slope The profiles of Fig 1, which demonstrate the change of slope of the Chalk surface, have been drawn with a vertical exaggeration of x 25. This amplifies considerably the magnitude of the change, the true dip of the slope down to the east being of the order of V2°-l°.

Although this slope is slight, a comparison of Maps 1 and 2 shows that it may be important in Controlling the distribution of the Tertiary sediments. At the western margin of the subcrop of the Tertiary the London Clay is missing, although the Woolwich and Reading Beds are still to be found in this area. This could be a result of offlap, but facies evidence indicates otherwise. According to Curry (1965), 'the whole pattern of occurence of the London Clay suggests that after Blackheath times there was a marine transgression on a wide front from the east over the whole of the area formerly occupied by the Lower London Tertiaries. . . . " It therefore seems probable that in Suffolk the London Clay overlapped the Woolwich and Reading Beds, and that the subcrops of those formations reflect primarily the effects of erosion. Some possible sequences of events concerning the change of slope are shown on Fig 2. If it occurred prior to Tertiary Sedimentation, then irrespective of

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OF

:

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POSSIBLE r

I Q

2 .

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KEY S E Q U E N C E S OF

EVENTS Reading

Beds

the position of the pinchout of the Woolwich and Reading B e d s , erosion would have to remove all the L o n d o n Clay to bring the Woolwich and R e a d i n g Beds in contact with the Pleistocene. H o w e v e r , if the change is due to post-deposit i o n a l warping, erosion could result in the Pleistocene directly overlying the Woolwich and R e a d i n g Beds, whilst still p r e s e r v i n g the L o n d o n Clay to the east. In addition, this w o u l d p r o d u c e a narrow subcrop of Woolwich and Reading B e d s , a p p r o x i m a t e l y l - 2 k m wide (assuming a dip of V20—1° a n d a thickness of 20m), parallel to the warp axis. This c o r r e l a t e s closely with the Situation shown on M a p 3. 1t is t h e r e f o r e c o n c l u d e d that the change of slope is a warp, and that most, if not all, of the warping took place after the d e p o s i t i o n of the L o n d o n Clay. T h e p r o j e c t e d positions of the w a r p axis are shown on Fig 1. O n each profile, the axis has b e e n p r o j e c t e d to + 2 0 m O . D . to remove the effects of a p p a r e n t displacement resulting f r o m lowering of the surface by e r o s i o n .

The Depressions T h e two depressions show several similar features. T h e y are e l o n g a t e d in a N N E - S S W direction, with wide floors and steep sides. B o t h are basinal, with the lowest points in a central e n c l o s e d a r e a , a n d filled with substantial thicknesses of Crag.

It is not possible to show Crag isopachytes, since C r a g and glacial sands and gravels are rarely distinguished in the drillers' logs; h o w e v e r , Fig 3 shows a typical well log from the

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ridge betvveen the two depressions compared w i t h a well log f r o m the eastern depression. These logs demonstrate that whereas the boulder clay varies little in thickness, the Crag is considerably thicker in the depression.

T h e o r i g i n o f the depressions is a matter of speculation. It is probable that their alignment is structurally controlled. since they are parallel, and their N N E - S S W trend is closely reflected by three other lineations, namely the N N E - S S W subcrop of

10 "27

2 i s : 12 "11

"20

0.

9 0.

NORTH SEA

0

12

.15

21

15

' 8 'Oldhaven 8eds 16

THICKNESS OF DEPOSITS

Map 3.

TERTIARY

(in metres)

KEY:

• Well encountering Tertiary (upper figure—London Clav lower figure — Woolwich & Reading Beds)

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the L o n d o n Clay and Woolwich and Reading Beds, the N N E - S S W warp axis, and the N-S strike of the Chalk zones (Boswell, 1916). T h e greater accumulation of Crag in the depressions could be due to either infilling of existing basins or preservation by post-depositional tectonic activity. Woodland (1946), using inconclusive hydrogeological evidence, suggests that the depressions are synclines, with an intervening anticline. H o w e v e r , equally satisfactory explanations could m 0 D 55 "

m.O D 50

boulder clay

boulder clay 34 30

34

glacial sands and gravels

m m

sands and gravels Crag

M 11 50i Crag

Chalk

-14

T M 169 727 (between

depressions)

(from I.G.S. w e l l logs)

- 3 9 5-

Chalk

T M 2 3 3 738

(eastern Eye depresslon)

( f r o m B e c k F u n n e l l and Lord

Fig 3.

1972)

TYPICAL WELL LOGS

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be provided by faulting, or karstic weathering exploiting structural lines of weakness, or a combination of mechanisms.

T h e problem cannot be resolved until more detailed investigations of the stratigraphic sequences within and between the depressions are carried out.

A n additional consideration which may be related to the formation of the depressions concerns the distribution of the Crag. M a p s 1 and 2 show that adjacent to the eastern Eye depression is a NE-SW trending ridge, over which the Crag is missing; the height of the Chalk surface in this area is about 20m above O . D . Further west, near Eye, the Chalk surface is at a similar height, but the Crag is present. The position of the ridge and its alignment sub-parallel to the depression suggest that there may be an underlying causal relationship between this anomaly of the Crag distribution and the origin of the depressions. This possible connection should be borne in mind in any examination of these features.

The Buried Valleys T h r e e narrow, steep sided buried Valleys have been identified f r o m M a p 2, at Diss, Haiesworth and possibly one on the eastern flank of the eastern Eye depression.

T h e Diss valley is typical of the 'buried tunnel Valleys' described by Woodland (1970). These are formed as the result of sub-glacial stream erosion, and are characterised by having steep sides, narrow bottoms, and undulating long profiles which often descend to below sea level. They are filled with fluvio-glacial sands, and are usually followed closely by present day rivers. Buried V a l l e y s of this type are quite c o m m o n in East Anglia, and are known elsewhere in Suffolk b e n e a t h the Gipping, Stour, Lark and Brett Valleys. As well as the obvious genetic consideration, the Diss valley can be shown to be post-Crag in age because of its stratigraphic relationships, since the western boundary of the Crag does not deviate into the valley. The fĂźll course of the valley extends outside the limits of the study area; Woodland (1970) shows it as passing beneath Diss for about lVikm to the north-west, beyond which it changes direction to the south-west (shown on M a p 2 by the two elongated zero metre contours) and continues off the study area for a further 2V2km.

T h e Haiesworth buried valley, whilst morphologically similar to that at Diss, is genetically different. A comparison of M a p s 1, 2 and 3 shows that the western limit of the Tertiary

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s u b c r o p , including both the L o n d o n Clay and the Woolwich a n d R e a d i n g Beds, is deflected into the Haiesworth valley.

T h i s suggests that these deposits either infilled an existing valley, or that Sedimentation was accompanied by penec o n t e m p o r a n e o u s subsidence. In either case, the formation of this valley is distinct in time and process f r o m the Diss valley.

T h e buried valley adjoining the eastern Eye depression is identified mainly f r o m the record of o n e well, at T M 246 670, w h e r e the C h a l k surface is a b o u t 20m lower than at a d j a c e n t wells to the north-east and south-west. A second well, at T M 257 669, shows a less significant d r o p in the level of the Chalk s u r f a c e of a b o u t 5m c o m p a r e d with the surrounding area.

T h e s e d a t a are t o o limited to form definite conclusions about the s h a p e or even the existence of the valley; if it d o e s exist, its age is p r o b a b l y post-Crag, since the C r a g b o u n d a r y does not a p p e a r to deviate into the valley ( c o m p a r e M a p s 1 and 2). T h e t h i c k n e s s of the b o u l d e r clay does not vary significantly over the p r o p o s e d valley, which suggests that an upper limit to the age of f o r m a t i o n would be pre-deposition of the b o u l d e r clay.

Conclusion T h e well records of the study area are to some extent limited b o t h in the a m o u n t of data they provide and in the reliability of t h e s e data. Within these constraints, it is still possible to establish a time sequence for the d e v e l o p m e n t of certain f e a t u r e s which characterise the concealed Chalk surface of mid-Suffolk: 1.

2.

3.

D e p o s i t i o n of the Woolwich and Reading Beds and L o n d o n Clay took place on a planatcd Chalk surface.

E v i d e n c e f r o m the Haiesworth buried valley indicates that this surface was probably already partly dissected.

Significant d o w n w a r p i n g of the Chalk surface east of H a i e s w o r t h occurred a f t e r deposition of the L o n d o n Clay a n d prior to the f o r m a t i o n of the Crag.

Subglacial erosion f o r m e d the Diss tunnel valley.

T h e r e are two omissions from this sequence. O n e is the p r o b a b l e f o r m a t i o n of the buried valley a d j a c e n t to the e a s t e r n E y e depression, which would a p p e a r to be post-Crag a n d p r e - d e p o s i t i o n of b o u l d e r clay in age. T h e second, more i m p o r t a n t omission, is the time of formation of the western a n d e a s t e r n E y e depressions, but there is insufficient i n f o r m a t i o n for any positive conclusions to be drawn c o n c e r n i n g this event.

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References Beck, R. B . , Funnell, B. M., and Lord, A. R. (1972). Correlation of Lower Pleistocene Crag at depth in Suffolk.

Geol.

Mag. 109, 137.

Boswell, P. G . H . (1913). On the Age of the Suffolk Valleys, with Notes on the Buried Channels of Drift. Quart. J. Geol. Soc. Lond. 63, 581.

Boswell, P. G . H . (1916). The Stratigraphy and Petrology of the Lower E o c e n e Deposits of the North-Eastern part of the L o n d o n Basin. Quart. J. Geol. Soc. Lond. 71, 536.

Curry, D. (1965). T h e Palaeogene Beds of South-East England. Proc. Geol. Ass. 76, 151.

Fisher, M. J., and Williams, G. M. (1975). A Palaeocene Foraminiferal Assemblage from Haiesworth, Suffolk.

Geol. Mag. 112, 47.

Whitaker, W. (1904). O n a Great D e p t h of Drift in the Valley of the Stour. Geol. Mag. 5, 511.

W o o d l a n d , A . W. (1946). Water Supply from Underground Sources of Cambridge-Ipswich District. Wartime Pamphlet No. 20.

W o o d l a n d , A. W. (1970). The Buried Tunnel Valleys of East Anglia. Proc. Yorks. Geol. Soc. 37, 521.

Acknowledgements I wish to thank Mr. N. J. D'Cruz, of Luton College of Higher E d u c a t i o n , for his advice in the preparation of this paper.

1 also wish to thank the Institute of Geological Sciences for making their well records available for inspection.

Department

Geoffrey, J. Notcutt, M.Sc., F.G.S., of Science, Luton College of Higher

Education