Analysis of the geological evidence for Holocene sea-level movements in southeast England

Analysis of the geological evidence for Holocene sea-level movements in southeast Englandt R. J. Devoy DEVOY, R. J . 1982 . Analy sis of the geological evidence for Holocene sea-level movements in southeast England , Proc. Geo!. Ass ., 93 (l l, 65-90. Sea-l evel indicators in the form of shells geos?ls, o~er-consolidated horizons and biogenic material interleaved in marine sediments: provIde . evidence of relative sea-level movements. Data collected largel y from studies undertaken since 1950, show the sea surface rising from below -30 m O .D. at c. 9300 BP to above + 0.5 ~ C?D. by c'. 1700 BP. D~tails of indi~idual studies are discussed on an areal basis. Ninety four I.ndlcator POints dated either by radiocarbon or relative pollen techniques have been ~~tabhshed .and pl?tted on a time-depth graph. Of these, 55 were taken as providing reliable C ~nd height ~vldence f~r the movement of MHWST. Statistical analysis of this data, using multiple regression .and series of events techniques, suggests that a smoothly rising, exponential form of sea .leve.l rise does not form the best solution to the data and that alterations in the speed and dlrectl~n of re.covery may ~a~e occurred. A non-random environmental process may ~a,:e been of . prlma~y Importance In Infl.uencing the development and timing of sea-level indicators. E.vldence IS presented for the Influences of climatic change and man in inducing coastal flooding after c. 3000 HP.

Department of Geography, University College, Cork, Ireland

1. INfRODUcnON

the essentially high energy marine environment of East Anglia maintains long stretches of an eroding and low-cliffed shoreline (Fig. 2). Sediment movement along the coast is mainly Sand SW towards the outer Thames estuary (Stride, 1973; Jansen, van Weering & Eisma, 1979 ). A product of this movement has been the formation of NE-SW trending depositional structures, frequently forming across the mouths of river estuaries, as in the case of the Orford Ness spit and bar complex (Carr, 1970 ; Carr and Baker, 1968) . South of Dunwich, Suffolk, and extending as far as the Thames estuary, intertidal flats become prominent features of the coastal zone and are exposed for several kilometers offshore at low tide. Along the south coast of England with its extensive tracts of high cliff (Fig. 2), sediment movement in the littoral zone is essentially eastwards. Areas of intertidal flat and quiet water deposition are invariably located behind or within the protective influence of spits and related barrier features. These features range in size from the major structures of Chesil, Hurst and Dungeness to smaller features as at Lymington, Poole and Portsmouth Harbours. Throughout the region extensive low-lying areas, often associated with the numerous river estuaries, form adjacent to the coastal zone and together with 2 the Fenland cover approximately 5000 km (Fig. 2). Much of this ground lies close to present springtide levels (Table 1) and has frequently been subjected to inundation by storms (Brookfield , 1951 ; Command, 1953; Grieve, 1959). Stratigraphic sections through these areas commonly show interleaving freshwater,

The coastline of southeastern England has been taken to include those areas between Ingoldmells, Lincolnshire (53°17' N, 00°32' E) and Poole , Dorset (50°43' N 01°55' W); excluding the estuaries some 750 km i~ length (Fig. 1). Geologically this composite region is d0r:tinated by Jurassic, Cretaceous and Tertiary strata. Pleistocene and Holocene sediments mantle these underlring. s~lid rocks , the former being predominantly glacigenic In East Anglia where they have an extensive though discontinuous outcrop. The region's structural history during and since Mesozoic times has been strongly influenced by the separation of the European and North American-Greenland crustal plates (Ziegler, 1975; Ziegler & Louwerens, ,1979 ; Kent 1975) and has witnessed various phases of subsidence, particularly evident in the North Sea Basin. Continued subsidence here during the Quaternary is indicated by the accumulation of nearly 1000 m of sediment (Caston , 1979) , underlining the longterm geological pattern of structural instability in the region and susceptibility to sea-level changes. The Holocene sediments constituting the coastal zone have been the subject of numerous geomorphological and sedimentary process studies (Kidson 1961 ; Hardy, 1964; Steers, 1964, 1981; Evans, 1965; Hey , 1967; Prentice, 1972; King, 1972; Jones, 1981; Straw & Clayton, 1979). Excepting the notable areas of sediment accumulation in the vicinity of The Wash, t A U.K. contribution to IGCP Project No. 61. 65

66

R. J. DEVOY

Channel Tunnel TUtino Green Langney Point Isle of Grain Leman a. Ower Banks Tilbury Halvergate Marshes, R. Yare Foulness Is. Hamworthy. Poole D8rtford Tunnel & Uttlebrook Aldeburgh Stone Marshes Tilbury Docks Broadness Marshes Eastborough Fm. West Thurrock Fawley lewes, Vale of the Brookes Crossness Keyworth. Poole Seleh Sandwich By-Pass Bowers Marshes Canvey Is. Sandwich By-Pass Witttersham Bridge Wlggenhall, St. Germans Colne Point Marsh CheaH Beach Chapel Point 31 Rotherhlthe "1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

32 arancaeter 33 Al1'Iberley Wild Brooks 34 Ingoldmen. 35 38 37 38

Wallasea Is. Scotney Court Fm. Lydd Dengle Flet Eaat Mersea

St'

-5-

Fathom 45 metre contour

lllIIl o 2'

l'

O'

,.

I o

mi. km.

land below mean sea level 2S

I

40

2'

Fig. 1. Southeastern England; showing the site location of sea-level indicators referred to in the text.

estuarine and marine sediments and it is the interpretation of these that form the basis for the reconstruction of Holocene sea-level movements.

2. LITERATURE REVIEW In this archaeologically rich and long settled region historical indicators of relative coastal movements have frequently been recorded (Codrington, 1915; Wheeler, 1929,; Warren, Piggott, Clark, Burkitt, Godwin & Godwin, 1936; Worsfold, 1943; Jenkins, 1954; Burchell, 1954; Green & Hutchinson, 1960; Mer-

rifield, 1965; Simmons, 1980). Coastal changes have been deduced from the occurrence of 'submerged forests' exposed on contemporary foreshores as intertidal peats (James, 1847; Whitaker, 1889; Spurrell, 1889; Reid, 1913). The advent of pollen and related biostratigraphic studies enabled such coastal sedimentary sequences to be set in a relative time framework (Godwin, 1975). They show the now well-established general Northwest European pattern of marine inundation of surfaces below c. -30 m during pollen assemblage zones (P.A.Z.) IV-V (pre-9000 BP). Submergence of higher levels at c. -5 m by late P.A.Z.

67

HOLOCENE SEA-LEVEL MOVEMENTS IN SOUTHEAST ENGLAND

Bill]

Areas of Holocene marine I eSlua ri ne aedlmentation., surfaces close to present "'.H.W.S.T.

[!l~~]

Zones 0' nearshore accretion

:::i.••

Shingle accumulation

..

llllW..

~

o l

Clills Clills Sand land

'ormed In ' solt rocks ' , < 25 m. 0 .0. formed In ' har d ' rocks Dunes slide teatures

,

Sand ridges

: ~. .. ,\ ~\

Sea - 1I00r hollows

FNN1 ~

Oc cur enc e or gravel

P'=l c::::::J

Clay & clayey sand s

~

Princ ipal sediment tran sp ort palhs

o

~:-

fj. .

J. -

-- -

Dungeness

..

1:/:

--/ o I o

ml krn.

25 I

40

Fig. 2. Distribution of Holocene coastal and offshore sediment types and structures.

VIla (5000 BP) was taken to indicate a rapid rise of relative sea-level, followed by a more gradual upward movement. Levels at a little above O.D. were submerged by mean high water mark of spring-tides (MHWST) during P.A.Z. VIII~IX (post-1700 BP). Later use of radiocarbon assays, particularly in the pioneering Fenland sea-level work of Godwin & Godwin (1933), Godwin & Clifford (1938) and Godwin (1940, 1943) supports this chronology. Detailed treatment of such data as a whole has been made in

Akeroyd (1966, 1972), Everard (1954), Jennings (1952), Churchill (1965, 1970), Godwin (1960, 1975, 1978), Lambert & Jennings (1960), Swinnerton (1931), D'Olier (1972) and Shephard-Thorn (1975). Recent studies have paid greater attention to examining the stratigraphy and pattern of events of small and more physiographically defined coastal units. Lithological and environmental quality changes have been used to determine the heights and dates at which sea-levels operated within such areas during any single phase in

68

R. J . DEVOY

TABLE 1. Present day tidal statistics for sea -level indicator sites (ordered from N to S) Time -depth MHWST Graph Nos. Locality of MTL Tidal Amplitude (Tables 3 & 4) (mO.D.) Indicarors (mO.D .) (MHWST-MLWST) (m) Chapel Point King's Lynn Gorle ston (R. Yare) Aldeburgh East Mersca Dengie Flats Foulness Is. Canvey Is. Tilbury Broadness Marsh Stone Marsh Crossness Point Sandwich Newhaven (Lewes) Arundel Fawley Poole Harbour

55 ,65,75 35,46,57,67, 83,87,88 9,86,90 14,64, 93,94 42 ,91 ,92 10.62 19 8,13,23,27,39,89 40 ,48,58,59,66,71 22 ,36,74 15,20,33, 41 43.54,73 30,53 38 68 78 25. 6 1 11.32,80

3.1 5 3.87

0.36 0.34

6 .0 5.9

0.84 1.2 2.56 2.62 2.55 3.15

-0.10 -0.09 0.4 0.17 0.13 0.27

1.9 2.3 4.6 4.8 4.8 5.7

3.38 3.42

0.35 0.36

6.0 6.2

3.5 1 3.6 2.51 3.08 ( ~2 . 0) 0.36 1.66 0.8/0 .3

0.36 0.38 0.08 no data no data -0.40 0.09

6.3 6.4 4.9 6.10 no data 3.8 1.8/1.3

Based on Admiralty Tide Tables (1980 ).

recovery. In Norfolk, wor k has focussed on Broadlan d (Green & Hutchinson, 1965; Funnell, 1979 ; Coles & Funnell, 1981) . Sedimentological and foraminiferal analyses indicate two marine incursions, the first indicating a MHWST level of c. -20 m O.D. at the coast. The onset of estuarine clay/silt deposition over salt marsh and Phragmites peat is dated to 7580±90 BP . TABLE 2. MaDma.I timings and variations in depth for the onset and closure of Holocene marine sedim entary sequences in the lower 'Thames estuary THAMES V

THAMES IV

THAMES III

THAMES II

THAMES ]

- 1700 (+0 .44 to - 0.75 m) 2600 (-0.8 to -1.8m) 3850 (- 1.9 to - 6.7 m) 6575 (-6.8 to - 12.3 m) 8200 (- 25.5 to - 13.2 m)

years BP no data (+ 0.4 to - 0 .9 m) 2800 (- 1.0 to - 2.0 m) 4930 (-3.0 to -6.9m) 6970 (- 8.0 to - 12.5 m)

years BP years BP years BP years BP

A second and similar incursion, reaching a level of c. -2 m O .D. in coastal areas , occurs after the widespread re-establishment of freshwater fe nwood pe ats between c. 4500-2000 BP. Discontinuous saltmarsh peats replace inorganic sedimentation after c. 1600 BP. Marine-estuarine conditions were established later some 20-23 km inland. At Aldeburgh Marshes, Suffolk (Fig. 1) (Carr & Baker, 1968 ) radiometric assays have been taken on peats, formed in an estuarine sediment series above basal sand. Occurring between -1.0 to -13.9 m O .D. , the biogenic strata are interpreted as having formed close to prevailing MHWST levels. In Gedgrave, King's & Sudbourne Marshes further south, peat horizons become irregular in occurrence and are intercalated with clay/silt , sands, shelly sands and shingle deposits. The diverse lithology may represent facies changes associated with alterations in the course of the river Orde and in op en coastal deposition . From a generally 'upward-coarsening' transgressive sedimentary sequence in the Foulness Island and Dengie Flats area, Essex , six phases of marine inundation are identified (Greensmith & Tucker, 1971a,b, 1973) . Recognition is based upon the height and timing of shell and sand ridges ('che niers'), marsh re treat fea tures and vertical changes in shell fauna. Removals of the marine influence are evidenced by geosols, peat horizons and formation of overconsolidated sediment

HOLOCENE SEA-LEVEL MOVEMENTS IN SOUTHEAST ENGLAND

layers, the latter possibly associated with salt marsh development. The opening phase of marine inundation, recorded at varying heights below -21 m a.D., is assigned to between 8900-7500 BP. Phase 2 ends by c. 4000 BP, with a removal of the marine influence identified by overconsolidated layers at -11 to -7 m a.D. Shell banks at -7.5 m a.D. indicate the beginning of marine phase 3. The lack of dateable material makes the placing of 14C limits on subsequent phases difficult. Phases 5 and 6 are thought to begin at c.1400 and 300 BP respectively. In the adjoining areas of the inner Thames estuary a similar general pattern of coastal marine inundation is recognised (Devoy, 1977, 1979). Partial disagreement on the timing, number and amplitude of the relative sea-level movements with those of Essex has been attributed to differences in local environments, sedimentology and data sources used. Biostratigraphic and sedimentological analyses have established a sedimentary sequence analagous with those of the river estuaries further north. Five main phases of marine incursion are identified (Table 2), detailing a relative rise of MHWST from c. -25.5 m at c. 8200 BP to +0.4 m at c. 1700 BP. Alternating organic and inorganic strata in the Medway and at Canvey Island, Cliffe, Cooling and St. Mary's Marshes occur on a comparable height and time framework with those of the inner estuary (Evans, 1953; Lake, Ellison, Henson & Conway, 1975). The biogenic levels in these areas are generally much thinner, between 10 and 30 ern thick. In the North Kent Marshes they are composed predominantly of monocotyledonous and herbaceous plant remains with high inorganic fractions present, indicative of saltmarsh accumulation (Devoy, 1980). Recent work on the east Kent and south coasts is limited. Boreholes and open sections in the Romney Marsh area record the formation offshore of sand spits and bars at an early stage in sea-level recovery after c. 9500 BP (Lewis & Balchin, 1940; Smart, Bisson & Worssam, 1966). These structures are associated with local river estuary deposition and are shown to have been later replaced to seaward by accumulation of SW-NE aligned shingle bars (Green, 1968). In the lagoonal environments behind the bars, sandy sediments are replaced by deposition of brackish water clays, containing the shells of Serobicularia sp. Termination of these marine-brackish phases at c. -1.5 m a.D. occurs with the extensive growth of freshwater oak-fenwood peat over the clay surface from c. 3500 BP onward. Return of estuarine and marine clay and silt deposition above the peat is tentatively dated to post-3020±94 BP (Callow, Baker & Pritchard, 1964). Given the nature of this protected bay-bar environment, the significance for sea-level movements of the recorded lithological and facies changes is open to debate. The evolution of the marsh coastline during pre-historic and historic times is discussed by Cunliffe (1980). Investigations in the nearby Ouse river valley

69

(Jones, 1971 & Thorley, 1971) have established that the basal Chalk surface is at -29.6 m a.D. in the Newhaven area, rising to -12.2 m a.D. near Lewes. Variable thicknesses of late Devensian sandy clay, sands and gravels mask the Chalk valley and slopes. Postglacial infilling of the valley by freshwater clays interbedded with thick oak-alder fenwood peats to c. 3190 BP or later, is seen as a response to rising sea-level at the coast. Replacement by estuarine sediments, recorded at levels of -6.5 to -2.3 m a.D. in the Lewes area, is marked by deposition of grey shelly clays and silts. Variability in the height of the contact is attributed to contemporary river erosion and later tidal scour, the freshwater deposits having perhaps formed to above a.D. in places before the estuarine incursion. Studies (Greenfield, 1960; Shephard-Thorn, 1975) from similar protected inland sites have provided data on the replacement of freshwater by brackish, estuarine sediments in the valleys of the Stour, Rother, Tillingham, Brede and Awn. Churchill's (1965) much discussed dating of biogenic material in tidal creek deposits at Fawley, Southampton Water, has provided a focus for re-vitalisation of work in this important area. To the north thin freshwater clays and peats are overlain by recent estuarine silt, whereas in the south thicker clay and silt sequences, containing shells of Macoma balthica, Scrobicularia plana and Hydrobia spp. overlie basal Pleistocene age gravels (-21 m near Calshot). These are interpreted as having formed in a shallowing estuary, with tidal flat sedimentation giving-way to saltmarsh deposits and in turn to the widespread growth of freshwater Betula-Phragmites fenwood peat (Hodson & West, 1972). The upper surface of the peat has been dated to 3689 ± 120 BP, at which time further marine inundation took place; sedimentation occurring in a saltmarsh or upper intertidal environment. Elsewhere in the Solent (Devoy, 1972Unpublished B.A. dissertation, Univ. of Durham) and from other sites in Poole Harbour (Gilbertson, 1967Unpublished B.A. dissertation, Univ. of Lancaster & Byrne, 1975-Unpublished B.A. dissertation, Univ. of Durham) biostratigraphic work upon estuarine sediments supports the evidence of similar coastal freshwater peat accumulations immediately prior to c. 3500 BP. This phase is followed by renewed marine inundation to levels a little above a.D. by c. 2500 BP. More intensive investigation of these sequences and of the earlier phases of sea-level recovery in the Solent, evidenced by submerged peat at levels of -36 m a.D. (Dyer, 1975), is being undertaken by F. Sutherland (Dept. of Geography; of Durham). A date at Hamworthy, Poole of 9289 ± 100 BP for the initial influence of marine conditions in the region (Godwin & Willis, 1959), has been given confusing and undue significance in the literature. Some authors (Bird & Ranwell, 1964; Godwin, Suggate & Willis, 1958) have cited the date 7348 B.C. as years BP. The radiocarbon

70

R . J . DEVOY

assay itself also appears to contradict the poll en assemblage result which places the level in PAZ VIb(8300-7700 BP). It is possible that accu mulat ion may have begun witho ut refer en ce to the co ntempor ary sea surface, whilst the contradictio n in dating may result fro m so me con ta mination of the sample a nd no reliance can be placed in the evidence . Recent commentar y on so me of the sea -leve l data from sou thea st a nd so uthe rn E ngland has been made by Jon es (1981) . Ters' (1973) 'oscill ating' pattern of sea-level reco very is seen as the most appropriate in resolving differences betw een the sea-level curves published for the reg ion. No an alysis of sea -level indi cat or s used in the sepa ra te studies is given, however, and the conclusio ns presented may owe mu ch to a desire for synthesis.

3. SELECTION OF SEA-LEVEL INDICATORS A number of conditio ns for da ta selection were es ta blished as a basis for the co nstru ction of relat ive sealevel graphs: (1) Direct access of the sea to each site at the time when the dated level , used to measure a sea-level pos ition , was be ing formed . Palaeogeographic information was used here when available, although specific site details were often lacking. Infere nces a bo ut the possible restriction of access by differ ential peat grow th or deposition al struc tures had to be based in such cases on single stra tigraphic records , micro -/m acro-fossil information and by anal ogy with th e co nte mpo ra ry coas ta l pro cesses. (2) The availability of palaeoenvironmental indicato rs, such as pollen , diatoms, foraminifera and she ll evidence, to show the presence of marine or brackish water conditions at the dated level. (3) Determination of the indicato r's relati onship to a specific palaeotidal level. Considerable discussion abo ut the re lationship betwe en the height of sea- leve l a nd the dev elopme nt of intercalated biogenic stra ta in coastal sedime ntary se quences has occurred , an d readers ar e referred for details to Godwin (1943) , Jelgersma (1961) , Tooley (1969, 1974, 1978), Devoy (1979), Plassche (198 1), Redfield (1967) and Johnson & York (191 5). The majorit y of -availa ble dat a points were taken from th e transition al zones of such seque nces. Palaeoe nviro nme ntal indicators show that whe re a cha nge from fresh to marine-brackish wat er co nditions is recorded and that subseque nt erosio n has not disturbed the contact, this zone app rox imates to the position of MHWST. Level s were used where this or a similar close relationship to MHWST could be determined. The absence of mean tide level (MTL) indicators or of palae otidal dat a for its recon struction , makes the recomm ended use of this level an unn ecessar y abstra ction. Many radi ocarb on dat ed samples in the regio n ha ve been taken from o rganic levels which do not . show a direct sea influence , although they are co n-

tained in stra ta int erleaved with marine sedimen ts. Often representing the growth of alder-oak fenwoods, these levels probably for m in response to the height of pre vailin g gro und wate r conditions . Th e relationsh ip of this water level to a sea surface po sitio n is extrem ely variable , but is likely to represen t the position between MHWST and MTL a t sites close to the coast (Jelgersma, 196 1, 1966 ; Plassche, 1981 ). Such levels have not been included in the reconstruct ion of sealevel movements, but are used to che ck the trend established by the mor e reliabl e data . Considering the tolerance of man y of the plant species invo lved to marin e inundati on (Jo hnson & Yor k, 191 5 ; Richardson, 1955 ; Martin , 195 9 ; Ad am , 1976 ; Tooley , 1978) and assuming co ntinued sea access to th e site, the posi tio n of MHWST need not have bee n grea tly different in some instan ces from the recorded level. (4) D etermination of the sea-level indicator's height to Ordnance Datum, Newlyn, by accura te surveyi ng techniqu es. In some instances heights have o nly be en estima ted by referen ce to the nearest ben chm ark or spo t hei ght o n the O rd na nce Survey 6"· map ser ies. Whe re th is has occ urred the maximum es timate of the probable error involved has been made. (5) Bo reh oles were assum ed to have been taken as close to the ve rtical as possible, allowing for erro r ±5° in calcul ating the height of the indicato r. Determination of the use of acceptable sampling methods and accuracy of the result ant informa tio n dep end lar gely on the pu blished record. Onl y le vels taken from clean ed open sections o r by coring methods pro viding undisturbed samples have been used (Wes t, 1977 ). (6) The age of an indic ator has been based whe rever possible upon a radiocarbon assay as published primarily in the journal Radiocarbon (1959-1 981). Standard errors of the result ha ve been expr essed at the level of ± Io. Poll en information whe re availabl e has been used as a crosschec k to th e 14C dat e . In th e ab sen ce of a radi ocar bon assay and if existing relia ble pollen informa tion could be referred to a regio nal chronozone system (West, 197 0 ; God win, 197 5), then a da te in years BP has been based o n pollen evidence alone . Cor rection of a sta ndard (5570 ± 30 yr)- radiocarbon date to tree-ring calendar yea rs has bee n made using data published by Switsur (1973). Graphing of all sea -leve l results has been expr essed in terms of both the sta ndard and corrected date. At a number of sites the 'yo unging' of dates derived from bio gen ic materi al thr ou gh autocompaction and particul arly by th e pen etration of Phragmites rhizomes from suprajacent levels has be en recognised (Kaye & Barghoorn, 1964) . Work published by Godwin, Willis & Switsur (196 5) and Godwin & Switsur (1966) has provided some est imate of age variatio ns involved in intru sion of Phra gmites ste ms and leaves from clay layers into und erl ying pe at in the Fenl and. In some similar sedime ntary an d stratig ra phic situa tions a n age e rror based o n these estima tes has been adde d to the date where suspicio n

71

HOLOCENE SEA-LEVEL MOVEMENTS IN SOUTHEAST ENGLAND

exists that a similar 'younging' may have occurred (Table 3). Dates have not been used where there is firm evidence of the operation of contamination processes, hard water error (Bowen, 1978) or an incompatability with pollen data.

ted on a time-depth graph (Fig. 3) (Tables 3 & 4). In each case an estimation is given of the possible margin of error associated with the recorded height and date. From these, 55 points are used to re-construct the direction and form of relative sea-level (MHWST) recovery. No accurate data are available from the early stages of the sea-level rise. Freshwater organic sediments, now submerged beneath marine sequences of the English Channel and North Sea, provide the only dated levels for this early phase (Table 4) (Godwin, 1943;

4. mE EVIDENCE FOR SEA SURFACE MOVEMENTS Based on the outlined criteria 94 data points from postglacial coastal sediment sequences have been plot-

n 0 0.

0

~i ,, L J

93

'0

ci

o ~

_1 5

a; E .!;;

1:

.Ql -

20

"

rt

:I:

I

~

Thames Ess ex & N. Ke nt ( to Sandwic h) E. Anglia S.E. England ( Pegw ell Bay to Selsey Bill ) Sou th ce ntral England

25

I

I

•

I

I

L... ...J

I

1- , 3

1 1 LJ

5

Dat es upon biogenic levels inte rlea ved in mar ine sediment s 30

Wj I

_ :. :

•

I

r -,

I

Moorlog

1

I

I

I

I

I

Shell

I

Cont act of biogenic mat er ial, subja c ent to inorga nic se diment s

I

I

I I

r -,

I

I

1

1

I

I

1

I, i f--1 i i i i

1

Con tact of biogenic mater ia l, supra ja cent t o i n or ga ni c s ed i me n t s

I

I I

I

I

I

I

I I

~ .~

L. _ ..J

- '0

1.000

2 ,0 00

3 ,0 00

. ,0 00

5 000

6,0 0 0

1, 0 00

8,0 00

i .OOO

10 .0 0 0

Years B.P.

Fig. 3. Time-depth graph of sea-level indicator points detailed in Tables 3 & 4. Defined by area, each point is accompanied by a plot of the estimated error in height and dating.

72

R. J. DEVOY

TABLE 3. Radiocarbon assays upon sea-level indicators identified in the Holocene coastal sediments of southeast England.

Graph Sire No. Name

Palaeoenv. Co-ordinates

Material

Monocot, peat with silt and wood fraction Phragmites peat with halophytes Silty peat

8

Tilbury

51°27'14"N 00°22'12" E

9

R. Yare Broadland

52°36' N 01°39' E

10

Foulness Is.

5e36'N 00°55' E

11

Hamworthy Poole

50 038'45"N 01°58'30"W

13

Tilbury

14

A1deburgh Marshes

52°05'55"N 01°35'25" E

15

Stone Marshes

51°27'41" N 00015'36"E

Woody peat with

19

Eastborough Fm. Stone Marshes

51°28' N 00°33' E

Silty monocot. peat Woody peat with

22

Broadness Marsh

51°27'36" N 00018'40"E

23

Tilbury

op. cit.

Detrital organic mud Silty monocot. peat Phragmites peat with wood

represented

Lab. Code

Tree Ring Correcrion Depth Radiocarbon (after from ±IO' Switsur, surface (Years) 1973) (metres)

HI. 0.0. afrer estimated compaction and HI. consolida.D. ation. (metres)

-13.23

-13.03

=19.3

-19.3

-18.0

8150

18.3

-16.5

-15.35

7500±350

8130

13.08 (min.)

-11.58

-11.08

01428

7050± 100

7790

12.48

-10.38

-9.48

13430

7010± 130

7760

10.6

-9.25

-8.65

01334

6970±90

7735

9.28

-8.82

-8.76

Estuary saltmarsh

SRR375

6690±80

7500

13.03

Estuary high marsh

01335

6680± 100

7490

9.08

-8.62

-8.17

Estuary tidal marsh Estuary tidal marsh Estuary reedswamp/ saltmarsh Estuary tidal marsh Estuary high marsh Estuary fenwoods Estuary high marsh

01339

6620±90

7440

12.68

-8.57

-8.22

01429

6575 ±95

7400

12.2

0834

6366± 124

7174

=7.00 t07.32

-7.32

-7.27

01430

6200±90

7046

8.52

-6.42

-5.62

01282

5640±75

6457

5.62

-4.99

-4.89

(pollen)

5500±300

6316

3.56

-3.01

-2.41

01285

5484±80

6290

6.38

-5.35

-4.65

Estuary tidal marsh

01427

7830± 110

8360

Estuary saltmarsh

HAR2535

7580±90

8180

Saltmarsh on tidal flat coast Estuary Brackish lagoon Estuary tidal marsh Estuary Brackish lagoon/ fen Estuary high marsh

BIRM 242

7516±250

(pollen)

15.33

monocots.

20

op. cit.

-11.2

-10.3

monocots.

op. cit.

Silty rnonocot. peat Silty monocot.

25

Fawley

50 049'15"N 01°19'30''W

peat Silty Phragmites

peat 27

Tilbury

30

Crossness

51°30'11" N 00008'05"E

32

Keysworth Poole Dartford Tunnel

50°41'25" N 02°04'25" 51°27'25"N 00015'12" E

op. cit.

Silty monocot. peat Woody peat with monocots.

33

Woody peat Alnus wood, in

woody peat with monocots.

-10.1

-7.27

73

HOLOCENE SEA-LEVEL MOVEMENTS IN SOUTHEAST ENGLAND

TABLE 3. (Continued.)

No.

Site Name

35

Set ch

36

Broadness

38

Sandwich

51 01T N 01°20' E

39

Bowers' Marsh

51°33' N 00°31 '

Peat

40

C anvey

5 1°31'35" N 00035'28"E

Cla yey

Is. 41

West Th urr ock

51°28'35 " N 00016'50" E

42

St. Pete r's Flat

5 1°44' N 00 °58 ' E

43

Stone Marshes R. Yare , BroadJand

Graph

46

48

53

East borough Fm . C rossness

Palaeoenu. Co- ordina tes

52°42' N 00025'E

op. cit.

op . cit.

Mal erial

Peat

Silty monocot. peat Woodpeat

monocot.

peat Cla yey monocot . peat with wood Mon ocot, peat with haloph ytes Silty m onocot .

op . cit.

Phragmiles peat

op . cit .

Silty m onocot ,

o p. cit.

pe at Silty monocot.

54

Stone Marshes

op. cit .

55

53°14'00"N 00 020'30''E 52°41'00"N 00021'35" E

58

Chapel Point Wiggenhall St. Germans T ilbury

59

Can vey

57

op . cit.

op . cit.

Is. 51

Fawle y

op. cit.

peat Silty monocot. pea t Wood peat Monocot, peat

Silty monocot. peat Silty monocot . peat PhraRmiles

fepre,"O enled

Lab. Cod e

T ree Ring Correction Radiocarbon (after ±Iu Switsur, (Yea rs) 1973)

Dept h from surface (metres)

H I. D.D . (metres)

Hr.D.D. aiter estimated com paction and consolidarion.

IGS-C14! 128

5440± 100

6245

7.3

-4.8

-4.2

01342

54 10 ±80

6219

8.92

- 4.8 1

-4.11

IGS -C 14! 115

53 15 ±100

6 16 1

7. 8

- 4.3

-3 .85

SRR 384

5180±70

600 7

= 3.2

-2.8

- 2.6

(polle n)

50 00± 300

5743

3.84

- 3.56

-3.01

IG S-C 14 152

497 5± 120

5708

6.4

- 3 .4

- 2 .8

Saltmarsh o n tidal fiat coast

SRR58

4959±65

5683

0 .06

- 2 .0

-2.0

Est uary tidal marsh Estuary reed swamp! saltrnars h Est uary saltmarsh

01336

4930±11O

5651

3.45

- 2.99

- 2.54

(po lle n)

4800±3oo

5488

-6.0

- 5.25

SRR 374

4700±50

5408

5.63

-3 .8

-3 .1

Estuary saltrnarsh

Q1 333

4195±100

4835

2.92

- 1.96

-1.51

Estuary tidal marsh Coastal fenwood Sedge fen! sa ltrnars h, fringing lagoon Es tuary tidal marsh Est uary fen ! saltm arsh Est uary reed swamp ! salrmarsh

Q1337

40 85 ±85

4685

1.62

-1.16

-0.81

Q 685

3943± 100

4480

nk

-1.83

-1.43

pollen

3900± 200

4377

3.66

-1.52

-0.92

Q1431

38S0±80

4305

7 .31

-5 .21

+4 .36

(po llen)

38oo±400

4292

3.00

-2.72

-2.32

Q831

3689± 120

4139

= 2 .38 to 2 .7

-2.7

-2.35

Fen , fringing estuary! lagoon Estuary tida l marsh Fe n behind pro tecte d coastestuary Est uary fen! saltrnarsh Est uary fenl saltmarsh Estuary fenwood! reed swamp

=6.0

74

R . J . DEVOY

TABLE 3. (Continued .)

nc o.o.

Graph Sire No . Nam e

Co -ordinate s

52

Foulne ss Is.

5 1°36'29" N 00°55' 35" E

64

Aldehurgh Marshes

52°08'38" N 01°34'37" E

65

Chapel Point

op. cit.

66

Tilbury

op. cit.

Marerial

ShellsCardium edule Mon ocot . peat

Mon ocot. peat with halophytes, Silty monocot.

67

Se tch

68

Lewes Vale of Brooks B.H .B I17 Tilbur y

71

op. cit.

50"52'OO" N 00°00' E op . cit.

73

Stone Marshes

op. cit.

74

Broadn ess Marsh

op . cit.

75

Chapel Point

op. cit.

pea t Peat

Wood y monocot . peat Silty monocot . peat Silty monocot. peat Silty monocot . peat Silty Phra ~-

mites

peat 78

Amberley Wild Brooks

80

Keysworth, Poole Wiggenhall St. Germans R. Yare, Broadland Setch

83

86 87

88

WiggenhaJJ St. Germans

50 055'20''N 00031'50'' W

op. cit. op . cit.

Silty Phragmites peat Monocot, peat Silty mono cot.

Palaeoen v. represenred

op. cit.

op . cit.

Monocot. peat Peat

Mon ocot , peat

Depth from surface (me tres)

Ht. D .D . (metres)

aft er estimated compact ion and consolid arion.

Open tidal flat coas t

BIRM 24 3

3580 ± 175

40 12

6.869.75

-5.0

-4.2

Est uarylagoon reed swamp! saltmarsh Open coast saltmarsh

12827

3460 ± 100

3837

3.7

- 3.4

- 2.85

0 686

3340 ± 110

3682

nk .

0.0

01432

3240±75

3576

4 .1

-2.0

-1 .45

IGS-CI4! 127

321 5±100

35 27

4.7

-2.2

-1.7

Thorley (1971)

3 190 ± 125

3494

3.5 to 3.65

- 2.3

-1.6

01433

3020±65 +40 0

3285

3.92

- 1.82

- 1.12

01338

2850 ± 65

3066

1.35

- 0.89

- 0.54

01340

2836± 85

3050

6.84

- 2.73

-1.83

0 844

2815± 100

3014

nk

+0 .13

+0.28

0 690

2620± 110

2802

2.5

+ 1.52

+1.57

(pollen)

2500±400

2636

1.52

- 0.97

- 0.57

poJJen+ 0489-490 0549-550 0805-807 SRR573

245 0± 250

2568

3.05

- 0.91

- 0.3 1

±50 1973 ± 400

1987

= 2.0

- 2.0

-1.45

IG S-C I41 126

1875 ± 100 ±400

1872

3.7

- 1.2

- 0.6

(po lle n)

1800±250

1795

1.22

+0 .91

+ 1.0 1

Estuary tidal marsh Fen fringing estua ry/ lagoon Estuary fen Estuary tidal marsh Estua ry tidal marsh Estuary tidal marsh Coastal reed swamp! saltmarsh Estuary reed swamp Estuar y fen Estua ry fen

peat op . cit.

Lab . Code

Tree Ring Correction R ad iocarhon (after ± l
Estuary reed swamp Fen fringing estuary! lagoon Estu ar y reed swamp! saltmarsh

+ 0.1

75

H O L O C E N E S EA-LEVEL MOVE MENTS I N SOUTH EAST E N GLA.'lD

TABLE 3. (Continued.)

Graph Sire No. N a me

Palaeoen v. Co -ordina res

Ma rerial

89

T ilbury

o p . cit.

90

R. Yare Broadland

op . cit.

91

Den gie Ra ts

51'41'N 00'56' E

92

D engie Rats

51'39' 1O"N 00'54'08" E

93

Eas t Mersea

5 1'46'N 00'59'E

Monocot . peat with haloph ytes Mo nocot. peat with haloph ytes Shell sCa rdium edu le ShellsCa rdi um ed ule Ha loph yte peat

94

East Me rsca

51"47' N 00"59' E

Hal ophyte pea t

re presented

Lab. Code

Tree Ring Correcrion (after R adiocarbon ±lcr Switsu r, (Years ) 1973 )

H r. 0 .0 . a fter estimated Depth from suriace (metres)

com paction and collSolid0 .0 . (metres ) a rion.

HI.

Es tuary saltmarsh

(po llen)

175 0 ±250

1744

1.7

+0.4

+0 .8

E stu ar y sa ltmarsh

SRR 57 5

1609±50

1593

= 0.5

- 0.5

-0.3

Open tida l flat

I G S-C I 41 138

1340± 100

1315

1.0

+1. 0

+ 1.05

Open tida l flat

BIRM 244

1265 ± 100

1241

1.0

+ 0.2

+0.25

Open tid al flatl saltmarsh Op en tidal flat l saltmarsh

SRR 158

173 ± 60

23 3

0.36

+ 1.14

+ 1.14

SRR56

118 ±48

190

0.06

+ 1.72

+ 1.72

nk = not known .

Godwin & Willis , 1959 ; Jelgersma , 1961 , 1979; Kooijmans, 1970; Ters, 1973 ; Shephard-Thorn, 1975). In accuracies in relating sample altitude to a.D. and uncertainties about the relation of the dated level to local stratigraphy, or to a sea-level position , renders such evidence of marginal use . The first available dates for MHWST levels (po ints 8,9, 10 and 11) show the replacement of reedswamp and saltmarsh by inorganic intertidal sediments at c. 7500 BP. At Tilbury (point 8) silty Phra gmites peat, containing low but consistent pollen frequencies of Che nopod iaceae and other sal tmarsh taxa , presages th e approaching marine conditions . The accompanying diatom assemblage shows the replacement of brackish water species by marine taxa in the overlying clay/silt. The assemblages are typified by C ye/otella striata, Campylodiscus echeneis, Synedra tabulata var. fasciculata, Navicula rostellata, Nit zschia navicularis, N. punctata and N. granulata, Melosira sulcata , C ymatosira belgica and Coscinodi scus excen tricus respectively. From Halvergate Marshe s (po int 9) Phragmires and saltmarsh peats are replaced by a gre y silt-fine sand, containing formainifera indicative of progressivel y deepening water conditions . Identification at succeed ing levels in the marine seque nce of an assemblage dominated by Prorelphidium angli cum, Elphidium magellanicum, E. excavatum, E. waddensis and Ammonia beccarii shows sed ime ntati o n in the low

intertidal zone or bel ow (Coles , 1977 ; Co les & Funnell , 1981). Point 10 is taken from a thin peat above o verconsolidated marine days and silts. An unworn fresh to brackish water shell fauna in th e pe at , including Spirorbis sp., Assiminea grayana , Pisidium sp ., Valvata macrostroma and V . piscinalis is succeeded by a fauna representative of a sub- to intertidal e nviro nment (Greensmith & Tucker, 1973). The en suin g marine phase is interru pted at points 13 a nd 15 by biogenic accumulation. D evelopme nt of silty monocotyledonous peat is foll owed at Stone Marshes (point 15) by the in situ growth of fre shwater alderoak fenwood. Continued impor ta nce of saltmarsh, aquatic and herb pollen taxa at Tilbury (poi nt 13), including Cyperaceae, Typha latifolia and Sparganium-type , at values > 25% total pollen (TP), suggests the persisten ce here of estuarine co nd itions. Shallowing water prior to peat gro wth, howe ver , is shown by the relative increase in mesoh alobi on and oligohalobion diatom frequen cies to > 30 % tot al diatoms (T D) in the subjacent d ay/silt. Phra gmites peat containing the brackish-water ostracod Cyprideeis tonosa and formainifera Elphidium excavatUI1l, gives a comparable radiocarbon assay for pe at form ation close to MHWST levels at point 14 . E arlier uninterrupted biogenic accumulation above shell y Cr ag probably represents growth under a fre shwater influence in a

76

R. J. DEVOY

TABLE 4. Radiocarbon assays upon strata forming within the Holocene coastal sediments of southeast England.

G raph N o.

Co-ordinates

Material

2

Channel Tu nnel B.H. P040 Tilling G reen

51°06 '04" N 01°24'08" N 50057' N, 00"44' E

3

Langn ey Point

4

Isle of Grai n

50047' N 00"19' E 51OZ6' 10" N 00"43'03" E

5

Leman and Ower Bank

53°10' N 02°00' E

6

Isle of Grain

7

Tilbury

51°31' N 00°3 1' E 5 1°27'14 " N 00022'12" E

Wood peat Silty wood peat Wood peat Silty organic mud Wood peat, with Betula & Pinus Peat

12

Dartford Tunnel

51OZ7'25" N 00015'12" E

16

Tilbu ry Docks

51°27'26" N 00021'54"E

17

Broadness Marsh

51°27'56" N 00018'40" E

18

Littlebrook RH. 3 Foulness Is. RH. R/11/1 West Thu rrock

51OZ7' N 00"15' E 51°36' N 00"54' E 5e28' N 00°17' E 50°42'00" N OooOO'E

21 24 26

28

Site Name

Lewes Vale of the Brooks B.H . B 123 Dartford Tunnel

op . cit.

Alnus & Q uercus fenwood peat Wood peat with A lnus and Qu ercus Phrugmites peatwitb organic mud Wood peat with A lnus and Que rcus Wood peat

Shells (Os trea) Silty wood peat Silty wood peat A lnus wood peat with

Palaeoeno. represented

R adiocarbon T ree Ring ± 10" (Years) Lab . Code Correction

Depth from surface (metre s)

Hr. O .D . (metres)

-36.5

9565± 120

2.06 (seabed) = 26.0

SRR379

8760 ±75

29.5

- 24.88

Coas tal fen/ lake

Q1286

85 10 ±1 10

8885

28.04

-26.44

Coastal fenwood

Q10 5

8422± 170

8750

nk

-36.6

Coas tal fen/ lake Estu ary fenwood

IGS- C14 / 88 Q1426

825 0 ± 100

8650

=20.0

8 170± 110

86 10

15.47

-13.37

Estuary

Q133 4

7140±110

7870

11.67

- 10.64

Estu ary fen

Q790

6940 ± 120

7710

15.42

-10.67

Estuary fen wood

Q 1283

6882 ± 90

7650

12.86

-8.75

Estuary fenwood Coas tal tidal flat Est uary fenwood Valley

SRR278

6820 ± 55

760 0

11.0

- 8.0

IGS- C14/ 139 IGS-C 14/ 153 BIRM 168

6620 ± 100

7440

-11 .5

6450± 120

7248

13.7215.24 11.45

6290± 180

7 121

9.5 to 9.8

- 8.2

Q1 284

5693± 80

6510

7.88

-6.85

Coastal tidal flat Est uary fen/ reedswamp

IGS-C I4 / 140 Q8 11

5650 ± 240

6467

5530± 100

6344

12.1913.72 8.82

Estuary fenwood

SRR 276

5460±60

6467

8.15

- 5. 15

Es tuary fenwood Estuary sedge fen-reed swamp

SRR 277 Q810

5370 ±60

6194

7.6

-4.6

4920 ±1 00

5640

6.39

- 5.49

Coas tal valley fen Coastal estuary fen

NPL 101

Coasta l fen

1GS-C14/ 116

9920 ± 120

-22.5

- 26 .36

fenw ood

fen wood

Estuary

-8.45

fenwood

monocot s.

29 31

Foulness Is. B.H. R/7/1 T ilbury Docks

51°36' N 00°57' E op. cit.

Shells (O strea ) Wood / Monocot. pea t Wood pea t with

34

Littlebrook B.H .3

op. cit.

37

Littiebroo k RH.3 Tilbury Docks

op , cit.

Wood pe at

op. cit.

Sedge,

-11.3 - 7.92

mon oco ts.

44

monocot.

peat

77

HOLOCENE SEA-LEVEL MOVEMENTS IN SOUTHEAST ENGLAND

TABLE 4. (Continued .) Depth Radiocarbon Graph No.

Site Name

Co -ordinates

Material

45

Wittersham, B1ackwall Bridge

51°00' N 00°41' E

Woo d peat

47

Wiggenhall St. Germans,

52°41' N 00"21'35 " E

Quercus wood in wood peat

49

Littlebrook B.H. 3

50

Caine Point

51

Chesil Beach

op . cit.

monocot.

51"47' N 01°05' E 50 039'N 02°36' E

52

Littlebrook B .H.4.

56

Ti lbury Docks West Thurrock

60

69

Blackwall Brid ge, Wittersham Rotherhithe

70

Brancaster

72

Rotherhithe

76

Littlebrook B.P. 3

63

51°27' N 00"15'E

op . cit. op . cit. op .cit.

51°21' N 00"03' W 52°57'30" N 00"40'50" E op. cit. op . cit.

53°14'OO"N 00020'30"E

Chapel Point

79

Littlebro ok R H. 4

op. cit.

81

Tilbury Docks

op . cit.

82

Ingoldmells

84

Wa llasea

85

Scotney Court Fm , Lydd

(Years)

IGS -C14/ 14

4845 ± 100

5545

5.895.97

-2 .8

Q31

4690 ± 120

5400

7.3

-5 .18

SRR275

4549±55

5275

6.95

-3.95

SRR970

4 277 ±45

4959

3.2

-3.28

Coastal fen

Q1028

4234±60

4 894

nk ,

-2.0

Estuary fen wood-reed swamp

SRR280

4220±50

4876

9.81

-3 .91

Estuary fenreed swamp Estuary fen

Q792

3940±110

4463

8.68

-4.88

IGS-C14/ 151 IGS-C14/ 13

3795± 115

4268

5.35

-2.35

3560±100

3992

4.27

-1.5

Estuary reed swamp Protected coastal fen op . cit.

SRR436

3180±80

3482

1.5

-0.1

(pollen)

3050±450

3348

0 .0

-0.45

SRR435

2947 ±50

3202

=2.0

- 0.6

Estuary fenreed swamp

SR R274

2650±50

2838

4 .7

-1.7

Open Coast tidal fiat

Q687

2630± 110

2814

nk

0.4

Estuary fen

SRR279

26 10 ± 50

2790

7.78

-1.88

Estuary reed swamp Coastal fen wood Estuarine sub -tida l location Fen on protected coast line

Q793

2467± 110

2595

6.39

- 2.59

Q81

2455± 110

2570

nk

0.0

IGS-C14/ 148

2300± 115

2423

7.0

- 3.4

NPL91

2050±90 +400

2090

0.61

+3 .0

River valley fenwood behind coastal lagoon Fenwood, behind coastal lagoon Estuary fenwood

peat Pea t Woo dy Phragmites peat Silty mo noco t peat with wood Phragmiles peat Silty peat with wood Pea t

Monocot . Peat Peat Monocot, peat Silty monocot.

77

nk = not known

Woody

peat Scrobicularia plana da Costa Silty

from Tree Ring Co rrection

Palaeoenu. represented

Estuary/ valley fen

±1
Lab . Code

surface (metres)

Ht .O.D . (metres)

mo nocot .

53°11' N 00"22 ' E 51°37' N 00"48'E 50°57' N 00°54' E

peat Phragmites pea t Wood,from wood pea t Ostrea edulis Linne Pea t

78

R. J . DEVOY

protected coastal location. Renewal of marine conditions is represented at points 19,20,22 and 23 with the re-establishment of Phragmites reedswamp and saltmarsh plant communities. The resulting monocotyledonous peats are overlain by grey blue, clay/silt, containing shells of Hydrobia spp. above points 19 and 5, with the latter also showing a rich brackish to increasingly marine diatom assemblage. A date for a MHWST level at Fawley (point 23) during this phase, is given by a radiocarbon assay on a thin Phragmites peat containing charcoal fragments, intercalated between intertidal clays. Shells of Hydrobia spp. and Scrobicularia plana occur immediately above the peat horizon. A major removal of marine conditions at c. 5000 BP is recorded by points 27,30,32,33,35,36,38-43 and 46 upon the base of Phragmites/monocotyledonous peats overlying marine-estuarine sediments. The final phase of clay/silt deposition at point 27 shows the rise to dominance of mesohalobion and oligohabobion diatom species, represented by Campylodiscus echeneis, Cyclotella striata, Achnanthes hauckiana, Nitzschia punctata, N. navicularis and Synedra tabulata var. fasciculata. These, together with the high frequencies of broken and abraded valves at 80% TD, indicate a high energy and shallowing water environment. Macrofossils and the pollen of Gramineae, Cyperaceae. Chenopodiaceae, Compositae Bellistype, Armeria-type and Plantago maritima from the transition zone into the overlying 1-3 m thick peats, record the initial growth of saltmarsh, reedswamp and sedge fen plant communities. These are replaced upward by alder carr and later by oak-hazel woodland, in which the record of high Thelypteris palustris and Pteridophyte values evidence damp shady conditions. Poor pollen preservation, with strong oxidation and corrosion of pollen grains at these and similarly dated levels, probably results from persistent exposure above the contemporary groundwater table. As recorded in the preceding biogenic level at Tilbury (point 27), the continuance of high Grarnineae, Cyperaceae, Sparganium-type, Ranunculus aquatilis-type, Chenopodiaceae and Compositae Bellis-type pollen values, suggest the persistence here of estuary conditions. The presence of brackish water foraminifera, pre-Quaternary spores and Hystrichospheres throughout the biogenic layer supports this interpretation (Devoy, 1979). Point 32 provides a date based upon pollen zonation (Gilbertson, 1967, unpublished B.A. dissertation, Univ . of Lancaster) for the growth of sedge fen peat, containing significant values of Chenopodiaceae, Gramineae and Cyperaceae pollen over thin blue clay. Deposited at the margin of an estuary the clay records a brackish/marine diatom flora , characterised by Diploneis spp. , Grammatophora d. oceanica, Plagiogramma sp. and Melosira sp. At point 46 grey silt/sand is succeeded by Phragmites peat with the characteristic assemblages of the for-

mainifera Elphidium williamsonii, Millammina [usca, Iadammina macrescens and Haplophragmoides sp. followed by J. macrescens and Trochammina inflata detailing the transition from a low to high saltmarsh. Inundation by marine sediments of these coastal biogenic sequences is recorded by points 48, 5355,58,59,61 and 64. The contact between Phragmites peat and overlying blue estuarine clay at Fawley (point 61) is dated to 3689± 120 BP. To the north of the region (point 55) a similar assay is given for the boundary between a woody detrital peat and overlying estuarine clay, containing macrofossil remains of the saltmarsh plants Triglochin maritimum, Juncus maritimus, Armeria maritima and Limonium sp. Possible erosion of the peat surface before clay deposition indicates the date should be regarded as maximal for the onset of marine conditions at the site . The remaining points taken from the Thames estuary, come from the junction between Phragmites and saltmarsh peats with an overlying estuarine clay/silt. Frequencies of Chenopodiaceae, Grarnineae, Cyperaceae, Armeriatype, Artemisia and Filipendula pollen rise over this transition . The seeds of Juncus maritimus, J. gerardii and Phragmites australis are also recorded from these levels at point 54 . The pattern suggests the existence of an upper saltmarsh environment prior to inundation, an interpretation supported by the diatom record from Tilbury (point 58) and the Dartford Tunnel (Devoy, 1979). From Wiggenhall St. Germans (point 57) saltmarsh peats are replaced upward by freshwater fenwood peat. This record is at variance with other data points from the region and may result from local changes in sedimentation. At Foulness (point 62) a shell beach ridge composed predominantly of the broken remains of Cardium edule records the height of prevailing high tide levels . Points 65,66,67,71 and 75 derive from the top and base of biogenic strata intercalating brackish water inorganic sediments. A monocotyledenous saltmarsh peat 0.18 m thick at Tilbury (points 65 and 66) contains macrofossil remains of Phragmites australis, Aster tripolium and Chenopodiaceae, the latter reaching pollen frequencies of >40% TP. Intertidal sedimentation at Lewes (point 68) may have caused some erosion of the underlying peat surface. The date of 3190± 125 BP for the ending of biogenic accumulation should therefore be regarded as maximal. Rising fre.quencies of Cyperaceae, Gramineae, Chenopodiaceae, Compositae and Plantago spp . pollen in the peat, however, presage the arrival of estuarine conditions. Further occurrence of biogenic levels in the estuarine/marine sediments dominating the coastal stratigraphic sequences after c. 3000 BP, are recorded by points 73 ,74,78,80,83,86-90, 93 and 94 . These are composed mainl y of thin silty peats containing the plant remains of Phragmites or solely saltmarsh species. Diatom and foraminferal assemblages from the inorganic sediments adjacent to points

HOLOCENE SEA-LEVEL MOVEMENTS IN SOUTHEAST ENGLAND

80,83, 86, 88-90 evidence accumulation in mudflat to upper saltmarsh environments. On the Essex coastline shell beaches formed at the margin of saltmarsh deposits and composed of a reworked inter- and subtidal fauna, mark high tide positions at c. 1300 BP (points 91 and 92). Work along the Brittany coast on analagous beach ridges (Larsonneur 1975) supports the environmental interpretation of these indicators.

5. DISCUSSION OF THE DATA In interpretations of the form and pattern of Holocene sea-level recovery, two main schools of thought have emerged. One would see a 'smooth' exponential curve for sea-surface change as the best resolution of sealevel data points (Jelgersma, 1961 , 1966; Kidson & Heyworth, 1973, 1976). The other viewpoint (Tooley, 1974, 1978; Morner, 1969, 1976; Ters, 1973) regards sea-level, however defined, as describing a series of oscillatory changes in height over time superimposed upon the longer term trend of sea-level rise . Explanations of such surface alterations refer to a glacioeustatic mechanism controlled fundamentally by climatic changes, alternatively to the influence of the geoid on the sea surface through time , or to a combination of both. Evidence to support the argument is often derived from the types of litho- and biostratigraphic changes in coastal sedimentary sequences identified in this survey. Palaeoecological techniques are used to demonstrate that such sequences reflect progressive and truly regional environmental changes and not the vagaries of local influences. Further support is gained from the tentative identification (Tooley , 1976 , 1978) of some inter-regional synchroneity in the development of coastal biogenic strata. Alternatively the proponents of a 'smooth ' pattern of sea-level reco ver y pro vide a variety of explanations for the occurrence of such multiple coastal peat layers. On permeable substrates, such as the Pleistocene sands and gravels of the Netherlands, the initiation of peat growth would be seen as a response to increases in the height of the freshwater table, consequent largely upon a rising sea-level. It is suggested that later inundation results from the rate of sea-level rise exceeding that of peat accumulation, with some authors (Kidson & Heyworth 1978 ) seeing the operation of storm surges as important in this process . The re currence of peat growth in the succeeding marine sediments may reflect growth during times of a reduced rate of rise or stillstand, or perhaps from development behind protective coastal barriers. A compaction/consolidation mechanism ma y further modify such a pattern, with high differential compaction of organic levels allowing marine incursion and inorganic sedimentation. A later decline in the rate of rise would facilitate the seaward plant recolonisation of the marine sediment surface. One of the major obstacles to such explanations lies

79

in the palaeoecological evidence for the nature of the environmental changes represented by the alternating strata. Those in support of the 'oscillating' sea -level hypothesis argue that such chan ges would not occur with the operation of a breaching barrier mechanism, or with a continuously rising sea-l evel over long time periods. Reference to sedimentological models for the development of sed imentary transgressive and regressive sequences (Ginsberg, 1975 ; Vail , Mitchum & Thompson, 1977 ; Streif, 1979) may, however, pro vide a tenable alternative . Changes in sediment particle size during a phase of marine inundation and deepening water in former coa stal zones, generally shows an upward-coarsening sequence (Reinec k & Singh , 1975 ; Selle y, 1970 ; Greensmith & Tucker, 1973 ; Kraft & Allen, 1975). Cla ys and silts formerly accumulating in saltmarsh and mudflat areas are pro gressively replaced by coarser-sized silts, sands and gravels representative of the higher energy low to subtidal zones. Conversely with shallowing water, due to pr ogradation and offiap of nearshore to terrestrial sediments, sands and gravels give way to an upward-fining sequen ce and the recurrence of clays and silts in an intertidal mudflat. Saltmarsh and later freshwater pe at accumulations are frequently observed (Evans, 1965 , 1975 ; Larsonneur, 1975 ; Harrison , 1975 ) as occurring in the final stages of such a marine sedimentary regression cycle . The sequential change in sedimentary facies and eventually depositional processes would probably allow for the recorded alterations in environmental quality indicators. Details of the sediment size pattern in each cycle would depend upon the compo sition and rate of sediment supply, coupl ed with the diurnal to annual energy status of the coastline. Th e frequency in alter ation of such sedimentary cycles, perhaps giving rise to peat growth with sufficient duration, will vary again with the rates of sediment supply, rate s of compaction/consolidation and sea -level rise , tectonic stability of the coastline and alteration s in tidal amplitude through time . At any single site, not und ergoing major subsidence, a constant sediment influx coupled with a uniform rate of sea-level rise might produce a regul ar spacing of biogenic strata through a sedimentary sequence. How ever, the likelihood of such parameters remaining unchanged for long periods of time is small. Viewed in the light of the probable importance of local en vironmental influences on sedimentation, an y cyclic pattern of biogenic growth may more probably be expected to appear at irregular intervals. Excluding a number of marine shell indicators, the data po ints identified in the region derive from the boundaries of such interleaved marine and biogenic sediments. Given the difficulties outlined in interpreting this sea-level data, a less intuitive and more objective approach must be used to establish the relationship and relevance of sea-level indic ators generall y and particularly of these biogenic levels to the pattern

80

R. J. DEVOY

.3 +-_--'---_-'--_---'--_--'-_---'_----'_ _'---_.l.-_+

used is: (1)

o.c

.

~ =:

". -6

\

,

-9

s:

~

c

,

-12

2, Y = -O·0067+0·00088X -OOO00ClQ34X

1 \

3, v , 2.4552-000283X+O-OClOO008X-oo::x:ocol -15

\ \

" \

\

4\

-18

-21

+---r----r--,----,-------y---,,---,---,---+ o Years B.P. )(10

3

Fig. 4. Fit of 2, 3 and 4th order curves by least-squares regressionto sea-level indicators representing MHWSTlevels in the Holocene. Data uncorrected for height errors and using the standard (5570±30yr) radiocarbon half-life.

of long term relative sea surface movements. In this review regression analysis (Davis, 1973; Miller & Kahn, 1962) was used initially to determine the general trend for relative sea-level recovery, represented by the distribution of MHWST levels given in Table 3. Curves for second, third and fourth order polynomial equations were fitted to the observations using leastsquare methods, with regression of height (y-axis) against time (x-axis) as the independent variable (Fig. 4). The generalised form of the polynomial expansion

Each of the three plots show a high degree of correlation with the data (Table 5). A significant improvement in the 'goodness-of-fit' occurs between the second and third order curves, but with no major improvement in the statistics for the third and fourth order terms. The cubic model may therefore reasonably be assumed to form the most satisfactory resolution of the data points. The resultant trend for relative sea-level indicates a steep rise to c. 5000 BP at a rate of ~0.5 cm", followed by an inflexion and levellingoff of the curve. At c. 3000 BP a faster rate of rise is re-established, but at the much lower value of I ~0.13 cm- , to achieve heights for MHWST well above D.D. in the present day. Considering the different interpretations of biogenic level development in coastal sediments, it would appear that both the sea-level 'oscillation' and possibly the sedimentary mechanisms argue for a single dominant process in controlling the height and timing of the derived sea-level indicators. Whatever their cause, the resulting pattern of indicator points in time and the signature of the curve fitted to the data must characterise the environmental processes in operation. Arguably the action of a number of mechanisms, each of which may be irregularly repeated as part of a wider complex of processes, operating under a continuous but diminishing rate of sea-level rise, would be likely to produce a random distribution of indicator points in time, best resolved by a curve of exponential form. Conversely, significant deviations from a random pattern, as perhaps indicated by the fit of a third-order polynomial to the data, could evidence the influence of an overriding or recurring environmental process. As

TABLE 5. Statistics of multiple regression analyses Data Uncorrected Time/ Uncorrected Depth

Uncorrected Time/ Corrected Depth

Corrected Time/ Corrected Depth

Order of equation

SST

SSR

SSD

R

R2

1 2 3 4 1 2 3 4 1 2 3 4

1068.23 1068.23 1068.23 1068.23 995.93 995.93 995.93 995.93 997.60 997.60 997.60 997.60

914.05 952.29 956.27 761.95 872.58 904.29 909.69 721.85 841.22 895.57 901.53

154.12 115.94 111.96 233.99 123.35 91.64 86.25 275.75 156.39 102.03 96.08

0.867 0.925 0.944 0.946 0.875 0.936 0.953 0.956 0.851 0.918 0.948 0.951

0.752 0.856 0.892 0.895 0.765 0.876 0.908 0.913 0.724 0.843 0.898 0.904

* Value significant at the 0.01 level.

F value 37.245* 16.820* 1.119 46.64* 17.652* 3.126 39.678* 27.169* 3.100

81

HOLOCENE SEA-LEVEL MOVEMENTS IN SOUTHEAST ENGLAND

an initial attempt in analysing the distribution of timedepth data, particularly for the existence of any regularity or periodic pattern and to further test the applicability of the regression model, the indicators were examined through a time series technique (Davis, 1973). It was also hoped that this type of analysis might serve to suggest a workable mathematical model for the geological development of the biogenic levels. Radiocarbon assays were taken to represent times for the growth of biogenic and essentially non-marine material in a time sequence otherwise dominated by inorganic marine sedimentation. As such they may be visualised as points or events occurring in a time (stratigraphic) continuum and are therefore treatable by the series of events approach (Cox & Lewis, 1966, Reyment & Collinson, 1971; Reyment, 1976, 1980). The indicators were grouped in 3 data sets representing; (1) 14C dates from all biogenic material forming within coastal sediment sequences (Tables 3 & 4); (2) 14C dates from the base of biogenic strata, timing the removal or regression of marine conditions; (3) 14C from the top of biogenic sequences, timing the onset of marine conditions. The data was analysed using established statistical techniques incorporated within an SASE computer programme (Birks, 1981; Lewis, Katcher & Weiss 1969). Results were obtained for both the standard (A) and tree-ring time corrected (B) radiocarbon assays in each set. Initial treatment, before further testing could proceed, required the determination of any trend (u), or change in the rate of occurrence of events with time and is defined by (Cox & Lewis, 1966);

u=

L ~/ii . to/2 to/ ( 1/1 2 n )

20

a 16

'·2

08 0

I 0::

00

-0-4

10

5

15

20

25

35

30

40

J

1·6 b

1·2 1·0

c

0·8

(2)

where n is the number of events, the series is observed over the interval (0, to) and the events occur at (t 1 ••• ~ ••• t,,). In the results (Table 6) only set 1A gave any indication of a significant trend (u = 2.41). More rigorous testing for a series of <100 values gave an adjusted u of 1.57 which is not significant and all the series can be regarded as statistically stationary. As the data appears therefore to be trend-free a check on any periodicity in events was next undertaken using serial correlation. This tests whether the intervals between successive events occur independently of each other. Results (Fig. 5) show that few of the serial correlation coefficients differ significantly from zero at the 5% level. This indicates that intervals between events are probably independently distributed in time and reflect the operation of some type of renewal process (Reyment & Collinson, 1971). Given such results the series were finally tested to see if they could best be represented by an exponential Poisson model. This assumes (a) that no trend occurs in a series of events, (b) intervals between events are independent, (c) that the probability of two events

04

0·4 0 I a::

0·0

I

I

I

I

0·0

-0-4 -0·6

-0·8 5

10

15

J - 1·2

5

10

J Fig. 5(a)-(c). Correlograms for data sets 3B(c); where RHO represents the value of tion coefficient and J the number of the coefficient in the sequence, e.g. 5 = 5th coefficient.

l Ala), 2A(b) & the serial correlaserial correlation serial correlation

82 TABLE 6. Statistical results for biogenic material formation in marine sequences

Biogenic series Regression series Transgression series

1A IB 2A 2B 3A 3B

n

u

84 84 22 22 27 27

1.11 2.41 * 0.56 1.04 0.65 0.98

Adjusted u

1.57

Mean

S.D.

c.v.

Sk ewness

Kurtosis

102.88 104.88 247.31 281.68 285.63 302.60

175.91 161.60 363.48 432 .62 425.67 428.46

1.71±0.26 1.54±0.23 1.47±0.43 1.54±0.45 1.49± 0.39 1.42±0.38

6.21 5.42 2.28 2.55 2.77 2.68

48 .15 38.83 7.59 9.15 9.65 9.38

n=number of events; u =value of trend statistic; S.D. = standard deviation ; c.V. = coefficient of variation ; * significant trend at 5% level. TABLE 7. Distribution-free tests for Poisson processes D'"n Biogenic series Regression series Transgression series

1A 1B 2A 2B 3A 3B

0.76 0.30 0.75 0.54 0.62 0.53

Untransformed data Dn D~ W~ 1.59* 1.5S* 0.94 1.13 0.61 0.83

1.59* 1.58* 0.94 1.13 0.62 0.83

3.03* 3.56* 0.99 1.21 1.09 1.31

D'" n 1.71* 1.44* 1.61* 1.55* 1.38* 1.29*

Transformed data Dn D~ 1.71* 1.44* 1.61* 1.55* 1.38* 1.29*

0.53 0.46 0.03 0.03 1.93* O.OS

~ 3.68* 3.04* 4.19 * 4.69* 2.99* 2.64 *

D-values = Kolmogorov-Smirnov statistics; ~ = Anderson-Darling statistic; * Rejection of the Poisson hypothesis at the 5% level.

occurring together is small and (d) the chance that an event will occur is proportional to the length of time preceding the event. Distribution-free procedures used to test this model measure essentially the degree of agreement or 'goodness-of-fit' between the theoretical Poisson distribution and the sample distribution values . Results of the tests are given in Table 7. In sets 2 and 3 none of the values from the un transformed data rise to significant levels, although significant deviations from the null hypothesis of an homogeneous Poisson distribution do occur in the transformed data. Support for the idea that the series reflect some type of renewal process is gaine d from examination of the plot of the log survivor function (Rho) and results of the periodogram analysis (Table 8)-(further details of the mathematical basis, application and interpretation of these techniques for series of events is given in Cox & Lewis, 1966). Both may be used as approximate checks for the applicability of the Poisson model with, in the former case, deviations from a straight-line plot indicative of disagreement with the Poisson mode. None of the graphs show a constant slope although some conformity with a straight-line does occur (Fig. 6). The periodogram results give no deviations from the hypothesis for either the operation of an homogeneous Poisson process or , that intervals between events are independeltly distributed . By comparison, statistics for the biogenic series underline the areas of doubt raised in the transformed data for sets 2

and 3 (Table 7). The values show strong devi ations from the predicted distribution in both the untransformed and transformed states. This conclusion is supported by the distinct departures from linearity shown in the plot of the log survivor function (Fig. 7). Further, all the coefficients of variation in the basic statistics (Table 6) give values> 1.0. These suggest an overdispersion of the data as a whole and probably indicates that the biogenic series (lA & B) at least do not conform with a Poisson process. The fit of a cubic rather than a quadratic curve to the data supports such a conclusion as does a graphical examination of the data, showing some tendency for the clustering of events (Fig. 8). TABLE 8. Tests of trend and independence in periodogram analysis

Biogenic series Regression series Transgression series

lA 1B 2A 2B 3A 3B

u

D;

D;;

Dn

~

0.79 0.44 0.10 0.65 0.48 0.66

0.48 0.42 0.56 0.55 0.33 0.33

0.41 0.41 0.46 0.43 0.33 0.37

0.48 0.42 0.56 0.55 0.33 0.37

0.25 0.24 0.45 0.35 0.31 0.39

u = trend value ; D-values = Kolmogorov-Smirnov Statistic and W~ = the Anderson-Darling statistic .

83

H OLO C ENE S E A- L E V E L M O VE."vlENTS IN SOUTH EAST ENG LAN D

-o
.1-_-+

_

-().44

TOp o f b iogen ic s t rat a ,

'cor r ecte d 14C dates x' x •

-084

Base of btogen lC strata, corrected '4c dates

x •

"'00 ~

~

- 1·2

1 00

0";

-2·8

• '. '

. ..',

Fig. 8. Graphical plot of intervals between events, indicative of the degree of clustering in the data,

-3 ·2

-3 ·5

-4 0 +-----r---.,..---.----.---.---~ 01

320

640

950

1300

1500

' 90 0

Ti m e Between Ev e nt s ( un sp eci f ied un it s )

Fig. 6. Logarithmic empirical survivor function, Log" R".,(X ) 1 showing plots for data sets 2B & 3B. -oo, -ir- - ----'- - - L-- - ...L..- - -L.- - ----'- - - I-,

':' -H)

-1 5

- 2·0

x 0

- 2·5

c

Q:

~

~

- 3.Q

-35

- 4.Q

- 4·5

- 5.Q

01

97

'90

290

390

4BO

5BO

Time Between Ev ents (unspec if ied unit s)

Fig. 7. Logarithmic empiricalsurvivor function, Log" R",,(X )l showing plot for data set lB.

The me anin g of the se results fo r the 'oscillatory' sea-level and sed ime ntological mod els of bioge nic level formation is equivoc al. Reliance in the interpre tation of such a tim e series must be influen ced by th e size of the da ta se t used , particularl y if the inform at ion is deri ved from radi ocarbon dating with all its inhere nt difficulti es in sa mple comparability, co nta minatio n, counting a nd correction err ors (She nnan, 1980). Based up on the ana lysis of < 100 values the conclusions here can o nly therefore be regarded as ten tati ve and a number of explan ations of the results ex ist. T he lack of sta tistical suppo rt for trend o r peri odicity in th e seri es seems co nsiste nt with th e operation of a sedimentar y o nla p/o ffiap mechan ism in for ming the bio genic levels. De spite differen ces in sed ime nt influx a nd local coas tal de pos itional histori es, so me co nsisten cy in th e timing of intersite pea t accumulatio n is to be expec te d, particularly between similar e nviro nments influen ced by the same pr evailin g rat es of sealevel rise . In thi s situatio n the mod el need no t be at variance with the evidence for th e non-rand om distribution of th e time series. Alternatively if the timing of events was due primarily to a glacio-eustatic or some other 'oscillatory' mechanism, similar sta tistical results might be expec ted if, in this case. th e result ant sea sur face movem ents occurred in respon se to climatic and rel ated cha nges operating o n d ifferen t a nd ove rla pping tim escale s. The influe nce of local ph ysiographic and de positional fact ors wo uld furthe r amplify the production of an apparently irregular pa ttern of relat ive sea -leve l movem ents. In this situation the intervals betw een even ts may not show any significant se ria l correla tion, whilst the pr ecise tim ing of onse t and rem o val of marin e co nditio ns (se ts 2 and 3) could displa y the appa re nt random pa tte rn identified. A combination of the sedimentary a nd 'oscillatin g' se a surface mechanisms could also be e nvisaged , particularl y if the ra tes of sea surface chan ge rem ain ed low.

84

R. J . DEVOY

Although estimation of the speed of some phases of Holocene marine incursion would dispute the feasibility of this alternative (Tooley, 1979), its operation might again produce a similar statistical outcome. Whatever the interpretation the statistical results suggest that a dominant environmental process has probably been instrumental in governing the distribution of indicators during sea-level recovery, at least prior to c. 3500 BP. Further, an uninterrupted or 'smoothly' rising exponential curve appears to form an inappropriate model for Holocene sea-level change. Independent support for at least one phase of stillstand , if not fall of the sea surface during recovery, is perhaps forthcoming in the fitted curve for relative sea-level trend (Fig. 4). The flattening of the curve at c.4500 BP coincides with the recording of poor pollen preservation and oxidation of levels in coastal biogenic material formed during this time. If these characteristics were caused by exposure above the ground water table, then a regional fall of sea-level might be invoked as a likely explanation for a seasonal, or perhaps more permanent fall of coastal freshwater tables. Despite the alternatives presented there seems insufficient grounds at this stage for constructing an 'oscillating' curve for the data. It is worth noting, however, that on a longer timescale firmer evidence exists for relative sea-level height oscillations, governed in the Pleistocene by primarily climatic or glacio-eustatic controls (Vail, Mitchum & Thompson, 1977). The question may be posed, why the absence of similar but perhaps smaller scale frequency sea-level variations in the Holocene too? The role of climate in influencing the formation of such coastal sedimentary sequences is evidenced at a number of sites after 3000 BP. From the Yare estuary, increased freshwater discharge toward the end of peat accumulation at 2000 BP is indicated by the enlargement and reincision of river channels through the peat (Coles & Funnell, 1981). Changes in plant communities due to waterlogging in a freshwater environment supports this finding. In nearby Fenland increased inland peat growth and apparent freshening of the environment at a comparable time, (Churchill, 1970; Godwin, 1978) might have resulted from a similar increase in freshwater throughput. Coles & Funnell (1981) suggest that in the Yare such an increase would have helped to maintain freshwater conditions and delay the onset of the marine influence recorded elsewhere. In the Thames the diatom flora from Tilbury shows a distinct 'freshening' after 3050 BP and the re-establishment of inorganic estuarine sedimentation. High mesohalobion and oligohalobion group values (>80% TD at maximum) are dominated by the species Fragilaria brevistriata, F. pinnata, F. construens var. venter, Navicula cryptocephalla and N. cincta. Following another phase of biogenic growth at c. 1700 BP the reappearance and increased dominance of these species evidences the

continued apparent reduction in the marine effect on this area. A similar timing for the rise in freshwater influence throughout the region may have its origins in the onset of increased storminess and a drop in average temperature over Europe by -2°C at about 2950 BP (Lamb, 1977). The influence of man in the coastal zone through embanking, drainage and agricultural practices must similarly have distorted the normal development of depositional systems, particularly by the production of increased sediment discharge and surface runoff. In Broadland, Fenland, Romney Marsh and the Thames frequent reference is made to alteration in areas of deposition and changes in coastal shape due to man (Jennings, 1952; Evans, 1953; Green & Hutchinson, 1960; Cunliffe, 1980). As has been pointed out (Jegersma, 1961; Plassche, 1977, 1980) sediment compaction and related consolidation factors form a major problem in the use of interleaved biogenic and inorganic sequences for the reconstruction of relative sea-level heights. In general terms the estimation of the amount of compaction which a level has undergone is based on (a) calculation of the effective vertical pressures acting on the layers beneath a stratum, both at the time of deposition and in the present day and (b) the change in thickness of the particular stratum due to the increase in overburden load . Details are thus required of the thickness, density and material composition of the layers , pore water pressure, sediment void ratio and liquid limits . Complications arise in a sedimentary sequence containing organic material, as continued oxidation and organic breakdown after initial compaction, necessitates the calculation of the subsequent changes in stratum thickness (Kaye & Barghoorn, 1964 ; Wilson, Radforth, MacFarlan & Lo, 1965 ; Hanrahan, 1980). Full discussion of the techniques involved is given in Skempton (1970), Marsland (1977) , Hanrahan (1980) and Terzaghi & Peck (1967). For most of the sites considered such details are lacking and specific compaction measurements cannot be given. Numerous soil mechanics studies have, however, been undertaken upon the interleaved organic, clay, silt and sand sediments from within the region and in analagous coastal areas (e.g. Skernpton & Henkel, 1953 ; Kidson & Heyworth, 1973). These have provided a generally consistent series of results on compaction values within such sequences at differing depths. Comparison of the data points used in this study with these results, has allowed a crude estimation ±50% of the likely compaction undergone at each level. The value of the estimation lies particularly in providing some explanation of the inconsistencies in height arising from the comparison of indicators developed upon relatively stable sand/gravel surfaces, with those levels forming at about the same time within more compressible silts and clays. The original height data have been transformed using this information (Table 3) and a curve plotted for the altered distribution of points (Figs. 9a

H OLOCENE SEA-LEVEL MOVEMENTS IN SOUTHEAST ENGLAND ·3

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Fig. 9a & b. Fit of 2, 3 & 4th order curves by least-squares regression to sea-level indicators repre sentin g MHWST levels in the Holo cene and corrected in height for sediment compaction/consolidation. (a) Data plotted on the standard Libb y radiocarbon half-life. (b) Dat a plotted using tree ring corrected radiocarbon values.

& b). A cubic model again forms the best resolution of the distribution , showing a small reduction in point disperis on. Another important difficulty in accurately assessing both the intersite relationship of se a-level data and the general form of sea-level recovery is the absence of reliable palaeotidal data. Sea-level indicators forming under the influence of a high MHWST regime must

85

show a gre ater height for a former sea-level, relative to a fixed datum, by compariso n with similar indicators forming at the same time , but und er relatively lower MHWST levels. Similar distortions will occur for indicators developing in relation to other tidal positions. Differences in fetch, coastal shape , water depth and cur rent movements through mu ch of ea rly and midHolocen e time , with MHWST levels still below - - 4.0 m a.D. at c. 5000 BP , must have resulted in differe nt tidal patterns . Failur e to reconstruct the tidal amplitudes in exis tence, at the time the sea-level indi cators used in the survey were formed, r aises the likelihood of height error in the co mpariso n of some of the data points. An atte mpt to estimate roughly the possible disparities between sites based on pr esent da y tid al inform ation (Devoy, 1979) ha s not been used . Changes in the location of amphidromic points and the physical parameters outlined would invalidate the use of this type of simplistic compar ison. After c. 4500 BP the influenc e of such distortions may , however, have been much reduced. In the Netherlands the present tidal ran ge was appare ntly established in central coa stal areas of the country by this time (Roep , Beets & Ruegg, 1975 ; Jegersma, 1979 ) and it is fea sible that the modern tid al regime had similarly be gun to operate in so utheast Engl and at a compara ble dat e. Such a conclusion may still onl y be regarded as tentat ive, for the construction of blockin g deposition al struc tures in narrow estuary zones (Coles & Funnell, 1981 ; Straw & Clayton, 1979) may have become of importance in this ph ase, causing temporary changes in tidal amp litud es. In analysing the graphs for sea -level change a distinct feature is the vertical hei ght disparit y in SOme of the dat a points of the same age. This is underscored by the high values for the sum of squares due to deviation for the fitted curves. In the Thames estuary data (Table 2) thes e height differences are spatially grouped into indi cators from the inner and outer estuary zones. It has been suggested (Devoy, 1977 , 1979), that these d ifferences may be resolved by the co nstructio n of two separate sea-level cur ves, with points from the inner estuary plotting at levels ab ove the others . In this instance explanation of the he ight differ ence probably centres upon cha nges in estu ar y shape over time , affecting the d ownstream-upstream differentials in tidal amplitude and also upon compaction/ consolidation factors. Elsewhere in the reg ion similar influences coupled with errors in height measurement, correct identifi cation of a sea -le vel indicator's position and rad iocarbon dating are prob abl y all of importance in explaining the differences identified. In this geologically longterm unstable region tectonic subside nce may also form a major and variably qu antified component in causin g differential chan ges in height between indicators. In the Th ames this factor has been suggested as an expl anation of some of the height differe nce (-1 .5 m at maximum) as yet una ccounted

86

R. J. DEVOY

for by the other two factors cited (cf. Greensmith & Tucker, 1980). A feature of the two curves tentatively identified is that the height gap is progressively reduced toward the present with their approximate merger by c. 2800 BP. This might be explained by increased sedimentation rates, the removal of flood depression areas by man in the outer estuary and a later decline in the rate of tectonic subsidence. Explanation of similarly separated sea-level curves from the Rhine-Meuse delta (Plassche, 1981) would dispute the operation of a major subsidence factor in the Thames and suggests a cause due solely to tidal differentials. In estuaries recording increases in tidal height upstream, early inundation of probably narrow channels would have resulted in the recording of a maximal tidal differential at the beginning and not at the end of the Holocene. Later development of flood depressions and increased frictional influences would have resulted in the progressive dissipation of the tidal wave upstream and allowed the eventual merger of the inferred relative sea-level curves. Application of this palaeotidal model from the Rhine-Meuse delta to other areas, must depend upon establishing similar estuary shape parameters, freshwater throughput and defining the influence of man in altering estuary form through time. In reviewing the operation of subsidence in the region a variety of information sources have been used (Valentin, 1953; Churchill, 1965; Smalley, 1967; Taylor & Smalley, 1969; Rossiter, 1972; West, 1972; Lennon, 1975; Caston, 1979). All the studies indicate a high degree of relative tectonic subsidence, apparently focusing offshore in the Thames/Essex area and increasing toward the southern North Sea Basin. In support of this trend, relative sea-level indicators from the outer Thames zone appear to generally plot at levels below others of the same age in the region (Fig. 3). Along the south coast data points are too disparate and sparse in number to help in establishing any distinctive areal subsidence trend. Those that do exist appear not to show any significant deviations or consistent pattern of variations from the regional data plot. Bearing in mind the inaccuracies inherent in the sea-level data, this survey does not dispute the trend established by the earlier findings. Any figures for inter-areal subsidence based upon sea-level data would be misleading at this stage.

6. CONCLUSIONS Analysis of sea-level indicators derived from estuary and nearshore open coastal environments provide evidence for the progressive marine inundation of levels above - - 30 m O.D. after 9000 BP. The recorded height and timing for formation of the indicators is shown not to be a simple function of this sea-level rise alone. Changes through time in local sedimentation, coastal shape, sediment compaction, tidal variations

and tectonic subsidence, have combined either to subsequently alter the height relative to Ordnance Datum at which the indicator was formed, or to provide between-site variations in the recording of a particular sea surface. An attempt at a quantitative estimation of some of the errors involved has resulted in establishing for some indicators the directional influence of a particular distorting factor, as well as a measure of improvement in overall data point dispersion. However, the establishment of an accurate timeheight framework for relative sea-level movements has not been achieved and must await at the least, (1) the provision of many more data points through coordinated programmes of coring and stratigraphic sectioning, (2) the related reconstruction of palaeogeographic changes and palaeotidal regimes throughout the region and (3) determination of specific compaction/consolidation values for each dated level. Use of mathematical techniques show that the trend of sea-level recovery probably does not conform to a simple pattern of exponential change. A time of possible stillstand or distinct reduction in the rate of rise appears to have occurred between c. 4500 to 3000 BP. Precise explanation of the formation of coastal interleaved biogenic and inorganic sequences remains unclear. Results of their analysis by series of events suggest that a single dominant environmental process, such as a sediment onlap/offlap mechanism, acting in conjunction with the longterm trend of Holocene sealevel rise is probably the primary cause. Support for actual oscillatory sea surface movements during the Holocene is not forthcoming, although their existence and influence upon the sedimentary record cannot be ruled out. Palaeoecological and sedimentary changes at many coastal sites do show the influence of a possibly worsening climate and greater freshwater throughput in estuary zones after 3000 BP, coupled with an increasing influence of man upon the depositional record of the coastal zone. Further investigation of these areas is required, particularly, (1) more complex modelling of coastal sedimentary data, (2) analysis of the inter-relationship between the record of climatic change and the impact of freshwater discharge on the coastal zone and (3) the influence of man in changing coastal form. Interpretation of the true pattern of sea-level recovery must probably therefore await the research into these areas, although its solution will perhaps not come from the results of any single regional record.

ACKNOWLEDGEMENTS The initial stages of organisation and data collection for this region in the UK's contribution to IGCP Project 61, was undertaken by Dr. T. Greensmith (QMC, London) with the assistance of Dr. V. Tucker (QMC, London). I very much appreciate their work,

H OLO C E N E SEA-LEVEL MOVEMENTS IN SOUTHE A ST ENGLAND

interest and ad vice on the project. I also wish to thank all those who helped in the draughting and production of this paper, particularly Ms. G . Houston (Dept. of Geograph y, U. C.C.) for drawing-up a number of the

87

figure s. To Dr. H. J . B. Birks (The Bot an y School, Uni ver sity of Cambridge) and Dr. K. Hourihan (Dept. of G eography, u.c.c.) my sincere thanks for their advice and help in data processing and computing.

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