Structure and development of shell cheniers in Essex, southeast England, investigated using high-frequency ground-penetrating radar

Structure and development of shell cheniers in Essex,southeast England, investigated using high-frequency

ground-penetrating radar

Adrian Neal a;�, Julie Richards b;1, Ken Pye b

a School of Applied Sciences, University of Wolverhampton, Wolverhampton WV1 1SB, UKb Geology Department, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK

Received 29 January 2001; accepted 6 August 2001

Abstract

The stratigraphy and internal structure of contemporary shell cheniers on the southern Essex coast, England wereinvestigated using a high-frequency (900 MHz) ground-penetrating radar (GPR) system. The cheniers lie on theseaward edge of laterally eroding saltmarshes, at the junction with the tidal flats. Four main types of chenier arerecognised, each with a characteristic morphology and dynamic status. Migrated radar reflection profiles obtainedfrom each of the chenier types achieved a sub-decimetre vertical resolution. Ground-truthing data demonstrate thatthe radar profiles accurately delineate the subsurface stratigraphy and sedimentary structure of the cheniers.Interpretation of the radar stratigraphy derived from the radar reflection profiles allows various deposits to beidentified. These have resulted from overwashing, overtopping, sedimentation across the whole of a seaward dippingbeachface or berm ridge welding onto the upper beachface. Each chenier is characterised by a different spatialarrangement of these four basic depositional units. The results from the GPR surveys support the hypothesis thateach of the chenier types represents a distinct phase in the development of these contemporary coastal landforms.However, a simple model involving a linear progression through chenier types 1^4 with time appears inappropriate.Instead, two main evolutionary pathways are proposed, with the sequence of development being principally controlledby the nature of the cheniers’ longer-term sediment budgets. 6 2002 Elsevier Science B.V. All rights reserved.

Keywords: ground-penetrating radar (GPR); shell chenier; beachface; washover; overtop; sediment budget

1. Introduction

Otvos (2000) de¢nes a chenier as a type ofbeach ridge that is largely wave built, but sur-

rounded to landward, to seaward and beneathby intertidal^supratidal mud£ats. Cheniers areshallow-based, generally anchored in the uppershoreface and composed of sand, shell and/orgravel that forms a sharp textural contrast tothe muds of the underlying tidal£at deposits.The relative importance of the main sedimentcomponents varies both between and within che-niers, but shells and shell debris are often domi-nant. Cheniers appear to be azonal landforms

0025-3227 / 02 / $ ^ see front matter 6 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 5 - 3 2 2 7 ( 0 1 ) 0 0 2 3 9 - 0

1 Present address: The Environment Agency, King¢sherHouse, Goldhay Way, Orton Goldhay, Peterborough,PE2 5ZR, UK.* Corresponding author.E-mail address: [email protected] (A. Neal).

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(Augustinus, 1989), with both active and relictexamples having been described from a widerange of locations including North America (Rus-sell and Howe, 1935; Schou, 1969; Penland andSuter, 1989), Central and South America (Augus-tinus, 1980; Meldahl, 1995; Vilas et al., 1999),Europe (Greensmith and Tucker, 1966, 1969,1975; Pontee et al., 1998), Africa (Anthony,1989), the Persian Gulf (Shinn, 1973), Australia(Chappell and Grindrod, 1984; Short, 1989;Lees, 1992; Woodro¡e and Grime, 1999), NewZealand (Woodro¡e et al., 1983), China (Xitao,1989; Cangzi and Walker, 1989; Chen, 1996; Sai-to et al., 2000) and Korea (Lee et al., 1994; Parket al., 1996).The internal structure of cheniers can be used

to help evaluate the processes responsible for theirformation and the nature of their longer-termevolution. Sedimentary structure may also forma means of distinguishing cheniers from othertypes of landform that can occupy similar strati-graphic settings, such as tsunami deposits (Bryantet al., 1992) or cultural deposits (Sullivan andO’Connor, 1993; Stone, 1995), and also facilitatetheir recognition in the geological record (Chen etal., 1990).Unfortunately, at present, data regarding the

internal structure of cheniers are limited. Relevantstudies have largely been con¢ned to those thathave obtained information from a small numberof trenches and cores or natural ¢eld exposures(Greensmith and Tucker, 1966, 1969; Hoyt,1969; Shinn, 1973; Gremillion and Paine, 1977;Rhodes, 1982; Augustinus, 1980; Cangzi andWalker, 1989; Penland and Suter, 1989; Xitao,1989; Lee et al., 1994; Park et al., 1996; Ponteeet al., 1998; Vilas et al., 1999). Trenching andcoring is often very di⁄cult to carry out, espe-cially in shell-rich cheniers where a lack of sedi-ment cohesion prevents formation of stable free-standing faces (e.g. Rhodes, 1982).Di⁄culty in obtaining data regarding the strat-

igraphy and internal structure of unconsolidateddeposits by traditional methods is common tomany studies undertaken in contemporary sedi-mentary environments. As a result, workers havelooked for new ways of obtaining subsurface in-formation and the use of the non-destructive geo-

physical technique ground-penetrating radar(GPR) is increasingly common. GPR has beensuccessfully used in a variety of sand and/or grav-el-dominated sedimentary environments to visual-ise internal structure. Within the coastal environ-ment, studies have largely been con¢ned to thoseutilising low-frequency radar (12.5^200 MHz) toinvestigate large-scale sand and/or gravel rich sys-tems in North America (e.g. Jol et al., 1996;Meyers et al., 1996; Van Heteren et al., 1996,1998; Van Heteren and van de Plassche, 1997;Smith et al., 1999), Europe (Van Overmeeren,1994, 1998; Tronicke et al., 1999; Bristow et al.,2000; Neal and Roberts, 2000, 2001; Neal et al.,2001) and elsewhere (Baker, 1991; Harari, 1996).However, a recent study by Zenero et al. (1995)has suggested that high-frequency radar (in theircase 500 MHz) can provide detailed images ofthe internal structure of shallow based (1^2 m)chenier deposits. This paper describes a furtherdeployment of high-frequency GPR to helpdetermine the internal structure of a variety ofshell-rich cheniers on the Essex coast, UK(Fig. 1), at sites not examined in detail since thework of Greensmith and Tucker (1966, 1969,1975).

2. Study sites

It is possible to divide the cheniers of the Essexcoast into two geographically distinct categoriesbased on their sediment characteristics ; sandand gravels dominate those on the northern Essexcoast, whereas the more southerly cheniers areprimarily composed of shell (Greensmith andTucker, 1975). This study examines the structureand development of active and recently activeshell-rich cheniers found on the southern Essexcoast, at Sale’s Point and St. Peter’s Way on theDengie Peninsula and around Foulness Island(Fig. 1).The Essex coast is macrotidal, being subject to

semi-diurnal tides with a spring tidal range ofbetween 4.8 and 5 m. The height of mean highwater spring (MHWS) tides varies between 2.6and 2.8 m above Ordnance Datum (O.D.). Theshelly cheniers of the southern Essex coast are

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supratidal, typically forming at the boundary be-tween the upper intertidal mud£ats and saltmarsh(Fig. 2) and occupying elevations between +2 and+4.5 m O.D. Contemporary chenier developmenthas been associated with recent erosion and land-ward retreat of the seaward edge of the salt-marshes (Fig. 3) (Pye, 2000).The cheniers are composed mainly of whole

and fragmented shells of Cerastoderma edule(the common cockle), but other mollusc speciespresent includeMytilus edulis, Crepidula fornicata,Buccinum undatum, Littorina spp. and Gibbulaspp. Greensmith and Tucker (1969) suggest thatthe shells be derived primarily from mass mortal-ity events. However, the scouring of subtidal andintertidal niches and the reworking of older de-posits also make a small, but signi¢cant, contri-bution. Greensmith and Tucker (1966, 1969)

make a distinction between ‘o¡shore’ shell banksand sheets, which form on the lower and middleintertidal £ats, and supratidal cheniers. The o¡-shore deposits are believed to act as a signi¢cantsource of sediment for the cheniers. However,sediment release and delivery to both the shellbanks/sheets and cheniers is likely to be controlledby complex interactions between short- and me-dium-term changes in hydrodynamic regime andstorminess, changes in foreshore gradient and lat-eral migration of major intertidal channels.Aerial photographs dating from 1970 to the

present-day and recent ¢eld surveying both sug-gest that the cheniers have a number of distinctmorphological forms (Types 1^4, Table 1, Figs. 2and 3) and that these may represent particularstages in an evolutionary sequence of chenier de-velopment. However, the limited spatial and tem-

Fig. 1. The location of the study area in southern Essex, southeast England. Also indicated are the positions of aerial photo-graphs that show site details for each of the chenier types investigated (Types 1^4, Fig. 2).

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Fig. 2. (a) Annotated aerial photograph showing the site characteristics associated with the Type 1 chenier examined and the lo-cation of radar re£ection Pro¢les 1a and b. (b) Annotated aerial photograph showing the site characteristics associated with theType 2 chenier examined and the location of radar re£ection Pro¢les 2a^d. (c) Annotated aerial photograph showing the sitecharacteristics associated with the Type 3 chenier examined and the location of radar re£ection Pro¢les 3a and b. (d) Annotatedaerial photograph showing the site characteristics associated with the Type 4 chenier examined and the location of radar re£ec-tion Pro¢les 4a and b. The aerial photographs were taken in 1997 and are reproduced with the permission of the EnvironmentAgency (Anglian Region). Environment Agency (Anglian Region) Copyright.

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Fig. 2 (Continued).

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poral resolution of the aerial photographs and¢eld surveys prevents a de¢nitive link betweenthe various types from being established. Theaim of this paper is to use sedimentological andstratigraphic data derived from GPR surveying

and trenching to help establish the mode of originand sequence of formation of the various mor-phological elements that comprise the di¡erentchenier forms. This information will then beused to try and ascertain whether the various che-

Fig. 3. (A) Arcuate Type 1 chenier lying on the seaward edge of vegetated saltmarsh. Note the gently landward dipping uppersurface and small wave-built ridge on its seaward side. (B) Central portion of a Type 2 chenier displaying a large medial ridge,lobate and stabilising landward side and steeply seaward dipping beachface. (C) Recurves at the lateral margin of a Type 2 che-nier. (D) A sequence of progradational chenier ridges associated with a Type 3 chenier. (E) Laterally continuous Type 4 cheniershowing a gently landward dipping upper surface and lobate landward margin. In all photographs ‘L’ and ‘S’ refer to the land-ward and seaward sides of the cheniers, respectively.

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nier types identi¢ed constitute points within acontinuum of natural chenier development.

3. Methods

The theoretical background to the GPR tech-nique and the practicalities of data collection arecomprehensively described in the current litera-ture (Davis and Annan, 1989; Conyers andGoodman, 1997; Reynolds, 1997; Neal and Rob-erts, 2000). Consequently, only those aspects ofthe GPR surveying that have direct relevance tothe interpretation of the resulting radar re£ectionpro¢les will be discussed in detail.In order to characterise the internal structure of

the four chenier types identi¢ed, 40 GPR surveytransects were performed using a PulseEKKO1000 GPR system, resulting in over 900 m of re-£ection pro¢le data. Initial trials at each of thestudy sites revealed that optimal imaging wasachieved with 900 MHz antennae, traces stackedfour times, a 50 ns time window and a step-size of0.05 m. At each survey site the average velocity ofthe electromagnetic waves in the subsurface wasestimated through a common mid-point (CMP)survey, using the principles outlined by Nealand Roberts (2000). The average velocities mea-sured varied between the pro¢les at the di¡erentlocalities (Table 1), but were consistently high.They are comparable to the higher of the veloc-ities typically associated with unsaturated sandand/or gravel (Neal and Roberts, 2000). The ve-locity information was subsequently used to con-vert two-way travel time to re£ections into depth/elevation.The topography of each of the GPR transect

lines was surveyed using a Sokkisha Set 4C Elec-tronic Total Station. The data were corrected toelevation above O.D. using benchmarks main-tained by the UK government’s EnvironmentAgency.Editing and processing of the GPR data was

carried out using version 4.2 of the Sensors andSoftware PulseEKKO 1000 system software (Sen-sors and Software, 1996). A low-frequency ‘De-wow’ ¢lter was applied to all the re£ection pro¢lesin order to remove noise associated with the large

transmit pulse of the GPR equipment. Two-di-mensional F^K migration was also carried outon the data, using the same procedure as Nealand Roberts (2001).During data processing, the return centre fre-

quency (the frequency of the electromagneticwaves recorded by the receiving antenna) was es-timated for each radar re£ection pro¢le. Valuesvaried between 535 and 720 MHz and, in combi-nation with the relevant subsurface radar wavevelocity, resulted in vertical resolution estimatesof between 0.03 and 0.08 m (Table 1). All radarre£ection pro¢les were then plotted in wiggle traceformat with no vertical averaging, a horizontalaveraging of three traces and an automatic gaincontrol with a maximum limiting value of be-tween 50 and 150.The radar re£ection pro¢les were interpreted

using the principles of radar stratigraphy, whichis based on seismic stratigraphy (Mitchum et al.,1977) and relies on the identi¢cation of systematicre£ection terminations (Beres and Haeni, 1991;Jol and Smith, 1991). These terminations havetraditionally been used to de¢ne ‘radar sequenceboundaries’, which in turn delineate packets ofre£ections termed ‘radar sequences’. However,since the advent of sequence stratigraphy duringthe 1980s, the term ‘sequence’ has developed avery speci¢c meaning related to relative changesin sea level (Van Wagoner et al., 1990; Emery andMyers, 1996). Consequently, in this paper theterm ‘radar surface’ will be used in preference to‘radar sequence boundary’ and ‘radar package’will be used instead of ‘radar sequence’. Such ter-minology is analogous to that used in modernseismic re£ection pro¢le interpretation (Emeryand Myers, 1996). ‘Radar facies’ are the two- orthree-dimensional sets of re£ections that lie be-tween the radar surfaces and are characterisedby their distinctive con¢gurations, continuity, am-plitude, radar wave velocity and external form.With respect to deposits formed exclusively ofcoarse clastic sediments, once the radar stratigra-phy has been de¢ned it can be interpreted geo-logically in terms of the nature of bounding sur-faces (radar surfaces), bed assemblages (radarfacies) and the overall geometry of the deposits(radar packages).

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Table 1Typical morphological characteristics, relevant radar re£ection pro¢les and GPR survey characteristics for the shell sheets and chenier Types 1^4 studied on thesouthern Essex coasta

Typical morphological characteristics Radar re£ection pro¢les Velocity Return centre frequency Vertical resolution(m ns31) (MHz) (m)

Shell sheets Thin layers of shell, up to 10 m wide, several 10’s m longand 0.5 m thick

Not suitable for radarpro¢ling

Type 1 chenier Small, discrete, arcuate cheniers. 10^20 m wide, 25^100 mlong and up to 1 m thick. Usually found at the limit ofshell dispersal by longshore drift

1a and b (Fig. 5) 0.140 535 0.07

Type 2 chenier Large, discrete cheniers. Up to 50 m wide, 150^250 m longand up to 2 m thick. Relatively narrow at their centre(10^20 m) and dominated by a single, large chenier ridge.Two or more recurved chenier ridges at their distal ends

2a^d (Fig. 6) 0.176 575^620 0.07^0.08

Type 3 chenier Large, laterally continuous (many 100’s m) cheniers. Setsof relict recurves to landward. Wide (many 10’s m) sequenceof parallel relict and active chenier ridges to seaward.Deposits can be over 2 m in thickness

3a and b (Fig. 7) 0.177 720 0.03

Type 4 chenier Laterally continuous (many 100’s m), with a low, singlechenier ridge to seaward. 20^25 m wide and up to 1.5 mthick. Periodically broken by large across-shore saltmarshcreeks

4a and b (Fig. 8) 0.130 544^650 0.05^0.06

a Aerial and ground photographs of chenier Types 1^4 are shown in Figs. 2 and 3.

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4. Radar re£ection pro¢les

For each chenier type identi¢ed, a representa-tive example from one of the three study sites hasbeen selected. Radar re£ection pro¢les from eachexample are presented in order to demonstrate thenature of their internal structure.At each of the study sites, ground-truthing was

attempted using either trenches or cores. This wasperformed in order to determine the cause of re-£ections in the radar pro¢les and validate the geo-logical interpretations of the radar stratigraphy.At St. Peter’s Way (chenier Type 2) and Sale’sPoint (chenier Type 4) the sediments often con-tained a signi¢cant proportion of sand and com-minuted shell ; free faces formed and it was pos-sible to observe the internal structure of thecheniers. At these locations, the internal structurewas shown to be largely imparted by centimetreto decimetre thick alternations of broken shell/sand and whole shells/valves (Fig. 4). Greensmithand Tucker (1969) noted similar bedding duringtheir studies of the cheniers at Sale’s Point. Var-iations in grain size and associated changes in thesediment:air :freshwater ratio have been identi¢edas important causes of radar re£ections in sandand/or gravel dominated sediments, along withvariations in grain shape and orientation, miner-alogy and organic content (Baker, 1991; VanDam and Schlager, 2000). The bedding identi¢edin the trenches showed excellent correspondenceto the form and orientation of re£ections on therelevant radar pro¢les (see Section 4.2 Type 2chenier and Section 4.4 Type 4 chenier).On Foulness Island (chenier Types 1 and 3),

ground-truthing was far less conclusive, as thecheniers were composed almost entirely of wholeshells and valves. Consequently, trenches werehighly prone to collapse, preventing the formationof free faces and the observation of any internalstructure. However, despite the lack of evidence

from ground-truthing, the radar re£ection pro¢lesfrom Foulness clearly indicate that the cheniershave a distinct internal structure (see Section 4.1Type 1 chenier and Section 4.3 Type 3 chenier).Furthermore, in some instances the structure isvery similar in character to that observed in pro-¢les from sites where ground-truthing was possi-ble. This strongly suggests that the origin of there£ections lies primarily in strati¢cation presentwithin the chenier sediments. This bedding ismost likely to be imparted by subtle variationsin valve size, packing and orientation.

4.1. Type 1 chenier

A chenier representative of Type 1 was locatedon the southeast coast of Foulness Island, at thesouthwestern limit of chenier formation (Fig. 2a).When surveyed it was relatively small (ca. 50 mlong, maximum width of 13 m) and low in eleva-tion (up to +3.4 m O.D., approximately 1 mabove the surface of the marsh, Fig. 3A). Wholevalves of C. edule dominated the chenier.Radar re£ection pro¢les are presented from two

transect lines, one through the centre of the che-nier and perpendicular to its crest (Pro¢le 1a,Fig. 5a) and one intersecting transect at right-an-gles (Pro¢les 1b, Fig. 5b). Despite some ‘ringing’,interpretation of the pro¢les reveals a relativelysimple radar stratigraphy.The base of the radar stratigraphy is marked by

a semi-continuous horizontal re£ection (radar sur-face A-1), against which overlying re£ections ter-minate with a downlapping relationship. Ground-truthing and topographic surveys con¢rm thatA-1 represents the marsh surface underlying thechenier. Lying above radar surface A-1 is radarfacies A-a, which characterises the main body ofthe chenier. In Pro¢le 1a (Fig. 5a), A-a consists oflandward dipping, sigmoidal shaped re£ectionsthat are laterally continuous over distances great-

Fig. 4. (A) Internal structure of seaward dipping beachface deposits in a trench at 24.5 m along Pro¢le 2d (see Fig. 6d). The sur-vey sta¡ is subdivided in 0.2 m increments. (B) Gently landward dipping beds formed of alternating layers of whole and brokenshell/sand a few centimetres thick, exposed in a trench 6 m along Pro¢le 4a (see Fig. 8a). The scale bar is 0.1 m long. (C) Gentlylandward dipping beds formed of alternating layers of whole valves and broken shell up to 0.1 m thick, exposed in a trench 10 malong Pro¢le 4a (see Fig. 8a). The scale bar is 0.1 m long. In all photographs ‘L’ and ‘S’ refer to the landward and seaward sidesof the cheniers respectively.

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er than 1 m and increase in dip in a landwardsdirection. In Pro¢le 1b (Fig. 5b), A-a displaysre£ections that are subparallel and gently undu-lating, de¢ning a number of broad depressions. Insome instances these depressions appear to con-tain low-angle divergent ¢ll re£ections (0^5 m,Pro¢le 1b, Fig. 5b). At the intersection betweenPro¢les 1a and 1b re£ections have a maximumtrue dip of approximately 9‡ to landward. How-ever, the re£ection con¢guration in Pro¢le 1b sug-gests that there are small, but signi¢cant, varia-tions in the true dip of re£ections forming A-athat de¢ne subtle landward directed radial dippatterns.The seaward margin of radar facies A-a is cut

by the seaward dipping radar surface A-2 (Fig.5a). A-2 has a true dip of 6‡ to seaward (basedon an intersection point at 7.8 m along Pro¢le 1awith a perpendicular pro¢le that has not beenpresented). The projected landward outcrop ofA-2 corresponds to a distinct break in slope onthe surface of the chenier (5.1 m, Pro¢le 1a, Fig.5a). Seaward of this point the chenier surface risesinto two distinct ridges, with crests at 6.1 and 7.8m. The radar structure of the ridge at 6.1 m isobscured by the air and ground waves, but thedeposits of the larger, more seaward ridge (7.8m) are characterised by radar facies A-b. On theseaward side of the ridge crest, A-b consists pre-dominantly of laterally continuous, seaward dip-ping re£ections that have true dip angles of up to14‡ seaward (again based on an intersection pointat 7.8 m along Pro¢le 1a with a perpendicularpro¢le that has not been presented). On the land-ward side of the ridge crest landward dipping re-£ections are present with apparent dips of up to11‡.The re£ections forming A-a are interpreted as

beds formed of whole shell. The low-angle land-ward dip, which increases in a landwards direc-tion, and sigmoidal form of the beds suggests thatthe chenier is largely composed of washover de-posits. Similar internal structures have been de-scribed from a variety of chenier deposits andascribed an overwash origin (Hoyt, 1969; Shinn,1973; Augustinus, 1980; Rhodes, 1982; Cangziand Walker, 1989; Penland and Suter, 1989; Xi-tao, 1989; Lee et al., 1994; Park et al., 1996). Thepresence of broad, regular undulations in beddingon the scale of several metres in the longshoredirection has not previously been described fromthe washover deposits of cheniers. These featuresare interpreted as being indicative of washoverlobe development.Radar surface A-2 is interpreted as a seaward

dipping beachface bounding surface that repre-sents either a period of non-deposition or minorerosion on the seaward side of the washover de-posits. The overlying, seaward dipping re£ectionsthat dominate radar facies A-b at lower elevationson the seaward side of the ridge crest are inter-preted as being the result of bedding formed bybeachface sedimentation. Similar deposits havepreviously been described from the seaward sideof active cheniers, cross-cutting washover depositsto landward (Hoyt, 1969; Shinn, 1973; Augusti-nus, 1980; Rhodes, 1982). Seaward and landwarddipping re£ections that characterise A-b beneaththe crest of the chenier ridge are interpreted asbedding resulting from overtopping, by analogywith very similar deposits described from sand-and-gravel-dominated coarse clastic beaches (Or-ford and Carter, 1982). Overtopping leads toridge development on the uppermost part of thebeachface on the seaward side of the chenier,largely through vertical aggradation.

Fig. 5. (a) Radar re£ection Pro¢le 1a, with an interpretation of its radar stratigraphy directly beneath. Pro¢le 1a was obtainedalong a transect perpendicular to the arcuate long-axis of the Type 1 chenier shown in Fig. 2a and Fig. 3A. The pro¢le runsfrom landward (WNW) to seaward (ESE). The intersection point with the perpendicular Pro¢le 1b and the position of a trench(T) are indicated. The trench revealed that the chenier was composed predominantly of whole shells and valves. Also indicatedare the positions of the air and ground waves, which are system noise associated with all radar re£ection pro¢ling, and the posi-tion of ‘ringing’, which is another form of noise that comprises multiple re£ection events of various origins. (b) Radar re£ectionPro¢le 1b with an interpretation of its radar stratigraphy directly beneath. Pro¢le 1b was obtained along a transect perpendicularto Pro¢le 1a. The interpretations of the two pro¢les indicate the location of two radar surfaces (A-1 and A-2) and two radar fa-cies (A-a and A-b).

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Fig. 5 (Continued).

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4.2. Type 2 chenier

A chenier indicative of Type 2 was surveyed atSt. Peter’s Way, on the Dengie Peninsula, approx-imately 2 km south of Sale’s Point (Fig. 1). Thechenier was around 150 m long, up to 30 m wideand reached an elevation of up to 4.3 m O.D.,approximately 2 m above the marsh surface(Figs. 2b and 3B,C). The chenier was primarilymade up of whole and broken valves of C. edule.Radar pro¢les 2a^c were obtained from transectsin the central portion of the chenier (Fig. 2b),where a large, single ridge was developed(Fig. 3B). Pro¢les 2a and 2b were acquired fromtransects 10 m apart and perpendicular to thecrest of the chenier ridge. These two pro¢leswere linked by Pro¢le 2c that ran along the crestof the chenier ridge. A fourth radar image (Pro¢le2d) was obtained from a transect across a se-quence of multiple recurved chenier ridges neartheir northern alongshore limit (Fig. 2b).In the central part of the chenier (Pro¢les 2a, b,

c, Fig. 6a^c) radar surface B-1 de¢nes the base ofthe radar stratigraphy, but is only visible in Pro-¢le 2a (Fig. 6a). B-1 is sub-horizontal and there£ections of overlying radar facies B-a downlapon to it. The £at-lying nature and elevation of B-1indicate that it represents the marsh surface be-neath the chenier ridge.Radar facies B-a directly overlies the marsh

surface on the landward side of the chenier andconsists of landward dipping, possibly sigmoidal-shaped, re£ections (Fig. 6a). These re£ections typ-ically display apparent dips of 10‡. Overlying B-ais radar surface B-2, which dips gently seaward(true dip of 3‡ at the intersection between Pro¢les2b and 2c). B-2 lies only on the seaward side ofB-a in Pro¢le 2a and erosionally truncates its re-£ections. By contrast, B-2 overlies much of B-a inPro¢le 2b and appears to show a more concor-dant relationship with it (Fig. 6b). B-2 is overlainby radar facies B-b. In Pro¢le 2b, B-b is welldeveloped and shows laterally continuous, gentlylandward dipping re£ections that display either aconcordant or low angle downlapping relation-ship to B-2. In Pro¢le 2a, B-b is much less welldeveloped, occurring only as a thin unit resting onthe seaward side of B-a. However, despite its lim-

ited extent, B-b can be seen to consist of bothgently landward and seaward dipping re£ectionsthat lie concordantly beneath the convex-up radarsurface B-3. B-3 dips gently seaward (true dip 4‡)in Pro¢le 2b and erosionally truncates the under-lying re£ections of B-b.In Pro¢le 2a, radar facies B-c consists of re£ec-

tions that dip to both landward and seaward,mimicking the convex-up form of radar surfacesB-3 and B-4 above and below (Fig. 6a). By con-trast, in Pro¢le 2b, B-c consists predominantly ofirregular seaward dipping re£ections, except at itsextreme landward margin where they are sub-hor-izontal (Fig. 6b). B-c is overlain by radar surfaceB-4, which is gently landward dipping in Pro¢le2b (true dip 4‡), but is convex-up in Pro¢le 2a.B-4 shows signi¢cant variation in elevation paral-lel to the present-day chenier ridge crest (Pro¢le2c, Fig. 6c). B-4 is overlain by radar facies B-d.Due to its very thin nature in Pro¢le 2a, B-d islargely obscured by the air and ground waves. InPro¢le 2b, however, B-d is thicker and can be seento consist of gently landward and seaward dip-ping re£ections whose form mirrors that of theoverlying chenier ridge surface.The re£ection con¢gurations of radar facies B-a

and radar facies B-b in Pro¢le 2b are interpretedas bedding associated with washover deposits,very similar in character to that described forType 1 cheniers. Radar surface B-2 is interpretedas a bounding surface that could be either slightlyerosional or non-depositional, depending on localcircumstances. Radar facies B-b in Pro¢le 2a isregarded as representing the bedding associatedwith minor overtop deposits. Radar surface B-3represents another bounding surface, which is in-terpreted as non-depositional in Pro¢le 2a, buterosional in Pro¢le 2b. Radar facies B-c in Pro¢le2b and radar facies B-d are both believed to rep-resent bedding associated with major overtop de-posits and consequent vertical aggradation of theseaward chenier ridge crest. The intervening radarsurface B-4 represents a bounding surface result-ing from either a period of non-deposition duringaggradation (Pro¢le 2a) or erosion (Pro¢le 2b).Radar facies B-c in Pro¢le 2b has clearly formedduring the same depositional episode as B-c inPro¢le 2a. However, overtopping of the chenier

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appears to have been prevented by the higher el-evation of pre-existing deposits in the area aroundPro¢le 2b (the maximum elevation associated withthe underlying radar surface B-3 in Pro¢le 2a isapproximately 3.1 m O.D., whereas in Pro¢le 2bit reaches an elevation of at least 3.5 m O.D.).This led to sedimentation only on the seawardside of the chenier in the area around Pro¢le 2band the formation of what appear to be a series ofupper beachface berm ridge deposits. Similar in-ternal structures have been described from bermridges on contemporary coarse clastic beaches(Maejima, 1982; Bluck, 1999).Pro¢le 2d, across the recurved ridges at the lat-

eral margins of the chenier, reveals a complex setof nine radar packages de¢ned by radar surfacesC1^C9 and characterised by radar facies C-a toC-i (Fig. 6d). The base of the radar stratigraphy isde¢ned by radar surface C-1, which dips gentlyseaward. The elevation and the lack of primaryre£ections beneath C-1, indicate that this is themarsh surface on which the sediments of the over-lying chenier are deposited.Radar facies C-a, which is found associated

with the most landward of the chenier’s recurves,consists of sigmoidal-shaped, landward dippingre£ections that downlap onto C-1 with apparentdips of up to 18‡. These are interpreted as beingthe result of cross-bedding associated with a seriesof washover deposits, similar in character to thoseidenti¢ed in Pro¢les 2a and 2b and in the Type 1cheniers.C-a is erosionally truncated on its seaward side

by radar surface C-2, which dips seaward (appar-ent dip of up to 9‡) and downlaps onto C-1 (Fig.

6d). The projected surface outcrop of C-2 corre-sponds to a major break in slope and the occur-rence to seaward of a recurved ridge with a di¡er-ent orientation to that associated with C-a. Theradar stratigraphy seaward of C-2, up to radarsurface C-6, consists of a set of seaward dipping,gently o¥apping radar surfaces that downlap onto C-1 and display largely concordant interveningradar facies (e.g. C-b, Fig. 6d). Sub-horizontal toslightly landward dipping sets of re£ections areoccasionally observed (e.g. C-c, Fig. 6d), althoughtheir nature is poorly de¢ned. At 16 m along Pro-¢le 2d another break in slope corresponds to thedevelopment of a another major recurved ridge,again with a signi¢cant change in the orientationof the crest relative to those lying to landward.The deposits of this ridge are characterised by theradar stratigraphy seawards of C-6, which againconsists of seaward dipping, o¥apping radar sur-faces. However, the main body of this ridge ischaracterised by two radar facies (C-f and C-g)that show both gently landward and seaward dip-ping re£ections. These radar facies are truncatedseawards by the more steeply dipping radar sur-face C-8 (Fig. 6d). C-h, the radar facies seawardof C-8, is then characterised by a series of sea-ward dipping re£ections.The radar stratigraphy above radar surface C-2

is interpreted as a sequence of seaward prograd-ing chenier ridge deposits, with the radar surfacesrepresenting signi¢cant hiatuses in progradationeither through non-deposition or erosion. Radarfacies characterised by concordant, seaward dip-ping re£ections are regarded as representing bedsresulting from accretion across the entire beach-

Fig. 6. (a) Radar re£ection Pro¢le 2a with an interpretation of its radar stratigraphy directly beneath. The pro¢le runs from land-ward (W) to seaward (E). The intersection point with the perpendicular Pro¢le 2c is indicated. (b) Radar re£ection Pro¢le 2bwith an interpretation of its radar stratigraphy directly beneath. The intersection point with the perpendicular Pro¢le 2c is indi-cated. (c) Radar re£ection Pro¢le 2c with an interpretation of its radar stratigraphy directly beneath. The locations of the inter-sections with Pro¢les 2a and 2b are indicated. The interpretations of radar Pro¢les 2a^c show the position of four radar surfaces(B-1 to B-4) and four radar facies (B-a, B-b, B-c and B-d). The various trenches (T) all revealed that this part of the chenier wascomposed predominantly of whole shells/valves. (d) Radar re£ection Pro¢le 2d with interpretations of its radar stratigraphy di-rectly beneath. Pro¢le 2d was obtained across the recurves of the Type 2 chenier shown in Fig. 2b. The pro¢les ran approxi-mately from landward (SW) to seaward (NE), although the orientation of the transect had to be changed at two points (OC) inorder to remain perpendicular to the crests of the recurves. The trenches (T) at 2, 17 and 20 m revealed that the majority of thechenier was composed of whole shells and valves. However, the trench at 24.5 m revealed seaward dipping beds of alternatingwhole shell and broken shell/sand (Fig. 4A). The interpretations indicate the positions of nine radar surfaces (C-1 to C-9) andnine radar facies (C-a to C-i).

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A. Neal et al. /Marine Geology 185 (2002) 435^469 451^452

Fig. 6 (Continued).

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A. Neal et al. /Marine Geology 185 (2002) 435^469453^454

Fig. 6 (Continued).

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A. Neal et al. /Marine Geology 185 (2002) 435^469 455^456

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A. Neal et al. /Marine Geology 185 (2002) 435^469457^458

face. A trench at 25 m along Pro¢le 2d con¢rmedthat the present-day beachface consisted of sea-ward dipping beds formed of alternating layersof whole and broken shells (Fig. 4A). The radarfacies characterised by gently landward and sea-ward dipping re£ections that onlap and downlaponto their underlying radar surfaces (C-f and C-g)are interpreted as deposits formed by berm ridgewelding onto the beachface (Maejima, 1982;Bluck, 1999).

4.3. Type 3 chenier

A chenier currently prograding seawards anddominated by a series of large, laterally continu-ous chenier ridges was surveyed on the north sideof Foulness Island (Figs. 1, 2c and 3D). The che-nier reached up to 4.4 m O.D., approximately2.4 m above the surface of the marsh, and hada maximum width of around 45 m. The chenierwas continuous alongshore for 800 m and wholevalves of C. edule dominated its sediment compo-sition.Pro¢le 3a was collected along a transect that

crossed two recurved ridges on the landwardside of the chenier and three continuous, shore-parallel ridges to seaward (Fig. 2c and 3D). Thebase of the radar stratigraphy is marked by thedistinct, gently seaward dipping radar surface D-1(Fig. 7a). Ground-truthing and the elevation ofthe re£ection con¢rm that D-1 is the marsh sur-face. The two recurved ridges display radar faciesthat are dominated by discontinuous, sub-hori-zontal to slightly landward dipping re£ection con-¢gurations (D-a and D-b, Fig. 7a). At the land-ward margin of D-a, steeper landward dippingre£ections are observed (1^3 m, Pro¢le 3a,Fig. 7a). The two radar facies are separated bya distinct, seaward dipping radar surface, D-2.The outcrop of D-2 corresponds with a break in

slope that marks the transition from one recurvedridge to another. Although the re£ections are notas clearly de¢ned as in previous examples, per-haps due to the complete dominance of wholeshells/valves in the cheniers, their con¢gurationagain suggests that they represent bedding result-ing from overwash sedimentation. The two phasesof overwash were separated by a distinct deposi-tional or erosional hiatus, leading to the forma-tion of the bounding surface represented by D-2.The three shore-parallel ridges seaward of the

recurves are associated with a very di¡erent radarstratigraphy (ridge crests at 21.5, 27.5 and 33.5 malong Pro¢le 3a, Fig. 7a). Due to the complexnature of the re£ection con¢gurations and the dis-tinct ridged topography (which rendered migra-tion only partially successful) clear radar surfacesand radar facies are di⁄cult to de¢ne in this partof Pro¢le 3a, even when using both migrated andunmigrated re£ection pro¢les. Pro¢le 3b (Fig. 7b),a perpendicular pro¢le along the ridge centred at27.5 m and suitable for migration due to its lim-ited topographic variation, helped clarify the ra-dar stratigraphy for the ridge, but only generalcomments about the other two ridges can bemade.Using Pro¢les 3a and 3b in conjunction, it is

possible to de¢ne ¢ve radar surfaces associatedwith the shore-parallel ridge at 27.5 m along Pro-¢le 3a. In Pro¢le 3a, radar surfaces D-4, D-5, D-6and D-7 all display convex-up con¢gurations,with their apex directly beneath the present-dayridge crest (Fig. 7a). Pro¢le 3b indicates thatD-4, D-5 and D-6 are laterally continuous overdistances of several tens of metres (Fig. 7b). Bycontrast, radar surface D-8 dips only seaward anderosionally truncates D-6, D-7 and radar faciesD-d, D-e and D-f (Fig. 7a). In Pro¢le 3a, radarfacies D-c, D-d, D-e and D-f display re£ectionsthat are laterally discontinuous, but also largely

Fig. 7. (a) Radar re£ection Pro¢le 3a with an interpretation of its radar stratigraphy directly beneath. Pro¢le 3a was obtainedfrom the Type 3 chenier shown in Figs. 2c and 3D. The pro¢le runs from landward (S) to seaward (N). (b) Radar re£ection Pro-¢le 3b with an interpretation of its radar stratigraphy directly beneath. Pro¢le 3b was obtained perpendicular to Pro¢le 3a alongthe crest of the chenier ridge at 27.5 m along Pro¢le 3a. The interpretations of the radar pro¢les indicate the positions of eightradar surfaces (D-1 to D-8) and seven radar facies (D-a to D-g). Surface exposures and trenches (T) revealed that the chenierwas composed predominantly of whole shells and valves.

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concordant with, or gently downlapping onto, theintervening radar surfaces. Consequently, theoverall re£ection con¢gurations are convex-upand mirror the form of intervening radar surfacesand the ridge itself. In Pro¢le 3b, parallel to theridge crest, the re£ections are again laterally dis-continuous, D-c, D-d and D-e showing a sub-hor-izontal, hummocky con¢guration. An irregulartrough-like feature is developed in the top ofD-e, 7^15 m along Pro¢le 3b (Fig. 7b). The baseof this feature is de¢ned by radar surface D-7 andoverlying radar facies D-f displays a well-devel-oped onlap ¢ll.The radar packages described for the shore-par-

allel ridge at 27.5 m along Pro¢le 3a are inter-preted as a vertically stacked sequence of overtopdeposits. Radar surfaces D-4, D-5 and D-6 repre-sent bounding surfaces formed due to deposition-al breaks during vertical aggradation. The re£ec-tions of radar facies D-c, D-d and D-e representbedding in overtop deposits, with each radarpackage representing the deposits of a distinctset of overtopping events. The irregular trough-like feature de¢ned by D-f is interpreted as repre-senting the development of a small washoverchannel in the chenier ridge crest. This channelwas subsequently plugged by sediments associatedwith radar facies D-f. Radar surface D-8 is inter-preted as a bounding surface formed by signi¢-cant erosion prior to the development of the che-nier ridge immediately seaward. Although theradar stratigraphy is less easily de¢ned for theother two shore parallel chenier ridges (Fig. 7a),their overall re£ection con¢gurations are broadlycomparable to those of the ridge centred at 27.5m, suggesting they were formed by similar depo-sitional processes.

4.4. Type 4 chenier

Examples of Type 4 cheniers were found exclu-sively at Sale’s Point, on the Dengie Peninsula(Figs. 1 and 2d). The chenier studied reached amaximum elevation of 3.6 m O.D., approximately1.5 m above the marsh surface (Fig. 3E). Themaximum width of the chenier was 22 m. Themorphology of the chenier was relatively simple(Fig. 3E), with a low, single chenier ridge on its

seaward side. The composition of the Type 4 che-nier di¡ered from the other types, as it consistedof whole and broken valves of C. edule, plus othermollusc species including B. undatum, C. fornica-ta,M. edulis, Gibbula spp. and Littorina spp., bro-ken shell debris and small amounts of sand. Aseries of radar pro¢les were obtained from thechenier, two of which are presented here fromtransects both perpendicular (Pro¢le 4a) and par-allel (Pro¢les 4b) to the chenier ridge crest (Fig. 8).The base of the radar stratigraphy is character-

ised by radar surface E-1, beneath which the ra-dar images are free of primary re£ections. E-1 isdistinctly concave-up in Pro¢le 4a. Trenches at 1.5and 6 m along Pro¢le 4a (Fig. 8a) con¢rm thatE-1 represents the saltmarsh surface beneath thechenier. Greensmith and Tucker (1969) have pre-viously noted signi¢cant depressions in the marshsurface beneath the cheniers at Sale’s Point andascribed them to a minor sediment loading e¡ect.Overlying radar surface E-1 is radar facies E-a

(Fig. 8). Perpendicular to the chenier ridge, E-a isdominated by laterally continuous, slightly con-cave-up, gently landward dipping re£ections(true dip ca. 3^4‡ at an intersection between apoint 7 m along Pro¢le 4a and another perpen-dicular pro¢le that has not been presented here).Lying above E-a is radar surface E-2. In much ofthe across-shore Pro¢le 4a (Fig. 8a) the re£ectionsof E-a are largely concordant with E-2. However,at the landward margin of the chenier the dip ofE-2 steepens to downlap onto E-1, erosionallytruncating the underlying re£ections of E-a.Where this occurs, the alongshore Pro¢le 4b(Fig. 8b) indicates that E-2 undulates, describinga series of broad troughs. These are divergently¢lled by the overlying radar facies E-b. In Pro¢le4a the re£ections of E-b are either concordant orvery gently onlapping with respect to E-2. Thegently landward dipping re£ections that charac-terise much of E-b steepen toward the landwardmargin of the chenier to reach true dips of ca. 20‡.Ground-truthing trenches at 6 and 10 m along

Pro¢le 4a indicated that the chenier deposits arecharacterised by centimetre to decimetre thick al-ternations of broken shell/sand and whole shells/valves (Figs. 4B,C and 8a). The form and orien-tation of this bedding shows excellent correspon-

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Fig. 8. (a) Radar re£ection Pro¢le 4a with an interpretation of its radar stratigraphy directly beneath. Pro¢le 4a was obtained from the central portion of the Type4 chenier shown in Figs. 2d and 3E. The pro¢le runs from landward (WNW) to seaward (ESE). The intersection point with the perpendicular Pro¢le 4b is indi-cated. The trench (T) at 1.5 m along the pro¢le revealed that the landward edge of the chenier was composed of whole shells and valves. At all other locations, ex-posures in the trenches displayed bedding formed of centimetre to decimetre thick alternations of whole shells/valves and broken shell/sand (see Fig. 4b,c), theform of which matched the re£ection con¢gurations on the radar pro¢le. (b) Radar re£ection Pro¢le 4b with an interpretation of its radar stratigraphy directly be-neath. Pro¢le 4b was obtained along a transect perpendicular to Pro¢le 4a. The interpretations of the radar pro¢les indicate the position of three radar surfaces(E-1, E-2 and E-3) and three radar facies (E-a, E-b and E-c).

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A.Neal

etal./M

arineGeology

185(2002)

435^469461

Fig. 8 (Continued).

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dence with the re£ection con¢gurations in radarfacies E-a and E-b. In addition, the re£ection con-¢gurations observed in Pro¢le 4a (Fig. 8a) matchthe internal structure of cheniers previously de-scribed from Sale’s Point and interpreted as wash-over deposits (Greensmith and Tucker, 1969). Thedivergent trough ¢lls associated with radar faciesE-b in Pro¢le 4b (Fig. 8b) are interpreted as re-sulting from the in¢lling of depressions betweenindividual washover lobes.Radar surface E-3 is seaward dipping and ero-

sionally truncates E-a, and possibly E-b, on itsseaward margin. It is interpreted as a either anerosional or non-depositional bounding surface.Overlying E-3 is radar facies E-c, which lies di-rectly beneath the chenier ridge crest and consistsof convex-up re£ections that dip gently to land-ward and seaward, broadly following the form ofthe overlying ground surface. These are inter-preted as minor overtop deposits that have ledto vertical aggradation on the seaward side ofthe chenier and development of a small ridge.

5. Discussion

This GPR investigation indicates that the shellycheniers of the southern Essex coast are formedfrom a limited number of morphostratigraphicelements resulting from either overwashing, over-topping, sedimentation across the entire seawarddipping beachface or berm ridge welding onto theupper beachface. These depositional elementsvary signi¢cantly in their importance between,and sometimes within, each of the chenier typesidenti¢ed.All four of the chenier types contain washover

deposits that occupy the most landward portionof the chenier, form the stratigraphically oldestmaterial and typically lie immediately above andbelow the level of MHWS. The washover depositstypically display low-angle, planar landward dip-ping strata that pass into more steeply dippingforeset strata at their landward margin. Such fea-tures are the most commonly described sedimen-tary structures from cheniers (Hoyt, 1969; Shinn,1973; Augustinus, 1980; Rhodes, 1982; Cangziand Walker, 1989; Penland and Suter, 1989; Xi-

tao, 1989; Lee et al., 1994; Zenero et al., 1995;Park et al., 1996; Vilas et al., 1999). Carter andOrford (1981) have described poorly developedtabular foresets at the distal edge of mixed sandand gravel overwash deposits and Schwartz (1982)has described very similar planar strati¢cationand foreset strata from small sandy washover de-posits at two sites in North America. Schwartz(1982) ascribed the planar strati¢cation to depo-sition on the upper wetted subaerial surface of thewashover fan and the foreset strata at the land-ward margin were interpreted as being the resultof deposition of overwash material into a stand-ing body of water, such as a tidal lagoon. Thepresence of well-developed foreset strata in thesouthern Essex cheniers supports the notion thatoverwash typically occurs during extreme tidalconditions e.g. high spring tides or storm surges,when the marsh areas behind the cheniers containa signi¢cant volume of tidal water.In some instances, a number of distinct phases

of washover can be identi¢ed in the cheniers.Where this occurs in the Type 2 chenier studied,the stratigraphically younger washover unit is de-posited against and seaward of the stratigraphi-cally older washover unit (B-b against B-a, Fig.6b). In addition, the younger deposits (B-b) aredominated by planar strati¢cation and foresetstrata are absent. Schwartz (1982) described thesame internal structure from sandy washover de-posits where the overwash failed to enter a signi¢-cant body of standing water. In the case of theEssex cheniers, this appears to have occurredwhen the stratigraphically older washover depos-its were not completely reworked by the next ma-jor overwash event, thus providing a barrier tothe landward movement of material during thatevent.In all four chenier types identi¢ed, stratigraph-

ically older washover deposits are overlain byyounger overtop deposits that occur further sea-ward and occupy higher elevations. In chenierTypes 1 and 4, these form a relatively small over-top ridge (up to 0.6 m in vertical extent), but inchenier Types 2 and 3, the ridges are much moresubstantial. Type 2 cheniers display a large, singleovertop ridge in their central portion, which canhave a vertical extent of at least 1.5 m and show

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multiple phases of vertical aggradation with sig-ni¢cant spatial variability. Type 3 cheniers consistof seaward progradational overtop ridges withvertical extents of up to at least 2.3 m and againshowing evidence for multiple phases of verticalaggradation and some spatial variability in theovertop process.Orford and Carter (1982) describe storm gener-

ated overtop and overwash sedimentation from asandy gravel barrier in southeast Ireland, notingthat overwashing and overtopping can interdigi-tate alongshore in either a random or rhythmicalmanner due to lateral variations in beach crestalheight, the height of breaking waves or wave run-up. Pro¢le 2c (Fig. 6c) indicates that there havebeen signi¢cant variations in the longshore heightof the crest during vertical aggradation of chenierType 2 and this appears to have helped controlthe extent of subsequent overtopping and over-washing.Beachface deposits occur both as thin veneer on

the seaward side of the cheniers (e.g. chenier Type1, Fig. 5a) or as more substantial deposits in pro-gradational settings (e.g. recurved portions of che-nier Type 2, Fig. 6d). Sedimentation takes theform of either: (1) accretion across the wholebeachface, resulting in sub-parallel, seaward dip-ping strati¢cation (e.g. C-h, Fig. 6d); or (2) thewelding of berm ridges onto the upper beachface,giving rise to discordant, gently seaward to gentlylandward dipping strata that onlap or downlaponto underlying bounding surfaces that displaya steeper seaward dip (e.g. C-f and C-g, Fig. 6d).Thin deposits of seaward dipping strati¢cation onthe seaward side of cheniers have been describedfrom a number of locations and are typically as-cribed an upper beachface origin (Shinn, 1973;Augustinus, 1980; Rhodes, 1982). More substan-tial progradational beachface deposits are lesswell described in the chenier literature, althoughdescriptions given by Hoyt (1969), Zenero et al.(1995) and Pontee et al. (1998) may be indicativeof their presence. In contrast to the internal struc-

ture displayed by the recurved portions Type 2cheniers in this study, Shinn (1973) describeswell-developed festoon bedding from the recurvesof carbonate chenier ‘spits’ in Qatar.Deposits resulting from berm ridge welding

onto the upper foreshore have been describedfrom a number of coarse clastic beach sequences(Maejima, 1982; Massari and Parea, 1988; Bluck,1999). These studies indicate that berm ridge de-posits may form under both fair-weather and im-mediate post-storm wave conditions. During thestorms themselves, erosion of the upper beachfaceand subsequent erosional bounding surface for-mation is generally favoured (Massari and Parea,1988; Bluck, 1999). The elevation of the bermdeposits in the Essex cheniers suggests that theyare most likely to be preserved when they formunder either: (1) fair-weather conditions associ-ated with high spring tides; or (2) immediatepost-storm conditions. In both cases, tidal heightsand/or wave heights would be insu⁄cient to gen-erate signi¢cant overtopping or overwashing.The spatial arrangement and relative impor-

tance of the various morphostratigraphic elementswithin the four chenier types identi¢ed supportthe concept that each type represents a distinctstage in an evolutionary sequence of chenier de-velopment. Combining the data from the GPRsurveys with results from the sequential aerialphotograph analysis and ¢eld surveys originallyused to identify each of the four chenier types,there appear to be two distinct evolutionary path-ways that the cheniers may follow, dependingupon local sediment budgets (Fig. 9). Cheniersof Type 1 typically develop at the longshore limitof chenier formation, often from low-lying shellsheets. After their initial formation Type 1 che-niers are likely to have a relatively low sedimentvolume and elevation. These attributes make themextremely susceptible to washover and landwardmigration during subsequent storm or extremehigh tide events. However, under a high positivesediment budget (Fig. 9) the cheniers will grow in

Fig. 9. Proposed evolutionary models that identify Type 1, 2, 3 and 4 cheniers as various stages in a natural sequence of chenierdevelopment under two broad sediment budget scenarios: (a) a high positive sediment budget; and (b) a low positive sedimentbudget. T1 to T4 are sequential, but arbitrary, times during chenier development.

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volume and elevation, making them less suscepti-ble to complete reworking by overwash. Eventu-ally a point is reached at which overtopping be-comes signi¢cantly more frequent thanoverwashing and a substantial overtop ridge isconstructed on the seaward side of the chenier.Orford and Carter (1982) have demonstratedthat vertical aggradation through overtoppingfurther increases resistance to overtopping andoverwashing during subsequent storm events.Consequently, with increasing chenier ridge eleva-tion, sedimentation that occurs entirely on thebeachface becomes increasingly common. Undersuch conditions, local interactions between themorphology of the chenier and incoming wavespectra favours dispersion of sediment alongshoreto its distal ends (see Augustinus, 1980). This re-sults in the development of a series of prograda-tional recurves characterised by beachface depos-its (Type 2 chenier, Fig. 9). Alongshore growth ofthe chenier continues until it is prevented eitherby amalgamation with other cheniers growing lat-erally or by a major physical barrier, such as amajor tidal creek or seawall. With continued highsediment supply, large-scale seaward progradationis then favoured and results in the development ofa series of chenier ridges largely formed by multi-ple overtopping events and separated by distinctphases of beachface erosion (Type 3 chenier,Fig. 9).Cheniers of Type 4 do not ¢t into the above

sequence, largely because they do not appear tohave experienced large, long-term, positive sedi-ment budgets. The presence of a signi¢cantamount of comminuted shell material re£ects thegreater age of the Type 4 cheniers at Sale’s Pointand also suggests that there has been considerablereworking of sediment, with only a limited inputof fresh shell material. The most likely scenariofor their development (unfortunately aerial photo-graphs do not extend far enough back in time torecord their inception) is that they form from theprogressive lateral extension and amalgamation oflandward migrating Type 1 cheniers and shellsheets, under a relatively low positive sedimentbudget. Under these conditions, new sedimentarymaterial is not supplied to the cheniers at a su⁄-cient rate to increase their volume and elevation

to a point where overtopping becomes signi¢-cantly more likely than overwashing. Conse-quently, overwashing remains the dominant pro-cess, with overtop and beachface depositsremaining poorly developed, forming only a thinveneer on the seaward side of the chenier. Com-parison of chenier morphologies mapped byGreensmith and Tucker (1975) for the period1952^1973 and an aerial photograph taken in1997 (Fig. 2d) indicates that in some areas theType 4 cheniers have broken down into Type 1cheniers and shell sheets. This local dissipation ofsome of the Type 4 cheniers suggests that sedi-ment supply is insu⁄cient in some areas to main-tain them (i.e. they have locally negative sedimentbudgets) and that they are reverting back to ear-lier morphological forms.

6. Conclusions

This study has demonstrated that high-fre-quency GPR can be used to provide detailed in-formation regarding the stratigraphy and internalstructure of thin, unconsolidated chenier depositscomposed predominantly of whole and commi-nuted shell. In this study, a sub-decimetre verticalresolution was achieved and ground-truthing con-¢rmed that the radar re£ection pro¢les show ex-cellent correspondence to the subsurface sedimen-tary structure.Four main chenier types (Types 1^4) can be

identi¢ed on the southern Essex coast based ontheir morphology. The GPR surveys reveal thatthe cheniers are composed of overwash, overtopand beachface deposits, which vary in their rela-tive importance between, and sometimes within,each of the chenier types. The deposits of thecheniers typically form under storm-related and/or high spring tide conditions, with antecedenttopography often exerting a strong control onthe style of sedimentation.Chenier Types 1^3 appear to represent various

evolutionary stages in a sequence of chenier devel-opment that occurs under a high positive sedi-ment budget. Within this sequence overwash isthe dominant process during the early stages ofchenier development, but is eventually replaced by

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overtop sedimentation and then lateral and sea-wards progradation across a well de¢ned beach-face. Type 4 cheniers do not ¢t into this evolu-tionary sequence, having developed directly fromType 1 cheniers and shell sheets under relativelylow long-term positive sediment budgets that fa-vour continued washover.Cheniers dominated by anything other than

overwash deposits are poorly described in the cur-rent literature. This may result from the fact thatcheniers rarely have su⁄ciently high positive sedi-ment budgets to inhibit washover under the waveregime that they typically experience. However, itmay equally re£ect the limited number of studiesthat have so far been completed due to the di⁄-culties in obtaining detailed subsurface informa-tion, such that the internal structure of cheniersglobally is currently poorly characterised. Conse-quently, high-resolution GPR surveying is poten-tially an important means of obtaining such in-formation and of thus gaining further insight intothe structure and development of both cheniersand other landforms constructed of coarse clasticsediments.

Acknowledgements

This research was supported by NERC Geo-physical Equipment Pool Loans 638 and 639.Martin Fenn, Simon Blott, Samantha Saye andHelen Jay provided ¢eld assistance. Kay Lancas-ter helped prepare some of the diagrams. J.R.’sPhD research is funded by an EPSRC researchstudentship. Additional funds were provided aspart of a Leverhulme Trust grant and by the Uni-versity of Wolverhampton. We thank DavidWelsh of the Environment Agency for benchmarkand aerial photograph data; COMAX for accessto Foulness Island and Robert Smith for access tosites on Dengie Peninsula. Peter Fenning andAndy Brown of Earth Science Systems, Kimptonand the sta¡ of the NERC Geophysical Equip-ment Pool, Edinburgh are thanked for technicaland logistical support. Harry M. Jol and PieterG.E.F. Augustinus provided excellent reviewsthat greatly enhanced the manuscript.

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Documents

Structure and development of shell cheniers in Essex, southeast England, investigated using high-frequency ground-penetrating radar