Marine Geology 355 (2014) 291–296

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Letter

Coastal lagoons: Geologic evolution in two phases

Kellie Adlam ⁎

Geocoastal Research Group, The University of Sydney, Australia

⁎ The School of Geosciences, Madsen Building F09, Th2006, Australia. Tel.: +61 433159444.

E-mail address: kellie.adlam@sydney.edu.au.

http://dx.doi.org/10.1016/j.margeo.2014.06.0050025-3227/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 February 2014Received in revised form 13 June 2014Accepted 13 June 2014Available online 20 June 2014

Keywords:coastal lagooncentral mud basinestuary evolutionestuary maturityestuary shorelinecoastal wetland

Estuaries formed globally when river valleys and bays were inundated by the rising sea towards the end of theHolocene post-glacial marine transgression and have been supplied with sediment ever since. Coastal lagoons,estuaries that are partially or wholly cut off from marine influence, are said to evolve through sedimentationfrom an immature, unfilled state to a mature, filled state at a rate dependent on sediment supply. The existenceof numerous examples of lagoons that remain unfilled worldwide, however, despite thousands of years ofsedimentation, suggests that a discontinuity in the geologic evolution of lagoons occurs before maturity isreached. Presented here is a hypothesis that geologic evolution takes place in two phases which are defined bychanges in physical processes. The early phase aligns with the traditional view but ends when a depth thresholdis reached. In the late phase, sedimentation is inhibited by the local energy regime and can only proceed if thelagoon surface area is reduced. Validation of the existence of these two distinct process-based phases of geologicevolution will improve the reliability of predictive models for lagoon shoreline changes and rates of basin fill aswell as reduce the risk of misinterpreting past conditions from the geological record.

© 2014 Elsevier B.V. All rights reserved.

1. Background

The low-relief shorelines of estuaries tend to support densely popu-lated coastal communities as well as important wetland habitats. Thelatter are often the focus of conservation efforts (Jennerjahn andMitchell, 2013), act as flood buffers during storm surges (Townendand Pethick, 2002), have the potential to store large quantities of green-house gases (Chmura et al., 2003; Saintilan et al., 2013), are underpressure due to human interventions (Elliott and Whitfield, 2011) andface future impacts from sea level rise and other climate change effects(Saintilan and Williams, 1999; McCarthy et al., 2001; Syvistki et al.,2005). Complexity and variability reign in estuarine environments(Roy et al., 2001) however, and this hampers efforts to understanddrivers of change in estuaries and on their shorelines. Coastal lagoons,a comparatively less complex and more narrowly defined category ofcoastal water body, are therefore the focus of the evolutionary modeldescribed herein.

Unlike many types of estuaries, coastal lagoons have only restrictedor intermittent connections to the ocean and, in order to maintain thatrestricted connection, also tend to have low freshwater input relativeto their size (Harris, 2008). Their energy regimes are dominated byinternally generated wind waves (Ward and Ashley, 1989; Bird, 1994;Harris, 2008). The low energy regime and reduced influence from

e University of Sydney, Sydney

ocean waves, tides and fluvial discharge provide an opportunityto study a reduced number of processes in an otherwise complex envi-ronment, so coastal lagoons are appealing natural laboratories forinvestigating processes operating in coastal water bodies. The formaldefinition of coastal lagoons adopted here is “inlandwater bodies… sep-arated from the ocean by a barrier, connected to the ocean by one ormore restricted inlets which remain open at least intermittently, andhave water depths which seldom exceed a few metres. A lagoon mayor may not be subject to tidal mixing, and salinity can vary from thatof a coastal fresh-water lake to a hypersaline lagoon, depending onthe hydrologic balance” (Kjerfve, 1994). Schematisation of an idealisedcoastal lagoon is provided in Fig. 1.

Coastal lagoons on the southeastern Australian coast occur on gentlysloping substrates where, following the stabilisation of sea level at least6000 years ago, transgressive coastal sand barriers were able to form atthemouths of flooded river valleys and coastal inlets, partially orwhollyisolating low energy, lagoonal environments (Roy, 1984; Bird, 1994). Inother parts of the world lagoons formed in similar circumstances,though sources of sediment for barrier formation vary (Bird, 1994). Aswith estuaries more broadly, coastal lagoons are considered temporaryfeatures over geologic timescales because they are naturally filled overtime through sedimentation (Ward and Ashley, 1989; Bird, 1994;Nichols and Boon, 1994). Lagoons intercept sediment conveyed by riv-ers from landwards catchments to the ocean until such time as accom-modation space within lagoon basins is exhausted. The lagoon systemthen acts as a sediment pathway rather than a sink, as sediment is deliv-ered directly to the coast, and the system is referred to as a delta (Roy,1984; Roy et al., 2001; Heap et al., 2004).

Alluvial plains

Mud basin & COE

Subaqueous river delta

Coastal barrier

Subaqueous tidal delta

Vegetated intertidal flatsX

Cut off embayment(COE)

Y

Fig. 1.An idealised coastal lagoon displayingmajor geomorphic units. The centralmudbasin and fringing intertidalflats are the focus of themodel of geologic evolution presentedhere. Cutoff embayments exhibit analogous behaviour on smaller scales. Other geomorphic units that make up coastal lagoons have the potential to act as system constraints whose position andextent influencewhere the threshold depth lies through their control onwind fetch, but are otherwise excluded from the present scope. Vertical line X–Y represents the cross section usedin Fig. 3. Figure adapted from source (Roy et al., 2001).

292 K. Adlam / Marine Geology 355 (2014) 291–296

Geologic evolution of coastal lagoons from unfilled to filled (delta)states has been described as a “seamless progression” that proceeds ac-cording to the rate of sediment supply (Roy et al., 2001). The persistenceof a large number of coastal lagoons along several coastlines despite upto 6000 years of sediment supply suggests that sediment supply is notthe only control on their geologic evolution. Examples are numerouson the southeastern Australian (Roy et al., 2001) and Texas coasts(Price, 1947). This paper presents the hypothesis that a threshold canbe reached in the natural geologic evolution of coastal lagoons thatchanges the trajectory of ongoing evolution. It is argued here that:(i) geologic evolution of coastal lagoons through sedimentation is divid-ed into two distinct phases, each affected by different factors; (ii) thethreshold between the two phases is directly related to the local windregime; and (iii) recognition of lagoon maturity in relation to thisthreshold will improve interpretation of past infill rates from the geo-logical record and prediction of future geologic evolution. The rationaleleading to the development of these arguments is presented and furtherresearch to validate the arguments is proposed.

2. Accommodation space within coastal lagoons

The geologic evolution of coastal lagoons is typically expressed interms of the rate of basin fill through sedimentation. It is thus helpfulto consider lagoon fill in terms of maturity (Roy et al., 2001). Immaturelagoons are newly inundated depositional basins in which the entirevolume of the water body is available to accommodate sediment;mature lagoons (or deltas) are entirely filled with sediment, accommo-dation space has been exhausted, and river discharge flows directly tothe coast. Most processes operating within lagoons affect maturitythrough the creation and consumption of accommodation space. Rela-tive sea level rise for example, creates accommodation space for

deposition; whereas reduced sediment supply will reduce the rate atwhich accommodation space is consumed (Nichol et al., 1994).

The term accommodation space, as used here, refers to the totalsubaqueous volume of coastal lagoon basins available for sediment de-position. Effective accommodation space, on the other hand, is the pro-portion of total accommodation space that is available for sedimentdeposition given the specific combination of sediment properties andenergy regime within a coastal lagoon. The volume of effective accom-modation space is less than the volume of total accommodation space.This is because once basin sedimentation comes near to occupying thetotal accommodation space, hydrodynamic processes such as wavesand currents periodically exceed some critical sheer stress andmay pre-vent deposition or, where the bed consists of unconsolidated sediment,cause entrainment (Roy and Peat, 1976). Effective accommodation spacemay be exhausted by ongoing sediment deposition but, in the absenceof any mechanism to alter sediment properties or energy regime,some total accommodation space remains because its consumption isinhibited by the energy regime. The rate of consumption of accommo-dation space in coastal lagoons and estuaries has previously been de-scribed as dependent on rates of sediment supply (Roy et al., 1980;Boyd et al., 1992; Dalrymple et al., 1992). The identification of a numberof coastal lagoons not actively infilling (Price, 1947; Roy and Peat, 1976)however, suggests that the assumption of supply-dependent fill is onlyvalid up to a certain point in the evolutionary trajectory. This point ap-pears to be reachedwell before total accommodation space is exhausted(i.e. before the system matures into a completely filled delta) and is as-sumed here to coincide with the exhaustion of effective accommodationspace.

For 31 oval, enclosed tidal basins on the Texas coast, maximumbasindepth has been correlated to averagewidth (Price, 1947). This dataset ispresented in Fig. 2. The depth–width ratios appear to have been main-tained over time, despite up to 10 ft of eustatic sea level rise, suggesting

0

1

2

3

4

5

6

0 5 10 15 20 25 30 35 40 45 50

Mea

n w

ater

dep

th (N

SW

) and

max

imu

dept

h (T

exas

) (m

)

Maximum fetch (NSW estuaries) and average width (Texas estuaries)(km)

NSW estuaries (Roy & Peat 1976)Non-humid Texas coast (Price 1947)Humid Texas coast (Price 1947)

NSW estuariesNon-humid Texas coastHumid Texas coast

Fig. 2. Scatter plot of wind fetch vs. water depth for twelve southeastern Australian estuarine basins (Roy and Peat, 1976) and 31 Texas water bodies (Price, 1947). Measurement param-eters differed between the two studies, as indicated on the axes (i.e.mean vs.maximumwater depth). Scatter among this data set is likely to be reduced by using awave energy parameterthat accounts for variation in the wind regime rather than simply basin width. A line of best fit has been drawn through the data points for each region to indicate where the thresholddepth might lie as a function of basin size, but had the basin width been replaced with wave energy a line representing the upper bound for each region would have better defined thethreshold depth than a line of best fit.

293K. Adlam / Marine Geology 355 (2014) 291–296

a dynamic equilibrium between basin form and energy regime. Depar-tures from observed correlations were within the range of incidentalscour and fill detected from historical depth soundings, and mainte-nance of the depth-width equilibrium was attributed by the author towave scour processes and wind fetch. The less regularly shaped shore-line of Myall Lake on the NSW coast, Australia, was also investigatedfor an equilibrium form, but in terms of point-specific wind fetch andwave energy rather than just physical dimensions (Shepherd, 1970).In that study, smooth bottom profiles were assumed to indicate thatbasin morphology was almost entirely controlled by local wave regimedespite heterogeneous wave exposure throughout the basin. The rela-tionship between depths at 33 stations throughout the lake and their re-spective energy regimes was, in fact, very strong (r= 0.91). The coastallakes of southeastern Australia have also been found to exhibit a depth-width correlation, as shown in Fig. 2 (Roy and Peat, 1976).

These examples suggest that once coastal lagoons fill to a certaindepth, their morphology (excluding non-erodible substrates) is in equi-librium with their time-averaged energy regime. The continuedprogression from this point towards absolutematurity appears to be ei-ther stalled or slowed. Further demonstration of this apparent equilibri-um is provided by numerous examples of coastal lagoons whose rate ofbasin accretion has kept pace with sea level rise, including the New Jer-sey coast (Carson et al., 1988), the Wadden Sea (Eysink, 1991), andmany more examples listed in Fig. 7.15 of Nichols and Boon (1994). Ifnot for the ongoing creation of accommodation space by rising sealevel it seems that basin sedimentation would have ceased entirely.

Originally, depth–width data for southeastern Australian coastallakes (Roy and Peat, 1976) were collated to test the authors' own hy-pothesis regarding the geologic evolution of coastal lagoons. They at-tributed the apparent equilibrium to the fact that the lagoon basin,through continued sedimentation, had reached a level at which windwaves interacted with sediment and prevented further deposition.Where an ocean connection allowed suspended sediment to beexported from the lagoon, infill appeared to have stalled despite contin-ued sediment supply. The depth at which this occurred was termed thethreshold depth, representing a threshold between supply-limited basin

sediment rates and sedimentation limited by energy regime. In terms ofaccommodation space, this depth represents the exhaustion of effectiveaccommodation space.Wave–sediment interactions as related to coast-al lagoon evolution have been described anecdotally elsewhere (Nicholsand Biggs, 1985; Nichols and Boon, 1994; Van Goor et al., 2003) buthave not formed the basis of formal classification schemes of coastal la-goonmaturity (Roy et al., 1980; Roy et al., 2001) that followed the orig-inal hypothesis. Since the threshold depth represents a quantifiabledistinction between two phases of geologic evolution, it is argued herethat maturity models could be strengthened if it was the basis onwhich phases of maturity were distinguished.

3. A two-stage model of geologic evolution

A model of geologic evolution of coastal lagoons is proposed hereand, it is hoped, will be the subject of thorough testing and validationin the future. The model consists of two distinct phases of evolution:the first reflects the traditional view of supply-limited sedimentation(Fig. 3a); whereas the second is constrained by additional factors thatact to prevent sedimentation (Fig. 3c). The threshold between the tworelates to depth and is defined as the depth at which the orbitalmotionsof wind waves are able to suspend sediments within the central mudbasin (Fig. 3b). A range of terminology has been used to describe thedepth where wave-sediment interactions occur including equilibriumdepth (Nichols and Biggs, 1985), base level (Nichols and Boon, 1994)and equilibrium volume (Van Goor et al., 2003). The term used here isthreshold depth after Roy and Peat (1976). It is also the depth at whicheffective accommodation space is exhausted.

Sediment mobilisation requires that wave orbital velocities exceedsome critical shear stress. Orbital velocities depend on wave lengthand period, and the critical shear stress depends on sediment properties(Blom et al., 1992), so both can vary between and within systems as afunction of catchment lithology and wind regime, respectively. Thethreshold between early and late stage geologic evolution is thereforebest described as a gradual transition. The term threshold depth in factthus covers a range of depths and may also be referred to as a threshold

Distance tothreshold depth

b

Total available accommodation space

Fetch distance

Total available accommodation space

Fetch distance

Infill has reached threshold depth and can only proceed as fetch is reduced

Total available accommodation space

Infill has reached threshold depth

Fetch distance

EARLY PHASE

THRESHOLD

LATE PHASE

YX

a

c

Fig. 3. Schematic, cross-sectional representation of two-phase geologic evolution forcoastal lagoons. During the early phase (a) basin sediments are not impacted by the orbitalmotions of wind waves and deposition can proceed uninhibited. At the threshold depth(b) wind waves are able to disturb sediments from the central mud basin. The frequencyof disturbance increases over time as infill continues and depths are reduced. If sediment isflushed through the lagoon entrance to the ocean, the threshold depth is maintained in-definitely. If sediment is retained, the late phase of evolution is entered (c) where sedi-mentation can occur incrementally if shoreline progradation or segmentation reducesfetch distances and creates a small amount of accommodation space. The location of thecross-section X–Y is indicated in Fig. 1.

294 K. Adlam / Marine Geology 355 (2014) 291–296

interval. At one extreme, the largest windwaves experienced by a givensystem, generated by high return period storms and acting to disturbthe finest particles on the bed, may have impacts at the greatest depthsbut would also have relatively rare and temporary influence on ongoingsedimentation. At the other extreme, the shallowest limit occurs whenwind waves associated with average climatic conditions are able to en-train all or most surface sediments, or at least prevent the deposition ofalready suspended sediments. As lagoons fill to shallower depthsthrough sedimentation, the influence of wave action increases and be-comes more frequent so the waters of very shallow coastal lagoonsare turbid for much of the time. As the basin accretes vertically towardsthe shallower end of the threshold interval, inhibition of basin sedimen-tation intensifies. Progression through this threshold interval is accom-panied by a gradual coarsening of surface sediments sincefine sedimentfractions are generally more easily entrained than coarser fractions, de-pending on the degree of cohesion (Christiansen et al., 2000). This effectwould bemanifest in the geological record as an upward coarsening se-quence. Of course, any non-erodible substrate shallow enough to besubjected to wave energy is unlikely to be affected to the same degreeas soft sediments, if at all.

By the time sedimentation is inhibited even under average climaticconditions, the second stage of geologic evolution is entered. Hence-forth, evolutionmay followoneof two trajectories on thebasis of lagoonentrance conditions. On the one hand, where entrance conditions allowefficient ocean exchange and regular flushing, often during river floods,suspended sediment is likely to be exported from the system and de-posited on the continental shelf (Roy and Peat, 1976; Nichols andBiggs, 1985; Nichols and Boon, 1994). In such cases, lagoon depths re-main relatively constant regardless of sediment supply (Fig. 3b), ashas been observed on the Australian and Texas coasts (Price, 1947;Roy and Peat, 1976). On the other hand, where ocean exchange is re-stricted and suspended sediment retention ismuch higher, creating tur-bid conditions, accelerated rates of sedimentation on lagoon shorelinesaremore likely. Shoreline accretion is necessary for ongoing infill due toits control on wind fetch. The coastal lagoon surface area is reducedthrough shoreline progradation, provided circulation patterns allowthe dispersal of sediment to shorelines (Roy et al., 1980; Bailey andHamilton, 1997; Green et al., 1997; Zhu et al., 2014), and the reducedwind fetch results in smallerwaveswith smaller and slowerwave orbit-al velocities (Fig. 3c). This effectively raises thewave base above the sur-face, creating a small amount of effective accommodation space withinthe central mud basin that can be consumed by vertical accretion, andso the cycle continues. LakeMacquarie in NSW, Australia, is an exampleof a coastal lagoon that is still deeper than the threshold depth due tovery low sediment supply (Roy and Peat, 1976). Nearby TuggerahLake, on the other hand, has filled to shallow depths and is frequentlyturbid (Roy and Peat, 1973), indicating that a threshold depth hasbeen reached. Wooloweyah Lagoon, part of the Clarence River systemin NSW, has reached a similar stage of evolution but, unlike TuggerahLake, has a much more restricted connection to the ocean that limitsflushing ability. It shows evidence of extensive shoreline progradation(White, 2009). Relict lagoons that have completely filled include thoseon the Tiber Delta coastline of Italy (Bellotti et al., 1995).

The existence of a natural, internal threshold between two phasesof behaviour does not make coastal lagoons unusual in coastalmorphodynamics. Feedback reversals, from self-forcing (positive) toself-regulating (negative) behaviour, can occur even in the absence ofchanging external conditions such as sediment supply, sea level rise orclimate in a range of coastal environments and processes (Cowell andThom, 1994). In the case discussed here, the reversal that occurs atthe threshold depth moves the coastal lagoon system from a situationwhere no feedbacks exist (accommodation space is consumed throughsedimentation, but as it does so there is no flow-on effect for furthersedimentation) to a situation that is defined by feedbacks (interactionbetween the wave base and the bed causes sediments to be entrainedand suspended so that they are available for shoreline progradationwhich, in turn, reduces the fetch distance, raises the wave base and al-lows further accretion to occur until the wave base is reached oncemore). The hypothesis that increased suspended sediment concentra-tion in the late stage of geologic evolution may accelerate shorelineprogradation is supported by the findings of Heap et al. (2004) who,based on facies mapping of hundreds of Australian estuaries, reportedacceleration in the rate of facies change at anunquantified, intermediatestage during the progression from wave-dominated estuary to wave-dominated delta that probably results from increased shorelineprogradation in the second phase of geologic evolution.

Independent of shoreline progradation, an additional fetch-reduction mechanism exists in coastal lagoons that can create ac-commodation space for sediment at any stage of geologic evolution,unaffected by the feedbacks described above. The mechanism isreferred to broadly as segmentation, a term which encompasses thegrowth of river deltas (Roy, 1984; Roy et al., 2001), cuspate spits(Zenkovich, 1967; Bird, 1994), wantide tidal shoals (Nichols andBoon, 1994) and reed swamp encroachment (Nichols and Boon,1994). In southeastern Australia, segmentation (or more specificallydelta growth), appears to dominate over shoreline progradation as a

295K. Adlam / Marine Geology 355 (2014) 291–296

mode of fetch reduction. River delta size has been correlated to over-all extent of infill for 68 estuaries in southeastern NSW (Roy et al.,2001) and classification of the shorelines of 20 southeasternAustralian coastal lagoons from aerial photographs indicates that87% of shoreline change is associated directly with delta growth(Edwards, Unpublished results, 1995). These data suggest that riverbed load is the primary limiting factor for fetch reduction and there-fore late stage evolution in this region. The role of shorelineprogradation on the southeastern Australian coastline remains sig-nificant, however, particularly given cut-off bays (Roy et al., 2001)can arise that do not receive fluvial deltaic input. The relative impor-tance of the two mechanisms of fetch reduction (shorelineprogradation and segmentation) in a worldwide context remains tobe seen, however these regional findings suggest that processes re-lating to both segmentation and shoreline progradation cannot beresponsibly ignored when characterising late stage evolution.

4. Further research

The model proposed here may be tested through the collation ofdata regarding the surface area dimensions of a large number of coastallagoons, along the lines of Price (1947) or Roy and Peat (1976) in con-junction with data concerning wind and wave energy, as in Shepherd(1970). It is possible that data based on energy regime would yield bet-ter correlationwith depth than is displayed in Fig. 2 and provide a betterapproximation of the threshold depth than the lines of best fit depicted,which were based on physical dimensions alone. Departures from thethreshold depth would likely be caused by a very deep lagoon basinwhere sedimentation so far has been insufficient to raise the bed tothe threshold depth (where data lies below the line) or by bedrock pro-trusions that prevent the energy regime from shaping the bed (wheredata lies above the line). Stratigraphic information is thus equally im-portant in such an analysis. The availability of bathymetric (OEH,2011), meteorological (ABM, 2012) and stratigraphic (NSW DoP,2001) data in Australia and elsewheremeans that a study of this natureon a large scale is feasible.

Coastal management and coastal lagoon research alike stand to ben-efit from the recognition of a lagoon's status relative to threshold depth.Coastal planning generally involves setback limits, among other deci-sions, and in order to define these some kind of prediction about the fu-ture position of the shoreline must be made. For the numerous lagoonsworldwide that appear to have reached a threshold depth, shorelinechange under the model presented here would either continue at his-torical rates where sediment is exported from the lagoon or accelerateif sediment retention is high. Interpreting historical sedimentationrates also relies on knowledge about the stage of evolution accordingto the threshold depth. Without recognition of the feedback reversalthat occurs at the threshold depth, the cause of decreasing sedimenta-tion rates in the geologic record could be incorrectly attributed tochanges in sediment supply or boundary conditions. Rates of changeon coastal lagoon shorelines are also of interestwherewetland regener-ation is concerned, for conservation of threatened species (Reed,1990), for the flood buffering properties of vegetation (Townendand Pethick, 2002) and in the burgeoning field of carbon sequestra-tion (Chmura et al., 2003; Saintilan et al., 2013). In all of thesecases, rates of shoreline accretion and habitat change within coastallagoons could be related to the phase of geologic evolution and theentrance conditions. Further, the model itself elevates the importanceof shoreline processes in the ongoing geologic evolution of coastallagoons and, if supported by further investigation, suggests that under-standing of complex and short-term processes operating on coastallagoon shorelines ought to be pursued in the interests of unravellingthe mechanisms of change at much larger timescales. The relativeimportance of shoreline progradation and fluvial delta growth in fetchreduction also deserves more attention.

5. Conclusion

A model of coastal lagoon maturity has been proposed in order toconsolidate and formalise previously published data and ideas. Themodel distinguishes between two distinct phases of geologic evolutionseparated by a threshold that relates to the local energy regime and isquantifiable. The first phase consists of lagoon basin infill whose rateis limited only by sediment supply. The second phase begins when thebasin is sufficiently shallow for wind wave orbital motions to preventdeposition or entrain sediments as a result of some threshold shearstrength being exceeded. Basin infill rates in this second phase areconstrained by wave energy which, in turn, depends on the wind re-gime and the fetch depth. As a result, lagoons in the late stage of evolu-tion are characterised by high turbidity, particularly where oceanconnection is restricted. In the latter case, suspended sediments are in-creasingly available for deposition on low energy shorelines resulting inshoreline progradation and fetch reduction. A feedback between lateralshoreline progradation (fetch reduction) and vertical accretion thuscontrols the rate of fill. If, on the other hand, ocean connection is rela-tively free and flushing efficiency is high, suspended sediment is unlike-ly to be retained within the lagoon and an equilibrium form may bemaintained. Segmentation of lagoon basins (through, for example, fluvi-al delta growth) also provides an additional and sometimes more prev-alent mechanism of fetch reduction, facilitating further basin accretion.While historically published data supports this model, further data col-lection would increase confidence in its validity. Data should concernthe dimensions of coastal lagoons and, most importantly, their windand wave regimes. Assuming the model is supported, future strati-graphic interpretations and morphological predictions ought to bemade in the context of lagoon basin maturity relative to the thresholddepth; the presence of a feedback between lateral and vertical changesdepends on whether the threshold depth has been reached.

Acknowledgements

I thank Dr. D. Penny, Dr. E. Bruce, J. Twyman and S. Adlam for theirearly stylistic reviews and Dr. Peter Roy for lending an expert's eye tothis manuscript. Comments from two anonymous reviewers also sub-stantially improved the work.

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