Estuaries of the past and present: A biofacies perspective

0 (2006) 289–298www.elsevier.com/locate/sedgeo

Sedimentary Geology 19

Estuaries of the past and present: A biofacies perspective

Claudia Schröder-Adams ⁎

Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada K1S 5B6

Abstract

Estuaries are complex sedimentary and ecological systems, where controlling factors are variable largely depending on wave vs.tidal dominance and fluvial processes. Paleoenvironmental reconstruction of their ancient counterparts in the form of coastal valleydeposits in the subsurface or outcrop requires a multidisciplinary approach. Microfossils can play an integral part in identifyingestuarine subenvironments. Foraminifera can be abundant in modern estuaries and resemble characteristics of brackish ichnofaunalcommunities in featuring low species diversity, but high abundances of opportunistic species, different feeding strategies andcommon infaunal species. Whereas sediment distribution is highly controlled by energy regimes, foraminifera seem to respond tosalinities and tidal exposure. Whereas individual taxa can widely range bathymetrically, the combination of certain taxa becomesdiagnostic for estuarine environments. Fossil marginal marine assemblages are dominated by agglutinated species due totaphonomic loss of the calcareous component that is often dominant in modern estuaries. When comparisons between fossil andmodern assemblages are undertaken it is advisable to compare with Recent subsurface or remnant assemblages for a more accuratebasis of paleoenvironmental interpretation. More integrated research with detailed taphonomic observations is needed on ancientcoastal valley deposits to find the paleoecological requirements of extinct taxa and their link to sedimentary facies and ichnofacies.© 2006 Elsevier B.V. All rights reserved.

Keywords: Estuary; Incised valley; Foraminifera; Taphonomy; Paleoecology

1. Introduction

Modern estuaries are extensively studied in theirsedimentological, physical and biological character. Estu-aries represent complex ecosystems forming an interfacebetween freshwater and marine environments. Theirfrequent close proximity to dense human populationmakes them ideal monitoring sites of anthropogenicimpact on natural biotopes (Alve, 1995; Daoust et al.,1996; Yanko et al., 1999; Scott et al., 2005). Ancientcoastal valleys can be sites of high preservation potentialfor their sedimentary fill and therefore might documentgeological history without major gaps (Dalrymple et al.,

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1992). During the last decade incised valley fills havereceived much attention from sedimentologists since theywere discovered as important petroleum exploration targets(Zaitlin et al., 1995). Ichnologists became successful inrecognizing incised valley fills and distinguishing estuarineenvironments from fully marine ones based on ichnofaciespatterns (e.g. Pemberton et al., 1992; MacEachern andPemberton, 1994). In modern and Pleistocene deposits ofWillapaBay,Washington for exampleGingras et al. (1999)were successful in recognizing five estuarine subenviron-ments based on ichnofacies pattern.

Benthic foraminiferal distribution patterns have beenwell documented from modern marginal marine settingsand a variety of shallow-water environments includingthose associated with estuaries have characteristic speciescompositions including numerous cosmopolitan taxa (e.g.

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290 C. Schröder-Adams / Sedimentary Geology 190 (2006) 289–298

Murray, 1991; Boyd andHonig, 1992; Culver et al., 1996;Hayward et al., 1996, 1999). However, despite the welldocumented clear relationships between modern assem-blages and their ecological controls and the abundance ofsedimentological and sequence stratigraphic data onancient coastal valley fills, marsh and estuarine forami-niferal assemblages are not commonly recorded from thePre-Cenozoic record.

This paper uses a few key studies on foraminiferalcompositions in estuaries of modern and Mesozoicsettings in order to demonstrate the complexity that isinvolved with applying modern ecological relationshipsto fossil assemblages and paleoenvironmental interpreta-tions. Since such a large sedimentological and sequencestratigraphic database has emerged during the last decadefrom Mesozoic strata in North America, chosen fossilassemblages are derived from studies addressing the Cre-taceous Western Interior Sea.

2. Sedimentological characteristic of estuaries

In 1992 Dalrymple et al. proposed a facies model forincised-valley estuaries and defined an estuary as ‘theseaward portion of a drowned valley system, which re-ceives sediment from both fluvial and marine sources andwhich contains facies influenced by tide, wave and fluvialprocesses. The estuary is considered to extend from thelandward limit of tidal facies at its head to the seawardlimit of coastal facies at its mouth’. Their tripartite sub-division into the flood-tide delta, central basin and bay-head delta facies (Fig. 1) has received much acceptance inthe literature, where lithofacies are largely controlled byenergy regimes that stem from waves and tidal currents,dominating the seaward outer zone and river currents onthe landward inner zone. These zones are separated by thecentral basin characterized by relatively small energy(Fig. 1).

Estuaries can be wave-dominated or tide-dominated orrepresent some variation of both. In a wave-dominatedestuary a variety of facies are formed due to the distri-bution of energy. At themouth highwave energy results incoarse-grained transgressive subtidal shoals, washoverdeposits, and beach and shoreface barriers cut by tidalinlets. The bay-head delta may also consist of sand andgravel delivered by the fluvial system. The central basin ischaracterized by deposition of organic-rich muds indeeper estuaries and salt marsh environments in shallowbasins. Tide-dominated estuaries are less common and arecharacterized by energy that reaches much further into theestuary than the wave energy resulting in a less pro-nounced tripartite facies distribution. Fine-grained sedi-ments accumulate mainly in tidal flats and marshes along

the edge, whereas the coarse-grained central part isdominated by tidal sand bars in the outer zone and upper-flow-regime sand flats in the inner zone (Nichols et al.,1991; Dalrymple et al., 1992; Zaitlin et al., 1995).

3. Ecological controls on biota in estuaries

Estuaries are home to a multitude of environments thatall have relatively shallow water in common, but differ inother physical, chemical and biological parameters.Typical estuarine habitats include low and high marsh,intertidal mud and sand flats, tidal channels, tidallythrough fluvially influenced point bars, quiet bays, andshoreface deposits, each with a set of ecological controlsto which biota respond (Fig. 1). These assemblages arecommonly distinctly different from shallow, open shelfenvironments. Climatic gradients impose another controlon estuarine flora and fauna.

Generally, estuaries are stressed environments for biotadue to variable physical parameters. Whereas lithofaciesin estuaries are mainly controlled by energy patterns,salinity gradients play a significant role in the distributionof biofacies in marginal marine environments (Haywardet al., 1999; Pemberton et al., 2001). Salinity variationsare controlled by tidal range, salinity content in adjacentopen ocean coastal waters, wind direction and strength,freshwater input from rivers, runoff from land, rainfall,evaporation, and morphology of the surrounding coastalarea (Pemberton et al., 2001). In addition habitat var-iability is imposed by ecological factors such as sedi-mentation rates, turbulence, substrate consistency, supplyof organic matter and nutrients, oxygen content, temper-ature and duration of aerial exposure in the tidal zone.Each faunal group reacts and adapts differently to thesefactors. Whereas generally species diversity of macro-and microorganisms is lower in brackish-water environ-ments due to their unstable and unpredictable conditions(Wolff, 1983), abundances of opportunistic species can behigh due to decreased competition.

Strategies for organisms to counteract extreme salinityand substrate fluctuations are deep infaunal habitats, wheresediments have a buffering effect. Other opportunistictraits are an omnivorous feeding strategy that is increas-ingly found in estuaries compared to freshwater or marinehabitats (Wolff, 1973). Individual animals are also adopt-ing several feeding strategies based on prevailing condi-tions (Goerke, 1971).

This paper looks at ecological controls on benthicforaminifera in estuaries. Foraminifera or other micro-faunal components have advantages and contribute valu-able information in integrated studies. They can be foundabundantly in core samples and might occur in sediments

Fig. 1. Estuary zonation showing in (A) energy distribution, (B) tripartite subdivision and (C) various estuarine subenvironments in plan view(modified from Dalrymple et al., 1992).

291C. Schröder-Adams / Sedimentary Geology 190 (2006) 289–298

where bioturbation has destroyed any diagnostic sedi-mentary structures. Although foraminiferal ecology canbe quite complex (Boltovskoy et al., 1991), it is re-markable that some genera of agglutinated foraminifera inestuarine and marsh environments have persistedthroughout the fossil record since Paleozoic time (Wight-man et al., 1992, 1994). It is, however, apparent that mostfossil marginal marine assemblages are characterized by

the distinct lack of calcareous foraminifera stressing theimportance of recognizing taphonomic loss in paleoen-vironmental interpretations.

4. Foraminiferal biofacies in modern estuaries

Benthic foraminifera are playing an integral part as alower trophic-level member in coastal andmarine settings


(Culver and Buzas, 1995). They are capable of a varietyof feeding mechanisms such as suspension feeding,grazing, carnivory, symbiosis, scavenging and uptakeof dissolved organics (Goldstein, 1999). On the otherhand they serve as food sources for a variety of highertropic-level organisms. Life positions of foraminiferainclude erect, epifaunal, infaunal and epifaunal-at-tached positions. These can be linked to various feedingstrategies and ecological habitats (Jones and Charnock,1985; Murray and Alve, 1994). Erect specimens areprimarily suspension feeders. Epifaunal taxa are pas-sive or active deposit feeders and can be herbivores,detrivores or omnivores. Infaunal species are detrital/bacterial scavengers and epifaunal-attached taxa areherbivores. Marginal marine settings are inhabited pre-dominantly by infaunal bacterial/detrital scavengers andmarshes contain abundant herbivores. In contrast, sus-pension feeders and epifaunal taxa prefer deeper waterenvironments with decreased energy and finer sediment(Jones and Charnock, 1985).

Benthic foraminifera have been widely used in Qua-ternary sediments to characterize marginal marine envi-ronments and interpret sea-level fluctuations (e.g. ScottandMedioli, 1980; Scott and Leckie, 1990). Bathymetricranges of individual benthic foraminifera can be large,but combined with other species become diagnostic for arestricted depth range (Johnson et al., 2005). Since thispaper makes the comparison between Quaternary andCretaceous estuarine fauna, taxa shall be named at ageneric level; evolutionary change does not permit com-parison at a species level.

Before Dalrymple et al. (1992) proposed the tripartiteestuarine subdivision Scott et al. (1980) divided estuariesinto upper estuary, transitional zone and open bayenvironments that roughly resemble the bayhead-delta,central basin and flood-tide delta respectively. Benthicforaminifera were studied in eastern Canada in theMiramichi River and Restigouche Estuary of NewBrunswick, and the Chezzetcook Inlet in Nova Scotiawhere water depths, temperature and circulation patternwere determined as controlling ecological factorsbesides salinity (Scott et al., 1980). Certain taxa indicatea clear correlation to ecological regimes. The upperestuary is characterized by Miliammina and the distinctlack of calcareous species. In north temperate latitudescalcareous foraminifera cannot reproduce in salinitieslower than 20‰. The genus Ammotium correlates to highconcentrations of suspended particulate matter indepen-dent from tidal exposure. Ammonia is related to waterdepth and occurs in the intertidal areas. Large occur-rences of Protelphidium indicate intertidal or shallowsubtidal conditions. Increased numbers of agglutinated

genera such as Saccammina, Reophax and Cribrosto-moides indicate deep water estuaries (Scott et al., 1980).

Boyd and Honig (1992) combined sedimentologicaland microfaunal analysis of cores in the small, wave-dominated estuary of Lawrencetown Lake along theEastern Shore of Nova Scotia. Generally, agglutinatedspecies dominate the marsh and more restricted areas ofthe estuary, whereas calcareous species reflect increasedmarine influence of normal salinity in an open baysetting. The following five biofacies were distinguishedbased on assemblage composition: (a) lower estuary; (b)mixed estuarine and marine; (c) open bay; (d) restrictedopen bay; and (e) mixed assemblage. Agglutinated taxasuch as Trochammina and Eggerella were abundant inmarsh deposits, whereas calcareous taxa such asElphidium,Haynesina,Buccella,Cibicides andRosalinarepresent open bay or mixed biofacies. Foraminiferalbiofacies were coupled with seven distinct lithofaciesallowing interpretations of Holocene coastal evolution.

Hayward et al. (1999) in his extensive study ofmodern New Zealand estuaries distinguished betweenthree types of estuaries including: (a) saltwedge estuariesthat are characterized by a strong halocline that can beseasonally variable; (b) partially mixed estuaries; and (c)well mixed estuaries with longitudinal and lateral salinitygradients, but not a vertical one. In these settings theyidentified numerous brackish habitats including subtidalchannel, subtidal and intertidal seagrass flats, intertidalsand andmud flats, mangrove forests, low salt meadows,salt marsh and high salt meadows. By measuring avariety of environmental proxies tidal exposure andsalinity were the most important controlling factors onthe distribution of benthic foraminifera with a somewhatreduced control of presence of intertidal vegetation inpositive correlation with organic carbon. Taxa withdecreasing tolerance to tidal exposure include Trocham-mina, Miliammina, Jadammina, Trochamminita, Elphi-dium, Ammotium, Ammonia and Haynesina. Generawith increasing tolerance to lower salinities includeAmmonia, Elphidium, Ammobaculites, Trochammina,Jadammina, Ammotium, Haplophragmoides, Miliam-mina and Trochamminita. The trend as observed byBoyd and Honig (1992) of a change from a dominantlyagglutinated to a dominantly calcareous assemblage andincreasing diversity from the inner to the outer estuarywas also observed in New Zealand's estuaries. Sedimentgrain size alone did not seem to play a large role incontrolling assemblage composition.

The fourth example of modern foraminiferal distribu-tion is a barrier island system of the southern DelmarvaPeninsula in Virginia, U.S.A. studied by Culver et al.(1996). Although this paper deals with estuaries this


example is chosen here because lagoonal faunas in thefossil record are often difficult to distinguish fromestuarine assemblages. Barrier lagoons are characterizedby similar subenvironments including marshes, tidal flats,subtidal flats, bars, tidal channels, quiet bays and inlets.These were differentiated by variables such as elevation,sediment grain-size distribution, tidal inundation, salinityand dominant flora. Salinities in barrier island systems canbe higher than in restricted estuaries and ranged in thiscase mainly from 28 to 32 ppt. Brackish salinities wereencountered near the heads of small streams. Overallspecies diversity and abundance is relatively high withlowest numbers occurring in outer tidal flats and innerfringe marshes. High numbers occur in the restricted bayand washover fan deposits. Taxonomic composition andspecies diversity data, statistically analyzed, resulted infour reliable subenvironments. The brackish marsh/channel biofacies (associated with a salinity of only9.4 ppt) is up to 98% dominated by agglutinated taxa suchas Trochammina, Arenoparrella,Haplophragmoides, andJadammina. Lagoonal tidal flats are dominated byElphidium and Ammonia. Lagoonal marshes/washoverfans are characterized by abundantAmmonia,Haynesina,Miliammina, Elphidium and Arenoparrelina. Channels/inlets/shoreface deposits contain Ammobaculites, Tro-chammina, Miliammina, Textularia, Quinqueloculina,Elphidium, Haynesina and Ammonia as dominant taxa(Culver et al., 1996). In contrast to New Zealand'sestuaries grain size was identified as having a significantinfluence on the abundance of Ammonia and Haynesina.

Most of these genera are also common elements ofestuaries, where diversities equally increase with greaterexposure to fully marine conditions and agglutinated taxabecome the dominant component of environmentallystressed marsh environments. Therefore the interpretationof lagoonal vs. estuarine paleoenvironments requires anintegrated approach addressing subsurface or outcropgeometry and sequence stratigraphic architecture.

5. Foraminiferal biofacies of Cretaceous marginalmarine settings

It is apparent that recent sedimentological research, ofwhich much is related to hydrocarbon explorationactivities, has unearthed abundant coastal valley depositsin particular within Mesozoic strata (e.g. Zaitlin et al.,1995, and references therein). At the same time work onestuarine ichnofacies has kept pace particularly due towork by Pemberton and co-workers. Micropaleontologi-cal research, however, on Pre-Cenozoic marginal marinestrata remains relatively scarce, although existing resultshave proven to be promising when using an integrated

approach. Several studies have demonstrated that mar-ginal marine foraminiferal assemblages have experiencedlittle change on generic level throughout the Phanerozoic(Wightman et al., 1992; Scott et al., 2003) and com-parisons to modern biofacies are frequently made use of.Cretaceous marginal marine settings are an ideal examplebecause of well documented higher frequency sea-levelcycles during times of extensive coastal onlap (Tibert andLeckie, 2004). Examples from the Western Interior Seawill be referred to here.

By using a generic comparison with modern estuarineassemblages as established by Murray (1971, 1973),Wall (1976) interpreted a salt marsh paleoenvironment inthe Campanian Bearpaw-Horseshoe Canyon transitionstrata as studied in a well of southern Alberta. Lowspecies diversity, but high abundances of the generaMiliammina, Ammotium, Trochammina, Verneuilinoidesand Eggerella were interpreted as representing marginalmarine settings. As confirmed by Scott et al. (1983) thesegenera with the exception of Verneuilinoides have theircounterparts in modern brackish marsh areas. Domi-nance of Trochammina favored the interpretation of asalt marsh over a hyposaline lagoon. A similar genericcomposition is associated with the estuary fill sedimentsof the Late Albian Paddy Member of the Peace RiverFormation in northwestern Alberta (Stelck and Leckie,1990).

The Albian Bow Island Formation in southwesternAlberta is a transgressive coastal succession with a seriesof stacked incised valley deposits in the upper member ofthe formation (Pedersen et al., 2002). Foraminifera as-semblage data were closely integrated with sedimento-logical analysis (Vorobieva, 2000; Pedersen et al., 2002)as demonstrated in a core at 6-32-13-27W4 that covers themiddle and upper Bow Island members overlain by themarine mudstones of the Westgate Formation (Figs. 2and 3). The coastal plain facies of the middle Bow Islandmember is barren of foraminifera. The upper Bow Islandmember is a complex series of facies representing thelagoonal estuarine facies association. Benthic foraminif-era are sparse and of low diversity, increasing inmudstone-rich layers including typical marginal marinegenera such as Miliammina, Gaudryina, Verneuilinoidesand Trochammina ofwhich the first three have an infaunallife habitat and the later one an epifaunal position. Theoffshore/lower shoreface assemblage of the WestgateFormation differs in composition where genera such asAmmobaculites, Haplophragmoides and the erect sus-pension feeder Bathysiphon occur.

In the U.S. part of theWestern Interior SeawayMorris(1971) worked on Upper Cretaceous marginal marinestrata in northwestern Colorado and distinguished three

Fig. 2. Legend for Fig. 3 (modified from Vorobieva, 2000).


assemblages based on species diversity, dominance, andcomparison to Recent assemblages and sedimentaryfacies characteristics. A Saccammina dominated assem-blage of low diversity is interpreted as a marsh orestuarine tidal creek environment. Tibert and Leckie(2004) suspected that the specimens of Saccammina asidentified by Morris (1971) might represent freshwaterthecamoebians. The second microfauna is dominated byVerneuilinoides and Trochammina, is of exclusivelyagglutinated composition, has a relatively low speciesdiversity, but high dominance and high abundance. It isassociated with brackish-water conditions in an estuaryor brackish bay supported by tidal indicators such asflaser bedding, where competition is kept low andopportunistic species can flourish. The third assemblageis characterized by relatively high species diversity and amixed calcareous/agglutinated species composition in-dicating deeper water offshore conditions.

Tibert and Leckie (2004) studied benthic microfossilsfrom the Turonian SmokyHollowMember of the StraightCliffs Formation in Utah. A total of eight lithofacies wererecognized and related to a tripartite estuarine subdivisionresembling the one proposed by Dalrymple et al. (1992)and Nichols et al. (1991) (Fig. 1). Trochammina andMiliammina were linked to a marsh environment in theriver-influenced proximal estuary; Ammobaculites was

prominent in the muddy central basin and the distalestuary under increased marine influence was representedby Verneuilinoides, Reophax and Textularia. Fluctuatingdominances of taxa, integrated with ostracod occurrencesand lithologies and compared to modern depth ranges ofthe same genera, resulted in a high frequency sea-levelcurve for the Smoky Hollow Member.

Johnson et al. (2005) studied the Upper CretaceousBlackhawk Formation in Utah, where the authorsdifferentiated between possible estuarine vs. backbarrierlagoonal settings based on the modern analog at thecoast of Virginia studied by Culver et al. (1996) andmentioned above. An entirely agglutinated assemblageconsisting of Textularia, Ammobaculites, Haplophrag-moides and Trochammina was interpreted as marginalmarine and brackish. In contrast a mixed assemblagewith an additional calcareous component including shelfand deeper water species was interpreted as a lagoonaldeposit behind a barrier island where mixing can occuras the result of overwash (Johnson et al., 2005).

Most published accounts of Cretaceous marginalmarine foraminiferal assemblages are almost exclusivelydominated by agglutinated taxa, although calcareousbenthic assemblages are prominent components of mo-dern estuaries and marginal marine settings. It becomesapparent that modifications of assemblages through tap-honomic processes have to be considered in paleoenvir-onmental interpretations.

6. The importance of taphonomy

The use of fossil assemblages for accurate paleoenvir-onmental interpretations cannot exclude the subject oftaphonomic alteration.We often rely on our knowledge ofmodern biofacies and their environmental significance tointerpret the past assuming that the relationship between aparticular taxon to its environment has not significantlychanged over geological time. It is, however, well knownthat the fossilization potential of benthic foraminifera ishighly variable (Smith, 1987). In addition to dissolution orbreakdown reworking through winnowing and bed-loadtransport can alsomodify the composition of assemblagesand affect paleoenvironmental interpretations.

By using Cenozoic foraminiferal assemblages in theBeaufort–Mackenzie Basin, where high sedimentationrates are prevalent, McNeil (1997) distinguished fourdiagenetic regimes that have an influence on foraminiferalpreservation. These are: (1) The early diagenetic regimeoccupies the upper 10 m of sediment, where aerobic andanaerobic bacteria contribute to the decay of organicmatter and increase of CO2 results in dissolution ofcalcareous foraminifera. Reduction processes of sulphate

Fig. 3. Core of the Late Albian Bow Island Formation in southwestern Alberta showing the Gamma Ray log, stratigraphy, facies, facies associations,lithology, trace fossils, and foraminiferal changes (modified from Vorobieva, 2000).


and iron lead to pyritization of foraminiferal tests, but alsocan contribute to the disaggregation of agglutinated tests(Schröder-Adams and McNeil, 1994). (2) The meteoricdiagenetic regime occurs in proximal areas of sedimen-tary basins, where corrosive, low salinity waters, over-saturated in respect to silica cause dissolution of calcareousforaminifera and result in secondary mineralization. (3)The burial diagenetic regime involves several km ofdepth and increased temperatures. Thermal alterationand colouration of agglutinated foraminifera takes place

and silicification of tests is common. (4) The overpres-sured diagenetic regime occurs where pore fluid pres-sures exceed the normal hydrostatic pressure, whichmight enhance silicification processes. Numerous stud-ies demonstrate the considerable modifications thatmarginal marine assemblages experience in the earlydiagenetic regime.

Several taphonomic experiments have been conductedin salt marsh environments that have epifaunal andinfaunal habitats. Goldstein and Harben (1993) studied


salt marshes of Georgia and documented a clear differencebetween surface and subsurface or remnant foraminiferalassemblages. The occurrence of infaunal species and pro-cesses of selective preservation had significantly alteredthe assemblage composition. In contrast, Tobin et al.(2005) play down the role of living infaunal foraminif-era and state that the upper 0–1 cm interval would berepresentative of the total assemblage downcore.

Marsh environments are indicated by low pH valuesin peat deposits that result in dissolution of calcareousforaminifera. Many calcareous benthic foraminifera areepifaunal, particularly miliolids, which are prone todissolution. Taxa with high preservation potentialincluded Arenoparrella, Haplophragmoides, Textularia,and Trochammina (Goldstein et al., 1995). Most ofthese are commonly found in the above discussed fossilassemblages of the Cretaceous Western Interior Sea.Agglutinated taxa of less fossilization potential werePseudothurammina, Ammotium, Ammobaculites andMiliammina showing that besides the common dissolu-tion of calcareous forms, agglutinated tests are alsoprone to taphonomic destruction (Goldstein et al.,1995). Ammobaculites and Miliammina, however, arecommonly found in Cretaceous assemblages.

A study of offshore shallow- to deep-water samples inthe Gulf of Mexico divides agglutinated species intodisaggregation-resistant taxa and disaggregation-pronetaxa (Denne and Sen Gupta, 1989). The first groupincludes typical shallow-water taxa with chambersarranged in series and calcareous cements such asGaudryina, Karreriella and Eggerella, of which Gau-dryina is frequently found in the Cretaceous fossilrecord. The latter group includes Trochammina, Ammo-baculites, Adercotryma, Recurvoides, Rhizammina andHaplophragmoides with organic cement and of whichseveral are also common in Cretaceous sediments. Thisshows how preservation potential differs even betweenbathymetric zones, as Trochammina is often wellpreserved inmarginal marine settings in the fossil record.

Buzas-Stephens and Buzas (2005) studied living anddead foraminiferal populations from the heavily anthro-pogenic impacted Nueces Bay in Texas and noted par-ticularly severe dissolution effects on the genusAmmonia.There, carbonate dissolution was partly linked to acidicconditions caused by pyritization of tests. The samemodification of Ammonia was observed in FullertonCove, New South Wales Australia (Schröder-Adams,unpublished data). Pyritization of foraminifera in oxy-genated sediments is regarded as a response to stress dueto heavy metal contamination, where bacteria mediatedbreakdown of protoplasm increases anoxic conditionswithin tests (Alve, 1991).

The at times total loss of calcareous components in theearly stage of the thanatocoenosis (Murray and Alve,1999) amplifies the notion that in paleoecological studiesbiofacies comparisons should be done with dead assem-blages that form the remnant after initial taphonomic lossof modern analogues (Murray, 2000). Using statisticalmeasures for diversity indices, as so often used in modernsettings, are also questionable taking the taphonomic lossof specimens into account.

7. Summary and future challenges

Estuaries and incised valley deposits are complexsystems that play an important role in the architecture ofmarginal marine depositional systems. Sediments aremainly controlled by energy conditions within the estuary.Foraminiferal distribution is also affected by energypatterns as tests can be transported and abraded by cur-rents. Foraminifera can be brought as suspended load fromthe exposed inner shelf to the low energy environment ofestuaries or quiet tidal inlets (Hayward et al., 1999).Estuaries with strong tidal currents may show inner shelfspecies carried into the estuary for quite a distance versustypical estuarine taxa are swept through the entrance intothe inner shelf region. However, whereas energy plays themost significant role in the distribution of sedimentaryfacies, biofacies is controlled by a multitude of factors.

Extensive work on the comparison of modern andMesozoic trace fossil communities has resulted in thebrackish-water model that proposes the following char-acteristics: (1) low ichnodiversity, (2) forms typicallyfound in marine environments, (3) dominance of infaunaltraces rather than epifaunal trails, (4) simple structures bytrophic generalists, (5) mixture of vertical and horizontaltraces from Skolithos and Cruziana ichnofacies, and (6)presence of monospecific suites (Buatois et al., 2005).When one compares these characteristics, predominantlyproduced by meio- and macrofaunal elements, with thoseof foraminiferal assemblages, certain similarities can beobserved. Brackish foraminiferal assemblages are alsocharacterized by: (1) low species diversity, (2) numerousgenera, that also occur in fully marine environments, (3)infaunal but also epifaunal taxa of which the infaunal taxaoften remain preserved, (4) bacterial/detrital scavengersand herbivores, and (5) presence of monospecificassemblages, particularly in the most stressed subenvir-onments such as marshes and tidally influenced brackishriver channels.

Data presented here clearly indicate that comparisonswith modern estuarine biofacies have their limitations.The overwhelming dominance of agglutinated taxa inancient marginal marine settings is clearly a remnant of


the once biocoenosis. Therefore more integrated studiesare needed that address ancient coastal valley systemsand clearly establish the relationship between sedimen-tary facies interpreted based on sedimentary structuresand tracefossils and foraminiferal biofacies. Taphonomiccriteria such as bioerosion, abrasion, dissolution,fragmentation, mineralization and sedimentary infillingin fossil faunas have to be carefully noted to increaseaccurate paleoecological observations (Herrero andCanales, 2002). Only then will the resulting databasenot entirely rely on comparisons to modern analogues,but establish relationships between fossil foraminiferalcompositions and their ancient marginal marine habitats.Many Mesozoic species and even genera are extinct andtheir paleoecological requirements need to be estab-lished independently from Recent assemblages. This canonly be achieved by detailed multidisciplinary studies ofPaleozoic and Mesozoic environments.

Acknowledgments

I like to thank Octavian Catuneanu for inviting me tocontribute to this volume. Ron Boyd and Tim Rolphintroduced me to their project on coastal valley evolutionin New South Wales, an opportunity I much appreciated.Reviews by Susan Goldstein and David Scott were muchappreciated. Financial support was provided by a NSERCDiscovery Grant to the author.

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Estuaries of the past and present: A biofacies perspective