Recognition of ancient carbonate wind deposits: lessons from amodern analogue, Chrissi Island, Crete

PIERRE LE GUERN and ERIC DAVAUDSection of Earth Sciences, Department of Geology and Paleontology, University of Geneva, 13 rue desMaraıchers, 1211 Geneva, Switzerland (E-mail: eric.davaud@terre.unige.ch)

ABSTRACT

Carbonate aeolian deposits are common along arid to semiarid, wind-exposed,

present-day coastlines bordered by productive carbonate ramps. Lithified

carbonate dunes (aeolianites) have been described around the world in marine

terraces of Quaternary age, but these deposits have seldom been identified in

the Pre-Quaternary record. Several authors have suggested that this scarcity

reflects that these deposits form and are preserved only during icehouse

periods characterized by high-amplitude sea-level changes. Others [e.g. McKee

and Ward Carbonate Depositional Environments (1983), AAPG Memoirs, Vol.

33, pp. 131–170] suggest that the scarcity of aeolianites in the Pre-Quaternary

record could reflect the ‘great difficulty in recognising wind blown

carbonate deposits and in differentiating between them [aeolianites] and

other carbonate sands of nearshore environments’. It has been considered that

carbonate shoreface/foreshore deposits are very difficult to discriminate

petrographically from backshore deposits. This petrographic study of recent

sediments from the shoreface to backshore along the northern coast of Chrissi

Island, Crete, confirms that carbonate aeolian sands can be very easily

misinterpreted as shoreface deposits. Textural examination of thin sections by

image analysis techniques indicates, however, that grain orientation patterns

differ between facies. Shoreface deposits exhibit a unimodal distribution of

grain orientation (flat rose diagram), whereas backshore deposits show a

tendency towards a bimodal distribution with a significant proportion of

vertical grains. This observation has been confirmed in Pleistocene aeolianites

from Tunisia and Western Australia. Grain verticality thus seems to be a

reliable criterion for discriminating wind-lain carbonate bodies from shoreface

deposits. Vertical grains in aeolian carbonate deposits could reflect gravity

effects (e.g. reorientation of grains because of meteoric water percolation and

air pull-up). Laboratory experiments conducted on carbonate sands under the

action of percolating waters confirm this hypothesis. This reorganization

process is preferentially developed in recently deposited and loosely packed

sands resulting from grainfall and/or grainflow. In addition, this suggests that

the presence of vertical grain orientation might be an indicator of the frequency

and intensity of rainfalls during deposition.

Keywords Aeolianites, Australia, coastal dunes, Crete, image analysis,petrotexture, Pleistocene, Tunisia.

INTRODUCTION

The term aeolianite was introduced by Sayles(1931) to describe lithified rocks of aeolian origin,

regardless of the dominant grain constituent.Some authors use this term only for aeoliandeposits lithified by carbonate cements, whereasothers use it solely for deposits of Quaternary age.

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The widely accepted use of the term, as proposedby Brooke (2001), defines aeolianites as ‘[…]coastal limestone that records the reworking ofvast quantities of shallow-marine biogenic car-bonate sediment into coastal dunes’. However,this term should not be restricted to coastaldeposits, as carbonate particles of marine origincan easily be transported far from the coastline, asdemonstrated by Goudie & Sperling (1977) in theThar desert, where foraminifers have been trans-ported over hundreds of kilometres inland. Forclarity in this study, the term ‘carbonate aeolia-nites’ is used to refer to lithified aeolian depositsin which carbonate grains constitute more than50% of the rock, as suggested by Abegg et al.(2001).Modern carbonate aeolian deposits are common

along arid to semiarid wind-exposed coastlinesbordered by productive carbonate ramps. Theirgeographic distribution reflects warm climatesand proximity to shorelines where abundantcalcareous sand accumulate. These deposits canform immense sandy accumulations and have ahigh preservation potential because of activeearly diagenesis.Shoreline settings, including the intertidal zone

and backshore environment, offer particularlyfavourable conditions for the precipitation of earlymarine and freshwater cements. Carbonate sandshave high permeability and porosity, allowingactive percolation of meteoric water. The abun-dance of aragonite and high-magnesium calciteparticles, vulnerable to undersaturated water,favours dissolution rapidly followed by precipita-tion from oversaturated percolating fluids. Menis-cus and microstalactitic cements precipitate andprotect the backshoredeposits from further erosionand sediment remobilization.Early meteoric cementation can be reinforced

when these deposits are flooded during transgres-sive episodes. The presence of a highly porous butprelithified framework and the intense circulationof supersaturated marine waters (driven by afrequent water-table fluctuation because of wavesand tides) promote rapid marine cementation.Although well-studied and recognized in the

Quaternary, ancient wind-lain carbonate depositsare seldom reported in the literature. A review ofPre-Quaternary aeolianite occurrence can befound in Gardner (1983), Loope & Haverland(1988), Handford & Francka (1991), Rice & Loope(1991), Hunter (1993), Brooke (2001) and Kindler& Davaud (2001).Fairbridge & Johnson (1978) suggest that low

sea level and exposure of the platform are

prerequisites for the formation of wind depositsof marine origin. These authors consider aeolia-nites to be closely related to glacial processes.The recent identification of aeolianites of Jur-assic age in Wyoming, USA (Kilibarda & Loope,1997) and of Lower Cretaceous age in the SaleveChain, France (Kindler & Davaud, 2001), how-ever, show that the formation of aeolianitesduring periods considered as greenhouse timesis possible.This scarcity of reported occurrences of Pre-

Quaternary aeolianites is probably because of thelack of recognition criteria rather than the rarity ofsuch deposits in the Earth’s history, as suggestedpreviously (e.g. Fairbridge& Johnson, 1978;McKee&Ward, 1983;White & Curran, 1988; Rice & Loope,1991; Kindler & Davaud, 2001). At the thin-sectionscale, aeolianites, which are mainly composed ofparticles of marine origin, could easily be misin-terpreted as shallow marine carbonate deposits.Carbonate grainstones are mostly considered to beof marine origin. Well-preserved marine bioclastsfound in aeolianites thus suggest a marine originand lead to a false palaeoenvironmental interpret-ation. The presence of diagenetic features relatedto the vadose zone (such as stalactitic or meniscuscements) or the presence of pedogenetic overprints(rhizoliths, alveolar structure, vadose pisoids),common in aeolianites, may also form in sub-tidalsands during a regressive episode.The only unambiguous criteria for recognizing

aeolianites are sedimentary structures, partic-ularly large-scale cross-stratifications and fineaeolian structures such as climbing translatentstratification (Hunter, 1977, 1993). Unfortunately,these macroscopic criteria, which are easilyidentifiable in Pleistocene and Holocene out-crops (because of selective cementation thatemphasizes grain-size changes), are often imper-ceptible in older deposits affected by morepervasive diagenesis. Inverse grading character-izing climbing translatent stratification (pinstripelamination) is due to the accumulation of veryfine sand or coarse silt in ripple troughs and theconcentration of larger grains on the crests ofwind ripples (Fryberger & Schenk, 1988). Whenthe particles in motion are mainly composed ofbioclasts of heterogeneous densities and shapes,this size sorting is poorly developed. As pointedout by Kindler & Davaud (2001), pinstripelamination is common in peloidal or ooliticBahamian aeolianites of Holocene (White &Curran, 1988) and Late Pleistocene age (Caputo,1993; Kindler & Strasser, 2000), but typicallyobscured in more cemented middle Pleistocene

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units and scarce in bioclastic aeolianites asyoung as Holocene age.Moreover, large-scale grainflow or grainfall

stratifications are often difficult to detect indeposits affected by diagenesis if detrital quartzis absent. When working on cores, the presence oflarge-scale grainflow or grainfall stratifications arenot reliable criteria for interpreting an aeolianorigin, as both occur also in sub-aqueous envir-onments.Aeolianites form huge sand bodies extending

over very long distances and are subject to earlyactive diagenetic processes that lead to partialcementation, which prevents burial compaction,favouring the development of moldic or vuggyporosity. Furthermore, during transgressivephases, submerged aeolianites form continuoushardgrounds on which bioherms may develop.For these reasons, aeolianites may constitutepotential reservoir rocks, and an ability to distin-guish them from sub-aqueous carbonates in thinsections and cores using unequivocal criteria isneeded. To that end, the aim of this paper is tocompare the petrographic and textural evolutionof recent sediments collected from shoreface tobackshore on the northern coast of Chrissi Island,Crete, and seek discriminant criteria.

CHRISSI ISLAND, CRETE

Geological setting

Chrissi Island is located in the Libyan Sea, about25 km south of Crete (Fig. 1). It is a topographic-ally low island and partly covered by an activedune field. It is bordered by a shallow carbonateplatform deepening gently towards the north. Thesemiarid conditions (precipitation <210 mmyear)1) and the distance separating the study areafrom Crete limit terrigenous inputs. Seawatersalinity varies from 38 to 39Æ5& and watertemperatures at the surface average ca 15� C inwinter and 25�C in summer. Prevailing windsblow from north to south. All of these conditionspromote a high production of carbonate particles,which are transported shorewards during stormevents and feed an active dune field in thenortheastern part of the island. These activedunes reach 12 m in height and lie on a poorlylithified aeolian substrate, which has an age ofbetween 2710 and 2315 radiocarbon years BP.The dunes are vegetated mainly by Juniperusoxycedrus and Pinus brutia. This vegetationcovers around 60–75% of the dunes, and can

effectively trap particles blown by the wind,stabilize the sand, and limit erosion. The dunesexhibit hummocky topography, with smoothstoss slopes and abrupt lee slopes. Erosionalblowouts showing marginal erosional walls arepresent behind the dune ridge, but their diameteris limited by the presence of the vegetation. Nodepositional lobe has been observed downwind.

Methods

Thirty-six box-cores were taken from shoreface tobackshore environments (Fig. 1). The deviceallowed the investigation of sands down to40 cm below the sediment surface and the pre-servation of millimetre-scale sedimentologicalfeatures. Sixty-six oriented sub-samples collected

Fig. 1. Location of Chrissi Island, south of Crete. Theline is the profile described in Fig. 2, and the numbersrefer to handcores plotted along the profile. Letters Aand B refer to the photographs shown in Fig. 3. Contourinterval is 2 m.

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from box-cores were dried, epoxy-impregnatedand made into large-scale thin sections(4 · 5 cm). Quantitative petrographic analysiswas carried out on thin sections and soft samplesby point counting. Grain-size and particle-orien-tation measurements were made using an imageanalysis software (Visilog 5.3, Noesis SA, France)coupled with a motorized microscopic stage.

Results

All the samples collected are pure grainstonescomposed of peloids, red algae, epiphytic foram-inifers (mainly Peneroplis planatus and Amphis-tegina lessonii), planktonic foraminifers(Globigerinoides trilobus) and skeletal grains.There are no evident facies changes or clearfaunal zonation between shoreface and aeoliandeposits (Figs 2 and 3A,B). Well-preserved plank-tonic foraminifers are present in aeolian dunesand most epiphytic foraminifers are better pre-served in dunes than in the shoreface, where theyare often intensively abraded and broken. Quan-titative petrographic studies, based on the point-counting method and image analysis (grain-size,porosity, orientation of the particles) suggest that:(1) the Shannon–Weaver heterogeneity index(Shannon & Weaver, 1949) computed for themain foraminifer species is slightly higher inaeolian dunes than in shoreface sediments; (2) theprimary porosity is slightly higher in shorefacesediments; and (3) the grain size of the aeoliandeposits is coarser than that of the upper shore-face area.Variance analysis and Scheffe’s test (Scheffe,

1953; Table 1) were carried out on the same dataset, which was subdivided into three majorgroups, according to the depositional environ-ment: group 1 represents the shoreface and thelower foreshore area, group 2 accounts for thebeach environment, and group 3 integrates datafrom the backshore area and the nebkhas.These statistical tests reveal that porosity is not

indicative of the depositional environment, or ofsampling depth down the box-core. Furthermore,they indicate that some of the main foraminiferpopulations are indicative of the depositionalenvironment (e.g. Elphidium), but cannot be usedas a discriminating criterion. Lastly, they showthat the faunal diversity (measured with theShannon–Weaver index) is significantly higherin aeolian deposits. This can be explained by themixing and concentration of species derived fromdistinct ecological environments. Discriminatingbetween shoreface and backshore deposits based

on this index is therefore possible but only inrelative term.These observations demonstrate that aeolia-

nites are very difficult to distinguish from theassociated sub-tidal deposits through petro-graphic analysis. The small differences men-tioned above, although statistically significant,cannot be used as reliable discriminant criteria.The presence of well-preserved and diversemicrofauna (including planktonic foraminifers)confirms that these deposits could very easily bemisinterpreted as marine deposits in the fossilrecord.Friedman (1961) and Valia & Cameron (1977)

suggest that a plot of mean grain size versusskewness ‘reflects the environment of depositionand is not altered by the mineralogy of the sand’.Grain-size measurements were made by imageanalysis, and the data obtained were subdividedinto quarter-phi size intervals. Using the methoddescribed by Friedman (1961, 1967, 1979), statis-tical measures were computed (Table 2), andplotted (Fig. 4). The scatter diagrams obtaineddo not allow for distinction between shoreface,beach and backshore deposits.Contrary to siliciclastic environments, carbon-

ate aeolian sands cannot be distinguished fromcarbonate sub-tidal sands on the basis of grainsize. This may be explained by the fact thatparticle-size distribution relates to particle beha-viour during transport. Sorting of carbonategrains is governed by grain shape and size,elongation and internal porosity, whereas sortingof homogeneous siliciclastic particles is primarilydependent on grain size.The few petrographic differences listed above

are applicable to studies in which comparisonsamong sediments from different environments arepossible. The data are thus comparative. Bycontrast, this method cannot be applied whendealing with carbonate deposits of unknownorigin. For aeolianite identification, sedimento-logists need absolute descriptive criteria thatare only diagnostic of aeolian sedimentologicalfabrics.Careful examination of grain arrangement, how-

ever, reveals a subtle textural difference betweenaeolian and sub-tidal sands. Sedimentary parti-cles lie frequently in sub-vertical position inaeolian sediments, whereas most particles restmore or less horizontally in sub-aqueous sedi-ments (Fig. 3A,B). This observation has beenconfirmed by measurements of particle orienta-tion in thin sections with image analysis. Bycalculating the width/length ratio (minimum

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Fig. 2. Profile from the lower shoreface to the backshore environment of Chrissi Island. Rose diagrams show grainorientation (bin size of 20�). Counting and measurements were performed by image analysis. Numbers of analysedparticles used for the rose diagrams: no. 2: n ¼ 1134; no. 4: n ¼ 787; no. 7: n ¼ 1441; no. 8: n ¼ 1177; no. 9:n ¼ 1317; no. 11: n ¼ 1032; no. 13: n ¼ 1159; no. 16: n ¼ 1140. The Shannon–Weaver heterogeneity index (H) iscomputed from H ¼ )

Ppi log pi, where pi is the proportion of the total number of individuals occurring in species i

(pi ¼ ni/N, ni being the number of individuals for each species and N the total number of individuals). Stacked-areasplot represents the population of the main foraminifers. Main species, such as Peneroplis, Amphistegina, Cibicides,Ophtalmidium, Elphidium, Sorites, Globigerinae, and Textulariidae are presented separately; less frequent ones aregrouped together according to the nature (H: hyaline; P: porcelaneous) and the form (LT: low trochospiral; T:trochospiral; HT: high trochospiral; Tu: tubular; S: spherical) of their test. Scale is in per cent.

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versus maximum Feret’s diameters) for eachparticle and eliminating those having a ratiolower than 0Æ5, only elongated particles wereconsidered. Particles with an apparent diameter<62 lm were removed from the data set, in orderto avoid noise from the pre-measurement imageprocessing.A rose diagram based on thousands of meas-

urements per sample confirms that there is asignificant difference between aeolian depositsand shoreface sands in grain orientation (Fig. 2).Aeolian deposits are characterized by a bimodaldistribution because of the presence of numerousvertical particles, whereas shoreface and fore-shore deposits present a unimodal distribution.Orientation of carbonate particles could, there-

fore, be a clue for recognizing sands of aeolianorigin. This hypothesis has been validated for

Pleistocene marine terraces from Western Austra-lia and Eastern Tunisia, in which well-preservedsedimentary structures allow the distinction ofaeolian from foreshore and shoreface deposits.

PLEISTOCENE OUTCROPS

Hergla, Tunisia

The Tyrrhenian formations of the eastern coast ofTunisia, near the town of Sousse, have beenstudied by Mahmoudi (1986), and the sectiondescribed in this paper has been studied byJedoui et al. (1987). The outcrop described hereis located ca 3 km south of the village of Herglaand shows two stacked regressive sequences(Fig. 5).

Fig. 3. Petrographic features. Samples top orientated towards top of page. (A) and (B) are from Chrissi Island (seeFig. 1 for location). (A) is from a sub-tidal environment (offshore distance is 170 m, depth is 3 m below mean sealevel), and (B) is from an aeolian dune (inland distance is 42 m, altitude is 2 m above mean sea level). Sample C isfrom the basal shoreface unit observed in the outcrop of Hergla (see Fig. 5). Sample D is from the Pleistocene aeolianunit III from Dirk Hartog Island (outcrop of Surf Point; see Fig. 6). For all these samples, the orientation of elongatedparticles is well-distributed. In aeolian deposits, particle orientation is largely vertical, whereas samples from sub-tidal depositional environments exhibit mostly horizontal particle orientation.

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This outcrop is ca 9 m high, and starts with aquartz-rich sandstone (Fig. 3C) showing cross-stratified laminations and herringbones. This firstfacies, which is scarce in fauna but containscoproliths and rare ooids, is interpreted as ashoreface deposit. The overlying facies is a lam-inated beach sandstone that shows crab burrowsand keystone vugs near the top. Highly biotur-bated aeolian deposits with rhizolites and helixshells overlie the beach facies. This discontinu-ous unit contains ooids, and abundant quartzgrains.A bioclastic sandstone rich in Cardium, Glycy-

meris andStrombus shells forms thefirst unit of thesecond stratigraphic sequence. The presence of

swaley cross-stratification indicates a shallowstorm-dominated environment. Beachrock slabsreveal the proximity of the palaeo-shoreline. Theuppermost unit is a fine ooid-bearing aeolianlimestone, exhibiting large tabular and angularcross-strata that are locally bioturbated by ancientvegetation. This facies is capped by a thin concre-tionary calcrete. The carbonate fraction increasesupwards in the section.The shoreface deposits (Fig. 3C) exhibit a flat

rose diagram (Fig. 5), similar to the ones observedin the present-day sub-tidal depositional envir-onments on Chrissi Island.The two aeolian units exhibit two distinctive

rose diagrams (Fig. 5). The first aeolian level (3 mabove present-day sea level) is highly bioturbated.Orientations are distributed asymmetrically, themajority between 0� and 40�. A vertical orienta-tion is nearly non-existent. The second andthicker aeolianite, which is also richer in carbon-ate particles, shows a rose diagram similar tothose observed in the modern dune system ofChrissi Island (i.e. a clear bimodal distribution ofthe grain orientation).

Dirk Hartog Island, Shark Bay, WesternAustralia

Shark Bay is a shallow embayment located in thesouthern Carnarvon Basin, on the western coast of

Table 1. Variance analysis and Scheffe’s test performed on the data set from Chrissi Island samples.

VariablesSub-tidal (G1)(mean)

Beach (G2)(mean)

Dune (G3)(mean) d.f. F-test P-value

Scheffe’s test –differencessignificant for:

Porosity (%) 33Æ89 31Æ02 33Æ50 2 0Æ898 0Æ4312Grain size (mm) 0Æ20 0Æ20 0Æ20 2 0Æ061 0Æ9407Elphidium 1Æ65 2Æ74 5Æ84 2 17Æ702 0Æ0002 G1 < G3; G2 < G3Cibicides 3Æ62 5Æ58 5Æ86 2 3Æ608 0Æ0567HLT 3Æ27 3Æ44 5Æ17 2 0Æ695 0Æ5167HT 0Æ79 0Æ45 0Æ64 2 0Æ173 0Æ8429Peneroplis 37Æ43 44Æ15 31Æ33 2 0Æ609 0Æ5585Amphistegina 21Æ36 10Æ38 5Æ57 2 0Æ930 0Æ4193Ophtalmidium 0Æ90 0Æ39 0Æ15 2 3Æ139 0Æ0772Miliolida 12Æ45 11Æ55 11Æ60 2 0Æ110 0Æ8966HHT 5Æ21 7Æ09 9Æ41 2 1Æ432 0Æ2741PTu 7Æ87 8Æ81 12Æ16 2 3Æ772 0Æ0511Globigerinae 2Æ04 0Æ89 7Æ56 2 3Æ000 0Æ8485PS 0Æ68 0Æ62 1Æ82 2 4Æ401 0Æ0347 G1 < G3Sorites 2Æ59 3Æ49 2Æ43 2 0Æ169 0Æ8461Textulariidae 0Æ14 0Æ22 0Æ46 2 1Æ568 0Æ2454Shannon Index 1Æ59 1Æ86 2Æ06 2 4Æ076 0Æ0422 G1 < G3

The data are subdivided into three major groups: group 1 represents the shoreface and the lower foreshore area(n ¼ 7); group 2 is the beach environment (n ¼ 2); group 3 combines data from the backshore area and the nebkhas(n ¼ 8). Marked effect for P < 0Æ05.

Table 2. Formulae used for statistical measures (fromFriedman, 1961, 1967, 1979).

Statisticalmeasure Symbol Formula

Mean �x/ 1/100P

fm/

Standarddeviation

r (P

f(m/ � �x/Þ2=100Þ1=2

Skewness a3 1/100r)3P

f(m/ � �x/)3

Mean-cubeddeviation

a3r3 1/100

Pf(m/ � �x/Þ3

fm/: frequency of each grain size class (expressed as /unit).

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Australia. The main geomorphological featuresare two peninsulas (Edel Land Peninsula to thewest and Peron Peninsula in the middle of thebay) and Dirk Hartog Island, an elongated island60 km long. Shark Bay is a ‘[…] terrain of dunesand interdune depressions that has been floodedby the postglacial (or Holocene) transgressionduring the last 10,000 years’ (Logan et al., 1970).Playford (1990) suggests that Pleistocene aeo-

lian limestone (Tamala Limestone) accumulatedduring a lowstand period ‘as enormous dunes[…], when the area was subjected to extremelystrong southerly winds’. In contrast, Logan et al.(1970) consider that the Tamala aeolianite wasprobably formed during a transgressive episode.

Present-day dunes are still active and aresubjected to southerly prevailing winds. Thetransported sand is mainly composed of carbon-ate grains of biogenic origin (fragments of shells,red algae, sea urchin spines and foraminifers).The active dune system is not discussed in thispaper.The outcrop described here belongs to the

Tamala Limestone, and is located south of DirkHartog Island, close to Surf Point (Fig. 6). Cliffsapproximately up to 40 m high form the coast andshow three main aeolian limestone units com-posed of skeletal grainstones. The basal aeolianunit (unit 1) is 10 m thick. It is slightly biotur-bated and mainly composed of quartz, porcela-neous and Elphidium-like foraminifers, and redalgae (Fig. 3D). It is overlain by a 4 m thickaeolian horizon (unit 2), which exhibits largecross-stratified strata with vadose pisoids and abioturbated layer at its top.The upper aeolian unit (unit 3) is up to 30 m

thick and comprises stacked dunes (1–4 m thick)separated by reactivation surfaces. Large-scalecross-stratifications show a wide range of dips(from 2� to 20�) and flow directions (from 118� to347�). This variety suggests that this unit hasrecorded several phases of aeolian activity andthat different parts of the palaeodunes have beenpreserved.Unit 1 shows a clear bimodal distribution of

grain orientation very similar to that of the present-day backshore deposits of Chrissi Island. For unit2, the distribution is identical, but vertical grainsare less abundant in the bioturbated, pisoid-richhorizon near its upper part. Unit 3 is bioturbated atits top and exhibits large-scale cross-laminationsand sandsheets. The distribution of grain orienta-tion is either bimodal or has concentrations ofparticles sloping nearly 45� (or its mirror equival-ent 135�), especially within the upper 6 m of theoutcrop. This asymmetrical distribution seems tobe closely related to the type of laminations anddiffers from that observed on Chrissi Island. Theseobservations indicate that subvertical particles arenot systematically present in aeolianite, and thatgrain orientation may vary as a function of theposition along the palaeodune.

DISCUSSION

Depositional or post-depositional processescould explain the presence of vertical grains.Depositional model implies that ellipsoidal par-ticles tend to settle more or less vertically in

Fig. 4. Plot of (A): third moment (skewness) andstandard deviation of the particle grain-size, and (B):third moment about the mean of the distribution(mean-cubed deviation) and standard deviation, usingphi (U) scale. Data are based on 62 thin sections andhave been grouped into three depositional environ-ments: shoreface [23 samples; mean number of parti-cles per thin section (l) ¼ 7743 particles], beach (sevensamples; l ¼ 7084), and dune (32 samples; l ¼ 7024)(see Friedman, 1961, 1967, 1979, for the method ofcalculation).

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unstable equilibrium. This situation is probablyattainable for very low-density particles, in low-gravity fields or when the density and theviscosity of the surrounding fluid is high. Noneof these conditions are realized in air withcarbonate grains. The lack of space betweenparticles in grainflows could also impede someof them from settling in a static stable position.If this were true, vertical particles should berestricted to grainflow deposits and should notoccur in the translatent-climbing-ripple facies orin the grainfall facies as observed in present-dayand Pleistocene aeolianites.Post-depositional processes related to percola-

tion of meteoric waters explain, in a more satisfac-tory way, the presence of vertical grains. Afterdeposition by wind and during periods of rainfall,particles of aeolian accumulations can rotateunderthe action of the meteoric water percolating down-wards through the pore system. This action wouldespecially affect elongated particles having a lowwidth/length ratio, such as Peneroplis planatusand shell fragments. The loose packing of recentaeolian deposits permits such movements.During heavy rainfall periods, some aeolian

sand layers could be saturated with water, cre-

ating temporary phreatic conditions. Particleswith a significant intra-skeletal porosity maycontain a large volume of air and could bepreferentially rotated as the air attempts to escapeupwards. This effect could reinforce the rotationtowards verticality that was initiated by thepercolating water. This process is confirmed bythe fact that foraminifers such as Peneroplisplanatus and some Miliolidae that are verticalcommonly show their largest chamber (the lastone) orientated towards the top (Fig. 3B).The rotation of particles could also be due to air

bubbles moving upwards through the pore net-work during heavy rainfalls, a process which wasevoked by Bain & Kindler (1994) to explain theoccurrence of irregular fenestrae in BahamianPleistocene aeolianites. The presence of aeolianhorizons with no detectable vertical particles maybe explained by arid conditions prevailing duringdeposition, or by sediment compaction that pre-vents particle rotation.The genetic hypothesis proposed above has

been confirmed by laboratory experiment. Twoconditions are required for re-orientating sedi-mentary particles: a very loose sand packing asobserved on the stoss-side of aeolian dunes

Fig. 5. Location of the village of Hergla, Tunisia. Letter C refers to the thin-section shown in Fig. 3. See text for theoutcrop description.

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(grainfall and grainflow), and the occurrence ofheavy rainfalls rapidly after deposition. Theseconditions are probably not realized in the case of

shoreface sands exposed to sub-aerial processesafter a eustatic sea-level drop. The loose sub-tidalsands may be reworked by wind as new aeolian

Fig. 6. Location and schematic view of the profile studied on Dirk Hartog Island, Shark Bay, Western Australia. Rosediagrams are based on n measurements per thin section.

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dunes, and those resistant to deflation are prob-ably not loose enough to be reorganized bypercolating meteoric water.

EXPERIMENTS

In order to test if the percolation of meteoricwaters through carbonate sand mainly composedof bioclasts has a direct effect on the internalframework of the sediment, a simple laboratoryexperiment was conducted.By decantation in air, two small tubes (5 cm in

height and 1Æ6 cm inner diameter) were filledwith sand from Chrissi Island. The bottom of onetube was riddled with small holes, and the tubewas placed under a tap delivering an outflow of0Æ34 l h)1. The outflow was set to deliver smalldrops, landing on a cotton wool protection inorder to prevent impact disturbance. This experi-ment was run for 3 h.Images of the internal framework of both sam-

ples were obtained by X-ray microtomographyusing a Skyscan device (Skyscan 1072, Skyscan,Aartselaar, Belgium). The limit of detection wasof 20 lm, based on the source spot size and ageometric magnification (16 times). Following 3Dmicrotomographical reconstruction based on theX-ray images, particle orientations for each sam-ple were measured by image analysis of fourvertical cross-sections separated by a 45� angle.Comparison of rose diagrams built using theseresults shows a higher proportion of verticalgrains in samples through which water percolated(Fig. 7). This clearly demonstrates that the perco-lation of meteoric water induces textural modifi-

cations at the surface of aeolian deposits, inparticular, a rotation of grains to new verticalorientations. Whether this process is able to affectthe depositional grain fabric several meters belowthe sediment surface remains unresolved and anissue that should be addressed in future experi-ments.

CONCLUSIONS

This study of recent sediments on Chrissi Islandconfirms that carbonate aeolian sands can havethe same petrographic and granulometric signa-ture as associated sub-tidal sands. Thus, aeolia-nites may be indistinguishable from sub-tidalmarine deposits in the fossil record when macro-scopic sedimentary structures such as very large-scale cross-beddings or pinstripe laminations arenot observed. The lack of these macroscopicstructures is often the case in pure bioclasticcarbonate rocks. The presence of an importantpopulation of vertical particles detected in pre-sent-day and Pleistocene aeolianites, however,provides a new criterion of subaerial exposure fordifferentiating aeolian from marine grainstones.As demonstrated by a laboratory experiment,

this textural fabric is caused by the percolation ofmeteoric water through very loose dry sands. Thisprocess is not expected to occur in sub-tidalsands exposed to sub-aerial conditions after adrop of the sea level. The reason is that theserelict deposits, if loose enough, will be reworkedas new regressive aeolian dunes, whereas sandsremaining in place, characterized by a tighterpacking or affected by early cementation, shouldnot be reorganized by the action of percolatingmeteoric water.This new criterion of sub-aerial exposure is

easily applied to thin section or core analysis ofancient deposits. Furthermore, it may be possiblethat this criterion can be used, in conjunctionwith other independent proxies of climate con-ditions, as an indicator of the frequency, intensityand distribution of rainfalls.

ACKNOWLEDGEMENTS

This research, part of the first author’s PhD thesis,was supported by the Swiss National Sciencefoundation (grant 2000-055599.98). The first draftof this manuscript was reviewed by Isabel Mont-anez, Gerald M. Friedman and David B. Loopewho proposed very helpful comments and sug-

Fig. 7. Rose diagrams showing the grain orientation ofexperimental results. Figure on left shows tube filledwith dry sand, whereas figure on right shows tube filledwith dry sand through which water was percolated for3 h (outflow was of 0Æ34 l h)1). The rose diagram on theright shows a higher proportion of vertical grains thanthat the one on the left. Bin size is 10�.

Recognition of ancient carbonate wind deposits 925

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gestions. We thank Milan Beres for his earlierreview of the English, which has greatlyimproved its readability.

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926 P. Le Guern and E. Davaud

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