Facies (2012) 58:509–522

DOI 10.1007/s10347-012-0298-0

ORIGINAL ARTICLE

Anatomy, internal heterogeneities, and early fracture network of a Pleistocene carbonate coastal dune (Rejiche Formation, southeastern Tunisia)

Claude-Alain Hasler · Gregory Frébourg · Eric Davaud

Received: 16 May 2011 / Accepted: 13 January 2012 / Published online: 19 February 2012© Springer-Verlag 2012

Abstract Although eolian deposits are known to recordthe dominant winds, secondary conditions such as windreversals during wintertime can also be observed in the pet-rographic composition and facies succession. Thus, eoliandeposits are used here as a local paleoclimatic proxy. Thespatial distribution of the depositional facies, early diage-netic imprints, and early fracture network of a coastal Pleis-tocene eolian ridge in southeastern Tunisia is describedusing a small-scale GIS model. Facies analysis indicatesthat coastal dune systems record seasonal cycles. The frac-ture density and directions are strongly inXuenced by thedepositional facies type. Laminated facies present a higherfracture density compared to more homogeneous facies andshow only one major fracture direction, while the morehomogeneous facies display a bimodal distribution. Such adiVerence between these two groups is explained by theheterogeneous distribution of the early calcite cementwithin the laminated facies. No tectonic activity or overly-ing strata have aVected the Pleistocene dunes under study.Therefore, the mechanism responsible for the fractures

could only be related to the own weight of the eolianite andto its internal or underlying lithologic heterogeneity.

Keywords Pleistocene · Eolianite · Seasonal model · Early diagenesis · Fracture network

Introduction

Eolianites are deWned as sedimentary accumulations ofwind-driven carbonate-dominated sand (Abegg et al. 2001;Sayles 1931). Carbonate eolianites can be found in inlanddepositional settings (Abegg and Robertson Handford2001; Goudie and Sperling 1977) but most of them arelocated in coastal environments (Abegg et al. 2001; Abeggand Robertson Handford 2001) as conWrmed by Quaternaryeolianites distributed along coastlines between latitudes50°N and 45°S (Brooke 2001 and references therein).Eolianites commonly form very extended sedimentary bod-ies, e.g., in Australia (Darwin 1851) or Morocco (Aberkan1989). Around the Mediterranean Sea, Quaternary eolia-nites are characterized by porous lithologies and are eco-nomically important as they form signiWcant aquifers(Tsoar 2000).

Although recent studies have shown that eolianites couldrepresent potential reservoir rocks in older formations(Frébourg et al. 2010a), carbonate eolian deposits are still veryseldom identiWed in pre-Quaternary sedimentary sequences(Fairbridge and Johnson 1978). This paradox led the latterauthors to consider that this facies is genetically related torapid glacio-eustatic Xuctuations of the sea level. However,the recent discoveries of Mesozoic (Kilibarda and Loope1997; Kindler and Davaud 2001) and Upper Permian(Frébourg et al. 2010a) eolianites demonstrate that this typeof deposit is more frequent than previously thought in the

C.-A. Hasler (&)Carbonate Research, Projects and Technology, Shell Global Solutions, Kessler Park 1, 2288 GS Rijswijk, The Netherlandse-mail: claude-alain.hasler@shell.com

G. FrébourgBureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, TX 78713-8924, USAe-mail: gregory.frebourg@beg.utexas.edu

E. DavaudEarth and Environmental Sciences, Department of Geology and Paleontology, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerlande-mail: eric.davaud@unige.ch

123

510 Facies (2012) 58:509–522

geological record and not necessarily related to ice ages. Formany authors, the scarcity of eolianites in the pre-Quaternaryrecord is due to the great diYculty in recognizing wind-blown carbonate deposits and in diVerentiating between themand other carbonate sands of nearshore environments (Fré-bourg et al. 2008, 2010b; Le Guern and Davaud 2005;McKee and Ward 1983; Rice and Loope 1991; White andCurran 1998). When large-scale sedimentary structures aremissing (which is often the case in ancient carbonate rocks)or cannot be observed (subsurface data), climbing translatentripples (pinstripe laminations) seem to be the only unequivo-cal petrographic criterion for discriminating eolian deposits(Frébourg et al. 2008, 2010b; Loope and Abegg 2001).Unfortunately, bioclastic particles often have an intraskeletalporosity that modiWes the original calcite or aragonite bulkdensity as well as their hydrodynamic and aerodynamicbehavior (Ginsburg 2005; Jorry et al. 2006) and, as a conse-quence, induce the deposition of heterogeneous assemblages.The presence of such assemblages may explain the rarity ofhigh-energy sedimentary structures such as pinstripes lami-nation in bioclastic pre-Quaternary eolianites. The absence ofrecurring and reliable petrographic criteria requires the use ofa combination of converging sedimentologic, stratigraphic,and diagenetic clues (Frébourg et al. 2008, 2010b; Loope andAbegg 2001). In the present study, the depositional faciesand the early diagenesis (cements and fractures) aredescribed and their interrelations are quantiWed.

Eolianites associated with alluvial or shallow-waterdeposits record the climatic and landscape evolution ofcoastlines through time. For instance, aridity favors wind-driven deposits and low-stand cliV-front dunes may help inpreserving pre-existing morphologies (Fornós et al. 2009).Numerous studies on Pleistocene eolianites have alsohelped to better understand and constrain the sea-level evo-lution curves during the last interglacial periods (i.e.,Hearty and Kindler 1995; Murray-Wallace 2002). At diVer-ent time and spatial scales, wind deposits also recordregional climatic conditions related to yearly trends. In theexample described here, from Tunisia, the seasonal cyclesrecorded in the coastal eolianite are described and com-pared to the present-day conditions.

Fracturing is the norm in carbonate reservoirs and maydevelop very early networks even when active tectonicactivity is absent (Guidry et al. 2007). Early fractures andfaults play an important role in the diagenetic sequence byacting as facilitators for dissolution of carbonate materialand precipitation of cements during the mineralogical sta-bilization process (Guidry et al. 2007; Kosa et al. 2003;Playford 1980; Warren 2000). Since each process in a dia-genetic sequence will aVect the following ones, the inXu-ence of the primary depositional facies on the Wrstdiagenetic events is the key to understanding and predictingthe petrophysical properties of carbonate rocks.

Geological setting

Pleistocene deposits from southeastern Tunisia can be sub-divided into two units (Jedoui 2000): a siliciclastic unit atthe base (quartz-rich unit), overlain by ooid- and peloid-rich deposits (carbonate unit). Both of these units showshallowing-upward cycles, from shallow-water marine atthe base, to wind-driven facies at the top. The carbonateunit is stratigraphically correlative to the Rejiche Formation(Jedoui 2000; Mahmoudi 1986, 1988). The base of thePleistocene shows an unconformity with the quartz-richunit lying directly over continental, poorly cemented,Mio-Pliocene units.

Along the Mediterranean coast, in the el Bibane lagoonarea, this formation builds a coastal belt, over 40 km longand several hundred meters wide, restricting access to theopen sea from the shallow-water lagoon (Jedoui and Bou-aziz 1997; Fig. 1). The quarry of Slob el Garbi within thecarbonate unit is located in the western part of this coastalbelt, about 50 km southeast from the Jerba Island (Fig. 1a).In Tunisia, and all over the Mediterranean Sea, the eolia-nites have been used to cut building stones since Romantimes.

Southeastern Tunisia has not been tectonically activeduring the last 130,000 years (Jedoui et al. 2003; PaskoVand Sanlaville 1983) and the relative sea-level changescould not be related to vertical tectonic movements(Bouaziz 1995). Thus, the carbonate unit must be associatedwith a eustatic sea-level Xuctuation (Jedoui et al. 1996) andhas been correlated using U-series analysis to the highstandof the last interglacial period (MIS 5e, Jedoui et al. 2003).

Dataset and methods

Sampling and basic analysis

The Slob el Garbi quarry has ideal exposures of the eoliandeposits that reveal numerous vertical sections (both per-pendicular and parallel to the elongation of the coastal belt)and one horizontal transect. Since classic sampling using ahammer is barely feasible on vertical walls and Xatsurfaces, small one-inch-diameter plugs were drilled. Thin-section-based, statistically signiWcant petrographic mea-surements were made for each of the depositional facies.The petrographic content of the latter has been describedusing image analysis software (Roduit 2005, 2007) onhigh-resolution scans of the thin-sections, using both nor-mal and polarized light. The plugging has been performedalong vertical sections in order to assess changes in the dia-genetic features from the base towards the top, as well as onthe horizontal transect so as to consider their lateral varia-tions. The facies sequence analysis performed on the Slob

123

Facies (2012) 58:509–522 511

el Garbi eolianite has been analyzed with the help of Strati-graphic Signal Analysis software (Ndiaye 2008) using themethod proposed by Harper (1984). Plug porosity was mea-sured using mercury (bulk volume) and acetone immersion(grain volume) using the modiWed method of the RussellPorosity Apparatus (Gealy 1929), while a PMI automatedgas permeameter was used to obtain the corresponding per-meabilities.

Geographic information system (GIS)

The exceptional horizontal surface of the quarry allowedthe precise mapping of the disconformities and the distribu-tion of the depositional facies within this fossil dune belt. Ageographic information system (GIS) was built using ESRIArcGIS 9.3 suite, on the basis of numerous pictures of thehorizontal transect of the quarry (Fig. 2). The oblique viewsof the pictures were transformed into a vertical and ortho-normal projection (Fig. 2a, b). This speciWc projection,which preserved distance and surface, enabled us to per-form not only qualitative but also quantitative analyses.The distribution of microfacies, the disconformities and thefracture network were all mapped.

Depositional setting

Depositional facies

The eolian deposits from the Slob el Garbi quarry can besubdivided into Wve facies according to their own deposi-tional process. All depositional facies are illustrated withpictures taken of the horizontal transect of the quarry(Fig. 3), where horizontal views allow a better observationof the sedimentary structures by stretching the millimeter-scale laminations (Loope and Abegg 2001).

Pinstripe lamination (Fryberger and Schenk 1988;White and Curran 1988) is extremely well preserved and is

underlined by sharp, millimeter-scale, cemented layers(Fig. 3a). The lamination records wind-ripples (Clemmen-sen and Abrahamsen 1983; Fryberger and Schenk 1981;Hunter 1977, 1981, 1993; Loucks and Ward 2001) andshows distinctive inverse grading (Fig. 4a), which reXectssubcritically climbing translatent strata. At the outcropscale, the laminae strike mainly parallel to the elongation ofthe coastal belt (in this case study, along the W–E axis) orperpendicular to the direction of migration of the dunes(i.e., towards the south).

GrainXow cross-stratiWcation, or sandXow cross-strati-Wcation (Abegg and Robertson Handford 2001; Clemmen-sen and Abrahamsen 1983; Fryberger and Schenk 1981;Hunter 1977, 1981) are common and well preserved in theeolianite of the Slob el Garbi (Fig. 3b). They are generatedby the avalanche of a cone-shaped mass of non-cohesivesand down the foreset of the slip faces of the dune (Loucksand Ward 2001). Commonly, a Wning-upward texture canbe observed, but generally, the grainXow is structureless,well sorted, and loosely packed (Fig. 4c). Basal contacts ofgrainXow deposits can locally truncate underlying strata,showing well-developed concave-up scours (Fig. 3b). Theavalanches show typical lenses with concave-up bases andconvex-up tops. The occurrence of grainXow depositsincreases towards the base of the slip face.

Grainfall lamination is more diVuse in appearance com-pared to pinstripe lamination (Fig. 4b). The laminaedevelop on the leeward side of the dunes, where Xow sepa-ration occurs (Clemmensen and Abrahamsen 1983; Fryber-ger and Schenk 1981; Hunter 1977, 1981; Loucks andWard 2001). They are associated with grainXow scars high-lighted by concave-up bases (Fig. 3c). These scars are notcompletely Wlled by grainfall deposits and unconnectedgrainXow inWlls may occur indicating that grainfall lamina-tion occurs mainly on the upper part of the lee-side of thedune.

Fadeout lamination was Wrst described by McKee et al.(1971) both in the Weld and in laboratory settings. These

Fig. 1 Geologic map of the el Bibane area and outcrop location (simpliWed after Jedoui 2000). Note the Rejiche Formation (plain dark grey),which forms the barrier between the Bahiret el Bibane and the Mediterranean Sea

n

ab

123

512 Facies (2012) 58:509–522

authors underlined that fadeout lamination occurs in drysand but not in cohesive wet sand. In our case study, fade-out lamination shows homogeneous decimeter-scale layers,in which lamination (mainly pinstripe) disappears laterally(Fig. 3d). Fadeout lamination also occurs due to changes inthe wind direction (Loope, pers. comm.). During windreversals, dune crests are strongly eroded and mounds ofwind-ripple laminae build up on the stoss surface of thelarge dune, in the lee of the highest points on the dune crest.These mounds build upward to the point that they developsmall (30–40 cm) slip faces. The pin-stripe laminae fadeout because as soon as the ripples get into the leeward sideof the mound, they are reworked into grainXows. When thenew, short slip-faces become established, the wind-ripplelaminae continue to feed the grainXows and the pin-stripesslowly prograde over the migrating slip face. GrainXows donot move far enough to undergo much sorting. When thewinds return to the dominant direction, portions of thesepackages may get preserved if the dune is growing verti-cally. The microfacies corresponding to the fadeout lamina-tion is homogeneous and often coarser than the otherdepositional facies (Fig. 4d).

Nebkha consist of lenticular bodies, characterized byintensely rhizoturbated sediment. This facies is by far therarest in the studied quarry. The plants stabilize the dunes,but dense root networks can completely destroy the original

sedimentary structures. These ichnofacies are not associ-ated with paleosoil in the quarry. Root traces, root moldsshowing uncemented tubular holes (Klappa 1980), and leafmolds can be found scattered in the quarry. Petrophysicalmeasurements were not performed on this facies, due to itsheterogeneity and excessively high permeability values.

Petrographic composition

All depositional facies consist of well-sorted grainstones.Their petrographic composition is mostly similar, with onlysmall relative abundance variations observed. Pellets arethe major sand constituent (over 30%). Coated grains areabundant, with well-developed ooids or with quartz grainsshowing only one or two cortical layers. Small, roundedbioclasts as well as foraminifera and foraminifera frag-ments (mainly miliolids) are present. Table 1 summarizesthe average petrographic composition for all depositionalfacies. The relative allochems content appears to be con-stant between facies and clearly indicates a marine originfor the sediment source, although the observation of“clean” quartz grains, with no evidence of marine alteration(ooid cortices) should be noted. The ratio between the“clean” quartz and the quartz showing marine alteration isnot constant: for the grainfall, grainXow, and pinstripefacies the ratio is close to 0.5 but for the fadeout facies, it

Fig. 2 Schematic representation of the geographic information sys-tem (GIS) built for this study. a View of the Slob el Garbi quarry. Noteperson for scale (black ellipse). Pictures of the horizontal layer of thequarry have been transformed into a vertical and orthonormal projec-tion (b) and merged in order to reconstruct the major horizontal sur-face of the quarry (c photomontage). Sedimentary structures have

been interpreted in terms of depositional facies (c facies). Pinstripelamination is highlighted in blue, grainfall and grainXow stratiWcationin green, and fadeout lamination in orange. Major disconformities areplotted in red and minor disconformities or reactivation surfaces inblack (c surfaces). The fracture network (c fractures) has also beenmapped and integrated to the GIS

a

b

c

123

Facies (2012) 58:509–522 513

reaches almost 0.9. The vegetal activity aVects all faciesand therefore does not possess its own petrographic signa-ture. Since, in addition, no petrophysical measurements areavailable for the nepkha facies (see previous section), thisfacies is not described in the table.

Unconformities

Eolian deposits present numerous surfaces showing angu-lar unconformities (Loucks and Ward 2001). When thedunes migrate, for instance, the lee-side lamination down-laps on the substrate. Reactivation surfaces are alsoformed whenever the wind regime increases or shifts.

Facies overlying each other conformably represent onlyhalf (53%) of the total contacts between observed facies inthe Slob el Garbi quarry. Three types of unconformitiesare present: major disconformities, which show trunca-tions and can be traced all over the quarry, and minor dis-conformities (with erosion) or reactivation (withouterosion) surfaces, ranging only up to a few meters inextent. The major disconformities form surfaces dippingtoward the south, i.e. in the same direction as the dunemigration, but with a lower angle. The vertical sectionsreveal well-developed foresets that record the angle ofrepose (about 40°), whereas the major disconformities dipwith an angle between 20° and 25°.

Fig. 3 Outcrop pictures of the four main depositional facies asobserved on the horizontal transect of the quarry (Fig. 2). a Pinstripelamination is the most abundant type of depositional facies. Lamina-tions are very well preserved and underlined by lighter, millimeter-scale, cemented layers. b GrainXow cross-stratiWcation consists ofstacked, concave-up base (black arrow) and convex-up top (white arrow)

lenses of structureless, well-sorted and loosely packed sand (morebrownish and darker than the laminated facies). c Grainfall lamination(white arrow) is more diVuse and is associated with grainXow scarsunderlined by concave-up bases (black arrow). d Fadeout laminationshows dark, homogeneous, decimeter-scale layers (black arrow), inwhich lamination (mainly pinstripe, white arrow) disappears laterally

a b

c d

123

514 Facies (2012) 58:509–522

Depositional cycle

A facies sequence analysis has been performed to deter-mine whether the chronological facies succession is ran-dom or follows a general trend. Five transects (avoidingnebkha patches and pedoturbated areas) were studied on thehorizontal surface where facies could be analyzed over alarge area. A part of one transect is interpreted in detail andplotted in Fig. 5a. These transects are oriented towards thesouth, along the direction of dune migration, recordingdepositional evolution through time. The sequences ofdepositional facies and disconformities have been noted onall transects and summarized into a transition matrix(Fig. 5b). Grainfall and grainXow facies are considered as

issued from the same depositional environment (lee-side ofa well-developed dune where the angle of repose is reachedand allows grainXow avalanches).

The transitions, which occur more frequently than arandom sequence, are then extracted using the transitionmatrix according to Harper’s method (Harper 1984). Themost signiWcant facies transitions are shown graphicallyin Fig. 5c and reveal a cyclic pattern. The cycle startswith a major disconformity, followed by the pinstripelamination facies. The grainXow/grainfall associationoverlies the pinstripe lamination separated by a minordisconformity. The last facies is the fadeout lamination,frequently showing a minor reactivation surface at thebase.

Fig. 4 Photomicrographs of the four main depositional facies. For thepurpose of comparison, all photomicrographs are at the same magniW-cation. All microfacies show the same grainstone texture, they onlydiVer in their grain packing. a Inverse grading, characteristic of pin-stripe lamination, which reXects climbing translatent ripples.

b Grainfall lamination showing laminae, thinner and more diVusecompared to the pinstripe facies. GrainXow (c) and fadeout(d) microfacies are dominated by homogeneous, well-sorted, andloosely packed grainstones. The latter shows a slightly coarser texturecompared to the others

a b

c d

123

Facies (2012) 58:509–522 515

Seasonal depositional model

The depositional cycle observed in the Slob el Garbi quarryeolianite is interpreted as recording seasonal trends. Sincethe exact determination of the local wind regime during theisotopic stage MIS 5e is impossible, present-day meteoro-logical datasets were used. In a Wrst step, we veriWed that allobservations made on the sedimentary record are not incontradiction to present-day conditions. Secondly, we usedthe present-day annual wind evolution as an aid to the inter-pretation of the depositional cycle.

Daily wind statistics for Djerba Airport, about 75 kmnorthwest from the studied area, are available (http://www.windfinder.com). Figure 6 shows the average intensityand direction of winds for each month during the past9 years (from March 2002 until April 2011). Two majorobservations can be made: the intensity of the winddecreases during spring and autumn and the direction of thewind shifts during winter. The average speed of 11 knots ismore or less constant during the year but the probability tohave winds over 4 on the Beaufort scale (over 16 knots) ishigher in the winters and summers. Throughout the year, thewind blows from the east–northeast (from the sea in Djerba),but it shifts and blows in the opposite direction (from thewest) between November and January, coming from theland. If these conditions were similar during the last intergla-cial, they should be conWrmed in the sedimentary record bychanges in dune migration dynamics and detrital content dueto the wind shifting directions twice a year.

The sedimentary stacking records shifts in wind direc-tion in diVerent ways. Small angle shifts are recorded byminor disconformities or reactivation surfaces. However,such minor reactivation surfaces could not be discriminated

from similar ones, which would be related to the 3D geom-etries of migrating dunes. Major wind shifts such as thewinter-wind reversal observed in the present-day record,will completely rework part of the dune Weld. Eolian dunespresent an asymmetric proWle with a steep angle corre-sponding to the angle of repose on the lee-side and a lowerangle on the stoss-side (Hunter 1977). A wind reversal willmainly rework the former lee-side, which becomes the newdown-wind side by decreasing the Xank’s angle. Major dis-conformities observed in the quarry match this explanation.

Grainstone composition is almost constant among alldepositional facies. Grain size and relative amounts of thediVerent types of grains are similar, with only one excep-tion: the content of “clean” quartz. Image analysis onscanned thin-sections and point counting measurementsshow that the ratio of “clean” quartz versus other quartzgrains is 50% higher in the fadeout depositional facies(Table 1). The “clean” quartz, contrary to the other quartzgrains observed within the depositional facies, does not dis-play any evidence of marine alteration such as ooid corticesor micritic coatings. These “clean” quartz grains have mostlikely a continental origin without having been in themarine environment. This interpretation is also supportedby the fact that wind blowing from the land will be drierthan breezes coming from the sea, which will also favor thefading of the depositional features (McKee et al. 1971).Winter sandstorms transport abundant sand in suspension.Fadeout lamination could also be due to the high amount ofsand incorporated by grainfall in the coastal dune complexduring and at the end of sandstorms.

The depositional cycle observed in the eolianite of theSlob el Garbi quarry can be explained by season-relatedwind conditions. At the base of the cycle, the pinstripe

Table 1 Petrographic composition, ratio of “clean” quartz (continental origin) versus quartz showing marine alterations, and petrophysical valuesfor the main depositional facies

Facies Samples Ooids Peloids Bioclasts Quartz Clean quartz Cement Porosity Perm. (Clean quartz)/(quartz)

% % % % % % % mD

Grainfall 2868C 4.78 29.88 0.80 11.16 6.77 22.71 23.90 650 0.61

GrainXow 2864A 9.20 37.20 1.20 8.00 4.80 14.40 25.20 930 0.60

GrainXow 2866C 10.40 29.20 2.00 12.80 6.00 13.20 26.40 830 0.47

GrainXow 2873B 10.40 33.60 0.80 6.80 4.80 14.80 28.80 1,350 0.71

Average 10.00 33.33 1.33 9.20 5.20 14.13 26.80 1,037 0.59

Pinstripe 2869C 4.80 34.40 3.60 10.40 5.20 24.40 17.20 450 0.50

Pinstripe 2863A 7.60 40.00 1.60 6.80 2.40 20.00 21.60 610 0.35

Pinstripe 2865B 8.40 36.80 2.40 10.80 4.80 19.60 17.20 720 0.44

Average 6.93 37.07 2.53 9.33 4.13 21.33 18.67 593 0.44

Fadeout 2869A 9.16 24.30 2.79 10.36 11.55 18.33 23.51 1,100 1.11

Fadeout 2870C 8.80 28.40 1.60 8.40 4.40 20.40 28.00 980 0.52

Fadeout 2871B 10.00 32.00 2.00 6.40 6.40 12.80 30.40 1,420 1.00

Average 9.32 28.23 2.13 8.39 7.45 17.18 27.30 1,167 0.89

123

516 Facies (2012) 58:509–522

lamination facies is dominant. The absence of grainfall orgrainXow depositional facies indicates that the dunes do notpresent well-developed slip faces (Fig. 7a). This part of the

cycle records small dunes, related to low-wind activitiesand interpreted as the transition from winter to spring(February and March, Fig. 6). During spring and until

Fig. 5 Facies succession. a Detailed portion of one of the Wveinterpreted horizontal transects. The surface types (D = major discon-formity, d = minor disconformity, r = reactivation surface) and deposi-tional facies (P = pinstripe lamination, GF = grainfall and grainXowstratiWcation, H = fadeout lamination) have been identiWed. b Faciestransition matrix corresponding to the Wve transects. The value in each

cell indicates the number of times the facies of a row switches to thefacies of the corresponding column. c Facies relationship diagram forthe eolianite of the Slob el Garbi quarry. The level of signiWcance is setat 0.2 (Harper 1984). The thickness of the arrows is proportional of thestatistical signiWcance of the transition. d Schematic genetic succes-sion corresponding to the facies diagram

a

b

c

d

Fig. 6 Present-day wind. a Wind statistics for the past 5 years (March2002 to April 2011) recorded at the Djerba-Melita airport. The Djerba-Melita weather station is the closest available wind survey in the area

(75 km). Average wind direction, average velocity, and probability ofstrong wind are given monthly. b Rose diagram for June and Decem-ber winds during the past 9 years (March 2002 to April 2011)

a b

123

Facies (2012) 58:509–522 517

autumn (April to September, Fig. 6), the wind blows con-tinuously from the sea and the probability of having windspeeds over 4 on the Beaufort scale (over 16 knots) ishigher. These conditions also support higher sand inputfrom the shoreline. This allows the development of propercoastal dunes with prograding slip-faces highlighted by thegrainfall and grainXow facies. Spring to autumn winds,coming from the sea, are the dominant conditions that arerecorded by the eolianite (Fig. 7b). During the Wrst part ofthe winter, the wind completely shifts its direction andblows from the continent (November to January, Fig. 6).The average wind speed is also stronger than that observedduring the autumn. The wind is able to rework the uncon-solidated sand bodies by Xattening the former slip face. Onvertical sections, the major disconformities due to the windreversal show a lower angle than the slip face. The fadeoutfacies is dominant, conWrming drier conditions and conti-nental input (increase of the “clean” quartz content) relatedto wind blowing from the land (Fig. 7c). Then, a new cyclestarts on a dune complex which was Xattened by the winterwinds, which also explains why grainfall and grainXow

facies corresponding to high angles are absent during thewinter to spring transition.

Post-depositional setting

Two major post-depositional diagenetic phases haveaVected the studied eolian deposits: an early meteoricvadose cementation followed by the development of adense fracture network.

Early cements

The Pleistocene eolianite of the Slob el Garbi quarry hasnever been submerged by the sea, since the sea level hasnever reached this altitude after the highstand of the lastinterglacial period. The altitude of the quarry is above the1 m isostatic rebound, which took place at the end of theHolocene (Jedoui et al. 1998). The cement types are nothomogeneously distributed within the dune complex andshow a vertical zoning.

Gypsum cement mainly occurs at the base of the quarryas large blade crystals, Wlling the porosity in well-packedlayers or as menisci cement in more open, loosely packedsediment (Fig. 8a). Early sulphate cements record evapo-ritic conditions and are common in both siliciclastic or car-bonate eolian deposits (Fryberger et al. 1988; Glennie andGödag 1998). In the present case, the source of brine feed-ing the gypsum precipitation has to be related to the vicinityof the el Bibane lagoon (Fig. 1). The gypsum cement isprobably recent and results from evaporative pumping ofbrines coming from the el Bibane lagoon through the highlyporous eolian sand.

Carbonate cements are more homogeneously distributedthan gypsum cements. They consist of equigranular calcitecement. They often show pendant and meniscus morpholo-gies made of equant-shaped crystals (Fig. 8b) or completelyWll the pores within the Wner layers of the pinstripe andgrainfall laminations, where capillary water is held and cangradually evaporate, indicating meteoric vadose conditions(Scholle and Ulmer-Scholle 2003). In the more homoge-neously packed grainXow and fadeout facies, the calcitecement does not have a uniform distribution but presents apreferential development around the “clean” quartz grains(Fig. 8c, d). This paradoxical cement location results fromhydrophilic properties of quartz and is interpreted as anindicator of vadose diagenesis (Frébourg et al. 2008,2010b; Hasler et al. 2007).

Early fractures

The Slob el Garbi eolianite is aVected by a dense fracturenetwork (Fig. 2). The Wssures remain open due to karstic

Fig. 7 Seasonal model for the succession of depositional facies. Thesymbols used in this Wgure are the same as those used in Fig. 5. See textfor a complete description of the model. Drawings are not to scale;average dune height is about 4 m. The distance between dunes cannotbe determined for the Slob el Garbi quarry

- Dunes have been flattened by inversed winter wind- No steep leeside flank (slipface)- Mainly wind ripples (pinstripe lamination)

a Winter-spring transition Wind

Sea

b Summer-autumn

- Proper dune morphology (presence of a slipface, stronger wind and/or more sand input of marine origin)- Mainly grainflow and grainfall facies

d Following summer

c Winter

- Erosion and flattened dunes due to inversed winter wind- Enriched in clean quartz (wind from continent)- Mainly fadeout lamination facies (dry condition)

Wind

Wind

Wind

GF

GF

GF

GF

P

P

P

P

H

P

H

123

518 Facies (2012) 58:509–522

dissolution, which is still under progress due to present-daymeteoric conditions. They appear early, since tectonicactivity or loading of overlying formations are absent, butafter the precipitation of the Wrst sulphate and calcitecements. The two major directions of fractures observed areboth parallel and perpendicular to the elongation of thedune belt (i.e., along the W–E and the N–S axis).

Relationship between depositional facies, early cements, and fractures

GIS allows the quantiWcation of the relative abundance ofeach facies, and the determination of the fracture density

and orientation for each depositional facies. By far themost abundant facies is the pinstripe lamination (account-ing for almost 50% of the surface, Fig. 9). This isexplained by the fact that the major disconformities are theresult of the winter-wind reversal and because winterwinds remove Wrst the grainfall and grainXow facieslocated on the leeside of the well-developed dunes(Fig. 7c) and tend to better preserve the pinstripe lamina-tion. The depositional facies can be merged into two dis-tinct groups according to the fracture density andorientation. The laminated facies (pinstripe and grainfall)present a high fracture density (about 1.2 cumulatedmeters of fracture per square meter) compared to that of

Fig. 8 Early cements. a Cross-polarized light photomicrograph show-ing gypsum cement (gc) Wlling the porosity in a ooid—(oo), peloid—(pe), and quartz—(qz)-rich grainstone. b Back-scattered SEM photo-micrograph of meniscus calcite cement (mc) growing between peloids(pe) and ooids (oo). c Cross-polarized light photomicrograph high-

lighting calcite cement (cc) growing preferentially around “clean”quartz (qz), i.e., not aVected by marine alteration (ooid cortices). Car-bonate grains such as peloids (pe) or ooids (oo) are only surrounded bya thin cement rim. d Back-scattered SEM photomicrograph showingcalcite cement located preferentially (cc) around the quartz grain (qz)

1mm

1mm

10mm

10mm

qz

qz

qz

qz

qz

qz

pe

pepe

pe

pepe

oo

oo

oooo

oo

oo

cc

gc

cc

mc

mc

mc

oo

qz

a

c

b

d

123

Facies (2012) 58:509–522 519

the more homogeneous grainXow and fadeout facies(about 0.8 mf/m

2). The laminated facies also display a uni-modal distribution of the fractures (perpendicularly to thedune migration) as opposed to bimodal fracture distribu-tion (both perpendicular and parallel to the migration ofthe dune belt) observed in the homogeneous facies(Fig. 9). These two distinct groups cannot be explaineddirectly by the texture of fabric of the sediment or thedepositional facies. The diVerence between the two groupsis explained by the heterogeneous distribution of the cal-cite cement within the laminated facies. As mentioned ear-lier, the calcite cement Wlls completely the Wner laminae(pseudophreatic conditions), but is present only as pendantand meniscus type in the coarse deposits. These diVerencesin cement types and abundance induce an anisotropic com-petence along the laminations that are mainly oriented par-allel to the dune belt. The homogeneous group, in contrast,does not exhibit this anisotropic cementation and shows asecond direction of fractures. As no recent tectonic activityis reported in this area, the fracturing is probably related tothe settling of the eolianite under its own weight and thecompaction of the underlying poorly cemented Miocene-Pliocene units.

Preservation potential

The Rejiche Formation has been used for building stonessince Roman times. Even if cements are not homoge-neously distributed therein, they represent, on average, 14–22% of the petrographic content of all depositional facies.The mineralogy of the allochems (aragonite vs. calcite)might be very important to the preservation potential ofeolianites (Loope and Abegg 2001). The scarcity of carbon-ate eolian deposits in the pre-Quaternary record is oftenattributed to the low preservation potential of these sandbodies (Hearty and O’Leary 2008; Longman et al. 1983),underlined by the scarcity of evidence of lithiWcation beforeburial (Loope and Abegg 2001). However, the example ofthe Slob el Garbi quarry, as well as the numerous records oflithiWed Holocene eolianites (Davaud and Septfontaine1995; Gardner and McLaren 1993; Halley and Harris 1979;Jedoui et al. 2002; Kindler 1992; Le Guern 2005) invalidatethis assumption. Eolianites are subjected to the percolationof meteoric water and contain signiWcant freshwater aqui-fers around the Mediterranean Sea (Tsoar 2000). Thus, theyundergo early vadose and phreatic cementation and mayform ridges resisting coastal erosion, even during transgres-

Fig. 9 Relationship between depositional facies, early cements, andearly fractures. Two groups are distinguished, based on the observationthat laminated facies (pinstripe lamination and grainfall lamination)are more cemented compared to the homogeneous facies (grainXowstratiWcation and fadeout lamination). The latter group (group 2) pre-

sents a bimodal distribution of the orientation of fractures (both paral-lel and perpendicular to the migration axis of the dunes). In contrast,group 1 mainly displays fractures along the direction of the coastalbelt, i.e., perpendicular to the direction of the dune’s migration

123

520 Facies (2012) 58:509–522

sive phases (Aberkan 2000; Frébourg 2010), meaning thatthey could resist mechanical compaction during burial.

Conclusions

This study of the Slob el Garbi quarry eolianite demon-strates a cyclic stacking of eolian facies and erosionalevents and is used as a local paleoclimate proxy. Themigration of the dune system is mainly controlled by windsblowing from the sea but the increase in quartz grainsshowing no evidence of marine overprints in the fadeoutfacies conWrms continental inputs related to periodic windshifts. The local climatic conditions prevailing during thelast interglacial period were therefore similar to thoseobserved nowadays, with seasonal wind reversals.

The presence of numerous vertical open fractures is dueto diVerential settling of the early lithiWed eolian grain-stones. The intensity and the spatial distribution of the frac-tures are controlled by the facies distribution and the relateddiagenetic heterogeneities. Considering that no tectonicactivity or overlying formations have aVected the Pleisto-cene dunes, the mechanism at the origin of the fractureswould only be related to the own weight of the eolianite andto its internal heterogeneity. Since this mechanism shouldbe similar in other coastal carbonate dunes, the Slob ElGarbi eolianite is proposed as a case study for early frac-tures and diagenesis in carbonate fossil dunes.

The potential of eolianites for preservation in the fossilrecord is probably higher than commonly accepted. Thisimplies that they could have a high-reservoir potential, notonly as aquifers as observed along the Mediterranean coast,but also as pre-Quaternary hydrocarbon reservoirs, whichseems to be conWrmed by recent studies (Frébourg et al.2010a).

Acknowledgments We would like to thank Luis Pomar and BobLoucks for their constructive reviews of an earlier version of the man-uscript. We also thank the two Facies reviewers, Stephan Jorry andDavid Loope, for the helpful remarks and comments. We are gratefulto the Swiss National Science Foundation for funding this work (Grantno. 200020-119777).

References

Abegg FE, Robertson Handford C (2001) DeXation origin of Missis-sippian carbonate eolianites of southwestern Kansas. In: AbeggFE, Harris PM, Loope DB (eds) Modern and ancient carbonateeolianites: sedimentology, sequence stratigraphy, and diagenesis.SEPM Spec Publ 71:183–203

Abegg FE, Loope DB, Harris PM (2001) Carbonate eolianites: deposi-tional models and diagenesis. In: Abegg FE, Harris PM, LoopeDB (eds) Modern and ancient carbonate eolianites: sedimentol-ogy, sequence stratigraphy, and diagenesis. SEPM Spec Publ71:17–30

Aberkan M (1989) Etudes des formations quaternaires des marges dubassin du Rharb (Maroc nord-occidental). PhD dissertation,Université de Bordeaux 1, Bordeaux

Aberkan M (2000) Précisions sur la chronostratigraphie des formationslittorals quaternaires au Nord-Est de Rabat (Maroc). Apport de lathermoluminescence. Géol Méd 27:27–31

Bouaziz S (1995) Etude de la tectonique cassante dans la plateforme etl’atlas sahariens (Tunisie méridionale): évolution des paléo-champs de contraintes et implications géodynamiques. PhD dis-sertation, Université de Tunis, Tunis

Brooke B (2001) The distribution of carbonate eolianite. Earth-Sci Rev55:135–164

Clemmensen LB, Abrahamsen K (1983) Aeolian stratiWcation andfacies association in desert sediments, Arran bassin (Permien),Scotland. Sedimentology 30:311–339

Darwin CR (1851) Geological observations on coral reefs, volcanic is-lands and on South America, being the geology of the voyage ofthe Beagle, under the command of Captain Fitzroy during theyears 1832 to 1836. Smith, Elder and Company, London

Davaud E, Septfontaine M (1995) Post-mortem onshore transportationof epiphytic foraminifera: recent example from the Tunisiancoastline. J Sediment Res 65:136–142

Fairbridge RW, Johnson DL (1978) Eolianite. In: Fairbridge RW,Bourgeois J (eds) Encyclopedia of sedimentology. Dowden, Stro-udsburg, pp 279–282

Fornós JJ, Clemmensen LB, Gómez-Pujol L, Murray AS (2009) LatePleistocene carbonate aeolianites on Mallorca, Western Mediter-ranean: a luminescence chronology. Quatern Sci Rev 28:2697–2709

Frébourg G (2010) Carbonate costal dunes: potential reservoir rocks?PhD dissertation 92. Terre & Environnement, Univ Genève, Gen-ève

Frébourg G, Hasler C-A, Le Guern P, Davaud E (2008) Facies charac-teristics and diversity in carbonate eolianites. Facies 54:175–191

Frébourg G, Davaud E, Gaillot J, Virgone A, Kamali M (2010a) Dis-covery of an eolianite in the Upper Dalan Member, KhuV Forma-tion, South Pars Weld, Iran. J Petrol Geol 33:141–154

Frébourg G, Hasler C-A, Le Guern P, Davaud E (2010b) Catastrophicevent recorded among Holocene eolianites (Sidi Salem Forma-tion, SE Tunisia). Sediment Geol 224:38–48

Fryberger SG, Schenk S (1981) Wind sedimentation tunnel experi-ments on the origins of eolian strata. Sedimentology 28:805–821

Fryberger SG, Schenk CJ (1988) Pinstripe lamination: a distinctivefeature of modern and ancient eolian sediments. Sediment Geol55:1–5

Fryberger SG, Schenk CJ, Krystinik LF (1988) Stokes surfaces and theeVects of near-surface groundwater-table on aeolian deposition.Sedimentology 35:21–41

Gardner RAM, McLaren SJ (1993) Progressive vadose diagenesis inLate Quaternary aeolianite deposits? In: Pye K (ed) The dynamicsand environmental context of aeolian sedimentary systems. GeolSoc Lond Spec Publ 72:219–234

Gealy WB (1929) Use of mercury for determination of volume of rockspecimens in Russell Porosity Apparatus. AAPG Bull 13:677–682

Ginsburg RN (2005) Disobedient sediments can feedback on theirtransportation, deposition and geomorphology. Sediment Geol175:9–18

Glennie KW, Gödag H (1998) Cemented Quaternary dune sands, RasAl Hamra housing area, Muscat, Sultanate of Oman. In: AlsharanAS, Glennie KW, Whittle GL, Kendall CGSC (eds) Quaternarydeserts and climatic change. A. A Balkema, Rotterdam, pp 109–116

Goudie AS, Sperling CHB (1977) Long-distance transport of forami-niferal tests by wind in the Thar Desert. J Sediment Petrol43:630–633

123

Facies (2012) 58:509–522 521

Guidry SA, Grasmueck M, Carpenter DG, Gombos AM Jr, Bachtel SL,Viggiano DA (2007) Karst and early fracture networks in carbon-ates, Turks and Caïcos Islands, British West Indies. J SedimentRes 77:508–524

Halley RB, Harris PM (1979) Fresh-water cementation of a 1,000-year-old oolite. J Sediment Petrol 49:969–987

Harper CW (1984) Improved methods of facies sequence analysis. In:Walker RG (ed) Facies models, 2nd edition. Geosci Can, Repr Ser1:11–13

Hasler C-A, Frébourg G, Davaud E (2007) Preferential cements aroundquartz grains in carbonate sediments: a new indicator of meteoricdiagenesis. In: 25th IAS meeting of sedimentology, Patras,abstracts, p 91

Hearty PJ, Kindler P (1995) Sea-level highstand chronology from sta-ble carbonate platforms (Bermuda and The Bahamas). J CoastalRes 11:675–689

Hearty PJ, O’Leary MJ (2008) Carbonate eolianites, quartz sands, andQuaternary sea-level cycles, Western Australia: a chronostrati-graphic approach. Quatern Geochronol 3:26–55

Hunter RE (1977) Basic types of stratiWcation in small eolian dunes.Sedimentology 24:361–387

Hunter RE (1981) StratiWcation styles in eolian sandstones: somePennsylvanian to Jurassic examples from Western Interior USA.In: Ethridge FG, Flores RM (eds) Recent and ancient nonmarinedepostional environments: models for exploration. SEPM SpecPubl 31:315–329

Hunter RE (1993) An eolian facies in the Ste. Genevieve Limestone ofsouthern Indiana. In: Keith Brian D, Zuppann Charles W (eds)Mississippian oolites and modern analogs. AAPG Stud Geol35:31–48

Jedoui Y (2000) Sédimentologie et géochronologie des dépôts litto-raux quaternaires: reconstitution des variations des paléoclimatset du niveau marin dans le Sud-Est tunisien. PhD dissertation,Université de Tunis, Tunis

Jedoui Y, Bouaziz S (1997) Ben Gardane, Carte géologique de laTunisie 1/100000, feuille 101 Ed Serv Geol, Tunis

Jedoui Y, Bouaziz S, Kallel N (1996) Les dépôts marins littoraux duSud-Est tunisien: indicateurs des variations eustatiques globales.In: 17th Regional African European meeting of sedimentology,Sfax, abstracts, pp 135–136

Jedoui Y, Kallel N, Fontugue M, Ben Ismail H, M’Rabet A, MontacerM (1998) A high relative sea-level stand in the middle Holoceneof southeastern Tunisia. Mar Geol 147:123–130

Jedoui Y, Davaud E, Ben Ismaïl H, Reyss J-L (2002) Analyse sédi-mentologique des dépôts marins pléistocènes du Sud-Est tunisien:mise en évidence de deux périodes de haut niveau marin pendantle sous-stade isotopique marin 5e (Eémien, Tyrrhénien). Bull SocGéol France 173:255–264

Jedoui Y, Reyss J-L, Kallel N, Montacer M, Ben Ismail E, Davaud E(2003) U-series evidence for two high Last Interglacial sea levelsin southeastern Tunisia. Quatern Sci Rev 22:343–351

Jorry SJ, Hasler C-A, Davaud E (2006) Hydrodynamic behaviour ofnummulites: implications fo depositional models. Facies 52:221–235

Kilibarda Z, Loope DB (1997) Jurassic aeolian oolite on a palaeohighin the Sundance Sea, Bighorn Basin, Wyoming. Sedimentology44:391–404

Kindler P (1992) Coastal response to Holocene transgression in theBahamas episodic sedimentation versus continuous sea-level rise.Sediment Geol 80:319–329

Kindler P, Davaud E (2001) Recognizing eolianites in thin section. Re-view and case study: the lower Cretaceous chambotte formation,Salève Chain, southeastern France. In: Abegg FE, Harris PM, Lo-ope DB (eds) Modern and ancient carbonate eolianites: sedimen-tology, sequence stratigraphy, and diagenesis. SEPM Spec Publ71:141–150

Klappa CF (1980) Rhizoliths in terrestrial carbonates: classiWcation,recognition, genesis and signiWcance. Sedimentology 27:613–629

Kosa E, Hunt D, Fitchen W, Bockel-Rebelle M, Roberts G (2003) Theheterogeneity of paleocavern systems developed along syndepo-sitional fault zones: the Upper Permian Capitan platform, Guada-lupe Mountains. In: Ahr W, Harris P, Morgan W, Somerville I(eds) Permo-Carboniferous carbonate platforms and reefs. SEPMSpec Publ 78:291–322

Le Guern P (2005) Caractérisation pétrographique et pétrotexturaled’éolianites pleistocène et holocènes. PhD dissertation 52. Terre& Environnement, Univ Genève, Genève

Le Guern P, Davaud E (2005) Recognition of ancient carbonate winddeposits: lessons from a modern analogue, Chrissi Island, Crete.Sedimentology 52:915–926

Longman MW, Fertal TG, Glennie JS, Krazan CG, Suek DH, TolerWG, Wiman SK (1983) Description of a paraconformity betweencarbonate grainstones, Isla Cancun, Mexico. J Sediment Res53:533–542

Loope DB, Abegg FE (2001) Recognition and geologic preservation ofancient carbonate eolianites. In: Abegg FE, Harris Paul M, LoopeDavid B (eds) Modern and ancient carbonate eolianites: sedimen-tology, sequence stratigraphy, and diagenesis. SEPM Spec Publ71:3–16

Loucks RG, Ward WC (2001) Eolian stratiWcation and beach-to-dunetransition in a Holocene carbonate eolianite complex, IslaCancun, Quintana Roo, Mexico. In: Abegg FE, Harris PM, LoopeDB (eds) Modern and ancient carbonate eolianites: sedimentol-ogy, sequence stratigraphy, and diagenesis. SEPM Spec Publ71:57–76

Mahmoudi M (1986) Stratigraphie, sédimentologie et diagenèse desdépôts Tyrrhéniens du Sahel tunisien. PhD thesis, Universitéd’Orsay, Paris

Mahmoudi M (1988) Nouvelle proposition de subdivisions strati-graphiques des dépôts attribués au Tyrrhénien en Tunisie (régionde Monastir). Bull Soc Géol France ser 8(4):431–435

McKee ED, Ward WC (1983) Eolian environment. In: Scholle PA,Bebout DG, Moore CH (eds) Carbonate depositional environ-ments. AAPG Memoir 33:132–170

McKee ED, Douglass JR, Rittenhouse S (1971) Deformation of lee-side laminae in eolian dunes. Geol Soc Am Bull 82:359–378

Murray-Wallace CV (2002) Pleistocene coastal stratigraphy, sea-levelhighstands and neotectonism of the southern Australian passivecontinental margin—a review. J Quatern Sci 17:469–489

Ndiaye M (2008) A multipurpose software for stratigraphic signalanalysis. PhD dissertation 71. Terre & Environnement, Univ Gen-ève, Genève

PaskoV R, Sanlaville P (1983) Les côtes de la Tunisie: variations duniveau marin depuis le Tyrrhénien. Ed. Maison de l’Orient, Lyon

Playford P (1980) Devonian ‘‘Great Barrier Reef’’ of the CanningBasin, Western Australia. AAPG Bull 64:814–840

Rice JA, Loope DB (1991) Wind-reworked carbonates, Permo-Pennsyl-vanian of Arizona and Nevada. Geol Soc Am Bull 103:254–267

Roduit N (2005) Two complementary eYcient methods to quantifyporosity types in digital images of thin sections with the softwareJmicroVision. In: 24th IAS Meeting, Oman, abstracts, p 142

Roduit N (2007) Jmicrovision: un logiciel d’analyse d’image polyva-lent. PhD dissertation 65. Terre & Environnement, Univ Genève,Genève

Sayles RW (1931) Bermuda during the ice age. Am Acad Arts Sci Proc66:381–468

Scholle PA, Ulmer-Scholle DS (2003) A color guide to the petrogra-phy of carbonate rocks: grains, textures, porosity, diagenesis.AAPG Memoir 77:1–474

Tsoar H (2000) Geomorphology and paleogeography of sand dunesthat have formed the Kurkar ridges in the coastal plain of Israel.Israel J Earth Sci 40:189–196

123

522 Facies (2012) 58:509–522

Warren J (2000) Dolomite: occurrence, evolution and economicallyimportant associations. Earth-Sci Rev 52:1–81

White B, Curran HA (1988) Mesoscale physical sedimentary struc-tures and trace fossils in Holocene carbonate eolianites from SanSalvador Island, Bahamas. Sediment Geol 55:163–184

White B, Curran HA (1998) Sedimentology and ichnology of Quater-nary carbonate eolianites. In: AAPG Annual Convention, SaltLake City, expanded abstracts

123