Holocene peritidal and evaporitic sedimentation in Southern Tunisia

Proceedings Chapter

Reference

Holocene peritidal and evaporitic sedimentation in Southern Tunisia

DAVAUD, Eric Jean, JEDOUI, Younes, STRASSER, Andréas

DAVAUD, Eric Jean, JEDOUI, Younes, STRASSER, Andréas. Holocene peritidal and evaporitic sedimentation in Southern Tunisia. In: Association internationale de sédimentologie. 17th Regional African European Meeting of Sedimentology: field trips guide book. 1996. p.

1-13

Available at:

http://archive-ouverte.unige.ch/unige:155582

Disclaimer: layout of this document may differ from the published version.

1 / 1

GEOL.

·-1

HOLOCENE PERITIDAL AND EVA-PORITIC SEDIMENTATION IN

SOUTHERN TUNISIA

E. DAVAUD, Y. JEDOUI & A. STRASSER

reprint from field trip book

17th Regional African European Meeting of Sedimentology,

Sfax, 1996

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HOLOCENE PERITIDAL AND EVAPORITIC SEDIMENTATION IN SOUTHERN TUNISIA

E. DAVAUD1, Y. JED0Ul² & A. STRASSER³

1 Departement de Geologie et de Paleontologie, 13 Maraichers, 1211 Geneva, Switzerland

2 Ecole Nationale d'lngenieurs, BPW, 3038 Sfax, Tunisia

3 lnstitut de Geologie, Perolles, 1700 Fribourg, Switzerland

Geology of the surroundings of Zarzis

On the coastal plain of Jeffara between Gabes and the Tunisian-Libyan border, only Mio-Pliocene and Quaternary sediments crop out. They mask the older Tertiary, Mesozoic and Paleozoic substrate which dips towards the sea in steps defined by NW-SE striking faults (Perthuisot et al. 1972). To the South of Jerba, this plain is characterized by low hills not exceeding 70 m, and by vast depressions which are occupied by internal seas (Bahiret el Bibane, Bahiret Bou Grara) and by sabkhas (Sabkha el Melah, Sabkha Bou Jemel; fig. 1 ).

t

^~

tm ^.

Upper Pleistocene:

coastal marine and aeolian deposits

Middle Pleistocene:

alluvial deposits

Pliocene/Lower Pleistocene:

alluvial and floodplain deposits; calcrete

1111

Miocene/Pliocene:

coarse alluvial and floodplain deposits

E3

Figure 1: Geological map of southeastern Tunisia (after Ben Hajali et al., 1985). The numbers in circles correspond to the localities visited during the fieldtrip.

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A system of SW-NE oriented ephemeral rivers drains this zone and sporadically delivers terrigenous material to the coastal depressions. The hills, mostly c9mposed of Mio-Pliocene deposits, are commonly affected by a pronounced calcretization at their tops. This calcrete of Villafranchian age is widespread in Tunisia. The sabkhas, lying at sea level or slightly below, reach far inland. They correspond to ancient Holocene lagoons which have been filled by mostly evaporitic deposits.

Along the recent coastline, oolitic and bioclastic limestones build a ridge of up to 6 m high (fig. 1 ). These limestones onlap an erosion surface cutting into Mio-Pliocene deposits, or they pass laterally into time-equivalent continental deposits. They correpond to coastal belts which developed during the transgressive maximum of the Tyrrhenian (125 ky B.P., interglacial of isotope stage Se; fig. 2). Locally, coastal aeolianites composed of marine sediment cover or juxtapose the Tyrrhenian carbonates. Although lithified, these aeolinites are younger in age (about 7000 years B.P.) and formed during the Holocene transgression.

The south-eastern part of the Pelagian Sea is rather shallow (fig. 1 ), allowing wide-spread meadows of seagrass to grow. In the photic zone, the ocean floor is covered by bioclastic calcareous muds, which progressively increase their clay content offshore. Close to the coast. the calcareous muds contain quartz in silt fraction of essentially aeolian origin.

Foreshore and shoreface facies display a high proportion of ooids which, however, are relictic and have ¹⁴C ages of 4 to 5 ky B.P. This age corresponds to the Flandrian climatic optimum and +2 m sea-level highstand (Paskoff & Sanlaville 1983).

Eustatic variations during the Late Pleistocene and Holocene

In the course of the last glaciation, global sea level dropped down to -120 m (fig.

2). This lowstand induced intense erosion especially of non- or weakly-lithified sediment (e.g.,

E'

'-- after Paskoff & Sanlaville (1983)

-

I ;:::.:::::::_;: -

. ----=<·

,

- - . ; --· ., - • -i.,-- - ,,I.I. ,, __

C1I ~

Cl) -2 JI 1 1 T 1 ~. 1 ~· 1 r" ^~

0

iii'

~ 4.0

...

'b

GO 5.0

1000 3000 5000 7000

yBP

after Shackleton ( 1987)

Sa

c!'t::~.:.

,;-',,! ....,.;.. 1 L: ' ~

I I I I I I I I I I I I I I I

0 20 60 1 00 140

kyBP

Figure 2: Isotopic changes during the Upper Pleistocene and the Holocene and recent sea-level changes in southern Tunisia.

lagoonal muds, floodplain silts and clays) and created local depressions. The coastal carbonate sands (beach and coastal dune facies) deposited during the Tyrrhenian sea-level highstand mostly resisted well to the erosion due to their early- d iagenetic lithification (fig. 1 ).

In the last 15 ky, sea level rose rapidly to a position slightly below the one reached during the last interglacial period (Fairbanks 1989; fig. 2). In many places around the world - and equally in the southern Mediterranean - a maximum of +2 m is reached at around 5500 years B.P., followed by a progressive stabilization at around present sea level. This evolution is considered to be the effect of a slow isostatic reaction after a rapid rise of sea level.

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STOP 1: A recent barrier island system in a microtidal realm

The southwestern part of the Pelagian Sea (southern Mediterranean, fig. 1) is characterized by a wide shallow ramp covered by dense Posidonia and Cau/erpa meadows.

These seagrasses favor a prolific biological activity and produce large quantities of carbonate particles. Because the climate is arid, there is no permanent terrigenous input. Silt-size siliciclastic material is sporadically brought into the sea through small wadis and by dust storms from the Southwest. In the Gulf of Gabes and in the study area, the tidal range is up to 1.5 m during spring tides and winter storms. Because the bathymetric gradient is very low, wide intertidal zones are often exposed.

The island of Jerba is connected to the coastline by a well-developed barrier- island system cut by two tidal channels (fig. 3). The southern part of this system described in Davaud & Septfontaine (1995) is favorable for studying shallow-subtidal, intertidal, supratidal, and sabkha deposits. The barrier island is composed mainly of bioclastic sands, whereas

Figure 3: Stop 1. SPOT image showing the barrier-island system connecting the island of Jerba (N) to the continent (S). The image width is 8 km.

lagoonal, supratidal, and shallow-subtidal facies commonly contain 50 % silty quartz.

The barrier island is lined seaward by a narrow and shallow submarine terrace.

This substrate consists of poorly cemented oolitic and peloidal grainstone and is interpreted as a Holocene aeolianite. It acts as a hydrodynamic barrier and probably controls the buildup of the present-day barrier island. The northeasterly prevailing winds are strong and maintain rough sea conditions during winter and spring.

The visited area can be subdivided into five sedimentary environments (fig. 4).

(1) The shoreface extends toward the NE with a very gentle slope that favors the development of a wide Posidonia and Cau/erpa meadow from fair-weather wave base (-4 m) down to 35 m, tens of kilometers away from the coastline (de Gaillande 1970). The green algae Halimeda is abundant in the Posidonia-dominated meadow, where they are fixed on the rhizomes (Poizat 1970). Because of the gentle slope and the direction of the prevailing winds (fig. 4), this environment is especially exposed to storm action.

(2) Intertidal sand flats fringe the barrier island and are well developed in the southeastern part of the study area.

(3) The barrier island itself is formed of steeply dipping beach deposits on the seaward side and of very low-angle washover fans on the lagoon side. Small-scale aeolian dunes are present on the lee sides of scattered halophytic bushes. The most interesting feature in this

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environment is the presence of thick accumulations of dead seagrass leaves forming a continuous band, locally up to 1.5 m thick, on the top of beach deposits. These organic remains accumulate during winter storms and are progressively disaggregated by the wind.

Thick organic layers may, however, remain trapped below washover deposits and partly preserved in the fossil record (Strasser et al. 1989).

(4) Supratidal marshes extend landward of washover deposits and form small flat islands in the lagoon. They consist of cyanobacterial mats and short halophytic vegetation (Sa/icornia) .

' ^,_

.... : ... "t

' ' "\, ' ' '\

When groundwater salinity ,.// t~~al channel

/

~ -· '\,.

1km increases, this environment grades into a sabkha.

t

'1 ...

"'~ meadow is present in the

\ western part of the study area.

\ 3. beach and washover Because of shallow depths and

) fans limited fetch, weak bottom-

/ water motions are maintained

,, ···,\ _,, ·-. 1. s oreface h even during strong wind perio s. . d

·v~ Petrography

'··-,,, reveals a drastic facies change ',,\ (fig. 5) between shoreface deposits, and intertidal and supratidal sands (Davaud &

Septfontaine 1995). Samples collected by diving at the upper bathymetric limit of the Posidonia meadows are Figure 4: Sedimentary environments of the barrier-island

system connecting the island of Jerba to the Tunisian coastline. This map is based on remote-sensing data and on field observations.

composed of very fine quartz and peloidal sands. Bioclastic components, counted in thin sections, constitute less than 20% of the sediment. Benthic foraminifera are rare and poorly preserved. On the other hand, the sediments of the intertidal sand flat and of the barrier island are made of coarse bioclastic particles. Most bioclastic debris consist of algae (broken Halimeda plates) and foraminifera. Peloids are also common but result mainly from abrasion and micritization of Halimeda plates. Twenty-one foraminifera taxa were identified; most of them belong to the epiphytic microfauna characteristic of Posidonia and Caulerpa meadows (Blanc-Vernet et al.

1979). The faunal assemblage is clearly dominated by miliolinids among which Peneroplis and Sorites are the most abundant. The absence of many common benthic mud- and sand-dweller foraminifera is surprising. Obviously all the epiphytic foraminifera are allochthonous, although their tests do not show clear evidence of transportation.

The small aeolian dunes consist of peloids, rounded micritized bioclasts, and foraminifera. The latter are mainly small subspherical forms. This shape sorting is probably due to preferential transport of more rounded grains, as noted in aeolian silts by Mazzullo et al.

(1992), and might be used as an additional criterion for identifying coastal dunes in the fossil record.

The lagoonal deposits are composed of highly bioturbated fine peloidal sands and quartz silts (fig. 5). Foraminifera are very rare and partly micritized. No living foraminifera were observed either on leaves of Cymodocea and Posidonia or in the sediment.

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foraminifera ~

I

quartz peloids micritized bioclasts halimeda

% 0 10

lagoon

I

20 30 40 50 0

beach and

washover fans shoreface

5

10 20 30 40 50 0 10 20 30 40 50

Figure 5: Stop1. Mean petrographical composition of surficia/ sediments. Micritized bioclasts and peloids originate mainly from micritized and abraded fragments of Ha/imeda plates (modified after Davaud & Septfontaine 1995).

The high concentration of well-preserved tests of epiphytic foraminifera in foreshore and backshore environments (including aeolian dunes) is not surprising and has been noted in other carbonate provinces (e.g., Moore 1957; Sneh and Friedman 1984). By contrast, the scarcity of the same epiphytic foraminifera in shoreface deposits, particularly in the upper part of the Posidonia meadow, is much more unexpected, because the meadows represent their biotope.

This paradoxical distribution suggests that epiphytic foraminifera are massively transported onshore and cannot accumulate on the wave- and storm-influenced shoreface.

Long-distance "post-mortem" transportation of such organisms has been reported by several authors (Moore 1957; Murray et al.1982; Murray 1987; Loose 1970; Seibold and Seibold 1981;

Sneh and Friedmann 1984). This process is usually attributed to the low density of these biogenic particles, which are characterized by a high and complex porosity (numerous chamber cavities associated with caniculate or porous walls).

The onshore enrichment in epiphytic foraminifera and Ha/imeda fragments, and the related impoverishment of shoreface deposits, can be explained as follows (Davaud &

Septfontaine 1995): during summer and autumn, the leaves and rhizomes of the Posidonia and Caulerpa meadows are intensively colonized by epiphytic foraminifera. Permanent fair-weather conditions maintain weak bottom-water motions, and no active transport occurs. When the leaves fall, they lose their epiphytic populations, which probably settle in situ. Winter storms generate strong turbulent bottom currents that transport the dead leaves and most of the epiphytic bioclasts onshore (and offshore; Poizat 1970). The heavier and denser bioclastic particles are transported as bed load over short distances or reworked in situ. The lighter and more porous ones are transported as suspended load far from their biotope. As a consequence of this differential hydrodynamical behavior, allochthonous microfauna often remain well- preserved while autochthonous ones may be deeply abraded. Furthermore, epiphytic faunal assemblages often reflect concentrations of forms with similar hydrodynamical properties rather than biocoenoses. These paradoxes should be kept in mind when interpreting the fossil record.

STOP 2: Holocene aeolianite and beachrock of Lalla Meriem

The rocky promontory of Lalla Meriem is composed of aeolian dunes belonging to the formation of Sidi Salem (fig. 6). These deposits form a coastal ridge stretchin~ from the South of the Gulf of Gabes to the Libyan border (Paskoff & Sanlaville 1983). ⁴C dating positions them around 7000 years B.P., when sea level was about 1 m lower than today (fig.

2). The sediment is dominated by aragonitic ooids with nuclei of well-rounded quartz grains, peloids, or bioclasts. Furthermore, there are fragments of bivalves, gastropods, echinoderms, bryozoans and red algae, as well as benthic and (more rarely) planctonic foraminifera. Some peloids and lithoclasts are present. The grains are rather well sorted and small (0.2 - 0.5 mm), and arranged in millimetric laminations.

-,

] ;

mht mlt

present-day wave-cut platform

2./::G{{. _{. ) l ' ·) ' ·· . . · ·}

Holocene wa~e~c~!

/ i ... :f... :.~:'.;.

platform (270~ }?t-:::·.:\: ...::

'·.:·.-.::)·./ ~~~ ~~ ~~~~ ~

· dunes (7000 BP)

6

petr,ographic compositTheion suggests a shallow-marine, agitated and fairly warm environment, where Bahama- type ooids could form. However, grain sizes, sorting and the strongly inclined, landward dipping bedsets demonstrate that the carbonate sands have been

mean high tide ..:··:...:...:.- deposited by wind action,

mean lowtide ..:.. .:::::_'.:·':::;}-)·/.:::":::::_. ::·.: forming a coastal aeolian

·· .. ····.^~ dune complex. The presence Figure 6: Stop 2. Holocene coastal dunes cut by wave

action near Lalla Meriem (Sidi Salem Fmt). This aeolianite developed 7000 years ago whensealevelwasabout 1 m lowerthan today. Beachrock formation isstill active.

of Helix and bioturbations attributed to land crabs and insects confirm this interpretation.

The formation of Sidi Salem is only weakly cemented. Calcite cements form small isometriccrystals, commonly as meniscus.This implies a fresh-watervadose diagenetic environment. Where the rock is exposed in the intertidalzone, marine peloidal cement may locallyfill the pores. 80 cm above today's sea level, a terrace is cut into the formation of Sidi Salem (fig. 6). It corresponds to the slightly elevated sea-level stand of 2700 years B.P. (Paskoff·& Sanlaville 1983).

Beachrock representing an ancient beach is observed in a small bay. It overlies the aeolianites of Sidi Salem and is situated at about the same level as the Recent intertidal zone.¹⁴C dating on bivalve shells cemented intothe beachrockindicates an age of 2000 years B.P. (Jedoui, unpubl.). The petrographic composition of the beachrock resembles that of the formation of Sidi Salem, but equally includes large lithoclasts of aeolianite. Fragments of pottery have also been found incorporated in the beachrock. The seaward-inclined coarse-fine grain-size alternations are typical of beach deposits (fig.6).

Micritic meniscus cements and, locally, peloidal cements imply a marine vadose diagenetic environment. Some beachrock slabs have been broken up due to storm-wave action.The blocks are accumulated in depressions where theyare partially re-incorporated into the beachrock through cementation which still continues today. These depressions are flooded at high tide and coated by algal-microbial mats. Feeding traces of fish can be observed on their soft surface.

STOP 3: The tidal flats around Bahar Alouane

The inlet of Bahar Alouane between Zarzis and Bahiret el Bibane represents an ancient channel linking the lagoon of Sebkha el Melah with the open ocean (fig. 1). It was cut during the last glaciation, then partly filled at the same time as the depression of Sebkha el Melah (Perthuisot 1975). At its mouth, the inletreaches a depth of 4 - 5 m. The rocky bottom of Tyrrhenian sediment is swept by tidal currents and colonized by sponges, ascidians and Halimeda. Towards the sabkha, depth decreases rapidly. The bottom turns muddy and is stabilized by Cymodocea. The Holocene sediment is composed of calcareous mud, silty quartz, carbonate bioclasts, and organic matter.

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. ,

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supratidal zone

sabkha {flooded during storms) tidal pond

pustular microbial thick microbial mats halophytic bushes films

l {---..~ ..

AA A A A A - --- - ---- - - --"'<;;td"-~~-~~-~-

-- - -- - ~ -- -- - :~n!phreatic

gypsum level

I

I

gastropods

Figure 7: Schematic profile across the supratidal zone near Bahar Alouane (see fig. 1 for location).

7

The supratidal zone is well developed and displays a characteristic zonation (fig.

7). Close to the coast, halophytic vegetation grows on low beach ridges, and thick algal- microbial mats colonize the shallow depressions. These mats show polygonal retraction cracks. Groundwater salinity increases rapidly inland, and the vegetation disappears. The microbial mats are much thinner and become pustular. Active evaporitic pumping during the summer months causes displacive gypsum to precipitate above groundwater level. This zone is flooded during storms, and the water extends up to the dam of the road leading from Zarzis to Ben Gardane.

STOP 4a: Red-algal bioherms of Bahiret el Bibane

Bahiret el Bibane is a shallow lagoon situated south of city of Zarzis (fig.1 ). It covers approximately 230 km², the greatest depth is 6.5 m. The lagoon is separated from the Mediterranean Sea by a narrow belt of well consolidated Tyrrhenian coastal deposits, which in their middle break up into a series of small islands. Strong tidal currents pass between the islands, but water exchange with the open sea is limited. No data exist on tidal amplitude and tidal cycles in the lagoon. At Zarzis tidal range reaches 80 cm. Tidal influence is attenuated inside the lagoon, and winds have an important impact on water level (Medhioub 1979). The climate is semiarid with mean temperatures of 28 °C in summer and 15 °C in winter (Perthuisot 1975). Mean annual rainfall is about 200 mm, the potential evaporation rate 2500 mm y(¹ (Medhioub & Perthuisot 1981 ). The lagoon is slightly hypersaline, with salinities ranging from 43 %0 to 51%o. After heavy rainfall it may temporarly become brackish. Elevated salinity leads to rather restricted faunal assemblages, and the resulting limitation of grazing organisms has permitted the development of a ridge of coralline algae along the northeastern shore (Thornton et al. 1978).

Neogoniolithon Halimeda.

reef Acetabulana

~J

serpulid biofierms

Cymodocea meadows with

Pinna

Figure 8: Stop 4a. Schematic cross section through the northwestem shore of the lagoon of Bahiret el Bibane. mht = mean high tide.

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The commonly mushroom-shaped bioconstructions reach 30 - 40 cm in height and up to 1 m in diameter. They exhibit a dish-shaped top, whereby tt:ie rim facing the lagoon is preferentially elevated. These bioherms are more or less amalgamated and form discontinuous ridges which emerge during low tide. They are composed principally of Neogoniolithon notarisii but also include some Tenarea at their base (Thornton et al. 1978). The fabric is porous, the cavities being locally filled with carbonate mud and quartz grains. Ostracods, serpulids, vermetid gastropods, and bivalves are found inside the bioherms.

The facies zonation is summarized in figure 8. A small lagoon between the coast and the Neogoniolithon reef is often filled by sediment. Seawards, small serpulid bioherms occur. Ha/imeda and Acetabu/aria grow at the base of the bioconstructions. Further out, Cymodocea meadows colonize the sediment, giving shelter to Murex. Shells of Pinna in life position offer a hard substrate for sponges, bryozoans, and ascidians. Oolitic dunes appear locally, but the aragonitic ooids with tangential cortical structure are relictic. They have been dated at 4750 years B.P. (Strasser et al. 1989) and formed when sea level was higher and the climatic and hydrological conditions more favorable (Paskoff and Sanlaville 1983).

STOP 4b: Early cementation of beach levees (Bahiret el Bibane)

Small beach ridges, usually inactive and covered by halophytic vegetation, separate the lagoon from coastal sabkhas or fields of relictic aeolian dunes. Consolidated beds of beach facies occur below and behind the beach ridges, at or above the water table (Strasser et al. 1989). Coastal erosion locally excavates these beachrock beds, and slabs of it accumulate on the beach and in the shallow subtidal zone.

The beach sediments are composed mainly of aragonitic ooids which develop around angular aeolian quartz grains or peloids. Some ooid surfaces are heavily corroded and suggest reworking, others are smooth and look very fresh. ¹⁴C dating, however, indicates an

age of about 4750 years.

The beach deposits overly a fine-grained lagoonal facies composed of silty quartz and ooids, and

aeolian deposits they are topped by aeolian

(silty quartz with gypsum crusts) .

---·=:---

·--.-_-_--- I

- Ii,. - - - - - -- -

• • - • • oolitic b~ach • - - - - Cemented beds occur

water table • - - -~:~~~its ...,. mht exclusively in beach facies

5 m [ 20cm

,___. quartz and biocla.stic

lagoonal deposits _3340t50

t

Figure 9: Stop 4b. Cross section through a beach levee from the northwestern Bahiret el Bibane. Superposed cemented crusts characterized by different ¹⁴C ages are interpreted as evidences for a sea-level drop following the Flandrian high- stand (after Strasser et al. 1989)

(fig. 9). The highest beachrock bed has been found 40 cm above the

groundwater level

corresponding to low tide. The lowest cemented layers are situated at the water table.

The beachrock beds are 2-15 cm thick. Some are continuous and can be followed over tens of meters along the beach where erosion has exposed them, others consist of isolated nodules. Their upper surface is slightly undulated and smooth, while the lower one can be very irregular and bumpy. This is largely due to preferential cementation around roots penetrating the beachrock layers.

The most common cements consist of aragonite needles which form isopachous rims around the grains. In many cases, a black layer covers the grains surfaces and forms the

1 I

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base of the aragonite cements. It is composed of cryptocrystalline aragonite and contains iron and sulfur. Strasser & Davaud (1986) and Strasser et al. (1989) .suggested that this layer originated from an organic mucus in which micritic aragonite was precipitated or trapped, and which then served as nucleation site for the larger crystals.

Two beachrock layers have been dated (fig. 9). The layer situated at groundwater level shows an apparent age of 3340 ± 50 B.P., while the upper layer gives 4370 ± 60 B.P. As the petrographic composition of the two samples is comparable, the older age of the higher bed may be explained by an older age of th.e cements. The elevated beachrock beds would therefore suggest a once-higher water table, and accordingly a higher sea level. Beachrock deposits in a similar, slightly elevated position, are found on the western shore of Djerba (2700 B.P., Dalongeville et al. 1980; Paskoff & Sanlaville 1983). This sea level high seems limited to southern Tunisia and may be of hydro-isostatic origin.

STOP 5 and 6b : Hypermagnesian carbonates, gypsum and halite of Sabkha el Melah

The Sabkha El Melah is a large evaporitic basin located on the southeastern coast of Tunisia, about 10 kilometers south of the city of Zarzis (fig. 1 ). The sabkha formed during the last 5000 years in a depression cut in late Tertiary (Pontian) and early Quaternary (Villafranchian) continental deposits (figs. 10, 11 ). Along the coast it is bordered by Tyrrhenian marine deposits (well cemented coastal dunes and beach ridges).

During the Flandrian sea level highstand (+ 2 m ), about 5500 years ago, the area of the present-day sabkha was occupied by a wide restricted lagoon, as deep as 30 m, connected with the Mediterranean sea through a narrow channel (fig. 10). This lagoon became hypersaline when sea level started to drop: hypermagnesian carbonates (huntite) which overlie the lagoonal deposits have been dated at 5300 B.P. (Perthuisot 1974). The lagoon was then rapidly filled with evaporites (mainly halite) and evolved into the present-day Sebkha el Melah.

5 km

Mediterranean r----Sea- -

after Perthuisot & Floridia (1973) Figure 10: Stops 5 and 6. Schematic map of the Sabkha el Me/ah (drastically simplified after Perthuisot & F/oridia 1973). The gypsum-rich layers crop out all around the sabkha between the relictic lagoonal coastal deposits and the halite infilling. During the Flandrian sea-level highstand this area was occupied by a wide lagoon reaching up to 30 m in depth.

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Hypermagnesian carbonates are well represented in the Sabkha el Melah; as a matter of fact, one of the world's largest known occurrence of huntite is found here (Perthuisot 1975). Precipitation of magnesite (MgC03) and huntite (CaMg3(C03)⁴⁾ is mediated by sulfate- reducing bacteria (Perthuisot et al. 1990 have documented the presence of bacteria inside the huntite). Precipitation of carbonates (CaC03) and sulfates (CaS0⁴⁾ increases the relative concentration of Mg which forms, together with C03 2-, hypermagnesian carbonates. The huntite is hydrated and appears as a white, sticky paste. Bacterial reduction thus destroys part of the initially formed gypsum and prolongs the formation of carbonates (Perthuisot 1975). H2S is liberated and, together with bacterially produced methane, can give rise to "will-o'-the-wisps"

which are occasionally seen at night by the local inhabitants.

2 CH20

+so/--->

+

+

+ co/-

Gypsum is present as needles, flat crystals, lenses, or small xenomorphic crystals (gypsum mush). It generally grows displacively inside the sediment. At the surface of the border of the Sabkha, swallow-tail gypsum crystals build up "mole hills". These crystals desintegrate easily and accumulate to form gypsum beaches. In the western part of the Sabkha, the gypsum layer also contains polyhalite (K₂Ca₂Mg(S0₄₎₄ * 2 H20; Perthuisot 1975).

Halitic sediment generally is non-consolidated and lacks stratification, indicating a continuous precipitation in the lagoon. Perthuisot (1975) has found dissolution cavities at the base of the halite layer which are due to vertical fluctuations of the brines. Collapse of such cavities is at the origin of "ai"ouns", natural wells filled with black mud and brines.

The surface of the Sabkha is now covered with a layer of halite-rich mud also containing black organic matter (fig. 11). This layer may reach 2 m in thickness (Perthuisot 1975). When it rains, the superficial crust of halite dissolves, only to re-precipitate when the sun appears again. The floating halite crystals are pushed by the wind and may accumulate to form "waves". After a storm, the Sabkha is covered by red-brown sands and silts of aeolian and fluvial origin.

Orn

10

20

30 NW

0 2km

HOLOCENE

g

~ ~

-

halite gypsum and hypermagnesian carbonates

!:f0Z'~~:f:]

D

SE

Mio-Pliocene substrate

PLEISTOCENE organic rich muds

oolitic/bioclastic sands ; stromatolites

D 00 fffl

lacustrine and continental deposits well cemented oolitic deposits (Tyrrhenian) fluvial/estuarine deposits

Figure 11: Stops 5 and 6. Cross section through the Sabkha et Me/ah based on eleven cores (after Perthuisot & Floridia 1973). The location of this profile is given on fig. 10.

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A4 11

STOP Sb and 6a: Cerebroid ooids and serpulid-stromatolite belt (Sabkha el Melah)

The Holocene lagoonal coastal deposits crop out in numerous pits around the Sabkha. They have been described in Davaud et al. (1990, 1994) and show the following stratigraphical succession of facies (fig.12):

1. Coarse cross-bedded gravels with scarce cerithid gastropods near the top; they represent pre-lagoonal alluvial deposits which have been locally reworked in a shoreface environment.

2. Oolitic and cerithid sands with planar lagoonward dipping stratification representing relictic beaches; they are commonly associated with poorly cemented slabs of similarly laminated oolitic and bioclastic sands. The slabs are coated by a thin stromatolitic crust and, on their lower surface, encrusted by Spirorbis worms. Similar slabs occur in the modern upper shoreface (Strasser et al. 1989) where they result from undermining of beach rock by wave action.

3. Serpulid bioherms forming discontinuous ridges parallel to the ancient coastline; they are anchored on stable substrates made of beachrock blocks or coarse gravel.

The bioherms are strongly cemented by botryoidal and spherulitic aragonite which commonly includes benthic foraminifera (Ammonia sp.). This indicates that cementation is synsedimentary in origin and took place while salinity conditions were still favorable to marine benthic life.

4. Serpulid bioherms and beachrock are overlain by domal and pseudo-columnar stromatolites which form a continuous belt along the ancient coastline (Davaud et al. 1994).

5. Red silts fill in the available space between bioherms; they contain abundant displacive gypsum crystals and scattered reworked cerithid shells. The silts still accumulate during winter floodings and tend to overlie the Flandrian foreshore and shoreface deposits.

beach rock

domal pseudo-columnar stromatolites . ~~

--:---:::----;-:::-=::~:-:-0:w~· ·-~ -:--:·i!l!i?'\..~'l. <51~!1§!!.~i'-:'"'":~.:-"'.·/ ~~:~·-;fi.jM. !la ..

~

e:

~1.r .. "'··. ^{' e}

present-day_ surface ofsaokha

· ,,; · ~ di . e · . · · bioclastic oolitic sands (cerebroid ooids)

coarse cross-bedded gravel Sm

I~

Figure 12: Stops 5b and 6a. Typical F/andrian shoreface-foreshore profile reconstructed from numerous pits dug along the borders of the Sabkha e/

Me/ah. The stromatolites developed all around the lagoon, commonly onlapping serpulid bioherms and beachrocks, when the salinity started to increase 5000 years ago (after Davaud et al. 1990).

The stromatolites reach up to 30 centimeters in height and half a meter in diameter. They frequently display loaf-like shapes, with the long axis arranged perpendicular to the Flandrian wave front. When water depth was reduced, the stromatolites tended to develop a pseudo-columnar morphology and grew horizontally . This led to mushroom shaped colonies and further to coalescent colonies which invaded the upper shoreface

Few similar serpulid/stromatolite co-occurrences have been described from the fossil record. Garwood (1931) mentioned the presence of algal stromatolites encrusted with spirorbids in the Lower Carboniferous of northwestern England. Jn the same area and the

-1

l

A4 12

same formations, Leeder (1973, 1975) observed lens-shaped serpulid bioherms reaching in average a few decimeters in height. More recently, Wright and Ma:icall (1981) described from the Upper Triassic in Southwest Britain stromatolites showing an alternation of flat or domal laminated algal units with layers composed of serpulids (Microtubus). It should be noted that all these occurrences are limited to the Carboniferous and the Triassic. Whether their absence in other time intervals results from lack of documentation or whether it is biologically or environmentally controlled, remains an open question.

Two types of ooid are dominant in beach deposits and beachrock slabs :

1. Tangential aragonitic ooids developed around nuclei of gastropod and serpulid fragments, or around spherulitic cores resembling the cements observed in serpulid bioherm cavities.

2. Abundant coarse cerebroid ooids. They are characterized by sectors made of tangentially arranged aragonite needles and micritic sectors consisting of random aragonite needles and aragonitic microspherulites. Similar microspherulites have been observed in pseudo-columnar stromatolites, where they grow around bundles of microbial filaments (Davaud et al. 1994). The random and microspherulitic crystals commonly grow from depressions on the nucleus surface, whereas the tangential pattern develops on nucleus protuberances. This indicates that areas on the grain surface protected from abrasion were favorable sites for microbial activity, which locally inhibited the precipitation of oolitic cortices. This interpretation has been proposed by Kahle (1974) and Richter (1983), and is supported by the frequent association of cerebroid ooids and stromatolites in pre-evaporitic environments (Kalkowsky 1908; Eardley 1938;

Sandberg 1975).

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