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Original article Occurrence of an in situ fern grove in the Aptian Douiret Formation, Tataouine area, South-Tunisia § Présence d’une forêt in situ de fougères dans l’Aptien de la formation Douiret, région de Tataouine, sud de la Tunisie Mohamed Ouaja a , Georges Barale b, * , Marc Philippe b , Serge Ferry b a Faculté des sciences de Gabès, département de géologie, cité Erriadh-Zrig, 6072 Gabès, Tunisia b UMR CNRS 5276, laboratoire de géologie de Lyon : terre, planètes, environnement, université Lyon 1, 43, boulevard du 11-Novembre-1918, 69622 Villeurbanne cedex, France Received 6 October 2010; accepted 3 January 2011 Available online 1 June 2011 Abstract A level with fern stumps was discovered in the Aptian Douiret Formation, South-Tunisia. These stumps are preserved as external moulds or casts, without any preservation of anatomical structures. These stumps are considered to be affiliated with the numerous fossil plants assigned to the fern genera Alstaettia and Piazopteris that are widely distributed in coeval strata from the same region, either as leaf imprints or as permineralized remains. The record of in situ fossil forests for the Southern Hemisphere reveals that their systematic components are different, i.e. mainly corystosperms and/or conifers, and rarely under tidal influence. The way this fern grove settled in a margino-littoral environment is discussed. # 2011 Elsevier Masson SAS. All rights reserved. Keywords: In situ stumps; Ferns; Aptian; Douiret Formation; Tunisia Résumé Première découverte d’un niveau à souches de fougères dans l’Aptien de la formation Douiret, sud de la Tunisie. Il s’agit de nombreuses souches en moulage externe ou interne sans structure conservée. Le matériel est comparé aux végétaux fossiles conservés en structure et empreintes connues dans la région et attribué aux fougères des genres Alstaettia et Piazopteris. Un aperçu des forêts fossiles in situ connues de l’hémisphère sud révèle que leurs compositions systématiques sont différentes, c’est-à-dire corystospermes et/ou conifères, et rarement sous influence tidale. Des explications sont données sur l’installation de cette forêt dans un environnement margino-littoral. # 2011 Elsevier Masson SAS. Tous droits réservés. Mots clés : Souches in situ ; Fougères ; Aptien ; Formation Douiret ; Tunisie 1. Introduction Middle to late Mesozoic rocks of Southern Tunisia, especially the Tataouine area, yield numerous plant impressions and silicified or haematized woody stems. Several early studies have documented Jurassic to Cretaceous examples of fossil woody structures with preserved anatomy from this area (Boureau and De Lapparent, 1951; Boureau, 1953; Koeniguer, 1966, 1971, 1973; Giraud, 1973, 1977). More recent works have surveyed palaeofloras of the Bathonian to early Albian age (Barale et al., 1997, 1998, 2000, 2007; Barale, 1999, 2007; Barale and Ouaja, 2001, 2002; Ouaja et al., 2004) that are dominated by pteridophyte and gymnosperm impressions with the addition of early Angiosperms in the Aptian and early Albian. Evidence of autochthonous herbaceous riparian communities was reported from levels characterized by the occurrence of Selaginellites, Equisetum, Daharia or Isoetites imprints (Barale et al., 1998; Barale, 2007). Here we report the example of an in situ fossil forest with stump bases and root systems in growth position. This permits Geobios 44 (2011) 473479 § Corresponding editor: Gilles Escarguel. * Corresponding author. E-mail address: [email protected] (G. Barale). 0016-6995/$ see front matter # 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.geobios.2011.01.005

Occurrence of an in situ fern grove in the Aptian Douiret Formation, Tataouine area, South-Tunisia

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Page 1: Occurrence of an in situ fern grove in the Aptian Douiret Formation, Tataouine area, South-Tunisia

Original article

Occurrence of an in situ fern grove in the Aptian Douiret Formation,Tataouine area, South-Tunisia§

Présence d’une forêt in situ de fougères dans l’Aptien de la formation Douiret,région de Tataouine, sud de la Tunisie

Mohamed Ouaja a, Georges Barale b,*, Marc Philippe b, Serge Ferry b

a Faculté des sciences de Gabès, département de géologie, cité Erriadh-Zrig, 6072 Gabès, Tunisiab UMR CNRS 5276, laboratoire de géologie de Lyon : terre, planètes, environnement, université Lyon 1, 43, boulevard du 11-Novembre-1918,

69622 Villeurbanne cedex, France

Received 6 October 2010; accepted 3 January 2011

Available online 1 June 2011

Abstract

A level with fern stumps was discovered in the Aptian Douiret Formation, South-Tunisia. These stumps are preserved as external moulds orcasts, without any preservation of anatomical structures. These stumps are considered to be affiliated with the numerous fossil plants assigned to thefern genera Alstaettia and Piazopteris that are widely distributed in coeval strata from the same region, either as leaf imprints or as permineralizedremains. The record of in situ fossil forests for the Southern Hemisphere reveals that their systematic components are different, i.e. mainlycorystosperms and/or conifers, and rarely under tidal influence. The way this fern grove settled in a margino-littoral environment is discussed.# 2011 Elsevier Masson SAS. All rights reserved.

Keywords: In situ stumps; Ferns; Aptian; Douiret Formation; Tunisia

Résumé

Première découverte d’un niveau à souches de fougères dans l’Aptien de la formation Douiret, sud de la Tunisie. Il s’agit de nombreusessouches en moulage externe ou interne sans structure conservée. Le matériel est comparé aux végétaux fossiles conservés en structure etempreintes connues dans la région et attribué aux fougères des genres Alstaettia et Piazopteris. Un aperçu des forêts fossiles in situ connues del’hémisphère sud révèle que leurs compositions systématiques sont différentes, c’est-à-dire corystospermes et/ou conifères, et rarement sousinfluence tidale. Des explications sont données sur l’installation de cette forêt dans un environnement margino-littoral.# 2011 Elsevier Masson SAS. Tous droits réservés.

Mots clés : Souches in situ ; Fougères ; Aptien ; Formation Douiret ; Tunisie

Geobios 44 (2011) 473–479

1. Introduction

Middle to late Mesozoic rocks of Southern Tunisia, especiallythe Tataouine area, yield numerous plant impressions andsilicified or haematized woody stems. Several early studies havedocumented Jurassic to Cretaceous examples of fossil woodystructures with preserved anatomy from this area (Boureau andDe Lapparent, 1951; Boureau, 1953; Koeniguer, 1966, 1971,

§ Corresponding editor: Gilles Escarguel.* Corresponding author.

E-mail address: [email protected] (G. Barale).

0016-6995/$ – see front matter # 2011 Elsevier Masson SAS. All rights reserveddoi:10.1016/j.geobios.2011.01.005

1973; Giraud, 1973, 1977). More recent works have surveyedpalaeofloras of the Bathonian to early Albian age (Barale et al.,1997, 1998, 2000, 2007; Barale, 1999, 2007; Barale and Ouaja,2001, 2002; Ouaja et al., 2004) that are dominated bypteridophyte and gymnosperm impressions with the additionof early Angiosperms in the Aptian and early Albian. Evidence ofautochthonous herbaceous riparian communities was reportedfrom levels characterized by the occurrence of Selaginellites,Equisetum, Daharia or Isoetites imprints (Barale et al., 1998;Barale, 2007).

Here we report the example of an in situ fossil forest withstump bases and root systems in growth position. This permits

.

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Fig. 1. Location map of the Oued Zefrat section (indicated by a star) inSouthern Tunisia.

Fig. 2. Lithostratigraphy of Jurassic to Cretaceous deposits in Southern Tuni-sia. Star indicates the approximate position of the studied level. Formation andmember names in italics are new or have been redefined by Ouaja (2003).

M. Ouaja et al. / Geobios 44 (2011) 473–479474

us to better reconstruct the Mid-Cretaceous local palaeoenvir-onment and vegetation.

2. Geographical and geological framework

The material studied here was discovered 120 km southwestof Tataouine, within a wadi, the Oued Zefrat (Fig. 1). It originatesfrom the Douiret Formation, dated as Aptian and consisting ofsands and clays (Fig. 2 and 3). Sandstones at the base of theformation are rich in fossil wood in association with vertebrateremains including chondrichthyans, Pycnodus or Lepidotus teethand Lepidotus notidanus scales (Bouaziz et al., 1988; Bentonet al., 2000; Cuny et al., 2004, 2010). Pervinquière (1912)assigned the formation to the Aptian on the basis of its sharkfossils. Its sandy-clayey or clayey-sandy lithology with Flaserbedding represents tidally influenced deposits (Ouaja, 2003).

Numerous Middle Jurassic Cretaceous units are representedin southeastern Tunisia, of which the Douiret Formation, AsferGroup, contains the in situ fossil flora studied herein (Baraleand Ouaja, 2002). Each of the three formations in the AsferGroup rests on an erosional unconformity (D1 to D3respectively; Fig. 2). The Douiret Formation is dividedinformally in to the Douiret sandstones at the base and theDouiret claystones member at the top. The sandstones containmany conifer trunks, in some cases more than 5 m long, androot systems fossilized horizontally on bedding surfaces.

3. In situ fossil forest description

The fossil forest soil crops out over ca. 400 square-meters(Fig. 4A), with about 100 in situ stumps. The stump sections are

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Fig. 3. Detailed Oued Zefrat section (modified from Ouaja et al., 2002 for theChenini Mb.). The exact position of the studied level, in the lower part of theDouiret Sandstones Member, is indicated by a star. D1-D5: unconformities.

M. Ouaja et al. / Geobios 44 (2011) 473–479 475

more or less circular, either isolated or associated in pairs, 30–

40 cm high and 5–15 cm in diameter. The spacing betweenstumps ranges from 20 cm to 1 m (Fig. 4E).

These stumps reveal the upper-most (proximal) part of theirroot system (Fig. 4C) and exhibit two types of preservation:casts of vertical axes with root bases commonly dichotomizing(Fig. 4B); and external moulds of axes displaying hollows ofeach axis unfilled by sediment (Fig. 4D). The root system isfilled of sediments and is similar in form as rhizoliths. There isno sign of axes penetrating from underlying layers and no signof roots growing through the tissues of older plants. It seemsthat the in situ community is constituted of plants of the samegeneration. Stems and roots are attached and sometimesslender axes appear very near a main stem but there is noprivileged direction of development. They may representresults of vegetative reproduction. The base of each stem isoften bulged and the roots are slender relatively to the diameterof the stem.

4. Material determination

The in situ stumps we observed lack preserved internalstructures and are difficult to assign to any plant group at firstright. The clays at the base of the Douiret Formation haveyielded a rich impression flora with pteridophytes andconifers (Barale and Ouaja, 2002). The flared base of severalof these stumps are typical of the numerous anatomically-preserved woody fern stems found within the same level, andpreviously assigned to Alstaettia Remy and Remy (Baraleet al., 1998). This genus was originally reported from Tunisiaby Koeniguer (1973) and was established for fern stems withnumerous meristelic bundles fused into concentric vascularrings. This assignment also conforms to the observations weperformed previously on a woody axis preserved as animprint at Bir Kharma (Ouaja et al., 2004). In turn, the fossilmaterial assigned to Alstaettia was in close relationship withthe foliage Piazopteris, a very common fern in SouthernTunisia at that time. Consequently, the stumps we observed inthe Douiret Formation are very probably those of Piazopterisand represent a rare example an in situ fossil fern grove.Ouaja et al. (2004) have described an axis of Alstaettiashowing a bulging base without sizeable roots. It is possibleto make the same observation on many stems of Oued Zefratforest. The presence of slender stems near to main stems ismore typical of ferns forest with vegetative reproduction thanof conifers forest.

Encasements of some rhizoliths transect several sets oftangential cross-beds (Fig. 4F), suggesting plant growth tookplace after deposition of these layers and probably reflectscolonization of an emergent tidal platform by riparianvegetation.

In correlative beds 20 km north in the Remada area, ahaematized stump in internal cast identical morphologically tothose of Oued Zefrat was found. This stump (internal cast) wasattributed to the fern genus Alstaettia on the basis of itsstructure (Ouaja et al., 2004), thus favouring attribution of theOued Zefrat forest stumps to ferns.

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Fig. 4. In situ fossil fern forest. A. General view. B. Cast of trunk showing the departure or radiation of roots. C. Lateral departure of roots. D. External encasement oftrunks (hollow core). E. Multiple stumps in close relationship. F. Lateral view of sandstone encasing stump. Casing retains successive sets of tangential cross-beds.

M. Ouaja et al. / Geobios 44 (2011) 473–479476

5. In situ fossil forests in the Mesozoic of SouthernHemisphere

Mesozoic in situ fossil forests with trees or stumps in growthposition are rare in the Gondwana, the best known being theJurassic Cerro Quadrado fossil forest of Patagonia. The treesencountered there were attributed to conifers with araucarioid,taxodioid or pinoid relationships (Wieland, 1935; Calder, 1953;Stockey, 1977).

An in situ Triassic forest deposit in the central TransantarcticMountains, with 99 silicified trunks, has been attributed byCuneo et al. (2003) to pteridosperms. The palaeoforest grewalong river banks and within proximal flood plain environ-ments. In Antarctica, in the Mid-Triassic of the central part ofthe Transantarctic Cordillera, about one hundred in situ? trunks

were observed and assigned to the Podocarpaceae (Del Fueyoet al., 1995).

Another Antarctic in situ fossil forest is that described byJefferson (1982) from Cretaceous strata of Fossil BluffFormation of Alexander Island, in which all the trees areconifers. Cretaceous strata of President Head, Snow Island,South Shetlands, Antarctica, host in situ roots, fern rhizomes,bennettitalean stems that characterize a flora deposited veryclose to its source area (Philippe et al., 1995).

In Mendoza province (Argentina) the first in situ forestdiscovered by Darwin in 1835 has been studied latter andinterpreted as a mixed forest associated with a palaeocommu-nity of horsetails (Brea et al., 2008). The in situ forest is amixture of corystosperms and conifers. It had a density of427–759 trees per hectare (Brea et al., 2009). The forest was

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M. Ouaja et al. / Geobios 44 (2011) 473–479 477

buried by pyroclastic flows (Poma et al., 2009). Three other insitu Triassic petrified forests dominated by corystosperms, havebeen discovered in Argentina (Zamuner, 1992; Spalletti et al.,1999; Artabe et al., 2001, 2007). Artabe et al. (2007) describe aforest density of 727–1504 trees per hectare.

Pole (1999) described an in situ mixed fossil forest ofconifers (Araucariaceae and Podocarpaceae), Pentoxylales andferns (Osmundaceae) from the Middle Jurassic of NewZealand. The conifers, about 100-year-old, were at least10 m high but some reached 200 years and probably exceeded30 m in height. Stand density was estimated to be 552–851 treesper hectare, whereas the Early Cretaceous fossil forest fromAntarctica was estimated to 558 trees per hectare. A MiddleJurassic fossil forest (conifers) is described from New Zealandby Thorn (2005). A total of 238 stumps were observed growingat high southern palaeolatitude, preserved as successive forestgenerations in a sandy braided river facies association.

Off the South African Namaqualand coast, Stevenson andBamford (2003) used a submersible to study in situ fossil treetrunks in exposed Late Cretaceous (late Coniacian) seaflooroutcrop over 2 km2. All trunks are from conifers related to thePodocarpaceae.

In conclusion, most of in situ fossil forests of Gondwanawere preserved in alluvial depositional settings (i.e., flood-plains, levees or swamps) with a systematic compositionmainly dominated by conifers or pteridosperms. At the OuedZefrat locality the density was estimated to be ca. 1000 stumpsper hectare. This estimation could be over-inflated as all castsand moulds were count yet some of them may have beenupturned roots and not stems. For his calculation, Pole (1999)took into account only those stumps which were more than80 mm in diameter, and Jefferson (1982) only those exceeding100 mm in diameter. Here, all vertical casts displayingperpendicular root bases were considered.

Although an autochthonous origin was hinted at for thefossil plants in Southern Tunisia (Barale, 2007), this is the firstreport of an in situ fern grove. Even though the measureddensity could appear slightly over estimated, it is clear thatecological conditions for plant growth were highly suitable. Sealevel fluctuations over this low-gradient coastal shelf probably

Fig. 5. Holocene root moulds very similar to those from south Tun

led to the temporary exposure of extensive substratesfavourable to the establishment of pioneer vegetation.Vegetative multiplication through the rhizome systems couldalso explain the high density of erect axes we observed. Thestump uniformity allows us to dismiss the idea that this forestcould have been of mixed type, with both ferns and conifers. Itis probable that ferns were able to colonize newly emergedareas with high water tables and unconsolidated substratesmuch easier than could conifers.

6. Palaeoenvironment of the fossil forest

Once judged as mostly fluvial, and belonging to the so-called‘‘Continental Intercalaire’’, Upper Jurassic to Lower Cretaceousdeposits of Southern Tunisia (Fig. 2) have been recently revisitedand reinterpreted during an extensive geological mappingprogram (Ouaja et al., 2002; Ouaja, 2003). Most, if not allsandstones are marine to paralic, being deposited as transgres-sive, tidal flat sequences resting on variably developed basalconglomerate bearing angular silcrete pebbles to blocks, roundeddolomite pebbles reworked from underlying successions,silicified wood debris, vertebrate bones, and shark teeth. Thesetransgressive lag deposits are the only depositional environmentnow recognized for the rich plant and dinosaur remains of thearea. The basal, rippled to megarippled sandstones of thetransgressive sequences typically grade up to green clays then tofull-marine, macrofossil-bearing dolomites representing themaximum flooding facies. However, some sequences lack theupper part of the deepening-upward facies succession. Tidalsandstones at base of the sequences commonly bear largesilicified conifer trunks. These trunks are interpreted to derivefrom the destruction by shoreline advance of the maturehinterland forests. Five major laterally-continuous transgressiveunconformities, once interpreted as fluvial channel lag deposits,have been recognized and used to demarcate the Upper Jurassicto Lower Cretaceous sedimentary Formations (Fig. 2). The basalSandstone Member of the Douiret Formation, Oued Zefratsection (Fig. 3) is a sandy/clayey heterolitic deposit interpreted tohave been laid down during the late stages of a short transgressiveinterval. Benefiting from the humidity and groundwater of the

isia (Boa Vista Island, Cape Verde Islands). Scale bar: 10 cm.

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coast, monospecific pioneer vegetation rapidly colonized theseemergent settings.

Cementation of sand by precipitation of calcite around theroots is clearly a post-depositional process since the primarysedimentary features such as oblique tangential cross-beds areundisturbed (Fig. 4F). A case of similar extant plant root systempreservation in the sub-actual sand dunes of Boa Vista Island(Caho Verde) was observed. The root casts and moulds found inthe Cretaceous of Tunisia strongly resemble the recentcounterparts found in the northeastern shore of Boa VistaIsland (Cape Verde Islands; Fig. 5) or rhizoliths described in aPleistocene Formation (Jones and Kwok-Choi, 1988). Thesecasts occur on a Holocene eolian dune which has beentemporarily covered by a vegetation, probably during anepisode of wetter climate. Trade winds have partially blown thesand of the dune, uncovering the root casts. These are sheaths ofcalcite-cemented sand surrounding a cylindrical void left bysand around the roots, of which there is no remain now withinthe axial hole of the moulds.

The xerophyte plants living in the sandy environmentsprotect their roots by the secretion of a mucilage that coheressediment particles together, generating living moulds aroundthem. Such roots are much more prone to the in situ goodpreservation of the system. The presence of in situ buriedrhizomes or roots is possible in a non-oxidizing environmentand with a high level of sedimentation (hydromorphic soils;Retallack, 1999).

7. Conclusion

The discovery of an in situ monospecific fern forest at OuedZefrat, Southern Tunisia, props up the previous observations ofautochthonous plants deposits of Lower Cretaceous. Thebibliography analysis shows the rarity of in situ Mesozoic fernforests in Gondwana land, and Oued Zefrat locality constitutesa link to the knowledge of the fern palaeoenvironment. Therarity of in situ fossil coastal fern forests may be explained bytheir position in newly exposed substrates at the top ofregressive deposits, a position that subjected them to erosion bysucceeding transgressive beach processes.

Acknowledgments

We are grateful to Sophie Cros for help with the figures. TheOffice National des Mines of Tunisia and UMR PEPS 5125 arethanked for their logistic support. This manuscript has benefitedfrom comments and reviews by S. McLoughlin and M. Bamford.

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