Keywords

1 Introduction

The Traras Basin (Fig. 1a) has been the subject of several studies: structural [1], biostratigraphic and sedimentological [2] which allowed it to be integrated palaeogeographically into the Tlemcenian field. During the Pliensbachian-Toarcian transition, the Traras basin was marked by major paleo-environmental and paleo-oceanographic changes. They were correlated with the events of the western Tethys. In particular, we cite sea level variations, carbonate production crisis, δ13Cand δ15N geochemical disturbances and an anoxic ocean event (T-OAE) [3–5]. In the present work, and through the study of two sections (Benzerka and Mellala, northern part of the Traras basin) (Fig. 1a), we provide information for the first time on the palynological components of the Lias in Algeria. The systematic inventory of the different palynomorphs (spores, pollen grains, acritarchs and algae) aims to characterize the Pliensbachian-Toarcian transition as far as possible.

Fig. 1
figure 1

Location map of the study area (a) (after [3]) and stratigraphic column of the Benzerka and Mellala sections (b and c)

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2 Stratigraphic Framework

As part of the Tlemcenian domain, the two selected sections (Figs. 1b, c) will be described in detail.

2.1 Benzerka Sect. (32, 65 m Thick) (Fig. 1b)

This section shows the following lithostratigraphic succession.

Tisseddoûra Limestone Fm. (5.90 m): it is represented by brecciated limestones, bioclastics, followed by clay limestones. Upwards, it changes to marl-limestone alternations. The macropaleontological content (Tetrarhynchia ageri and Spiriferina sp.), on the one hand, and micropaleontological ones (namely the benthic foraminifera Lingulina tenera carinata, Lenticulina acutiangulata mg. Lenticulina, Lenticulina, Lenticulina gottengensis mg. Lenticulina, Marginulina prima, Spirillina infima, Bolivina liasica, Lingulina tenera tenera), on the other hand, give a Late Pliensbachian age (Algovianum Zone).

Bayada Fm. (26.75 m): it is essentially a marl-limestone alternation (Late Pliensb chian-Early Toarcian) (Emaciatum-Levisoni Zones).

2.2 Mellala Sect. (47 m Thick) (Fig. 1c)

Located a few meters from the road (Ghazaouet-Nadroma), the Mellala section is represented essentially by the “Bayada Beds” which consist of marl and limestone alternation. The lithostratigraphic succession shows from bottom to top:

  • 6.20 m: alternation of marls and decimeter-thick beds of well-stratified marly limestone, very rich in ammonites, belemnite rostra and brachiopods.

  • 7.80 m: alternation of marls and centimeter-thick beds of marly limestone with nodular appearance. The rich macrofauna is mainly represented at the top by the ammonite, Paltarpites paltus and Dactylioceras (Eodactylites) pseudocommune indicating the transition between the Late Pliensbachian (Emaciatum Zone) and the Early Toarcian (Polymorphum Zone).

  • 25.25 m: alternation of marls and centimeter to decimeter-thick beds of marly limestone with a nodular appearance. Note the presence at the base of zoophycus and some brachiopods and ammonites.

  • 07.75 m: marls and centimeter to decimeter-thick beds of marly limestone.

Note that the stratigraphy of this section was described by Elmi et al. (2009). For example, brachiopod-associated ammonites and foraminifera were used to date the Late Pliensbachian (with the Emaciatum Zone) and the Early Toarcian (with the Polymorphum and Levisoni Zones).

3 Analysis of Palynological Assemblages

Quantitative analysis showed a predominance of the terrestrial elements represented by spores and pollen grains along the Late Pliensbachian- Early Toarcian interval (Figs. 2 and 3). Thus, we have been able to note the following quantitative variations:

Fig. 2
figure 2

Distribution of palynomorphs in the Benzerka section

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Fig. 3
figure 3

Distribution of palynomorphs in the Mellala section

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  • Of the Late Pliensbachian (Algovianum-Emaciatum Zones): the “Tisseddoûra Limestones” (Algovianum Zone), the ten (10) samples of the Benzerka section yielded spores and pollen grains consisting of: Rotverrusporites tenuis, Reticulatisporites sp., Zonapollinites sp., Araucariacites liasicus, Lycopodiumsporites reticulatum, Classopollis sp., Inaperturopollenites sp., Spheripollenites subgranulatus, Spheripollenites sp., Exesipollenites sp., Cerebropollenites sp., associated with an acritarch, Micrhystridium sp. and the algae: Tasmanites sp., Tytthodiscus sp., Prasinophytes sp. and forms whose determination remains unfortunately very difficult (Incertae Sedis). In addition, the palynological residues of eighteen (18) samples from the Bayada Beds (Emaciatum Zone) contained palynological assemblages consisting of spores and pollen grains (Varirugosisporites sp., Leiotriletes sp., Lycopodiacidites sp., Calamospora sp., Piloseporites sp., Deltoidospora sp., Callialasporites segmentatus, Bennettitaepollinites sp.). Acritarchs and algae are represented (Tasmanites puntatus, Leiosphaeridia sp., Crassosphaera sp., Tapajonites sp., Umbellasphaeridium sp., Baltisphaeridium sp., Acanthodiacrodium sp.).

  • In the Early Toarcian (Polymorphum-Levisoni Zones): the “Bayada Beds”, in the Polymorphum Zone produced a relatively poor palynological assemblage, which included some spores and pollen grains (Reticulatisporites sp., Verrucosisporites sp., Lycopodiumsporites sp., Rotverrusporites tenuis, Lycopodiacidites sp., Spheripollenites sp., Classopollis sp., Inaperturopollenites sp., Araucariacites sp.), associated with scanty and diversified algae (Tasmanites puntatus, Tasmanites sp., Leiosphaeridia sp., Botryococcus sp., Prasinophytes sp.) and indeterminate forms (Incertae Sedis). Acritarchs are completely absent. Moreover, in the Levisoni Zone, the palynological association is dominated by spores and pollen grains: Cyathidites minor, Cyathidites australis, Reticulatisporites castellatus, Reticulatisporites sp., Lycopodiumsporites clavatoides, Lycopodiumsporites sp., Classopollis sp., Classopollis torosus, Araucariacites sp., Spheripollenites subgranulatus, Spheripollenites sp., Exesipollenites sp., Cerebropollenites sp., Inaperturopollenites sp. accompanied by acritarchs (Micrhystridium sp., Umbellasphaeridium sp., Baltisphaeridium sp., Acanthodiacrodium sp.) and algae (Tasmanites sp., Tytthodiscus sp., Leiosphaeridia sp., Prasinophytes sp.).

4 Conclusion

The qualitative and quantitative results of the palynological assemblages found in the Pliensbachian-Toarcian interval show that the boundary between the Algovianum Zone and the Emaciatum Zone is marked by the appearance of a group of sporomorphs consisting of Cyathidites minor, Verrucosisporites sp., Monosulcites sp., Lycopodiumsporites clavatoides, Zonalappollenites sp., Classopollis sp., Araucariacites liasicus, Araucariacites sp., Monosulcites sp., Reticulatisporites castellatus. The boundary between the Polymorphum Zone and the Levisoni Zone is distinguished by the appearance of taxa: Cyathidites australis, Lycopodium sp., Classopollis torosus and Botryococcus sp. This palynological study, carried out for the first time in Algeria, allowed us to identify 14 (fourteen) pteridophyt spores, 15 (fifteen) gymnosperm pollen genera, 4 (four) acritarchs and 08 (eight) algae that show a great similarity with palynomorphs found in some northern and southern Tethyan basins.