Abdessamed Mahboubi

Raimund Feist

Jean-Jacques Cornee

Abdelkader Ouali Mehadji

Catherine Girard
email: catherine.girard@univ-montp2.fr

Jean-Jacques Cornée

Frasnian (Late Devonian) conodonts and environment at the northern margin of the Algerian Sahara platform: the Ben Zireg section

Keywords: Frasnian, conodont biostratigraphy, biofacies, depositional environment, northwestern Algerian Sahara

Introduction

The Ben Zireg area at 50 km NE of Bechar township constitutes the north-eastern most exposure of Saharan Palaeozoic deposits adjacent to the Main Atlas fault. They differ from all neighbouring outcrops of the Bechar basin such as Soltane el Betoum and Maider el Mhadjib to the N and W of Djebel Antar [START_REF] Weyant | Relationship between Devonian and Carboniferous strata near the northern confines of the Bechar basin, Algeria[END_REF] (Fig. 1a) in comprising a well exposed, almost complete succession of marine sediments from Cambrian-Ordovician Page 2 of 33 Proof For Review through Early Carboniferous. As such the Ben Zireg section is a reference for the northern margin of epicontinental Sahara in SW Algeria, at 200 km in distance from the Palaeozoic of the Ougarta region further to the South (Fig. 1a). A major contribution to its structure and stratigraphy by [START_REF] Pareyn | Les Massifs Carbonifères du Sahara Sud-Oranais[END_REF] was mainly devoted to the Carboniferous, whereas apart from a few scattered data, the Devonian, and Frasnian in particular, remained largely unstudied regards biostratigraphical subdivision and sedimentary development.

After the first recognition of Devonian fossils at Ben Zireg by [START_REF] Flamand | Recherches géologiques et géographiques sur le Haut-Pays de l'Oranie et sur le Sahara (Algérie et Territoires du Sud)[END_REF], [START_REF] Menchikoff | Etude géologique sur les confins algéro-marocains du Sud[END_REF] established the existence of Late Devonian series based on cephalopods. Among them Pharciceras was considered at that time representative of the early Frasnian. This taxon, however, is now restricted to the Latest Givetian since the definition of Frasnian stage boundaries by SDS/IUGS [START_REF] Klapper | Decision on the boundary stratotype for the Middle/Upper Devonian Series boundary[END_REF], 1993). The presence of Frasnian strata was definitely established when [START_REF] Massa | Observations sur les séries Siluro -Dévoniennes des confins algéro-marocains du Sud[END_REF] recovered the first conodonts, notably Ancyrognathus asymmetricus and Ancyrodella curvata.

Since then, and in contrast to numerous conodont-based biostratigraphical investigations in the neighbouring Tafilalet area (e.g. Belka et al., 1999), no more work was carried out to evaluate the extent and completeness of the early Upper Devonian in the Algerian part of northern Sahara.

This study aims establishing fine-scaled conodont biostratigraphy and biofacies as well as sedimentary dynamics of the Ben Zireg section to emphasise the importance of its pivotal position between Frasnian sections on both sides of the Main Atlas fault, i.e. the allochthonous Hercynian Meseta of Central Morocco, and their eastern prolongation into the Algerian Hercynian Tifrit Horst, and cratonic Gondwana sections both in SW Algeria (Marhouma) and Tafilalet / Maider in SE Morocco (Fig. 1a).

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Geological Setting

In contrast to Palaeozoic rocks north of the alpine Main Atlas fault which were affected by complex tangential tectonics and deformation during the Variscan orogeny (e.g. [START_REF] Allary | Etude tectonique et microtectonique d'un segment de chaine hercynienne dans la partie sud-orientale du Maroc central[END_REF], the Palaeozoic deposits of the western Sahara platform north of the Precambrian Reguibat shield (Fig. 1a) remained generally only weekly deformed to wide synclinal and narrower anticlinal structures, where Devonian sedimentation rates reflect basin and ridge depending depositional environments [START_REF] Elmi | Quelques environnements des faciès noduleux mésogéens[END_REF][START_REF] Conrad | Late Devonian regression and Early Carboniferous transgression on the Northern African platform[END_REF][START_REF] Wendt | Sedimentary evolution of a Palaeozoic basin and ridge system: the Middle and Upper Devonian of the Ahnet and Mouydir (Algerian Sahara)[END_REF]. Exposures of the north-eastern Bechar basin close to the Atlas fault are more intensely folded to form narrow W-E trending anticlinal structures, often over-thrusted to the N, and these features along with the occurrence of synorogenic wildflysch sediments with olistolithes of various ages characterising the Dinantian of Ben Zireg [START_REF] Pareyn | Les Massifs Carbonifères du Sahara Sud-Oranais[END_REF] might indicate dynamic relations to the Maghrebian variscan belt in the north.

In this context the Ben Zireg Palaeozoic forms an acute, narrow anticlinal structure with subvertical axial plane, opening to the ENE under 50°, before being unconformably covered by continental Permo-Triassic sediments and marine Liassic and Cenomanian-Turonian deposits (Fig. 1b). The structure is crossed by numerous vertical faults that run sub-parallel to the Main Atlas fault and, affecting also the Cretaceous cover, may belong to the alpine transform system. The eastern end of the structure is framed by a prominent ridge of some 200m of Middle through Late Devonian carbonates easily accessible from the S where the road-truck from Ben Zireg station to Taghit runs along the Oued Ben Zireg shortly before it bents to the S (Fig. 1b). The investigated section in the Frasnian is situated on the steep southern flank of the Devonian ridge along a southerly directed, deeply incised gorge (coordinates: N 31° 54' 39.4" W 001° 47' 58.8", Fig. 1b).

Material and Methods

One single section was logged in detail and thirty-seven oriented samples were taken both for sedimentological investigations and conodont research. Thin slices from all samples were cut to investigate the corresponding microfacies. Microfacies were determined by using optical observations under photonic microscope. Depositional settings result in zonations proposed by [START_REF] Wright | Shallow-water carbonate environments[END_REF] and [START_REF] Flügel | Microfacies of carbonate rocks. Analysis, Interpretation and Application[END_REF]. Noticeable is a general recrystallization of micrite into microsparite in all samples, and the presence of pressure-dissolution surfaces indicative of a discrete cleavage. These feature are related to the Variscan deformation and do not allow observations of the primary matrix of the rocks.

For conodont extraction, the calcareous samples were dissolved in 10% acetic acid. The sulphic acid (10%) was used to dissolve iron-and pyrite-rich samples (beds BZ1A and 1C and BZ2). The insoluble residue was then rinsed through two sieves (1mm and 100µm). Residue smaller than 1 mm were dried and handpicked under a stereomicroscope. Specimens were photographed with SEM (JEOL 5600). All platform (P1) elements were picked (2660 elements)

(Table 1). The limestone samples yielded relatively abundant conodont elements (mean of 115/kg). Some of these elements were identified at the specific level for biostratigraphy. In this paper the biostratigraphic zonations are those of [START_REF] Klapper | The Montagne Noire Frasnian (Upper Devonian) conodont succession, In Devonian of the World[END_REF]Klapper ( , 2007a) ) for the Frasnian and Klapper (2007b) and [START_REF] Schülke | Early Famennian conodont biostratigraphy of the stratotype area (Montagne Noire, Southern France)[END_REF] for the lowermost Famennian.

Among the platform P1 elements, different genera can be recognised that are supposed to have different ecological preferences (e.g. [START_REF] Seddon | An ecologic model for conodonts[END_REF][START_REF] Sandberg | Conodont biofacies of late Devonian Polygnathus styriacus Zone in western United States[END_REF]. The changing distribution patterns of conodont abundance (called biofacies) in space and time can therefore provide some information about the depositional environment. The biofacies is [START_REF] Sandberg | Conodont biochronology, biofacies, taxonomy and event stratigraphy around Middle Frasnian Lion Mudmound (F2h), Belgium[END_REF]Sobstel et al., 2006). The variations of relative abundances of these 5 different genera were investigated through the Ben Zireg section. The relative abundances of the four groups (Palmatolepis, Icriodus, Polygnathus and ancyrodellids) are represented as percentage of the total assemblage.

Results

4.a. Lithostratigraphy

The Frasnian deposits at Ben Zireg section are 26.5 metres thick, between sample BZ1

(Givetian) and sample BZ16 (early Famennian) (Fig. 2). Sedimentary rocks display a rhythmic pattern characterized by cm to dm-thick alternations (couplets) of hard, fine-grained limestones and softer, argillaceous fine-grained limestones or claystones. In the field, these alternations clearly display a rather constant organization which indicates sedimentary cycles and subcycles (Fig. 3). Sedimentary cycles are limited at top and bottom by thick, argillaceous carbonates or dark claystones. Between, twenty couplets are generally found, organized into four lithological subcycles, each composed of five couplets. Sedimentary

Page 6 of 33 Proof For Review subcycles are defined by rhythmic facies and bed-thickness changes. In the present-day of our knowledge, it is not possible to assign a clear significance to these sedimentary cycles.

The Frasnian deposits comprise 15 sedimentary cycles. From bottom to top, a general increase in thickness of sedimentary cycles is observed through the Frasnian deposits: cycles 1 to 9 and 11 to 12 are 1 to 2m thick, whilst the cycles 10 and 13 to 15 are 2.5 to 3.5 m thick.

The facies are rather homogenous with rare macrofossil remains, but slight differences can be depicted, from bottom to top: 1) Above sample BZ1 is a cm-thick iron crust. Between 0.6 and 4m above the base of the section, there are interbedded cm to dm thick massive, greyish to brownish micritic limestones and cm to dm thick argillaceous limestones. The first beds above the hard-ground are cherty and ferruginous (BZ 1b). The massive limestones are tentaculid-rich wackestones.

The top of some beds can be coated by iron hydroxydes. Limestones display a discrete nodular structure and some pyrite cristals.

2) Between 4 and 10.7 m above the base of the section are interbedded ferruginous limestones and argillaceous limestones (Fig. 4a). Ferruginous limestones display a pronounced pseudo-nodular structure underlined by iron films around micritic nodules because of pressure-dissolution process of tectonic origin or/and bioturbation (=bioerosion; [START_REF] Elmi | Quelques environnements des faciès noduleux mésogéens[END_REF]. Limestones are mudstones to wackestones with tentaculids (Fig. 4d), some entomozoan crustaceans and pelagic molluscs ("filaments"). Pyrite crystals are present and sparse phosphate grains occur. Most of the beds are coated by thin iron coatings.

Argillaceous limestones also display nodular structure and iron films.

3) Between 10.7 and 15.5 m above the base of the section, massive and argillaceous limestones are found. These are bioturbated mudstones with rare pelagic organisms

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Proof For Review (tentaculids, entomozoan crustaceans, pelagic molluscs). They also yielded foraminifers in residues obtained for conodont extraction. Some cm-thick blackish shales are also interbedded.

4) Between 15.5 and 19.5 m above the base of the section are massive, greyish pseudonodular limestones. These are mudstones with some pelagic organisms (tentaculids, entomozoan crustaceans, pelagic molluscs) and some benthic ostracods (Fig. 4e).

5) Between 19.5 and 26.5m above the base of the section deposits are dominant pinkish to greyish pseudo-nodular limestones and interbedded argillaceous micrites (Fig. 4b).

Limestones are tentaculid mudstones, with some entomozoan crustaceans, pelagic molluscs and goniatites. They also yielded rare benthic ostracods, foraminifers, and rare fragments of brachiopods and trilobites; the latter exclusively occur in dish-like flattened mud pebbles occurring on the surface of a single bed (Bed 14 d with Acuticryphops acuticeps; Fig. 4f). The uppermost part of the Frasnian deposits consists of a 30 cm thick black shale level overlain by 35 cm bed of plane-laminated, grain-sorted, pinky calcisiltite between samples BZ15c and BZ16 (Fig. 4c). The significance of this reddish calcisiltite remains enigmatic. It could have resulted in turbidite current activity carrying detrital carbonates and iron oxides of possibly of continental origin into the offshore domain.

4.b. Biostratigraphy

Distribution of conodont platform elements is given on Fig. 2, and some biostratigraphic species are presented on the Fig. 5. Genera abundance per level is listed on Table 1.

The first level (BZ1) delivered Polygnathus linguiformis linguiformis (gamma morphotype), Po.

linguiformis mucronatus, Po. linguiformis weddigei, Po. pseudofoliatus and Po. latifossatus indicating the Givetian [START_REF] Aboussalam | Das "Taghanic-Event" im höheren Mittel-Devon von West-Europa und Marokko[END_REF].

The following level BZ1A indicates MN Zone 5 due to the occurrence of Palmatolepis punctata. This level coincides with the base of the Middle Frasnian (Becker & House, 2000a).

Polygnathus dubius and Po. decorosus also occur within this zone.

The MN Zone 6 is identified with the entry of Ancyrodella gigas form 2 in bed BZ2. Icriodus symmetricus, Po. decorosus, Po. dubius are also present in this zone.

The occurrences of both An. lobata and An. curvata (early form) in bed BZ4B.1 mark the base of the MN Zone 7.

The MN Zone 8 is identified by the co-occurrence of Pa. hassi and Ag. coeni.

An undifferentiated MN

4.c. Temporal calibration of terminal Frasnian black shales

Temporary dysoxic events are recorded by several thin intervals of black shale deposits that occur throughout the section, especially at the top of the succession where a 30 cm thick discrete black shale level (between BZ 15 C and 15D) is developed. The Lower Kellwasser is known to occur at the boundary between the MN zone 12 and MN zone 13 [START_REF] Girard | Quantitative conodont-based approaches for correlation of the Late Devonian Kellwasser anoxic events[END_REF]. At Ben Zireg the Lower Kellwasser event is located at the end of sedimentary cycle 12 and beginning of cycle 13, coinciding with the entry of the marker Palmatolepis bogartensis. It is not defined by a distinct horizon but rather constitutes a two metres-thick succession of dark micritic limestones and few black shales. In this interval the percentage of Icriodus elements increases (Fig. 6). The Upper Kellwasser horizon that comprises at Ben Zireg black shale deposits in its lower part and reddish calcisiltites in its upper part (BZ 15C and D), represents the uppermost part of MN Zone 13 before being superseded by basal Famennian strata (BZ 16). As the most basal part of the lower triangularis Zone has not been established, a small depositional gap might exist. This gap might be related to a sea-level instability, materialized by the pinkish detrital level. Such instability is often recorded at the top of the Upper Kellwasser horizon event (i.e. [START_REF] Girard | Quantitative conodont-based approaches for correlation of the Late Devonian Kellwasser anoxic events[END_REF][START_REF] Schindler | Die Kellwasser-Krise (höhe Frasne-Stufe, Oberdevon)[END_REF].

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4.d. Biofacies

Numerous studies using the biofacies approach were performed through the Frasnian, from very different parts of the world, showing that the assemblages were dominated by Polygnathus during the early and middle Frasnian and then by Palmatolepis at the end of the Frasnian (e.g. Morocco: [START_REF] Lazreq | Biostratigraphie des conodontes du Givétien au Famennien du Maroc central -Biofacies et événement Kellwasser[END_REF]China: Ji & Ziegler 1993; USA: [START_REF] Sandberg | Upper Devonian of Western United States. In Devonian of the World[END_REF]Russia: Ovnatanova et al., 1999). At Ben Zireg, three biofacies have been distinguished from bottom to top (Fig. 6): polygnathid-icriodid, polygnathid-ancyrodellid and then palmatolepidpolygnathid. Several peaks of Icriodus occur successively in Zone MN5 and through MN8 to MN10 zones. Ancyrodellids are never dominant but their percentage can reach more than 20% of the assemblage at the end of Zone MN11 and during Zone MN12.

The change from relatively shallow-water polygnathid-icriodid and polygnathid-ancyrodellid to deep-water palmatolepid-polygnathid biofacies through the Frasnian correlates with conclusions of [START_REF] Johnson | Devonian eustatic fluctuations in Euramerica[END_REF] and trends in conodont biofacies that have been observed globally.

Interpretations and Discussion

5.a. Biostratigraphy and depositional environment

The fine-scaled conodont stratigraphy in the Ben Zireg section bears witness to a considerable stratigraphic hiatus at the base of the section. The underlying bed (BZ1) yields a Givetian, probably late Givetian conodont association where no typical Frasnian elements are found. This age attribution is emphasised by the occurrence of Pharciceras at Ben Zireg mentioned by [START_REF] Menchikoff | Etude géologique sur les confins algéro-marocains du Sud[END_REF] but not recovered since [START_REF] Massa | Observations sur les séries Siluro -Dévoniennes des confins algéro-marocains du Sud[END_REF]. As the superseding bed (BZ1B) already yields Pa. punctata, the entire early Frasnian is missing (i.e.

MN Zone 1 to MN Zone 4).

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12

The interval of Zones MN5 to MN10 is only 9 m thick, attesting to severe condensation of deposits under slow sedimentation rates. Conversely, sedimentation rates increased considerably thereafter. Indeed, sedimentary cycles 10 to 15 comprise an interval of 17 m in thickness that represents two-thirds of the entire Frasnian deposits. The sedimentary cycles increase in importance from bottom to top of the section. Deposits of Zone MN13 reach a thickness of 9 metres that correspond to one third of the total thickness of the section.

The resumption of the Middle Frasnian sedimentation is neither marked by erosional feature nor transgressive deposits. Noticeable is only the presence of ferruginous cherty beds just above the iron crust (bed BZ 1b). Sedimentary rocks are condensed in the lower part of the section (cycles 1 to 6 with frequent iron coatings as micro-hard-grounds, phosphatic grains).

They display a rhythmic pattern dominated by micritic deposits. Detrital deposits are restricted to a few shaly interbeds. Sediment texture is widely dominated by mudstones and fewer wackestones. The fauna is poorly diversified, mostly represented by pelagic organisms (tentaculites, pelagic bivalves, entomozoan crustaceans, goniatites and conodonts) associated with some autochtonous benthic foraminifers and benthic ostracods, and rare fragments of trilobites and brachiopods in flat mudstone pebbles. These criteria indicate that the Frasnian deposits were emplaced on a distal ramp or distal platform setting (e.g. [START_REF] Franke | Pelagic carbonates in the Variscan belt -their sedimentary and tectonic environments[END_REF], below wave action base and very far from emerged areas. Condensation in intervals 1) and lower 2) could indicate a distal submarine rise. The interval 3) between 10.7 and 15.5m yield bioturbation features, rare pelagic organisms, abundant foraminifers, benthic ostracods and numerous Icriodus. These indicate that the corresponding sediments were probably deposited at shallower depth than both the overlying and underlying ones.

The interval 4) during the MN 13 Zone interval indicates a return to deeper outer ramp Page 12 of 33 Proof For Review setting. Noticeable is the presence of some soft pebbles levels composed of mudstones with trilobites, indicative of downslope transport.

5.b. Conodont distribution patterns

In order to elucidate the significance of the conodont biofacies, assemblages from Ben Zireg were compared with those established from the Mahrouma section (Ougarta area) by Mahboubi et al. (2013a and b;Fig. 6).

The Marhouma section is distinct from Ben Zireg in the lower part of the Middle Frasnian where a pure Polygnathus biofacies prevails. Thereafter, the Marhouma pattern is consistent with the trends of the Ben Zireg section: first by a clear increase of the percentage of ancyrodellids during the MN11 Zone (base of the Upper Frasnian), second by an obvious increase of Palmatolepis till the Frasnian/Famennian boundary.

In detail, a slight shallowing is indicated during early late Frasnian (MN Zones 11-12) in both sections due to the obvious increase of Ancyrodellids. As depositional environments are different, i.e. submarine rise vs subsiding basin, this trend seems independent from local setting dynamics.

5.c. Gaps at the Givetian/Frasnian transition

Occurrences of gaps in the later Devonian, and at the Givetian-Frasnian transition in particular, are quite common in sections South of the Atlas Fault. This is most obvious in Devonian outcrops of the Bechar Basin N and W of Ben Zireg where Lower Visean conformably supersedes various levels from Tournaisian to Late Emsian [START_REF] Weyant | Relationship between Devonian and Carboniferous strata near the northern confines of the Bechar basin, Algeria[END_REF], bearing witness of the marked diachronism of the omission. Whether the latter is due to Page 13 of 33

Proof For Review 14 syntectonic mouvements as [START_REF] Weyant | Relationship between Devonian and Carboniferous strata near the northern confines of the Bechar basin, Algeria[END_REF] claims or results from current induced erosion or influence of differentiated submarine scenery into sea-mounts and depressions is up to debate.

The Ben Zireg section can be compared with other conodont-dated sections in Algeria and Morocco (Fig. 6). Whereas in the basinal Marhouma section the Givetian-Frasnian transition is presumably complete [START_REF] Göddertz | Devonische Goniatiten aus SW-Algerien und ihre stratigraphische Einordnung in die Conodonten-Abfolge[END_REF]Mahboubi et al., 2003 a), various sections on the Tafilalet platform exhibit hiatuses (Becker & House, 2000b): Jebel Mech Irdane (latifossatussemialternans -MN 4 zones, [START_REF] For Review | Globale events im Grenz-Bereich Mittel-/Oberdevon[END_REF][START_REF] Walliser | The Jebel Mech Irdane section[END_REF][START_REF] Aboussalam | Das "Taghanic-Event" im höheren Mittel-Devon von West-Europa und Marokko[END_REF], Jebel Amelane (MN 1-6 zones, Becker & House 2000a), Bou Tchrafine (lower part of the MN zone 5, Becker & House, 2000b;[START_REF] Aboussalam | Das "Taghanic-Event" im höheren Mittel-Devon von West-Europa und Marokko[END_REF] and Hamer El Khdad section (disparilis-MN zone 4, [START_REF] Gouwy | Conodont-based graphic correlation of upper Givetian-Frasnian sections of the Eastern Anti-Atlas (Morocco)[END_REF]. In these areas, the extent of the gap is variable and may range between the Givetian disparilis zone and the Frasnian MN zone 6. Hiatuses in the same time interval were also noted in northern Morocco, France, Germany and Austria, implying a supra-regional distribution (summarized in Table 1 of [START_REF] Hüneke | Erosion and deposition from bottom currents during the Givetian and Frasnian: Response to intensified oceanic circulation between Gondwana and Laurussia[END_REF]. These hiatuses have been interpreted as the result of a strong current-induced erosion, often occurring within lower parts of fossil contourite units [START_REF] Hüneke | Early Devonian (Emsian) to Late Devonian (Famennian) stratigraphy and conodonts of the Antoinettenweg section in the Lower Harz Mountains (Germany)[END_REF][START_REF] Hüneke | Erosion and deposition from bottom currents during the Givetian and Frasnian: Response to intensified oceanic circulation between Gondwana and Laurussia[END_REF].

However, possible contourite deposits were not yet identified at Ben Zireg, and current activity is not obvious as no sorting of conodont assemblages were observed, and accumulations of benthic biodetritus do not occur. Instead, we favour the interpretation that these hiatuses and condensation features resulted from depositional conditions on off-shore submarine rises under slow sedimentation rates with numerous episodes of nonsedimentation, sub-marine dissolution and abrasion of bedding surfaces marked by residual iron-rich incrustations and hard-grounds.

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5.d. Accumulation rates

Interbasinal correlation with the South Marhouma section reveals that the Middle Frasnian interval is at least four times thicker than the studied section (Fig. 6). This suggests an important accumulation rate that reflects high subsidence in the Ougarta trough, while the Bechar basin suffered condensation (Fig. 7). This is presumably due to voluminous finegrained detrital influx from the far highlands (i.e Reguibat shield) into the Ougarta sill.

At Ben Zireg condensation progressively reversed during Zones 11-13. This was also the case in Marhouma and in the Anti-Atlas and Meseta domains (Wendt andBelka, 1991, Gouwy, Haydukiewicz &[START_REF] Gouwy | Conodont-based graphic correlation of upper Givetian-Frasnian sections of the Eastern Anti-Atlas (Morocco)[END_REF][START_REF] Lazreq | Biostratigraphie des conodontes du Givétien au Famennien du Maroc central -Biofacies et événement Kellwasser[END_REF] (Fig. 7). At the end of the Upper Frasnian, deposits tend to homogenize in thickness over wide areas, which may reflect more uniform depositional conditions and less differentiated subsidence rates, both in platform and basin domains.

5.e. Late Frasnian Kellwasser facies

The terminal Frasnian Upper Kellwasser horizon is defined by an outstanding level of oxygen depleted carbon-rich black shales that occur intercalated in a succession of normally oxygenated sediments. This is generally the case in European sections of offshore cephalopod limestones [START_REF] Schindler | Die Kellwasser-Krise (höhe Frasne-Stufe, Oberdevon)[END_REF] but also in sections characterising the Central Moroccan Hercynian Meseta near Mrirt [START_REF] Lazreq | Biostratigraphie des conodontes du Givétien au Famennien du Maroc central -Biofacies et événement Kellwasser[END_REF].

In contrast, South of the Main atlas Fault, the upper Frasnian both in platform and basin settings of the Tafilalet and Maider regions as well as in the South Marhouma area are exclusively composed of black cephalopod-rich limestones and shales. In these, the two Kellwasser horizons are lithologically difficult to discriminate but are discernable by Page 15 of 33 Proof For Review conodont and goniatite biostratigraphy. This lithological unit of Kellwasser-like facies that starts diachronously at the base or in the lower part of the late Frasnian and extends into the lower Famennian is termed by [START_REF] Wendt | Age and depositional environment of upper devonian (early Frasnian to early famennian) black shales and limestones (Kellwasser facies) in the eastern Anti-Atlas, Morocco[END_REF] "Upper Kellwasser member".

The Ben Zireg section differs from all other known sections of SE Morocco and adjacent northwestern Algerian Sahara in the persistence of well-oxygenated carbonate sedimentation throughout the Late Frasnian and Early Famennian in which, similarly to the situation in the Moroccan Meseta, the intercalation of a clearly defined Upper Kellwasser horizon is obvious.

Additionally, the presence of the trilobite Acuticryphops acuticeps, index of pre-Upper Kellwasser MN-Zone 13 in European and Central Moroccan sections, is exceptional as it is unknown from contemporaneous dysoxic deposits of Kellwasser facies in Southern Morocco and Algeria. Its occurrence shortly before the Upper Kellwasser horizon witnesses for normal oxygenated level bottom conditions at Ben Zireg.

These "Hercynian" aspects of late Frasnian lithology and fauna bear evidence of closer palaeogeographic relations to the Frasnian of the Maghrebid hercynides than any other area South of the Atlas Fault.

Conclusions

The Ben Zireg section is representative for Frasnian deposits on the northern edge of the Algerian Sahara platform where the early Upper Devonian is generally largely incomplete or even absent in the Bechar Basin. Though the entire early Frasnian is missing at Ben Zireg, this section is the most complete as, conformably superseding early mid-Devonian, all conodont zones of the middle and late Frasnian are represented below the earliest Famennian. Such a fine-scaled biozonation representing MN zones 5-13 is obtained for the first time in Frasnian sequences of the Algerian Sahara.

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The lithological succession is one of largely oxygenated, fine-grained carbonates of the offshore realm that are condensed in its lower part to continuously gain in importance when sedimentation rates increase.The latest Frasnian MN Zone 13 is the thickest one. Initial depositional environment was that of an offshore submarine rise subsequently smoothed to bottom level at the top of the section.

Both the Lower and Upper Kellwasser events are recorded, the latter being represented by an outstanding horizon of anoxic black-shale deposits intercalated in the normally oxygenated carbonate sequence. This disposition is unique among all other known terminal Frasnian sections S of the Atlas Fault where the Kellwasser horizons cannot be distinguished lithologically within a uniform shale/limestone succession of Kellwasser-like kind. Conversely, it is identical with equivalent sections characterizing terrains of the European and Maghrebian Variscids. In particular, evidence of instability in the topmost part of the Upper Kellwasser horizon has also been observed in many European sections.

Frasnian conodont biofacies patterns at Ben Zireg, i.e. the notable increase of Ancyrodellids at the beginning of the late Frasnian, follows the global trend independently from local conditions linked to subsidence or paleogeographic constraints. (location see Fig. 1b): Bou Terga, Jebel Atrous II [START_REF] Gouwy | Conodont-based graphic correlation of upper Givetian-Frasnian sections of the Eastern Anti-Atlas (Morocco)[END_REF];

Figure and table captions

Bou Tchrafine (Bultynck & Jacob, 1981[START_REF] Bultynck | Accuracy and reliability of conodont zones: the Polygnathus asymmetricus "zone" and the Givetian-Frasnian boundary[END_REF], 1987;Becker & House, 2000b);

Ouidane Chebbi [START_REF] Wendt | Age and depositional environment of upper devonian (early Frasnian to early famennian) black shales and limestones (Kellwasser facies) in the eastern Anti-Atlas, Morocco[END_REF]Belka et al. 1999), Oued Rheris [START_REF] Wendt | Age and depositional environment of upper devonian (early Frasnian to early famennian) black shales and limestones (Kellwasser facies) in the eastern Anti-Atlas, Morocco[END_REF], Anajdam, Bou Alzaz Nord [START_REF] Lazreq | Biostratigraphie des conodontes du Givétien au Famennien du Maroc central -Biofacies et événement Kellwasser[END_REF], South Marhouma (Mahboubi et al. 2013 a, b;Mahboubi & Gatovsky, submitted).

  Zone 9-10 is discriminated from bed BZ7 to bed BZ7A by the presence of Pa. ljaschenkoae together with Pa. kireevae. The conodont association is composed by Pa. amplificata in bed BZ7, Po. zinaidae and Po. aequalis in bed BZ 7A which have their first occurrences during this interval. The base of the MN Zone 11 is defined by the first occurrence of the marker Pa. feisti in bed BZ8 and corresponds to the base of the Upper Frasnian substage (Becker & House 1998). The presence of the MN Zone 12 is attested in bed BZ12A with the first appearance of Pa. winchelli, index-species of the zone. The presence of the marker Pa. bogartensis in bed BZ13 allows discriminating the basis of MN Zone 13. Thereafter, the level BZ13A.1 yields Pa. ederi and Ag. asymmetricus, and Pa. brevis and Pa. boogaardi occur in bed BZ14. The co-occurrence of Pa. beckeri and Pa. juntianensis is recorded in bed BZ15 at MN Zone 13. Page 9 of 33 Proof For Review The occurrences of Pa. ultima and Pa. triangularis in Bed BZ16 indicate the Famennian stage (early triangularis Zone) (Klapper, 2007b). Other species present are Pa. subperlobata, Pa. sp. B, Pa. quadrantinodosalobata praeterita, Po. angustidiscus, Po. cf. politus and Ic. alternatus. In our record, Pa. cf. spathula occurs slightly before the first known appearance of Pa. spathula (late triangularis zone, Schülke 1999).

Figure 1 .

 1 Figure 1. Main Palaeozoic outcrops in north-west Africa. (a) location of selected Frasnian

Figure 2 .

 2 Figure 2. Stratigraphic log with sedimentary cycles, Kellwasser events and distribution of

Figure 3 .

 3 Figure 3. Exposure of the Ben Zireg succession from the Givetian to the Famennian showing

Figure 4 .

 4 Figure 4. Upper Devonian facies types of Ben Zireg section (Algeria). Os -benthic ostracods;

Figure 5 .

 5 Figure 5. Significant conodonts from the Ben Zireg section:

Figure 6 .

 6 Figure 6. Percentage of conodont genera at Ben Zireg (left) and at the South Marhouma

Figure 7 .

 7 Figure 7. Correlation of selected Frasnian sections from both sides of the Main Atlas Fault

Page 5 of 33 Proof For Review named

  after the one or the two most abundant genera, and trends in the variations of the relative proportion of the different genera are assumed to provide indirect water depth proxy

	through time.
	During the Frasnian period, five conodont genera dominated the assemblages. Icriodus is
	considered as characteristic of shallow environments, whereas Palmatolepis should prefer
	deeper conditions. Ancyrodella and Ancyrognathus are grouped under the term
	ancyrodellids. Together with Polygnathus they are considered as ubiquitous, with a slight
	preference for deep environments for Polygnathus whereas Ancyrodellids would prefer
	shallow ones

Table 1 .

 1 Numbers of platform conodont genera at the Ben Zireg section. The low conodont

abundances (<20P1) in italic are not included in the biofacies analyses. Pa-Palmatolepis; Po-Polygnathus; An-Ancyrodella and Ancyrognathus; Ic-Icriodus. Page 24

of 33 Proof For Review Page 25 of 33 Proof For Review

				Proof For Review			Page 26 of 33
	Sample	Weight	Depth	Pa	Po	An	Ic	Total
		(g)	(m)
	BZ16B	200	27.6	52	2		2	56
	BZ16A	1138	27.25	193	25		1	219
	BZ16	990	26.7	178	31			209
	BZ15D	200	26.5	26	5	8		39
	BZ15C	827	26	70	18	9	1	98
	BZ15A	100	25	21		2	1	24
	BZ15	873	24.1	52	5	2	9	68
	BZ14D.1 1000	23.5	296	73	9	16	394
	BZ14C	90	22.5	4	8	1		13
	BZ14	1205	22	80	22			102
	BZ13B	210	21	35	13	1	4	53
	BZ13A.1 1260	20.1	160	24	19	42	245
	BZ13	879	18.5	62	22	1	23	108
	BZ12A	751	17.6	5	37	8	4	54
	BZ11	230	16	1	14	13	2	30
	BZ10B	230	15.5	3	13	4	3	23
	BZ10A	200	13.65	7	9	3	3	22
	BZ10	671	12.82	15	32	7	9	63
	BZ9	456	11.85		44	4	21	69
	BZ8A.1	747	11.5	13	54	5	43	115
	BZ8A	140	11.25	8	4		5	17
	BZ8	875	8.75	12	20	7	43	82
	BZ7A	170	9	4	7	1	3	15
	BZ7	834	8.1	12	41	4	28	85
	BZ6B	400	7.25	6	35	1	5	47
	BZ6	620	7	4	33		8	45
	BZ5B	150	5.25		4	4		8
	BZ5	730	5.75		66		4	70
	BZ4B.1	1120	5.25		79	6	40	125
	BZ4B	267	5		14		1	15
	BZ3C	580	3.25		6		1	7
	BZ3	610	3	2	12		12	26
	BZ2C	347	2.25		2		3	5
	BZ2	1083	1.75	2	13	2	17	34
	BZ1C	1037	1		3			3
	BZ1A	715	0.5	1	48	1	4	54
	BZ1	800	0		27		1	28

Acknowledgements

The authors are very grateful to Abess Kada, M'hamed Mahboubi, Tadjedinne Hassen, Brigitte Meyer Berthaud for their help in the field. Gilbert Klapper is deeply acknowledged for checking some Frasnian conodonts. The first fieldwork was supported by the ANR Palasiafrica