Middle Jurassic Zoophycos and Chondrites from the Mélah Formation of Saharan Atlas, Algeria

The ichnogenera Zoophycos and Chondrites are described for the first time from the Middle Jurassic of Algeria. The trace fossil assemblage recovered from the Mélah Formation contains seven ichnospecies (Chondrites targionii, C. ?intricatus, Zoophycos brianteus, Z. cauda-galli, Zoophycos isp. A, Zoophycos isp. B and Zoophycos isp. C), which all represent the Zoophycos ichnofacies. Chondrites and Zoophycos in the Mélah Formation were made by unknown worm-like invertebrates in a deeper offshore or a shallow bathyal environment (below storm weather wave base) and may indicate oxygen depletion within


INTRODUCTION
The Mesozoic deposits are widely distributed in northern Africa along the southern margin of the former Tethys Ocean. On the Algerian territory, the Jurassic has been mapped repeatedly over the past 60 years, particularly those of the Maghribides chain in the Tell Atlas and the Saharan Atlas, which links from west to east the Moroccan-Algerian and the Tunisian-Algerian basins. Only a few studies address the Jurassic of the Saharan Atlas, e.g. Flamand (1911), Cornet (1952) and unpublished theses (Bassoullet 1973;Douihasni 1976;Ait Ouali 1991). They are only available in unpublished theses (e.g. Flamand 1911;Cornet 1952;Bassoullet 1973;Douihasni 1976;Ait Ouali 1991). The most recent work goes back to the thesis of Mekahli (1995), subsequently published in the Documents des Laboratoires de Géologie de Lyon (Mekahli 1998).
Chondrites has a stratigraphic range from the Ordovician to the Pliocene. It is found in modern deepsea muds, but also in turbidite series as well as in shallow-marine shales and even in storm sands (Seilacher 2007). Different forms of Chondrites share a plant-like kind of branching along bedding planes (often termed 'fucoids'), but they also show a preference for relatively quiet and low-oxygen environments (Seilacher 2007). Chondrites clearly belongs to the category of feeding burrows (fodinichia), but details of the probing, feeding and backfill processes are less uniform among different ichnospecies (Seilacher 2007).
The aims of this paper are (1) to describe Zoophycos and Chondrites for the first time from the lower Middle Jurassic of Algeria and (2) to discuss their stratigraphic and environmental distribution and palaeoeocology.

GEOLOGICAL SETTING
The Saharan Atlas domain extends some 1200 km along a SW-NE direction, from the Moroccan High Atlas in the west to the Tunisian Atlas in the east. To the north, the Saharan Atlas range is juxtaposed with a platform domain of the High Plateaus. Its southern boundary is a major tectonic dislocation, which is termed the South Atlas Fault, along which the Saharan Atlas is overthrusted on the Saharan Platform. The Saharan Atlas range includes a series of subranges, from west to east: the Ksour Mountains, the Djebel Amour, the Ouled-Naïl Mountains, the Aurès Mountains, the Nememcha-Mzab Mountains and the Melegue Mountains (Fig. 1A). The studied section is located in the Ksour Mountains (Fig. 1A, B).
The Lower Jurassic deposits of the carbonate platform sealed the sediments infilling the initial Triassic rift. The Mesozoic sedimentation was initiated within small Triassic rift basins that were filled with varicoloured clays, evaporites, interrupted by occasional basaltic lava flows (e.g. diaper of Aïn Ouarka). During the Early and Middle Jurassic, marine conditions were prevailing and the Ksour Basin was then filled in with a thick sedimentary succession. The renewing of rifting processes increased the accommodation space once again in the middle Early Jurassic, and the carbonate platforms were drowned and disrupted by subsidence. Thick successions of limestones, calciturbidites (e.g. Brèche de Raknet El Kahla Formation), sandstones and marls accumulated during the Early and Middle Jurassic (Bassoullet 1973;Mekahli 1998). These sediments are overlain by a ca 3000 m thick series of mostly continental Upper Jurassic-Lower Cretaceous siliciclastics (Djara, Aissa, Tiloula and Tiout formations). The Mesozoic succession had been uplifted and deformed during the Eocene and the Oligocene (Dewey et al. 1973). Fold and fault deformations affected the Jurassic and Cretaceous deposits. Jurassic units formed ridges separated by broad synclines composed of Lower Cretaceous formations. The Cenomanian-Turonian limestone ledges (Rhoundjaïa Formation) occupy the top of synclines (Benyoucef et al. 2017).
The section studied herein (coordinates 33°1′6.16″N; 0°23′45.99″W) is situated about 3 km south of the national road n°6 on the southern slope of the Djebel Souiga, a 15 km long hill, close to the water source commonly known as 'Aïn Dehara' (Fig. 1B).

STRATIGRAPHY AND SEDIMENTOLOGY
The southern flank of the Djebel Souiga provides wellexposed and laterally continuous outcrops to study lower Middle Jurassic marl-limestone alternations representing the Mélah Formation (Mekahli 1998). The Mélah Formation can be subdivided into two informal members ( Fig. 2): Zoophycos-bearing marl-limestone member and marl-argillaceous limestone member.
The Zoophycos-bearing marl-limestone member rests on the 'Marno-calcaires d'Aïn Beida' Formation (Toarcian) (Fig. 2). The member is 30 m thick and is represented by regular alternation of thin-to mediumbedded (0.03-0.20 m thick) limestone and light grey marls (0.15-0.80 m). Limestone beds are bluish-grey in colour, apparently massive and contain numerous ammonites, nautiloids and belemnites. The limestone beds are commonly rich in trace fossils, represented by Zoophycos and Chondrites. The beds have sharp and slightly undulating contacts. Microscopic analysis of limestone facies documents the presence of skeletal components, including Posidonomya, siliceous sponge spicules and radiolarians, in addition to rare planktic foraminifers. Occasional planktic single-celled green algae (Globochaete sp.) are also found. Textures of the limestones range from mudstone to wackestone (Bassoullet 1973;Mekahli 1998).
The marl-argillaceous limestone member rests on the Zoophycos-bearing marl-limestone member and is accessible in all studied sections, either completely or partially due to a cover of younger sediments. It is up to 42 m thick (Fig. 2) and consists of irregular alternation of soft, greenish marls and blue to grey argillaceous limestones. The limestones occur as centimetre-to decimetre-thick beds containing abundant ammonites and less common belemnites. The interbedded marls yielded abundant thin-shelled bivalves (Bositra buchii Roemer, 1836). The upper boundary of the member is a very sharp erosional surface. The member is followed by the Tniet El Klakh Formation, a mixed siliciclasticcarbonate unit of Bajocian-Bathonian age. Bassoullet (1973) cited in the studied section several ammoniterich beds indicative of Upper Aalenian to Lower Bajocian age. The textural characteristics, the pronounced abundance of ammonites and pelagic microfauna such as radiolarians suggest open marine normal salinity environment. The scarcity of benthic elements, and the absence of high-energy hydrodynamic structures indicate that both members of the Mélah Formation were deposited below mean storm weather wave base. Trace fossils are not common in the marl-argillaceous limestone member, but some bioturbation occurs in its upper part (Bassoullet 1973;Mekahli 1998).

TRACE FOSSIL DESCRIPTIONS
Chondrites targionii (Brongniart, 1828) Figure 3A Description. Tree-like, slightly curved, branched, flattened tunnels, with three orders of branches. Secondorder branches are dominant. The angle of branching ranges from 20° to 45°. Tunnels are about 1 mm wide. The whole burrow system is about 8 cm wide. The branches are slightly curved and filled with lighter material than the host rock. The traces are preserved in concave or convex epirelief. Traces are preserved at the bedding planes.
Chondrites ?intricatus (Brongniart, 1828) Figure 3B Description. Small, tree-like branching, downward radiating, straight and short hypichnial burrows. The branches are filled with lighter material than the host rock. The width of the burrow system is smaller than 50 mm and the width of the funnel is about 1 mm.
Discussion. Chondrites ?intricatus and C. targionii co-occur in the Mélah Formation (Fig. 3B). The larger C. targionii cross-cut the smaller C. ?intricatus. The smaller Chondrites deserves a separate name as there seems to be a size break between smaller and larger Chondrites specimens in our sample. The described specimens are assigned only tentatively to C. intricatus due to their rather poor preservation.
Zoophycos brianteus Massalongo, 1855 Figure 3C, E Description. Preserved in full relief, endichnial, horizontal to inclined, spiralling, probably helicoidal J-shaped spreiten burrows. The specimens are more than 30 cm wide. Primary and secondary lamellae are distinguishable. Primary lamellae are 1-2 mm wide; they start from a central tube and bend slightly towards the external part of the spreite.
Zoophycos cauda-galli (Vanuxem, 1842) Figure 3D Description. Endichnial, full relief, U-shaped helicoidal spreiten burrows. The outline is cock-tail shape and the primary and secondary lamellae are distinct. The specimens are more than 30 cm wide. Primary lamellae are 3-4 mm wide.
Discussion. Trace fossil morphology classified today as Zoophycos cauda-galli was first found in calcareous and clay-rich sandstone of the Devonian Hamilton Group in New York and Ohio, USA and described under the name Fucoides cauda-galli (Vanuxem 1842). It was later described as fossil seaweed under the name Spirophyton cauda-galli (Hall 1863). The specimens studied here are similar to Z. cauda-galli in the overall shape, dimensions of the spreite and primary lamellae, and the described ichnospecies does not exhibit any obvious morphological differences from the type material (Vanuxem 1842, pp. 128-129, fig. 30).
Zoophycos isp. A Figure 3E Description. Hypichinial, horizontal, flat, arcuate Ushaped spreiten burrows with a visible marginal tunnel. The marginal tunnel is less than 1 cm wide. The primary and secondary lamellae are not easily distinguishable. The maximum width of the specimens is 8.6 cm.
Discussion. The specimens described here should be referred to U-shaped Zoophycos. Their traces are quite abundant and diverse on the sole surfaces of marly limestone from the Mélah Formation (Fig. 3E). The morphology is treated in open nomenclature because of the poor state of preservation and limited number of specimens.
Zoophycos isp. B Figures 3F, 4 Description. Endichnial, horizontal, spiralling, J-shaped spreiten burrows. The outline is cock-tail shape and the primary and secondary lamellae are visible. The primary lamellae are 3-5 mm apart. The burrow system diameter is about 30 cm.
Discussion. The lobes resembling the tail of a cock are diagnostic for Z. cauda-galli, but Zoophycos isp. B differs from that ichnospecies in having J-shaped spreiten, a slightly smaller diameter of the burrow system and a larger maximal diameter of primary lamellae. The morphology is treated in open nomenclature because of the poor state of preservation and limited number of specimens.  Zoophycos isp. C Figure 3G Description. Hypichnial, horizontal, helicoidal, arcuate U-shaped spreiten burrows. The spreite lamina are arranged around a central point. The primary and secondary lamellae are clearly distinguished. The burrow system diameter is about 30 cm. The primary lamellae are 8-9 mm apart.
Discussion. Zoophycos isp. C resembles Zoophycos cauda-galli in having a similarly U-shaped spreiten burrow, but differs from the latter ichnospecies in much thicker primary lamellae. It differs from the other described Zoophycos ichnospecies also in much thicker primary lamellae. The morphology is treated in open nomenclature because of the poor state of preservation and limited number of specimens.

DISCUSSION AND CONCLUSIONS
The trace fossil assemblage from the Middle Jurassic Mélah Formation of the western Saharan Atlas contains two ichnogenera and probably up to seven ichnospecies which represent the Zoophycos ichnofacies. The described Zoophycos traces show variable morphologies (Z. brianteus is most common); some were left in open nomenclature (Zoophycos isp. A, Zoophycos isp. B and Zoophycos isp. C). The observed morphological variation may in some instances reflect various section views of the trace fossil rather than true morphological differences. Moreover, as it has been demonstrated in Zoophycos rhodensis (Bromley & Hanken 2003) and in Zoophycos described by Pervesler & Uchman (2004), the morphologies typical of distinct ichnospecies (e.g. Z. brianteus and Z. insignis) may meet in one structure. The Zoophycos observed during our study is indicating a relatively quiet, nutrient-rich, offshore environment (Seilacher 2007;Zhang et al. 2015). Similarly to the Mélah Formation, Zoophycos is characteristic of certain Jurassic and Cretaceous basinal deposits of southeastern France (Olivero 1996(Olivero , 2003.  (Bromley 1990;Zhang et al. 2015). Zoophycos indicates that the sediments forming the seafloor could have been slightly dysoxic or, alternatively, Zoophycos could have occurred in oxic deposits but in deep tier (Ekdale 1992). Chondrites has also often been considered to be an indicator of anoxia in sediments (Bromley & Ekdale 1984;Gong & Droser 2001). This could suggest oxygen deficiency in the sediments during the deposition of the Zoophycosbearing member of the Mélah Formation.
Zoophycos occurs usually in deep-sea sediments since the Jurassic (Bromley 1990;Seilacher 2007;Zhang et al. 2015), but is also present in shallower marine deposits in the German Triassic (Knaust 2004). The abundance of ammonites and pelagic microfauna and the scarcity of benthic faunal elements suggest that Zoophycos-bearing marl-limestone member of the Mélah Formation could be deposited in offshore, normal sea salinity settings. The ichnospecies of Zoophycos from the Mélah Formation are also similar in morphology to those described from the deep-sea environments (Zhang et al. 2015) and do not resemble nearshore representatives of Zoophycos (Knaust 2004).
It is accepted that Zoophycos and Chondrites are more related to the oxygen level within the substrate than to bathymetry. What seems to be most important here is that Zoophycos and Chondrites are the only trace fossils in the formation, all shallower traces are missing and sedimentary structures are well preserved (i.e. primary lamination). When shallow tier trace fossils are missing and the sedimentary structures are preserved, the redox boundary could be close to the sediment-water interface and therefore any colonization by oxygen-dependent burrowers would be excluded. Hence, the settings could be deep as the oxygen level was low. Alternatively, this may suggest weak ventilation, infrequent storms, or no storms and no normal weather wave action, but any geological structures prohibiting normal wave activity are unknown from the study region.
Most likely the studied trace fossils were made by unknown worm-like invertebrates in a deeper offshore or a shallower bathyal environment (Zhang et al. 2015). Chondrites is likely a feeding system of unknown tracemakers related to infaunal deposit-feeders such as sipunculoids and/or annelids (Osgood 1970). This is in agreement with the other available palaeontological data, indicating a relatively deep-water environment for the lower, Zoophycos-bearing member of the Mélah Formation.