First record of trace fossils from the Oxfordian Argiles rouges de Kheneg Formation (Tiaret, northwestern Algeria)

Three main facies associations FA-1 to FA-3 occur in the Oxfordian Argiles rouges de Kheneg Formation in northwestern Algeria. They correspond respectively to the deeper part of a mixed siliciclastic-carbonate shelf, upper shoreface and offshore transition-lower offshore. The trace fossil association of the Argiles rouges de Kheneg Formation contains fifteen ichnogenera and is moderately diverse for the Upper Jurassic. The formation contains diverse and abundant deep water or dominantly deep water trace fossils (i.e. Belorhaphe, Chondrites, Helminthopsis, Nereites, Megagrapton). They indicate that a part of the formation was deposited in offshore transition to lower offshore environments. comprising young mountains formed during the Alpine orogeny. These domains are, from north to south: (i) The Tellian Atlas (knappe domain, composed of sedimentary rocks ranging from the Jurassic to Miocene in age); (ii) The High Plateaus (the foreland of the Alpine range bearing a thin sedimentary cover of Jurassic age). In the western part, there are a series of mountain ranges such as the Saïda Mountains and the Frenda Mountains; (iii) The Saharan Atlas (formed from an elongated trough pinched between the High Plateaus and the Saharan Platform, infilled with a thick sedimentary sequence during the Mesozoic times). From the palaeogeographic point of view, Alpine Algeria was a part of north-western Africa during the Mesozoic, and was characterized by the individualization of two different paleogeographic domains: the autochthonous intracontinental chains (the Atlas s.s., FRIZON DE LAMOTTE et al,. 2008) and the Rif Range with the Tellian-Kabylian belt (Maghrebides), composed of parautochthonous to allochthonous units (CHALOUAN et al., 2008). The history of these domains is contemporary with the opening of the central Atlantic and Neo-Tethys oceans (BREDE et al., 1992; GOMEZ et al., 2000) during the Late Jurassic, on the southern shelf of Western Tethys (e.g. DERCOURT et al., 1993; BERRA & ANGIOLINI, 2014). However, the study area was likely to have bathyal water depths (TCHOUMATCHENCO & KHRISCHEV, 1992). The Argiles rouges de Kheneg Formation contains the Upper Jurassic marine deposits outcropping in the southern Tell Atlas. These deposits are mostly of pelagic origin, with subordinate yet locally important detrital beds at the midUpper Oxfordian ‘’Grès intercalaires’’. In the uppermost Late Oxfordian, intermittent siliciclastic deposits have been described as the Argiles de Saïda facies (subunit C-2 in this study) and interpreted as a result of intra-Oxfordian tectonic movements (ATROPS & BENEST, 1981). In the Middle Oxfordian to Late Oxfordian, of the Argiles de Saïda Formation in the Saïda Mountains and the Frenda Mountains, the depositional environment was characterized by shallow Article history: Manuscript received April 25, 2020 Revised manuscript accepted June 30, 2020 Available online June 29, 2020


INTRODUCTION
The studied Upper Jurassic outcrops are located to the north of the town of Tiaret on the boundary of the southern Tellian domain (Fig. 1). The uppermost Jurassic deposits are distributed in North Africa along the southern margin of the former Tethys Ocean. In northwestern Algeria, the Oxfordian strata stretch over the Tellian Atlas and are named as the Ammonitico rosso de Frid and Argiles rouges de Kheneg (ATROPS & BENEST, 1994), in the Oran and Arzew mountains (northern Tell) they are known as the Upper Oxfordian "Grès chocolatés" and eventually as "l'ensemble détritique à ostracodes" in the Tessala Mountains (FENET, 1975). In the High Plateaus Oxfordian rocks have been either referred to the comprising young mountains formed during the Alpine orogeny. These domains are, from north to south: (i) The Tellian Atlas (knappe domain, composed of sedimentary rocks ranging from the Jurassic to Miocene in age); (ii) The High Plateaus (the foreland of the Alpine range bearing a thin sedimentary cover of Jurassic age). In the western part, there are a series of mountain ranges such as the Saïda Mountains and the Frenda Mountains; (iii) The Saharan Atlas (formed from an elongated trough pinched between the High Plateaus and the Saharan Platform, infilled with a thick sedimentary sequence during the Mesozoic times).
From the palaeogeographic point of view, Alpine Algeria was a part of north-western Africa during the Mesozoic, and was characterized by the individualization of two different paleogeographic domains: the autochthonous intracontinental chains (the Atlas s.s., FRIZON DE LAMOTTE et al,. 2008) and the Rif Range with the Tellian-Kabylian belt (Maghrebides), composed of parautochthonous to allochthonous units (CHALOUAN et al., 2008). The history of these domains is contemporary with the opening of the central Atlantic and Neo-Tethys oceans (BREDE et al., 1992;GOMEZ et al., 2000) during the Late Jurassic, on the southern shelf of Western Tethys (e.g. DERCOURT et al., 1993;BERRA & ANGIOLINI, 2014). However, the study area was likely to have bathyal water depths (TCHOUMATCHENCO & KHRISCHEV, 1992).
The Argiles rouges de Kheneg Formation contains the Upper Jurassic marine deposits outcropping in the southern Tell Atlas. These deposits are mostly of pelagic origin, with subordinate yet locally important detrital beds at the mid-Upper Oxfordian ''Grès intercalaires''. In the uppermost Late Oxfordian, intermittent siliciclastic deposits have been described as the Argiles de Saïda facies (subunit C-2 in this study) and interpreted as a result of intra-Oxfordian tectonic movements (ATROPS & BENEST, 1981). In the Middle Oxfordian to Late Oxfordian, of the Argiles de Saïda Formation in the Saïda Mountains and the Frenda Mountains, the depositional environment was characterized by shallow water mixed carbonate-siliciclastic deposition, as evidenced by frequent tidal and tempestite structures (CHERIF et al., 2015(CHERIF et al., , 2018HALAMSKI & CHERIF, 2017).

FACIES CHARACTERISTICS AND STRATIGRAPHY
The Argiles rouges de Kheneg Formation was defined by AT-ROPS & BENEST (1994), in order to accommodate a series of marine deposits above the Middle Oxfordian Ammonitico rosso de Frid Formation and overlain by the Kimmeridgian Calcaires du Bou Rheddou Formation. These deposits display a wide array of different lithologies, such as marly and nodular/pseudo-nodular limestones, and bioturbated sandstones. However, the formation can be subdivided into three informal units, based on their respective faunal and lithologic features (corresponding to the facies-type, namely F1-F4) (Fig. 2).

Unit A (Lowermost part of the formation -lower Ammonitico rosso)
This first unit is up to 25 m thick in the Aïn El Hamra area, and about 15 m thick in Oued Kheneg. It consists of the alternation of red claystone /marlstone interlayers (Facies F1) 0.10 to 0.50 m thick (displaying planktonic foraminifers and ostracods) and red to green nodular limestone (Facies F2), occurring in 0.20 to 3 m thick packages. Both F1 and F2 commonly contain ammonites, echinoids, crinoids, bivalves and brachiopods. The microscopic scale of F2 (ATROPS & BENEST, 1984BENEST, , 1986 shows skeletal and intraclast components with non-skeletal grains (ooids and pellets) forming wackestone and packstone-textured carbonates. The fossil allochems include echinoid debris, crinoid fragments, bivalves, and planktonic foraminifers.

Unit B (Middle part of the formation -middle sandstone)
This 5-15 m thick unit is easily distinguished from the upper part of the lower unit by the presence of a 0.20 to 1 m thick sandstone (Facies F3), separated by centimetre thick red to green claystone interlayers. The middle sandstone unit shows a large variation in thickness from 15 m in the Oued Kheneg section to 5 m in the Aïn El Hamra section. The sandstone beds are yellow to brown when weathered, grey to greenish in fresh cuts. They show sharp erosive bases, pebble impressions, are channelized (Fig. 3C), and show sole marks (flute-casts). The wave/current structures are usually represented by horizontal and sigmoidal bedding, thick sets of hummocky-cross stratification (HCS) and large wavy cross-bedding (Fig. 3D). Commonly, the beds show unidirectional, linguoid ripple-marks ( Fig. 3E) or mega-ripples ( Fig. 3F). At the top of this unit, the upper surface of the sandstone is highly bioturbated with large Thalassinoides (Fig. 5D). No body fossils have been recorded in this unit.

Unit C (Uppermost part of the formation -upper Ammonitico rosso)
This 40-45 m thick unit is divided into two subunits:

Subunit C-1 (lower part of the unit)
As the lower part of the formation, this 22-30 m subunit is mostly composed of alternations of interlayered decimetre thick red (rarely green) claystone/marlstone (F1) (0.60-1 m) and red, pink to green micritic limestone (F2) (0.10-0.20 m). The limestone is nodular to pseudo-nodular and bioturbated. The bioturbation is generally represented by filled burrows. In the lower part, the limestone beds are rich in ammonites, echinoids, crinoids, echinoderm debris, and brachiopods. Broadly, the lithostratigraphic characteristics as well as the fossil content and the microscopic texture are similar to the lower unit.

Subunit C-2 (upper part of the unit)
This subunit is 12 to 15 m thick, cropping at the upper part of the Argiles rouges de Kheneg Formation (Fig. 3G). It is mostly composed of green shales to red claystone (F1) and thin sand-and siltstones (Facies F4). F4 embrace fine-grained sandstones (subfacies F4-a) and millimetre to centimetre thick siltstone laminae (subfacies F4-b). The F4-a and F4-b are brown in colour, and are 0.02-0.15 m thick, show channelized and sharp erosive bases, contain septarian nodules (lower part of the succession). The beds display abundant flute-cast, groove-mark, load-cast, horizontal lamination, small scale HCS, unidirectional, wavy and linguoid ripple-marks (Fig. 3H).

FACIES INTERPRETATION
The table below summarizes the main facies (F1, F2, F3 and F4) with their principal lithological, sedimentological and ichnological features:

FACIES ASSOCIATION AND PALAEOENVIRONMENT
Three facies associations have been distinguished, namely FA-1 to FA-3

Facies association (FA-1): The deeper part of platform
FA-1 comprises mudstone facies (F1) and limestone facies (F2), F1 includes red claystone (subfacies F1-a) and marlstone interlayers (subfacies F1-b). F1 and F2 are rhythmically interbedded and correspond to the Ammonitico rosso facies of the lower unit and the subunit C-1 of the upper unit and locally to some parts of the subunit C-2. Sediments of FA-1 were likely deposited in the deeper part of the carbonate shelf.

Facies association (FA-2): Upper shoreface
FA-2 occurs exclusively in the middle unit and is composed of thick sandstone (F3), mostly thinning upward. It contains subordinate thin green claystone interlayers (F1-a). It may be attributed to the upper shoreface deposits and is the shallowest facies of the Argiles rouges de Kheneg Formation.

Facies association (FA-3): Offshore transition-lower offshore
The upper part of the Argiles rouges de Kheneg Formation is composed of the facies association FA-3, which is made of interbedded fine-grained sandstones (F4-a), siltstones (F4-b) and claystone intervals (F1-a) (Fig. 3G). The shallow-water and high-energy conditions are proven by several wave/current sedimentary structures, and the recorded trace fossils belong to the Cruziana ichnofacies-Nereites ichnofacies transition. This facies association represents deposition in offshore transition-lower offshore environments.

ICHNOLOGY
Trace fossils are abundant and diverse in the subunit C-2 (uppermost part of the formation), but less frequent in the middle sandstone. Fifteen ichnotaxa are recognized, presented in alphabetical order.
Belorhaphe zickzack HEER, 1877 (Fig.4A) This trace fossil is hypichnial zigzag and meanders in fine grained sandstones. Most of the zigzags are with an apical angle cf. Bergaueria isp. (Fig. 4B) This trace is a hypichnion perpendicular to the stratification plane. It occurs mainly in the upper part of the second and third units. Bergaueria is a cylindrical structure, with an apical depression. It is considered as a resting or dwelling trace, produced by suspension feeders (FÜRSICH, 1975;PEMBERTON et al., 1988) and reported from shallow to deeper marine environments (CRIMES & ANDERSON, 1985;UCHMAN, 1998;CHERIF et al, 2015).
Chondrites isp. (Fig. 4C) This trace is preserved as full relief, composed of a branching dendritic burrow network, mostly horizontal to the bedding. The burrows are from 0.5 to 1 mm wide, less than 30 mm long. Chondrites is frequent in the bioturbated limestone of the lower and upper Ammonitico rosso facies. It is considered as a fodinichnion (RICHTER, 1927) of marine annelids (SIMPSON, 1956), and is classified as a chemichnion (BROMLEY, 1996). Generally, Chondrites is reported from offshore (CHERIF et al, 2015(CHERIF et al, , 2018 to deep-sea deposits or in deep tier (BROMLEY, 1990), often considered to be an indicator of anoxia in sediments (BROMLEY & EKDALE 1984) Helminthopsis isp. (Fig. 4D) This trace is a hypichnial meandering ridge 0.2 to 2 mm wide and 100 mm long. It is attributed to polychaete worms (KSIĄŻKIEWICZ, 1977); mostly reported from deeper facies (CHAMBERLAIN;1971;WETZEL et al., 2007).
Megagrapton irregulare KSIĄŻKIEWICZ, 1968 (Fig. 4E) This trace fossil is preserved as a system of hypichnial, winding branched semi-cylindrical ridges forming an irregular net. The ridges are 1-2 mm wide, the branching points are 30-150 mm apart. M. irregulare has been observed in deposits of a wide spectrum of environments (BUATOIS et al., 2017), but is considered to be typical of turbiditic deposits (KSIĄŻKIEWICZ, 1977;UCHMAN, 1998). AZPEITIA MOROS, 1933 (Fig. 4F) These are hypichnial, winding branched ridges forming net with meshes bordered by strings, which are 1.0-1.7 mm wide and about four meshes are preserved. The maximum width of the mesh is 30 and 50 mm. M. submontanum occurs exclusively in deep water, mainly in flysch deposits (UCHMAN, 1998).
Nereites irregularis SCHAFHÄUTL, 1851 (Fig. 5B) This trace is preserved as an epichnial meandering burrow, made of a shallow furrow, 4-5 mm wide. Nereites are locomotion traces of annelid worms (SEILACHER, 2007), commonly occurring in deep-sea deposits. It is composed of a median faecal string and bounding zones, which indicate sediment reworking (UCH-MAN, 1995). Ophiomorpha isp. (Fig. 5C) This consists of a horizontal, cylindrical burrow 5 to 10 mm wide, and at a least of 70 mm long, covered by irregular pellets and co-occurs with Planolites.
cf. Ophiomorpha annulata KSIĄŻKIEWICZ, 1977 (Fig.  5D) occurs as a straight to slightly curved, horizontal tunnel, preserved in epirelief, on the sandstone bed, 2.5-8 mm in diameter, and about 250 mm long. O. annulata maybe smooth or showing arranged granules as a meniscate shape.
Ophiomorpha rudis KSIĄŻKIEWICZ, 1977 (Fig. 5D) It co-occurs with Palaeophycus and Planolites as simple cylindrical burrows, straight or slightly curved, horizontal to bed-ding plane. This trace is 2.5-5 mm wide on average and 300 mm long, showing scratches or small to strong nodular external surfaces. Ophiomorpha is produced by shrimp-like crustaceans, living in burrows made in high-energy coastal marine sand environments (FREY et al., 1978), but it has also been reported from deep-sea deposits (KERRN & WARME, 1974;UCHMAN, 1995;UCHMAN, 2009).
Palaeophycus isp. (Fig. 5C-D and E) Slightly sinusoidal horizontal and unbranched cylindrical burrows, 5 to 10 mm in diameter, a few centimetres to 150-200 mm-long; studied specimens are preserved as positive/negative epi-or hyporeliefs. This trace characterizes mainly the upper part of the third unit. Palaeophycus is an extremely eurybathic faciescrossing trace, interpreted as a mostly dwelling burrow of suspension feeding or predatory worms (PEMBERTON & FREY, 1982).

DISCUSSION AND CONCLUSIONS
Three main facies associations (FA-1 to FA-3) occur in the Oxfordian Argiles rouges de Kheneg Formation in northwestern Algeria. Two facies (FA-2 and FA-3) associations characterize shelf environments from the shallowest (FA-2) to deeper ones (FA-3), while FA-1 could be interpreted as the deepest part of the shelf, attributed to the intra-Oxfordian tectonic movements (BENEST & ATROPS, 1998;CHERIF et al., 2015). The transition from FA-1 (lower Ammonitico rosso unit) to FA-2 (middle sandstone unit) have been interpreted as an intermittent sandstone discharge caused by tectonic instability. The passage from FA-1 to FA-3 was attributed to an important SW current provenance of clastic influx, infilling and reducing the bathymetry, namely the Argiles de Saïda facies (ATROPS & BENEST, 1984). Such evolution is indicated by stratal colour variations and attributed to temporal changes in sediment composition (e.g., clastic vs. carbonate content, SAVRDA, 2007). Hence, the Oxfordian Argiles rouges de Kheneg Formation is characterized by a diverse trace fossil association (15 ichnotaxa), occurring in a wide range of marine environments (i.e. Bergaueria, Ophiomorpha, Palaeophycus, Planolites, Thalassinoides). However, deep water traces or predominantly deep water traces (i.e. Belorhaphe, Chondrites, Helminthopsis, Nereites, Megagrapton) are also diverse and abundant in the Argiles rouges de Kheneg Formation, indicating that this part of the formation was deposited under deep water conditions. They are interpreted as the Cruziana ichnofacies-Ne-reites ichnofacies transition, and the absence of the typical turbiditic sequence excludes the really deep flysch deposits, whereas the frequent wavy/storm structures could correspond to the offshore transition-lower offshore environment.
In well-oxygenated substrates, the ichnofossil component is characterized by high diversity assemblages that contain forms of large diameter up to several centimetres (SAVRDA, 2007). The presence of infaunal communities and traces of their activities is dependent on bottom-water oxygenation. The less diverse bioturbation is related to lower oxygen concentrations (e.g., RHOADS AND MORSE, 1971;SAVRDA et al., 1984). Therefore, the occurrence of Chondrites without the other trace fossils in the Ammonitico rosso facies (F2) from the lower and upper part of Argiles rouges de Kheneg Formation have been regarded as an indicator of the low oxygen concentrations in bottom waters (BROMLEY & EKDALE, 1984;SAVRDA and BOTTJER, 1989;WIGNALL, 1991), or dysoxic conditions representing a probably pipe zone (SAVRDA, 2007) in deep water sediments during particular time intervals. It can also occur in well oxygenated deposits in deep tier, where oxygenation of pore waters is already low. The intense bioturbation in the uppermost part of the Argiles rouges de Kheneg Formation may be attributed to the increasing supply and higher food availability caused by oxygen levels related to storm events.
Definite shallow water traces are only represented by Protovirgularia. This is somewhat surprising considering the occurrence of shallow water facies in the Argiles rouges de Kheneg Formation.