Sedimentology and ichnology of the mid-Cretaceous succession of the Ouled Nail Mounts (Eastern Saharan Atlas, Algeria)

Shallow marine deposits characterize the upper Albian – lower Cenomanian deposits of Northern Algeria. In Djebel Azzeddine (Ouled Nail Mounts), the corresponding sediments have been subdivided into three distinctive units A to C. The first discovered ammonite fauna from the Bou Saada area allowed the attribution of a part of the mid-Cretaceous post-Continental Intercalaire deposits to the upper Albian. The ammonite-bearing level indicates a maximum flooding surface and could be correlated with similar levels from Northern Algeria. The studied succession is characterized by a low ichnodiversity containing eight ichnotaxa with abundant Thalassinoides, common Skolithos, and rare Gyrolithes, Oichnus, Planolites and cf. Tisoa. This ichnoassemblage is dominated by domichnion, fodinichnion and praedichnion trace fossils, and is attributed to the Skolithos and Glossifungites ichnofacies. These traces are produced mainly by decapod crustaceans, polychaetes and naticid gastropods. The sedimentological and ichnological data suggest shoreface to backshore environments with mixed tide/storm energy, and long subaerial exposures indicated by Lofer cyclothems in the lowermost part and dinosaur footprints in the upper part of the section. 2. GEOLOGICAL SETTING The Algerian Atlasic system consists of the Saharan Atlas to the west, and the Aures, Nementcha, Negrine and Tebessa Mountains to the east (e.g., DJEBBAR, 2000). Their equivalents are the High and Middle Atlas in Morocco, and the Tunisian Atlas in Tunisia, forming, together with the Tell-Rif system to the North, the Atlas Mountains belts sensu lato of northwestern Africa (HALAMSKI & CHERIF, 2017), considered as part of the west Mediterranean alpine system (Fig. 1A). The Ouled Nail Mounts represent the eastern part of the Saharan Atlas (Fig. 1B), which corresponds to an intracratonic autochthonous chain located in northern Algeria, belonging to the Atlasic system (DJEBBAR, 2000; NAIMI & CHERIF, 2021a; NAIMI et al., 2021a). The Algerian Saharan Atlas extends SWNE over about 650 km in length and 90 to 140 km wide between the Moroccan High-Atlas and the Zibane Mountains (or Biskra promontory) (GUIRAUD, 1973). This chain was developed in a subsiding intra-plate asymmetric basin, in existence since the Triassic, located between two stable domains, the Oran Meseta (High Plateaus) in the North, and the Saharan Platform in the South, from which it is respectively isolated, by the South Mesetan and the South Atlasic Faults (KAZI-TANI, 1986). The stratigraphic series (Fig. 1C) of the study area (the northeastern part of the Ouled Nail Mounts) begins with Triassic strata cropping out in diapirs (Kerdada and Ain Ograb), represented by purplish clays, gypsum, dolostones and doleritic ophites. The Triassic rocks are overlain by a 6000 m-thick Cretaceous (Valanginian to Maastrichtian) succession. The Cenozoic (Paleogene to Quaternary) continental deposits unconformably overlie the Mesozoic sediments (EMBERGER, 1960). The mid-Cretaceous sedimentary succession cropping out in the investigated area is characterized by lower Albian continental sandstones of the Continental Intercalaire, rich in the reArticle history: Manuscript received December 15, 2020 Revised manuscript accepted May 31, 2021 Available online October 15, 2021


INTRODUCTION
The Cretaceous deposits are widely distributed in the Algerian Saharan Atlas. In the last few years, several sedimentological, bio-lithostratigraphic and palaeontological studies were focused on the mid-Cretaceous strata of the western (Ksour Mounts) and mains of vegetation (EMBERGER, 1960), overlain by shallow marine carbonate platform deposits. The lower part (300-400 m) of this sequence is dated as upper Albian, consisting of marlydolostone alternations rich in fragments of oyster shells. The palae ontological content of these facies suggests a very shallow marine environment under rough-water conditions .
The lower part of the overlying 460-735 m-thick Cenomanian strata is characterized by shallow-water marlstone-limestones. They are similar to the underlying upper Albian deposits,  (B) Field photography of the studied succession: 1, dolomitic limestones of the lowermost part of the section; 2, Thalassinoides-rich beds; 3, algal limestones of the uppermost part of Unit A (black arrow shows the chert level); 4, micritic limestones of Unit B; 5, gastropod-rich limestones; 6, the lowermost part of Unit C (white arrow indicate ammonites-bearing limestones; black arrow indicate dinosaur tracks-bearing dolostones); 7, dinosaur tracks-bearing surface; 8, marls-shelly limestones alternation from the uppermost part of the section. rich in oysters and echinoderms, and overlain by lagoonal marlstone-dolostone alternations and thick gypsum beds with subordinate limestone interlayers rich in foraminifera (EMBERGER, 1960). The uppermost part of the Cenomanian beds consists of massive mudstones, nodular and bioclastic limestones and black shales (GROSHENY et al., 2008).

MATERIAL AND METHODS
Two field expeditions (December 2019 and March 2020) were conducted. During these missions the mid-Cretaceous succession cropping out in Djebel Azzeddine near the city of Bou Saada was sampled and described bed-by-bed for lithological changes, colour, composition, geometry, sedimentary structures and palaeontological content. The fossils (bivalves, gastropods, ammonites and brachiopods) as well as trace fossils were photographed in situ, collected and stored in the Géologie du Sahara laboratory (Kasdi Merbah University) to be identified and investigated for their palaeoenvironmental interest.
A new dinosaur tracksite was discovered in the studied succession. However, further studies on these footprints are required.

LITHOSTRATIGRAPHIC FRAMEWORK AND PALAEOENVIRONMENT
The Upper Albian -Lower Cenomanian deposits of Djebel Azzeddine were framed as Vraconnian -Cenomanian, corre-sponding to a megasequence, divided into two fourth-order sequences (HERKAT, 1999). In the present work, the studied interval has been subdivided into three informal units (Fig. 2).

Unit A: Marlstone-algal bioturbated limestones unit (upper Albian)
This 52 m-thick unit constitutes the base of the marine mid-Cretaceous deposits outcropping near the city of Bou Saada. Its lower limit has been hidden due to recent urbanization. EMBERGER (1960) indicates that Djebel Azzeddine marine carbonates overlie Albian sandstones of the Continental Intercalaire. The dominant stacking pattern of this unit is represented by an obvious rhythmicity expressed by discrete bed packages (0.6 -6 m thick) of limestones and dolomitic limestones intercalated with greenish to grayish soft, occasionally foliated, fossiliferous marlstones (0.6 -3 m) (Fig. 2). The limestones are hard, highly burrowed with large Thalassinoides isp. (Fig. 7A) and organized in shallowingupwards wackestone to packstone. They are massive, sub-nodular ( Fig. 3A), rarely laminated, yellow, light to dark brown in colour when weathered, white to light gray in cross-section and mostly with sharp erosive bases. The fossil components are dominated by oysters, gastropods and echinoids. These dolomitic limestones show a red loferitic breccia, mud cracks, parallel laminations, micro-HCS (hummocky-cross stratifications), stromatolitic laminae, silex layers, paleosol, teepee structures and shrinkage pores (Fig. 3B,D and E). The upper contact of this unit corresponds to a hardground with oxidized dolo-mudstones characterized by condensed gastropod levels dominated by the species Actnonella delgadoi, as well as vertical borings (Fig. 3F).

Unit B: Lower marlstone-shelly limestone unit (upper Albian)
This 22.5 m-thick unit comprises white and green marls (0.2 -2.5 m) alternating with grayish to yellowish massive, shelly and sandy limestones. The limestone beds are 0.05 to 2.5 m thick, broadly pseudo-nodular to nodular, bioturbated, channelized, white to dark gray weathering coloured, gray to yellowish in cross-section, showing noteworthy densely packed thin bioclasts of benthic fauna, organized in packstone to grainstone textures (Fig. 4A). The middle part of this unit exhibits many subordinate shell beds (0.2 -2.5 m thick), thinning upwards, amalgamated and wave rippled, showing a rapid transition into an overlying marly lithofacies, namely: Cucullaea-rich limestones corresponding to bioturbated limestone, composed of monotaxic bivalves (Cucullaea sp.) (Fig. 4B), and polytaxic gastropod-rich limestones with fragmented and randomly oriented shells (Fig.  4C). A scarce brachiopod fauna is also present.
Internally, the limestone beds of this unit contain small scale hummocky-cross stratifications, lenticular, flaser to wavy bedding, internal mud drapes, tidal rhythmites, unidirectional, linguoid, wavy ripple marks and mega ripples ( Fig. 4D-E). The ichnotaxa of this unit are represented by Gyrolithes isp. (Fig. 6A), Oichnus isp. (Fig. 6B) and cf. Tisoa siphonalis (Fig. 7E). The uppermost part of this unit is representedby a concentration of large-sized ammonites Mortoniceras sp. and Pervin quieria sp., arranged in single post-mortem disposition , and small fragments of Engonoceras sp. which co-occur with bivalves and gastropods.
The 2 -3 m-thick dolomite in the middle part of this unit is characterized by dinosaur footprints (Figs. 8 and 9) associated with vertical burrows attributed to Skolithos (Fig. 6D), as well as a rich assemblage of worn and recrystallized molds of bivalves and gastropods. The uppermost part of the studied succession consists of regular alternations of light green to white soft marls ( Fig. 5E-F) and mollusk rich limestones, composed of abundant disarticulated and fragmented ( Fig. 5C) or whole mollusk shells including bivalves and gastropods (Fig. 5D).

Facies analysis
On the basis of sedimentological and palaeontological characteristics such as lithology, sedimentary structures, fossils and/or trace fossils, bed thickness and taphonomy of shell beds, fifteen distinctive sedimentary facies types (FT1 to FT15) have been identified, described, interpreted and presented in Table 1 and Figures 3-5.

Age of the succession
Despite the extension of the mid-Cretaceous succession of the Ouled Nail Mounts, no detailed bio-and lithostratigraphic investigations have been previously carried out on these deposits. EM-BERGER (1960), based only on lithological criteria, such as the occurrence of rich oyster shell fragment-limestones, assigned a latest Albian (Vraconnian) and early Cenomanian age to the mid-Cretaceous marine sediments of the Djebel Azzeddine section. Furthermore, HERKAT (1999) assigned the same deposits to a whole Vraconnian-Cenomanian mega-sequence encompassing three successive sequences.
Our new findings indicate a late Albian-lower Cenomanian age for the mid-Cretaceous deposits of the Ouled Nail Mounts. No biostratigraphic fossils have been recorded in the lowermost part of the analyzed succession. The last bed of the first unit contains a condensed gastropod shell-bearing level of Acteonella  Sub-nodular dolomitic limestones Marlstone-algal bioturbated limestones unit They consist of yellowish to brownish, hard, massive or scarcely laminated, poorly fossiliferous, 0.6 to 3 m-thick, fine-grained dolomitic limestone beds. The main faunal components are gastropods, rare echinoids, and highly fragmented and disarticulated oysters, which are oriented horizontally to the bedding. The sedimentary structures are represented by horizontal laminations and micro hummocky-cross stratifications. The beds are intensively bioturbated. However, the locally sub-nodular to nodular aspect ( Fig. 3A) is due to abundant large Thalassinoides (T. isp. and T. suevicus) ( Fig. 7A and D). The biological component and ichnological association of this facies supports middle shoreface environment (e.g., HOWARD & FREY, 1984), with well-oxygenated water above the sea floor . The presence of hummocky-cross stratifications with oriented, fragmented and disarticulated benthic fauna such as oysters and echinoids points to high energy deposits related to periodic storm events.

Marlstone-algal bioturbated limestones unit
The loferites are the most widespread facies in the analyzed succession. They consist of light yellow to dark brown, 0.8 to 6 m-thick dolomitized beds, made by complete Lofer cycles, represented by: (i) dolomitic limestones, similar to that of the FT1, with reworked and transported bioclasts and shell debris, showing in-situ slumped brecciation (Fig. 3C); (ii) horizontal, irregularly undulating and laterally continuous, stromatolitic cryptoalgal laminae (Fig. 3D); and (iii) red soil with red loferitic breccia, tepee structures, shrinkage pores containing internal sediment and millimeter-to centimetersized gypsum crystals and mud cracks (Fig. 3B).
The gypsum crystals grew displacively as lenticular crystals within the algal mats. Furthermore, the last bed belonging to this facies shows discontinuous blackish chert band which displays a nodular and cauliflower-shaped pattern and co-occurs with stromatolitic laminae (Fig. 3E). These cycles may be correlated with Lofer-type facies of the Triassic of the Austrian Alps (FISCHER, 1964). They indicate a regressive shallowing-upward trend, and may represent ideal elementary cyclothems (sensu D' ARGENIO, 1974 andSTRASSER, 1991). However, they are meter-scale, corresponding to three successive members: (i) Member C: subtidal dolostone beds represented by dolomitic limestones which yielded benthic foraminifera (miliolids) and encrusted algae (EMBERGER, 1960);(ii) Member B: stromatolitic dolostones associated with tidal-flat features (teepees, shrinkage pores, gypsum crystals and mud cracks), and demonstrate a restricted intertidal zone (SHINN, 1983); and (iii) Member A: red paleosols and loferitic breccia (supratidal soil conglomerates),considered as diagnostic of a subaerial exposure related to pedogenesis process in nearby emerged areas, and they are compatibles with the Member A of the Lofer cyclothem.
The in-situ brecciation and slump breccia are interpreted as local collapses of the carbonate platform related to active tectonics (IANNACE et al., 2014). Unfortunately, no syn-sedimentary faults or other palaeotectonic features were detected. However, HERKAT & GUIRAUD (2006) evidenced a tectonic instability during the late Albian in other localities from Ouled Nail basin, to the south of our study area.

Thalassinoides-rich beds Marlstone-algal bioturbated limestones unit
This facies corresponds to yellowish sandy nodular sandy limestones, 0.25 to 1.2 m thick, rich in Thalassinoides paradoxicus. The nodular appearance of these beds is due to the high density of these trace fossils (Fig. 7C). T. paradoxicus suggests a low energy environment (MÁNGANO & BUATOIS, 1991). Furthermore, a similar facies from the Lower Cretaceous of Argentina has been interpreted as a discontinuity surface, representing a change of local environmental conditions and a decrease of sedimentation rate (MÁNGANO & BUATOIS, 1991).

Marlstone-algal bioturbated limestones unit
This facies corresponds to reddish massive dolomitized limestones, 2 m-thick on average, including a monotaxic condensation of Acteonella delgadoi Choffat, 1901 and vertical borings filled with yellowish sandy material. These gastropods are randomly oriented, moderately sorted, densely packed are relatively fragmented (Fig. 3F). Also, they are neither bored nor encrusted. Acteonella occurs in shallow marine or full marine lagoons (KOWALKE & BANDEL, 1996), with an infaunal way of life (SOHL & KOLLMANN, 1985). Furthermore, the taphonomic characteristics of A. delgadoi shells indicate an in situ preservation succeeding short post-mortem period.

FT5. Micritic limestones Lower marlstone-shelly limestones unit
This facies consists of whitish to greenish fine-grained micritic massive, sub-nodular and tabular limestones ( Fig. 4A), 3 -4 m-thick, showing an intermittent horizontal lamination, containing bivalve fauna, highly bored by circular to subcircular borings assigned to Oichnus isp. (Fig. 6B). In thin sections, the micritic limestones facies shows benthic and planktonic foraminifera embedded in a mudstone-wackestone texture. This facies was deposited in an open marine setting, under low energy conditions, below storm wave base . Oichnus is considered as predatory gastropod borings, which occur on brachiopod, echinoid, and molluscan shells. In open marine setting, it has been recorded from offshore sediments (GRUN et al., 2017).

FT6.
Bioclastic pseudo-nodular limestones Lower marlstone-shelly limestones unit FT6 corresponds to pseudo-nodular to nodular, bioclastic, channelized, grayish to yellowish, 0.05 -2.5 m thick limestones. The bioclastic content consists generally of fragments of monospecific oyster shells fragments randomly oriented, moderately to highly fragmented, abraded and relatively poorly sorted, embedded in a packstone to grainstone cement. Echinoid spines are also present. Some rare trace fossils such as cf. Tisoa siphonalis has been observed in this lithofacies (Fig. 7E). In very shallow marine setting, Tisoa supports high energy conditions (BOCKELIE, 1991;KNAUST, 2019), which can be suggested in the studied succession by the fragmented and disoriented oyster within a limestone matrix. The presence of benthic bioclasts is related to storm events, in a shoreface depositional environment.

FT7.
Cucullaea-rich limestones Lower marlstone-shelly limestones unit and Upper marlstone-shelly limestones unit The shell beds are made of a monotaxic concentration of the bivalves Cucullaea sp. These limestone beds are whitish, laterally continuous, 0.2 -0.4 m, matrix supported, showing internal erosion-sedimentation surfaces. FT7 is very rich in shells which are loosely fragmented and abraded or mostly complete and well-preserved, oriented parallel to bedding (Fig. 4B). Thereby, they are neither encrusted nor bioeroded. Cucullaea bivalves are well known in the mid-and upper Cretaceous deposits from several south Tethyan regions as well as Algeria, Egypt, Jordan, Morocco and Tunisia, occurring in shallow marine limestones (e.g., NAGM & BOUALEM, 2019). The studied specimens are considered to constitute the first record from Algeria. The low degree of fragmentation, the lake of bioerosion and encrustation indicate very limited transport. Cucullaea-rich limestones has been considered as channel lags of laterally migrating subtidal channels, developing at estuarine to shallow marine off the mouth of a wave and tide influenced estuary (MARENSSI et al., 1998) during a slow transgressive event.

Description and range
Interpretation and environmental significance FT8.

Lower marlstone-shelly limestones unit
This facies corresponds to grayish amalgamated limestone beds, 0.2 -0.5 m-thick, showing sharp erosive bases, composed mainly of abundant polytaxic gastropods. They are densely packed, randomly oriented, highly fragmented and flattened by compaction, and exhibit signs of abrasion ( Fig. 4C). The top of the beds show wave ripples and scarce trace fossils. The characteristics of this facies as well as the sharp erosive base, the high degree of fragmentation and dense packing of bioclasts indicate storm-induced currents transporting gastropod. The sedimentological and taphonomic data suggest wave to storm dominated platform, above the fair-weather wave base (shoreface).

FT9. Laminated limestones Lower marlstone-shelly limestones unit
The laminated limestones are whitish to yellowish beds, 0.1 -0.6 m-thick, containing small scale HCS (hummocky-cross stratifications), lenticular, flaser to wavy bedding, internal mud drapes, and vertically stacked bundles of alternating sandstone/mudstone parallel laminations (tidal rhythmites) ( Fig. 4D-E). The top surfaces of the beds are commonly characterized by unidirectional, linguoid and wavy ripple marks and mega ripples. This facies can also contain abundant disarticulated and fractured or whole mollusk shells (thick-shelled oysters) (Fig. 4F), and spiral burrows perpendicular to the bedding, assigned to the ichnogenus Gyrolithes isp. (Fig. 6A). The recorded sedimentary features of this facies suggest a tidal flat environment, characterized by an alternation of low and high energy periodic tidal flat deposits (CHERIF et al., 2018). The presence of the trace fossil Gyrolithes indicates a shallow marine environment (intertidal zone), with stiff and/or firm substrates (NETTO et al., 2007). The intense fragmented bioclasts and hummocky-cross stratifications provide evidence of periodic storm events and deposition in a wave/tide-dominated zone (lower foreshore to upper shoreface environment).

FT10.
Ammonites-bearing limestones Lower marlstone-shelly limestones unit points to post-mortem drifting. The non fragmentation of mortoniceratid shells could be related to calm-water conditions and rapid burial by the decantation of the fine particles in suspension. Also, the presence of glauconite in such shallow environment is related to upwelling phenomena (BRANDANO et al., 2020). Consequently, the transport of mortoniceratids could be the result of these processes.

FT11. Structureless limestones Upper marlstone-shelly limestones unit
This facies is composed of hard limestone beds, whitish weathering color-grayish in fresh, 0.2 -0.3 m thick, displaying strong concretionary vertical burrows filled with yellow-red coarser-grained sandy material, associated with Planolites isp. (Fig. 6C). The internal face of the limestone beds includes HCS, SCS and parallel and low-angle cross laminations. The trace fossil Planolites characterizes all aquatic environments (KNAUST, 2017). The hummocky cross stratifications indicate storm wave action dominated platform and the presence of low-angle cross bedding suggests wave swash zone , reflecting an upper shoreface depositional environment.

FT12. Dinosaur tracks-bearing dolostones Upper marlstone-shelly limestones unit
The FT12 corresponds to hard brownish to reddish dolostone beds, 2 m-thick, including small-sized tridactyl dinosaur footprints, which comprise traces of digits II, III and IV, preserved in concave epirelief. Some vugs are present on the track-bearing surface and they are mineralized with calcite.
The track-bearing surface contains Skolithos isp. burrows (Fig. 6D) associated with scarce Thalassinoides isp. (Fig. 7B). These footprints are very poorly preserved due to weathering processes and they are associated with oyster and gastropod remains. The trace fossils Skolithos and Thalassinoides co-occur in very shallow marine environments, influenced by tides and storms (BENYOUCEF et al., 2014). A subaerial exposure is evidenced by the presence of dissolution-vugs, red detrital material, and oxidized dolostone. The co-occurrence of marine bivalve and gastropod fauna with dinosaurs suggests marginal-littoral environment. The sedimentological analysis together with palaeontological and ichnological data indicate an intertidal environment with periodic storm-generated episodes.

FT13. Shelly limestones Upper marlstone-shelly limestones unit
FT13 is represented by biodetrital and amalgam molluscan packstone-wackestone beds, 0.15 to 0.8 cm-thick, whitish, composed of complete or fragmentary bivalves (pectinids and oysters) and gastropods shells, randomly oriented parallel to bedding ( Fig. 5C-D). The sedimentary structures are represented by rare hummocky-(HCS) and swaley-cross stratifications (SCS) (Fig. 5A-B). The microfacies, the distribution of molluscan shells as well as the sedimentological features indicate tempestite deposits referred to a lower shoreface environment, between fair weather wave and storm wave basis.

Marlstone-algal bioturbated limestones unit, Lower marlstone-shelly limestones unit and Upper marlstone-shelly limestones unit
This facies corresponds to glauconitic greenish to grayish, soft, and occasionally foliated, 0.6 -7 m-thick marlstones (Fig. 5E), including rich foraminifera, abundant oyster and gastropod bioclasts. In this marly facies, no sedimentary events have been recorded, but the presence of reworked bioclasts could be related to two processes: (i) the slight reworking of autochthonous elements such as oyster shells; or (ii) the sedimentation of transported bioclasts from proximal areas. Thus, FT14 reflects a shoreface environment, under storm influence.

FT15. Whitish marls Lower marlstone-shelly limestones unit and Upper marlstone-shelly limestones unit
The FT15 consists of whitish to light gray, soft marls, 0.2 -2.5 m-thick (Fig. 5F). The main components of this facies are bivalves, gastropods and rare small brachiopods.   (KIEKEN, 1974). Consequently, the maximum flooding surface related to the mid-Cretaceous transgression in northern Algeria (Frenda-Tiaret, Ouled Nail and Hodna basins) is characterized by condensed Mortoniceratinae-beds, which are diachronous, pointing to a late Albian age sensu lato.

INVERTEBRATE TRACE FOSSILS
The mid-Cretaceous succession of the Djebel Azzeddine section records a low diversity assemblage of trace fossils. Six ichnogenera were recognized with abundant Thalassinoides, common Skolithos, and rare Gyrolithes, Oichnus, Planolites and Tisoa. Except for Thalassinoides and Tisoa, it was impossible to identify specimens in the ichnospecies level. Gyrolithes isp. (Fig. 6A) Description: Bioturbation structures described herein consist of vertical, sinistrally or dextrally spiraled burrows, corkscrewshaped, preserved as epichnial. These burrows are smooth and filled by dark and fine sediment in comparison with the host sediment. They are perpendicular to the bedding. Coil diameter is 40 -50 mm, and shaft diameteris 5 -8 mm.

5.1.
delgadoi (Fig. 3F). The studied specimens are considered to constitute the first record from Algeria. This Actaeonellid gastropod is a widespread middle to upper Albian taxon, recorded from Egypt, France, Morocco and Portugal (SOHL & KOLLMANN, 1985). A. delgadoi occurs in the Dipoloceras (D.) cristatum and

Oichnus
Occurrence: Lower marlstone-shelly limestone unit. Remarks: Bioerosion structures or borings occur in shallow marine biogenic substrates such as bivalve, brachiopod and echinoid shells (e.g., NAIMI et al., 2021c;VINN et al., 2021a). Many small round holes (or drill holes) in shells are assigned to the ichnogenus Oichnus which is produced essentially by predatory gastropods, particularly naticid gastropods (MÜLLER, 1969), known from the Cambrian  to the Holocene (NIELSEN & NIELSEN, 2001), and belonging to the ichnofamily Oichnidae (WISSHAK et al., 2019). Oichnus is interpreted as an example of Praedichnium (predation traces) with or without signs of attachment (WISSHAK et al., 2015;VALLON et al., 2016). Planolites isp. (Fig. 6C) Description: Epichnial burrows preserved in positive epirelief and oriented more or less parallel to the bedding. They consist of simple, unlined, straight, unbranched, slightly inclined burrows, 6 mm wide and 35 mm long. Planolites isp. burrows are filled with yellow-red coarser-grained sandy material different from that of the host rock, which is finer and lighter, and co-occur with strong concretionary vertical undetermined burrows characterized by a similar fill.

Remarks:
The post-depositional trace fossil Planolitesis interpreted as a feeding trace of vermiform deposit-feeders (UCH-MAN, 1995), arthropods and bivalves (KNAUST, 2017). It is considered as a cosmopolitan trace fossil known from the Ediacaran, occurring in different aquatic environments in softgrounds (e.g., UCHMAN, 1995;KNAUST, 2017;BELAID et al., 2020). In a shallow marine setting, Planolites commonly occurs in the Cruziana ichnofacies (BUATOIS & MÁNGANO, 2011). Skolithos isp. (Fig. 6D) Description: Vertical to subvertical, unbranched, cylindrical and tabular burrows, preserved as endichnia. The burrow apertures at the bedding plane surface are circular to slightly oval. Skoli thos isp. burrows usually completely penetrate the rock and are filled with a brownish sandy material with small recrystallized bioclasts. They are 2-13 mm in diameter, with a maximum length of about 120 mm. Skolithos isp. co-occurs with Thalassinoides isp. and dinosaur footprints.
Remarks: Thalassinoides burrows occur in a shallow marine setting and represent a common constituent of the Cruziana ichnofacies (BENYOUCEF et al., 2012(BENYOUCEF et al., , 2019CHERIF et al., 2015CHERIF et al., , 2018BELAID et al., 2020). They are known from the Ordovician (EKDALE & BROMLEY, 2003) to the Holocene (NICKELL & ATKINSON, 1995), and seem to be abundant within Mesozoic and Cenozoic strata (EL-SABBAGH et al., 2017). Thalassinoides is considered as a fodinichnion-domichnion trace fossil produced by decapod crustaceans (FREY et al., 1984). Furthermore, Palaeozoic Thalassinoides may be produced by non-crustacean tracemakers (CARMONA et al., 2004). WOODWARD, 1830 (Fig. 7C) Description: T. paradoxicus is recorded for the first time from Algeria. It is preserved in positive epichnia and hypichnia, mostly as hypichnia on the sole of the beds. T. paradoxicus is densely branched, subcylindrical to cylindrical burrows, highly irregular in size and morphology. The burrow system is multidirectional and oriented at various angles with respect to bedding, 20 -80 mm in diameter, occurring as contorted nodules. The tunnels are horizontal, straight to slightly curved, whereas the bifurcations consist mostly of T-shaped intersections than Y-shaped. The burrow filling is similar to that of the host material.

T. paradoxicus
Occurrence: Marlstone-algal bioturbated limestones unit. Remarks: T. paradoxicus differs from the recorded T. sue vicus by its complex irregularly branching system, as well as the predominance of T-branches rather than Y-shaped bifurcations. The studied T. paradoxicus branched system resembles that described in the middle Miocene of Egypt (EL-SABBAGH et al., 2017). It occurs in shallow siliciclastic deposits (KNAUST, 2020), especially in the middle shoreface (HOWARD & FREY, 1984) to foreshore (CHRZASTEK et al., 2018), and suggests a low energy environment (MÁNGANO & BUATOIS, 1991). T. para doxicus burrows are domichnion, documented in firmgrounds characterizing the Glossifungites ichnofacies. They probably required firm, at least semi-consolidated substrates to prevent burrow collapse (MYROW, 1995). RIETH, 1932 (Fig. 7D) Description: The studied Thalassinoides suevicus are preserved in epichnia and endichnia, characterized by their complex irreg-ularly branching system. Tunnels and shaft diameters vary from 5 to 24 mm, filled with a fine brown sandy material. Thereby, dichotomous bifurcations are more common than T-shaped branches.

T. suevicus
Occurrence: Marlstone-algal bioturbated limestones unit. Remarks: Thalassinoides suevicus burrows support a subtidal environment (middle shoreface) (e.g., HOWARD & FREY, 1984). They characterize the soft grounds (MYROW, 1995), within a shallow marine setting, with well-oxygenated water above the sea floor . , 1840 (Fig. 7E) Description: It consists of a cylindrical, vertical U-shaped burrow, showing laminations within the passive burrow fill, the tube is filled by micritic material. The remaining portion of the wellpreserved tube is 30 mm long and 6 mm wide, constituting the long axis of a cylindrical calcareous concretion.

cf. Tisoa siphonalis DE SERRES
Occurrence: Lower marlstone-shelly limestones unit. Remarks: The difference between Arenicolites and Tisoa was discussed by KNAUST (2019). Despite the close affinity between these trace fossils, the studied burrow has been attributed to cf. T. siphonalis due to the high length-width ratio and the presence of the calcareous concretion. The studied cf. Tisoa siphona lis resembles the trace fossil Annerepichnites walakhavasensis (sensu KULKARNI & GHARE, 1991), recorded from shallow marine Bathonian -Kimmeridgian sediments from India, and which has been recently attributed to Tisoa siphonalis (KNAUST, 2019). The key feature of this trace is the presence of a laminated fill, which has been observed in the studied burrow. Tisoa sipho nalis occurs in shallow to deep marine environments (KNAUST, 2017(KNAUST, , 2019CHERIF et al., 2021a, b), from the lower Ordovician (PICKERILL & KEPPIE, 1981) to the Holocene (BADVE & GHARE, 1984). ++ is interpreted as the result of dwelling activity (domichnion) of polychaetes (KNAUST, 2017), related to widespread authigenic seep carbonate formation (VAN DE  et al., 2010), and it is common in quasi-anoxic organic-rich and in cold seep deposits (KNAUST, 2019). In Algeria, this ichnospecies has been reported from the lower Miocene Tiaret Marl Formation  and the early Cretaceous of the Ouarsenis Range .

Ichnological analysis
The ichnoassemblage of the studied succession is composed of horizontal, vertical and inclined trace fossils constituting an impoverished example of the Skolithos -Glossifungites ichnofacies. It is dominated by domichnion, fodinichnion and praedichnion trace fossils produced mainly by worms, decapods and naticid gastropods.
Trace fossils of the lower part of the section correspond to a firmground suite of the Glossifungites ichnofacies and they are represented essentially by Thalassinoides paradoxicus. The T. paradoxicus rich bed (unit A) is characterized by low ichnodiversity, high abundance, and intense bioturbation which destroyed the primary sedimentary structures and the presence of branched burrow systems. These characteristics are typical of the substrate-controlled Glossifungites ichnofacies (BUATOIS & MÁNGANO, 2011). However, firmground burrowers may produce Tisoa (KNAUST, 2017) and Gyrolithes (NETTO et al., 2007). The typical examples of the Glossifungites ichnofacies (archetypal Glossifungites ichnofacies) are recorded in shallowto marginal marine environments; furthermore, surfaces containing this ichnofacies indicate transgressive events (BUATOIS & MÁNGANO, 2011). The Glossifungites ichnofacies occurs as a result of intense erosion in the zone of maximum wave energy of wave-dominated tidal flats (YANG et al., 2009).
The Skolithos ichnofacies is well represented in the upper part of the section, mainly dominated by vertical, cylindrical, simple dwelling burrows of suspension-feeders, and characterized by the abundance of three-dimensional burrow systems dominated by vertical components, the absence of horizontal trace fossils produced by a mobile fauna, low ichnodiversity and variable abundance. Skolithos constitutes the most common ichnogenus of the Skolithos ichnofacies, well-known in nearshore settings. The dominance of vertical dwelling structures of infaunal suspension-feeders such as Skolithos isp. indicates the high abundance of organic particles that are kept in suspension in the oxygenated water column by currents and waves (BUATOIS & MÁNGANO, 2011). The predominance of vertical components over horizontal components indicates relatively high wave energy (HOWARD & FREY, 1984) related to stressful conditions. Such situations can be indicated by the low ichnodiversity and the mono specific occurrences of Skolithos isp. (MÁNGANO & BUATOIS, 2004). In shallow marine water, the Skolithos ichnofacies is typical of foreshore to upper-and middle-shoreface environments, and it occurs in lower-intertidal flats depending on the tidal regime (BUATOIS & MÁNGANO, 2011).
Several dinosaur footprints have been recorded in a similar setting. Marginal marine carbonate sediments of a large innershelf environment, characterized by dolomitic sedimentation related to a warm and dry climate yielded theropod and ornithopod footprints from the Barremian of Portugal (SANTOS et al., 2013). Furthermore, tridactyl footprints which co-occur with bivalves and gastropods have been documented in dolomitic facies from the early Jurassic of France (MOREAU et al., 2018). These tracks are associated with desiccation cracks and they indicate deposi-tion within a periodically emergent environment. The invertebrate trace fossils and mud volcanoes recorded from these deposits allowed attribution of these track-bearing deposits to a subtidal to inter-supratidal flat marsh. In northern Africa, a similar ichnoassemblage including Skolithos and dinosaur footprints (theropod, sauropod and ornithischian), reported torepresent a tidal flat, has been described from the mid-Cretaceous of Morocco (IBRA-HIM et al., 2014). Such a vertebrate-invertebrate ichnoassemblage has also been documented in a shallow-marine carbonate setting in the middle Jurassic of Wyoming (KVALE et al., 2001). Invertebrate trace fossils are dominated by vertical and cylindrical burrows attributed to the ichnogenus Skolithos, indicating a soft-ground typical of an intertidal onshore facies persistent during formation of the dinosaur trackway. In the lower Cretaceous of Texas, dinosaur footprints are associated with a shallow invertebrate ichnofauna, suggesting a supratidal to shallow subtidal environment (FARLOW et al., 2012).

The mid-Cretaceous transgression and palaeogeography
The Djebel Azzeddine mid-Cretaceous series could be correlated with the upper unit of the Rhelida Formation, which crops out in the Ksour and Djebel Amour Mounts, respectively in the western and central parts of the Algerian Saharan Atlas. The Rhelida Formation transgressive deposits directly overlay the Continental Intercalaire and have been attributed firstly to the Vraconnian (uppermost Albian) (BASSOULLET, 1973). On the basis of new biostratigraphic data, as well as vertebrate remains from the Rhelida Formation equivalents in Morocco (e.g., CAVIN et al., 2010), Egypt (e.g., LE LOEUFF et al., 2012), and the Guir basin (southwestern Algeria) (BENYOUCEF et al., 2014, this Formation has now been dated as lower -middle Cenomanian .  , 1967). These fluvio-deltaic sediments are overlain by the late Albian -Cenomanian Djebel Tenfeld carbonate Formation (AUCLAIR & BIEHLER, 1967;CISZAK, 1993), which yielded new ostracod species (DAMOTTE, 1984). The Albianlower Cenomanian strata of the Tellian Atlas (northwestern Algeria) are represented by turbidite-deposits and deep marl-limestone alternations (e.g., CISZAK, 1993). It is concluded that the mid-Cretaceous transgression is diachronous across northern Algeria. It is precocious (late Albian) in the eastern part of the Saharan Atlas (Ouled Nail basin) and the northern border of the Oran High Plateaus (Daïa and Frenda-Tiaret basins), and more recent in the central and western parts of the Saharan Atlas (Djebel Amour and Ksour basins).

CONCLUSIONS
New insights on the lithostratigraphy and the palaeoenvironment have been provided from the upper Albian -lower Cenomanian marine succession overlying the Continental Intercalaire in the eastern part of the Ouled Nail Mounts (eastern Algerian Saharan Atlas). The studied succession has been subdivided into three distinctive units: The Marlstone-algal bioturbated limestones (unit A), lower marlstone-shelly limestones (Unit B) and upper marlstone-shelly limestones (unit C). Units A and B have been attributed to the upper Albian on the basis of new recorded fossils. A typical Lofer cyclothem with in situ slumped brecciation has been observed from the lowermost part of the section, reflecting local collapses of the carbonate platform. An ammonite-rich bed, discovered at the uppermost part of unit B including mortoniceratids and engonoceratids belonging to Mortoniceras sp., Pervinquieria sp. and Engonoceras sp. has been recorded for the first time in the Bou Saada area. These ammonites indicate a maximum flooding surface of the mid-Cretaceous transgression and could be correlated with similar levels from other Algerian basins such as the Frenda-Tiaret and the Hodna basins. Unit C is barren of any biostratigraphic fauna, but has been assigned to the upper Albian -lower Cenomanian based on its position in the succession.
The studied succession is characterized by a low ichnodiversity containing eight ichnotaxa such as: Gyrolithes isp., Planolites isp., Skolithos isp., Thalassinoides isp., T. paradoxicus, T. suevi cus, cf. Tisoa siphonalis, and the boring Oichnus isp. T. paradoxi cus and Oichnus isp. are recorded for the first time from Algeria. Thalassinoides burrows are abundant, with common Skolithos, and rare Gyrolithes, Oichnus, Planolites and cf. Tisoa. This ichnoassemblage is dominated by domichnion, fodinichnion and praedichnion trace fossils, and attributed to the Skolithos and Glossifungites ichnofacies.These trace fossils are produced mainly by decapod crustaceans, polychaetes and naticid gastropods. The sedimentological, palaeontological and ichnological data suggest an environment ranging from backshore (supratidal) to shoreface with a mixed (tide/storm) energy source. Also, new small-sized tridactyl dinosaur footprints have been observed in Unit C. Considering the scarcity of diagnostic characters available we refrain from assigning these ichnites to specific ichnotaxa, and more accurate studies are required to proceed in that direction.