2021 | 74/3 | 209–223 | 7 Figs. | 1 Tab. | www.geologia-croatica.hr Journal of the Croatian Geological Survey and the Croatian Geological Society 1. 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 central Saharan Atlas (Djebel Amour Mounts) (MEBARKI et al., 2016; BENYOUCEF et al., 2017; FERRÉ et al., 2017; MENNAD et al., 2020; SALHI et al., 2020; ÖZER & BENYOUCEF, 2021). In the Ouled Nail Mounts (eastern part of the Saharan Atlas), the corresponding deposits are represented by a succession made of marly-limestone/dolostone alternations which display a rich mac- rofauna (bivalves, gastropods, ammonites), as well as trace fos- sils. The best exposures of this interval crop out in the Djebel Azzeddine, Djebel Amrane and Djebel Tsegna, and were first extensively explored by BROSSARD (1866), PÉRON (1883) and RITTER (1902). The studied interval has been mapped and in- vestigated since the late mid 20th century (EMBERGER, 1960; GUIRAUD, 1973; HERKAT, 1999). Unfortunately, with the ex- ception of the previously cited works, no comprehensive revision of the upper Albian-lower Cenomanian succession has been car- ried out in recent years. The late Albian (Vraconnian) – lower Cenomanian interval is thereby considered as an important eu- static event, corresponding to the global and greatest mid-Creta- ceous transgression (HANCOCK & KAUFFMAN, 1979; AMÉDRO, 2008). This paper aims to provide the first sedimentological and ichnological study of the upper-Albian transgressive marine de- posits exposed in Djebel Azzeddine (Ouled Nail Mounts). Our contribution provides the first ammonite-bearing level, ichnotaxa inventory, detailed facies analysis and new dinosaur footprint re- cord that facilitates identification of the depositional environment and the palaeobiogeography in the Saharan Atlas during the mid- Cretaceous. Sedimentology and ichnology of the mid-Cretaceous succession of the Ouled Nail Mounts (Eastern Saharan Atlas, Algeria) Mohammed Nadir Naimi and Amine Cherif Université Kasdi Merbah, Laboratoire de Géologie du Sahara, Ghardaïa Road, Ouargla, 30000, Algeria; (naimi.mohamed_nadir@univ-ouargla.dz (Mohammed Nadir Naimi), acherif11@gmail.com (Amine Cherif)) doi: 10.4154/gc.2021.15 Abstract Shallow marine deposits characterize the upper Albian – lower Cenomanian deposits of North- ern 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 crus- taceans, polychaetes and naticid gastropods. The sedimentological and ichnological data sug- gest shoreface to backshore environments with mixed tide/storm energy, and long subaerial ex- posures 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 Sa- haran Atlas (Fig. 1B), which corresponds to an intracratonic au- tochthonous chain located in northern Algeria, belonging to the Atlasic system (DJEBBAR, 2000; NAIMI & CHERIF, 2021a; NAIMI et al., 2021a). The Algerian Saharan Atlas extends SW- NE 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 Mese- tan and the South Atlasic Faults (KAZI-TANI, 1986). The stratigraphic series (Fig. 1C) of the study area (the north- eastern 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 Tri- assic rocks are overlain by a 6000 m-thick Cretaceous (Valangin- ian to Maastrichtian) succession. The Cenozoic (Paleogene to Quaternary) continental deposits unconformably overlie the Meso- zoic sediments (EMBERGER, 1960). The mid-Cretaceous sedimentary succession cropping out in the investigated area is characterized by lower Albian conti- nental sandstones of the Continental Intercalaire, rich in the re- Article history: Manuscript received December 15, 2020 Revised manuscript accepted May 31, 2021 Available online October 15, 2021 Keywords: Trace fossils, Transgression, Albian – Cenomanian, Ouled Nail Mounts, Algeria G eo lo gi a C ro at ic a Geologia Croatica 74/3210 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 marly- dolostone alternations rich in fragments of oyster shells. The palae ontological content of these facies suggests a very shallow marine environment under rough-water conditions (NAIMI et al., 2021b). The lower part of the overlying 460–735 m-thick Cenoma- nian strata is characterized by shallow-water marlstone-lime- stones. They are similar to the underlying upper Albian deposits, Figure 1. Location map of the study area. (A) Location of central Algeria in the western Mediterranean; (B) The main structural domains of central Algeria; (C) Sim- plified geological map of the Bou Saada area (modified after the geological map of Bou Saada 1/200.000); (D) Late Albian global palaeogeography and location of the Ouled Nail basin (map after SCOTESE, 2013). G eologia C roatica Naimi and Cherif: Sedimentology and ichnology of the mid-Cretaceous succession of the Ouled Nail Mounts (Eastern Saharan Atlas, Algeria) 211 Figure 2. The main lithostratigraphic characteristics of the upper Albian – lower Cenomanian of Djebel Azzeddine. (A) Lithostratigraphic column of the Djebel Azzeddine section; (B) Field photography of the studied succession: 1, dolomitic limestones of the lowermost part of the section; 2, Thalassinoides-rich beds; 3, al- gal 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. G eo lo gi a C ro at ic a Geologia Croatica 74/3212 rich in oysters and echinoderms, and overlain by lagoonal marl- stone-dolostone alternations and thick gypsum beds with subor- dinate 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). 3. 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, col- our, composition, geometry, sedimentary structures and palae- ontological 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 suc- cession. However, further studies on these footprints are re- quired. 4. 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 se- quences (HERKAT, 1999). In the present work, the studied in- terval has been subdivided into three informal units (Fig. 2). 4.1. Unit A: Marlstone-algal bioturbated limestones unit (upper Albian) This 52 m-thick unit constitutes the base of the marine mid-Cre- taceous 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 rhyth- micity 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 shallowing - upwards wackestone to packstone. They are massive, sub-nodu- lar (Fig. 3A), rarely laminated, yellow, light to dark brown in col- our when weathered, white to light gray in cross-section and mostly with sharp erosive bases. The fossil components are domi- nated by oysters, gastropods and echinoids. These dolomitic limestones show a red loferitic breccia, mud cracks, parallel lami- nations, micro-HCS (hummocky-cross stratifications), stroma- tolitic laminae, silex layers, paleosol, teepee structures and shrinkage pores (Fig. 3B, D and E). The upper contact of this unit Figure 3. Field photographs of Unit A. (A) Pseudo-nodular limestones; (B) Intertidal to supratidal limestones with algal laminae, paleosol, shrinkage pores (black arrows), and loferitic breccia (white arrow); (C) Subtidal limestones with in-situ slumped breccia; (D) Stromatolitic limestone; (E) Dolomitic limestone showing algal laminae including chert nodules; (F) Top surface of dolomitic limestone showing hardground with abundant Acteonella delgadoi. G eologia C roatica Naimi and Cherif: Sedimentology and ichnology of the mid-Cretaceous succession of the Ouled Nail Mounts (Eastern Saharan Atlas, Algeria) 213 corresponds to a hardground with oxidized dolo-mudstones chara- cterized by condensed gastropod levels dominated by the species Actnonella delgadoi, as well as vertical borings (Fig. 3F). 4.2. 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 corre- sponding to bioturbated limestone, composed of monotaxic bi- valves (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). Figure 4. Field photographs of Unit B. (A) Micritic limestone bed; (B) Cucullaea-rich limestones; (C) Gastropod-rich limestones, with sharp erosive base and ripple- mark in top surface; (D) Small-scale hummocky-cross stratification in fine limestone bed at the top of the unit, with light micritic laminae and dark sandy-micritic laminae; (E) Tidal rhythmites from the top of the unit; (F) Limestone bed with robust bioclasts of oysters and wavy rippled upper surface; (G-H) Large size Mortoni- ceracidae of the ammonites-bearing bed. G eo lo gi a C ro at ic a Geologia Croatica 74/3214 The uppermost part of this unit is representedby a concen- tration of large-sized ammonites Mortoniceras sp. and Pervin­ quieria sp., arranged in single post-mortem disposition (Fig. 4G-H), and small fragments of Engonoceras sp. which co-occur with bivalves and gastropods. 4.3. Unit C: Upper marlstone-shelly limestone unit (upper Albian – lower Cenomanian) The lower part of this 18 m-thick unit is composed of an alterna- tion of white marls (2 – 3 m) and whitish massive limestones with sporadic, thin, bivalve shell beds of Cucullaea sp. These lime- stone beds are hard, display yellow-red sandy inclined burrows as Planolites isp. (Fig. 6C), hummocky-cross stratifications (HCS), swaley-cross stratifications (SCS) and parallel and cross laminations (Fig. 5A-B). 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). 4.4. Facies analysis On the basis of sedimentological and palaeontological character- istics 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. 4.5. Age of the succession Despite the extension of the mid-Cretaceous succession of the Ouled Nail Mounts, no detailed bio- and lithostratigraphic inves- tigations 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 con- tains a condensed gastropod shell-bearing level of Acteonella Figure 5. Field photographs of Unit C. (A) Limestone bed with low-angle cross lamination; (B) Limestone bed showing hummocky cross-stratification (HCS) and swaley cross-stratification (SCS); (C) Oyster-rich bioclastic limestone (black arrows indicate echinoid spines); (D) Limestone bed rich in gastropod and pectinid shells; (E) Greenish marls interlayered with limestone beds; (F) Uppermost part of the succession showing rhythmic whitish marls-dolomitic limestones alternations. G eologia C roatica Naimi and Cherif: Sedimentology and ichnology of the mid-Cretaceous succession of the Ouled Nail Mounts (Eastern Saharan Atlas, Algeria) 215 Ta bl e 1. D es cr ip tio n an d se di m en to lo gi ca l a tt rib ut es o f l ith of ac ie s i de nt ifi ed in th e up pe r A lb ia n – lo w er C en om an ia n de po si ts o f D je be l A zz ed di ne (O ul ed N ai l M ou nt s, A lg er ia ). Fa ci es ty pe (F T) D es cr ip tio n an d ra ng e In te rp re ta tio n an d en vi ro nm en ta l s ig ni fic an ce FT 1. Su b- no du la r d ol om iti c lim es to ne s M ar ls to ne -a lg al b io tu rb at ed li m es to ne s u ni t Th ey c on si st o f y el lo w is h to b ro w ni sh , h ar d, m as si ve o r s ca rc el y la m in at ed , p oo rly fo ss ili fe ro us , 0 .6 to 3 m -t hi ck , fi ne -g ra in ed d ol om iti c lim es to ne b ed s. Th e m ai n fa un al c om po ne nt s a re g as tr op od s, ra re ec hi no id s, an d hi gh ly fr ag m en te d an d di sa rt ic ul at ed o ys te rs , w hi ch a re o rie nt ed h or iz on ta lly to th e be dd in g. T he se di m en ta ry st ru ct ur es a re re pr es en te d by h or iz on ta l l am in at io ns a nd m ic ro hu m m oc ky -c ro ss st ra tifi ca tio ns . T he b ed s a re in te ns iv el y bi ot ur ba te d. H ow ev er , t he lo ca lly su b- no du la r t o no du la r a sp ec t ( Fi g. 3 A) is d ue to a bu nd an t l ar ge T ha la ss in oi de s ( T. is p. a nd T . s ue vi cu s) (F ig . 7 A a nd D ). Th e bi ol og ic al c om po ne nt a nd ic hn ol og ic al a ss oc ia tio n of th is fa ci es su pp or ts m id dl e sh or ef ac e en vi ro nm en t ( e. g. , H O W A RD & F RE Y, 1 98 4) , w ith w el l-o xy ge na te d w at er a bo ve th e se a flo or (N A IM I e t a l., 2 02 0) . T he p re se nc e of hu m m oc ky -c ro ss st ra tifi ca tio ns w ith o rie nt ed , f ra gm en te d an d di sa rt ic ul at ed b en th ic fa un a su ch a s o ys te rs a nd ec hi no id s p oi nt s t o hi gh e ne rg y de po si ts re la te d to p er io di c st or m e ve nt s. FT 2. Lo fe rit es M ar ls to ne -a lg al b io tu rb at ed li m es to ne s u ni t Th e lo fe rit es a re th e m os t w id es pr ea d fa ci es in th e an al yz ed su cc es si on . T he y co ns is t o f l ig ht y el lo w to da rk b ro w n, 0 .8 to 6 m -t hi ck d ol om iti ze d be ds , m ad e by c om pl et e Lo fe r c yc le s, re pr es en te d by : ( i) do lo m iti c lim es to ne s, si m ila r t o th at o f t he F T1 , w ith re w or ke d an d tr an sp or te d bi oc la st s a nd sh el l de br is , s ho w in g in -s itu sl um pe d br ec ci at io n (F ig . 3 C) ; ( ii) h or iz on ta l, irr eg ul ar ly u nd ul at in g an d la te ra lly c on tin uo us , s tr om at ol iti c cr yp to al ga l l am in ae (F ig . 3 D ); an d (ii i) re d so il w ith re d lo fe rit ic br ec ci a, te pe e st ru ct ur es , s hr in ka ge p or es c on ta in in g in te rn al se di m en t a nd m ill im et er - t o ce nt im et er - si ze d gy ps um c ry st al s a nd m ud c ra ck s ( Fi g. 3 B) . Th e gy ps um c ry st al s g re w d is pl ac iv el y as le nt ic ul ar c ry st al s w ith in th e al ga l m at s. Fu rt he rm or e, th e la st b ed b el on gi ng to th is fa ci es sh ow s d is co nt in uo us b la ck is h ch er t b an d w hi ch d is pl ay s a n od ul ar an d ca ul ifl ow er -s ha pe d pa tt er n an d co -o cc ur s w ith st ro m at ol iti c la m in ae (F ig . 3 E) . Th es e cy cl es m ay b e co rr el at ed w ith L of er -t yp e fa ci es o f t he T ria ss ic o f t he A us tr ia n A lp s ( FI SC H ER , 1 96 4) . T he y in di ca te a re gr es si ve sh al lo w in g- up w ar d tr en d, a nd m ay re pr es en t i de al e le m en ta ry c yc lo th em s ( se ns u D ’A RG EN IO , 19 74 a nd S TR A SS ER , 1 99 1) . H ow ev er , t he y ar e m et er -s ca le , c or re sp on di ng to th re e su cc es si ve m em be rs : ( i) M em be r C: su bt id al d ol os to ne b ed s r ep re se nt ed b y do lo m iti c lim es to ne s w hi ch y ie ld ed b en th ic fo ra m in ife ra (m ili ol id s) a nd en cr us te d al ga e (E M BE RG ER , 1 96 0) ;(i i) M em be r B : s tr om at ol iti c do lo st on es a ss oc ia te d w ith ti da l-fl at fe at ur es (te ep ee s, sh rin ka ge p or es , g yp su m c ry st al s a nd m ud c ra ck s) , a nd d em on st ra te a re st ric te d in te rt id al z on e (S H IN N , 19 83 ); an d (ii i) M em be r A : r ed p al eo so ls a nd lo fe rit ic b re cc ia (s up ra tid al so il co ng lo m er at es ),c on si de re d as d ia gn os tic of a su ba er ia l e xp os ur e re la te d to p ed og en es is p ro ce ss in n ea rb y em er ge d ar ea s, an d th ey a re c om pa tib le s w ith th e M em be r A o f t he L of er c yc lo th em . Th e in -s itu b re cc ia tio n an d sl um p br ec ci a ar e in te rp re te d as lo ca l c ol la ps es o f t he c ar bo na te p la tfo rm re la te d to ac tiv e te ct on ic s ( IA N N AC E et a l., 2 01 4) . U nf or tu na te ly , n o sy n- se di m en ta ry fa ul ts o r o th er p al ae ot ec to ni c fe at ur es w er e de te ct ed . H ow ev er , H ER KA T & G U IR AU D (2 00 6) e vi de nc ed a te ct on ic in st ab ili ty d ur in g th e la te A lb ia n in o th er lo ca lit ie s f ro m O ul ed N ai l b as in , t o th e so ut h of o ur st ud y ar ea . Th e lo fe rit es (F T2 ) s ug ge st a su pr at id al (b ac ks ho re ) t o a su bt id al (s ho re fa ce ) d ep os iti on al e nv iro nm en t, w ith a pr ol on ge d ex po su re o n th e tid al fl at s. FT 3. Th al as sin oi de s- ric h be ds M ar ls to ne -a lg al b io tu rb at ed li m es to ne s u ni t Th is fa ci es c or re sp on ds to y el lo w is h sa nd y no du la r s an dy li m es to ne s, 0. 25 to 1 .2 m th ic k, ri ch in Th al as sin oi de s p ar ad ox ic us . T he n od ul ar a pp ea ra nc e of th es e be ds is d ue to th e hi gh d en si ty o f t he se tr ac e fo ss ils (F ig . 7 C) . T. p ar ad ox ic us su gg es ts a lo w e ne rg y en vi ro nm en t ( M Á N G A N O & B U AT O IS , 1 99 1) . F ur th er m or e, a si m ila r f ac ie s f ro m th e Lo w er C re ta ce ou s o f A rg en tin a ha s b ee n in te rp re te d as a d is co nt in ui ty su rf ac e, re pr es en tin g a ch an ge o f l oc al en vi ro nm en ta l c on di tio ns a nd a d ec re as e of se di m en ta tio n ra te (M Á N G A N O & B U AT O IS , 1 99 1) . FT 4. Ac te on el la -r ic h be d M ar ls to ne -a lg al b io tu rb at ed li m es to ne s u ni t Th is fa ci es c or re sp on ds to re dd is h m as si ve d ol om iti ze d lim es to ne s, 2 m -t hi ck o n av er ag e, in cl ud in g a m on ot ax ic c on de ns at io n of A ct eo ne lla d el ga do i C ho ffa t, 19 01 a nd v er tic al b or in gs fi lle d w ith ye llo w is h sa nd y m at er ia l. T he se g as tr op od s a re ra nd om ly o rie nt ed , m od er at el y so rt ed , d en se ly pa ck ed a re re la tiv el y fra gm en te d (F ig . 3 F) . A ls o, th ey a re n ei th er b or ed n or e nc ru st ed . Ac te on el la o cc ur s i n sh al lo w m ar in e or fu ll m ar in e la go on s ( KO W A LK E & BA N D EL , 1 99 6) , w ith a n in fa un al w ay o f l ife (S O H L & KO LL M A N N , 1 98 5) . F ur th er m or e, th e ta ph on om ic c ha ra ct er is tic s o f A . d el ga do i s he lls in di ca te a n in si tu pr es er va tio n su cc ee di ng sh or t p os t- m or te m p er io d. FT 5. M ic rit ic li m es to ne s Lo w er m ar ls to ne -s he lly li m es to ne s u ni t Th is fa ci es c on si st s o f w hi tis h to g re en is h fin e- gr ai ne d m ic rit ic m as si ve , s ub -n od ul ar a nd ta bu la r lim es to ne s ( Fi g. 4 A) , 3 – 4 m -t hi ck , s ho w in g an in te rm itt en t h or iz on ta l l am in at io n, c on ta in in g bi va lv e fa un a, h ig hl y bo re d by c irc ul ar to su bc irc ul ar b or in gs a ss ig ne d to O ic hn us is p. (F ig . 6 B) . I n th in se ct io ns , t he m ic rit ic li m es to ne s f ac ie s s ho w s b en th ic a nd p la nk to ni c fo ra m in ife ra e m be dd ed in a m ud st on e- w ac ke st on e te xt ur e. Th is fa ci es w as d ep os ite d in a n op en m ar in e se tt in g, u nd er lo w e ne rg y co nd iti on s, be lo w st or m w av e ba se (B EN YO U CE F et a l., 2 01 7) . O ic hn us is c on si de re d as p re da to ry g as tr op od b or in gs , w hi ch o cc ur o n br ac hi op od , ec hi no id , a nd m ol lu sc an sh el ls . I n op en m ar in e se tt in g, it h as b ee n re co rd ed fr om o ffs ho re se di m en ts (G RU N e t a l., 20 17 ). FT 6. Bi oc la st ic p se ud o- no du - la r l im es to ne s Lo w er m ar ls to ne -s he lly li m es to ne s u ni t FT 6 co rr es po nd s t o ps eu do -n od ul ar to n od ul ar , b io cl as tic , c ha nn el iz ed , g ra yi sh to y el lo w is h, 0 .0 5 – 2. 5 m th ic k lim es to ne s. Th e bi oc la st ic c on te nt c on si st s g en er al ly o f f ra gm en ts o f m on os pe ci fic o ys te r sh el ls fr ag m en ts ra nd om ly o rie nt ed , m od er at el y to h ig hl y fra gm en te d, a br ad ed a nd re la tiv el y po or ly so rt ed , e m be dd ed in a p ac ks to ne to g ra in st on e ce m en t. Ec hi no id sp in es a re a ls o pr es en t. So m e ra re tr ac e fo ss ils su ch a s c f. Ti so a sip ho na lis h as b ee n ob se rv ed in th is li th of ac ie s ( Fi g. 7 E) . In v er y sh al lo w m ar in e se tt in g, T iso a su pp or ts h ig h en er gy c on di tio ns (B O CK EL IE , 1 99 1; K N AU ST , 2 01 9) , w hi ch c an b e su gg es te d in th e st ud ie d su cc es si on b y th e fra gm en te d an d di so rie nt ed o ys te r w ith in a li m es to ne m at rix . T he pr es en ce o f b en th ic b io cl as ts is re la te d to st or m e ve nt s, in a sh or ef ac e de po si tio na l e nv iro nm en t. FT 7. Cu cu lla ea -r ic h lim es to ne s Lo w er m ar ls to ne -s he lly li m es to ne s u ni t a nd U pp er m ar ls to ne -s he lly li m es to ne s u ni t Th e sh el l b ed s a re m ad e of a m on ot ax ic c on ce nt ra tio n of th e bi va lv es C uc ul la ea sp . T he se li m es to ne be ds a re w hi tis h, la te ra lly c on tin uo us , 0 .2 – 0 .4 m , m at rix su pp or te d, sh ow in g in te rn al e ro si on -s ed i- m en ta tio n su rf ac es . F T7 is v er y ric h in sh el ls w hi ch a re lo os el y fra gm en te d an d ab ra de d or m os tly co m pl et e an d w el l-p re se rv ed , o rie nt ed p ar al le l t o be dd in g (F ig . 4 B) . T he re by , t he y ar e ne ith er en cr us te d no r b io er od ed . Cu cu lla ea b iv al ve s a re w el l k no w n in th e m id - a nd u pp er C re ta ce ou s d ep os its fr om se ve ra l s ou th Te th ya n re gi on s a s w el l a s A lg er ia , E gy pt , J or da n, M or oc co a nd T un is ia , o cc ur rin g in sh al lo w m ar in e lim es to ne s ( e. g. , N AG M & B O U A LE M , 20 19 ). Th e st ud ie d sp ec im en s a re c on si de re d to c on st itu te th e fir st re co rd fr om A lg er ia . T he lo w d eg re e of fra gm en ta tio n, th e la ke o f b io er os io n an d en cr us ta tio n in di ca te v er y lim ite d tr an sp or t. Cu cu lla ea -r ic h lim es to ne s h as be en c on si de re d as c ha nn el la gs o f l at er al ly m ig ra tin g su bt id al c ha nn el s, de ve lo pi ng a t e st ua rin e to sh al lo w m ar in e off th e m ou th o f a w av e an d tid e in flu en ce d es tu ar y (M A RE N SS I e t a l., 1 99 8) d ur in g a sl ow tr an sg re ss iv e ev en t. G eo lo gi a C ro at ic a Geologia Croatica 74/3216 Fa ci es ty pe (F T) D es cr ip tio n an d ra ng e In te rp re ta tio n an d en vi ro nm en ta l s ig ni fic an ce FT 8. G as tr op od -r ic h lim es to ne s Lo w er m ar ls to ne -s he lly li m es to ne s u ni t Th is fa ci es c or re sp on ds to g ra yi sh a m al ga m at ed li m es to ne b ed s, 0. 2 – 0. 5 m -t hi ck , s ho w in g sh ar p er os iv e ba se s, co m po se d m ai nl y of a bu nd an t p ol yt ax ic g as tr op od s. Th ey a re d en se ly p ac ke d, ra nd om ly o rie nt ed , h ig hl y fra gm en te d an d fla tt en ed b y co m pa ct io n, a nd e xh ib it si gn s o f a br as io n (F ig . 4 C) . T he to p of th e be ds sh ow w av e rip pl es a nd sc ar ce tr ac e fo ss ils . Th e ch ar ac te ris tic s o f t hi s f ac ie s a s w el l a s t he sh ar p er os iv e ba se , t he h ig h de gr ee o f f ra gm en ta tio n an d de ns e pa ck in g of b io cl as ts in di ca te st or m -in du ce d cu rr en ts tr an sp or tin g ga st ro po d. T he se di m en to lo gi ca l a nd ta ph on om ic da ta su gg es t w av e to st or m d om in at ed p la tfo rm , a bo ve th e fa ir- w ea th er w av e ba se (s ho re fa ce ). FT 9. La m in at ed li m es to ne s Lo w er m ar ls to ne -s he lly li m es to ne s u ni t Th e la m in at ed li m es to ne s a re w hi tis h to y el lo w is h be ds , 0 .1 – 0 .6 m -t hi ck , c on ta in in g sm al l s ca le H CS (h um m oc ky -c ro ss st ra tifi ca tio ns ), le nt ic ul ar , fl as er to w av y be dd in g, in te rn al m ud d ra pe s, an d ve rt ic al ly st ac ke d bu nd le s o f a lte rn at in g sa nd st on e/ m ud st on e pa ra lle l l am in at io ns (t id al rh yt hm ite s) (F ig . 4 D -E ). Th e to p su rf ac es o f t he b ed s a re c om m on ly c ha ra ct er iz ed b y un id ire ct io na l, lin gu oi d an d w av y rip pl e m ar ks a nd m eg a rip pl es . T hi s f ac ie s c an a ls o co nt ai n ab un da nt d is ar tic ul at ed a nd fra ct ur ed o r w ho le m ol lu sk sh el ls (t hi ck -s he lle d oy st er s) (F ig . 4 F) , a nd sp ira l b ur ro w s p er pe nd ic ul ar to th e be dd in g, a ss ig ne d to th e ic hn og en us G yr ol ith es is p. (F ig . 6 A) . Th e re co rd ed se di m en ta ry fe at ur es o f t hi s f ac ie s s ug ge st a ti da l fl at e nv iro nm en t, ch ar ac te riz ed b y an a lte rn at io n of lo w a nd h ig h en er gy p er io di c tid al fl at d ep os its (C H ER IF e t a l., 2 01 8) . T he p re se nc e of th e tr ac e fo ss il G yr ol ith es in di ca te s a sh al lo w m ar in e en vi ro nm en t ( in te rt id al z on e) , w ith st iff a nd /o r fi rm su bs tr at es (N ET TO e t a l., 2 00 7) . T he in te ns e fra gm en te d bi oc la st s a nd h um m oc ky -c ro ss st ra tifi ca tio ns p ro vi de e vi de nc e of p er io di c st or m e ve nt s a nd de po si tio n in a w av e/ tid e- do m in at ed z on e (lo w er fo re sh or e to u pp er sh or ef ac e en vi ro nm en t). FT 10 . A m m on ite s- be ar in g lim es to ne s Lo w er m ar ls to ne -s he lly li m es to ne s u ni t FT 10 c on si st s o f c on de ns ed a m m on ite b ed , f or m ed b y fin e- gr ai ne d ye llo w is h to g ra yi sh sa nd y an d gl au co ni tic li m es to ne s, 0. 8 cm th ic k, ri ch in M or to ni ce ra tin ae a nd E ng on oc er at id ae , a rr an ge d in si ng le po st -m or te m d is po si tio n, a ss oc ia te d w ith a bu nd an t b iv al ve s C uc ul la ea sp ., oy st er s, ra re in oc er am id s an d ga st ro po ds . I t c on st itu te s t he o nl y fa ci es c on ta in in g am m on ite fa un a al on g th e se ct io n. T he se am m on ite s b el on g to M or to ni ce ra s s p. , P er vi nq ui er ia sp . a nd E ng on oc er as sp . Pe rv in qu ie ria sp . s pe ci m en s a re a bu nd an t o n th e to p of th e be d, a nd th ey a re c ha ra ct er iz ed b y th ei r la rg e di am et er o ft en a bo ut 2 5 cm (F ig . 4 G -H ). H ow ev er , E ng on oc er as sp . s am pl es a re fr ag m en te d an d sh ow w el l-p re se rv ed su tu re s. En go no ce ra tid ae o cc ur in sh al lo w m ar in e en vi ro nm en ts , i n pa rt ic ul ar in g la uc on iti c se di m en ts , s om e of th em pr ef er rin g tid al a nd la go on al w at er s ( BU TJ O R, 2 01 0) . T he c o- oc cu rr en ce o f i no ce ra m id s w ith g as tr op od s i nd ic at es sh al lo w m ar in e se tt in gs , w ith w ar m w at er s a nd w el l o xy ge na te d en vi ro nm en t d ur in g tr an sg re ss iv e ph as es (B O U A LE M , 2 01 8) . M or to ni ce ra tin ae su gg es t a n op en m ar in e se tt in g, a nd th ei r p re se nc e in sh al lo w m ar in e de po si ts po in ts to p os t- m or te m d rif tin g. T he n on fr ag m en ta tio n of m or to ni ce ra tid sh el ls c ou ld b e re la te d to c al m -w at er co nd iti on s a nd ra pi d bu ria l b y th e de ca nt at io n of th e fin e pa rt ic le s i n su sp en si on . A ls o, th e pr es en ce o f g la uc on ite in su ch sh al lo w e nv iro nm en t i s r el at ed to u pw el lin g ph en om en a (B RA N D A N O e t a l., 2 02 0) . C on se qu en tly , t he tr an sp or t of m or to ni ce ra tid s c ou ld b e th e re su lt of th es e pr oc es se s. FT 11 . St ru ct ur el es s l im es to ne s U pp er m ar ls to ne -s he lly li m es to ne s u ni t Th is fa ci es is c om po se d of h ar d lim es to ne b ed s, w hi tis h w ea th er in g co lo r- gr ay is h in fr es h, 0 .2 – 0 .3 m th ic k, d is pl ay in g st ro ng c on cr et io na ry v er tic al b ur ro w s fi lle d w ith y el lo w -r ed c oa rs er -g ra in ed sa nd y m at er ia l, as so ci at ed w ith P la no lit es is p. (F ig . 6 C) . T he in te rn al fa ce o f t he li m es to ne b ed s i nc lu de s H CS , SC S an d pa ra lle l a nd lo w -a ng le c ro ss la m in at io ns . Th e tr ac e fo ss il Pl an ol ite s c ha ra ct er iz es a ll aq ua tic e nv iro nm en ts (K N AU ST , 2 01 7) . T he h um m oc ky c ro ss st ra tifi ca tio ns in di ca te st or m w av e ac tio n do m in at ed p la tfo rm a nd th e pr es en ce o f l ow -a ng le c ro ss b ed di ng su gg es ts w av e sw as h zo ne (B EN YO U CE F et a l., 2 01 7) , r efl ec tin g an u pp er sh or ef ac e de po si tio na l e nv iro nm en t. FT 12 . D in os au r t ra ck s- be ar in g do lo st on es U pp er m ar ls to ne -s he lly li m es to ne s u ni t Th e FT 12 c or re sp on ds to h ar d br ow ni sh to re dd is h do lo st on e be ds , 2 m -t hi ck , i nc lu di ng sm al l-s iz ed tr id ac ty l d in os au r f oo tp rin ts , w hi ch c om pr is e tr ac es o f d ig its II , I II an d IV , p re se rv ed in c on ca ve ep ire lie f. So m e vu gs a re p re se nt o n th e tr ac k- be ar in g su rf ac e an d th ey a re m in er al iz ed w ith c al ci te . Th e tr ac k- be ar in g su rf ac e co nt ai ns S ko lit ho s i sp . b ur ro w s ( Fi g. 6 D ) a ss oc ia te d w ith sc ar ce T ha la ss in oi - de s i sp . ( Fi g. 7 B) . T he se fo ot pr in ts a re v er y po or ly p re se rv ed d ue to w ea th er in g pr oc es se s a nd th ey a re as so ci at ed w ith o ys te r a nd g as tr op od re m ai ns . Th e tr ac e fo ss ils S ko lit ho s a nd T ha la ss in oi de s c o- oc cu r i n ve ry sh al lo w m ar in e en vi ro nm en ts , i nfl ue nc ed b y tid es a nd st or m s ( BE N YO U CE F et a l., 2 01 4) . A su ba er ia l e xp os ur e is e vi de nc ed b y th e pr es en ce o f d is so lu tio n- vu gs , r ed d et rit al m at er ia l, an d ox id iz ed d ol os to ne . T he c o- oc cu rr en ce o f m ar in e bi va lv e an d ga st ro po d fa un a w ith d in os au rs su gg es ts m ar gi na l-l itt or al e nv iro nm en t. Th e se di m en to lo gi ca l a na ly si s t og et he r w ith p al ae on to lo gi ca l a nd ic hn ol og ic al d at a in di ca te a n in te rt id al e nv iro nm en t w ith p er io di c st or m -g en er at ed e pi so de s. FT 13 . Sh el ly li m es to ne s U pp er m ar ls to ne -s he lly li m es to ne s u ni t FT 13 is re pr es en te d by b io de tr ita l a nd a m al ga m m ol lu sc an p ac ks to ne -w ac ke st on e be ds , 0 .1 5 to 0 .8 cm -t hi ck , w hi tis h, c om po se d of c om pl et e or fr ag m en ta ry b iv al ve s ( pe ct in id s a nd o ys te rs ) a nd ga st ro po ds sh el ls , r an do m ly o rie nt ed p ar al le l t o be dd in g (F ig . 5 C- D ). Th e se di m en ta ry st ru ct ur es a re re pr es en te d by ra re h um m oc ky - ( H CS ) a nd sw al ey -c ro ss st ra tifi ca tio ns (S CS ) ( Fi g. 5 A -B ). Th e m ic ro fa ci es , t he d is tr ib ut io n of m ol lu sc an sh el ls a s w el l a s t he se di m en to lo gi ca l f ea tu re s i nd ic at e te m pe st ite de po si ts re fe rr ed to a lo w er sh or ef ac e en vi ro nm en t, be tw ee n fa ir w ea th er w av e an d st or m w av e ba si s. FT 14 . G re en is h m ar ls M ar ls to ne -a lg al b io tu rb at ed li m es to ne s u ni t, Lo w er m ar ls to ne -s he lly li m es to ne s u ni t a nd U pp er m ar ls to ne -s he lly li m es to ne s u ni t Th is fa ci es c or re sp on ds to g la uc on iti c gr ee ni sh to g ra yi sh , s of t, an d oc ca si on al ly fo lia te d, 0 .6 – 7 m -t hi ck m ar ls to ne s ( Fi g. 5 E) , i nc lu di ng ri ch fo ra m in ife ra , a bu nd an t o ys te r a nd g as tr op od b io cl as ts . In th is m ar ly fa ci es , n o se di m en ta ry e ve nt s h av e be en re co rd ed , b ut th e pr es en ce o f r ew or ke d bi oc la st s c ou ld b e re la te d to tw o pr oc es se s: (i) th e sl ig ht re w or ki ng o f a ut oc ht ho no us e le m en ts su ch a s o ys te r s he lls ; o r ( ii) th e se di m en ta tio n of tr an sp or te d bi oc la st s f ro m p ro xi m al a re as . T hu s, FT 14 re fle ct s a sh or ef ac e en vi ro nm en t, un de r st or m in flu en ce . FT 15 . W hi tis h m ar ls Lo w er m ar ls to ne -s he lly li m es to ne s u ni t a nd U pp er m ar ls to ne -s he lly li m es to ne s u ni t Th e FT 15 c on si st s o f w hi tis h to li gh t g ra y, so ft m ar ls , 0 .2 – 2 .5 m -t hi ck (F ig . 5 F) . T he m ai n co m po ne nt s of th is fa ci es a re b iv al ve s, ga st ro po ds a nd ra re sm al l b ra ch io po ds . Ba se d on th e lit ho lo gi ca l a nd fo ss il co nt en t, th is fa ci es is a tt rib ut ed to a n op en m ar in e se tt in g, w ith lo w e ne rg et ic co nd iti on s ( BE N YO U CE F et a l., 2 01 7) . Ta bl e 1. C on tin ue d G eologia C roatica Naimi and Cherif: Sedimentology and ichnology of the mid-Cretaceous succession of the Ouled Nail Mounts (Eastern Saharan Atlas, Algeria) 217 area), including Hysteroceras orbignyi, Pervinquieria perinflata var. crassissima, Scaphites hugardianus, Stoliczkaia dovsedensis st. notha and Turrilites tuberculatum, and indicating a late Albian age (M. (Mortoniceras) inflatum Zone) (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. 5. INVERTEBRATE TRACE FOSSILS The mid-Cretaceous succession of the Djebel Azzeddine section records a low diversity assemblage of trace fossils. Six ichnogen- era were recognized with abundant Thalassinoides, common Skolithos, and rare Gyrolithes, Oichnus, Planolites and Tisoa. Except for Thalassinoides and Tisoa, it was impossible to iden- tify specimens in the ichnospecies level. 5.1. Gyrolithes isp. (Fig. 6A) Description: Bioturbation structures described herein consist of vertical, sinistrally or dextrally spiraled burrows, corkscrew- shaped, preserved as epichnial. These burrows are smooth and filled by dark and fine sediment in comparison with the host sedi- ment. They are perpendicular to the bedding. Coil diameter is 40 – 50 mm, and shaft diameteris 5 – 8 mm. delgadoi (Fig. 3F). The studied specimens are considered to con- stitute 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 Mortoniceras (M.) inflatum Interval Zones (EL QOT, 2018). In the uppermost part of the second unit, an ammonite-bear- ing level has been discovered for the first time in this part of Ouled Nail Mounts, including Mortoniceras sp., Pervinquieria sp. and Engonoceras sp. (Fig. G-H). Mortoniceras and Pervinquieria species indicate the upper Albian sensu lato, and co-occur in the M. (Mortoniceras) pricei, M. (Mortoniceras) inflatum and M. (Mortoniceras) fallax Zones (MONOD ET AL., 2000; KENNEDY ET AL., 2008; GALE & KENNEDY, 2020). On the basis of this association (Acteonella delgadoi, Mortoniceras sp. and Pervinquieria sp.), a part of the mid-Cretaceous post-Conti- nental Intercalaire marine deposits should correspond to the late Albian. A similar ammonite-bearing bed has been documented in the late Albian of the Frenda-Tiaret Mounts (BOUALEM, 2018), 300 km to the northwest of the Djebel Azzedine section. The am- monite fauna which yielded this level indicates the M. (Mortoni- ceras) pricei and M. (Mortoniceras) fallax condensation Zones, named the Mortoniceras event. The same glauconitic ammonite- bearing level was recorded in the Hodna (to the east of our study Figure 6. Invertebrate trace fossils from the upper Albian – lower Cenomanian of Ouled Nail Mounts. (A) Gyrolithes isp. (black arrow); (B) Bivalve shell showing abundant Oichnus isp.; (C) Planolites isp. (black arrow) associated with undetermined concretionary burrows; (D) Abundant Skolithos isp. G eo lo gi a C ro at ic a Geologia Croatica 74/3218 Occurrence: Lower marlstone-shelly limestones unit. Remarks: The ichnogenus Gyrolithes constitutes a domich- nion trace fossil produced by crustaceans in intertidal and shal- low subtidal environments (GERNANT, 1972; DWORSCHAK & RODRIGUES, 1997; NETTO et al., 2007). It can also be pro- duced by capitellid polychaetes (POWELL, 1977). Gyrolithes oc- curs from the Ediacaran – Cambrian boundary (LAING et al., 2018) to the Holocene (WETZEL et al., 2010), and indicates a marginal marine environment (GERNANT, 1972; POWELL, 1977; WETZEL et al., 2010). This trace fossil is attributed to the Skolithos and Cruziana ichnofacies (PEMBERTON et al., 2001). Morphological features of the studied burrows resemble that of the ichnospecies G. lorcaensis UCHMAN & HANKEN (2013) and G. polonicus FEDONKIN (1981). 5.2. Oichnus isp. (Fig. 6B) Description: Circular, sub-circular, oval to weakly elliptical, mil- limetre-sized borings in the tests of undetermined bivalves. They are perpendicular to the surface of the substrate, and shallower than wide. These borings are over 2 mm in maximum diameter. Occurrence: Lower marlstone-shelly limestone unit. Remarks: Bioerosion structures or borings occur in shallow marine biogenic substrates such as bivalve, brachiopod and echi- noid shells (e.g., NAIMI et al., 2021c; VINN et al., 2021a). Many small round holes (or drill holes) in shells are assigned to the ich- nogenus Oichnus which is produced essentially by predatory gas- tropods, particularly naticid gastropods (MÜLLER, 1969), known from the Cambrian (VINN et al., 2021b) 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). 5.3. 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 character- ized by a similar fill. Occurrence: Upper marlstone-shelly limestones unit. Remarks: The post-depositional trace fossil Planolitesis in- terpreted 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 Edi- acaran, occurring in different aquatic environments in soft- grounds (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). 5.4. 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. Occurrence: Upper marlstone-shelly limestones unit. Remarks: The ichnogenus Skolithos characterizes the litto- ral to shallow sublittoral Skolithos ichnofacies (SEILACHER, 1967). It is created by suspension-feeding organisms such as an- thozoans, crustaceans, holothurians, phoronids, polychaetes and priapulids for dwelling (domichnia) (KNAUST, 2017; KNAUST et al., 2018). Skolithos burrows are generally associated with high hydrodynamic energy within shallow water environments (VINN & WILSON, 2013). This trace fossil is known from the Ediaca- ran (MCCALL, 2006) through to the Holocene (DASHTGARD & GINGRAS, 2012). 5.5. Thalassinoides isp. (Fig. 7A and B) Description: Systems of burrows consisting of horizontal tun- nels and vertical or inclined cylindrical shafts. Diameters of tun- nels and shafts range from 10 to 30 mm. Thalassinoides isp. bur- rows show Y- and T-shapes, and are filled with a brownish detrital material. Occurrence: Marlstone-algal bioturbated limestones unit and Lower marlstone-shelly limestones unit. Remarks: Thalassinoides burrows occur in a shallow ma- rine setting and represent a common constituent of the Cruziana ichnofacies (BENYOUCEF et al., 2012, 2019; CHERIF et al., 2015, 2018; BELAID et al., 2020). They are known from the Or- dovician (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-domichn- ion trace fossil produced by decapod crustaceans (FREY et al., 1984). Furthermore, Palaeozoic Thalassinoides may be produced by non-crustacean tracemakers (CARMONA et al., 2004). 5.6. T. paradoxicus 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 bur- row filling is similar to that of the host material. 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 de- scribed 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 en- ergy environment (MÁNGANO & BUATOIS, 1991). T. para­ doxicus burrows are domichnion, documented in firmgrounds characterizing the Glossifungites ichnofacies. They probably re- quired firm, at least semi-consolidated substrates to prevent bur- row collapse (MYROW, 1995). 5.7. T. suevicus RIETH, 1932 (Fig. 7D) Description: The studied Thalassinoides suevicus are preserved in epichnia and endichnia, characterized by their complex irreg- G eologia C roatica Naimi and Cherif: Sedimentology and ichnology of the mid-Cretaceous succession of the Ouled Nail Mounts (Eastern Saharan Atlas, Algeria) 219 ularly branching system. Tunnels and shaft diameters vary from 5 to 24 mm, filled with a fine brown sandy material. Thereby, di- chotomous bifurcations are more common than T-shaped branches. 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 wa- ter above the sea floor (NAIMI et al., 2020; NAIMI & CHERIF, 2021b). 5.8. cf. Tisoa siphonalis DE SERRES, 1840 (Fig. 7E) Description: It consists of a cylindrical, vertical U-shaped bur- row, showing laminations within the passive burrow fill, the tube is filled by micritic material. The remaining portion of the well- preserved tube is 30 mm long and 6 mm wide, constituting the long axis of a cylindrical calcareous concretion. Occurrence: Lower marlstone-shelly limestones unit. Remarks: The difference between Arenicolites and Tisoa was discussed by KNAUST (2019). Despite the close affinity be- tween these trace fossils, the studied burrow has been attributed to cf. T. siphonalis due to the high length-width ratio and the pres- ence 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, 2019; CHERIF et al., 2021a, b), from the lower Ordovician (PICKERILL & KEPPIE, 1981) to the Holocene (BADVE & GHARE, 1984). ++ is interpreted as the result of dwelling activ- ity (domichnion) of polychaetes (KNAUST, 2017), related to widespread authigenic seep carbonate formation (VAN DE Figure 7. Invertebrate trace fossils from the upper Albian – lower Cenomanian of the Ouled Nail Mounts. (A) Thalassinoides isp. from Unit A; (B) Thalassinoides isp. associated with Skolithos isp. from Unit C; (C) Thalassinoides paradoxicus network; (D) Thalassinoides suevicus; (E) cf. Tisoa siphonalis. G eo lo gi a C ro at ic a Geologia Croatica 74/3220 SCHOOTBRUGGE et al., 2010), and it is common in quasi-an- oxic organic-rich and in cold seep deposits (KNAUST, 2019). In Algeria, this ichnospecies has been reported from the lower Mio- cene Tiaret Marl Formation (CHERIF et al., 2021a) and the early Cretaceous of the Ouarsenis Range (CHERIF et al., 2021b). 6. DISCUSSION 6.1. Ichnological analysis The ichnoassemblage of the studied succession is composed of horizontal, vertical and inclined trace fossils constituting an im- poverished 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 ichnodi- versity, high abundance, and intense bioturbation which de- stroyed 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 pro- duce Tisoa (KNAUST, 2017) and Gyrolithes (NETTO et al., 2007). The typical examples of the Glossifungites ichnofacies (archetypal Glossifungites ichnofacies) are recorded in shallow- to marginal marine environments; furthermore, surfaces contain- ing 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 characteri- zed 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 ich- nogenus of the Skolithos ichnofacies, well-known in nearshore settings. The dominance of vertical dwelling structures of infau- nal 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 ichno- facies 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 inner- shelf environment, characterized by dolomitic sedimentation re- lated 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 inverte- brate trace fossils and mud volcanoes recorded from these depos- its allowed attribution of these track-bearing deposits to a subtidal to inter- supratidal flat marsh. In northern Africa, a similar ich- noassemblage including Skolithos and dinosaur footprints (thero- pod, 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 ichnoassem- blage 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 cylindri- cal burrows attributed to the ichnogenus Skolithos, indicating a soft-ground typical of an intertidal onshore facies persistent dur- ing formation of the dinosaur trackway. In the lower Cretaceous of Texas, dinosaur footprints are associated with a shallow inver- tebrate ichnofauna, suggesting a supratidal to shallow subtidal environment (FARLOW et al., 2012). 6.2. 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 For- mation 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 Rhe- lida Formation equivalents in Morocco (e.g., CAVIN et al., 2010), Egypt (e.g., LE LOEUFF et al., 2012), and the Guir basin (south- western Algeria) (BENYOUCEF et al., 2014, 2015, 2016), this Formation has now been dated as lower – middle Cenomanian (BENYOUCEF et al., 2017). Further north of the Saharan Atlas in the Frenda-Tiaret Mounts (northern border of High Plateaus), similar deposits defined as the Mcharref Formation dated as up- per Albian (BOUALEM & BENHAMOU, 2017) overlie the Sidi Ouadah Formation, considered as the equivalent of the Continen- tal Intercalaire (PEYBERNÈS et al., 1986). The condensed am- monite bed [Mortoniceras event sensu BOUALEM (2018)] indi- cates a maximum flooding surface related to the late Albian transgression (NAGM & BOUALEM, 2019). Further west, in the Daïa Mounts, the equivalent of the Albian Continental Intercal- aire consists of the Grès de Bossuet Formation (AUCLAIR & BIEHLER, 1967). These fluvio-deltaic sediments are overlain by the late Albian – Cenomanian Djebel Tenfeld carbonate Forma- tion (AUCLAIR & BIEHLER, 1967; CISZAK, 1993), which yielded new ostracod species (DAMOTTE, 1984). The Albian – lower Cenomanian strata of the Tellian Atlas (northwestern Al- geria) are represented by turbidite-deposits and deep marl-lime- stone alternations (e.g., CISZAK, 1993). It is concluded that the mid-Cretaceous transgression is diachronous across northern Al- geria. It is precocious (late Albian) in the eastern part of the Sa- haran 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 (Dje- bel Amour and Ksour basins). 7. 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 G eologia C roatica Naimi and Cherif: Sedimentology and ichnology of the mid-Cretaceous succession of the Ouled Nail Mounts (Eastern Saharan Atlas, Algeria) 221 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 fos- sils. A typical Lofer cyclothem with in situ slumped brecciation has been observed from the lowermost part of the section, reflect- ing local collapses of the carbonate platform. An ammonite-rich bed, discovered at the uppermost part of unit B including mor- toniceratids 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 ichnodiver- sity 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 gastro- pods. 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 availa- ble we refrain from assigning these ichnites to specific ichno- taxa, and more accurate studies are required to proceed in that direction. ACKNOWLEDGMENT We thank the managing editor T. FLUKSI (Zagreb), the associ- ate editor A. MEZGA (Zagreb), M. 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