The investigated section cropping at Djebel Bourzal represents the best exposures for the Cenomanian-Turonian marlstone-limestone strata(UTM coordinates: 31N, 743665.210E 3865187.326N). It consists essentially to fossiliferous marlstone-limestone-dolostone (125-235 m) alternations, with a gypsum beds (0-125 m).The lower boundary of the studied formationconsists of the underlying Cenomanian dominated marl-limestone- to gypsum and dolostone alternations, while the upper boundary is traced with the disappearance of the last massive limestone bed typical of the Bourzal Formation and apparition of a thick Cenomanian marls and limestone overlaid by Turonian dominated marl-limestone and dolostone alternations. Based on lithofacies, thickness, colors, sedimentological characteristics, fossil content, the Bourzal Formationhas been subdivided into two informal units (Figs. 2, 3):
Figure .2 near here
Unit A
This 125 m-thick first unit is divided into two sub-units:
Sub-unit A1: it is up to 70 m thick, consisting of alternation of limestone and marlstone interlayers. The lowermost part of this unit corresponds to light gray (Facies BZ 1-3-4) limestone beds (0.3-1 m), showing planar-, cross-bedding and ripple marks. It is composed of predominantly well-sorted, spherical to ellipsoidal ooid grains, cemented by sparry calcite, displaying benthic foraminifera, bivalves, gastropods, algaes, echinoderms and bryozoans. The uppermost part of this sub-unit shows fine-grained limestone, exhibiting benthic fauna (Facies BZ 6), algal laminations (Fig.4A) and fenestral structures (Facies BZ 2-5-7).
Sub-unit A2: it is thick of about 55 m, composed of marl-limestone- to gypsum and dolostone alternations (Facies BZ 8 to 15). The dominated limestone beds (1-10 m) are whitish- to yellowish, containing black chert nodules and mud crucks (Fig. 4B and C), benthic fauna, stylolite, associated with small-scale vugs with soft sediment deformation (convolute lamination) and fluide escape structures (Fig. 4D and E), and calcite-filled fracture. The summit of this sub-unit is represented by microcrystalline dolomies, brown weathering and gray color in fresh-cut, displaying wavy bedding and mud-cracks. It consists of dolomicrite to microdolosparite, generally lacking of fossils, screening fenestral fabrics, and some trace fossils
Figure 3 near here
of Thalassinoides isp. (Fig. 4F). The sub-unit A shows sometimes abundant rudists, gastropods, bivalves and oysters, attributed previously by Laffitte (1939) to Cenomanian age. Similar assemblages have been recorded from the Cenomanian of Algeria (e.g, Zaoui et al. 2016; Bensekhria et al. 2019).
Figure .4 near here
Unit B
This 110 m-unit B is conspicuously divided into two sub-units:
Sub-unit B1: made of 35 m-thick fossiliferous greenish-gray marls, overlying the massive limestone and gypsum layers of the underlying unit A, containing abundant bivalves, in particular Ostra columba Laffitte (1939).
Sub-unit B2: it is thick of about 75 m, consisting of dominated yellowish-whitish massive limestones (0,2-1 m), admitting yet greenish marly interlayers. The most part of this sub-unit shows fine grains dominanted by bioclasts (algaes, echinoderms), peloids and ooids, cemented by microsparte (Facies types BZ 23 and 24), or occasionally with fenestral structure and dolomicrosparite (Facies types BZ 25 to 32). It can be also represented by a finely textured limestone, made up of calcite particles, demonstrating laminations, fenestral structures, and little of microfauna (categorised as Facies types BZ 16 to 22).
Microfacies analysis (Table. 1)
The most important depositional features perceptible in studying thin sections of Bourzal limestone rocks are textures, biogenic (microfossils) and non-biogenic allochems (mainly oolites) and sedimentary structures, allowed to the detection of eight (08) microfacies, their description and interpretations are given below. They emphasize three facies belts, labeled FBA to FBC, which are: FBA: tidal flat environment microfacies, FBB: lagoon environment microfacies, FBC: shoal environment microfacies. These microfacies belts reflect the sea-level change during the Cenomanian-Turonian interval.
Table .1 near here
1. Facies Belt A (FBA): Tidal flat environment microfacies
Two main microfacies (FBA1 to FBA2) are present in this facies belt
1.1. Fenestral wackstone microfacies (FBA1) (Fig. 5A)
This microfacies is showing fine-grained limestones (predominantly micritic mudstones), and large-size diagenetic dolosparite or sparse dolomite rhombs. This corresponds to fenestral fabric with cryptalgal structures, irregularly formed and distributed, within cyanobacterial mats indicating a peritidal environment. This microfacies contains a little of bioclasts, excepting some scarce foraminifera (small biserial agglutinated or textularia and miliolinid), fragments of bivalves and undeterminable bioclasts. Tiny cracks are also present, filled with calcite and organic material.
Environment: The fenestral structures are commonly the result of gas bubble process, contraction and expansion, and air seepage during flooding, or even referred to burrowing action of worms or insects (Shinn 1983). These structures are characteristics of a tidal flat zone, therefore; the microfacies FBA1 corresponds to the MFS21 of Flügel (2010), common in intertidal pools or platform interior (FZ8, p 663).
1.2. Dolomicrite-microsparite microfacies (FBA2) (Fig. 5B)
The microfacies is characterised by a dense packed, composed primarily of fine- to medium light- to gray crystalline dolomite grains ranging from 20 to 100 µm. The allochems are represented by very fine peloids, some quartz grains, microbial filaments, accompanied with coarse-crystalline gypsum casts with occasional desiccation cracks. This microfacies is almost devoid of skeletal grains, while, a laminar structure and fenestral fabrics can be observed.
Environment: The existence of small crystals, the preservation of the original texture, and the absence of fossils evidence that this has been developed within a supratidal sub-environment.
The microfaciesFBA2 corresponds to MFS25, dominated by dolomitic facies, including terrigenous quartz and evaporitic crystals, without indigenous biota. Such microfacies has been deposited likely in beside carbonates within supratidal sabkha (FZ 9A of Flügel 2010).
2. Facies belt B: Lagoon environment microfacies
Two main microfacies (FBB) constituting the facies belt B, which are;
2.1. Bioclastic packstone microfacies (FBB1) (Fig. 5C and D).
This microfacies displays a grain-supported texture, bioclasts of benthic foraminifera (orbitoids, alveolinids and miliolids), micritized algaes and fragments of bivalve shells, with lesser number of nummulites.
Environment: The abundant orbitoids and miliolids, and a lesser number of nummulites in mud-supported facies, suggests a deposition in a lagoonal environment. This facies is equivalent to RMF-20 (Restricted Marine Facies 20) of Flügel's facies belt. The lagoon had medium to low energy levels and shallow water depth, with deposition below the fair-weather wave base in the photic zone. The lagoon's salinity and temperature were comparable to those of the open ocean, and it had an excellent link to the open marine environment.
2.2. Bioclastic wackstone-packstone microfacies (FBB2) (Fig. 5E and F)
This microfacies includes skeletal grains embedded in a grey micritic matrix. The allochems comprise fragments of echinoderms, bivalves, pelecypods, orbitolinids, miliolids, textularia, rotaliida, ostracods, dasyclad and red algae, peloids and rare extraclasts (siliciclast grains).
Environment: The minor amount of non-skeletal grains (peloids) and the diversified benthic foraminifera are characteristics for unrestricted tidal pond environment under moderate energy conditions (FT5 of Flügel 2010). This microfacies can be compared to SMF 10 (bioclastic packstone and wackestone with abraded and worn skeletal grains), indicating transport from high-energy to low-energy environments, above fair-weather wave base within shelf lagoon (FZ 7).
Figure . 5 near here
3. Facies belt C: Shoal microfacies
This is the more frequent facies recorded in the investigated section, made of two main microfacies (FBC1, FBC2 and FBC3):
3.1. Cortoidal-ooidal grainstone microfacies (FBC1) (Fig. 6A, B, C and D).
The dominant grain types are cortoid grains embedded in sparitic cement with sparse ooids (exhibiting distinct concentric lamination and radial fibrous structures, and can show transitions from cortoids to ooids). The oncoids are conspicuously rounded to rarely elongate, sometimes fragmented, with mostly debris of bivalves or algal as nuclei (Fig.6H). Additional grains are also present as well as peloids, intraclasts and aggregates (Fig.6G). The skeletal grains are represented mainly by transversal section of bivalves or complete shell, showing geopetal fabrics, echinoderms or rare echinoid spines, gastropods and algaes (Fig.6E and F), as well as benthic foraminifera, displaying sporadic parallel inclined arrangement.
The cortoids consist of non-laminated micritized edges, which are the most important composing of this microfacies and are related to constructive and destructive micritization of microboring of fungi and microbes (Bathurst 1966; Reid and Macintyre 2000). The processes of carbonate grain micritization are documented in warm tropical shallow marine, caused by cyanobacteria and microbes. The presence of inclined and parallel arrangement of the bioclasts induced a current transport. Two sub-types are majority observed:
3.1.1. Radial ooid bioclastic grainstone microfacies (FBC1a) (Fig. 6A and B).
The FBC1 is composed of an abundant ooids of almost radial-concentric type (0,5-1mm), which are moderately to well-rounded and well-sorted. with radial-fibrous textures. They show mostly bioclastic grains of calcite or micritical nuclei, tubules caused by algal perforations of Girvanella sp, and little ooid having relatively large nuclei composed of either peloids or bioclasts. Surrounding these large nuclei consists of thin outer cortical layers, and ranging from poorly to moderately sorted, and from moderately to well-rounded, depending on the original shape of the nucleus particle. Some ooids show evidence of broken and then a new cortical layer regenerated around the fragmented pieces (oval or spherical peloids grains and scarce aggregates, cortoids and bioclasts). The overall allochems are linked with fibrous and granular sparite cement.
Environment: The radial-fibrous ooids can be compared to SMF 15-R (ooid grainstone with radial ooids), which are common in shallow low-energy, corresponding to restricted near coast marginal-marine parts of a carbonate platform (FZ 8). The transition to tangential concentric microfabric suggests the change to settings agitated water energy.
3.1.2. Micritized-ooid grainstone microfacies (FBC1b) (Fig. 6C and D)
The predominant grain types in this microfacies sub-type are small superficial micritized ooids (150-300 µm), with a few peloids and bioclast fragments, micritized partially or completely. The original cortical structure is selectively replaced by the micrite due to biological processes, and an obviously transition from cortoids and micritized bioclasts is also observed. The ooid nuclei often consists of gastropods or bivalve’s fragment.
Environment: The features of this facies are similar to those of SMF 15-C (ooid grainstone with concentric ooids), indicating deposition under high energy shallow, reworking bioclasts and producing ooids. These sediments have accumulated in high-energy, ooid shoals situated near the outer margins of the carbonate platform (FZ 6), with a low sedimentation rate suggested by the presence of micritization.
3.2. Ooidal-peloidal grainstone-packstone microfacies (FBC2) (Fig. 6E and F).
It is composed of often well-sorted rounded to subrounded ooids and bahamite peloids, not exceeding 0.5 mm, associated with microsparite cement or micritic matrix. The whole microstructure of ooids is either entirely or partially micritized, with bioclastic or quartz grains nuclei. Some of them can show relics of the primary structure, but the main features correspond to round micrite grains pseudopeloids within the grainstone texture (GT) or packstone (PA). The bioclasts are represented by bivalves and gastropods, even echinoderms and algal fragments.
Environment: The ooidal grains are organised either in weakly compacted grainstone, showing a partial dissolution of ooid cortex or with tangential fabric, characterising ooid formation in high-energy zones. The FBC2 is compared with SMF 15-C: oolite shoal, inner ramp environment.
3.3. Rudist boundstone microfacies (FBC3) (Fig. 6G and H).
This microfacies is characterised by dominant rudist fragments, recorded in the uppermost of the studied section. The interspaces between the shells are infilled with dark micritic mud, containing benthic foraminifera and scattered small fragments.
Environment: The areas between the rudist shells have been filled with mud, connecting the individual rudist shell into a single massive. Generally, these organisms colonised shallow marine depth, where existed a suitable circumstances of food source, as well as light, circulation and low energy level (Aly et al. 2005). The rudist boundstone microfacies are suggestive of existence of a barrier reef biotopes, corresponding to facies belt 5 of Wilson, and SMF 7 of Flügel's classifications.
Figure .6 near here