The Paleocene shallow-marine carbonates of the Upper Sabil Formation are dated using larger benthic foraminifera (mainly nummulitids, alveolinids and orthophragminids). For the Paleocene and Eocene, a continuous system of numbered units (shallow benthic zones—SBZ) was established (Serra-Kiel et al. 1998). Serra-Kiel et al. (2020) recently recalibrated the Paleocene SBZ. The SBZ scheme is characterized mainly by oppelzones, identified by the co-occurrence of phyletically unrelated taxonomic groups (Pignatti and Papazzoni 2017) that benefit from the increasing knowledge of shallow benthic foraminifera such as Miscellaneidae (Hottinger 2009; Di Carlo et al. 2010; Sirel 2012; Benedetti et al. 2018), Rotaliidae (Hottinger 2014; Benedetti et al., 2018; Vicedo et al., 2021) and other as yet poorly known taxa (e.g. Benedetti et al. 2018, 2020). The Middle to Late Paleocene shallow-marine carbonate deposits of the Sirt Basin are dominated by Ranikothalia, miscellaneids and rotaliids, thus they can be successfully dated using Shallow Benthic Zonation.
Larger benthic foraminifera (LBF) from the Upper Sabil Formation were recently described by Vršič et al. (2021). Biostratigraphically important species are illustrated in Fig. 7. Their stratigraphic ranges are outlined in Fig. 8. Several index species of SBZ 3 were determined. A single specimen of Kathina cf. aquitanica was found. Kathina aquitanica Hottinger is an index taxon of SBZ 3 (Hottinger 2014). Serra-Kiel et al. (2020) extended the range of Kathina aquitanica from SBZ 2 to SBZ 4. Elazigina lenticula (Hottinger) is a widespread taxon, but has a wide stratigraphic range (SBZ 3–6, according to Hottinger 2014). Glomalveolina primaeva (Reichel) is the most widely recognized index taxon of the SBZ 3 among the presented foraminiferal assemblage (Serra-Kiel et al. 1998). Larger benthic foraminifera of the family Miscellaneidae Sigal are abundant constituents of the Middle–Late Paleocene shallow-marine environments. Two species were found: Miscellanea yvettae Leppig (sensu Hottinger 2009) and Miscellanites primitivus (Rahaghi) (see Hottinger 2009). Both taxa are usually considered index fossils of SBZ 3 (Hottinger 2009), although Miscellanites primitivus has been recently reported by Consorti and Köroğlu (2019) from SBZ 2. Akbarina primitiva, erected by Sirel (2009), is considered a junior synonym of Miscellanites primitivus (e.g. Serra-Kiel et al. 2020) and is a marker of SBZ 2 in Turkey. Therefore, the most probable range of M. primitivus is within SBZ 2 and SBZ 3. Ranikothalia solimani Butterlin in Butterlin and Monod is the most abundant species and enables the correlation between the investigated wells. However, this taxon not yet has an unambiguous stratigraphical distribution. Rotorbinella hensoni (Smout) is constrained to SBZ 2 and SBZ 3 (Hottinger 2014), although Vicedo et al. (2021) suggested that R. detrecta (Hottinger) should be synonym of R. hensoni, thus extending its range up to SBZ1. Representatives of the family Discocyclinidae widely occur in the investigated foraminiferal assemblages. The Discocyclinidae first appeared in SBZ 3 (Less et al. 2007) and are essential constituents in the Eocene strata. The occurrence of unidentifiable Discocyclinidae in our samples is sufficient to support age determination as Selandian–Early Thanetian and refute the possibility that Upper Sabil Formation could be older than SBZ 3.
Microfacies of the Upper Sabil Formation
The petrographic analysis distinguished 13 microfacies types (MFT) labelled as MFT 1 to MFT 13 (Table 1).
MFT1 is a planktonic foraminiferal wackestone (Fig. 9a, b). The main bioclasts are planktonic foraminifera (Fig. 9a), small benthic foraminifera (Fig. 9b), ostracods and fine-grained unidentifiable bioclasts.
MFT 2 is a fine-grained, foraminiferal–microbiodetritic packstone (Fig. 9c, d) consisting of LBF debris (mainly nummulitids), fine-grained unidentifiable bioclasts, planktonic foraminifera, small benthic foraminifera and rare echinoid spines.
MFT 3 is a foraminiferal–echinodermal packstone (Fig. 10a, b) and consists of echinoderms, small rotaliids, LBF and rare planktonic foraminifera. LBF are commonly fragmented and contribute to sediment accumulation as fine-grained debris. Coralline algal crusts are mostly fragmented and rarely form bindstones with intergrowing acervulinids, planorbulinaceans, cyclostome bryozoans, serpulids, and peyssoneliacean algae.
MFT 4 is a foraminiferal–algal packstone (Fig. 11a, b) composed of LBF (nummulitids, orthophragminids, larger rotaliids and rare Glomalveolina), abundant fragmented coralline branches and rare coralline crusts. Lenticular orthophragminids are abundant, whereas flattened discoidal forms are scarce. Foraminiferal debris is coarser grained. Corals occur as fragments and are occasionally encrusted by thin microbial crusts. Sometimes, foraminifera also exhibit micrite envelopes.
MFT 5 is a poorly sorted echinodermal–algal floatstone (Fig. 11c, d) with echinoderm debris, coralline algal branches, algal crust fragments, peysonneliaceans and rare LBF (orthophragminids and larger rotaliids).
MFT 6 is an unsorted orthophragminid–algal floatstone (Fig. 12a–h) and was encountered only in the upper parts of wells 1 and 5 (Fig. 4). It consists of LBF (mainly orthophragminids), red algae (corallinaceans and peyssoneliaceans) and corals. Corals are primarily represented by the massive and encrusting plocoid/cerioid forms. Phaceloid/dendroid corals occur only in cases as isolated corallites (Fig. 12d). Thin crusts of coralline algae (mainly genus Sporolithon) (Fig. 12e) and peyssoneliacean algae form a bindstone (Fig. 12c). Frequently, they are associated with acervulinid foraminifera (Fig. 12g), plocoid/cerioid corals (Fig. 12g), serpulids (Fig. 12f) and cyclostome bryozoans (Fig. 12h). Cerioid/plocoid corals exhibit macroborings, which are mostly filled with micrite. Microborings within the coralline algae are mostly hollow or cemented. In bioconstructor-dominated parts, the diversity of LBF is low.
MFT 7 is a poorly sorted, foraminiferal–bryozoan–algal floatstone (Fig. 13a–c), occasionally also bryozoan floatstone (Fig. 13d). The LBF assemblage consists of Ranikothalia solimani and Miscellanea yvettae; most of them are over 2 mm in size. Dendroid cyclostome bryozoans, echinodermal debris and fragmented coralline algal branches are subordinate bioclasts.
MFT 8 is an unsorted microbialite–coral boundstone. Corals play a minor role as bioconstructor. Abundant sedimentary matrix occurs between the boundstone. Microbialite microfabric terminology follows Zamagni et al. (2009). The following microbialite microfabrics were recognized: dense micrite (Fig. 14a), agglutinated microfabric (Fig. 14a, b), and clotted/peloidal microfabric (Fig. 14c). The agglutinated components consist of fine-grained debris and various larger bioclasts, e.g. LBF, echinoid spines, bryozoans and echinoderms. The microbialites are enveloped by encrusting laminar, plocoid/cerioid corals (Fig. 13a). Corals are also embedded in dense micrite (Fig. 14b). The microbialites are very rarely encrusted by intergrowing, thin coralline crusts and acervulinids (Fig. 14d).
Different coral morphotypes are intergrowing with microbialites. The most common are massive and encrusting cerioid/plocoid corals of Actinacis-type (Fig. 14e) and Goniopora-type assemblages. Phaceloid/dendroid corals are represented mainly by Oculina-type assemblage (Fig. 14f). The sedimentary matrix represents a volumetrically significant component of the boundstone microfacies. The contact between the sedimentary matrix and microbialites is mostly gradual. Occasionally, it is difficult to distinguish between sedimentary matrix and microbialites as the latter may agglutinate large bioclasts, e.g. LBF.
MFT 9 is densely packed, well to moderately sorted miscellaneid–foraminiferal–algal packstone (Fig. 15a, d) with miscellaneids, Ranikothalia solimani, lenticular orthophragminids (Fig. 15d), larger rotaliids, miliolids and textulariids. Other bioclasts are coralline branches and rare coralline crusts, rare peyssoneliacean crust fragments, oyster shells and echinoderms. Some bioclasts exhibit micrite envelopes.
MFT 10 is a moderately to poorly sorted rotaliid–algal packstone (Fig. 16a–c) consisting of larger rotaliids, non-geniculate and geniculate coralline algae, peyssoneliacean algae, solenoporacean algae Parachaetetes asvapati Rao and Pia (Fig. 16c), serpulids, miliolid foraminifera, echinoderms, cidaroid spine and rare fragments of plocoid/cerioid corals. Bioclasts commonly exhibit a micritic envelope.
MFT 11 is a moderately sorted rotaliid–algal grainstone (Fig. 16d) with micritized grains, larger rotaliids, geniculate and non-geniculate coralline algae. Circumgranular, scalenohedral calcite cement represents the first generation of calcite cement followed by pore-filling blocky calcite cement (Fig. 16d).
MFT 12 consists of a moderately sorted, Glomalveolina–bioclastic grainstone (Fig. 17a–c) with Glomalveolina, large and small porcelaneous taxa, rotaliids, micritized grains, aggregate grains and Distichoplax biserialis (Fig. 17b). A high proportion of the grains exhibit a thin micrite envelope. The grains are rimmed by first-generation circumgranular cement consisting of equidimensional calcite crystals. However, this cement is poorly preserved and overprinted by a later pore-filling drusy to blocky calcite spar. Echinoderms are overgrown by syntaxial calcite cement, which also engulfs other grains as poikilotopic cement.
MFT 13 is a moderately to poorly sorted Glomalveolina–bioclastic packstone (Fig. 17d–h), consisting of Glomalveolina, large and small porcelaneous taxa and rotaliids. The foraminifera are associated with bivalves and gastropods. Glomalveolina and other bioclasts may act as the nucleus for oncoidal coating (Fig. 17f).
Middle Paleocene to Middle Eocene, shallow-marine carbonate platform/ramp systems are characterized by distinct, mainly depth-controlled facies distribution as demonstrated by various investigations (Ghose 1977; Hottinger 1997; Ćosović et al. 2004; Zamagni et al. 2008; Afzal 2010; Bagherpour and Vaziri 2012). Important criteria for the characterization of different facies zones are texture, sorting, diagenetic features, and the environmental distribution of the fossils involved, especially different groups of larger benthic foraminifera. The main drivers of the environmental distribution are the depth of the photic zone, water temperature, nutrient supply, and substrate type (Hottinger 1983, 1997; Renema and Troelstra 2001). The variation of the foraminiferal test morphology is a vital depth criterion (Hottinger 1997). Lenticular LBF tend to reduce their axial diameter with depth, i.e. become thinner and more flattened/discoidal with the progressive depth of the photic zone (Beavington-Penney and Racey 2004; Eder et al. 2018). The reason for this adjustment lies in larger benthic foraminifera hosting algal symbionts, which are dependent on photosynthesis. In the lower photic zone, LBF tend to reduce the wall thickness and increase the relief of the ornamental sculptures to reduce the amount of total light reflection (Hottinger 2006). Flat discoidal foraminifera with thin transparent wall can place their symbionts beneath the test wall (Hohenegger 2009). According to Machaniec et al. (2011), orthophragminids show a particular dependence of the shell morphology on access to light and indirectly on depth, where flat, discoidal tests suggest low energy/low light/increased depth.
In this study, such a trend was observed mainly in the orthophragminids and, to a lesser extent, in Ranikothalia. With the progressive depth of the photic zone, the test morphology of the orthophragminids changes from robust lenticular forms to thin flattened discoidal forms. Robust lenticular forms occur primarily in the coarse-grained MFT 9 (Fig. 15b) and MFT 5 (Fig. 11a, b). The delicate, flattened forms (Fig. 12a, b) occur in more fine-grained textures, especially in the orthophragminid–algal floatstone (MFT 6) of the core 1 (Fig. 6) in well 1. Thick, lenticular orthophragminids indicate a shallower environment, while flattened discoidal ortophragminids point to a deeper environment with low energetic conditions (e.g. Hottinger 1997).
Another critical criterion are coralline algae. Paleogene coralline red algae occur together with corals in the meso-oligophotic zone (Pomar et al. 2017). Different coralline genera occur in different environments and water depths, respectively. According to Aguirre et al. (2007), the sporolithaceans dominate the deeper outer platform facies. Solenoporaceans and geniculate corallines are characteristic of the shallow inner shelf shoals. Zamagni et al. (2008) reported the occurrence of thin algal crusts together with flattened discoidal orthophragminids in a deeper mid ramp setting. Towards shallower water, the degree of fragmentation of coralline algae increases, as observed in our material. The orthophragminid–algal floatstone (MFT 6) is characterized by abundant thin algal crusts (Fig. 12c) accompanied by flattened discoidal orthophragminids. In the shallower miscellaneid–foraminiferal–algal packstone (MFT 9), the degree of fragmentation is much higher, and coralline algae occur exclusively as fragmented bioclasts (Fig. 15).
Cortoids are important paleoenvironmental indicators. Grainstones with a high percentage of cortoids and other coated grains, together with rounded bioclasts, characteristically form in areas of constant water action, at or above wave base (Flügel 2004). These conditions occur in the ‘winnowed platform edge sands’ of carbonate platforms as well as in current-washed sand shoals of inner ramp settings (Flügel 2004). Cortoids are especially abundant in MFT 12–13 in the inner ramp. Micrite envelopes developed around grains are also related to more protected seagrass-dominated environments (Perry 1999). Some cortoids are the product of destructive micritization by microendolithic organisms. Simultaneously, constructive micrite envelopes related to epilithic organisms have been observed, which eventually led to the formation of aggregate grains by microbial binding. Cortoids have been frequently observed also in MFT 10–11. Rare coated grains within the MFT 4 and MFT 9 of the mid ramp under mesophotic/oligophotic conditions are considered reworked from the inner ramp.
Based on the microfacies and micropaleontological data, a paleoenvironmental model with the distribution of foraminiferal assemblages and algae was reconstructed (Fig. 16). The paleoenvironmental model is mainly based and adapted from Zamagni et al. (2008), Bagherpour and Vaziri (2012), and Afzal (2010) and shows a carbonate ramp with inner ramp, mid ramp and outer ramp setting with different facies belts characterized by differentiated assemblages.
The outer ramp is the zone below the storm wave base (SWB). Typical lithologies are argillaceous mudstones and wackestones, frequently associated with marl or shale beds (Flügel 2004). Outer ramp deposits (MFT 1 and MFT 2) were encountered only in well 9 adjacent to the domal structure. The complete absence of shallow-water biota, e.g. LBF or coralline algae, points to deposition below the rhodocline (sensu Liebau 1984), which marks the oligophotic/dysphotic boundary (Pomar et al. 2017). Common planktonic foraminifera and small benthic foraminifera (Midway-type fauna sensu Berggren and Aubert 1975) are characteristic of deep-marine settings. The age relation of the outer ramp deposits with the domal structures is not established in terms of biostratigraphy (e.g. planktonic foraminifera). However, the occasional occurrence of packstones with fine-grained LBF debris (MFT 2) points to a reworking of shallow-water material from adjacent shallow-water environments. Therefore, the outer ramp deposits could be isochronous with the domal structures.
The mid ramp is the zone between the fair-weather wave base (FWWB) and the storm wave base (SMB). Fairweather phases are dominated by suspension fall-out, consisting largely of lime or terrigenous mud, and are commonly bioturbated. Associated grainstone or packstone sediments consist primarily of autochthonous bioclasts (Burchette and Wright 2002).
LBF (orthophragminids, Ranikothalia), coralline and peyssoneliacean algae, small rotaliids, bryozoans, and echinodermal debris are the most common organisms of the mid ramp depositional setting (Zamagni et al. 2008). These organisms were prolific carbonate producers in the mesophotic–oligophotic depths (Pomar et al. 2017). LBF, red algae and echinoderms are occurring in packstones/floatstones/rudstones. Apart from differences in their quantitative distribution, the studied successions exhibit different grain sizes reflecting different water-energy conditions, i.e. shallowing (shift to coarser grain size) or deepening (shift to more fine grain size). This simple trend becomes overprinted by resedimentation of coarse-grained material into deeper water (Fig. 11a, b).
MFT 3 is a fine-grained packstone deposited in a low-energetic deeper mid ramp. Rare thin coralline crusts (mainly Sporolithon sp.) suggest deposition under oligophotic conditions above rhodocline (Pomar et al. 2017). The presence of few planktonic foraminifera points to a deeper mid ramp.
Coarse-grained bioclasts consisting of coralline algae, LBF and echinoderms (MFT 4–5) are transported into the deeper parts of the ramp, intercalated between predominately fine-grained limestones. Occasionally, bioclastic material originating from the shallower inner ramp was redeposited into the mid ramp. Dasycladalean green alga, Neomeris aff. avellanensis, has been interpreted as allochthonous, i.e. being transported from the inner ramp. Glomalveolina found in the MFT5 is interpreted as allochthonous, i.e. being transported from the inner ramp (MFT 12–13).
LBF biodiversity in MFT 6 is limited chiefly to flattened orthophragminids. This group of flattened foraminifera generally occupies the deepest-water environment among the larger foraminifera, down to the lower limit of the photic zone (Zamagni et al. 2008; Beavington-Penney and Racey 2004). Abundant coralline and peyssoneliacean algal crusts often form bindstones, which give evidence of the lateral or ramp-upward occurrence of bionconstructions. Genus Sporolithon points to deeper conditions (Aguirre et al. 2007). The deeper environment is also documented by a few planktonic foraminifera (Fig. 12e) occurring in the very fine-grained textures.
MFT 7 is a floatstone associated with MFT 8 and MFT 9. Zamagni et al. (2009) described similar microfacies with bryozoans, algae and bioclastic debris as lateral skeletal deposits associated with microbialite–coral mounds. Baceta et al. (2005) documented comparable microfacies as inter-reef and fore-reef facies.
Microbialite–coral boundstone (MFT 8) was described by Zamagni et al. (2009) on a late Paleocene carbonate ramp from SW Slovenia as microbialite–coral mounds associated with lateral skeletal deposits and abruptly overlain by stratified foraminiferal–algal packstones. In Sirt Basin, Bebout and Pendexter (1975) described coral–algal micrite as “a loose framework of branching and laminated corals with a micrite matrix”. Bebout and Pendexter (1975) most likely recognized the micrite matrix as a depositional matrix with embedded corals and not as microbial in origin. Zamagni et al. (2009) pointed out that the micrite matrix is most likely of microbial origin. The microbialite–coral boundstone alternates with MFT7 and MFT 9 in a low-energetic mid ramp under mesophotic conditions. Most of the micrite was microbially trapped. According to Zamagni et al. (2009), early lithification of the microbial crusts provided a hard surface for encrusting corals to grow on it.
MFT 9 is dominated by miscellaneids, Ranikothalia, and larger rotaliids. Such foraminiferal assemblages were also described from other Tethyan Paleocene localities, e.g. Galala Mountains in Egypt (Scheibner et al. 2003), Taleh Zang Formation in the Zagros Mountains (Bagherpour and Vaziri 2012) and Indus Basin in Pakistan (Afzal 2010). According to Ghoose (1977), miscellaneids occur mainly in the near-reef zone of the back reef. Miscellaneids have been described from inner ramp together with porcelaneous taxa (Zamagni et al. 2008). They have also been recovered from deeper carbonate settings associated with coral–algal patch reefs under mesophotic conditions (Consorti and Köroğlu 2019). Rare Glomalveolina is considered allochthonous, i.e. displaced from proximal environments.
The inner ramp comprises the euphotic zone sensu Pomar (2001) between the upper shoreface and the fair-weather wave base (Flügel 2004). The inner ramp consists of bioclastic sand shoals which form sheet-like grainstones. Lagoonal sediments range from mud-, wacke- to packstone with restricted biota (Burchette and Wright 1992).
MFT 10 is a packstone with occasionally poorly washed texture and was deposited in a protected back-shoal environment. MFT 11 is a rotaliid algal grainstone with common rotaliids, geniculate coralline algae and solenoporacean algae, and points to deposition on an inner ramp shoal environment. Geniculate corallines and solenoporaceans have been reported from the lower euphotic depth (Pomar et al. 2017). Both algal types are common constituents of back-reef sands (Baceta et al. 2005). Coralline algal grainstone with geniculate corallines has been described from the high energetic inner platform (Aguirre et al. 2007; Scheibner et al. 2007) and lagoon behind an inner ramp shoal (Bagherpour and Vaziri 2012).
MFT 12 and MFT 13 represent the shallowest facies belt preserved on the top of the Intisar domal structure (Fig. 5). Both MFT’s mark a significant shift in the foraminiferal assemblages from predominately hyaline tests of the mesophotic–oligophotic mid ramp to predominately porcelaneous taxa (Fig. 17) of the euphotic inner ramp. Porcellaneous foraminifers inhabit shallow-water, protected environments, but they also occur reworked in sand shoals (Španiček et al. 2017; Benedetti 2018). MFT 12 (Fig. 17a–c) represents the highest energetic environment. Very shallow conditions are evident by abundant micritization of the bioclasts, oncoid coatings and aggregate grains (Fig. 17a). MFT 13 (Fig. 17d–h) was deposited in a protected low-energy lagoonal/back-shoal environment behind a grainstone shoal barrier. Mresah (1993) described MFT12 and MFT 13 as “alveolinid wackestone–packstone” in a narrow facies belt in the “barrier reef” area sensu Gumati (1992) (Fig. 2).