Biofacies analysis and depositional environments of Mid-Eocene Larger Benthic Foraminifera-rich deposits in Northern Tunisia PDF

Biofacies analysis and depositional environments of Mid-Eocene Larger Benthic Foraminifera-rich deposits in Northern Tunisia Sirine Chouat University of Carthage Mohamed Slim El Ayachi University of Carthage Kamel Boukhalfa University of Carthage Rabeh Alouani University of Carthage Mabrouk Boug...

Biofacies analysis and depositional environments of Mid-Eocene Larger Benthic Foraminifera-rich deposits in Northern Tunisia Sirine Chouat University of Carthage Mohamed Slim El Ayachi University of Carthage Kamel Boukhalfa University of Carthage Rabeh Alouani University of Carthage Mabrouk Boughdiri (  [email protected] ) University of Carthage Research Article Keywords: Mid-Eocene, Northern Tunisia, “Reineche Limestone”, carbonate ramp, Larger Benthic Foraminifera Posted Date: March 21st, 2023 DOI: https://doi.org/10.21203/rs.3.rs-2698146/v1 License:   This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License Additional Declarations: No competing interests reported.  1 Biofacies analysisand depositional environments of Mid-Eocene Larger 2 Benthic Foraminifera-rich deposits in Northern Tunisia 3 4 Sirine Chouat1, Mohamed Slim El Ayachi1, Kamel Boukhalfa1, 2, Rabah Alouani1, 5 Mabrouk Boughdiri1,* 6 7 1 8 University of Carthage, Faculty of Sciences of Bizerte; 7021 Zarzouna, Tunisia 2 9 LR18ES07; University of Tunis El Manar, Faculty of Sciences of Tunis; 2092 Tunis, Tunisia 10 11 12 * Corresponding author: [email protected] (M. Boughdiri) 13 14 15 16 17 18 1 19 Abstract 20 In NW Tunisia, the lateral equivalents of the Mid-Eocene “Reineche Limestone” member of the 21 Souar Formation (Bartonian) are still poorly known. Sedimentological investigations in the 22 “Reineche Limestone” type locality in NE Tunisia; and three correlatives, first described here 23 from NW Tunisia, allow the reconstruction of their depositional settings and to propose a 24 regional geodynamic context. Fossil assemblages, rock texture and fabrics plead in favor of eight 25 micro-biofacies (Mf1 to Mf8) indicating “shoal” inner ramp to outer ramp depositional settings. 26 The Bartonian carbonates of NE Tunisia bear LBF-dominating assemblages and subsidiary 27 planktic and small benthic foraminifera, gastropods, algae and echinids indicating a progressive 28 marine ramp context under oligotrophic conditions. However, correlative successions from NW 29 Tunisia are represented by relatively thinner carbonate intervals including two main facies: 1) 30 bioclastic limestone facies and 2) phosphorite-rich carbonates. The first is dominated by LBF 31 assemblages suggesting the same depositional conditions as the “Reineche limestone”. However, 32 phosphorite-rich carbonate facies is mainly characterized by peloids, bone fragments and 33 lithoclast components with subsidiary nummulitids, planktic and small benthic foraminifers, 34 echinids, oyster shells, brachiopod fragments and fine siliciclastic components, all suggesting 35 oxic-suboxic to anoxic conditions for phosphorite genesis. This would have implied microbial 36 decomposition of organic compounds and/or bacterial reduction of sulfate in shallow marine 37 environment. These phosphorite-rich carbonate of northwestern Tunisia may serve as an example 38 of phosphatic sediment production and accumulation during the latest episode of the Paleogene 39 phosphatogenesis first described here from the south-Tethyan margin of Tunisia. 40 41 Keywords: Mid-Eocene, Northern Tunisia, “Reineche Limestone”, carbonate ramp, Larger 42 Benthic Foraminifera. 2 43 Introduction 44 45 During the Middle Eocene (Bartonian), wide shallow-marine carbonate platforms have been 46 developed under the main control of atransgressive eventwell recorded in the circum-Tethyan 47 domain (Fig.1A, B) (Martin-Martin et al. 2021,and referencestherein). In these broad carbonate 48 shallow-marine environments, larger-benthic-foraminifera-(LBF)-rim facies evolved. 49 50 51 Fig. 1 (near here) 52 53 This biophase was dominated by nummulitids, alveolinids and orthophragminids that required 54 favorable ecologicalconditions including euphotic and oligotrophic marine habitats and tropical 55 to subtropical water temperature(Hottinger 1983; Hallock 1985, 2000). This transitional period 3 56 with high abundance of K-strategist LBF taxa is dated as Lower Bartonian on the basis of the 57 First Occurrence(FO) of the genus Heterostegina (Less et al. 2008; Less and Özcan 2012). It 58 corresponds to the onset of a new global community maturation cycle (Hottinger 2001) and 59 coincides with the transient warming during the Middle Eocene Climatic Optimum (MECO; 60 Bohaty and Zachos 2003; Bijl et al. 2010). In this context, the Bartonian time interval in Tunisia 61 is characterized by the development of a shallow marine carbonate platform (Fig.1C) with a 62 remarkable variability of carbonate facies locally bearing diversified LBF assemblages. Three 63 correlative lithostratigraphic units in the lower Souar Formation(Fm) are known: the “Reineche 64 Limestone” (Burollet 1956), the “Siouf” and the “Dolomite” members (Comte and Dufaure 65 1973) were described along a NE-SW trend from theTunisian Atlas as they correspond 66 toregional marker beds (Burollet 1956; Comte and Dufaure 1973; Bismuth andBonnefous1981; 67 Fournié 1978; Ben Ismail-Lattrache and Bobier 1984; Fakhfakh Ben Jemai 2001; Amami-Hamdi 68 et al. 2013, 2014, 2016; Haj Messaoud et al. 2021). The “Reineche Limestone” and “Siouf” 69 members consist of bioclastic limestone horizons corresponding to a thick nummulite-rich 70 carbonate accumulation considered as good hydrocarbon reservoirs throughout the offshoreof 71 Tunisia and Libya (Klett 2001; Taktak et al. 2010; Njahi-Derbali et al. 2017).In contrast, 72 previous thematic studies in northwestern Tunisia (Gottis and Sainfeld 1956; Kujawski 1969; 73 Rouvier 1977; Erraoui 1994; Alouani et al. 1996; Boukhalfa et al. 2009; Tlig et al. 2010) did not 74 focus on a detailed biostratigraphic frame and no detailed biostratigraphic data are reported from 75 coeval deposits of the “Reineche Limestone” marker carbonate levels. Their depositional setting 76 and paleogeographic context is still the object of controversies (e. g. Bonnefous and Bismuth 77 1982; Ben Ismail-Lattrache 2000; Fakhfakh Ben Jemai 2001). 78 This work aims at attempting to seal debates on the matter on the basis of micro- and macro- 79 facies analyses, regional correlations and paleoenvironmental interpretations of middle Eocene 80 deposits from northwestern Tunisia. Beyond regional investigations, our study attempts replacing 4  81 the Middle Eocene carbonate platform of northern Tunisia in its southern Tethys context, and 82 discussing major controlling factors of the analyzed carbonate deposits. 83 84 1. Material and methods 85 86 1.1. Location andGeological setting of the study sections 87 88 The study successions are located in northern Tunisia. Four Bartonian-aged sections are sampled 89 (Fig. 1C).The Damous Quarry section (D section; 20m) was investigated in the Cap Bon 90 peninsula of NE Tunisia,at thewestern flank of the Jebel Abderrahman anticline, ca50 m to the 91 west of the road joining Menzel Bouzelfa to Oum Dhouil localities.The Sidi N’sir section (SN; 92 1.20m) is located to the SE part of the structural thrust zoneof NW Tunisia, ca500 m to the East 93 of the road deserving Mateur to Bejalocalities.The Djebba section (DJ; 4m) is located in the salt 94 Dome structural zone, at northwestern flank of the Goraa plateau near Djebba village which is 95 situated about 5.5 km to the South of Thibar and 13km to the west of Teboursouk.The Oued 96 Hassene section (OH; 0.60m) was sampled in the Beja area, ca10 km to the North-west of Beja 97 town, and 3 km to the West ofAmdoun locality. 98 Our study is focused on the limestone packages intercalated within the Souar (sections D, SN 99 and Cherahil (sectionsDJ, OH) Formations. 100 101 1.2. Reference stratigraphic scale 102 103 The stratigraphic reference chart of the Eocene corresponds to that published in the International 104 Subcommission of Paleogene Stratigraphy homepage (ISPS; www.paleogene.stratigraphy.org) 105 where the Eocene series (-56 to-33.9 My) comprises the Ypresian, Lutetian, Bartonian and 5 106 Priabionian stages. For the Bartonian, its base is defined by the abundance of the larger benthic 107 foraminifer species Nummulites prestwichianus; its top approximates the base of the Priabonian 108 marked by the successive extinction of large acarininids and the species Morozovelloides 109 crassatus. 110 In Tunisia, the lithostratigraphical chart of the Eocene series (Fig. 1D) shows that marly to 111 limestone sequences of Middle to upper Eocene age in central and northern Tunisia are included 112 in the Souar or Cherahil Formations, both considered as lateral equivalents of the Jebs evaporitic 113 Formation (Bishop 1988) towards the South. 114 Since pioneer works on the middle-late Eocene in Tunisia, the proposed biostratigraphic schemes 115 based on benthic and planktic foraminifers and ostracods has been the subject of controversies 116 and continuous improvements (Burollet 1956; Bismuth and Bonnefous 1981; Bonnefous and 117 Bismuth 1982). The nineties of the last century are characterized by notable advances in 118 approaching a relatively stable age assignments to the reference Souar and Cherahil Fms (Ben 119 Ismail-Lattrache 2000; Amami-Hamdi et al. 2013, 2014, 2016 and references therein). These 120 Formations are composed of shallow to deep marine clays with thin limestone intercalations, 121 assigned to the Lutetian-Priabonian interval (Morozovella lehneri biozone (P12) - Turborotalia 122 cerroazulensis (P16/P17) biozones, Ben Ismail-Lattrache 2000). They intercalate carbonate 123 marker levels referred to as the “Reineche” and “Siouf” members within the lower third of the 124 Souar and Cherahil formations, respectively. 125 Based on nannofossil biozonation and 13Corg chemostratigraphy, recent improvement of the 126 previous charts by Haj Messaoud et al (2021, 2023) provide a high resolution correlation table 127 used here. They confirm that the most part of “Reineche” and “Siouf” members in carbonate 128 platforms lie within the Lower Bartonian (lower CNE 15 Zone of calacarous nannofossils, 129 correlated with the E12 Zone of Planktic Foraminifers and the SBZ 17 of Small Benthic 130 Foraminifers). A partly equivalent interval in the reference section of the Souar Fm (Zaghouan 6 131 area) includes a radiolarian-rich biosiliceous interval encompassing the Lutetian-Bartonian 132 transition. 133 134 1.3. Field prospecting and laboratory analyses 135 136 Field geological investigations include bed-by bed sampling with thorough observations of 137 lithological and sedimentological features. Analytical tasks in laboratory aimed at defining the 138 petrographic texture of bioclastic limestone beds and their micropaleontological content inLBF 139 assemblages. A total of 33samples were collected for thin section observations under a“Nikon 140 Eclipse E 200”optical microscope for the identification of main skeletal and non-skeletal 141 components, carbonate grains, matrix, and mineral compounds. Photographs were taken by 142 means of a digital camera (Leica M80), transferred to the computer using a Nikon’s Digital Sight 143 DS-U3 microscope camera controller and treated with the imaging software Nikon NIS Elements 144 F4. The microfacies analysis and lithology description followed the methodology of Flügel 145 (2010) and the terminology of Dunham (1962). In order to differentiate microfacies assemblages, 146 all the allochem components and matrix were characterized and visually estimated in thin 147 sections. A particular attention has been paid to the Bartonian phosphorite-rich lithofacies which 148 is first described here. 149 150 151 152 153 154 155 7 156 2. Results 157 158 2.1. Identified bio-microfacies 159 On the basis of the content in skeletal elements and foraminiferal assemblages, eight main bio- 160 microfacies have been recognized (Tab.1). 161 162 163 Tabl. 1 (near here) 164 - Foraminiferal/red algal biofacies (MF1): occurs only at the middle part of the Damous section. 165 The skeletal assemblage is dominated by alveolinids, associated with micritized nummulites, 166 small benthic foraminifera, miliolids, amphisteginids and, orthophragminids. 167 168 - Ostracod biofacies (MF2): is observed in both Sidi N’sir and Oued Hassene sections. It is 169 dominated by ostracods with subordinate rare Nummulitids, and brachiopod shells. 8 170 171 Fig. 2 (near here) 172 - Nummulitid biofacies (MF3): identified in all studied sections. The foraminiferal assemblage is 173 dominated by larger and flat nummulites. It is associated with common orthophragminids, 9 174 alveolinids, amphistegina and small benthic foraminifera at the Damous section, while 175 brachiopods, ostracods, echinoderms, and small benthic foraminifera are usually common in 176 Oued Hassene and Djebba sections. 177 178 - Nummulitid and orthophragminid biofacies (MF4): described in Damous, Sidi N’sir and 179 Djebba successions. It is predominated by larger and flat nummulites and orthophragmines, 180 associated with rare small benthic and planktic foraminifera, brachiopods and echinoderms. 181 - Operculina biofacies (MF5): recorded only in Sidi N’sir section. The skeletal assemblage is 182 largely dominated by Operculina and large flat nummulites, associated with common 183 orthophragminids, rare planktic foraminifera, echinoderms, and ostracods. 184 185 - Orthophragminid biofacies (MF6): observed at the base of both Damous and Sidi N’sir 186 sections. It is dominated by orthophragminids associated with common nummulites, 187 Amphistegina, planktic foraminifera and rare echinoderms, bryozoans, brachiopods, and small 188 benthic foraminifera. 189 190 - Globigerinid biofacies (MF7): characterizes the upper part of Damous section. The skeletal 191 assemblage is largely dominated by globigerinids. Small benthic foraminifers are usually 192 common and green-glauconite grains can be relevant. 193 194 - Phosphorite microfacies (MF8): characteristic of Sidi N’sir, Oued Hassene and Djebba 195 successions. It is mainly composed of peloids, bone fragments, and lithoclasts associated with 10 196 197 Fig. 3 (near here) 198 199 common small benthic foraminifera, nummulitids, ostracods, brachiopods, and echinoderms 200 fragments and rare globigerinids. 201 202 203 11 204 2.2. Facies distribution in the study sections 205 206 2.2.1. The Damous section 207 208 The section is made of 20-m-thick limestone beds of the “Reineche” member intercalated 209 between the clayey “Souar A” member, to the base, and the silty-clay interval of the “Souar B” 210 member, to the top (Fig. 3). Microfacies, characteristic sedimentological features and 211 macrofossil content allowed subdividing the Damous section into five intervals (Figs. 2, 3). 212 213 -Interval M1 (4m; orthophragminid biofacies, Mf6): composed of marls and thin-beddedmuddy 214 limestone intercalations. The limestone bedsshow packestonetexture dominated by 215 orthophragminids (Fig. 3A-B) associated with nummulitids, planktic and small benthic 216 foraminifera, echinoids, bryozoans and amphistegina. Rare coral fragments, miliolids and red 217 algae are also present. 218 219 - Interval M2 (4.5m; orthophragminid biofacies, Mf6): represented by fossiliferous chalky 220 limestoneshowing an irregular base (Fig. 2). It consists of wackestone dominated by 221 orthophragmines and globigerinids (Fig. 3C-D) associated with nummulitids, alveolinids,red 222 algae and crinoids. 223 224 - Interval M3 (4m; nummulitid biofacies, Mf3): starts witha 3m-thick limestonepackage of 225 packestone texture yielding small nummulitids and alveolinids (Fig. 3E-F) associated with 226 common orthophragminids as well as planktic and small benthic foraminifera and red algae. This 227 limestone level is overlain by a 1 m-thick limestone level with a grainstone texture (Mf1) 12 228 displaying abundant small benthic foraminifera, red algae, nummulitids, common alveolinids 229 (Fig. 3G), miliolids (Fig. 3H) and amphisteginids (Fig. 3I). 230 231 - Interval M4 (2 m; nummulitid and orthophragminid biofacies, Mf4): composed of a thick 232 limestone level with a thin intercalated clayey layer. The limestone package is of a packestone 233 texture showing nummulitid and orthophragminid-dominating biofacies (Fig. 3J-K). Planktic 234 and small benthic foraminifera, amphisteginids, miliolids, and alveolinids are also present. 235 236 - Interval M5 (5.5 m; globigerinids biofacies, Mf7): represents the uppermost part of the 237 sectionand consists of limestone/shale couplets. The unit is characterized by abundant planktic 238 foraminifera (Fig. 3L) and overlain by the globigerina-rich shales of the Souar “B” member. 239 240 2.2.2. The Sidi N’sir section 241 242 The Sidi N’sir section is represented by a 1.1 m-thick limestone package of the “Reineche” 243 member underlain by the clay dominated succession of “Souar A” and covered by silty-clays of 244 “Souar B” (Figs. 5, 6A-B). As for the Damous section, the Sidi N’sir succession can be 245 subdivided into four intervals (Figs. 5, 6, 7). 246 247 -Interval M1 (0.4m; nummulitid and orthophragminid biofacies; Mf4) consists of a 0.4 m-thick 248 limestone bed of a packestone texture dominated by large flat nummulites and orthophragminids. 249 It is underlined by an irregular surface showing poorly sorted polygenic conglomerates (black 250 and light gray lithoclasts). Thin section analyses indicate that the skeletal assemblage is 251 dominated by nummulitids and orthophragminids (Fig. 4A) associated with rare planktic 13 252 253 Fig. 4 (near here) 254 255 foraminifers, echinoids, and brachiopod fragments. Lithoclasts represent a very significant 256 fraction and glauconite grains are present. 14 257 258 Fig. 5 (near here) 259 260 -Interval M2 (0.4m; Operculina biofacies, Mf5): composed of a 0.4 m-thick limestone bed of a 261 packestone texture dominated by Operculina and large flat nummulites (Fig. 4B) associated with 15 262 rare orthophragminids, planktic foraminifera and ostracods. Lithoclasts are common and green 263 glauconite grains also occur. 264 265 -Interval M3 (0.15m; nummulitid facies, Mf3): composed of a 0.15 m-thick bed composed of big 266 sized nummulites up to several centimeters in diameter (Fig. 4C). 267 268 -Interval M4 (0.15 m; ostracod biofacies, Mf1): composed of a 0.15 m-thick limestonebed 269 topped by rounded calcareous lithoclasts. The skeletal assemblage includes ostracods and rare 270 small nummulites (Fig. 4D). This interval comprises a very thin layer (0.01m) showing 271 phosphorite-rich microfacies (Mf8). Thin section analyses of this phosphorite horizon show 272 peloids, bone and shell fragments and small nummulites (Fig. 4E-F). 273 274 2.2.3. The Oued Hassene section 275 276 This section consists of a 0.60 m-thick fossiliferous limestone bed overlain by “Cherahil B” 277 marl/limestone alternations (Figs. 8, 9). It represents a lateral equivalent unit of the “Reinèche 278 Limestone” of the Souar Formation (Figs. 8, 9, 10).These deposits of the“Reineche Limestone” 279 lateral equivalent can be subdivided into three intervals (Figs. 5A, 6). 280 281 -Interval M1 (0.2m; globigerinidbiofacies, Mf7): composed of 0.2 m thick, muddy limestoneof a 282 mudstone texture with rare Globigerinids (Fig. 6A-B) associated with brachiopod shells (Fig. 283 6C). 16 284 285 Fig. 6 17 286 -Interval M2 (0.3m; phosphorite-rich microfacies, Mf8): constituted of a 0.3 m-thick, 287 phospharenite (Fig. 6D) with peloids (Fig. 6E), bone and shell fragments (Fig. 6F) associated 288 with planktic and benthic foraminifera,mollusk fragments and rare small nummulites (Fig. 6D). 289 290 -Interval M3 (0.10 m; nummulitid facies, Mf3): composed of a 0.10 m-thick fossiliferous 291 limestone bed yielding abundant big sized nummulites up to several centimeters in diameter. The 292 skeletal assemblage includes nummulites (Fig. 6G) and rare ostracods and molluscan shell 293 fragments (Fig. 6H). Peloids, lithoclasts and glauconite grains are also present (Fig. 6G-H). 294 295 4.2.3. The Djebba section 296 297 The“Reineche Limestone” lateral equivalent of the Djebba section is represented by a 4-m-thick 298 succession made of thin fossiliferous limestones, clays, and phosphorite-rich beds. This 299 succession is intercalated between the clay/limestone couplets of “Cherahil A”, to the base, and 300 the Clays and silty-clays interval of “Cherahil B”. This section can be subdivided into three main 301 intervals. 302 303 -Interval M1 (2m; nummulitid and orthophragminid biofacies, Mf4): composed of clayswiththin- 304 bedded limestone intercalations. The limestone beds are of apackestone texture dominated by 305 nummulitids and orthophragminids (Fig. 7A), associated with nummulithoclast, red algae (Fig. 306 7B) and planktic and small benthic foraminifera (Fig. 7A-C). The terrigenous fraction consists of 307 mainly fine subangular quartz grains (Fig. 7A, B). 308 309 -Interval M2 (1.5 m; phosphorite-rich microfacies, Mf8): constituted of a 1.5 m-thick, 310 phospharenite with peloids, bone and shell fragments (Fig. 7D-E). The skeletal assemblage 18 311 312 Fig. 7 (near here) 313 314 consists of small benthic foraminifera, echinoids, brachiopod fragments (Fig. 7D) and molluscan 315 shells (Fig. 7E). 316 19 317 -Interval M3 (0.5 m; nummulitid facies, Mf3): consists of a 0.5 m-thick fossiliferous limestone 318 bed yielding very abundant large flat nummulites (Fig. 7F). The skeletal assemblages include 319 nummulites associated with rare ostracods, small benthic foraminifera, echinoids and 320 brachiopods and molluscan shell fragments. Peloids, lithoclasts and quartz grains are also present 321 (Fig. 7F). 322 323 3. Facies interpretation and paleoenvironmental reconstruction 324 325 A generalized Cenozoic carbonate ramp model,main depositional environments and associated 326 faunas were outlined by Buxton and Pedley (1989).Based on modern assemblage analyses, this 327 basic model previously proposed in the ramp profile of Read (1982),has been further refined by 328 several authors (Van der Zwaan et al. 1990; Hohenegger 1994, 2000, 2004; Hohenegger et al. 329 1999, 2000; Geel 2000; Racey 2001; Pomar 2001; Renema and Troelstra 2001; Beavington- 330 Penney and Racey 2004; Renema 2006, 2018; Mateu-Vicens et al. 2009; Pomar et al. 2017; 331 Boudaugher-Fadel 2018). From outer to inner ramp settings, a diverse array of faunas responds 332 to environmental constraints.Flat and thin large rotaliids (Operculina, Heterostegina, 333 Orthophragminids) dominate lower photic-zone assemblages and are associated with common 334 planktonic foraminifera. Thick and robust nummulitids and amphisteginids (e.g. Nummulites, 335 Amphistegina) thrive in middle-shelf environments, and occupy niches close to the inner shelf. 336 Very robust and large rotaliids (several species of Amphistegina, Miogypsina) dominate reef 337 settings, whereas in shallower waters, miliolids (Alveolinids, Soritids) are more abundant and 338 can dominate in restricted environments (Martin-Martin et al. 2021; Coletti et al. 2021). 339 340 Within this framework, our data on foraminiferal assemblages serve to constrain the depositional 341 environments of identified biofacies. Through the Damous and Sidi N’sir sections, the various 20 342 biofacies characterize a ramp profile with a gradual transition from a depositional environment to 343 another (globigerinid, operculina, orthophragminids, Nummulitid and alveolinid biofacies). In 344 the Djebba and Oued Hassene sections,occurrences of phosphorite-rich facies associated with 345 terrigenous input and a decrease inLBF assemblages, indicate depositionalconditionchanges 346 between these two studied sectors. 347 348 In this study, seven biofacies and one phosphorite-rich microfacies are described from the 349 different analyzed sections (Tab. 1).For each section, the paleoenvironmental interpretation will 350 be based on the main bio- and microfacies characteristics from proximal to distal settings as 351 follows (Fig. 8): 352 353 3.1.1. Inner ramp 354 355 -Mf1: Foraminiferal/red algae grainstone microfacies characterize thin-bedded limestones in the 356 middle part of the Damous section which shows abundant LBF and calcareous red algae, and a 357 sparitic cement. LBF assemblages,mainly including nummulitids, orthophragminids and 358 alveolinids, calcareous red algae, echinoderm fragments and miliolids also occur. Grainstone 359 texture and fossil content indicates a high-energy shoal environment (Flügel, 2010) (Fig. 8). 360 - Mf2: (ostracod biofacies) wackestone muddy-limestone. This microfacies is dominated by 361 wackestone with scattered ostracods and subordinate rare nummulitids, and brachiopod shells set 362 in micritic matrix. Ostracods typically occur as major components in stressed brackish, 363 hypersaline, or freshwater environments (Flügel 2010). Ostracods, small nummulites, and 364 brachiopod fragments are consistentwith euphotic subtidal seagrass environments of the inner 365 ramp (Fig. 8). 366 21 367 368 Fig. 8 (near here) 369 3.1.2. Middle ramp 370 371 - Mf3: Nummulite-rich packestone microfacies is recorded in all studied sections. This 372 microfacies is characterized by packestone texture dominated by larger and flat nummulites 373 associated with common orthophragminids, alveolinids, Amphistegina and small benthic 374 foraminifera at the Damous section. However, brachiopods, ostracods, echinoderms, and small 375 benthic foraminifera are usually common in the Oued Hassene and Djebba sections. Larger and 376 flat nummulites characterize the mesophotic zone (up to 40-m-deepwater mass) with a 377 moderately low-energy. 378 379 - Mf4: Nummulitid and orthophragminid packestone microfacies is described in the Damous, 380 Sidi N’sir and Djebba successions. It is dominated by larger flat nummulites and 381 orthophragmines, associated with rare small benthic and planktic foraminifera, brachiopods, and 22 382 echinoderms. This microfacies indicates a mesophotic zone, within a slightly deeper marine 383 setting than Mf3. 384 385 - Mf5: Operculina packestone microfacies is observed only in the Sidi N’sir section. This 386 microfacies is largely dominated by Operculina and largeflat nummulite specimens, associated 387 with common orthophragminids, rare planktic foraminifera, echinoderms, and 388 ostracods.Operculina associated with large flat nummulite indicate the relatively deep part of the 389 mesophotic zone with a moderately low-energy. 390 391 - Mf6: Orthophragminid packestone microfacies is characteristic of the lower part of both 392 Damous and Sidi N’sir sections. It is dominated by orthophragminids associated with common 393 nummulites, Amphistegina, planktic foraminifera and rare echinoderms, bryozoans, brachiopods, 394 and small benthic foraminifera. The orthophragminid fauna characterize the deepest part of the 395 mesophotic zone, just above the storm wave base. 396 397 3.1.3. Outer ramp 398 399 Mf7: Globigerinid mudstone-wackestone microfacies is observed in both the upper part of 400 Damous section and the lower part of the Oued Hassene section. This microfacies is dominated 401 by a mud-wackestone micritic matrix with scattered globigerinids and small benthic 402 foraminifers. Green-glauconite grains also occur. This facies indicates an oligophotic zone with 403 relatively low-energy, below the storm wave base within the outer ramp setting (Fig. 8). 404 405 -Mf8: Phospharenite microfacies is observed in the study northwestern sector including the Sidi 406 N’sir, Oued Hassene and Djebba successions. It is mainly composed of peloids, bone fragments, 23 407 and lithoclast grains associated with common small benthic foraminifera, nummulitids, 408 ostracods, brachiopods, and echinoderm fragments and rare globigerinids. In both the Oued 409 Hassene and Djebba sections, large flat nummulites are characterized by a partial 410 phosphatization of their tests. The characteristic faunal assemblage indicates that this 411 phospharenite microfacies onset within the mesophotic zone of the middle ramp setting. 412 413 4. Discussions 414 415 4.1. Regional geodynamic approach 416 417 The above-mentioned descriptions allow us to correlate the study sections with equivalents in 418 central Tunisia aiming at their replacement in a local geodynamic context and the elucidation of 419 major factors that would have controlled deposition. The W-E correlations of our study sections 420 exhibit an abrupt thickness variation towards higher values in the North East (Damous section) 421 with an intermediate NE-SW elongated band corresponding to thinner coeval deposits of the 422 Cherahil Fm (sections SN and OH). Works by Amami-Hamdi et al. (2016) and Ben Ismail- 423 Latrache (2000) identified a 5-m thick LBF- rich limestone level of the Siouf member in the J. 424 Jebil section. In the Siliana area (J. Bargou and J. Serj sections), this carbonate marker beds are 425 only 0.3-0.4m in thickness. These thickness shifts are consistent with the general paleogeography 426 based on the facies distribution (Fig. 1). 427 428 This particular regional paleogeography, exhibiting an elongated “finger-shaped” distribution of 429 facies, follows the general NE-SW trends of the known major faults of Tunis-Ellès and El Alia- 430 Teboursouk. In fact, these faults acted as transcurrent accidents during Jurassic and Cretaceous 431 continuous opening of the western-Tethys margin of Tunisia and constituted the SE and NW 24 432 borders of the Tunisian Trough. After the filling of the initial basin by thick Upper Cretaceous to 433 Paleocene sediments, the inherited basin architecture still shows the imprints of this fault- 434 bordered structure. The Africa-Eurasia plate convergence was active since late Cretaceous as 435 recorded by obvious deformations observed in Upper Cretaceous and Paleogene-Lower 436 Paleogene deposits (Guiraud 1998). The early Eocene compressive event led to the initiation of 437 an inversion in the movement of these bordering major NE-SW accidents. Hence, the structuring 438 of the Tunisian Trough consisted of a mosaic of uplifted blocs and small intercalary depressions 439 surrounded by the deeper “Dorsale” (NE Tunisia) and Tellian basins (NW Tunisia). The 440 continuous exhumation of Triassic domes would have participated as a relevant controlling 441 factor of this architecture. To the South-West, in the north-central Tunisia basin, the facies 442 distribution consists of rather NW-SE trending bands, sub-parallel to the direction of ancient 443 major faults that had structured southern Tunisia basins since the Paleozoic. In this area, During 444 the Middle Eocene times, the consequent NE extension is still active, but rather limited; the basin 445 polarity remaining constant. This can be explained by the play reversal of these ancient faults 446 with a dextral strike-slip component during the initial stages of the Eocene compression. Further 447 to the Eastern Tunisia Pelagian block, signatures of deep subsidence can be followed along a N-S 448 adjacent band, parallel to the NS major fault that underlines the N-S Axis of central Tunisia. 449 Hence, we interpret the onset of the limestone “Reineche” and “Siouf” members as the product 450 of a major sea level high that interplayed with regional tectonics implying mainly N-S, NE-SW 451 and NW-SE ancient accidents.This regional geodynamic context is to consider in the wider 452 frame of the initial echoes of the Atlasic compressive Eocene event that affected all the South- 453 West Tethys margin of the Maghreb. In this same line, the Paleogene is considered as a time 454 interval when major tectonic activities has taken place throughout the Tethyan domain due to 455 the closing of the Neo-Tethys Ocean as the Afro-Arabian and Eurasian plates converged (Cohen 456 et al. 1980; Ben Ayed 1986; Turki et al. 1988; Bedir et al. 1992; Guerrera et al. 2019). In the 25 457 Tunisia Tellian domain, the Middle to Late Eocene “Atlas event” (Frizon de Lamotte et al. 2000, 458 2009; El Ghali et al. 2003; Khomsi et al. 2009, 2016; Leprêtre et al. 2018) is further relayed by 459 the overthrust event of the Numidian Flysch sequences (Oligocene to Late Burdigalian) over 460 Early Miocene (Burdigalian to Langhian) foredeep sedimentary successions (Khomsi et al. 2009; 461 2016; Boukhalfa et al. 2009, 2020; Melki et al. 2011; Riahi 2015). 462 463 4.2. Ecological considerations and main controlling factors 464 465 Through the studied sections, the microfossil assemblages include a mixture of mainly euphotic 466 LBF and coralline/red algae elements, and heterotrophic components (small benthic and planktic 467 foraminifers, ostracods, echinids, mollusks, etc). These dominating-heterotrophic components 468 suggest oligo- to mesotrophic marine warm waters at low latitudes. Local mesotrophic ecological 469 niches where the organic matter would have been recycled by LBF burrowing activities can be 470 evoked. Furthermore, the increase of nutrient supply by continent currents from the SW adjacent 471 areas of the Kasserine island (and related emerged lands) towards the NE marine environments is 472 also consistent with the described mixture of biofacies elements. In fact, affine associations are 473 also known from other Mid-Eocene section in the Tethyan domain (Betic ranges, Geel 2000; 474 Jabaloy Sanchez et al. 2019; Moroccan Rif, Maaté et al. 2000; Pyrenean foreland basin of SE 475 France, Serra Kiel et al. 2003a,b; Anatolian domain, Ozcan et al. 2010; Egypt, Tawfik et al. 476 2016, among others). All these deposits can be included in the “forealgal facies” type of Wilson 477 and Vecsei (2005), mainly characterized by photophile LBF and corallian/red algae. In our study 478 sections, as in coeval levels from the above-cited sectors, LBF associations were identified at 479 different depths, from euphotic to oligophotic conditions, in line with a progressive marine ramp 480 where mainly oligotrophic conditions reign. 481 26 482 4.3. Mid-Eocene phosphorite occurrences 483 484 The main Tethyan phosphorites onset during the Cretaceous-Eocene time interval that coincides 485 with Neo-Tethys closure event as a result of the Afro-Arabian and Eurasian plate convergence 486 (Frizon de Lamotte 2009; Khomsi et al. 2009, 2016; Leprêtre et al. 2018; Guerrera et al. 2019). 487 The appropriate tectonic activities are at the origin of the basin structuring for the major 488 phosphatic settings throughout the Tethys realm (Sassi 1974; Zaier 1984; Chaabani 1995; 489 Baioumy and Farouk 2022 and references therein). In Tunisia, phosphorites are reported in three 490 sectorswhere the Upper Paleocene-Lower Eocene Chouabine Fm is well developed. These 491 sectors are referred to as:The Eastern Basins, the Northern Basins, and the Gafsa-Metlaoui Basin 492 (Sassi 1974; Beji-Sassi 1985; Zaier 1984; Kocsis et al. 2013, 2014; Ounis et al. 2008; Garnit et 493 al. 2012, 2017). The similarity between various Tethyan phosphorites (in terms of mineralogy 494 (francolite) and constituents (peloids, fish bones, shark teeth and fossils)), suggests that these 495 phosphorite genesis took place in comparable conditions with affine causal mechanisms (Sassi 496 1974; Beji-Sassi 1985; Zaier 1984, 1999; Ben Hassen 2007; Ounis et al. 2008; Kocsis et al. 497 2013, 2014; Garnit et al. 2012, 2017): the pristine phosphatic mud had deposited in a relatively 498 deep marine environment, under the influence of strong upwelling and consequent high 499 productivity. These primary phosphorites have been further reworked landwards by wave actions 500 during marine transgressive phases and then accumulated in shelf environments (Zaier 1984). 501 The Mid-Eocene phosphorites of NW Tunisia, show various types of grains with arenite- 502 dominating grain-size (less than 2mm). The main fine-grained components are composed of 503 peloids, coprolites, bone fragments, fossils (large and small benthic foraminifers, echinoids, 504 mollusks). The origin of the phosphorus of these deposits can be attributed to the microbial 505 breakdown of organic compounds under conditions of marine upwellings that brought nutrient- 506 rich deep ocean water in the Tethys basin, with a subsequent liberation of organic phosphatic 27 507 compounds into pore waters (Zaier 1984): the abundant fossil content and phosphatized bioclasts 508 being consistent with this interpretation. The distribution of the carbonate factories throughout 509 northern Tunisia domains is mainly controlled by the sea-water temperature (Middle Eocene 510 Climatic Optimum, Zachos et al. 2008), nutrient availability (Upwelling), and varying 511 terrigenous supply from neighboring Paleo-highs. Two major carbonate factories, both yielding 512 larger benthic foraminifera, can be distinguished in the study area. In both Damous (Cap bon 513 peninsula) and Sidi N’sir (Imbrication structural zone) sections, the carbonate factory, 514 consistently dominated by LBF, associated with echinoderms, mollusk, small benthic 515 foraminifera, miliolids and algae, is developed under suitable conditions in the mesophotic zone. 516 In turn, this might suggest a situation with limited relief around the basin during the development 517 of the “Reineche Limestone” member. Towards northwestern Tunisia, in both Djebba (Domes 518 zone) and O. Hassene (Tellian domain) sections, the carbonate factory is characterized by 519 restricted water circulation with anoxic conditions. This is consistent with the decrease in LBF 520 skeletal assemblages (mostly nummulitid resisting taxa) and precipitation, formation and 521 accumulation of phosphorite deposits. The lack of evaporites and presence of abundant small 522 benthic foraminifera indicate that the Mid-Eocene phosphorite rocks were precipitated in the 523 middle ramp (oligotrophic zone) and possibly accumulated by periodic energetic storm currents. 524 The Mid-Eocene phosphatic basin of the northwestern Tunisia (Domes zone and Tellian domain) 525 ismainly controlled by the holokinetic activities and the phosphorite deposits are accumulated in 526 narrow inter-diapiric shaped basins. 527 528 529 530 531 28 532 5. Conclusion 533 534 This study proposes the first reconstruction of depositional environmemnts of Mid-Eocene 535 (lower Bartonian) deposits in NW Tunisia. Based mainly on large benthic foraminiferal 536 assemblages, the Bartonian “Reineche Limestone” and coeval deposits from northern Tunisia 537 include eight micro-biofacies that onset in a progressive shallow-marine carbonate platform, 538 contemporaneous with the Mid-Eocene global transgressive event. Mid-Eocene phosphorites are 539 first described in the Tellian basin northwestern Tunisia. Within the study area, two major LBF 540 carbonate factories are distinguished: the first is dominated by LBF and developed in the 541 mesophotic zone (NE Tunisia); the second is characterized by restricted water circulation with 542 anoxic conditions (NW Tunisia). This is consistent with a decrease in the LBF skeletal 543 assemblages, and the precipitation, formation and accumulation of phosphorite deposits in 544 narrow inter-diapiric shaped basins. In this same context, the onset of the limestone “Reineche” 545 and “Siouf” members is the product of a major sea level high that interplayed with regional 546 tectonics implying mainly N-S, NE-SW and NW-SE ancient accidents. This regional 547 geodynamic context remains under-constrained: future works may focus on multidisciplinary 548 investigations of new sections in NW Tunisia aiming at more precise dating by means of 549 nannofossils biozonations, and magneto- and chemostratigraphic calibrations. Extended regional 550 correlations would provide the necessary improvement regarding spatio-temporal distribution of 551 the various facies and adjacent basin delimitation. This would allow long distance correlations as 552 a relevant support for replacing the Mid-Eocene deposits of northern Tunisia in their Neo- 553 Tethyan geodynamic context. Eventually, the other controlling events for depositional systems 554 need to be interpreted as interdependent factors with more or less effects. In this line, future 555 paleocirculation models would identify the main gateways at a large scale considering the whole 556 geodynamic context, and then the part of the MECO control and related signatures. 29 557 558 559 Declaration of competing interest 560 The authors declare that they have no known competing financial interests or personal 561 relationships that could have influenced the work reported in this paper. 562 563 Acknowledgements 564 The authors are grateful to reviewers for their constructive and helpful revisions. 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 30 582 References 583 584 Alouani R, Ben Ismail-Lattrache K, Melki F, Talbi F (1996) The UpperEocene prograding folds 585 in the northwestern of Tunisia: stratigraphic records and geodynamic significance. 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Basin Research 13:313–334. 783 Pomar L, Baceta JI, Hallock P, Mateu-Vicens G, Basso D (2017) Reef building and 784 carbonateproduction modes in the west-central Tethys during the Cenozoic. Marine and 785 Petroleum Geology 83:261–304. 786 Racey A (2001) A review of Eocene nummulite accumulations: structure, formation and 787 reservoir potential. J. Pet. Geol. 24 (1):79–100. 788 Read F (1982) Carbonate platforms of passive (extensional) continental margins: type, 789 characteristics, and evolution. Tectonophysics, 81:195-212. 790 Renema W, Troelstra SR(2001)Larger foraminifera distribution on a mesotrophic carbonate shelf 791 in SW Sulawesi (Indonesia). Palaeogeography, Palaeoclimatology, Palaeoecology 175:125–146. 40 792 Renema W (2006) Habitat variables determining the occurrence of large benthic foraminifera in 793 the Berau area (East Kalimantan, Indonesia). Coral Reefs 25:1- 351. 794 Renema W (2018) Terrestrial influence as a key driver of spatial variability in large benthic 795 foraminiferal assemblage composition in the Central Indo-Pacific. Earth-Science Reviews 796 177:514–544. 797 Riahi S, Soussi M, Ben-Ismail-Lattrache K (2015) Age, internal stratigraphic architecture,and 798 structural style of the Oligo-Miocene Numidian Formation of northernTunisia. Ann. Soc. Geol. 799 Pol. 85:345–370. 800 Riahi S, Soussi M, Stow D (2021) Sedimentological and stratigraphic constraints on Oligo– 801 Miocene deposition in the Mogod Mountains, northern Tunisia: new insights 802 for paleogeographic evolution of North Africa passive margin. International Journal of Earth 803 Sciences. https://doi.org/10.1007/s00531-020-01980-z. 804 Rouvier H (1977) Géologie de l'Extrême nord-tunisien: tectoniques et paléogéographies 805 superposées à l’extrémité orientale de la chaîne nord maghrébine. Thèse ès Sciences,Université 806 Paris VI : 1-703. 807 Sassi S (1974) La sédimentation phosphatée au Paléocène dans le Sud et le centre ouest de la 808 Tunisie. Thèse Doc. Es-Sci.Univ. Paris Sud Orsay :1-292. 809 Serra-Kiel J, Hottinger L, Caus E, Drobne K, Ferràndez C, Jauhri AK, Less G, Pavlovec R, 810 Pignatti J, Samsó JM, Schaub H, Sirel E, Strougo A, Tambareau Y, Tosquella J, Zakrevskaya E 811 (1998) Larger foraminiferal biostratigraphy of the Thetian Paleocene and Eocene. Bull. Soc. 812 Géol. Fr. 169 (2):281–299. 41 813 Taktak F, Kharbachi S, Bouaziz S, Tlig S (2010) Basin dynamics and petroleum potential of the 814 Eocene series in the Gulf of Gabes, Tunisia. J. Petrol. Sci. Eng. 75:114–128. 815 Tlig S, Sahli H, Alouani R, Mzoughi M (2010) Depositional environment controls on petroleum 816 potential of the Eocene in the North of Tunisia. J. Petrol. Sci. Eng. 71: 91–105. 817 Turki MM, Delteil J, Truillet R, Yaich C (1988) Les inversions tectoniques de la Tunisie centro- 818 septentrionale. BSGF 3 (8):399–406. 819 Van der Zwaan GJ, Jorissen FJ, De Stigter HC (1990) The depth dependency of 820 planktonic/benthic foraminiferal ratios: constraints and applications. Marine Geology 95:1–16. 821 Zachos JC, Dickens GR, Zeebe RE (2008) An early Cenozoic perspective on greenhouse 822 warming and carbon-cycle dynamics. Nature 451: 279–283. 823 Zaier A (1984) Etude stratigraphique et tectonique de la région de Sra Ouertane (Atlas tunisien 824 central). Lithologie, pétrographie et minéralogie de la série phosphatée. Thèse de doctorat 3ème 825 cycle, Faculté des Sciences de Tunis : 1-163. 826 Zaier A (1999) Evolution tectono-sédimentaire du bassin phosphaté du centre-ouest de la 827 Tunisie : minéralogie, pétrographie, géochimie et genèse des phosphorites. Thèse de doctorat, 828 Faculté des Sciences de Tunis. 829 830 831 832 833 834 42 835 836 837 Figure captions 838 839 Fig. 1: Geographical and geological setting of the studied area. 840 A. Location of the study area within the Mediterranean region. B. Mid-Eocene 841 palaeogeographical map of southern Tethyan margin (Meulenkamp and Sissingh, 2003). See 842 the same used colours in Fig. D for stratigraphic attributions. C. Location of the main studied 843 sections throughout the different palaeogeographical domains of Tunisia during the Mid- 844 Eocene (Ben Ismail-Lattrache, 2000). D. Synthetic lithostratigraphic chart of the Paleogene 845 showing main columnar sections, and stratigraphic nomenclature from southern to northern 846 Tunisia (Bismuth and Bonnefous, 1981). 847 Fig. 2: Stratigraphic logs of Damous (A) and Sidi N’sir (B) sections with information on 848 facies, texture, and skeletal assemblages. 849 Fig. 3: Skeletal assemblage and microfacies in the Damous section. 850 A-D. Orthophragminid biofacies (Mf6) associated with small benthic foraminifera (A-B) and 851 globigerinids (C-D). E-F. Nummulitid biofacies (Mf3) associated with alveolinids (F). G-I. 852 Foraminiferal/Algal biofacies (Mf1) showing alveolinids (G), Miliolids (H), small benthic 853 foraminifera and amphisteginids (I). J-K. Nummulitid and orthophragminid biofacies (Mf4) 854 associated with alveolinid (J) and planktic foraminifera (K). L. Globigerinid biofacies (Mf7). 855 Fig. 4: Skeletal assemblage and microfacies in the Sidi N’sir section. 43 856 A. Nummulitid and orthophragminid biofacies (Mf4). B. Operculina biofacies (Mf5). C. 857 Nummulitid biofacies (Mf3). D. Ostracod biofacies (Mf2). E-F. Phospharenite microfacies 858 (Mf8) showing peloids and quartz grains and rare nummulites (E) and molluscan shells (F). 859 Fig. 5: Stratigraphic logs of Oued Hassene (A) and Djebba (B) sections with information on 860 facies, texture, and skeletal assemblages. 861 Fig. 6: Skeletal assemblage and microfacies in the Oued Hassene section. 862 A-C. Globigerinid biofacies (Mf7) showing planktic foraminifera (PF) (A-B) and 863 Brachiopod fragment (C). D-F. Phospharenite microfacies (Mf8) showing planktic (PF) and 864 small benthic forams (SBF), nummulites (N) and molluscan shells (M). E. peloids. F. Bone 865 fragment. G-H. Nummulitid biofacies showing large flat nummulites (G) and molluscan 866 shells (M) and ostracods (Os) (H). The matrix contains phosphatic peloids grains. 867 Fig. 7: Skeletal assemblage and microfacies in the Djebba section. 868 A-C. Nummulitid and orthophragminid biofacies (Mf4) associated with planktic forams (PF) 869 (A), Red algae (RA) (B) and small benthic forams (SBF) (C). D-E. Phospharenite microfacies 870 showing (D) small benthic forams (SBF), Echinids (Ech), and Brachiopod fragment (Br), (E) 871 molluscan shells (M). F. Nummulitid biofacies (Mf3) showing large flat nummulites within 872 phosphatic dominated matrix. 873 Fig. 8: Environmental microfacies distribution for the Middle Eocene “Reineche Limestone” 874 member and their coeval deposits in northern Tunisia, arranged from proximal to distal 875 depositional environments: Mf1, Inner ramp shoal,Mf2, Inner ramp sea grass, euphotic 876 subtidal environment; Mf3 Proximal middle ramp LBF accumulations (nummulitids), 877 mesophotic environment; Mf4, Mid- middle ramp mesophotic environment; Mf5,Distal 44 878 middle ramp LBF accumulations (Operculina), mesophotic environment; Mf6,Distal middle 879 ramp LBF accumulations (orthophragminids), mesophotic environment; Mf7, Outer ramp 880 lacking LBF, globigerinid microfacies, oligophotic environment; Mf8, Middle ramp, 881 phospharenite microfacies. Ramp subdivision is based on Burchette and Wright (1992), and 882 photic zones are analogous to those described by Pomar et al. (2017). 883 Tab.1: Mf1 to Mf8 bio-microfacies showing their occurrences and main fossil content and 884 non-skeletal grain components. 45