Microfacies analysis, sequence stratigraphy, and depositional environment of Eocene deposits: A case study from Samawa desert, Southwestern Iraq

Introduction

The Eocene epoch represents a significant chapter in Iraq's geological history, characterized by major global geological changes. During this time, the Neo-Tethyan Ocean came to an end due to the convergence of tectonic plates. The Arabian Plate, which underwent subduction, collided with the Turkish and Iranian plates, triggering important geological processes (Numan 1997). Bramkamp (1946, as cited in Van Bellen et al. 1959) was the first to identify the Rus and Dammam formations from its type section located on the southeastern margin of the Dammam Dome in Saudi Arabia. Subsequently, Van Bellen et al. (1959) reported findings related to the Dammam Formation at Zubair 3, which is recognized as its type locality. The well in this study was drilled to the depth of 136 m, and the complete cores were obtained for the following depths:136–121 m and 111–69.5 m. (see Fig. 1).

The Rus Formation is characterized by substantial beds of anhydrite interspersed with non-fossiliferous dolomite, exhibiting grey to dark grey colors (Tamar-Agha 2021; see Fig. 2-A). It also features partially silicified claystone lithofacies (Fig. 2-B) and dolomitized limestone containing gypsiferous, fossiliferous miliolid dolostone, which includes nodules of anhydrite and marl. The distribution of the Rus Formation is somewhat restricted, primarily observed in subsurface sections across stable shelf areas. While, the Dammam Formation consists of limestone types (chalky, organodetrital, or dolomitic), dolomites, marls, and shales. The formation is predominantly made up of neritic shoal limestones that are frequently recrystallized and/or dolomitized, featuring nummulitic structures in the lower and middle sections and miliolid fossils in the upper sections (Jassim and Goff 2006) (Fig.2 C-F).

The Eocene deposits in Iraq indicate a warm climate that ranges from tropical to subtropical, accompanied by a rich marine ecosystem (Maziqa et al. 2023). Additionally, Maziqa et al. (2024a) pointed out that the presence of nummulites and various marine fossils implies the existence of lagoons, shallow marine habitats, and adjacent marine zones. During the Eocene, shallow seas covered a sizable portion of the continents. Nummulite fossils have been found worldwide, demonstrating their extensive dispersion. Most of the dolomitized and/or recrystallized limestone in the studied formation is miliolid-bearing in the upper part and nummulitic in the lower part. The boundaries between the Paleocene and Eocene are marked by an earlier sequence of layers that is both conformable and transgressive, whereas the Eocene–Oligocene boundaries are missing from the examined section because of disconformities shown by the presence of basal conglomerate (Maziqa et al. 2024a). Many researchers, including Al-Hashimi and Amer (1985); Tamar-Agha and Saleh, (2016); Al-Jibouri, (2003); Al-Samarraie, and Al-Dulaimy, (2015); Kadhim (2015); Al-Waely (2016); Jassim et al. (2018), Tamar-Agha (2021), as well as several researchers from GEOSURV mentioned in Maziqa et al. (2023), have conducted extensive investigations on the Rus and Dammam formations in Iraq. Locally, the objective of this study is to conduct various microfacies analyses, sequence stratigraphy assessments, and palaeoenvironmental interpretations of carbonate systems located in southwestern Iraq. Also, the Dammam Formation is considered very important because the water stored between its rocks is used in the special injection process for oil reservoirs, especially in the Mishrif reservoir (Lazim et al. 2024). This study is significant for understanding the geological events that occurred during the Eocene period, particularly the notable disappearance of the Neo-Tethyan Ocean, which resulted from the convergence of tectonic plates. The Arabian Plate experienced subduction and collided with the Turkish and Iranian plates, resulting in significant geological transformations (Al-Mutury and Al-Asadi 2008).

Fig 1- Geographical and geological map of the studied well (B.H.S.7) Samawa area, southwest Iraq

Fig 2- Representative core samples from Samawa B.H.S-7: A- Anhydrite rock (depth 75 m Rus Formation), B- claystone lithofacies (depth 69.5 m Dammam Formation); C-Nummulitic limestone (depth 58 m Dammam Formation), D- Diagenetic processes (silicification) (depth 45.5 m Dammam Formation), E and F- Diagenetic processes (dissolution) (depth 67 m and depth 68 m Dammam Formation)

 Geological setting

The Eocene epoch, particularly between approximately 56 million and 33.9 million years ago, was marked by significant geological transformations across the globe. Notably, the convergence of the Arabian and Iranian tectonic plates during this time led to the disappearance of the Neo-Tethyan Ocean. The interaction of the Turkish and Iranian plates with the subducting Arabian Plate initiated crucial geological processes (Numan 1997). In contemporary geological classifications, Iraq is divided into two primary regions: The Inner and Outer Platforms (Fouad 2015). The active Abu Jir-Euphrates Fault Zone is situated in the southwestern part of the country, while the Anah Graben Fault, located to the west, serves as the main geological division between the outer and inner Arabian platforms. The contact zone created by the intersection of these platforms encompasses the borehole examined in this study (Figure 3) and enhances our understanding of the geological dynamics in this area, with the current investigation focused on the vicinity of the inner platform. According to Fouad (2015) and Al-Kaabi et al. (2023), these fault zones are crucial in defining the limits and topographical characteristics of Iraq's Arabian platforms. The Arabian continental plate's edge bends around the outer platform and undergoes uplift and extension shortly before subduction into the descending slab (Al-Mutury and Al-Asadi 2008). This event has a significant effect on the formation and distribution of different stratigraphic successions in the area and acts as an early warning of the coming collision during the Eocene period. Denudation processes have revealed a stratigraphic succession of marine and continental deposits that date from the Paleocene to the Pleistocene. The majority of the Southern Desert is blanketed by shallow marine deposits from the Paleogene period. The Umm Er-Radhuma Formation, which is found along the Iraqi–Saudi Arabian border, is the oldest identified formation in the region. There is a lateral connection between the carbonate facies of this formation and the anhydrite facies of the Rus Formation (Maziqa et al. 2024a). Scarps are prominent features in the southern region, with each notable scarp corresponding to a distinct layer of the Dammam Formation. The Dammam rocks are significantly eroded by three wide, deeply carved wadis, each measuring over 50 km in width, that flow toward the Euphrates River (Maziqa 2024).

Fig 3- Stratigraphic column of the borehole, Samawa (B.H.S-7), southwest Iraq, the ages of units were determined by Maziqa et al. (2024b)

 Methodology

This study is based on fifty-eight core samples taken from a 141-meter succession obtained from borehole S-7 (44° 59' 58" E and 30° 54' 54" N), drilled by the Iraq Geological Survey in 2011 as part of a comprehensive geological mapping project in the southern Samawa region of the Southern Desert of Iraq (Figure 3). A total of 40 thin sections were prepared for the Dammam Formation and 15 for the Rus Formation, along with two slides specifically focused on the contact with the Umm Er-Radhuma and four slides for the Euphrates formations. The classification of the carbonate rocks within these formations follows the criteria established by Dunham (1962) and the modified scheme of Embry and Klovan (1971), based on their depositional textures. This study also employs the revised classification from Dunham (1962), as referenced by Raymond (1995). Furthermore, the well-recognized Ramp Standard Microfacies Types (RMF) are compared to the carbonate microfacies recognized in these deposits, drawing on the works of Wilson (1975) and Flügel (2010). The prevalence of mud-supported shallow shelf open marine facies, along with limited lateral variation in these facies, suggests the presence of a gently sloping ramp in the research area (Davis et al. 2019; Mahdi et al. 2022). Insights into the petrographic characteristics of the carbonate depositional environments are derived from Flügel (2010) as well as the research of Al-Hashimi and Amer (1985). Additionally, larger benthic foraminifers from the Eocene are identified and classified according to the taxonomic frameworks of Boudaugher-Fadel (2018).

 Microfacies analysis

Nine microfacies were identified as a result of the petrographic examinations.  Specific depositional settings are linked to the identified microfacies. The following sections discuss the broad environmental interpretations of these microfacies.

 Rus Formation:

Laminated evaporite-carbonate Microfacies: (mf1)

This microfacies was found at the following depths: 75, 79, 84, 97.5, 107, 122, 124, 126, 129, 133 and 136 meters. The Miliolids with their biomolds are dispersed in this microfacies. The presence of nodular and fibrous aggregate gypsum defines this microfacies petrographically (Figure 4-A). Aphanocrystalline dolomite is found between the fibrous gypsum that fills veins and cracks. Gypsum has formed, and dolomite has supplanted anhydrite in these microfacies. This microfacies was deposited in a confined (lagoonal) environment, this type of unlaminated sub-microfacies is believed to have developed on a low-energy, restricted circulation marine substrate (Flügel 2010). Numerous studies have identified two predominant environments within the Rus Formation of the deposit: the sabkha and lagoon environments. However, this study, anchored in the notable presence of various organisms particularly miliolids proposes a close  relationship, suggesting that the lagoon environment that situated in closer proximity to the sabkha than previously thought (Tamar-Agha and Saleh 2016; Tamar-Agha 2021; Maziqa et al. 2024 a+ b).

Dammam Formation:

Mudstone:

There are two sub-microfacies inside this microfacies:

 Fine lime mudstone (mf2):

The depths of these sub-microfacies are 7, 23.5, 49, and 27.5 m. Regarding its composition, micrite that has completely recrystallized through diagenetic processes constitutes the composition of this sub-microfacies. The identified fossils consist of small Miliolids, Nummulite striatus, Nummulite sp., and fragments of shell material. Additionally, scattered rhomboid dolomite crystals are present in the micritic matrix. Notably, dolomite may form through the diagenetic alteration of limestone, as described by Pomar (2001). Dissolution is the most characteristic feature of this microfacies, some of which are joined by minor calcite dissolution. This sub-microfacies contains several of porous such as vugs and intracrystalline pores. In the peritidal inner ramp setting, the deposition resembles that of the typical RMF22 microfacies, as described by Flügel (2010). This similarity suggests comparable depositional environments and sedimentary processes, highlighting the consistency of microfacies characteristics in these specific geological settings (Fig. 4- B).

Non-Burrowed lime-mudstone (mf3):

The borehole at the following depths 21.5, 25 and 72 m, reveal that the rocks in this sub-microfacies consist of fine to very fine crystalline dolomite, according to petrographic analysis (Fig. 4C). In some samples, rhomboid dolomite crystals may be present; dedolomitization replaces part of these crystals with calcite. Trace fossils were found in some samples, and calcite is present in the voids left by crystals in the cement and calcareous dolomite. Intraclasts, which are rare and found in a limited number of irregular samples, exhibit a form similar to dolomudstone. Iron oxides and clay particles are rare and occur in patches and mixed forms within the groundmass. Gypsum is present in minute amounts as cement. The most prevalent pore types in these sub-microfacies include veins, biomolds, intercrystalline pores, and vugs. This sub-microfacies, a non-burrowed calcareous mudstone, is located in the peritidal (tidal-flat) platform region and correlates to the standard microfacies RMF19 (Flügel 2010).

Wackestone microfacies: Two sub-microfacies comprise this microfacies including bioclastic-foraminiferal wackestone and intraclast wackestone.

Bioclastic-foraminiferal wackestone (mf4):

At depths of 9, 10, 13, 14.5, 17.5, 19, 29, 33.5, 35, 36, 37, 38.5, 39.5, 43 and 46 m, this sub-microfacies is found in drill cores. Its composition results from significant dolomitization and consists of fine to extremely fine crystalline dolomite. It comprises bioclasts and is surrounded by a micrite that has been petrographically substituted by dolomite. The skeletal elements of this microfacies include algae, echinoderm fragments, mollusca debris, bryozoan skeletons, ostracoda, N. bayhariensis, N. elevate, Nummulite sp., and miliolids (Fig. 4 D). These fossils have broken down, leaving the rocks with plenty of biomolds and large vugs. The most common pore types in these sub-microfacies are biomolds, intraparticle, intracrystalline pores, and veins. Similar to the RMF20 ramp microfacies (Flügel 2010), algae and miliolid were discovered in the lagoon environment, and deposited within the inner ramp setting. The imperforate foraminifers of these facies, which are abundant in the mud and thrive in relatively moderate water turbulence, suggest lagoonal shallow subtidal habitats (Romero et al. 2002). The lagoon ecosystem was impacted by the occurrence of dolomitic layers; however, an excess of algae might periodically dominate (Flügel 2010).

Intraclas wackestone (Mf5):

This sub-microfacies exist in borehole B.H.S-7 at 69, 69.5 and 71.5 m. In terms of petrography, it is composed of fine crystalline dolomite that is aphanitic. Moreover, scattered rhomboid dolomite crystals are present in the facies (Fig. 4E). Its color varies from yellowish-grey to brown, and the dolomitization gives it a ghostly appearance. It is also quite durable. In these sub-microfacies, the most common forms of pores include veins, intraparticle pores, fractures, and vugs. This type of sub-microfacies resembles the typical RMF24 microfacies, which are deposited in the peritidal platform region (Flügel 2010)

 Packstone microfacies:

Foraminifera-Nummulites packstone (mf6):

This sub-microfacies occurred at certain depths (112, 114, 118 and 120 m). It is distinguished by the presence of nummulites. Based on petrographic analysis, the known fossils include bryozoan skeletons, echinoderm plates, algae, Operculina Libya, Nummulites fraasi, N. planulatus, N. murchisoni, N. globulus, and Nummulites sp. Serra Kiel (1998) states that Nummulites fraasi is one of the essential genera (Nummulites), belonging to the Ypresian and its typical shallow benthic zones. The bioclasts have been largely replaced by coarse calcite. The foraminifer-nummulite packstone resembles the ramp microfacies RMF 13 in that it contains a variety of nummulites and bioclasts (Flügel 2010) (Fig. 4F). On the other hand, bryozoans show evidence of open sea circulation. Racey (2001) reported that Nummulites were discovered in a variety of open marine settings on both shelves and ramps. Pomar (2001) detailed the complex interactions among the prevalent larger foraminifers of the early Cenozoic carbonate platforms.

Rudstone microfacies: Based on the percentages of the three different types of textural components, three sub-microfacies are identified. They are as follows:

Bioclastic floatstone to rudstone (mf7):

This sub-microfacies were identified at depths 53, 58 and58.5 m. Petrographic analysis reveals that these sub-microfacies consist of macrofossils, secondary calcite, and biomolds. The groundmass is predominantly composed of crystalline dolomite, which exhibits euhedral to rhombohedral shapes and varies in size from fine to medium grains. The average size of the fossils found is greater than 2.5 mm, including bivalves, bryozoans, algae, ostracods, N. beaumonti, N. gizehensis, and N. bayhariensis (Fig. 4G). Several varieties of cement, such as blocky and granular cement, are present. In these sub-microfacies, biomolds, vugs, and intercrystalline pores are the most prevalent pore types. According to Flügel (2010), this sub-microfacies correspond to the RMF18 ramp microfacies that are deposited in a marine (restricted) setting. The variation of the foraminiferal assemblage indicates that it is distinctive of a marine environment in the photic zone. Compared to the less constrained open shallow subtidal inner ramp environment described by Abdel-Fattah (2022), it is thought to be more energetic. Large benthic foraminifera with perforated walls, corallinaceans, echinoids, bryozoans, and a mud-micrite matrix are indicators of deposition in a shallower environment, according to Pomar (2001) and Cosovic et al. (2004).

Nummulites rudstone (mf8): 

These sub-microfacies are located in B.H. S-7 at depths of 57, 59, 60, 64, 67 and 68.5 m. Petrographically, a range of foraminifers, both larger and smaller, is observed in the final microfacies, along with other fossils, most of which are larger than 2 mm (Fig. 4H). The identified fossils include echinoid plates, shell fragments, Linderina chapmani, L. brugesi, Linderina sp., Orbitolina sifini, Nummulites gizehensis, N. bayhariensis, N. planulatus, N. globulus, and Nummulites sp. Diagenetic processes found in this sub-microfacies include recrystallization and dissolution (including vug, intraparticle, intercrystallite, and fracture), which contribute to major porosity, as well as cementation by calcite. It is composed of partly dolomitized microsparite or, in some cases, preferentially dolomitized micrite that has been recrystallized. In nummulitic biomolds, some samples' fossils dissolve entirely and remain as biomolds, while other samples contain secondary calcite clasts that hold the fossils in situ. This sub-microfacies presents RMF27, which is located in the bioclastic shoal of the inner ramp (Flügel 2010). According to Reekman and Friedman (1982), the bigger fossil size (specifically Nummulites sp.) would be the best indicator of a shoal or barrier.

Euphrates Formation:

Bioclastic foraminiferal floatstone/rudstone (mf9):

The facies, which represent the Euphrates Formation, is found in B.H. S-7 at depths of 2, 4, 6, and 6.75 meters. In this formation, there are numerous mollusks with an average size of more than 1.5 cm, miliolids, gastropods, and Peneroplis sp. (Fig. 4I). Veins and vugs filled with secondary calcite are rare in samples. Petrographical analysis showed a high concentration of bioclasts located in a microsparitic groundmass that is partially replaced by extremely fine dolomite. Partial dolomitization, neomorphism (recrystallization and inversion), the formation of porosity (intraparticle biomolds, vugs, intercrystalline, and veinlets), and cementation by secondary calcite are the main diagenetic events that have impacted this facies. RMF20 presents this microfacies that were deposited in a shallow to restricted inner ramp (lagoon environment) (Flügel 2010). Babazadeh (2003) states that the presence of microfacies, mollusks, and bryozoans within the lagoonal deposits indicates that these sediments were formed in lagoons characterized by relatively shallow water conditions. This finding emphasizes the ecological dynamics of the environment, as the diverse assemblage of organisms suggests a productive shallow-water ecosystem conducive to the deposition of such sediments.

Depositional environments of study interval

Based on the identified microfacies and fossil assemblages, the depositional environments of the studied formations have been analyzed. The Rus and Dammam formations are extensively distributed in the south and southwest regions of Iraq, displaying a variety of facies throughout their sequences as delineated by the microfacies descriptions. The characteristics of these depositional setting and their associated sediments are summarized in Figure 5 (Jassim and Goff 2006).

Rus Formation

The Rus Formation can be categorized into two members (the Lower and Upper members). The Lower Member is characterized by flaky yellowish marl interbedded with thin layers of nummulitic limestones and anhydrites, featuring gastropods near the top; it indicates the presence of sabkha environments. In contrast, the Upper Member is associated with miliolids limestone beds that were formed in a lagoonal setting on the Shelf (Jassim and Goff 2006; Tamar-Agha and Saleh 2016). There is only one identified environment in the borehole being studied for the Rus Formation. The interfingering zone between the Dammam and Rus formations observed in this study is indicative of predominantly lagoonal environments rather than shallow sabkha settings, as there were no distinct anhydrite layers identified. The boundary between the Rus Formation and the underlying Dammam Formation is indicated by the shift from dolomitic limestones that are abundant in algae and small benthic foraminifera to shaly and fossil-rich limestones that contain a high concentration of large benthic foraminifera, including types such as Nummulites and Alveolina (Al-Saad 2005).

Dammam Formation

In the studied borehole, five distinct environments have been identified. They are as follows: The investigation of microfacies within the formation reveals intriguing insights into the depositional environments. First, the presence of fine-grained lime mudstone (Mf2), non-bedded mudstone (Mf3), and lithoclastic wackestone-floatstone (Mf5) suggests a peritidal environment. The notable occurrence of microcrystalline dolomite coupled with the complete absence of fossils supports this interpretation, as highlighted by Mazzullo and Reid (1988) and further elaborated by Mazzullo (2000).

Additionally, the identification of wackestone-packstone sub-microfacies (Mf4) within bioclastic-foraminiferal microfacies relationships, now transitioned into dolomicrite, signifies the lagoon of an inner ramp sub-environment. This particular association was deposited in the upper portion of the formation while still submerged in water, as indicated by Alsuwaidi et al. (2020).

Furthermore, the presence of large foraminiferal species, such as Nummulites gizehensis, along with shell fragments, is indicative of shoal environments. Rhombic dolomite crystals, varying from medium to coarse sizes, are sporadically distributed throughout the Nummulites Rudstone sub-microfacies (Mf8). These microfacies, deposited above the typical wave base, thrive in a moderate to high-energy shoal environment, according to the observations of Wilson (1975), Racey (2001), and Al-Menoufy et al. (2021). The bioclastic floatstone-rudstone sub-microfacies (Mf7) point towards a restricted internal platform environment, as discussed by Akhzari et al. (2015) and Rineau et al. (2021).

Lastly, the occurrence of Foraminiferal-nummulitic packstone (Mf6) hints at an interior environment of an open marine platform. The presence of various bioclasts including Lindrina sp., Rotalia sp., bryozoans, and small foraminifers suggests open marine conditions. Their association with coralline algae, bryozoans, and echinoderms further indicates the presence of forebays or shallow open marine habitats. These open marine facies association aligns with Flügel's (2010) delineations of open marine to mid-ramp facies zones, and it intersects with Pomar et al. (2017) and Burchette and Wright's (1992) classifications of shallow subtidal ramp environments, as illustrated in Figure 5.

Euphrates Formation:

The existence of microfacies associations of Bioclastic foraminiferal floatstone/rudstone microfacies (Mf9) indicates that it is an interior habitat, or lagoon inner ramp (Al-Ghreri, et al. 2016; Dawood and Al-Hetty 2024).

Fig 4- A) Laminated evaporite- carbonate microfacies: (mf1); depth 104 m Rus Formation; B) Fine mudstone sub-microfacies (mf2); depth 36 m Dammam Formation;  C) Non-burrowed lime-mudstone sub-microfacies (Mf3); depth 25 m, Dammam Formation; D) Bioclastic- foraminiferal wackestone sub-microfacies (mf4); depth 29 m, Dammam Formation; E) lithoclastic wackestone - floatstone sub-microfacies (Mf5): depth 69.5 m, Dammam Formation; F) Foraminifera-nummulitic packstone sub-microfacies (mf6); depth 118 m, Dammam Formation, G) Bioclastic floatstone-rudstone sub-microfacies (mf7); depth 58  m, Dammam Formation; H) Nummulites rudstone sub-microfacies (mf8); depth 60 m Dammam Formation; I) Bioclastic  foraminifera floatstone-rudstone sub-microfacies (mf9); depth 6 m, Euphrates Formation(mi=micrite, D=dolomite, p=pellet).

Fig 5- Proposed depositional environment model for Lower Eocene–Lower Miocene of the studied borehole in southwestern Iraq (modified after Burchette & Wright 1992).

Sequence stratigraphy

Sequences are described by Van Wagoner et al. (1988, 1990) as a conformable succession of genetically linked strata that are bounded by unconformities and their corresponding conformities at both the top and bottom. Nonconformities are surfaces that have a discernible temporal gap and show degradation or non-deposition. Because benthic foraminifera are highly sensitive to environmental changes, they provide particularly reliable data in this context (Mahdi et al. 2022). The validity of this idea is supported by an examination of the distribution of benthic foraminiferal associations in deposits where the cycles of eustatic rise and fall of sea level have been extensively documented (Cubaynes et al. 1989). Six third-order depositional sequences are identified in the studied interval.

Sequence A:

The strata in depositional sequence A date to the Lower Eocene. The age of these sequences was determined by Maziqa et al. (2024 a and b). This depositional sequence has a thickness of 15.5 m, and its microfacies are linked with Highstand Systems Tracts (HST). The bottom portion of depositional sequence A is conformably positioned within the Umm Er-Radhuma Formation. A marine microfacies identified as a lagoon (mf1) marks the Maximum Flooding Surface (MFS). It is believed that these sediments characterize the Early HST.

Sequence B:

The sediments of depositional sequence B also date to the Lower Eocene after comparing it with the Range biozone N. deserti- N. fraasi (Early Ypresian) age (Maziqa et al. 2024b). The microfacies relations of this 43.5 m thick depositional sequence may be defined as Transgressive and Highstand System Tracts (TST and HST). Benthic foraminifera limestone alternates with marly limestone in the bottom portion of depositional sequence B (TST). The MFS, identified by shallow open marine microfacies (mf6), separates TST and HST. The MFS is overlaid by laminated evaporite-carbonate microfacies (Mf1). These sediments are thought to represent the Early HST. Lagoonal microfacies constitute the majority of early HST deposits, while late HST deposits are indicated by peritidal (tidal-flat) platform deposits. The presence of more shielded sediments (i.e., imperforate foraminifera in wackestone-packstone) seen in late HST indicates that the accommodation space is filling up. The contact between the depositional sequences B and C is located at the top of MFS (Fig. 6). This sequence's upper boundary (SB2) is dated to the most recent Lower Eocene, coinciding with the Lower to Middle Eocene border. The Middle Eocene boundary of the sequence boundary with age 30.0 Ma (Haq et al. 1988), and the isotopic event known as OCi-1 at 28.4 Ma (Abreu & Haddad 1998. Hardenbol et al. 1998), all appear to overlap with this sequence boundary.

Sequence C:

Sequence C corresponds to the Middle Dammam (Middle Eocene) based on the N. gizehensis-N. planulatus-N. discorbinus Assemblage Zone (Maziqa et al. 2024b). The first notable index fossils of Nummulites, specifically Nummulites gizehensis, emerged in the Middle Dammam, signifying a new cycle of transgressive sedimentation as outlined by Al Hashimi and Amer (1985) and Maziqa et al. (2024a). According to Ben Smail-Lattrache et al. (2014), N. gizehensis is recognized as an excellent guide species for the Middle Eocene period. These fossils exhibit sizes exceeding 25 mm (Fig. 2C) and have been documented in several thin sections. Depositional sequence C has a thickness of approximately 25.5 meters.  The transition from shoal microfacies to restricted peritidal facies indicates an upward trend in microfacies within the TST of depositional sequence C. The wackestone containing perforated large benthic foraminifera serves as the MFS marker for this depositional sequence. Gradual microfacies transitions from shoal to sheltered (peritidal) environments characterize the upper section of the depositional sequence (HST), reflecting decreases in water depth. The floatstone, characterized by a limited diversity of imperforate foraminifera and identified as an SB2 type, represents the sequence boundary. Several notable references pertain to this sequence boundary. Hardenbol et al. (1998) identified an SB at 25.1 Ma, while Van Buchem et al. (2010) noted TB1.3 (26.5 Ma) of Haq et al. (1988) in the Late Eocene, accompanied by an isotopic event referred to as OCi-3 at 25.2 Ma (Abreu & Haddad 1998). Our interpretation of the sea-level changes through the deposition of the Dammam Formation reveals geometric parallels with the global sea-level curve for the Middle to Upper Eocene established by Haq et al. (1988). However, some discrepancies can be attributed to the local geological environment. While Haq et al. (1988) reported three third-order depositional sequences during the Rupelian–Chattian stage, we identified only one third-order depositional sequence in the studied area. This study proposes that these differences are linked to sediment supply variations within the study area and the influence of regional tectonic conditions.

Sequence D:

By the end of the Middle Eocene and the beginning of the early Late Eocene, as revealed by the Miliolids-Peneroplids Assemblage Zone (Maziqa et al. 2024b), the studied region was predominantly characterized by shallow, restricted lagoonal environments (Fig. 6). The marine transgression of the Eocene period advanced into an Upper Eocene limited marine platform, where diverse facies emerged within the Dammam depositional zone. This area is particularly noted for its abundant occurrences of peneroplids and miliolids, as well as dolomitic limestone. Depositional sequence D corresponds to the Upper Eocene Upper Dammam, which signifies a new phase of transgressive sedimentation, characterized by the final appearance of Nummulites gizehensis (Al Hashimi & Amer 1985; Maziqa 2024; Maziqa et al. 2024a). Depositional sequence D exhibits a thickness of approximately 22.5 meters. The shift from restricted microfacies to peritidal facies highlights the upward trends in microfacies within the TST of this depositional sequence. Notably, depositional sequence D in the studied borehole (B.H.S-7) is symmetrical, indicating a well-balanced development of these marine conditions.

Sequence E:

Depositional sequence E represents the Upper Eocene (Upper Dammam). According to Al Hashimi & Amer (1985), Maziqa (2024), and Maziqa et al. (2024a), this depositional sequence has a thickness of approximately 14.5 m. The shift from lagoonal to peritidal facies indicates a deepening trend corresponding to the TST of this depositional sequence. The MFS of this depositional sequence was identified by packstone and wackestone, characterized by dolomite and dominated by miliolids and peneroplids occurrences. Depositional sequence E in the study area (B.H.S-7) is asymmetrical and is truncated by the upper unconformable surface (SB1) of the Dammam Formation.

Sequence F:

Depositional sequence F was assigned to the Lower Miocene Lower Euphrates Formation. This depositional sequence has a thickness of over six meters. Lagoonal microfacies exhibit deepening upward trends characterizing the TST of this depositional sequence. A dolomitic limestone showing rudstone texture dominated by Peneroplis sp., miliolids, gastropods, and mollusks, served as the MFS for this depositional sequence. Information within this depositional sequence is limited due to the scarcity of rock samples, as it represents a critical contact line with the formation currently under investigation.

Fig 6- Lithostratigraphic logs of Eocene strata with sequence stratigraphy subdivision and depositional environments of borehole B.H.S-7, southwestern Iraq.

 Discussion

The Dammam Formation is encountered at depths ranging from 120 to 112 m, corresponding to the Lower Eocene; from 69.5 m to 43 m, representing the Middle Eocene; and from 43 to 7 m, indicating the Upper Eocene.

Maziqa (2024) noted that the Dammam Formation predominantly comprises shoal limestone, which is frequently recrystallization and/or dolomitization. The dolomitic limestones show  nummulitic bioclasts that are present in the lower and middle sections and miliolid in the uppermost layers. In this study, the Dammam Formation is composed of dolomitized limestone, intraclastic dolomitized limestone, nummulitic limestone, and dolomitized nummulitic limestone, which may be partially or fully silicified (Fig. 2C-D). The Nummulitic limestone facies change laterally towards the south and southeastern area into anhydrite (Sabhka) facies during the Lower Eocene. Significantly, the formation in the study area has undergone extensive diagenetic processes, including pronounced dolomitization and, in particular, dissolution (Maziqa et al. 2024). These processes have resulted in the dissolution of numerous fossils within the formation, contributing to the development of secondary porosity (Fig. 2E-F). The Dammam Formation overlies the Rus Formation unconformably, with the contact defined by greenish-grey claystone beds (Tamar-Agha and Saleh 2016). According to Van Bellen et al. (1959), Maziqa et al. (2023), Maziqa (2024b), and Maziqa (2024), the upper boundary with the overlying Euphrates Formation may also be disconformable. In this study, the middle section of the Dammam Formation is identified as the contact, characterized by a dark brown claystone bed situated at a depth of 69.5 m (Figure 2B). However, the presence of breccia lithofacies indicates the contact line between the two formations (Dammam and Euphrates Formations). According to Martn-Martn et al. (2020), the Eocene deposits in Iraq indicate a warm, tropical to subtropical climate alongside a diverse marine ecology. The presence of Nummulites and other marine fossils suggests the existence of lagoons, shallow marine environments, and marginal marine areas. The distinction between these habitats is particularly pronounced before and after the Middle Eocene Climatic Optimum, a time marked by peaks in Nummulitic carbonate production on the platform and a decline in carbonate production in deeper basinal sections (Messaoud et al. 2021). Numerous authors have described the depositional environment of the Middle Member of the Dammam Formation across different regions. This formation was deposited in shallow, warm water, neritic sublittoral forereef and shoal zone (Al Samarraie & Al-Dulaimy 2015; Al Wa'aly 2016; Jassim et al. 2018).

Research on sequence stratigraphy of this formation remains quite limited, with the majority of existing studies focusing on paleontology and sedimentology, with the notable exception study by Al-Waely (2016). Al-Waely categorized the Dammam Formation into three sequences: early, middle, and late Eocene. In contrast, the current study provides a more comprehensive analysis, detailing the sections in greater depth. Specifically, the early Eocene encompasses depositional sequence A and B, while the middle Eocene is represented by depositional sequence C. The late Eocene includes depositional sequence D and E. The decrease in eustatic sea level during the Late Eocene was associated with the prevalence of miliolids, Textularids, and small Nummulitids within this studied succession, alongside a significant extinction event affecting planktonic foraminifera (Berggren et al. 1995; Anan 2007). According to Salama, et al. (2021), both the drop in sea level and tectonic disturbances may have contributed to this global regression. Changes in sea level, which were partly related to tectonic activities such as uplift and subsidence or the formation of mountains, could have been one way this global decline was expressed in the research area.

The sedimentation patterns of the Dammam Formation sequences are significantly influenced by eustatic sea-level fluctuations during the Eocene, while tectonic activity also contributed to the development of the late Eocene sequences.

Conclusion

Core samples from the B.H.S-7 well have been used to analysis of general microfacies and sequence stratigraphy of the Rus and Dammam formations in southwest Iraq. The Rus and Dammam formations are arranged in the following ascending order:

1. During a relatively stable phase of the Eocene and minor fluctuations in sea level led to alternating transgressive and regressive cycles, facilitating the deposition of the Rus evaporites along with the Dammam limestones and dolomites. However, by the late Eocene, a notable decline in sedimentation activity was observed, marking a significant shift in depositional conditions.
2. Nine microfacies associations representing various depositional environments have been identified based on the diversity of microfacies.

Within the Rus Formation, a distinct lagoon ecosystem has been identified. In contrast, the Dammam succession encompasses five unique habitats including (1) the peritidal environment, represented by dolomite formations. (2) The lagoon setting is characterized by the presence of miliolids and algae. (3) The shoal habitat is distinguished by rudstone-like associations, including the Nummulites rudstone sub-microfacies (mf8) and larger Nummulites species. (4) A restricted environment, which may or may not feature Nummulites species, indicated by the presence of miliolids and characterized by diagenetic limestone. Finally, (5) the small-scale inner ramp settings, which showcase Nummulites species, contain packstone microfacies rich in bioclastics, Nummulites fragments, and Echinodermata fragments. Additionally, a lagoon environment has also been documented within the Euphrates Formation. This detailed classification highlights the diverse sedimentary environments and biotic interactions present in these geological formations.

3. Research on the microfacies of the Rus and Dammam formations reveals a cyclical pattern of sedimentation with upward inclinations indicating shallowing or deepening. The distinction of depositional series is facilitated by the vertical stacking of facies associations. However, in general, it is getting shallower into the top due to the closure of the Neo-Tethys Ocean.
4. The Paleocene-Eocene boundaries are characterized by an earlier bed succession that is both conformable and transgressive, whereas the Eocene-Oligocene boundaries are absent from the section under study due to nonconformities indicated by basal conglomerate.
5. The study identifies five third-order depositional sequences and two sequence boundaries in the Eocene succession of the Dammam Formation, revealing the effects of eustatic sea-level fluctuations and tectonic activity. Sequence A (Lower Eocene) with lagoon microfacies at the Maximum Flooding Surface (MFS), Sequence B (Lower Eocene) marked by alternating limestones and shallow open marine microfacies, Sequence C (Middle Eocene) showing a transition from shoal to peritidal facies with Nummulites gizehensis fossils, Sequence D (end of Middle Eocene to Early Upper Eocene), Sequence E (Upper Eocene) transitioning from lagoonal to peritidal microfacies, and Sequence F (Lower Euphrates, Lower Miocene) with lagoon microfacies and upward-sinking trends. These findings illustrate the complex interactions between sea-level changes and tectonic processes impacting the Dammam Formation's sedimentary architecture.

Acknowledgments

We would like to thank the Geological Survey of Iraq for their generous provision of samples and for facilitating our field visits.