Biostratigraphyand Paleoenvironments of Benthic Foraminifera From Lower Part of the Damlouk Member, Western Desert, Iraq

Age and paleoenvironment of part of the Damlouk Member, Ratga Formation from a surface section in the Iraqi Western Desert are investigated. Twenty-nine species of Larger Benthic Foraminifera (LBF) belonging to 13 genera are recognized from the studied section. The LBF assemblage is dominated by the following groups, Nummulites, Alveolina , Rotalia , and Lockhartia . Two species of corals with fragments of bryozoan, red algae and mollusca had also been identified. The palaeontological investigation aimed at identification of LBF assemblages to evaluate their abundance and distribution with respect to sedimentary environment and to determine its age. Based on the recognized Larger Benthic Foraminifera of the studied part of the Damlouk Member, it is subdivided into two biozones: Nummulites gizhensis -Nummulites moculatus Assemblage Zone and Assilina spira-Lokharatia hunti Concurrent Range Zone. Both biozones suggest a Middle - Late Lutetian to Early Bartonian age. The LBF are concentrated around shallow marine carbonate bank-controlled limestone unit and the associated ramp facies. Two basic environmentally significant assemblages were recognized based on the vertical distribution and the relative abundance of the LBF. The Nummulites bank assemblage is characterized by lensoidal and robust Nummulites of different species. The other group is the fore-bank assemblage which shows relatively higher diversity, including flat LBF such as: Nummulites , Assilina , Lockhartia , Heterostigina and Operculina . The third back-bank assemblage is not well presented in the studied section, shows the limited distribution, and is considered part of the back-bank lagoon facies. This group is characterized by the occurrence of Alveolina and other imperforated forams. The recognized assemblages represent the transition from inner to middle ramp facies with a water depth not exceeding 50m in relatively low energy conditions.


Introduction
The Cretaceous-Paleogene stratigraphic succession of the Western Desert of Iraq is interesting for two reasons; the first because it includes the economically attractive phosphate horizons of Iraq, and the second reason is the stable tectonic setting of the area as a part of the Arabian plate interior [1]. This conceivably displayed the eustatic controls on stratigraphy in a better resolution than the plate margin sequences. Thus, special attention is growing and given to this sequence, and this work represents part of continuous interest in the stratigraphic development of the area. (see [2] ) .
Studies on the western desert were reactivated during the seventies by the geologists of the Geological Survey of Iraq to map the area and evaluate its potential mineral resources, including phosphates and uranium [3]. This attempt is followed by extensive works of teams from the geological survey to revise the stratigraphic status and relations of the Cretaceous-Paleogene phosphatic-rich strata [3,4]. [5] finish the first report of that revision attempt by revising the stratigraphic status by introducing new units. The Eocene strata of the study area are now considered the Ratga Formation [3]. The argillaceous lime mud of the Damlouk Member in the Ratga Formation rich in planktic foraminiera [6]. [5] gave the stratigraphic formal and detailed description of the Ratga Formation type section, subdivided into three lithologic members from bottom to top: Swab, Damlouk, and Mugur Members. The Formation represents the phosphatic carbonate equivalent unit to part of the platform carbonates of the Dammam Formation of the Southern Desert. Outcrops of these members are distributed along valleys such as Swab, Akashat and Ratga, which drained down towards the Ga'ara Depression to the southeast.

Al-Qayim and Ghafor
Iraqi Journal of Science, 2022, Vol. 63, No. 11, pp: 4799-4817 4801 The study area is located in the northwestern part of the Iraqi Western Desert about 80 km from Al-Rutba town in an area called Akashat, between 40 o 00' and 40 o 15' Long and 34 o 00' and 33 o 45' (Figure 1).

Geologic Setting
The area is part of the stable platform of the Arabian Plate, which is controlled by subdued tectonic features, including the Hail-Rutba uplift [7]. This uplift and the succeeding erosion expose Paleozoic strata in a sub-circular depression called Gara Depression. Strata at the shoulders of this depression belong to the Mesozoic Era, followed by the Cenozoic strata. To the north and northwestern part of this depression, strata are dipping gently in that direction with dry valleys such as Al-Mana, Al-Ratga, and Akash cutting into the Paleogene strata and draining into the Euphrates valley ( Figure 1). The examined section of the Damlouk Member is exposed along these valleys and their tributaries. To the East and central Iraq, the Ratga Formation is replaced by the equivalent basinal marly limestone sediments of the upper part of the Aaliji Formation (Paleocene -Early Eocene) and the Jaddala Formation (Middle-Late Eocene) Formation. Further northeastwards, towards the northeastern margin of the sedimentary basin, these basinal facies are replaced by the Kolosh\ Sinjar, Gercus formations and the platform carbonates of the Pilaspi \ Avanah formations ( Figure 2). The age of the Ratga Formation is Lower-Upper Eocene [3]. It is widely distributed in the Western Desert, with a thickness of 200-220m. The Formation consists mainly of cyclic alternation of Nummulitic limestone, phosphatic limestone, crystalline chalky limestone with irregular chert horizons and nodules, and basinal marly limestones. The lower and upper boundaries conform to the Paleocene Akashat Formation and the Oligocene Sheikh Alas/ Shurau formations [3]. They subdivided the formation into three lithologic members from bottom to top: Swab, Damlouk, and Mugur Members. These members display similar cyclic alternations of similar lithologic associations in each member. The Damlouk Member includes two cycles of similar facies sequence and is often called Damlouk (A) for the lower cycle and Damlouk (B) for the upper one ( Figure 3). The type locality of the Damlouk Member is selected from Wadi Akash for Damlouk (A) (28m), and wadi Halgum for Damlouk (B) (52m) [3,5 ].
Each cycle consists of two distinctive lithologic units. The lower unit represents the shelfdeep marine facies, and the upper part represents the shallow marine facies and commonly consists of Nummulitic limestone and/or shelly bioclastic limestone making the basic components of a carbonate bank.
The examined section is 32 meters thick and includes the complete lower cycle (Damlouk A) and part of the upper cycle (Damlouk B). Therefore, the examination focuses on the lower cycle where most samples were collected. The cycle is 11 m. thick, and includes both shallow marine and deep marine units. The lower deep marine unit (A1) is about 9 m. thick, and usually consist of chalky limestone and phosphatic limestone (occasionally phosphorite), which is infrequently silicified (Figure 4-A, B, and D). The phosphatic limestone becomes marly limestone towards the bottom of the sequence, forming the overall deep marine part of the cycle (Figures 4-A-C).
The shallow marine unit (A2) consists of a 2 m thick horizon of Nummulitic-shelly limestone of the carbonate bank. This limestone is often hard, protrudes out and is rich in varieties of Nummulites.

Material and Methods
Eight samples (with 1 -1.5 m spacing ) were collected from the lower sequence (Damlouk A) of the Damlouk Member in the studied section, which reaches 32 meters in thickness. Sampling cover both units of the sequence: unit (A1), which represents the shallow marine Nummulitic bioclastic limestone facies, and the deep marine chalky, phosphatic, and marly limestone unit (A2) ( Figure 3). Thin section for each sample was prepared and stained to differentiate between variable carbonate minerals following [8]. Identification of taxa followed by quantitative analyses of fossils based on percentage and distribution of different species of larger foraminifera. Representative taxa and other recognized microfossils are studied using polarizing microscope photograph images. The identification of the recognized LBF and the biostratigraphic analysis of benthic foraminifera is assisted by using [9 and 10].

Microfacies
A detailed microfacies analysis and facies interpretation of the Damlouk Member from the Western Desert of Iraq is well presented by [1]. This presentation is good enough and needs to be reviewed here to shed some additional details on facies type. Each complete sedimentary cycle of the Damlouk Member includes the following basic microfacies: a) Nummulitic Grainstone -Packstone microfacie: It is characterized by the dense occurrence of generally robust Nummulites of different genera amongst Nummulites fabiani, Nummulites striatus, Nummulites bahyanesis, Nummulites Sabatacicus and Nummulites sp., in addition to the occurrence of N. gizehensis gizehenses, N. discorbinus, N.beaument. Other LBF includes Alveolina, Lokhartia, and Elphidium. These large benthic foraminifera are often embedded in a partly recrystallized micritic matrix. According to [11], this microfacies is commonly associated with the Nummulitic limestone bank ( Figure 4). b) Nummulitic Shelly Bioclastoic Packstone microfacies: Mixed bank Nummulites, pelecypods shells and their fragments are the common components of this microfacies ( Figure  4B). Other bioclastic elements include red algae and gastropods. All are embedded in a bioclastic micritic matrix. This microfacies is commonly associated with both the carbonate bank's shelly -Nummulitic limestone horizons [1]. c) Shelly Packstone to Wackestone microfacies: This microfacies is characterized by whole shells of pelecypods and gastropods often embedded in a lime mud matrix. Shells sometimes are fragmented, forming bioclastic shelly microfacies . Pelecypods are commonly dominated by the dense occurrence of large Oysters or dwarf Nuculana shells. These microfacies characterize the shelly or Oyster beds, which are an occasional part of the carbonate bank horizon [12]. These three microfacies are the most common at the carbonate bank and make the back-bank lagoon the basic parts of the inner ramp facies ( Figure 5).

d) Phosphatic Packstone to Wackestone microfacies:
This microfacies is associated with samples that have phosphatic grains of different kinds and origin. The common type of phosphatic grains is oolites, in addition to peloides, bone fragments, and phosphatic intraclasts. Matrix either lime mud or phosphatic mud, occasionally replaced by calcite cement. It is occasionally associated with bioclasts of different molluscs, indicating active agitation. This microfacies is associated with the phosphatic limestone horizons of the middle ramp [1] (Figure5).

Biostratigraphy
Ranges and relative abundances of the identified LBF from the studied section is shown in (Figure 6). The larger benthic foraminiferal assemblages has low diversity. A total of twentynine species of LBF belonging to thirteen genera have been identified in addition to bioclasts of Mollusca, corals, red algae, and bryozoan (Figures 7 -9). Based on the identified LBF, the studied section is subdivided into two biozones, from the lower part to the upper part of the section:

a-Nummulites gizhensis -Nummulites moculatus Assemblage Zone Definition: Biostratigraphic interval of this zone is characterized by the association of taxa (Nummulites gizhensis -Nummulites moculatus).
Zone boundary: This zone is represented by the First Appearance Datum (FAD), and dominance of the large benthic foraminifera is represented by (Nummulites gizhensis -Nummulites moculatus).

Paleoenvironmental Implications
Extensive Cretaceous and Cenozoic shelf carbonate platforms were produced by Larger Benthic Foraminifera (LBF). This group of foraminifera with algal symbionts are restricted to shallow euphotic-mesophotic areas. Differences in light attenuation by the water column are reflected by different forms of wall structures [16]. Nummulitic banks are common in Early and Middle Eocene Tethyan shallow-marine carbonates, especially in oligotrophic settings [11].
The type, distribution, and relative abundance of the Larger Benthonic Foraminifera assemblages of the lower cycle of the Eocene Damlouk Member show two important groups of benthic foraminifera associated with distinctive sedimentary facies and environmental conditions. The general environmental setting seems to be controlled by a Nummulites bank and the associated environments. These environments represent an Eocene gradually steepened carbonate ramp [1].
Based on that, the examined samples from the studied section were grouped into two facies, each including a different assemblage with different environmental settings. These assemblages and their environmental setting were correlated with other Paleogene Zagros carbonate ramps [17][18][19][20][21]. Similarities were recognized and thus support the interpretation of paleoecological distribution.
The upper part (A2), where the two samples (8 and 7) are taken from, represents the nummuliticshelley bank facies of the inner ramp [1]. The indicative LBF assemblage of this facies is generally includes Robust Nummulites such as ( Nummulites fabiani, Nummulites striatus, Nummulites bahyanesis, Nummulites sabatacicus and Nummulites sp.). Other associated components include Mollusca shells, red algae fragments, occasional miliolids, and other imperforated benthic foraminifera. The latter components might be derived from the back-bank and associated environment, which is located next to the bank in a landward position but not well presented in the studied section ( Figure 11). Bioclasts of corals, Bryozoa are infrequently noticed. These assemblages often occur in a very shallow and highly agitated carbonate bank within the mesophotic environment [21]. Miliolids occurs at depths of 0-100 m and Nummulites at depth up to 200m. The rough environmental conditions of this facies yields dense but low diversity assemblage, which thrives during high stand conditions of the marine transgression of the cycle [1].
The second assemblage is associated with the deep marine facies of the rest of the samples with proximal, middle ramp facies or fore-bank assemblages. The sediment of this facies is often thicker and characterized by the middle-outer ramp facies of light gray chalky and marly limestone. It occasionally becomes phosphatic limestone upward the cycle [1]. Diagnostic LBF elements of this assemblage include discoidal and flat Nummulites typified by the characteristic occurrence of the very large Nummulitse gezihensis foraminifera. In addition Nummulites brongniari, Nummulites moculatus, Nummulites beaumonti, Nummulites schaubi, and Nummulites bahariyaensis also occurred. Other abundant foraminifera of this assemblage includes species of: Operculina, Assilina, Amphistegina, Heterostegina, Lokhartia, and Lenticulina ( Figure 11). Bioclasts of bryozoan, red algae, and Mollusca are infrequently recognized associated with the assemblage. Components of this facies reflect a relatively quiet environment with deeper conditions in the Oligophotic zone [21]. The large and flat characters of these LBF groups suggest a middle ramp setting of relatively deeper water and oligotrophic conditions often associated with fore-bank facies [22,11,23,24]. [25] pointed out that the Operculina species occur in low energy and medium light within the lagoon-shelf region. The presence of Nummulites suggests shallower inner ramp/ shelf settings [26,16]. The facies of this assemblage is developed within the middle ramp association during the early stage of transgression as part of the transgressive system tract [1]. The middle ramp environment is characterized by relatively deep water conditions occasionally interrupted by fair weather storms [27]. The storm effect contributes to the localized and dense accumulations of these flat, large, and discoidal benthic foraminifera ( Figure 11).

Conclusions
This study has the following conclusions: 1-The lower cycle Damlouk (A) of the Damlouk Member, Ratga Formation in the study area is characterized by the diverse and abundant occurrence of larger benthic foraminifera and bioclasts of mollusc, coral, red algae, and Bryozoa. 2. Two biozones have been recognised based on the proposed stratigraphic ranges of the 29 identified larger benthic foraminifera species and calcareous algae of the studied section. These are: Nummulites gizhensis -Nummulites moculatus Assemblage Zone, and the Assilina spira-Lokhartia hunti Concurrent Range Zone, which is extended in age from the Middle to Late Lutetian for the lower unit, and Early Bartonian for the upper unit. 3. Using more considerable foraminifera diversity, distribution, abundance, and association with the different sedimentary facies support the paleoecological interpretation of the innermiddle ramp setting. Robust Nummulites dominate the carbonate bank of the inner ramp assemblage. The lensoidal and flat LBF of Nummulites and Assilina, Heterostegina, Lokhartia, Operculina, Amphistegina and Lenticulina dominate the fore-bank assemblages of the middle ramp.