Microfacies analysis of the Late Maastrichtian- Danian Phosphatic Succession in the H3-Trebeel district, Western Desert of Iraq

The Late Maastrichtian–Danian phosphatic succession prevails as a deposit to the west of Rutbah region, Western Iraq. This is manifested through the lithostratigraphic sections of boreholes (K.H5\6 and K.H 5\8) drilled previously in the area. The succession is mainly composed of phosphate, shale, porcelanite, oyster and foraminiferal carbonate lithofacies belonging to Digma and Akashat formations. Three facies associations are distinguished during the study: the phosclast planktonic (FA1) that dominates the outer ramp, the phosclast foraminiferal (FA2) that dominates the mid ramp, and the quartz dolomitic phosclast (FA3) present in the inner ramp. These facies’ associations are differentiated into seventeen microfacies types. Microfacies analysis and fauna contents have shown gradual facies variation grading from a high energy inner ramp environment in the east to a low energy deep water ramp environment in the west.

reported that, during the Late Campanian and Maastrichtian periods, the climax of obduction and closure of the Southern Neo-Tethys has led to a significant transgression across Iraq as a whole. Due to deformation that occurred along the NE Tethyan margin, the Cenomanian Sea transgressed onto the Rutbah uplift which led to the reactivation of longitudinal ridges and transversal blocks. NW-SE trending ridges divided the main basin into smaller sub-basins, causing facial variations. The tectonic activity at the margins of the Arabian Plate and the narrowing of the Tethys Sea led to the development of phosphorites within the Late Cretaceous formations [15]. The supplementary type section of Latest Maastrichtian Digma Formation is suggested by [2] at Um Er Radhuma Al-Safra, about 10 Km southeast of Akashat mine, while the original type locality is in the Anah oil well 1 [4]. This Formation is exposed at Marbat Al-Hasan hill and continues westwards, along the northern rim of Ga`ara Depression, then it extends along the western rim of Ga`ara Depression, farther southwards crossing the Highway No.1, to the Iraqi -Saudi Arabian borders. The Digma Formation is underlain conformably by the Tayarat Formation [2,7] and overlain conformably by Akashat Formation in the western and northern rims of Ga`ara Depression [6]. The type locality of the Akashat Formation is located in wadi Samhat, along the western rim of Ga'ara Depression, which is located 7 Km south of Akashat phosphate mines [2]. The Akashat Formation is exposed, only in the western and middle parts of the Iraqi Western Desert. In the subsurface, the Akashat Formation almost has a uniform extension, where it extends north and westwards. Towards north, it passes to Aaliji Formation, whereas towards east and northeast, it is replaced by Umm Er Radhuma Formation, to the east and northeast of Rutbah Uplift [15].

Materials and Methods
The subsurface Upper Cretaceous -Paleocene stratigraphic succession at the Western part of Rutbah province, Western desert, is studied using core samples obtained from two boreholes drilled previously in the area. The K.H5\6 borehole is bounded by (38° 56' 13"E, 32° 42' 15"N) coordinate and the K.H5\8 borehole is bounded by (39° 29' 52"E, 33° 11' 11"N) coordinate ( Figure-1). 184 samples were collected within a space interval that ranged between 0.5 and 1.0 m, depending on lithological variations. Samples were used to study petrography and microfacies of the succession. The distinction between calcite and dolomite minerals was accomplished by using alizarine staining method to thin sections. The staining affects calcite, where it changes the original colour to red or purple while, on the contrary, dolomite is not affected. Thin sections were investigated by applying Denham classification of carbonate rocks [16,17] to determine microfacies characteristics.

Lithostratigraphy 4.1 Lithostratigraphy of K.H 5\6 section
The studied section consists of three formations: Upper part of Tayarat Formation which represents the Early Maastrichtion age, Digma Formation which represents the middle Late to late Late Maastrichtion age, and Akashate Formation which refers to the Danian age [1, 2, 9, and 14]. The latter formation is subdivided into three members: Traifawi, Hiri, and Dwaima [2]. In the borehole K.H 5\6, only the 13 m sections were described from the upper part of Tayarat Formation, which consists of dolostone. Digma Formation in K.H 5\6 comprises 64 m thick sections of limestone and marly limestone, interbedded with shale and dolomitic shale overlying the dolomitic units of Tayarat Formation and underlying the Akashat Formation. This Formation attains a thick succession of limestone characterized by moderately fine to medium-sized euhedral floating rhombic dolomite scattered in shale beds. In the studied section, the Formation can be divided into 3 lithological units from the base to the top, as described below (Figure-2). 1-The lower unit consists of fossliferous limestone, with depth interval of 197-184 m (thickness 13 m), which is characterized by the presence of planktonic foraminifera and phosphate clast. 2-The second unit consists of shale with micro rhombic dolomite filled with organic materials. It is located at a depth interval of 183-172 m. Rare planktonic foraminifera exist; it was difficult to identify them, due to the dolomitization processes that altered the rocks. 3-The upper unit consists of marly limestone interbedded with shale that is rich in planktonic foraminifera, few benthic foraminifera, and bioclast, along with few phosphate clasts. The upper part of this interval (171-133 m) is characterized by the presence of organic material in shale. In the K.H5\6, only the lower member was studied. In the studied section, the Trifawi Member was introduced to describe a 34 m section, which can be divided into 3 lithological units from the base to the top, as described below. 1-The first 10 m consist of the basal laminated grey to black shale directly overlying the Digma Formation, with a depth interval of 133-123 m. It is characterized by the existence of a moderate percentage of planktonic foraminifera and few benthic foraminifera. This interval is characterized by the presence of organic material in shale, and it is assigned to the earlist Danian age. 2-The second unit consists of 5 m of limestone with few dolomitic rhombic crystalls, with a depth interval of 123-118 m, which is characterized by the existence of a moderate to high percentage of planktonic foraminifera and few benthic foraminifera, along with few phosphate clasts. It is assigned to the early Danian age. 3-The upper unit of the Trifawi Member consists of 6 m of dolomitic limestone, from a depth interval of 118 to 112 m, overlaid by 12 m of limestone which is rich in planktonic foraminifera, few benthic foraminifera, and few bioclasts that occur within an interval of 112 to 99 m.

Lithostratigraphy of the K.H 5\8 section
The studied section consists of three formations: the Upper part of Tayarat, Digma, and Akashate. Digma Formation in K.H 5\8 comprises 51 m thick limestone overlying the dolomitic units of Tayarat Formation, which consists of dolomitic limestone at a depth interval of 190-160 m. This unit is characterized by dolomite with medium to coarse subangles, with bad sorted quartz. The depth interval is 170-164 m, characterized by silicified limestone, whereas the interval of 164-160 m is marked by crystalline limestone. The Digma Formation underlies the Akashat Formation. Digma Formation is formed of a thick succession of limestone. In the studied section, the formation can be subdivided into 3 lithological units from the base to the top, as follows ( Figure-3). 1-The lower unit consists of limestone at a depth interval of 156-160 m, which is characterized by abundant phosphate clasts and bone fragments, with rare existence of planktonic foraminifera in the upper part of this unit. 2-The second unit consists of limestone at a depth interval of 156-143 m, which is characterized by the existence of few bone fragments that are scattered in micritic groundmass. These bone fragment have large size and elongated shape, making a floatstone texture. 3-The upper unit consists of limestone at a depth interval of 156-110 m, which is characterized by the existence of planktonic foraminifera, few benthic foraminifera, and bioclast. The upper part of this unit is characterized by the presence of phosphate clasts and bone fragments, in addition to planktonic foraminifera.

Mousa et al.
Iraqi Journal of Science, 2021, Vol. 62, No. 4, pp: 1188-1203 1192 The Akashat Formation belongs to the Paleocene age and is subdivided into three members based on fossils content, as follows.
Trifawi Member: The term Trifawi Shale Member was introduced to describe 7 m, of which 3 m are characterized by index planktonic foraminifera, which is overlaid by 5m of crystalline limestone. The basal laminated black shales is overlying the Digma Formation. It consists of a grey to black shale. It is assigned to the early Paleocene age (Danian). This member is characterized by the existence of index fossils of Guemblitria cretacea related to the Danian age Cushman, which in turn is overlaid by the Hiri Member of the same formation, which is characterized by thick units rich in shell fragments of mollusca.

Carbonate and non-carbonate consistency
Carbonate grains can be divided into non-skeletal and skeletal grains. The main skeletal grains are composed of planktonic foraminifera and few benthonic foraminiferas in addition to bioclasts of echinoderm and fossil fragments that appear in small amounts in some intervals within the Digma and Akashat formations in K.H 5\6 and K.H 5\8 wells (boreholes). Shell fragments are represented by mollusk shells, such as pelecypods and gastropods, which are recognized in some intervals. The non-skeletal grains in this study are represented by peloids, which were found within Digma Formation in KH 5\6 in some intervals. The non-carbonate component is expressed by phosclasts and bone fragments, which represents dominant components within the Digma and Akashat formations in the borehole K.H 5\8. It also occurs in the borehole K.H 5\6 section. Phosclast is defined as a phosmud (pristine) that has been subjected to fragmentations, winnowing, reworking, and redeposition as phosphate grains. Another source for lithoclast is coprolite fragments. These terms followed the suggestions of earlier studies [1,18]. Silica bioclast that contains mainly sponge spicules within siliceous facies was recorded in boreholes K.H 5\6 and K.H 5\8. Quartz was observed in the lower part of early Late Masstrichtion succession in the K.H 5\8 section.

Facies associations
Microfacies identification, classification, and analyses based on sedimentological, petrographical, and paleontological criteria are directed to predict the sedimentary system of the studied area. Mixed different rock and allochems types have been together subjected to phosphatization. The Late Maastrichtian-Danian phosphatic mixed lithofacies are composed of genetically related microfacies (MF), which interfere with each other to re-build three main association facies (FA) represented by five textural types: mudstone, wackestone, packstone, grainstone, and floatstone ( Figures-2 & 3). This gives a detailed picture of their facies distribution and depositional environment that is ranging from basinal-distal outer ramp to proximal inner ramp. There microfacies and facies association are distinguished, as described below. H5\8. This microfacies corresponds to RMF5 [19]. Ramp Microfacies Types (RMF) correspond in their criteria to Standard Microfacies Types of Wilson for carbonate platforms SMF [19]. The planktonic assemblages with a micrite matrix indicate a deeper subtidal distal-outer ramp to basinal facies below the storm wave base (SWB). The warm water provinces are characterized by species with thick-walls and ornamented by keels. These taxa include Marginotruncana, Globotruncuna, Globotruncanita, and Gansserina, that occupy the Tethyan province [20]. The same species are recognized in these microfacies within K.H 5\6. Occurrences: This microfacies was introduced to describe five m thickness recognized in the upper part of the Middle late Maastrichtian within Digma Formation in K.H5\6. These microfacies are recorded in ten m within the lower part of Akashat Formation in K.H5\6. This microfacies may correspond to the Biozone VII in Syria [21] and may be equivalent to RMF 5 [19]. Black shale\marl rich in planktonic foraminifera indicates the deposition in a deeper subtidal distal-outer ramp to the basinal environment. Occurrences: This microfacies is observed in two intervals, the first is in the lower part of Digma Formation represented by three m, while the second is introduced to describe four m thickness recognized in the upper part of the Middle late Maastrichtian within Digma Formation in K.H5\6, along with six m thickness in K.H5\8. This microfacies corresponds to RMF 5 [19]. Mohammed (1993), depending on XRD, SEM, and chemical analysis, proved that some beds of planktonic mudstones are porcelanitic [1]. Planktonic spiculetic porcelanite indicates the deposition in the deeper subtidal distal-outer ramp to the basinal environment.

Phosbioclast planktonic mudstone\wackstone (MF4):
This microfacies occurs in grey mudstone\ wackstone and consists of planktonic foraminifera such as Pseudoguembelina hariaensis (Pl.3I), Globotruncana sp., Rugoglobigerina sp., Heterohelix sp., and Archaeoglobigerina carteri, in addition to few to moderate percentages of phosbioclast, which occur as an accumulation of skeletal fragments that have been subjected to phosphatization. Planktonic Foraminifera in the lower part of Digma Formation are represented by small-sized biserial and low trochospiral spired planktonic foraminifera. Heterohelicids show high abundance in several marine environments, suggesting the ability to adapt to a wide range of water column conditions [22]. Occurrences: This microfacies is observed in the lower part of Digma Formation in K.H5\6 and is present in nine m thickness of the succession. This microfacies corresponds to RMF 5 [19]. Phosbioclast planktonic mudstone\wackstone indicates deposition in the proximal outer ramp of paleoenvironment.

Phosclast planktonic wackestone\ packstone (MF5):
This microfacies consists mainly of planktonic foraminifera and few to moderate percentages of phosclast which has small size. Occurrences: Phosclast planktonic wackestone that are recorded in the middle part of Akashat Formation in K.H5\6 occur in four m thickness (Pl. 1D). Phosclast planktonic wack-packstone that are recorded in Digma Formation occur in five m within the middle part. Phosclast planktonic packstone are recognized through four m in the upper part of Digma Formation in K.H5\6 (Pl. 1E). This microfacies may correspond to Biozone II in Syria [21]. This microfacies may also correspond to RMF 5 [19]. It suggests deposition in the proximal outer ramp paleoenvironment. 6.1.6 Phosclast planktonic foraminifera black shale\marl (MF6): It is characterised by the presence of high organic material of shale\marl with high proportions of planktonic species (up to 90%) and less than 10% of phosoclast (Pl. 1F). All the individuals of planktonic foraminifera include keeled species which are assigned to the deep-water form group [19] that increases in abundance towards the upper part of the Digma Formation, indicating the presence of a deep deposition basin during that time. Deepening of the basin, it is associated with the appearance of planktonic forms, such as Plummerita hantkeninoides (Pl.3E), Globotruncana, and Globotruncanita. These genera dwell among moderate to deep water zones and geographical environments, associated with the tropical-subtropical Tethyan realm, during Cretaceous [23]. This complex assemblage of species happened as the result of an increase in water depth that took place during the middle-late Late Maastrichtian. Occurrences: This microfacies is represented by fifteen m thickness, recognized in the upper and middle parts of Digma Formation in K.H5\6. This microfacies is equivalent to RMF 5 [19]. This microfacies suggests deposition in the proximal outer ramp paleoenvironment. Occurrences: This microfacies is present among four m thickness, recorded in K.H5\6, that represents the late Late Maastrichtian within Digma Formation. This microfacies corresponds to RMF 3 [19]. Phosclast planktonic peloidal packstone microfacies suggest deposition in the proximal outer ramp paleoenvironment.

Phosclast foraminiferal -dominated mid ramp facies association (FA2)
These facies associations are graded from wackestone to packstone, including three microfacies which are MF8, MF9, and MF10. . This microfacies may correspond to the Biozone I in Syria [21] and to RMF 5 [19]. Foraminiferal packstone indicates deposition in high energy distal mid ramp paleoenvironment.

Planktonic phosclast packstone (MF10):
This microfacies consists mainly of phosclast in different sizes and shapes, which are mostly rounded to subrounded, with the presence of bone fragments and few planktonic foraminiferas with low percentages of benthic foraminifera (Pl.2C). Phosphate grains are very common in limestones through most of the intervals of the studied succession, especially in the middle part of Digma Formation in K.H5\6, as well as in the lower, middle, and upper parts of Digma Formation in K.H5\8. Abundant phosphate grains generally indicate very specific geological and paleoenvironmental conditions, such as oxygen deficiency, upwelling conditions, and transgressive intervals [24,25,26,27]. Occurrences: Planktonic phosclast packstone were recognized in two intervals within the lower and upper parts of Digma Formation in K.H5\8 (22 m thick). The matrix of this microfacies in the lower intervals was seen as a micro sparry calcite. This microfacies could be equivalent to Biozones II and III in Syria [21], and may be corresponding to RMF 5 [19]. Planktonic phosclast packstone indicates deposition in proximal mid ramp facies association.

Quartz phosclast dolomitic-dominated inner ramp facies associated (FA3)
These Occurrences: This microfacies is recognized in three m in the upper part of Tayarat Formation. This microfacies corresponds to RMF29 [19]. It represents an effect of shallowing. Quartz-phosclast grainstone suggest deposition in shallow back-shoal inner ramp facies association. Also the same microfacies occurs in five m in the upper part of Hirri Member of Akashat Formation in K.H5\8, but in Hirri Member, this microfacies does not contain quartz grains and suggest deposition in shoal mid ramp facies association .

Shelly-phosclast floatstone (MF12):
This microfacies consists of mollusca shell fragments and occurs at the upper interval of borehole K.H 5\8. It contains an abundance of phosclast (Pl. 2E). This microfacies comprises seven m thickness. It is recognized as Hirri Member of Akashat Formation within borehole K.H5\8. This microfacies is equivalent to RMF 30 and interpreted to be deposited in a shallow back-shoal inner ramp facies association [19].

Dolomitic shale\marl (MF14):
This microfacies is represented by three m within the middle part of Digma Formation in borehole K.H5\6 and consists of shale\marl that is filled by organic material with a moderate percentage of scattered rhombs of subhedral to euhedral small-sized dolomite crystals. It contains very few planktonic components which are difficult to be identified (Pl.1H). This microfacies is equivalent to RMF22 [19]. Dolomitic shale\marl is interpreted to be deposited in a semi-restricted subtidal -lagoon inner ramp facies association.

Quartz dolomitic mudstone\ wackestone (MF15):
This microfacies consists of micrite matrix. Quartz mudstone is barren of any grains and contains channel porosity which is filled by quartz cement (Pl. 2G). This microfacies was introduced to describe three m thickness recognized in the upper part of the Tayarat Formation within K.H5\8. The microfacies is topped by farm ground with Thalassinoides burrows. Quartz dolomitic wackestone was recognized in K.H 5\8 within Tayarat Formation at a depth interval of 190 to160 m. This unit is characterized by dolomite with medium to coarse subangular-rounded bad sorted quartz. This microfacies can be equivalent to RMF 22 [19].It indicates deposition in peritidal inner ramp facies association.

Dolostone inner ramp facies (MF16):
This microfacies consists of dolomite only (Pl. 2H) and comprises13 m thickness. It represents the upper part of the Tayarat Formation, along with other 6 m in the middle part of Akashat Formation within K.H5\6. This microfacies is equivalent to RMF 22 [19] and indicates deposition in peritidal inner ramp facies association.

Conclusions
A detailed study of the Late Maastrichtian-Early Paleocene facies association was conducted to provide the basis for palaeoenvironmental interpretation of the studied successions. Seventeen microfacies types were classified genetically to yield three facies associations that built the lithostratigrphy of Tayarat, Digma, and Akashat formations; (1) Phosclast planktonic outer ramp facies association (FA1), which is exemplified in seven microfacies, (2) Phosclast foraminiferal mid ramp facies association (FA2), which included three microfacies, (3) Quartz phosclast dolomitic inner ramp facies association (FA3). The inner ramp facies association is exemplified in seven microfacies. The important microfacies present in borehole K.H 5\6 succession are composed of black shale\marl rich in planktonic foraminifera (MF2) and Black shale\marl with phosclast planktonic foraminifera (MF6), because these microfacies are considered the index fossils of Maastrichtian and Dainan age, and are exemplify by Plummerita hantkeninoides and Guemblitria cretacea, respectively. Planktonic phosclast packstone (MF10) is the essential microfacies in borehole K.H 5\8 succession because it represents the index fossils of the Late Maastrichtian (Plummerita hantkeninoides). The Cretaceous/Palaeogene (K\Pg) boundary is suggested as conformable contact, as there are no breaks in sedimentation and biotic content continues along the successions.The first two facies represent deposition within moderate deep to deep water environment in the mid and outer ramp settings; the latter facies reflected an effect of shallowing and deposition in inner ramp setting. The petrographic analyses of 184 samples were focused on allochem percentage and matrix type. They show that the Maastrichtian-Danian succession deposition took place in a wide variety of depositional environments, ranging from pretidal, lagoon, barrier shoal, and inner to middle ramp environments. The deeper environment is represented by the outer ramp to basinal.
Based on the study of planktonic foraminifera genera at the K.H5\6 and K.H5\8 sections, the identified planktonic foraminifers within the sediments of the Digma Formation are represented by Globotruncana sp. and Globotruncanita genera, which dominated these sections. It is correlated to Globogerinoides sp. and Hedbergella sp. that reflect a warm water seaway, in connection with the Tethyan realm during the Upper Cretaceous. Therefore, these findings suggest that this section has a close similarity with the planktonic foraminifer's genera in the Tethyan provinces. In the Lower part of Digma Formation of the middle Late Maastrichtian, the presence of Heterohelix ssp. and Rugoglobigerina spp. can be correlated to the presence of oligotrophic that indicate warm environmental conditions in the lower part of the succession described in borehole K.H5\6.