Planktic Foraminiferal Biostratigraphy of the Upper Part of the Damlouk Member, Ratga Formation, Western Desert, Iraq

The basinal part of the Damlouk Member upper sedimentary cycle of the Ratga Formation exposed in the Qaim area of the Iraqi western desert is examined. The studied section consists of marlstone, marly and phosphatic limestone, and planktic foraminiferarich limestone beds. Detailed study of the planktic foraminiferal assemblages of these rocks revealed the occurrences of (30) species belonging to (11) genera. The stratigraphic distribution of these species permits the recognition of three biozones. These are from the lower to upper part of the section: Acarinina bullbrooki Zone, (Middle Lutetian), Morozovelloides lehneri Zone, (Late Lutetian), Globigerinatheca semiinvoluta – Hantkenina alabamensis Zone, (Early Bartonian). These zones indicate that the studied section of the Ratga Formation (Damlouk Member) is of Middle-Late Lutetian to Early Bartonian age. These biozones are correlated with different local and regional studies.


INTRODUCTION
The Paleogene succession of the Western Desert of Iraq is an interesting sequence for two reasons; it includes the economically attractive phosphatic horizons of Iraq. The second reason is due to the stable tectonic situation of the area, which will preserve the Paleogene sequence as compared to the Arabian active plate margin of NE Iraq. Based on that interest, further study and understanding the origin and the history of this sequence are needed. This work aims to contribute to the revision of the chronostratigraphic framework of the Paleogene successions in the Western Desert. The stratigraphic description of the Paleogene sequence of the Western Desert is subject to active and detailed works by the geologists of the Geological Survey of Iraq to evaluate its potential mineral resources including phosphates and Uranium (Al-Mubarak and Ameen 1983;Hagopian 1979;Karim and Al-Bassam, 1997). These efforts followed the detailed work of (Bellen et al., 1959) who had established the early stratigraphic subdivisions of the area. Followed by the work of (Al-Naqib, 1967) who had reviewed the general geology of the Western Desert. The work of (Hagopian, 1979) is considered an important stratigraphic contribution. He subdivided the Eocene Dammam Formation exposed at the edge of the western desert into five lithologic units based on lithostratigraphic characteristics. Extensive works of teams follow this attempt from the geological survey in detail to study the stratigraphic relations of the Paleogene phosphatic rich strata. The Eocene strata of the study area are considered now to be included in the Ratga Formation, which is introduced informally for the first time by (Jassim et al., 1984;and Al-Bassam et al., 1986). Karim and Al-Bassam (1997) gave the stratigraphic formal and detailed description of the Ratga Formation type section, which is subdivided into three lithologic members and they are from bottom to top as follows: Swab, Damlouk, and Mugur Members. Outcrops of these members are distributed along valleys such as Swab, Akashat, and Ratga, which are drained down towards the Ga'ara Depression to the southeast. The selected section of the Damlouk Member for this study is located along Wadi Akashat at the northwestern part of the Iraqi Western Desert, about 80 kilometers north of Rutba Town. The investigated area is called the Akashat area, which runs between 40 o 00' and 40 o 15' Long., and 34 o 00' and 33 o 45' Lat. (Fig. 1). Karim and Al-Bassam (1997) formally describe the Ratga Formation of Lower-Upper Eocene age with details. It is geographically cropped out and distributed in the Western Desert with a thickness range between 200 -220 m. This unit consists mainly of cyclic alternations of marl and marly limestone. The lower basinal unit of the pelagic marly limestone is often recognized as partly phosphatic, intensively recrystallized to chalky limestone with silicification as chert horizons and nodules. The upper unit is composed of shallow marine Nummulitic limestone variably intermixed with shelly bioclastic limestone. It is subdivided into three lithologic members from the bottom to the top: Swab, Damlouk, and Mugur Members (Fig. 2).   -Bassam, et al., 1986;and Karim and Al-Bassam, 1997).

LITHOSTRATIGRAPHY
These members display cyclic alternations of similar lithologic units (described above). The Lower boundary is conformable with the Paleocene Akashat Formation. The upper boundary is also conformable with Sheikh Alas/ Shurau Formations of Lower Oligocene age (Karim and Al-Bassam, 1997). The Damlouk Member is usually recognized with two distinctive cycles informally called Damlouk (A) for the lower cycle and Damlouk (B) for the upper cycle. The type locality of the Damlouk Member is selected from Wadi Akash for Damlouk (A) (28 m), and Wadi Halgum for Damlouk (B) (52 m) (Jassim et al. 1984;Al-Bassam et al. 1986;and Karim and Al-Bassam, 1997). The selected studied section is 32 meters thick and includes the complete lower cycle (Damlouk A) in addition to the basinal marly limestone part of the upper cycle (Damlouk B). The sampling of this section is focused on the basinal part of the upper cycle (B1) due to its fresh exposure and relatively thick section. The lower cycle (Damlouk A) is 11 meters thick and includes both shallow marine and deep marine sediments (Jassim et al. 1984;Al-Bassam et al., 1986;and Karim and Al-Bassam, 1997). The deep marine unit (A1) is about 9 meters thick and usually consists of phosphatic limestone and chalky limestone (occasionally phosphorite), with occasional silicification (Fig. 3). The phosphatic limestone part of (A1) is becoming marly limestone towards the bottom of the sequence, forming altogether the deep marine part of the cycle. The shallow marine unit of this cycle (A2) consists of 2 meters thick horizon of Nummuliticshelly limestone. This unit represents part of a shallow marine carbonate bank ( Karim and Al-Bassam, 1997;Al-Qayim et al., 2019). This limestone is often hard, protrudes out, and contains abundant and diversified assemblages of Nummulites.
The basinal deep marine sediments of the upper cycle (Damlouk B) are the only representative part of the upper cycle (B1), whereby the upper shallow marine part is removed by recent erosion. The exposed and sampled section of this basinal unit (B1) is 21 meters thick, which is considered relatively the thickest in the area. The lower part of this unit is about 10 meters thick buff relatively soft phosphatic limestone. The upper part of this unit is 11 meters thick and consists of white chalky limestone, which displays silicification in the form of dark brown thin cherty layers or nodules (Fig. 3).

MATERIALS AND METHODOLOGY
Field description of the studied section includes lithological documentation and stratigraphic relations. Nine samples are collected from the basinal unit of the upper cycles (Damlouk B). Three samples are from the phosphatic limestone part and six samples are from the silicified chalky limestone part (Fig. 3). The samples are collected in uniform spacing and chosen from hard parts to be suitable for thin sections that are made at the workshop of the Department of Geology, Sulaimani University, and examined under Leitz polarizing microscope for identification of the different species of the planktonic foraminifera. Identification and classification of the examined foraminifera are done following the works of (Pearson and Wade, 2015;Loeblich and Tappan, 1988;Loeblich and Tappan, 2015;and Wade et al., 2011).

PLANKTIC FORAMINIFERAL ASSEMBLAGES
The first description of Eocene planktic foraminifera was made during the nineteenth Century during early geological exploration in Europe (e.g., Gümbel, 1868;Hantken, 1875;Terquem, 1882). This study is based on an analysis of nine samples taken from the studied section that led to the identification of 30 species belonging to the 11 genera of planktic foraminifera, which are distributed from the lower to the upper part of the section. The planktic foraminifera species recognized within the studied section are: (Turborotalia cerroazulensis (Cole); Turborotalia cocoaensis (Cushman); Turborotalia frontosa Subbotina; Turborotalia prolata; Turborotalia pseudoampliapertura Bolli; Turborotalia sp.; Globigerinatheca semiinvoluta (Keijzer) ; Globigerinatheca lutherbacheri Bolli;

BIOSTRATIGRAPHY
The studied section of the Ratga Formation yielded rich planktic foraminiferal assemblages of good preservation. Thirty planktic foraminiferal species belonging to eleven genera had been identified (Fig. 4). These planktic foraminiferal assemblages are typical of tropicalsubtropical Tethyan character (Berggren and Pearson, 2005). The stratigraphic distribution of the planktic foraminiferal species is shown in Figure (5). The stratigraphic distribution of these planktic foraminifera permits the recognition of three biozones (Fig. 6). These are from the oldest: Acaranina bullbrooki Zone, (Middle Lutetian), Morozovelloides lehneri Zone, (Late Lutetian) Globigerinatheka semiinvoluta-Hantkenina alabamensis Zone (Early Bartonian). These zones are correlated with similar zones which were established by other authors as shown in figures (7and 8). Below is the description of each of these biozones.