Iraqi Geological

Abstract


Introduction
The Eocene Epoch witnessed the development of a carbonate platform covering a huge area of Iraq (Al-Hashimi, 1972;Al-Kubaysi, 2013;Al-Dulaimi and Al-Wa'aly, 2016). The foraminiferal contents of the Eocene platform were the target for many previous studies in the Western-South Desert of Iraq (Amer, 1980;Mohmood, 1983;Al-Mutter, 1984). From its typical type section in the Dammam area in Saudi Arabia, Sandrey (1952) in Bellen et al. (1959) provided the initial description of the Dammam Formation. Owen and Nasr (1958) documented a supplemental-type section in Iraq from B.P.C. Oil Well Zubair 3 in the Basra area of southern Iraq. The following coordinates define it: 30°23'01" latitude; 47°43'29" longitude. According to lithology, the additional type section's formation is composed of whitish-gray, porous, dolomitized limestone, which can occasionally be chalky (Owen and Nasr, 1958).
Thirty-seven samples were taken from borehole No. Kr-1, in North Karbala City in the Western Desert of Iraq, was used to study the Dammam Formation paleontologically, at 32° 33' 05" N and 43° 20' 21" E, (Fig.1), which GEOSURV drilled in 2009-2010 while carrying out a thorough geological survey in Karbala city (Mohammad, 2010).
From the oldest to the youngest, the investigated samples revealed the presence of the Dammam Formation (Middle Eocene) and the Euphrates Formation (Early Miocene).  (Sissakian, V., 2000).
The aim of the study is to determine the biostratigraphy of the studied section inaccurate way, depending on identified the important fossils and microfacies.

Methodology
This study is based on thirty-seven samples collected from the borehole (Kr-1). The carbonate rocks of this formation is classified after Dunham (1962) with modification of Embry and Klovan (1971), depending on the depositional texture of the rocks. Grain types such as bioclasts and intraclasts may qualify for this classification. The modified Dunham classification (1962) by Raymond (1995) is also adopted in this work.
The identified carbonate microfacies of the formation is compared with Standard Microfacies Types from well-known environments (e.g., Wilson, 1975 andFlugel, 2010). Depositional environments of the carbonate microfacies are discussed mostly according to their petrographic characteristic (Flugel, 2010) and also according to Al-Hashimi and Amir (1985).
Work takes two directions: • In order to achieve the aim of the present study, soft samples were collected from the borehole Kr-1.
At least 250 gm was taken from each sample to extract the microfaunal contents. The samples were soaked in a hydrogen peroxide solution concentration of 30% or acetic acid that a concentration of 60 %. The samples were washed under running water and sieved using a 63 μm sieve (Al-Shawi et al., 2019;Al-Ali et al., 2020). The residue was dried and separated into several fractions to facilitate the picking of different microfossils using a binocular microscope. The picked foraminiferal tests were identified to species level and photographed. • More core samples collected from borehole no. Kr-1 drilled by GEOSURV, made thin sections in this work at their laboratories.

Stratigraphy of Dammam Formation
During the Eocene, the initial collision stage happened, when the Arabian continental plate's edge starts to rise and stretch as it bends around the outer swell just before being drawn into the subduction system by the descending slab, this is the first sign that a collision is about to happen (Al-Muturi and Alasadi, 2008;Handhal and Mahdi, 2016;Al-Kaabi et al., 2023). In the borehole, Kr-1 is represented by 64 m thick (drilling depth 41-105 m) of the Dammam and Euphrates formations. Nummulites sp. first appearance as broken fragments at depth of 105 meters, and its last appearance was at a depth 49.5 meter. Dammam Formation has been divided paleontologically into one member in the study area:

Middle Member
It consists mainly of light grey to white, medium hardness to hard and Nummulitic limestone, alternating with white massive recrystallized sugary and dolomitic limestone followed by whitish grey, hard, fossiliferous dolomitic limestone (Figs. 2A, B and 4).   (Jassim et al., 1984).
The lowermost of the upper part of the Dammam Formation consists of grey, thick-bedded, very hard, recrystallized and dolomitic limestone, whereas the uppermost of the upper Dammam Formation consists of grey massive, fossiliferous, dolomitic limestone, with silicified bands, with a thickness of 55.5 m (drilling depth 49.5-105 m) in well Kr-1. Nummulites sp. the last appearance in Dammam Formation is at depth of 49.5 meters and its first appearance is at a depth of 105 meters.

Upper contact of the Dammam Formation
The upper contact of the Middle Member of the Dammam Formation is unconformable with the Euphrates Formation. It is sharp contact represented by the disappearance of Nummulites gisehensis which is considered an index fossil to the Middle Member and larger Foraminifera depending on the paleontological study and the basal conglomerate (Brecciated limestone or dolomitic limestone lithofacies ( Fig. 2 C) at the bottom of the Euphrates Formation which was studied approximately 5 meters thickness and indicating of Oligocene absence. The thickness of the basal conglomerate is 3.5 meters at a study area above the Nummulitic packstone. Van Bellen et al. (1959) and Al-Hashimi (1973) suggested that the upper contact with overlain of Euphrates Formations may be unconformable.

Microfacies
Vertical alterations in lithology and faunal assemblages define it depending on the skeletal and non-skeletal components. As a result, many facies are seen during the formation's succession. In this investigation, two main microfacies were recorded, these are:

Packstone microfacies
this facies divided into two submicrofacies, these are:

Nummulitic packstone microfacies
This microfacies is characterized by prevailed of Foraminifera (Fig. 9A). The colors of these rocks are whitish grey, pale brown, brownish grey, and medium tough to tough. The recognized fossils are Nummulites gizehensis zeitteli, Nummulites gizehensis (FRSKAL), Nummulites bayhariensis, N. elevata, N. planulatus, N. preforates, N. murchisoni, N. atacicus, N. globulus, N. millecaput, N. beaumonti, Nummulites sp., Linderina bruge, Rotaliids, with a little percent of echinoid spines, Gastropod, and shell fragments. It consists of bioclasts partially replaced by calcite crystals. The bearing microfacies for this assemblage is at a depth of 79.5 -82.5 m, The bioclasts are badly preserved as a result of severe silicified and remain as ghosts and biomoldic. These molds are thought to represent the space left by most of the completely dissolved fossils (Nummulites-shaped molds) (Fig. 4-c). The presence of larger foraminifera, miliolid, rotaliid, ostracods or maybe pelecypods, and quartz grains within the fossiliferous packstone is similar to ramp microfacies RMF16, which is packstone with abundant miliolid that deposited at characterizes restricted environment, which is deposited within the restricted platform (Flugel, 2010). Also, the Nummulites-gastropod association indicates the proximity of relatively restricted settings (Moody, 1987).
The texture is a mixture of these bioclast presented as packstone (Fig.10), the matching of microfacies with ramp microfacies RMF 20 that deposited at restricted environments, it could be shallowed sometimes with abundant algae, in addition to the presence of dolomitic beds that effected with lagoon environment (Flugel, 2010).

Nummulitic Floatstone microfacies
The final microfacies presents a collection of different types of foraminifera (larger and smaller) with other fossils, most of the fossils exceed the 2mm size ( Fig. 11(A, B and C) ). The rocks of these facies are represented by white to whitish gray to gray, fine to medium crystalline, and medium tough to tough. The recognized fossils are Nummulites gizehensis zeitteli, N. gizehensis, N. elevata, N. bayhariansis, N. planulatus, N. murchisoni, Nummulites sp., Linderina chapmani, Li. brugesi, Linderina sp., ostracods, echinoids, and shell fragments. The observed digenetic processes in these microfacies are: recrystallization, dissolution (vugs, intraparticle, intercrystallite, and fracture), and effected in the cementation by calcite. This facies is equivalent to the ramp microfacies RMF 26, which is deposited at open marine. The larger Nummulitic Floatstone at a depth of 63.5 m and a depth of 94.5 m are dominant skeletal grains. Reekman and Friedman (1982) suggested that the larger size of the grain (Nummulites sp. in the current study) would be a perfect indicator of shoal or barrier.

Depositional environment of the Dammam Formation
The depositional environment of Nummulites accumulations is often challenging to interpret because Nummulite's paleoecology is poorly understood (Racey, 2001), details gained from textural and faunal properties helped in the designation of microfacies zones leading to environmental subdivisions for suggested ramp platform. For the Dammam formation: restricted, shoal, and open marine environments.
Nummulies sp. is the index fauna of the Dammam Formation. It is found both as whole tests and broken ones. Nummulites sp. is found within different assemblages of fossils. Nummulites sp. It characterizes shallow waters both inner and outer platform or ramp (Flugel, 2010). Sartorio and Ventarini (1988) suggested a platform-edge environment. Larger foraminifera has been a common constituent of shallow-marine shelf carbonates formed in warm water environments since the Late Paleozoic. Extensive Cretaceous and Cenozoic shelf carbonates were produced by large benthic foraminifera (Hallock 1981).
Depending on the samples of a borehole (Kr-1) in deducing the microfacies. Different authors in various locations have provided descriptions of the depositional environment that resulted in the Middle Member of the Dammam Formation. According to Henson (1950), Porkony (1958), and Al-Hashimi (1973and 1974, it deposited in a neritic sublittoral fore reef shoal zone of shallow warm water temperature. Amer (1980) noted that the Middle Dammam Formation's sediment and accompanying fauna were laid down in a neritic sublittoral fore reef shoal zone of shallow warm water temperature. Buday (1980) came to the conclusion that the Western and Southern Deserts' creation took place predominantly in a shallow neritic environment. In the southwest of Busaiya, it is tropical and subtropical marine with a depth of not more than 20 m (Al-Sharbati and Ma' ala, 1983), according to Al-Mubarak and Amin's (1983) description of the Middle Member as shallow marine tropical-subtropical with a depth of not more than 100 m. It is tropical and subtropical marine to the south of Samawa (Al-Ani and Ma'ala, 1983). According to Jassim et al. (1984), the environment consisted of an inner shelf to shelf depth shallow marine environment that was followed by well-defined warm normal marine conditions that gave rise to vast shoals of enormous nummulites and Mollusca fauna. Miliolids and alveolinids were in large quantities, which suggested that these conditions were more salinized. Al-Hashimi and Amer (1985) hypothesized that the region of the Dammam Formation deposition, which is distinguished by dolomitic limestone and predominate occurrences of miliolids and Peneroplis, was covered by generally confined maritime platform (lagoon) facies. Mahdi and Youkhanna (1996) identified the tropical-subtropical shallow marine depositional environment of the Middle Member of the Dammam Formation as having a depth of not more than 100 m. In this study, the Middle Member of the Dammam Formation is dated to the Middle Eocene. The presence of Nummulites gizehensis and large Nummulites sp., which were deposited in shallow warm tropical water and marine environments, as well as the lack of planktonic foraminifera.
This biozone indicates the Middle Eocene (Late Lutetian) age for the Middle Member of the Dammam Formation.Three main and secondary microfacies were recognized in the Dammam Formation from Borehole (Kr-1), these are: Nummulitic packstone, Dolomitic and Bioclastic Packstone, and Floatstone. Depending on microfacies and biostratigraphy, the Dammam Formation was deposited in different environments including a restricted-marine platform, open interior platform, and platform margin sand shoals or lagoon. In the current study, the Miogypsina sp. (index fossil of the Euphrates Formation).