Calcareous Nannofossil Biostratigraphy and Ostracoda Paleoecology of Hartha Formation from Balad (1) well, Central Iraq

Seventeen samples of Hartha Formation in Balad (1) well, central Iraq, are studied on the basis of stratigraphic ranges of the recorded calcareous nannofossils for twenty species belonging to twelve genera. The studied section reveals three biozones arranged from oldest to youngest as follows; (1) Calculites ovalis Interval Biozone (CC19), (2) Ceratolithoides aculeus Interval Biozone (CC20), (3) Quadrum sissinghii Interval Biozone (CC21). These Biozones are correlated with other calcareous nannofossils biozones from both local and regional sections, leading to conclude the age of the Middle Campanian. Rerecorded eighteen ostracode species that belong to eleven genera are identified, all of which were previously recorded from Iraq and adjacent regions. The occurrence of these species leads to conclude a continental shelf environment, while they are typical of inner shelf-outer shelf depth.


Introduction
The Hartha Formation was first described By Rabanit, who described the formation at the type section in well Zubair-3 well, between depths 1704.3-1833.2 m. and about 129 m. in thickness. It consists of organic detrital glauconitic limestone with grey marl and green shale, where the limestone is strongly dolomitized in some places [1].
The Hartha Formation was deposited in the Upper Cretaceous sedimentary cycle within a shallow marine to lagoon environment. The sediments were spread in central and southern Iraq [2,3]. Hartha Formation (Late Campanian-Maastrichtian) is one of the important formations deposited during the upper Cretaceous. The formation includes important carbonate reservoirs that are producible in central and southern Iraq. It acquired its importance because of the presence of quantities of hydrocarbon. The study area is located in Balad Oilfield within Salah Al-Din governorate in central Iraq, 9 km south of Balad and about 80 km northwest of Baghdad ( Figure 1). The location, identified with the UTM, has the coordinates of N: 3755 165.5 M and E: 418 349.5 M. Tectonically, the formation was first described, in its type section, in the well of Zubair (3) in the Zubair field, southern Iraq. It consists of fossiliferous limestone, marl, and shale and composed of organic detritaland glauconitic limestones with grey and green shaly interbeds [2]. In a study of sediments and reservoir of the Hartha and Saadi formations in the Qayarah field in northern Iraq, the formation was divided into three major microfacies and deposits in the inner, middle, and outer shelf environments [1]. The studied section is located in central Iraq (Figure 1), lying at the Unstable Shelf of the Iraqi tectonic units [4]. The studied section was obtained from Balad well No.1, Central Iraq, being between 1672 and 2115 m in depth and about 443 m in thickness.

Materials and methods
Seventeen samples of limestone were selected for studying the calcareous nannofossils, using the thin section which was examined under transmitted light microscope. The calcareous nannofossils were extracted by using the method (H) described previously [5]. It is an extraction method for microfossils that can be properly examined after being extracted from the rocks. Each paleontologist tends to have favorite methods for these purposes. The sample preparation involved decanting and preparation of smear slides of calcareous nannofossils using a small amount of the disaggregated sample. The sample was placed in distilled water and a drop of cellosize was added to act as a dispersant. The cover slip was left to dry on a warm hotplate. To make permanent mounts, the slide and residue were allowed to dry at a low temperature away from possible sources of contamination. A drop of mounting medium (e.g. Canada balsam) was placed on a clean cover slip and the residue was removed. The sample was allowed to dry before examining it with transmitted light. The Ostracoda were studied by reflected light microscope.

-Systematic Paleontology
The aim of systematic classification is to provide images of the notable calcareous nannofossils from the Hartha Formation in Ba(1) well, Central Iraq, and to describe the twenty species. The material and images are stored in the Department of Geology, Science College, University of Mosul ( Figure 2).

II-Ostracoda 1 -Systematic Paleontology
The aim of systematic classification is to provide images of the notable Ostracoda from the Hartha Formation in Ba (1) well, Central Iraq, and to describe the eighteen species. The material and images are stored in the Department of Geology, Science College, University of Mosul ( Figures. 5 and 6).

-The ostracoda assemblages as environmental indicators
a-The occurrence of the assemblage Cytherella and Pontocyprella indicates outer shelf environment [25]. b-The association of Cytherella, Bairdia, and Paracypris indicates shelf environment [26]. c-The presence of Cytherella, Bairdia, and Pontocyprella indicates middle shelf environment [27]. d-The assemblage of Cytherella, Bairdia, Hermanites, and other ornamented species indicates outer shelf environment [17,28]. According to Singh [29], the assemblage of Cytherella, Bairdia, Hermanites, and Uroleberis with ornamented species is a characteristic to inner shelf taxa. e-The association of Bairdia and Parakrithe indicates shallow marine inner shelf to outer shelf depth [17] and inner shelf-outershelf [10]. f-The occurrence of the assemblage of Krithe, Parakrithe, and Pontocyprella with ornamented species is a characteristic to outer shelf environment [10]. g-According to Morsi and Wendler [10], the assemblage of Cytherella, Bairdia, Paracypris, and Peloriops is characteristic to open marine shelf texa. Ahmad et al. [30] stated that when the ecology at the species level is unknown or overlapped, a certain limitation is placed upon the interpretation of paleoecology based on the generally accepted habitat of certain genera. The diversity of ostracods is an important indicator for determining the paleoecology in marine settings. Higher ostracod diversities are more common in the shelf environment [12]. h-The occurrence of the smooth species of Cytherella, Bairdia, Bairdoppilata, Bythocypris, Pontocyprella, and Parakrithe generally indicate deep marine environment and outer shelf to upper slop. The association with the ornamented and eye-tubercle species of Brachycythere , Hermanites, and Peloriops generally refers to middle to outer shelf environment; therefor, the assemblages of ostracoda from Hartha Formation were found to be characteristic for continental shelf environment and they are typical for inner shelf-outer shelf environment.

Conclusions
On the basis of stratigraphic ranges of the recorded calcareous nannofossils, three biozones were determined in Hartha Formation. They are arranged from the oldest to the youngest as follows: 1-Calculites ovalis Interval Biozone (CC19) Part, 2-Ceratolithoides aculeus Interval Biozone (CC20), 3-Quadrum cf. sissinghii Interval Biozone (CC21) Part. These Biozones are correlated with other calcareous nannofossil biozones from both local and regional sections, leading to conclude the age of Middle Campanian. Also, eighteen ostracode species that belong to eleven genera were identified, all of them previously recorded from Iraq and adjacent regions, leading to conclude a continental shelf environment. It is also concluded that they are typical of inner shelf-outer shelf environment.