Late Cretaceous ostracods in Shushud section (North of Birjand)

The flysch basin of eastern Iran is in the interval between two faults, Harirud fault in the east and Nehbandan fault in the west, consisting of thick deposits of flysch- like sediments (Aghanabaty, 2004). The study area locates in the flysch Basin of eastern Iran, on the southwest of Shushud village and in the north of  Birjand (South Khorasan Province). This research carried out with the aim of identifying and introducing of ostracods and comparing the proposed biozones based on them with the provided biozones based on calcareous nannoplanktons in this region (Jalili et al., 2011). The studied section that called “Shushud section” due to its place in the southwest of Shoshud village, locates 35 kilometers far from the north of Birjand. The thick of the studied sequence is 480 m and has a lower fault boundary with a gray limestone unit. The upper boundary is covered by limestone deposits conformably.

The sampling of this section was carried out with a focus on shale and marl beds. The samples size is about 500 gr. Samples are prepared in the following way: At first, they were placed in water for 24 hours that has 10 ccs of 15 percent hydrogen peroxide for every liter of water. Subsequently, they were washed with 30 and 60 mesh sieves and then the remaining deposits were discharged into separate containers and were dried. In the next step, the ostracods in each sediment sample were transferred to a slide by a 00 size brush and were studied by a reflected microscope.

Ostracods are one of the most useful calcareous crustaceans that contain shells composed of calcareous and chitinous valves (Hadavi, 1998). Ostracods are able to live in all ecosystems and marine ostracods can live from low to high depths (about 2800 m). This microfossil group is found abundantly in marl, shale, calcareous marl, sand, silt and clay (Cohen, 2007). The Grosdidier (1973) report is one of the relatively comprehensive reports about ostracods of Iran that describes the ostracods in the Persian coastal region of Iran. The study of Shushud deposits has led to introduce 36 species belonging to 16 genera of ostracods. In this study, based on the occurrence of the identified species, three interval biozones have been recognized which are as follows: Limburgina Formosa Zone: This biozone, which covers the first 140 meters of Shushud section is defined with the first appearance of the species Limburgina Formosa to the first appearance of the species Hermanites sagittal.  Hermanites sagittal Zone: The range of this biozone is characterized by the first appearance of the species Hermanites sagittal to the first appearance of the species Schuleridea sp.1 and including the thickness of 110 m of the studied section. Schuleridea sp.1 Zone: The first appearance of the Schuleridea sp.1 species indicates the beginning of this biozone and it is the last identified biozone in the Shushud section which continues up to the end of the section and its thickness is 230 m. In the current research, the ostracods of the formation compared with the calcareous nannofossils in the Shushud section in similar horizons in terms of age (Jalili et al., 2014). After studying the nanofossils of the section, five CC20-CC26 nanofossil biozones of the Sissingh (1977) zoning were identified and the results of the study of ostracods were compared with the nannofossils. The Limburgina formosa biozone is almost equivalent to the Cratolithoides aculeus Zone (CC20) and has an age equivalent to the end of early Campanian. The Hermanites sagittal biozone is almost equivalent to the Quadrum sissinghii Zone (CC21) and is a part of the Quadrum trifidum Zone (CC22) and is equivalent to the end of late Campanian. It is worth noting that the Tranolithus phacelosus Zone (CC23) and the Reinhardtites levis Zone (CC24) are defined in the age range of the Late Maastrichtian, but due to the studied sandstone beds did not have any nannofossil, the separation of these biozones was not possible. Therefore, Hermanites sagittal biozone is about equivalent to a part of the Quadrum trifidum Zone (CC22), the Arkhangelskiella cymbiformis Zone (CC25) and Nephrolithus frequens Zone (CC26) and the age range of this biozone is the end of  Late Maastrichtian.

The introduction of 16 genera and 36 species of ostracods was identified. The identified ostracods in this section have low diversity and variety because of the unfavorable environmental conditions of the basin, as well as the effect of diagenesis in the studied section which is evident due to the hardness of most of shale and Marl deposits. Based on the detected ostracods, 3 biozones have been identified for the studied section which are as follows: Hermanites sagittal Zone, Limburgina Formosa Zone and Schuleridea sp.1 Zone.According to the introduced biozones and their correlation with the results of the study of calcareous nannofossils, the age range of the Shushud section in the north of Birjand is suggested from the end of Campanian to Late Maastrichtian.