Ostracoda of Chaman-Bid Formation in Ghoroneh section (Binalud Mountains)

The Binaloud Mountains, with an almost eastern-western trend, is extended from the Jajarm area in the Northern Khorasan to the Torbat-e-Jam region. In the Binalud zone, Jurassic succession in many places covered Paleozoic rocks and metamorphic assemblages, known as Mashhad Phyllite. Chaman-Bid Formation is one of the most famous Jurassic deposits in the northeastern. The studied sequence in the Ghroneh section is located at 43 kilometers north of Neyshabur and northeast of Ghroneh village (580 45' 36'', 360 45' 05''), respectively contains siliciclastic rocks of Kashafrud Formation, shale, marl, limestone, and sandstone beds of Chaman-Bid Formation, and thick to massif limestones of the Mozduran Formation. Most paleontological studies on the Chaman-Bid Formation are done based on ammonites (Majidifard, 2003, 2008; Raoufian et al., 2011, 2014; Seyed- Emami et al., 2013, 2018), palynomorphs (Mafi et al., 2014a,b), and more recently nannofossils by Amani et al. (2016). Based on calcareous nannofossils, the Early Bajocian–Early Berriasian age was suggested for the mentioned deposits in the Ghroneh section. In the present study, for the first time, ostracods of the Chaman-Bid Formation have been investigated. The present study introduces the Chaman-Bid Formation ostracods and compares this fauna within the calcareous nannofossils biozones.

  The thickness of the Chaman-Bid Formation in the studied section is 715 meters and consists of shale, marl, and limestone, interbedded with some sandstone beds. Sixty-four samples of the Chaman-Bid Formation with intervals of 10 to 15 meters were collected. 300 grams of each sample were soaked in water and hydrogen peroxide for 24 hours and washed in 30 and 60 mesh sieves. The residue was dried, and all ostracods in each sample were separated. The ostracods were studied with a Zeiss ZH55 microscope at 40x magnification and photographed in scanning electron microscopy (SEM) at the central laboratory of the Ferdowsi University of Mashhad.

Ostracoda is one of the most valuable crustaceans in paleontological and paleoecological studies. The remains of these small and microscopic crustaceans are widely distributed through Phanerozoic, and individuals belonging to this subgroup are characterized by their particular morphological and physical characteristics (Van Morkhoven, 1963). In the present study, 45 genera and 59 species of Ostracoda have been identified in the Chaman-Bid Formation in the studied section. Previous studies of Ostracoda have been compared with ammonite zones. The current study is important since there are no reports on Iran's Jurassic ostracods. Therefore, introducing ostracods and determining their age range is a significant objective of this study; however, their preservation is relatively poor. The study of the Chaman-Bid Formation ostracods in the Ghroneh section led to identifying of 59 species belonging to 45 genera. For relative age dating in the present study, calcareous nannofossils have been used and identified ostracods correlated with them. These assemblages have been compared with 12 previously reported nannofossil zones from the Ghroneh section. Nannofossil zones (Amani et al., 2016) belong to NJ9 to Early CC1 and confirm the age of Early Bajocian to Berriasian for the discussed ostracod assemblages.