Biostratigraphy and paleoecology of the calcareous nannofossils in the upper part of Neyzar Formation and the base of Kalat Formation in the Tang-e-Neyzar section, East of the Kopet-Dagh basin

In the Kopet Dagh sedimentary Basin in north east of Iran a full record of Mesozoic and Cenozoic strata is present. In the Kopet Dagh Basin, Neyzar and Kalat formations belongs to the upper Cretaceous strata. Neyzar Formation is located on the Ab-Talkh Formation and is overlain by the Kalat Formation and mainly consists of fine to medium grain sandstone in the lower part and limy shale and marl at the upper part of the section (Darvishzadeh, 1370). The main aim of the present study are to investigate on the calcareous nannofossils at the upper part of Neyzar Formation and the lower part of Kalat Formation in order to determine the exact age of the strata in Tange-Neyzar section. For determining the paleoenvironment, the calcareous nannofossils paleoecology has been investigated.

Fourteen samples from the upper part of Neyzar Formation and 6 samples from the lower part of the Kalat Formation have been studied. Systematic sampling in every mater has been done. For studying calcareous nannofossils regarding biostratigraphy and paleoecology simple smear slides (Perch-Nielsen, 1985) were prepared. For calcareous nannofossil biostratigraphy Sissingh (1977) biozonation emended by Perch-Nielsen (1985) and Burnett (1998) zonation have been applied.

From the base of the section, species such as Reinhardtites Levis, Ceratolithoides aculeus, Uniplanarius sissinghii, Eiffellithus parallelus are present. The first occurrence (FO) of Reinhardtites levis is a marker for the middle part of UC14dTP, the FO of C. aculeus indicates the boundary of UC15aTP and UC15bTP subzones, the FO of U. sissinghii marks the boundary between UC15bTP and UC15cTP, and the FO of E. parallelus shows the boundary between UC15cTP and UC15dTP subzones. From the early parts of the section, E. examine is present that its LO indicates the boundary between UC15 and UC16 biozones and is observed up to the end of the section. According to the mentioned taxa, calcareous nannofossil subzone UC15eTP with early late Campanian age was identified at the studied interval. The presence of taxa with cold water affinity like M. decussata, Prediscosphaera spp., Eiffellithus spp., A. cymbiformis and A. octoradiata and the absence of warm water taxa indicate decrease in temperature and cool greenhouse condition. Micula decussata which marks cool, oligotroph and stressful condition (Pospichal and Wise, 1990; Thibault and Gardin, 2006; Watkins and Self-Trail, 2005) is dominant at the studied interval. Prediscosphaera spp. is also abundant and is a marker of cold (Watkins and Self-Trail, 2005) and oligotroph (Friedrich et al, 2005) surface waters. The highest relative abundance of W. barnesae which is a warm and oligotroph taxa (Thierstein, 1981; Lees, 2002; Sheldon et al, 2010) is recorded at the lower part of the section, Lithraphidites carniolensis with warm and oligotrophic affinity is common is stratified photic zones (Friedrich et al, 2005) that its highest relative abundance is recorded at the lower part of the interval. Eiffellithus spp. with the highest relative abundance of E. turriseiffelii at the lower part of the section is considered as an oligotroph taxa (Friedrich et al, 2005). The highest relative abundance of A. cymbiformis with cold surface water affinity (Thierstein, 1981; Barrera, 1994; Watkins and Self-Trail, 2005) is recorded at the lower part of the section. The highest relative abundance of B. bigelowii is also recorded at the bottom of the section. In the Zagros sedimentary Basin, C. ehrenbergii is considered as cold water taxa, but a decrease in the depth is also important (Razmjooei et al, 2020).

According to the calcareous nannofossil taxa at the upper part of the Neyzar Formation and the lower part of the Kalat Formation, UC15eTP subzone equivalent with the upper part of CC22 biozone of Sissingh emended by Perch-Nielsen has been identified. Because of the week preservation of the calcareous nannofossils, U. trifidus was not recorded. Species such as E. eximius and R. anthophorus that are markers of the uppermost part UC15 and CC22 are recorded up to the top the section. Regarding these data early late Campanian age was determined for the studied interval. The dominance of cold water taxa like M. decussata, Prediscosphaera spp., Eiffellithus spp. and A. cymbiformis and the absence of warm water taxa indicate cool greenhouse condition at this time interval similar to other parts of the world.