Investigation of the Early Cretaceous Calcareous Nannofossils in Rumeshtik section (Southeast Sarayan)

IntroductionThe Lut Block is alsom called the "Median Mass of East Iran". This basin is a region characterized by Jurassic-Cretaceous-Tertiary sequence (Stocklin and Nabavi, 1973). In this research, to determine the age of these sequence in the Sarayan region (Rumeshtik section), a detailed study of calcareous nannofossils under optical microscope was performed. This section, based on the structural classification of Iran, is located in the east margin of Lut block. Previous nannofossils studies of Cretaceous deposits in Lut Block were done by Hadavi et al. (2012) in Gazak section (East Birjand). In the present study, for the first time the nannofloras of the Rumeshtik section were discussed and biostratigraphy of these deposits were presented. The thickness of the studied sequence is 690 meters, and consists of limestone and shale. Ninety eight samples at intervals from 4-10 meter were collected. For the nannofossils, smear-slides were prepared using the technique of Bown and Young (1998) and examined under a light microscope at 1000 magnification by both cross-polarized and phase-contrast methods.
DiscussionIn the present study, 33 species belong to 18 genera of calcareous nannofossils in Rumeshtik section were identified. These nannofossils are well to moderately preserved. Some species are abundant in the lowermost part of the studied sections, such as Nannoconus dolomiticus, Nannoc¬onus quadrates and Nannoconus bonetii, while other species are abundant in the uppermost part of the studied sequence such as Nannoconus abundans, Nannoconus circularis, Nannoconus borealis, Lithraphidites bollii and Assipetra terebrodentariu. According to the First occurrence (FO) of marker species, in Rumeshtik section, three calcareous nannofossil biozones (CC3-CC5) were recognized.
Calcicalathina oblongata zone (CC3): This zone explained as the interval from the FO of C. oblongata to FO C. loriei by Sissingh (1977). The age of this zone is Late Valanginian. This is the oldest identified zone in the studied section. Thickness of this biozone is about 370 meter.
Cretarhabdus loriei zone (CC4): This zone explained as the interval from the FO of C. loriei to last occurrence (LO) Speetonia colligata by Sissingh (1977). The age of this zone is Hauterivian. Thickness of this biozone is about 220 meter.
Lithraphidites bollii zone (CC5): This zone explained as the interval from the LO S. colligata to LO C. oblongata by Sissingh (1977). The age of this zone is Late Hauterivian to Barremian. Nannoconus abundans is an index species for Late Hauterivian (Taylor, 1979). Because of the continued presence of C. oblongata, and present of N. abundans, we located lower part of Lithraphidites bollii zone at the end of the studied section. Therefore age of the uppermost part of the studied section is Late Hauterivian. Thickness of this biozone is about 100 meter.
According the calcareous nannofossil biostratigraphy, age of the sequence in the studied region is suggested to be Late Valanginian- Late Hauterivian.
In the current study, in addition to biostratigraphy, investigation of temperature changes was done too. During the Early Cretaceous time, some nannofossils such as Lithraphidites bollii, Nannoconus spp., and Watznaueria barnesae indicate the warm surface water and more related to lower paleolatitudes (Thibault & Gardin, 2007º Street & Bown, 2000). The common occurrence of species belong to the genus Watznaueria, Lithraphidites, and Nannoconus are indicative not only of a warm climate, but also of low latitude setting.
ResultsIn this study, 33 species with relatively well to moderately preservation were identified in the Rumeshtik section. The studied sediments belong to the zones CC3-CC5 of Sissingh (1977). These biozones attribution allow us to access an age of Late Valanginian- Late Hauterivian for the studied section. Index calcareous nannofossil species at the studied sediments indicate that the basin for deposition of these sediments located in low latitude with warm temperature.