Biostratigraphy and paleoecology of the Asmari Formation at northwest of Deris Village, west of Fars Province

Introduction The Asmari Formation at Deris section has been studied in order to biostratigraphy and paleoecology observations. This section with the thickness of 460 m is located at northwest of Deris village, in interior Fars zone of the Zagros Basin with coordinates of E: 51° 32' 26'', N: 29° 41' 59''. The Asmari Formation deposits in this section consists of interbedded gray and cream to gray thin, medium, thick and massive limestone, with slightly dolomitic nodular and marl. Based on field studies and according to the thickness of the layers, color and lithology, 5 lithostratigraphic units have been nominated for the mentioned section. Method and Materials Using regional geological maps, the study section was identified and measured. Field observation focused on detailed description of bed-by-bed thickness, color, lithology and biotic components systematically. 450 samples from the Asmari Formation were collected. Field works were completed with the study of more than 270 thin-sections for identification of skeletal debris, fabric characteristics and microfacies interpretation. Biostratigraphy results have been determined and concluded based on Ehrenberg et al., (2007) and recent biozones introduced by Laursen et al., (2009) and van Buchem et al., (2010). Result and Discussion Biostratigraphy study based on the distribution of benthic foraminifera, led to identification of 19 genera and 19 species, accordingly two assemblage zones have been recognized and Oligocene (Rupelian-Chattian) age was introduced for the Asmari Formation in the Deris section. The suggested assemblage zones are: 1- Nummulites vascus - Nummulites fichteli Assemblage Zone (Rupelian) This collection has been deployed from the base of the section to the thickness of 66 m and consists of Nummulites vascus, Nummulites fichteli, Nummulites sp., Operculina sp., Amphistegina sp., Heterostegina sp., Elphidium sp.1, Elphidium sp.14, Dendritina rangi, Planorbulina sp., Neorotalia viennoti, Haplophragmium slingeri, Nephrolepidina tournoueri, Nephrolepidina sp., Sphaerogypsina sp., Eulepidina sp., Subterranophyllum thomasi, corralinacean algae, miliolids and textularids. Based on strontium isotope study by Ehrenberg et al. (2007), because of the presence of Nummulites spp. this assemblage indicates the Rupelian stage. On the other part, it is equivalent to the assemblage zone number two of Laursen et al., (2009) (Nummulites vascus - Nummulites fichteli Assemblage Zone) and van Buchem et al., (2010). The age is Rupelian stage. 2- Archaias asmaricus - Archaias hensoni - Miogypsinoides complanatus Assemblage Zone (Chattian) This association is introduced for the thickness of 66-460 m and consists of Operculina sp., Amphistegina sp., Heterostegina sp., Elphidium sp.1, Elphidium sp.14, Dendritina rangi, Planorbulina sp., Neorotalia viennoti, Haplophragmium slingeri, Sphaerogypsina sp., Eulepidina sp., Nephrolepidina tournoueri, Nephrolepidina sp., Reussella sp., Valvulinid sp., Borelis cf. pygmaea, Triloculina trigonula, Triloculina cf. tricarinata, Pyrgo sp., Archaias asmaricus, Archaias operculiniformis, Archaias sp., Peneroplis thomasi, Peneroplis evolutus, Peneroplis sp., Austrotrillina howchini, Austrotrillina sp., Meandropsina iranica, Discorbis sp., Ditrupa sp., Spirolina sp., Spiroloculina sp., Bigenerina sp., Miogypsinoides cf. complanatus, Subterranophyllum thomasi, corralinacean algae, miliolids and textularids. It is equivalent to the assemblage number four of Laursen et al., (2009) (Archaias asmaricus - Archaias hensoni - Miogypsinoides complanatus Assemblage Zone) and van Buchem et al., (2010). So, it is considered as Chattian in age. In this study, the Asmari Formation biozones of Deris section also have been correlated with two other sections in the Internal Fars zone and Dezful Embayment zone of Zagros Basin. This comparison confirms that the foreland basin of Zagros becomes deeper in SE to NW protraction. Some ecological factors such as sedimentary basin substrate, depth and hydrostatic pressure, water energy, temperature, turbidity and light influence the distribution and frequency of organisms specially benthic foraminifera (Hottinger, 1983; Romero et al., 2002; Bassi et al., 2007; Flugel, 2010). By study of skeletal elements and accumulation of carbonate grains, it was found that the Oligocene Asmari Formation carbonates have been deposited in the waters of the tropics. Moreover, detailed analysis of microfacies and paleoecology shows that the association of skeletal debris is heterozoan (foralgal, rodalgal and foramol) in this section.