Biostratigraphic correlation and foraminiferal associations of the Oligocene successions in Fars Basin

The Oligocene-Miocene shallow marine limestone, Asmari Formation, from the Zagros Basin, SW Iran, constitutes one of the main hydrocarbon reservoirs in the world. This succession shows a variety of facies patterns and depositional architectures. Also these strata contain a rich fauna of planktonic and benthic foraminifera. Larger benthic foraminifera are considered to be good indicators of shallow marine environments. Distribution of this group in addition to other biogenic components (mollusk, echinoid, bryozoan, coral, corralinacean, brachiopod, worm tube, etc.) and sedimentary structure, along the stratigraphic sequence of the Asmari Formation is used in this research as a tool for introducing the biostratigraphic correlation and foraminiferal associations. In order to study the foraminiferal associations and biostratigraphic correlation of the Asmari Formation in the Fars Basin, four stratigraphic sections were selected, include: Shool, Kaftarak, Darengoon, and Noorabad sections. This study is based on more than 270 thin sections derived from surface outcrops.
According to the distribution of planktonic and benthic foraminifera, three assemblage zones were recognized I-Nummulites vascus-Nummulites fichteli, II- Lepidocyclina-Operculina-Ditrupa and, III-Archaias asmaricus/hensoni-Miogypsinoides complanatus. The oldest recognized biozone (biozone no. I, which is determined according to the presence and absence of Nummulites spp.) is reported from Shool and Kaftarak sections. Biozone no.II is recognised at Noorabad Section and represents undifferentiated Rupelin-Chattian time. Biozone no.III which represents Chattian time is reported from Noorabad and Darengoon sections. The Asmari Formation is time equivalent to Oligocene-Miocene in different parts of the Zagros Basin. The recognised biozones in the Fars Basin confirm the age of Rupelian onto Chattian for the Asmari Formation, in the study area.
Three foraminiferal associations are recognized in the investigated sections according to the test shape of the larger benthic foraminifera and sedimentary texture. The identified foraminiferal associations represent the middle and inner parts of a homoclinal ramp. The association no. 1 consists of planktonic foraminifera together with thin and thick tests of perforate larger benthic foraminifera. Perforate tests are apparent as nummulitid (Nummulites, Operculina and Heterostrgina) and lepidocyclinid (Eulepidina and Nephrolepidina). The association of planktonic foraminifera and thin and flat test of nummulitid and lepodocyclinid representing the deeper part (about 150 m) of the Asmari platform in the study area. Increasing in the thickness ratio to the diameter of the test is the sign of proximal middle ramp. This foraminiferal association is presented thorough the Rupelian-Chattian time in the Noorabad Section and indicated salinity value of 34 to 40 psu. The shallowest depth (40 m) of the proximal middle ramp is coinciding with the presence of thick rotallid tests. The association no. 2 is composed of the mix of perforate and imperforate tests of larger foraminifera (Operculina, Heterostegina, Eulepidina, Nephrolepidina, Neorotalia, Sphaerogypsina, Planorbulina, Archaias, Peneroplis, Austrotrillina, Borelis, Triloculina, Praearchaias, Sivasina, and milliolid). This is formed in the semi-restricted lagoon in the inner ramp and is reported from the Noorabad Section (Rupelian-Chattian), Kaftarak Section (Rupelian) and Darengoon Section (Chattian). The presence of the Borelis genus confirms the salinity value of 40-50 psu for this association. High diversity imperforate tests of larger foraminifera are the main maker of the third association. This indicates the hypersaline-restricted lagoon and salinity value higher than 50 psu. The association no. 3 is the sign of shallowest depth of sedimentary basin. The dominance of Peneroplis, Austrotrillina tests is the indicator of depth lower than 30m. This is formed throughout Late Rupelian and Chattian in the Shool, Noorabad, and Darengoon sections. The end Chattian sea level falling controls the formation of the association no. 3 in these sections. This is resulted in the deposition of the clastic and evaporate sediments of the Razak and Gachsaran formations over the Asmari Formation.