Lurestan Basin; Shahbazan Formation; larger benthic foraminifera; paleoenvironment; ramp

Introduction Zagros foreland basin in Iran on the basis of sedimentary history is divided into five provinces including Lurestan, Fars, Zagros, Dezful embayment and Izeh Zone (Farzipour-Saein et al. 2009). In northeast Lurestan, the exposure of Mesozoic-Cenozoic stratigraphic column consists of about 4-5 km of pre-orogenic strata and about 6 km of synorogenic deposits. The Mesozoic pre-orogenic succession is mainly composed of passive margin carbonate units. The overlying synorogenic deposits include, from bottom to top, the Amiran- Kashkan detrital sequence, the Shahbazan-Asmari shallow-marine platforms, the Gachsaran evaporitic Formation, and the Agha Jari and Bakhtyari detrital formations.
Materials and Methods In order to determine the age and paleoenvironment of the Shahbazan Formation, one surface section measured and sampled at the Kuh e Chenareh anticline. Chenareh stratigraphic section is located near the last tunnel of the Khorramabad – Andimeshk road about 5 km east of Zal city, at 32°47′N, 48°18′E.
Results We have identified 20 genera and 26 species of larger foraminifera in the studied sediments of the Shahbazan Formation. In the study area, four foraminiferal assemblages identified ascribed to the Shahbazan Formation span time with the Lutetian–Bartonian (SBZ13-SBZ18?) age according to Serra-Kiel et al. (1998).
The following six foraminiferal assemblages are arranged on a palaeobathymetric profile, ranging from the shallow-marine inner ramp to the deep-marine basin including: (1) soritid-miliolid assemblage, (2) alveolinid assemblage, (3) nummulitid-alveolinid assemblage, (4) nummulitid assemblage, (5) nummulitid-discocyclinid assemblage and (6) planktic foraminifera assemblage. The transitions between the individual foraminiferal assemblages are gradual and occasionally interfere with each other.
Discussion The modal analyses of the bioclastic components and composition of foraminifera assemblages allowed six facies (assemblages) to be recognised. These facies can be interpreted as having accumulated in open marine, fore-bank, bank and lagoonal environments (back bank setting). The nummulitic packestone/flotestone with abundant B-forms and minor A-forms and the nummulitic packestone with abundant A-forms and minor B-forms dominates in the lower and upper part of the bank, respectively. The Discocyclinid-Nummulitid facies is observed at the basal parts of the section. The presence of elongate Discocyclina in this facies may reflect an increases in water depth, although they may also be allochthonous (Beavington-Penney and Racey, 2005). The nummulites facies overlain by nummulitid-alveolinid packestone facies and this facies also overlain by alveolinid facies. Anketell and Mriheel (2000) in their study of the Jdeir Formation infer that the orbitolites-alveolina facies accumulated in a back-bank lagoonal setting. Wackeston-packestone facies with soritids and miliolids accumulated in the uppermost parts of this formation. The gradual variations in facies from shallow to deep photic zone deposits in this succession is more compatible with a suggested ramp model by Anketell and Mriheel (2000) and Höntzsch (2011). This model (in the northern areas of the Lurestan basin), in comparison with their counterparts in the most southern parts of the Arabic platform (Seeb Formation in the north of Oman and Dammam Formation in Saudi) (Boukhary et al., 2006), shows that there is a broad correlation between these sedimentary environments.