Microfacies characteristics, sedimentary environments and sequence stratigraphy of Upper Cretaceous deposits in northwest of Nehbandan (Basiran section)

The Basiran stratigraphic section is located about 160 km northwest of Nehbandan.The section was measured in detail at 59 06 30 N and 31 52 50 E. Nehbandan area with respect to fourfold geological subdivision of Iran is part of Central Iran that is located in the eastern flank of Lut Block which first time are studied by Stocklin et al.in 1972. Gorgij (2001) stratigraphically and paleontologically investigate Upper Cretaceous deposits in Mighan and Basiran sections. Upper Cretaceous deposits in in this area consists of 275m conglomerate,alternation of conglomerate-sandstone, sandy limestone -marl and limy marl, marl with intercalation of limestone-sandy limestone thin beds and medium-bedded to massive limestone. Microfacies analysis led to the recognition of 9 microfacies that are related to 5 belts; Coast, tidal flat, lagoon, shoal, shallow open marine and deep open marine environments. Main part of the section are deposited in the open marine environment that consist of marl,marly limestone and limestone. The doals of this study are: (1) describing and determining main carbonate and siliciclastic microfacies of late Cretaceous deposits (2) interpreting and providing depositional model for reconstruction of its paleoenvironmental setting based on microfacies characteristics (3) dividing the section based on lithostratigraphic principles and (4) recognizing a sequence stratigraphic model of this successions based on the vertical variation of facies,stratal key beds and stratal packing pattern. The Basiran section as a complete stratigraphic section was measured and described. Up to 68 samples (indicated by KB1 to KB68) were collected and 170 thin sections are prepared. Based on field observations, sedimentological characteristics, parasequence stacking patterns, sequence boundary types and other key stratigraphical surfaces are identified and were obtained. Scheme of Dunham (1962) and Embry and Klovan (1971) for carbonate rocks and for samples descriptions from Flugel (2004) and Wilson (1974) methods were used. Discussion of Results and The Basiran section predominantly composed of limestone - marl in the middle and upper part and conglomerate – sandstone in the lower part. The most prominent component of carbonate sediments are larger foraminifera such as Orbitoides, Lepidorbitiodes, Siderolites, Omphalocyclus and some of pelagic foraminiferas. Systematic determination of those benthic foraminiferas suggests Late Campanian-Maastrichtian age for Basiran section. Microfacies analysis led to the Recognition of 9 microfacies that are related to 5 belts; Coast, tidal flat, lagoon, shoal, shallow open marine and deep open marine environments. With respect to gradual shallow water facies changes, absence of barrier reef, sliding and slumping structure, cortoids, oncoids, pisoids and aggregate grains, that are abundant in rimmed shelves, several evidences shows that above mentioned carbonate are deposited in the ramp environment (Burchette & Wright 1992). Furthermore, based on high diversity and abundance of benthose foraminiferas this environment can be considered as foram dominated carbonate ramp system (Boxton & Pedley 1989). Based on field observation, microfacies analysis and sequence stratigraphic studies, fourth third order depositional sequence (DS) in the Basiran section are identified. These depositional sequences are separated by type1 sequence boundary (lower boundary of first depositional sequence and upper boundary of fourth depositional sequence) and type 2 sequence boundary (lower boundary of 2 nd and 3 rd depositional sequence). First depositional sequence consists of LST, TST and HST and contains tidal flat, lagoon, shoal, shallow open marine and deep open marine microfacies. Lower boundary is of type 1 sequence boundary (SB1). 2 nd and 3 rd depositional sequences consist of TST and HST. Therefore contains shallow open marine and shoal microfacies and have been separated by sequence boundary type 2 (SB2). Fourth depositional sequence contains of TST and HST. Upper boundary is of type 1 sequence boundary (SB1) and terminates to with clear unconformities to Paleocene conglomerate. It is seems that both local tectonic activity and global eustasy may have controlled the third and forth order sequences and cyclicities of the Upper Cretaceous sediments in Basiran section. The SB1 in the top of the fourth depositional sequence, which shows subaerially exposed the carbonate platform, possibly related to the widespread glacio-eustasy sea-level drop could be linked to the cooling event in the Cretaceous-Tertiary boundary(KTB).