Facies and sedimentary environment of the Lower Cretaceous transgressive siliciclastic and carbonate succession in the southwestern margin of central Iran, northeast of Isfahan

Abstract

Introduction In this study, successions including red beds siliciclastic and the carbonate rocks of Lower Cretaceous sequence is studied at Kuh-e-Bejareh in the south western margin of Central East Iranian Microcontinent. The Lower Cretaceous red beds represents the first deposits after the Late Cimmerian phase and unconformably overlies the dark green sandstones and shales of Nayband Formation and overlain conformably by cliff-forming carbonate deposits of the Aptian Orbitolina Limestone. The Jurassic is gap in between and sediments are missing in the study area. Mazroei-Sebdani (2016) and Mazroei-Sebdani et al. (2018) studied in detail the provenance and diagenetic events of similar succession. In this research, recognition of clastic lithofacies and carbonate microfacies would result to the interpretation of depositional environments and to reconstruction of palaeoenvironment and palaeogeography of the south western margin of Central East Iranian Microcontinent during Early Cretaceous (Barremian?-Aptian). Material & Methods The studied outcrop section at Kuh-e-Bejareh represent the Lower Cretaceous siliciclastic red beds (base coordinate: N 33⁰ 05′ 33.7″, E 51⁰ 57′ 34.9″) in western margin of Central East Iranian Microcontinent, 47 km northeast of Isfahan, central Iran. The interval were logged bed-by-bed using Jacob Staff. The succession have 138 m thickness in that nine lithostratigraphic sub-units were defined. Lithofacies types were determined using the modified lithofacies classifications of Miall (2006). Palaeocurrent indicators were measured at 74 locations. Stereonet version 9.8.3 were used to construct rose diagrams. Total of 75 samples were collected from the outcrop section in the field. The petrographic classification for sandstone were based on Folk (1974) and carbonates rocks is based on Dunham limestone classification (Dunham, 1962). Wilson (1975) and Flügel (2010) facies belts and sedimentary models were used to interpret the carbonate microfacies. Discussion of Results & Conclusions Lithostratigraphic sub-units 1-6 are grouped into the K1 unit (92 meters) and 7-9 (46 meters) into lower part of the K2 unit. The K1 unit composed of red clast-supported conglomerate, fine-grained cross-bedded sandstone and siltstones. The K2 unit composed of sandy limestone and fossiliferous (Orbitolina) limestone with verity of rocks textures. Lithofacies analysis led to identification of four coarse-grained (Gcm, Gp, Gh, Gt), five medium-grained (St, Sm, Sh, Sp, Sr), two fine-grained (Fl, Fm) and an intermediate facies (Sh and Fl). Sandstones of the Lower Cretaceous siliciclastic red beds plot into quartz-rich sublitharenite field of a QFL ternary plot of Folk (1974). Recalculating the rock fragments to 100% and plotting them on rock fragment triangles indicates that these sandstones are sub-chert arenites and sub-phyllarenites. Identified lithofacies, geometry of beds, horizontal and vertical stacking patterns, stratal surfaces, palaeocurrents as well as the fossil and trace fossil contents of the strata led to identification of three siliciclastic and one mixed siliciclastic-carbonate facies associations represents a terrestrial to shallow-marine succession from braided and meandring fluvial, supratidal to tidal flat environments. The Lower Cretaceous clastic sequence represents palaeocurrent direction from the south to the north. Based on the measured palaeocurrent, the Sanandaj–Sirjan Zone have been considered as a main source for the siliciclastic rocks. The investigation of carbonate thin sections led to the identification of five microfacies with a verity of mudstone to garinstone texture. The identified microfacies comprises sandy mudstone, peloid grainstone, bio-/peloid wack-, pack-, and grainstone, and intraclastic, echinoid, Orbitolina garinstone. The identified carbonate microfacies were deposited in the mixed siliciclastic-carbonate tidal flat, restricted and open lagoons toward bioclastic shoal of the inner part of likely an epiric carbonate platform. This carbonate sequence has been deposited in the near shore area of such a shallow-water carbonate platform that covered some parts of central Iranian Block, as northern margin of Neotethys Ocean, during Early Cretaceous.

Keywords

• Silisiclastic and Carbonate Facies
• Sedimentary Environment
• Early Cretaceous
• Central Iran
• Northeast of Isfahan