Facies analysis and depositional environment of a late Cambrian mixed carbonate-siliciclastic ramp from the Arabian Plate (Zagros Basin, Southwestern Iran)

Abstract

Early Palaeozoic represents a crucial period in Earth history, marked by intervals of life spread alternated with short-lasting crisis in biological activity. The Zagros Basin (SW Iran) could be a key area for the study of this time interval, due to the widespread and well-exposed Cambrian-Ordovician successions; nevertheless, these are still poorly constrained from a sedimentological and stratigraphic point of view. This study, carried out through a high-resolution facies and microfacies analysis of the Zard Kuh section, aims to give a contribution to the palaeogeographic evolution of this area. The studied section (296 m thick) includes the upper Mila C Member and Ilebeyk Formation, referred to the late Cambrian (Furongian) based on acritarch assemblages. The upper Mila C Member (23 m thick) is mainly characterized by medium to thin-bedded bioclastic packstones to mudstones grading upward into a nodular limestone unit. A sharp horizon marks the contact with the overlying Ilebeyk Formation, represented by a shale-dominated interval in the lower part. These mudrocks grade upward into heterolithic lithofacies, generating a coarsening upward sequence with an increase of siltstone and sandstone intercalations in the lower-middle part. The coarser-grained layers display scoured basal surfaces, with common gutter casts, capped by plane parallel lamination and hummocky cross-stratification (HCS). In the middle part of Ilebeyk Formation, siliciclastic deposits are alternated with relatively thick carbonatic intervals, showing similar features with Mila C Member. The upper part of the Ilebeyk Formation is dominated by shales, overlain at the topmost by a 28 m thick unit of micaceous sandstone and siltstone. The sandstone layers include plane-parallel bedding, trough cross-stratification and long-wavelength HCS. Based on the facies and microfacies analysis, the depositional system of the studied section is interpreted as a mixed carbonate-siliciclastic ramp. In the Mila C Member and carbonatic unit of Ilebeyk Formation, small cycles characterized by erosive surfaces and fining upward fossiliferous biomicrites suggest deposition in middle ramp setting, affected by storm events. On the other hand, the nodular limestone intervals at the top of the carbonatic units point to deposition below the SWB under a transgressional regime. The shale to heterolithic parasequences from the Ilbeyk Formation are interpreted as autocyclic fluctuations of the depositional system from lower to upper offshore environment. In the uppermost part, the sandstone-dominated interval is attributed to shoreface setting, marking an abrupt change in the bathymetric conditions after a forced regression. The sedimentological features of the studied area can be correlated with other coeval depositional events at both regional (Arabian plate) and global (Laurentia and Baltica) scales, providing new constraints for the lower Palaeozoic evolution of the northeastern Gondwanan margin.