Depositional history and sequence stratigraphy of central Tethyan from the Upper Triassic Nayband Formation, Central Iran

摘要

To progress in the knowledge of Upper Triassic evolution of the central Tethys realm, an integrated approach which includes new sedimentological and sequence stratigraphy has described in central Iran. The Nayband Formation of Late Triassic age in central Iran is composed of various rocks of siliciclastic and mixed carbonate-siliciclastic deposits and represents an example of storm-dominated shelf deposition. Based on texture and sedimentary structures of the siliciclastic deposits, 11 lithofacies were identified and classified into three categories, i.e., conglomerate (Gms, Gcm), sandstone (Sp, Se, Sm, Sr, Hcs, Scs, Sh) and mud rock (Fm, F). These mainly consist of alternations of sandstone and shale, and constitute the lower and upper units of the studied section. The mixed carbonate-siliciclastic sediments crop out in the middle unit of the section. Based on field observations and petrographic studies, 12 microfacies were recognized which can be grouped into three depositional environments: shoreface (lower, upper), offshore-transition (proximal, distal) and offshore (upper, lower) on a storm-dominated shelf. Seven 3rd-order depositional sequences have been identified in this section based on field observations, facies analysis and sequence stratigraphy studies. The lower and upper boundaries of this succession are type 1 sequence boundaries (SB1); whereas, other boundaries are type 2 sequence boundaries (SB2). Depositional sequences are composed only of TST, MFS and HST, whereby the transgressive sequences (TST) mainly consist of deeper facies, and regressive sequences (HST) consist of shallower facies. Shelf transport was driven by the available accommodation space on the shelf and therefore was a function of eustatic sea-level fluctuations, but local tectonic activity has also controlled the thickness of the deposits.