Palaeoenvironment and basin architecture of the Lower Jurassic Ab-Haji Formation, east-central Iran

The Lower Jurassic Ab-Haji Formation of the Kalmard, Tabas and Lut blocks, east-central Iran, has been studied using an integrated stratigraphic-sedimentologic approach. The Ab-Haji Formation is mostly composed of greenish sandstones and siltstones and locally contains thin coal seams. Four well exposed sections were measured and studied in order to identify lithofacies and facies associations and to interpret the palaeoenvironment. This formation reaches a thickness up to 350 m in northern Tabas Block but may locally be reduced to few tens of meters or even missing. Field studies have led to the recognition of sixteen lithofacies grouped into five facies associations: fluvial plain, coastal plain, delta front, prodelta and shallow siliciclastic shelf. Reconstruction of the palaeogeography of east-central Iran marks a west–east continental-to-marine gradient. Thickness variations, lateral facies changes and basal unconformity of the siliciclastic rocks of the Ab-Haji Formation on the Kalmard, Tabas and Lut blocks show palaeo-relief on the fault-bounded Yazd Block in the west and the Shotori Swell at the eastern edge of the Tabas Block. The pattern of thickness variations and rapid EW facies changes is best explained by a tectonic model showing large tilted fault blocks in an extensional basin. The obtained results are important for palaeogeographic and palaeoenvironmental reconstructions of the east-central Iran since its sediments record the geodynamic history of this region, as well as an even larger area, during and in the aftermath of the main Cimmerian event, from the beginning of the Early Jurassic.