An overview of the stratigraphy and facies development of the Jurassic System on the Tabas Block, east-central Iran

Abstract

Abstract The Tabas Block of east-central Iran shows very thick and well-exposed Upper Triassic–Jurassic sequences, which are crucial for the understanding of the Mesozoic evolution of the Iran Plate. The succession is subdivided into major tectonostratigraphic units based on widespread unconformities related to the Cimmerian tectonic events. As elsewhere in Iran, there is a dramatic change from Middle Triassic platform carbonates (Shotori Formation) to the siliciclastic rocks of the Shemshak Group (Norian–Bajocian), reflecting the onset of Eo-Cimmerian deformation in northern Iran. Following the marine sedimentation of the Norian–Rhaetian Nayband Formation, the change to non-marine, coal-bearing siliciclastic rocks (Ab-e-Haji Formation) around the Triassic–Jurassic boundary is related to the main uplift phase of the Cimmerian orogeny. Condensed limestones of the Toarcian–Aalenian Badamu Formation indicate widespread transgression, followed by rapid lateral facies and thickness variations in the succeeding Lower Bajocian Hojedk Formation. This tectonic instability culminated in the middle Bajocian compressional–extensional Mid-Cimmerian event. The resulting Mid-Cimmerian unconformity separates the Shemshak Group from the Upper Bajocian–Upper Jurassic Magu (or Bidou) Group. The succeeding Late Bajocian–Bathonian onlap of the Parvadeh and Baghamshah formations (Baghamshah Subgroup) was caused by increased subsidence of the Tabas Block rather than a eustatic sea-level rise, followed by the development of a large-scale platform–basin carbonate system (Callovian–Kimmeridgian Esfandiar Subgroup). Block faulting starting in the Kimmeridgian (Late Cimmerian event) resulted in the destruction of the carbonate system, which was covered by Kimmeridgian–Tithonian limestone conglomerates, red beds and evaporites (Garedu Subgroup or Ravar Formation). Virtually the same pattern of relative sea-level change, facies development and succession of geodynamic events is recorded from the Late Triassic–Jurassic of northern Iran (Alborz Mountains), suggesting that the Iran Plate behaved as a single structural unit at that time.