Platform carbonates of the Upper Jurassic Mozduran formation in the Kopet Dagh Basin, NE Iran—facies, palaeoenvironments and sequences

Abstract

The Mozduran Formation, a thick carbonate succession (over 1200 m) of the Upper Jurassic (Callovian-Kimmeridgian) in northeast Iran, has been studied to determine its palaeoenvironments and sequences. Detailed petrographic investigations have led to the recognition of several microfacies and microfacies groups which constitute four palaeoenvironmental associations: tidal flat/beach, lagoonal, platform-margin and open-marine. Most of the Mozduran Formation sediments were deposited on a rimmed carbonate platform adjacent to a deeper marine environment.The Mozduran carbonate succession consists of five, thick shallowing-upward sequences (“3rd-order” cycles). No major hiatus has been recognized between these cycles. Therefore, the contacts are considered subtidal conformities (drowning unconformities). These “3rd-order” cycles can be correlated with those recognized in Upper Jurassic strata in other parts of the world; thus, they are considered largely eustatic (mainly tectono-eustatic) in origin. Numerous small-scale shallowing-upward cycles (parasequences) are contained within the major shallowing-upward cycles. Both subtidal and peritidal parasequences, showing an abrupt deepening of microfacies, have been recognized. A combination of episodic subsidence and autocyclic processes could have been responsible for the formation of these small-scale cycles.Rapid carbonate accumulation reflected in rare first-generation marine cements, including meteoric cements, resulted in grain-to-grain pressure solution and sutured contacts during burial, with late cements filling the remaining pores. Many of the platform margin grainstones/packstones have been replaced by coarsely crystalline dolomite, possibly through seawater pumping and Kohout convection during the transgressive and highstand phases of sea level.