FACIES ANALYSIS AND STRATIGRAPHIC SEQUENCE ARCHITECTURE IN A BACK-ARC BASIN IN CENTRAL IRAN: A CASE STUDY FROM THE EARLY MIOCENE OF THE QOM FORMATION

Abstract

Marine deposits of the Qom Formation, which is an important gas reservoir in Central Iran with the age of Early Oligocene to Early Miocene, is studied to determine facies, sedimentary paleoenvironment, and depositional sequences. The primary lithology is limestone, which is accompanied by a conglomerate, sandy marl, marl, and sandy limestone. Based on siliciclastic content, textural analysis, and the biotic constituents, ten facies have been identified. These facies belong to five depositional settings, including delta, a tidal inlet, lagoon, shoal, and open marine. According to the absence of continuous and large barrier reefs, gradual vertical variation in facies from the transitional environment (delta) to shallow open marine, the absence of oncoid, pisoid and aggregate grains that are mostly present in rimmed carbonate shelf environments, the absence of calciturbidites and slump and slide structures, the Qom Formation has been deposited in a homoclinal ramp setting (inner, middle and outer ramp). Field studies and vertical facies variation architecture in the framework of depositional system tracts led to the recognition of two 3rd order depositional sequences in the Early Miocene (Aquitanian) time. Sedimentary facies in the Qom Formation that mainly occurred in the middle ramp setting reveal a mostly aggredational stacking pattern in depositional sequences. The Early Miocene sequences stratigraphic architecture of the Qom Formation based on correlation charts are similar to the regional sequences of the Arabian plate and Zagros basin.