Early Carboniferous siliciclastic-carbonates in east-central Iran versus coeval carbonates of north Iran: record of latest Tournaisian-early Visean tectonic events

Three stratigraphic sections of the Early Carboniferous Gachal Formation in the Kalmard area, central Iran were studied. The Gachal Formation shows alternation of siliciclastics and carbonates in repeated cyclic arrangements. The microfacies and depositional environment are described and reconstructed. Facies analyses indicate the presence of seven carbonate and one siliciclastic facies types, which are indicative of back-, inner- and middle-ramp settings. A mixed siliciclastic-carbonate ramp setting is proposed for the depositional environment of the Gachal Formation. This mixed lithology has only been reported from that central Iranian formation. The Mobarak Formation, coeval deposits of the Gachal Formation, in Alborz (northern Iran) is mainly composed of carbonates. The facies analysis of the carbonate portion of the Early Carboniferous Gachal Formation shows some resemblance with those of the Mobarak Formation, even though middle- and outer-ramp facies are more abundant in the Mobarak Formation. During the latest Tournaisianearliest Visean third-order, shallowing-upwards depositional sequences were recognized in the Gachal Formation in the study area. Tectonic activity (fault movements) was the main reason for thickness variations of the Gachal Formation members in these three studied section. Synsedimentary tectonic activity must have been responsible for the platform rising, leading to the erosion that created the siliciclastic influx into the Carboniferous basin in the Kalmard area. Our results show the importance of synsedimentary tectonics on the development of carbonate facies.