Sedimentary environment, sea level changes and mixing mechanism in mixed siliciclastic-carbonate ‌sequence of Qom Formation in Dastjerd area

The Qom Formation with Oligocene-Miocene age which is important due to its hydrocarbon reservoirs in some areas, is outcropped with a 330 meters thick mixed siliciclastic-carbonate sequence in Dastjerd area that uncomfortably overlain the Lower Red Formation and is conformably underlained by Upper Red Formation. In this study, the deposits were studied for petrography, microfacies and sedimentary environment, sea-level changes and mixing mechanism. Based on the frequency and distribution of the skeletal and other constituents of the carbonate deposits of Qom Formation in the studied section, six microfacies associated with three facies belt (open marine, shoal & lagoon) were identified related to a carbonate ramp. Mixing and hybridization of carbonate deposits of Qom Formation with different amounts of siliciclastic inputs in sand, silt size in through the thickness and laterally in the depositional basin of this sediments, is related to the tectonic activity and erosion of older rocks such as the Lower Red Formation and associated Paleogene volcanic rocks, deposited in the backarc basin of a subduction system. According to the microfacies analysis of the studied sediments, two cycles of sea level changes were recognized. The first cycle is associated with unstability of the basin floor and fluctuations in the relative sea-level, which leads to the deposition of a thick layer of sandstone regarded to the high tectonic activity and the second cycle is coincident with more stability of the basin which ultimately tends to a shallowing condition.