Facies changes and sedimentary environment in a pre-arc basin (Nazarkardeh Formation, Kopeh Dagh sedimentary basin, northeastern Iran)

A unique basement of Kopeh  Dagh sedimentary basin is located and investigated in Agh-darband area, northeastern Iran. The second formation of the Agh- darband group, the Nazarkardeh Formation is cropped out in the area by carbonate and volcanogenic successions. This Formation with Anisian age is overlaid by the Sina Formation, Ladinian age with a sharp boundary and is underlaid by the Sefidkuh Formation with late Scythian in age by a gradual boundary. During this research, four stratigraphic sections are measured from petrology, sequence stratigraphy, microfacies and sedimentary basin points of view. Next this measurement, 300 samples are token and 100 thin sections are made. 11 microfacies with 1 hybrid facies and 3 terrestrial facies are determined. By high resolution studies on these data, this Formation is deposited in lagoon, shoal and open marine which is deposited between the Sefidkuh Formation (carbonate ramp) and the Sina Formation (marginal shelf), so that the depositional environment of the Nazarkardeh Formation is determined as a transitional environment from carbonate ramp to marginal shelf. The existence of hybrid facies show the volcanic activities during deposition of this Formation. the existence of intraformation and monomictic conglomerate in the last depositinal stages of this formation, shows the falling and collapse facies next to the deep floor changes of the sedimentary basin. The sea level change graphs in A, B, C and D stratigraphic sections in contrast with sea level change graphs of Anisian (middle Triassic) shows a third level sequence stratigraphy and is markable by a non-erosional boundary (SB2) on the basis on top of the A, B, C and D stratigraphic sections.