Sedimentary facies and depositional environment of the Aitamir Formation in the Gorgan Plain: A wave-dominated shoreface model

The siliciclastic sediments of the Aitamir Formation (Albian–Cenomanian) at the Oghchi locality of the Gorgan Plain consist of a thick succession of the black-grey shales and thin to very thick-bedded sandstone, deposited in a shoreface to offshore depositional system. Four facies associations including offshore, offshore transition, lower and mid-upper shoreface associations are identified in this succession. Offshore facies association consisting of a thick package of the fissile dark shales with the abundance of the framboidal pyrites displays deposition below the SWWB and dysoxic condition. The offshore transition facies association includes the heterolithic sediments of the shale and very thin-bedded sandstones/siltstones. The lower and mid-upper associations are characterized by an increase in abundance and the thickness of the sandstone layers and display a generally thickening and coarsening up-ward trend of a classical shoreface depositional system. The lateral correlation of the studied section and central sections of the Aitamir Formation of the Kopet-Dagh Basin indicates an increase in subsidence effect in the creation of the accommodation space and a decrease in the sediment supply of the N-NW parts of the basin. The presence of a thick package of dark shales with abundant framboidal and chamber filling pyrites of the Aitamir Formation suggests these sediments as a prone candidate for hydrocarbon exploration.