Sequence stratigraphy of a southern Paleotethys distally-steepened ramp during the Early Carboniferous (Alborz Basin, Iran): Deciphering Tectonic and Eustatic controls

Abstract

ABSTRACT The Lower Carboniferous carbonate sediments of the Mobarak Formation have been analysed to evaluate the role of allogenic and autogenic factors on the development of a distally steepened carbonate platform. Thirteen facies are identified in the studied succession and are grouped into four facies associations: inner ramp FA‒FB; middle ramp FC; ramp slope FD; and deep ramp FE. The rifting event at the northern margin of the Gondwana supercontinent caused a remarkable change in the depositional system so that the precursor homoclinal ramp (Khosheilagh Formation, Devonian) turned into the distally steepened ramp of the Mobarak Formation. Six depositional sequences are identified in the succession, characterized by varieties the normal transgression‒regression cycles, the TST‒HST, or the transgression‒forced regression cycles consisting of the TST‒HST‒FSST. The main controlling elements in the stacking pattern of sedimentary packages, relative sea-level changes and the formation of depositional sequences were tectonic activity and eustatic sea-level changes. Development of the FSST package was caused by the increasing tectonic activity and the formation of turbiditic sediments, amplified by wave-induced currents as the autogenic factor on formation of the re-sedimented deposits. Development of the HST deposits displays a relative balance between sedimentation rate and the creation of accommodation space. Thick packages of deep ramp calcareous shale were developed during quiescence in the tectonic activity and increase in accommodation space, created by thermal subsidence and rise in eustatic sea-level.