Transition from the Hajir Formation to the Muaydin Formation: A facies change coinciding with extensional, syndepositional faulting (Ediacaran, Jabal Akhdar Dome, Central Oman Mountains)

Abstract

In the core of the Jabal Akhdar Dome, an irregular horst composed of carbonates of the Hajir Formation is bounded by syndepositional extensional faults on either side. The western, SSW-dipping fault is an extensional growth fault associated with rollover anticline and syncline within syndepositional siliciclastic strata of the Muaydin Formation. Syndepositional extension is indicated by increasing thickness of the involved strata of the Muaydin Formation towards the fault. The eastern, ESE-dipping fault, exhibits a larger displacement than the western fault. Both the western and the eastern fault are blanketed by postdeformational Muaydin deposits. Taking into account that the strike of both faults differs significantly (see above), we suggest that the eastern fault represents the main fault of a relay ramp system whereas the western one is the fault of a breached relay ramp with a dip angle of 65°. Compaction is able to explain some particular stratal patterns of the Muaydin Formation. The eastern fault may provide insights as to possible similar Ediacaran movements along the NNE-striking Proto-Semail Gap (i.e. the Proterozoic Semail Gap). Ediacaran vertical block movements at the latter may explain why the Muaydin Formation is present in the Jabal Akhdar Dome in the west but absent in the easterly located Saih Hatat Dome. The tectonic setting is characterized by WNW-ESE-oriented extension. Immediately west of the irregular horst we found a seemingly complete stratigraphic transition from the Hajir Formation to the Muaydin Formation. On top of this structure is a minor stratigraphic hiatus at the contact between both formations. Although the observed features are all outcrop-scale phenomena, we suggest that they provide insights into the general Ediacaran tectonic and depositional setting. This setting is characterized by complete and incomplete stratigraphies and by syndepositional extension which may be held responsible for the abrupt change from carbonate to siliciclastic facies as well as common slumps in the lower part of the Muaydin Formation.