Early Carboniferous events along the north-eastern Gondwanan margin: an example from the Mobarak Formation of the Central Alborz Mountains, Northern Iran.

The transition from the Late Devonian to the Early Carboniferous was marked by tectonic events, environmental change and global oceanic biological turnover that influenced sedimentary regimes. The end-Devonian faunal extinction event, for example, eliminated most of the reef building taxa, which, coupled with an increasing rate of subsidence in response to the opening of the Paleo–Tethys and a sea-ward shift of carbonate factories during the Lower Carboniferous, led to the formation of extensive carbonate ramp platforms along the Laurasian and Gondwanan margins. This development has already been reported from the western Gondwanan and southern Laurasian margins but not from northern to north-eastern Gondwanan margin yet. We aim to improve the knowledge on the main factors controlling the development and growth of carbonate ramps during the Lower Carboniferous by examining the Alborz basin along the northern margin of Gondwana. The Lower Carboniferous Mobarak Formation records the development of a storm-sensitive pervasive carbonate factory on the southern Paleo-Tethys passive margin following the opening of the Paleo-Tethys Ocean into the Alborz basin along the northern margin of Gondwana. Four outcrops in the central Alborz Mountains were studied: the Jaban, Aroo, Shahmirzad and Labnesar sections. Four events have been described as the principal factors controlling carbonate platform evolution in the Lower Carboniferous: (1) A transgression linked to global temperature rise in the Early Tournaisian (Middle Hastarain) resulted in the formation of thick-bedded argillaceous limestones. This interval is represented by massive beds of dark mudstones–wackestones to packstones that laterally interfinger with finely laminated limestones, and with bed thickness ranging on the centimeter to meter scale. This interval correlates with Lower Tournaisian nodular to argillaceous limestones of the Moravia Basin (Czech Republic), the Rhenish Slate Mountains (Poland) and the Dinant Basin (Belgium). (2) Late Hastarian–Early Ivorian glaciations were identified in Southern Gondwana but had not been evidenced in Northern Gondwana yet. This glaciation regime is recorded through the appearance of inner-ramp channel-form facies in mid– and outer-ramp settings. (3) During Late Ivorian–Early Visean?, a differentiation block faulting regime along the basin’s margin caused uplift of the westernmost parts of the Alborz basin and resulted in a sub-marine collapse in the eastern part of the central basin. This caused vast sub-aerial exposure and brecciation the top of the Mobarak Formation at the Jaban and Aroo sections. The sub-marine collapse was recorded through the occurrence of Zoophycos bearing fine-grained limestones in the top of the Labnesar and Shahmirzad sections. (4) Tectonic activity that coincided with considerable and abrupt sea level falls as an indirect consequence of the Viséan and Serpukhovian glaciations phases. This progressive sea level drop led to the stagnation of the carbonate factory, which is expressed as an erosional surface at the top of the Mobarak Formation