Mixed carbonate-siliciclastic platforms: a case study from the Hauterivian-Barremian Gadvan Formation in the Interior Fars area, southern Iran

The Hauterivian-Barremian Gadvan Formation represents a depositional sequence in the central and eastern regions of the Zagros Mountains. Field surveys and petrography of 30 thin-sctions of this Formation in the Khaneh-Kat anticline togheter with wireline logs (gamma-ray and sonic) and 25 thin-sections analysis of two drilled wells in the Dashtak and Bandubast gas fields (Interior Fars area) indicate important events in the basin evolution, especially the changes in the sedimentation regime and platform evolution during the time period. The key point is upward grading of the thick-bedded layers, feature forming Berriasian-Valanginian carbonates into shale, marl and argillaceous limestone interbeds of the Gadvan Formation. The vertical changes is consisten with a prominent decrease in carbonate production and a definite change in the nature of the Fars platform. The intense bioturbation and mostly mud-dominated texture of skeletal limestone microfacies with significant occurrence of molluscs, green algae and benthic foraminifera in the cyclic alternations of shales and limestones indicate that the Gadvan Formation was deposited in a mixed carbonate-siliciclastic platform. The platform formed as the result of increased siliciclastics influx from the southern areas of the Zagros fold-thrust belt (clastic drowning of the platform) and the change of the Fahlyian oolithic-algal ramp into an environment consisting of an open lagoon and a muddy shelf. In the drilled wells, these changes correspond to a significant increase in gamma-ray and sonic wireline logs. The Khalij Member shows the regional continuity, implies a decrease of the (relative) siliciclastics supply and increase of carbonate production in the mixed platform. Considering the relative tectonic quiescence of the Interior Fars area during the Early Cretaceous, the mentioned changes can be the result of the interplay between tropical climate and short-term fluctuations in the context of global sea level rise.