Carbonate factories and their critical control on the geometry of carbonate platforms (mid-Cretaceous, southern Iran)

Tropical rimmed platforms represent a much more efficient protection from coastal erosion than carbonate ramps. Understanding the controlling factors of platform geometry is therefore crucial to predict the fate of rimmed platforms under global-warming conditions. Here, we present detailed biostratigraphic, sedimentological, geochemical, and high-resolution carbon-isotope analyses of mid-Cretaceous (Albian-Cenomanian) carbonate rocks well exposed in the Khormoj section located in the Zagros Mountains of southern Iran. Sixteen microfacies were identified, allowing us to define the superposition of different platform geometries, a carbonate ramp in the Albian and a rimmed platform in the Cenomanian. Biostratigraphic and carbon-isotope analyses allowed us to identify the OAE1b, OAE1d, and Middle Cenomanian events in the Khormoj section. The paleo-depth changes documented by carbonate microfacies testify to two transgressive events that can be correlated with long-term eustatic curves during the ramp stage. The Albian ramp was formed by orbitolinids under high siliciclastic input whereas the Cenomanian rimmed platform was built by rudists during clear water conditions. These two carbonate factories have drastic different carbonate sedimentation modes. The orbitolinid factory is characterized by a low carbonate production rate and fine carbonate grains resulting in ramp geometry, whereas the rudist factory is characterized by a high carbonate production rate and coarse carbonate grains leading to a rimmed platform geometry.