Vertical transition from ramp to rimmed shelf: example of the Turonian carbonate platforms in Central Tunisia

Abstract

The Gattar and Bireno carbonate platforms developed in Central Tunisia during early and middle Turonian, respectively. Most previous geological works consider these carbonate platforms as rimmed shelves, that developed up on a substratum arranged into a series of blocks bounded by major E-W faults. The present work aims through field observation and facies record microscopic analysis to show that the Gattar platform was not a rimmed platform as mentioned in previous works but it rather matches with a homoclinal ramp whereas the Bireno platform consists of a proper rimmed shelf. Based on the sequence stratigraphy principles, it will be shown that each of these two platforms is made of a carbonate succession that developed during the highstand of a third order eustatic cycle. The Gattar carbonate ramp is overlain by the Annaba member marls that accumulated during the following trangressive cycle. On the top of the marls accumulations, the Bireno platform rudist shoal occured during the following highstand separating between an outer shelf to the north and an inner shelf to the south. The vertical transition from the Gattar ramp to the Bireno rimmed shelf seems to have been most controlled by sea-level changes and local paleotopography rather than the play of active faults and resulting tilted blocks, as reported in the previous works.