Cenomanian oyster communities from a tide-dominated epeiric ramp in the southern Tethys: A sediment-fauna relationship

Abstract

The Cenomanian shallow marine succession in the Southern Galala Plateau of Egypt contains rich benthic fauna and preserves enormous information on how paleo-communities respond to fluctuating environmental conditions. Based on rock and fossil data, the Galala Formation was interpreted to be the result of deposition on a tide-dominated homoclinal ramp. Three main facies associations representing three depositional environments (intertidal, shallow subtidal, and deep subtidal) were recognized. Three distinct macroinvertebrate associations corresponding to these environments and dominated mainly by oysters were quantitatively identified. They show a gradational vertical increase in diversity as well as in the infaunal and mobile species. This trend can be attributed to the Cenomanian sea-level rise, where harsh marginal marine environments in the middle Cenomanian became more stable toward the latest Cenomanian. Sea flooding may have eliminated barriers, enhanced water circulation, and stopped hyperpycnal flow and/or terrigenous flux. Consequently, oxic mesotrophic conditions may have prevailed and siliciclastic starvation may have concentrated fossil materials, and therefore, diversity increased. Based on the vertical and lateral changes of the facies stacking pattern, two main parasequence types were recognized, the first includes deep subtidal deposits changed upwards to shallow subtidal and the second includes shallow subtidal deposits that pass upward into intertidal. Moreover, two and one-half third-order mixed sequences were identified and correlated to their regional and global counterparts.