Chlorozoan versus foramol lithofacies in Upper Cretaceous rudist limestones

Abstract

The study of Upper Cretaceous limestones of the Nurra region (northwestern Sardinia, Italy) that crop out with open shelf, rudist-rich deposits has shown that during the early Senonian (Coniacian-Santonian interval) sediments characterized by assemblages dominated by molluscs, with variable amounts of red algae, bryozoa and echinoids, replaced sediments including hermatypic corals, green algae and variable amounts of non-skeletal grains. These latter components were largely dominant in the previous carbonate-shelf contexts of the Nurra during Jurassic-early Cretaceous times, becoming less numerous until they disappeared in the transgressive deposits that followed the tectonically induced middle Cretaceous emersion phase. A rhodalgal association (typical of anomalous tropical areas or transitional ones) has been recognized in the replacing limestones that may be more generically considered foramol-type deposits. This seems to document conditions of stress in the waters impinging on the Nurra carbonate platform during the Coniacian-Santonian transition. The observation that, in the late Cretaceous, a similar evolutionary trend seems to have characterized many shallow neritic successions, from many different areas, leads us to hypothesize more general causes of stress (e.g. induced upwellings on the marginal sectors of the shelves, increased oceanic overturns and hyperproductivity due to terrestrial runoff on post-emersive transgressed substrata). Peculiar oceanic conditions were able to create depositional contexts that were considerably changed with respect to the previous ones, such that the more opportunistic forms of the foramol assemblages flourished. The change in dominant assemblages resulted in a different organization of the sediment as a response to the different producing communities. The resulting temperate-type carbonate shelves were characterized by a low growth potentiality essentially due to a higher dispersion rate and this could have facilitated their drowning.