The role of biogenic silica in the formation of Upper Cretaceous pelagic carbonates and its palaeoecological implications

摘要

During the Late Cretaceous due to high global sea-level the coastlines were shifted far towards the land and pelagic carbonates dominated over siliciclastic sedimentation. In most parts of the epicontinental European Basin, during that time, the huge amounts of pelagic chalk were formed, while in the NE part of the European Basin similar carbonates with biogenic silica (opoka) were deposited. The main mineralogical component of both lithofacies is calcium carbonate, which is derived from coccoliths (skeletal remains of planktonic algae called coccolithophorids). The opoka is harder than chalk and contains a significant amount of biogenic silica. Detailed petrographic studies of the opoka and its comparison with chalk show the role of biogenic silica in the formation of both lithofacies deposited in an epicontinental sea. The studies indicate that the difference between two lithofacies are primary in origin and are triggered by the presence of a significant amount of sponge spicules occurring only in opoka. The spicules contribute in opal-A input to the calcareous-mud-precursor, which during diagenesis recrystallized into more stable opal-CT, responsible for the macroscopic difference between opoka and chalk. A different scenario occurs in chalk lithofacies, where the sponge spicules are rare and opal-A comes from radiolarian skeletons. The primary difference in compositions of the calcareous-precursor of opoka and chalk are discussed in light of palaeoecological factors during deposition (depth, rate of sedimentation and nutrient influx from currents). The calcareous ooze is considered the modern equivalent of the Cretaceous chalk, but its mineralogical composition is more similar to the opoka mud-precursor. (C) 2018 Elsevier Ltd. All rights reserved.