Relative bathymetric position of opoka and chalk in the Late Cretaceous European Basin

Abstract

Opoka is a distinctive, highly fossiliferous marine spiculitic limestone typical of the Upper Cretaceous epicontinental successions of Poland, western Ukraine and some other parts of Europe. Several recent works have suggested that opoka was deposited in pelagic offshore settings, more distal and deeper water than those of the chalk. We oppose this view based on facies characteristics and lateral relationships of selected Campanian and Maastrichtian successions of Poland and Ukraine, as well as on palaeoecology and taphonomy of fossil assemblages preserved in these deposits. Our conclusions are as follows: 1) when opoka and chalk occur in the same facies tract, opoka occupies a more proximal position than the chalk; 2) opoka could have been deposited in a variety of settings, ranging from relatively deep-water to shallow-water ones; 3) chalk was usually deposited at greater depth than opoka, but some intervals of chalk appear to have been laid down at the same depth as some opokas. Both opoka and chalk are important sources of data on Late Cretaceous marine communities of Europe. Therefore, a proper interpretation of the water depth relations between these facies is of key importance for our knowledge of these communities.