Rudist Mud Mounds and Biostromes from Upper Cretaceous of Istria (Croatia)

In addition to biostromes which are the most common paleo-enviromental appearance of rudist congregations, in Upper Cretaceous of Istria, rare rudist mud-mounds were investigated. The aim of this work is to present the appearance of rudist congregations with respect to the lithofaces characteristics of paleo-environment. Rudist congregations could appear within both mud mounds and biostromes in relatively shallow subtidal. They are the result of the interaction of the accommodation space adequate to the thriving for radiolitids and carbonate sedimentation rate. The shallowing and the decrease of accommodation space are indicated in mud mounds by thin sheets of densely packed F–R with fragments of radiolitids at the top of beds together with lateral migration of congregations within mud mound marked by flanks. They are visible as lamination. In case when radiolitid congregations throve in shallower subtidal paleo-environment with less accommodation space, they appear through biostromes.