Microstratigraphy And Taphonomy Of Rudist Shell Concentrations In Upper Cretaceous Limestones, Cilento Area (Southern Italy)

Rudist bed type and distribution has been investigated in Upper Cretaceous limestones cropping out in the northern Cilento area (southern Italy). These limestones are dominated by fine-grained, peloidal, silty packstone in which rudist rich beds intercaled. An inner shelf environment may be inferred on the basis of the recognized sedimentary and taphonomic features. The rudist shell beds are characterized by low species diversity, with slight differences in abundance of a few species belonging to the Durania, Bournonia, Sauvagesia, Gorjanovicia and Biradiolites genera, that usually form oligo- or monospecific congregations. The internal fabric of these levels (i.e. orientation, arrangement, packing and sorting of the skeletal elements; internal microstatigraphy) has permitted to distinguish two broad shell bed categories: a) shell beds considered as “Primary Shell Concentration” in which the shell concentration is essentially created by the behaviour of local shell producers, preserved in situ and in growth position; b) shell beds considered as “Hydraulic Shell Concentration”, that were deposited under the influence of hydraulic processes and/or input of surrounding bioclastic sediments. The taphonomic analyses allowed us to highlight the role of some of the biotic and abiotic factors which controlled the distribution of the rudists in the various habitats. The increase of physical disturbance (expecially hydrodynamism) is the primary difference between these shell bed categories. The establishment and development of the densest rudist congregations appear to be related to the accomodation space made available my means of relative sea level rise. The lowering of the sea-level was often accompanied by the increased influence of waves and/or currents on the sea bed and the consequent sediment disturbance and demise of the rudist lithosome, although other factors cannot be excluded.