Larger foraminiferal and coralline algal facies in an Upper Eocene storm-influenced, shallow-water carbonate platform (Colli Berici, north-eastern Italy)

Abstract

Facies analysis of Priabonian shallow-water carbonate successions in the outcrops near Mossano (eastern Colli Berici, north-eastern Italy) and the subsequent integration of palaeoecological data have been used to produce a detailed palaeoenvironmental model. The facies distribution and the processes, which controlled the larger benthic foraminiferal and coralline algal assemblages in the study Priabonian storm-influenced, carbonate ramp, are discussed in detail. In the middle-ramp trough, cross-bedded small Nummulites and rhodolith packstones pass laterally into coralline algal crust wackestones. These rhodoliths exhibit two well-differentiated growth-phases: the first occurring during low-water energy periods, followed by a second developed during high-energy events during which they were reworked and partially destroyed. These facies pass laterally into orthophragminid rudstones and packstones characterised by orthophragminid tests and channel structures. The latter were formed by storm-induced return currents, which swept the middle-ramp creating distal middle-ramp channel structures. Further toward the distal part of the middle-ramp the return currents decreased in energy and discharged nutrients allowing the coralline crustose pavement to develop. This is characterised by rhodoliths typically discoidal in shape (up to 10 cm in diameter) with a loose inner arrangement consisting of encrusting-to-foliose growth-forms. The occurrence of the channel structures in orthophragminid rudstones and in the coralline crustose pavement appears to be unique in Upper Eocene carbonate ramp depositional systems. During the erosive events associated with the return currents, these shallow-water carbonates were unlithified or only weakly lithified and were, therefore, easily transported into deeper water ramp settings. The model suggests that the significant changes of the middle-ramp benthic communities with depth could have formed, at least in part, by offshore return currents, which played an important role in both controlling substrates and in supplying nutrients.