The Upper Eocene crustose coralline algal pavement in the Colli Berici, north-eastern Italy

Abstract

A crustose coralline algal pavement, identified in Upper Eocene (Priabonian) shallow water, middleramp carbonates in north-eastern Italy (Colli Berici, Southern Alps), represents a rare example of this facies. The crustose pavement consists of a coralline crust bindstone with a wackestone-packstone matrix, and is characterised by the dominance of crustose coralline thalli composed primarily of melobesioids (Lithothamnion and Mesophyllum) and mastophoroids (Spongites, Lithoporella, Neogoniolithon). In places the coralline bindstone can be seen to develop from isolated encrusting-to-foliose thalli which bifurcate and join to form an open framework interbedded with matrix debris from crusts. Various forms of rhodoliths occur commonly within this facies. The largest discoidal rhodoliths (up to 12 cm of large diameter) show an inner arrangement consisting of loosely packed laminar (encrusting-to-foliose) coralline thalli with a high percentage of constructional voids (50-63%). Accessory components are represented by larger hyaline perforated foraminifera such as nummulitids and orthophragminids. This facies formed in a ramp palaeoenvironment characterised by relatively low hydrodynamic energy and low rates of sedimentation. Channelised structures present within the facies were formed by return currents which swept the middle ramp creating such distal structures. Further toward the distal middle-ramp the return currents decreased in energy and discharged nutrients allowing the mesotrophic crustose coralline algal pavement to develop.