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.