Icehouse, cool-water carbonate ramps: the case of the Upper Pliocene Capodarso Fm (Sicily): role of trace fossils in the reconstruction of growth stages of prograding wedges

Abstract

An integrated approach, based on the use of trace fossils combined with analysis of physical and biogenic structures, identification of key surfaces, and reconstruction of stratigraphic architecture, proved to be of critical value in defining the depositional environments, elucidating the dynamics of progradation, and characterizing the various systems tracts of Upper Pliocene progradational wedges (Capodarso area, Sicily) generated by cool-water carbonate ramps. The studied succession consists of a stack of six cycles, consisting of terrigenous mudstones passing upwards into biocalcarenite wedges with distinct clinoforms. The prograding biocalcarenite bodies show seaward-increasing height and steepness of the clinoformed front as a result of development into increasingly deeper water, and may be regarded as the record of distally steepened ramps, dominated by storm-induced downwelling flows. Evidence of forced-regressive progradation is provided by stratal geometries, physical structures, and trace fossil assemblages existing at the top of the bodies, which attest to a gradual increase in energy level at the top of the ramp, concurrently with the progression of seaward outbuilding of the bodies. Three trace-fossil suites (Thalassinoides/Piscichnus, Scolicia, Dactyloidites) are shown to be linked with the successive growth stages of individual prograding wedges, whereas abandonment stages, characterized by starvation of sediment input, are marked by the Ophiomorpha suite. The Capodarso ramps, like other Plio-Pleistocene equivalents of the Mediterranean area, are small, high-gradient ramps, with stratigraphical architecture controlled by high-amplitude, orbitally driven glacio-eustatic changes. An ecologically and bathymetrically based subdivision of the ramps into inner, mid and outer ramp environments is hardly applicable, as most of the wedge progradation occurs in highly dynamic conditions, dominated by physical processes of transport and resedimentation of skeletal material, which result in faunal composition dominated by allochthonous material. The use of trace fossils is of critical value in this context, due to the scarcity of ecological information provided by faunal elements.