Alternation of ecologic regimes in a deep-marine carbonate basin: calciturbidite trace fossils from the Cretaceous Scaglia Rossa, northeastern Italy.

The mostly pelagic Scaglia Rossa limestone exposed in the Vich quarry near Belluno contains numerous light-colored turbidites, consisting of calcareous sandstone and mudstone derived from the adjacent Friuli Platform. The turbidites contain an ichnofauna that records short-lived departures from background conditions in this part of the Belluno Basin in the Late Cretaceous. At least seven distinctive ichnotaxa were recognized, some of which could be the parts or extensions of more complex burrow systems of deposit feeders that exploited the disturbances. Earliest exploitation of the individual turbidites is indicated by nondescript burrow mottling in the lowest sandstone divisions. This is followed by an ensemble of comparatively well preserved sand-filled and typically mud-lined/-mantled burrows produced as the mud fraction began to accumulate gradually from suspension. All of these structures are crosscut by a latest series of mud-filled tunnels. This is not exactly the pattern of siliciclastic turbidites of flysch successions, having post-event ichnoassociations consisting of traces that are more nearly contemporaneous and record either depth of penetration or post-depositional movement of the redox boundary. Because the mud component in the Vich turbidites may have required several months to settle out of suspension, the patterns of bioturbation were extended over months to years. But as in the case of siliciclastic turbidites, emplacement of the calciturbidites suppressed the background fauna and supported a post-event association of different endobenthic animals, by changing temporarily the physical properties of the substrate, providing a windfall of organic particles, and by ventilating the near-bottom water.