A Jurassic “Tongue of the Ocean” infilled with oolitic sands: The Belluno Trough, Venetian Alps, Italy

Abstract

The Belluno Trough is a narrow and elongate basin on the northwestern corner of the Apulian Plate continental margin. It acquired topographic identity in Early Liassic time, during the breakup of a widespread Upper Triassic carbonate shelf. At the beginning (Liassic), the Belluno Trough was a starved depression accumulating euxinic mudstone and bounded by two shallow-water carbonate banks (Trento and Friuli Platforms). This typical Bahamian physiography lasted until Late Liassic time, when the shallow-water Trento Platform collapsed tectonically and suddenly drowned, tilting northward and breaking along an old tectonic line; it became therefore a submerged pelagic plateau. In the meantime, the western edge of the Friuli Platform, now the windward margin of an oceanic platform, turned into a prolific, super-producing, oolite “factory”. Quite soon the oolite sands of the Friuli margin started to flow down the slope, reaching the basin through numerous entry points and gradually building a complex of coalescent deep-sea fans, which, prograding westward, infilled the Belluno Trough. The fans terminated westward against the faulted edge of the Trento Plateau, on which above a karst-like surface, locally encrusted with manganese, red nodular ammonitic limestone was slowly accumulating. Northward, where the topographic relief decreased, the oolite fans could spill over the escarpment onto the edge of the plateau. By the end of Middle Jurassic time, the Belluno Trough was no longer a submarine depression but a gentle slope connecting the Friuli Platform to the deep, pelagic Trento Plateau. In Callovian time, probably because of a short drop in sea level, large parts of the Friuli Platform were exposed to subaerial diagenesis. During the subsequent Oxfordian transgression, the resulting rocky bottom became the substratum for an extensive growth of reefal organisms (hydrozoans, corals, rudists), which fringed the Friuli Platform for the whole Malm and Cretaceous times.