Oncoids and Stromatolites in the Rosso Ammonitico Sequences (Middle-Upper Jurassic) of the Venetian Alps, Italy

A specific type of oncoids, mostly found in association with stromatolitic domes in decimetric cycles, is quite common in red nodular limestones (Rosso Ammonitico) of the Trento Plateau area (Southern Alps). This was a pelagic horst block of the southern continental margin of the Tethys (Fig. 1) which evolved from a strongly subsiding Liassic carbonate platform. Thin Bajocian sequences of clean-washed Bositra-bearing coquinas and crinoidal sands (the “Posidonia alpina” beds) commonly underlie — and in places interfinger with the lowermost part of — the “Rosso Ammonitico” pelagic limestones (Sturani 1971). Theseform condensed and discontinuous sequences in the Trento Plateau area, and are usually represented by two members. The lower member (Rosso Ammonitico inferiore, Upper Bajocian to Middle Callovian) is often dominated by an oncolitic-stromatolitic facies and is more massive; moreover, in places it includes sparse interbeds of clean-washed and rippled microcoquinas of thin-shelled pelecypods. The upper member (Rosso Ammonitico superiore, Upper Oxfordian to Tithonian) grades upwards into Majolica limestones and shows a lower frequency of cryptalgal structures as well as the presence of sparse thin shaly or marly interbeds; it is most commonly characterized by a flaser-nodular facies in which evidence of exposure of the nodules at the depositional interface is often lacking. The two members in addition are locally separated by some meters of cherty limestones.