The drowning of the Trento Platform: facies characterization and depositionalchanges across the San Vigilio Oolite - Rosso Ammonitico Veronese transition (Lower-Middle Jurassic, Venetian Southern Alps, N Italy).

The drowning of Jurassic shallow-water carbonate platforms is a widespread phenomenon across the Tethys realm in a context of extensional tectonics manifested with the deposition of red colour, hemi-pelagic nodular limestone, rich in ammonites. The study of the facies deposited during the stages preceding and following platform drowning contributes to the understanding of the causes, modes and timing of carbonate platform demise. The Lower-Middle Jurassic carbonate succession of the Trento Platform (Venetian Southern Alps, N Italy) is a field laboratory for studying the sedimentary record of the onset of platform drowning and demise of a carbonate platform consisting of peritidal facies (Monte Zugna Formation, 120 m thick; Hettangian-Sinemurian), high-energy ooidal grainstone (Loppio Oolitic Limestone, 40 m thick; Sinemurian) and low-energy subtidal lagoon (Rotzo Formation, 200 m thick; Sinemurian-Pliensbachian). The San Vigilio Oolite (10 m thick; Toarcian-lower Aalenian) represents the last shallow and high-energy deposit before drowning and consists of oolitic grainstone with intraclasts and peloids deposited above fair-weather wave base. The upper portion of the San Vigilio lithostratigraphic unit is devoid of ooidal grainstone (20 m thick) and consists, from base to top, of: 1) bioclastic packstone with lithistid sponges, oncoids and coated grains, 2) oncoidal packstone-rudstone with coated grains and bioclasts, 3) coated grain skeletal packstone-grainstone. The abundance of siliceous sponges and crinoids, the lack of ooids and the reddish colour of the upper San Vigilio unit are indicative of a gradual deepening of the depositional environment, compatible with the onset of carbonate platform drowning. A regional drowning unconformity marked by an erosional surface and a hardground composed of mm to cm thick Fe-Mn nodules separates the San Vigilio Oolite from the overlying hemi-pelagic Rosso Ammonitico Veronese (upper Bajocian-Tithonian). The hardground consists of mm to cm thick Fe-Mn polymetallic nodules favoured by low sedimentation rates, enhanced by the sweeping action of bottom currents, removing fine sediment and oxygenating the seafloor. The lower Rosso Ammonitico Veronese (5-11 m thick) overlying the hardground consists of superimposed: 1) skeletal packstone with protoglobigerinids and peloids, 2) bioclastic packstone with Bositra thin-shelled bivalves and peloids, 3) intraclastic packstone with bioclasts. This facies superposition demonstrates a gradual deepening of depositional environments from above (San Vigilio Oolite) to below fair-weather wave base (sponges and echinoderms rich deposits; upper San Vigilio) followed by the abrupt reduction in sedimentation rate (hardground polymetallic nodules) and by the establishment of a hemi-pelagic sedimentation (drowning succession; Rosso Ammonitico Veronese) on bioturbated oxygenated seafloors below the photic zone, as confirmed by a drop in the benthic faunal diversity.