How a carbonate platform drowns: facies changes at the transition between shallow-water carbonates and Ammonitico Rosso facies (San Vigilio Oolite-Rosso Ammonitico Veronese, Lower-Middle Jurassic, Trento Platform, N Italy).

The drowning of Jurassic shallow-water carbonate platforms is a widespread phenomenon across the Tethyan realm in a context of extensional tectonics manifested with the deposition of nodular Ammonitico Rosso facies. The sedimentological and stratigraphic analyses of the facies deposited during the stages preceding and following the Jurassic Trento Platform drowning (Southern Alps of Italy) contributes to the understanding of the causes, modes and timing of carbonate platform demise. The pre-drowning shallow-water facies (San Vigilio Oolite, Toarcian-lower Aalenian, ca. 30 m thick) consist of the association of: 1) ooidal grainstone with intraclasts, peloids and micrite coated grains (ooidal shoals above fair-weather wave base); 2) micrite coated grains packstone-grainstone (current-washed sand shoals at or above fair-weather wave base); 3) sponge and echinoderm packstone with oncoids (skeletal shoal) and 4) oncoidal packstone-rudstone with bioclasts (oncoidal shoal). The top of the shallow-water carbonate platform is represented by a disconformity marked by a sharp erosional surface cutting grains of the underlying cemented packstone-grainstone. The sedimentation above the erosional surface is characterized by Ammonitico Rosso facies (Rosso Ammonitico Veronese, upper Bajocian-Tithonian, 20 to 30 m thick) consisting of basal Protoglobigerina packstone with thin-shelled bivalves (Bositra) and peloids with mm- to cm-scale lithoclasts of the underlying shallow-water carbonates passing upward to intraclastic packstone with Bositra and peloids and peloidal packstone with radiolarians and calcisphaeres. Sedimentological evidence indicates that the erosional surface marking the boundary between the shallow-water carbonate platform and the hemi-pelagic Ammonitico Rosso facies is related to subaerial exposure. The following Bajocian abrupt creation of accommodation space controlled by the extensional tectonics prevented the recovery of the carbonate platform production, leading to the deposition of the Rosso Ammonitico Veronese, which marks the definitive drowning of the Trento Platform.