Facies changes in shallow-water carbonates on the rifted Adria margin: Non-skeletal facies as tectonic and environmental markers on the Lower Jurassic Trento Platform (Southern Alps)

Carbonate platforms represent excellent archives for understanding changes in biological evolution and in climatic and paleoceanographic conditions, which are expressed by abrupt facies and carbonate production changes. In the Early - Middle Jurassic interval, the shallow-water tropical carbonate systems of the western Tethys experienced discrete and diverse stepwise changes, marked by major shifts in carbonate factories and depositional systems or drowning events. On the resilient Trento Platform (Southern Alps), two regionally extensive lithostratigraphic units (Loppio Oolitic Limestone in the mid-late Sinemurian and San Vigilio Oolite in the mid Toarcian) record major depositional phases dominated by ooid and non-skeletal production. Facies analysis was carried out across the stratigraphic boundaries marking the onset of dominant ooidal facies to investigate the controlling factors. Facies changes from the upper subtidal unit of Monte Zugna Formation (facies association FA1) to the Loppio Oolitic Limestone (FA2) are characterized by sharp transition between peloidal-micritic and almost exclusive ooidal sedimentation. FA1 comprises very shallow (restricted) inner platform facies dominated by strongly bioturbated mudstone and thin peloidal-oncoidal packstone and grainstone, possibly arranged within a complex facies mosaic. FA2 represents an extensive high-energy, shallow (passing from below to above FWWB) and subtidal shoal complex (strand-plain), dominated by micritized ooids, rarely displaying relict radial structures. Facies changes across the upper Rotzo Formation (FA3) and the San Vigilio Oolite (FA4) are marked by a transitional lithozone, made up of micrites with low-diversity biota and clay layers. FA3 includes mostly low-energy shallow lagoonal facies, represented by bioclastic (bivalves, large benthic foraminifers and algae) packstone and wackestone, associated with oncoidal packstone. FA4 represents a high-energy, shallow (above FWWB) and subtidal shoal complex, constituted by dominant (radial) ooidal grainstone and abundant crinoidal grainstone, both with very common micrite-coated grains, passing laterally to lower energy sponge-bearing facies, deposited in back-shoals settings. The onset of Sinemurian and Toarcian non-skeletal carbonate production is indicative of rapid regional environmental changes, able to hinder skeletal and microbial productions. These changes may be controlled by the interplay between climatic oscillations (short-lasting cooler-warmer and humid-dry intervals) and the isolated physiography of the Trento Platform, exposing the platform-top to high-energy current circulation and open-marine conditions during tectonic quiescence. Although similar evolutions are reported from other Tethyan platforms, controlling factors are still debated: the ooidal limestones of the Trento Platform may represent key marker facies for understanding the dynamic environmental conditions in the Lower Jurassic western Tethys.