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Abstract

The Ammonitico Rosso of the western Venetian Alps is a 10- to 25-m-thick, red nodular limestone that overlies thick Late Triassic to Middle Jurassic shallow-water carbonates that form the South Trento Platform. Deposition of the Ammonitico Rosso is thought to represent a Middle-Late Jurassic drowning event whereby the South Trento Platform became a deeply submerged plateau. The Ammonitico Rosso is problematic in that it: (1) overlies a platform-wide unconformity that contains complex brecciated fabrics filled by pelagic-rich "Posidonia alpina" sediment and cement; (2) is rich in ammonites and other pelagic fauna; and (3) contains stromatolites, oncolites, and wave-rippled coquinas.

Based on new data from the eastern margin of the South Trento Platform, the drowning succession is interpreted to have a shallow origin. These data include: (1) the discovery of two sponge-coral-stromatoporoid patch reefs within oolitic and peloidal grainstone below the unconformity; (2) cavities and fill associated with the unconformity; and (3) diagenetic fabrics and transition from platform interior to platform margin facies in both the Lower and Intermediate members of the Ammonitico Rosso. Faunal and lithologic similarity of sponge-coral-stromatoporoid reefs with other Lower Jurassic reef complexes suggest that these reefs are Late Pliensbachian in age. Cavities and neptunian dikes within back-reef, reef, and fore-reef sediments are filled by P. alpina sediment, rounded lithoclasts, fibrous cement, and crystal silt. Pendant cement and crystal silt found within reef cavities and neptunian dikes overlap deposition of internal sediment.

In a west-to-east transect above the unconformity, stromatolitic and oncolitic mudstone/wackestone in the Lower Ammonitico Rosso grades first into thrombolites and stromatolites, then into nodular burrowed wackestone and packstone/grainstone. Packstone and grainstone contain well-preserved ammonites, pelagic bivalves, peloids, belemnites, gastropods, solitary corals, and fibrous cement. In the Intermediate Member, thin-bedded chert-rich limestone grades into event strata (i.e., tempestites) composed of limestone gravel and well-sorted sand, and pelagic-dominated mudstone/wackestone. Gravels are poorly sorted, sometimes imbricated and contain lithoclasts derived from underlying sediments. These lithologies overlie truncation surfaces that include deep irregular excavations, rounded gutters, and gently scoured surfaces and grade upward into sands that possess hummocky, low-angle, and planar cross-stratification. Sands are composed of coarse- to fine-size lithoclasts, peloids, and skeletal grains. Peloidal mud-stone and wackestone contain protoglobigerinids, radiolarians, ammonites, pelagic bivalves, belemnites, crinoids, and solitary corals. Solitary corals are found in growth position on ammonite and belemnite substrates.

The drowning succession of the South Trento Platform correlates with long-term eustatic rises and falls of sea level and includes: (1) Upper Pliensbachian deposition and tectonism; (2) a transgressive systems tract and high-stand systems tract (Toarcian); (3) a small-scale type 1 sequence boundary (late Toarcian-Lower Bajocian?); (4) a drowning sequence (Aalenian-Upper Bajocian); and (5) a composite condensed section (Upper Bajocian-Tithonian).

The appearance of pelagic organisms on the South Trento Platform and a biotic succession from (1) sponge-coral-stromatoporoid reefs to (2) bioeroded sponge and hermatypic coral reefs to (3) grainstone composed of ahermatypic suspension/detrital feeders and planktic organisms to (4) "stunted" pelagic and benthic faunas to (5) microbial mat (stromatolite) structures indicates progressive paleoecologic deterioration of shallow-water environments. Analogy of these transitions with those observed on modern "drowned" platforms suggests that the demise of carbonate producing benthos was caused by increasing amounts of nutrients and organic matter (i.e., trophic resources) and establishment of oxygen-deficient environments. Faunal transition is coincident with the breakup of Pangea during the Lower-Middle Jurassic, the deposition of organic-rich shale and manganese-rich limestone in periplatform and basinal settings, and eustatic sea-level rise. This suggests that influx of trophic resources was associated with changes in regional circulation patterns and upwelling. Drowning is interpreted to have occurred gradually over time through a combination of eustatic sea-level rise and environmental change.

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