Cyclical variation in paleoenvironments of the Rotzo formation (Lower Jurassic, Lessini Mts., N Italy)

The Trento Platform is a paleogeographic unit that in the early Jurassic represented a relative high on the southern margin of the Tethys. It was bordered eastward by the Belluno Basin and westward by the Lombardy Basin. We can distinguish two main phases during Jurassic times: the first is represented by a very shallow water paleoenvironment in which the deposition of the Calcari Grigi Group (Hettangian- Pliensbachian) and the Oolite di San Vigilio Group (Toarcian - Aalenian) took place; the second phase corresponds to the drowning of the Trento Platform during the middle and late Jurassic, with the deposition of the deep-water limestones of the Rosso Ammonitico Veronese (Bajocian – Titonian). The Calcari Grigi Group is composed by three formations: the Monte Zugna Formation (Hettangian-Sinemurian), Oolite di Loppio Formation (Sinemurian-Pliensbachian) and the Rotzo Formation (Pliensbachian). The Calcari Grigi Group is overlain either by the Oolite di San Vigilio or directly by the Ammonitico Rosso (Posenato & Masetti, 2012; Masetti et al., 2012; Bosellini & Broglio Loriga, 1971). The Rotzo Formation is the most fossiliferous one, well known for its terrestrial flora, already described by De Zigno in the 19th century and by Wesley in the 20th century (Avanzini et al., 2006). The Rotzo Formation has been interpreted as deposited in shallow tropical lagoons, only a few metres deep. These lagoons were closed seawards by oolitic shoals and bars, and bordered landwards by marshes and emerged areas (Posenato & Masetti, 2012; Dal Corso et al., 2013). The discovery of amber in the clay layers of the Rotzo Formation in the Bellori section (Lessini Mountains, Verona) attracted again the attention to this Lower Jurassic unit (Neri et al., 2013). The Bellori outcrop displays about 20 m of limestones with intercalated clays and marls rich in organic matter and sometimes fossil wood (coal) and amber. The limestones are well stratified, with beds 10 cm to more than one metre thick, whereas the clayey levels range between 3 and 40 cm in thickness. In the Bellori section two biozones were recognized: a lower Orbitopsella zone and an upper Bosniella zone, allowing to date the strata to the Pliensbachian. Five different facies were recognized (Fig. 1): A: clayey and carbonaceous layers; B: fine-grained limestone; C: limestone with bivalves (Lithiotis); D: limestone with Orbitopsella; E: oolitic limestone. The facies are repeating cyclically with two distinct patterns: in the lower part of the stratigraphic column we observe typically the succession D-C-A, whereas in the upper part the Orbitopsella limestone (Facies D) disappear and the fine-grained limestone (Facies B) become common, giving rise to the characteristic succession C-B-A. Inside the A Facies is sometimes visible a microcyclicity with the succession Lithiotis beds-coal-clay repeated. The D-C-A cycles record changing environments from the well oxygenated open lagoon, close to the open sea, up to the continental environment. The C-B-A cycles testify the transition from a protected lagoon to low-oxygen conditions maybe in nonmarine environment. The upper cycles could be linked to the establishment of Lithiotis mounds restricting the water circulation and giving rise to protected eutrophic facies. Close to some of the clay levels we observed plant root traces, testifying the cyclical emersion of at least part of the platform. To explain the observed cyclicity we can advance different hypothesis: 1)high-frequency eustatic oscillations; because the glacio-eustatic cycles are to be excluded during the Jurassic, we consider this explanation very unlikely. 2)Local subsidence, maybe linked to locally high sedimentation rate; this should imply cyclical variations of the organic productivity. 3)Local tectonics; this is coherent with the generalized subsidence due to the rifting phase related to the opening of the Tethys ocean. References Avanzini, M., Piubelli D., Mietto P., Roghi G., Romano R., Masetti D., 2006. Lower Jurassic (Hettangian-Sinemurian) dinosaur track megasites, southern Alps, northern Italy.C Harris et al., eds., 2006, The Triassic-Jurassic Terrestrial transition. New Mexico Museum of Natural History and Science Bulletin 37: 207-216. Bosellini A. & Broglio Loriga C., 1971. 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