Rotzo Formation

The Rotzo Formation (also known in older literature as the Noriglio Grey Limestone Formation) is a geological formation in Italy, dating to roughly between 192 and 186 million years ago and covering the Pliensbachian stage of the Jurassic Period in the Mesozoic Era. Has been traditionally classified as a Sinemurian-Pliensbachian Formation, but a large and detailed dataset of isotopic ¹³C and ⁸⁷Sr/⁸⁶Sr data, estimated the Rotzo Formation to span only over the Early Pliensbachian, bracketed between the Jamesoni-Davoei biozones, marked in the Loppio Oolitic Limestone–Rotzo Fm contact by a carbon isotope excursion onset similar to the Sinemu-Pliens boundary event, while the other sequences fit with the a warm phase that lasts until the Davoei biozone. The Rotzo Formation represented the Carbonate Platform, being located over the Trento Platform and surrounded by the Massone Oolite (marginal calcarenitic bodies), the Fanes Piccola Encrinite (condensed deposits and emerged lands), the Lombadian Basin Medolo Group and Belluno Basin Soverzene Formation (open marine), and finally towards the south, deep water deposits of the Adriatic Basin. The Pliensbachian Podpeč Limestone of Slovenia, the Aganane Formation & the Calcaires du Bou Dahar of Morocco represent regional equivalents, both in deposition and faunal content.

Fossil prosauropod tracks have been reported from the formation. This formation was deposited within a tropical lagoon environment, similar to modern Bahamas which was protected by oolitic shoals and bars from the open deep sea located to the east (Belluno Basin) and towards the west (Lombardia Basin). It is characterized by a rich paleontological content. It is notable mostly thanks to its great amount of big aberrant bivalves, among which is the genus Lithiotis, described in the second half of the nineteenth century. The unusual shape of Lithiotis and Cochlearites shells, extremely elongated and narrow, characterized by a spoon-like body space placed in a high position, rarely preserved, seems to suggest their adaptation to soft and muddy bottoms with a high sedimentation rate. 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.

The sedimentary cover of the Southern Alps has been recognized as a well-preserved section of the Mesozoic Tethys' southern continental margin, featuring a horst and graben structure linked to the rifting associated with the opening of the central North Atlantic that in the Late Triassic and Early Jurassic, created elevated blocks separated by troughs. While the western margin (Piedmont and Lombardy) quickly submerged in the Early Jurassic (As seen by the Saltrio Formation & Moltrasio Formation), the eastern regions maintained shallow water sedimentation, including the Friuli and Trento Platforms, this last one latter evolving into a pelagic plateau, and separated from the Lombardian basin by the Garda escarpment fault system.

The Early Jurassic Calcari Grigi Group represents the shallow-water sedimentation phase of the Trento Platform, revealing several sites over an area of about 1,500 km². The continuity of dinosaur tracks from the Hettangian-Pliensbachian interval indicates a stable connection between the Southern Alps' carbonate tidal flats and nearby vegetated lands and freshwater sources, although the exact locations of these lands remain uncertain. Detailed sedimentological studies of the Calcari Grigi Group, particularly the Rotzo Formation, describe it as a shallow subtidal platform with an inner lagoon bordered by oolitic shoals.

The Coste dell’Anglone ichnosite for example, situated on the margin of this lagoon within a sandy barrier complex, was influenced by pioneer plants like Hirmeriellaceae in semi-arid conditions. Sedimentary structures indicate a shallow water tidal environment with heterolithic stratification pointing to steady flows at low current velocities. The presence of dinosaur tracks and supratidal markers suggests repeated subaerial exposure, contrasting with previous interpretations of the site as fully subtidal.

These findings align with the lagoon-barrier island complex scenario, featuring a subtidal ramp gently inclined to the west and an intertidal-supratidal barrier island complex trending approximately N-S, now corresponding to the Mt. Brento-Biaina and Mt. Baldo chains.

The presence of the families Centropyxidae and Difflugiidae testifies the presence of a mixed marine-terrestrial depositional system, lacking large bodies of water.

Microfossils of the Rotzo Formation consist of benthic foraminifera, calcareous algae, Ostracoda and coprolites. Foraminifera are mainly benthic agglutinated species belonging to the superfamily Lituolacea (suborder Textulariina), while lamellar and porcellaneous-walled species are very rare. The bivalve Opisoma excavatum is very common.

The Rotzo Formation is known mostly due to its massive bivalve associations of the genera Lithiotis, Cochlearites and Lithioperna that extended all along the Pliensbachian Trento Platform forming mass accumulations of specimens that formed Reef-Like structures. This fauna appeared after the early Pliensbachian C-cycle perturbation, that triggered the diffusion of the Lithiotis Fauna, noted on the rapid widespread of this biota after the event layers. All of the genera related with this fauna appeared on the lower Jurassic, and all but one became extinct before the Middle Jurassic. This "Reefs" had a strong zonation, starting with the bivalves Gervilleioperna and Mytiloperna, restricted to intertidal and shallow-subtidal facies. Lithioperna is limited to lagoonal subtidal facies and even in some low-oxygen environments. Finally Lithiotis and Cochlearites are found in subtidal facies, constructing buildups. This sections formed various kinds of ecosystems on the Trento platform, where it appeared in branched corals filled with (Spongiomorpha), Domal corals (Stromatoporida), tubular corals, Styllophyllidae corals, unidentified Cerioidea colonial corals, regular echinoid debris, sponges, and the solitary coral Opelismilia sp., with also aggregated snail shells.

In the Western Venetian Prealps a shallow-water, oceanic carbonate platform system, the Trento platform, developed on the Early Jurassic, producing a large succession of massive to well-bedded white Limestones, several 100 m (330 ft) thick that are part of the Calcari Grigi Group, where the Rotzo Formation is the Upper Member. On the local limestone of the Rotzo Formation deep burrowing is a very common type of biogenic activity, as is shown due to the presence of a large characteristic network of burrows which reach down to the lagoonal, marly-clayey assigned strata, suggesting intense bioturbation by large unknown organisms, perhaps giant decapod crustaceans (Probably members of the family Erymidae), although, the burrows found are not closely related to the ones of Shrimps or other decapods, but resemble those of Stomatopoda and Malacostraca. Other includes abandoned burrows, vertical biogenic action and infilling on the sea substrate.

Episodic surficial bioturbation is common on the Rotzo Formation, due to invertebrates or fishes which alter intensely but rapidly the substrate for many cm in depth. It this case the Bioturbation is assigned to mollusc predatory Chondrichthyes, such as Hybodontidae and Heterodontidae. It also resemble traces left by present day flat angel sharks or Squatinidae and Guitarfish such as Rhinobatos.

Unidentified fish scales are known from the formation.

On the Inter-supratidal levels show that on the Rotzo Formation the Tracksites were rarely hit by Storm Waves. Bella Lastra Tracksite recovers this environment, where the shales present (Where Fish & Crocodrylomorph Remains where found) are filled with plant roots, pollen grains, spores, freshwater ostracodes and the bivalve Eomiodon. This was deposited mostly on a Lagoonar environment with abundant shed vegetation. The main local Track record recovers specially Theropoda and Sauropoda, where the Sauropods are the most abundant tracks present (70%), moving the Otozum-like Sauropodomorphs of lower levels, with the climate changing from arid to humid. The Coste dell’Anglone ichnosite is considered as derived from semi-arid tidal flat deposits, due to the abundance of Cheirolepidiaceae Pollen. As the Pliensbachian Trento Platform is considered to be formed by a channelized barrier formed by sand, with reiterate tide emersions. The dinosaurs living here probably trampled on the subtidal flats looking for fishes trapped on tidal-derived ponds.

Rotzo Formation nearby land hosted Bahamian-type biomes (San Salvador Island Mangroves in the picture) with nearby "Taxodium swamp"-like coniferous associations dominated by the Pagiophyllum producer

The Rotzo Formation was deposited on a Lagoon on the emerged Trento Platform, leading to a well preserved fossil flora record, collected and studied since the 19th century. The great level of floral fossilization has even allow to discovery fossil amber on the Bellori section. This amber has allowed to determine that the environment was a shallow tropical lagoon, only a few metres deep, closed seawards by oolitic shoals and bars. This levels are dominated by a high abundance of Classopollis sp. (Cheirolepidiaceae), associated with dry and wet climates in coastal areas. The abundance of this group of conifers is also proven by the high presence of cuticles of Pagiophyllum cf. rotzoanum. Beyond this genera, spores are highly diversified, including from Sphenophyta, Selaginellales to Ferns, with abundance (more than 50%) of trilete spores (Deltoidospora), what suggest a good freshwater availability corresponding to a wet climate, proven also by the presence of aquatic miospores of algae such as Botryococcus and Pseudoschizaea. The climate was arid on some seasons with monsoon months. The abundance of marine fauna on this sediments, including fragments of corals, bryozoans, bivalves, echinoids, and foraminifera, suggest transport from brackish lagoons and marshes, probably occurred during storm events. Overall data points to a marshy and/or submerged paleoenvironment, comparable to the present-day Taxodium swamp or cypress swamp and a Bahamian-type marine environment in a rather wet monsoonal climate as in the modern southeastern Asia. The abundant presence of glossy black charcoal little fragments point to wildfires being a consistent local landscape feature.

The Rotzo Formation records one of the few Early Jurassic assamblages with Amber in the world, the nicknamed "Bellori amber" found near the village of the same name. Made mostly of small droplets of less than 1 mm with exceptionally preserved morphology its likely the amber producing plants were likely not stressed or affected by disease. Due to the small size animal inclusion have not been found. However various plant materials, identified “mummified wood” and wood tissue are known. Additionally large amounts of Circumpolles Cheirolepidiaceous pollen, and occasional freshwater algae Pseudoschizaea remains are included. Several cuticle fragments are attributed to the araucariaceous or Hirmeriellaceae genus Pagiophyllum. Those lived on a coastal and wet palaeoenvironment similar to the present-day Taxodium swamps with monsoonal seasons as in the modern southern Asia.