The Moltrasio Formation (also known as the Lombardische Kieselkalk Formation, Saltrio Formation, Broccatello Formation and Alpe Perino Limestone) is a geological formation in Italy and Switzerland. This Formation mostly developed in the Sinemurian stage of the Lower Jurassic, where on the Lombardian basin tectonic activity modified the current marine and terrestrial habitats. Here it developed a series of marine-related depositional settings, represented by an outcrop of 550–600 m of grey Calcarenites and Calcilutites with chert lenses and marly interbeds, that recovers the Sedrina, Moltrasio and Domaro Formations. This was mostly due to the post-Triassic crisis, that was linked locally to tectonics. The Moltrasio Formation is considered a continuation of the Sedrina Limestone and the Hettangian Albenza Formation, and was probably a shallow water succession, developed on the passive margin of the westernmost Southern Alps. It is known due to the exquisite preservation observed on the Outcrop in Osteno, where several kinds of marine biota have been recovered.
Utilities
The first extraction activities of the famous Saltrio & Moltrasio stone give back to the times of the ancient Romans, with modern reports of activity in Salnova quarry since 1400.
In the Monte Oro area, on the southern slope of Monte Orsa, there were numerous trench quarries which were used to extract this precious rock, used both for structural constructions and for the production of artefacts and artistic works. In more recent times the mining activity has been transformed and we have moved from the extraction of stone for construction to the extraction for the production of stabilized and split crushed stone, useful for the production of motorway foundations and mixtures for the production of asphalt. To date it is the only active quarry where Saltrio stone is extracted. Above, the Moltrasio limestone emerges, a greyish-brown limestone composed of biocalcarenite and containing widespread nodules of spongolitic silica. This rock is rarely fossiliferous except in the contact areas between the Formations. At the roof of the Moltrasio Fm, a whitish yellow limestone emerges, again of marine-pelagic origin, where there is a lot of micro-diffused silica within the sediment. Since the early 1900s, fossil finds have been known in the Salnova Quarry and in the various quarry sites present in the surroundings of this site. The first written testimonies, and subsequent revisions, are reported starting from the sixties by Giulia Sacchi Vialli. The scholar describes the fossil faunas of Saltrio by listing and detailing various taxa belonging to ammonoids, nautiloids, gastropods, crinoids, brachiopods and bivalves. In that period, the great phase of extraction of ornamental stone using manual-mechanical methods had just ended in the quarry. Paleontologists could only recover fossils from the waste flakes near the quarry and therefore the possibility of seeing more specimens was limited to the length of manual operations. In those years, however, the quarry was acquired by Salnova SPA (1969): the purpose of the extracted material, and therefore the extraction method and processing, changed. From classic and manual extraction we move on to the use of heavy mechanical means and extraction with explosives: the moved rubble increases considerably, making it easier to observe other specimens, new lithologies and above all different faunas. The fauna present at the base of the Moltrasio Formation is condensed and includes ammonoids of species attributed to the entire Sinemurian. The taxa attributable to the Lower Sinemurian found in the Saltrio quarries probably come from the base of the formation or have been reworked. The Formation includes taxa indicative of all the biozones between the Bucklandi Zone (Lower Sinemurian) and the Obtusum Zone, and possibly also of the Oxynotum Zone of the Upper Sinemurian, present at the base of the Formation. The contact between the Main Dolomite and the Moltrasio Formation also contains selachian teeth, glauconite and phosphated internal models of ammonites.
Description
The Moltrasio Limestones record a transgressive episode during which the sea flooded a fault-segmented carbonate platform. Sedimentation was slow and often interrupted, producing condensed successions with bored hardgrounds, glauconite coatings, and local phosphatic grains. On structural highs, crinoid-rich grainstones and packstones formed encrinitic carpets that were later bored and glauconitized. In nearby grabens, coarse epiclastic calcarenites and rudites accumulated, composed of reworked Triassic platform grains later mixed with crinoid debris. Lower-lying areas preserve thin condensed horizons overlain by dark laminated micrites with sponge spicules, scattered bioclasts, and minor terrigenous and phosphatic material. Along the southern margin, the transgression is marked by reddish calcarenites and litharenites derived from basement rocks. Basinward, these deposits grade into the spiculitic Kieselkalk, a sponge-rich lime mudstone with interbedded bioclastic and fine terrigenous layers. The Saltrio environment was complex, with different layers showing distinct conditions. In some areas, the Saltrio layers blend with the "Broccatello d'Arzo", a related limestone formation, but they can still be separated based on differences in their structure and fossil content. The region also experienced sedimentary discontinuities, where layers were not deposited continuously, likely due to tectonic activity or submarine erosion. The stratigraphic sequence at the Galli quarry, located at an elevation of approximately 700 meters on the southeastern flank of the ridge above Saltrio, represents one of the most detailed exposures of the Saltrio Formation. This section, reaching a total thickness of about 17 meters, begins atop underlying dolomite and consists of a series of carbonate-dominated layers that reflect varying depositional conditions in a marine setting.
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