Moltrasio Formation

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.^[7] 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.^[7][8]

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.^[8]

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.^[9]

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.^[7]

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.^[9] 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.^[9] The contact between the Main Dolomite and the Moltrasio Formation also contains selachian teeth, glauconite and phosphated internal models of ammonites.^[10]

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.^[11]

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.^[11] 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.^[11]

Basinward, these deposits grade into the spiculitic Kieselkalk, a sponge-rich lime mudstone with interbedded bioclastic and fine terrigenous layers.^[11] 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.^[11]

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.^[12] 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.^[12]

At the base, a thin dolomitic breccia layer (up to 1 meter thick) contains angular dolomite fragments embedded in a lighter calcareous-dolomitic matrix. This is overlain by a 0.3-meter-thick marly limestone with minor detrital components, displaying an olive-gray to greenish hue and iron oxide stains. Next is a 0.8-meter saccharoidal limestone with sparse marl, glauconite, and quartz grains, followed by a thin (0.1–0.2 meter) reddish-brown clay horizon.^[12]

Above this, a 1-meter oolitic limestone features intact and broken ooids in a compact calcareous cement, grading from white to grayish-yellow. This transitions into a 3-meter unit of finely to coarsely detrital marly limestone rich in organic fragments, shifting from gray-pink at the base to yellowish upward, with iron oxide patches. A 0.8-meter calcareous breccia with diverse clasts and ooids follows, exhibiting irregular surfaces.^[12]

The upper part includes a 5.5-meter marly limestone with minimal detritus, progressing from gray-ashy at the base to dark smoky due to bituminous content. The sequence concludes with a 5-meter dark limestone containing chert nodules, which become more abundant and marly toward the top, before passing into overlying cherty limestones approximately 200 meters thick.^[12]

During the late Hettangian to early Sinemurian, the western Lombardy Basin formed part of the Southern Alps area, passive margin of Adria and formed part of an evolving rift system linked to the western Tethys, where horst-and-graben tectonics created alternating shallow platforms and subsiding troughs.^[13] Structural highs such as Monte Campo dei Fiori, the Varese-Arbostora swell, and Monte Nudo defined the basin architecture, producing shallow carbonate platforms, emergent land, and subsiding depocenters.^[13][14][15] Nearby emerged areas are known at Briançonnais with the bauxite rich “Siderolitico” linked to long emersion phase (Ladinian-Bathonian) and Western Tauern with graphitic schists and quartzites & sandy marble, overall pointing to the Lombardian sector acting as a possible land bridge with Laurasia, connected with the Calcari Grigi di Noriglio Formation in the Trento carbonate Platform, based on Theropod tracks of similar size to Saltriovenator.^[16] The main inner land was the Malossa-Zandobbio palaeohigh system in the Po Plain, tied with the Saltrio area by a regional belt of positive blocks.^[17] This Highs, if assumed as a single unit probably got 1,000-3,000 km² for the inner lands and 22,500 km² in total of intermittently exposed terrain (similar in size to modern Sicily).^[18] Indicators of subaerial conditions are seen at Castello Cabiaglio-Orino, thick "terra rossa" paleosols developed directly above the Rhaetian Zu Limestone, showing rhizoliths, alveolar structures, and meteoric diagenesis.^[13] Palynological assemblages from these horizons indicate Hirmeriellaceae-dominated forests and understory of Lycopsid-Ferns, as well potential Characeae, adapted to marsh or ponded settings in a Tropical subhumid climate.^[13][19] These emerged lands bordered a gulf-shaped embayment, open northward, where shallow-marine carbonate platforms alternated with rapidly subsiding basins.^[20] Towards the Early Sinemurian the Arbostora swell submerged into a shallow open sea (ramp-slope), still bordered south and southwest by emerged land supported by terrigenous sands from eroded igneous/metamorphic rocks and terrestrial plants in the limestones.^[13][21]

Within the Moltrasio Formation, the shallowest deposits belong to the "Alpe Perino Limestone" ("Gozzano-type" marginal onlap), a small carbonate platform developed on structural highs and fault-bounded grabens. Its basal beds of Ostracod-rich mudstones and marls, with reworked Triassic lithoclasts and local plant remains (Castello Cabiaglio–Orino section), reflect restricted lagoons or marsh-like ponds on the inner platform.^[13][22] Then the facies evolve in repeated shallowing-upward cycles with stromatolitic-rich tidal flats and fossil-rich (gastropods, bivalves, echinoids, Dasycladales, and Foraminifera) shoals, marking a dominance of shallow subtidal to intertidal settings in the Monte Nudo basin margin, with limited terrigenous input but clear evidence of proximity to land.^[22] This unit is either part of the "Late Hettangian hypothesis" (shallower section being flooded by the Saltrio Beds) or is part the "Early Sinemurian hypothesis" (overlap with earliest Saltrio Beds on tectonic blocks and be diachronous shallow-platform vs. outer-ramp).^[13]

Early marine flooding in fault-bounded grabens produced the “Viggiù-type” facies, consisting of cross-bedded coarse epiclastic calcarenites-rudites, rich in rounded ooids, algal-encrusted bioclasts, dolomitic pelites, and lithoclasts derived from exposed Triassic platforms (Hauptdolomit), later mixed with echinoderm debris.^[11] These record short-transport input from adjacent structural highs during the first phase of transgression.^[11]

On neighboring highs, pale crinoidal packstones-grainstones (“Saltrio-type”) formed as autochthonous crinoid meadows, with encrusting sponge reefs, bivalvia, brachiopods, bryozoans, and foraminiferans.^[11] Fragmentary Ichthyosaur remains and bioeroded dinosaur bones (e.g., Saltriovenator zanellai) suggest transport from nearby terrestrial sources into a proximal slope or ramp, that is, an open subtidal zone some dozen of meters depth reached by the effects of storm waves and with constant bottom currents.^[23] Pauses in sedimentation generated bored hardgrounds coated with glauconite and phosphatic crusts, indicating slow accumulation under open-marine conditions.^[11]

In intervening lows, condensed horizons (“Poaggia-type”) developed on firmgrounds, hosting abundant ammonites and other pelagic organisms, overlain by thin encrinitic calcarenites and laminated spiculitic micrites that contain resedimented crinoid grains, sponge spicules, fish-phosphate, and fine terrigenous silt, reflecting hemipelagic deposition and early slope development. At the platform margins, such as Gozzano and Monte Fenera, reddish calcarenites and litharenites with basement-derived quartz and rhyolite mark true transgressive onlap onto exposed highs.^[11] These later give way to sponge-dominated carbonate mounds (locally “Broccatello-type”), reflecting a benthic, sponge-reef-dominated carbonate factory in deeper, low-light environments during progressive platform drowning.^[24]

Regional studies link this to platform drowning amid rifting, with carbon-isotope excursions implying volcanic influences and ocean perturbations. A modern analogue is the Bahama Banks, featuring oolitic shoals and lagoons in a subtropical passive-margin setting.^[12]

• List of fossiliferous stratigraphic units in Italy
• Calcare di Sogno, Toarcian fossiliferous formation of Lombardy
• Rotzo Formation, Pliensbachian fossiliferous formation of Veneto
• Coimbra Formation, Sinemurian fossiliferous formation of Portugal
• Aganane Formation, Pliensbachian Formation of Morocco