Three important Alpine pelagic lithologic types are ammonitico rosso, Radiolarite, and Maiolica, which range in age from late Early Jurassic to Early Cretaceous.
Ammonitico rosso is in many places red nodular limestone, lying above continental crust. This pelagic deposit bears some resemblance to some Holocene nodular deposits in the eastern Mediterranean. A somewhat restricted Middle Jurassic Alpine Tethys sea has been postulated to account for the occurrence of the “home-made” bottom currents that led to partial dissolution of aragonite and precipitation of calcite nodules.
The sudden appearance of the radiolarite facies in Late Jurassic time may signify a major tectonic event that sufficiently altered the Tethyan paleogeography to permit the intrusion of an equatorial current into the Alpine realm. The radiolarite facies is commonly believed to have been deposited beneath the calcite compensation depth, but calcitic aptychi have been found in radiolarite, and Aptychus Limestone is intimately associated with this mainly siliceous formation. The depths of the Radiolarite sea during Late Jurassic time were probably closer to the lysocline for aragonite. Because the Radiolarite is the first sediment deposited on ophiolite, one might postulate that the Tethys at the time of Radiolarite deposition had an oceanic depth comparable to that in regions of newly formed crust. Both lines of reasoning have led me to suggest that the Tethys in Late Jurassic time had a depth of 2,500 ± 200 m, not 4,000 to 5,500 m as postulated previously by some authors.
The Maiolica Formation is pelagic limestone with slump interbeds and intraformational chert nodules. The radiolarite-limestone succession in the Alpine Tethys is exactly the opposite of the simple model constructed by plate stratigraphers, who usually found a limestone-radiolarite-red clay succession on an aging crust. It is possible that excessive blooms of siliceous faunas may have created a silica crisis at the end of Jurassic time. The Alpine Tethys, being disadvantageously situated at the end of the Jurassic equatorial current system, could no longer be given enough silica to produce radiolarians. In their place, a calcareous flora of nannofossils flourished, resulting in the change from Radiolarite sedimentation to Maiolica sedimentation.
Considering the rate of the silica transport, chert in the Maiolica deposit could not have been formed by submarine diagenesis. Nodules took millions of years to grow in formations in which concentration gradients existed between the domain where amorphous biogenic silica was being dissolved and the nuclei around which quartz or cristobalite was being precipitated.