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New radiolarian data from CHENAILLET-MONTGENEVRE ophiolite (French-Italian Alps) : click here

Radiolarians and siliceous rocks

Radiolarians Polycystines are unicellular planktonic marine organisms belonging to the class Actinopoda. They absorb silica from the marine environment and construct tiny skeletons (diameter 0.1-0.2 mm) according to well-defined geometric patterns. Radiolarian fossils are known from the early Phanerozoic to the present (550 million years) and, because an immense diversity of taxons evolved rapidly, they have a significant stratigraphic interest. Unlike calcareous skeletons, they are not subject to greater dissolution with water depth and can accumulate differentially on deep seafloors. They are considered as the background pelagic sediment at certain time periods of the Phanerozoic Eon, only diluted by calcareous biogenic or terrigenous inputs.

As a result of their deposition, radiolarians have generated accumulations of biogenetic oozes that have evolved through diagenesis into sedimentary rocks called radiolarian cherts or radiolarites. Twenty-five years ago, these rocks were still considered not datable as paleontologists could not find a way to extract siliceous shells from an also-siliceous matrix: the chemical process was discovered in the late 1960s by processing these rocks with low-concentrated solutions of hydrofluoric acid (HF).

As radiolarian-bearing rocks are widely distributed in mountain ranges exposing geological units of oceanic affinities, the tool represented by radiolarian micropaleontology has become significant in tectonic studies. Plate tectonic movements such as subduction/accretion processes, have thrust layers of oceanic crust capped by radiolarian-rich sediments into mountain ranges. Radiolarian biostratigraphy provide a mean of controlling the age of these sediments, and therefore the age of oceanic crust itself. In some orogens formed of a patchwork of various lithospheric fragments such as the North-American Cordillera, radiolarians and radiolarian cherts directly reveal to us the existence of ancient ocean basins. In doing so, they provide a mean to predate the timing of closure and deformation of these basins, a key to the understanding their geodynamic history.

Because radiolarians are also present in lithologies such as siliceous shales, mudstones, argillites, siltstones and pelagic carbonates, they provide a regional stratigraphic and structural tool used in Cordilleran geological mapping, mining exploration, as well as academic research related to oceanic environments.