Spherulitic and Cone-in-cone Concretions in Cambro-Ordovician Black Shales, Île-aux-Grues/Île-aux-Oies, Quebec, Canada: An Interpretation of Their Origin

English Abstract:

Spherulitic concretions are an exception among carbonate concretions which are a trademark of organic-matter rich sediments and usually consist of micritic carbonate. The occurrence in Cambro-Ordovician black shales of unknown stratigraphic age on mid-channel islands in the St. Lawrence Estuary in Quebec is among four known examples of spherulitic carbonate concretions. To interpret their origin is the objective of this contribution. The concretions studied occur in close association with and show various transitions to cone-in-cone structure. A phenomenological theory of spherulitic crystallization relates the thickness δ of an impurity-rich layer in front of impurity-rejecting growing crystals to the impurity-diffusion coefficient D and the growth velocity G of the crystal by δ=D/G. In spherulite-forming environments, extremely small values of δ (on the order of <10^-4 cm) in con junction with cellulation lead to spherulitic fibre growth.The Quebec occurrence consists of intergrown fine fibers of ferroan calcite and quartzine, pointing to the formation of the concretions below the sulfate reduction zone. Most spherules precipitated experimentally in the laboratory crystallized in a gelatinous medium in the presence of impurities. Silica gel is the most common medium used. The occurrence of silica gel has not been reported from clastic or biogenic siliceous pelagic deep-sea sediments, not even in hydrothermally sourced ponds of mid-oceanic ridges. However, experimental spherulite precipitation has also been achieved from alkaline aqueous solutions. This opens the possibility to explain the intimate intergrowth of calcite and quartzine in the concretions. Alkaline conditions are established during early diagenesis in the lower sulfate reduction zone so that fibrous calcium carbonate precipitation in the presence of impurities may start in the carbonate-reduction (methane-generation) zone. The switch from carbonate precipitation to silica precipitation into the pore spaces of coarse, open-texture fibrous carbonate spherulite may be achieved by the dissolution of sponge spicules or radiolarians which would lead to low supersaturation allowing the direct precipitation of quartzine.