Spherical concretions : understandings and applications

Abstract

Spherical concretions found in sedimentary rocks are fascinating natural objet trouvés because of their rounded shapes and distinct boundaries. They consist of several minerals, including carbonate minerals, silicate minerals, and iron oxides. Well-preserved fossils are often found in concretions, particularly those composed of calcium carbonate. Concretions are thought to form by diffusion and the development of a syn-depositional reaction front that travels rapidly from the center of the concretion toward its outer margins. Based on the examination of several hundred spherical calcium carbonate concretions, we developed a diffusion-based model to represent the generalized growth conditions of spherical concretions. This model shows that spherical concretions grow rapidly during the first few years of diagenesis. In particular, carbonate concretions consist mainly of CaCO₃, and their permeability is greatly reduced by cementation and sealing by calcite. As a result, any fossils inside the concretion are well preserved, as water is prevented from penetrating the concretion after its formation. This sealing can provide strong resistance to weathering for more than a million years. Based on this model, we have developed synthetic concretion-forming solvents. To test the effectiveness of these solvents in sealing groundwater flow paths, we conducted an in situ experiment in an underground laboratory in Horonobe, Hokkaido. In the experiment, groundwater flow paths in the excavation damaged zone around an underground gallery were successfully sealed. The experiment showed a decrease in permeability by a factor of 1/100 to 1/1,000 over one year. Here we present a detailed model of the concretion formation process and our conclusions about the sealing process. This sealing process can be applied to activities that require long-term containment of material underground; for example, the geological disposal of nuclear waste and underground carbon dioxide storage. These applications will become increasingly important in the near future.