Early marine diagenesis in calcium carbonate rich sediments: A review of implications for geotechnical systems

Soils rich in skeletal calcium carbonate are well known to undergo diagenesis under shallow seawater environment early after deposition. Numerous diagenetic processes are driven or catalyzed by microorganisms, e.g., algae and fungi, if sunlight and organic matter are bio-available. The objectives of this paper are to review the effects that such diagenesis has on the geotechnical properties of skeletal carbonate strata, specifically a phenomenon known as "micrite" formation, and to point out the potential relationship of micrite formation with the unusual very soft or very hard mechanical response of the Savannah River Site's Santee Formation. Micrite formation is the bio-geochemical replacement of skeletal carbonate by a new carbonate called "micrite". The micrite mineral makeup is aragonite or high-Mg calcite, but its isotopic ratios and strontium content are markedly different from those of the original skeletons. Furthermore, the micrite mineral is much finer (e.g., higher specific surface), less porous, and less soluble than its skeletal counterpart, and commonly becomes the dominant soil matrix. Upon freshwater flush, micrite tends to lithify (harden), in contrast with skeletal carbonates which are susceptible to dissolution. The diagenetic mechanism of micrite formation thus may explain atypical mechanical behavior within carbonate-rich soils in the US Southeast.