what is the geochronological significance of Glauconite?

Answer 1

Normally, glauconite is considered diagnostic of continental shelf marine depositional environments with slow rates of accumulation. Typically, it appears in Jurassic/lower Cretaceous deposits of Greensand, so-called after the colouration provided by the glauconite. It can also be found in impure limestones, such as Kentish Rag and in Chalk. It develops as a consequence of diagenetic alteration of sedimentary deposits, changes in the biotite micas, for example, being influenced by the decaying process of the organic matter in animal shells. Glauconite forms under reducing conditions in sediments and such deposits are commonly found in nearshore sandstones, open oceans and the Mediterranean Sea but not in the Black Sea or in fresh-water lakes. It oxidises on contact with air; the resulting deoxygenated air is a hazard to miners.