An interval of time in Earth history (Cambrian Period) and its rock record (Cambrian System). The Cambrian Period spanned about 60 million years and began with the first appearance of marine animals with mineralized (calcium carbonate, calcium phosphate) shells. The Cambrian System includes many different kinds of marine sandstones, shales, limestones, dolomites, and volcanics. Apart from the occurrence of an alkaline playa containing deposits of trona (hydrated basic sodium carbonate) in the Officer Basin of South Australia, there is very little provable record of nonmarine Cambrian environments.