Microcrystalline carbonate rock. Described in the Fossil Lake region as kerogen-rich laminated micrite (KRLM) or kerogen-poor laminated micrite (KPLM) it is primarily in this rock that the majority of Fossil Lake fossils are found.

Limestone consisting of lime mud (i.e., very fine-grained limestone).

A micro-crystalline calcite, with grain size less than 4µm (aka 'lime mud')