These beds are sandwiched between units comprising interbedded sandstones, siltstones and shales with microbial mats, where present, usually containing the fossils.
In modern microbial mats, debris from the surrounding habitat can become trapped within the mucus, which can be cemented together by the calcium carbonate to grow thin laminations of limestone.
The combination of mild hypersalinity and low oxygen content is believed to have inhibited the formation of microbial mats and prevented the invasion of bottom dwelling organisms.