The Bitter Springs preservational mode is the preservation of microorganisms in silica, in shallow Precambrian waters.

Taphonomic processes

Silica is emplaced around the organisms very rapidly, before their cells can even collapse; the strong silica is able to withstand great pressures and preserves three dimensional detail even after metamorphosis. Microbial mats, as they decayed, acted as nucleii for silica precipitation.

Occurrence

The preservational mode is only present in the Precambrian

Precambrian

The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...

, and is restricted to shallow or tidal waters.
After this point, organisms such as sponges extracted silica from the oceans in order to construct tests (skeletons), with the result that the surface oceans no longer became supersaturated with respect to silica when evaporation condensed the mineral.

Constraints

Because microbial mats and shallow, supersaturated seas were necessary for the formation of this form of deposit, larger eukaryotic organisms could not survive in the regions in which these deposits were formed; hence the record is mainly restricted to bacteria.