Depositional environments, ichnology, and rare soft-bodied preservation in the Lower Cambrian Latham Shale, east Mojave

The Lower Cambrian Latham Shale contains very rare soft-bodied preservation of members of the Burgess Shale Fauna. Soft-bodied preservation is unusually common in Lower and Middle Cambrian strata, however, the majority of deposits which bear this type of preservation, like the Latham Shale, contain only occasional examples. The depositional environments of the Latham Shale were investigated in order to understand the conditions under which rare Burgess Shale-type preservation occurred. Mudstones of the Latham Shale consist of amalgamated event beds deposited in a shelfal setting, under fluctuating bottom water oxygen conditions which, at times, precluded infaunal activity. Soft bodied preservation occurs in only the distal, most fine-grained portions of the Latham Shale, which lack bioturbation. While frequency of burial events, lack of coarse grained sediments from some facies, and proximity of the oxycline were conducive to soft bodied preservation, the depositional setting of the Latham Shale was more typical of distal shelf settings than those of Cambrian lagerstätten, such as the Burgess Shale. Ichnologic evidence from the Latham Shale suggests that the low depth of bioturbation may have been important in soft-bodied preservation in this unit, and that the subsequent secular increase in depth of bioturbation may have restricted soft-bodied preservation in comparable environments.