Trace fossil biozonation based on first appearances is a key element of Neoproterozoic-Cambrian biostratigraphy. Three zones have previously been recognized: I) simple burrows (Neoproterozoic); II) Phycodes pedum (basal Cambrian); and III) Rusophycus avalonensis (Lower Cambrian). However, the relative roles of evolution, ecology, and taphonomy in producing this zonation have not previously been tested.

Neoproterozoic-Cambrian strata in the Mackenzie Mountains, northwestern Canada (Ingta, Backbone Ranges, and Vampire Formations) are 1.2 km thick and contain ten siliciclastic facies associations, representing depositional environments ranging from terrestrial to offshore shelf. The succession can be subdivided stratigraphically into eighteen simple sequences, each approximately one million years in duration. Trace fossil occurrences were digitized at the level of simple sequences and were plotted on a matrix with facies associations (environment) shown on the x-axis and stratigraphic sequences (time) shown on the y-axis. Trace fossil assemblages can be used to recognize the Precambrian-Cambrian boundary with a high degree of consistency in this succession. Cambrian-type trace fossils do not occur in any facies in any of the four Neoproterozoic simple sequences. In contrast, they occur in all fourteen Lower Cambrian simple sequences and are present in 87% (26 of 30) of the fossiliferous Cambrian domains within this matrix. Several of the ichnotaxa crucial to the biozonation (e.g. Phycodes, Rusophycus) show broad environmental tolerance and occur in most or all trace fossil-bearing facies associations. Subdivision of the subtrilobite Lower Cambrian into three zones is readily evident in sublittoral facies but is more equivocal in marginal marine facies.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah