JUST HOW COMPLETE IS THE SEDIMENTARY RECORD AT BIOMERE EXTINCTIONS? AN EXAMPLE FROM THE LOWER-MIDDLE CAMBRIAN BOUNDARY IN LAURENTIA

The causes of biomere extinctions and the nature of the ecological record is one of the central issues in reconstructing the Cambrian world. It has long been recognized that the amount of sedimentary record available for sampling and the relative stratigraphic completeness may bias perceptions of biodiversity, estimates of taxonomic rates of evolution, and abruptness of extinctions. High-resolution stratigraphy of sections along a transect from the cratonal and inner shelf to the outer shelf and slope should provide enough information to observe the details of spatial and temporal faunal variability and detect even the most cryptic of unconformities.

We have chosen to study the olenelloid extinction and recovery fauna across the traditional Lower-Middle Cambrian boundary in the region from the Grand Canyon to the Sierra Nevada. Comparison of over two dozen sections along a shelfal transect revealed a pattern of fourth- or fifth-order stratigraphic sequences, probably resulting from rapid fluctuations of relative sea level within the boundary interval. The sequences are relatively thin (less than 20 meters), have cryptic boundaries, generally beginning with a very thin (less than 0.5 meter) carbonate (sideritic pebble bed or silty ribbon limestone), overlain by a coarsening-upward clastic sequence beginning with a regionally extensive, fissile shale interval.

Detailed comparison of the stratigraphic sequences across the shelf demonstrates a coincidence between sequence boundaries and faunal change within the Lower-Middle Cambrian boundary interval. In the outermost shelf, the facies are so condensed that the extinction, survival, and initial recovery faunas occur within a few meter thick interval, which needs to be studied on a centimeter scale. In conclusion, it appears that the olenelloid extinction event may not be preserved in the stratigraphic record in most shelfal sections, and that other biomere extinctions need to be studied in much greater detail, to make sure that they are not an artefact of an imperfect sedimentary record.