THE CAMBRIAN-ORDOVICIAN ADVENT OF INTERTIDAL BIOTURBATION: TIDAL FLATS BEFORE AND AFTER

The history of tidal-flat sedimentation can be subdivided into two periods separated by the onset of intertidal bioturbation in the Cambrian. Tidal flats are complex systems subject to marked environmental stresses, including subaerial exposure, extremes of salinity, and locally high sedimentation rates. Bioturbators in the intertidal zone appear to have overcome problems associated with subaerial exposure in two stages. Beginning in the Early Cambrian, organisms evolved the ability to bioturbate intertidal substrates during high water, while surviving low water by retreating into burrows or shells, or by moving to a submerged region. Bioturbation led to disruption of depositional textures, sediment mixing, enhanced sediment irrigation and oxygenation, and the ingestion and excretion of sedimentary particles.

The second stage is associated with the Late Cambrian-Early Ordovician advent of animals capable of surviving subaerial exposure, as is recorded by tracks and trails of potential and known subaerial origin in the Mt. Simon-Wonewoc and Nepean formations of the North American midcontinent. The ability of organisms to survive and subsequently thrive in subaerial conditions added new ecological and sedimentary dynamics to tidal-flat settings in the Late Cambrian-Early Ordovician. Onset of subaerial activity magnified the effects of subaqueous burrowing in these environments.

The advent of bioturbation profoundly changed the physical, chemical, and biological characteristics of sediments in the intertidal zone. Sediment reworking substantially reduced the preservation potential of primary sedimentary structures. Reworking and grazing by metazoans probably led to the demise from the intertidal zone of microbially dominated "matground" sedimentation more typical of the Precambrian. Such factors must be taken into account when using Recent analogues to interpret tidal-flat successions of Cambrian age and older.