USING TRACE FOSSILS TO RECOGNIZE MARINE TRANSGRESSION: TIME DOES MATTER

Regardless of age, a transition from non-marine to marine deposition seemingly should be easily recognizable. A transgressive facies shift is accompanied by a marked change in sedimentation and geochemistry. In addition, with the advent of metazoans in the Precambrian, biological evidence such as the record of behaviour preserved in trace fossils is present. However, contrary to the Uniformitarian approach applied to the interpretation of physical sedimentary structures and cement mineralogy, the use of trace fossils may require a different strategy because of the time-dependent nature of tracemakers and their behaviour. Two concerns are: 1) the absence of exclusively non-marine animals and traces prior to the Ordovician and 2) the continuing evolution of new organisms and behaviours in both the marine and non-marine realms.

Examples of time dependency are illustrated by the environmentally important marine trace fossils, Skolithos and Cruziana. Skolithos, a simple, vertical dwelling tube, has been noted in deposits ranging in age from Ediacaran through Recent. Using the presence of Skolithos to recognize Lower Paleozoic marine strata is clearly less problematic than doing so in younger deposits, especially after the appearance of Devonian terrestrial insects. Similarly, the marine, feeding-locomotion trace Cruziana, if attributed exclusively to trilobites, is properly restricted to the Paleozoic. However, if this trace fossil is treated as a form genus, its inferred behaviour is succeeded by the activities of isopods and other potentially non-marine arthropods and continues into the Recent, e. g., Isopodichnus. Consequently, as with Skolithos, the interpretation of Cruziana-bearing strata as marine loses certainty through geologic time and probably cannot be done with complete confidence much beyond the Devonian.