The Stratigraphical Significance of Trace Fossils

Trace fossils have many uses in stratigraphy. Short-ranging forms can date otherwise unfossiliferous successions, as exemplified here by lower Paleozoic successions in Europe and North America. Trace fossils, including trilobite tracks, also commonly extend beneath the lowest fossiliferous Phanerozoic horizons. This distribution suggests that (1) trilobites may have evolved rapidly at this time fwm soft-bodied ancestors and (2) trace fossils should be considered in defining a base to the Cambrian system. Trace fossils are of value in environmental stratigraphy, including forms characteristic of rocky shores, sandy beaches, the neritic zone, and the bathyal-abyssal zone. Trace fossils are also useful in constructing paleogeographical syntheses, as demonstrated by examples from Paleozoic, Mesozoic, and Tertiary strata. Trace fossils can further aid the stratigrapher in tectonically complex areas, especially in determination of way-up and in quantitative estimation of tectonic compression. Finally, trace fossils are of value in global tectonics, as illustrated by particular reference to the North Atlantic. An example is the Arenig sequence: at this level, shallow-water Cruziana bearing sandstones in Wales give way northwestward, across the Irish Sea fault zone, to deep-water Nereites-bearing turbidites; in Newfoundland, a similar change occurs across a fault line between the “Avalon platform” and “central mobile belt.” This similarity suggests initial continuity of the faults, and allows equation of the Welsh sequence with that on the “Avalon platform”—not the “central mobile belt” as has generally been claimed.