Trace Fossils In Sediments

Trace fossils as environmental indicators_

The discovery that electrified ichnologists in the 1960s was that certain trace fossils were reliable instant guides to ancient sedimentary environments. Identify a particular trace fossil, or trace fossil assemblage, and you have pinned the water depth, tide and storm conditions, salinity and oxygen levels. And this works whatever the age of the rocks, whether Cambrian or Cretaceous. The trace fossils remained remarkably constant in appearance, even if their producers might have been quite different.

This paleoenvironmental scheme of trace fossils presented by Seilacher (1964, 1967b) has been modified and enlarged since then (Frey et al. 1990), but in principle it divides trace fossil assemblages into a number of ichnofacies (Fig. 19.12). The ichnofacies are named after a characteristic trace fossil, and they indicate particular sedimentary facies (Box 19.6). The ichnofacies is identified on the basis of an assemblage of trace fossils, and it may be recognized even if the name-bearing form is absent.

The classic marine ichnofacies, those named for Nereites, Zoophycos, Cruziana and Skolithos, are not simply depth-related, as Seilacher first proposed, but are associated with particular sedimentary regimes, combining aspects of water energy, bottom sediment type, temperature, chemistry and food supply. These four ichnofacies include assemblages of trace fossils typical of fair-weather, normal conditions of deposition, and those characteristic of exceptional storm and turbidite event beds. The complexity of controls on the marine ichnofacies is shown in many field-based studies where alternations between ich-nofacies may be found at a single location (Box 19.7).

The Scoyenia ichnofacies is one of several continental trace fossil facies, and depends on the presence of shallow freshwater, while the Psilonichnus ichnofacies is controlled by coastal marine influence on a terrestrial setting. Not included here are some additional terrestrial ichnofacies (see Mcllroy 2004). Since 1990, several ichnologists have proposed ichnofacies in ancient soils, paleosols, to characterize different kinds of insect burrows, nesting chambers and the like, and

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