Preservation of Trace Fossils

Trace fossils are normally preserved at or near interfaces, as a result of burrowing or crawling activity along the junction of successive beds of different lithology. The traces may be enhanced by diagenetic concentration of such minerals as calcite, chert, and pyrite, and hence may appear as concretionary nodules. The scour-fill activity of turbidity currents on the seabed may also play a preservational role in flysch sequences, and in some instances, a difference in orientation of grains within beds of uniform lithology may be sufficient to reveal trace fossils.

A variety of formal terms has been proposed to describe trace fossil preservation. Among the more useful are full reliefs and semi-reliefs, the latter being subdivisible into epireliefs and hyporeliefs, depending on their relationship to the main casting medium. Alternative terms are epichnia, endichnia, hypichnia, and exichnia.

Characteristics of the sedimentary matrix control the style of preservation to a considerable extent; for instance, compact clay retains finer impressions than does unconsolidated sand. Variations in sedimentation rate can affect the morphology of burrows, and the character of whole assemblages of trace fossils—as for instance in turbidite sequences—may be strongly influenced by the sedimentary bottom conditions.