The mode of preservation of trace fossils can show whether they were produced before or after a major sedimentary event, such as a turbidite flow. Turbidites are underwater avalanches, or gravity flows, that may transport vast quantities of sediment rapidly into deeper waters. The Lower Silurian mudstones and sandstones of central Wales and the Welsh coast have long been known as a source of trace fossils that belong to different environments within the deep-ocean Nereites ichnofacies. Crimes and Crossley (1991) identified 25 ichnogenera from the sandstone turbidites of the Aberystwyth Grits Formation, the commonest forms being Helminthopsis, Paleodictyon and Squamodictyon (Fig. 19.6a, b). The finer-grained sediments yielded different ichnofaunas, consisting mainly of Nere-ites, Dictyodora, Gordia and Helminthoida (Fig. 19.6c, d).
One clear distinction in the Welsh Basin ichnofaunas was probably the result of minor turbidite activity at the toe of spreading fans. Pre-turbidite and post-turbidite assemblages have been identified, representing the trace fossils that are formed in normal background times, and those that were formed after a turbidity flow event. Before the flow, Orr (1995) identified an assemblage of surface trails and shallow burrows. After the passage of a low-energy turbidite flow, the top layers of the existing sediment were stripped off, casting the deeper pre-turbidite burrows as convex hyporeliefs on the sole of the turbidite sand. After the flow had waned, a post-turbidite trace fossil assemblage was developed within the turbidite sand (Fig. 19.6e).
These insights allow a clearer interpretation of the trace fossil assemblages: although Helminthopsis and Paleodictyon may be found together in the sandstones, Helminthopsis is an opportunistic post-turbidite form that colonized the sands soon after the turbidite flow had ceased. Only later did Paleodictyon and other forms move in to occupy the stable sediment.
turbidite erosion and deposition post-turbidite assemblage
turbidite erosion and deposition pre-turbidite assemblage
Figure 19.6 Typical trace fossils of the Lower Silurian sediments of the Welsh Basin (Nereites ichnofacies): (a) Helminthopsis, (b) Paleodictyon, (c) Nereites, (d) Gordia, and (e) the pre- and post-turbidite trace fossil assemblages. (Courtesy of T. P. Crimes.)
removes the top layers of sediment above a burrow, the burrow may be seen as a semirelief. Trails are formed on the top of the sediment pile, and are thus exogenic (exo, outside) structures, typically seen as semireliefs. Under-tracks are impressions formed on sediment layers below the surface on which the animal was moving, and it is important to distinguish these from the true track as the morphology may be different. The shapes of tracks and undertracks have been investigated in numerous experimental studies on modern animals (Box 19.3).
Interpreting ancient behavior_
Trace fossils can plug gaps in knowledge when body fossils are rare. Two environments where
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