530 TRACE FOSSILS components of the same tier In practice, deviations from the random cross-cutting of older structures by younger can occur; these include preferential reexploitation as well as avoidance of earlier formed structures Complete reworking of sediment at depth will obliterate the record of earlier activity in shallower tiers, and thus reduce the diversity of the trace fossil community (the palaeoichnocoenosis) The preservation of a complete tiering profile requires the intensity of bioturbation to decline with depth; earlier formed parts of the ichnofabric, including occasionally the mixed layer ichnofabrics, occur as relict patches between later structures The deposition of an event bed will result in a rapid upward relocation of the infauna; if this event bed is sufficiently thick, the base of the transition layer will be moved above the older sediments, leaving a frozen tiering profile preserved within them; the distance from the sediment surface to the base of the transition layer indicates the thickness of sediment occupied (Figure 11B) The upper part of the sediment column is often a soupground, and thus remobilized relatively easily; the upper parts of the frozen tiering profile, notably the mixed layer, may be eroded during the deposition of the next bed (Figure 11C) Evacuation of the sediment column following a rapid deoxygenation event, in which the redox threshold boundary is moved above the sediment–water interface, will leave a frozen profile preserved below the succeeding sediments (Figure 11D) Mixed layer ichnofabrics, rare in the geological record, are more likely to be preserved in this manner than by burial below an event bed Other means of reconstructing the tiering profile include the identification of the depth to which an intergenic postdepositional trace fossil penetrated the substrate In a sequence of event beds of different thickness, the components of shallower tiers occur only on the soles of thinner beds (Figure 12A) Secondary trace fossils (see above) occur within a ‘piped zone’ (Figures 12B and 12C) The thickness of the piped zone indicates the volume of ecospace used by the endobenthic community; components of progressively deeper tiers occur closer to its base In the simple model in Figure 12B, the secondary trace fossils were sourced from the immediately overlying layer whilst it was being deposited, and the piped zone is contained entirely within one layer The example in Figure 12C shows a more complex situation; the lowest bed was rebioturbated during deposition of the second, not the first, overlying layer, but the latter was too thin to contain the piped zone entirely In this idealized example, the colour of each layer is different, and the source of sediment in the piped zone is obvious In practice, however, many hemipelagic Figure 12 Endobenthic tiering revealed by the ichnofabrics produced (A) The base of the event bed cannot be reached by the infauna of the shallower tiers; the trace fossils at its sole will comprise those of tier and, if the accretion of sedi ment subsequently is gradual, tier (B, C) Emplacement of secondary trace fossils defines a piped zone (pz), the thick ness of which is a measure of the volume of ecospace utilized For clarity, earlier formed parts of the ichnofabrics have been omitted and pelagic sequences are characterized by alternating lithologies, the differences in colour and composition of which can be the result of orbital forcing In such cases, trace fossils within the piped zone may be actively infilled by the same sediment as the host lithology Natural systems are obviously more complex, but more dynamic models can be produced by allowing variables, such as the rate of sediment accretion, including negative values (erosion), or the levels of oxygenation, to fluctuate over time Not all circumstances will satisfy the assumptions within the model Thin-bedded siliciclastic turbidites often exhibit a bipartite division into a sand-rich lower part and a mud-rich upper part, and infauna can position themselves with respect to the interface between the two, independent of its depth below the sediment–water interface Cross-cutting relationships can also be generated by the superimposition of two successive