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across a range of the depth zones represented by Seilacher's classic bathymetric sequence of ichnofacies. In fact, they are mostly restricted to marginal marine, intertidal and shallow shelf zones, but that is related to the commonest occurrences of the required substrates.

Organisms in sediments

Trace fossils depend on sediments. The ichno-facies scheme highlights the important roles of broad sedimentary environment (marine or continental, deep oceanic, shelf or intertidal,

Figure 19.12 The major ichnofacies, and their typical positions in a hypothetical diagram of marine and continental environments. Typical offshore marine soft-sediment ichnofacies, from deep oceanic and basinal locations to the intertidal zone, include the Nereites (N), Skolithos (Sk), Zoophycos (Z) and Cruziana (Cr) ichnofacies, which may occur in various water depths and in different conditions of sedimentation. A storm-sand fan and a turbidite fan are indicated. The Psilonichnus (Ps) ichnofacies occurs in supratidal marshes and the Scoyenia (Sc) ichnofacies includes all lacustrine and related continental settings. The Glossifungites (G) ichnofacies is typical of firmgrounds, the Trypanites (Tr) ichnofacies consists of borings in limestone, and the Teredolites (Te) ichnofacies consists of borings in wood. (Modified from Frey et al. 1990, and other sources.)

Figure 19.12 The major ichnofacies, and their typical positions in a hypothetical diagram of marine and continental environments. Typical offshore marine soft-sediment ichnofacies, from deep oceanic and basinal locations to the intertidal zone, include the Nereites (N), Skolithos (Sk), Zoophycos (Z) and Cruziana (Cr) ichnofacies, which may occur in various water depths and in different conditions of sedimentation. A storm-sand fan and a turbidite fan are indicated. The Psilonichnus (Ps) ichnofacies occurs in supratidal marshes and the Scoyenia (Sc) ichnofacies includes all lacustrine and related continental settings. The Glossifungites (G) ichnofacies is typical of firmgrounds, the Trypanites (Tr) ichnofacies consists of borings in limestone, and the Teredolites (Te) ichnofacies consists of borings in wood. (Modified from Frey et al. 1990, and other sources.)

lake or terrestrial), salinity and sedimentation rate. Sediments affected the ancient burrow-ers and crawlers (biological effects), but the sediments also affect how trace fossils look to us today (preservation effects).

The physical properties of sediments can exert controls on trace fossil distributions, and four factors are particularly important:

1 The average grain size affects sedimentingesting burrowers, organisms that require particular sediment sizes to line their burrows, and filter feeders which must avoid fine suspended sediment.

2 Sediment stability, particularly in the Glossifungites and Trypanites ichnofacies, which depend on firm and lithified substrates, respectively. Some organisms build burrows of different morphology, depending on the stability of the sediment.

3 Water content, producing sediments that range from soupy in consistency to totally lithified sediments with zero porosity, whose sole trace fossils are borings (Trypanites ichnofacies). Firmgrounds contain relatively little water and are character ized by particular burrows of the Glossi-fungites ichnofacies. 4 Chemical conditions in sediments, particularly oxygen levels. Trace fossils are rare or absent in completely anoxic situations, but a surprising variety of animals can survive in dysoxic (very low oxygen) conditions. Generally, the smaller the burrow, the lower the oxygen level.

Burrowing organisms divide up the different strata of unconsolidated sediment in rather precise ways, a phenomenon called tiering. The top few centimeters of sediment on the seafloor, the mixed layer, may be a mixture of water and sediment, either loose sand that moves with the currents or soupy mud. Deeper down is the historical layer, the older consolidated sediments from which water has been squeezed, and between the two is the transitional layer. Each burrower is restricted to a particular depth of burrowing, some exploiting the near-surface oxygenated zone, and others extending ever deeper into the sediment. Interpretation of tiering can be difficult because the mixed layer is readily disturbed,

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