Box Alternating ichnofacies

Many sedimentary sequences show a mix of ichnofacies, as would be expected, since no ichnofacies is exclusive to a single location or water depth. The Cardium Formation of Alberta, Canada has produced abundant trace fossils from a sequence of muds and sandstones (Pemberton & Frey 1984) (Fig. 19.14). The normal quiet-water sedimentation produced mud, silt and fine sand layers, and diverse trace fossils of the Cruziana ichnofacies, mainly representing the activities of mobile carnivores and deposit feeders exploiting relatively nutrient-rich, fine-grained sediments. These sediments were interrupted sporadically by storm beds, thick units of coarse sand washed back from the shore region into deeper water by storm-surge ebb currents. The trace fossils of these units are Skolithos, Ophiomorpha, Diplocraterion and various fugichnia, all typical elements of the Skolithos ichnofacies.

One view of this alternation between trace fossils of the Cruziana and Skolithos ichnofacies might be that there had been repeated changes in sea level from deep to shallow offshore conditions. However, the control is more probably the dramatic changes in energy of deposition. The opportunistic members of the Skolithos ichnofacies colonized the storm beds, probably having been washed down from the intertidal zone, and they were able to cope with the rapid fluctuations in unconsoli-dated sediment depth. The storm events doubtless killed off most of the members of the Cruziana ichnofacies, or displaced them to the margins of the affected area. After the storm-surge ebb currents waned, and slow sedimentation resumed, the surface-feeding organisms recolonized the whole area.

Ichnofacies

Figure 19.14 Sediments and trace fossils in the Late Cretaceous Cardium Formation of Alberta. Normal, fine-grained sediments (A, C) are associated with Cruziana ichnofacies trace fossils, while intermittent, coarse, sandstone, storm beds (B) show trace fossils of the Skolithos ichnofacies. 1, Chondrites; 2, Cochlichnus; 3, Cylindrichnus; 4, Diplocraterion; 5, Gyrochorte; 6, Paleophycus; 7, Ophiomorpha; 8, ?Phoebichnus; 9, Taenidium; 10, Planolites; 11, Rhizocorallium; 12, Rosselia; 13, Skolithos; 14, Thalassinoides; 15, Zoophycos. (Based on Pemberton & Frey 1984.)

Figure 19.14 Sediments and trace fossils in the Late Cretaceous Cardium Formation of Alberta. Normal, fine-grained sediments (A, C) are associated with Cruziana ichnofacies trace fossils, while intermittent, coarse, sandstone, storm beds (B) show trace fossils of the Skolithos ichnofacies. 1, Chondrites; 2, Cochlichnus; 3, Cylindrichnus; 4, Diplocraterion; 5, Gyrochorte; 6, Paleophycus; 7, Ophiomorpha; 8, ?Phoebichnus; 9, Taenidium; 10, Planolites; 11, Rhizocorallium; 12, Rosselia; 13, Skolithos; 14, Thalassinoides; 15, Zoophycos. (Based on Pemberton & Frey 1984.)

and traces are lost or distorted, and the historical layer may contain traces of many different generations, perhaps representing many months or years of erosion and deposition.

Most burrowers are restricted to the mixed layer, as this minimizes the physical effort for animals that are simply moving from A to B. Organisms that feed on organic matter also favor the surface layers. Deeper burrowers are mainly those forming domichnia, where the body of the organism is large, or where it possesses long siphons, in order to keep contact with oxygenated waters above. Deeper layers are also safer from predators, whether those operating from the surface, or other burrowers. There are also feeding opportunities at depth, at the redox layer, where the oxygenated surface sediments meet the deeper anoxic sediments - a horizon that is characterized by unusual shelly faunas and sulfur-oxidizing bacteria.

Such tiering patterns in any particular environment increase in complexity through time. In a Middle Ordovician example (Fig. 19.15a), the subsurface layers are filled with simple horizontal burrows, Planolites. These are cut by branching fodinichnia, Chondrites, exploiting an organic-rich layer at a depth of 2030 mm below the surface of the sediment. The deepest burrows may be Teichichnus, showing spreiten structure (multiple ghosts of previous burrow positions), and extending down to 100 mm at the deepest. An Early Jurassic example (Fig. 19.15b) shows a substantial increase in depth burrowed, to perhaps 0.5 m, with small Chondrites in the upper layers, a new large Chondrites extending to lower layers, and domichnia, Thalassinoides, at the deepest levels. Finally, in a Late Cretaceous example (Fig. 19.15c), there are at least nine tiers, three horizons of shallow burrows near the surface, Planolites, Thalassinoides, Tae-nidium, Zoophycos, and large and small Chondrites going deepest, perhaps to a maximum depth of 1 m.

Trace fossils and time_

Trace fossils do not evolve in the way body fossils do, and they generally cannot be used for dating rocks. This is mainly because of their rather peculiar properties; as we have seen, trace fossils are excellent indicators of sedimentary environments just because they

(a)
(b)
Ichnofacies

Figure 19.15 Examples of trace fossil tiering, in which burrowers choose specific depth horizons below the sediment-water interface. (a) In the Middle Ordovician limestones of Oland, Sweden, there are three tiers. (b) In the Early Jurassic Posidonienschiefer of Germany, there are also three tiers. (c) In the Late Cretaceous Chalk of Denmark, there are at least nine tiers. (Based on Ekdale & Bromley 1991, and other sources.)

Figure 19.15 Examples of trace fossil tiering, in which burrowers choose specific depth horizons below the sediment-water interface. (a) In the Middle Ordovician limestones of Oland, Sweden, there are three tiers. (b) In the Early Jurassic Posidonienschiefer of Germany, there are also three tiers. (c) In the Late Cretaceous Chalk of Denmark, there are at least nine tiers. (Based on Ekdale & Bromley 1991, and other sources.)

Unit

Characteristics or first appearance of ichnotaxa

General characteristics

Unit

Characteristics or first appearance of ichnotaxa

General characteristics b mb w o L

Cruziana tenella

Plagiogmus

Arthropod furrowing traces. Large back-filled burrows

Rusophycus avalonensis

Taphrhelminthopsi

s Rusophycus

Arthropod resting traces. Large bilobed furrowing traces

Treptichnus pedum

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