Ichnofacies
An ichnofacies is an assemblage of trace fossils that provide an indication of the conditions that their formative organisms inhabited.
Concept
Trace fossil assemblages are far from random; the range of fossils recorded in association is constrained by the environment in which the trace-making organisms dwelt.[1] Palaeontologist Adolf Seilacher pioneered the concept of ichnofacies, whereby the state of a sedimentary system at its time of deposition could be deduced by noting the fossils in association with one another.[1]
Significance
Ichnofacies can provide information about water depth, salinity, turbidity and energy. In general, traces found in shallower water are vertical, those in deeper water are more horizontal and patterned.[1] This is partly because of the relative abundance of suspended food particles, such as plankton, in the shallower waters of the photic zone, and partly because vertical burrows are more secure in the turbulent conditions of shallow water. In deeper waters, there is a necessary transition to sediment feeding (extracting nutrients from the mud).[1] Food availability, hence trace type, is also controlled by energy: high energy environments keep food particles suspended, whereas in lower energy areas, food settles out evenly, and burrows will tend to spread out to cover as much area as economically as possible.[1]
Ichnofacies have a major advantage over using body fossils to gauge the same factors; body fossils can be transported, but trace fossils are always in situ.[1]
Recognised ichnofacies
| Ichnofacies[2][3] | |||
|---|---|---|---|
| Ichnofacies name | Constituent fossils | Usual matrix | Implied environment |
| Cruziana | horizontal repichnia and cubichnia; vertical burrows | Well sorted sands and silts | Mid to distal continental shelves. Below normal wave base, but not necessarily below storm wave base |
| Glossifungites[1] | Omission surfaces[1] | Erosion surfaces on semi-consolidated muds, which suspension feeders dug shelters into.[1] Usually shallow water, but channelisation may allow to occur in deeper water.[1] | |
| Nereites | pelagic muds between turbidites[1] | ||
| Psilonichnus | |||
| Scoyenia[1] | Skolithos, Cruziana, Diplichnites, Rusophycus[4] | Variable - typically sandstones; red beds may be nearby | Diagnostic of a terrestrial[1]/freshwater[4] facies. |
| Skolithos | littoral sands[1] | Shallow water,[1] above wave base (?) | |
| Teredolites | |||
| Trypanites | |||
| Zoophycos | impure sands and silts[1] | Deeper water, bottom of shelf; turbidite facies.[1] | |
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12 1.13 1.14 1.15 1.16 Seilacher, A. (1967). "Bathymetry of trace fossils". Marine Geology 5 (5-6): 413–428. doi:10.1016/0025-3227(67)90051-5.
- ↑ "Ichnofacies". UCL.
- ↑ Benton, M.J.; Harper, D.A.T. (1997). Basic palaeontology. Longman Harlow, Essex, England. ISBN 9780582228573.
- ↑ 4.0 4.1 Woolfe, K.J. (1990). "Trace fossils as paleoenvironmental indicators in the Taylor Group (Devonian) of Antarctica". Palaeogeography, Palaeoclimatology, Palaeoecology 80 (3-4): 301–310. doi:10.1016/0031-0182(90)90139-X.