Ichnofacies
Introduction
Ichnofacies are the palaeoenvironmental tool that emerges when trace fossils are studied as assemblages rather than as individuals. A single burrow type tells you relatively little; an assemblage of burrow types, occurring together repeatedly across many geological periods and many locations, is a powerful environmental fingerprint. The recurrence of the same assemblage across geological time means that the environmental conditions that produced it - substrate type, water depth, energy, oxygen level - have been reproduced exactly, and the ichnofacies concept codifies that recurrence.
The Ichnofacies Concept
From a sedimentological standpoint, study of trace-fossil assemblages has proven more useful than study of individual ichnogenera or ichnospecies. A trace-fossil assemblage embraces all trace fossils present within a single rock unit. Among assemblage groupings, ichnofacies have particular significance in palaeoenvironmental studies. Ichnofacies describe associations of trace fossils that are recurrent in time and space and that reflect environmental conditions such as water depth (bathymetry), salinity, and the nature of the substrate on which they formed (e.g., mud vs. sand bottom). Fundamentally, ichnofacies are sedimentary facies defined on the basis of trace fossils, and each ichnofacies may include several ichnogenera. [1]
Nine principal ichnofacies are currently recognised (Table 3). Four of these - Skolithos, Cruziana, Zoophycos, and Nereites - were established on the basis of marine water depth. [1]
The Nine Principal Ichnofacies
Skolithos Ichnofacies
The Skolithos ichnofacies is characterised especially by vertical, cylindrical or U-shaped burrows - including Ophiomorpha, Diplocraterion, and Skolithos. Overall diversity of ichnogenera is low and few horizontal structures are present. The ichnofacies develops primarily in sandy sediment where relatively high wave or current energy is typical. Organisms construct deep burrows to protect against desiccation or unfavourable temperature and salinity changes during low tide, and to escape the shifting substrate. The Skolithos ichnofacies is typical of sandy shoreline environments but may grade seaward into shallow shelf environments. It has also been reported from some deeper-water environments such as deep-sea fans and bathyal slopes. [1]
Cruziana Ichnofacies
The Cruziana ichnofacies commonly occurs in somewhat deeper water than the Skolithos ichnofacies - in subtidal zones below fair-weather wave base but above storm wave base - typical of the middle and outer shelf. It is characterised by a mixed association of traces that may include nearly vertical burrows, inclined U-burrows (Rhizocorallium), horizontal structures (Cruziana), surface traces (Thalassinoides), and forms with star shapes (Asteriacites) or C-shapes (Arenicolites). The Cruziana ichnofacies commonly has high diversity and abundance of traces - a profusion of burrows may be present. It is typically developed in well-sorted silts and sands but may occur in muddy sands or silts. [1]
Zoophycos Ichnofacies
The Zoophycos ichnofacies is most typical of quiet-water environments with moderately low oxygen levels and muddy bottoms, but can occur in other substrates. It is characterised by traces ranging from simple to moderately complex, such as Spirophyton. Individual traces may be abundant, but overall diversity is low. Sediments of the Zoophycos ichnofacies may be totally bioturbated. Although commonly considered indicative of deeper water, it is known to occur also in shallow water. Its value as a palaeodepth indicator is therefore problematical - its distribution appears tied more closely to oxygen levels and bottom sediment type than to water depth. [1]
Nereites Ichnofacies
The Nereites ichnofacies is characteristic of deep water and is apparently restricted to turbidite deposits. It is distinguished by complex horizontal crawling and grazing traces and patterned feeding/dwelling structures. The ichnogenera are ornate and complicated - including Paleodictyon, Spirorhaphe, and Nereites. Total diversity of traces is high, but the abundance of individual traces is low. The Nereites ichnofacies develops initially in sandy (turbidite) substrates but may later colonise parts of muddy (pelagic) deposits that form on top of sandy turbidites. [1]
Five Additional Ichnofacies
The Psilonichnus ichnofacies is a softground ichnofacies developed under nonmarine to very shallow marine or quasi-marine conditions. It is characterised by J-, Y-, or U-shaped burrows of marine organisms, vertical shafts, and horizontal tunnels of insects and tetrapods; tracks and trails of insects, reptiles, birds, and mammals; and root traces. [1]
The Scoyenia ichnofacies occurs in both terrestrial and aquatic environments and is characterised by diverse traces including small horizontal, curved, or tortuous feeding burrows, sinuous crawling traces, tracks, trails, and vertical cylindrical to irregular shafts. The Trypanites ichnofacies develops in fully lithified marine substrates - beachrock, rocky coasts, hardgrounds, and reefs - and is characterised by cylindrical, tear-, or U-shaped borings commonly vertical to branching, mostly dwelling structures for suspension-feeding organisms; it also includes rasping and scraping traces and microborings by algae and fungi. The Glossifungites ichnofacies develops in firm but unlithified marine substrates typically consisting of dewatered cohesive muds, and is characterised by vertical, cylindrical, U- or tear-shaped borings and/or densely branching burrows of suspension feeders or carnivores such as shrimp, crabs, worms, and pholadid bivalves - individual structures may be abundant but diversity is low. The Teredolites ichnofacies is restricted to woody substrates (woodground) in estuarine or nearshore environments, and consists of profuse club-shaped borings that may be stumpy to elongate and subcylindrical to subparallel. [1]
Limitations as Palaeodepth Indicators
Although each marine ichnofacies tends to be characteristic of a particular bathymetric zone, individual trace fossils can overlap depth zones. No single biogenic structure is an infallible indicator of depth and environment. The basic controls on trace fossil formation are not simply depth but include substrate nature, water energy, deposition rates, water turbidity, oxygen and salinity levels, toxic substances, and food availability. [1]
The Zoophycos ichnofacies is the clearest illustration of this limitation. It is so closely tied to oxygen levels and substrate type that it occurs across a wide depth range - from shallow to deep - wherever those conditions are met. Using it as a depth indicator without considering these other variables leads to systematic errors in palaeoenvironmental reconstruction.
Related Topics
Sedimentary Structures
Sedimentary structures are large-scale features of sedimentary rocks - including parallel bedding, cross-bedding, ripples, and mudcracks - that form as a direct result of depositional or...
Trace Fossils
Trace fossils are the geological record of life's behaviour - burrows dug, surfaces crawled across, sediment searched for food. They are not body fossils: no skeleton was converted to mineral, no...
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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