Trace Fossils and Sed/Strat

Ichnofacies represent recurring groupings of trace fossils, and are the backbone of many sedimentological and stratigraphic applications of trace fossil studies. Initial identification of recurring ichnofossil assemblages lead Adolf Seilacher to propose several distinct ichnofacies, and relate them to specific marine depositional environments (Seilacher, 1953; Seilacher, 1967). The fact that many of these ichnofacies seemed to be depth dependent caused quite a stir; after all, infallible bathymetric data would considerably simplify much of our interpretations of marine depositional environments. This simple relationship was shown to be incorrect, however (Frey et al., 1990). Various environmental factors, including energy (which controls sedimentation rate, turbidity, and substrate characteristics), the availability of light and oxygen, and the salinity of the ambient water all plays an important role in the occurrence of critters and their traces (Pemberton and MacEachern, 2006).

Reproduced here is Figure 1 (page 156) from Frey et al. (1990), showing the distribution of various ichnofacies as representative of, but NOT exclusive to, specific suites of depositional systems. For instance, the diagrammatic sandy submarine fan system on the right side of the diagram is shown as being typified by the Skolithos ichnofacies, which is also shown as occurring in the nearshore (foreshore) environment on the left side of the diagram. This illustrates that the creatures are more attuned to the exploitation of high energy, sandy subtrates, rather than being restricted by some bathymetric concern.

Although this more complicated view of ichnofacies pretty much dashes our hopes of a simple bathymetric evaluation of marine rocks based on critter tracks, it does provide us a very useful way to track changing energy conditions in a succession of rocks. Additionally, the time scale at which animals produce their tracks is considerably finer than any resolution we can have in the rock record. Most trace-makers have life spans in the range of months, allowing us to compare cross-cutting and tiering relationships, or the relative timing of emplacement (particularly where escape structures are preserved).

Distinguishing event beds in the rock record is an area where trace assemblages have been shown to help. Frey (1990) identified an assemblage unique to storm beds; as one might expect, the background quiet-water conditions were typified by lots of delicate surface traces, and are juxtaposed with hardier and deeper-burrowing forms in the sandy storm beds.

The picture above, a heavily bioturbated sandstone from the Blackleaf Formation in Montana, shows almost no distinguishing morphologies (possible Ophiomorphia can be distinguished if you are up-close and lucky); however, because of the preferential diagenetic alteration of the tunnel networks, we can see pretty clearly distinct horizons of trace-maker activity, suggesting that there are cryptic surfaces of nondeposition preserved within lower shoreface sand. One can trace these surfaces out laterally, where they sometimes grade into bounding surfaces for packages of swaley-cross stratified sands.

Another ichnofacies with time-stratigraphic importance is the Glossifungites icnhofacies. This is an assemblage of burrows formed in a firmground, often compacted and dewatered muds, and dominated by vertical burrows (like Skolithos or Diplocraterion) and box-work tunnel systems (like Thalassinoides) (Pemberton and Frey, 1985). Commonly, many of these traces are passively infilled, often by coarser material than the substrate. In siliciclastic settings, most firmgrounds are often associated with erosionally exhumed substrates that have been inundated by marine waters. These firmgrounds indicate the presence of a depositional discontinuity between initial erosional exposure of the substrate and later sedimentation (Pemberton and MacEachern, 2006). It is during this discontinuity that the surface is colonized and bioturbated. Thus, the presence of this ichnofacies can have important implications for demarcating erosional surfaces, and therefore has implications for genetic stratigraphic or sequence stratigraphic frameworks.

REFERENCES

Frey, R.W., 1990, Trace fossils and hummocky cross-stratification, Upper Cretaceous of Utah:
Journal of Sedimentary Petrology, v. 43, p. 203-218.

Frey, R.W., Pemberton, S.G., and Saunders, T.D.A., 1990, Ichnofacies and bathymetry: a passive
relationship: Journal of Paleontology, v. 64, p. 155-158.

Pemberton, S.G., and Frey, R.W., 1985, The Glossifungites ichnofacies: modern examples from
the Georgia coast, USA in Curran, H.A. (ed), Biogenic Structures: Their Use in Interpreting Depositional Environments, SEPM Special Publication 35, p. 237-259.

Pemberton, S.G., and MacEachern J.A., 2006, Applied Ichnology Short Course: The Use of Trace
Fossils in Sequence Stratigraphy, Exploration and Production Geology: SEPM Short Course 18, Houston, TX, 274 p.

Seilacher, A., 1953: Studien zur palichnologie. I: Uber die methoden der palichnologie: Neues
Jahrbuch fur Geologie und Paleontologie, Abandlungen, 98, p. 87-124.

Seilacher, A., 1967, Bathymetry of Trace Fossils: Marine Geology, v. 5, p. 413-428.