ICHNOFACIES


Basic Definition of Ichnofacies

Assemblages of trace fossils, in association with body fossils and lithologic information, provide excellent clues to parameters of ancient environments. This use of all preserved aspects of an ancient sedimentary deposit for interpreting its original environment of deposition is called facies analysis. If dealing simply with the trace fossils in a sedimentary deposit, you would be interested in describing its environmentally related and contemporaneous trace fossils; in a modern environment, this assemblage is an ichnocoenose. The preserved record of the original ichnocoenose is an ichnofacies.

The ichnofacies concept, since its inception in geology by Adolph Seilacher, has become perhaps the best paleontological tool for interpreting ancient environments. Ichnofacies were originally defined as archetypal and recurring assemblages with reference to a bathymetric profile, but subsequent work has shown that water depth is only one facet of ichnofacies. Ichnofacies are named after one distinctive ichnogenus that is commonly (but not necessarily) present in the assemblage.

Nine ichnofacies have been named so far. These ichnofacies, their general environmental association, and representative ichnogenera are (from Pemberton et al., 1992):

Development and refinement of the ichnofacies concept is still ongoing, especially with increased recognition of the diversity of nonmarine trace fossil assemblages (which will necessitate subdivision of the Scoyenia ichnofacies).


Applications of Ichnofacies


The ichnofacies concept has been applied to petroleum exploration as an aid to interpreting depositional environments. The interpretation of sedimentary environments helps to assess potential petroleum reservoirs and source rocks on the basis of stratigraphic architecture; laterally adjacent facies succeeding one another vertically (Walther's Law) can be better discerned if the facies are distinctive and identifiable. Trace fossil assemblages provide an excellent supplementary tool for facies analysis, especially when body fossils are lacking.

A promising area for future applications of ichnofacies in facies analysis is in hydrogeology and other aspects of environmental geology. Hydrogeologists often must approximate many of the same parameters (porosity, permeability, facies architecture) sought by petroleum geologists, hence ichnofacies present another important set of data for these geoscientists.

Ichnofacies also indicate the evolution of paleocommunities throughout geologic time. Trace fossils are evident in rocks from the Proterozoic Eon to the Pleistocene Epoch, hence their assemblages record organismal behavior and the evolution of behavior. This information is particularly valuable for interpreting the behavior of organisms that rarely have body parts fossilized, adding another dimension to the paleontological data set for evolutionary theory.


Quantification of Bioturbation and Use in Ichnofacies


Many workers have attempted to quantify the amount of biogenic disturbance of sediment in a given stratigraphic sequence. Most schemes for classification of amounts of bioturbation involve semiquantitative categories, such as those used by Reineck (1963), Howard and Frey (1972), Droser and Bottjer (1986), and Martin (1993). These categories are divisions between two extremes: 0% bioturbation (no evidence of biogenic mixing of sediments) and 100% bioturbation (complete biogenic mixing of sediments). Thus, categories between these end members in a continuous spectrum of bioturbation could be ranges of biogenic disturbance, such as 1-10% bioturbation, 10-40%, 60-90%, and so on. Names have been given to such categories reflecting some ordinal scale ("ichnofabric index 2," "ichnofabric index 3") or general description that shows the relative progression of bioturbation ("very slightly bioturbated,"slightly bioturbated").

The ichnofacies concept has incorporated concepts regarding amounts of bioturbation, although only in an imprecise way. This imprecision is probably more representative of the great variety of expression of bioturbation in different environments rather than faulty work by ichnologists. Nevertheless, some ichnofacies have been noted as containing relatively more or less evidence of bioturbation in addition to qualitative information regarding trace fossil assemblages. For example, the Skolithos ichnofacies typically shows less evidence of bioturbation than the Cruziana ichnofacies, although some exceptions occur.

The "ichnofabric" approach has been advocated in recent years as an alternative or modification to the ichnofacies concept and has been the main topic of discussion at the last three International Ichnofabric Workshops (1991, 1993, 1995). Additionally, traditional ichnofacies have been regarded by some workers as taphonomically biased and are thus not representative of original ichnocoenoses (Bromley and Asgaard, 1990). Further testing of the ichnofacies concept should resolve its utility as a tool for the interpretation of ancient environments.


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