Trace fossils are widely used environmental indicators in marine sediments, where they are most abundant. They have one advantage over body fossils in that they are almost invariably in place; derived and transported trace fossils are
very rare. They are well preserved and readily studied in drill core, and have therefore made a substantial contribution to subsurface environmental analysis Trace fossils may be used in two ways: (1) to indicate local sedimentation and erosion patterns and (2) to indicate the depositional environment. Trace-fossil assemblages have become valuable tools in the analysis of discontinuities and their allostratigraphic signif- icance. The study of ichnofacies has become a specialized form of paleoecology. It is widely used in subsurface studies because of the ease with which trace fossils may be studied in cores. Many recent review articles and textbooks have been written on this subject, among the most useful of which is that by MacEachern et al. (2010).
There are various ways in which trace fossils may be used to evaluate local erosion and sedimentation. The density of bioturbation in a bed varies inversely with the rate of Fig. 3.56 Depth distribution of
common types of fossil invertebrates (Heckel1972b)
sedimentation, so that where sediment supply is low and invertebrate life abundant primary structures such as bedding may be completely destroyed MacEachern et al. (2010, Fig. 3) illustrated a bioturbation index, which is an attempt to quantify the intensity of alteration. Beds formed by rapid sedimentation, such as the passage of a turbidity current or a storm, may contain little or no penetration by organisms, except perhaps for escape burrows made vertically through the bed. Types of bioturbation may vary from one lithology
to the next, because of different behavior patterns of the organisms in response to different sediment types. Ichnofa- cies are useful in the examination of cores and outcrops for the analysis of discontinuities, including the various bounding surfaces used in sequence-stratigraphic analysis (MacEachern et al.2010, p. 45).
Important and widely quoted work by Seilacher (1967) showed how trace fossils could be used to interpret paleo- bathymetry. A set offive biofacies (now termed ichnofacies) Fig. 3.57 Vertical variations in
fossil assemblages of Niagaran reefs, Great Lakes (Lowenstam et al.1956)
were erected. These have now been expanded to nine (MacEachern et al.2010; see Fig.2.26in this book). In the shallowest waters waves and currents keep nutrients in suspension. Animals are subject to violent conditions and build deep vertical burrows such as Skolithus. In less tur- bulent waters faunal diversity increases, sediment feeding becomes more important and a variety of crawling and grazing feeding trails appear (Cruziana biofacies). Below wave base sedimentation is slower, the oxygen content of the sediments is lower and nutrient supply more sparse. Sedi- ment mining organisms of the Zoophycos and Nereites biofacies are characteristic. The latter, with its complex but highly systematic grazing patterns, is particularly typical of abyssal submarine-fan deposits. Other assemblages include a nonmarine Scoyenia biofacies, a“softground” Psilonichnus assemblage, in addition to the Skolithus and Cruziana assemblages, and the Glossifungites biofacies, which develops onfirm but unlithified substrates. The details of the biofacies distributions varies from basin to basin and depend on sediment type, water temperature, salinity and circulation patterns. Frey and Pemberton (1984) pointed out that
local sets of environmental factors are most important in con- trolling the distribution of tracemakers, whether or not these parameters occur at specific water depths. For instance, many of the estuarine point bars… exhibit a high-energy, channelward side typified by a Skolithus association and a low-energy bankward side typified by a Cruziana association… The respective associations occur in close proximity, at the same stratigraphic or bathymetric datum.
Also, as Howard (in Basan 1978) pointed out, different organisms may make very similar structures, and conversely, the same species may make different structures when engaged in different activities or when interacting with substrates of different composition. Interpretations must therefore always be made in conjunction with other facies studies, and the geologist would be wise to consult published work on trace fossil assemblages in other basins of a similar age. Given these caveats,“ichnofacies are facies models that address animal-sediment responses in the depositional environment” (MacEachern et al. 2010, p. 29). The term facies model is used here with explicit reference to the term as defined by Walker (1976).
In carbonate environments, algal mats and stromatolites generally are excellent indicators of intertidal deposition, although recent discoveries of stromatolites in subtidal set- tings has indicated that an environment hostile to browsing organisms may be the critical control. According to James (1984b), their“upper limit is controlled by climate; in arid areas they cannot grow above the high intertidal into the supratidal zone, whereas in areas of high rainfall where the supratidal zone is moist orflooded for days at a time, mats are prolific. The lower limit is more variable and appears to be controlled by the presence of gastropods that eat algae.”
In hypersaline zones, gastropods cannot survive and mats grow into the subtidal zone. After deposition and burial, mats commonly rot away, but they leave voids as a result of the disappearance of organic materials, entrapped gas, or shrinkage. The resulting distinctive type of porosity has been referred to as laminoid fenestrate, loferite, or birds-eye. Stromatolites are present well back into the Precambrian and may have lived in similar environments throughout this time (see also Pratt2010).
MacEachern et al. (2010, p. 31) report on recent advances in the analysis and understanding of ichnofacies in conti- nental environments.