Shallow Marine Terrigenous Environments
Published:January 01, 1984
In marine settings seven recurring ichnofacies are recognized, each named for a representative ichnogenus: Trypanites, Teredolites, Glossifungites, Skolithos, Cruiziana, Zoophycos and Nereites. These trace fossil associations reflect adaptations of benthic trace-makers to such environmental parameters as substrate consistency, hydrodynamic energy level, depositional processes, hydrography and food supply. The traces in the marine softground ichnofacies (i.e., Skolithos, Cruziana, Zoophycos and Nereites) are distributed according to numerous environmental factors, including especially bathymetry; the traces in the hardground (Trypanites), firmground (Glossifungites) and woodground (Teredolites) ichnofacies are distributed on the basis of substrate type and consistency (Fig. 15-1).
The bathymetric zonation of softground ichnofacies is imperfect. Particular combinations of trace fossils vary according to local conditions and age relationships, and there are numerous examples of bathymetric displacement of many of supposed facies-characteristic ichnotaxa (e.g., shallow-water occurrences of Zoophycos and deep-water examples of Skolithos). Therefore, extreme caution must be exercised when applying bathymetric generalizations to rock sequences based solely on individual trace fossils. However, as Howard (1978) has pointed out, in spite of these limitations, zonations based on the energy-depth ichnofacies model (Fig. 15-2) are real. Seilacher's (1967a) model continues to stand as an excellent indicator of general depositional conditions.
One of the best examples of the energy-depth zonation of trace fossils is the classic beach-to-offshore sequence, which is characterized by a relatively simple energy gradient. Shallow, nearshore zones are typified by high-energy conditions and are dominated by physical sedimentary structures. Deeper, lower-energy, offshore environments display increasing biogenic influence (e.g., see Howard and Reineck, 1972
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Ichnology: The Use of Trace Fossils in Sedimentology and Stratigraphy
Ichnology is a fascinating field of endeavor. As with science in general, it is a process of solving mysteries–in this case, mysteries of fossil behavior. In a very real sense the ichnologist is Sam Spade or Sherlock Holmes–following footprints, searching for traces of dastardly deeds, studying artifacts, attempting to reconstruct a sequence of events from subtle clues, pursuing the identity of someone (or something) long dead. Who was the culprit? What was he/she doing? Where was he/she living, working or going? Not only intellectually intriguing, ichnology also has practical application and economic importance. In today’s frenzied quest for energy and mineral resources, exploration geologists value every tool that aids their search. Ichnologic observations and analyses can help the sedimentologist reconstruct ancient depositional environments, help the stratigrapher correlate sedimentary strata, help the paleontologist determine the nature of fossil communities, and help the geochemist determine the effect of organisms on sediment composition. This publication was written to serve as a comprehensive and intelligible introduction to ichnology for anyone with even rudimentary geologic training, whether or not that person enrolls in a formal course on the subject. The book emphasizes sedimentologic, stratigraphic and paleoecologic al aspects of ichnology.