Continental Subaerial Environments
Published:January 01, 1984
Continental subaerial (i.e., non-aquatic) deposits can be conveniently grouped into two main categories: eolianites and soil zones. A common misconception regarding such deposits is that they are devoid of fossils. Although their paleontologic record is indeed meager, distinct suites of trace fossils are present that ultimately may prove useful as diagnostic tools in identifying these environments in the rock record.
By far the most extensive geologic record of continental non-aquatic environments is that characterized by eolianites (i.e., sand dune deposits). Modern eolian sediments are associated with two major areas: (1) sandy deserts (or ergs) and (2) coastal dunes. Their identification depends primarily on the recognition of features that can be attributed to the transportation, deposition and erosion of sediment by wind.
Recent reviews on the physical characteristics of eolianites (Bigarella, 1972; Collinson, 1978c; Walker and Middleton, 1977; Ahlbrandt and Fryberger, 1982 have suggested that the following features may be diagnostic of wind-derived deposits: (a) large-scale, high-angle cross-strata (up to 35 m thick), which commonly are planar-tabular or trough to wedge-shaped; (b) high ripple indices with large bedforms showing a consistent ripple index from base to top; (e) sedimentary structures related to the process of sand avalanching down dune slip faces; (d) minor sedimentary features, including raindrop imprints, vertebrate tracks and deformation of lee-side laminae; (e) intercalated interdune deposits and/or poorly sorted lag deposits along erosional bounding surfaces; (f) frosting of sand grains; (g) large and small-scale deformation features; (h) characteristic light and heavy mineral separation ratios.
Although such sedimentologie features
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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.