Trace Fossils as Indicators of Sedimentologic Processes
Published:January 01, 1984
1984. "Trace Fossils as Indicators of Sedimentologic Processes", Ichnology: The Use of Trace Fossils in Sedimentology and Stratigraphy, A. A. Ekdale, R. G. Bromley, S. G. Pemberton
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Preferred orientation or alignment of traces is seen occasionally, but it is not really a common phenomenon. When observed, however, oriented or aligned burrows can be important pieces of evidence to indicate current strength and direction; usually such patterns occur in fairly high-energy conditions, because there seemingly is no need for an animal to respond directly to the azimuth of low-energy currents.
The agglutinated tube of the polychaete Diopatra cuprea, for example, typically exhibits a current-influenced orientation (Myers, 1970, 1972). The tube itself is vertical in the sediment, but the short tube cap that protrudes above the water-sediment interface usually is bent over in a horizontal position, so that the shape of the whole strucutre is an upside-down “L”. The animal inhabits shifting substrates in sandy tidal channels and estuarine point bars, where current strengths can be rather high. The tube cap is oriented perpendicular to the dominant current direction (i.e., neither directly into nor away from the current), so that the worm may sit in its burrow and grab food particles or construction materials that pass by. Preferred orientation of Diopatra tubes enabled Kern (1974) to interpret paleocurrent directions in Middle Eocene marginal marine deposits in southern California.
Not only do individual Diopatra tubes exhibit special orientation, but also dense populations of the tubes may be spatially arranged in linear patterns. Along the eastern U.S. coast it is not uncommon to see large numbers of Diopatra tubes lined up in rows on point bars, the rows being oriented perpendicular
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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.