Deep-sea Pelagic Environments
Published:January 01, 1984
In the deep sea, benthic macro-organisms leave few or no body fossils, owing to both calcite dissolution on the abyssal ocean floor and the predominance of soft-bodied organisms in abyssal communities. Deep-sea trace fossils provide geologists with virtually the only fossil evidence of the benthic macroinvertebrate community in ancient abyssal environments. In fact, their trace fossils often yield important ecologic and ethologic information, such as the relative importance of suspension feeders and deposit feeders in the bottom community, that body fossils may not provide even if present.
Deep-sea sediments at all latitudes and water depths in the world's oceans today exhibit intense bioturbation caused by the burrowing activities of benthic organisms (Berger, Ekdale and Bryant, 1979; Ekdale and Berger, 1978; Scholle, Arthur and Ekdale, 1983; Ekdale, Muller and Novak, 1984. The relatively low depositional rates of pelagic material (generally less than 3 cm per thousand years) allow even a very low abundance of burrowers to mix the sediment so effectively that primary structures and stratification are not preserved. Thus, biogenic sedimentary structures and complex ichnofabrics are the general rule in pelagic ooze and clay; primary depositional structures are the exception. On the other hand, land-derived material in turbidites typically reaches deep-water settings in a rapid and spasmodic manner. These deposits generally are well-bedded and display a variety of physical structures and bed forms as well as biogenic structures related to bioturbation in the upper portions of such units.
Deep-sea organisms that are potential producers of trace fossils include both eurybathic and stenobathic forms. Among the former are various genera of coelenterates, sipunculan, brachiopods, decapods, scaphopods and ophiuroids, which have broad depth tolerances. Among the latter are various genera of sponges, echiurans, pogonophorans, isopods, amphipods and asteroids, which have narrow depth tolerances. Unfortunately, very little information is available on which kinds of animals make which kinds of burrows in the deep sea today. Moreover, there are numerous types of ancient deep-sea trace fossils that have no recognized modern representatives. It would be inaccurate, therefore, for sedimentologists to base their paleodepth interpretations of trace fossils solely on the depth zonation of modern
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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.