Deep-water Slope Environments
Published:January 01, 1984
Continental margins, whether “active” or “passive” in the tectonic sense, usually are very dynamic environments of deposition. Deep-water slope environments in the marine realm, taken in the broad sense to include the outer continental shelf, continental slope and continental rise, exhibit considerable variation in sedimentary facies (Fig. 18-1). Deposition may be by sliding (e.g., glide or slump), gravity transport (e.g., grain flow or debris flow) or gravity-induced currents (e.g., turbidity currents). Sedimentation is spasmodic, with long periods of non-deposition punctuated by short bursts of rapid sediment influx.
Turbidites, or flysch deposits in general, are spasmodically deposited slope sediments (Bouma, 1962; Mutti and Ricci Lucchi, 1972; Middleton and Hampton, 1973; Walker and Mutti, 1973; Walker, 1979b). They may occur along oceanic continental margins, within isolated marine basins or even in large lakes, although it is only the marine deposits that contain well-known ichnofaunas. In the strict sense, turbidites are packages of sediment deposited by gravity-driven, fluidized turbidity currents. Typically, but certainly not invariably, such deposition occurs in fairly deep water (i.e., bathyal and abyssal depths).
Turbidite packages usually exhibit a characteristic cyclicity of microfacies, known widely as the “Bouma sequence” (Fig. 18-2). An ideal Bouma sequence contains the following succession: Ta, massive or graded sand layer; Tb, planar parallel-laminated sand layer; Tc, ripple cross-laminated or convolute-laminated fine sand or silt layer; Td, planar parallel-laminated silt layer; Te, hemipelagic mud layer, which may be finefy laminated, bioturbated or apparently structureless. Many turbidites do not contain such an idealized sequence at all, and
Figures & Tables
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.