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15.1: Introduction

  • Page ID
    4246
  • The topic of deposition is an important one, because, obviously, every sedimentary sequence was deposited somehow. This section is meant to serve as background for our consideration of the current-generated physical sedimentary structures in real sedimentary deposits.

    Let me pose a question for you: Why does deposition happen? I wonder whether this strikes you as a trivial question or as a difficult question. In one sense, we can supply a simple answer: sediment is carried by a flow, and when the conditions are such that the flow becomes overloaded, the sediment is deposited. But in another sense, this is a superficial answer, because it does not account for the conditions under which a flow becomes overloaded, and we have to look for a more fundamental answer. (By overloading I mean that the flow, at a given time, is transporting a greater sediment load than what it would be transporting if the sediment transport were in equilibrium with the given flow.)

    The most straightforward process involved in deposition is settling: the downward fall of sediment particles through the surrounding fluid by the pull of gravity (see Chapter 3). Keep in mind, however, that there is far more to deposition than just settling of sediment particles: you have to worry about where the sediment came from, how it got to the site of deposition, and why it was that more sediment was falling out of suspension than was being resuspended at the site of deposition. Considerations like this are absolutely critical to a really fundamental understanding of sediment deposition, but in my opinion not nearly enough attention has been given to such matters in the literature on sedimentation, either by hydraulic engineers or by sedimentary geologists. This chapter makes only the barest start on addressing such matters.