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Introduction
Marine sediments are formed by several processes. Detrital fragments of rocks and minerals can be carried to the sea from distant, upland sources. Alternately, they can be formed in place by biological or chemical processes operating either at the site or very close by. Marine sediments can be grouped into three categories, based on their composition and mode of origin.
- Terrigenous - grains which have been eroded from the land and carried to the marine environment, typically by rivers, wind, glaciers, slumping and mass wasting (clastic).
- Biogenic - Fragments derived from biologically precipitated skeletal material, usually broken down by physical and biological erosion. These are mainly calcium carbonate but a minor fraction can be siliceous (i.e, sponge spicules).
- Authigenic - Chemical deposits which are usually formed by precipitation from solution in the environment of deposition.
- Evaporites, which are precipitated under hypersaline conditions (above normal marine salinities), have been included in this group.
Each sedimentary suite carries a record of both its origin and its ultimate environment of deposition. Sediment composition provides important clues about the location of origin of the individual grains, and textural characteristics provide information about energy levels during transport and deposition, especially in areas dominated by clastic sediments of terrigenous origin.
This chapter has two primary goals.
- The first is to help the reader understand how different sedimentary assemblages are formed and how to use these patterns to interpret the conditions that are responsible.
- The second is to consider how the oceanographic processes discussed in the previous chapters act on marine sediments to transport them from site to site and to distribute them as the deposits we observe in modern and ancient marine settings.
The discussion starts with textural characteristics common to all sediment types regardless of their origin. These sediment properties have been discussed in a number of textbooks dedicated solely to the subject of sedimentation. They are described only generally below, and the student seeking more detailed descriptions is referred to Friedman, et al. 1992 for an excellent treatment of the subject. Building on this, each major sediment type and the rocks they ultimately form are considered. Once the groundwork has been laid for understanding the characteristics of marine sediments, sampling and analytical methods are briefly outlined. Finally, the mechanisms most important in transporting sediments in modern marine systems are considered.