A revised textural classification of gravel-free muddy sediments on the basis of ternary diagrams

A revised textural classification of gravel-free muddy sediments based on the adaptation, modification and expansion of previous schemes using ternary plots is presented. The new approach increases the range of application and the environmental sensitivity of textural sediment classification. In the case of simple two-component sand/mud mixtures, six sediment types based on mud (silt+clay) or sand content are distinguished: sand (<5% mud), slightly muddy sand (5–25% mud), muddy sand (25–50% mud), sandy mud (50–75% mud), slightly sandy mud (75–95% mud), and mud (>95% mud). The class names accurately describe a sediment within defined textural limits, and the scheme can thus be used to divide a depositional environment into textural sub-environments or facies. By diagonally subdividing the two-component system, a more complex three-component classification scheme based on sand/silt/clay ratios has been generated. In this scheme, 25 sediment classes are distinguished, each defined by a generic name and a letter–number code. It not only allows a more detailed textural subdivision of sedimentary environments than the two-component system, but also incorporates a genetic element by distinguishing between different hydrodynamic regimes. Thus, sandier and more silty sediments, reflecting deposition under higher energy conditions, are progressively segregated from muddier and more clayey sediments which reflect deposition under lower energy conditions. This is illustrated with examples from a variety of intertidal and open shelf environments. In addition, the letter–number codes can be used to label sediment facies maps, annotate stratigraphic sections and structure data banks. The two classification schemes are complementary, offering two levels of resolution and fulfilling the basic requirements of practicality, ease of use and global applicability.