8 - Allochthonous Sediments

Abstract

Natural processes tend to separate the various products of weathering and, after erosion and transportation, deposit sand, mud, and carbonates in many different environments. Thus sediments are grouped and segregated spontaneously on the earth's surface. This chapter begins with the discussion on how the various sedimentary rocks are named and classified. It first describes the problems of sediment classification in general, and then the classification of the allochthonous rocks in particular. Sediment is what settles at the bottom of a liquid. Five main genetic classes of sediment can be recognized: chemical, organic, residual, terrigenous, and pyroclastic. These five main genetic classes of sedimentary rocks can be divided into two separate types: the allochthonous and the autochthonous deposits. The allochthonous sediments are those that are transported into the environment in which they are deposited, whereas the autochthonous sediments are those that form within the environment in which they are deposited. The end-member concept is one of the most useful ways of classifying rocks. Sediments containing three constituents can be classified in a triangular diagram in which each apex represents 100% of one of the three constituents. Four component sediment systems can be plotted in three dimensions within the faces of a tetrahedron. Because sedimentary rocks contain many components, neither of these systems is entirely satisfactory.