Physical Properties of Sedimentary Rocks
Objective: To familiarize you with a description of the following elements of sediment texture in unconsolidated and lithified sediment samples: grain size, sorting, sphericity, rounding, grain shape, composition estimation, and textural maturity.
Background: Boggs, Chapter 3: Physical Properties of Sedimentary Rocks
Introduction
Sediment characteristics can provide information about source materials, the depositional environment (how much energy there is in waves and currents), and other physical and chemical factors. When rocks are broken down into fragments, either through the mechanical means of weathering, or through chemical reactions, the fragments are called sediment. When that sediment is compacted or cemented together, it forms a sedimentary rock. Sediments are either clastic or chemical. That is, rocks are broken down through either mechanical or
chemical means.
Clastic sediment
Clastic sediment is what one usually thinks of when speaking of sediment. From the Greek word klastos (broken), it refers to the broken remains of rocks of all types, broken and altered by weathering processes such as wind, water and ice. Clastic sediment is also known as detrital sediment.
Chemical sediment
Chemical sedimentary rocks may contain fossils and other sedimentary characteristics, but their components were not broken up mechanically. Rather, rocks were dissolved in solution (as salt can dissolve in water) and transported, then precipitated chemically (as salt can precipitate out of a saturated solution).
This lab will investigate unconsolidated sediments. You will learn how to characterize the particles or grains that are present, the size and size distribution of those grains, and then make some interpretations from these observations. In addition, you will learn some fundamentals of statistics.
Texture refers to properties of a sediment such as particle size, shape, roundness, and sorting. A well sorted sediment is one in which the grains are all about the same size. In contrast, a poorly sorted sediment contains a chaotic mixture and large, intermediate, and small grains. Shape is a measure of the sphericity of a grain. Some grains are almost spherical, whereas others may be elongate or flattened. Particle roundness refers to the smoothness of a grain, regardless of its shape. Grains may be rounded (i.e., no sharp corners), subangular or angular. The concepts of roundness, shape, and sorting are illustrated in Figures 1 and 2.
