The origin of terriginous clastic grains

Abstract

Terriginous clastic (detrital) grains are those fragments of rocks or minerals derived by physical or chemical break-down of source rock. Every near-surface rock or surface outcrop is a potential source of such grains. Discontinuities such as joints, rock cleavage, crystal/grain boundaries and crystal cleavage are acted upon by chemical atmospheric and soil weathering and by physical weathering. Rocks are broken down into their constituent crystals or into small rock fragments, and these grains are attacked chemically to give characteristic breakdown products and solutions. Transport by gravity, wind and water (see Part 2) will further break down and change the grains, such changes being particularly effective on easily cleft or soft minerals. After deposition the particles may be further changed chemically in the diagenetic realm (see Part 8). Therefore, we may be sure at the outset that the chemical and physical nature of a deposit of grains will differ radically from that of the source rock. For example, the average feldspar content of igneous and metamorphic rocks is around 60% whereas that of sandstones is only about 12%. On the other hand, sandstone is usually greatly enriched in quartz as compared to igneous and metamorphic rocks. We shall now discuss some of the reasons why such differences should occur.