Terrigenous Sediments and Rocks

Sources

Most terrigenous sediments are ultimately derived from the breakdown of crystalline igneous rocks . Although sedimentary rocks are also eroded to provide grains, their constituents are generally derived from the prior erosion of igneous or metamorphic rocks. Notable exceptions include cherts, carbonates and volcaniclastic grains. The principal source of terrigenous marine sediments is river discharge. Rivers transport more than 18 x109 metric tons of suspended solids to the world's oceans annually. Holeman, 1968 This amounts to a 100-km high column of sediment occupying the area of two football fields. More than one-third of this is carried by about ten rivers , with the Ganges and Amazon Rivers alone carrying 20 percent of the worldwide total.

Sand and gravel rolling or bouncing along the bed (bed load) are mainly deposited in estuaries and on beaches or offshore bars. The muds that travel in suspension are deposited in estuaries and coastal wetlands, largely due to a process called flocculation (the wholesale "clotting" of finer particles at the point where fresh water from the rivers meets the saline waters of the ocean).

As sediment is delivered to the open ocean, most of it is trapped by wave action in the nearshore environment and moves parallel to the coast, with less than ten percent of modern river suspension sediment load ever reaching the deep ocean. Friedman, et al., 1992 During major storms, however, offshore-flowing currents can reach a magnitude sufficient to move clastic sediments through the energy barrier of the surf zone and away from the shore. In some cases, these form into shore-parallel sand bars and migrate back onto the beach. In other instances, sediment is lost to the shelf until the next sea-level drop.

Terrigenous Sand

The assortment of minerals occurring in a sediment is the key to identifying the site from which it was derived. While a tedious task, identifying all the mineralogical components and their likely sources is necessary if a study of provenance is to be quantitatively useful. Unfortunately, the work expands rapidly as more sophisticated techniques are employed. Blatt, et al. 1980 point out that the typical approach is a necessary compromise between the need for detailed analysis and the amount of time available.

Although hundreds of minerals have been identified, only a few are considered to be common rock-forming minerals. Most of the common minerals in terrigenous sediments are silicates, which are made up of silicon and oxygen with strong atomic bonds. The common silicate minerals include quartz, feldspars, micas, pyroxenes, amphiboles, micas and olivine. Except for quartz (Si02), these minerals have similar chemical composition of calcium (Ca); sodium (Na), potassium (K) and iron (Fe) bound in a silicate tetrahedron (Si04).

Techniques for identification of mineral and rock grains in a sediment sample range from hand lens examination to the use of petrographic microscopes and Scanning Electron Microprobes. Identification of terrigenous rock forming minerals is not easy, but some general characteristics such as color, hardness and cleavage help in the identification.


















The table provides a few basic characteristics that are useful in identifying some of the more common constituents using a hand lens.

Once reduced to grains composed of individual minerals, the durability of each mineral will dictate its stability. The composition of sediment near to its original source will largely reflect the mineralogical composition of the parent rock. Even at its source, however, the sedimentary suite will also reflect the erodability of the underlying rocks under the influence of local processes. In high-latitude glacial climates, processes such as frost cracking and glacial scour may produce a different sediment assemblage when compared to the same rock type exposed to arid desert conditions, including intense heat and sand-blasting by wind-blown sand. The further away from the source, temporally or spatially, the more important the more durable minerals become. As an illustration of this process, consider that the "average" sand or sandstone contains about 65 percent quartz with a mean size of 2f (0.25 mm), and less than 15 percent feldspar. In contrast, the igneous rocks from which they are derived contained more than 60 percent feldspar. Blatt, et al.,1980 The disparity in composition is a reflection of the greater resistance of quartz to physical and chemical weathering in a surface environment. Feldspar is less resistant and is rapidly converted to silt and clay. Thus, the relative abundance of quartz versus feldspars and other, less-stable minerals is a reflection of the intensity and duration of the weathering processes. Quartz, orthoclase feldspar and the micas will dominate near the source. Over time, quartz becomes the most common sedimentary mineral because of its high stability to physical and chemical weathering. Feldspars are found in areas of rapid deposition or proximal to the source area.

Terrigenous Muds

Over time, less durable minerals will break down to form silt and clay. Silts are comprised of mainly quartz, feldspar, and carbonate grains that fall into the size range of 4 to 9 f (0.063 to 0.002 mm). Friedman, et al., 1992 The term clay refers to a specific group of minerals with a basic silicate structure and composition which is combined with metallic ions to form the different clay minerals (chlorite, illite, kaolinite, and montmorillonite). The particles are all smaller than 9 phi and can be seen only with an electron microscope. clay minerals The composition is similar to the micas and other silicate minerals, but the structure is unique in having two different types of layers combined in different patterns with layers of water molecules.

Classification of Terrigenous Rocks

The underlying sedimentary character is the cornerstone for classifying terrigenous rocks, with the grain size of sediments being generally translated into a corresponding rock type. Claystones, siltstones, shales (mixed clay and silt), sandstones and conglomerates (gravels) represent rocks made up of increasingly larger grains.










Because the classification of a particular rock is in part related to the distribution of its composite grains, conglomerates and sandstones can be further classified on the basis of mineralogical components. Most schemes use a ternary diagram with quartz, feldspar and metamorphic/extrusive rock fragments as the three end members to develop a classification. These end members are plotted by percentage to determine the basic rock type. On the ternary diagram , arenites are those rocks with sand-sized grains, predominantly composed of quartz. Arkosic rocks contain much higher proportions of feldspar and are often poorly sorted. Graywackes are usually darker rocks comprised with a greater percentage of angular rock fragments. The fine-grained sediments and rocks may be divided on the basis of the dominant mineral composition (kaolinite, illite, and montmorillonite). Accessory minerals such as glauconite or pyrite are often used as modifiers in the rock name, i.e glauconitic quartz sandstone.