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Unformatted text preview: Sediments and Sedimentary Rocks I- Introduction and origin of sedimentary rocks: Although sedimentary rocks constitute 5% or less of the earth's crust, they cover 75% of the surface of continents. As such, they provide us with important industrial material, and are host to some of the most important economic deposits such as oil and gas. Studying sedimentary rocks is also essential to understanding surface processes and ancient environments. Sedimentary rocks form by deposition of sediments and their lithification (i.e. transformation to rock) at surface or near surface conditions. As such, there are four processes which are commonly involved in the formation of sedimentary rocks: (1) weathering, (2) erosion or transportation of the weathered material, (3) deposition usually in a basin of some sort, and (4) lithification, which involves the transformation of loose unconsolidated sediments into a rock. We have already discussed in detail the process of weathering in the last chapter, and will discuss the transportation of the weathered material in forthcoming chapters dealing with the action of wind, running water and ice. Deposition will be discussed in detail in a later section, but we must first understand the types of sediments deposited and learn how to describe them. II- Types of sediments 1- Detrital sediments : Also known as clastic sediments. These form by the fragmentation of the parent rock by weathering, followed by erosion or transportation of these fragments over variable distances to their place of deposition. Detrital sediments are classified according to their size into 7 groups (Table 1). Because large-sized fragments are more difficult to transport, they tend to occur closer to their source area than smaller-sized ones produced from the same rock. Clastic sediments are also described according to their shapes , and range of different sizes in an area (known as sorting , see below). Two criteria are used to describe the shape of a sedimentary particle: roundness and sphericity . Roundness describes the angularity of a particle, whereas sphericity describes the degree to which the shape of this particle approaches that of a sphere. Therefore, an elongated particle may be very well rounded, but will have low sphericity (Fig. 1). A rounded fragment was probably transported over a longer distance compared to one that is angular. Sorting is a measure of the degree of separation of sediments into different size groups. Clastic sediments commonly have a variety of different sized material mixed together. A sediment with a narrow range of sizes is described as "well-sorted", one with a very wide range of sizes is considered "poorly sorted" (Fig. 2). Sorting is a function of the transporting agent as well as the depositional environment....
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This note was uploaded on 02/27/2012 for the course GEOLOGY 110 taught by Professor Staff during the Fall '08 term at Marshall.
- Fall '08