GEOL 3015_Sedimentary Rock Lab.pdf

GEOL 3015_Sedimentary Rock Lab.pdf - GEOL 3015 Introductory...

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G EOL 3015 : Introductory Geology For Engineers Sedimentary Rocks E XERCISE 3: Sedimentary Rocks INTRODUCTION The sedimentary rocks include all those rocks that are formed as a result of the deposition, or accumulation, of particles or fragments. Such deposition may be mechanical or chemical. The great bulk of sedimentary rocks originated as deposits on the floor of the sea, but some were laid down in fresh water, and a few on the surface of the land. Because the mechanisms of transportation and deposition do not operate absolutely uniformly at a given place over a period of time, sedimentary rocks almost invariably possess a layered structure or stratification . Each layer, or stratum, has a great horizontal extend compared with its thickness. The thicker units of stratification are termed bedding and the finer units are termed lamination . The complexities and variability of the sedimentary processes make it difficult to develop a single, simple, coherent, and logical classification of sedimentary rocks. One useful way is to classify the rocks according to the chemical composition of the rocks as a whole, e.g., carbonaceous, siliceous, calcareous, but this obscures common features related to their origin. Other classifications concentrate on the origin or environment of deposition, but this is difficult to determine for individual specimens. Although realizing its defects, particularly in relation to deposits or organic, we shall adopt here a single primary division into (a) clastic and (b) chemical deposits. However, as the limestones involve both types of origin, they will be considered as a separate group of calcareous rocks . Clastic rocks are made up of fragments, originally separate but now bound together as a result of compaction or, more usually, by the addition of a minor amount of cementing material which is deposited in the pore space between the fragments. Examination of such a rock with a lens, or a microscope if it is very fine-grained, will reveal its granular nature. When the fragments are predominantly the remains of organisms, we may employ the term bioclastic . Chemical rocks are deposited by precipitation of soluble salts from water. Examination of such rocks will reveal that the bulk of the material is crystalline, although fragments may also be present. Microscopic organisms are often involved in the precipitation mechanism, in which case the rock should properly be called biochemical , but this is usually very difficult to determine, especially as some recrystallization usually occurs in ancient rocks.
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G EOL 3015 : Introductory Geology For Engineers Sedimentary Rocks CLASTIC ROCKS In the naming of clastic rocks, the principal variables to be considered are the composition and size of the particles, their shapes and degree of sorting, and the nature of the cementing material.
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  • Fall '17
  • Dr. Conly
  • Limestone, Introductory Geology For Engineers

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