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sedrx - Sedimentary Rocks Page 1 of 10 EENS 1110 Tulane...

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This page last updated on 01-Feb-2012 EENS 1110 Physical Geology Tulane University Prof. Stephen A. Nelson Sediment and Sedimentary Rocks Sedimentary Rocks Rivers, oceans, winds, and rain runoff all have the ability to carry the particles washed off of eroding rocks. Such material, called detritus , consists of fragments of rocks and minerals. When the energy of the transporting current is not strong enough to carry these particles, the particles drop out in the process of sedimentation . This type of sedimentary deposition is referred to as clastic sedimentation . Another type of sedimentary deposition occurs when material is dissolved in water, and chemically precipitates from the water. This type of sedimentation is referred to as chemical sedimentation . A third process can occur, wherein living organisms extract ions dissolved in water to make such things as shells and bones. This type of sedimentation is called biochemical sedimentation. The accumulation of plant matter, such as at the bottom of a swamp, is referred to as organic sedimentation. Thus, there are 4 major types of sedimentary rocks: Clastic Sedimentary Rocks , Chemical Sedimentary Rocks , Biochemical Sedimentary Rocks , and Organic Sedimentary Rocks . Clastic Sediments and Sedimentary Rocks The formation of a clastic sediment and sedimentary rocks involves five processes: 1. Weathering - The first step is transforming solid rock into smaller fragments or dissolved ions by physical and chemical weathering as discussed in the last lecture. 2. Erosion - Erosion is actually many process which act together to lower the surface of the earth. In terms of producing sediment, erosion begins the transpiration process by moving the weathered products from their original location. This can take place by gravity (mass wasting events like landslides or rock falls), by running water. by wind, or by moving ice. Erosion overlaps with transpiration. 3. Transportation - Sediment can be transported by sliding down slopes, being picked up by the wind, or by being carried by running water in streams, rivers, or ocean currents. The distance the sediment is transported and the energy of the transporting medium all leave clues in the final sediment that tell us something about the mode of transportation. 4. Deposition - Sediment is deposited when the energy of the transporting medium becomes too low to continue the transport process. In other words, if the velocity of the transporting medium becomes too low to transport sediment, the sediment will fall out and become deposited. The final sediment thus reflects the energy of the transporting medium. 5. Lithification (Diagenesis) - Lithification is the process that turns sediment into rock. The first stage of the process is compaction. Compaction occurs as the weight of the overlying material increases. Compaction forces the grains closer together, reducing pore space and eliminating some of the contained water. Some of this water may carry mineral components in solution, and these constituents may later precipitate as new minerals in the pore spaces. This causes cementation, which will then start to bind the individual
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