Sed%20Strat%20lab%201b%20Textures%20by%20Touch -...

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Sedimentology and Stratigraphy Laboratory Exercise 1b Sediment Textures by Touch Purpose: To acquaint you with techniques and criteria for field categorization of sedimentary textures based on relative percent of silt, clay, and sand. Procedure: Commonly, you will be called upon to make quickjudgments regarding the texture of unconsolidated sediments in the field, without the advantage of lab equipment. This is typically done using sense of touch. Because this technique has inherent inaccuracies, we do not concern ourselves with attaining exact measurements. Instead, we attempt to place samples within broad fields depicting relative percent of silt, clay, and sand content. A common standard for categorizing these textural fields is the USDA texture triangle (Below). A. o ~ . ~ ~ H ~ » m ro 0 - PERCENT SAND C08'AIISOII Gf 'AI1IQI SID 5W.1S SieYe 0, .. w. . 1- . ~ I~ I . SILT ICLAY SILT OR CLAY
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Field assignments of sediments into the textural categories on the USDA triangle are accomplished by rubbing moist sediment samples between one's fingers. Which category the sample falls into can be assessed using the following criteria. Clay (C): Can be molded into thin ribbons by smearing samples between one's fingers. Smearing samples between your fingers will produce smooth surfaces with well- developed slickensides having good striations. Note that the C category may still contain up to 60% silt. Don't be surprised if the sparkles of tiny silt grains are still quite visible across the slickensided surface. Note as well that sometimes samples may contain concretions, fossils, or other stray grains that give the clay a "lumpy' form, but such stray grains will not alter the textural classification unless they are a major component of the sample, and are not judged as secondary contamination or diagenesis. Silty Clay (SiC): Samples in this category will appear the same as samples in the C
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Sed%20Strat%20lab%201b%20Textures%20by%20Touch -...

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