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ANSWERS TO STUDY QUESTIONS CHAPTER 7 - Sedimentary Rocks

1. Quartz and clay are most common minerals in detrital sedimentary rocks because they are more stable than most other minerals in the surface environment.

2. Clay can imply a group of minerals (i.e. kaolinite, smectite, montmorillenite) or size of particle which is smaller than silt.

3. Conglomerate, sandstone, and shale are all detrital rocks but differ as to their grain size. Conglomerate has the largest grains and therefore formed from sediment deposited by the most rapid currents. Shale is made of the smallest particles, which can only deposit where currents are relatively calm.

4. Evaporite deposits form from the evaporation of seawater.
    Rock salt and rock gypsum are the most common evaporites.

5. Compaction is an important lithification process with silt and clay (mud).

6. Calcite cement can be detected by reaction to hydrochloric acid (fizzes).
Iron oxide cement can be detected by it’s red to brown color.
Silica cement is difficult to detect, but it’s presence is suggested by the absence calcite and iron oxide. 

7. Clastic textures consist of pre-existing rock fragments of any size or shape bound together by compaction and/or some kind of cement. Nonclastic textures consist of networks of intergrown crystals as would be the case for salt crystallizing from evaporated seawater.
All detrital rocks have a clastic texture.
All evaporites have a nonclastic texture.

8. Probably the single most characteristic feature of sedimentary rocks is bedding (stratification).

9. (1.) CHALK - Composed of intact (non-recrystallized) microscopic shells made of calcium             carbonate.
    (2.) LIMESTONE - Recrystallized chalk or any sedimentary rock made predominately of calcium 
            carbonate.
    (3.) DIATOMITE - Composed of intact (non-recrystallized) microscopic shells made of silica.
    (4.) CHERT - Recrystallized diatomite; a form of microcrystalline silica (quartz).
    (5.) COQUINA - A limestone composed almost entirely of visible shell fragments.
    (6.) COAL - Carbon deposits formed from the remains of plants that did not decay.

10. (1.) MUD CRACKS (desiccation cracks) - Indicate that the sediment must have been exposed         above sea level prior to lithification.
    (2.) CROSS BEDDING (cross stratification) - Indicate that the sediment was deposited by some         kind of current - either water or air.
    (3.) RIPPLE MARKS - Symmetric imply wave action; asymmetric imply current action.

Last Updated: 01/13/2015
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