Sedimentary Rocks From the Latin sedimentum - settling Show Sedimentary rocks make up only 5% (by volume) of the upper 10 miles of the crust, but about 75% of the outcrops on the continents. Two primary types of sedimentary rocks:
Sediments become rocks by lithification
Walther's Law of Facies: A conformable vertical sequence of facies generated by a lateral sequence of environments. It can be paraphrased as what might be found vertically should also be found horizontally. DETRITAL SEDIMENTARY ROCKS Keys to identification and classification of sedimentary rocks:
If we had three separate samples of silt, sand, and gravel, each very well sorted in their respective containers, but the mixed all three together, we would end up with a very poorly sorted mixture.
Particle sizes for the classification of detrital rocks Conglomerate - clastic rock in which the fragments are larger than 2 mm in size; fragments have rounded sides and corners. Breccia - clastic rock in which the fragments are larger than 2 mm in size; fragments have angular sides and corners. Siltstone vs. Mudstone vs. Claystone - siltstone has visible silt particles; mudstone feels gritty when chewed; claystone feels smooth when chewed; all are generally non laminated. Shale - general name for all of the very fine-grained siltstones, mudstones, and claystones that are laminated or fissile. Arenite - rock composed of any material that is sand sized (e.g., a quartz arenite is composed of sand-sized particles of quartz). Arkose - poorly sorted, clastic rock composed primarily of quartz and more than 25 percent of potassium feldspar giving it a pinkish color; usually formed from granitic rocks. Greywacke - gray to black sandstones that have a fine-grained clay, mud, or silt matrix. CHEMICAL SEDIMENTARY ROCKS Derived by the precipitation of minerals from solution (e.g., calcite or quartz). Precipitation may be caused by an inorganic process (e.g., decrease in temperature or pressure) or by interaction with an organic process. Useful chart for chemical sedimentary rocks
Limestone - most abundant of the chemical sedimentary rocks; formed either by inorganic precipitation or by biochemical processes.
Chert - a variety of cryptocrystalline (no crystal structure) silica; formed by the precipitation of silica from solution.
Coal - organic sedimentary rock composed of the remains of plant material. Various grades of coal include peat, lignite, bituminous coal, and anthracite coal. DEPOSITIONAL ENVIRONMENTS Continental: on a land mass; usually detrital.
SEDIMENTARY STRUCTURES A sedimentary structure reflects the depositional environment under which the rock was deposited. Bedding - sediments are arranged into layers defined by bedding planes; usually horizontal, but may also be subhorizontal or curved.
Surface features - surficial features that indicate the depositional environment. Ripple marks - wavy feature formed by wind, wave, or current.
Mudcracks - cracks formed by the desiccation, or drying, of the mud; indicative of an environment that was wet and has now dried. Can be used as an indicator of stratigraphic up; wider portion of crack will point up. Rain-drops - small imprints of rain-drops may also be found on some sedimentary rock surfaces; can be used to indicate which way is up in a stratigraphic section. Is gypsum A chemical sedimentary rock?Gypsum is a chemical sedimentary rock.
It forms when large bodies of water are rich in calcium and sulfate.
Which is a example of chemical sedimentary rock?The most common chemical sedimentary rock, by far, is limestone. Others include chert, banded iron formation, and a variety of rocks that form when bodies of water evaporate. Biological processes are important in the formation of some chemical sedimentary rocks, especially limestone and chert.
What are chemically sedimentary rocks?Chemical sedimentary rocks
Chemical sedimentary rock forms when mineral constituents in solution become supersaturated and inorganically precipitate. Common chemical sedimentary rocks include oolitic limestone and rocks composed of evaporite minerals, such as halite (rock salt), sylvite, baryte and gypsum.
What are bedded sedimentary rocks?Bedding (also called stratification) is one of the most prominent features of sedimentary rocks, which are usually made up of 'piles' of layers (called 'strata') of sediments deposited one on top of another.
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