Sedimentary Rock PDF EAG142/3 Geology

Sedimentary Rock Formed from sediments which have been transported & deposited Also from remains of plants & animals which have been lithified under t...

Sedimentary Rock Formed from sediments which have been transported & deposited Also from remains of plants & animals which have been lithified under the tremendous heat & pressure of overlying sediments or by chemical reactions EAG142/3 – Geology for Civil Engineer Origin Soil particle resulting from the decay of rock masses or from By chemical precipitates, deposited in sedimentary basins in Muhd Harris Ramli increasing thicknesses, eventually lithify into rock strata from School of Civil Engineering heat, pressure, cementation, and recrystallization. Universiti Sains Malaysia 3 Processes : Mechanical Weathering The rock mass is broken into fragments as the joints react to Materials Resulting freeze-thaw cycles in cold climates, expansion-contraction, and The residue can include rock fragments of various sizes, the expansive power of tree roots. consisting essentially of unaltered rock; particles of various sizes, consisting of materials resistant to chemical Processes : Chemical Weathering decomposition, such as quartz; and clays or colloidal particles, which are insoluble products of chemical decomposition of less- The rock mass is acted upon chemically by substances dissolved resistant rocks such as feldspar and mica. in water, such as oxygen, carbon dioxide, & weak acids, causing the conversion of silicates, oxides, & sulfides into new compounds such as carbonates, hydroxides, & sulfates. 4 5  Clastic Sediments (Detritus) The particle products of weathering are transported primarily by flowing water to be deposited eventually in large water bodies or basin Mechanical The products are generally segregated by size; boulders, cobbles, Weathering pebbles, granules, sand, silt, and clay. by Wind Wind currents provide transport for finer sand grains and silt sizes. 6 7 Chemical Precipitates (Nondetrital) Rock forms by lithification, which occurs as the thickness of the Materials are carried in solution in flowing water to the sea or overlying material increases. other large water bodies where they precipitate from solution. The detritus or precipitate becomes converted into rock by Chemical precipitates include the immense thicknesses of marine compaction, the deposition of cementing agents into pore spaces, carbonates (limestone and dolimites) & the less abundant and physical and chemical changes in the constituents. evaporites (gypsum, anhydrite and halite) At the greater depths “consolidation” by cementation is a In addition to being formed from physical-chemical processes, common process, caused by the increase in the chemical activity many nondetrital rocks are formed from the dissolved matter of interstitial water that occurs with the increase in temperature precipitated into the seas by the physiological activities of living associated with depth. organisms. 8 9  Detrital Group (Clastic Sediments) Rock Type Material Diameter Composition Depositional (mm) Environment Classified by particle size as conglomerate, sandstone, siltstone, Along Stream and shale. Boulders >256 bottoms. Seldom found in rock masses Same as rock source Along stream bottoms. Deposited as alluvial Cobbles 256 – 64 Nondetrital Group Conglomerate fans and in river channels Includes chemical precipitates & organics. Chemical precipitates As for cobbles; also are classed by texture, fabric, & composition. Pebbles 64 – 4 deposited in beaches As for cobbles or sand As for pebbles and Granule 4–2 sand 10 11 Rock Material Diameter Composition Depositional Type (mm) Environment Rock Type Material Composition Depositional Environment Sandstone Sand 2 – 0.02 Primarily quartz; also All alluvial deposits; stream Massive Calcite felspar, garnet, channels, fans, floodplains, Limestone Deep, quite water (CaCO3) magnetite. beaches, deltas. Some locales; Chalk Cemented Shells Along beaches, warm water hornblende, Calcareous Dolomite Seawater precipitation or pyroxene, shell, Dolomite precipitate [CaMg(CO3)2] alteration of limestone fragments Gypsum Siltstone Silt 0.02 – As for sand; often Deltas and floodplains Gypsum Saline water 0.002 some clay particles [CaSO4·2H2O] Shale Clay