Sedimentary rocks (2)

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Sedimentary Rocks Transportation of sediments and formation of sedimentary rocks by mode of river water- deposition on the continent and on the ocean floor Fluvial deposits Marine deposits SEDIMENTARY ROCKS: are the secondary rocks which are formed from the loose fragments or detrital or clastic sediments produced by weathering of older rocks • Almost 90% of earth crust is made up of igneous rocks • 75% of land surface on the earth is covered by thin veneer of sediments or sedimentary rocks • These sediments are transported and deposited by river water, wind or by movement of glacial ice Transportation is either in suspension or in solution • When settle down on the beds of ocean, river and lakes undergo compaction/cementation for millions of years to form SEDIMENTARY ROCKS IMPORTANCE OF SEDIMENTARY ROCK “Present is the key to the past” • Helps in knowing depositional environment viz marine (ocean deposits), fluvial (river deposits), aeolian (wind deposits), glacial, estuarine, Lacustrine (lake deposits) etc • Helps in knowing the provenance (i.e source area of the sediments); change in climatic conditions i.e in knowing and understanding old climate=paleoclimate TYPES OF SEDIMENTARY ROCKS Clastic rocks • • • • Chemical & Organic rocks Sandstones Conglomerates Breccia Shale/mudstones Evaporitic rocks These rocks are formed due to evaporation of saline water (sea water) eg Gypsum, Halit (rock salt) Carbonate rocks Organic rocks Form basically from CaCO3 – both by chemical leaching and by organic source (biochemical) eg Limestone; dolomite Form due to decomposition of organic remains under temperature and pressure eg Coal/Lignite etc CLASTIC ROCKS • formed from broken rock fragments weathered and eroded by river, glacier, wind and sea waves These clastic sediments are found deposited on floodplains, beaches, in desert and on the sea floors solidify Clastic rocks • Clastic rocks are classified on the basis of the grain size: conglomerate, sandstone, shale etc GRAIN SIZE Gravel >256-2 mm Boulder: >256mm Cobble: 64-256 mm Pebble: 4-64 mm Granule: 2-4mm Fine gravel Sand 2- 0.062 mm Silt 0.062-0.004 mm Clay mm=Pebble) Cement: dissolved substance that bounds the sediments Calcareous siliceous Fine-gravel/ Granule (size cm • Laminae < cm • Orientation of bedding helps in knowing the paleo-current direction of the old rivers GRADED BEDDING Medium-coarse sandy lithounit (cross stratified) Fine gravelly lithounit Laminated layers of fine silt and clay Cross-stratified sst Paleo-flow from right to left Sand Dunes Typical cross stratification in Ripple marks Biogenic structures Foot prints Snake Trails Mud cracks [...]... energy conditions of the river DIFFERENT CATERGORIES OF CLASTIC ROCKS • RUDACEOUS ROCKS: made up of rounded or subrounded Pebbles and cobbles eg conglomerate • ARENACEOUS ROCKS: made up of mainly sand eg Sandstone These rocks are either accumulated by wind action or deposited under water action or marine or lake environment • ARGILLACEOUS ROCKS: made up of clay size sediments eg Shale, mudstones, siltstones... constructed on argillaceous rocks sunk by 20 feet Srisailam Dam in Andhra Pradesh (One of the 12th largest Hydroelectric Projects in the India) faced similar problem, however, precautions were take by grouting to stop the seepage along the weak zones Because of its impermeable and porous nature it acts as cap rocks in the occurrence of Oil and Gas EVAPORITIC ROCKS These rocks are formed within the... ferruginous sst Different type of Sst (based on their composition) Quartz Sst Arkose (Feldspar) Graywacke/lithic arenite SHALE Shale-fissile • • Shales are clastic rocks, made up mainly fine silt/clay • They are most abundant sedimentary rocks, accounts for about 80% of them • Often contain fossils • Mostly hydrous aluminum silicate in composition = from weathered feldspars • Deposition takes place... Cementing material is usually secondary Silica (Siliceous cement), Calcium carbonate (Carbonate cement), Iron rich (ferruginous cement) • Cement itself to some extent is the source of weakness in the sedimentary rocks • Because cementing material and the clastic sediments are usually of different composition, leading to heterogeneity in their physical characteristics • Hence such rock will not behave homogeneously... reduce the porosity and permeability in the rock mass and increase competence • Shape of grains: i.e if coarser grains are rounded or subrounded=cement material will not have firm grip=such rocks behave as incompetent rocks • SANDSTONE: • Made up of sand grains dominantly of Quartz and Feldspars, where quartz is highly resistive to weathering • Cementation plays similar role in this rock as seen in conglomerate... salt for cooking, • • • • • it is used For production of Paper, Soap Detergents Antiseptics As chemical for dyeing etc • GYPSUM: is used for plaster and in manufacturing construction materials CARBONATE ROCKS • Limestone: It is a non-clastic rock formed either chemically or due to precipitation of calcite (CaCO3) from organisms usually (shell) These remains will result in formation of a limestone • Limestones... lime, carbon dioxide, and cement • Massive and compact lst Are competent to support CE-structure • However, if it occur in huge thickness then it is not advisable, because of its typical CAVING character Sedimentary structures • Bedding is most imp Feature of a sed Rock • Beds are usually > 1 cm • Laminae < 1 cm • Orientation of bedding helps in knowing the paleo-current direction of the old rivers GRADED
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