Uploaded byDr Sarath Babu
Sedimentary rocks
Sedimentary rocks form from the compaction and cementation of sediments such as sand, mud, and minerals. There are two main types - clastic rocks which form from solid particle sediments, and chemical rocks which form from mineral precipitation. Common sedimentary rocks include conglomerate, sandstone, limestone, shale, and laterite. Sedimentary structures within the rocks like ripple marks, cross-bedding, and mudcracks provide clues about the depositional environment.
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Sedimentary rocks
- 1. SEDIMENTARYSEDIMENTARY ROCKSROCKS Dr. P. SarathbabuM.Sc. B.Ed. Ph.D. Department of Geology Acharya Nagarjuna University
- 2. What type ofrock is formed when weathering and erosion cause sediments to press together in layers? Characteristics Tough rock with layers (foliation) of light and dark minerals, often curved; various colors; often glittery from mica
- 4. The Making ofSedimentary Rocks
- 6. Sedimentary Rocks • sediment:unconsolidated (“soft-rock”) • sedimentary rock: consolidated (“hard-rock”) • Sedimentary Rocks are rocks formed when particles of sediment build up and are “cemented together” by the effect of pressure and minerals. sea Fragments washed to the sea Sedimentary rocks Rocks are broken up by the action of weather Gettingolder
- 7. • Form atthe Earth’s surface by the lithification of sediments deposited from their transporting medium (water, wind, or ice).
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- 9. Sediment in aStream
- 10. Transport: rounding, andsize reduction
- 11. Sedimentary structures *Records thesedimentary history and depositional environment; provides valuable clues as to deposition. Most basic is simple bedding (layering effect of sedimentary rocks). 1. Ripple marksRipple marks - undulations on a sand surface produced by wind or water (asymmetrical or symmetrical) 2. Cross-beddingCross-bedding - inclined layering produced in sand by ripples or dunes at an angle to the horizontal 3. Mudcracks (desiccation cracks)Mudcracks (desiccation cracks) - polygonal pattern of cracks produced on the surface of mud as it dries. 4. Raindrop imprintsRaindrop imprints - circular pits produced by the impacts of rain on soft mud 5. Graded beddingGraded bedding - progression of grain sizes from coarser at the bottom to finer on top (or vice-versa) 6. Flute marksFlute marks - scoop-shaped depressions preserved on the bottom surface of muddy beds. Good indicator of top and bottom of bed. 7. Tool marksTool marks - ridges or discontinuous marks with a preferred orientation on bottom surfaces of beds. Indicates current direction. 8. TracksTracks-footprints of organics, dinosaur footprints. TrailsTrails are caused as organism crawls through mud or sediment. BurrowsBurrows - excavations made by organisms in soft sediment commonly filled with different sediment 9. StromatolitesStromatolites-mound-like structures formed when sediment is trapped by blue-green algae. Characteristic of limestones. Oldest life form on Earth. 10. StylolitesStylolites-pressure solution cracks formed as pressure squeezes solution through fractures in carbonates.
- 12. 1. Ripple marksRipplemarks - undulations on a sand surface produced by wind or water (asymmetrical or symmetrical)
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- 14. Asymmetric and SymmetricRipples river or wind currents (uni- directional) tidal currents (bi- directional)
- 15. • 2. Cross-beddingCross-bedding- inclined layering produced in sand by ripples or dunes at an angle to the horizontal
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- 17. Typical cross stratificationin Sand Dunes
- 18. 3. MudcracksMudcracks -polygonal pattern of cracks produced on the surface of mud as it dries.
- 19. Origin of MudCracks
- 20. 4. Raindrop imprintsRaindropimprints - circular pits produced by the impacts of rain on soft mud
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- 24. Typical cross stratificationin Sand Dunes
- 25. Formation of Coalfrom Swamp Deposits
- 27. 6. Flute marksFlutemarks - scoop-shaped depressions preserved on the bottom surface of muddy beds. Good indicator of top and bottom of bed. 7. Tool marksTool marks - ridges or discontinuous marks with a preferred orientation on bottom surfaces of beds. Indicates current direction. 8. TracksTracks-footprints of organics, dinosaur footprints. TrailsTrails are caused as organism crawls through mud or sediment. BurrowsBurrows - excavations made by organisms in soft sediment commonly filled with different sediment 9. StromatolitesStromatolites-mound-like structures formed when sediment is trapped by blue-green algae. Characteristic of limestones. Oldest life form on Earth. 10. StylolitesStylolites-pressure solution cracks formed as pressure squeezes solution through fractures in carbonates.
- 28. There are two(2) types of sedimentary rocks, based on their texturestextures: 1.1. ClasticClastic (also called “Detrital”)(also called “Detrital”)-- form from deposition of solid grains; classified based on grain size: conglomerate, sandstone, and shale 2. ChemicalChemical -- form from minerals precipitating out of water and usually involves some sort of chemical reaction; classified based on mineral content: limestone, dolomite are examples.
- 29. ClasticClastic Sedimentary RocksClassification based on grain size 1. Conglomerate (rounded grains) -- more than 30% of grains are gravel-sized (> 2mm) 2. Breccia (angular grains) -- more than 30% of grains are gravel- sized (> 2mm) 3. Sandstone -- grain bits. 1/16mm-2mm 4. Shale -- made of mud-sized grains (<1/16 mm) ChemicalChemical Sedimentary Rocks classified by composition 1. Limestone -- made mostly of calcite 2. Dolomite (dolostone) - made mostly of dolomite 3. Evaporites -- various mineral salts that precipitate from evaporating water (ex: halite, gypsum) 4. Bituminous Coal: organics compressed into peat then lithified into coal
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- 37. Sandstone Grain size :coarse grained Usual Colour : Brown Shape: subangular- subrounded Structure: crude bedding Composition : Quartz, feldspar
- 38. Sedimentary rocks 2.SANDSTONE Grain size: 1 – 2mm Usual Colour : Light brown Structure : stratification, current bedding, ripples marks, rain prints Texture : well sorted grains, >2mm-coarse 0.5 – 0.25- medium 0.25-0.1- fine Composition : quartz as chief mineral, feldspars, mica,garnet as minor amount. Occurrence: fine grains indicate high energy of deposition and rounded shape indicate long transportation
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- 40. Limestone Grain size :fine grained Usual Colour : grey Shape: laminate Structure: crude bedding Composition : calcite
- 41. Sedimentary rocks 3.LIMESTONE Grain size: Fine grained, compact Usual Colour : Grey Structure : crude bedding Texture : It is fine grained rock. It is commonly compact and massive. Some Lst; may have organic structure. Composition : Calcite, magnesite is also present is variable amounts, some contains calcareous shells Occurrence: These occurs as non-detrital rocks. They formed either chemically, due to precipitation of calcium carbonate from surface water, or organically, due to the accumulation of hard parts of organisms.
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- 43. Shale Grain size :Fine grained Usual Colour : Yellow- black Shape: rounded Structure: Lamination, closed fabric Composition : clay minerals, silicates
- 44. Sedimentary rocks 4.SHALE Grain size: Fine grained Usual Colour : variables Structure : Lamination, ripple marks and organic structures Texture : very fine grained <0.01mm Composition : Its composed of kaolinite, montmorillonite and illite,qtz,mica,chlorite are in minor amount Occurrence: Shales formed out of mechanically transported and deposited sediments. These are made up of fine grained i.e; silty and clay, and occurrs as laminated structures.
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- 46. Sedimentary rocks 5.LATERITE Grain size: Fine grained Usual Colour : Red Structure : Lamination, ripple marks and organic structures Texture : porus and concretionary Composition : These are essentially clay rich in alluminium and iron hydroxides with minor amounts of silica, alluminimum hydroxides are called bauxites(pisolitic texture). Occurrence: Its occurs as residual deposits due weathering in hot humid climate, and occurs as mantle over bed rocks.
Editor's Notes
- #5 Every rock has a history, and we saw that many rocks are made of smaller pieces of other rocks that we called “grains.” Rocks that are made up of other rocks are part of a class of rocks called “sedimentary rocks.” Each of those grains goes through its own adventure on its way to becoming part of the final rock that we pick up. It starts off as part of another rock. Wind, rain, and other agents break the rock down into smaller pieces. They often move the pieces far away from the original rock, until eventually they settle down somewhere else. Eventually, a pile of individual grains will get cemented together into one solid rock. Each of these stages affects the way the final rock looks. Let’s focus on a few of them.