Sedimentary Rocks Deposited on or Near Surface of Earth by Mechanical or Chemical Processes
What Rocks Tell Us Rock TypeHow ClassifiedWhat it Tells Us Igneous CompositionTectonic Setting TextureCooling History Sedimentary Chemical Composition Surface Environment Grain Size Energy of Environment Metamorphic CompositionOriginal Rock Type Mineral Makeup Temperature, Pressure TextureDegree of Change
Clastic (terrigenous or detrital) –Conglomerate or Breccia –Sandstone –Siltstone –Shale Chemical/biochemical –Evaporites –Carbonate sedimentary rocks (limestones and dolostone) –Siliceous sedimentary rocks Organic (coals) –Other - ironstones Types of Sedimentary Rock
Sedimentary Rocks are the Principal Repository for Information About the Earth’s Past Environment Depositional environments in ancient sediments are recognized using a combination of sedimentary facies, sedimentary structures and fossils
Environmental Clues in Sedimentary Rocks Grain Size - Power of Transport Medium Grading - Often Due to Floods Rounding Sorting Cross-bedding - Wind, Wave or Current Action Transport, Reworking }
Environmental Clues in Sedimentary Rocks Fossils –Salt Water - Corals, Echinoderms –Fresh Water - Insects, Amphibians –Terrestrial - Leaves, Land Animals Color And Chemistry –Red Beds - Often Terrestrial –Black Shale - Oxygen Poor, Often Deep Water –Evaporites – Arid Climates
Sedimentary Rocks Clastic Rocks Made of Fragmentary Material Deposited by –Water (Most Common) –Wind –Glacial Action –Gravity Biochemical Sedimentary Rocks Evaporation Precipitation Biogenic Sediments
Clasts (larger pieces, such as sand or gravel) Clasts and matrix (labelled), and iron oxide cement (reddish brown color)
Terrigenous (also called detrital or clastic) Terrigenous sedimentary rocks are classified according to their texture (grain size): Gravel: Grain size greater than 2 mm If rounded clasts = conglomerate If angular clasts = breccia Gravel: Grain size greater than 2 mm 1. If rounded clasts = conglomerate 2. If angular clasts = breccia
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. CLASTIC ROCKS (mudstone) Clastic rocks solidify Clastic rocks are classified on the basis of the grain size: conglomerate, sandstone, shale etc.
Degree of roundness helps in knowing the distance of transportation (method of erosion) Angular clasts- short distance transport from the source Rounded clasts- long distance transport
Sediment Sizes and Clastic Rock Types Sedimentary rocks made of silt- and clay-sized particles are collectively called mudrocks, and are the most abundant sedimentary rocks. Rock TypeSedimentGrain Size ShaleClayless than 0.001 mm SiltstoneSilt.001-0.1 mm SandstoneSand.01-1 mm ConglomerateGravel1mm +
Bedding or Stratification Almost Always Present in Sedimentary Rocks Originally Horizontal Tilting by Earth Forces Later Variations in Conditions of Deposition Size of Beds (Thickness) –Usually 1-100 Cm –Can Range From Microscopic to 50m
Diagenesis Alteration Limestone - Dolomite Plagioclase – Albite Recrystallization Limestone Compaction + Cementing Quartz Calcite Iron Oxide Clay Glauconite Feldspar Diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification.
Cementation Clastic particles ranging from silt- size to boulder-size may be deposited on the sea floor. As they are buried, ion-laden sea water may deposit minerals in the pore spaces between the grains, thus effectively cementing them together. By this process the sediments become rocks such as siltstone, sandstone and conglomerate. Compaction Clastic particles smaller than silt, such as mud are deposited on the sea floor. As they are buried, the weight of overlying sediments presses downward on the mud particles and compacts them, resulting in the formation of rocks such as claystone,mudstone or shale.
Sandstones Conglomerates Breccia Shale/mudstones TYPES OF SEDIMENTARY ROCKS Clastic rocksChemical & Organic rocks Evaporites rocks These rocks are formed due to evaporation of saline water (sea water) eg. Gypsum, Halit (rock salt) Carbonate rocks Form basically from CaCO 3 – both by chemical leaching and by organic source (biochemical) eg. Limestone; dolomite Organic rocks Form due to decomposition of organic remains under temperature and pressure eg. Coal/Lignite etc.
Chemical Sediments Evaporites -Water Soluble Halite Gypsum Calcite Precipitates Example: Ca(sol'n) + SO 4 (Sol'n) = CaSO 4 Gypsum Limestone Iron Formations Alteration After Deposition Dolomite Biogenic Sediments Limestone - Shells, Reefs, Etc. Organic Remains Coal Petroleum
EVAPORITIC ROCKS These rocks are formed within the a depositional basin from chemical substances dissolved in the seawater or lake water. Gypsum Halite (NaCl) CaSO 4.2H 2 0
Economic importance of Evaporites SALT: other then daily use of 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.
Biogenic Sediments Non-Clastic Sedimentary Particles I Broken fragments of calcite, mostly from algae. Shallow sub tidal sediments from the Yucatan, Holocene. Largest fragments are about 1 mm in length. Chalk –Chalk: which is made up of foraminefera is very fine grained
Biogenic Sediments Non-Clastic Sedimentary Particles Broken shell fragments of calcite frombivalve molluscs. These shell fragments accumulated on a beach and are cemented together. This rock is almost 100 percent shell fragments, and is therefore called a coquina. Largest fragments about 2 cm in length.
Limestone: It is a non-clastic rock formed either chemically or due to precipitation of calcite (CaCO 3 ) from organisms usually (shell). These remains will result in formation of a limestone. CARBONATE ROCKS Limestones formed by chemical precipitation are usually fine grained, whereas, in case of organic limestone the grain size vary depending upon the type of organism responsible for the formation –Fossiliferous Limestone: which medium to coarse grained, as it is formed out of cementation of Shells.
COALS: Organic Remains Coals are carbon-rich rocks that are composed of the altered remains of woody plant debris. The two principal types of coals are: lignite (brown coal): composed of loosely bound (friable) organic detritus, including some clearly recognizable plant remains bituminous coal: highly compacted black coal composed of recrystallized carbon
Characteristics and names of some common clastic sedimentary rocks. Particle SizeRock NameRock Characteristics mud (see below) Shalesmooth feel, layered appearance mud (mud sized particles: < 0.063 mm) Mudstone smooth feel, massive to layer silt (silt sized particles: 0.063 - 0.004 mm) Siltstone slightly gritty feel, may have layered appearance sand (sand-sized particles: 0.0625-2.0 mm) Sandstone rough gritty feel, constituent grains clearly visible, including quartz, feldspar, other minerals, and rock fragments. granules, pebbles, cobbles, boulders (granule to boulder sized particles: 2 mm - > 256 mm) Conglomerate large rounded fragments composed of older rock materials granules, pebbles, cobbles, boulders (granule to boulder sized particles: 2 mm - > 256 mm) Breccia large angular fragments composed of older rock materials
Characteristics of common non-clastic sedimentary rocks that will not react with dilute HCl. Grain size Rock NameRock Characteristics very fine grained: can't see constituent particles with naked eye Chert hard, scratches glass, typically white, green, or red; tends to have conchoidal fracture variable grain size Rock Gypsum soft, can be scratched with fingernail; may be translucent or opaque
Characteristics of common non-clastic sedimentary rocks that will react with dilute HCl. Grain SizeRock NameRock Characteristics very fine grained, can't make out particles with naked eye Chalk Pure white, powdery, light-weight, will write on sidewalks or walls variable in grain sizeLimestone dense and soft, with a crystalline or dull (earthy) luster medium grained, particles commonly visible with naked eye Coquina fragments of fossils, usually shells of invertebrates variable in grain sizeFossil Limestone dense and soft, may be crystalline or dull, with visible fossils, such as snail or clam shells or other taxa
Sedimentary Rock Review Of Sedimentary Processes This chart is a review of the various steps involved in the formation of sedimentary rocks that have been discussed above.
Landforms Associated with Sedimentary Rocks Mesa Flat-topped hill capped with hard rock Cuesta Gently-tilted layer of hard rock: Door Peninsula The gentle upper slope, on top of the layer is called the dip slope Hogback A sharp ridge of hard rock, edge of a steeply- dipping layer