The Importance of Sedimentary Rocks 1 Sediments and sedimentary rocks cover approximately 75 of Earth 2 They comprise about 5 by volume of Earth s outer 10 miles 3 Contain evidence about past environments 4 Sedimentary rocks are an important resource a Coal oil and other fossil fuels b Groundwater resources Origins of Sedimentary Rock 1 Sedimentary rocks are products of mechanical and chemical weathering a Sediments and soluble constituents are typically transported downslope by gravity b The sediments are then deposited and subsequently buried c As deposition continues the sediments are lithified into sedimentary rocks 2 There are three types of sedimentary rocks a Detrital chemical and organic sedimentary rocks Detrital Sedimentary Rocks transported and micas 2 Shale 1 Detrital sedimentary rocks form from sediments that have been weathered and a The chief constituents of detrital rocks include clay minerals quarts feldspars b Particle size is used to distinguish among various rock types a Silt and clay sized particles b Form from the gradual settling of sediments in quiet non turbulent environments c Sediments form in thin layers that are called laminae d Has fissility meaning the rock can be split into thin layers e Crumbles easily and tends to form gentle slopes f Most abundant sedimentary rock 3 Sandstone a Sand sized particles b Forms in a variety of environments c Second most abundant sedimentary rock d Quartz is the most abundant mineral i Quarts sandstone is predominately composed of quartz ii Arkose sandstone contains appreciable quantities of feldspar iii Graywacke contains rock fragments and matrix in addition to quartz and sandstone e The particles in sandstone vary and are classified by their sorting and shape i Sorting is the degree of similarity in particle size in a sedimentary rock 1 If all the grains in a rock are of similar size the rock is well sorted 2 If the grains in a rock are different sizes both large and small grains the rock is poorly sorted 3 Sorting can help decipher the depositional environment of the rock f The particles in sandstone vary and are classified by their sorting and shape i Particle shape varies from rounded to angular ii The degree of rounding is indicative of how far the sediments have been transported 1 Rounded sediments are typically transported great distances while angular sediments are only transported a short distance 4 Conglomerate and Beccia a Conglomerate consists of rounded gravel sized sediments b Breccia consists of angular gravel sized sediments c Both types of rocks are poorly sorted Chemical Sedimentary Rocks 1 Chemical sedimentary rocks form from precipitated material that was once a solution a Precipitation of materials occurs by Inorganic processes such as evaporation i ii Organic processes from water dwelling organisms form biochemical 2 Limestone sedimentary rocks a Most abundant chemical sedimentary rock b Mainly composed of the mineral calcite c Can form from inorganic and biochemical origins d Biochemical limestone originates from the shells of marine organisms i Large quantities of marine limestone are formed from corals 1 Corals secrete a calcium carbonate skeleton and create reefs a Australia s Great Barrier Reef is the largest coral reef on Earth e Coquina is composed of cemented fragments of shell material f Chalk is composed of the tests of microscopic marine organisms g Inorganic limestone forms when chemical changes increase the calcium carbonate of the water until it precipitates i Travertine is a type of limestone found in caves 1 Is precipitated when the water in the cave loses carbon dioxide ii Oolitic limestone is composed of small spherical grains called ooids 1 Ooids form as tiny seeds roll in shallow marine water supersaturated with calcium carbonate 3 Dolostone a Similar to limestone but contains magnesium b Origin of dolostone is unclear i Significant quantities of dolostone are created when magnesium rich waters circulate through limestone 4 Chert 5 Evaporites a Composed of microcrystalline quartz b Forms when dissolved silica precipitates i Flint jasper and agate are varieties of chert a Form when restricted seaways become oversaturated and salt deposition starts b Rock salt and salt gypsum are two common evaporates c Occasionally evaporates form on salt flats when dissolved materials are precipitated as a white crust on the ground Coal An Organic Sedimentary Rock 1 Organic sedimentary rocks form from the carbon rich remains of organisms 2 Occasionally plant structures leaves bark and wood are identifiable in coal 3 Stages of coal formation a Accumulation of plant remains b Formation of peat c Formation of lignite and bituminous coal d Formation of anthracite coal Turning Sediments into Sedimentary Rock Diagenesis and Lithification 1 Many changes occur to sediment after it is deposited a Diagenesis chemical physical and biological changes that take place after sediments are deposited and buried i Occurs within the upper few kilometers of Earth s crust ii Recrystallization of more stable minerals from less stable ones aragonite 2 Many changes occur to sediment after it is deposited to calcite a Lithification unconsolidated sediments are transformed into sedimentary rocks i Compaction as sediments are buried the weight of the overlying material compresses the deeper sediments ii Cementation involves the crystallization of minerals among the individual 1 Sedimentary rocks are classified according to the type of mineral sediment grains Classification of Sedimentary Rocks a Two major groups i Detrital together ii Chemical organic 1 Has clastic texture composed of discrete fragments cemented 1 Has nonclastic or crystalline structure where the minerals form patterns of interlocked crystals Sedimentary Rocks Represent Past Environments 1 An environment of deposition or a sedimentary environment is a geographic setting where sediment is accumulating 2 Determines the nature of the sediments that accumulate grain size grain shape etc 3 Types of sedimentary environments a Three broad categories i Continental ii Marine iii Transition 4 Continental environments a Dominated by stream erosion and deposition i Streams are the dominant agent of landscape alteration b Glacial i Deposits are typically unsorted mixtures of sediments that range from clay to boulder sized c Wind eolian i Well sorted fine fragments 5 Marine environments a Shallow marine to about 200 meters i Borders the world s continents ii