Geology - Sedimentary Rocks - Quick Guides

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Study GuideGeology–Sedimentary Rocks1. ClasƟc Sedimentary RocksClastic sedimentary rocksform from pieces of pre-existing rocks that were broken apart,transported, deposited, and then compacted and cemented together. Geologists classify these rocksmainly bygrain sizeand by thetypes of rock fragmentsthey contain.1.1 Grain Size, Distance, and TransportGrain sizedepends largely onhow far sediment traveledbefore it was deposited.•Short travel distance→particles staylarge and angular•Long travel distance→particles becomesmaller and more roundedAs rock fragments move, they bump into each other and wear down. This smoothing process is calledrounding.

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Study Guide1.2 SorƟng of SedimentSediment is also affected bysorting, which describes how well sediment grains are separated bysize.•Large, coarse, angular fragmentssettle close to the source•Sand-sized particlestravel farther before settling•Silt, mud, and claytravel the farthest and settle in calm waterIn general,longer transport = better sorting and smaller grains.1.3 Coarse-and Medium-Grained ClasƟc Rocks1.3.1 Breccia•Made oflarge, angular gravel-sized fragments•Depositedvery close to the source•Angular shape shows little transport1.3.2 Conglomerate•Made oflarge, rounded gravel-sized fragments•Indicateslonger transportthan breccia1.3.3 Sandstone•Amedium-grainedrock•Made mostly ofsand-sized particles•Most grains arequartz1.3.4 Special Types of Sandstone•Quartz sandstone: at least90% quartz

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Study Guide•Graywacke: a “dirty” sandstone withmore than 15% silt orclay, giving it a darker, speckledlook•Arkose: a sandstone withover 25% feldspar grains1.4 Fine-Grained ClasƟc RocksFine-grained clastic rocks form from very small particles that usually settle inquiet water.1.4.1 Shale•Made ofsilt and clay•Thinly layeredand very smooth•Fissile, meaning it splits easily along layers1.4.2 Siltstone•Made mostly ofsilt-sized grains•Looks similar to shale•Not as fissile(does not split as easily)1.4.3 Mudstone•Finest-grainedclastic rock•Contains moreclay•Poorly layeredcompared to shale and siltstoneMost shale, siltstone, and mudstone aretan, brown, gray, or black.2. Chemical Sedimentary RocksChemical sedimentary rocksform whenminerals crystallize out of water. This can happen whenwater evaporates or when chemical and biological processes cause minerals to settle and harden.

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Study GuideUnlike clastic rocks, these rocks arenot made from broken rock fragments, but fromdissolvedmaterials.2.1 LimestonesLimestoneis themost abundant chemical sedimentary rock. It is made mostly of the mineralcalcite (CaCO₃)and usually forms inshallow, warm seawater.Most limestone forms throughbiochemical processes, meaning living organisms play a big role:•Corals and algaeare especially important limestone builders.•Many marine organisms use calcite to make shells or skeletons.•When these organisms die, their remains settle on the sea floor and slowly turn intolimestone.Special Types of Limestone•OoliticlimestoneForms in shallow ocean water where waves roll tiny grains of calcite intosmall, sand-sizedspheres called oolites. These build up in tidal zones.•CoquinaMade fromlarge, broken shell fragmentsthat have been cemented together.•Bioclastic and skeletal limestoneContain a wide mix of shell fragments and fossils, ranging from fine to coarse grains.•ChalkA veryfine-grained limestonemade from the microscopic skeletal remains of tiny marineorganisms that slowly settled to the ocean floor.Because these limestones form from shell fragments, they can be thought of aschemical, clastic,and organic sedimentary rocksat the same time.

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Study Guide2.2 Dolomites (Dolostones)Many limestones are later changed intodolomite, also calleddolostone.Thistransformation happens during the early stages of:•Compaction•Loss of water (dewatering)•Lithification (turning sediment into rock)The process is calleddolomitization:•Calciumin limestone is partially removed•Magnesium-rich fluidsreplace it•The new mineral formed isdolomite, with the formulaCaMg(CO₃)₂2.3 ChertsChertis ahard, glassy rockmade ofsilicathat crystallized from water.Common varieties of chert include:•Flint•Agate•JasperChert often forms as:•Nodules, also calledgeodes, inside limestone•These nodules fill cavities that once held gas or organic material•Chert can also form asthin layerswithin sedimentary rock bedsChert is also a major part ofiron formations, layered rocks that formed in shallow seas long ago.These rocks are extremely important because they are amajor source of ironworldwide.

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