2 sedimentary

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1、2DescriptionofSedimentaryRocksInthecaseofsedimentaryrockswecanstartwithabroadtwo-foldclassification.Thefirstgroupconsistsofthoserocksinwhichthecomponentmaterialshavebeenderivedfrompreexistingrocks,andhavebeenmoved-perhapsgreatdistances-totheultimatesiteofdeposition①.Rocksoft

2、histypearecommonlyreferredtoasclasticordetrital.Asecondbroadgroupofsedimentaryrocksconsistsofthoserockswhosecomponentmaterialshavebeenformedeitherinorveryclosetotheultimatesiteofdeposition.Suchrocksincludechemicalprecipitatesandcertaintypesofsedimentswhichconsistlargelyofth

3、ebrokenshellsoforganismsthatlivedinornearthedepositionalenvironment.Thissecondgroupofrocksincludesthelimestonesanddolomitesandiscommonlyreferredtoaschemical.Clasticrocksaregenerallysubdividedlargelyonthebasisofthegrainsizeofthecomponentmaterial.Thecommonlyusedtermsare:CONGL

4、OMERATESANDBRECCIAS.Thesearerockswhichconsistofpebble-andcobble-sizedmaterial.Inaconglomeratethegrainsarereasonablysmoothandrounded.Inabrecciathegrainsareirregularandangular.Conglomeratesandbrecciasgenerallydevelopinareasquiteclosetotheirsourcerocks.SANDSTONES.Sedimentsconsi

5、stingprimarilyofsand-sizedmaterialsconstituteapproximatelyone-fourthofthesedimentaryrocksexposedonthecontinents.Thepredominantmineralsofsandstonesarequartzandfeldspars,andinmanysandstonesquartzconstitutesvirtuallytheentiresediment.Mineralssuchasbiotite,hornblende,andaugite

6、,whicharecommoninigneoussourcerocks,arecomparativelyunstableincontactwiththeairandwateroftheearth'ssurface.Hencetheproportionofquartzandfeldsparisincreasedinsedimentaryrocksoveritspercentageinigneousrocks.Geologistscommonlysubdividesandstonesintothreedifferentgroups:orthoqu

7、artzites,arkosesandgraywackes.SILTSTONES.Thesearerocksthatarecomposedofhardenedsilt.SHALE.Shaleisbestdefinedasconsistingofextremelyfine-grainedmineralsinthesizerangedesignatedasclay.Inadditiontothissizedefinitionforshaleparticles,manyofthemineralsthatarefoundintheshaleare

8、commonlyreferredtoasclayminerals.Infact,fine-grainedmuscoviteisoneofthemajo