Stromatactis cavities in sediments and the role of coarse-grained accessories

Bulletin of geosciences

Koptíková L¹  Hladil J¹  Růžička M¹ 

关键词: sedimentation experiments;    polydisperse suspensions;    stromatactis cavities;    crinoid columnals;    carbonate sediments;   

DOI  :  

来源: Czech Geological Survey

【 摘 要 】

Theanalysisofpreviousexperimentalworkmadeonthesimulationofstromatactis-likecavitiesinrapidlysettlingsuspensionsofparticulatemattersubstantiatesthecontinuationofexperimentstowardidentifyingtheconditionsofcavityformation.Ithasbeensuggestedthatthemostpromisingdirectionsinthisexperimentalworkinvolvethesimplificationofcomplexfactors(e.g.,variablesthatderivefromthepropertiesofparticlesandmedia,aswellasthesedimentationdynamicsofslurries).Ournewhydrodynamicconceptofstromatactisformationaddressesthetraditionalkeyargumentsofpreviousauthorsontheoriginsofstromatactissystems.Thedirectproductionofstromatactis-typecavitiesduringthesedimentationoffineparticulate,polydisperse,multimodalaquaticsuspendedmattercanbecharacterizedintermsofcompetitionbetweenfluidsescapingfromcompressed,diluteddomains,andthedynamiceffectsofthedensepackingofsolidparticlesontheirboundaries,thelattergraduallyovertakingfromtheformer,untilamiddlelayerofsedimentissufficientlystabilizedandthefirstinternalsedimentationfromresidualsuspensionsbegins.Withtheearlieststabilizationofthegrain-supported,skeleton-likestructuresinthesediment,lowdomicalbutsurprisinglystablevaultsdevelopabovethecavityzones.Underneaththecoalescingarchedstructures,thereoftenremainplacesinwhichgrainscanstillbefluidized,andwhichconsequentlyenablethefurtherwideningoftheseprimarycavities.Thespecificgrainsizedistributionisderivedfromnaturalcounterparts,anattributecombinedwiththehighinternalfrictionangle,andincreasesthefinalsedimentcohesionandstability.Thisprocessisparticularlyeffectivewhenhighlypolydisperse-multimodalsedimentmaterialshavehighlyangular,rugged,orpotentiallycohesivegrains.Whentherelativelycoarsest-andfinest-grainedfractionsarepresentinincreasedamounts,theenergydissipationofthedenseturbulentslurryisenhanced,andthestromatactis-producingmid-layerisgraduallysealedbyarelativelyimpermeable,non-stromatactis,veryfine-grainedcoverintheupperpartofthedeposit.Twocategoriesofexperimentsarediscussedinthispaper.Thefirstisaimedatexplaininghowmoderatelylargeparticlescaninteractwitheachother.Inthiscategoryofexperiments,moderatelygradedtridispersemixturesofangularorhighlytexturedparticlestendedtoproducefirmlypackedclusterswithensuingdomicalvaultingabovecavities.Thesecondgroupofexperimentsisbasedoncombinationsofbidispersemixturesoflargegrainswithpolydispersenearly-unimodalmatricesofsmallangulargrains.Thesetwocomponents,ifusedseparately,haveclosetozerocapacityforproducinganytypeofstromatactis-likecavities.However,oncetheywerecombined,evenmodestamountsoftheselargegrainsledtothegrowthofspaciouscavitysystems,particularlyifartificialcrinoidcolumnalswerepresent.Thecomparisonofourexperimentalresultswithnaturalexamplessuggestthatcrinoidcolumnalsmustberegardedasanimportant,althoughnotindispensableacceleratorofstromatactiscavityproduction.

【 授权许可】

Unknown