Sedimentary!Rocks! T. Perron – 12.001 We’ve!heard!about!igneous!rocks.!Formed!by!cooling!& !crystallization!of!melts.!If! you!sample!a!random!part!of!Earth’s!interior,!odds!are!pretty!good!you’ll!get!an! igneous!rock.!What!about!Earth’s!surface?![Q:!What!do!you!hit!if!you!dig!in!your! yard?!Soil,!which!is!made!largely!of!rock!fragments.]!Usually!you’ll!hit!sediment,!and! those!can!be!transformed!back!into!rock.! ! Toda y:! Fa t e! of! rocks! at!the!surface,!and!the!processes!they!r ecor d!along!the!way ! (we’ll !ta lk !l a ter,!in!t he!l ectures!on!plate!tectonics,!about !how !they!get !to!the!surfa ce)! ! Let ’s! st art ! w ith!a! story!about!some!enigmatic!features!in! a!~500!Ma!rock!ou tcrop !in! Wis co ns in ! (PPT) ,!from!a!paper!by!Whitey!H agadorn!et!al.!(Geology,!2002).! Lik e!al l! good!stor ies,!it!is!a!tragedy.! ! “The%follow ing%scenario%for%medusoid%stranding,%decay,%and%burial%is% hypothesized.%Jellyfish%were%blown%or%swam%into%a%shallow%embayment.%The%tide% ebbed%and%escape%behaviors%were%initiated%…%During%mass%strandings,%most% jellyfish%settle%on%the%surface%with%their%subumbrella%down,%and%they%commonly% pump%their%bells%in%an%attempt%to%escape%from%stranding%…%This%escape%behavior% often%compounds%their%fate,%because%they%pump%much%of%their%stomach%and% internal%cavities%full%of%sand,%thus%exacerbating%their%fate%through%extreme%sand% loading.%…%The%only%structure%often%preserved%within%the%umbrellar%center%is%a% convex%sediment%mound%formed%either%through%pulsing%of%the%dying%jellyfish%or% decomposition%of%the%sediment Aladen%gastrovascular%cavity.”% ! Tragic!indeed .!But!this!is!a!great!example!of!the!rich!information!stored!in! sedimentary!rocks.!! ! We !co uld !als o!us e!the !gr ain!s iz e!and!ripple!wavelength !o f!th e !sand stone!to!infer! wate r!d epth !and !wav e !ene rgy .!In!fact,!my!group!has! bee n!d o ing !experiments!to!try! to!interpret!ripple!patterns!preserved!in!sedimentary!rocks![PPT:!Myrow!photo!of! outcrop!with!defects,!and!time!lapse!of!experiment].!! ! Let’s!make!a!table!of!the!different!stages!that!different!kinds!of!sedimentary!rocks! pass!through.!This!will!help!you!interpret!the!samples!you!examine!in!the!lab.! ! 1!
! Source!Rock! ! (Silici)clastic*sedimentary*rocks* Chemical/Biological*sed*rocks* ! Physical!processes! Chemical!processes! Me chanical!weathering!–! • Chemical!weathering! • fragmentation!of!bedrock! o Dissolution!–!usually!in! H2O! Er osion! –!detachment!from! • o Alteration!–!to! surfa ce!in! sou rce!area! Breakdown! different!minerals!by! • Transport ! –!moved!away!from! chemical!reactions! sou rce! a rea !b y!fl u id! (water,! • Transport!–!in!solution! wind ,!ic e )!or !gr av i ty ! • Finin g !–!breakdown!i nto! smaller!particles!during! transport! Deposition! Precipitation! • Me ch ani c al! –!du e! to!drop!in! • Inor ganic!d ue!to !satur ati on! trans po r t!e ne rgy ! addi tio n!o f!s olute ! o loss!of!s olv ent!by !e v ap! Sedimentation! o change!in!sol ub ility ! o Biotically!mediated!(e.g.!shells! • of!sil ica !or!carbonate )! Burial*! In!a!d epression!or !bas in!(ocean,!lake,!valley) ! • In!a!place!wher e!Ear th’s !surface!is!sinking!(Gulf !of!Mexico)! • More!sediment!deposited!on!top! • Diagenesis!(alteration!of!rock)! ! !Lithification!(transformation!into!rock)! Pressure,!heat,!different!fluids!lead !to…! • Lithification! • Compaction! • Dew at erin g ! Chem! rea ct ion s! • Addition!of!miner als,!including!cementation! • ! Sedimentary!Rocks! ! *N ote!the!pr es ervation!bias .!In!gener al,!s ed !r ocks !r ecor d !what!is !happening!in! bas ins,!which !o fte n!contain!bo dies!of !wate r .!T he!s o il!yo u!d ig!up!i n!your!suburban! back!yard!in!western!MA!will!probably!not!form!a!sedimentary!rock! in!that!location,! because!it!is!eroding!rather!than!being!deposited!&!buried.! ! Differen t!kinds! of! sed!rocks!result!from! Differen t !source!material!(rock)! • Different!pathways!through!the!process!we!just!sketched!out! • ! ! 2!
Knowing!the!names!& !categories!that!have!been!devised!to!describe!sed!rocks!is! valuable ,!and!your !textbook!chapter!will!help!with!that.![PPT:!textbook!tables!on! clastic,!b i o/chem]! ! Equally !important! is!knowing!how!a!rock!records!different!parts!of!this!pathway,! especial ly:! ! 1. Which!transport!agent(s)?!H ow!strong?!If!water,!how!much,!and!what!kind!of! flow?! 2. Tectonic/geographic!settings!of!source!& !sink.!“Active”!margin!like!the! Pacific!rim,!or!“passive”!like!eastern!N.!America?!Steep!river!exiting! mountains,!large!river!floodplain,!beach,!or!sea!floor?! 3. Climate.!Glacial?!Tropical?!Desert?! 4. Biological!activity.!Production,!deposition,!alteration!of!sediments.! ! How !ca n!w e!look ! at ! a!sed!rock !a nd!deduce!t hese!t hings?!Rest !of!l ect ure: ! Ind icator s !of !thes e!four! fa ctors!in!cla stic!and!chem /bio! sed! rock s ! • • Prominent!recent!examples!of!geologists!applying!these!ideas! ! ! 3!
! Clastic( Chemical/Biological( 1.! Typ e! &! Gra in!si ze! –! pr op!to!tr ans por t!ener gy.! Reworking!of!chem./ bi o !s ed!r ocks,! • • st rengt h! of! Bould er /gr ave l/s and /s ilt/clay! –!256/2/0.05/0.005!mm! can!apply!clastic!methods ! trans po r t!age nt! Gra in!sha pe! –!tr ans po rt!d istanc e.!L arger!grains!round! Diagenetic!textures,!e. g .!ooids ! • • more!easily! Grain!sorting!–!change!in!transport !energy!in!space! • (river!profile)!or!time!(e.g.!fining!up)![PPT]! • Bedforms! (shape!of!sed!surfa ce!due!to!transport! processes)![hand!samples ,!recall!ripple!example]! o Symmetry! ! !bi /uni !d ir e c ti o nal!f lo w ! o Amplitude! ! !fl ow !energy,! w ind! vs.! w ater!(eolian! dunes!make!thick!XQ be d s ) ![P P T ] ! o Orie ntati on! !tr ans po r t!d i r e c ti o n! ! 2.! Tect onic! &! Mineralogy!–!source!rock!type!varies!with!tec!setting! Species!composition! • • geogr aphic! • Quar tz!content!–!mafic!minerals,!feldspar!break!down! • Presence!of!clastic!component! ! ! set t ing! of! faster,!so!distal!sediments!more!qtz Qrich! higher!energy,!near!shore! sou rce! &! sink ! • Grain!size!–!proxy!f or !topograph ic!s lope !and !tec!ac ti v ity! • Rock!type! –!limestone!(car bo nate )! Ind irect,!via!tr ans por t!agent!& !ener gy !(st eep!coa sta l ! made!of!foraminifera! [PPT] ,!cher t! • river! vs.!large!del t a )! (silica)!made!of!diatoms,! radiolarian ![PPT]! ! !CCD ! 3.!Climate! Chem!weathering!of!gra ins! Redox!state! –!BIFs! • • • Sequence!of!deposits! ! ! climate!change! • Presence!of!evaporit e! Coarser!grains! ! !colder ! • Species!composition! • Indirect,!via!transport!medium!(e.g.,!glacier!vs.!wind!vs.! C hemical!composition!–!climate! • • river)! pr oxies!(lecture!later!in!course)! 4.! B iol ogica l! • Bi oturbati on ! • Species!composition! acti vi ty ! C lasts!made!of!biogenic!material! Mi neralogy!(microbial!role!in! • • dolomitization?)! Sed!str uctur es,!esp!fossils ! • Sed!str uctures!–!reefs,! • stromatolites,!fossils!in!general!! ! 4!
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