lecture 14 feb - Dr. Hoyle
Transcript of lecture 14 feb - Dr. Hoyle
SedimentaryRocks
Therockcycleandsedimentaryrocks• Weathering• Erosion• Transportation• Deposition• Burial• Compaction• Cementation• Lithification
PhysicalGeologybyStevenEarleusedunderaCC-BY4.0
RockOutcrop- source
• Theoutcropofrockcanbe• Igneous• Metamorphic• Sedimentary
•Allcangiveanewsedimentaryrockgoingthroughthismultistepprocess
Nowsedimentisformedbyweathering,erosion,transportationanddeposition…..
WeatheringI• Whenrocksareexposedatthesurface
• Temperaturesvarywidely• Pressureisreduced• Theyareexposedto20%oxygen(andothergases)• Usuallywaterisabundant
• Weatheringincludestwomainprocesses• Mechanicalweathering- breakdownofrockintosmallerfragments(freeze/thawcycles)• Chemicalweathering- changeofmineralstonew,morestableforms
• Mechanicalweatheringgivesnewsurfacesforattackbychemicalprocesses• Chemicalweatheringweakensrock– moreeasilyweatheredmechanically• Weatheringcreates:
• Soil- maybemorelaterinsemester
• Sedimentaryrock– fromclastsandionsinsolution
WeatheringII- mechanicalweathering
• Decreasedpressurewhenoverlyingrockisremoved• Decreasedconfiningpressurecausescrackingorexfoliationofrock
• Freezing/thawingofwaterrockcracks• Formationof‘salt’crystalsinrockcracks• Crackingcausedbyplantroots• Exposurebyburrowinganimals(removessoilandaffectsweathering)
Severalprocesses….
WeatheringIII- chemicalweathering1
• Chemicalchangestomineralswhenexposedtosurfaceconditions• water(intheairandonthegroundsurface)• abundanceofoxygen• presenceofcarbondioxide(formscarbonicacidwhencombinedwithwater)
• Veryspecifictoboththemineralandtheenvironmentalconditions• e.g.1– Quartzisvirtuallyunaffectedbychemicalweathering• e.g.2– Feldspariseasilyaltered.
• Generallychemicalweatheringis:• greaterinwarm/wetclimates• muchlessincold/dryclimates
• Theprocessthatismostimportanttoalmostallchemicalweathering is:• H2O+CO2 àH2CO3 followedbyH2CO3 à H+ +HCO3
-
WeatheringIV- chemicalweathering2
• 2maintypesofchemicalweathering• Hydrolysiswheresomemineralsarealteredtootherminerals• Completedissolution
• Forexample,feldsparundergoeshydrolysis—tokaoliniteviz.• CaAl2Si2O8 +H2CO3 +🁬O2 à Al2Si2O5(OH)4 +Ca2++CO3
2- J
• andCalcite(e.g.limestone)completelydissolvesviz.• CaCO3(s) +H+ +HCO3
- à Ca2+(aq)+2HCO3- (aq)
WeatheringV– productsofweathering
PhysicalGeologybyStevenEarleusedunderaCC-BY4.0
ErosionandTransportation• Mechanicalweatheringisgreatlyfacilitatedbyerosion• Erosion istheremovalofweatheringproductscausingtheexposureofmorerock• Transportation isthemovementofsediment/dissolvedionsfromerosiontodeposition.Itcanoccurby:• Wind• Flowingwater• Glacialice• Massmovementdownaslope
• Fasterflowrates(windorH2O)causelargerpiecestobetransported
Deposition• Occursasconditionschangeandtransportnolongeroccurse.g.watercurrentslowsdown/stops
• SeeTable6.3intext–• Environment• Transport• Sedimentformed
• Sedimentaryrockisthenformedbyburialandlithification…..
PhysicalGeologybyStevenEarleusedunderaCC-BY4.0
Principlesofsedimentaryaccumulation• Principlesof…..1. Superposition - sedimentarylayersaredepositedinorder,andthat
unlesstheentiresequencehasbeenoverturned,thelowerlayersareolderthantheupperones
2. Originalhorizontality- sedimentsaccumulateinhorizontallayers,bygravity.• Thisimpliesthatobservedtiltedsedimentarylayersmusthavebeenmoved(plates?)
3. Inclusions - allrockfragmentsinasedimentarylayermustbeolderthanthelayer(fromLyell- 1830s)
4. Faunalsuccession- thereisaspecificorderthatwidelydistributedorganismshaveevolvedthroughtime,• thustheIDofspecificfossilscanbeusedtodeterminearock’sage
(possiblythemostcontroversial)
Burial,cementationandlithification• Lithification isthetermusedtodescribetheprocessesthattakeplacewithinasedimenttoturnitintosolidrock(atdepthsof100-1000sm)• Burial byothersedimentsleadstocompactionofthematerialandremovalofsomeoftheinterveningwaterandair.• Afterthisstage,theindividualclastsarealltouchingoneanother.• Cementation istheprocessofcrystallizationofmineralsinporesbetweenthepieces,andatpointsofcontactbetweenlargerclasts(sandsize+)• DependingonP,T&chemicalconditions - cementcrystalsmaybe:• calcite,hematite,quartz,clayminerals,thereareothers
Nowontosedimentarystructures…..
Sedimentarystructures- bedding
• Bedding istheseparationofsedimentsintolayers• Layersdifferfromoneanotherby• texture,composition,colour,orweatheringcharacteristics
• Bedsseparatedbypartings—narrowgapsbetweenadjacentbeds• Beddingshowschangesindepositionthatarerelatedto• seasonaldifferences,climatechange,locationschangeofrivers/deltas,tectonicchanges.
• Partingsmayrepresentperiodsofnon-depositionfor10-100sy• Beddingcanforminalmostanydepositionalenvironment
Sedimentarystructures– graded&cross-bedding
• Gradedbeddingisseenasarangeofgrainsizefrombottomtotopwithinasinglebed• Normally- coarseatthebottomandfinertowardthetop.• Somearereversed
• Mostgradedbedsforminasubmarine-fanenvironment(fig6.17intext)• sediment-richflowsdescendperiodicallyfromshallowshelfandontoseafloor.
• Cross-bedding isbeddingthatcontainsangledlayersandformswhensedimentsaredepositedbyflowingwaterorwind.(seefigs6.1&6.20intext)• Cross-bedssizes…
• fromwatertendtobe1-10cm• fromwinddeposits(aeolian)<5m
• Cross-bedsformbydepositonleadingedgeofadvancingripple/dune• ‘Eachlayerisrelatedtoadifferentripplethatadvancesintheflowdirection,andispartiallyerodedbythefollowingripple’(fig6.21intext,nextslide)
PhysicalGeologybyStevenEarleusedunderaCC-BY4.0
Cross-beds(fig6.21)PhysicalGeologybyStevenEarleusedunderaCC-BY4.0
Cross-beddingYouTubevideo
Sedimentarystructures- ripplesandmudcracks
• Ripples associatedwithformationofcross-beddingmaybepreservedonthesurfacesofsedimentarybeds.• Clastsinstreamstendtotiltwiththeirupperendspointingdownstreambecausethisisthemoststablepositionwithrespecttothestreamflow
• Mudcracksformwhenmuddysedimentaredepositedinashallowwaterthendriesup(e.g.atidalflatsorpond)• Thishappensbecausetheclayintheuppermudlayertendstoshrinkondrying,andsoitcracksbecauseitoccupieslessspacewhenitisdry.
Andnowsedimentaryrockclassification….
Classificationofsedimentaryrock• Thereareseveralpossiblewaystosubdividesedimentaryrocks• Usuallytheclassificationsisinto:
• Clastic• Nonclastic (chemicalandbioclastic/fossiliferous)
• Clasticrefersto- solidfragmentsthataretransportedtotheburialsite• Nonclastic/chemicalrefersto– transportedmaterialisinsolution• Thereissomeoverlapbetweenthetwoas
• almostallclasticsedimentaryrockscontaincementformedfromdissolvedions• manychemicalsedimentaryrocksincludesomeclasts
• Mostsedimentaryrocksareoneofthese2typeswithabitoftheotherJ
ClasticRocks– features/characteristics
PhysicalGeologybyStevenEarleusedunderaCC-BY4.0
Clasts…….
• Smallerclaststendtobesinglemineralcrystals• Largerclastsaretypicallypiecesofrock• Bysize…..• Mostclasts<0.063mmareactuallycomposedofclayminerals• Mostsand-sizedclasts (0.063-2mm)arequartz• Commonasquartzresistsweatheringmorethanothercommonminerals
• Mostclasts>2mmarerockfragments• fine-grainedrocklikebasaltorandesite• coarse-grainedrocklikegraniteorgneiss.
Udden-Wentworthgrainsandsizescale
Unitsinmm Unitsinmmµm
PhysicalGeologybyStevenEarleusedunderaCC-BY4.0
Namingsandstone
• Sandstoneisthemostimportantclasticrock• Sandstoneiseither• <15%siltandclay- Arenite - cleansandstoneOR• >15%silt/clay- Greywacke(“wacka”buttextsays“wackie”!)
• Namesforarenites:consideringthesand-sizedgrainsonly• >90%quartzthencalledquartzarenites• >10%feldsparand feldspar>rockfragments,thencalled:
• feldspathicarenitesorarkosic arenitesorjustarkose• >10%rockfragments,and rockfragments>feldspararelithicarenites
Chemicalsedimentaryrocks
• Chemicalsedimentaryrocksaremadefrommaterialtransportedasionsinsolution• Notpossibletorelatechemicalsedimentsbacktosourcerocks• Themostcommonchemicalsedimentaryrock,byfar,islimestone• Butotherrocksarechert,evaporites andbandedironformation(BIF)• Biologicalprocesses- importantinformationofsomenonclasticrocks,e.g.• Limestone
• limestoneismostlyfragmentsofmarineorganismsthatmakecalcitefortheirshells• Chert
• mostchert includesatleastsomeofthesilicatests(shells)ofdiatomsandothermicroscopicorganisms
Chert andBIFDalesGoge byGraemeChurchard underCCBY2.0.
Howevaporites formHassanZ.Harrazh [email protected]
Precipitationsequencefrom(sea)waterwithincreasingevaporation:
1.CalciteCaCO32.GypsumCaSO43.HaliteNaCl4.Sylvite KCl +otherK&Mgsalts
(leastsolublefirst)
Evaporite – [email protected]
Evaporite - bigdepositinCanada
• World’slargestreservesof‘potash’occurinSaskatchewan• Oreisexceptionallyhighgrade(25-30%K2O)• atdepthsof950-1100mincreasingto>3500m• Uniformmineralizationandthickness(2.4-3m)• nostructuraldeformations• MostlySylvinite (NaCl+KCl)• carnallite - hydratedKMgCl)• andclay
Publicdomain
• NowaYouTubevideoonSedimentaryRockIdentification(7mins)
• Reading forafterstudybreak182-194,202-210
•TEST• Iwillsendyousomequestionsbytheendoftheday.
• Thesewillbequestionsontheactualtest.• ThankyoutothosewhosubmittedaquestionforconsiderationJ• THEEND