5 - Diagenesis

DiagenesisChapters 5, 6, 7Diagenesis• Definition• Controls on diagenesis• Zones, processes and products• Porosity• Organic matter• SummaryDiagenesis• Physical and chemical changes taking placein a sediment or sedimentary rock betweendeposition and either: a) metamorphism, orb) uplift and weathering• Sediment converted into consolidatedsedimentary rock• Low temperature near-surface processes tohigher temperature subsurface processes(

Diagenesis vs. MetamorphismControls on Diagenesis• Movement of pore fluids• Meteoric/surface waters into sedimentarybasins• Potentiometric head defined by ground watertable – above sea level, pore fluids will readilyflow into marine sedimentary basins• Thermal convection• Inverse density gradient caused by thermalexpansion of water (batholiths, salt domes, etc.)• Compaction• Porosity reduction drives interstitial watersupwardShallow Carbonate DiagenesisBoggs 2001

Boggs 2001BioturbationCarbonates• Micritization• Carbonate grains may be bored by fungi,bacteria, algae• Fine-grained (micrite) carbonate(aragonite, high-mag calcite) may thenprecipitate in holes• In some cases, only exteriors of grainsaffected – micrite rims/envelopes• In other cases, grains may be completelymicritized

Micrite envelopeshttp://geology.uprm.edu/Morelock/GEOLOCN_/7_image/micrit.jpgDiagenetic Processes• Mesodiagenesis: four main processes:• Compaction• Dissolution• Precipitation• RecrystallizationPress and Siever 2001

Diagenetic ProcessesMcIlreath and Morrow 1990Compaction• Loosely packed sand porosity approaches25%; saturated mud 60-80% water. Porosityreduced during burial due to overburdenpressure• Fabrics may form identifiable in thin sectionincluding: deformation, distortion, flattening• Pseudomatrix formation when rock fragmentsalter to clays under pressure – looks like aprimary clay matrix• Pressure solution where grain boundariesundergo dissolution and crystallizationCompactionBoggs 2001

Concavo-Convex Contact0.27 mmwww.gly.uga.edu/railsback/PDFimage0208a.htmlSutured/Concavo-convex contacts2.4 mmwww.gly.uga.edu/railsback/PDFimage0212.htmlDissolution• Silicate and carbonate mineralsdissolved under conditions that are theopposite for cementation• Calcite and silicates show oppositebehaviour – conditions for precipitation ofthe one are favourable for dissolution ofthe other

Factors influencing the solubility ofCaCO3 http://www.usask.ca/geology/classes/geol243/243notes/243week10a.htmlhtmlFeldspar dissolution and calcite cement(high-pH conditions)http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStratL1/slideshow_1_7.htmCementation• Development of new precipitates in pore spaces• Carbonates (calcite) and silicates (quartz) mostcommon, also clays in siliciclastic rocks• May be in response to groundwater flow,increasing ionic concentration in pore waters, andincreased burial temperatures• Overgrowths or microcrystalline cement whenhigh pore-water concentrations of hydrous silica• Iron oxide (hematite, limonite) determined byoxidation state

Calcite cementPress and Siever 2001Quartz overgrowths – Dakota Fm.Quartz overgrowths followed by calcite cementhttp://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStratL1/slideshow_1_16.htm

Chlorite CementBoggs 2001Illite cementwebmineral.com/specimens/picshow.php?id=1284Cementation• Cementation of carbonates may takeplace in a variety of realms• Meteoric – vadose/phreatic• Marine (phreatic) - seawater• “Subsurface” - basinal brines• Use fabric to help infer origin

Carbonate CementsPendant calcite cementhttp://sheba.geo.vu.nl/~imma/Project3.htmlBladed calcite cement followed by coarse sparhttp://web.umr.edu/~greggjay/Carbonate_Page/LSGallery/pages/c-TF(F)b_10.htm

Mineral Replacement• Dissolution of one mineral is replacedby another, simultaneously• No volume change• Carbonate replacement bymicrocrystalline quartz; chert bycarbonates; feldspars and quartz bycarbonates; feldspars by clay mineralsSericitewww-geoazur.unice.fr/PERSO/verati/Mineral Recrystallization• Existing mineral retains originalchemistry but increases in size• Volume change• Amorphous silica to coarse crystallinequartz; fine lime mud into coarse sparrycalcite

Burial Dolomitization• Dolomite may form as a replacement of aprecursor limestone• Use textural relationships to determine origin• Certain types of calcium carbonate may bepreferentially dolomitized• Dolomite may be a fracture/void spaceinfill• Problem: need mechanism for circulatinglarge volumes of Mg-rich waterDolomite replacing matrix around micritized ooidsweb.umr.edu/~greggjay/Carbonate_Page/DoloGallery/Saddle (“Baroque”) Dolomitehttp://www.uky.edu/KGS/emsweb/trenton/fieldwork.html

Diagenetic Structures• Liesegangen bands - result from groundwaterprecipitates in porous sandstones• Concretions - nucleated, regular shapedrounded objects• Nodules - irregularly shaped rounded objects• Calcite, siderite, pyrite authigenesis aroundan organic nucleus• Geodes - concentric layers of chalcedony withinternal crystals of euhedral quartz or calciteIndicators of Diagenetic Histories• Conodont color alteration (Harris, 1979) -Cambrian-Triassic phosphatic fossils from paleyellow (1; 300C)• Vitrinite Reflectance - resistant plant cells alteredunder T&P, and reflect more light the higher therank (100-240C)• Clay Mineral Transformation - stability of certainclay minerals (>100C smectites form mixedlayerclays; >200C become illites; >300C onlymica remains)Indicators of Diagenetic Histories• Zeolite facies - hydrous aluminosilicatesalteration (150C prehnite & pumpellyite)• Stable isotope ratios – see next slide

Isotopicsignature ofcarbonatecan indicatediagenetichistoryDiageneticZones -ShaleDiagenesis• Where multiple diagenetic episodes haveaffected a rock, it can be important toestablish the paragenetic sequence• Detailed thin section observations• SEM images• Isotopic analyses of diagenetic phases• Etc.• Use to define burial history, fluid flowepisodes, etc.

SEM image of quartzovergrowth (Q),chlorite (C), andframboidal pyrite (P)Almon&Davies 1981Diagenesis of Qtz Arenite

Porosity/Permeability• Characterization of porosity andpermeability may be a an importantpart of thin-section description• How much?• What is origin?• Is porosity connected? (impliespermeability)Burial and PorosityBoggs 2001Primary Porosity• Amount of void spaces within a rock• Primary porosity: a function of grain size, sorting,and packing

Secondary Porosity• Development of pore spaces in rockthrough diagenesis• Deep diagenetic fluids dissolve lessstable framework grains or cement suchas carbonate, plagioclase, pyroxene,amphiboles, and rock fragments• Compression produces fracturesSecondary Sandstone PorosityPorosity & Permeability• Porosity: % of void space in rock/sedimentthat may contain fluids• Total porosity – all pore spaces• Effective porosity – connected pores• Permeability: ability to transmit fluids• Units – Darcies• Absolute/relative permeability a function ofporosity, texture, diagenesis, etc.

Press and Siever 2001Small pores, butinterconnected –high permeabilityLarge pores, butnot connected –low permeabilityMoldic porosity

Secondary porosity – Dakota Fm.Interparticle porosity – Dakota Fm.Porosity and PermeabilitySediment Porosity (%) PermeabilityGravel 25 to 40 excellentClean Sand 30 to 50 good to excellentSilt 35 to 50 moderateClay 35 to 80 poorGlacial Till 10 to 20 poor to moderateRock Porosity (%) PermeabilityConglomerate 10 to 30 moderate to excellentSandstone, Well-sorted, little cement 20 to 30 good to very goodAverage 10 to 20 moderate to goodPoorly sorted, Well cemented 0 to 10 poor to moderateShale 0 to 30 very poor to poorLimestone, dolomite 0 to 20 poor to goodCavernous limestone up to 50 excellentCrystalline rockUnfractured 0 to 5 very poorFractured 5 to 10 poorVolcanic Rocks 0 to 50 poor to excellent

Organic Diagenesis• Buried organic matter also undergoesdiagenetic transformations• Peat -> coal (increase in wt % carbon,decrease in “volatiles”)• Hydrocarbon generation• Type (gas or oil) depends on temperature andkerogen type• Kerogen – set of complex organic compounds,composed of varying proportions of C, H, and Ohttp://www.uky.edu/KGS/coal/coal_information.htm

van Krevelen plotSummary• Diagenesis: Physical and chemical changestaking place in a sediment or sedimentaryrock between deposition and either: a)metamorphism, or b) uplift andweathering• Affects all sedimentary deposits• Siliciclastics, carbonates, organic, othersSummary• Three zones:• Eodiagenesis – early/shallow diagenesis• Mesodiagenesis – deep burial• Teleodiagenesis – uplifted• Key results:• Compaction• Dissolution• Precipitation• Replacement

Summary• Porosity• May be destroyed (compaction, cementation)or created (dissolution, fracturing) during burial• Characterization of porosity type andconnectivity can be important• Diagenesis of organic deposits leads toformation of coals and hydrocarbons