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Diagenesis

Chapters 5, 6, 7

Diagenesis

DefinitionControls on diagenesisZones, processes and productsPorosityOrganic matterSummary

Diagenesis

Physical and chemical changes taking place in a sediment or sedimentary rock between deposition and either: a) metamorphism, or b) uplift and weathering Sediment converted into consolidated sedimentary rock Low temperature near-surface processes to higher temperature subsurface processes (<300C and 1-2 kb)

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Diagenesis vs. Metamorphism

Controls on DiagenesisMovement of pore fluids

Meteoric/surface waters into sedimentary basins

Potentiometric head defined by ground water table – above sea level, pore fluids will readily flow into marine sedimentary basins

Thermal convectionInverse density gradient caused by thermal expansion of water (batholiths, salt domes, etc.)

CompactionPorosity reduction drives interstitial waters upward

Shallow Carbonate Diagenesis

Boggs 2001

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Boggs 2001

Bioturbation

Carbonates

MicritizationCarbonate grains may be bored by fungi, bacteria, algaeFine-grained (micrite) carbonate (aragonite, high-mag calcite) may then precipitate in holesIn some cases, only exteriors of grains affected – micrite rims/envelopesIn other cases, grains may be completely micritized

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http://geology.uprm.edu/Morelock/GEOLOCN_/7_image/micrit.jpg

Micrite envelopes

Diagenetic Processes

Mesodiagenesis: four main processes:CompactionDissolutionPrecipitationRecrystallization

Press and Siever 2001

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Diagenetic Processes

McIlreath and Morrow 1990

Compaction

Loosely packed sand porosity approaches 25%; saturated mud 60-80% water. Porosity reduced during burial due to overburden pressureFabrics may form identifiable in thin section including: deformation, distortion, flattening Pseudomatrix formation when rock fragments alter to clays under pressure – looks like a primary clay matrix Pressure solution where grain boundaries undergo dissolution and crystallization

Compaction

Boggs 2001

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Concavo-Convex Contact

www.gly.uga.edu/railsback/PDFimage0208a.html0.27 mm

Sutured/Concavo-convex contacts

www.gly.uga.edu/railsback/PDFimage0212.html2.4 mm

Dissolution

Silicate and carbonate minerals dissolved under conditions that are the opposite for cementation

Calcite and silicates show opposite behaviour – conditions for precipitation of the one are favourable for dissolution of the other

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Factors influencing the solubility of Factors influencing the solubility of CaCO3 CaCO3 http://www.usask.ca/geology/classes/geol243/243notes/243week10a.http://www.usask.ca/geology/classes/geol243/243notes/243week10a.htmlhtml

Feldspar dissolution and calcite cement (high-pH conditions)

http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStratL1/slideshow_1_7.htm

Cementation

Development of new precipitates in pore spaces Carbonates (calcite) and silicates (quartz) most common, also clays in siliciclastic rocks May be in response to groundwater flow, increasing ionic concentration in pore waters, and increased burial temperatures Overgrowths or microcrystalline cement when high pore-water concentrations of hydrous silica Iron oxide (hematite, limonite) determined by oxidation state

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Press and Siever 2001Calcite cement

Quartz overgrowths – Dakota Fm.

Quartz overgrowths followed by calcite cement

http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStratL1/slideshow_1_16.htm

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Chlorite Cement

Boggs 2001

webmineral.com/specimens/picshow.php?id=1284

Illite cement

Cementation

Cementation of carbonates may take place in a variety of realms

Meteoric – vadose/phreaticMarine (phreatic) - seawater“Subsurface” - basinal brines

Use fabric to help infer origin

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Carbonate CementsCarbonate Cements

http://sheba.geo.vu.nl/~imma/Project3.html

Pendant calcite cement

http://web.umr.edu/~greggjay/Carbonate_Page/LSGallery/pages/c-TF(F)b_10.htm

Bladed calcite cement followed by coarse spar

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Mineral Replacement

Dissolution of one mineral is replaced by another, simultaneously No volume change Carbonate replacement by microcrystalline quartz; chert by carbonates; feldspars and quartz by carbonates; feldspars by clay minerals

www-geoazur.unice.fr/PERSO/verati/Sericite

Mineral Recrystallization

Existing mineral retains original chemistry but increases in size Volume change Amorphous silica to coarse crystalline quartz; fine lime mud into coarse sparrycalcite

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Burial DolomitizationDolomite may form as a replacement of a precursor limestone

Use textural relationships to determine originCertain types of calcium carbonate may be preferentially dolomitized

Dolomite may be a fracture/void space infillProblem: need mechanism for circulating large volumes of Mg-rich water

Dolomite replacing matrix around micritized ooids

web.umr.edu/~greggjay/Carbonate_Page/DoloGallery/

Saddle (“Baroque”) Dolomite

http://www.uky.edu/KGS/emsweb/trenton/fieldwork.html

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Diagenetic Structures

Liesegangen bands - result from groundwater precipitates in porous sandstones Concretions - nucleated, regular shaped rounded objects Nodules - irregularly shaped rounded objects Calcite, siderite, pyrite authigenesis around an organic nucleus Geodes - concentric layers of chalcedony with internal crystals of euhedral quartz or calcite

Indicators of Diagenetic Histories

Conodont color alteration (Harris, 1979) -Cambrian-Triassic phosphatic fossils from pale yellow (1; <80C) to black (5; >300C) Vitrinite Reflectance - resistant plant cells altered under T&P, and reflect more light the higher the rank (100-240C) Clay Mineral Transformation - stability of certain clay minerals (>100C smectites form mixed-layer clays; >200C become illites; >300C only mica remains)

Indicators of Diagenetic HistoriesZeolite facies - hydrous aluminosilicatesalteration (<100C heulandite & analcime; 100-150C laumontite; >150C prehnite & pumpellyite) Stable isotope ratios – see next slide

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Isotopic signature of carbonate can indicate diagenetichistory

DiageneticZones -Shale

Diagenesis

Where multiple diagenetic episodes have affected a rock, it can be important to establish the paragenetic sequence

Detailed thin section observationsSEM imagesIsotopic analyses of diagenetic phasesEtc.

Use to define burial history, fluid flow episodes, etc.

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Almon&DaviesAlmon&Davies 19811981

SEM image of quartz overgrowth (Q), chlorite (C), and framboidal pyrite (P)

Diagenesis of Qtz Arenite

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Porosity/Permeability

Characterization of porosity and permeability may be a an important part of thin-section description

How much?What is origin?Is porosity connected? (implies permeability)

Burial and Porosity

Boggs 2001

Primary PorosityAmount of void spaces within a rockPrimary porosity: a function of grain size, sorting, and packing

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Secondary Porosity

Development of pore spaces in rock through diagenesisDeep diagenetic fluids dissolve less stable framework grains or cement such as carbonate, plagioclase, pyroxene, amphiboles, and rock fragmentsCompression produces fractures

Secondary Sandstone Porosity

Porosity & Permeability

Porosity: % of void space in rock/sediment that may contain fluids

Total porosity – all pore spacesEffective porosity – connected pores

Permeability: ability to transmit fluidsUnits – DarciesAbsolute/relative permeability a function of porosity, texture, diagenesis, etc.

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Press and Siever 2001

Small pores, but interconnected –high permeability

Large pores, but not connected –low permeability

Moldic porosity

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Secondary porosity – Dakota Fm.

Interparticle porosity – Dakota Fm.

Sediment 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 moderate

Rock Porosity (%) PermeabilityConglomerate 10 to 30 moderate to excellentSandstone, Well-sorted, little cement 20 to 30 good to very good

Average 10 to 20 moderate to goodPoorly sorted, Well cemented 0 to 10 poor to moderate

Shale 0 to 30 very poor to poorLimestone, dolomite 0 to 20 poor to goodCavernous limestone up to 50 excellentCrystalline rock

Unfractured 0 to 5 very poorFractured 5 to 10 poor

Volcanic Rocks 0 to 50 poor to excellent

Porosity and Permeability

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Organic Diagenesis

Buried organic matter also undergoes diagenetic transformations

Peat -> coal (increase in wt % carbon, decrease in “volatiles”)Hydrocarbon generation

Type (gas or oil) depends on temperature and kerogen typeKerogen – set of complex organic compounds, composed of varying proportions of C, H, and O

http://www.uky.edu/KGS/coal/coal_information.htm

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van Krevelen plot

Summary

Diagenesis: Physical and chemical changes taking place in a sediment or sedimentary rock between deposition and either: a) metamorphism, or b) uplift and weatheringAffects all sedimentary deposits

Siliciclastics, carbonates, organic, others

Summary

Three zones:Eodiagenesis – early/shallow diagenesisMesodiagenesis – deep burialTeleodiagenesis – uplifted

Key results:CompactionDissolutionPrecipitationReplacement

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Summary

PorosityMay be destroyed (compaction, cementation) or created (dissolution, fracturing) during burialCharacterization of porosity type and connectivity can be important

Diagenesis of organic deposits leads to formation of coals and hydrocarbons