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Diagenesis
Chapters 5, 6, 7
Diagenesis
Definition
Controls on diagenesis
Zones, processes and products
Porosity
Organic matter
Summary
Diagenesis
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(
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Diagenesis vs. Metamorphism
Controls on Diagenesis
Movement of pore fluids Meteoric/surface waters into sedimentary
basins Potentiometric head defined by ground water
table above sea level, pore fluids will readilyflow into marine sedimentary basins
Thermal convection Inverse density gradient caused by thermal
expansion of water (batholiths, salt domes, etc.)
Compaction Porosity reduction drives interstitial waters
upward
Shallow Carbonate Diagenesis
Boggs 2001
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Boggs 2001
Bioturbation
Carbonates
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
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http://geology.uprm.edu/Morelock/GEOLOCN_/7_image/micrit.jpg
Micrite envelopes
Diagenetic Processes
Mesodiagenesis: four main processes:
Compaction
Dissolution
Precipitation
Recrystallization
Press and Siever 2001
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Diagenetic Processes
McIlreath and Morrow 1990
Compaction
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 fragments
alter to clays under pressure looks like aprimary clay matrix
Pressure solution where grain boundariesundergo 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 mineralsdissolved under conditions that are theopposite for cementation
Calcite and silicates show oppositebehaviour conditions for precipitation ofthe one are favourable for dissolution ofthe other
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Factors influencing the solubility ofFactors influencing the solubility of
CaCO3CaCO3 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) 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
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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 takeplace in a variety of realms
Meteoric vadose/phreatic
Marine (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 replacedby another, simultaneously
No volume change
Carbonate replacement bymicrocrystalline quartz; chert bycarbonates; feldspars and quartz bycarbonates; feldspars by clay minerals
www-geoazur.unice.fr/PERSO/verati/Sericite
Mineral Recrystallization
Existing mineral retains originalchemistry but increases in size
Volume change
Amorphous silica to coarse crystallinequartz; fine lime mud into coarse sparrycalcite
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Burial Dolomitization
Dolomite may form as a replacement of aprecursor limestone
Use textural relationships to determine origin Certain types of calcium carbonate may be
preferentially dolomitized
Dolomite may be a fracture/void spaceinfill
Problem: need mechanism for circulatinglarge 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
Liesegangenbands - 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 calcite
Indicators 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 certain
clay minerals (>100C smectites form mixed-layer clays; >200C become illites; >300C onlymica remains)
Indicators of Diagenetic Histories
Zeolite facies - hydrous aluminosilicatesalteration (150C prehnite & pumpellyite)
Stable isotope ratios see next slide
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Isotopicsignature ofcarbonatecan indicatediagenetichistory
DiageneticZones -Shale
Diagenesis
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.
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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 andpermeability may be a an important
part of thin-section description How much?
What is origin?
Is porosity connected? (impliespermeability)
Burial and Porosity
Boggs 2001
Primary Porosity
Amount of void spaces within a rock
Primary porosity: a function of grain size, sorting,and packing
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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 fractures
Secondary Sandstone Porosity
Porosity & 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.
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Press and Siever 2001
Small pores, butinterconnected high permeability
Large pores, butnot connected low permeability
Moldic porosity
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Secondary porosity Dakota Fm.
Interparticle porosity Dakota Fm.
Sediment Porosity (%) Permeability
Gravel 25 to 40 excellent
Clean Sand 30 to 50 good to excellentSilt 35 to 50 moderate
Clay 35 to 80 poor
Glacial Till 10 to 20 poor to moderate
Rock Porosity (%) Permeability
Conglomerate 10 to 30 moderate to excellent
Sandstone, Wel l-sor ted, l it tle cement 20 to 30 good to very good
Average 10 to 20 moderate to good
Poorly sorted, Well cemented 0 to 10 poor to moderate
Shale 0 to 30 very poor to poor
Limestone, dolomite 0 to 20 poor to good
Cavernous limestone up to 50 excellent
Crystalline rock
Unfractured 0 to 5 very poor
Fractured 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 undergoesdiagenetic transformations Peat -> coal (increase in wt % carbon,
decrease in volatiles)
Hydrocarbon generation Type (gas or oil) depends on temperature and
kerogen type
Kerogen 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 changestaking place in a sediment or sedimentaryrock between deposition and either: a)metamorphism, or b) uplift andweathering
Affects all sedimentary deposits
Siliciclastics, carbonates, organic, others
Summary
Three zones: Eodiagenesis early/shallow diagenesis
Mesodiagenesis deep burial
Teleodiagenesis uplifted
Key results: Compaction
Dissolution
Precipitation
Replacement
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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