Diagenesis
Chapters 5, 6, 7
Diagenesis
„
„
„
„
„
„
Definition
Controls on diagenesis
Zones, processes and products
Porosity
Organic matter
Summary
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)
1
Diagenesis vs. Metamorphism
Controls on Diagenesis
„
Movement of pore fluids
„
Meteoric/surface waters into sedimentary
basins
„
„
Thermal convection
„
„
Potentiometric head defined by ground water
table – above sea level, pore fluids will readily
flow into marine sedimentary basins
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
2
Boggs 2001
Bioturbation
Carbonates
„
Micritization
„
„
„
„
Carbonate grains may be bored by fungi,
bacteria, algae
Fine-grained (micrite) carbonate
(aragonite, high-mag calcite) may then
precipitate in holes
In some cases, only exteriors of grains
affected – micrite rims/envelopes
In other cases, grains may be completely
micritized
3
Micrite envelopes
http://geology.uprm.edu/Morelock/GEOLOCN_/7_image/micrit.jpg
Diagenetic Processes
„
Mesodiagenesis: four main processes:
„
„
„
„
Compaction
Dissolution
Precipitation
Recrystallization
Press and Siever 2001
4
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
pressure
Fabrics 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
5
Concavo-Convex Contact
0.27 mm
www.gly.uga.edu/railsback/PDFimage0208a.html
Sutured/Concavo-convex contacts
2.4 mm
www.gly.uga.edu/railsback/PDFimage0212.html
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
6
Factors influencing the solubility of
CaCO3
http://www.usask.ca/geology/classes/geol243/243notes/243week10a.html
http://www.usask.ca/geology/classes/geol243/243notes/243week10a.html
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
7
Calcite cement
Press and Siever 2001
Quartz overgrowths – Dakota Fm.
Quartz overgrowths followed by calcite cement
http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStratL1/slideshow_1_16.htm
8
Chlorite Cement
Boggs 2001
Illite cement
webmineral.com/specimens/picshow.php?id=1284
Cementation
„
Cementation of carbonates may take
place in a variety of realms
„
„
„
„
Meteoric – vadose/phreatic
Marine (phreatic) - seawater
“Subsurface” - basinal brines
Use fabric to help infer origin
9
Carbonate Cements
Pendant calcite cement
http://sheba.geo.vu.nl/~imma/Project3.html
Bladed calcite cement followed by coarse spar
http://web.umr.edu/~greggjay/Carbonate_Page/LSGallery/pages/c-TF(F)b_10.htm
10
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
Sericite
www-geoazur.unice.fr/PERSO/verati/
Mineral Recrystallization
„
„
„
Existing mineral retains original
chemistry but increases in size
Volume change
Amorphous silica to coarse crystalline
quartz; fine lime mud into coarse sparry
calcite
11
Burial Dolomitization
„
Dolomite may form as a replacement of a
precursor limestone
„
„
„
„
Use textural relationships to determine origin
Certain types of calcium carbonate may be
preferentially dolomitized
Dolomite may be a fracture/void space
infill
Problem: 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
12
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 mixedlayer clays; >200C become illites; >300C only
mica remains)
Indicators of Diagenetic Histories
„
„
Zeolite facies - hydrous aluminosilicates
alteration (<100C heulandite & analcime; 100150C laumontite; >150C prehnite & pumpellyite)
Stable isotope ratios – see next slide
13
Isotopic
signature of
carbonate
can indicate
diagenetic
history
Diagenetic
Zones Shale
Diagenesis
„
Where multiple diagenetic episodes have
affected a rock, it can be important to
establish the paragenetic sequence
„
„
„
„
„
Detailed thin section observations
SEM images
Isotopic analyses of diagenetic phases
Etc.
Use to define burial history, fluid flow
episodes, etc.
14
SEM image of quartz
overgrowth (Q),
chlorite (C), and
framboidal pyrite (P)
Almon&Davies 1981
Diagenesis of Qtz Arenite
15
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 Porosity
„
„
Amount of void spaces within a rock
Primary porosity: a function of grain size, sorting,
and packing
16
Secondary Porosity
„
„
„
Development of pore spaces in rock
through diagenesis
Deep diagenetic fluids dissolve less
stable framework grains or cement such
as carbonate, plagioclase, pyroxene,
amphiboles, and rock fragments
Compression produces fractures
Secondary Sandstone Porosity
Porosity & Permeability
„
Porosity: % of void space in rock/sediment
that may contain fluids
„
„
„
Total porosity – all pore spaces
Effective porosity – connected pores
Permeability: ability to transmit fluids
„
„
Units – Darcies
Absolute/relative permeability a function of
porosity, texture, diagenesis, etc.
17
Press and Siever 2001
Small pores, but
interconnected –
high permeability
Large pores, but
not connected –
low permeability
Moldic porosity
18
Secondary porosity – Dakota Fm.
Interparticle porosity – Dakota Fm.
Porosity and Permeability
Sediment
Gravel
Clean Sand
Silt
Clay
Glacial Till
Porosity (%)
25 to 40
30 to 50
35 to 50
35 to 80
10 to 20
Permeability
excellent
good to excellent
moderate
poor
poor to moderate
Rock
Conglomerate
Sandstone, Well-sorted, little cement
Average
Poorly sorted, Well cemented
Porosity (%)
10 to 30
20 to 30
10 to 20
0 to 10
Permeability
moderate to excellent
good to very good
moderate to good
poor to moderate
Shale
Limestone, dolomite
Cavernous limestone
Crystalline rock
Unfractured
Fractured
Volcanic Rocks
0 to 30
0 to 20
up to 50
very poor to poor
poor to good
excellent
0 to 5
5 to 10
0 to 50
very poor
poor
poor to excellent
19
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 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
20
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
weathering
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
21
Summary
„
Porosity
„
„
„
May be destroyed (compaction, cementation)
or created (dissolution, fracturing) during burial
Characterization of porosity type and
connectivity can be important
Diagenesis of organic deposits leads to
formation of coals and hydrocarbons
22