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Diagenesis

Diagenesis is the term used for all the 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 temperaturenear-surface processes to higher temperature subsurface processes (


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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 affectedmicrite rims/envelopes In other cases, grains may be completely micritized

Diagenetic Processes

The two most important diagenetic processes are compaction, and lithification, the term used

for the complex of processes including compaction by which a loose sediment is converted

into a solid sedimentary rock.

Four main processes of Mesodiagenesis are,

Compaction Dissolution Precipitation Recrystallization

Compaction


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Most sediments deposited under normal surface conditions have primary porosities ofon the order of 30% to 70%.

The lower values are more typical of coarser, sandy sediments, and the higher initialporosities are more typical of finer-grained, clay-rich sediments.

Porosity in carbonate rocks varies widely, depending upon sediment type: theporosity of well-sorted carbonate sands is in the same general range as that of the

corresponding siliciclastic sands, whereas reef carbonates commonly have much

higher initial porosities.

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 lookslike a primary clay matrix

Pressure solution where grain boundaries undergo dissolution and crystallization

Because of the difference in compaction between mudrock and sandstone, a sandstone dike

emplaced vertically upward or downward before much compaction can become contorted

because the mudrock undergoes so much more compaction than the sandstone

Deformation of vertical sandstone dyke by compaction of mudrock, A) Before compaction ;

B) After compaction


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Dissolution

Silicate and carbonate minerals dissolved under conditions that are the opposite forcementation

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

aCementation

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 porewaters, and increased burial temperatures

Overgrowths or microcrystalline cement when high pore-water concentrations ofhydrous silica

Iron oxide (hematite, limonite) determined by oxidation statectorsFactors influencing the solubility

Cementation

Cementation of carbonates may take place in a variety of realms

Meteoricvadose/phreatic Marine (phreatic) - seawater Subsurface - basinal brines

Use fabric to help infer origin

Mineral Replacement

Dissolution of one mineral is replaced by another, simultaneously


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No volume change Carbonate replacement by microcrystalline quartz; chert by carbonates; feldspars and

quartz by carbonates; feldspars by clay minerals

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

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

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 frompale yellow (1; 300C)

Vitrinite Reflectance - resistant plant cells altered under T&P, and reflect more lightthe higher the rank (100-240C)


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Clay Mineral Transformation - stability of certain clay minerals (>100C smectitesform mixedlayer clays; >200C become illites; >300C only mica remains)

Zeolite facies - hydrous aluminosilicates alteration (150C prehnite & pumpellyite)

Stable isotope ratios

Diagenesis of Sandstones

All changes, physical, chemical, and biological, that occur in a sediment after deposition and

before metamorphism ( 60% water, which can be squeezed out by exerting little

pressure

Muds can be compacted because grains are ductile (flexible) and can pack easily

Compaction of Sands.

Sands are not easily compacted because they are supported by grain-to-grain


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contacts

Quartz and feldspar are not ductile at diagenetic P and T

Modern sands 45+5% porosity

Compacted quartz sandstone ~30% porosity

Ductile lithic fragments can be squeezed into pore spaces, so lithic sandstones can

be compacted more

Can compaction alone convert sand into sandstone? Sometimes

Quartz + Feldspar + water squeezed to the limit of sedimentary conditions, still

loose grains

80% Quartz + 20% schist or mudstone fragments yields multigrain aggregates

100% Mud yields mudrock

Compaction alone can produce a rock from a sediment with high content of

ductile lithic fragments or mud

Cementation

Growth of new authigenic minerals from pore fluids

Authigenic = grown in the sediment after deposition (as opposed to detrital)

Cements precipitate in pores: usually coat grains, increase areas of grain-grain

contact, decrease pore space (porosity)

Most common Cements are:

Quartz - SiO2

Calcite - CaCO3

Hematite - Fe2O3

Clay - kaolinite, illite, montmorillonite, chlorite (not really a clay mineral)

Quartz (SiO2) Cement

Quartz cement commonly nucleates on quartz grains, is optically and

crystallographically continuous with detrital grain

Quartz cement is most common where quartz grains are abundant

SiO2 must come from pore waters that move through the sandstone

Quartz cemented quartz arenites (Tuscarora Ss.) are very resistant to weathering

Dust rings may show detrital grain boundaries

Calcite (CaCO3) Cement


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Very common

Reacts with acid

Requires permeability for CaCO3 saturated waters (with [Ca++] x [CO32-] above a certain

value)

Calcite is orders of magnitude more soluble than Quartz, may form and laterdissolve

Often discontinuous

May form concretions = locally cemented areas in friable Ss, typically around fossils

Calcite-cemented Sandstone

Calcite (and dolomite) Cement, stained

Hematite (Fe2O3) Cement

Forms in oxidizing environment

Makes red beds red

only about 1% Fe2O3 required to make red color

Fe2+ dissolved from ferromagnesian minerals during diagenesis gets oxidized to Fe3+ and

precipitated as hematite cement

Hematite Cement

Clay Cement

Some clay in sandstones is detrital

Some clay is authigenic

Clay cement coats sand grains

Clay plates grow perpendicular to surface and form honeycomb texture

Clay coatings can prevent quartz cement from growing and preserve porosity

Clay Coatings

Authigenic clay is perpendicular to grain boundaries

Diagenesis is complex

Compaction depends on mud content, sorting, % ductile fragments, angularity of

grains, depth of burial (pressure)


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Cementation depends on chemistry and amount of pore fluid

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Diagenesis of Sandstones n