Lec 6 - Sandstone Diagenesis

what is diagenesis

physical and chemical changes which take place after the deposition of a sediment

Diagenetic processes include

• compaction and pressure dissolution

• dissolution of existing grains

• precipitation of new materials in pore spaces

• solid state replacement of existing minerals by new minerals

what happens during diagenesis

• unstable minerals may be destroyed

• new minerals may grow

• existing crystals may become larger

cementation

dissolution and recrystallization. Solid phases are in disequilibrium; chemical reactions occur to reestablish equilibrium. Requires the presence of water.

precipitation

• re-precipitation forms cement

• pores become infilled by chemical precipitation of stable minerals

• cementation reduces porosity and permeability

What pressure and temperature conditions are ideal for diagenesis

<300 C, <2 kilobars

what are the diagenetic factors

• temp <300 C

• pressure <2 kilobars

• ability of water to pass through sediment

• salinity: generally increases with depth

• pH (acid, neutral, or alkaline)

• Eh (redox potential)

• decaying organic matter tends to create local reducing conditions

Shallow Burial refers to

Eodiagenesis

Eodiagenesis

• compaction and fluid expulsion

• formation of Fe-oxides or pyrite

• Precipitation of mineral overgrowths (quartz, calcite, k-spar)

• dissolution of soluble phases

When does Eodiagenesis occur?

lasting 1000 to 1M years at depths of 1-100 meters.

what happens with the pore water during shallow burial

pore waters are soon modified by breakdown of organic matter and bacterial activity

Deep Burial

Mesodiagenesis

What occurs during Mesodiagenesis

• pore waters become saline

• compact: grain contacts etch

• cementation: calcite precipitates at high Temp, quartz precipitates at low temperatures

• dissolution: continues removal of unstable phases

• replacement: one mineral replaces another

• dewatering: smectite to illite transition

what happens to the pore waters during mesodiagenesis

pore waters are further modified by reactions w/clay minerals, dissolution of unstable grains, precipitation of authigenic minerals and mixing w/water from other sources.

what are the conditions in which mesodiagenesis takes place?

operates for tens of millions of years, affects sediments at depths of around 10,000m where temp are in region of 100-200 C

Uplift

Telodiagenesis

Telodiagenesis refers to

exposure to meteoric rain water following uplift

what occurs during telodiagenesis (uplift)

• pore water is flushed out with fresh water

• Low T and P minerals are favored

• cements and deep burial minerals may dissolve

what are the effects of compaction and pressure dissolution

causes dewatering and closer packing. Compaction reduces porosity. Further compaction leads to local fracturing and bending of weak grains.

what are the types of grain contacts

point contacts, long contacts, concavo-convex, sutured contacts

how are contacts changed?

contacts are changed with compaction. Dissolution of minerals at contact points due to increase local pressure.

Silica Cementation

one of the most common types of cement is quartz overgrowth. In many cases, shape of the original grain is delineated by a thin Fe oxide-clay coating between the overgrowth and the grain. Dust line

Fe-oxide clay coating between overgrowth and the grain (quartz)

dust-line

sources of silica cement

Due to pressure dissolution. Pore solutions become enriched in silica which is then reprecipitated as overgrowths when supersaturation is achieved.

What are the different carbonate cements

calcite, dolomite, siderite

Two types of calcite cement

poikilitic crystals and drusy calcite spar

Poikilitic crystals

large single crystals up to several cms across which envelop many sand grains

Drusy calcite spar

forms mosaic of equant crystals which fill the pores between grains, and typically show an increase in crystal size towards the center of the original cavity

Characteristics of Calcite cement

• commonly calcite is th efirst cement

• early precipitation of calcite inhibits quartz overgrowth formation and feldspar alteration to clay

• decreases porosity and permeability

Dolomite cements

vary from pore filling microcrystalline rhombs to coarse anhedral mosaics and large poikilitic crystals . Most common are well formed mm-sized rhombohedral. Commonly Fe rich indicating ppt in reducing condition s

Siderite cements

occur in some sandstones as microcrystalline and fibroradiate mosaics

Feldspar overgrowths

sometimes do grow on detrital feldspar - most common on K-feldspar but also occur on detrital albite

Authigenic clays

illite and kaolinite are common

montmorillonite, mixed illite-montmorillonite, mixed montmorillonite-chlorite

earliest, often predating quartz or calcite cementation

authigenic clays occur as what?

pore-filling cements and clay rims that reach up to 50 microns

where do authigenic clays precipitate from

precipitated from solutions that leached unstable minerals in the shallow burial environment.

Hematite Cementation and Pigmentation

many clastics are red due to hematite. In many cases, these rocks were deposited in terrestrial environments

How does hematite typically occur?

they typically occur as a very thin coating around grains.

Absence of hematite at grain contacts indicate what?

Diagenetic origin

Origin of hematite pigment? Detrital

Detrital origin through lateritic weathering of Fe hydroxides stains of detrital sand grains

origin of hematite pigment — diagenetic

Diagenetic Fe is supplied by intrastratal dissolution of detrital silicates such as hornblende, augite, olivine, chlorite, biotite, and magnetite. If the diagenetic environment is oxidizing, then the iron is reprecipitated as hematite.