diagenesis

diagenetic items include

• 1. cementation
• 2. compaction
• 3. recrystallization,
• 4 oil formation
• 5. new minerals,
• 6. ores

diagenesis is an open system

metamorphism is a closed system

stages

diagenesis terminology

• eo - early or shallow
• meso - middle or deeper
• tele - late

Compaction

• shale
• sands
• CO³ rocks

allochems

General Statements about diagenesis

• equilibrium
• diagenetic minerals/authigenic minerals
• preservation of original grain
• affects most sedimentary rocks
• order of events
• older rocks have more diagenetic silica
• younger rocks have more diagenetic calcite
• vertical water movement

Proof of Diagenesis

• silica replacement of shells
• devitrification of glass shards
• microstylolites
• fossil outline in coarse grained rock
• pore filling cement
• euhedral faces on detrital grains (authigenic growth)
• concretions & nodules
• clear overgrowths on quartz
• authigenic tourmaline & other diagenetic minerals
• gypsum in sand
• dolomite fossils

Physical Changes from diagenesis

bacterial diagenesis

environmental conditions

early diagenesis

2% of diagenetic minerals

• hematite
• feldspar
• dolomite
• siderite
• anhydrite
• gypsum
• barite
• Ti oxide minerals
• zircon
• tourmaline
• garnet
• zeolite
• clays
• micaceous minerals
• sulfides

diagenetic quartz & calcite

mineral stability fields - geothermal gradient

zeolite facies

5 chemical processes occur simultaneously during diagenesis

• 1. precipitation
• 2. cementation
• 3. alteration
• 4. recrystallization
• 5. dissolution

pressure solution

*

2 types of dissolution

• congruent
• incongruent (alteration)

intrastratal dissolution

replacement & replacement textures

decementation

pH & temperature are main controls

Silica Cement

early or late

precipitated or from pressure solution

6 SiO2 ppt. mechanisms:

• 1. cooling a hot solution
• 2. lowering pore pressure
• 3. mixing SiO2 saturated & saline water
• 4. lowering pH of saturated solution
• 5. evaporating an undersaturated solution
• 6. semimembrane osmosis through a clay-rich shale layer

solubility of SiO2 is essentially unaffected by pH <9

sources could be:

• 1. pressure solution
• 2. meteoric water
• 3. radiolarian ooze & clay diagenesis
• 4. volcanic activity in sea water
• 5. clay diagenesis & hydrolysis reactions
• 6. pore water circulation

Problems:

• 1. cement solubility
• 2. cement volume
• 3. solubility of silica increases with pressure and temperature,
• 4. meteoric water

Calcite Cement

very early

related to unconformities

lithic & quartz arenites and arkoses

precipitated under complex conditions

• lower solubility in hot water than in cold water
• solubility also decreases at higher pressures

• 1. CaCO3 is pH susceptible
• 2. CaCO3 solubility decreases with temp & pressure
• 3. salt sieving
• 4. later diagenesis - secondary porosity

sources of CaCO3 for cement

• 1. marine waters
• 2. pressure solution
• 3. meteoric water movement

SS cementation trend young CaCO3 cement old silica cement

*

cement depends on what ground water is saturated with + pH, temp etc.

if CaCO3 then calcite cement ppt - easy to redissolve

if SiO2 then silica cement ppt - hard to recycle

pressure solution

Other Cements

most common sandstone cements are silica, calcite, & clay

• sulfates - gypsum, anhydrite, barite
• iron oxides - hematite, siderite, limonite/goethite
• authigenic feldspar
• dolomite
• zeolite
• salt
• pyrite
• zircon
• tourmaline
• rutile

time

Gel3510 Sedimentation page