Diagenesis of Carbonate Rocks

Question 1

How do early cements appear in carbonate rocks and what are they composed of?

Answer 1

Fibrous and line pore margins as a fringe of equal thickness. Made of HMC, LMC or aragonite

Question 2

Why do thousands of pore volumes worth of seawater need to be pumped through rocks for complete cementation?

Answer 2

Low concentrations of Ca and HCO3 in the sea

Question 3

In what environments are early cements thickest?

Answer 3

Shallow wave and current swept environments

Question 4

Where do carbonate factories occur and what can happen if sea level drops?

Answer 4

In shallow water

Subjected to fresh water, carbonate factories can be exposed

Question 5

What happens when a carbonate is exposed to fresh water?

Answer 5

Extensive dissolution, especially of aragonite, more soluble in fresh water.
Calcium and bicarbonate then crystallise as LMC cements.
Can turn carbonate cement into carbonate rock.

Question 6

What effect does mechanical compaction have on limestones?

Answer 6

Produces brittle fracturing e.g. spalling of thin fringes of early cement, dealamination of ooids.

Question 7

How and where do styolites develop?

Answer 7

> 100m burial depth pressure, forms by chemical compaction produced by pressure

Question 8

What happens when aragonite becomes unstable during burial?

Answer 8

Dissolved leaving pore, which is later cemented by calcite. Usually after early marine cementation.
Aragonite replaced by calcite on a very fine scale

Question 9

What happens to HMC during burial?

Answer 9

Replaced by dolomite or LMC

Question 10

What is the chemical formula for dolomite?

Answer 10

CaMg(CO3)2

Question 11

Why are dolostones common over geological record and absent in recent sediments?

Answer 11

All dolomitization takes place during burial

Question 12

Where can dolomite be found on the surface?

Answer 12

Dogs gall stones

Highly saline tidal flats (sabkhas)

Question 13

What may inhibit dolomitization?

Answer 13

Highly ordered structure of dolomite crystals

Chemistry of seawater e.g. presence of sulphate anion

Question 14

What does dolomite modify during burial to overcome the inhibition of dolomitization?

Answer 14

Modifications in temperature, pressure and fluid composition

Question 15

How much dolomite is produced when a carbonate sediment is recrystallised with HMC?

Answer 15

30% Dolomite

Question 16

What is required to produce thick dolostones?

Answer 16

A mechanism for flushing Mg rich fluid through the limestone

A source of Mg from water with high Mg conc and Mg/Ca ratio - seawater is most likely source.

Question 17

Dolomitization occurs best in evaporated seawater, what does the precipitation of gypsum/anhydrite inhibit?

Answer 17

SO4 2-

Question 18

Where does most recent dolomite form and how does this happen?

Answer 18

Arid intertidal environments e.g. tidal flats in Australia, Persian Gulf
Seawater floods onto carbonates of the tidal flats, percolates through then is drawn up by evaporation

Question 19

How can mudrocks undergoing compaction liberate Mg2+?

Answer 19

Intergranular seawater buried along with the mudrocks

Intragranular (structural) water expelled during the smectite to illite transformation

Question 20

What is a chert?

Answer 20

Replacement of limestone by quartz during diagenesis

Question 21

Name three kinds of chert?

Answer 21

Microquartz (replacive)
Megaquartz (pore-filling)
Chalchareous quartz