Carbonates 4

Question 1

how do dolomites and limestones react differently to acid?

Answer 1

dolomites react slowly

limestones react quickly and have a strong fizz

Question 2

Name the 3 different terminologies for fabrics

Answer 2

nonplanar: crystals all fit together
Planar E: kite shaped dolomite crystals with porosity between it
Planar S: Subhedral, sometimes see crystal boundaries.

Question 3

When is dolomite most common in terms of stratigraphical age?

Answer 3

It is most common in older rocks, it is rare in holocene and younger rocks.

Question 4

what are the requirements for dolomitisation?

Answer 4

• Sufficient Mg and Co coming into a limestone; this can be from marine or burial fluids.
• Must be active setting with long lasting hydrological drive to deliver Mg, Co and export Ca
• Sufficient CaCo3 to be replaced

Question 5

Give reasons why a limestone may not be dolomitised?

Answer 5

• Insufficient flow, not enough Mg coming in
• Fluid temperature may be too low
• Conditions suitable for dolomitisation dont last long enough

Question 6

Discuss dolomitisation in hypersaline environments

Answer 6

Dolomite is formed from seawater which is undergoing evaporation. During evaporation it increases the Mg/Ca ratio which encourages dolomitisation.

Question 7

what does the reflux model suggest?
What depositional setting are these located in?
what are the types of reflux? How much can both of these dolomitise?

Answer 7

The model suggests that dolomitisation occurs in layers controlled by permeability.
Lagoon and marine settings

Active and Latent Reflux.

Active reflux can dolomitise entire platforms
Latent reflux can only have partial dolomitisation as seawater is drawn into the top of the platform

Question 8

Discuss Sabkha dolomitisation, how is this related to the reflux model?

Answer 8

Dolomitisation occurs in inter/supratidal areas with episodic flooding and it is synsedimentary. The increasing Mg/Ca ratio comes from storm flooding bringing in Mg and seawater evaporation pulls out calcium.
Hydrologically and chemically related to the reflux model.

Question 9

Give 2 examples of burial diagenesis occurring in a burial environment.

Answer 9

Stylolites, saddle dolomites, petroleum inclusions

Question 10

Burial models require flow; name 3 potential types of these flows.

Answer 10

Compaction, thermal convection, hydrothermal or hydrofrigid, topographic or tectonic driven flow.

Question 11

Thermal convection models are driven by what?

Answer 11

Variations in temperature and thermal conductivity of rocks, along with elevated heat. This means thermal cells can be open, closed or mixed.

Question 12

Discuss open convection cells and give an example of one.

Answer 12

Platforms that are open to seawater recharge, the convection can operate to 2-3km depth.
Example: Bahamas

Question 13

What is the most dominant fluid for dolomitisation?

Answer 13

seawater

Question 14

Discuss seawater dolomitisation

Answer 14

Seawater is the main dolomitisation fluid which occurs in reflux models and is to do with thermal convection during shallow-intermediate burial.

Question 15

Discuss Hydrothermal Dolomites (HTD)

Answer 15

They have this name due to saddle morphology and their association with base metal mineralogy. They also need temperature data from fluid inclusions or isotopes.

Hydrothermal dolomites have higher temperature water than regional temperatures.

Question 16

Discuss HTPD

Answer 16

High temperature pressure dolomites.
These are associated with petroleum and exotic minerals which cause unusual sedimentation to occur. They form at temperatures >80 degrees and there are no controls on them.

Question 17

Why is there a lack of dolomite in recent environments?

Answer 17

Because it is complex and is likely due to the long nucleation periods that are required.