Lecture 13: Dolomite

Postdepositional dolomite

(Different place, different time) forms in an environment that is different than the original environment of the limestone, and at a later time Most of these dolomites form after burial Almost all regional-scale dolostones are postdepositional In these cases, Mg2+ is usually supplied from evaporated, old seawater that may have come from within the rock itself (connate water), or have been transported there by fluids flowing through some sort of conduit

Penecontemporaneous dolomite

(Same place, same time) Forms in the same sedimentary environment as the original carbonate, and at approximately the same time Mostly form in settings that have: High evaporation rates of sea water Abundant microbes

Sources of Mg2+

- Seawater is the most common source of abundant Mg2+ - Flooding - Evaporation/reflux - Compaction - Subsurface fluid flow

What is needed for dolomitization?

- Settings where limestones form - Mg2+, or by Mg2+ - CO32- (calcite) - Temperatures above 50°C - Relatively more Mg2+ than Ca2+ needed (but not too much calcium) - Sufficient time (less important) - A long-lasting and efficient pathway for Mg2+ containing fluids to infiltrate the limestone - Within the rock itself - Along faults - Within conduits like burrows - settings where there is a supply of CO3 ions (alkaline fluids) or CO2 gas (hydrothermal)

T or F: Dolomite is stable

False, it is metastable

Are hydrothermal dolomites local?

Yes they are usually local as they usually follow paths like faults

Reflux

abnormal backward flow where magnesium-rich brines tend to be flushed downward owing to their high density

Saddle dolomite

an indicator of high temperature diagenesis

Dolostones are...

limestones that have been ~75% replaced with dolomite