Carbonates 3 Flashcards
Define diagenesis
Diagenesis is the process which changes sediment into a rock
Why are some bivalves calcitic and some aragonitic?
The invertebrates pick up the most dominant carbonate mineralogy at the time they evolved.
name some inorganic precipitates and what controls them
Ooids and marine cements, controlled by ocean chemistry
Aragonite and high mg calcite are unstable through geological time, discuss what happens because of this.
They will dissolve and re precipitate and create pore space which is then infilled by low mg calcite. This low mg calcite drives diagenesis in carbonates
what mineral drives diagenesis in carbonates
low mg calcite
Discuss cementation and when it occurs.
Cementation occurs when fluids are supersaturated in CaCo3, this causes cements to precipitate
Discuss dissolution. what environment are these important in? What may you see in thin section?
Sediments or lithified rocks may undergo dissolution on large or small scales and happens when the pore fluids are undersaturated. This is important in the near surface meteoric environment.
May see micrite envelope with nothing inside it showing that the grains dissolved.
Discuss mechanical and chemical compaction. What is a typical feature you see from chemical compaction?
Mechanical compaction: loser packing of the grains and grain breakage during burial.
Chemical compaction: Occurs in previously lithified limestones, with increased depth and pressure it encourages chemical compaction to occur. Typically see stylolite and dissolution seams.
Do ramps and platforms have the same diagenetic processes acting on them?
Yes they do
In the marine environment what diagenetic processes occur?
Cementation (isopachous rims or bladed crystals) and microbial micritisation (micrite envelopes)
In the meteoric environment what diagenetic processes occur? what will you see in thin sections as proof of these processes?
Dissolution and reprecipitation of CaCo3. The dissolution of high mg calcite and aragonite creates secondary porosity.
Will see isopachous rims and pore filling cements as a result of reprecipitation and meniscus cement created by dissolution
Why do limestones and dolomites lose porosity at different rates?
Limestones lose porosity quickly as they are soft and more reactive so compact easily. Dolomites lose porosity slower because they are rigid due to interlocking crystal structures so retain porosity to greater depths.
In the shallow - deep water environment what diagenetic processes occur?
Burial, cementation, compaction (chemical and mechanical), late stage dissolution
define Karst, what kind of features does it create?
Features produced by the flow of meteoric water through carbonates during subaerial exposure. Can create fractures, vugs and cements, can also get open channel flows within cavities.
what are some of the controls on karst?
Relative sea level, climate, mineralogy, duration of exposure, burial history
