Lecture 12 - Diagenesis Flashcards
More notes needed on the last few slides
What is diagenesis
The chemial and phsyical processses which act upon sediment grains in the subsirface” they give way to metamorphism but there is no clear line
Diagenitc processe: compaction
massof overlying sediment, water squeezed out and sediment compacted together
Process recrystalisation
Unstable minerals change to their crystal forms
Dissolution
Unstable minerals may dissolve
Replacement
Usntale minerals may be replaces
Authigenesis
Precipitation of new minerals within pores
Cementaion
Sediment cemeted by authgetic precipitaion
bacterial action
degradation of organic matter
where can we look at diagensis
mudrocks
sandstones
carbonates
Diagenesis in mudrocks, main process?
The main diagentic process is Compaction
effect on thickness of mudrock
it can be compacted up to ten times
effects of explusion of water on mudrocks
At despotion mudrocks are 70% water, at depths of 1km they are 30%, much iof the water here is locked up. at 100km heat is also needed for the desaturation of these mudrocks
What are the changes in the clay minerolfy
The main change is
smectities + potassium -> illites + water
When does this smectities change happens
2-3km deep and 70-90degrees C
Sandstone diagensis what can efect it
depositional enviroment sediment comp sediment texture pore water chem burial and uplift histrory
Properties of sand compared to mud (water props)
the sediment grains in sand such as quartz are not as ductile and therefore not ductile under temps and pressures
what are the four factors that compation of sandstone depends on
1) clay content ( average is 5%)
2) Perecenatge of ductile grains (such as mud most sandstones only contain 15% of non ductile materials)
3) sorting of the sand grains
4) burial depth (pressure)
What forms concavo convex contacts
If one grain dissolves preferintally
What forms sutured contacts
if grains are of a similar soulbitlty
What are the four main cements in sandstones
1) quatz
2) calcite
3) haematite
4) clay mineral cement
Quatz cement proprs
Pressure-solution can provide silica in the system for cermentss
calcite cement properties
Calcite cements can redissolve if acidic water passes through them.
Haematite cement
Most of the iron is supplied as a sissoultion of other rocks. only a tiny amount is needed to make the rocks RED (IRON Fe2O3)
Clay cement props
Most modern sands have little caly matrix. etiher clay matrix is primary or the clay matrix is diagentic
Diagensis in carbontaes info
Begins as soon as the carbonate is precipatated or deposited. alage sponges worms etc bore into the carbonate
Cemenation in limestone
cemenation is the main diagentic process that turns loose sediment into hard limestone
what are hard grounds
Typically when sedimenation has ceased for 1000s of years
Crusts on the sediment surface
explain stylo bites
These are sutured surface which cut grains, cements and matrix as a result of chemicak compation
