Lecture 12 - Diagenesis

Question 1

What is diagenesis

Answer 1

The chemial and phsyical processses which act upon sediment grains in the subsirface” they give way to metamorphism but there is no clear line

Question 2

Diagenitc processe: compaction

Answer 2

massof overlying sediment, water squeezed out and sediment compacted together

Question 3

Process recrystalisation

Answer 3

Unstable minerals change to their crystal forms

Question 4

Dissolution

Answer 4

Unstable minerals may dissolve

Question 5

Replacement

Answer 5

Usntale minerals may be replaces

Question 6

Authigenesis

Answer 6

Precipitation of new minerals within pores

Question 7

Cementaion

Answer 7

Sediment cemeted by authgetic precipitaion

Question 8

bacterial action

Answer 8

degradation of organic matter

Question 9

where can we look at diagensis

Answer 9

mudrocks
sandstones
carbonates

Question 10

Diagenesis in mudrocks, main process?

Answer 10

The main diagentic process is Compaction

Question 11

effect on thickness of mudrock

Answer 11

it can be compacted up to ten times

Question 12

effects of explusion of water on mudrocks

Answer 12

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

Question 13

What are the changes in the clay minerolfy

Answer 13

The main change is

smectities + potassium -> illites + water

Question 14

When does this smectities change happens

Answer 14

2-3km deep and 70-90degrees C

Question 15

Sandstone diagensis what can efect it

Answer 15

depositional enviroment
sediment comp
sediment texture
pore water chem
burial and uplift histrory

Question 16

Properties of sand compared to mud (water props)

Answer 16

the sediment grains in sand such as quartz are not as ductile and therefore not ductile under temps and pressures

Question 17

what are the four factors that compation of sandstone depends on

Answer 17

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)

Question 18

What forms concavo convex contacts

Answer 18

If one grain dissolves preferintally

Question 19

What forms sutured contacts

Answer 19

if grains are of a similar soulbitlty

Question 20

What are the four main cements in sandstones

Answer 20

1) quatz
2) calcite
3) haematite
4) clay mineral cement

Question 21

Quatz cement proprs

Answer 21

Pressure-solution can provide silica in the system for cermentss

Question 22

calcite cement properties

Answer 22

Calcite cements can redissolve if acidic water passes through them.

Question 23

Haematite cement

Answer 23

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)

Question 24

Clay cement props

Answer 24

Most modern sands have little caly matrix. etiher clay matrix is primary or the clay matrix is diagentic

Question 25

Diagensis in carbontaes info

Answer 25

Begins as soon as the carbonate is precipatated or deposited. alage sponges worms etc bore into the carbonate

Question 26

Cemenation in limestone

Answer 26

cemenation is the main diagentic process that turns loose sediment into hard limestone

Question 27

what are hard grounds

Answer 27

Typically when sedimenation has ceased for 1000s of years

Crusts on the sediment surface

Question 28

explain stylo bites

Answer 28

These are sutured surface which cut grains, cements and matrix as a result of chemicak compation