Sedimentary Rocks Lecture 8

Front 1

What is diagenesis

Back 1

Any physical, chemical or biological alteration of sediment after initial deposition and during and after its lithification.

Front 2

What is lithification

Back 2

Conversion of sediment into rock.

Front 3

What is the upper boundary on temperature associated with diagenesis?

Back 3

< 300 C

Front 4

What is the geothermal gradient?

Back 4

temperature change per unit length of depth into the earth.

Front 5

What is the geostatic or lithostatic gradient?

Back 5

the rate of increasing P with respect to increasing depth in Earth’s interior.

Front 6

What is the hydrostatic gradient?

Back 6

the rate of increasing P with respect to increasing depth in water.

Front 7

What is the minimum burial depth and time associated with the onset of diagenetic processes?

Back 7

1-10^5 years and <1-100m

Front 8

What is mechanical compaction?

Back 8

compaction—the loss of porosity by rearrangement of the grain framework due to overburden pressure.

Front 9

What is the connection between compaction and thinning of deposits?

Back 9

the more compact a deposit becomes the thinner it gets.

Front 10

What is the connection between compaction and expulsion of pore fluid?

Back 10

compaction require sediment deposit to expel water that is filling pore space. this causes thinning of the deposit.

Front 11

What is the connection between under-compacted deposits and excess hydrostatic pore pressures?

Back 11

under-compacted sedimentary deposits ten to have pore fluid pressure greater than the hydrostatic pressure. these excess pressures drive subsurface fluid flow and destabilize deposits.

Front 12

12. How does cementation affect porosity?

Back 12

cementation decreases porosity.

Front 13

What is pressure solution? How does pressure solution at grain contacts affect porosity?

Back 13

pressure concentrated at points of contact between grains. causes solution and migration of ions away from points of contact. to lower pressure areas were the dissolved phase can be reprecipitated.

Front 14

What causes grain deformation and fracturing?

Back 14

high pressures.

Front 15

What are concavo-convex grain contacts?

Back 15

grains that start to dissolve in the pore waters at their contacts.

Front 16

What are sutured grain contacts?

Back 16

irregular grain boundaries.

Front 17

What is a styolite?

Back 17

A Stylolite is a sutured boundary made visible by insolube material trapped and concentrated in the zone of pressure solution. More common in carbonates.

Front 18

What are Cementation, Dissolution, Authigenesis, Recrystallization, and Replacement?

Back 18

cementation: nucleation and precipitation of a mineral the cements the rock.
Dissolution: a mineral dissolves, creating secondary porosity
Authigenesis: a 'new' mineral that grows in a pore space is authigenic
Recrystallization: the crystal favric changes, but the mineralogy stays the same. Usually calcite.
Replacement: one mineral dissolves, another forms in its place. Usually taking the old shape.

Front 19

What are the two most common pore-filling cements?

Back 19

Carbonate minerals, (calcite & aragonite) 2) silica.

Front 20

Under what pH conditions does carbonate dissolve and precipitate?

Back 20

carbonates precipitate from alkaline solutions greater than 7

Front 21

Under what pH conditions does silica dissolve and precipitate?

Back 21

Silica cements precipitate from acidic solutions with a pH less than 7.

Front 22

What are common sources of ions for calcite cement?

Back 22

calcium carbonate

Front 23

What are common sources of ions for silica cement?

Back 23

dissolution of silica dust and biogenic silica.

Front 24

What is the relationship between temperature and silica solubility?

Back 24

as temperature increases amorphous silica increases

Front 25

What are quartz overgrowths?

Back 25

the development of quartz cement around quartz minerals

Front 26

What are the most common authigenic clay minerals?

Back 26

Clay minerals that precipitate in place from pore fluids. Most common, illite and kaolinite

Front 27

What are clay rims?

Back 27

clay cements typically formed around mineral grains. typicaaly and early or first diagenetic event.

Front 28

What does occlude mean?

Back 28

to completely fill pore space

Front 29

What do smectite and kaolinite alter to with burial depth?

Back 29

with increasing depth of burial and temperature, the dehydration of smectites and kaolinite causes alteration to illite and chlorite.

Front 30

What is the most common clay mineral found in deeply buried and very old sedimentary rocks?

Back 30

illite

Front 31

What is the significance of an “embayed” sand grain?

Back 31

It is partially dissolved away

Front 32

What is secondary porosity?

Back 32

inhibition of cementation with the dissolution of minerals. This creates a new porosity in the sandstone deep underground.

Front 33

Dissolution of grains of what mineral produces the greatest amount of secondary porosity?

Back 33

feldspar

Front 34

What is vitrinite reflectance?

Back 34

resistant plant cells altered under T&P, and reflect more light the higher the rank. Coal.

Front 35

What are zeolites?

Back 35

hydrous aluminosilicates alteration

Front 36

What are fluid inclusions and how are they used to assess diagenetic conditions?

Back 36

fluid inclusion preserves a sample of the pore fluid at the time of cement or overgrowth formation. analyzed to find out diagenetic processes and rates.

Front 37

What is differential compaction?

Back 37

spacially varying compaction over a layer of inconsistent thickness.