Sedimentary Classification & Diagenesis, Ch 5

Front 1

What are the QFR rock names going clockwise, starting with feldspar?

Back 1

Arkose, Subarkosic, Quartz, Sublithic, Lithic

Front 2

Short Defs:
1) arenite
2) wacke
3) argillaceous

Back 2

1) arenite - sand
2) wacke - dirty (clean sand w/ clay & mud)
3) argillaceous - clay

Front 3

What is compositional maturity?

Back 3

Compositional maturity refers to the relative abundance of stable and unstable framework grains in a sandstone.

Front 4

What is textural maturity?

Back 4

Textural maturity is determined by the relative abundance of matrix and the degree of rounding & sorting of framework grains.

Front 5

What class of textural maturity is most permeable, and therefore, makes a good resvoir?

Back 5

Supermature, where there is little or no clay, grains are well sorted and well rounded.

Front 6

What is diagenesis?

Back 6

Diagenesis is a chemical, biochemical or physical change undergone by a sediment after its initial deposition and during/after lithification.

Front 7

Diagenesis takes place at temperatures higher than those of the near-surface weathering environment, but lower than those producing metamorphism. Generally, the agreed upon boundary is:

Back 7

Generally, the agreed upon boundary is < 300 degrees C. The overall constraints are about: depths < 20km, temp < 250 C, pressure < 5 kbars. Diagenesis begins early (1-10^5 y) and at shallow depths (<1-100 m).

Front 8

Chemical (and biochemical) diagenetic processes take place when pore fluids pass through the pore network of the deposit/ rock. Minerals may react with the pore fluids in five ways:

Back 8

cementation
dissolution
authigenesis
recrystallization
replacement

Front 9

What is cementation?

Back 9

Cementation is the nucleation and precipitation of a mineral that cements the rock; cements are precipitated in pores.

Front 10

What is dissolution?

Back 10

Dissolution is when a mineral dissolves, creating secondary porosity.

Front 11

What is recrystallization?

Back 11

Recrystallization is what happens when the crystal fabric changes, but the mineralogy stays the same. The most common mineral to recrystallize is calcite.

Front 12

What is replacement?

Back 12

Replacement is what happens when one mineral dissolves, another forms in its place. In many cases, the new mineral will mimic the original shape and morphology of the mineral that was replaced. Silica is commonly replaced by calcite and vice versa.

Front 13

What is the process that is directly related to replacement?

Back 13

Authigenesis is the “new” mineral that grows in a pore space.

Front 14

What are the 3 main results of diagenesis?

Back 14

1) Lithification, 2) Reduction of primary porosity, and 3) Production of secondary porosity

Front 15

What is the main physical process associated with diagenesis?

Back 15

Mechanical compaction, which is the loss of porosity by rearrangement of the grain framework due to overburden pressure with an increase in depth (except with secondary porosity)

Front 16

What does compaction require?

Back 16

Compaction requires sediment deposits to expel water that is filling pore space. This causes a thinning of the deposit.

Front 17

What 4 factors contribute to the porosity of a rock?

Back 17

1) sorting, 2)composition, 3) depth w/in ground, 4) environment & history

Front 18

What is Pressure Solution?

Back 18

Pressure solution is a process which occurs as pressure is concentrated at the point of contact between two grains in the sediment. This causes solution and subsequent migration of ions or molecules away from the point of contact, towards an area of lower pressure where the dissolved phase can be reprecipitated

Front 19

Stylolites are composed of _____ and are common in _____.

Back 19

aThe stylolite is composed of insoluble material (often organic-rich) that became concentrated along the boundary in the zone of rock dissolution. Stylolites are more common in CARBONATE than in siliciclastic rocks.

Front 20

Clay minerals that form in the diagenetic process of replacement are called what?

Back 20

Authigenic clay minerals are minerals that precipitate in place from pore fluids. Clay mineral cements typically form rims around grains, but can also completely fill (occlude) the porosity. Authigenic clays require Al, which is commonly supplied from alteration of feldspars during burial, or from detrital clays.

Front 21

Authigenic clay minerals that form at shallow depth may alter to other clay minerals as the ________ increases and ________ change composition with burial.

Back 21

temperature AND pore fluids

Front 22

What the most common cements in sedimentary rocks and under what chemical concondition do they precipitate?

Back 22

1) carbonate minerals (e.g., calcite & aragonite) precipitate from alkaline solutions, pH greater than 7;
2) silica cements precipitate from acid solutions, pH less than 7.

Front 23

Calcium carbonate (calcite) solubility decreases with:

Back 23

1) increases in pH, temp, pressure; 2) marine waters; 3) rainwater movement; 4) pressure solution.

Front 24

The source of silica for cementation is:

Back 24

1) Dissolution of silica dust & biogenic silica; 2) dissolution of feldspars, amphiboles, & pyroxenes; 3) transformation of montmorillonite to illite & feldspar to kaolinite; 4) pressure solution.

Front 25

Solubility of quartz and amorphous silica increases with:

Back 25

higher temperature AND with hydrogen-ion concentration (pH) (effect is insignificant at pH values below about 9 for amorphous silica and 10 for quartz)

Front 26

What is embayed plagioclase?

Back 26

The plagioclase sand grains no longer have their original roundness. Instead the grains are 'embayed' (partially dissolved away) and calcite cement has filled in the embayments. This happens under high pH conditions, where silicates (e.g., feldspar) are dissolved and calcite is precipitated. Imagine there are high pH conditions, dissolving the feldspar grains and inhibiting carbonate cementation. The net result is development of porosity in the sandstone while it was buried underground. This 'secondary porosity' can dominate the porosity structure of deeply buried sandstones.

Front 27

What is secondary porosity?

Back 27

Secondary porosity is a subsequent or separate porosity system in a rock, often enhancing overall porosity of a rock. This can be a result of chemical leeching of minerals or the generation of a fracture system. This can replace the primary porosity or coexist with it

Front 28

What are the first two indicators of Diagenetic Histories?

Back 28

1) Vitrinite Reflectance - (100-240˚C) resistant plant cells altered under T&P, and reflect more light the higher the rank. 2) Clay Mineral Transformation - stability of clay minerals (>100˚C smectites transform to mixed-layer clays; >200˚C become illites; >300˚C only mica remains)

Front 29

What are the last two indicators of Diagenetic Histories?

Back 29

3) Zeolite facies - hydrous aluminosilicates alteration (<100˚C heulandite & analcime; 100-150˚C laumontite; >150˚C prehnite & pumpellyite). Fluid inclusion analysis: determining temp of cement formation, fluid salinity, hydrocarbon fluid saturation and maturity at timing of cementation.