Classification of Sedimentary Rocks Questions and Answers

Q: The peculiar structure of a particular type of nodular anhydride that consists of slightly elongated, irregular polygonal masses of anhydrite separated by thin dark stringers of carbonate or clay minerals is referred to as :

The structure is called chickenwire structure.Nodular anhydrite in which slightly elongated, irregular polygonal nodules are pressed together and separated by thin dark films of carbonate or clay looks like the mesh of chicken-wire fencing. It forms by displacive growth of anhydrite within sediment, typically in supratidal sabkha settings, and is a classic indicator of evaporitic conditions.'Mesh wire structure' is not the established term, framboidal refers to raspberry-like clusters of pyrite microcrystals, and so the option combining the first two is also wrong.

Q: ................................ is one of the allochemical constituent of carbonate rocks that is formed within the depositional basin by fragmentation of penecontemporaneous, weakly cemented carbonate sediment.

The constituent is an intraclast.In Folk's classification of limestones, intraclasts are fragments of penecontemporaneous, weakly consolidated carbonate sediment that were broken up and redeposited within the same basin, for example by storms tearing up a lime-mud crust. They are one of the four allochems, with ooids, fossils (bioclasts) and peloids.Extraclasts (lithoclasts) come from older rocks outside the basin. Peloids are small, structureless grains, often faecal pellets, and bioclasts are skeletal fragments of organisms.

Q: In Dunham's (1962) classification of limestone ....................... represents limestone containing more than 10% grains in carbonate mud.

The limestone described is a wackestone.Dunham's 1962 classification first asks whether the rock is mud-supported or grain-supported. Among mud-supported limestones, a mudstone has less than 10% grains and a wackestone has more than 10% grains still floating in carbonate mud.A packstone is grain-supported, the grains touching one another with mud filling the spaces between them, and a grainstone is grain-supported with no mud. A boundstone is a limestone whose components were bound together during deposition, as in a reef, and is not defined by its grain percentage at all.

Q: Pettijohn proposed the "Fluidity Index" based on :

Pettijohn's fluidity index is based on matrix-rich wacke versus matrix-poor arenite.In Pettijohn's sandstone classification, the proportion of clay matrix reflects the fluidity (viscosity) of the depositing medium: turbid, dense flows leave matrix-rich wackes, while clean, fluid currents winnow matrix away and leave arenites.The other options are his other two indices. The ratio of quartz to feldspar plus rock fragments is the maturity index (option C), and the ratio of rock fragments to feldspar is the provenance index (option B). Accessory minerals against micas and clays is not one of his indices.

Q: A limestone classified as oomicrite by Folk's classification but when combining Folk's and Dunham's classification it can be classified as :

The key's answer is Oomicrite Packstone.Folk's name describes the components: ooids (oo-) in a matrix of lime mud (micrite). Dunham's name describes the texture, especially whether grains or mud support the rock. An oomicrite has plenty of ooids, more than a quarter of the allochems, and a rock with abundant grains in contact, with mud filling the spaces between them, is a grain-supported packstone, so the combined name is oomicrite packstone.A mudstone in Dunham's scheme has less than 10% grains, which contradicts the many ooids an oomicrite contains. A boundstone is bound together by organisms during deposition, as in a reef. Crystalline limestone is Dunham's term for rock whose depositional texture has been destroyed by recrystallisation.

The peculiar structure of a particular type of nodular anhydride that consists of slightly elongated, irregular polygonal masses of anhydrite separated by thin dark stringers of carbonate or clay minerals is referred to as :

A.Mesh wire structure
B.Framboydal frame structure
C.Chickenwire structure
D.Both (A) and (B) are correct

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: C

The structure is called chickenwire structure.

Nodular anhydrite in which slightly elongated, irregular polygonal nodules are pressed together and separated by thin dark films of carbonate or clay looks like the mesh of chicken-wire fencing. It forms by displacive growth of anhydrite within sediment, typically in supratidal sabkha settings, and is a classic indicator of evaporitic conditions.

'Mesh wire structure' is not the established term, framboidal refers to raspberry-like clusters of pyrite microcrystals, and so the option combining the first two is also wrong.

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................................ is one of the allochemical constituent of carbonate rocks that is formed within the depositional basin by fragmentation of penecontemporaneous, weakly cemented carbonate sediment.

A.Extraclast
B.Peloid
C.Bioclast
D.Intraclast

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: D

The constituent is an intraclast.

In Folk's classification of limestones, intraclasts are fragments of penecontemporaneous, weakly consolidated carbonate sediment that were broken up and redeposited within the same basin, for example by storms tearing up a lime-mud crust. They are one of the four allochems, with ooids, fossils (bioclasts) and peloids.

Extraclasts (lithoclasts) come from older rocks outside the basin. Peloids are small, structureless grains, often faecal pellets, and bioclasts are skeletal fragments of organisms.

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In Dunham's (1962) classification of limestone ....................... represents limestone containing more than 10% grains in carbonate mud.

A.Wackestone
B.Packstone
C.Mudstone
D.Boundstone

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: A

The limestone described is a wackestone.

Dunham's 1962 classification first asks whether the rock is mud-supported or grain-supported. Among mud-supported limestones, a mudstone has less than 10% grains and a wackestone has more than 10% grains still floating in carbonate mud.

A packstone is grain-supported, the grains touching one another with mud filling the spaces between them, and a grainstone is grain-supported with no mud. A boundstone is a limestone whose components were bound together during deposition, as in a reef, and is not defined by its grain percentage at all.

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Pettijohn proposed the "Fluidity Index" based on :

A.Matrix-Rich Wacke Versus Matrix-Poor Arenite
B.Ratio of Rock Fragments to Feldspar
C.Perentage of Quartz or Ratio of Quartz to Feldspar + Rock Fragments
D.Accessory Mineral Versus Mica and Clay Minerals

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: A

Pettijohn's fluidity index is based on matrix-rich wacke versus matrix-poor arenite.

In Pettijohn's sandstone classification, the proportion of clay matrix reflects the fluidity (viscosity) of the depositing medium: turbid, dense flows leave matrix-rich wackes, while clean, fluid currents winnow matrix away and leave arenites.

The other options are his other two indices. The ratio of quartz to feldspar plus rock fragments is the maturity index (option C), and the ratio of rock fragments to feldspar is the provenance index (option B). Accessory minerals against micas and clays is not one of his indices.

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A limestone classified as oomicrite by Folk's classification but when combining Folk's and Dunham's classification it can be classified as :

A.Oomicrite Mudstone
B.Oomicrite Boundstone
C.Oomicrite Packstone
D.Oomicritic crystalline limestone

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: C

The key's answer is Oomicrite Packstone.

Folk's name describes the components: ooids (oo-) in a matrix of lime mud (micrite). Dunham's name describes the texture, especially whether grains or mud support the rock. An oomicrite has plenty of ooids, more than a quarter of the allochems, and a rock with abundant grains in contact, with mud filling the spaces between them, is a grain-supported packstone, so the combined name is oomicrite packstone.

A mudstone in Dunham's scheme has less than 10% grains, which contradicts the many ooids an oomicrite contains. A boundstone is bound together by organisms during deposition, as in a reef. Crystalline limestone is Dunham's term for rock whose depositional texture has been destroyed by recrystallisation.

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