conglom&brecc

8/12/2019 conglom&Brecc

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Chapter 5:

Sandstone and Conglomerates


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Terrigenous

From the Earth

Terrigenous clastic sedimentary rocks are

composed of clasts of pre-existing rocks

and minerals.

Also referred as:

detrital (detached from pre-existing rocks by

erosion or weathering), or;

Epiclastic (derived from the surface).


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Silisiclastic

Because most terrigenous clastics areespecially rich in quartz and other silicatesminerals, the term si l is ic last icis also

used.Clast formed by physical weathering are

eroded and transported by mass wasting,wind, water, and ice and are deposited asdiscrete, unconsolidated fragments thatare eventually lithified.


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Terrigeneous sediments and sedimentary

rocks are defined on the basis of clast

diameter.

Three distinct groups are recognized:

Conglomerates and breccias

Sandstones

mudrocks


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Conglomerates and Breccias

Conglomeratesis lithified gravel made up

of rounded to subrounded clasts whose

diameters exceed 2 mm.

They are also called roundstone or

puddingstone.

Breccias is lithified rubble made up of

angular clasts coarser than 2 mm. They are also called sharpstone.


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The roundness (angularity) of the grains is

measured using standard grain silhouettes.

Very coarse clastic rocks are collectively

referred to as rudites or rudaceous

sedimentary rocks.


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CompositionMost clasts on conglomerates and

breccias are fragments of rocks andminerals produced by the disintegration of

bedrock.

These occur both as coarser-grainedfragments and finer-grained matrix filling

the space between fragments.

Clasts are typically glued together by asmall amount of siliceous, calcareous, or

ferruginous cement.


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Three principal categories of clasts are

distinguished:

Mineral fragments that occur as major

components,

Mineral fragments that occur as accessory

constituents, and

Fragments of rock.


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Mineral fragments occurring as

major constituents (5% or more)

Clasts of a single mineral such as quartz orfeldspar tend to be less abundant inconglomerates and breccias than in sandstonebecause few igneous, metamorphic, orsedimentary rocks have original grains coarseenough to disintegrate into pebbles and coarserdetritus.

Source rocks with mineral grain diameterscoarser than 8 mm (fine pebble) include quartzveins, pegmatites, deep-seated plutons, high-grade metamorphic rocks, breccias andconglomerates.


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Quartz is the most abundant major mineralin conglomerates and breccias.

It is harder than other rock-forming mineral

Has no cleavage

Practically insoluble

Large clasts of K-feldspar, plagioclase

feldspar, and mica can also be abundantbut seldom last as long as quartz becausethey corrode, disaggregate, and abrade

with transport. The sand matrix is similar in composition

to sandstone interbedded with theconglomerate or breccia.


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Mineral class occurring as accessory

constituents (less than 5%)

Other fragments occur as accessory, theirpresence is incidental.

They occur as accessory minerals either

because their original abundance insource rocks is low or because they areeasily destroyed by weathering.

Micas and heavy minerals such as: olivine,pyroxene, amphibole, zircon, magnetiteand hematite.


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Rock fragments

Rock fragments are typically the most

abundant component in very coarse-

grained terrigenous rocks and are

invariably the most interesting.

Careful analysis of their composition

provides us with direct information on

provenance.


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Texture

Conglomerate and breccia textures are studiedat the outcrop using methods of quantitativegrain size analysis that differ from those used forsandstone.

Grain diameters of particles coarser than sand(>2mm).

The interstitial space between framework grains

can be empty; filled with finer-grained detritalmatrix; or occupied by cement, fluid (oil, water)or natural gas.


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Two distinctive varieties of

conglomerates (and breccias) are

defined on the basis of texture: Orthoconglomerates (literally, true

conglomerates) consist mainly of gravel-sized

framework grains. Matrix (sand or finer) is less

than 15%. So, grain-supported framework. Paraconglomerate have a matrix of sand and

finer clasts. Matrix is at least 15%; most have

50% matrix and are actually sandstone and

mudrocks in which pebbles, cobbles and

boulders are scattered.


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Extraformational or intraformational

They are separated by comparing the

composition of framework and matrix grains. Intraformational- conglomerates and breccias

have an interior (intrabasinal) source: that is;they are eroded from the same sedimentary rock

unit they are a part, rather than being derivedfrom rocks located outside the depositionalbasin. Consequently in t raformat ionalconglomerates and breccias have framework

grains identical in composition to those in thematrix.


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Extraformational or intraformational

Extraformational- conglomerates and

breccias are derived from sources areas

outside the depositional basin. Detritus

weathered from external sources is carriedaway and deposited elsewhere. As a result

framework clasts differ markedly in

composition from matrix.Framework matrixis exotic; that is, not derived by the erosion

and redeposition of matrix material.


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Orthoconglomerate and

Paraconglomerates

(Orthobreccias and Parabreccias)

They are separated by examining

the proportion of matrix.

Orthoconglomerate are matrix

poor, Paraconglomerates are

matrix rich.


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Orthoconglomerates

They are matrix-poor(80% or more frameworkgrains) and have anintact, stable, grain-

supported fabric. Theyare transported anddeposited on a grain-bygrain basis by fluids,specifically water or air.

Oligomict or petromict-cngl are further dividedinto these on the basis offramework graincomposition.


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Oligomict or petromict

In oligomict(orthoquartzose) conglomerates (orbreccias), more than 90% of the frameworkclasts consist of fragments of only a fewvarieties of resistant rocks and minerals as

metaquartzite, vein quartz, and cherts. Inpetromict(polymict) clasts of many different

composition of metastable and unstable rocksare abundant; for example, basalt, slate, and

limestone.


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A more precise classification can be given

by specifying predominant clast size and

lithology:

quartz pebble oiligomictic orthoconglomerate

slate coble petromictic parabreccia


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Oligomict orthoconglomerates imply

wholesale decomposition and

disintegration of immense volumes of

rocks, reflecting climate and topography

that promote chemical decomposition andphysical disintegration of all but the most

resistant components.

Typically stream channels deposits andbars deposits, or near shore marine

settings.


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Petromic are much more abundant than

oligomict orthoconglomerates and are

mainly alluvium eroded from high-relief

areas.


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Paraconglomerates


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Paraconglomerates Paraconglomerates and parabreccias are

further divided on the basis of theirinferred origin as well as the size andinternal organization of their matrix.

Is the matrix sand or mud? Is the matrix internally laminated or

chaotic?

Is the framework imbricated, sorted, andvertically graded?

Is the deposit sheetlike or lenticular?

With what other types of sediment is the

deposit associated?


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Paraconglomerates

Paraconglomerates containinga matrix of

delicately laminated mudrocks in which

coarser framework grains float are called

laminated pebbly (or cobbly, or bouldery)mudrock.

Dropstone- ice rafting


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Paraconglomerates

Paraconglomerates in which the matric is

disorganized and non-laminated are either

tillite (only if glacial origin can be inferred)

or tilloid (deposited by mass movement).


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