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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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Conglomerates and Breccias
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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Conglomerates and Breccias
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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