Transcript Chapter 5: Sandstone and Conglomerates

Chapter 5:
Sandstone and Conglomerates
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).
Silisiclastic
 Because
most terrigenous clastics are
especially rich in quartz and other silicates
minerals, the term silisiclastic is also
used.
 Clast formed by physical weathering are
eroded and transported by mass wasting,
wind, water, and ice and are deposited as
discrete, unconsolidated fragments that
are eventually lithified.
 Terrigeneous
sediments and sedimentary
rocks are defined on the basis of clast
diameter.
 Three distinct groups are recognized:



Conglomerates and breccias
Sandstones
mudrocks
Conglomerates and Breccias
 Conglomerates
is 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.
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.
Composition
 Most
clasts on conglomerates and
breccias are fragments of rocks and
minerals produced by the disintegration of
bedrock.
 These occur both as coarser-grained
fragments and finer-grained matrix filling
the space between fragments.
 Clasts are typically glued together by a
small amount of siliceous, calcareous, or
ferruginous cement.
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.
Mineral fragments occurring as
major constituents (5% or more)

Clasts of a single mineral such as quartz or
feldspar tend to be less abundant in
conglomerates and breccias than in sandstone
because few igneous, metamorphic, or
sedimentary rocks have original grains coarse
enough to disintegrate into pebbles and coarser
detritus.
 Source rocks with mineral grain diameters
coarser than 8 mm (fine pebble) include quartz
veins, pegmatites, deep-seated plutons, highgrade metamorphic rocks, breccias and
conglomerates.
 Quartz
is the most abundant major mineral
in 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 abundant
but seldom last as long as quartz because
they corrode, disaggregate, and abrade
with transport.
 The sand matrix is similar in composition
to sandstone interbedded with the
conglomerate or breccia.
Mineral class occurring as accessory
constituents (less than 5%)
 Other
fragments occur as accessory, their
presence is incidental.
 They occur as accessory minerals either
because their original abundance in
source rocks is low or because they are
easily destroyed by weathering.
 Micas and heavy minerals such as: olivine,
pyroxene, amphibole, zircon, magnetite
and hematite.
Rock fragments
 Rock
fragments are typically the most
abundant component in very coarsegrained terrigenous rocks and are
invariably the most interesting.
 Careful analysis of their composition
provides us with direct information on
provenance.
Texture

Conglomerate and breccia textures are studied
at the outcrop using methods of quantitative
grain size analysis that differ from those used for
sandstone.
 Grain diameters of particles coarser than sand
(>2mm).
 The interstitial space between framework grains
can be empty; filled with finer-grained detrital
matrix; or occupied by cement, fluid (oil, water)
or natural gas.
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.

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 derived
from rocks located outside the depositional
basin. Consequently intraformational
conglomerates and breccias have framework
grains identical in composition to those in the
matrix.
Extraformational or intraformational
 Extraformational-
conglomerates and
breccias are derived from sources areas
outside the depositional basin. Detritus
weathered from external sources is carried
away and deposited elsewhere. As a result
framework clasts differ markedly in
composition from matrix.Framework matrix
is exotic; that is, not derived by the erosion
and redeposition of matrix material.
Orthoconglomerate and
Paraconglomerates
(Orthobreccias and Parabreccias)
They are separated by examining
the proportion of matrix.
Orthoconglomerate are matrix
poor, Paraconglomerates are
matrix rich.
Orthoconglomerates


They are matrix-poor
(80% or more framework
grains) and have an
intact, stable, grainsupported fabric. They
are transported and
deposited on a grain-by
grain basis by fluids,
specifically water or air.
Oligomict or petromictcngl are further divided
into these on the basis of
framework grain
composition.
Oligomict or petromict

In oligomict (orthoquartzose) conglomerates (or
breccias), more than 90% of the framework
clasts consist of fragments of only a few
varieties of resistant rocks and minerals as
metaquartzite, vein quartz, and cherts.
 In petromict (polymict) clasts of many different
composition of metastable and unstable rocks
are abundant; for example, basalt, slate, and
limestone.
 A more
precise classification can be given
by specifying predominant clast size and
lithology:
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quartz pebble oiligomictic orthoconglomerate
slate coble petromictic parabreccia
 Oligomict
orthoconglomerates imply
wholesale decomposition and
disintegration of immense volumes of
rocks, reflecting climate and topography
that promote chemical decomposition and
physical disintegration of all but the most
resistant components.
 Typically stream channels deposits and
bars deposits, or near shore marine
settings.
 Petromic
are much more abundant than
oligomict orthoconglomerates and are
mainly alluvium eroded from high-relief
areas.
Paraconglomerates
Paraconglomerates
 Paraconglomerates
and parabreccias are
further divided on the basis of their
inferred origin as well as the size and
internal organization of their matrix.
 Is the matrix sand or mud?
 Is the matrix internally laminated or
chaotic?
 Is the framework imbricated, sorted, and
vertically graded?
 Is the deposit sheetlike or lenticular?
 With what other types of sediment is the
deposit associated?
Paraconglomerates
 Paraconglomerates
containing a matrix of
delicately laminated mudrocks in which
coarser framework grains float are called
laminated pebbly (or cobbly, or bouldery)
mudrock.
 Dropstone- ice rafting
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).