Download Conglomerates

Department of Petroleum Geology & Sedimentology,
Faculty of Earth Sciences,
King Abdulaziz University,
Jeddah, Saudi Arabia
EPS 421
CLASTIC SEDIMENTARY ROCKS
Lecture 08: Composition, textures and
Classification of conglomerates and breccias;
part I
Prof. Dr. Mahmoud A. M. Aref
The Classification of Clastic Sedimentary Rocks
A very basic
classification of all
sedimentary rocks
is based on the type
of material that is
deposited and the
modes of
deposition.
EPS 421 Lecture 08
Conglomerates
„
The term conglomerates is used as a general class name for
sedimentary rocks that contain a substantial fraction (at least 30
percent) of gravel-size (> 2 mm) particles.
„
Conglomerates are common in stratigraphic successions of all
ages but probably make up less than 1 percent by weight of the
total sedimentary rock mass.
„
They are closely related to sandstones in terms of origin and
depositional mechanisms, and they contain some of the same
kinds of sedimentary structures (e.g., tabular and trough cross
bedding, graded bedding).
EPS 421 Lecture 08
Gravel-sized rocks
Gravel Sized Detrital Rocks are Subdivided Based on Grain
Roundness into:
„
Conglomerate:
„
Breccia:
„
rounded clasts
„
angular clasts
general term for lithified gravel
that has been eroded and
transported
EPS 421 Lecture 08
"non-transported" fragments
(faults, solution collapse,
volcanoes, meteorite impacts)
Particle Composition of conglomerate-1
„
Conglomerates are composed of gravel-size pieces of:
1.
Framework grains
2.
Matrix
3.
Cement.
F
C
M
1) Framework grains composed of:
C
A) Most of the gravel-size framework grains are rock fragments
(clasts).
- Any kind of igneous, metamorphic, or sedimentary rock may be
present in a conglomerate, depending upon source rocks and
depositional conditions.
B) Individual minerals such as quartz
EPS 421 Lecture 08
Particle Composition of conglomerate-2
„
„
The matrix of conglomerates commonly consists of various
kinds of clay minerals and fine micas and/or silt-or sand-size
quartz, feldspars, rock fragments and heavy minerals.
The matrix may be cemented with quartz, calcite, hematite,
clay, or other cements.
EPS 421 Lecture 08
Rounding of Gravel Particles
Talus slope – Angular
= 5 km of transport
River gravels – Sub-rounded
Beach gravels
Effective rounding by waves
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Rounding of Gravel Particles
Little change in sphericity
Rapid initial
change in
roundness
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Surface Textures of Gravel
Ventifacts –
wind sculpturing
Striated clasts –
glacial grinding
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a, b = clast axes
p = parallel
i = imbricated
Gravel Fabric
Imbricated gravel
Gravel carried in
suspension:
streamlined transport
with a-axis parallel to
flow
Gravel carried as
bedload: rolling
or sliding with aaxis transverse to
flow
EPS 421 Lecture 08
Gravel particles not
able to adjust
individually to flowing
medium, e.g. encased
in ice, or in debris flow
Gravel Fabric:
Imbrication
A-axis transverse to flow
B-axis dips upflow
scour
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Gravel Fabric: Unordered
EPS 421 Lecture 08
Classification of Conglomerates
„
„
Conglomerates can originate by several
processes (See next table).
The present course is interested mostly
in epiclastic conglomerates, which form
by breakdown of older rocks through the
processes of weathering and erosion.
Major types
Epiclastic
conglomerate
and breccia
Volcanic
breccia
Cataclastic
breccia
Solution
breccia
Meteorite
impact breccia
EPS 421 Lecture 08
Fundamental Genetic Types of Conglomerates and Breccias
Major types
Epiclastic
conglomerate
and breccia
Volcanic
breccia
Cataclastic
breccia
Subtypes
Origin of clasts
Extraformational
conglomerate
and breccia
Breakdown of older rocks of any kind through the processes of
weathering and erosion; deposition by fluid flows (water, ice) and
sediment gravity flows
Intraformational
conglomerate
and breccia
Penecontemporaneous fragmentation of weakly consolidated
sedimentary beds; deposition by fluid flows and sediment
gravity flows
Pyroclastic breccia
Explosive volcanic eruptions, either magmatic or phreatic
(steam) eruptions; deposited by air-falls or pyroclastic flows
Autobreccia
Breakup of viscous, partially congealed lava owing to continued
movement of the lava
Hyaloclastic breccia
Shattering of hot, coherent magma into glassy fragments owing
to contact with water, snow, or water-saturated sediment
(quench fragmentation)
Landslide and slump
breccia
Breakup of rock owing to tensile stresses and impact during
sliding and slumping of rock masses.
Tectonic breccia: fault,
fold, crush breccia
Breakage of brittle rock as a result of crustal movements
Collapse breccia
Breakage of brittle rock owing to collapse into an opening created by
solution or other processes
Solution
breccia
Insoluble fragments that remain after solution of more soluble
material; e.g., chert clasts concentrated by solution of lime-stone
Meteorite
impact breccia
Shattering of rock owing to meteorite impact
Classification of epiclastic conglomerate
????
origin
Extraformational
I. Intraformational
texture
II. Orthoconglomerate
III. Paraconglomerate
Intact, grain-supported fabric
Unstable, matrix-supported fabric
composition
A. Oligomict
Only a few kinds
of resistant rocks
and mineral clasts
A. Oligomict
Orthoconglomerate
B. Petromict
A wide variety of
Unstable rock and
mineral clasts
A. Laminated
Matrix
B. Petromict
Orthoconglomerate
Laminated
Conlomeratic
mudrock
EPS 421 Lecture 08
B. Unlaminated
Matrix
1. Tillite
Glacial
deposition
2. Tilloid
(a) Debris flow
(b) Grain flow
CLASSIFICATION OF CONGLOMERATES AND
BRECCIAS
„
On the basis of sediment fabric, there are two types: clast-supported and
matrix-supported conglomerates. The latter have been termed diamictites.
„
Two particular types of breccia are slump breccias, consisting of broken and
brecciated beds derived from downslope slumping, and solution breccias,
resulting from the dissolution of evaporites and the collapse of overlying
strata.
EPS 421 Lecture 08
1. Epiclastic conglomerate and breccia
1. 1. Extraformational conglomerate and breccia
Breakdown of older rocks of any kind through the processes of weathering and
erosion; deposition by fluid flows (water, ice) and sediment gravity flows
1.2. Intraformational conglomerate and breccia
Penecontemporaneous fragmentation of weakly consolidated sedimentary
beds; deposition by fluid flows and sediment gravity flows
EPS 421 Lecture 08
Epiclastic conglomerate
Tectonic breccia
Tectonic breccia
Collapse breccia
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Conglomerates Based on Composition
Oligomictic Conglomerates:
A conglomerate in which the
clasts are made up of only one
rock type.
Polymictic Conglomerates
A conglomerate in which clasts
include several different rock
types.
EPS 421 Lecture 08
Oligomictic Conglomerates
„
They are composed of only the
most stable and durable kinds of
clasts (quartzite, chert, veinquartz).
„
Stable conglomerates composed
mainly of a single clast type are
referred to by Pettijohn (1975) as
oligomictic conglomerates.
EPS 421 Lecture 08
Oligomictic Conglomerates
„
Most oligomictic conglomerates were probably derived from
mixed parent-rock sources that included less stable rock types.
„
Continued recycling of mixed ultra-stable and unstable clasts
through several generations of conglomerates ultimately led to
selective destruction of the less stable clasts and concentration
of stable clasts.
EPS 421 Lecture 08
Polymictic
Conglomerates
„
Conglomerates that contain an assortment of many kinds of clasts are
polymictic conglomerates.
„
Polymict conglomerates that are made up of a mixture of largely unstable or
metastable clasts such as basalt, limestone, shale, and metamorphic phyllite
are commonly called petromict conglomerates (Pettijohn, 1975) Almost any
combination of these clast types is possible in a petromict conglomerate.
EPS 421 Lecture 08
Oligomictic
conglomerate
Polymictic
conglomerate
A conglomerate in which
the clasts are made up of
only one rock type.
Suggests that the source area was nearby or
source rock extended over wide geographic area.
A conglomerate in which
clasts include several
different rock types.
Conglomerates that include clasts from a widevariety of source rocks, possibly derived over a
wide geographical area or a smaller but
geologically complex area.
EPS 421 Lecture 08
Conglomerates Based on Origin
Extraformational Conglomerates
A conglomerate in which clasts are exotic (i.e., derived
from outside the depositional basin). Clasts are normally
very well rounded and well sorted.
Intraformational Conglomerates
A conglomerate in which clasts are derived locally
from within the depositional basin (e.g., clasts composed
of local muds torn up by currents; such clasts are
commonly termed "rip-up clasts" or "mud clasts").
EPS 421 Lecture 08
Extraformational
Conglomerate
A conglomerate in which clasts
are exotic (i.e., derived from
outside the depositional basin).
Clasts are normally very well
rounded and well sorted.
EPS 421 Lecture 08
Clasts derived from a distant
source.
Intraformational A conglomerate in which
conglomerate clasts are derived locally
from within the
depositional basin (e.g.,
clasts composed of local
muds torn up by
currents; such clasts are
commonly termed "ripup clasts" or "mud
clasts").
Deposition in an environment where muds
accumulated. Muds were in very close
proximity to the site of deposition as the
clasts would not withstand considerable
transport.
Mudstone fragments in river sands – derived from collapse of muddy
river bank
EPS 421 Lecture 08
Conglomerates Based on Fabric
„
If clasts touch, 60-65% clasts by volume
(depends on sorting and shape)
„
Clast-supported ("orthoconglomerate")
pores filled with:
cement (openwork)
matrix (closed-work)
= bedload transport
„
Matrix-supported ("paraconglomerate" or
"diamictite")
= suspension in turbulent flow, or glacial ice
EPS 421 Lecture 08
Conglomerates Based on Texture-1
Orthoconglomerates
Epiclastic conglomerates that are so
rich in gravel-size framework
grains that the gravel-size grains
touch and form a supporting
framework are called clastsupported conglomerates
Paracnglomerates
It is a Clast-poor conglomerates that
consist of sparse gravels
supported in a mud/sand matrix.
they are called matrix-supported
conglomerates (diamictite).
EPS 421 Lecture 08
Conglomerates Based on Texture-2
„
Conglomerates and diamictites can be further divided
on the basis of clast stability into quartzose
conglomerate/diamictite and petromict
conglomerate/diamictite on the basis of relative
abundance of these clast types.
„
Further classification on the basis of clast type
(igneous, metamorphic, sedimentary) can be made if
desired; however, such classification may not be
necessary in many cases.
EPS 421 Lecture 08
Classification of Conglomerates and Diamictites on the
Basis of Clast Stability and Fabric Support
Percentage of
ultrastable clasts
Type of fabric support
Clast-supported
Matrix-supported
>90
Quartzose conglomerate
Quartzose diamictite
<90
Petromict conglomerate
Petromict diamictite
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Classification of conglomerate on the basis of clast lithology
and fabric support. (From Boggs, 1992)
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