Transcript Lab6Siliciclastic14

Siliciclastic Rocks
Francis. 2014
Eolian Dune
Cross-Bedding
Siliciclastic rocks are the lithified accumulations of clastic grains of silicate minerals and
rocks that have typically been deposited by water. The classification of siliciclastic rocks is
based on grain size:
Conglomerates > 30% gravel (>2 mm) and larger clastic
Grains & Breccias:
(< 5 %)
Sandstones:
(~30%)
> 50% sand-sized (0.06 - 2 mm) clastic grains
Mudstones:
( 65 %)
> 50% silt (0.06 - 004 mm) and
/ or clay (< 0.004 mm)
Grain-Size versus Transport
Mechanism
There is a rough correspondence between the major grain size
divisions and the transport mechanism, which is in return
responsible for their physical separation during the fluid
transport process, and the final distinction between sedimentary
rocks.
• pebbles and larger are carried in the ‘bed load’
by traction - conglomerates
• sands are carried in ‘bed load’ by intermittent
saltation and suspension - sandstones
• silts and clays are carried in suspension in the
‘wash load’ - shales
Conglomerates and Breccias (> 30% gravel (>2 mm) and larger clastic grains):
Conglomerates and breccias can be distinguished by the sphericity of the clasts in the rock: if the clasts
are rounded the rock is referred to as a conglomerate, if they are angular it is a breccia. With both
conglomerates and breccias, grain-size, shape and orientation can be measured accurately in the field and
may provide valuable information about the depositional environment. It is also important to note the
‘maximum clast size’, since it is often a reflection of the competency of the flow (i.e. a measure of the
hydraulic energy of the transport process).
Polymictic
Conglomerates
Monomictic Quartz-Pebble Conglomerate
Conglomerate Beds
Paraconglomerates Diamictites
Paraconglomerates Diamictites
Intraformational
Breccias
commonly monomictic
Magmatic
Diatreme Breccia
Wawa, Ontario
typically matrix-supported
&
diamictic
Meteorite
Impact Breccias
Manicouagan Crater
impactite
commonly matrix-supported
&
diamictic
impactite
anorthosite
impact
breccia
Sandstones
( > 50% sand-sized (0.062 - 2 mm) clastic grains ):
Sandstones are classified according to the types of clastic grains present
(quartz, feldspar, & lithic fragments) and the presence (wackes) or absence
(arenites) of significant fine-grained matrix material (< 0.03 mm).
After this subdivision, they are described in terms of the types of preserved
sedimentary structures, using terms like cross-bedded sandstone and relative
maturity using criteria such as degree of sorting, roundness of the clasts,
diversity of clast types, etc.
greywacke
Arenites /
Sandstones
quartz arenite
greywacke
lithic arenite
15%
Arenites:
fine-grained matrix not visible to naked eye (<10%).
quartz arenite:
(~ 35 %)
quartz grains  90 %.
Rare in modern environment, but quite common in late
Precambrian and Paleozoic. Tend to be relatively mature,
and may represent end-product of several cycles of erosion,
transport, and deposition. Abundance of quartz arenites in
Archean may reflect more intensive early Earth weathering
conditions. Commonly silica cement predominates.
synonym = orthoquartzite
feldspathic arenite:
(~ 15 %)
visible feldspar / (felds + rock frag.)  50 %.
Commonly developed in granitic terranes
and therefore restricted to local basins, but may also
develop in cold or arid climates where feldspar is relatively
resistant to decomposition, or in areas of high erosion rates.
Typically cemented by calcite.
synonym = arkose, if felds is K-spar
lithic arenite:
(~ 20 %)
visible rock fragments / (felds + rock frag.)  50%
The most abundant sandstone, as the sand-sized sediment
loads of most modern rivers are dominated by lithic
clasts. Furthermore, if greywackes are derived from the
decomposition of lithic and feldspar clasts, then lithic arenites
comprise  50 % of all arenites. Tend to be immature, poorly
sorted. Typically cemented by calcite.
synonym = sub-greywacke
Quartz Arenite
Feldspathic Arenite
Lithic Arenite
Greywacke:
fine-grained (<0.03 mm) matrix visible to the naked eye (> 10-15%).
(35%)
The presence of the matrix gives the rock a dark grey colour. The clastic grains
are polymictic and commonly relatively angular. The matrix is composed
finely crystalline chlorite and sericite developed during diagenesis, along with
silt-size quartz and albite. This fine-grained matrix has reacted with and
obliterated the original outline of the clastic grains, acting as the cementing
agent.
There are two hypotheses for the origin of the matrix:
1. diagenetically altered interstitial silt and clay.
2. diagentically altered lithic and feldspar grains of a former lithic arenite.
Most true greywackes are Paleozoic, or older in age, occurring as
‘flysch’ sequences of marine turbidites in response to orogenic events.
Greywackes are not found in fluviatile or any other continental environment.
Few modern sediments or sandstones, including marine turbidites, contain
significant fine-grained matrix. The question is thus are all greywackes
produced by diagenesis of lithic arenite protoliths, or was there something
different about the transport and depositional mechanism of greywackes in the
past which is not operative today.
Relative abundance:
lithic wacke
feldspathic wacke
quartz wacke
typical
less common
rare
Mud Rocks:
( > 50% silt (0.062 - 004 mm) and / or clay (< 0.004 mm))
Mud rocks are composed of silt-sized quartz and feldspar grains and much smaller clay mineral particles.
Depending of the relative proportions of these two types of grains, mud rocks range from siltstones to shales,
mudstones, and claystones.
Siltstones can be distinguish from shales and mudstones by
biting a piece between your teeth. If it feels "gritty" then it
is a siltstone, if it feels smooth or slick, then it is a shale or
claystone.
One of the most important features of mud rocks is
their colour, an indication of their oxidation state and
the paleo-environment of their deposition:
Red shales are oxidized and typically represent sub-aerial detritus derived from the continents. They
may represent in actual sub-aerial deposits, but also are formed by continental dust settling into
organic-poor deep marine environments.
Green shales are relatively reduced, and common in the shallow submarine environments depleted in
oxygen by the decay of organic matter.
Black shales are rich in organic matter and highly reduced, typically deposited in anoxic environments.
They sometimes act as source rocks from which oil and gas are released during burial and diagenesis.