Siliciclastic Rocks Francis. 2014 Eolian Dune Cross-Bedding.
Embed Size (px)
Transcript of Siliciclastic Rocks Francis. 2014 Eolian Dune Cross-Bedding.
Siliciclastic RocksFrancis. 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 clasticGrains & Breccias:
(< 5 %)
Sandstones: > 50% sand-sized (0.06 - 2 mm) clastic grains (~30%)
Mudstones: > 50% silt (0.06 - 004 mm) and ( 65 %) / 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).
Monomictic Quartz-Pebble Conglomerate
Magmatic Diatreme 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
Arenites: fine-grained matrix not visible to naked eye (<10%).
quartz arenite: quartz grains 90 %.
(~ 35 %) 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: visible feldspar / (felds + rock frag.) 50 %.
(~ 15 %) 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: visible rock fragments / (felds + rock frag.) 50%
(~ 20 %) 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
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
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 typical feldspathic wacke less common
quartz wacke 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
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.