Download Petrology Lab 6: Siliciclastic Rocks

Francis, 186-212, 2014
Petrology Lab 6: Classification of Siliciclastic Rocks
Siliciclastic rocks are the lithified accumulations of clastic grains or particles of silicate minerals
and/or rocks that have typically been deposited by water.
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:

silts and clays are carried in suspension in the ‘wash load’

sands are carried in ‘bed load’ by intermittent saltation and suspension

pebbles and up are carried in the ‘bed load’ by traction
1
Francis, 186-212, 2014
The classification of siliciclastic rocks is based on grain size:

Conglomerates & Breccias:
(< 5 %)
> 30% gravel (>2 mm) and larger clastic grains

Sandstones:
( 20 %)
> 50% sand sized (0.062 - 2 mm) clastic grains

Mud Rocks:
( 65 %)
> 50% silt (0.062 - 004 mm) and/or clay (< 0.004 mm)
2
Francis, 186-212, 2014
Station A1 & A2:
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.
A conglomerate or breccia may be either monomictic (all the clasts are of the same type) or
polymictic (different types of clasts). 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). In addition you should examine the clast-matrix relationships in these
samples: clast-support fabric is typical of fluvial and beach gravel, whereas matrix-support
fabric is typical of debris flow deposits and glacial tills.
Classify each of the samples at this station according to the scheme (as far as is possible in the
absence of other data, such as field relationships) in your handouts.
3
Francis, 186-212, 2014
Station B1 % B2:
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 such as cross-bedded sandstone, and relative maturity using criteria such
as degree of sorting, roundness of the clasts, diversity of clast types, etc.
Classify the sandstones at Station B according to the following scheme:

Arenites:
fine-grained matrix not visible to naked eye (<10-15%).
quartz arenite:
(~ 35 %)
quartz grains  90 %. Rare in the
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. 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
4
Francis, 186-212, 2014

Greywacke:
sandstone with a fine-grained matrix visible to the naked eye (> 1015% matrix with < 0.03 mm grain-size). Commonly the presence of this
matrix gives the rock a dark grey colour. The clastic grains are typically
polymictic and commonly relatively angular. The matrix is composed of
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 silt and clay that were initially present between
the coarser sand-sized grains.
2. diagentically altered lithic, and feldspar, clastic 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
relatively rare
5
Francis, 186-212, 2014
Station C:
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.
This station contains a number of different types of mud rock. 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 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.
Examine the different types of shale and siltstone at this station and categories them as
best you can.
6
Francis, 186-212, 2014
Station D: Grain Size Analysis
Each of the clastic sediment samples listed represents one of the following environments:

glacial river sand:
poorest sorting, bimodal grain size
distribution.

meandering stream sand:
better sorting and finer grain size than braided
stream sands.

braided stream sand:
poorer sorting and coarser grain size than
meandering stream sands.

turbidite sand:
poorer sorting than meandering river sands, but
finer grain size that braided stream sands.

beach sand:
best sorting.
Plot frequency and arithmetic cumulative percent diagrams for each sample and
determine the modal and mean grain sizes, and the graphical standard deviation,
skewness, and kurtosis.
Plot a cumulative percent probability diagram and determine the  grain sizes
corresponding to transitions in transport mechanism from traction to saltation and
suspension.
Using the above characteristics, chose the most likely type of sand for each sample.

0
1
2
2.5
3
3.5
4
4.5
Sample #1
25 gms
2.4
8.6
6.9
4.5
1.5
1.1
0.0
Sample #2
25 gms
0
0.1
0.6
2.4
18.5
2.1
0.8
0.5
Sample #3
25 gms
0.2
0.5
4.4
2.1
4.6
7.4
4.3
1.5
7
Sample #4
25 gms
0.0
0.5
2.2
7.4
8.2
5.7
1.0
0.0
Sample #5
25 gms
0.0
0.05
0.6
12.8
11.05
0.5
0.0
0.0