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 bywater.

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

The classification of siliciclastic rocks is based on grainsize:

  • Conglomerates & Breccias: > 30% gravel (>2 mm) and larger clastic grains

(< 5 %)

  • Sandstones: > 50% sand sized (0.062 - 2 mm) clastic grains

( 20 %)

  • Mud Rocks: > 50% silt (0.062 - 004 mm) and/orclay (< 0.004 mm)

( 65 %)


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 clastsize’, 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.


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-grainedmatrix not visible to naked eye (<10-15%).

quartz arenite:quartz grains  90 %. Rare in the

(~ 35 %)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:visiblefeldspar / (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 areasof 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:sandstone with a fine-grained matrix visible to the naked eye (> 10-

15% 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.
  1. 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 wacketypical
  • feldspathic wacke less common
  • quartz wacke relatively rare

Station C:

Mud Rocks: ( > 50% silt (0.062 - 004 mm) and/orclay (< 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 fromshales 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.

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.

 / Sample #1
25 gms / Sample #2
25 gms / Sample #3
25 gms / Sample #4
25 gms / Sample #5
25 gms
0 / 2.4 / 0 / 0.2 / 0.0 / 0.0
1 / 8.6 / 0.1 / 0.5 / 0.5 / 0.05
2 / 6.9 / 0.6 / 4.4 / 2.2 / 0.6
2.5 / 4.5 / 2.4 / 2.1 / 7.4 / 12.8
3 / 1.5 / 18.5 / 4.6 / 8.2 / 11.05
3.5 / - / 2.1 / 7.4 / 5.7 / 0.5
4 / 1.1 / 0.8 / 4.3 / 1.0 / 0.0
4.5 / 0.0 / 0.5 / 1.5 / 0.0 / 0.0

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