Practical 6: Subject: Sedimentary Rocks - Part 1 Aims
Practical 6: Subject: Sedimentary Rocks - Part 1 Aims
Practical 6: Subject: Sedimentary Rocks - Part 1 Aims
Fig. 1. Flow chart showing the various paths that sediment can take from its source in the
zone of weathering to its final site of deposition where it is turned into a solid sedimentary
rock (from Klein & Philpotts, 2013)
SUPPORTING CONCEPTS
1. Sedimentary rocks are composed of material, that formed by mechanical and/or chemical
weathering. The material is transported by various media (water, ice, wind) and finally
deposited (Fig. 1).
2. The most common minerals in sedimentary rocks are quite stable under conditions of the
Earths surface.
3. The main minerals / petrographic componentes in sedimentary rocks are: (a) quartz, (b)
calcite (c) clay minerals, (d) rock clasts and, subordinate, (e) feldspars.
4. The most important marine depositional environments for sedimentary material are: (a)
deltas, (b) shallow-marine, (c) shores, (d) deep-marine and (e) reefs. Important non-marine
(terrestrial) environments are: (f) fluviatile (rivers), (g) glacial (glacier), (h) alluvial (scree
etc.) and (i) arid (desert).
COMPOSITION OF SEDIMENTARY ROCKS
Only those minerals which are resistant to weathering and transport are preserved.
1. Quartz
Quartz is the most common mineral in siliciclastic sediments, because it is very common in
many source rocks and has high hardness and weathering resistance. In contrast, Fe-Mg-Ca
silicates like olivine, pyroxenes, amphiboles, biotite, but also feldspars, are more easily
affected by weathering processes. Quartz can also be dissolved and re-precipitated as SiO2cement.
2. Calcite
Calcite is the main mineral in limestone and the most common cement mineral in siliciclastic
rocks. Calcite (CaCO3) forms either by anorganic chemical or biochemical precipitation (e.g.,
corals). Dolomite (CaMg(CO3)2) forms during diagenesis of carbonate sediments.
3. Clay minerals
These very fine-grained sheet-silicates form due to chemical weathering of silicates,
especially of feldpars. They are concentrated in fine-grained siliciclastic sediments (e.g.,
mudstones)
4. Rock fragments and fossils
620.010
and
620.002,
Lab
to
Introduction
to
Mineralogy
and
Petrology
for
PE
Clasts of various rocks are a further components in siliciclastic rocks (e.g., in conglomerate,
wacke). Bioclasts are relicts of organisms (fossils). Corals can form thick reefs.
5. Other minerals
Feldspars and micas may become important in some sedimentary rocks. Dolomite, gypsum,
anhydrite and halite are important in evaporites. From fossilised organic material coal and
petroleum (oil, gas) can form.
Minerals
remaining
in
sediment
INTENSITY
OF
WEATHERING
Low
Medium
Quartz
Quartz
Feldspar
Feldspar
Mica
Mica
Pyroxene
Clay
minerals
Amphibole
High
Quartz
Clay
minerals
Table. 1 Minerals in clastic sediments derived from an average granite outcrop under varying
intensities of weathering (after Press & Siever, 1995)
SILICICLASTIC SEDIMENTS
A. General comments
1. Composition
The distinction of components (clasts), matix and cement is essential for the description and
classification of clastic sedimentary rocks.
Components: coarser material > 0,02 mm.
Matrix: fine material <0.02 mm) that was deposited (together with coarser components).
Cement later mineral precipitates (carbonates, quartz, etc.) in the pore spaces during
diagenesis.
2. Roundness
Larger particles (clasts) are affected by physical abrasion transport. They break up, decrese in
size and become rounded (longer transport > increased roundness). Roundness can be
described with the terms angular, subangular, subrounded and rounded (Fig. 2).
Fig. 3. Illustration of sorting
matrix-supported
grain-supported
(gravel)
(sand)
(mud: silt, clay)
>2.0 mm
0.02-2.0 mm
<0.02 mm
Sandstone s.str. (arenite) have <15% matrix (grain size of matrix < 0.03 mm); Wacke (also
greywacke) have >15% matrix. For further subdivision of sandstones the abundance of quartz
feldspar rock fragments is used (Fig. 5).
Fig. 5. Classification of sandstone (after McBride 1963, from Klein & Philpotts 2013).
PRACTICAL 6 - TASKS
SUBJECT: SILICICLASTIC SEDIMENTARY ROCKS
1. Describe one coarse-grained clastic sedimentary rock according to the following
parameters:
a. Estimate the average grain-size of the detrital components.
b. How is the shape of the detrital components characterised? (look at the roundness and
compare with Fig. 2!)
c. How ist the sorting and textural marturity of the rock (compare with Fig. 3!) ?
d. How many distinct detrital components can you observe? (look at the color, hardness etc.!).
Try to identify the detrital components (use also the diluted hydrochloric acid!)
e. Determine whether matrix, or cement can be observed! Estimate the ratio between
matrix/cement and the detrital components! Is the fabric matrix-supported or grain-supported?
(see Fig. 4).
f. Is the matrix made up by siliceous or carbonate material? Use also the diluted hydrochloric
acid!
e. Is the rock solidified? Name the rock!
2. Sandstone and Wacke: Compare samples RS25 with RS26 (or RS31)
a. Estimate the average grain-size of the detrital grains.
b. Which sample has more matrix? Why is RS25 of lighter colour? Is the matrix carbonatic?
c. Which rocks is better sorted?
d. Can you identify quartz in the samples (start with RS25 and use a hand lens!); Can you
observe rounded quartz grains? Why would this observation be important?
e. Try to name the rocks; e.g., use Fig. 5 for the matrix-poor sample.
3. Describe and identify specimen RS21 (S175). See specimen S253 for comparison.
Consider the following parameters:
PRACTICAL 7
SUBJECT: SEDIMENTARY ROCKS - PART 2
CHEMICAL AND BIOGENIC SEDIMENTS
1. Classification
The principal classification is based on chemical composition (Table 3).
Tabelle 3.
Rock
Notes
Limestone
Mineralogy
Calcite, CaCO3
Dolostone
Dolomite, CaMg(CO3)2
see limestones
Chert
cryptocrystalline, high
hardness
Evaporite
Gypsum, CaSO4.2H2O;
Halite (NaCl)
Iron-formations
10
When living evertebrate organisms (molluscs, corals etc.) produce their shells they precipitate
Ca-carbonate through the same reaction. The precipitated calcium carbonate is calcite but in
some cases it is (metastable!) aragonite.
Carbonate components
In contrast to siliciclastic rocks the petrographic componentes of carbonate rocks are mostly
produced at the depositional sites (not transported very far).
1) Allochems (Carbonate components)
Bioclasts: fossils, fossil fragments
Ooids: round, often concentric growth
Intraclasts: irregular shaped
Mineral grains (quartz, white mica etc.)
2) Matrix
fine-grained carbonate mud (= Mikrite)
3) Cement
coarser-grained diagnetic carbonate material filling pore spaces (= Sparite)
Classification of limestones
Fig. 6. Classification of limestones after Dunham (1962). Scale bar in photos s 1 mm.
11
Dolostone
Dolostones are present in essentially the same environments as limestones. The main mineral
is dolomite. Dolomite is not directly precipitated from seawater. It forms during diagenesis
(see lecture notes). For distinction of limestone and dolostone do the HCl-test.
Marls
Marls are mixtures of fine-grained (silt, clay) siliciclastic material with carbonate material.
With HCl calcite reacts (fizzing r.) and the fine clay material is left forming a very thin film
of insoluble material.
3. Evaporites
Marine evaporites are formed by evaporation of seawater. Rocks have relatively simple
mineralogy forming beds of gypsum, anhydrite and salt (halite). In the field evaporites are
often associated with carbonate rocks. Halite (salty taste!) has low density (D=2.16) and is
easily deformed by flow -> salt domes, salt diapirs.
4. Cherts
Rocks composed of very fine-grained amorphous/cryptocrystalline silica. These rocks are
harder than carbonate rocks and break with conchoidal fracture. Flint: chert nodules in
carbonate rocks (-> stone tools).
5. Other chemical and biogenic sedimentary rocks
Iron-formations: banded or laminated iron ores (BIF; banded iron formations); >15 wt% Fe;
chert layers.
Phosphorite: rocks with >18 wt.% P2O5; main mineral is apatite.
Coal: combustible sedimentary rock formed from fossilized plant material under reducing
conditions; main coal formation was in the Carboniferous period (~300 m.yr. ago). Coals are
subdivided according to their rank into low-rank lignite (brown coal), bituminous coal
(intermediate rank) and anthracite (high-rank).
Oil and natural gas: not solid; form when kerogen in organic-rich sediment, on being buried
and heated, is converted to oil and natural gas. Black shales are the most common source
rocks for these hydrocarbons. Hydrocarbons migrate into reservoir rocks, which must have
high porosity and permeability. They are less dense than water and therefore tend to rise
towards the surface. For them to be kept in a reservoir rock they must be confined by an
impermeable cap rock in a natural trap (Fig. 7).
12
Fig. 7. Possible traps for oil and gas (from Klein & Philpotts, 2013).
13
PRACTICAL 7 - TASKS
SUBJECT: CHEMICAL AND BIOGENIC SEDIMENTARY ROCKS
1. Sample S253. There is a detailed petrographic description of this rock in you lab handout
(page 8). Read it carefully and try to confirm the observations, understand the terminology
and follow the interpretations. This is how a short petrographic description of a sedimentary
rock could look like!
2. Try yourself and describe sample S252. Note and follow the systematic grouping. Use
the back side if you need more space.
General aspect:
Mineralogy and clast/component features:
Petrogenesis:
14