Sedimentary Rocks
Sedimentary rocks are classified into two main categories: clastic
(mechanical) and nonclastic (chemical) rocks. The classification scheme is
based on the processes that form these two distinct categories. Each category
is subdivided into more detailed sections. The final rock name is derived from
“hands on” examination and the “process of elimination” of the possible
categories and sub-categories. It is not enough to simply memorize the
appearance of a particular specimen. The samples that you will identify are
merely representatives of a particular category of sedimentary rock that has
thousands of examples in nature. You must learn the philosophy of the
classification scheme. Constantly ask yourself, “Why is this rock X?”
Sedimentary rocks are composed of various percentages of the primary rockforming minerals: quartz, calcite, dolomite, gypsum, halite, potassium
feldspar, plagioclase feldspar, biotite mica, muscovite mica, hornblende,
augite, and olivine. Some minerals are more common than others in
sedimentary rocks due to their ability to resist physical and chemical
weathering at the Earth’s surface. Recall Bowen’s Reaction Series. Those
igneous minerals that crystallized last from a magma are the most stable at the
Earth’s surface. This is why our beaches are composed primarily of quartz
sand, not olivine sand. Not all of these rock-forming minerals have an igneous
origin. Some form at the Earth’s surface through various chemical reactions.
Clastic Sedimentary Rocks:
Clastic sedimentary rocks (sometimes referred to as mechanical sedimentary
rocks) are composed of pieces of rocks and minerals that have been weathered
and eroded, deposited, and lithified. As such, clastic sedimentary rocks are
easily identified by their granular appearance. Remember that weathering is
the process by which rocks and minerals are broken down at the Earth’s
surface. Erosion is the transport of sediments, either by wind, water, glaciers,
or gravity. Lithification is the process by which loose sediments become
sedimentary rock. Lithification involves two main processes: compaction and
cementation. Compaction is caused by the overlying weight of new sediment
being deposited on old sediment. The overlying weight compacts the
sediment, forces out water and air, and reduces the amount of free space
(porosity) between the grains. Cementation is the process of binding sediment
particles together. As water passes through the pore spaces between sediment
grains, minerals precipitate on the grains and eventually cement them together.
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The most common types of cement include calcium carbonate (calcite), silicon
dioxide (quartz), and various types of iron oxides (e.g. hemtite and goethite).
Calcium carbonate cements will fizz in weak hydrochloric acid (HCl), whereas
quartz and iron oxide cements will not. Iron oxide cements commonly give the
clastic rock a reddish or yellowish appearance. Clastic sediments are found in
every environment on the Earth’s surface. Common environments include
rivers, beaches, deserts, and glaciated areas.
Clastic rocks are classified based on the size of their original fragments, and by
the shape and mineralogy of the fragments. The size ranges are as follows:
2mm+ = gravel
1/16 mm – 1/256 mm = silt
2mm – 1/16 mm = sand
< 1/256 mm = clay
Nonclastic Sedimentary Rocks:
Nonclastic sedimentary rocks (sometimes referred to as chemical sedimentary
rocks) are formed by the direct precipitation of minerals from water (the
inorganic rocks), or by the biological activity of certain organisms (the organic
rocks). These types of sedimentary rocks are most commonly formed in or
around large bodies of water, either marine or nonmarine. Each type of
nonclastic rock has a distinct set of physical and mineralogical characteristics.
You learned most of these characteristics during the mineral identification lab
earlier in the semester. The same rules apply, but the minerals will now be too
small to see. Only a few of the physical properties of minerals will be
identifiable, but they will be enough. Lithification eventually turns nonclastic
sediments into rock. Nonclastic rocks commonly appear to have an
interlocking mosaic of mineral grains, or no grain structure whatsoever
(massive appearance).
The inorganic nonclastic rocks are commonly composed of only one of four
main minerals: calcite, dolomite, gypsum, and halite. The organic nonclastic
rocks are composed of a mixture of plant and animal parts (fossils), with some
percentage of inorganic components. This commonly creates a “gray area” in
our nonclastic rock classification scheme, as you will see.
Section 1: Rock Classification:
Use the identification flowchart in conjunction with the sedimentary rock
classification chart to identify the sedimentary rocks. Verify your
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identifications with your lab instructor before proceeding to the next section.
Use the blank identification chart to record your observations. Use a pencil to
fill in the chart.
Section 2: Interpretations:
Modern day sediments are deposited in particular environments. These
environments are characterized by distinct grain sizes, and in some case
distinct mineral suites. The three most common depositional environments
are marine, transitional, and nonmarine. Marine environments include shallow
to deep water, transitional environments represent the interface between the
land and sea (deltas, tidal flats, beaches), and nonmarine environments
represent those on continents (rivers, lakes, glaciers, and deserts). Geologists
study these modern environments and use the information to interpret the
ancient environments within which sedimentary rocks formed. Sedimentary
rocks also commonly have fossils within them. These are also a function of
ancient depositional environments. Consider a rock with clam shells and coral
fragments versus a rock with dinosaur bones. Which one formed in an ocean
environment? Which one formed on land?
The size of the sediment particles can also be used to infer the nature and
distance of transport. Simply put, larger particles require more energy to be
transported. Larger particles tend to remain close to their source, whereas
smaller particles tend to be carried further. This is called sorting. Well-sorted
deposits are those of uniform grain sizes. In addition, particles display a
greater degree of roundness with transport due to abrasion. Abrasion is the
act of smoothing and rounding particles during transport. Angular particles
tend to predominate close to their source. Well-rounded particles suggest a
great degree of transport.
Using the concepts of depositional environments, sorting, roundness, mineral
composition, and lithification, answer the following questions.
1. Which of the clastic sedimentary rocks displays the greatest amount of
sorting and roundness? Support your answer.
What are the dominant minerals in this rock? Explain your answer.
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2. Which of the clastic sedimentary rocks displays the least amount of sorting
and roundness? Support your answer.
What are the dominant minerals in this rock? Explain your answer.
3. Place all of the clastic sedimentary rocks in order from shortest to longest
distance of transport. List their names and support your choice of order.
4. Examine the sandstones. What is the cement for each type? How did you
determine this?
5. Compare the coquina and fossiliferous limestone. Which represents a
higher energy environment? Why?
6. The University President has just assigned YOU the task of evaluating
sedimentary rocks that will be used for a new building on campus. Which
of the rocks that you have studied would be better suited for use in
construction? Which should be avoided? Compose your answer as a
paragraph that outlines your recommendations and supporting data.
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