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 rock- forming 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. 37 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 2mm – 1/16 mm = sand 1/16 mm – 1/256 mm = silt < 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 38 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.
39 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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