Part A
Sediment & Sedimentary Structures
The process of making sedimentary rocks starts with the weathering of rocks at the Earth's surface. Weathering can either be mechanical (physically breaking rocks into smaller pieces) or chemical (changing the composition of a rock by chemical processes). After the rock is weathered, it can then be eroded (moved by wind, water, or ice) and deposited (settleing of material). Sedimentary structures, like those in Figure 7-2, are commonly preserved in sediment (e.g., ripple marks, mud cracks, etc.). Sedimentary rocks can form from loose sediment of various sizes (i.e., mud, sand, gravel), from mineral crystals (like silica or calcite) that evaporate or precipitate from water, or from the accumulation of organic material. Over a relatively long period of time, the sediment undergoes lithification (literally, turned to stone), which may involve compaction, cementation, evaporation, and/or precipitation. A few simple observations of sediment and sedimentary structures can yield important information about the environment in which they were deposited. Because we can observe how sediment and sedimentary structures form today, we can thus interpret how they formed in the geologic past.
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Figure 7-2. Sediment and sedimentary structures in Grand Canyon NP and White Sands NP. Dessication cracks forming from wet mud in Ryder Canyon of Grand Canyon NP (left). |
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Clastic Sediment
Let's first look at clastic sedimentary rocks: ones made of sediment or broken pieces (clasts or fragments) of other rocks. Clasts can range in size (fine to coarse), shape (rounded to angular), and sorting (poor to well).
Clast Size
Sediment particle (clast or grain) sizes include gravel-sized (>2 mm), sand-sized (2 mm to 1/16 mm), and mud-sized (<1/16 mm). Each of these size categories can be subdivived into even smaller units (e.g., silt = 1/16 mm to 1/256 mm, clay = <1/256 mm, etc.). Clast size is related to the distance sediment traveled prior to deposition. Typically, the farther the sediment has traveled, the smaller the clast size due to abrasion. Clast size can also be related to current energy, with movement of larger clasts requiring stronger current. Conversely, smaller clasts require lower energy environments for them to settle out of suspension.
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Figure 7-3. Clast sizes: gravel (>2 mm), sand (1/16 mm to 2 mm), and mud (<1/16 mm). |
Clast Rounding
Rounding of sediment refers to the relative smoothness of the particle corners. Rounding can be related to distance of transportation or the amount of current energy. For example, well-rounded sediment reflects long transportation distances or high-energy currents (like wave action along a beach) because of greater abrasion.
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Figure 7-4. Clast rounding: well rounded (left) and angular (right). |
Clast Sorting
Sorting refers to the range of clast sizes in sediment. Well-sorted sediment has equal-sized clasts, whereas poorly-sorted sediment has both large and small clasts. Sorting is related to the distance sediment traveled prior to deposition. Typically, the farther the sediment has traveled, the sediment becomes more well-sorted with abrasion and as larger particles drop out. Sorting is also related to the type of current transportation, with wind- and water-carried sediment (sand and mud) more likely to be well sorted than sediment carried by ice.
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Figure 7-5. Clast sorting: well-sorted sediment where most of the clasts are similar in size (left) and poorly-sorted sediment, where there is a range of clast sizes (right) |
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Example 1 |
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What is the size of most of the clasts in the image above? |
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Gravel (dominantly >2 mm) |
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Example 2 |
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Are the clasts in the image above rounded or angular? |
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Rounded (edges are relatively smooth and rounded) |
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Example 3 |
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Describe the sorting of the clasts in the image above. |
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Poorly sorted (there's a range of clast sizes) |
Because sedimentary rocks were once sediment, they record the surface conditions of the past, such as where they originated, how far they were transported, when and where they were deposited, etc. Thus, studying sediment is very important in the understanding the sedimentary rocks they become.
Reference the previous discussion and the sediment panels shown in Figure 7-6 for questions A01 through A10. Pay attention to the scale!
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Figure 7-6. Sediment panels showing different clast sizes, rounding, and sorting. |
Sedimentary Structures
Sedimentary structures are commonly preserved in sediment (for example, ripple marks or mud cracks). These structures can be used to describe the type of depositional environment in which the rock formed.
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Figure 7-7. Charts of common sedimentary structures that form in soft sediment and are found in sedimentary rock (courtesy Hayden-McNeil Publishing). |
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Figure 7-8. Images of common sedimentary structures. Symmetrical ripple marks (top left), asymmetrical ripple marks (top right), mud cracks (or dessication cracks) (middle left), cross bedding (middle right), trace fossil crawling traces (bottom left), and a trace fossil track (bottom right). |
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Example 4 |
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Name this type of sedimentary structure. |
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Cross bedding (the angled lines in the rock bounded by horizontal top and bottom boundaries) |
Reference the Figures 7-7 and 7-8 to answer Quiz Me! questions A11 through A20.