Lab 8

EASC-101 Lab 8 - Geological Structures Adapted by Fedora Gonzalez-Lucena (2021) MacEwan University from the adaptation by Joyce M. McBeth, Tim C. Prokopiuk, Karla Panchuk, Lyndsay R. Hauber, & Sean W. Lacey (2018) University of Saskatchewan from Deline B, Harris R & Tefend K. (2015) "Laboratory Manual for Introductory Geology". First Edition. Chapter 12 "Crustal Deformation" by Randa Harris and Bradley Deline, CC BY-SA 4.0. 8.1 INTRODUCTION Earth is an active planet shaped by dynamic forces. Forces generated by plate tectonics and other geological processes can build mountains, and crumple and fold rocks. As rocks respond to these forces, they undergo deformation, which results in changes in shape and/or volume of the rocks. The resulting features are termed geologic structures. This deformation can produce dramatic and beautiful scenery; for example, in Figure 8.1 the originally flat (horizontal) rock layers were deformed to form folds in the rocks. Structural geology is the subfield of geology in which Figure 8.1 | Deformed rocks along the coast of Italy. Source: Randa Harris (2015) CC BY-SA 3.0 view source

scientists study the relationships between geological structures (such as folds and faults) and the processes (such as plate tectonics) that have shaped Earth's crust through time. Why is it important to study structures and deformation within the crust? These studies can provide us with a record of the geologic history in a region, and also give us clues to the broader geological processes happening globally through time. This information can be critical when searching for valuable mineral resources. The correct interpretation of features created during deformation helps geologists find oil and valuable metal ores in the petroleum and mining industry, respectively. It is also essential for engineers to understand the behavior of deformed rocks to create and maintain safely engineered structures (e.g., in open and underground mines, and for roads). When engineers do not adequately consider geology in their planning - for example by excluding consideration of geological structures - disaster can strike. An example of this is the disaster that occurred at the Vajont Dam, Monte Toc, Italy in the early 1960s. The location was a poor choice for a dam: the valley was steep and narrow with undercut riverbanks at the base and the area surrounding the dam was prone to large landslides due to solution cavities in the limestone canyon walls which could fill with water and interbedded claystones that generated zones of structural weakness in the rocks. Thorough geological tests were not performed prior to construction. Shifting and fracturing of rock that occurred during the filling of the reservoir and faster downhill movement of surface geological deposits were warning signs that went unheeded. In 1 9 6 3 , a m a s s i v e landslide in the area displaced much of the water in the dam, causing it to override the top of the dam and flood the many villages downstream, resulting in the deaths of almost 2,000 people (Figure 8.2). Figure 8.2 | An image of the Vajont reservoir shortly after the massive landslide (landslide scar at right, dam located in foreground on the left). Source: Unknown (1963) copyright expired. view source

This lab will cover the methods geologists use to describe geological structures, including strike and dip measurements, representations of geological structures on maps and how to construct geological cross-sections. 8.1.1 Learning Outcomes After completing this lab, you should be able to: • Demonstrate an understanding of the concepts of strike and dip • Interpret geologic features using block diagrams • Interpret a geologic map • Create a geologic cross-section from a geologic map • Recognize different types of folds and faults 8.1.2 Key Terms • Contact • Strike • Dip • Geological cross-section • Geological map • Monocline • Anticline • Syncline • Dome • Basin • Joints • Faults • Dip-slip faults • Foot wall • Hanging wall • Normal fault • Reverse fault • Thrust fault • Horst and graben • Strike-slip fault 8.2 STRIKE AND DIP To learn many of the concepts associated with structural geology, it is useful to look at block diagrams and block models. Block diagrams are images based on three-dimensional (3-D) block models, which are blocks of wood or paper with geological structures marked on them. Block models and block diagrams assist in visualizing how 3-D geological structures in the real world can be represented in two dimensions on a map or in a geological cross-section. As you examine the block diagrams in the figures in this section, note the different ways that you can view them: from above, or from the sides. If you look at a block from along the side, you are seeing the cross-section view. This is the view of geological structures you also see when you drive through the mountains and the roads have been cut through the rocks, exposing structures in the rock that you wouldn't see otherwise. If you look at the block from directly above, you are looking at the map or plan view (Figure 8.3). As you look at geological maps and drawings and try to figure out about how rocks have changed when they are tilted and/or deformed, it is useful to remember how they were deposited

in the first place. Let's briefly review some of the geological laws that you learned in the geologic time lab. Sedimentary rocks, under the influence of gravity, will deposit in horizontal layers (principle of original horizontality). The oldest rocks will be on the bottom (because they had to be there first for the others to deposit on top of them) and are numbered with the oldest being #1 (law of superposition). The wooden block in Figure 8.4 (a cross-section view of sedimentary layers) provides an example of the principle of original horizontality and the law of superposition. Each of the boundaries between the colored rock units in Figure 8.4 represents a geological contact , which is the planar surface between two adjacent rock units. Earth’s rock layers are often complicated: rock layers are often tilted at an angle, not horizontal - this indicates that Figure 8.3 | Map or plan view vs cross-section view. The top block in this image is an area viewed in map view, which is the view from directly above the block. The lower block is from the same rock layers, and you are viewing it in cross- section (or from the side). Note that when you view the rocks in cross-section, you can see how the rock layers are tilted. Source: Randa Harris (2015) CC BY-SA 3.0 view source

changes have occurred since deposition (e.g., the rocks have been uplifted by tectonic activity and tilted). Figure 8.5 is a block model example of tilted rocks. Which color bed in the block model is the oldest? Given the law of superposition and the principle of original horizontality, it is more likely that the gray bed on the bottom left side of the block was the bottom bed during deposition, and therefore the oldest. In some circumstances, beds can be completely overturned (for example in recumbent folds); if this was the case, the grey bed would be the youngest bed in figure 8.5. In the lab exercises for this week, we will not have any exercises with overturned beds. Figure 8.4 | Horizontal sedimentary layers viewed in cross-section. In this image, di ff erent rock types are given di ff erent colors. The oldest rock, on the bottom, is labelled #1. The youngest rock in this image is #4. Source: Randa Harris (2015) CC BY- SA 3.0 view source Figure 8.5 | Tilted rocks in a block model. Source: Randa Harris (2015) CC BY-SA 3.0 view source