Unconformities are gaps in the rock record that occur because part of the record has been removed (as with erosion), or because no record was created (as with non-deposition). The surface along which erosion has occurred or along which no sediments have been deposited for a geologically significant length of time is called an unconformity. Igneous intrusions and faults are two ways in which cross-cutting may occur. From these observations, we can say that the beds in Figure 2.3 were deposited in the order F, E, D, C, B, A (superposition) faulting occurred along H (horizontal continuity,cross-cutting relationships) and intrusion of G occurred (cross-cutting relationships).įigure 2.3. Notice that there is also an igneous intrusion ( G) that is not cut by the fault. Sometimes a lot of evidence is required to demonstrate that offset layers are part of the same bed. We assume the beds labelled C on each side of the fault (and D)are the same beds on either side of the fault, because we know that they should not end abruptly, and because they have the same composition, as indicated by the patterns. This is an indication that they are cut by a fault ( H), which has moved beds C and D next to beds A and B. For example, in Figure 2.3 below, beds A through F appear to end abruptly. This principle states that geologic features are always younger than the rocks or surfaces that they cut across. The principle of horizontal continuity is often used with the principle of cross-cutting relationships. Beds do not end abruptly-they either change composition slowly where there is a change in environment, or they become thinner and thinner until they “pinch out.” The principle of horizontal continuity states that beds tend to extend laterally over long distances.Taken together, these two rules permit us to say that the beds in Figure 2.2 were deposited in the order E, D, C, B, A, and that the beds were folded. Older materials are more likely to be at the bottom of piles, while more recent additions are likely to be near the top. If you are looking for an item on your desk, the more recently you last handled it, the closer to the top of a stack you will look for it. This is not unlike having a stack of books and papers on your desk. The principle of superposition states that in a sequence of beds that has not been overturned, the beds get progressively younger from bottom to top. This principle implies that if beds are not horizontal, something happened to them after they were deposited to disturb them from a horizontal position.įigure 2.1. Once the irregularities are covered, however, the top of the sediment layer will be basically horizontal (e.g., beds C and D, Fig. 2.1). The bottom of the beds may be irregular if sediments have been deposited on an irregular surface, such the contact between beds A and B in Figure 2.1 below. The principle of original horizontality states that beds form in horizontal layers.The surface shared between the top of one bed and the bottom of the next is called a contact. Sedimentary rocks include rocks whose particles were erupted from a volcano, such as volcanic ash. Sedimentary rocks are deposited in layers, called beds or strata. These rules are called the principles of stratigraphy. Geologists keep in mind four basic rules of thumb when they try to figure out the history of a particular group of rocks. use sedimentary and volcanic way-up indicators to determine the stratigraphic facing of beds. explain what an unconformity is, list the three kinds of unconformities, and describe how they are different.use the four principles of stratigraphy to interpret the geological history of a cross-section.ObjectivesĪfter studying this unit, you should be able to Lesson 3 covers way-up indicators in lava flows, and Lesson 4 covers way-up indicators in pyroclastic rocks. In Lesson 2 you will learn how primary structures in sedimentary rocks can be used to determine way-up, the direction in which rocks get younger. It also covers gaps in the rock record, called unconformities. Lesson 1 covers the principles of stratigraphy-the “rules of thumb” that geologists apply when attempting to create geological histories. In Unit 2 you will learn how to determine and describe the geological history of a group of rocks, including the order in which the rock beds formed and the sequence of events, such as folding and faulting, that may have occurred afterward. Geology 319 Structural Geology: The Architecture of Earth’s Continental Crust Study Guide :: Unit 2 Basic Techniques for Interrogating the Rock Record Overview
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