15.4B| Applying Relative Dating to Geologic History Diagrams

Relative dating is an important tool for geologists to reconstruct a series of events quickly, especially in the field. In the following section, apply what you have learned regarding relative time to the questions below.

Instructions

Complete all of the proceeding assignments so you have the background information needed to answer interpret the illustrations and answer the questions. Below, read the instructions on "Applying the Geologic Principles to a Geologic History Diagram".

Study each illustration and consider what you have learned about relative dating techniques, i.e. superposition, cross-cutting relationships, etc. Answer each question accordingly. You have no time limit and three attempts. 

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Time Needed

60-90 minutes.

Learning Objectives

• Use the principles of relative dating to arrange the sequence of geologic events in geologic cross-sections 
• Identify unconformities and geologic structures in photos, illustrations, cross-sections, and block diagrams  

Applying the Geologic Principles to a Geologic History Diagram

Using the geologic principles of relative dating, we can examine a series of rock layers, determine their relative ages, and establish an order or series of events that must have occurred. Common events that are often recognized can include 1) Deposition of sedimentary layers, 2) Tilting or folding of rocks, 3) Uplift and erosion of rocks, 4) Intrusion of liquid magma, and 5) Fracturing or breaking of rock (faulting). Figure 14.14 below shows a stack of strata, A-G, with A at the bottom. A-D are intruded by unit H. There is an unconformity between D and E. From this information the sequence of events is, first (1) deposition of A-D, (2) intrusion of H, (3) erosion to make the disconformity, (4) deposition of E-G, and (5) uplift and erosion. 

Figure 14.15 shows deformed layers A-F. These layers are offset by a fault and an unconformity cuts off the layers and the fault. Tilted layers G-J are above the unconformity. This information allows us to determine that the first event (1) was the deposition of layers A-D, (2) folding layers A-D, (3) faulting layers A-D, (4) erosion to make the angular unconformity, (5) deposition of layers G-J, (6) tilting of G-J, and (7) uplift and erosion. 

Figure 14.15: Steps to consider when approaching a geologic history diagram. (CC-BY 4.0, Chloe Branciforte, own work)