USING RELATIVE DATING AND UNCONFORMITIES TO DETERMINE SEQUENCES OF EVENTS

Transcription

1 EARTH AND ENVIRONMENT THROUGH TIME LABORATORY- EES 1005 LABORATORY THREE USING RELATIVE DATING AND UNCONFORMITIES TO DETERMINE SEQUENCES OF EVENTS Introduction In order to interpret Earth history from rock layers, it is necessary to know which rock layers are older and which rock layers are younger. Absolute ages have long been measured radiometrically for igneous and metamorphic, but radiometric age dating has only been recently done for sedimentary rocks. Methods of absolute dating are 1. Principle of Original Horizontality discussed in the lecture. In the lab we will deal instead with relative dating. In relative dating, events are established as 1) older than, 2) younger than, or 3) the same age as other geologic events. Relative geologic ages are determined using the three following fundamental concepts: Sedimentary rocks are originally deposited in a horizontal position. Most non-horizontal rocks have been altered by forces in the Earth's crust 2. Principle of Superposition In an undisturbed stratigraphic sequence (layers of rocks), the last rocks to be deposited are the youngest rocks (at the top) and they get progressively older going down the stratigraphic sequence. 3. Principle of Cross-Cutting Relationships Geologic structures or bodies are younger than the features that they cut. A is younger than B if it cuts across or displaces B.

2 Block Diagrams and Geologic Events In Laboratory 2, you utilized the data displayed in block diagrams to successfully identify geologic structures. In today's laboratory you will be looking at block diagrams to successfully identify the sequence of geologic events. Use the fundamental concepts (p. 12), the terms used to describe geologic events (below and correctly identify any unconformities using diagrams on the next page. Use the block diagram below as a tool for success. Notice that the rock layers are numbered from oldest (1) to youngest (3) and geologic events are lettered. Ask yourself what happened first, what happened next, and what was the last geologic event? In relative dating, we want to establish a sequence of events for a given area, as seen in crosssection. These events may include: - Deposition: sediments get deposited from the top and are lithified - Erosion: a portion of the rock bed is removed by wind or water - Folding: stress forces rock to bend (plastic deformation) - Faulting: stress forces rock to break (brittle deformation) and become displaced along a plane, sometimes in conjunction with tilting - Intrusion: intrusive igneous bodies (dikes, sills, or plutons) rise from deep in the Earth and cool before reaching the surface. Explanation B cuts across rock layers 1-3 and Dike A, so it must be younger than rock layers 1-3 and Dike A. B does not cut across C, therefore it is older than C. Sequence of Events Cross-sectional view - igneous intrusive sequence 1. Deposition of rock layers Intrusion of Dike A 3. Intrusion of Dike B 4. Fault Z a. Intrusion of Dike C OR 4. Intrusion of Dike C a. Fault Z

3 Unconformities Significant breaks in the stratigraphic (rock) record which represents a gap in geologic time are called unconformities. There are three main types of unconformities: 1. Disconformity- The erosional surface separates rock beds which are parallel, but there is a significant lapse of time between the two sets of beds. 2. Angular unconformity- The erosional surface separates rock beds which are at an angle to each other. For example, a horizontal sequence will lie unconformably upon a tilted sequence.

4 3. Nonconformity- The erosional surface separates younger sedimentary rock beds from underlying older igneous or metamorphic rocks.

5 Laboratory Exercise For each block diagram, label rock units with numbers (1 being the oldest) and geologic events with letters (A being the oldest). Then explain the sequence of events starting with the oldest. Use a piece of notebook paper to give the sequence of events. Remember to correctly identify all unconformities.

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