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
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:
1. Principle of Original Horizontality
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.
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 1-3
2. 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
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.
3. Nonconformity- The erosional surface separates younger sedimentary rock beds from
underlying older igneous or metamorphic rocks.
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.