GEO 101 Lecture Notes - Lecture 16: Structural Geology, Inclinometer, Sedimentary Structures
23 Jun 2018
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Interpreting Structures
Understanding the formation of geologic structures in a region is important in
reconstructing its geologic history. Generally, the greater the number of structures, the
more complex the geologic history. Structures often offset, rearrange, or bury blocks of
bedrock, making geologic interpretation more difficult. Understanding geologic
structures is important not only to those in academic fields, but to those in industrial and
engineering fields as well. Knowing the structural history of an area is important for
finding petroleum and metallic ore bodies and for determining rock stability (for
example, in order to locate dams and nuclear reactors on structurally stable bedrock).
Structural events are often inferred from how the bedrock has moved. For example, the
law of original horizontality suggests that sedimentary rocks were deposited as flat lying‐
layers on a relatively horizontal surface. If these rocks are now exposed at the surface
and are still flat lying, it can be concluded that they were uplifted by an evenly applied ‐
tectonic force. If they are tilted in one direction, it can be concluded that the uplifting
stress was greater at one end and pushed the layers into an inclined position.
Occasionally, although the bedrock is horizontally layered, the sedimentary structures
and age relationships shown by fossils indicate the top layer is the oldest—this
arrangement indicates that somehow the entire sequence has been overturned by
tectonic forces and what was the oldest layer on the bottom is now on top.
Mapping in the Field
The ease with which structural geology can be understood is largely dependent on how
much of the bedrock is available for study. In areas such as northern Canada, where
much of the bedrock has been exposed by glaciation, as much as 75 percent of the
bedrock can be walked on and studied. Alternatively, in the southeastern United States,
often less than 10 percent of the bedrock is exposed because of abundant weathering,
soil cover, and vegetation. Reconstructing the geologic history of an area can be
especially challenging (and creative) if little rock is exposed.
Geologists try to find all the bedrock exposures, or outcrops, in an area to construct a
geologic map. They identify rock types, relationships, textures, features (such as cross ‐
bedding), and structures (such as folds and faults) as well as cross cutting relationships ‐
of intrusive rocks, rock mineral contents, and fossils. Detailed directional measurements
along structures, when plotted on a map, can reveal a bigger picture of how the rocks
have been folded and faulted.
One of the most useful measurements is the strike and dip of a tilted rock unit (Figure
1). The strike of the unit is the direction (compass bearing) of the line formed by the
intersection of the tilted bedding plane with the horizontal plane. The dip angle is the
angle between the horizontal plane and the tilted bedding plane. Compasses equipped
with a device called an inclinometer can determine the dip angle. The direction of dip is
always perpendicular to the strike direction. For example, in Figure the rock strikes
north south and dips 25 degrees to the east. A rock that is perfectly flat lying has no ‐ ‐
strike direction and no dip. A rock unit that has been tilted into a vertical position has a
maximum dip of 90 degrees.
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