STRATIGRAPHY: MAKING SENSE OF CHAOS
What is Stratigraphy?
Stratigraphy- The branch of geology
that seeks to understand the geometric relationships between different rock
layers (called strata), and to interpret the history represented by
these rock layers.
Image by the US Dept. of Interior.
Contact- A boundary that separates different strata or rock units.
Laws of Stratigraphy
P. Trap: berømte danske mænd og kvinder, 1868
Nicholas Steno (1638-1686) was a Danish-born
pioneer of geology, and is considered to be the father of stratigraphy.
Nicholas Steno's observations of rocks layers
suggested that geology is not totally chaotic. Rather, the rock layers preserve a chronological
record of Earth history
and past life.
developed three fundamental principles of stratigraphy, now known as
1) Law of Original
Horizontality– Beds of sediment deposited in water form as horizontal (or
nearly horizontal) layers due to gravitational settling.
2) Law of Superposition–
undisturbed strata, the oldest layer lies at the bottom and the youngest
layer lies at the top.
Law of Lateral Continuity–
Horizontal strata extend laterally until they thin
to zero thickness (pinch out) at the edge of their basin of deposition.
Other Important Principles of Stratigraphy
Law of Cross-Cutting Relationships–
An event that cuts across existing rock is younger than that disturbed rock.
This law was developed by Charles Lyell (1797-1875).
5) Principle of Inclusion–
Fragments of rock that are contained (or included) within a host rock are
older than the host rock.
A surface that represents a
very significant gap in the geologic rock record (due to erosion or long periods of
There are 3 main types of
1) Disconformity –
representing missing rock between sedimentary layers that
are parallel to each other. Since disconformities are parallel to bedding
planes, they are difficult to see in nature.
2) Angular Unconformity – A
contact in which younger strata overlie an erosional surface on tilted or folded
rock layers. This type of unconformity is easy to identify in nature.
by FCIT. Original image from Textbook of Geology by Sir Archibald Geikie
3) Nonconformity – A contact
in which an erosion surface on plutonic or metamorphic rock has been covered by
younger sedimentary or volcanic rock.
Paraconformity- A contact between parallel layers
formed by extended periods of non-deposition (as opposed to being formed by
erosion). These are sometimes called "pseudounconformities").
Unconformities VS Bedding Planes
Unconformities represent huge gaps in time! The
nonconformity between the Vishnu Schist and overlying sedimentary layers in
the Grand Canyon represents 1.3 billion years of missing rock record.
Bedding planes, or planes separating
adjacent sedimentary layers, also represent gaps in the rock record but on a
much smaller scale than an unconformity.
Relative age dating is a way to
use geometric relationships between rock bodies to determine the
sequence of geologic events in an area. Relative dating is different from
absolute dating in which specific dates are assigned to geologic
events (we will discuss absolute dating techniques later).
Relative dating is based on
the five principles of stratigraphy discussed above.
Historical Perspective on the Origin of
Werner's Concept of Neptunism
(1749-1817), a German geologist,
proposed that Earth’s crust originated
in ocean water through the process of precipitation. This idea
became known as Neptunism, in reference to the Roman God of the sea.
classified rocks into 4 categories, as shown in the diagram below:
Figure by RJR
(red)– Granite and metamorphic rock were precipitated from oceans.
(light brown)– Next, fossil-rich sedimentary rocks were precipitated.
These rocks are tilted due to deposition on the non-horizontal surfaces of primitive rocks.
This aspect of Werner's model was useful for explaining the origin of tilted sedimentary rocks.
Flat lying sedimentary rocks were eventually precipitated. The secondary rocks were thought to include interlayered basalts, which Werner
thought formed by combustion of buried coal layers.
Tertiary (or alluvial) rock
(yellow)– Finally, after the
ocean receded, recent erosion and deposition
created a thin veneer of overlying sediment.
Today we know that Werner's basic assumption
that granite precipitated from seawater is incorrect. We also know that
basalt is not the product of coal combustion.
concept of Neptunism was influential because:
1) Werener was right that some sedimentary
rocks, such as limestones, do precipitate from ocean water.
2) Werner was
not a catastrophist and did not need to make his interpretation of rock layers
consistent with scriptual teachings.
relative age assignments represents an early attempt to determine Earth's
Historical Perspective on the Origin of Rocks:
Hutton's Concept of
The Scottish geologist James Hutton (1726-1797)
granite and basalt by solidification within the earth (as opposed to
precipitating in from oceanwater). This idea is known as Plutonism, in
reference to the God of the deep underworld.
This concept of plutonism
was supported by basalt melting/cooling experiments Sir James Hall
conducted in 1792. These experiments showed that the basalts form by the
solidification of liquid magma.
Hutton viewed tilted strata as having been initially deposited horizontally, and
then were subsequently deformed (tilted and folded) by the forces of Earth's internal heat engine.
He would argue that these forces gave rise to mountains.
Furthermore, he suggested that the mountains eroded to
produce the sedimentary rocks we find in the rock record.
Hutton viewed the
earth continually recycling itself with a balance between destruction and
rejuvenation. Mountains are created, eroded, and reformed.
were not well received by people in the early 1800’s because he was a poor
writer, and because his science was anti-catastrophic and did not support the scriptures.