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.
Public Domain Image by the US Dept. of Interior.
Contact- A boundary that separates different strata or rock units.
Steno's Laws of Stratigraphy
Image from J. 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.
He developed three fundamental principles of stratigraphy, now known as Steno's Laws:
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– In undisturbed strata, the oldest layer lies at the bottom and the youngest layer lies at the top.
3) 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
4) 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.
Unconformities
Unconformity – A surface that represents a very significant gap in the geologic rock record (due to erosion or long periods of non-deposition).
There are 3 main types of unconformities:
1) Disconformity – A contact 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.
Image provided by FCIT. Original image from Textbook of Geology by Sir Archibald Geikie (1893).
3) Nonconformity – A contact in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.
4) Paraconformity- A contact between parallel layers formed by extended periods of non-deposition (as opposed to being formed by erosion). These are sometimes called "pseudo unconformities").
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
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 Rocks: Werner's Concept of Neptunism
Abraham Werner (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.
Werner classified rocks into 4 categories, as shown in the diagram below:
Figure by RJR
1. Primitive rock (red)– Granite and metamorphic rock were precipitated from oceans.
2. Transition rock (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.
3. Secondary rock (dark brown)– 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.
4. 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.
Nevertheless, Werner's 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.
3) Werner’s relative age assignments represents an early attempt to determine Earth's sequential history.
Historical Perspective on the Origin of Rocks: Hutton's Concept of Plutonism
The Scottish geologist James Hutton (1726-1797) argued that 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.
Hutton’s ideas 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.
