EPSc 230 Introduction to Astrobiology
Spring 2006
Stratigraphic Superposition
At the close of the 18th century, careful studies by scientists showed that rocks had
diverse origins. Some rock layers, containing clearly identifiable fossil remains of fish
and other forms of aquatic animal and plant life, originally formed in the ocean. Other
layers, consisting of sand grains winnowed clean by the pounding surf, obviously formed
as beach deposits that marked the shorelines of ancient seas. Certain layers are in the
form of sand bars and gravel banks -- rock debris spread over the land by streams. Some
rocks were once lava flows or beds of cinders and ash thrown out of ancient volcanoes;
others are portions of large masses of once molten rock that cooled very slowly far
beneath the Earth's surface. Other rocks were so transformed by heat and pressure during
the heaving and buckling of the Earth's crust in periods of mountain building that their
original features were obliterated.
Between the years of 1785 and 1800, James Hutton and William Smith advanced the
concept of geologic time and strengthened the belief in an ancient world. Hutton, a
Scottish geologist, first proposed formally the fundamental principle used to classify
rocks according to their relative ages. He concluded, after studying rocks at many
outcrops, that each layer represented a specific interval of geologic time. Further, he
proposed that wherever uncontorted layers were exposed, the bottom layer was deposited
first and was, therefore, the oldest layer exposed; each succeeding layer, up to the
topmost one, was progressively younger.
Today, such a proposal appears to be quite elementary but, nearly 200 years ago, it
amounted to a major breakthrough in scientific reasoning by establishing a rational basis
for relative time measurements. However, unlike tree-ring dating -- in which each ring is
a measure of 1 year's growth -- no precise rate of deposition can be determined for most
of the rock layers. Therefore, the actual length of geologic time represented by any given
layer is usually unknown or, at best, a matter of opinion.1
Stratigraphic Principles and Relative Time
Much of the Earth's geology consists of successional layers of different rock types, piled
one on top of another. The most common rocks observed in this form are sedimentary
rocks (derived from what were formerly sediments), and extrusive igneous rocks (e.g.,
lavas, volcanic ash, and other formerly molten rocks extruded onto the Earth's surface).
The layers of rock are known as "strata", and the study of their succession is known as
"stratigraphy". Fundamental to stratigraphy are a set of simple principles, based on
elementary geometry, empirical observation of the way these rocks are deposited today,
and gravity. Most of these principles were formally proposed by Nicolaus Steno (Niels
Steensen, Danish), in 1669, although some have an even older heritage that extends as far
back as the authors of the Bible. A few principles were recognized and specified later.
An early summary of them is found in Charles Lyell's "Principles of Geology", published
in 1830-32, and does not differ greatly from a modern formulation:
EPSc 230 Introduction to Astrobiology
Spring 2006
1. The principle of superposition - in a vertical sequence of sedimentary or
volcanic rocks, a higher rock unit is younger than a lower one. "Down" is older,
"up" is younger.
2. The principle of original horizontality - rock layers were originally
deposited close to horizontal.
3. The principle of original lateral extension - A rock unit continues laterally
unless there is a structure or change to prevent its extension.
4. The principle of cross-cutting relationships - a structure that cuts another is
younger than the structure that is cut.
5. The principle of inclusion - a structure that is included in another is older
than the including structure.
6. The principle of "uniformitarianism" - processes operating in the past were
constrained by the same "laws of physics" as operate today.
Note that these are principles. In no way are they meant to imply there are no exceptions.
For example, the principle of superposition is based, fundamentally, on gravity. In order
for a layer of material to be deposited, something has to be beneath it to support it. It
can't float in mid-air, particularly if the material involved is sand, mud, or molten rock.
The principle of superposition therefore has a clear implication for the relative age of a
vertical succession of strata. There are situations where it potentially fails -- for example,
in cave deposits. In this situation, the cave contents are younger than both the bedrock
below the cave and the suspended roof above. However, note that because of the
"principle of cross-cutting relationships", careful examination of the contact between the
cave infill and the surrounding rock will reveal the true relative age relationships, as will
the "principle of inclusion" if fragments of the surrounding rock are found within the
infill. Cave deposits also often have distinctive structures of their own (e.g., spelothems
like stalactites and stalagmites), so it is not likely that someone could mistake them for a
successional sequence of rock units.
These geological principles are not assumptions either. Each of them is a testable
hypothesis about the relationships between rock units and their characteristics. They are
applied by geologists in the same sense that a "null hypothesis" is in statistics -- not
necessarily correct, just testable. In the last 200 or more years of their application, they
are often valid, but geologists do not assume they are. They are the "initial working
hypotheses" to be tested further by data.
Using these principles, it is possible to construct an interpretation of the sequence of
events for any geological situation, even on other planets (e.g., a crater impact can cut
into an older, pre-existing surface, or craters may overlap, revealing their relative ages).
The simplest situation for a geologist is a "layer cake" succession of sedimentary or
extrusive igneous rock units arranged in nearly horizontal layers. In such a situation, the
"principle of superposition" is easily applied, and the strata towards the bottom are older,
those towards the top are younger.
This orientation is not an assumption, because in virtually all situations, it is also possible
to determine the original "way up" in the stratigraphic succession from "way up
EPSc 230 Introduction to Astrobiology
Spring 2006
indicators". For example, wave ripples have their pointed crests on the "up" side, and
more rounded troughs on the "down" side. Many other indicators are commonly present,
including ones that can even tell you the angle of the depositional surface at the time
("geopetal structures"), "assuming" that gravity was "down" at the time, which isn't much
of an assumption.
In more complicated situations, like in a mountain belt, there are often faults, folds, and
other structural complications that have deformed and "chopped up" the original
stratigraphy. Despite this, the "principle of cross cutting relationships" can be used to
determine the sequence of deposition, folds, and faults based on their intersections -- if
folds and faults deform or cut across the sedimentary layers and surfaces, then they
obviously came after deposition of the sediments. You can't deform a structure (e.g.,
bedding) that is not there yet! Even in complex situations of multiple deposition,
deformation, erosion, deposition, and repeated events, it is possible to reconstruct the
sequence of events. Even if the folding is so intense that some of the strata is now upside
down, this fact can be recognized with "way up" indicators.
No matter what the geologic situation, these basic principles reliably yield a
reconstructed history of the sequence of events, both depositional, erosional,
deformational, and others, for the geology of a region. This reconstruction is tested and
refined as new field information is collected, and can be (and often is) done completely
independently of anything to do with other methods (e.g., fossils and radiometric dating).
The reconstructed history of events forms a "relative time scale", because it is possible to
tell that event A occurred prior to event B, which occurred prior to event C, regardless of
the actual duration of time between them. Sometimes this study is referred to as "event
stratigraphy", a term that applies regardless of the type of event that occurs (biologic,
sedimentologic, environmental, volcanic, magnetic, diagenetic, tectonic, etc.).
These simple techniques have been widely and successfully applied since at least the
early 1700's, and by the early 1800's, geologists had recognized that many obvious
similarities existed in terms of the independently-reconstructed sequence of geologic
events observed in different parts of the world. One of the earliest (1759) relative time
scales based upon this observation was the subdivision of the Earth's stratigraphy (and
therefore its history), into the "Primary", "Secondary", "Tertiary", and later (1854)
"Quaternary" strata based mainly on characteristic rock types in Europe. The latter two
subdivisions, in an emended form, are still used today by geologists (although very
recently there has been a movement to lump together the Tertiary and Quaternary. The
earliest, "Primary" is somewhat similar to the modern Paleozoic and Precambrian, and
the "Secondary" is similar to the modern Mesozoic. Another observation was the
similarity of the fossils observed within the succession of strata.
EPSc 230 Introduction to Astrobiology
Spring 2006
Example:
What has happened here, and what is the order of events? The figure above shows
several different rock types. Some of these rocks are sedimentary rocks (rocks originally
formed in horizontal layers as layers of sediments, say from a lake or stream). Some of
these rocks are volcanic. Write down an a sheet of paper (without looking at the answer
below): what are the order of events that happened here, and what are the observations
you made that allow you to come to that conclusion? Then, check yourself with the
answers below:
The oldest identifiable bed of rock is layer A. It was likely originally formed in a
horizontal position, and has subsequently been folded at some later date. After layer A
was formed, then layers B through F formed on top of layer A (all of these also formed as
horizontal layers, therefore the folding must have occurred some time after F was
deposited, right?). Then volcanic rock G intruded into layers A through F. It probably
intruded upwards because hot liquid magmas like to travel upwards (they are less dense
than surrounding cold solid rock so tend to rise). Now what happened?? Did the folding
of rocks A through F happen next?? Look at rock G. If volcanic rock G intruded before
the folding, then we might expect to see G perpendicular to layers A through F. We
don’t see that. Rather, we see that G is perpendicular to the absolute/true horizon.
Therefore, the folding of rocks A through F probably happened before volcanic rock G
intruded layers A through F. Now what happened?? Layers H, I, and J were deposited as
horizontal layers. This deposition happened after G intruded and after the folding of
layers A through F. How do we know that? Because layer H is horizontal and it crosscuts F and G. Lastly, a fault (right hand side of the figure) came along and offset all the
layers. The fault is the last thing to happen here, because the fault cross-cuts all layers it
touches. See how easy it is to put a complex set of geologic events in order by looking at
cross-cutting relationships and using the principle of superposition?
Credits: Example figure came from “Life on a Young Planet” by Andrew Knoll, 2003.
The introductory text came from www.leeric.lsu.edu/bgbb/1/strat_measuring.html.
EPSc 230 Introduction to Astrobiology
Spring 2006
Your Assignment:
1. Look very closely at the attached Viking orbiter image of Ravi Valles (on Mars –
this can be downloaded from the web site under the Weeklies page).
2. Now, we haven’t talked much about the surface of Mars (that’s OK, we will
soon), but try to identify as many different types of features as you can. Make a
list.
3. Try to propose a hypothesis of how each feature happened. There may be
multiple explanations for how each feature formed – these are competing
hypotheses. If there are many competing hypotheses, which do you think is the
better hypothesis? What evidence suggests that this hypothesis is better?
4. Now, try to find cross-cutting relationships and use the principles of superposition
to come up with the sequence of events that happened to form the various features
on this image. Which events happened early? Which happened late?
5. What implications do your observations and hypotheses have toward the history
of water on the surface of Mars? Does this surface tell you that water might have
been present early in Mars’ history or late?