The study of rock strata or layers. Stratigraphy
is concerned with aspects of the rock layers such as their
succession, age relationships, lithologic composition, geometry,
distribution, correlation, fossil content, and all other
aspects of the strata. The main aim of stratigraphy is to
understand and interpret the rock record in terms of paleoenvironments,
mode of origin of the rocks, and the causes of
similarities and differences between different stratigraphic
units. Because sedimentary rocks are laid down one on top of
another, we can look at a thick pile of sedimentary rocks,
such as those in the Grand Canyon, and as we look lower
down, we look further back in time. The time difference
between the rocks at the top of the Grand Canyon and those
at the base is nearly two billion years. Thus, by looking at the
different layers, we can reconstruct the past conditions on the
planet at this particular place.
These relationships are expressed in several laws of
stratigraphy. The first, known as the Law of Original Horizontality,
states that water-laid sediments form horizontal
strata, parallel to the Earth’s surface. So if sedimentary rocks
are now found inclined, we can infer that they have been
deformed. The second law, the Principle of Stratigraphic
Superposition, states that the order in which the strata were
deposited is from bottom to top, assuming that the strata
have not since been overturned.
From the Principle of Stratigraphic Superposition, we
can define the relative ages of two different sedimentary
units. The older rocks are below the younger ones—this is
useful for correlating geologic strata from well-exposed to
poorly exposed areas, for once the relative age of a unit is
known, then you know which rocks are above and below
it. Where rocks are folded or tectonically deformed, some
may be upside down, and simply knowing which unit you
are looking at may not be enough to tell you which units
might be found underground beneath this particular outcrop.
One way to tell if rocks are upside down or not is to
use the geometry of sedimentary features that formed
when the rock was a sediment. For instance, if the original
rock showed graded bedding, from coarse-grained at the
bottom to fine-grained at the top, and now the rock has
fine material at the base and coarse at the top, it may be
upside down. If sand ripples or cross laminations are
found on the bottoms of the beds instead of the tops, that
would be additional evidence that the strata are now upside
down.
Although the relative ages of strata can be defined by
which is on top of which, the absolute ages can not be determined
in this manner, nor can the intervals of time between
the different units. One reason for this is that deposition is
not continuous, and there may be breaks or discontinuities in
the stratigraphic record, represented by unconformities.
Stratigraphic Classification
Because rocks laid down in succession each record environmental
conditions on the Earth when they were deposited,
experienced geologists can read the record in the stratigraphic
pile like a book recording the history of time. Places like the
Grand Canyon are especially spectacular because they record
billions of years of history.
Classical stratigraphy is based on the correlation of distinct
rock stratigraphic units, or unconformity surfaces, that
are internally homogeneous and occur over large geographic
areas. The formation is the basic unit of rock stratigraphy
and is defined as a group of strata which constitutes a distinctive
recognizable unit for geologic mapping. Thus, it must be
thick enough to show up on a map, must be laterally extensive,
and must be distinguishable from surrounding strata.
Formations are named according to a code (the Code of
North American Stratigraphic Nomenclature), using a prominent
local geographic feature. Formations are divided into
members and beds, according to local differences or regionally
distinctive horizons. Formations may be grouped together
with other formations into groups.
A more recent advance in stratigraphy is time stratigraphy,
that is, the delineation of certain time-stratigraphic
units. Units divided in this way have lower and upper
boundaries that are everywhere the same age but may look
very different and be comprised of very different rock
types. Time-stratigraphic units may be identified by using
fossils known to occur only during a certain period, or by
correlating between unconformities (erosional surfaces)
that have about the same age in different places. The primary
unit of time stratigraphy is the system, which is an
interval so great that it can be recognized over the entire
planet. Most systems represent time periods of at least tens
of millions of years. Larger groups of systems are called
erathems or eras for short. Time units smaller than system
are the series, and stage, which are typically used for correlation
on a single continent or within a geographic
province.
Time Lines and Diachronous Boundaries
In many sedimentary systems, such as the continental shelf,
slope, and rise, different types of sediments are deposited in
different places at the same time. We can draw time lines
through these sequences to represent all the sediments deposited
at a given time, or to represent the old sediment/water
interference at a given time. In these types of systems, the
transition from one rock type to another, such as from the
sandy delta front to the marsh facies, will be diachronous in
time (it will have different ages in different places).
Correlation of Rocks
If a geologist has studied a stratigraphic unit or system in one
location and figured out conditions on the Earth at that point
when the rock was deposited, we may wonder how this can
be related to the rest of the planet, or simply to nearby areas.
In order to accomplish this task, the geologist first needs to
determine the relative ages of strata in a column, then estimate
the absolute ages relative to a fixed timescale. Stratigraphic
units may be correlated with each other locally using
various physical criteria, such as continuous exposure where
a formation may be recognizable over large areas. Typically, a
group of characteristics for each foundation is amassed such
that each formation can be readily distinguished from each
other formation. These include gross lithology or rock type,
mineral content, grain size, grain shape, color, or distinctive
sedimentary structures such as cross-laminations. Occasionally,
key beds with characteristics so distinctive that they are
easily recognized are used for correlating rock sections.
Most sedimentary rocks lie buried beneath the surface
layer on the Earth, and geologists and oil companies interested
in correlating different rock units have to rely on data
taken from tiny drill holes. The oil companies in particular
have developed many clever ways of correlating rocks with
distinctive (oil rich) properties. One common method is to
use well-logs, where the electrical and physical properties of
the rocks on the side of the drill hole are measured, and distinctive
patterns between different wells are correlated. This
helps the oil companies in relocating specific horizons that
may be petroleum rich.
Index fossils are those that have a wide geographic distribution,
commonly occur, and are very restricted in the time
interval in which they formed. Because the best index fossils
should be found in many environments, most are floating
organisms, which move quickly around the planet. If the
index fossil is found at a certain stratigraphic level, often its
age is well known, and it can be correlated with other rocks
of the same age.
See also MILANKOVITCH CYCLE.
stratosphere See ATMOSPHERE.
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