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Stratigraphy and Lithological Correlation
Laboratory - Theory
In this lab you will learn about sequences of sedimentary rocks
and how they may
be correlated, or traced between outcrops. Ideally, the rocks
may be correlated
directly by walking along the contacts between adjacent rock
units, across the
countryside. This is seldom the case, however, particularly
where vegetation and
soil cover make rock exposures poor (as in humid areas, such as
the eastern
United States). In other situations, geologists are interested
in beds deep below
the Earth's surface, and they must use drill hole and core data
to correlate the
rocks.
Geologists study rocks in outcrops (natural or man-made
exposures such as road
cuts or quarries), or in drill cores. When studying an outcrop
of sedimentary rock,
the most obvious feature is bedding (also called strata or
layers). Although the
rocks may be tilted or folded, the sediments were originally
laid down in horizontal
beds, which extended as continuous layers in all directions
(such as a layer of mud
on the sea floor), with the oldest layers on the bottom and the
youngest layers on
top. A sequence of sedimentary rocks may be divided up into a
number of
lithostratigraphic units of various sizes.
Providence Canyon, Georgia. Photo by Pamela Gore.
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STRATIGRAPHY
Walking along cliffs that were cut by ocean waves or rivers have
always made
humans wonder how they were formed. The time required to create
such majestic towers has created serious debate, both scientific
and religious.
Geologic time was very difficult for scientists to "discover. It
was not until the mid 18th century that James Hutton, a Scottish
geologist, realized that the Earth was many millions of years old.
This was an unimaginable idea because people in his
day believed the Earth was only a few thousand years old.
Hutton tried to develop scientific methods to determine the time
required for every
day geologic processes and compare with the past. For example he
tried to calculate mud accumulating in the ocean today, to figure
out how much time had
passed since the formation of the Earth. He used the term
uniformitarism to compare the present day rock cycle with the past
rock cycle. From these comparisons you can interpret how rock
layers or strata were formed but not the
length of time. You can determine which stratum is younger or
older, just by the position of the strata.
Since most rocks on the surface of the Earth are sedimentary,
early geologists used them to look for answers to the age of the
Earth. The birth of stratigraphy
has its roots in scientists trying to determine the age of the
Earth. They made simple predictions by looking at sedimentary
processes going on today.
Geologists started to realize that you can trace certain strata
by comparing the
fossils that it contains. The use of fossils became an important
tool to unravel the history of the Earth.
Nicholas Steno, a Danish physician living in Italy in 1669
proposed that the Earths strata accumulated with three basic
principles. Steno pointed out obvious, but overlooked principles of
sediment accumulation. They included the Principle of
Original Horizontality, Principle of Superposition, and
Principle of Original Continuity. If sediments accumulate in a
large basin, the laws of gravity will
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deposit the beds, horizontal to the surface of the Earth. Beds
can pinch out along the sides of the basin as in the figure
below.
The Principle of Superposition states that in a sequence of
sedimentary rock
layers, the bottom layers are older than the top layers. The
bottom layers were deposited first. In the figure below A is the
oldest bed and G is the youngest.
The Principal of Original Continuity states that the beds can be
traced over a long
interval if the basins were open. For instance, Bed F can be
traced continuously to the smaller basin in the figure below. The
other beds below F can then be
correlated to Beds A-E.
The Principle of Faunal Succession was later added by William
Smith in the late 1700's who observed and studied fossils embedded
in rock layers. This principle
states that the oldest fossils in a series of sedimentary rock
layers will be found in the lowest layer (layer A). Progressively
younger fossils occur in higher layers
(layer B). This is the same concept as superposition, but it
helped geologists realize that you can look at the age of these
layers and assign relative dates. This
parallels evolution. Younger organisms replace older organisms
as the older ones become extinct.
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Since organisms change through time, it allows correlation of
beds far apart. If the
layers have similar fossils, one can deduce that the layers are
the same age.
The principles of stratigraphy help to develop a sequence of
rock layers. In the figure below, the oldest rocks are on the
bottom (sandstones). The sandstones
represent rocks deposited in a shallow marine environment. The
younger rocks reveal an environment change into a tidal area.
Through time the tidal area
evolves into a lagoon and then a swamp.
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The sequence provides information on changing environments
through time. Then you can determine the sequences in other places
and then correlate one rock type
with another.
Stratigraphy is important to understand events that happened
over time and over a large area, However, to interpret these events
you require slices of rocks
through time commonly referred to as or cores. Ships can core
rock layers from the ocean bottom. Cores would be taken at
intervals that can help us correlate and
interpret how the rocks were laid down.
In the figure of cores, each core represents a slice of the
Earth. In A, the green shells are the oldest and the blue seastars
are the youngest. You can see that as
you go from cores A to D the fauna adds snails to the region. A
stratigrapher would determine what caused this sequence of events.
Stratigraphers also look at
the rocks, the fossils, and other evidence to make these
conclusions.
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LITHOSTRATIGRAPHIC UNITS
A lithostratigraphic unit is defined as a body of sedimentary,
extrusive igneous,
metasedimentary, or metavolcanic strata which is distinguished
on the basis of
lithologic characteristics and stratigraphic position (position
in the rock sequence).
The smallest lithostratigraphic rock unit is the bed.
A formation is a set of similar beds, and formations are the
fundamental
units of stratigraphy.
By definition, formations are:
1. Lithologically homogeneous (all beds are the same rock type
or a distinctive set
of interbedded rock types).
2. Distinct and different from adjacent rock units above and
below.
3. Traceable from exposure to exposure, and of sufficient
thickness to be
mappable (formations are commonly hundreds of feet thick, but
may be thinner or
thicker).
4. Formations must have names. Formations are usually named for
some
geographic locality where they are particularly well exposed.
(This locality is
referred to as the type section.) If the beds are dominated by a
single rock type,
this may appear in the name. (Also, to be valid, the name of a
formation must be
published in the geological literature.)
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Subdivisions within formations are called members. Members also
have names. A
formation, however, does not have to contain members. Members
may be
designated to single out units of special interest or economic
value, such as coal
beds or volcanic ash layers.
The lithostratigraphic terms (bed, member, formation, and group)
refer to
sedimentary, volcanic, metasedimentary, and metavolcanic rocks
only. Intrusive
and highly deformed and metamorphosed rocks are called
lithodemic units.
The rocks in the Piedmont (which includes the Atlanta area) are
mostly
metamorphic and intrusive igneous rocks, and should therefore be
called
lithodemic units. The fundamental lithodemic unit is the
lithodeme (roughly
equivalent to a formation). The term "formation" should not be
used for intrusive
and metamorphic rock (according to the North American
Stratigraphic Code of
1983, revised 2004) http://www.agiweb.org/nacsn/code2.html.
STRATIGRAPHIC SECTIONS
Geologists study sequences of sedimentary rocks on a bed-by-bed
basis. They
measure the thickness of each bed, record the physical,
chemical, and biological
characteristics of the rock, and note the nature of the contacts
(or bedding
planes) between beds. Using these data, the geologist draws up a
stratigraphic
section for a particular sequence of rock. A stratigraphic
section is a graphical
or pictorial representation of the sequence of rock units.
Standard symbols
(called lithologic symbols) are used to refer to each rock
type:
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DRAWING A STRATIGRAPHIC SECTION
To draw a stratigraphic section, you must have data from a
sequence of rocks. You
will need to have data on the thickness of each bed, and all of
the physical,
chemical, and biological characteristics of that bed, as well as
the character of its
contacts. Before you start, you need to examine your data to
determine the total
thickness of the section you plan to draw. Then, determine a
proper scale so that
the entire section will fit on your paper (such as, 1" =
100').
Draw a vertical column in which you will plot your data, and
then mark off the
thickness of each bed or formation using the scale you
established. Draw in the
contacts between units; if the contacts are erosional, you
should use a wavy line.
Once you have drawn in contacts, draw in the lithologic symbols
for each unit.
Information on fossils and sedimentary structures, etc. may be
placed within the
unit, or beside it using a special symbol or small sketch. Color
may be illustrated
with a special symbol, or by coloring your section. There are
standard symbols
which have been established by oil companies and core logging
companies. You
may use theirs (see a reference book), or create your own. Your
instructor may
give you further instruction on this.
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Once you have drawn several stratigraphic sections for an area,
you may begin to
correlate them.
LITHOLOGIC CORRELATION
Geologists can draw stratigraphic sections for several outcrops
(or cores) in an
area, and then trace beds from one section to another. This is
called lithologic
correlation. Basically, correlation demonstrates the equivalency
of rock units
across an area. The sections being correlated may be miles
apart. Basically, a
correlation is a hypothesis that units in two widely separated
sequences are
equivalent. Clearly, the more unique characteristics that two
sections share, the
greater the probability three is that the correlation is
correct.
Illustration of lithologic correlation
Correlation may be performed in several ways. Distinctive beds
(called key beds or
marker beds), distinctive sequences of beds, bed thicknesses,
and unconformities
may be traced between sections.
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Key beds or marker beds tend to have some unusual,
distinguishing feature
which allows them to be readily identified, such as a bed of
volcanic ash in a
sedimentary sequence, or a bed of conglomerate in a sandstone
sequence, or a
bed of fossil shells or bones, or a bed of limestone in a shale
sequence. Key beds
or marker beds should also be laterally extensive, to aid in
correlation over a large
area.
Distinctive sequences of beds are also useful in correlation.
For example, the
sequence "limestone - dolostone - limestone" may be found within
a thick unit of
shales and siltstones, and correlated between sections.
In some cases, beds can be correlated between sections based on
their
thicknesses. One of the best examples of this is the correlation
of laminations in
cores from the evaporites of the Castile Formation in the
Permian of western Texas
and New Mexico. Cores were drilled about 9 miles apart, and the
thickness of the
laminations matches almost exactly.
UNCONFORMITIES
Sometimes, one or more rock units are missing from the middle of
a sequence.
Close examination of the outcrop shows a sharp or irregular
contact where the
missing rocks should be. This contact is called an unconformity.
Unconformities
are surfaces which represents a gap in the geologic record,
because of either
erosion or non-deposition. Unconformities can be traced between
stratigraphic
sequences miles apart. Although unconformities may truncate
rocks of many
different ages, the sediments directly overlying the
unconformity are roughly the
same age.
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Illustration of an unconformity causing beds to be missing from
a sequence
There are four basic types of unconformities:
1. Angular unconformities
2. Nonconformities
3. Disconformities
4. Paraconformities
1. ANGULAR UNCONFORMITIES
Angular unconformities are characterized by an erosional surface
which truncates
folded or dipping (tilted) strata. Overlying strata are
deposited basically parallel
with the erosion surface.
Angular unconformities
2. NONCONFORMITIES
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Nonconformities are characterized by an erosional surface which
truncates igneous
or metamorphic rocks.
Nonconformities
Photo of a nonconformity, with rounded gravel overlying
weathered metamorphic
rock. Route 52 near Ellijay, GA. Photo by Pamela Gore.
3. DISCONFORMITIES
Disconformities are characterized by an irregular erosional
surface which truncates
flat-lying sedimentary rocks.
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Disconformities
4. PARACONFORMITIES
Paraconformities are characterized by a surface of
non-deposition separating two
parallel units of sedimentary rock, which is virtually
indistinguishable from a sharp
conformable contact; there is no obvious evidence of erosion. An
examination of
the fossils shows that there is a considerable time gap
represented by the surface.
Paraconformity
SEDIMENTARY FACIES
A facies is a unit of sedimentary rock deposited in a particular
sedimentary
environment. A facies has distinctive physical, chemical, and
biological
characteristics which serve as clues that help the geologist to
interpret the
environment in which the rock was deposited. (Examples of
sedimentary
environments include beaches, rivers, lakes, deserts, alluvial
fans, deltas, reefs,
lagoons, tidal flats, etc.)
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You might refer to a red sandstone facies, or a mud cracked
limestone facies, or a
fossiliferous black shale facies.
LATERAL FACIES CHANGES
Beds may change laterally in thickness or in rock type, as a
result of differences in
the sedimentation rate, or environment of deposition. In these
cases, a bed of
rock may be in the same position in the sequence, but it is
somewhat different in
thickness or rock type. For example, a lateral change in rock
type is caused by a
lateral change in depositional environment; you could envision
the deposits of a
river passing laterally into the deposits of a floodplain, or
possibly a delta. Or, you
could envision beach sands passing laterally into deeper water
silts, muds, and
clays.
Illustration of lateral changes in bed thickness
In some cases, a bed thins progressively in one direction until
it pinches out. A
pinch-out may or may not be accompanied by the increase in
thickness of an
adjacent unit. In some case, the entire sedimentary section
thins in a certain
direction.
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Illustration of pinch-out of a limestone bed and a sandstone
bed, with three
sections drawn to show the different facies that would be
present in each.
WALTHER'S LAW AND VERTICAL FACIES CHANGES
The sedimentary sequence seen in outcrops is the result of
different types of
sediment being deposited in different sedimentary environments
over time,
producing a vertical sequence of different facies.
Lateral changes in facies are relatively easy to understand.
Vertical facies changes
may initially be somewhat puzzling. How does one layer of
sedimentary rock come
to overlie another? The vertical relationships between facies
are explained by
changes in sea level, or changes in subsidence and sedimentation
rates.
As laterally-adjacent sedimentary environments shift back and
forth through time,
as a result of sea level change, facies boundaries also shift
back and forth. Given
enough time, facies which were once laterally adjacent will
shift so that the
deposits of one environment come to overlie those of an
adjacent
environment. In fact, this is how many (if not most) vertical
sequences of
sedimentary rocks were formed. This concept was first stated by
Johannes Walther
in 1894, and is called Walther's Law. Basically, in a
conformable sedimentary
sequence (i.e., one without unconformities), sedimentary units
which lie in vertical
succession represent the deposits of laterally adjacent
sedimentary environments
migrating over one another through time.
At any one time, sediment of different types is being deposited
in different places.
Sand is deposited on the beach, silt is deposited offshore, clay
is deposited in
deeper water, and carbonate sediment is deposited far from shore
(or where there
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is little or no input of terrigenous sediment). Sedimentary
environments (and
facies) move as sea level changes, or as a basin fills with
sediment.
Distribution of sedimentary facies
A sea level rise is called a transgression. A transgression will
produce a vertical
sequence of facies representing progressively deeper water
environments (a
deepening-upward sequence). As a result, a transgressive
sequence will have
finer-grained facies overlying coarser-grained facies
(fining-upward from sand at
the bottom, and then to silt, and then to shale). Transgressions
can be caused by
melting of polar ice caps, displacement of ocean water by
undersea volcanism, or
by localized sinking or subsidence of the land in coastal
areas.
Transgressive Sequence
A sea level drop is called a regression. A regression will
produce a sequence of
facies representing progressively shallower water environments
(shallowing-
upward sequence). As a result, a regressive sequence will have
coarser-grained
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facies overlying finer-grained facies (coarsening-upward).
Regression can be
caused by a buildup of ice in the polar ice caps, or localized
uplift of the land in
coastal areas.
Regressive Sequence
We can easily see how transgressive and regressive sequences
form. First, start
with this basic situation:
Illustration of the formation of REGRESSIVE (A - D) and
TRANSGRESSIVE (E - G)
sequences.
Assume that sea level drops and the beach moves seaward.
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Repeat for another sea level drop.
Now notice how the facies have migrated to keep their proper
position relative to
sea level.
Now begin again with the same basic situation.
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Assume that sea level rises and the beach moves landward.
Repeat for another sea level rise.
Now notice again how the facies have migrated to keep their
proper position
relative to sea level.
The figure below illustrates a transgression followed by a
regression, or a
transgressive-regressive sequence. The part of the record
deposited during
the transgression is marked by an arrow labeled "T", and the
part deposited during
the regression is marked by an arrow labeled "R". Four facies
are shown: a
sandstone facies, a siltstone facies, a shale facies, and a
limestone facies. Note
that the facies pattern produces a broad V shape in vertical
section.
Three "time lines" are shown. (In the geologic record, a "time
line" could be
represented by a thin volcanic ash bed, representing one
particular eruption
event.) Note that the lithologic units cut across the time
lines. The facies are time-
transgressive or diachronous. Note that the time line marked
"Time 2" bisects
the V shape of the transgressive-regressive sequence. The point
of sea level high
stand (maximum transgression) in a transgressive-regressive
sequence is always
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a time line, marking the time of maximum transgression.
(Similarly, the point of
sea level low stand (maximum regression) in a
transgressive-regressive sequence
is always a time line, marking the time of maximum
regression.)
Transgressive-regressive sequence