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.

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

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.

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.

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.

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.)

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:

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.

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.

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.

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

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.

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.)

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.

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

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

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.

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.

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

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