1. GEOLOGICAL TIME
a. Age of the Earth
In the western tradition, one of the earliest and most influential figures in the
interpretation of geological time was James Ussher (1581 - 1665), Archbishop
of Armagh. An important historical figure in his own right, Ussher also
published a chronology of Earth's history using all dates mentioned in the
Bible to establish a timeline. Using this technique, he established the first
day of creation to be October 22, 4004 BC. This date would make the Earth a
little over 6,000 years old!
Figure 1.1 James Ussher (1581 - 1665), Archbishop of Armagh. Image from
As science continued to develop during the 1700s, people started to become
dissatisfied with Ussher's estimated age for the planet. One such scientist
and notable natural historian was George Louis De Buffon (1707 - 1788).
Believing the Earth to have been initially as a hot molten mass, Buffon
heated iron spheres (which he thought was a reasonable model for the
structure of the planet) and calculated the time they took to cool. Using this
method Buffon believed the Earth to be around 75,000 years old.
Figure 1.2 George Louis De Buffon (1707 - 1788). Image from Wikipedia.
Irish Geologist John Joly published a paper in 1899 in which he estimated the
Earth's oceans (which he believed to be the same age as the planet) to be
about 90 million years old. He calculated this by estimating how long it would
take for the oceans to reach their current salinity (from an original fresh water
state) as salt is added via the erosion of minerals in rocks. (Today we
understand that the Earth's oceans have not been getting increasingly salty
with time. As such, present day salinity levels cannot be used as a gauge to
estimate the passage of geological time.)
Later in his career Joly was to work with Ernest Rutherford using radioactive
decay in minerals to estimate the age of rocks. This technique provides us with the current age of the Earth at 4.6 Ga (Giga-anum: billions of years).
Figure 1.3 Ocean water. Photo by S. Sutherland.
b. Deep Time
4.6 Ga is a vast amount of time, especially when the oldest recorded human
being was only 122 years old when he died. It is this vast amount of time that
the geologist and paleontologist must consider when trying to understand the
evolution of the Earth and its biological systems. This is often referred to as
the concept of deep time.
Analogy is often used to help people grasp the vast tracts of time that have
passed since our planet formed. If you compress all of Earth's history into
one year, these are some of the significant events in our planets history:
January 1: Earth accretes out of Solar disk
February 1: Formation of oldest rock (preserved until today)
November 15: Creatures with shells first appear
December 15: Dinosaurs evolve
December 26: Dinosaurs become extinct
December 31, 23:59:18: end of last Glaciation
December 31, 23:59:46: Birth of Christ
Figure 1.4 An artist's rendition of Geological Deep Time. Image from
2. GEOLOGICAL CONCEPTS AND
Like any science, geology and paleontology has its own terminology and
fundamental concepts. Before we can explore the evolution of Earth through
deep time, you need to become familiar with these concepts and the language we use to describe them.
Rocks and Minerals
As cells and tissues are some of the fundamental units to a biologist, so
rocks and minerals are the fundamental units to a geologist.
A mineral can be defined as "a naturally occurring crystalline solid with a
characteristic chemical composition, a highly ordered atomic structure, and
specific physical properties."
Figure 1.5 Quartz (left) and feldspar (right) are the most common minerals in
the Earth's crust. Image from Wikipedia.
A rock on the other hand is an aggregate of minerals. Rocks fall into a basic
three fold classification:
b. Metamorphic; and
a. Igneous Rocks
Igneous [audio] rocks (derived from the Latin "ignis" meaning fire) are rocks
that have formed by the cooling of magma or lava. Igneous rocks are
generally composed of interlocking crystals of varying sizes.
Figure 1.6 An image of granite [audio], an igneous rock composed of the
minerals quartz, feldspar, and mica, as seen from under a microscope.
Image from Wikipedia.
If a magma cools within the Earth's crust it is referred to as an intrusive [audio] igneous rock. This term is derived from the manner in which magmas
physically work their way through or ‘intrude' into the surrounding rock as they
rise from their point of generation. As they cool slowly they often develop
large crystals (like you commonly see in kitchen counter tops).
If a magma escapes via volcanic activity and forms a lava the igneous rock
is called an extrusive [audio] igneous rock. These rocks cool quickly and so
the crystals that form are very small and often invisible to the human eye.
Another form of extrusive igneous rock, volcanic ash, often forms as magma
fragments explosively during a volcanic eruption.
Figure 1.7 Igneous rock types and examples. Images from Wikipedia and the
US Geological Survey.
b. Metamorphic Rocks
Metamorphic [audio] rocks form as the result of the transformation of an
existing rock via heat, pressure and/or the action of fluids. Such conditions
commonly occur deep in the Earth's crust. The original rock could be
igneous, sedimentary, or an older metamorphic rock. The study of
metamorphic rocks can help provide information about the processes that
occur deep in the sub surface.
Figure 1.8 A gneiss [audio], such as this outcrop found near
Geirangerfjord, Norway, is a metamorphic rock that forms at high
temperatures and pressures deep in the Earth's crust. It is exposed
here at the surface following millions of years of erosion of the rock
that originally covered it. Image from Wikipedia.
c. Sedimentary Rocks
Sedimentary [audio] rocks are probably the most useful rocks for the
determination of Earth's ancient climate and biology. Sediments form in
response to particular environmental conditions and as such, provide clues to
Earth's past including climate, ancient geography, and life forms. Sedimentary rocks form via the sedimentation (or deposition) of materials at
the Earth's surface and within water bodies, commonly in layers sometimes
called strata[audio] or sedimentary beds. Sedimentary rocks can be
generated by physical, chemical, and/or biological processes. Diagenesis
[audio] is a term used to refer to the transformation of sediments into a
sedimentary rock via a collective variety of chemical, physical, and
sometimes biological processes. Diagenesis commonly occurs as sediments
are buried below successively younger strata.
Figure 1.9 Layers of red sandstone, a sedimentary rock, found in Lower
Antelope Canyon, Arizona. Image from Wikipedia.
d. Clastic Sedimentary Rocks
Most sedimentary rocks are produced by the erosion of pre-existing rocks
producing fragments or grains that are transported and deposited at various
distances from the site of erosion. Such rocks are called clastic [audio]
sedimentary rocks. Clastic sediments can be deposited anywhere they can
settle out of by gravity from the media that is transporting them (water or air).
The largest, but by no means exclusive, depositional area (or basin) for
sediments are the oceans.
Figure 1.10 Examples of environments where sediments are being deposited.
Click on 'Zoom-in' for a more detailed view. Background image by G. Lascu,
foreground image from the US Geological Survey.
Part of the aim of this course is to provide some insight into how we can
interpret ancient processes and environments. Clastic sedimentary rocks
provide a simple tool that can be used to determine the source, origin, and
length of transport of a particular sediment prior to it becoming a sedimentary
rock. The composition of the grains or the larger fragments (clasts [ audio]) of
a sediment can help in the determination of the original source of that
sediment and perhaps the pattern of ancient drainage and river systems.
If a sediment is deposited close to the rocks from which it was originally
eroded, it will have number of characteristics that will uniquely identify it as
such. It would probably contain a large number of coarse/angular grains and clasts (greater than 4 mm) as the process of transport has not had sufficient
time to fragment and round off the particles in the sediment.
The sediment would also tend to have a higher proportion of unstable
minerals and fragments of rock. Quartz is the most stable mineral at surface
temperatures and pressures. Other minerals eroded from older rocks are
destroyed with long-term transport.
In addition, the rock would generally exhibit poor sorting. Sorting refers to the
variation in clast/grain sizes in a sediment, which is often large in sediments
that have not traveled far from their original source. A sediment with
characteristics as described above is called an immature sediment.
Given this, a clastic sediment that has undergone extensive transport (for
example down a long river system or blown around a desert for many years or
washed continually back and forth across a shallow ocean) will tend to be fine
grained and composed mostly of well-rounded and well-sorted quartz grains.
A sediment with these characteristics is called a mature sediment.
Figure 1.11 Correlating sediment maturity and transport distance. Images
from Wikipedia and Northern Virginia Community College.
e. Calcium Carbonate
Although clastic sedimentary rocks form the majority of the sedimentary
rocks on the Earth's surface, biologically precipitated sediments are also an
important source of sedimentary rocks. The most obvious is the precipitation
of calcium carbonate by various creatures including corals and mollusks to
form thick deposits of limestone.
Figure 1.12 Satellite picture of the Atafu coral atoll in Tokelau in the Pacific
Ocean and a fossil coral (inset). Images from Wikipedia.
Equally important are microplankton calcium carbonate producers such as
coccolithophores. During the Cretaceous (145 - 65 Mya, million years ago),
warm shallow tropical oceans covered much of the Earth's continents
providing perfect conditions for the proliferation of coccolithophores and ultimately the generation of vast thicknesses of the fine chalk (a fine grained
Figure 1.13 The famed White Cliffs of Dover in SE England is almost
100% coccolithophore-produced chalk, another type of calcium
carbonate rock. Inset: a coccolithophore, about 2 µm (micrometres
or microns) in diameter. Images from Wikipedia.
In addition to clastic and biologically precipitated sediments, the intense
evaporation of water can also precipitate sediments in the form of salt
crystals. Rocks formed in this manner are called evaporites [audio].
Evaporites can form in a number of ways including the evaporation of an
inland sea or part of an ocean with restricted contact to the wider ocean. This
may occur in arid areas with limited freshwater input. As the salinity of the
water increases, crystals of evaporite minerals such as halite [ audio] (table
salt) and gypsum [audio] crystallize out of the water body and settle in layers
on the ocean floor.
Evaporite formation can occur on a vast scale. The Mediterranean Sea for
example, has on occasion been closed off at the Strait of Gibraltar and the
entire sea has evaporated away. This accounts for the thick deposits of salt
that can be found just below the ocean floor of the Mediterranean.
Figure 1.14 (top) The process of evaporite formation in a shallow
ocean basin, such as the Mediterranean Sea. (bottom) Salts
precipitating around the edge of the Dead Sea, the deepest
hypersaline lake in the world. Images from Wikipedia.
3. STRATIGRAPHY Now that we have a basic understanding of rocks and minerals we can now
consider how geological features (and in particular sedimentary rocks) are
arranged in the real world. This is the science of Stratigraphy [audio].
Stratigraphy studies how rock layers (beds or strata) are arranged and is a
key concept in the interpretation of the passage of geological time. We will
examine 4 Basic Principles of Stratigraphy:
a. Principle of Superposition
b. Principle of Original Horizontality
c. Principle of Lateral Continuity
d. Cross-Cutting Relationships
The first three of these principles were outlined by Nicholas Steno, a Danish
anatomist and geologist, in the late 1600s.
Figure 1.15 Nicholas Steno (1638 - 1686), a Danish pioneer in anatomy and
geology. Image from Wikipedia.
a. Principle of Superposition
The Principle of Superposition states that in layered strata (sedimentary
rocks or lava flows), the oldest layer will be at the bottom of the exposed
strata and the youngest at the top. Effectively " what's on top is youngest”.
Although this might appear to be somewhat obvious it is important to
remember that this principle was being introduced to a world that believed
that everything around us had been created over a period of days. To suggest
that a vast pile of strata had not simply been called into existence but had
developed over a very long period of time was a new paradigm.
Figure 1.16 Strata in Isfjorden, Svalbard, Norway. Image from Wikipedia.
b. Principle of Original Horizontality
According to the Principle of Original Horizontality , sediments are
deposited horizontally. After they have been transformed into rock it is
possible for these strata to become tilted by various tectonic movements. Basically, " tilted or folded layers used to be flat”.
Figure 1.17 Dipping (tilted) chalk strata, Cyprus. Image from Wikipedia.
c. Principle of Lateral Continuity
The Principle of Lateral Continuity states that a stratum (layer) of
sedimentary rock will continue in all directions until it thins, grades into
another type of sediment, or comes against the edge of the depression into
which it is being deposited.
Figure 1.18 A schematic drawing that shows an application of the Principle of
Lateral Continuity in assessing the evolution of the present (bottom) from its
d. Cross-cutting Relationships
The principle of Cross-cutting Relationshipswas developed by James
Hutton, a Scottish geologist, physician, chemist, and experimental farmer.
Hutton believed that the relationships between stratigraphic units indicated
that Earth was very old. The famous geological section that Hutton used to
demonstrate this is Siccar Point in Scotland. We will discuss this next.
e. James Hutton at Siccar Point, Scotland
At Siccar Point, Scotland you can see vertical sediments cross-cutby an
erosion surface called an unconformity [audio]. On top of this surface
younger sediments have been deposited. See figure below.
Hutton realized that this relationship of stratigraphic units could have only
developed as a result of Earth processes operating over many millions of
years. The character of the rocks on either side of the unconformity also
speak to the vast changes in the environment that can occur over any one
given point on the Earth's surface. At Siccar Point, the rocks below the
unconformity were originally deposited in a deep ocean but the rocks now on
top of them were deposited in a hot desert on land! Figure 1.20 Siccar Point, Scotland, where James Hutton and his
colleagues found strong evidence that confirmed his ideas about
unconformities and the relationship between the rock layers. Image
from Wikipedia , annotations by S. Sutherland.
With knowledge of the 4 basic stratigraphic principles, we can now unravel
the history of a geological section such as Siccar Point (refer to figure below):
i. Sediments below the unconformity originally deposited
horizontally in a deep ocean (Principle of Original
ii. Sediments buried, transformed into rock via diagenesis,
tilted, and folded by tectonic activity;
iii. Rocks uplifted by tectonic activity and exposed at the
surface by erosion producing an erosion surface;
iv. Desert sediments deposited horizontally on top of the
erosion surface producing the unconformity (Cross-cutting
Relationships and Principle of Superposition);
v. Section buried again and desert sediments transformed
into rock via diagenesis;
vi. Section tilted again by tectonic activity;
vii. Section uplifted by tectonic activity and exposed at the
surface by erosion.
Figure 1.21 The "geological history" of Siccar Point, Scotland.
Unconformities represent periods of non-deposition of sediment or active
erosion of strata. They help us appreciate that the geological record in any
one location is NOT complete but contains gaps. Unconformities may
represent important periods of activity in Earth history such as mountain building events where strata are being actively uplifted and eroded.
Figure 1.22 Schematic of selected types of unconformity.
Several types of unconformity are recognized (refer to figure above):
i. Disconformity: exists where the layers above and below
an erosional boundary have the same orientation
ii. Nonconformity: develops where sediments are deposited
on top of an eroded surface of igneous or metamorphic
iii. Paraconformity: strata on either side of the unconformity
are parallel, there is little apparent erosion
iv. Angular unconformity: strata is deposited on tilted and
eroded layers (such as at Siccar Point)
Here are some examples of unconformities from around the world.
Figure 1.23 Taum Sauk Precambrian - Cambrian nonconformity
indicating a 1 Ga gap in the geological record. Image fromWikipedia,
annotations by S. Sutherland.
Figure 1.24 Permian - Cretaceous disconformity, Texas showing a
165 Ma (million years) gap in the geological record. Image from
Wikipedia, annotations by S. Sutherland. Figure 1.25 Angular unconformity between the folded Entrada
Formation (Middle Jurassic) and overlying Morrison Formation (Late
Jurassic) strata, Utah. Image copyright © by Thomas McGuire,
made available via the Earth Science World Image Bank.
Unconformities are just one type of cross-cutting relationship. Intrusive
igneous rocks may also demonstrate cross-cutting relationships in a
geological outcrop indicating how they physically intruded into pre-existing
Figure 1.26 Igneous dike cross-cutting horizontal strata, Grand Canyon,
Faults (planes along which movement have occurred) also demonstrate
cross-cutting relationships with the fault disrupting the strata it cuts through.
Figure 1.27 Faulted Rocks, Guatemala. Photo courtesy of the
Geology and Geophysics Science Centre, US Geological Survey .
h. Relative Dating
Working out the history of a geological section in the manner above is called
relative dating. Relative dating in this instance does not provide us with any
information of the actual age of this section (for example, in millions of
years). Rather it provides us with the relative sequence of events that led to
Relative dating can be used to provide a roughguide as to the amount of
geological time that may have passed. For example: if in general, muddy
sediments accumulate at a rate of 10 m (metres) per million years, then it
can be surmised that 100 m of mudstone will take 10 million years to accumulate.
Obviously there are inherent errors with this estimate:
• The rate of accumulation of sediment might not be constant;
• Periods of non-deposition or active erosion may have occurred;
• During diagenesis, sediments are compressed and compacted so that
the vertical extent of sedimentary rocks may not represent the original
vertical extent of sediment.
Although this method of calculating geological time would be useless for
relatively most outcrops of stratigraphy, the errors tend to average
themselves out with large packages of sediment and may provide a very
rough estimate of geological time. Although more accurate or "absolute"
dates for Earth time would have to wait for the development of our
understanding of radioactive [audio] or radiometric [audio] dating, the
stratigraphic principles detailed in this lesson can be used extensively in the
unraveling of the Earth's geological history.
Figure 1.29 The process of deducing the history of a succession of
sediments, such as that shown above, using relative dating, would
be a very complex task. Photo courtesy of Ian West, as published in
"Geology of the Wessex Coast of Southern England".
With their understanding of stratigraphy, geologists in the late 1700s started
to get the impression that the Earth was much older than Ussher's estimate
of 6,000 years. Geologists started to see the world through the eyes of
James Hutton who believed that the Earth's history as recorded in the rocks
could be interpreted by observing the slow methodical process of erosion and
deposition that we see occurring around us every day.
In short, the statement " The present is the key to the (understanding of the)
past", a summary of Hutton's Uniformitarianism as it became known, formed
the basis for the understanding of the Earth and of the science of modern
geology. Uniformitarianism [audio] is neatly summarized by the rock cycle
(see below), which describes the slow dynamic transitions through geological
time of the three main categories of rock. Note however, that the rock cycle does not imply that every rock must pass through every stage represented in
Figure 1.30 The Rock Cycle describes the slow dynamic transitions through
geological time of the three main categories of rock. Figure by G. Lascu.
Today the term actualism is commonly used, a term that acknowledges that
although most geological processes (like the uplift or erosion of mountains)
are very slow, some geological processes (such as volcanic eruptions or the
impacts of meteorites) can cause changes that are relatively geologically
Figure 1.31 Charles Lyell (1797 - 1875 ), an influential geologist and
close friend of Charles Darwin. His insights into cross-cutting
relationships and geological time were extremely important in our
modern understanding of Earth history. Image from Wikipedia.
1. SEDIMENTARY FACIES
Figure 2.1 The Badlands-Drumheller, Alberta. Image from Wikipedia.
Different types of sediments are being deposited in different areas due to
differing conditions of deposition. Areas where sediments are being deposited
are often called sedimentary environments. Sedimentary environments can
be found all over the surface of the planet including in high mountains, along
river plains and lakes, in hot arid deserts, along coast lines, and in the deep
ocean. Each of these sedimentary environments will be characterized in the
geological record by particular groups and association of sediments.
a. Near Shore Sedimentary Environment Figure 2.2 A simplified diagrammatic representation of a near shore
sedimentary environment. Figure by S. Sutherland.
Consider an example of a near shore sedimentary environment where a river
meets the ocean and starts to deposit its sediment load. The coarsest (most
dense) portion of the sediment from the river is deposited close to the
shoreline where the water is the most turbulent. The finer (least dense)
material stays in suspension until it settles out of the water where conditions
are less turbulent ("quieter").
Beyond the area where all the material from the river has been deposited,
biological processes by calcium carbonate-secreting organisms provide the
main sediment type: carbonate mud. This accounts for the changing lateral
character of the sediments in this environment from coarse sandy material
close to the shore line passing into mud/clay and ultimately carbonate mud
further off shore.
Figure 2.3 Figure by S. Sutherland. Inset image of coccolithophore
In the cross section of a near shore environment (see figure above) we see
the same pattern of deposition from coarse to fine clastic sediments and
eventually carbonates, once all the clastic material has settled out of the
water column. As you can see, changing environmental conditions
(particularly the turbidity of water in this case) across this sedimentary
environment are controlling the character of the sediments that are being
deposited laterally. By understanding the controls on sediment deposition, a
geologist examining sedimentary rocks can interpret and reconstruct ancient
sedimentary environments and so build a picture of the planet as it existed
many millions of years ago.
These environmentally controlled sediment differences are called facies
A facies refers to all of the characteristics that can be used to define a
particular sedimentary unit. These characteristics are: sediment type (e.g.
sandstone, mudstone, limestone), presence of any sedimentary structures (example: ripples, cross bedding), and sometimes the fossil content.
Since depositional environments grade laterally into other environment, facies
changes are gradual. Facies are given names that attempt to represent the
defining characteristics of a particular unit. For example, the facies pictured
below could be described as fossiliferous mudstone facies (1), cross-bedded
sandstone facies (2), and rippled siltstone facies (3).
Figure 2.4 Examples of sedimentary facies. Images from Wikipedia.
b. The Temporal (time) - Sea Level Component
The naming model discussed previously however, gets a little more
complicated when we have to consider geological time. This is because
over time, the distribution of sedimentary facies is controlled by changes in
sea level. As shown in the plot below, sea level has changed globally many
times – it is not just a recent phenomenon. It is also important to remember
that sea level can change locally as well as globally.
Figure 2.5 Global sea level change over the past 542 million years.
Image created by Robert A. Rohde, Global Warming Art.
Scenario 1: No Sea Level Change
Let's take the same simplified model of a nearshore environment shown
earlier, and try to determine what would happen to the pattern of facies we
would record as sea level changes over time.
Consider a first case scenario where the rate that sediments are deposited
keeps pace with changes in sea level. This means that facies get built on top
of each other, producing a pattern where facies boundaries and the location of
the shoreline will not change location over time. Figure 2.6 Stacking of sediments with no relative change in sea
level. Figure by S. Sutherland.
If we were to extract a core of sediments from the area at A' – A (see figure
above), a sedimentary core would contain the same facies from the bottom to
the top of the core. As the sedimentary facies are the same over time from
the bottom of the core (oldest) to the top of the core (youngest), then the
environmental conditions (sea level) must have been the same over time. A
diagram of the core is shown below.
Figure 2.7 Core of sediment taken from A' – A in previous diagram
for the case where the rate that sediments are deposited keeps pace
with changes in sea level. Figure by S. Sutherland.
Scenario 2: Rising Sea Level - Transgression
Now, let us consider what would happen if a near shore sedimentary
environment experiences a rise in sea level. At a certain Time T1, sediments
are deposited into the near shore as shown in the figure below.
Figure 2.8 Facies changes with increasing sea level at Time 1 (T1). Figure by
Now with a rise in sea level between Time 1 and Time 2, the shore line
moves back towards the land. A new package of sediment (between T1 and
T2 on the diagram below) has been deposited. Note that the boundaries
between the facies also get pushed back towards the land as they follow the
changing environmental conditions. This movement of the ocean onto the
land in this manner is called a marine transgression [audio]; facies move
towards the land.
Figure 2.9 Facies changes with increasing sea level at Time 2 (T2). Figure by
S. Sutherland. Sea level continues to rise with the deposition of the sediment between T2
and T3 on the diagram below. The shore line and the boundaries between
facies continue to move towards the land.
Figure 2.10 Facies changes with increasing sea level at Time 3 (T3). Figure
by S. Sutherland.
Sea level continues to rise with the deposition of sediments between T3 and
T4 (figure below). We can now see that facies are describing a particular
pattern in response to sea level change.
Figure 2.11 Facies changes with increasing sea level at Time 4 (T4). Figure
by S. Sutherland.
NOTE: if we were to map the same sediment type (i.e. facies; yellow line in
figure above) we would becross-cuttingthe Time lines (red lines at T1, T2,
T3, T4) and would not be following the same time plane. In effect, we would
be mapping the change in environmental conditions over time as
demonstrated by the facies. Because of this characteristic, facies are often
referred to as diachronous (or time crossing).
If we were to follow the same time horizon (for example the package of
sediment deposited between times T3 and T4), we would not remain in the
same facies as we move laterally from the shore line but would pass into
differing facies representing sediments deposited at the same time but under
differing environmental conditions.
These concepts are extremely important to consider when attempting to
correlate rocks of the same geological age across large distances. This will
be covered in the next lesson.
Figure 2.12 Core of sediment taken from A' – A in previous diagram
for the case where the near shore sedimentary environment
experiences a rise in sea level. Figure by S. Sutherland. Now if we were to extract a core of sediment from A' – A in the previous
diagram (see figure above), it would have recorded changes in facies
vertically. This is because over time, facies have been migrating in space
over this location in response to sea level change. Note that the bottom part
of the core (older part) has near shore sediments while the top part (younger
part) has offshore carbonate muds. This means that over time, sea level
must have beenincreasing and a transgression must have occurred.
Scenario 3: Falling Sea Level - Regression
In the case when sea level falls over time, a different pattern of facies will
occur. Shoreline and facies move away from the land – this is called a
Figure 2.13 Facies changes recording sea level fall. Figure by S. Sutherland.
If we were to extract a core of sediment from A' – A in the previous diagram,
it would have recorded a vertical pattern of sediments that is the opposite of
that recorded for a marine transgression. The core (see figure below) goes
from offshore carbonate muds at the bottom to near shore sediments at the
top, which means that over time,sea level must have been decreasing and a
regression must have occurred.
Figure 2.14 Core of sediment taken from A' – A in previous diagram
for the case where the near shore sedimentary environment
experiences a fall in sea level. Figure by S. Sutherland.
Figure 2.16 Facies changes during marine transgression and regression.
The figure above summarizes the concepts we just learned in this section: i. Facies are distributed based on the changing conditions of deposition over
the surface of our planet. In the example discussed here, shallow high
energy, near shore conditions are characterized by sand while deeper, quieter
conditions are characterized by finer grained sediments.
ii. As conditions change (such as sea level) facies will appear to migrate,
"following" the particular environmental conditions under which they are
iii. Over time this leads to patterns of facies as one facies migrates over
This means that when we recover a vertical succession of rock like the one
we used in our example we can deduce a couple of things...
• As we go from coarse, near-shore facies at the bottom of the core to
off-shore, deeper water carbonate sediments at the top of the core, sea
level (in this example) must have been rising over time.
• The facies that we are looking at in a vertical sense have lateral
equivalents that were deposited in different environments but at the
same time as the observed core. With this information (and if we have
access to more than one core) we can predict lateral changes in facies
from a vertical core of rock and test our prediction by finding their lateral
Figure 2.17 Lateral changes in facies. The uneven boundaries
between the facies are an attempt to demonstrate that facies
gradeinto each other just as environments grade into each other.
Figure by S. Sutherland.
All this is summed up in Walther's Law:
"Facies that occur in a conformable (i.e. demonstrating no unconformities)
vertical succession of strata
were deposited in laterally adjacent depositional environments."
in other words, "Adjacent sedimentary environments (facies) will end up
overlapping one another over time". Figure 2.18 Facies changes in a near shore sedimentary environment.
It is important to realize that changing sea level will not just control the
distribution of sediments in a near shore environment. Changing sea level and
the changing position of shore lines will have a major effect on sedimentary
facies from the tops of the mountains to the deepest oceans.
Consider times of very high sea level as existed during the Cretace