STRATIGRAPHIC CORRELATION, FOSSILS, FACIES, AND SEA LEVEL CHANGE
Correlation of Strata
The need to classify and organize rock layers
according to relative age led to the geologic discipline of stratigraphy.
Rocks at different locations on Earth give different
"snapshots" of the geologic time column. At a particular location, the
rocks never fully represent the entire geologic rock column due to extensive
erosion or periods of non-deposition or erosion.
The thickness of a particular rock layer (representing
a particular time period) will vary from one location to another or even
The process that stratigraphers use to understand these relationships between
strata at different localities is known as "correlation".
For example, rocks named Juras (for the Juras
Mountains) in France and Switzerland were traced northward and found to overlie
a group of rocks in Germany named Trias. The Trias rocks in turn,
were found to underlie rocks named Cretaceous in England (the chalky
“White Cliffs of Dover”).
Based on these relationships, is the Juras older or
younger than the Cretaceous? What are the two possible scenarios?
The location where a particular rock layer was discovered is called a "type
locality". Most of the “type localities” of
the geologic time column are located in Europe because this is where the science
of stratigraphic correlation started.
The Sedgwick/Murchison Debate
In 1835, Adam Sedgwick (Britain) and Roderick
Murchison (Scotland) decided to name the entire succession of sedimentary rocks
exposed throughout Europe. They were geology colleagues and friends, but
they had a famous argument over the division between the Cambrian and Silurian
Sedgwick’s topmost Cambrian overlapped with
Murchison’s lowermost Silurian. Eventually the disputed rock layers were
assigned the age “Ordovician”.
Rocks Divisions versus Time Divisions
It is important to remember that the rock record
is an incomplete representation of real geologic time due to the presence of unconformities.
geologists are careful to distinguish geologic time from the rocks that
represent snapshots of geologic time:
CORRESPONDING ROCK DIVISIONS
Formations (The main stratigraphic unit)
Rock divisions, such as the Cambrian System, can
be correlated worldwide based on fossils. In contrast, rock units such as
groups, formations, and members are localized subsets of systems. Rock
units depend on the environment of deposition, which varies from one
location to another.
Stratigraphic Rock Units
The rock divisions (Eonothem, Erathem, and System) simply divide rocks into the
appropriate time eon, era, or period. Obviously,
all Cambrian System rocks are from the Cambrian regardless of their location on
In contrast, the rock units (Groups, Formations, Members) are localized features
(of limited regional extent) that depend on the local environment of deposition.
The main rock unit of stratigraphy is the formation,
a localized and distinctive (easily
recognizable) geologic feature (i.e., the Chinle
Formation of Late Triassic lake and river deposits in Arizona, Nevada, Utah, and
Different formations are distinguished and correlated based upon lithology
(overall rock characteristics), which includes:
1) Composition of mineral grains
3) Texture (grain size, sedimentary structures)
Formations are “clumped” into groups and
divided into members.
Datum- In correlation, a datum is a line of equivalent age.
The ideal datum is a stratigraphic marker that is both geographically extensive
and represents an instantaneous moment in geologic time. A good example is
a volcanic ash layer that formed by a specific volcanic eruption followed by
worldwide dispersal by atmospheric currrents.
Fossils for Strata Correlation
Sedimentary rocks that date from the same age can be
correlated over long distances with the help of fossils.
Principle of Fossil Correlation- Strata
containing similar collections of fossils (called fossil assemblages) are of similar age. Also,
fossils at the bottom of the strata are older than fossils closer to the top of
Index Fossils- Index fossils are the main type
of fossil used in correlation. To be an index fossil, a fossil species
1) Easily recognized (unique).
2) Widespread in occurrence from
one location to another.
3) Restricted to a limited
thickness of strata (limited in age range).
The limited life-spans of these organisms allows us
to easily constrain the age of rocks in which they occur.
The best index fossils are those that are free
floating and independent of a particular sedimentary environment. For
example, organisms that are attached to one particular type of sediment are
going to have limited geographic extent and will not be found in many rock
types. By contrast, organisms that are “free floaters” or “swimmers”
will have a wider geographic extent and be found in many different rock types
A fossil zone is an interval of strata
characterized by a distinctive index fossil.
Fossil zones typically represent packets of 500,000
to 2,000,000 years. Fossil zones boundaries do not have to correlate with
rock formation boundaries. Fossil zones may be restricted to a small portion of a
formation or they may span more than one formation.
A fundamental assumption in fossil correlation is that
once a species goes extinct, it will never reappear in the rock record at a
Fossil types that are generally restricted to just one
type of sediment are called facies fossils. They are not very useful in
correlation, but are extremely useful for reconstructing paleoenvironments.
What is a Fossil?
Some examples of fossils are:
1) The preservation of entire organisms or body parts.
This includes the preservation of actual body parts (mammoths in tundra), as
well as morphological preservation via the replacement of biological matter by minerals
A petrified log in Petrified Forest National Park,
2) Casts or impressions of organisms.
Eocene fossil fish Priscacara liops from Green River
Formation of Utah
Trackways from ''Climactichnites'' (probably a
slug-like animal), in the Late Cambrian of central Wisconsin.
Thalassinoides, burrows produced by crustaceans, from
the Middle Jurassic of southern Israel.
5) Fecal matter (called coprolites).
dung found in southwestern
Saskatchewan, USGS Image.
Theories on The Origin of Fossils
At one time, fossils were
considered to be younger than the rocks in which they occurred. People
speculated that fossils formed when animals crawled into preexisting rock, died,
and became preserved in stone.
interpreted the widespread occurrence of fossilized marine organisms on land
as support for a world-wide flood as described in scripture.
Leonardo da Vinci’s (1452 - 1519) Interpretation of
Self-portrait of Leonardo da Vinci,
Regarding fossils that occur in
strata many miles from the sea, da Vinci argued that:
The fossils could not have been washed in during a "Great Deluge" because they could not have traveled
hundreds of miles in just 40 days.
unbroken nature of the fossils suggest that they were not transported by
violent water; instead the fossils represent formerly living communities of organisms that were
preserved in situ.
3) The presence of fossil-rich
strata separated by fossil-poor strata suggests that the fossils were not the result of
a single worldwide flood, but formed during many separate events.
Lateral Variations in Formations
Historically, geologists initially believed that the layer-cake sequence
of sedimentary rocks existed worldwide (i.e., that the layers extended indefinitely
By the late 1700’s people began to realize that
formations had a limited extent both vertically (up and down) and laterally
(horizontally across Earth's surface).
People also began to realize that lithologic variations (changes in texture, color, fossils, etc)
can occur laterally within formations themselves.
Today we interpret such variations in the context of modern
depositional environments. For example:
ENVIRONMENT OF DEPOSITION
Near shore marine- The energy is high
due to rough waters at the water-land interface.
Coarse sediments, and
fossils of robust
organisms that can withstand high energy environments.
Deep ocean- The energy is low
due to the general calmness of water away from land.
Fine sediments, and
fossils of more fragile
Note that the two different
lithologies can be deposited simultaneously (representing the same
moment in geological time) so long as they are
deposited at different locations.
Different lithologies grade
laterally into one another in a manner called intertonging. An example is
the way that the Old Red Sandstone of Wales (a terrestrial deposit) grades laterally into marine
sediments of Devonshire to the south (both are Devonian).
reflects the changes in depositional environments that occur over space and time
(lateral and temporal variations). Often these changes in environment are
linked to shoreline migrations resulting from sea-level
changes over time.
Depositional Environments and Sedimentary Facies
Depositonal environments are characterized initially
by the sediments that accumulate within them, and ultimately by the sedimentary
rock types that form. For example, a reef environment is characterized by
carbonate reef-building organisms. Ultimately, the sediments become lithified to form fossiliferous limestone.
A sedimentary facies is a three-dimensional body of sediment
(or rock) that contains lithologies representative of a particular depositional
environment. For example,
Mudstone and shale with
Laminated pelagic clays, cherts,
and possible limestone.
Well-sorted, well-rounded, and
possibly cross-bedded sandstone.
Analysis of sedimentary facies helps geologists to reconstruct
past geologic environments and paleogeography.
Transgressions vs. Regressions
The sea-level has fluctuated throughout geologic
history, and these changes have a profound effect on the geologic rock record.
A transgression is an advance of the sea over
A regression is a retreat of the sea from land
A transgressive facies pattern is characterized by:
1. The movement of marine facies landward over
2. A fining-upward sequence (the new marine
environment is lower energy than the prior terrestrial environment).
3. A basal, erosional unconformity (erosion was more
profound before the seas advanced).
A regressive facies pattern is characterized by:
1. The movement of terrestrial facies seaward and over
2. A coarsening-upward sequence.
3. An erosional unconformity at the top.
Walther’s Law- Over time, the lateral changes in sedimentary facies due to transgressions and
regressions will also produce vertical changes in sedimentary facies:
1. A transgressive facies sequence fines in the
direction of the transgression, and also fines upward.
2. A regressive facies sequence coarsens in the
direction of the regression, and also coarsens upward.
What causes transgressions and regressions?
1. Worldwide rises and falls in sea level (eustatic
changes), perhaps related to climatic change.
2. Tectonic uplift, isostatic rebound, or crustal
3. Rapid sedimentation.
It is often difficult or impossible to determine the exact cause of
a transgression or regression seen in the geologic record. The cause may be
worldwide or local. The fact that there is a
transgression or regression indicates an “apparent” sea-level change.
The Stratigraphy of Unconformities
Recall that unconformities represent missing time due
Periods of non-deposition.
Periods of erosion.
The main types of unconformities are:
2. Angular unconformity
Unconformities vary from one location to another (just like
rock formations and
sedimentary facies). In other words, some locations along the unconformity
surface will represent more missing geologic time than others.
Unconformities may eventually disappear laterally and
transition into a conformable sequence of strata.
Oil companies use large scale, unconformity bounded
rock units called sequences to correlate rocks in a
process called sequence stratigraphy.
Six major unconformity-bounded sequences are
recognized worldwide in the Phanerozoic. These sequences are not restricted to
period or era boundaries.
The major sequences are believed to represent
worldwide fluctuations in sea-level.