Biostratigraphy and Correlation
Geologic background
 Most fossils are preserved in sedimentary rock
 That is rock that is formed from sediment, sand, clay, silt,
even gravel and boulders.
 The sediment goes through a process called lithification
which results in the transformation of loose sediment into
solid rock
 Lithification is a three step process:
 First the sediment must be deposited by wind, water or ice. The
place it is deposited is called the envronment of deposition.
 Over time the sediment is buried deeper and deeper until,
eventually, it is compacted into a more uniform mass.
 Then ground water moving through the sediment deposits a
“binder” or “cement” – usually CaCO3 and the sediment becomes
solid.
 Sedimentary rocks that contain fossils are typically sandstones
made up of sand sized particles; shales, or mudstones, made up
of clay and finer particles; and limestones.
 These sedimentary rocks are the most common rock types on
the earth’s surface.
 Ocean basins, low lying coastal areas and inland basins receive
most of the sediment destined to be lithified.
 In warm climates, specifically in shallow seas without much
terrestrial (land derived) sediment, we often find “chemical”
sedimentary rocks. Typically limestone.
 Limestones often preserve excellent samples of marine fossils
through recrystallization.
 Any organic remains buried in the sediment have the potential to
become fossilized
 Fossil remains can help us determine what the environment of
deposition was when the sediment was deposited.
 Different environments are also characterized by different types
of sedimentary rock. The term used for this is sedimentary
facies.
 Sedimentary facies are illustrated well in the figure on page 47 of
your text.
 If we follow the sediment as it travels from the mountains to the
coast you will see how the concept of sedimentary facies is
used.
 We would not expect to find the same living organisms in a
desert as we find in the ocean. Likewise, we would not expect to
find the same assemblages of fossils in a marine shale that we
would find in a desert sandstone or a stream delta.
 So let’s look at the diagram and see what we might expect to
find……..
There are other factors that influence what types of fossils might
be found.
 Latitude also plays a role in what fossils may be found.
 Cold water faunas differ quite a bit from those in warm water.
 Warm and cold currents also affect the distribution of organisms
allowing them to be productive well outside their normal limits.
 So fossils can be used to show changes in the relative position
of landmasses and ocean basins.
 Another factor in fossil distribution is water depth. Some
Foraminifera (single celled animals with a “shell”) are found in
very specific depth ranges.
 Reefs: Reefs are a specific environment that generally contain a
good record of the organisms that make them up.
 While we tend to think of reefs as surrounding tropical islands,
there are reefs in many marine areas, even the arctic.
 Reefs today are generally the home of corals, but they are found
in the ancient environment made up of sponges, bryozoans and
other animals that found that niche productive.
 Deep sea environments are potentially the best for preserving
organic remains. They are quiet, generally without scavengers
or predators to disturb the remains, and they receive a lot of
sediment to insure burial.
 Remains of both pelagic (floating) and nektonic (swimming)
marine organisms fall through the water and are preserved in
marine sediments, as are the remains of organisms that are
carried along with the sediment and are deposited on the ocean
floor.
 The one drawback to preservation in the deep sea environment
is that of going too deep. At about 4000m the CaCO3 will begin
to go back into solution, so only a fossil preserved with silica will
survive.
 So now we have ancient organisms preserved in rock made from
the sediment that buried them. We know that sedimentary rock
is indicative of the environment present when the sediment was
deposited.
 How do we use that information to help us:
 Locate and identify fossils
 Explain events in earth’s history
 Put those events in chronological order
 Biostratigrahy is the subspeciality of Geology that uses fossils to
identify and date rock strata
 Correlation is the process of equating different rock units across
geographical areas.
Biostratigraphy and Correlation
 “Each objectively definable extinct fossil taxon divides
geologic time into three segments – the time before it
appeared, the time during which it existed, and the time
since its disappearance.” Shaw,1964
 D’Orbigny first used fossil assemblages to divide strata
into stages. The stages then became units independent of
the lithology and could be used over wide areas.
 Oppel followed with his idea of overlapping range zones
defined by appearances and disappearances of fossils in
aggregates.
 We know that there are many factors involved in the
distribution of fossils – not all relating to their life style.
 Facies fossils are those that are known to inhabit narrow
s e d i m e n ta r y f a c i e s a n d th e y a p p e a r a n d d i s a p p e a r i n th e
stratigraphic column on that feature alone
 Some fossil species such as Globotruncana
truncatulinoides are right coiling in warm waters and left
coiling in cool waters. This gives a record of global climate
throughout their range.
 The first occurrence of a fossil in the local stratigraphic
record may be the result of :
 Evolution
 Migration
 Habitat change
 Likewise the final occurrence may not be due to extinction
but rather to migration out of the area.
 First and last occurrence may also be the result of how
well the depositional environment preserves the remains.
 Lazarus taxa are those that appear to have become extinct
only to reappear in a later stratagraphic levels.
 Elvis taxa are those that, through convergent evolution
appear very much like earlier forms- but indeed are not.
 Another problem is fossils being reworked, as sediment,
from earlier beds and included in later ones. This has
been referred to as the “zombie effect” as the fossils “lurk
i n l a te r s e d i m e n ts ..”
To review Index Fossils:
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Distinctive (easily identified)
Geographically widespread
Plentiful
Facies independent
Rapidly evolving
Short lived (geologically)
 When using index fossils it is important:
 To sample the entire section and determine the range of the
fossil within it.
 Don’t imply that the rock units are time-equivalent – especially if
the index fossil isn’t present throughout
 Don’t recognize the rock units by the fossil – use the lithologies
 Use global rather than local indexes
 Some exercises in correlation…….