Geosciences 308 Correlation and
Biostratigraphy
Lithostratigraphy
- subdividing rocks based on their lithology
Formations: groups, members
Biostratigraphy
- subdividing rocks based on their fossil content
Zones - strata that contain diagnostic
fossil species
Correlation
rock-correlation - same rock unit, no
implication of temporal equivalence
time-correlation - rocks deposited at
same time
Time-correlation
using physical evidence:
isotopic dating, marker beds, position in
cycle, varves, paleomagnetism
Stratigraphic
ranges controlled by evolution, environment, migration, quality of record
Time-correlation
using fossils:
Fundamental unit-the range of a species;
before, during, after
Controls on species ranges: evolution,
environment, migration
Controls on species ranges: sampling,
unconformities, diagenesis
Single species approaches
"index" or "guide"
fossils
Multi-species approaches
assemblage zones
concurrent range zones
graphic correlation
The
development of the Geologic Time Scale
Superposition and early subdivisions
based on lithology
William "Strata" Smith and the
use of fossils
Type areas and type sections
Correlation
to the type are
Correlation
and Biostratigraphy
Last time I
talked about some approaches to dating that used fossils and came up with
estimates of the absolute age of the fossils (and the rocks that contain
them). Now, I want to consider the use
of fossils in correlation: determining that two bodies of rock were deposited
at the same time (more or less).
Relative dating.
Important contrast
between lithostratigraphy and biostratigraphy
Lithostratigraphy: subdivision of rocks based on their
lithology. Basic unit is the Formation (Groups and members)
example: Bisbee Group, which includes the:
Cintura
Formation
Mural
Limestone
Morita
Formation
Glance
Congomerate
in contrast,
Biostratigraphy
- the study of the subdivision of rocks based on their fossil content.
--this is in
contrast to lithostratigraphy -
Basic
unit is the zone: interval of rock
characterized by diagnostic fossils
Correlation:
term is often used in two ways in geology
1. Physical correlation - determining the
physical continuity of rock bodies
2. Temporal correlation - determining that two
or more rock bodies were deposited at the same time.
--we are concerned
here with temporal correlation, and even if I don't modify the word
correlation here, I do mean temporal correlation.
How to correlate
1. Physical methods
(won't consider much here)
marker
beds a volcanic ash fall, for example, will
blanket the ground over a large area.
That ash fall can get preserved, and, if recognized in two or more local
sections, can be used as a time line.
K/T
boundary clay a global marker bed
position
in a cycle - (on
handout) among several sections in a
transgressive-regressive sequence, the line connecting the deepest environment
of deposition will be a time line (assuming no local tectonic activity).
Use
Bisbee group example
varves - sediments in some lakes, and in some
seasonally fluctuating marine basins, are deposited in couplets of light and
dark layers.
In
glacial lakes, for example, dark layers represent months in which the lake was
frozen, and only the very fine organic matter and clays settle out. Lighter, thicker layers, represent summer
months, in wich sediment is added to the lake by streams.
Like
tree-rings, series of these couplets can be very distinctive and can be used to
correlate over the whole lake basin.
paleomagnetic
methods - changes in
polarity of the earth's magnetic field.
HANDOUT Polarity recorded in
lava flows (where isotopic dating can be used). Polarity also recorded in sediments (if little or no
post-depositional mixing) with fossil zones.
NOTE THEREFORE, the two step calibration with radiometric dates.
2. Using fossils in
correlation: some basic principles
The basic
biostratigraphic principle:
-every species
divides strata and geologic time into three intervals:
1. The interval before which the species occurs
2. The interval during which the species occurs
3. The interval after which the species
occurred
The interval of
strata in which a species occurs is termed its stratigraphic range.
the
interval between the first appearance of a species and the last
appearance of that species.
Let's consider
what controls the stratigraphic range of a species.
There are two
basic categories of controls: Biotic
controls and sampling controls.
A. Biotic controls.
1.
Evolution. The time of
origination and the time of extinction
(this is the fundamental issue)
2.
Ecology. Environmental conditions must be such to
allow the existence of the species.
Example: polar bears do not
occur in the desert. Saguaro cactus
have southern Az as their northern limit.
They can't tolerate extended periods of freezing. As climate changes in the future, they may
extend their range to the north and east (into S. New Mexico and west Texas,
for example). Conversely, if the
climate cools, saguaros will disappear locally.
If
a species’ first appearance (or last appearance) is controlled ecologically,
then the first appearance should not be used in correlation.
Ecologic
controls may be recognized by lithologic changes in the rocks.
3.
Biogeography.
A species could, in principle, live in a habitat, but not be able to get
there: For example, penguins might be
able to live in the Arctic, but they can’t get there. So the environment is OK, but biogeography controls their
distribution.
Species may migrate into or out of a
region as geographic conditions change.
The possum, for example arrived in
North America about 2 million years ago, when the isthmus of Panama
became dry land. It was present in S
America long before that time.
Migration. Recognized by
coincident appearance of a number of new forms, though can be difficult to
distinguish from a true evolutionary first appearance.
B. Uncertainty about true stratigraphic range:
1. Unconformities. Intervals of time represented either by erosion or
non-deposition. Last appearance because
of unconformity, first appearance above and unconformity?
2. Diagenesis.
Post-depositional destruction of fossils by dissolution or
recrystallization.
3. Collection failure. Poor sampling. Shorter range
4. Reworking
- higher last appearance than expected
Three of these
factors (ecology, biogeography, uncomformities, diagenesis, collection failure)
work so that a species' stratigraphic range is a minimum estimate of the actual
temporal range of the species.
Reworking of fossils is the exception to this).
Now, on the the use of fossils:
1. Single species approaches
a. The basics.
The basic problem then is this:
Is the species absent because it has gone extinct, has yet to migrate
in, the environment isn't appropriate, or what?
b. Index or guide fossils. Single species (or higher taxa) that have
proven to be especially useful in correlation:
Attributes:
1. short strat range (rapid
evolution)
2.
broad geographic distribution
3. Broad ecological tolerance
Examples: (often
planktic or swimming) Cretaceous
ammonites, Conodonts,
Cambrian and Ordovician trilobites, Ordovician graptolites, Cenozoic planktic foraminifera
2. Multi-species approaches. Because more than
one species is involved, this approach may be less sensitive to problems of
sampling or environment.
a. Assemblage zones. Intervals of rock based on the overlapping occurrence of three or
more species (Zone usually named after one of those species). Deals with some of the problems of
interpreting range limits.
b. Concurrent range zones. Zone based on the overlap of the end of the
range of one species and the start of the range of another species. A particularly distinctive combination of
species.
c. Graphic correlation. Uses the bottom and top of several species
to relate one or more stratigraphic sections to another. – as in lab
Correlation and geologic time
I've talked now
about correlation and how fossils are used in correlation - that's the business
of biostratigraphy. Were the rocks
deposited at the same time??
But I haven't
yet talked explicitly about how fossils are used to estimate the age of
sedimentary rocks. This requires a
historical treatment because the geological time scale, as you have come to
know and memorize it, is not the product of some scientific commission that
came up with the whole thing all at once.
Giovanni
Arduino, working in northern Italy in the late 18th century devised a local
scheme for classifiying rocks accoridng to their relative age:
Used
principle of superposition to develop a three-fold scheme
Primary rocks - igneous and metamorphic rocks at the
cores of the mountains
Secondary rocks - sedimentary rocks along the flanks of
the mountains, often no longer horizontal
Tertiary
rocks - poorly
consolidated or unconsolidated rocks along the coastal plain (lying above
the
Secondary rocks)
Note that in
this early scheme, fossils were not used to distinguish one group fo rocks from
another. Indeed, lithology and
superposition were the key ways to get at the relative age of the rocks.
Other similar
schemes were developing throughout Europe during the late 18th century and
early and middle 19th century.
One difficulty
became pretty clear pretty soon:
lithology was a poor guide to the age of the rocks. If you could find rocks of the same
lithology some distance above or below the ones you were looking at, how could
you use rock type as a good guide to age?
This problem was
solved by the use of fossils and involved the work of two very different people
around 1800
1. William Smith.
Surveyor for canals then being built all over England. He noted that each group of rocks could be
ca\haracterized by their fossil content, and that you could recognize that
ionterval in different places -- even if the lithology was not the same.
2. Georges Cuvier.
A French scientist working in the Paris area, also noted that intervals
of the sections he was working on were characterized by distinctive assemblages
of fossils. He used those fossil
assemblages to map the area.
The
important point here is that strata could be distinguished based on their
fossils.
Why this was the case was not clear at the
time. Cuvier's explanation called for a
series of catastrophes wiping out one fauna and allowing another to migrate in
afterwards. Smith was an eminently
practical man and didn't care much.
The
point is this: the use of fossils in
correlation did not and does not require evolution. Darwin's theory, after all, did not get published until the 1850s
- long after fossils were used for correlation.
All you need to use fossils is to recognize that species have different first appearances and different last appearances: there was no single episode of the creation of all species, and extinction happened throughout the history of life.
In the late 18th
and early 19th century, before the formulation of a coherent theory of evolution,
fossils were used to subdivide rock bodies in Europe (and to a much lesser
extent in North America). And
furthermore, the same sequence of fossils was found from place to place. In other words, the same relative order
appeared from place to place. The
principle of superposition then suggested that this was a temporal sequence.
Local schemes
were developed, for example in the chalks of northern Europe, for rocks we now
call Cretaceous (meaning chalk-bearing).
These chalks could be subidivided based on their fossils. This region is now termed the type area, and individual lithologic
sections for subdivisions of the Cretaceous, for the Maestrichtian, for
example, are called type sections.
Superposition
showed that Cretaceous rocks were stratigraphically above Jurassic rocks and
below Tertiary rocks.
So, when a
paleontologist picks up an ammonite and declares the fossil and thus the rock
to be of Cretaceous age, he or she is correlating that local section to the
type section (often have intermediate
regional, or continental type sections
Then, the
absolute age (in millions of years) could determined by dating, using
radioisotopes, lava flows that occur within, say, Cretaceous rocks.