Download Correlation and Biostratigraphy

Geosciences 308 Nov. 4, 2004
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
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, preservation
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 area
Correlation and Biostratigraphy
The last lecture concerned 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) – a mappable lithologic unit
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.
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 too much here)
a. 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. Isochronous (same time). K/T boundary clay (fallout from impact
of asteroid) is a global marker bed.
Cretaceous-Tertiary
boundary clay at
Woodside Creek, New
Zealand. Note
extensive drillholes
for samples from
above and below clay
layer. Image from
rwww.peripatus.gen.nz/
paleontology/extinction.html
b. position in a cycle - (see figure) 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).
c. 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 which 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.
d. Paleomagnetic methods - changes in polarity of the earth's magnetic field. (see figure)
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 at
that particular place/environment. 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 precise 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 an unconformity?
2. Preservation. Post-depositional destruction of fossils by dissolution or
recrystallization.
3. Collection failure. Poor sampling. Shorter range
4. Reworking - higher last appearance than expected
Five of these factors (ecology, biogeography, uncomformities, preservation, collection failure)
work so that a species' stratigraphic range is a minimum estimate of the actual temporal range of
the species. Reworking of fossils can cause species last appearances to occur after their time of
extinction.
Now, on to 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 stratigraphic 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.
Benton and
Harper, 1997.
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.
Raup and Stanley
c. Graphic correlation. Uses the bottom and top of several species to relate one or more
stratigraphic sections to another. – as in lab exercise this week.
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 of 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 characterized by their fossil content, and that you could recognize
that interval 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.