Russel T. Arndts (rta@TIGGER.STCLOUD.MSUS.EDU) wrote:
...
|I have been told that the geologic collum is found (and is thus
|repeatable) in England and in South Dakota.
And Alberta, and parts of Australia, and many other places. Gaps
abound in the Earth's geology at every conceivable scale at an individual
location, but reasonably complete vertical sections with representation of
a portion of every major geological period exist in many places.
|However, **then** I found that there are different index fossils used in
|each column.
In some intervals, for some faunas and floras, yes. This is
inevitable because organisms do not necessarily extend globally. If you
want global correlation of an interval of time by a single fossil, you
have to find one which was not limited to (for example) the Atlantic
Ocean, and even then it is not likely to extend onto land :-) This is a
tall order, due to the natural distribution of life -- not many life forms
are ubiquitous, and those which are tend to have relatively long ranges.
However, you are greatly exaggerating the effect of this problem.
There are index fossils which can be traced globally in many intervals of
the geological column. For example, graptolite zones in the Ordovician
and Silurian are virtually global in extent. Correlation of these zones
to areas without graptolites is facilitated by comparison to other fossils
and to other methods of physical correlation (e.g., volcanic ash beds and
geochemical fluctuations). Claiming that non-global index
fossils/biozonations means the entire subdivision could be inaccurate is
an exaggeration. It could, at most, introduce imprecision and small
miscorrelations. In fact, this is a major issue in biostratigraphy. But
there is no confusion about, for example, the distinction between
Cretaceous and Tertiary rocks. Not only do many types of organisms (not
only the ones used as index fossils) become extinct at the end of the
Cretaceous, and many new species first appear above that point, but there
is a blatently-obvious geochemical anomaly which is correlatable at a
global scale down to a precision of centimetres of thickness.
|IF the gelogic column is repeatable, then the rules and
|definition for the choice of an index fossil (for example for the period
|310 million to 300 million years ago) is crucial or we simply have two
|piles of rock that the evolutionists claim to explain.
You are making a mountain out of an issue which only introduces
small imprecisions in the relative time scale. Miscorrelation of a few
zones, when there are dozens within the geological time periods most
people are familiar with, is not going to fundamentally change the
observations, and these imprecisions are continually being resolved and
the zonation refined as more data becomes available. Back in the 19th
century, your question would have been completely legitimate. These days,
a significant amount of imprecision could conceivably occur in
biozonations, but it would still not change the fundamental pattern
because the biozonation is so fine. Most people do not even know the six
stages of the late Cretaceous (Cenomanian, Turonian, Coniacian, Santonian,
Campanian, and Maastrichtian), let alone the twenty or so zones subividing
them even further. The Jurassic is subdivided into about 70 ammonite
zones. Flipping a few of these around or miscorrelating by a few zones
from Europe to North America is not going to change the broad pattern at
all, which is all that many people consider. To suggest there are larger
miscorrelations than this would have to ignore the many other ways in
which stratigraphic sections can be correlated.
In my own field area, I correlate between sections using four
biozones established on the basis of microfossil content. A single
volcanic ash bed which occurs in the same stratigraphic sections tens of
kilometres apart, and the occurrence of volcanic lava flows at the top of
almost all the sections, confirms the correlation suggested by the
fossils. Otherwise I would have to propose the occurrence of 4 different
ash beds at as many locations, and 4 different eruption episodes amounting
to hundreds of metres of lava flows which are chemically similar. This is
a possibility, I suppose, but it seems rather unlikely, particularly when
the fossils I observe fit perfectly into the age interval expected from
the fossils in units even further above and below. Every fossil and every
stratigraphic section is a test of that hypothesis of correlation. There
are innumerable such tests, for more than 200 years, on the global
correlation represented by the conventional geological time scale. The
substantial refinement of the scale over that time reflects the testing
which has occurred. The chances the geological time scale is
fundamentally wrong at a *broad* scale are very small.
Your skepticism is worthwhile, and is justified -- miscorrelations
do occur for the reasons you have stated -- but if you look at the details
of the situation, you will quickly realize that at the scale of detail the
geological column is known, it does not matter for the level of detail
*usually* considered by critics of evolutionary theory.
|What are the rules for determining the ages of index fossils?
Their vertical position in a stratigraphic column. Basically, it
is determined by geometrical relationships and the principle of
superposition.
|Certainly, fossil species are not used to date index fossils.
Actually, fossils can be used to date other fossils, and those
"other fossils" can be used for local correlation, but this is
once-removed from the original fossil succession, so it is less precise.
It may provide a local correlation which is very precise, but integrating
that into the global scale would be more difficult.
|By the way, the concept of index fossils predates radiometric dating.
Yes, but upon the establishment of a succession of fossils on a
global scale based on stratigraphic relationships, radiometric dating
serves as a test of the hypothesis that the fossils really are
successional and really do correlate laterally. To a first approximation
(taking into account the vagaries of samples and analytical uncertainty),
the results are completely consistent. Units considered to be late
Cretaceous in age in western North America and late Cretaceous in Europe,
based on correlation of ammonite zones between the two areas, yield
approximately the same radiometric dates.
Here is a useful reference about the geological time scale and how it is
established:
Harland, W.B.; Armstrong. R.L; et al., 1990. A Geologic Time Scale, 1989.
Cambridge University Press: Cambridge, p.1-263. ISBN 0-521-38765-5
The discussion is quite technical, and not particularly useful for
biozones/index fossils, but should give the reader an appreciation for the
current state of the time scale, and how finely it has been subdivided and
how numerical calibration (i.e. radiometric dates, usually) are applied.
Note that Harland et al., 1989 is by no means definitive. It just
represents one of several syntheses to that point. Parts of the scale
have been revised by continual subsequent work.
-Andrew
macrae@geo.ucalgary.ca
home page: http://www.geo.ucalgary.ca/~macrae