Paleontology

J Bret Bennington

Biostratigraphy

Biostratigraphic Units

The basic unit in biostratigraphy is the biozone. Unfortunately, the exact definition of a biozone is somewhat nebulous.

The NACSN defines a biozone as any body of rock defined or characterized by its fossil content. This indicates that a biozone need not have any specific time significance. However, as used by most biostratigraphers, biozones are implied to at least approximate being equivalent to unique time intervals.

Overhead: Biozone types

In biostratigraphic practice there are several ways that biozones are defined. The most conceptually simple is to define a biozone as equivalent to the time interval over which a particular species existed. This is called a taxon-range-zone. A taxon-range-zone is named for the species or taxon that defines it.

The temporal resolution of a biostratigraphic zonation is limited by the length of the time intervals occupied by the species being used. If the species that you have chosen to define a time interval lived for ten million years, then any strata that contain that species can be said to have formed within at most ten million years of one another. They may have formed within 100 years of each other, but you cannot be certain below ten million.

One way to improve your resolution is to use the interval of time during which the ranges of two species overlap. This is called a concurrent-range-zone or range-overlap-zone. Thus if species A has a ten million year range and species B has a ten million year range, but both species only existed together for 2 million years, then you could define a range-overlap-zone with a resolution of 2 million years.

It is also possible to define a range zone based on the interval of time when two species did not exist together. Thus if species A exists for ten million years, but one million years into its span species B evolves and exits for 10 million years, then not only can you define a concurrent-range-zone of 8 million years (the A - B biozone) but you can also define a one million year interval when A existed but not B, and a one million year interval when B existed but not A. These are called consecutive-range-zones.

There is one problem with using consecutive-range-zones – they are based on the absence of a species. Using negative information is always a tricky proposition because you can never really be certain if the species in question does not exist in the rock at a particular stratigraphic level, or if it is just locally absent, or if you just failed to find it in your sampling.

In addition to basing biozones on the ranges of one or two overlapping species, one might also designate a biozone based on the overlapping ranges of a group of species. Such a biozone is called an assemblage zone.

In most cases, assemblage zones define a relatively broad interval of time during which a characteristic group of species lived. Not all species need to be present in a stratum for it to be identified as belonging to a particular biozone.

Assemblage biozones have the disadvantage of being less precise than range biozones, but the advantage of increasing the number of representative species and therefor the probability of finding fossils in a stratum indicative of that biozone.

It might also be possible to base a biozone on the time of maximum abundance of a particular species. This is called an abundance or epibole biozone. Abundance biozones are not used extensively for correlation because it is very difficult to show that a species reached its maximum abundance at the same time in all places. Abundance is probably more tied to the attainment of a set of local conditions than it is to any one time in the life of a species.

Difficulties in the practice of Biostratigraphy

To understand the difficulties on encounters trying to use fossils to mark intervals of time in rocks, it is best to begin by discussing the ideal case.

A fossil species that is used as a biostratigraphic marker because it is indicative of a particular interval of geologic time is called a guide fossil or an index fossil.

The ideal index fossil should have the following attributes:

1. It should be abundant and easily collected, easily recognized, and easy to distinguish from similar species.

2. Its span of existence should be short so as to maximize stratigraphic resolution.

3. It should have a wide geographic distribution to permit correlations all over the world.

4. It should be found in a variety of different facies (not having been restricted to a single depositional environment.).

5. The rate of dispersal of the index fossil should be rapid so that as soon as it evolves it occupies a wide geographic area.

Unfortunately, the ideal index fossil probably has never existed. The most problamatic of the above criteria is number 4 facies independence. Most organisms are rather closely tied to particular environments and are thus found as fossils in only a limited range of facies. An extreme example of the facies problem in correlation is the difficulty in correlating terrestrial strata with marine strata. Few, if any marine organisms, which are often the principle biostratigraphic fossils in use, end up being entombed in terrestrial facies. Fortunately, terrestrial organisms are frequently washed out to sea, or drift with the wind into marine environments (pollen and spores) allowing terrestrial biozones to be correlated with marine biozones.

Planktonic or nectonic organisms sometimes come close to being facies independent, not because they lived in a variety of environments, but because after they die their remains can drift into a variety of depositional settings. This is one reason why graptolites in the Ordovician and Silurian, Ammonoids in the Mesozoic, and foraminifera in the Cenozoic are often utilized as biostratigraphic index fossils.

Another difficulty is the fact that many species with short spans of existence also tend to be endemic (restricted to a specific geographic area). The converse is also true. Widely distributed species tend to resist going extinct.

For many biostratigraphic applications microscopic fossils such as spores, pollen, foraminifera, and marine algae are the organisms of choice for several reasons.

a. They are small and a large number can be extracted from small amounts of sample. This is particularly important where drill core is the sole source of sample.

b. Many microfossils are either windblown or planktonic, giving them wide occurrence in a number of different facies.

c. Windblown and planktonic species are often rapidly and widely geographically dispersed.

Graphic Correlation

Graphic correlation is a method for making very precise correlations between measured sections of sedimentary strata. It was developed by Alan B. Shaw in the 1950's while he was working for the petroleum industry and is sometimes referred to as Shaw's Method.

Graphic correlation is particularly useful for correlating sections of approximately similar age within a depositional basin. This method requires that the stratigraphic sections be measured and described in great detail and that large numbers of fossils be collected at regular intervals. The data collected for graphic correlation are amenable to database management and computer analysis, and the method enjoys wide use in the petroleum industry. Microfossils such as spores, pollen, and marine algae, which can be extracted in large number from small drillcore samples, are often used to make graphic correlations.

How to make a graphic correlation

Fossils from samples collected at regular intervals through a section are identified to the species or genus level and their occurrences are plotted beside a columnar diagram of the measured section. For each species in common between the two sections to be correlated, the lowest occurrence (base of range) and highest occurrence (top of range) are recorded as measurements in feet or meters above the base of the section.

Rare species and species that appear to be highly restricted to particular facies (depositional environments as represented by rock types) are usually eliminated from further consideration, because their ranges in the sections probably do not represent their true range through time.

The two sections are used to create an X-Y graph. Generally, the thicker or more complete section is used as the X axis. Both base and top measurements are plotted for each species (use a separate symbol for each), with the distance from the base of each section giving the X-Y coordinates for each base and top point.

The line formed by the plotted range bases and tops shows the approximate relationship between rates of rock accumulation in each section. Statistical regression can be used to plot the best fit line through segments of points arranged along a linear trend. This line can be used to relate any horizon in one section to the temporal equivalent in the other. Changes in relative rate of accumulation between sections will cause a change in the slope of the correlation line. A hiatus in deposition or a disconformity will manifest itself as a plateau of points in the Y section or a vertical line of points in the X section.

Making a composite reference section

Shaw's method works because it bases the correlation on a maximum amount of data. Inaccuracies in the determinations of ranges probably vary randomly and tend to cancel each other out in producing the line of correlation, which can be thought of as a best fit or concensus correlation. Having established the line of correlation between two sections it can be used to "correct" the ranges by adjusting points that do not lie along the line of correlation. Generally, the most complete section available is chosen as the reference section and as other sections are correlated to it the bases and ranges of the reference section are adjusted by moving bases downward and tops upward until they lie along the line of correlation.