Biostratigraphy is the traditional use of paleontological data since the
time of Smith and Cuvier. The aim is to use fossils to make correlations,
that is, placing rocks in a relative time context. the basic notion is that
any fossil existed for a limited time interval so that all rocks containing
that fossil formed during the specific time interval. You have two reading
selections. Raup and Stanley provide some insight to some of the techniques
and questions. Clarkson more briefly summarizes the current status (and
terminology). The major topics are stratigraphy, correlation, quantitative
correlation, and precision of correlation.
Stratigraphy (Raup and Stanley, p. 197-207, 222-225) and Clarkson (p.
20-23)
A few basic points to keep in mind as you read over this section: (1) there
are several different types of geological units - each of which is defined
in a particular way; (2) there are rules regarding how this is all done
- and embedded in the rules are certain concepts.
The "North American Stratigraphic Code" (AAPG Bull., 1983, v.
67, p. 841-875 with periodic refinements) established the formal rules on
the types of units, how to define them, etc. The different types of units
are: lithostratigraphic (rock-stratigraphic), biostratigraphic, chronostratigraphic
(time-rock), and geochronologic (time) units. Clarkson outlines the formal
nomenclature, so you may want to read it first. Raup and Stanley discuss
some of the controversies (the nature of chronostratigraphic units) and
why some units are limited in practice. As you read these sections, please
make a list of the attributes of the following types of units: lithostratigraphic
(rock-stratigraphic), biostratigraphic, chronostratigraphic (time-rock),
and geochronologic (time) units. You also may want to consider why we
have these varied types of geological units.
Here are a couple of things to consider about biozones and correlation.
(1) You will notice that there are several types of biozones (Clarkson)
- we need not worry about all the details. The zones that are used are range
zones, including the concept of overlapping zones. An important point mentioned
by Raup and Stanley is the notion that a biozone has inherent limits on
its extent. Consider the following sketch:
A barrier divides a region into two depositional
basins for most of the history of fossil X. During that time, A has the
index fossil of interest, but B does not. Within A, there will be barren
zones without fossils. Furthermore, within A the fossil has a point and
place where it evolved and where the last population survived. The first
and last occurrences of the fossil are not exactly simultaneous across A
(let alone B). Biostratigraphic units would not include all of the rocks
in both basins, but chronostratigraphic units would - of course, recognizing
them is another matter.
(2) Raup and Stanley mention the philosophical shift toward defining chronostratigraphic
units by defining boundaries. This has become quite important in the last
few decades and this approach is replacing the earlier practice of defining
a chronostratigraphic unit by a type section approach (to see why, see Raup
and Stanley, p. 222-224). But, how does this actually take place? International
commissions meet for a decade or two to consider such boundaries. They attempt
to select the most useful faunal group and to identify the best (continuous)
section. A specific horizon in a specific section is selected by vote after
years of debate (and lots of ink) to be the stratotype (reference) point
for the boundary. The horizon is often marked by a spike (a.k.a. the "golden
spike"). Notice that the basal boundaries of systems, series, and stages
are being defined in this manner. The tops are defined by the base of the
next unit. (This ensures stability in case of missing time or overlap.)
Correlation (Raup and Stanley, p. 207-216)
This section is fairly straightforward (I hope) - if not, ASK. You should
be familiar with the basic idea of using index fossils. The text goes beyond
this to consider other techniques for determining equivalency. We could
also add other features: ash beds, regional unconformities, etc.
Quantification of correlation (Raup and Stanley, p. 216-222)
Please carefully read over this section on Shaw's method of quantitative
correlation. The importance of this method grows every year. I will review
this topic in some detail during class with some real databases. This will
(hopefully) help clarify this - it seems a bit mysterious the first time
through!
Precision of correlation (Raup and Stanley, p. 225-229)
Finally, we need to consider what level of precision should we expect in
biostratigraphy? The text suggests (p. 225-226) that long-range (across
continents or between continents) correlations based upon fossils may have
a considerable error (several millions of years or more). This has been
the topic for considerable debate in recent years as there has been a international
effort to delineate global stages (not just series). The general consensus
is considerably more optimistic than the text. Efforts to define boundary
stratotypes and new models for stratigraphy have forced biostratigraphers
to focus their efforts on improving the precision of correlation. Precision
has improved due to several strategies: careful examination of evolving
lineages, the use of multiple fossil groups, and quantitative correlation.