Biostratigraphy:

Except locally, lithostratigraphic units are not generally reliable as time markers. This is due to variations in facies as well as the progradation of units. Good lithologic time markers might include event beds, like tempestites, glacial diamictites or ash fall deposits formed during a single episode. However, the potential cyclicity of these events makes their worldwide use problematic. Today our primary sources of temporal correlation come from:

Now, we consider the latter.

Index Fossils, Correlation, and the birth of Biostratigraphy:

  • In 1796 William Smith, a British civil engineer, addressed this, adding the principle of Faunal succession to those of Steno and Hutton. Essentially, Smith noted that:

    By noting the fossils present, it became possible to:

    In 1815, Smith published the first geologic map of England.


    The great thing: These correlations made the association of formation scale units into larger ones (the stages and systems on which the Geologic Time Scale is based) possible. Thus, essentially all stratigraphy above the formation scale is biostratigraphy.

    Which fossils do we use?


    Rock units are not time units!

    With Steno's and Smith's principles as a basis, geologists define a heirarchy of higher order rock units, including:

    Larger units need not be contiguous in space but are assumed to be contiguous in time. Their upper and lower boundaries must be instantaneous and isochronous.

    From these, we derive the Geologic Time scale, in which geochronologic Periods correspond to lithostratigraphic systems. The numerical dates that we place on their upper and lower boundaries are secondary to the identity of the rock units.

    Subsequent to Smith:

    Biozones

    These are still rock units!

    Primary data of biostratigraphy: presence or absence of a fossil taxon in a geologic horizon

    Last Appearance Datum (LAD): either local or global

    First Appearance Datum (FAD): either local or global

    Biozone (often just "zone"): Rock unit characterized by one or more taxa that permit it to be distinguished from adjacent rocks.

    Consider the hypothetical data at right. As we explore these, note that the definition of most biozones requires some element of uncertainly or inferrence.


    Types of Biostratigraphic units (and thus rock units):

  • Abundance Zone (also called Peak Zone, Acme Zone): Subset of teilzone where index species reaches some higher level of abundance: useful locally, but almost certainly environmental rather than time-related. That isn't to say that there is no time signal necessarily - Note examples of global changes in abundance due to global environmental changes. E.g.:

    Reasons for caution

    Biostratigraphy opened the door to global correlation of strata, but is, nevertheless subject to biases and filters that make it most reliable on a local scale than a global one.

    Mass Extinctions Abrupt simultaneous extinctions of numerous taxa, if real, are powerful biostratigaphic markers and are interesting in their own right. Alas, identifying them isn't straightforward. Identifying a truly abrupt extinction requires continuous deposition and a reasonably dense fossil record.

    Even in continuous deposition with a good record, the taxa can be deceptive.

    Mindful of these considerations, we see why biostratigraphers employ a variety of zone definitions despite their invocation of conjecture and assumptions: In many circumstances, the ability to bring more data to bear on a problem is simply more important than the avoidance of the fuzziness that follows from inference an conjecture. The biostratigrapher seeks the optimal tradeoff for the specific situation.

    Biostratigraphic nomenclature and golden spikes:

    Biostratigraphy is the principal determinant of such important things as period boundaries. Boundaries between periods are arbitrarily decided, but usually involve biostratigraphic markers. Some conventions:

    E.G. the Silurian-Devonian boundary (right) was decided to be the base of the Monograptus uniformis (a graptolite) zone. Over forty such golden spikes have been established.

    Quantitative Biostratigraphy Besides hopefully constraining their age and sequence, does biostratigraphy add to our kowledge of the deposition of sediments? Actually, yes.

    Graphic correlation: method for stratigraphic correlation based on statistical correlation of first and last appearances, but not biozone terminology. Facilitates comparison of locality sections containing local FADs and LADs of the same taxa. Used to:

    Composite standards: The examples above correlate teilzones from pairs of localities. On a larger scale, the data used to achieve this can be combined into substantial composite standarddatabases that: Comparison of information from a particular locality with the composite standard facilitates robust identification of range extensions, hiatuses, and outliers.

    Biochronology: When biostratigraphic data is combined with numeric age information we can use biozones as the basis for biochrons, time units (as opposed to rock units).

    One famous application: Land Vertebrate Ages. Originally just North American Land Mammal Ages (for Cenozoic), then extended into mid-Late Cretaceous, then became Land Vertebrate Ages. Now practiced for many different continents.

    Ironically, land mammal assemblages were used as the basis for biochronology because they were too sparse and localized to be useful in identifying biostratigraphic units. Sequence sometimes established by evolutionary grade rather than by any explicit reference to stratigraphy.

    Over a century of development, competing criteria have been used in defiintions of ages. Today, biostratigraphers must formally resolve contradictions that arise as new information becomes available. E.G: The Chadronian was originally defined by the last appearance of titanotheres and the top of the Chadron formation. Alas, titanotheres are now known from above the Chadronian. Which criterion do we use?

    All rest on the assumption that biostratigraphic units are good proxies for time. As a first order approximation, this is so, but again, caution is necessary.

    The bad news:


    The good news:

    Final thoughts

    References:

    • Prothero, D. and R., and F. Schwab. 2003. Sedimentary Geology: An Introduction to Sedimentary Rocks and Stratigraphy, 2nd Edition, W. H. Freeman and Company.