Biostratigraphy I: Rock units and Biozones

Stratigraphy without fossils:

Using the stratigraphic principles of Nicholas Steno: (1668) and the uniformitarian principle of James Hutton (1795), Geologists of the early 19th century could establish the relative ages of formation scale rock units.

Contact of the Coconino (above) and Hermit Shale formations - Grand Canyon
Formations: the smallest mappable units. E.G. the Chinle Formation. Remember, formations must:

However to identify formations based strictly on lithostratigraphy, there are difficulties:

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:

    William Smith's geologic map from
    University of Nebraska Omaha - GEOL1170
    By noting the fossils present, it became possible to: In 1815, Smith published the first geologic map of England (right).

    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 depends on biostratigraphy.

    The ammonite Sphenodiscus, restricted to Maastrichthian Stage of Cretaceous.
    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.

    In GEOL 331 we assume that all students, regardless of background, are have memorized the The Geologic Time Scale - simplified version, and are able to interpret a detailed version.

    Remember, the numerical dates that we place on their upper and lower boundaries are secondary to the identity of the rock units. For example, the formal base of the Jurassic is a stratotype - a specific contact in the Kuhjoch section of the Karwendel mountains in Austria, between rock units dated to 201.3 Ma. If we were to discover that Triassic rocks below that contact were actually only 200 Ma, we would not say, "Oops. I guess these rocks are Jurassic after all." Rather, we would reassess the age of the boundary between the Triassic and Jurassic.

    Subsequent to Smith:

    Biozones - 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.

    The following examples are drawn from the hypothetical microcosm 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 of taxon B
  • 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.:

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