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:

Taken these in order of discovery, we start with fossils - the realm of biostratigraphy.

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 anything - England as it happened.

    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?

    Robert Bakker illustration of Carnian vertebrates from Kheper.

    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:


    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 more reliable:

    Depositional Caveats: