GEOL 331 Principles of Paleontology

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:

Have a distinct lower and upper boundary
Have a type section: a particular spot where it is well exposed and can be compared to other localities
Have been laterally continuous during formation (even if its surface exposure isn't)
Be stratigraphically continuous from top to bottom (except for intertonguing)
Be mappable on a standard quadrangle-scale map

Coconino Formation
Typically have a charactieristic range of lithologies. E.G. the Permian Coconino Formation of northern Arizona. This mature eolian quartzarenite is exposed both in the Grand Canyon (right) and in nearby Walnut Canyon, and is continuous between them beneath the surface.

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

Physical proximity: Units have to be seen in direct association.
Diachronous (i.e. Time transgressive) units representing facies might overlap temporally with adjacent units causing confusion over whether superposition of units actually signified relative age.

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:

Rock units were characterized by unique sets of fossil taxa.
These sets of fossil taxa - faunas presumably represent the diversity of things living at the time the sediment was laid down.
That the occurrance of many fossils was independent of the lithology of the rock.

William Smith's geologic map from
University of Nebraska Omaha - GEOL1170

By noting the fossils present, it became possible to:

correlate rock units of varying lithologies across vast distances
establish time horizons in lithologically uniform or diachronous rock units.

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?

Index fossils: Fossils useful in biostratigraphy.
- Common
- Geographically widespread
- Easily preservable
- Diagnosable
- Found in multiple environments (when dead)
- Short species duration
Example: Ammonoids (right), Shelled cephalopods that evolved quickly, so each species lasted only a few million years, but whose remains were distributed worldwide in many environments.

Lingula from SUNY Cortland - Tutorial - Brachiopoda
Facies fossils: In sharp contrast. Fossils of organisms that endured for long periods of geologic time but were linked to a specific environment. Example: Lingula (right), a brachiopod living only in lagoonal mud-flats that has changed very little in the last 500 million years.

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:

Stages: Groups of formations characterized by distinct faunal assemblages. E.G. the Carnian Stage of the Upper Triassic Series.
Series: Groups of stages. E.G. the Upper Triassic Series
Systems: Groups of series. E.G. the Triassic System

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:

1842 - Alcide d'Orbigny (1802-1857) published analysis of Jurassic system of France. He established the concept of stages which he supports by identifying fossil assemblages. He regarded each stage as an interval between separate creations and flood-induced extinctions.
Roughly contemproaneously, Friedrich Quenstedt (1809-1889) established his own system based on detailed data-bases recording first and last appearance of individual species. Quendstedt rejected d'Orbigny's assemblages as too vague.
Quendstedt's student Albert Oppel (1831-1865) studied biostratigraphy of France, Switzerland, and England, augmenting Quendstedt's system and developing a system of dignostic aggregates identified by overlapping range zones called Biozones that discriminate fossil faunas on a much finer scale. Oppel is widely viewed as the founder of modern biostratigraphy.

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):

Teilzone of taxon A at locality I.

Teilzone: Between local FAD & LAD of that taxon. - The truly unambiguous observations!

Taxon Range Zone of taxon A
Taxon Range Zone: Between global FAD & LAD of that taxon. - Requires some inferrence - Global correlation of horizons bearing the fossil taxon.

Concurrent Range Zone of taxa A - D
Concurrent Range Zone: Intersection of the taxon range zones of two or more taxa.

Interval Zone of taxon A and B FADs
Interval Zone: Interval (global) between two successive FADs or two successive LADs

Assemblage Zone of taxa A - D
Assemblage Zone: Characterized by 3 or more taxa in natural assemblage. Biostratigraphers like assemblages because they are free from some of the complications that plague biozones based on a single taxon. (See next lecture) Alas, with assemblages, they must put up with fuzzy boundaries, as the FADs and LADs of the constituent taxa aren't simultaneous.
- Special Case: Oppel Zone: Addresses the shortcomings of the assemblage zone by being defined by the FAD or LAD of one taxon, but characterized by additional taxa of an assemblage. Named for Albert Oppel, the first to use non-arbitrarily defined biozones (1858).

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.:

The coccolithophorid Braarudosphaera which bloomed in times of global environmental stress.
Changes in coiling direction of the foram Globotruncana truncatulinoides in response to global temperature changes.

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