Oxfordian Age: The Dawn of the Late Jurassic
The Oxfordian represents a pivotal chapter in Earth's history, marking the beginning of the Late Jurassic Epoch. As the lowest stage of the Upper Jurassic Series, it provides geologists and paleontologists with a critical window into the marine environments and biological evolution of the Mesozoic Era.
Spanning from approximately 161.5 ± 1.0 million years ago (Ma) to 154.8 ± 0.8 Ma, the Oxfordian sits chronologically between the Callovian age and the Kimmeridgian age. Its study allows scientists to map the transition of global ecosystems during a time when the planet's geography and climate were in significant flux.
[ไม่มีภาพประกอบ]
Key Facts
- Time Span: 161.5 ± 1.0 Ma to 154.8 ± 0.8 Ma.
- Position: Earliest age of the Late Jurassic Epoch.
- Defining Fossils: Defined by the first appearance of specific ammonite species.
- Global Standard: Recognized by the International Commission on Stratigraphy (ICS).
- Primary Lithology: Often characterized by limestone, marl, and clay layers.
Etymology and Historical Context
The term "Oxfordian" was introduced in 1844 by Alcide d'Orbigny. The name is derived from the city of Oxford, England, where the geological beds of this age are prominently developed. However, these formations are not limited to one city; they crop out almost continuously from Dorset to the coast of Yorkshire, typically forming low, broad valleys.
Before the formal adoption of the term Oxfordian, early geologists used various descriptions. William Smith referred to these layers as "Clunch Clay and Shale" between 1815 and 1816, while W. Buckland described them in 1818 as "Oxford, Forest or Fen Clay." Today, these rocks are well-exposed in several locations, including Weymouth, Oxford, Bedford, Peterborough, the cliffs of Scarborough, Red Cliff, Gristhorpe Bay, and parts of Uig and Skye.
Stratigraphic Definitions
In stratigraphy—the study of rock layers (strata)—the boundaries of an age are defined by the First Appearance Datum (FAD), which is the earliest occurrence of a specific fossil in the geological record.
The Lower Boundary
The base of the Oxfordian Stage is defined by the FAD of the ammonite species Brightia thuouxensis. While a Global Boundary Stratotype Section and Point (GSSP)—a physical reference point in the rock record—had not been assigned as of 2009, candidate sections include Redcliff Point in Dorset, UK, and Savouron in Provence, France. The horizon of the ammonite Cardioceras redcliffense is also considered a candidate for this boundary.
The Upper Boundary
The top of the Oxfordian Stage, which marks the beginning of the Kimmeridgian, is defined by the FAD of the ammonite species Pictonia baylei. The GSSP for this boundary is located at Flodigarry, Skye, Scotland (57°39′40″N 6°14′44″W), and was officially ratified in 2021.
Biozones of the Tethys Domain
Within the Tethys domain—the ancient ocean that existed between the supercontinents of Gondwana and Laurasia—the Oxfordian is further subdivided into six distinct ammonite biozones. These zones allow for high-precision dating of marine sediments:
- Zone of Epipeltoceras bimammatum
- Zone of Perishinctes bifurcatus
- Zone of Gregoryceras transversarium
- Zone of Perisphinctes plicatilis
- Zone of Cardioceras cordatum
- Zone of Quenstedtoceras mariae
These alternating layers of limestone (which are more competent or hard) and marl or clay provide a cyclical record of the environmental changes occurring during this period, as seen in locations like Péry-Reuchenette in the Jura Mountains of Switzerland.
Oxfordian Summary Table
| Feature | Details |
|---|---|
| Epoch | Late Jurassic |
| Time Range | 161.5 ± 1.0 to 154.8 ± 0.8 Ma |
| Lower Boundary Fossil | Brightia thuouxensis |
| Upper Boundary Fossil | Pictonia baylei |
| Upper GSSP Location | Flodigarry, Skye, Scotland |
| Preceding Age | Callovian |
| Succeeding Age | Kimmeridgian |
Frequently Asked Questions
What is the Oxfordian?
The Oxfordian is the earliest age of the Late Jurassic Epoch and the lowest stage of the Upper Jurassic Series, occurring between approximately 161.5 and 154.8 million years ago.
How is the start of the Oxfordian defined?
The base of the Oxfordian is defined by the first appearance of the ammonite species Brightia thuouxensis in the stratigraphic record.
Where can Oxfordian rocks be found today?
They are widely found in England, cropping out from Dorset to Yorkshire, with notable exposures in Oxford, Weymouth, Bedford, and Scarborough, as well as in Skye and Uig in Scotland.
What is a GSSP and does the Oxfordian have one?
A GSSP (Global Boundary Stratotype Section and Point) is an internationally agreed-upon reference point in a rock sequence. The Oxfordian's upper boundary has a ratified GSSP at Flodigarry, Skye, Scotland, while the lower boundary has several candidate sections.
Which fossils are used to divide the Oxfordian in the Tethys domain?
The Oxfordian is divided into six biozones based on the presence of specific ammonites, including Epipeltoceras bimammatum, Perishinctes bifurcatus, and Quenstedtoceras mariae, among others.