Seeing the connections

06.01.2014
Distance need not be a barrier in geological interpretation. This point is borne out by a thin layer of rock in south-east France, which marks an important point in geological time on the NCS.
  • Robert W Williams and Bjørn Rasen (photos)
 

The author studying the 168-million-year-old “limestone bed RB071”, the layer which defines the boundary between the Bajocian stage (left) and the Bathonian stage (right). This layer was designated a “golden spike” in 2008.

The author studying the 168-million-year-old “limestone bed RB071”, the layer which defines the boundary between the Bajocian stage (left) and the Bathonian stage (right). This layer was designated a “golden spike” in 2008.

 

How could the fossil content of rocks in the French region of Provence have anything to say about the way geologists interpret a particular depositional stage off Norway?

Moreover, why is this special point in time named not after the village where it is defined but for the city of Bath in the UK?

Curiously, the answers have to do with the slow northward drift of Africa and the birth of geology as a field of scientific inquiry.

The excruciatingly slow collision of the African and European continents created the Alps, and exposed an extraordinary record of Europe’s geological past.

That in turn was crucial to the development of geology in the 19th century. These mountains lay bare a vast expanse of the Earth’s history in the form of folded, buckled and fractured strata.

Some of the rocks forming magnificent Alpine vistas from the Gorenjska region of Slovenia to the Provence Alps of France were once soft mud on the floor of an ocean now squeezed into oblivion.

The colossal energy of a continental mash-up has compressed crystalline basement rocks and overlying sediments like an accordion, and thereby given birth to a mountain chain.

 

The Provence Alps of France were once soft mud on the floor of an ocean.

Rising landscape.
The Provence Alps of France were once soft mud on the floor of an ocean.

 

Sun-washed

Ancient decor

< Ancient decor.
Public spaces in Barrême are ornamented with fossil ammonites (extinct shelled octopi) around 127 million years old.


The Asse de Clumanc and Asse de Blieux waterways in south-east France merge to form the river Asse. At their confluence stands Barrême, a quiet Provençal village of centuries-old buildings built of pastel masonry with a sunwashed patina.

A curious feature of Barrême’s public spaces is decorative displays of C-shaped, horn-like fossils almost a metre tall. They were once ammonites, shelled octopi which gradually died out toward the end of the Cretaceous.

Fifty-nine million years before the ammonites went extinct, this C-shaped species was common when the Alpes-de-Haute- Provence region was a flat, muddy sea floor.

The rocks under Barrême received international recognition 140 years ago, because these fossils showed that the layers there were a previously unknown section of the Lower Cretaceous.

That is why the name of this medieval town is familiar worldwide to geologists in its adjectival form of “barremian”.

Le Barrêmien is a group of strata defined in 1873 by the French geologist Henry Coquand, and ranks as the fourth of six stages in the Lower Cretaceous series.

 

Fossil ammonite on the boundary between Bajocian and Bathonian.

Valuable.
Fossil ammonite on the boundary between Bajocian and Bathonian.

Set

To a geologist, a stage is a set of rock beds which contain specific fossil species, while a series represents a group of such stages. This classification scheme began when early 19th-century geologists showed that the vertical and horizontal distribution of fossil species and rock types was the key to unlocking Earth’s history.

Geological mapping proved an essential tool, which transformed geology into a mainstream science.

But 19th-century geologists employed only the relative ages of rocks to decipher deep time. There was no knowledge about or technology available to measure absolute ages.

Early stratigraphic nomenclature dealt with rock characteristics and the chronologic succession of fossil species – parameters which geologists could observe in the field.

Erathems, systems, series and stages became standard terms for discussing sedimentary rocks. The first to discover and define new rock intervals such as series and stages also had the honour of naming them.

These designations were often derived from geographical regions, or even from cities and villages which lay near the rock sections.

The 20th century’s advances in nuclear physics would provide the technology to measure the ages of rocks in millions of years. Absolute age dating opened a window to Earth history which allowed more detailed mapping.

Geological time units (eras, periods, epochs and ages) became meaningful units as geophysicists steadily improved the accuracy of radioisotope age dating.

Enhanced data quality over the past 50 years prompted a revision of the nomenclature used for rock and time subdivisions. This process became the mandate of the International Commission on Stratigraphy (ICS).

Through specialist subcommittees, the ICS refines global rock units (systems, series and stages) to make them applicable over the entire Earth.

To define a stage boundary, geologists nominate several candidate sedimentary sections to an ICS subcommittee. The requirements are stringent.

For the ICS to approve a section, the defining layer must hold at least one fossil event (for example, the oldest appearance of a particular microfossil) which occurs at the same time in deposits worldwide.

The transition to the older stage below must also have no missing layers, and volcanic ash must be present because it contains unstable elements which permit absolute ages to be determined for the fossils.

Finally, the candidate section must lie in an area which has political stability and infrastructure, and which allows access to the rocks.

 

Defining

The ICS calls the defining layer a global stratotype section and point (GSSP). A geologist once described a GSSP as a “golden spike” because it is metaphorically like a nail hammered into the rock to mark a boundary between two geological units.

With their definitions based on global fossil events and absolute dates, these spikes provide geoscientists with a global standard for geological mapping.

Previously, for example, geologists working on the NCS used the definition of both the Volgian and the Portlandian stages to denote the uppermost section of the Jurassic.

Fossil ammonites found in northern Europe and Asia prove effective markers for local Volgian beds. Unfortunately, these ammonite species are absent in southern latitudes.

In the UK, the Portlandian stage covers almost the same interval as the Volgian, but the definition is based on ammonites which did not mix with their contemporaries in other realms.

Ammonites which preferred the southern Jurassic ocean (later eliminated by the collision between Africa and Europe) define the Tithonian stage in low latitudes.

In Europe alone, therefore, the Volgian, Portlandian and Tithonian stages are overlapping but not identical intervals of rock and time. To make matters worse, the European stages overlap with others throughout the world.

So what happened to Henry Coquand's Le Barrêmien after the advent of golden spikes? His strata near the village actually fails to satisfy all the criteria for a GSSP.

While the Barremian stage will retain its name, however, the ICS will probably award the golden spike to a rock outcrop in the Spanish province of Murcia.

But its rich Provencal geology has nevertheless ensured that the village of Barrême won gold after all – but just not for the Barremian stage.

In 2008, the ICS awarded it a completely unrelated GSSP located in the nearby Ravin du Bès, where the steep cliffs comprise thin layers of light and dark marl.

These alternating shades of grey form a parallel striped pattern which resembles a gigantic geological bar code, and the golden spike layer is 168 million years old.

Thirty-nine million years older than the Barremian stage, it defines the base of the Bathonian stage in the Middle Jurassic.

Belgian geologist d'Omalius d'Halloy proposed the name Bathonian in 1843 after the “Bath Oolite” limestone near the city of that name in south-west England.

This 167-million-year old limestone gives the local buildings their characteristic warm colour. It represents a group of marine beds containing a characteristic fossil fauna.

The Bathonian stage plays an important role on the NCS, since it lies in the middle of the very sandy series called the Middle Jurassic.

These sands contain more than 60 per cent of Norway’s petroleum reserves, and it is fascinating to think that this stage was discovered as far back as 1843.

Originally identified in the UK, it is now defined officially by a thin layer of marl in a rugged ravine five kilometres downstream from Barrême.

Because of its wide-ranging geology, France has been awarded six GSSPs by the ICS, including two in the Cretaceous (with the ICS currently assessing three additional candidates there).

In addition come one in the Jurassic (with two additional candidates), one at the Devonian- Carboniferous boundary and two more for the uppermost stages of the Devonian.

If all the candidate sections are also accepted, France will rank alongside Italy and the UK as the leading countries for accommodating golden spikes.


Topics: Geology