Alongside 3-D seismic, long-reach horizontal drilling and the application of plate tectonics to exploration, sequence stratigraphy has been one of the greatest scientific advances within the petroleum geology industry of the last 50 years.
Although practiced by Peter Vail and his colleagues at Exxon from the 1960s onward, sequence stratigraphy came to the wider attention of the industry with the publication of the seminal AAPG Memoir 26, “Seismic Stratigraphy: Applications to Hydrocarbon Exploration,” in 1977. Since then, it has been embraced by the industry because the integration of time-significant surfaces with stratigraphic architecture provides great predictive insight at a variety of scales and extracts value from many data types.
Sequence stratigraphy includes the interpretation and prediction of sedimentary facies and architecture resulting from changes in relative sea level (the interplay of sediment supply, tectonics and eustasy). If the eustatic component can be isolated, then this provides an enhanced ability to correlate and to predict facies geometries. The validity of eustatic sea-level curves remains controversial, both in terms of demonstrating their proof and of attributing driving mechanisms; nonetheless, however, a growing consensus exists in the stratigraphic community that there are synchronous global sea-level events through geological time. Yet their frequency, amplitude and pace remain much debated.
Cuvier and Catastrophism
The debate around eustasy strongly echoes a debate that engaged the geological pioneers of the 19th century. “Catastrophism” was the term used to describe the notion that Earth’s history had been punctuated by major events that had caused extinctions and affected sedimentation patterns. The validity of this concept was a question that vexed the early geologists of Britain and mainland Europe (the distinction of Britain versus mainland Europe is worth making because catastrophism had more opposition in Britain and more support in mainland Europe).
The primary author of catastrophism was the brilliant French scientist, Baron Georges Cuvier. The noted natural history writer, Stephen Jay Gould, described Cuvier as “perhaps the finest intellect in 19th century science.” Based at the Natural History Museum in Paris, Cuvier made immense contributions to zoology, paleontology and geology. He recognized that the form of animals could be reconstructed by understanding the function of bones and organs, a discipline now termed “comparative anatomy.” A gallery he created in the museum that housed more than 16,000 zoological specimens, illustrating form and function, was one of the scientific sensations of the early 19th century.
Having studied living animals, Cuvier became increasingly interested in fossils and applied comparative anatomy to determine the form of creatures very different from anything living. He recognized a huge Cretaceous marine lizard (now called a “Mosasaurus”) and a flying reptile that he called a “petro-dactyle.” These and other seemingly fabulous creatures gave Cuvier cause to reason that worlds must have existed in the past, inhabited by extinct creatures very different to those around us. This conclusion then led to a need to understand the history of the Earth, providing a rallying call for geology to develop a purpose that would enable geoscience to match the achievements of physics and astronomy.
Accordingly, Cuvier’s geological interests rapidly expanded beyond the determination of the nature of fossils. In 1811, in collaboration with the mineralogist Alexandre Brongniart, he presented “Essai sur la géographie mineralogique des environs de Paris,” or “Essay on the mineralogical geography of the surroundings of Paris,” which contained not only a geological map of the Paris Basin, but a stratigraphic synthesis in the form of a novel sedimentary log. In the manner of William Smith in Britain, Cuvier and Brongniart used the fossil content of the strata for subdivision and correlation. Moreover, they determined a series of alternating freshwater and marine environments, representing a pioneering attempt to reconstruct the geological history of a rock succession.
This type of analysis led Cuvier to conclude that Earth’s history had been episodically interrupted by sudden “revolutions” that caused faunal extinctions.
In 1812, he wrote, “Life upon the Earth in those times was often overtaken by these frightful occurrences. Living things without number were swept out of existence by catastrophes. Those inhabiting the dry lands were engulfed by deluges, others whose home was in the waters perished when the sea bottom suddenly became dry land; whole races were extinguished leaving mere traces of their existence, which are now difficult to recognize, even by the naturalist. The evidences of those great and terrible events are everywhere to be clearly seen by anyone who knows how to read the record of the rocks.”
The comparison with sequence stratigraphy is obvious: sea-level changes profoundly affect the rock record.
Cuvier’s description of Earth history as a series of former worlds inhabited by exotic extinct creatures, with each world in turn being destroyed by catastrophe, generated a popular, romantic interest in geology and paleontology. Here was a scientific challenge for the age – to reconstruct and place in order these lost worlds; in other words – to determine the history of our planet back into deep time.
This challenge was picked up by many of Cuvier’s disciples. Foremost among these was Alcide d’Orbigny, a pioneering micropaleontologist and scientific explorer. He integrated catastrophism with the formal subdivision of geological time by introducing the concept of stages (e.g., Cenomanian) linked to global events. For d’Orbigny, these were “the expression of the boundaries which Nature has drawn with bold strokes across the globe.” Stages were viewed as the results of transgressions separated by unconformities created by sea-level fall, once again drawing a comparison with modern sequence stratigraphy.
‘The Present as the Key to the Past’
However, there were those who vehemently opposed catastrophism. Foremost among these was Sir Charles Lyell, whose “Principles of Geology” (1830-33) provided a manifesto for a gradualistic view of Earth history and uniformitarianism: a steady-state Earth. Like Cuvier, Lyell was obsessed with making geology more scientific. Geology needed a set of laws by which to operate in order to be judged as a true science. This need meant that “All theories are rejected which involve the assumption of sudden and violent catastrophes and revolutions of the whole Earth, and its inhabitants.” It also meant that “No causes whatever have from the earliest time … to the present, ever acted, but those now acting and that they have never acted with different degrees of energy from which they now exert.” Apparent sudden changes can be explained by the imperfect nature of the geological record, he argued. Such a doctrine had also been expressed by one of the founding fathers of geology, James Hutton, in the late 18th century.
For much of the rest of the 19th and 20th centuries, it seemed as though Lyell and uniformitarianism prevailed in the debate. Students were taught “the present is the key to the past” and that the remarkable should be dismissed as an explanation for geological phenomena. However, there were those who persisted in recognizing the importance of “events” in Earth history. This included the great Austrian geologist Eduard Suess, who introduced the concept of eustasy to describe global transgressions and regressions.
The New Catastrophism
In the first half of the 20th century, the notion of episodic global or regional changes in sea-level found favor with an influential set of stratigraphers, including T.C. Chamberlin, E.O. Ulrich, Eliot Blackwelder, Joseph Barrell, Amadeus Grabau, Johannes Umbgrove, Harry Wheeler and, eventually, Larry Sloss, who pioneered the use of sequences in their modern sense. Grabau was happy to describe “The Rhythm of the Ages,” a reflection of his views on stratigraphy reflecting eustasy.
By the late 1960s and ‘70s, iconoclastic geologists, such as Derek Ager, talked of “The New Catastrophism” and stressed the importance of events in the stratigraphic record. For Ager, geological history could be likened to the life of a soldier: “Long periods of boredom and short periods of terror.” Gradualistic processes occurred, but were punctuated by the unusual, be those asteroid impacts or events originating much closer to home, such as intense volcanicity, brief periods of global anoxia, or submarine landslides.
Against this background of neocatastrophism, the concepts of sequence stratigraphy and eustasy could be considered as viable. Relatively short-term sea-level fluctuations were likely, and some might well be global in nature (their local expression modified by tectonics and sediment supply). The relatively recent acceptance of astronomical forcing of cycles in the sedimentary record provides a hybrid of uniformitarianism and catastrophism. The processes are ongoing, yet can lead to sudden, dramatic shifts in sedimentation patterns.
In this context, could global sea-level changes be related to the definition of chronostratigraphic units, as originally envisaged by Cuvier’s disciple, d’Orbigny?
Stages are now defined by the Global Boundary Stratotype Section and Point methodology. A GSSP is an internationally accepted reference section in which the lower boundary of a stage is defined by a specific stratigraphic horizon. This horizon is generally associated with an event in the rock record – typically an inception or extinction of a key fossil species. The choice of horizon/location and associated event can be the subject of contentious debate; as such, there are a significant number of stages that are currently not defined by a GSSP. Even those that are remain the subject of ongoing discussion.
Therefore, a case can be made for linking a global sea-level event to a stage boundary definition. Peter Vail commented on this possibility in 1977: “Using global cycles with their natural and significant boundaries, an international system of geochronology can be developed on a rational basis.”
The scientific community still remains wary of this approach because the reality of eustasy continues to be debated. Nonetheless, Cuvier appears to have been vindicated. Sudden transgressions and regressions are a part of Earth history, aligning sequence stratigraphy with some of the earliest interpretations of the rock record.