Cambrian stratigraphy

From Palaeos.org

Stages of the Cambrian

Current Status of Early & Middle Cambrian Stratigraphy

The stratigraphy of the Cambrian - particularly the Early and Middle Cambrian - has always been chaotic. A brief survey of recent work suggests that it is likely to remain that way. The ICS has outlined its proposal for these epochs, but it has been impossible to obtain consensus. The heart of the problem lies, not with some innate intractability of the stratigraphic psyche, nor with administrative bungling by the ICS. The source of the confusion is, rather, the creativity of stratigraphic science.

In the XIXth and XXth Centuries, the Lower Cambrian could not be subdivided with any confidence because none of the usual markers were present. There were few well-mapped Early Cambrian sites, and no Early Cambrian conodonts, ammonites, graptolites, readable magnetic polarity reversals, or even (thankfully!) any iridium anomalies. Nevertheless, the problem in the XXIst Century is the opposite. We are now awash in data from new sources using new methods which have not been adquately standardized, correlated in different regions, or securely nailed down with radiometric dates.

One excellent example is the base of the Cambrian itself. Only a few years ago, the stratigraphic community coalesced around the idea that, although there were essentially no Earliest Cambrian fossils, except the unclassifiable Ediacaran fauna, the Cambrian should begin with the first macroscopic trace fossils - simple bioturbation by bilaterian "worms." In 1992, the ICS selected a suitable global stratotype section and point in Newfoundland, Canada, to mark the first clear appearance of bilaterian animals at about 542 Mya. Now (2006), only 14 years later, it is obvious that the Bilateria date well into the Ediacaran, as evidenced by fossils like Kimberella (Fedonkin & Waggoner, 1997), bilaterian, possibly arthropod, trace fossils (Martin et al., 2000), and - yes - even trace fossils of bilaterian worms (Dornbos et al., 2005).

Meanwhile, new, but rather unstandardized, stable isotope techniques (e.g. ⁸⁷Sr/⁸⁶Sr, δ¹³C) have added layers of new stratigraphic information. See, generally, Knoll (2000) (we, too, have had a shot at explaining δ¹³C measurements (scroll to near bottom of page) and discussing some of their problems). Previously "enigmatic" Ediacaran and Early Cambrian fossils are also being gradually tied to conventional taxa, so that biostratigraphic schemes and correlations can be extended into previously dark corners of the Cambrian. Knoll (2000). For examples, see Jago et al. (2002); Gubanov & Peel (2003). Finally, as in many other areas of paleontology, political changes in the last two decades have opened new localities to exploration (and closed others).

The ICS Position

Meanwhile, the International Subcommission of Cambrian Stratigraphy (ISCS) at the ICS has been, ever so slowly, working away at the insurmountable problem of imposing an "official" stratigraphy on ground which is both literally and figuratively shifting. On one side, they face increasing pressure and high-volume whining from ill-mannered people, including ourselves, who need a stable, well-characterized outline for the Cambrian. On the other side lies the abyssal certainty that, whatever they decide, it will be obsolete within a few years and everyone will laugh at them. So, like Frankenstein (who was sort of a committee himself) the ISCS has slowly retreated before an angry mob of geological peasants armed with with torches and pitchforks, shuffling up the narrow mountain defile towards an inevitable doom. Occasionally, the ISCS howls with rage and frustration, and tosses down a partial decision or two to discourage the more aggressive challengers.

In this curious manner have we made halting, and somewhat limited, progress toward an official stratigraphy of the Cambrian. Actually, a comparison of the annual reports of the ISCS for 2001 and 2005 suggests that not a lot has been accomplished in the last few years, and the ISCS website is in serious disrepair. Nevertheless, the promises made by the Subcommission have at least become much more specific, due largely to actions taken in the last year (2005).

Here's the current status. The Early Cambrian will be divided into two epochs ("series"), each containing two ages ("stages"). The earlier of the two epochs is the Terreneuvian. The general effect is to collapse the top of the Early Cambrian into the Botomian (!). The Middle Cambrian and Furongian will each have three ages. The impact of tripartite divisions is hard to assess, since almost every regional system uses only two stages for each of these epochs. The ISCS has picked out some likely trilobites to mark these later stages. Thus, we expect that all this will be filled out with trilobite zonation in the fullness of time.

For the lower reaches of the Cambrian, the obvious and sensible thing would be for the ISCS to adopt the stages from the Siberian Platform, like everyone else. The probability that this will happen approaches zero. However, this type of decision sometimes becomes an intensely political business. We try our best to steer clear of political issues. Accordingly, we will refrain from further comment or speculation. The current (2009) overall picture looks like this:

The '~' indicates an estimated date or duration, while '?' indicates unofficial status of the underlying stage.

As we write (2009), there is furious re-dating going on, with some boundaries (e.g. Tommotian, Paibian) seeming to change from month to month. Thus, while we are current to a certain point, the dates may be slightly obsolete by the time you refer to this page. While this is awkward, it is a distinct improvement over the stasis of the pre-2006 era.

ATW080426, wikified HAJ090121