The newest geological time period: The Ediacaran
|Category||Paleontology and Stratigraphy|
|Location||International Geological Congress,oslo 2008|
|Holding Date||23 September 2008|
The Ediacaran, agreed and ratified in 2004, is the newest period-scale addition to the Geological Time Scale. It represents 60-90 Myr of geological time, and is remarkable in a number of ways. It is the first period to be defined within the Precambrian, and the first to be defined upon essentially non-biological stratigraphic proxy data. It was established as a single, undivided unit and one whose lower boundary has relatively broad numerical time constraints. Its definition, moreover, is predicated upon the interpretation that the collapse of the Marinoan Glaciation and the inception of cap carbonate deposition in the Ediacara Hills represents an effectively synchronous, globally traceable event; this contrasts with the more typical diachronous distribution of glacial deposits in Phanerozoic glaciations. The cap carbonate, found in several regions worldwide, is distinctive lithologically and records an extraordinary C-isotope excursion that acts as a chemostratigraphic marker.
The recently proposed subdivision of the Ediacaran Period into five stages in the Yangtze Platfom successions of China (based largely upon secular changes in carbon and strontium isotopes) is promising but has yet to be tested worldwide. Few of the remarkable fossils associated with the (later part of the) Ediacaran Period look likely to be of great value biostratigraphically, other than at very coarse levels of resolution. The frondose Ediacaran biota has now been found widely (England, Newfoundland, Russia, Australia, China etc); however, this biota seems simultaneously too facies-controlled and too slowly evolving to be biostratigraphically useful in practice, while such as the Weng’an Biota (embryos and such) of the Doushuantuo Formation requires exceptional circumstances for its preservation. The acritarchs are promising but not yet widely tested, while the more widespread shelly macrofossils (e.g. Cloudina) were confined to the latest part of the period.
Despite these uncertainties (and the competing suggestion of formalizing the Vendian), the Ediacaran has been generally accepted by geologists as an effective and useful formal division of geological time. The rapid advances made in deciphering its geological and palaeoenvironmental history suggests that its lower boundary and any potential subdivisions will become increasingly well constrained. Furthermore, the success of this proposal makes it increasingly likely that the preceding Cryogenian will emerge as a formal period, defined by a GSSP to coincide with the inception of the late Proterozoic glaciations.Much further work is necessary, however, before this will come to fruition.