Vendian Period

Abstract

Keywords: stratigraphy, Vendian Period, Vendian biota, fossil record, evolution, extinction

Introduction

The Vendian System and Period were first proposed by Sokolov 1952, from drill core sequences on the Siberian Platform (Sokolov & Fedonkin 1984). Although the Vendian was not embraced quickly, and was not adopted by the Subcommission on Precambrian Stratigraphy, it came into almost universal de facto usage. A new stratotype in Belarus was proposed (Makhnach & Veretennikov 2001) to address the perceived shortcoming of the original type section in a drill core.

There is much confusion over the status of the Vendian, so it is worth further detailing its status, particularly in relation to the “new” Ediacaran Period (Knoll et al. 2004). The Vendian is an Eurasian geochronologic and chronostratigraphic unit, just as the Sinian is recognised in China. It was never recognised by the Subcommission on Precambrian Stratigraphy which had, in 1991, opted instead for a “Neoproterozoic III” interval defined “strictly by chonometric age, without reference to events recorded in sedimetary rocks” (Knoll et al. 2004, p. 621). The recent recognition of a properly (GSSP) defined Ediacaran System and Period by the Subcommission on Precambrian Stratigraphy is certainly a step in the right direction, and it probably means that new publications will increasingly refer to the Ediacaran rather than the Vendian, and references to the latter outside of the Siberian Platform will eventually disappear. However, it does not mean, as some commentators have suggested, that the Vendian is suddenly invalidated as a legitimate geochronologic and chronostratigraphic unit, or that it has “gone away” in any sense.

The absolute age constraints on the Vendian interval have ebbed and flowed over the past few years; perhaps the best current estimate is from ~605 to 541 million years (Ma) ago, somewhat shorter than current estimates for the Ediacaran (~635 to 541 Ma; Cohen et al. 2015). Possibly, however, this is simply because nobody is bothering to revise the absolute age estimates for the base of the Vendian as new data and techniques come to light. To a first order approximation, it is probably reasonable to simply think of the Vendian as the Siberian equivalent to the Ediacaran. Much of the following text will take this approach.

Throughout the extent of the period, dominant organisms were simple, entirely marine, and for the most part soft-bodied: hard-bodied organisms did not occur until nearly the beginning of the Cambrian Period when the so-called “small shelly faunas” appeared.

Stratigraphy

Type Section/Sections

The Vendian Period and System were first proposed by Sokolov 1952, from drill core sequences on the Siberian Platform. More recently, a new stratotype in Belarus was proposed by Makhnach & Veretennikov 2001.

Lower (Sturtian-Vendian) Boundary

The Sturtian is another geochronologic unit which is no longer in widespread use; it encompassed the Sturtian glaciation (one of the “snowball” events) and was defined in Australia.

Placing the base of the Vendian is a somewhat pointless exercise today; probably nobody will ever complete the necessary work to provide an accurate chronology.

Upper (Vendian-Cambrian) Boundary

Since 1947, when H.E. Wheeler initiated debate with the suggestion that the Precambrian-Cambrian boundary should be based upon the first appearance of trilobites, much has ensued. Progress has largely been facilitated by the International Geological Congress (IGC) and the establishment in 1960 of a Subcommission on Cambrian Stratigraphy. The classical idea of placing the boundary at an unconformity has been displaced by the search for monofacial, continuous deposition sequences across the boundary, with the view to selecting a stratotype.

The search itself produced a wealth of data from around the world – including the Palaeotethyan Belt, Siberian Platform, and England – eventually focusing upon south-east Newfoundland.

Prior to 1990, the boundary was generally placed at the base of the Tommotian Stage, a rock unit known from the Siberian Platform. However, in 1991 the International Subcommission on Cambrian Stratigraphy made a decision to draw the base on the Cambrian at the first appearance datum (FAD) of the distinctive horizontal burrow ichnofossil, Treptichnus (formerly Phycodes and Trichophycus) pedum (Seilacher 1955; Fig. 1), in the reference section at Fortune Head. This horizon correlates with the base of the Siberian Nemakit-Daldynian Stage, ~13 Ma earlier than the Tommotian (included within the Vendian System in some older literature).

Both of the Siberian stages have since been abandoned by the ISCS, however. Presently, the early Cambrian is represented by the Terreneuvian Series, which is divided into the Fortunian Stage (basal) and “Stage 2”. This is unlikely to be the final word on the matter.

(1) 

Fig. 1: The horizontal burrow trace fossil, Treptichnus (formerly Phycodes and Trichophycus) pedum defines the lower boundary of the Cambrian in the reference section at Fortune Head, southeastern Newfoundland. [Image courtesy of Dr. Gerd Geyer, Institut für Paläontologie, Bayerische Julius-Maximilians-Universität, Würzburg, Germany.]

Chronology

Paleogeography

Paleontology

New Zealand Occurrences

Precambrian rocks are almost unknown in New Zealand. Possibly the most likely contender is parts of the Balloon Formation, which is exposed in the Cobb Valley area: see Cooper & Grindley 1982, p. 50.

References

Cohen, K.M.; Finney, S.C.; Gibbard, P.L.; Fan, J.X. 2015: The ICS international chronostratigraphic chart v 2015/01. Episodes 36: 199-204.

Cooper, R.A.; Grindley, G.W. 1982: Late Proterozoic to Devonian sequences of southeastern Australia, Antarctica and New Zealand and their correlation. Geological Society of Australia Special Publication 9: 1-103.

Knoll, A.H.; Walter, M.R.; Narbonne, G.M.; Christie-Blick, N. 2004: A new period for the geologic time scale. Science 305: 621-622. Science.

Makhnach, A.S.; Veretennikov, N.V. 2001: The Vendian of Belarus as one of the priority stratotypes of the Vendian System of the East European Platform. Doklady of the National Academy of Sciences of Belarus 45: 123-126.

Ogg, J.G.; Ogg, G.; Gradstein, F.M. 2008: The concise geologic time scale. Cambridge University Press: 1-177.

Seilacher, A. 1955: Spuren und fazies im Unterkambrium. In Schindewolf, O.; Seilacher, A. (ed.) 1955: Beiträge zur Kenntnis des Kambriums in der Salt Range (Pakistan). Akademie der Wissenschaften un der Literatur, Mainz, Abhandlungen der mathematisch-naturwissenschaftlichen Klasse 10 10: 261-446.

Sokolov, B.S. 1952: On the Age of the Oldest Sedimentary Cover of the Russian Platform. Izvestiya Acad. Nauk SSSR, Geol. Ser. 5: 12-20. .

Sokolov, B.S.; Fedonkin, M.A. 1984: The Vendian as the Terminal System of the Precambrian. Episodes, 7: 12-19. .

Wheeler, H.E. 1947: Base of the Cambrian System. Journal of Geology 55: 153-159.