CAMBRIAN STRATIGRAPHY

The Cambrian is a short period in earth history, starting approximately 545 m.y. ago and ending about 490 m.y. ago. Nevertheless, it was without any doubt one of the most important and dramatic periods. The lower boundary of the Cambrian is not only the beginning of a new system but also the start of the Paleozoic and the Phanerozoic. And the Early Cambrian saw the extremely rapid diversification of multicellular animals, the Cambrian Explosion, that determined the animal evolution and is indirectly responsible for the present-day wildlife.

Stratigraphical units

Mostly for practical reasons, any system in earth history needs to be subdivided into smaller portions. However, any period has different aspects: The subdivision of the rock successions leads to the distinction of Groups, Formations, and Members, which are lithostratigraphic units. Short-lived fossils permit to recognize rocks of certain ages. They establish Biozones, which are biostratigraphic units. The pure time aspect leads to chronostratigraphy and a subdivision of systems into Series and Stages. The choice of subdivision at a particular locality or in a certain region is normally without any problem. Globally useful units, however, are generally extremely difficult to establish. All these hierarchically ordered units need to be fixed by sound boundaries that should be easily correlatable over large distances. Ideally, a boundary should be recognizable on a global scale by the sudden and synchronous appearence of a significant and short living fossil that occurs abundantly in the rocks and/or by a significant change of physical parameters in a rock succession (e.g., a dramatic change in the oxygen or carbonate isotopic composition). However, those are lucky events and rarely found, or they are at places in the successions that are unsuitable for global subdivisions for historical and other reasons. Stratigraphical subdivision is hence a difficult task, and this is the reason why the International Subcommission on Cambrian Stratigraphy (see I.U.G.S.) exists.

Cambrian stratigraphy

As it seems, the subdivision of the Cambrian is particularly difficult. The Cambrian System is currently without formally agreed international subdivisions, such as stages. This partly reflects the scarcity of suitable biostratigraphic markers for intercontinental correlation at the stage level and a strong faunal provincialism. However, research in progress on trilobites and conodonts (for the latter half of the Late Cambrian) shows promise for long range correlation and definition of stages. The Cambrian time interval is of growing international interest and research is being actively pursued by ISCS members. A number of suggested correlations was published in recent years by members of the Cambrian Subcommission, but usually they differ from each other in several aspects. Nevertheless, they depart markedly from correlations suggested some twenty or even ten years ago and indicate a steady progress in stratigraphical research.

The base of the Cambrian

The choice of a formal Cambrian lower boundary started in the 1970s and finally ended by the ratification of the Proterozoic-Cambrian boundary stratotype in 1991. The decision was officially driven forward by the I.G.C.P. Working Group 29 on the Precambrian-Cambrian boundary, which became the longest active of all I.G.C.P. working groups. The major reason for this delay of a formal decision was the growing amount of data on strata and fossils from the Proterozoic-Cambrian transition that lead to steady change of the opinions where the boundary should be drawn. Starting in the 1970s with the correlation of strata the earliest shelly macrofossils, small shelly fossils of Tommotian aspect came into focus shortly after. However, this episode ended when it became clear that the correlation potential of the earliest SSFs is insufficient. Finally, it became evident that the stratigraphical occurrence of trace-fossils depicts an evolution to more complicated traces, which, in turn, proves the progressive evolution to more anatomically complicated animals that were able to perform a progressively complex behaviour. The first trace with a somewhat complicated pattern is Trichophycus pedum (formerly known as "Phycodes pedum"). It occurs nearly worldwide, and its first occurrence is with the late fossils of Ediacaran aspect or, usually, in strata above them, whereas the first shelly fossils appeared clearly later. Hence, the ichnofossil assemblage with Trichophycus pedum marks the first occurrence of well-developed, fairly complex metazoan animals, and this is today regarded as the most useful landmark to characterize the Cambrian lower boundary. The best section to study the stacked appearance of various ichnofossils, SSFs, and finally shelled macrofossils is the section at Fortune Head, southeastern Newfoundland, which belonged to the Cambrian continent Avalonia. Accordingly, the International Subcommission on Cambrian Stratigraphy (through its Working Group on the Precambrian-Cambrian Boundary) made the official decision in 1991 to draw the base on the Cambrian at the first appearence date (FAD) of Trichophycus pedum in the reference section at Fortune Head.

The Proterozoic-Cambrian boundary GSSP:

The Fortune Head section near Fortune, southeastern Newfoundland.

Exposed are rocks of the Chapel Island Formation, members 1 and 2.

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The top of the Cambrian

Definition of the Cambrian-Ordovician boundary was a basic problem dealt with by the Cambrian-Ordovician Boundary Working Group for a long period. Traditional concepts of the Cambrian-Ordovician boundary based on the occurrence of the graptolite Rhabdinopora flabelliforme, which has a limited regional distribution. Strata with Rhabdinopora flabelliforme are often difficult to correlate precisely so that different species (and subspecies) of conodonts were favored for definition of the Cambrian-Ordovician boundary. The Cambrian-Ordovician Boundary Working Group finally decided in 1998 by majority that the base of the Ordovician should be placed at the base of the zone with Iapetognathus fluctivagus, which approximates the Cordylodus lindstromi Zone, the Rhabdinopora flabelliforme Zone and the FAD of the trilobite Jujuyaspis. The GSSP for this boundary was chosen at the Green Point section, Newfoundland.

Geochronological framework

The restriction of faunal groups to particular climatic belts and facies confined many genera and species of Cambrian organisms to single Cambrian continents, and diachronous occurrences of key groups between the continents often limits the precision the precision of biostratigraphy in global correlation, especially in the Early Cambrian. Correlations based on international comparison of improved geochronologic dating techniques will improve correlations, particularly of the uppermost Proterozoic to sub-trilobitic Lower Cambrian interval.

About a decade ago it became evident that most of the published age data on the Cambrian were irrelevant because they were based on Rubidium-Strontium ages that deviated from the true values and gave ages that were up to 50 m.y. too high. This created a remarkable changes in the age assignment of the Cambrian in general, particularly in its lower boundary as portrait in the figure below.

Changing views of late Neoproterozoic to earliest Ordovician time. The figure shows estimated dates from Harland et al., 1982, the Decade of North American Geology (DNAG) issues (1983), Harland et al., 1990, the International Union of Geological Sciences (IUGS) table generated by Cowie and Brasier (1989), and the data presented by Bowring & Erwin, 1998. Open circles denote poorly constrained geochronologic tie-points and the black circles better contrained tie-points. Error bars are shown for the Harland et al., 1982 and 1989 time scales. The 543.9±1.0 m.y. at the Proterozoic-Cambrian boundary cames from ashes in the Rusophycus avalonensis assemblage zone of southeastern Newfoundland so that the base of the Cambrian is slightly older than shown in the figure and would be roughly at 545 m.y.The Manykaian stage of Siberia and its equivalents were added to the Cambrian in 1992. Note that Neoproterozoic subdivisions are not yet firmly established. Slightly modified after Bowring & Erwin (GSA today, vol. 8, September 1998, Fig. 2).

New datings focus particularly on the latest Proterozoic-earliest Cambrian interval, from which no readily correlatable biostratigraphic data are available. A number of recent age data with relatively reliable numbers are listed below.

Latest Proterozoic and base of the Cambrian

595±15 m.y. mid-Dahai Mb., Meishucun, South China (Rb-Sr whole rock age; Zhang et al., 1984)

575±7.6 m.y.             volcanics, Carolina Slate Belt, eastern United States (U-Pb age, Kozuch et al., 198)
550±26 m.y.              volcanics, Puncoviscana Foldbelt, northwestern Argentina (K-Ar age, Omarini et al., 1996)
551.4±5.8 m.y.         rhyolite flow, Mooring Cove Fm., Fortune Bay, Nfld. (isotope dilution U-Pb age; Tucker and McKerrow, 1995)
543.6±0.24 m.y.       volcanic breccias, Lessyusa Fm., Nemakit-Dal'dyn Stage, Khorbusuonka, Olenek uplift, Siberia (U-Pb zircon age; Bowring et al., 1993)

535±7 m.y. granitoids, Puncoviscana Foldbelt, northwestern Argentina (U-Pb age, Bachmann et al., 1987)
534.6±0.4 fluvial conglomerates, between Nemakit-Dal'dyn and Tommotian stages, Kharaulakh Mountains, Siberia (U-Pb zircon age; Bowring et al., 1993)

525±7 m.y., max. 539±34 m.y. K-bentonite, Zhongyicun Mb., Dengying Fm., Meishucun, South China (SHRIMP zircon ages; Compston et al., 1992)

Middle Early Cambrian
530.7±0.9 m.y. felsic volcanic ash, near top of the Chapel Island Fm., R. avalonensis Zone, New Brunswick (U-Pb dilution age; Isachsen et al., 1994)

521±7 m.y. tuff layer fom the upper Lie de vin Formation, Morocco (SHRIMP zircon age; Compston et al., 1992)

522±1 m.y. ash layer fom the upper Lie de vin Formation, Morocco (isotope dilution U-Pb age; Landing et al., 1998)

Late Early Cambrian
526±4 m.y. felsic tuff, Heatherdale Shale, mid-Botomian, Sellick's Hill, Fleurieu Peninsula, South Australia (SHRIMP zircon ages, Cooper et al., 1992)
525±8 m.y. felsic tuff, Cymbric Vale Fm., western New South Wales (SHRIMP zircon ages, Zhou and Whitford, 1994)

517±1 m.y. ash layer from uppermost Issafen Formation, Sectigena Zone, Morocco (isotope dilution U-Pb age; Landing et al., 1998)

511±1 m.y. volcanic ash layer from "Protolenus Zone", southern New Brunswick (isotope dilution U-Pb age; Landing et al., 1998)

Latest Middle Cambrian
505.1±1.3 m.y. ignimbrites and crystal tuffs, Taylor Formation, probable equivalent of Floran/Undillan, Antarctica (U-Pb zircon age; Encarnación et al. 1999)
494.4±3.8 m.y. Comstock Tuff, Lejopyge laevigata Zone, mid-Boomerangian, Tasmania (U-Pb zircon age; Perkins and Walshe, 1993)

Latest Late Cambrian
501±7 m.y. mudstones, base of Cordylodus proavus Zone, Xiaoyangqiao section, Dayangcha, Northeast China (Rb-Sr age; Chen et al., 1988)
491±1 m.y. ash bed, top of Upper Cambrian, North Wales (U-Pb zircon age; Davidek et al., 1998)

.A diagrammatic summary of biostratigraphically and geochronologically well constraint samples of late Vendian and Cambrian age was provided by Sam Bowring and Doug Erwin (GSA today, vol. 8, September 1998, Fig. 3), which is presented below. This figure presents data from Bowring et al., 1993, Grotzinger et al., 1995, Compston et al., 1995, Landing et al., 1998, and Davidek et al., 1998. MIT IDTIMS data were isotope dilution-thermal mass spectrometry data from the Massachusetts Institute of Technology; ANU SHRIMP are super-high resolution ion microprobe data from the Australian National University; ROM IDTIMS are isotope dilution-thermal mass spectrometry data from the Royal Ontario Museum, Toronto. Note that the stages Nemakit-Daldynian, Tommotian, Atdabanian and Botoman used in the figure are Siberian regional stages. The 543.9±1.0 m.y. at the Proterozoic-Cambrian boundary cames from ashes in the Rusophycus avalonensis assemblage zone of southeastern Newfoundland so that the base of the Cambrian is slightly older than shown in the figure and would be roughly at 545 m.y.

Isotopic studies and paleomagnetism

Significant changes of physical parameters in a rock succession theoretically have a great potential, and studies on the oxygen, carbon and strontium isotopes have growing value, especially for regional studies of facies and stratigraphy. This is particularly interesting, when faunal groups are restricted to climate belt and magnafacies and are confined to single Cambrian continents, or when the first occurrence of key faunal groups is diachronous. Comparison of stable isotopic signatures and paleomagnetic variations have already improved correlations of the uppermost Proterozoic and sub-trilobitic Early Cambrian and will allow a precise intercontinental correlation when more data and improved techniques are available. Especially paleomagnetic data still suffer from uncertainties, such as diagenetic overprint, rates of sedimentation or paleolatitude.

Isotopic research has made good progress in the last decade, mainly studies on carbon and strontium isotopes, and plenty of data are now available for the Cambrian lower and upper boundary intervals (further reading).

For most of the Cambrian, no meaningful paleomagnetic data have been published (further reading). The latest Cambrian appears to be characterized by mainly reversed polarity. The earliest Ordovician, by contrast, is characterized by periods of normal polarity (Ripperdan and Kirschvink, 1992).

Kirschvink (1976, 1978a, 1978b) investigated polarity fluctuations in the Early Cambrian of the Amadeus Basin, Australia and showed that the Cambrian portion of the Arumbera Sandstone is an interval of mixed polarity following normal polarity in the Late Proterozoic lower Arumbera Sandstone. This interval of mixed polarity extend into the Todd River Dolomite and Eninta Sandstone. Biostratigraphic data give evidence for an Early Cambrian age of the Todd River Dolomite. Data from the Early Cambrian of South Australia and from the Middle and Late Cambrian of central Australia summarized by Klootwijk (1980) suggest that the Lower Cambrian Ajax Limestone (Flinders Ranges) indicates a mixed polarity while the coeval Wilkawillina Limestone and Oraparinna Shale has a reversed. The Middle Cambrian Wirrealpa Limestone, the Moodlatana Formation and parts of the Balcoracana Formation appear to have a reversed, but the Pantapinna Sandstone has a mixed polarity. The upper Giles Creek and the lower Shannon formations have a reversed polarity (see Shergold, 1995).

Latest update: March 13, 2001