The maximum age of the base of the Cambrian is now reason- ably well constrained. A high-quality207Pb/206Pb date of 543±
1 Ma on volcanic ashes in the upper Spitskopf Member of the Schwarzrand Subgroup in Namibia, is assigned to the latest Ediacaran (Grotzinger et al., 1995). The Spitskopf Member is overlain, with erosional contact, by the Nomtsas Formation,
207Pb/206Pb dated at 539.4± 1 Ma, the basal beds of which
contain Tricophycus pedum. Interestingly, some elements of the globally distributed Ediacara fauna are found stratigraphically immediately above the dated ash bed in the Spitskopf Mem- ber, indicating that this fauna, characteristic of the Ediacaran, ranged into the base of the Cambrian (Grotzinger et al., 1995).
Similar faunal relationships have been found in South Australia (Jensen et al., 1998).
The age of the base of the Cambrian is thought to be just younger than 543 Ma (Brasier et al., 1994; Grotzinger et al., 1995), an age consistent with other zircon dates, stratigraphi- cally less-well constrained, from Siberia (Bowring et al., 1993), and from the late Ediacaran (Grotzinger et al., 1995; Tucker and McKerrow, 1995).
Recently, the Precambrian–Cambrian boundary has been identified in drill cores in Oman with tuffs on either side (Bowring et al., 2003; Amthor et al., 2003; Fig. 11.4). Chemostratigraphic and paleontologic data are interpreted to indicate the simultaneous occurrence of an extinction of Precambrian mineralized skeleton fossils (Namacalathus and
485 490 505 510 515 520 525 530 495 500 535 540 545 Series/Stage AGE (Ma)
Ordovician
Ediacaran
Early Middle Furongian 3rd stage 4th stage 1st stage 2nd stage Paibian −4 −2 0 2 4 6 8 6th stage no stages designated (0/00 PDB)Figure 11.5 Strontium and carbon isotope trends through the
Cambrian Period from published data on marine carbonate rocks and fossils. Strontium curve for the lower Paibian stage to the upper third stage is from Monta ˜nez et al. (2000), and for Late Cambrian and Early Cambrian is from Ebneth et al. (2001). The carbon-13 curve for the Late and Middle Cambrian is from Monta ˜nez et al. (2000) and for Early Cambrian is from Kirshvink & Raub (2003).
Cloudina) and a large-magnitude, short-lived negative excur- sion in carbon isotopes, which is widely equated with the boundary (Grotzinger et al., 1995; Bartley et al., 1998; Kimura and Watanabe, 2001). The ash bed immediately below the boundary yielded 543.2±0.5Ma,andtheashbedatthebound- ary 542.0± 0.3 Ma (2-sigma). Including external radiogenic factors the authors prefer the quoted uncertainty to be 1 myr (S. Bowring, pers. comm., 2003). The 542± 1 Ma date then is the best estimate for the age of the Precambrian–Cambrian boundary and the base of the Phanerozoic (Fig. 11.5).
The difference between the age of the top of the Cambrian (i.e. of the base of the Ordovician), here taken as 488.3 Ma, and the bottom, 542± 1 Ma, gives 54 myr as the duration of this period.
Ages of the base of the Late Cambrian and base of the Middle Cambrian are not well constrained. The Taylor For- mation in Antarctica (Encarnaci´on et al., 1999) has yielded zircons with a weighted mean age of 505.1± 1.3 Ma on ashes
We think 503 Ma is a better estimate based on this date. The Late Cambrian, or Upper Cambrian in this context, has a lower boundary equivalent to the base of the Agnostus pisiformis Zone or the base of the Mindyallan Stage of Australia. This level lies three zones below the base of the Furongian Series, the youngest division of the Cambrian. The base of the Furongian Series is therefore estimated at 501 Ma.
Ash beds associated with “upper Lower Cambrian” pro- tolenid trilobites in southern New Brunswick (Landing et al., 1998) have yielded zircons, which give a composite age based on three samples, of 511± 1 Ma. The beds are correlated with the middle Botoman to Toyonian Stages of Siberia. The date was taken by (Landing et al., 1998) to indicate an age of 510 Ma for the base of the Middle Cambrian. This translates to 513 Ma for the base of the Middle Cambrian as applied in Australia. The Early Cambrian, therefore, occupies over half of Cambrian time.
The ages of boundaries between these levels are poorly constrained, and caution should be used when using the nu- merical scale in Figs. 11.1 and 11.2. In the latest Late Cam- brian, a volcanic sandstone in North Wales gives a maxi- mum age for the Lower Peltura scarabaeoides Zone of 491 ± 1 Ma (Davidek et al., 1998). Ash beds from Morocco, taken as representing the “middle Botoman to Toyonian,” help constrain the age of these stages. Five single-grain zir- con analyses cluster at 517± 1.7 Ma (Landing et al., 1998). Ash beds in New Brunswick with an age of 530 ± 2.5 Ma (Isachsen et al., 1994) only weakly constrain the age of the Tommotian.
Within the Late Cambrian, which is finely zoned by trilo- bite biostratigraphy, stages are here proportioned according to the number of trilobite zones they contain. This method is also used for the late Middle Cambrian, where agnostoid trilo- bites provide reliable zonation and inter-regional correlation. The method, however, assumes a more or less constant rate of evolutionary turnover and a uniformity in paleontological practice in zonal designation, which are not only unproven, but are unlikely to be true.
Fossil diversity and abundance become increasingly rare passing downwards through the early part of the Middle Cam- brian and Early Cambrian; and, as a result, the biostratigraphic framework becomes increasingly vague. In the early part of the Early Cambrian, resolution of the time scale is limited as much by lack of biostratigraphically useful fossils as by lack of
radio-isotopic data. Our estimates of stage durations become correspondingly intuitive and the age of stage boundaries in the Early Cambrian shown in Figs. 11.1 and 11.2 should be regarded as highly approximate.
To summarize, the duration of the Cambrian is almost 54 myr, ranging from 542.0 to 488.3 Ma. The base of the Middle Cambrian is near 513 Ma, and that of the Furongian
Series, near 501 Ma. Hence the Early Cambrian lasted 29 myr, the Middle Cambrian 12 myr, and the Furongian approxi- mately 13 myr. Because ages for the Early–Middle and Middle Cambrian– Furongian boundaries are approximate estimates, the durations of intra-Cambrian divisions are equally ten- tative. More intra-Cambrian radiometric dates are urgently required.
Darriwilian Tremadocian 5th stage (unnamed) 2nd stage (unnamed) 469 Ma (mid-Ordovician)
Geographic distribution of Ordovician GSSPs that have been ratified (diamonds) on a mid Ordovician map (status in January,
2004; see Table 2.3). Four of the seven Ordovician stages are not yet named, including two that have formalized GSSPs.
Rapid and sustained biotic diversification (“Ordovician radiation”) to reach highest diversity levels for Paleozoic; prolonged “hot-house” climate punctuated by “ice-house” intervals and oceanic turnover; strong fluctuations in eustatic level, global glaciation, and mass ex- tinction at end of period; appearance and evolution of pandemic plank- tonic graptolites and conodonts important for correlation; moderate to strong benthic faunal provincialism; re-organization and rapid mi- gration of tectonic plates surrounding the Iapetus Ocean; migration of South Pole from North Africa to central Africa, all characterize the Ordovician period.
1 2 . 1 H I S T O RY A N D S U B D I V I S I O N S
Named after the Ordovices, a northern Welsh tribe, the Or- dovician was proposed as a new system by Lapworth in 1879. It was a compromise solution to the controversy over strata in North Wales that had been included by Adam Sedgwick in his
A Geologic Time Scale 2004, eds. Felix M. Gradstein, James G. Ogg, and Alan G. Smith. Published by Cambridge University Press. c F. M. Gradstein, J. G. Ogg, and A. G. Smith 2004.
Cambrian System but which were also included by Murchison as constituting the lower part of his Silurian System. Although it was initially slow to be accepted in Britain, where it was instead generally called Lower Silurian well into the twentieth century, the Ordovician was soon recognized and used else- where, such as in the Baltic region and Australia. The name Ordovician was officially adopted at the 1960 International Geological Congress in Copenhagen.
Black graptolite-bearing shales are widely developed in Or- dovician sedimentary successions around the world. Lapworth (1879–80) described the stratigraphic distribution of British graptolites at the same time that he proposed the Ordovician System, and graptolites have played a major role in the recogni- tion and correlation of Ordovician rocks since that time. Lap- worth demonstrated as long ago as 1878, in southern Scotland, the fine biostratigraphic precision that can be achieved with this group. In the last several decades, conodonts have proved to be of similar global biostratigraphic value in the carbonate facies. In the shelly facies developed mainly on the continental shelf and platform, trilobites and brachiopods are used extensively
for zonation, and coral–stromatoporoid communities enable biostratigraphic subdivision in the Late Ordovician. Chitino- zoan and acritarch zonations are still being developed and both groups hold promise of providing long-range correlation with good precision.
Subdivision of the Ordovician into Upper and Lower, or Upper, Middle, and Lower, parts has been very inconsistent (Jaanusson, 1960; Webby, 1998). The International Subcom- mission on Ordovician Stratigraphy voted to recognize a three- fold subdivision of the System (Webby, 1995), which is used here.
Because of marked faunal provincialism and facies differ- entiation throughout most of the Ordovician, no existing re- gional suite of stages or series has been found to be satisfactory in its entirety for global application. The Ordovician subcom- mission therefore undertook to identify the best fossil-based datums, wherever they are found, for global correlation, and to use these for definition of global chronostratigraphic (and chronologic) units (Webby, 1995, 1998). In this respect, it has deviated from the course followed by the Silurian and Devo- nian subcommissions, both of which have recommended the adoption of pre-existing (regional) stage or series schemes for global use.
In 1997, the Ordovician subcommission agreed to subdi- vide the period into three primary divisions, each to comprise two stages; and, in 2003, an uppermost seventh stage “Hir- nantian” was added. It is not yet decided whether the primary subdivisions will be formally designated as Early, Middle, and Late, or will carry locality names. It is not yet decided whether they will be given the status of series, as preferred by the In- ternational Commission on Stratigraphy (ICS), or of “sub- periods” (subsystems). They are referred to here as the Early, Middle, and Late Series.
During the early 1990s, the Ordovician subcommission es- tablished a number of working groups to investigate and rec- ommend levels within the period suitable for international correlation, and therefore for defining international stages (Webby, 1995, 1998). Seven general chronostratigraphic lev- els have been certified as primary correlation levels for the seven international stages. They are based on the first ap- pearance of key graptolite or conodont species. At present, four boundaries have been formally voted on and are de- fined by a global stratigraphic section and point (GSSP). Only three have been formally named: the Tremadocian (Stage 1), the Darriwilian (Stage 3), and the Hirmantian (Stage 7).
The seven stages are referred to here informally, as Stages 1 through 7 (Fig. 12.1).