Geochronology of the Cambrian: a precise Middle Cambrian U–Pb zircon date from the German margin of West Gondwana

Abstract

A volcanic tuff 1.0 m above the base of the Triebenreuth Formation in the Franconian Forest provides the first precise and biostratigraphically bracketed date within the traditional Middle Cambrian. The first illustration of fossils from the Triebenreuth Formation in this report and their discussion allow a more highly refined correlation within the Middle Cambrian. A weighted mean 206Pb–238U date of 503.14±0.13/0.25/0.59 Ma on zircons from this subaerial pyroclastic tuff was determined by U–Pb chemical abrasion isotope dilution mass spectrometry (CA-TIMS) techniques. At c. 6.0–7.0 Ma younger than the base of the traditional Middle Cambrian in Avalonia, the new West Gondwanan date from east-central Germany suggests that estimates of 500 Ma for the base of the traditional Upper Cambrian and 497 Ma on the base of the Furongian Series may prove to be too ‘old’. Biostratigraphically well-bracketed dates through most of the Middle Cambrian/Series 3 and below the upper Upper Cambrian/upper Furongian Series do not exist. An earlier determined 494.4±3.8 Ma date from the Southwell Group of Tasmania may actually prove to be a reasonable estimate for the age of the base of the traditional Upper Cambrian. Until high precision dates are determined on the base of the traditional Upper Cambrian and base of the Furongian Series, the rates of biotic replacements and geological developments and the durations of biotic zones in the Middle/Series 3 and Upper Cambrian/Furongian Series remain as ‘best guesses’.

Diversity partitioning during the Cambrian radiation

“…23). This problem raises the question whether our results might change with further revisions of the timescale.…”

Improved Geochronologic Accuracy and Precision for the ICS Chronostratigraphic Charts: Examples from the late Cambrian–Early Ordovician

“…As discussed by Landing et al (2014), Pigage et al's (2012) Lower Ordovician date is surprising for its great age-its reported 491 ± 0.13 Ma date is even older than a recalibrated U-Pb zircon date of 488.71 ± 1.17 Ma from Cambrian Stage 10 (discussed above). The consequences of assigning such seemingly "old" dates to the Cambrian, which are contrary to recently revised dates on the Ogof ddu and Bryn-llyn-fawr ashes (discussed above) and lead to their estimate of a very "old" Cambrian-Ordovician boundary age, were not by discussed in detail by Pigage et al (2012).…”

“…An illustration that seemingly precise dates on the ICS Chart are estimates that may obscure arguably "better" alternative estimates can be taken from the late Cambrian. As discussed by Landing et al (2014), the base of the traditional Upper Cambrian, or the part of the Guzhangian Stage immediately under the Furongian Series (Fig. 1) has long been estimated as 500 Ma, with 500.5 Ma appearing on the recent Charts .…”

“…With the age of the base of the "traditional" Upper Cambrian commonly estimated at 500 Ma, but perhaps better estimated at ca. 494 Ma (see discussion in Landing et al, 2014), Pigage et al's (2012) estimate of >493.3 Ma on the base of the Ordovician seems to leave very little or essentially no time available for the faunal, eustatic, and geological history of the late Cambrian.…”

“…492 Ma) zircons from the Yukon tuff may be zircons formed during earlier activity at the volcanic center. Indeed, all of the zircons from the Yukon tuff may have been reworked or formed early in the magma chamber, and the Pigage et al (2012) report may not be relevant to refining the age the Cambrian-Ordovician boundary (Landing et al, 2014). With the age of the base of the "traditional" Upper Cambrian commonly estimated at 500 Ma, but perhaps better estimated at ca.…”