Australia: The Land Where Time Began

A biography of the Australian continent 

Cryogenian Dating

Great environmental and biological change took place in the Neoproterozoic but complete integration of these records has not been possibly because of the lack of direct, precise constraints on the deposits. The authors have dated rocks of Proterozoic age in northwestern Canada to high precision by the U-Pb dating, the resulting ages constraining the large perturbations in the carbon cycle, a major diversification and depletion in the microfossil record, as well as the onset of the Sturtian Glaciation. Interbedded with glacial deposits from the Sturtian Glaciation are a volcanic tuff, dated to 716.5 Ma, is synchronous with the age of the Franklin Large Igneous Province and palaeomagnetic poles that place Laurentia in equatorial regions. As a result of this location ice was grounded below sea level at palaeolatitudes that were very low, implying that the extent of the Sturtian Glaciation was global.

There is evidence of the breakup of Rodinia, glaciation that was widespread (Kirschvink, 1992; Hoffman et al., 1998), high-amplitude fluctuations in geochemical proxy records Halverson, 2006) and early eukaryote radiation (Knoll et al., 2006). A better understanding of the nature and interrelationships of these events was previously prevented by age uncertainties, both relative and absolute.

As a response to strong palaeomagnetic evidence of low latitude glaciation found in the Elatina Formation in Australia (Sohl et al., 1999; Evans, 2000) the snowball Earth hypothesis was developed (Kirschvink, 1992; Hoffman et al., 1998). According to the authors the Elatina Formation, as well as its cap carbonate, have been chemically and lithographically correlated to glacial deposits of Marinoan age in the Ghaub Formation, Namibia that dates to 635.5 ± 0.6 Ma (Hoffman et al.,2004) in South China, the Nantuo formation, 636.3 ± 4.9 Ma (Zhang et al., 2008), underlying the cap carbonate of the basal Doushantuo Formation, 635.2 ± 0.2 Ma (Condon, 2005), as well as other glacial deposits from around the world, such as the Ice Brook Formation in northwestern Canada (Hoffman & Li, 2009). There are problems with dating sites from the Sturtian Glaciation because of the small number of robust palaeomagnetic poles and age constraints that are precise obtained from volcanic rocks that are interbedded directly with the glacial deposits from the Cryogenian. Glacial deposits found stratigraphically below diamictite units from the Marinoan (10),and banded iron formation within these deposits (Kirschvink, 1992) have been found to have a global distribution, inferring the global nature of the Sturtian Glaciation.

From the intrusive and volcanic rocks of northwestern Canada within strata of Neoproterozoic age, the authors present 4 high precision U-Pb isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) dates. Coupled with high resolution δ13C profiles these dates have allowed the synthesisation of geological, geochemical, palaeomagnetic, and palaeontological data on a regional and global scale. The accurate integration of these records has allowed the placing of hard constraints on the timing and extent of the Sturtian Glaciation, and its relationship to the Franklin large igneous province (LIP), and the microfossil record from the Cryogenian.

In northwestern Canada there are strata of Neoproterozoic age exposed in erosional windows (inliers) through Palaeozoic carbonate rocks that extend more the 1,500 km, from the Alaska Border east through the Ogilvie and Wemecke Mountains of Yukon to the Mackenzie Mountains in the Northwest Territories and north to Victoria Island of Nunavut. In the central Ogilvie Mountains the Coal Creek Inlier has exposures that consist of mixed carbonate and siliciclastic rocks from the upper Fifteen Mile Group and the Lower Mount Harper Group (LMHG), bimodal volcanic rocks of the Mount Harper Volcanic Complex (MHVC) (Mustard & Roots, 1997), as well as glacial diamictite of the Upper Mount Harper Group (UMHG). Drop stones that penetrate the bed with impact margins and outsized clasts in fine, laminated beds, and striated clasts seen in exposures in the Hart River Inlier in the eastern Ogilvie Mountains, all infer a glacial origin for the UMHG. Glacial push structures and deformation of soft sediments indicate grounded ice. In the western Ogilvie Mountains the UMHG and the unit 2 of the Upper Tindir Group, that is iron-rich, correlate with the Sayunei Formation of the Rapitan Group in the Mackenzie Mountains (Macdonald et al., 2010; Yeo, 1981). Proximity to a marine ice-grounding line is suggested by massive diamictite and stratified glacial deposits that contain ice-rafted debris, that is coarse-grained, in the Rapitan Group in the Northwest Territories (Yeo, 1981; Eisbacher, 1978), and its correlatives in Yukon and Alaska.

Dating of the MHVC by multigrain U-Pb ID-TIMS analysis gave a result of 751 +26/-18 Ma (Roots & Parish, 1988). The authors1 used samples of quartz-phyric rhyolite from the same site in member D obtaining a weighted mean of 206Pb-238U zircon date of 717.43 ± 0.14 Ma, which they interpreted as the age of eruption of this unit. The authors suggest the age they obtained of about 33 My younger than the original dates for the site is probably due to inherited cores in the multigrain zircon fractions in the previously dated samples that resulted in an age that was artificially old. Situated below the MHVC a green, flinty, bedded tuff within the allodapic dolostone beds near the top of unit PF1a of the Fifteen Mile Group yielded a weighted mean  206Pb-238U zircon date of 811.51 ± 0.25 Ma, which was interpreted as deposition time. Within glacial deposits of the UMHG a brecciated tuff, that is green to pink, situated above the MHVC, gave a weighted mean of 206Pb-238U zircon date of 716.47 ± 24 Ma, that was interpreted as the age of deposition.

Zircon and baddeleyite samples from gabbroic sills and dykes from the Franklin LIP in the MintoInlier on Victoria Island were dated previously at 723 ± +4/-2 Ma and 718 ± 2 Ma (Heaman et al., 1992). The sample used by the authors1 came from a coarse-grained diabase sill that was more than 20 m thick that intruded into the middle of the Wynniatt Formation (Jones, 2009), for which the weighted mean 206Pb-238U baddeleyite date was 716.33 ± 0.54 Ma. The authors1 interpreted the apparent discrepancy between the previously obtained date and their new date as an artefact introduced by comparing upper intercept and  206Pb-238U dates with their  206Pb-238U dates, when considering systematic errors that were recently recognised in the U decay constant ratio (Schoene et al., 2006).

The extent of the Franklin LIP is revealed by geochronology to be more than 3,000 km, about 716.5 Ma, from the Yukon-Alaska border to Elsmere Island, where there are mafic dykes dating to about 716 ± 1 Ma (Denyszn et al., 2009). Though on Victoria Island no evidence has been found of prevocalic extension and rifting (Rainbird1993), within the LMHG and the lower suite of the MHVC (Mustard & Roots, 1997) normal faulting is conspicuous that temporarily link the Franklin LIP to extension on the northwestern margin of Laurentia.

The Franklin LIP has been demonstrated by several palaeomagnetic studies on strongly magnetised mafic dykes, sills and lavas to have been emplaced at a time when the northwestern part of Laurentia was within 10o of the equator (Evans, 2000; Park, 1994; Palmer et al., 1983). Palaeomagnetic data that have been obtained from the dated sill, as well as the sediments that it intrudes into on Victoria Island are consistent with the previous low-latitude results (Jones, 2009). The tuff that is interbedded with glacial deposits in the UMHC has been dated to 716.47 ± 0.24 Ma, which is indistinguishable from a date obtained from the franklin LIP of 716.33  ± 0.54 Ma. This indicates grounded ice was present about 716.5 Ma on the northwestern margin of Laurentia, a time when it was situated in equatorial latitudes. According to the authors1 climate models have been predicting for a long time that if ice was present any closer to the equator than about 30o-40o the ice albedo feedback would lead to global glaciation (North et al., 1981; Bendtsen, 2002). This leads the authors1 to conclude that at about 716.5 Ma the Sturtian Glaciation was on a global scale.

It remains uncertain whether the Sturtian was comprised of a single, discrete glaciation lasting tens of millions of years or multiple episodes of glaciation that included the low-latitude glaciation that apparently occurred at about 716.5 Ma.  Previous to this study age constraints of the Sturtian Glaciation were obtained from a minimum, 662.9  ± 4.3 Ma from South China (Zhou, 2004), and maximum 726  ± 1 Ma from Leger Granite, Oman (Bowring, 2007). Based on their age determination from member D of the MHVC of 717.43 ± 0.14 Ma the authors1 suggest it provides an maximum constraint on the glaciation in low latitudes in the Sturtian as glacial deposits are not known of below member D, as well as because it is predicted by models that once ice has advanced past 30o the advance to the equator would be extremely fast (North et al., 1981; Bentsen, 2002), equatorial latitudes being glaciated synchronously around the world. Some evidence has been reported of a possible pre-Sturtian glaciation at about 750 Ma, the Kaigas Glaciation, in southern Namibia (Frimmel et al., 1996), Zambia (Key et al., 2001) and northwestern China (Xu et al., 2009), though the authors1 suggest that the contact relationships between the purported glacial deposit and the dated unit, as well as the glacial origin of the deposit, are suspect (Hoffman & Li, 2009). The authors1 suggest it is possible the Kaigas Glaciation was a regional glaciation. At about 685 Ma previous synglacial constrains have been reported from Idaho (Lund et al., 2003; Fanning & Link, 2004), though these results have been questioned as it is uncertain  if they are in  fact glacial deposits, contact between volcanic rocks that have been dated and diamictites are tectonic, repeated analysis giving different results (10). A 206Pb-238U ID-TIMS date of 711.52 ± 0.20 has been reported for volcaniclastic rocks that were interbedded with glaciogenic deposits of the Ghubrah Formation in Oman (Bowring et al., 2007). If the same glacial period is being recorded in the Ghubrah Formation as by the UMHG, it would indicate that the length of the Sturtian Glaciation would have been at least 5 My

The record of eukaryote evolution can be placed in the context of geochemical perturbations and glaciation on the global scale by using a δ13C record that is recalibrated and expanded. A maximum constraint on the Bitter Springs isotopic stage is provided by the 811.5 Ma age of the tuff (Halverson, 2006), and it is also a  useful benchmark for the calibration of the microfossil record of the Early Neoproterozoic. An example is the chemostratigraphic position of the mineralised scale microfossils from the Lower Tindir Group in the western Ogilvie Mountains above the Bitter Springs isotope stage and below deposits of glacial origin with banded iron formation that have earlier been correlated with the Rapitan Group (Macdonad et al., 2010). The microfossils of the Tindir Formation are therefore broadly coeval with the Chuar Formation microbiota in the Grand Canyon that have been dated to more than 742 ± 6 Ma, in Death Valley the Beck Spring Formation, that was formed pre-glacial, and in Spitsbergen, the Svanbergfjellet Formation. A large number of major eukaryotic crown groups, including members of the Rhizaria, Amoebozoa, green algae, red algae and vaucheriaceaen algae had diverged and diversified between the onset of the Bitter Springs isotope stage, about 811.5 Ma and the beginning of the Sturtian Glaciation, about 716.5 Ma, as is indicated by the calibration, collectively of these diverse microfossil records. Between the Sturtian Glaciation and the Marinoan Glaciation, between about 716.5 Ma and about 635 Ma, the record of the microfossils is depauperate, with only simple acritarchs of unknown phylogenetic affinity being described from this time (Knoll et al., 2006). The authors1 suggest this apparent bottleneck might be due, at least in part, to a limited number of samples and poor preservation, and/or the possible survival of some groups as cryptic forms. According to the authors1 a diverse biosphere clearly persisted throughout the glaciations of the Neoproterozoic (Knoll et al., 2006), though the impact on eukaryotic evolution of the global glaciation remains to be resolved.

The task of addressing the mechanisms responsible for environmental change in the Neoproterozoic with the aid of high-precision ages tied directly to the stratigraphic record. In the pre-Sturtian LMHG the presence of the Islay δ13C anomaly suggests there may be a relationship between global cycling of carbon and degradation of the climate. There is synchrony among continental extension, Franklin LIP, and the Sturtian glaciation, which is consistent with the hypothesis that a climate state could be produced that was more susceptible to glaciation by the CO2 drawdown via rifting and the weathering of the Franklin Basalts at low latitudes (Bendsten, 2002; Donnadieu et al., 2004). It is not clear whether the main part of the magmatism occurred before or during the glaciation, in spite of the new age constraints.

Sources & Further reading

  1. Macdonald, Francis A., Mark D. Schmitz, James L. Crowley, Charles F. Roots, David S. Jones, Adam C. Maloof, Justin V. Strauss, et al. "Calibrating the Cryogenian." Science 327, no. 5970 (March 5, 2010 2010): 1241-43.
Author: M. H. Monroe
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Last updated 29/12/2012 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading