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The East European Platform in the late Ediacaran: new paleomagnetic and geochronological data
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ScienceDirect Russian Geology and Geophysics 54 (2013) 1392–1401 www.elsevier.com/locate/rgg
The East European Platform in the late Ediacaran: new paleomagnetic and geochronological data N.M. Fedorova a,*, N.M. Levashova a, M.L. Bazhenov a, J.G. Meert b, N.D. Sergeeva c, I.V. Golovanova c, K.N. Danukalov c, N.B. Kuznetsov a, A.F. Kadyrov c, M.M. Khidiyatov c a
b
Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017, Russia Department of Geological Sciences, University of Florida, 274 Williamson Hall, Gainesville, FL 32611, United States c Institute of Geology, Ufa Science Center, Russian Academy of Sciences, ul. Karla Marksa 16/2, Ufa, 450000, Russia Received 25 December 2012; accepted 21 February 2013
Abstract The paleogeography of the Earth, including the East European Platform, is very inaccurately defined for the interval 500–700 Ma. The quantity and quality of Late Precambrian–Cambrian paleomagnetic data on this platform are absolutely insufficient for reliable paleogeographical or paleotectonic reconstructions. Since there are almost no unstudied objects in the platform that could be used for paleomagnetic studies, it seems reasonable to consider the deformed platform margins. Of particular interest is the Bashkir anticlinorium (South Urals) with numerous Ediacaran sedimentary sections, some of which contain tuff beds suitable for isotope dating. We present paleomagnetic and geochronological data on the Upper Ediacaran Zigan Formation, sampled in the western part of the western limb of the Bashkir anticlinorium. The East European Platform must have been at near-equatorial latitudes at ~550 Ma. © 2013, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: paleomagnetism; Ediacaran; paleogeography; East European Platform; South Urals
Introduction The interval from Ediacaran to Early Ordovician appears one of the most mysterious and interesting epochs in the Earth’s history (Sokolov, 2011). The early Ediacaran coincided with the end of the Global Ice Age (Evans, 2000; Hoffman et al., 1998), followed by a dramatic increase in biodiversity. This event is usually called “the Cambrian explosion.” The same time interval is charatcerized by oxygenation of the atmosphere and the oceans as well as a dramatic increase in bioturbation (Crimes, 1992; Knoll, 2000; McCall, 2006). The formation of the Gondwana supercontinent ended in the cambrian (Li et al., 2008; Meert and Tamrat, 2004). As presumed by J.G. Meert et al. (1993), the continental drift was considerably more rapid in the Ediacaran–Cambrian than later. For the same period, D.A. Evans (1998) put forward a hypothesis of unusually high rates of the true polar wander and J.L. Kirschvink et al. (1997) presume that the true polar wander was reciprocal at that time. It is hard to find another time interval so interesting to researchers and inviting
* Corresponding author. E-mail address: [email protected] (N.M. Fedorova)
so many contradictory and, sometimes, very bizarre hypotheses. These phenomena are largely controlled by the position and drift of the main continental masses and the formation and closure of the oceans. However, the existing views on the Earth’s paleogeography and plate tectonics in the Precambrian, after the breakup of Rodinia and the formation of Gondwana (from 800–750 to 550–530 Ma), are uncertain and contradictory. Paleomagnetism remains the main instrument for paleogeographical and paleotectonic reconstructions. Unfortunately, exactly for the time interval of interest, the quantity and quality of paleomagnetic determinations are obviously not enough for definitive paleogeographical and paleotectonic reconstructions (Metelkin et al., 2012). This problem is especially acute for North America (Laurentia) and the East European Platform. According to different authors, Laurentia and the East European Platform (EEP) might have been located at different latitudes, from South pole to Equator, in the Ediacaran (Abrajevitch and Van der Voo, 2010; Evans, 1998; Kirschvink et al., 1997; McCausland et al., 2007; Meert, 1999, 2013; Meert et al., 2007; Pisarevsky et al., 2008; Schmidt and Williams, 2010).
1068-7971/$ - see front matter D 201 3, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2013.10.003 +
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The existing paleomagnetic data on the Ediacaran rocks of the EEP are not enough for the determination of its paleogeographic position. It is clear that new data are required for the reconstruction of the EEP paleogeography. Unfortunately, almost all the sites suitable for paleomagnetic research in the platform itself have already been studied. Studying the deformed platform margins appears to be the most promising issue. First and foremost, they include the Bashkir uplift, which contains thick, mainly sedimentary sections of the Late Ediacaran suitable for paleomagnetic studies. The tuff interbeds present in some of them permit isotopic and geochronological dating. The new paleomagnetic and geochronological data on the Late Ediacaran sedimentary rocks presented in this paper permit a more reliable location of the EEP at ~550 Ma. Geological summary and sampling The thick Upper Proterozoic sediments of the Bashkir anticlinorium (Fig. 1) make up one of the most complete Meso- and Neoproterozoic sections in the world. The stratotypes of many units of the regional stratigraphic scale for 1500–500 Ma are localized here (Keller and Chumakov, 1983). The uppermost Neoproterozoic unit in the western part of the anticlinorium is the Ashin Group, which consists of terrigenous sediments overlying eroded Mesoproterozoic sediments. The Ashin Group is overlain by the Lower Devonian Takata Formation with stratigraphic unconformity in the western part of the anticlinorium and by Middle Ordovician sediments in its southern part. The Ashin Group is divided into five formations (Bekker, 1988; Keller and Chumakov, 1983). Two lower ones (Bakeevo and Uryuk) are of quartz-arkose composition, and three upper ones (Basu, Kukkarauk, and Zigan) are polymictic (Fig. 1b). The group is considered a molasse (Bekker, 1988), but the polymict composition, which is specific to molasses, is observed only in its upper part. The Basu and Zigan Formations are compositionally almost the same. They are both dominated by gray-brown and gray-green sandstones and siltstones, with subordinate mudstones and gravelstones. In the field, one can differentiate between them only in sections containing conglomerate outcrops of the Kukkarauk Formation. The section of the Kukkarauk Formation is dominated by coarse-grained red-colored sandstones with a bed of red-brown polymictic conglomerates in the middle part (Kozlov, 2002). These conglomerates make up a regional marker bed separating the Basu Formation from the Zigan Formation. The conglomerates are a few tens of meters thick in the south, a few meters thick in the central part of the area, and absent in the north. The age of the Ashin Formation remains disputable, but the prevailing view is that the four upper formations belong to the Ediacaran and the lower Bakeevo Formation belongs to the late Cryogenian. The sediments of the Zigan Formation, which ends the Ashin Group, are the main subject of our study. The formation mainly consists of middle- and very fine-grained greenish
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gray, gray, and brownish gray polymictic sandstones interbedded with polymictic or, more rarely, quartzose siltstones as well as mudstone beds and members. The total thickness of the sediments is up to 500 m (Bekker, 1988). The Zigan Formation underwent paleomagnetic and geochronological sampling in several sections of the western Bashkir anticlinorium (Fig. 1a): Z1 (53.97 ºN; 56.89 ºE). Along the Mendym–Tolparovo road, one outcrop of the Zigan Formation (~30 m thick) and six isolated outcrops above in the section (each 4–6 m thick) were sampled. The rocks mainly consist of alternating brown, gray-brown, and gray-green sandstones belonging to the lower Zigan Formation. In total, 66 oriented samples were taken here. Z2 (53.91 ºN; 56.82 ºE). Six isolated outcrops of the Zigan Formation (each from 2 to 4 m thick), consisting of only gray-green sandstones, were sampled from the sides of the Mendym Valley. In total, 42 oriented samples were taken. Z3 (53.56 ºN; 56.66 ºE). The brown and gray-brown sandstones of the lower Zigan Formation were sampled from an interval of 50–60 m in the cliffs on the bank of the Zigan River. Also, several isolated outcrops of gray-green sandstones were studied in the middle and upper parts of the formation. In total, 125 oriented samples were taken. Z4 (54.56 ºN; 58.10 ºE). The brown, gray-brown, gray, and green sandstones and siltstones of the upper Ashin Group outcrop on the western margin of the Ust’-Katav Village. The section here is 70–75 m thick. The absence of the Kukkarauk Formation in this outcrop makes it impossible to determine whether the section belongs to the Zigan or the Basu Formation. Several unlithified tuff interbeds from 1 to 10 cm thick are present in the lower third part of the section, among gray-brown sandstones and siltstones (Grazhdankin et al., 2011). A sample for U–Pb zircon dating of the rocks was taken from one of these interbeds. No paleomagnetic sampling was carried out in this section.
Age of the Zigan Formation Uranium–lead dating was conducted for zircon from the tuffs taken from section Z4. Slurry was prepared by soaking an almost unlithified tuff sample, and zircon was extracted from it using heavy liquids and electromagnetic separation. Zircon grains were handpicked under a binocular microscope and then mounted in epoxy together with those from the FC-1 standard. Uranium–lead isotope data were obtained at the University of Florida by LA-MC-ICP-MS (laser ablation with a multicollector ICP-MS). The equipment and procedure for the LA-MC-ICP-MS method are described in (Koler and Sylvester, 2003; Simonetti et al., 2005). The error was estimated from the reproducibility of the isotope analysis of standard zircon FC-1 (Paces et al., 1993), attributed to individual results. The 206Pb/238U values were corrected for common Pb by the 207Pb-based method (Williams, 1998). The 207 Pb/235U values, corrected for common Pb, were calculated from the known 238U/235U = 137.88, corrected 206Pb/238U,
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Fig. 1. a, Sketch map of the southwestern Urals (simplified after (Kozlov, 2002)). b, Simplified stratigraphic column of Ediacaran sediments in the western South Urals. 1, Paleozoic sediments of the East European Platform and western Urals; 2, Paleozoic sediments of the eastern Urals; 3, Ediacaran; 4, Neoproterozoic; 5, upper Mesoproterozoic; 6, lower Mesoproterozoic; 7, metamorphic complexes; 8, intrusions; 9, thrusts; 10, other faults; 11, studied Ediacaran sections, marked like in the text. Heavy line indicates the Zil’merdak thrust, to the west of which there is no angular unconformity between the Ediacaran and Paleozoic rocks.
and normalized 207Pb/206Pb ratios. A conventional concordia diagram was plotted using the Isoplot/Ex software, v. 3.09a (Ludwig, 2004). In total, 51 zircon grains (80–100 µm) were studied. Discordance was less than 10% for the 206Pb/238U and 207 Pb/235U ratios in 40 grains, which permitted determining the U–Pb concordant age of the studied tuffs as 548.2 ± 3.5 Ma (Fig. 2). Note that this determination has been already mentioned by D.V. Grazhdankin et al. (2011) but without a description of the result. As pointed out above, there is no way of telling whether the tuff interbeds sampled in section Z4 belong to the Basu or Zigan Formations, but the isotope dating obtained for the tuffs is a key to solving this problem. The Basu Formation contains Ediacaran fauna: Pseudorhizostomites howchini Sprigg, Protodipleurosoma paulus Beck., Paliella patelliformis Fed., and Medusinites sp. as well as burrow marks (Palaeopascichnus delicatus Palij, Neonereites uniserialis Seilacher, and Catellichnus octonarius Bekker) (Bekker, 1992,
1996). This assemblage constrains the age of the Basu Formation at 575–560 Ma. The U–Pb age of the studied tuffs (548.2 ± 3.5 Ma) suggests that section Z4 is younger than the Basu Formation. The Basu Formation and the Devonian sandstones of the Takata Formation are separated only by the clastics of the Zigan Formation and the coarse-grained sandstones and conglomerates of the Kukkarauk Formation. Since section Z4 differs dramatically from the Kukkarauk Formation, the only possibility remains: The section is composed of the Zigan Formation, and the isotopic age obtained can be extended to the entire formation, presumably aged ~548 Ma.
Paleomagnetic studies Hand specimens, oriented with a mining compass, were selected for paleomagnetic studies. The samples were taken in a stratigraphic order, with each 6–8 samples regarded as a
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Fig. 2. Concordia diagram for a sample from a tuff interbed in section Z4. Inset shows a cathodoluminescence photograph of one of the studied zircon grains.
site. The sampling interval for one site was usually 3–4 to 6–7 m. Cubes with an edge of 20 mm were cut from the samples. All the samples underwent stepwise (up to 20 stages) temperature demagnetization to 700 ºC in the Paleomagnetic Laboratory of the Geological Institute (Moscow). The demagnetization was carried out in a self-made furnace with a two-layered permalloy shield (remanent field, ~10 nT). Magnetization was measured on a JR-4 spinner magnetometer with 0.05 mA/m noise, placed in Helmholtz coils, which reduced the effect of the Earth’s magnetic field by several times. The results of the heating were analyzed using orthogonal diagrams (Zijderveld, 1967). The directions of the magnetization components were determined using path segments including no less than three dimensions (Kirschvink, 1980). Data on samples from one site were averaged, and the average vectors for the sites were used to calculate the total average direction of the entire sedimentary unit. The data were processed in the software of Cogné (2003). In most of the samples from the Zigan Formation, there is the low-temperature component of magnetization, removed at temperatures of up to 200 ºC. This component, which has a direction close to that of the present-day field, will not be considered. Temperatures higher than 200 ºC do not permit detecting stable magnetization components in the green and gray-green sandstones, which make up ~60% of the collection (Fig. 3a).
Analysis of some samples at temperatures of 300 to 575 ºC reveals the medium-temperature component of magnetization in the gray-brown and brown rocks (Fig. 3b). However, its directions are not grouped within the sites or in the entire collection. We presume that this component is a result of the overlapping of the spectra for the blocking temperatures of the low- and high-temperature magnetization components. Therefore, these directions carry no useful paleomagnetic information and will also not be considered. Approximately in two-thirds of the gray-brown and brown sandstone samples from the Zigan Formation, the high-temperature component (HTC) was distinguished. It decreases toward zero (Fig. 3c, d) and has blocking temperatures of up to 670–675 ºC (in some samples, they are no more than 580 ºC). It is likely that the main carrier of magnetization is hematite (considerably more rarely, magnetite). The directions of the HTC are well-grouped both within the sites and in the entire collection (Table 1). The fold test (McElhinny, 1964) is uninformative for the HTC because of too narrow bedding variations. There are some arguments for the primary character of the HTC. First, both normal and reversed polarities are observed in the collection. The angle between the average vectors of different polarities γ = 8.7° is smaller than the critical value γcrit = 10.5° (McFadden and McElhinny, 1990), so that the reversal test is positive (Fig. 4).
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Fig. 3. Examples of Zijderveld diagrams for samples from the Zigan Formation in stratigraphic coordinates. Filled (open) symbols, projection onto a horizontal (vertical) plane. Temperatures are given in ºC, and the magnetization intensity along the axes is given in mA/m. Magnetization components cannot be distinguished in sample M1696 (a). Solid line indicates the medium-temperature component (Fig. 4b), and the dashed line indicates the high-temperature component (HTC). a–d, See explanations in the text.
Second, several zones of different polarities were distinguished in the outcrops; for example, the presence of ~20 zones in section Z3 along the Zigan River suggests the primary character of the HTC. Third, if the studied late Ediacaran rocks had been remagnetized later, the resulting pole would coincide with the corresponding region of the Phanerozoic apparent polar wander path for the EEP (Torsvik and Cocks, 2005). The pole obtained in the South Urals is far away from the Phanerozoic curve at any polarity (Fig. 5). We presume that the most important fact is the agreement between the poles for coeval rocks from other EEP parts and
that for the South Urals. Some paleomagnetic poles agreeing with our result were obtained from the Upper Ediacaran rocks (550–555 Ma) of the southeastern shore of the White Sea (Iglesia-Llanos et al., 2005; Popov et al., 2002, 2005). The pole for volcanics from South Volhynia belongs to the same group (Elming et al., 2007). The southeastern shore of the White Sea is more than 1500 km away from the Bashkir anticlinorium, and the distance between the South Urals and Volhynia is no shorter. It is quite hard to imagine a geologic event causing simultaneous remagnetization of rocks at such long distances. Therefore, the positive test of external convergence indicates the primary character of magnetization at all
Table 1. Directions of the high-temperature magnetization in the sediments of the Zigan Formation Site
DAº
Dº
N/No
Geoghraphic coordinates
Stratigraphic coordinates
Dº
Iº
k
α95º
Dº
Iº
k
α95º
Section Z1 P1147 P1163
303 290
21 24
16/7 16/10
281.1 101.8
40.7 –42.7
18.7 21.1
14.3 10.8
285.4 103.6
20.8 –19.3
17.6 18.1
14.8 11.7
Section Z3 N5061 M1674 P1187 M1681 N5069 M1695 Mean (sites) Mean (samples)
297 291 279 286 282 283 – –
26 23 26 28 26 26 – –
15/6 7/6 8/6 14/11 15/10 15/13 (8) 106/69
279.3 91.7 285.7 289.1 304.5 120.2 286.9 293.0
43.3 –44.0 48.6 46.1 36.3 –43.7 43.7 38.4
26.5 56.5 90.7 20.6 15.1 14.2 83.4 10.4
13.3 9.0 7.1 10.3 13.1 11.4 6.1 5.6
283.5 96.2 283.9 288.3 300.4 116.0 287.2 291.7
18.0 –22.0 22.6 18.0 12.2 –18.5 19.1 13.8
26.5 39.1 80.8 22.5 13.6 12.7 103 10.4
13.3 10.8 7.5 9.8 13.8 12.1 5.5 5.6
Note. GC, Geographic coordinates; SC, stratigraphic coordinates; DA, dip azimuth; D, dip angle; N/No, selected to used samples (the number of sites is in parentheses); D, declination; I, inclination; k, concentration parameter; and α95, radius of confidence circle (Fisher, 1953).
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Fig. 4. Stereograms of the directions of the high-temperature magnetization component for samples of sedimentary rocks from the Zigan Formation. GC, Geographic coordinates (a). SC, Stratigraphic coordinates (b). Filled (open) circles show the directions of normal (reversed) polarity. Stars indicate the average HTC directions for two groups of different polarities with confidence circles. Filled (open) symbols and solid (dashed) lines are projections onto the lower (upper) hemisphere.
three sites. All these facts suggest the primary character of the HTC of magnetization in the Zigan Formation.
Overview of paleomagnetic data on the Ediacaran complexes of the East European Platform Before we discuss the paleogeographic significance of the data obtained, it makes sense to consider published paleomagnetic data on the Ediacaran rocks of the EEP (Fig. 6). The only early Ediacaran pole was detected from the Egersund dike complex, southwestern Norway (Walderhaug et al., 2007). The rocks were reliably dated by the U–Pb zircon method (616 ± 3 Ma), and the paleomagnetic result was confirmed by the test for burned contacts Carbonatite intrusive complexes aged ~585 Ma were studied in two places: the Fen complex of Southern Norway (Meert et al., 1998; Piper, 1988) and the Alnø complex of Central Sweden (Meert et al., 2007; Piper, 1981). The paleomagnetic data on the Fen complex suggest that the EEP is located at middle latitudes, but many authors point out the agreement of this pole with Permo-Triassic data on the same platform. Also, they presume remagnetization of the Fen complex during magmatic activity in the nearby Oslo graben. Two magnetization components were detected during the paleomagnetic studies of the Alnø complex. According to one of them, the EEP must have been located at the pole, and according to the other, in the tropics; this does not permit using these data for paleogeographical reconstructions (Meert et al., 2007). The Ediacaran rocks in the southwestern EEP (Volhynia) are lavas and tuffs a few hundred meters thick, overlain by a 200-m-thick sedimentary cover. Unfortunately, the Ediacaran volcanic sections of Volhynia are known almost exclusively
from drilling data. Four lava flows were stripped in two small mines in northern Volhynia, and two more were stripped in the southern part of the region. For lack of other objects, these flows were studied several times (Elming et al., 2007; Iosifidi et al., 2001; Nawrocki et al., 2004). Elming et al. (2007) combined all the data and made the following conclusion: The data from North Volhynia suggest that the EEP was located near the pole at 560–580 Ma, whereas those from South Volhynia suggest that this part of the platform was located at a latitude of ~20º. The sedimentary section overlying the volcanics was studied by A.G. Iosifidi et al. (2005), who distinguished six magnetization components in these sediments. Unfortunately, the impossibility of determining the age or exact direction of these components does not permit using these data. Ediacaran rocks are known in a vast area of northern Russia but almost exclusively from drilling data. These rocks were dated by the U–Pb zircon method (550–555 Ma) based on the tuff interbeds from sections of the southeastern shore of the White Sea (Iglesia-Llanos et al., 2005); also, reliable paleomagnetic data were obtained from some outcrops (Iglesia-Llanos et al., 2005; Popov et al., 2002, 2005). As was shown above, the pole obtained for the upper ediacaran rocks (Z) agrees with the coeval poles from the southeastern shore of the White Sea (W) (Iglesia-Llanos et al., 2005; Popov et al., 2002, 2005) and with that for the upper Ediacaran lavas of South Volhynia (V) (Elming et al., 2007). All these poles form a dense cluster (ZWV) (Fig. 5), and the average pole for this group has a latitude Plat = 26.9 ºN and a longitude Plong = 299.2 ºE (concentration parameter k = 35, radius of the confidence circle A95 = 11.4º, and N = 6 poles). The dispersion within the ZWV group of poles might be due to slight age differences between the studied rocks.
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Fig. 5. Apparent polar wander path for the East European Platform (diamonds and dotted line) from Early Ordovician (480 Ma) to present (Torsvik and Cocks, 2005), with some Ediacaran data on this platform. Stars with confidence circles show the data on sedimentary rocks from the Zigan Formation for two versions of polarity (ZN and ZR, see explanations in the text). Crosses indicate poles aged 550–555 Ma from the southeastern shore of the White Sea (Iglesia-Llanos et al., 2005; Popov et al., 2002, 2005), and the circle shows a pole aged ~550 Ma from the Ediacaran lavas of South Volhynia (Elming et al., 2007). Numbers near some poles on the polar wander path indicate age, Ma.
Discussion The above data suggest that the Zigan Formation accumulated at ~550 Ma and the HTC of magnetization is primary in these rocks. This means that the Urals edge of the EEP was located at near-equatorial latitudes in the late Ediacaran.
Fig. 6. Distribution of Vendian paleomagnetic poles in the East European Platform (all the poles are projected onto the Northern hemisphere). Square shows the Egersund dike complex, southwestern Norway (Walderhaug et al., 2007); diamond, the Fen complex of Southern Norway (Meert et al., 1998; Piper, 1988); sidelong cross, the Alnø complex of Central Sweden (Meert et al., 2007; Piper, 1981): Alnø-1 and Alnø-2, poles for the high- and low-angle components, respectively; straight crosses, basalts from North and South Volhynia (Elming et al., 2007); circles, the southeastern shore of the White Sea (Popov et al., 2002, 2005); and star, the South Urals (present paper). Confidence circles around the poles are not shown.
How justified is it to extrapolate the data on the western Urals to the entire platform? The rocks of the western Urals Fold Belt are known to be very similar to the sediments of the platform itself. The Meso- and Neoproterozoic sedimentary rocks of the Bashkir anticlinorium are reliably compared to the sediments stripped by deep boreholes in the Cisuralian foredeep and in the eastern EEP. These correlations are confirmed by a set of seismic profiles (Keller and Chumakov, 1983). Thus, the western Urals Fold Belt is a platform margin, though deformed, and there were no significant displacements of the Bashkir anticlinorium with respect to the platform. The same is evidenced by the above-mentioned convergence between the magnetic poles for the South Urals, White Sea, and Volhynia. Undoubtedly, local rotations of individual structures are possible in the deformed platform margin. The main folding took place in the Permian in connection with the formation of the Urals orogen (Puchkov, 2003). The lack of angular unconformities in the sections of the western South Urals from early Ediacaran to late Early Permian suggests that of pre-Permian deformations in the region. That is to say, local rotations, if any, must have occurred in the Permian, thus affecting all the pre-Permian paleomagnetic data. Studies aimed at the search for local rotations in the western South Urals relative to the EEP have been conducted before (e.g., (Shipunov, 1998)). It turned out that a vast majority of the studied sections are not rotated with respect to the EEP within the error of the method and the very scarce rotations are, most likely, related to small local structures. Although this issue calls for additional studies, it is likely that most of
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Fig. 7. Determination of the inclination decrease in the rocks of the Zigan Formation. a, Theoretical comparison of the inclination distribution in the case of a dramatic inclination decrease. b, c, Initial theoretical distribution (b) and that after the decrease (c), both reduced to the pole of the sphere. For clarity, gray areas show different parts of the distributions. d, Distribution of individual HTC directions in the Zigan Formation, reduced to the pole of the sphere and the same polarity. Dotted circles are shown to emphasize the almost circular shape of this distribution.
the blocks of the Bashkir anticlinorium did not undergo any significant displacement relative to one another or the EEP. Thus, it appears reasonable to extrapolate the paleomagnetic data on the Ediacaran of the Bashkir anticlinorium to the EEP. Another important problem is the possible effect of the inclination decrease on the paleomagnetic data on the sedimentary rocks. Since this phenomenon is frequent, its possibility cannot be ignored. We tried to estimate the possible decrease for our collection. If the directions initially (before the decrease) showed an F- or nearly F-distribution (Fisher, 1953), it became elongated after the decrease (Bazhenov, 1981). The steeper the initial inclination and the greater the decrease, the more dramatic the change in the distribution shape (Fig. 7). Let us assume that the EEP was located at high latitudes (e.g., ~55º N, inclination ~70º) in the Ediacaran. The observed inclination of 15º in this case must result from a great decrease. However, at a decrease from 70º to 15º, the initial circular distribution with an elongation equal to unity will become strongly elongated (8–10) (Fig. 7a–c). The observed distribution is almost circular, with an elongation <<2, which is statistically insignificant (Fig. 7d). Therefore, there was no
dramatic decrease. A slight inclination decrease of 5º–10º is possible, but an effect of such extent cannot change the conclusion that the Zigan Formation accumulated at nearequatorial latitudes. So, the poles of the ZWV group show that the EEP was located at the near-equatorial latitudes of the Northern or Southern hemispheres in the late Ediacaran. An attempt can be made to determine the polarity of the paleomagnetic data using the paleogeographic position of the other plates in the late Ediacaran. It was reliably established that the opening of the Iapetus Ocean began between Laurentia, Gondwana, and the EEP at 615–550 Ma. This is evidenced by rift complexes on the British isles, in Norway, and in the Northern Appalachians (Cawood et al., 2001; Cocks and Torsvik, 2011; McCausland et al., 2007). The views on the opening of the Iapetus are contradictory, but two paleomagnetic results with ages of 550–555 Ma for Laurentia suggest its equatorial position (McCausland and Hodych, 1998; Van Alstine and Gillett, 1979). If the EEP were located in the Northern hemisphere in the late Ediacaran, the distance between two sides of the Iapetus would be more than 5000 km (Fig. 8, version A). This
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References
Fig. 8. Possible positions of the East European Platform (Baltica) in accordance with the results. Letters A and B indicate possible positions of Baltica depending on selected polarity. Areas with known geologic indicators of the opening of the Iapetus Ocean are in gray (Van der Voo, 1993). Star shows the study area in the South Urals.
configuration appears hardly possible; it is much more likely that the EEP was at the near-equatorial latitudes of the Southern hemisphere (Fig. 8, version B). Conclusions The above facts invite the following conclusions. (1) The age of the Zigan Formation is ~550 Ma; (2) High-temperature magnetization in the Zigan Formation can be considered primary; (3) The pole obtained can be extrapolated to the entire East European Platform; (4) The Zigan Formation showed no dramatic inclination decrease; (5) The mean direction of the high-temperature magnetization of the Zigan Formation corresponds to a paleolatitude of ~10º ± 3º. Consequently, the East European Platform was located near the Equator in the late Ediacaran. We thank our colleagues from the Institute of Geology (Ufa) for valuable geological data on the Bashkir anticlinorium and help with field studies. Also, we are grateful to N.Ya. Dvorova and O.A. Krezhovskikh for the demagnetization of paleomagnetic collections. Finally, our thanks go to D.V. Metelkin and the anonymous referee for the enormous work of improving the paper. This work was supported by the Russian Foundation for Basic Research, grants 11-05-00037 and 11-05-00137, Program No. 10 of the Department of Earth Sciences RAS, and the US National Science Foundation, grant EAR11-19038.
Abrajevitch, A., Van der Voo, R., 2010. Incompatible Ediacaran paleomagnetic directions suggest an equatorial geomagnetic dipole hypothesis. Earth Planet. Sci. Lett. 293 (1–2), 164–170. Bazhenov, M.L., 1981. Inclination decrease in the Paleogene sandstones of southern Darvaz. Dokl. AN SSSR 260, 1336–1339. Bekker, Yu.R., 1988. Precambrian Molasses [in Russian]. Nedra, Leningrad. Bekker, Yu. R. 1992. The Oldest Ediacaran biota of the Urals. Izv. RAN, Ser. Geol., No. 6, 16–24. Bekker, Yu.R., 1996. Discovery of Ediacaran fauna in the uppermost Vendian sediments of the South Urals. Regional’naya Geologiya i Metallogeniya, No. 5, 111–131. Cawood, P.A., McCausland, P.J.A., Dunning, G.R., 2001. Opening Iapetus: Constraints from the Laurentian margin in Newfoundland. GSA Bull. 113 (4), 443–453. Cocks, L.R.M., Torsvik, T.H., 2011. The Palaeozoic geography of Laurentia and western Laurussia: A stable craton with mobile margins. Earth Sci. Rev. 106 (1–2), 1–51. Cogné, J.P., 2003. PaleoMac: a Macintosh application for treating paleomagnetic data and making plate reconstructions, Geochem. Geophys. Geosyst. 4 (1), 1007, doi:10.1029/2001GC000227. Crimes, T.P., 1992. The record of trace fossils across the Proterozoic-Cambrian boundary, in: Origin and Early Evolution of the Metazoa. Plenum, New York, pp. 177–202. Elming, S.-Å., Kravchenko, S.N., Layer, P., Rusakov, O.M., Glevasskaya, A.M., Mikhailova, N.P., Bachtadse, V., 2007. Palaeomagnetism and 40Ar/39Ar age determinations of the Ediacaran traps from the southwestern margin of the East European Craton, Ukraine: relevance to the Rodinia break-up. J. Geol. Soc. (London) 164 (5), 969–982. Evans, D.A., 1998. True polar wander, a supercontinental legacy. Earth Planet. Sci. Lett. 157 (1–2), 1–8. Evans, D.A., 2000. Stratigraphic, geochronological, and paleomagnetic constraints upon the Neoproterozoic climatic paradox. Am. J. Sci. 300 (5), 347–433. Fisher, R.A., 1953. Dispersion on a sphere. Proc. R. Soc. London, Ser. A, 217 (1130), 295–305. Grazhdankin, D.V., Marusin, V.V., Meert, J., Krupenin, M.T., Maslov, A.V., 2011. Kotlin regional stage in the South Urals. Dokl. Earth Sci. 440 (1), 1222–1226. Hoffman, P.F., Kaufman, A.J., Halverson, G.P., Schrag, D.P., 1998. A Neoproterozoic snowball Earth. Science 281 (5381), 1342–1346. Iglesia-Llanos, M.P., Tait, J.A., Popov, V., Abalmasova, A., 2005. Palaeomagnetic data from Ediacaran (Vendian) sediments of the Arkhangelsk region, NW Russia: An alternative apparent polar wander path of Baltica for the Late Proterozoic–Early Palaeozoic. Earth Planet. Sci. Lett. 240 (3–4), 732–747. Iosifidi, A., Bachtadse, V., Khramov, A., Kuznetsova, A., 2001. Paleomagnetic data for Vendian basalts from Ukraine, 3rd Int. Conf. on Problems of Geocosmos, Abstracts Volume. St. Petersburg, pp. 74–75. Iosifidi, A.G., Khramov, A.N., Bachtadse, V., 2005. Multicomponent magnetization of Vendian sedimentary rocks in Podolia, Ukraine. Russ. J. Earth Sci. 7 (1), 1–14. Keller, B.M., Chumakov, N.M. (Eds.), 1983. Riphean Stratotype: Stratigraphy and Geochronology [in Russian]. Nauka, Moscow. Kirschvink, J.L., 1980. The least-square line and plane and the analysis of palaeomagnetic data. Geophys. J. R. Astron. Soc. 62 (7), 699–718. Kirschvink, J.L., Ripperdan, R.L., Evans, D.A., 1997. Evidence for a large-scale reorganization of Early Cambrian continental masses by inertial interchange true polar wander. Science 277 (5325), 541–545. Knoll, A.H., 2000. Learning to tell Neoproterozoic time. Precambrian Res. 100 (1–3), 3–20. Kozlov, V.I. (Ed.), 2002. Geological Map of Russia and the Adjacent Part of Kazakhstan, Scale 1 : 1,000,000 (New Ser.). Sheet N-40 (41) (Ufa). VSEGEI, St. Petersburg. Koler, J., Sylvester, P.J., 2003. Present trends and the future of zircon in geochronology: Laser ablation ICPMS, in: Hanchar J.M., Hoskin, P.W.O. (Eds.), Zircon. Rev. Mineral. Geochem. 53 (1), 243–275.
N.M. Fedorova et al. / Russian Geology and Geophysics 54 (2013) 1392–1401 Li, Z.X., Bogdanova, S.V., Collins, A.S., Davidson, A., De Waele, B., Ernst, R.E., Fitzsimons, I.C.W., Fuck, R.A., Gladkochub, D.P., Jacobs, J., Karlstrom, K.E., Lu, S., Natapov, L.M., Pease, V., Pisarevsky, S.A., Thrane, K., Vernikovsky, V., 2008. Assembly, configuration, and break-up history of Rodinia: A synthesis. Precambrian Res. 160 (1–2), 179–210. Ludwig, K.R., 2004. User’s Manual for ISOPLOT, a Geochemical Toolkit for Microsoft Excel, Version 3.09a. McCall, G.J.H., 2006. The Vendian (Ediacaran) in the geological record: enigmas in geology’s prelude to the Cambrian explosion. Earth-Sci. Rev. 77, 1–229. McCausland, P.J.A., Hodych, J.P., 1998. Paleomagnetism of the 550 Ma Skinner Cove volcanics of western Newfoundland and opening of the Iapetus Ocean. Earth Planet. Sci. Lett. 163 (1–4), 15–29. McCausland, P.J.A., Van der Voo, R., Hall, C.M., 2007. Circum-Iapetus paleogeography of the Precambrian–Cambrian transition with a new paleomagnetic constraint from Laurentia. 156 (3–4), 125–152. McElhinny, M.W., 1964. Statistical significance of the fold test in palaeomagnetism. Geophys. J. R. Astron. Soc. 8 (3), 338–340. McFadden, P.L., McElhinny, M.W., 1990. Classification of the reversal test in palaeomagnetism. Geophys. J. Int. 103 (3), 725–729. Meert, J.G., 1999. A paleomagnetic analysis of Cambrian true polar wander. Earth Planet. Sci. Lett. 168 (1–2), 131–144. Meert J.G., 2013. Ediacaran–Early Ordovician paleomagnetism of Baltica: A review. Gondwana Res., doi:10.1016/j.gr.2013.02.003. Meert, J.G., Tamrat, E., 2004. A mechanism for explaining rapid continental motion in the Late Neoproterozoic, in: Eriksson, P.G., Altermann, W., Nelson, D.R., Mueller, W.U., Catuneanu, O. (Eds.), The Precambrian Earth: Tempos and Events. Developments in Precambrian Geology. Elsevier, Amsterdam, Vol. 12, pp. 255–267. Meert, J.G., Van der Voo, R., Powell, C.M., Li, Z., McElhinny, M.W., Symons, D.T.A., 1993. A plate-tectonic speed limit. Nature 363, 216–217. Meert, J.G., Torsvik, T.H., Eide, E.A., Dahlgren, S., 1998. Tectonic significance of the Fen Province, S. Norway: Constraints from geochronology and paleomagnetism. J. Geol. 106 (5), 553–564. Meert, J.G., Walderhaug, H.J., Torsvik, T.H., Hendriks, B.W.H., 2007. Age and paleomagnetic signature of the Alnø carbonatite complex (NE Sweden): Additional controversy for the Neoproterozoic paleoposition of Baltica. Precambrian Res. 154 (3–4), 159–174. Metelkin, D.V., Vernikovsky, V.A., Kazansky, A.Yu., 2012. Tectonic evolution of the Siberian paleocontinent from the Neoproterozoic to the Late Mesozoic: paleomagnetic record and reconstructions. Russian Geology and Geophysics (Geologiya i Geofizika) 53 (7), 675–688 (883–899). Nawrocki, J., Boguckij, A., Katinas, V., 2004. New Late Vendian palaeogeography of Baltica and the TESZ. Geol. Quart. 48 (4), 309–316. Paces, J.B., Miller, J.D., Jr., 1993. Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift System. J. Geophys. Res. 98 (B8), 13,997–14,013. Piper, J.D.A., 1981. Magnetic properties of the Alnøn complex. Geol. Foeren. Stockholm Foerh. 103 (1), 9–15.
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Piper, J.D.A., 1988. Palaeomagnetism of (late Vendian—earliest Cambrian) minor alkaline intrusions, Fen Complex, southeast Norway. Earth Planet. Sci. Lett. 90 (4), 422–430. Pisarevsky, S.A., Murphy, J.B., Cawood, P.A., Collins, A.S., 2008, Late Neoproterozoic and Early Cambrian palaeogeography: models and problems, in: Pankhurst, R.J., Trouw, R.A.J., Brito Neves, B.B., de Wit, M.J. (Eds.), West Gondwana: pre-Cenozoic Correlations Across the South Atlantic Region. Geol. Soc., Spec. Publ. 294, pp. 9–31. Popov, V., Iosifidi, A., Khramov, A., Tait, J., Bachtadse, V., 2002. Paleomagnetism of Upper Vendian sediments from the Winter Coast, White Sea region, Russia: Implications for the paleogeography of Baltica during Neoproterozoic times. J. Geophys. Res., Solid Earth 107 (B11), 10.1029/2001JB001607. Popov, V.V., Khramov, A.N., Bachtadse, V., 2005. Palaeomagnetism, magnetic stratigraphy, and petromagnetism of the Upper Vendian sedimentary rocks in the sections of the Zolotitsa River and in the Verkhotina Hole, Winter Coast of the White Sea, Russia. Russ. J. Earth Sci. 7 (2), 1–29. Puchkov, V.N., 2003. Uralides and Timanides: their structural relationship and position in the geologic history of the Ural-Mongolian fold belt. Geologiya i Geofizika (Russian Geology and Geophysics) 44 (1–2), 28–39 (27–38). Schmidt, P.W., Williams, G.E., 2010. Ediacaran palaeomagnetism and apparent polar wander path for Australia: no large true polar wander. Geophys. J. Int. 182, 711–726. Shipunov, S.V., 1998. Folding history of the South Urals, from paleomagnetic data, in: Rock Paleomagnetism and Magnetism [in Russian]. OIFZ RAN, Moscow, pp. 69–71. Simonetti, A., Heaman, L.M., Hartlaub, R.P., Creaser, R.A., MacHattie, T.G., Böhm, C., 2005. U-Pb zircon dating by laser ablation-MC-ICP-MS using a new multiple ion counting Faraday collector array. J. Anal. At. Spectrom. 20 (8), 677–686. Sokolov, B.S., 2011. The chronostratigraphic space of the lithosphere and the Vendian as a geohistorical subdivision of the Neoproterozoic. Russian Geology and Geophysics (Geologiya i Geofizika) 52 (10), 1048–1059 (1334–1348). Torsvik, T.H., Cocks, L.R.M., 2005. Norway in space and time: a Centennial cavalcade. Norw. J. Geol. 85, 73–86. Van Alstine, D.R., Gillett, S.L., 1979. Paleomagnetism of Upper Precambrian sedimentary rocks from the Desert Range, Nevada. J. Geophys. Res. Solid Earth 84 (B9), 4490–4500. Van der Voo, R., 1993. Paleomagnetism of the Atlantic, Tethys and Iapetus Oceans. Cambridge Univ. Press, Cambridge. Walderhaug, H.J., Torsvik, T.H., Halvorsen, E., 2007. The Egersund dykes (SW Norway): a robust Early Ediacaran (Vendian) palaeomagnetic pole from Baltica. Geophys. J. Int. 168 (3), 935–948. Williams, I.S., 1998. U-Th-Pb geochronology by ion microprobe, in: McKibben, M.A., Shanks, W.C. III, Ridley, W.I. (Eds.), Applications of Microanalytical Techniques to Understanding Mineralizing Processes. Rev. Econ. Geol. 7, 1–35. Zijderveld, J.D.A., 1967. A.C. demagnetization of rocks: analysis of results, in: Collinson, D.W., Creer, K.M., Runcorn, S.K. (Eds.), Methods in Paleomagnetism. Elsevier, Amsterdam, pp. 254–286.
Editorial responsibility: D.V. Metelkin
ScienceDirect Russian Geology and Geophysics 54 (2013) 1392–1401 www.elsevier.com/locate/rgg
The East European Platform in the late Ediacaran: new paleomagnetic and geochronological data N.M. Fedorova a,*, N.M. Levashova a, M.L. Bazhenov a, J.G. Meert b, N.D. Sergeeva c, I.V. Golovanova c, K.N. Danukalov c, N.B. Kuznetsov a, A.F. Kadyrov c, M.M. Khidiyatov c a
b
Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017, Russia Department of Geological Sciences, University of Florida, 274 Williamson Hall, Gainesville, FL 32611, United States c Institute of Geology, Ufa Science Center, Russian Academy of Sciences, ul. Karla Marksa 16/2, Ufa, 450000, Russia Received 25 December 2012; accepted 21 February 2013
Abstract The paleogeography of the Earth, including the East European Platform, is very inaccurately defined for the interval 500–700 Ma. The quantity and quality of Late Precambrian–Cambrian paleomagnetic data on this platform are absolutely insufficient for reliable paleogeographical or paleotectonic reconstructions. Since there are almost no unstudied objects in the platform that could be used for paleomagnetic studies, it seems reasonable to consider the deformed platform margins. Of particular interest is the Bashkir anticlinorium (South Urals) with numerous Ediacaran sedimentary sections, some of which contain tuff beds suitable for isotope dating. We present paleomagnetic and geochronological data on the Upper Ediacaran Zigan Formation, sampled in the western part of the western limb of the Bashkir anticlinorium. The East European Platform must have been at near-equatorial latitudes at ~550 Ma. © 2013, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: paleomagnetism; Ediacaran; paleogeography; East European Platform; South Urals
Introduction The interval from Ediacaran to Early Ordovician appears one of the most mysterious and interesting epochs in the Earth’s history (Sokolov, 2011). The early Ediacaran coincided with the end of the Global Ice Age (Evans, 2000; Hoffman et al., 1998), followed by a dramatic increase in biodiversity. This event is usually called “the Cambrian explosion.” The same time interval is charatcerized by oxygenation of the atmosphere and the oceans as well as a dramatic increase in bioturbation (Crimes, 1992; Knoll, 2000; McCall, 2006). The formation of the Gondwana supercontinent ended in the cambrian (Li et al., 2008; Meert and Tamrat, 2004). As presumed by J.G. Meert et al. (1993), the continental drift was considerably more rapid in the Ediacaran–Cambrian than later. For the same period, D.A. Evans (1998) put forward a hypothesis of unusually high rates of the true polar wander and J.L. Kirschvink et al. (1997) presume that the true polar wander was reciprocal at that time. It is hard to find another time interval so interesting to researchers and inviting
* Corresponding author. E-mail address: [email protected] (N.M. Fedorova)
so many contradictory and, sometimes, very bizarre hypotheses. These phenomena are largely controlled by the position and drift of the main continental masses and the formation and closure of the oceans. However, the existing views on the Earth’s paleogeography and plate tectonics in the Precambrian, after the breakup of Rodinia and the formation of Gondwana (from 800–750 to 550–530 Ma), are uncertain and contradictory. Paleomagnetism remains the main instrument for paleogeographical and paleotectonic reconstructions. Unfortunately, exactly for the time interval of interest, the quantity and quality of paleomagnetic determinations are obviously not enough for definitive paleogeographical and paleotectonic reconstructions (Metelkin et al., 2012). This problem is especially acute for North America (Laurentia) and the East European Platform. According to different authors, Laurentia and the East European Platform (EEP) might have been located at different latitudes, from South pole to Equator, in the Ediacaran (Abrajevitch and Van der Voo, 2010; Evans, 1998; Kirschvink et al., 1997; McCausland et al., 2007; Meert, 1999, 2013; Meert et al., 2007; Pisarevsky et al., 2008; Schmidt and Williams, 2010).
1068-7971/$ - see front matter D 201 3, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2013.10.003 +
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The existing paleomagnetic data on the Ediacaran rocks of the EEP are not enough for the determination of its paleogeographic position. It is clear that new data are required for the reconstruction of the EEP paleogeography. Unfortunately, almost all the sites suitable for paleomagnetic research in the platform itself have already been studied. Studying the deformed platform margins appears to be the most promising issue. First and foremost, they include the Bashkir uplift, which contains thick, mainly sedimentary sections of the Late Ediacaran suitable for paleomagnetic studies. The tuff interbeds present in some of them permit isotopic and geochronological dating. The new paleomagnetic and geochronological data on the Late Ediacaran sedimentary rocks presented in this paper permit a more reliable location of the EEP at ~550 Ma. Geological summary and sampling The thick Upper Proterozoic sediments of the Bashkir anticlinorium (Fig. 1) make up one of the most complete Meso- and Neoproterozoic sections in the world. The stratotypes of many units of the regional stratigraphic scale for 1500–500 Ma are localized here (Keller and Chumakov, 1983). The uppermost Neoproterozoic unit in the western part of the anticlinorium is the Ashin Group, which consists of terrigenous sediments overlying eroded Mesoproterozoic sediments. The Ashin Group is overlain by the Lower Devonian Takata Formation with stratigraphic unconformity in the western part of the anticlinorium and by Middle Ordovician sediments in its southern part. The Ashin Group is divided into five formations (Bekker, 1988; Keller and Chumakov, 1983). Two lower ones (Bakeevo and Uryuk) are of quartz-arkose composition, and three upper ones (Basu, Kukkarauk, and Zigan) are polymictic (Fig. 1b). The group is considered a molasse (Bekker, 1988), but the polymict composition, which is specific to molasses, is observed only in its upper part. The Basu and Zigan Formations are compositionally almost the same. They are both dominated by gray-brown and gray-green sandstones and siltstones, with subordinate mudstones and gravelstones. In the field, one can differentiate between them only in sections containing conglomerate outcrops of the Kukkarauk Formation. The section of the Kukkarauk Formation is dominated by coarse-grained red-colored sandstones with a bed of red-brown polymictic conglomerates in the middle part (Kozlov, 2002). These conglomerates make up a regional marker bed separating the Basu Formation from the Zigan Formation. The conglomerates are a few tens of meters thick in the south, a few meters thick in the central part of the area, and absent in the north. The age of the Ashin Formation remains disputable, but the prevailing view is that the four upper formations belong to the Ediacaran and the lower Bakeevo Formation belongs to the late Cryogenian. The sediments of the Zigan Formation, which ends the Ashin Group, are the main subject of our study. The formation mainly consists of middle- and very fine-grained greenish
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gray, gray, and brownish gray polymictic sandstones interbedded with polymictic or, more rarely, quartzose siltstones as well as mudstone beds and members. The total thickness of the sediments is up to 500 m (Bekker, 1988). The Zigan Formation underwent paleomagnetic and geochronological sampling in several sections of the western Bashkir anticlinorium (Fig. 1a): Z1 (53.97 ºN; 56.89 ºE). Along the Mendym–Tolparovo road, one outcrop of the Zigan Formation (~30 m thick) and six isolated outcrops above in the section (each 4–6 m thick) were sampled. The rocks mainly consist of alternating brown, gray-brown, and gray-green sandstones belonging to the lower Zigan Formation. In total, 66 oriented samples were taken here. Z2 (53.91 ºN; 56.82 ºE). Six isolated outcrops of the Zigan Formation (each from 2 to 4 m thick), consisting of only gray-green sandstones, were sampled from the sides of the Mendym Valley. In total, 42 oriented samples were taken. Z3 (53.56 ºN; 56.66 ºE). The brown and gray-brown sandstones of the lower Zigan Formation were sampled from an interval of 50–60 m in the cliffs on the bank of the Zigan River. Also, several isolated outcrops of gray-green sandstones were studied in the middle and upper parts of the formation. In total, 125 oriented samples were taken. Z4 (54.56 ºN; 58.10 ºE). The brown, gray-brown, gray, and green sandstones and siltstones of the upper Ashin Group outcrop on the western margin of the Ust’-Katav Village. The section here is 70–75 m thick. The absence of the Kukkarauk Formation in this outcrop makes it impossible to determine whether the section belongs to the Zigan or the Basu Formation. Several unlithified tuff interbeds from 1 to 10 cm thick are present in the lower third part of the section, among gray-brown sandstones and siltstones (Grazhdankin et al., 2011). A sample for U–Pb zircon dating of the rocks was taken from one of these interbeds. No paleomagnetic sampling was carried out in this section.
Age of the Zigan Formation Uranium–lead dating was conducted for zircon from the tuffs taken from section Z4. Slurry was prepared by soaking an almost unlithified tuff sample, and zircon was extracted from it using heavy liquids and electromagnetic separation. Zircon grains were handpicked under a binocular microscope and then mounted in epoxy together with those from the FC-1 standard. Uranium–lead isotope data were obtained at the University of Florida by LA-MC-ICP-MS (laser ablation with a multicollector ICP-MS). The equipment and procedure for the LA-MC-ICP-MS method are described in (Koler and Sylvester, 2003; Simonetti et al., 2005). The error was estimated from the reproducibility of the isotope analysis of standard zircon FC-1 (Paces et al., 1993), attributed to individual results. The 206Pb/238U values were corrected for common Pb by the 207Pb-based method (Williams, 1998). The 207 Pb/235U values, corrected for common Pb, were calculated from the known 238U/235U = 137.88, corrected 206Pb/238U,
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Fig. 1. a, Sketch map of the southwestern Urals (simplified after (Kozlov, 2002)). b, Simplified stratigraphic column of Ediacaran sediments in the western South Urals. 1, Paleozoic sediments of the East European Platform and western Urals; 2, Paleozoic sediments of the eastern Urals; 3, Ediacaran; 4, Neoproterozoic; 5, upper Mesoproterozoic; 6, lower Mesoproterozoic; 7, metamorphic complexes; 8, intrusions; 9, thrusts; 10, other faults; 11, studied Ediacaran sections, marked like in the text. Heavy line indicates the Zil’merdak thrust, to the west of which there is no angular unconformity between the Ediacaran and Paleozoic rocks.
and normalized 207Pb/206Pb ratios. A conventional concordia diagram was plotted using the Isoplot/Ex software, v. 3.09a (Ludwig, 2004). In total, 51 zircon grains (80–100 µm) were studied. Discordance was less than 10% for the 206Pb/238U and 207 Pb/235U ratios in 40 grains, which permitted determining the U–Pb concordant age of the studied tuffs as 548.2 ± 3.5 Ma (Fig. 2). Note that this determination has been already mentioned by D.V. Grazhdankin et al. (2011) but without a description of the result. As pointed out above, there is no way of telling whether the tuff interbeds sampled in section Z4 belong to the Basu or Zigan Formations, but the isotope dating obtained for the tuffs is a key to solving this problem. The Basu Formation contains Ediacaran fauna: Pseudorhizostomites howchini Sprigg, Protodipleurosoma paulus Beck., Paliella patelliformis Fed., and Medusinites sp. as well as burrow marks (Palaeopascichnus delicatus Palij, Neonereites uniserialis Seilacher, and Catellichnus octonarius Bekker) (Bekker, 1992,
1996). This assemblage constrains the age of the Basu Formation at 575–560 Ma. The U–Pb age of the studied tuffs (548.2 ± 3.5 Ma) suggests that section Z4 is younger than the Basu Formation. The Basu Formation and the Devonian sandstones of the Takata Formation are separated only by the clastics of the Zigan Formation and the coarse-grained sandstones and conglomerates of the Kukkarauk Formation. Since section Z4 differs dramatically from the Kukkarauk Formation, the only possibility remains: The section is composed of the Zigan Formation, and the isotopic age obtained can be extended to the entire formation, presumably aged ~548 Ma.
Paleomagnetic studies Hand specimens, oriented with a mining compass, were selected for paleomagnetic studies. The samples were taken in a stratigraphic order, with each 6–8 samples regarded as a
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Fig. 2. Concordia diagram for a sample from a tuff interbed in section Z4. Inset shows a cathodoluminescence photograph of one of the studied zircon grains.
site. The sampling interval for one site was usually 3–4 to 6–7 m. Cubes with an edge of 20 mm were cut from the samples. All the samples underwent stepwise (up to 20 stages) temperature demagnetization to 700 ºC in the Paleomagnetic Laboratory of the Geological Institute (Moscow). The demagnetization was carried out in a self-made furnace with a two-layered permalloy shield (remanent field, ~10 nT). Magnetization was measured on a JR-4 spinner magnetometer with 0.05 mA/m noise, placed in Helmholtz coils, which reduced the effect of the Earth’s magnetic field by several times. The results of the heating were analyzed using orthogonal diagrams (Zijderveld, 1967). The directions of the magnetization components were determined using path segments including no less than three dimensions (Kirschvink, 1980). Data on samples from one site were averaged, and the average vectors for the sites were used to calculate the total average direction of the entire sedimentary unit. The data were processed in the software of Cogné (2003). In most of the samples from the Zigan Formation, there is the low-temperature component of magnetization, removed at temperatures of up to 200 ºC. This component, which has a direction close to that of the present-day field, will not be considered. Temperatures higher than 200 ºC do not permit detecting stable magnetization components in the green and gray-green sandstones, which make up ~60% of the collection (Fig. 3a).
Analysis of some samples at temperatures of 300 to 575 ºC reveals the medium-temperature component of magnetization in the gray-brown and brown rocks (Fig. 3b). However, its directions are not grouped within the sites or in the entire collection. We presume that this component is a result of the overlapping of the spectra for the blocking temperatures of the low- and high-temperature magnetization components. Therefore, these directions carry no useful paleomagnetic information and will also not be considered. Approximately in two-thirds of the gray-brown and brown sandstone samples from the Zigan Formation, the high-temperature component (HTC) was distinguished. It decreases toward zero (Fig. 3c, d) and has blocking temperatures of up to 670–675 ºC (in some samples, they are no more than 580 ºC). It is likely that the main carrier of magnetization is hematite (considerably more rarely, magnetite). The directions of the HTC are well-grouped both within the sites and in the entire collection (Table 1). The fold test (McElhinny, 1964) is uninformative for the HTC because of too narrow bedding variations. There are some arguments for the primary character of the HTC. First, both normal and reversed polarities are observed in the collection. The angle between the average vectors of different polarities γ = 8.7° is smaller than the critical value γcrit = 10.5° (McFadden and McElhinny, 1990), so that the reversal test is positive (Fig. 4).
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Fig. 3. Examples of Zijderveld diagrams for samples from the Zigan Formation in stratigraphic coordinates. Filled (open) symbols, projection onto a horizontal (vertical) plane. Temperatures are given in ºC, and the magnetization intensity along the axes is given in mA/m. Magnetization components cannot be distinguished in sample M1696 (a). Solid line indicates the medium-temperature component (Fig. 4b), and the dashed line indicates the high-temperature component (HTC). a–d, See explanations in the text.
Second, several zones of different polarities were distinguished in the outcrops; for example, the presence of ~20 zones in section Z3 along the Zigan River suggests the primary character of the HTC. Third, if the studied late Ediacaran rocks had been remagnetized later, the resulting pole would coincide with the corresponding region of the Phanerozoic apparent polar wander path for the EEP (Torsvik and Cocks, 2005). The pole obtained in the South Urals is far away from the Phanerozoic curve at any polarity (Fig. 5). We presume that the most important fact is the agreement between the poles for coeval rocks from other EEP parts and
that for the South Urals. Some paleomagnetic poles agreeing with our result were obtained from the Upper Ediacaran rocks (550–555 Ma) of the southeastern shore of the White Sea (Iglesia-Llanos et al., 2005; Popov et al., 2002, 2005). The pole for volcanics from South Volhynia belongs to the same group (Elming et al., 2007). The southeastern shore of the White Sea is more than 1500 km away from the Bashkir anticlinorium, and the distance between the South Urals and Volhynia is no shorter. It is quite hard to imagine a geologic event causing simultaneous remagnetization of rocks at such long distances. Therefore, the positive test of external convergence indicates the primary character of magnetization at all
Table 1. Directions of the high-temperature magnetization in the sediments of the Zigan Formation Site
DAº
Dº
N/No
Geoghraphic coordinates
Stratigraphic coordinates
Dº
Iº
k
α95º
Dº
Iº
k
α95º
Section Z1 P1147 P1163
303 290
21 24
16/7 16/10
281.1 101.8
40.7 –42.7
18.7 21.1
14.3 10.8
285.4 103.6
20.8 –19.3
17.6 18.1
14.8 11.7
Section Z3 N5061 M1674 P1187 M1681 N5069 M1695 Mean (sites) Mean (samples)
297 291 279 286 282 283 – –
26 23 26 28 26 26 – –
15/6 7/6 8/6 14/11 15/10 15/13 (8) 106/69
279.3 91.7 285.7 289.1 304.5 120.2 286.9 293.0
43.3 –44.0 48.6 46.1 36.3 –43.7 43.7 38.4
26.5 56.5 90.7 20.6 15.1 14.2 83.4 10.4
13.3 9.0 7.1 10.3 13.1 11.4 6.1 5.6
283.5 96.2 283.9 288.3 300.4 116.0 287.2 291.7
18.0 –22.0 22.6 18.0 12.2 –18.5 19.1 13.8
26.5 39.1 80.8 22.5 13.6 12.7 103 10.4
13.3 10.8 7.5 9.8 13.8 12.1 5.5 5.6
Note. GC, Geographic coordinates; SC, stratigraphic coordinates; DA, dip azimuth; D, dip angle; N/No, selected to used samples (the number of sites is in parentheses); D, declination; I, inclination; k, concentration parameter; and α95, radius of confidence circle (Fisher, 1953).
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Fig. 4. Stereograms of the directions of the high-temperature magnetization component for samples of sedimentary rocks from the Zigan Formation. GC, Geographic coordinates (a). SC, Stratigraphic coordinates (b). Filled (open) circles show the directions of normal (reversed) polarity. Stars indicate the average HTC directions for two groups of different polarities with confidence circles. Filled (open) symbols and solid (dashed) lines are projections onto the lower (upper) hemisphere.
three sites. All these facts suggest the primary character of the HTC of magnetization in the Zigan Formation.
Overview of paleomagnetic data on the Ediacaran complexes of the East European Platform Before we discuss the paleogeographic significance of the data obtained, it makes sense to consider published paleomagnetic data on the Ediacaran rocks of the EEP (Fig. 6). The only early Ediacaran pole was detected from the Egersund dike complex, southwestern Norway (Walderhaug et al., 2007). The rocks were reliably dated by the U–Pb zircon method (616 ± 3 Ma), and the paleomagnetic result was confirmed by the test for burned contacts Carbonatite intrusive complexes aged ~585 Ma were studied in two places: the Fen complex of Southern Norway (Meert et al., 1998; Piper, 1988) and the Alnø complex of Central Sweden (Meert et al., 2007; Piper, 1981). The paleomagnetic data on the Fen complex suggest that the EEP is located at middle latitudes, but many authors point out the agreement of this pole with Permo-Triassic data on the same platform. Also, they presume remagnetization of the Fen complex during magmatic activity in the nearby Oslo graben. Two magnetization components were detected during the paleomagnetic studies of the Alnø complex. According to one of them, the EEP must have been located at the pole, and according to the other, in the tropics; this does not permit using these data for paleogeographical reconstructions (Meert et al., 2007). The Ediacaran rocks in the southwestern EEP (Volhynia) are lavas and tuffs a few hundred meters thick, overlain by a 200-m-thick sedimentary cover. Unfortunately, the Ediacaran volcanic sections of Volhynia are known almost exclusively
from drilling data. Four lava flows were stripped in two small mines in northern Volhynia, and two more were stripped in the southern part of the region. For lack of other objects, these flows were studied several times (Elming et al., 2007; Iosifidi et al., 2001; Nawrocki et al., 2004). Elming et al. (2007) combined all the data and made the following conclusion: The data from North Volhynia suggest that the EEP was located near the pole at 560–580 Ma, whereas those from South Volhynia suggest that this part of the platform was located at a latitude of ~20º. The sedimentary section overlying the volcanics was studied by A.G. Iosifidi et al. (2005), who distinguished six magnetization components in these sediments. Unfortunately, the impossibility of determining the age or exact direction of these components does not permit using these data. Ediacaran rocks are known in a vast area of northern Russia but almost exclusively from drilling data. These rocks were dated by the U–Pb zircon method (550–555 Ma) based on the tuff interbeds from sections of the southeastern shore of the White Sea (Iglesia-Llanos et al., 2005); also, reliable paleomagnetic data were obtained from some outcrops (Iglesia-Llanos et al., 2005; Popov et al., 2002, 2005). As was shown above, the pole obtained for the upper ediacaran rocks (Z) agrees with the coeval poles from the southeastern shore of the White Sea (W) (Iglesia-Llanos et al., 2005; Popov et al., 2002, 2005) and with that for the upper Ediacaran lavas of South Volhynia (V) (Elming et al., 2007). All these poles form a dense cluster (ZWV) (Fig. 5), and the average pole for this group has a latitude Plat = 26.9 ºN and a longitude Plong = 299.2 ºE (concentration parameter k = 35, radius of the confidence circle A95 = 11.4º, and N = 6 poles). The dispersion within the ZWV group of poles might be due to slight age differences between the studied rocks.
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Fig. 5. Apparent polar wander path for the East European Platform (diamonds and dotted line) from Early Ordovician (480 Ma) to present (Torsvik and Cocks, 2005), with some Ediacaran data on this platform. Stars with confidence circles show the data on sedimentary rocks from the Zigan Formation for two versions of polarity (ZN and ZR, see explanations in the text). Crosses indicate poles aged 550–555 Ma from the southeastern shore of the White Sea (Iglesia-Llanos et al., 2005; Popov et al., 2002, 2005), and the circle shows a pole aged ~550 Ma from the Ediacaran lavas of South Volhynia (Elming et al., 2007). Numbers near some poles on the polar wander path indicate age, Ma.
Discussion The above data suggest that the Zigan Formation accumulated at ~550 Ma and the HTC of magnetization is primary in these rocks. This means that the Urals edge of the EEP was located at near-equatorial latitudes in the late Ediacaran.
Fig. 6. Distribution of Vendian paleomagnetic poles in the East European Platform (all the poles are projected onto the Northern hemisphere). Square shows the Egersund dike complex, southwestern Norway (Walderhaug et al., 2007); diamond, the Fen complex of Southern Norway (Meert et al., 1998; Piper, 1988); sidelong cross, the Alnø complex of Central Sweden (Meert et al., 2007; Piper, 1981): Alnø-1 and Alnø-2, poles for the high- and low-angle components, respectively; straight crosses, basalts from North and South Volhynia (Elming et al., 2007); circles, the southeastern shore of the White Sea (Popov et al., 2002, 2005); and star, the South Urals (present paper). Confidence circles around the poles are not shown.
How justified is it to extrapolate the data on the western Urals to the entire platform? The rocks of the western Urals Fold Belt are known to be very similar to the sediments of the platform itself. The Meso- and Neoproterozoic sedimentary rocks of the Bashkir anticlinorium are reliably compared to the sediments stripped by deep boreholes in the Cisuralian foredeep and in the eastern EEP. These correlations are confirmed by a set of seismic profiles (Keller and Chumakov, 1983). Thus, the western Urals Fold Belt is a platform margin, though deformed, and there were no significant displacements of the Bashkir anticlinorium with respect to the platform. The same is evidenced by the above-mentioned convergence between the magnetic poles for the South Urals, White Sea, and Volhynia. Undoubtedly, local rotations of individual structures are possible in the deformed platform margin. The main folding took place in the Permian in connection with the formation of the Urals orogen (Puchkov, 2003). The lack of angular unconformities in the sections of the western South Urals from early Ediacaran to late Early Permian suggests that of pre-Permian deformations in the region. That is to say, local rotations, if any, must have occurred in the Permian, thus affecting all the pre-Permian paleomagnetic data. Studies aimed at the search for local rotations in the western South Urals relative to the EEP have been conducted before (e.g., (Shipunov, 1998)). It turned out that a vast majority of the studied sections are not rotated with respect to the EEP within the error of the method and the very scarce rotations are, most likely, related to small local structures. Although this issue calls for additional studies, it is likely that most of
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Fig. 7. Determination of the inclination decrease in the rocks of the Zigan Formation. a, Theoretical comparison of the inclination distribution in the case of a dramatic inclination decrease. b, c, Initial theoretical distribution (b) and that after the decrease (c), both reduced to the pole of the sphere. For clarity, gray areas show different parts of the distributions. d, Distribution of individual HTC directions in the Zigan Formation, reduced to the pole of the sphere and the same polarity. Dotted circles are shown to emphasize the almost circular shape of this distribution.
the blocks of the Bashkir anticlinorium did not undergo any significant displacement relative to one another or the EEP. Thus, it appears reasonable to extrapolate the paleomagnetic data on the Ediacaran of the Bashkir anticlinorium to the EEP. Another important problem is the possible effect of the inclination decrease on the paleomagnetic data on the sedimentary rocks. Since this phenomenon is frequent, its possibility cannot be ignored. We tried to estimate the possible decrease for our collection. If the directions initially (before the decrease) showed an F- or nearly F-distribution (Fisher, 1953), it became elongated after the decrease (Bazhenov, 1981). The steeper the initial inclination and the greater the decrease, the more dramatic the change in the distribution shape (Fig. 7). Let us assume that the EEP was located at high latitudes (e.g., ~55º N, inclination ~70º) in the Ediacaran. The observed inclination of 15º in this case must result from a great decrease. However, at a decrease from 70º to 15º, the initial circular distribution with an elongation equal to unity will become strongly elongated (8–10) (Fig. 7a–c). The observed distribution is almost circular, with an elongation <<2, which is statistically insignificant (Fig. 7d). Therefore, there was no
dramatic decrease. A slight inclination decrease of 5º–10º is possible, but an effect of such extent cannot change the conclusion that the Zigan Formation accumulated at nearequatorial latitudes. So, the poles of the ZWV group show that the EEP was located at the near-equatorial latitudes of the Northern or Southern hemispheres in the late Ediacaran. An attempt can be made to determine the polarity of the paleomagnetic data using the paleogeographic position of the other plates in the late Ediacaran. It was reliably established that the opening of the Iapetus Ocean began between Laurentia, Gondwana, and the EEP at 615–550 Ma. This is evidenced by rift complexes on the British isles, in Norway, and in the Northern Appalachians (Cawood et al., 2001; Cocks and Torsvik, 2011; McCausland et al., 2007). The views on the opening of the Iapetus are contradictory, but two paleomagnetic results with ages of 550–555 Ma for Laurentia suggest its equatorial position (McCausland and Hodych, 1998; Van Alstine and Gillett, 1979). If the EEP were located in the Northern hemisphere in the late Ediacaran, the distance between two sides of the Iapetus would be more than 5000 km (Fig. 8, version A). This
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References
Fig. 8. Possible positions of the East European Platform (Baltica) in accordance with the results. Letters A and B indicate possible positions of Baltica depending on selected polarity. Areas with known geologic indicators of the opening of the Iapetus Ocean are in gray (Van der Voo, 1993). Star shows the study area in the South Urals.
configuration appears hardly possible; it is much more likely that the EEP was at the near-equatorial latitudes of the Southern hemisphere (Fig. 8, version B). Conclusions The above facts invite the following conclusions. (1) The age of the Zigan Formation is ~550 Ma; (2) High-temperature magnetization in the Zigan Formation can be considered primary; (3) The pole obtained can be extrapolated to the entire East European Platform; (4) The Zigan Formation showed no dramatic inclination decrease; (5) The mean direction of the high-temperature magnetization of the Zigan Formation corresponds to a paleolatitude of ~10º ± 3º. Consequently, the East European Platform was located near the Equator in the late Ediacaran. We thank our colleagues from the Institute of Geology (Ufa) for valuable geological data on the Bashkir anticlinorium and help with field studies. Also, we are grateful to N.Ya. Dvorova and O.A. Krezhovskikh for the demagnetization of paleomagnetic collections. Finally, our thanks go to D.V. Metelkin and the anonymous referee for the enormous work of improving the paper. This work was supported by the Russian Foundation for Basic Research, grants 11-05-00037 and 11-05-00137, Program No. 10 of the Department of Earth Sciences RAS, and the US National Science Foundation, grant EAR11-19038.
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Editorial responsibility: D.V. Metelkin