B. V. Burov
Kazan State University, Kazan, Russia
The Early Triassic rocks of the Moscow Syneclise are similar in many respects to the deposits that had accumulated there during the Tatarian Stage, especially in terms of its continuing molasse sedimentation and also in terms of its geomagnetic field. These rocks were always hard to be distinguished. Moreover, the tracing of a boundary between the Permian and Triassic rocks is complicated by the existence of unconformities, their volumes differing in different areas of the basin. Yet, these unconformities are reflected in the paleomagnetic data [Burov and Boronin, 1977]. The Triassic rocks may rest on the directly or inversely magnetized rocks of the Vyatka Stage and also on the rocks of the North Dvina Stage. Although directly magnetized rocks are usually found at the base of the Triassic rock sequence, in some areas they are replaced by inversely magnetized rocks, and one cannot always prove that a given magnetic zone of reversed polarity continues the Triassic rock sequence ( R0T ), or was produced by the erosion of the direct-polarity zone ( N1T ). The extent of the synchronous death of various groups of living organisms on the land and in the sea is still unknown. The Permian-Triassic boundary in the continental rocks is usually placed using the remains of terrestrial vertebrates, the first appearance of Lystrosaurus, or the presence of palynological remains showing a typical Triassic appearance. In marine deposits this boundary is usually located by the first appearance of Otoceras ammonites. It has been suggested recently that this boundary should be placed somewhat higher, at the base of the Hindeodus parvus conodont zone. It should be noted that in addition to the marine fossils the Otoceras beds are usually characterized well by palynological complexes [Balme, 1979], this allowing their correlation with continental rock formations.
The Lower Vokhma horizon has been identified in the Triassic rocks using the stratotypes of the Vetluga River basin and, proceeding from its rhythmic structure, has been subdivided in detail into the Astashikha, Riabina, and Krasnaya Bakovka members. The differentiation of the Triassic rocks in this area was based on the fauna of terrestrial vertebrates, as well as on phyllopodia, chonkhostracia, ostracodes, flora, and some spore and pollen complexes.
It has been found recently [Lozovskii et al., 2001a, 2001b] that the Astashikha fossil-bearing rocks overlie, in the Kichmenga River basin, another older, Triassic, Nedubrovo rhythmic rock member. Also traceable here is the contact between the Vokhma rocks and the underlying Vyatkan rocks of the Tatarian series of the Permian rock series. The Nedubrovo member differs from the enclosing rocks by the diversity and abundance of various terrestrial biota groups [Lozovskii, 2001a, 2001b].
1. Thin (to lenticular) intercalation of greenish gray and gray thin-plated claystone and siltstone beds with their bedding planes showing inclusions of charred plant remains (Figure 3). The bottoms of some interbeds show a violet-pink color, the tops including an interlayer of light gray marl nodules ranging from 3 cm to 5 cm in size. Occasional lenses and concretions of fine-grained argillaceous sandstone occur in the middle of this layer. Also found in this layer were numerous dispersed phytoleims, Otynisporites megaspores, pollen grains and spores, ostracodes, conchostracans, and insects. According to the data reported by V. R. Lozovskii, the argillaceous constituent of the rocks is represented mainly by smectite, the product of volcanic ash alteration. In terms of its origin, this layer represents the estuarine deposits that had accumulated under oxygen-free conditions with the markedly seasonal supply of the rock material. The thickness of this layer is 5-6 m.
2. Brownish-red thin-bedded clay with marl concretions, up to 5 cm in size, and two interbeds of gray clay, 0.3 m and 0.1 m thick. The top of the layer includes, locally, lenticular interlayers (0.1-0.2 m thick) and nodules of greenish-gray and pinkish fine-grained sandstone and marl nodules (Figure 3). The thickness of this layer is 6-7 m.
These rocks are overlain, with traces of erosion, by greenish-gray alluvial polymictic sand interbedded by layers of poorly cemented sandstone and conglomerate which represent the basal layers of the Astashikha member, with a visible thickness of about 2.5 m, in which a Tupilakosaurus sp. vertebra and unidentified remains of Triassic procholophones were found. The upper part of the slope is composed of Quaternary deluvial sandy loam and loam with gravels and pebbles
Measured at the laboratory were remanent magnetization using a JR-4 spin-magnetometer, and magnetic susceptibility using a KLY-1 instrument. All rock samples had been subject to magnetic cleaning using a temperature of 250o C.
Many rock sequences of the Russian Platform show a specific feature consisting in the fact that the Permian-Triassic boundary coincides with a depth level marked by an abrupt change in the magnetic properties of the rocks [Burov, 2004].
Like the similar rocks of the Vyatkan member, the Permian rocks exposed in the Yug River drainage basin in the vicinities of the Medvezhii Vzvoz, Gavrino, Rogovik, and other villages show the low values of magnetic susceptibility ( c av = 57 10-5 SI in the Glebov Gully) and of natural remanent magnetization ( Jnav =7.3 10-5 Am m-1 ). The transition to the Triassic rocks is accompanied by a high growth of magnetic susceptibility up to ~1000 10-5 SI, (the average value being 623 10-5 SI), typical mainly of Layer 1 composed of greenish-gray, argillaceous siltstone and claystone residing at the base of the Nedubrovo Member (Figure 1).
Beginning from the base of the 2nd layer, magnetic susceptibility declines to 100-400 10-5 SI, the average value being 159 10-5 SI, that is, an order of magnitude higher than the value obtained for the Permian rocks, except for some depth levels where magnetic susceptibility was found to be close to the Permian values.
Our study of magnetic minerals in the Triassic rocks suggest that even the finely dispersed rocks at the lower horizons of the Vetluga Series are enriched almost everywhere, irrespective of their facies, in magnetite, maghemite, and hematite, which control the high magnetization of the rocks (Figure 6b). A similar magnetic mineral association is widely developed and has been studied in detail [Burov et al., 1986] in the products of low-temperature titanomagnetite oxidation. In our opinion, volcanic ash contributed to the formation of the petromagnetic characteristics of the Early Triassic rocks in the study area.
The characteristic effect of the growth of magnetization at the temperature of 180o C and its substantial decline after the first heating to 600o C suggests the presence of maghemite in association with magnetite in the rocks.
The fact that almost all of the levels shown in the figure had several duplicates allowed us to estimate, though with a not high accuracy, the mean direction of the old NRM component (D = 229.2o, J=-47.3o, K = 6, and a95=15.9o ), the paleolatitude (28.5o N), and the position of the geomagnetic pole ( q = 44.4o N and l = 157.2o E).
The Permian rocks of the sequence were characterized merely by a few oriented rock samples, the NRM component of which, though being thermally stable, varied greatly along the section in terms of its direction, and did not correspond to both the direct and inverse polarity of the geomagnetic field.
The normally magnetized Astashikha deposits, ranked as the oldest Triassic rocks of the Moscow Syneclise, are underlain in this sequence by the reversely magnetized rocks of the Nedubrovo member. It should be noted that the nearest potential heat sources, namely, the basalt traps of the Adz'va River basin in the Pechera area bordering the Ural region begin the sequence of the Lower Triassic rocks and their primary magnetization has a reversed polarity either [Balabanov, 1988].
It can thus be concluded that the Nedubrovo rock sequence is unique in the study region because of its most complete Lower Triassic rocks of the Russian Platform. The high magnetization values obtained for the first layer are anomalous and not characteristic of normal sedimentary rocks. It is not inconceivable that this is just the level where not only ferromagnetic but also some other traces can be found for the onset of a new Mesozoic Era.
The almost ubiquitous participation of maghemite in the formation of the ferromagnetic habit of the almost all rocks of the Indian Stage, especially in its lower layers, irrespective of their facies, allows one to suggest that the genetic roots of this admixture had been associated with the air transportation and addition of the volcanic material. If our assumption of the airborne ferromagnetic ash material addition to the Triassic sediments is correct and that this material was voluminous enough, a similar effect can be expected in other sedimentary basins. Of particular interest are the stratons recorded at the Permian-Triassic boundary in South China. In contrast to those mentioned above, they consist of marine rocks. Their magnetic characteristics have been studied fairly well [Heller et al., 1995]. The authors of this paper emphasized that the magnetization intensity of the Permian marine limestones in all rock sequences of South China is much lower than that of the Triassic ones. As follows from the results of their study, the Permian rocks of the Wulong and Shuijiang rock sequences show their average natural remanent magnetization to be as low as 0.353 mA m -1, whereas this value was found to be as high as 4.85 mA m -1 for the Triassic rocks. These authors believe that the Triassic limestone contains single-domain magnetite and/or maghemite.
The problem of the high content of the volcanic material in the Triassic sedimentary rocks is of great interest not only in terms of new opportunities in the potential discretization and correlation of the events. The effect of the inferred high reactivation of volcanic activity in the Early Triassic time suggests the higher global dusting of the atmosphere, as well as changes in its gas content, in solar luminance, in the temperature, and in some other side effects of the environmental transformations. It is possible that the magnitudes of these events at the beginning of the Triassic epoch turned out to be sufficient to disturb and change the climate of the planet and its biota at the transition from the Paleozoic to the Mesozoic Era.
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