RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES6001, doi:10.2205/2006ES000206, 2006
[4] The authors analysed data on the mode of occurrence, lithology and facies characteristics of Maastrichtian and Danian sediments of the vast areas of North Eurasia encompassing the territories of Lithuania, Belarus, Ukraine, lands along the Volga (Povolzh'e), the Cis-Caspian region, the Crimea, the Caucasus and Turkmenistan.
[5] At the end of Cretaceous and in the Paleocene, several large sedimentary basins were located there with individual structure-facies characteristics. The common feature of the territories is their spatial remoteness from the North Atlantic and Hindustan. Data from summarizing reports [Nikishin et al., 1999] and author's data of the Caucasus, Povolzh'e, and West Turkmenistan are used in this paper.
[7] In Chechnya and North Dagestan, Danian sediments are missing and Maastrichtian sediments are overlain transgressively by marls of foraminefera series of the Upper Paleocene. In South (Limestone) Dagestan the boundary between Maastrichtian white limestone and Danian gray pelitomorphic limestone is uniform and pronounced. Danian base in G. taurica zone is not established in these sections. In Nagornyi Karabakh and East Azerbaijan, Upper Paleocene marl overlay transgressively Maastrichtian limestone.
[8] In the southern slope of West Caucasus, Upper Cretaceous and Paleogene flysch is abundant in the seaside zone from Anapa to Sochi. Maastrichtian part of flysch (Snegurovskaya suite) is composed of rhythmically alternating marl, siltstone and limestone. The boundary of the Paleogene and the Cretaceous is not marked lithologically and is only distingushed by foraminifera fauna of 50 m below the suite top. Danian lower part is not reliably established in Novorossiysk flysch.
[9] In Gornyi Crimea, Danian limestone and marl overlay Maastrichtian stage represented by sandy marl and calcareous sandstone with underwater erosion traces. In eastern Gornyi Crimea the stage boundary is concordant.
[10] In western Central Asia, sedimentation of carbonaceous type prevailed at the end of Cretaceous - the beginning of Paleogene. Maastrichtian and Danian sediments are commonly represented by limestone, marl and locally by calcareous clay.
[11] In West Kopetdag, Bolshoy Balkhan and Tuarkyr, Danian sediments overlay transgressively Maastrichtian and Campanian sediments. In Malyi Balkhan, Central Kopetdag, in Mangyshlak and Ustyurt, sequences prevail that show traces of "hard ground" type interruption with insignificant stratigraphic interval. Less common are full sequences where stage boundary is marked by a thin (1-3 cm) bed of montmorillonite-hydromica brown clay. Highly metamorphosed quartz and high concentrations of Ir, Ni, Cr, Sc and platinoids were revealed in boundary clays. This layer is associated with a great impact at the end of Cretaceous - beginning of the Paleogene [Nikishin et al., 1999; Veymarn et al., 1998].
[13] In Prichenomorskaya and Dnieper-Donetsk basins and Donbass, the deposition of chalk and marl beds went on at the end of Cretaceous and the Paleocene. The boundary of the Maastrichtian and Danian is commonly erosional with missing Maastrichtian stage top completely or partially.
[14] In the central part of the Cis-Caspian basin, the Upper Cretaceous and the Paleocene are represented with limestone-marl formation. Cretaceous upper horizons are commonly washed out there and Danian sediments overlay transgressively by Lower Maastrichtian substage sediments.
[15] Against the general relatively uniform background, the Cis-Caspian region and Povolzh'e are distinguished where a specific structure-facial zone is pronounced while it missed in other sedimentary basins.
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Figure 1 |
[17] A distinguishing feature is the location of siliceous beds at large disjunctive zones: sub- meridional Volga-Kama fault, Pachelmskiy aulakogen and a series of sublatitidinal step-like plunges (Zhadovskiy ledge), which bound the northern slope of the Cis-Caspian basin. Their intersection and triple junction with Pachelmskiy aulakogen is outlined at Volsk-Balakovo area. Rikhter [2003] was the first to determine this tectonic zone as a complex mosaic-block junction zone of three paleorifts (Figure 1). In this structure-facial zone, Cretaceous-Paleogene boundary clearly marks the end of carbonaceous sedimentation and the beginning of the Paleogene siliceous sedimentation.
[18] The first stage of siliceous sediments intense accumulation is confined to the beginning of the Santonian. Its product was Lower Santonian "banded series" represented with alternating opoka and combined clayey-siliceous, carbonaceous-siliceous and zeolitic-carbonaceous-clayey-siliceous rocks of the total thickness of up to 40 m. Rock interlayers of small thickness and mixed composition are locally encountered in the Upper Santonian, Campanian, and Maastrichtian but the bulk of them are concentrated in the Lower Santonian.
[19] The second outburst of siliceous accumulation is confined to the Zealandian. It is positioned in sections with opoka and siliceous clays of Nizhnesyzranskaya suite whose thickness reaches 50-75 m. Opoka and mixed carbonaceous-siliceous-terrigenous rocks occur as individual interlayers in Upper Paleocene and Eocene sequences [Akhlestina and Ivanov, 1998, 2000].
[20] The major fields of siliceous deposition are structurally attracted to the southeastern area of Ryazan-Saratov depression (Pachelmskiy aulacogen) and western and northern onboard zones of the Cis-Caspian basin. Toward the eastern slope of Voronezh massif, the opoka is replaced by terrigenous formations and in the central part of the Cis-Caspian basin by carbonaceous-terrigenous formations.
[21] In this structure-facial zone three types of boundary sections are known. The first type is well expressed in the section near village Belgorodnya, where Upper Maastrichtian limestone with sharp gap is overlapped by twelve-meter band of complex alternation of glauconite calcerous-siliceous sandstone and siltstone with gruss and rock debris basal horizon. These deposits known as Belgorodnya layers up to the section gradually change into dark gray massive opoka of Nizhnesyzrznskaya sub-suite. On a vase of nannoplankton finding of zone NP3, Musatov et al. [2004] refers the Belgorodnya layers and Syzranskaya suite to the Zealandian.
[22] In the sections of the second type, Belgorodnya layers are missing and rocks of Syzranskaya suite overlay transgressively different horizons of the Maastrichtian.
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Figure 2 |
[26] In the lower part of layer 2 besides Maastrichtian nannoplankton forms, species typical of the lower part of NP2 zone and are noted in some samples of zones NP1 (definitions by N. A. Savitskaya and P. G. Kalinichenko). Since the zone index species Cruciplacolithus tenuis was not detected in the samples, L. I. Ermokhina (preprint, 1990) dates this part of the section as the upper part of zone NP1 - the lower part of zone NP2. Nannoplankton of zones NP4-NP5 is defined from the upper part of layer 2. Danian section begins with the zone Globorotalia pseudobulloides - Globoconusa daubjergensis by the composition of plankton foraminifera complex. L. I. Ermokhina distinguished Danian sediments of layer 2 as Klyuchevskaya suite.
[28] A similar section was studied in the vicinity of village Teplovka of Novo-Burasskiy district of Saratov region. There in the mountain slope the Upper Cretaceous-Paleocene contact is exposed in a large rain channel. The section is composed from bottom to top (Figure 2a).
[31] The literature on the problems of opal-cristobalite rock formation and mixed diversities closely related to them in Povolzh'e is quite extensive. Earlier, researchers associated the periodic bursts of silicic accumulation with transgressive stages of shelf basins development in the north-eastern periphery of the Tethys. It was assumed that they were caused by active displacements of oceanic water masses and the rise to near-surface layers of deep water enriched with silica. Climate warming and tectonic volcanic activation favorable for plankton growth with silicic function were noted as accompanying features [Akhlestina and Kurlaev, 1979, 1988 and others]. Subsequently owing to evident correlation between the areas of silicic accumulation and fracture zones their formation was associated with the inflow of thermal water enriched with silica coming by deep-seated faults at the moment of their activity [Akhlestina and Ivanov, 2000].
[32] The association of lithotypes in the contact of Maastrichtian and Klyuchevskaya suite testify to a rapid change in sedimentation conditions with a minimum time interval or without an actual time gap. It is suggested by the marked boundary between the Maastichtian and Danian, weak traces of underwater rewashing of the soft floor, increased carbonate content of Danian low horizons with rapid growth of silicification up the section.
[33] All these features allow us to assume hidden events underlying the rapid change of sedimentation conditions in the marginal structure-facial zone. The hypothesis of tectonic activaty in the junction zones of deep-seated faults seems to be the most credible, however the initial cause of the activation has not been established.
[34] Chronologically the pulses of silicic accumulation in Povolzh'e coincided with Senomanian and Maastrichtian plumes but it can hardly testify to direct effect of the latter on the disjunctive tectonics of such a remote area. In the context of possible effects of the plumes on sedimentation, data on small pyroclastics presented in Santonian and Syzran opoka of Povolzh'e with absolute missing of it in carbonaceous parts of the section seem to bear more information [Akhlestina and Ivanov, 1998, 2000]. Fresh ash horizons of the type that is known in Veshenskaya suite [Muraviev et al., 1997] are not of particular interest in this case because they are located close to the Caucasus and the Carpathian volcanogenic belts.
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Figure 3 |
[36] The presence of metallic iron spherules of cosmic origin in sediments of different ages is a well-known phenomenon. But the our case accurately stratified isolated horizons in sections set apart for more than 30 km. Besides, data were published on the enrichment with spherules composed of hydrous ferric oxides near the Maastrichtian-Danian border in section Abat in Oman [Ellwood et al., 2003]. The authors associated their appearance with a great impact event. However the interval of enrichment with spherules embraces approximately 50 cm of sediment thickness, i. e. many thousands of years and their number maximum does not coincide with K/T boundary and is 10-20 cm below it. Besides it is not clear why no spherule of metallic iron was discovered though in similar sections metallic iron spherules were repeatedly encountered. Thus in Maastrichtian-Danian sections of Koshak (Mangyshlak) and Gams (Austria) particles of metallic iron were discovered at different levels of the sections irrespective of the age and lithology of sediments [Grachev et al., 2005; Pechersky et al., 2006]. In these cases a relation is more likely between metallic iron particles and space meteoric dust, which subsided at different times. The data given on sections Klyuchi and Teplovka and their likely relation to impact events, owing to their spatial proximity to Cretaceous-Paleogene Kamenskaya and Puchezh-Katomskaya astroblemes, should be considered as preliminary and more detailed further studies of boundary layers and search for minerals with traces of shock metamorphism and others are required.
Citation: 2006), Sedimentogenesis in Maastrichtian-Danian basins of the Russian plate and adjacent areas in the context of plume geodynamics, Russ. J. Earth Sci., 8, ES6001, doi:10.2205/2006ES000206.
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