Lower Paleozoic Deposits in the Transition Zone between the Carbonate Platform and Black Shale Basin (Eastern Taimyr)

Lithology and Mineral Resources - Sedimentological features of the Upper Cambrian–Middle Ordovician rocks exposed in eastern Taimyr in the Faddey Gulf area are discussed. The studied sections...     

The Cambrian of Bennett Island (New Siberian Islands)

The paper presents new data on the Cambrian stratigraphy of Bennett Island, one of the least explored East Arctic islands. The section, about 500 m of total thickness, comprises four lithological units that store a record of the deposition history: (1) clastic sediments including storm sandstones; (2) shallow-marine mudstone; (3) lagoonal variegated mudstone and limestone; (4) black shale. It is suggested to classify the units as formations with their proper names. The section spans all epoches of the Cambrian stratigraphy constrained by trilobite fossils. In the Cambrian, territory of the island belonged to Siberia rather than to some exotic terrane, judging by abundant endemic Siberian trilobite species in the Bennett section. This inference is supported by synchronicity in recorded deposition events of Bennett Island and northeastern Siberia (Kharaulakh Mountains). The Cambrian sediments of the two areas were deposited in different parts of a single shallow sea which extended as far as Taimyr.     

Paleogene-Neogene sediments of Bel’kov Island (New Siberian Islands): Characteristics of sedimentary cover in the eastern Laptev shelf

This work is aimed at characterizing the 40-m-thick Paleogene-Neogene sedimentary sequence of Bel’kov Island, which was accumulated in the marginal part of a fluvial plain. On the basis of the plant megafossils and spore-pollen data, the studied sediments were formed in the Late Eocene-Early Miocene with a probable sedimentation gap at the Eocene-Oligocene boundary. It was substantiated that the Paleogene-Neogene sediments were developed as a continuous cover over the entire area of Bel’kov Island. Later, they were eroded away from the island, but presumably preserved on the Bel’kov Horst and Zarya Strait. This suggests that the pre-Quaternary seismic complex, which continuously covers the Bel’kov Horst, also has Late Eocene-Early Miocene age. Selective dating of nonrounded detrital zircons from Lower Miocene conglomerates allowed us to substantiate the transportation of sediments from the Verkhoyansk-Kolyma Fold System along the Bel’kov-Svyatoi Nos Graben. The crystalline basement similar to the Ust’-Lena inlier was likely exposed on the East Laptev Horst and served as the source of metamorphic detritus, including zircons dated within 1850–2000 Ma.     

Aptian-Albian coaliferous sediments of Kotel’nyi Island (New Siberian Islands): New data on the section structure and ignimbrite volcanism

The Aptian-Albian sediments of Kotel’nyi Island are represented by a terrigenous coaliferous complex with the apparent thickness of approximately 700 m. The upper two thirds of their section enclose ignimbrites and rhyolitic ash tuffs. The integral thickness of volcanics is 170 m. A new sequence composed largely of acidic volcanics and sedimentary rocks is defined in the upper part of the Cretaceous section. The K-Ar age estimated for ignimbrite glasses is 110–107 ± 2.5 Ma, which corresponds to the first half of the Albian. The fossil flora list is added by several previously unknown forms. The macroflora of Kotel’nyi Island is similar to its Albian counterpart from the Kolyma-Indigirka region and allows Cretaceous sediments from the lower part the Kotel’nyi Island section to be dated back to the Aptian (?)-Albian (except for the terminal Albian). The palynological characteristic of rocks immediately contacting the dated volcanics appeared to be untypical of Albian sediments of Siberia and similar to that of the Late Neocomian palynocomplexes. This is partly explained by erosion and reworking processes. The examined continental sediments accumulated in post-orogenic extension settings. They constitute the lower strata of the Aptian(?)-Tertiary post-orogenic complex filling riftogenic depressions in the New Siberian Islands and Laptev Sea.     

New data on the stratigraphy and depositional environment for Upper Jurassic and Lower Cretaceous deposits of the Stolbovoi Island (New Siberian Islands)

The section of Mesozoic terrigenous deposits over 1200 m thick has been described, and a new geological map of the Stolbovoi Island southern half has been compiled. The inference has been made that the sedimentary sequence represents a single uniform turbidite complex, which is indivisible into lithological units and which does not show transition to shallow-marine facies either in the visible foot section or visible roof section. The complex accumulated in the foreland basin emerged on the margin of the Novosibirsk-Chukchi continental block during the Anyuian orogeny. The presence of upper Volgian deposits (Upper Jurassic) with a visible thickness of 640 m, Berriasian (Ryazanian) deposits about 100-m thick, and lower Valanginian deposits with a visible thickness of ～200 m has been established on the basis of identification of fossils represented by remains of bivalve mollusks (the genus Buchia). Finds of upper Volgian buchias in the southeastern part of the island are inconsistent with field geological observations of the sequence of deposits. Paleontological data acquired for the island southern part suggest the presence of a transverse thrust fault, along which upper Volgian rocks were thrust over lower Neocomian rocks. The possibility of another interpretation of the data has also been considered. The correlation of the Buchia Beds of the Stolbovoi Island, Nordvik Peninsula, the basin of the Anyui River, and northern California has been carried out. The inference about a close relationship between the biota of the Stolbovoi Island and the North Pacific paleobiogeographic realm has been drawn.     

Upper Devonian depositional system of Bel’kov Island (New Siberian Islands): An intracontinental rift or a continental margin?

The archipelago of New Siberian Islands situated on the northeastern continental shelf of Eurasia is considered a part of an exotic terrane that collided with Siberia in the Early Cretaceous. Bel’kov Island is located close to the inferred western boundary of this terrane and thus should demonstrate attributes of its localization at the margin of the Paleozoic oceanic basin. The Upper Devonian section on Bel’kov Island is a continuous sequence of deepwater terrigenous rocks, which indicates a tendency toward deepening of the basin previously revealed on adjacent Kotel’ny Island. The lowermost Upper Devonian unit on Bel’kov Island is represented by thin Domanik-like strata resting on the Middle Devonian carbonate platform. The main body of the Upper Devonian sequence, more than 4 km in total thickness, is made up of gravity-flow sediments including turbidites, clay and block diamictites, and olistostromes in the upper part of the section, which accumulated at the slope of the basin or its rise. At many levels, these sediments have been redeposited by along-slope currents. The uppermost unit of organogenic limestone is evidence for compensation of the trough. According to conodont assemblages, the deepwater terrigenous rocks were deposited from the early Frasnian to the early Tournaisian. This time is known for extensive rifting in the eastern Siberian Platform. The data obtained allowed us to reconstruct a NNW-trending Late Devonian rift basin on the Laptev Sea shelf similar to other rifts at the eastern margin of the Siberian Platform.     

Neoproterozoic island arcs in East Sayan: duration of magmatism (from U–Pb zircon dating of volcanic clastics)

Two island arcs of different ages have been reconstructed in the Neoproterozoic history of southeastern East Sayan: Dunzhugur and Shishkhid. According to earlier concepts, the Dunzhugur arc formed at ～1020 Ma and underwent collision with the Siberian(?) continent at ～810 Ma. The Shishkhid arc formed somewhat earlier than 800 Ma and existed till the end of the Late Baikalian (～600 Ma, from indirect data). This primitive geologic history, when each arc existed for 200 Myr, was suggested because of the deficit of direct data, and its reconstruction cast doubt. In this work we present results of preliminary dating of detrital zircons separated from the volcaniclastic rocks composing the above arcs. We analyzed 12 zircon crystals from the Dunzhugur volcanic clastics, whose ²⁰⁶Pb/²³⁸U age varies from 844 ± 8 to 1048 ± 12 Ma (1σ). Five most ancient zircons form a concordant cluster with an age of 1034 ± 9 Ma (2σ). Hence, the arc formed earlier than it was assumed and existed for a long time, most likely, till its collision with the continent. We also studied two zircon samples from the volcaniclastic rocks of the Oka accretionary prism, which probably formed in the Shishkhid arc. All ten crystals of the first sample form a concordant cluster with an age of 813 ± 7 Ma (2σ). The analyzed zircons of the second sample arrange in two clusters, with an age of 775 ± 8 Ma (2σ, nine crystals) and 819 ± 17 Ma (three crystals). Thus, the Shishkhid arc formed earlier than it was assumed, at the end of the Early Baikalian, and underwent active volcanism at least till 775 Ma. Dating of detrital zircons from the volcaniclastics generated at the mature stage of the Shishkhid-arc evolution will help to reconstruct partly or completely its history in the period 775–600 Ma.     

Deep-seated pegmatites of the Emiytas mafic-ultramafic complex on Big Lyakhov Island,New Siberian Islands,and their age: <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar and SHRIMP data

Pegmatites of the Emiytas basic-ultrabasic metamorphic complex have a granodiorite-granite composition. Their genetic relations with the host amphibolites follow from the low initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7028 and from the P-T conditions (650°C and 10 kbar), which are close to those of the host rocks. Amphibole, biotite, and muscovite megacrysts analyzed by the ⁴⁰Ar/³⁹Ar method yielded plateau ages of 209.7 ± 0.9, 203.0 ± 0.9, and 178.5 ± 1.5 Ma, respectively. The former two minerals contain excess Ar, whereas the K-Ar system of the muscovite is undisturbed. The cooling of the complex to the closure temperature of this system was likely controlled by its exhumation to a shallower depth level. Zircons from the Emiytas pegmatites occur as polyhedral equant crystals with weakly contrasting sectorial zoning, very low concentrations of U (4–8 ppm in the enriched domains), and low Th/U ratios (0.002–0.003), which suggest that the mineral crystallized at significant depth in the presence of fluid. Tentative SHRIMP II measurements (five analyses) yielded a zircon age of 202 ± 17 Ma. The applying a specialized approach to the analysis of young low-U zircons on an ion probe is discussed. In spite of the small number of analyses, new geochronologic data leave no doubt that the crystallization age of the pegmatites is Late Triassic-Early Jurassic and invalidate earlier ideas that the Emiytas complex is Precambrian-Early Paleozoic. This conclusion makes the Emiytas amphibolites to be one of the various oceanic and suprasubduction complexes related to the Mesozoic South Anyui suture, which is important for reconstructions of the tectonic evolution of the East Siberian Arctic shelf.     

Mid-Cretaceous Tuor-Yuryakh Section of Kotelnyi Island,New Siberian Islands: How Does the Probable Basement of Sedimentary Cover of the Laptev Sea Look on Land?

The model of geological structure of sedimentary cover of the Laptev Sea accepted by most geologists suggests that the lower seismic complex of the cover begins by the Aptian–Albian sedimentary rocks. They can be studied in natural outcrops of Kotelnyi Island. The section of the Tuor-Yuryakh Trough, which exposes the lower part of the Cretaceous complex, is described in the paper. It is composed of continental coaliferous rocks ~100 m thick. The marking beds divide it into five members, which are traced along the western wall of the trough at the distance up to 3 km. The spore–pollen complexes and plant megafossils indicate that almost the entire visible section of the mid-Cretaceous is Albian. Only its lower part no more than 14 m thick can probably belong to the Aptian. Marine facies with Albian foraminifers were found 15 m above the bottom of the Cretaceous complex. The section of the Cretaceous rocks is underlain by the Lower Jurassic marine clays and siltstones. The foraminifer assemblages of this part of the section are typical of the upper Sinemurian–Pliensbachian and fossil bivalves indicate late Sinemurian age of the host rocks. The hiatus ~70 Ma duration has no expression in the section and this boundary can de facto be substantiated only by microfossils. This vague contact between the Lower Jurassic and mid-Cretaceous rocks does not correspond to geophysical characteristics of the bottom of the lower seismic complex of the cover of the eastern part of the Laptev Sea. The latter is described as the most evident seismic horizon of the section of the cover, suggesting unconformable occurrence of the lower seismic complex on a peneplenized surface of lithified and dislocated rocks. This is mostly similar to the bottom of the Eocene sediments, which were observed on Belkovsky and Kotelnyi islands. The paper discusses possible application of our land results for interpretation of the shelf seismic sections of the Laptev Sea. It is concluded that local reasons are responsible for a vague boundary between the Lower Jurassic and mid-Cretaceous sequences in the section studied. Our observations support ideas on possible Aptian–Albian age of the rocks of the basement of the lower seismic complex; however, it is proposed to use also the previously popular idea on the Eocene age of the lower seismic complex of sedimentary cover of the eastern part of the Laptev Sea as one of the possible working scenarios.     

Where does the South Anyui suture go in the New Siberian islands and Laptev Sea?: Implications for the Amerasia basin origin

The South Anyui suture is a major tectonic boundary in NE Asia, believed to represent the remains of an ocean basin which separated Siberia from North America in Jurassic time. Its history also figures prominently in the Mesozoic reconstruction of the Arctic and the origin and evolution of the Amerasia basin. Three types of proposed trends of the South Anyui suture are evaluated. 1) The suture ends near the Kolyma River mouth where it meets the rotational transform. The paper, however, proves that the suture continues further westward up to Big Lyakhov Is. Consequently, a simple geometric rotational model for Amerasia basin origin must be rejected. 2) The suture trends from Big Lyakhov to the Anjou islands. The Anjou islands geology is examined, and it is concluded that the suture could not go through them. Hence, all proposed versions of the rotational hypothesis of the Amerasia basin opening are claimed to be invalid. 3) A proposed Taimyrian connection of the suture is examined, and it is concluded that this model must be rejected as well. The failure of all previously suggested models for the suture extent through the New Siberian islands and Laptev Sea means that in Early Mesozoic there was no oceanic basin that separated the New Siberian–Chukotka terrane from Siberia. Thus Siberia and North America formed a continuous continent in Jurassic time. This paper presents evidence that the South Anyui suture has instead turned back from Big Lyakhov island and followed a sinuous path designated as the Chroma Loop before connecting with the Kolyma Loop suture. On this model the South Anyui suture can be interpreted as a small segment of extensive boundary which separated the Amerasia Jurassic margin and terranes accreted to it from the Pacific. The modern boundary around the North Pacific is also quite sinuous. It was suggested that in Jurassic time it was straighter and the Amerasia ocean was originated as a common back-arc basin. Finally a new two-pole parallelogram hypothesis for the Amerasia basin opening is suggested and the approaches to its verification are outlined.