| Origin of recent deposits in the Barents shelf |
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KRAPIVNER R.B. The genesis and stratigraphic nomenclature of recent deposits of the Barents shelf are still controversial; the only thing which is quite clear is that they accumulated during the neotectonic period or certain part of it.The thickness of the deposits is insignificant, varying from 2-5 to 100-150 m. or more.In spite of this, they form an independent structural-formational stage separated from underlying rocks by the boundary, in the literature named as the upper regional unconformity. Most widespread lithotypes of recent deposits on the shelf main area are clayey, silty-clayey, and silty-sandy-clayey mud of the upper sedimentary sequence and their underlying diamicton, locally subdivided into two horizons. Mud is shown by an acoustically transparent (uniform or layered) seismogram, substantiated by a low degree of its consolidation characterized by the liquidity index IL (Krapivner, 2008). In thick (many tens of meters) sections of the upper sedimentary sequence, the acoustically transparent seismogram, at a certain depth (at 0.25 < IL < 0.5) is changed (with unchanged lithology) into chaotic, which is regarded to be typical of diamicton, though virtually it is a sign of consolidated homogeneous rocks of the clayey composition. As the thickness of mud does not commonly exceed 5-6 m (up to 10-25 m, occasionally higher, in troughs), their base is in most cases recorded by the reflecting boundary, which coincides with jumps in the values of density, porosity, and the liquidity index. It is one of the most significant characteristics indicating that on the entire area of the shelf, the mud and its underlying deposits belong to different sedimentary complexes. A long hiatus in sedimentation, separating them, is confirmed by the thermoluminescence and paleomagnetic datings of diamicton and by 14C age, as well as by the deposition conditions and facies composition of sediments of the upper complex (Krapivner, 2006, 2008). Most complete sections of this complex are assigned to buried river valleys, fragments of which, preserved in landforms, are represented by marginal and transverse troughs (except neotectonic graben-troughs: St. Anne, Franz-Victoria, and Medvezhinsk, in which the presence of such valleys is problematic). The slopes and bottom of those troughs are covered with sediments of the upper acoustically transparent sedimentary complex. Bodies of river bed alluvium, lenticular in cross section, buried under the complex, are recorded by seismoacoustical profiles at absolute marks going far beyond the limits accepted by the glacio-eustatic conception. River bed alluvium, from 5-6 to 10 m thick, assumed to occur in the Central trough and proved in the Pechora--Kanin shallows, has been penetrated by boreholes. The diamicton, occurring on the elongated low-angle slopes of paleovalleys, and alluvial sand enclosed in it at their bottom, are overlapped with fine-bed clayey mud, which belong to facies of estuarine tidal rhythmites (Middleton, 1984). They deposited in a shallow-water environment characterized by highly turbid water and with periodically varying freshening that affected the composition of the foraminiferal fauna, represented merely by few species. Predominant are arctic eurybiontic species, which sustain a low salinity and (or) high turbidity of bottom waters: Retroelphidium clavatum (= Elphidium excavatum f. clavata) and Cassidulina reniforme. Shallow-water species that withstand freshening and elevated hydrodynamic energy are also present. Tidal rhythmites have the highest thickness (from 7--9 to 62 m) in deep transeverse troughs, which cut the belt of shallow-water banks, extending between Novaya Zemlya and the Kola monocline, and northwest and southeast of it. Such geomorphological position is favorable for high tides, which follow far up river streams. Anomalously high thickness of tidal rhythmites here is caused by the fact that their accumulation has compensated irregular tectonic subsidence of the area. Mud has usually brownish colors, being composed by 70-90% of pelitic material, almost devoid of grains larger than 0.1 mm with Corg content that are common for recent sediments of the Barents Sea (1-2%). Farther north, the thickness of tidal rhythmites varies from parts of a meter to several meters, not exceeding possible height of the spring tide. This is due to the subsidence rate of recent deepwater areas of the shelf that is much higher than that of the accumulation of tidal rhythmites. As a consequence, littoral facies in the vertical section were being rapidly replaced by the neritic, being accompanied by a sharp (several times) increase in the number of foraminiferal species and the appearance of stenohaline forms. Involving extrapolations 14С age the base of littoral sediments varies from 12 to 14.5-16 ka, and the surface from 7.7 to 11.5 ka years (Krapivner, 2006). In the Pechora Sea that was not reached by tides, the riverbed sands of drowned river valleys are overlain with liman sediments, i.e., dark-gray clayey, silty--clayey, and silty mud, up to 15--20 m thick. The carbon-14 age of the sequence embraces an interval of 14.5-4.5 ka years ago, when microfaunal assemblages do not exhibit signs of periglacial conditions (Krapivner, 2006, 2008). Liman facies, like estuarine, are overlain with neritic sediments, which beyond paleovalleys and their slopes, overlie recent consolidated deposits (usually diamicton), and, in some cases, Mesozoic or Paleozoic rocks. On the surface of the sea floor, below the wave base level, among the sediments, there are abundant gray and dark-gray mud of the polymodal silty-sandy-clayey composition, massive structure with negligible admixture (usually, about 2-4%) of coarse-grained (more than 0.25 mm) sand fractions, as well as angular fragments of Paleozoic and pre-Paleozoic rocks (parts of a percent or several percents). Fragments of local Mesozoic clay and sandstone also occur. The median diameter (Md) of mud particles ranges from 0.004-0.005 to 0.07-0.1 mm, their granulometric sorting (S0) is poor: usually 4 < S0 < 7. The poor sorting causes high density of mud, which at a depth of 0.1-0.2 m usually amounts to 1.7-1.8 g/cm3. Since the matrix of the mud and diamicton are virtually identical, differing only in their physical state, they were named diamictonous (Krapivner, 2008). This mud covers the surface of submarine uplands, forming a sheet of acoustically transparent sediments, through complete thickness (usually from 2-3 to 5 m). The formation of their granulometric composition mostly depends on removal of suspended in water particles in the flat turbulent flow (waves and currents) with a decreasing velocity and on thawing of coarser fractions in the floating ice. Small fields of diamictonous mud are known in the Pechora--Kanin shallows, somewhat above the wave base level, where the conditions of their formation are analogous to those discussed earlier (Krapivner, 1973). Pelitic material removed from submarine uplands, precipitates in conjugated depressions; as a result the sedimentation rate here increases, whereas the role of a random factor (precipitation of ice rafted large fractions) becomes less significant. Moreover, the major part of the fast ice, which supplies these fractions, is discharged above submarine uplands, which surround the island border of the shelf. Hence, the depressions accumulates homogeneous clayey mud with a high content of pelite particles, to 70--80% and more, which is 2-3 times as great as in diamictonous mud, Md = 0.0015-0.0045 mm and S0 < 3--3.5. Well sorted sediments cause their low density, which near the bottom is no more than 1.4-1.5 g/cm3. Silty-clayey mud occurs in the broad transitional zone between diamictonous and clayey mud, bear intermediate values of granulometric indexes: Md = 0.005-0.01 mm, 3 < S0 < 5. The composition of foraminifers in all types of the neritic mud suggests the normal conditions of marine sedimentation, not related to deglaciation processes (Krapivner, 2008).
The glacial genesis of diamicton in lowlands bordering the Barents shelf in the south has been disputed for more than 100 years. By their lithological--paleontological characteristics, they are identical to diamictons of the shelf, the glacial--marine origin of which was substantiated by works of the Arctic marine geology-engineering expedition (Rokos and Lyusternik, 1990). Two lithologically identical diamicton horizons are distinguished in the Russian sector of the shelf. The lower one is developed at the Pechora--Kanin shallows, as well as in erosion-tectonic depressions of the Central Basin and the Admiralty Upland. It usually rests on Paleocene, Mesozoic and pre-Mesozoic layers, their thickness varies from 15--20 to 50--60 m, occasionally, higher. The upper diamicton is widespread everywhere except the Pechora-Kanin shallows, in which it is not encountered but in southern environment of the Kolguev Island. In large submarine uplands, its thickness does not usually exceed 5-10 m, while in the Central Basin, in some cases, it amounts to 50 m or more. In the areas, devoid of lower diamicton, the upper diamicton overlies directly on Mesozoic or pre-Mesozoic rocks. Only in Northern Kanin Bank, in the Kolguev Island and its surroundings, the two diamicton horizons are separated by deposits of a different composition, which includes marine fauna, whereas in the rest of the area, where both horizons occur in the same section, the upper diamicton overlies directly the lower, in some places being separated from it by a thin outwash horizon or by a member of sand. The lithological features of diamicton: massive texture, poor granulometric sorting, presence of erratic and local clastic material, and elevated density, which allow assign it to the glacial till, are also characteristic of the diamictonous mud. The cumulative curves of average grainulometric compositions of diamicton and diamictonous mud virtually coincide. In addition to redeposited nodules, diamicton thin sections contain autigenic ones (Rokos and Lusternik, 1990). Slight differences between the lithological compositions of diamicton and diamictonous mud are due to their different age and deposition conditions. In most cases, diamicton rests on slightly lithified Mesozoic rocks, and therefore, it contains their fragments, while the composition of its clay fraction, particularly in the basal part of the section, is more associated with the mineralogy of Mesozoic clays. It is commonly explained by glacial exaration, though destruction and redeposition of Mesozoic rocks might be a result of submarine washout and activity of fast ice on the tidal coasts (Dionne, 1989). The Corg average content in diamicton is twice less than in recent marine sediments, because it is fairly ancient and its diagenesis is more profound. The reduced content of easily soluble salts in pore extracts is due to a long subaerial interval that preceded the accumulation of sediments in the upper sedimentary sequence, during which diamicton underwent infiltration washing with meteoric water, which changed the cation composition of the absorbed complex of its clay fraction. In the vertical section both diamicton and diamictonous mud underwent diagenetic compaction, which is manifested in the dependence of density (?) upon the liquidity index (IL). The statistical selection (about 1000 laboratory samples) included deposits with poor granulometric sorting, which is defined by a ratio of Md and S0: if Md = 0.01-0.1 mm S0 > 4, if Md < 0.01 mm S0 > 5 (Lisitsyn, 1966). The law of diamicton and diamictonous mud compression is described by the same regression equation: ? = 2.105 – 0.29 . IL ± 0.13. For well-sorted clayey mud and clay, this dependence is represented as follows: ? = 1.99 – 0.22 . IL ± 0.19. The cited equations suggest that if IL> 1.6, diamictonous mud and diamicton have a higher density, than clayey mud and consolidated clay, and this discrepancy grows as the dewatering increases (IL declines). Therefore, the elevated density of diamicton and diamictonous mud are accounted for by the common cause, i.e., compact grain packing substantiated by a poor granulometric sorting, and there are no reasons to explain this property of diamicton by glacial effects. The presented data indicate that in the process of consolidation, the diamictonous mud turns into diamicton (just like clayey mud gets transformed into clay). This is confirmed by a gradual transition between them through rare thick (about 50 m and more) sections of seismic gravitites, the genesis of which is discussed in a separate communication. Virtually, all diamicton samples selected for the microfaunal analysis (36 boreholes south of 760) contain foraminifers, numbering from several tens to several hundreds, in some cases to 1-2 thousand and more per 100 g of air-dry rock. Shells are usually sufficiently or well-preserved. The assemblages (from 10-12 to 40-60 species) are represented by recent species with admixture of boreal forms and those that became extinct in the Pliocene. Coexistence of cold-resistant and thermophilic forms, as well as almost persistent presence of allochthonous Mesozoic foraminifers allow some authors to consider all diamicton fauna as redeposited. The first point is not correct, as foraminiferal fossil assemblages are tapho- rather than biocenoses; the second is quite common for deposits of any genesis, which contain material redeposited from the underlying rocks. The average content of Mesozoic foraminifers in diamicton is 14%, which increases in troughs and in the Central depression to 24.5% and decreases on submarine rises to 8%. Irrespective of amounts of the redeposited Mesozoic microfauna, Late Cenozoic foraminifers do not form random collection of species, but complexes characteristic of the shelf basin with normal salinity that is located in the subarctic temperature zone. Different ratios of elphidiids and cassidulinas, a relative role and composition of planktons, boreal species and those that became extinct in the Pliocene, as well as forms, which indicate deepening or shallowing and freshening of the paleobasin, can be interpreted in terms of the facial analysis of marine deposits, and when combined with geological data, they allow to differentiate the upper diamicton from the lower. Disturbances in the normal bedding of Mesozoic rocks and the diamicton base, identified in cores and commonly considered to be glaciotectonic events, are represented by fragments of secondary deformation structures in the areas of dynamic effects of neotectonically active faults identified by geophysical evidence in the basement of the Barents--Kara plate (Krapivner, 2007, and communication in the present volume). Their tectonic types were studied in details on a long (30 km) coastal outcrop of the Kolguev Island (Krapivner, 1986). In the shelf, these disturbances are in some cases observed topographically as small ridges. Larger ridges and rises, assumed to be ice accumulation forms, were cut out in diamicton by erosion, after its deposition, in the period of subaerial exposure of the recent shelf, prior to accumulation of sediments in the upper sedimentary sequence, which overlap those projections in the floor landforms. High neotectonic activity of the Barents shelf is associated with the development of the Eurasian basin of the Arctic Ocean, one of world’s youngest oceanic basins. It is this phenomenon, rather than continental glaciation, that accounts for a peculiar nature of the Late Cenozoic geological history and paleogeography of the western Arctic continental margin, rather than continental glaciation. References |