Home – Home – Jornals – Russian Geology and Geophysics 2008 number 9

Sea-level change has been commonly interpreted to be of eustatic origin, and many eustatic events were hypothesized for the Phanerozoic, including several 1-3 Myr long cycles in the Ordovician with magnitudes up to 100 or 200 m. However, sea-level change modeling using stratigraphic data from Northern Estonia, which was an area of slow shallow-marine (≤10 m) deposition through most of the Ordovician, indicates fluctuations of no more than 20 m. In the Late Ordovician the sea level fell only twice for ~100 m within 1 Myr during the Gondwanian glaciation. Although the sea level remained relatively stable, there were frequent 100-200 m changes of sea depths we inferred with reference to the time spans of stratigraphic units and intervals between tectonic events estimated reliably against stable durations of East Siberian chronozones (biochrons) of the Ordovician. In the absence of eustatic events, the sea-depth changes most likely resulted from rapid crustal uplift and subsidence. According to correlated well-documented Ordovician sections from East Siberia, the rate of crustal subsidence changed rapidly in different periods and in different places of the area, thus being of a regional scale. The controversy between the sea-level stability and the regional-scale variations in sea depths controlled by rates of crustal uplift and subsidence can be resolved assuming a model of variable eclogitization rates in the lower crust caused by lithospheric stress change. Our inferences undermine the traditional petroleum prediction approach implying formation of depositional traps due to rapid eustatic sea-level change.

Apatite fission-track (AFT) thermochronological modeling as a diagnostic tool for periods of stability (peneplanation) and tectonic activity (orogeny) has been broadly used in tectonic studies of Central Asia in recent years. We discuss more than 100 AFT ages of samples from the Kyrgyz Tien Shan and Altai and compare them with AFT data from northern Kazakhstan. Geological, geomorphological, and AFT data indicate intense activity in the Late Cenozoic Eurasian continental interior. The impact from the India-Eurasia collision on the northern Tien Shan, Altai, and northern Kazakhstan regions showed up at 11, 5, and 3 Ma, respectively, as a result of stress propagation into the continent, with the ensuing reactivation and mountain growth. We hypothesize that a distant effect of the Late Cenozoic India-Eurasia collision was to rejuvenate Paleozoic fault zones and to deform the Mesozoic sedimentary cover north of the collision front as far as the West Siberian Plate. The reactivation facilitated formation of tectonic oil and gas traps. The activity in northern Central Asia under the effect of the Indian indentation into Eurasia appears to continue and may evolve to include uplift of southern West Siberian plate with uplift.

We present results of study of the specific composition and parageneses of pyrope-almandine garnets from alluvial deposits of the Muna-Markha interfluve drainage in the Yakutian diamondiferous province. The data are correlated with the specific features of garnets (tentatively crustal) from Yakutian and Guinean kimberlites and their crustal xenoliths and from metamorphic rocks of the Anabar Shield. The conclusion is drawn that most of orange pyrope-almandine garnets from the alluvial deposits were generated from kimberlites with varying contents of disintegrated crustal material. This gives grounds to include the studied garnets into a complex of indicator minerals of kimberlites and use them in search for diamonds in the Yakutian diamondiferous province.

The influence of 6.75 m thick dolerite dike on the sheet coal of the Kaierkan deposit (northwestern Siberian Platform) was studied by organogeochemical methods. It is shown that initial bituminous coal was transformed into anthracite in the immediate vicinity of the dike. Chemical kinetic modeling of the dike-induced cracking of coal's organic matter was performed, and the maximum paleotemperatures were estimated.

In situ data on the daily dynamics of pH, [O₂], and Eh values at five water layer depths were obtained. Some regularities have been established: All these parameters are characterized by significant diurnal fluctuations and their daily cyclic recurrence; the diurnal cycles of pH and [O₂] nearly coincide (they show close extreme points on the time scale); the Eh cycles are less distinct and antisymbatic to the pH and [O₂] cycles, but typical Eh extremes sometimes lag by 2-3 h. These data were analyzed using the principle of equilibrium locality and partiality. The main flows of O₂ and CO₂ in the system have been revealed with regard to the water layer-atmosphere gas exchange, absorption of CO₂ by phytoplankton in the photic layer, breathing of phyto- and zooplankton, birth and die-off of biota, settling of mortomass, and redistribution of detailed components within the water layer as well as between it and the bottom sediments. On the basis of the results obtained, the known technique for estimating the content of primary photosynthesis products (Bruevich's method) was corrected. The used generalized algorithm of chemical thermodynamic analysis of the substance state in aqueous systems can be applied to waters of different classes as well as any surficial waters if Eh, pH, and [O₂] are taken into account.

We revised geological data substantiating the unified 1983 Regional Stratigraphic Chart of Gorny Altai Quaternary deposits. Based on our own and literature data, we showed that Lower and Middle Quaternary glacial horizons are erroneously distinguished in the Yaloman-Katun' zone of southeastern Altai. A new correlation is proposed, according to which the glacial complex of the maximum glaciation (MIS-6) corresponds to the Inya catafluvial series and the glacial complex of the first postmaximum glaciation (MIS-4 unit), to the Sal'dzhar catafluvial series. The lectostratotypes of both series are described. The event history of the second half of the Late Neopleistocene in Gorny Altai (MIS-3 and MIS-2) was less catastrophic for ancient biota and Paleolithic man than it was believed earlier.

We suggest a method of multi-element low-frequency acoustic logging data processing which combines advantages of the semblance and phase velocity processing (PVP) techniques, namely, the interference resistance in the former and better accuracy in the latter. The new method is based on point (windowless) semblance by the Hilbert transformation. The introduced concept of phase semblance makes it possible to estimate the signal distribution semblance and the phase. Also suggested are an algorithm for preliminary estimation of formation S-wave slowness from signal group power variations and improved forms of processed data imaging.

We discuss historical evidence for seismicity on the southern margin of the Siberian craton collected from old local newspapers. The reported earthquakes vary in magnitude from M = 2.5 to 4.5, and their macroseismic locations agree well with the regional tectonic framework. The new data prove seismic activity in the area and can be used in seismic risk assessment.