MORPHOTECTONICS OF THE BASEMENT SURFACE OF THE WEST SIBERIAN BASIN WITHIN NADYM-TAZ INTERFLUVE

The map of reflecting horizon A, plotted on the basis of regional geophysical materials, yieldsinformation about the bottom topography of sedimentary cover of the West-Siberian Basin within Nadym-Taz interfluve. Through morphotectonic analysis about this surface, the authors are able tounderstand the distribution characteristics of the main topographic irregularities, with the blockstructure of the lower cover floors and underlying complexes as the focus.The largest irregularities are caused by the presence of three regional altitude steps, which show a regional tendency of basement being submerged in the north-north-east direction. Within these irregularities, the authors have distinguished some regions, which can be divided, based on the characteristics of morphostructural distribution, into two groups, i.e., areal (Nadym, Tol'ka, Urengoy)and belt (Tarko-Sale, Russko-Chasel'ka and Messoyakha). The first ones represent quasihomogeneous blocks, whose surfaces are complicated by irregular low-amplitude elevations and troughs and low-gradient zones with gradient values being up to 0.03, or they submerge stepwise, with the leveled areas being separated by zones with high gradients (0.07-0.15). The morphostructure of the other group of regions is characterized by the en-echelon distribution of contrastingly positive and negative elements with high values of slope gradients (higher than 0.1-0.2).

Postglacial incision-infill cycles at the Borisoglebsk Upland: Correlations between interfluve headwaters and fluvial network

The article discusses postglacial landscape transformation in the Northern Hemisphere Middle Pleistocene glaciation area located in the center of the Russian Plain.We attempted to verify the regional paleogeographic model by reconstructing the Late Pleistocene incision-infill cycles at the Eastern Borisoglebsk Upland based on a comparison of inactive interfluves headwaters and actual fluvial network palaeoarchives.The study was also aimed to determine the past extent of fluvial systems.Interdisciplinary research of the actual and buried topography,Ethology and pedogenic properties of surface deposits was carrid out with remote sensing data interpretation,DGPS survey,and detailed description of geological cores involved.The study was followed by analysis of grain size,chemical and organic contents,microstructure,and numerical dating.Integrating the available results,we propose a scenario of the fluvial network transformation at the Eastem Borisoglebsk Upland over the last 150 ka.At least four fluvial incision stages were determined while network extent has significantly changed through the Late Pleistocene.Three can be generally associated with the regional base level decrease-Late Moscow,Late Valdai and Late Holocene and accompanying isolation of the Nero Lake terraces of 130 m,100-105 m and 95-98 m.Incision stages were separated by landscape stability or aggradation periods those were asynchronous at the middle and upper parts of the fluvial network.The main agent of initial valley infill appears to be local lacustrine sedimentation altered by alluvial and colluvial deposition towards the second half of Valdai.Revealed landscape conditions variability emphasize the importance of comprehensive local correlations for regional retrospective models.