Transgressive Stage Research Articles

Depositional Environment and Stratigraphic Sequence of the Oligocene Succession in Fallujah and Melh-Tharthar Oilfields, Central Iraq

The current study dealt with the facies and stratigraphic sequence of the Oligocene succession in the Fallujah and Melh-Tharthar oilfields, which are located in the most part the northern of the Mesopotamian Zone at central Iraq. The studied Oligocene succession consists of Bajawan, Baba, Sheikh Alas and Plani formations in the Fallujah oil field, while in Melh-Tharthar oil field, consists of Anah, Azkand, Sheikh Alas and Palani formations. The facies analysis of the studied Oligocene sequence was described according to the carbonate fabric and the particle types in the Fallujah oil field that showed eleven micro-facies for seven facies associations (back-reef, shallow open, shoal, fore-reef, reef, slope and deep shelf), while in the Melh-Tharthar oil field, the Oligocene succession was characterized by thirteen micro-facies for six facies associations (back-reef, reef, fore-reef, shoal, slope and deep shelf). Four depositional cycles were diagnosed through facies analysis of the studied sequence, the first sedimentary cycle of Palani Formation represents a transgression stage (TST) of the deep-sea facies and ends with the slope facies during the highstand system tract stage (HST). The second depositional cycle of Sheikh Alas Formation represents by progressive stage (TST), which is characterized by deepening upward for fore-reef association facies in Fallujah oil field and reef/fore-reef facies associations in Melh-Tharthar oil field. The third depositional cycle represents Baba Formation which was deposited during three stages of the HST in a succession of fore reef/reef and shoal facies associations. While in Melh-Tharthar oil field represent by Azkand Formation which characterized by the similar facies associations of Fallujah succession, with exception of presence the fourth HST stage that represented by fore-reefs and coral reef facies associations. The high stand stage continued during the deposition of the Bajawan Formation, as a fourth depositional cycle, which showed the deposition in shallow open marine and back-reef associated facies, whereas in Melh-Tharthar oil field such cycle is characterized by alternative of reef and Backreef facies association which deposited during HST stage. The fourth cycle ended with a regional regression stage to form SB1 which marking the end of the Oligocene.

Characteristics of the hydrological regime of the periglacial moraine lakes in the Altai

In the last decade glaciers of Inner Asia have been shrinking with acceleration, and the number of glacial lakes has been growing significantly, including those with a high probability of outburst and high-water discharge. The hydrological regime of lakes in the Altai is understudied in comparison with lakes in other high mountain countries. The article is based on our extensive field material on hydrological and morphometric characteristics of moraine lakes collected in the Russian and Mongolian parts of the Altai Mountains and combined with the Earth remote sensing data. We have proposed detailed indicators (criteria) of transgressive, regressive and post-regressive stages of lake development. It allows allow to determine at a qualitative level at what stage a lake is at and to assess its further development. The characteristics of hydrological regime of Altai lakes at different stages of development are described for the first time. Based on the criteria proposed, the classification of glacial and moraine lakes in the Altai high mountains was carried out. It shows that at present most of the lakes are in the transgressive stage. Over the last 22 years, the number of lakes has increased significantly: on the territory of the North Chuya Ridge – from 28 to 60, on the South Chuya Ridge – from 39 to 73, on the Katun Ridge – from 57 to 89, on the territory of the Tavan-Bogdo-Ola massif – from 11 to 19, and on the territory of the Mongun-Taiga massif – from 8 to 11 lakes.

A novel approach to understanding the genesis and distribution of widespread reservoirs at the basin edge: A case study of gas-bearing sandstone in the Sangonghe Formation, Junggar Basin, Western China

A 20 m thick sandstone drilled in the second member of the Sangonghe Formation in the Qianshao area of the Junggar Basin has been identified to be a high-quality gas-bearing reservoir. However, recent missions reveal an uncertainty in reservoir distribution and bring high risk to the subsequent gas development, although the gas-bearing reservoir has been encountered by several drillings and proved to be widespread. To investigate the accurate distribution and reveal the evolution of the reservoir, we reconstruct the stratigraphic framework and depositional stages by a high-revolution sequence stratigraphic method, effectively identify the gas-bearing reservoir by seismic attributes refusion and seismic stratigraphic slices, and finally map the distribution strata distribution at different stages. The results indicate that the landward onlap phenomenon in the seismic data indicates a retrogradational stratigraphic structure during the water level rise rather than the equal-thickness stratigraphic pattern under stable water conditions. According to the retrogradational stratigraphic structure, the real driver for the widespread characteristic of a gas-bearing reservoir is the backward staggered movement of multistage sand bodies during the transgressive process rather than single-stage deposition. The fusion of seismic acoustic impedance and main frequency attributes provides an effective survey to identify the accurate distribution of the reservoir, by which interpretation of a stratigraphic slice along the vertical centerline of the reservoir extracted from the multiattribute fusion survey can present the distribution and evolution of the three stages on only one map. Achieved by an appropriate interpretation on the map, two sandy debris flow bodies are identified as being deposited at the bottom successively during the first two transgressive stages. They are completely staggered in the source direction with a clear boundary between well Qs9 and Qs7. Subsequently, the subsequent rise of the water level led to the deposition of a large-scale multibranch shallow-water delta, which exhibits the characteristics of multistage, multisource, and multibranch. The results not only advance our comprehension of widespread sand bodies but also furnish fundamental insights for the development of lithologic condensate gas reservoirs in the Qianshao area.

Autogenic shrinkage and channel destabilization of an overexpanded downstream alluvial system under steady rise of relative sea level: An experimental study

Autogenic shrinkage and channel destabilization of an overexpanded downstream alluvial system under steady rise of relative sea level: An experimental study

Holocene records of oyster reefs in a shallow semi-enclosed island embayment of the Aegean Sea

Holocene records of oyster reefs in a shallow semi-enclosed island embayment of the Aegean Sea

Development and chronology of the Late Jurassic shallow-water carbonate deposits of the Holy Cross Mountains area, central Poland

The Late Jurassic shallow-water carbonates with intervening clayey-marly deeper-water deposits in the Holy Cross Mts. area formed over large bank of the elevated part of the Northern Tethyan Shelf during about 12 myr. They comprise three main successions (I, II and III) deposited partly in different environmental conditions, controlled by tectonic and climatic factors, and still preserved in the north-eastern margin, the north-western margin and the south-western margin of the Holy Cross Mountains. The history of sedimentation is presented according to the concept of the large tectono-stratigraphic units COK, LUK and KVB, which owe their origin to variable rates of tectonic subsidence, as introduced by Kutek (1994) for the area of central Poland. The studied deposits of the COK megasequence corresponding to the Upper Oxfordian and the Lower Kimmeridgian up to the Hypselocyclum Zone consist of coral limestones, various grained (including oolitic) limestones, and micritic limestones formed over the gradually enlarging shallow-water carbonate platform of the Holy Cross Mts. This platform was subsequently subdivided into two elevated areas, separated by a depressed zone in the middle, bounded by the Nowe Miasto–Iłża–Bałtów Fault Zone in the north-east and the Holy Cross Fault System in the south. The younger megasequence LUK with it strongly transgressive character marks the successive stages of the marine transgression which entered the central, lowered part of the area of the Holy Cross Mts. from the west, where it appeared already in the early Hypselocyclum Chron. It successively spread across the Holy Cross Mts. area towards the north-east and south bringing everywhere the deposition of various oyster lumachelles and marls with ammonites at the end of the Hypselocyclum Chron and during the Divisum Chron of the Early Kimmeridgian to the Acanthicum/Mutabilis Chron of the earliest Late Kimmeridgian. The following megasequence KVB is represented by the detrital lumachelles and chalky limestones with nereineids of the Eudoxus Chron of the Late Kimmeridgian marking the development of still younger shallow-water carbonate platform in the uplifted areas in the north-eastern and possibly the south-western margins of the mountains, allegedly subdivided by a deeper area of sedimentation of marly deposits. The youngest Late Jurassic deposits of the Holy Cross Mts., are very fragmentarily preserved, mostly because of Early Cretaceous uplift and erosion. They suggest an initial episode of complete drowning of the carbonate platform which became covered by marly deposits during the Early Tithonian, and the subsequent restoration of shallow-water carbonate sedimentation at the end of the Early Tithonian.

Evolution of Coniacian facies and environments in the Iberian basin: a longshore current controlling siliciclastic sand distribution on a carbonate platform

The Coniacian carbonate sediments of the Iberian basin were deposited on a homoclinal ramp that grades upwards into a distally steepened ramp, with a major shoreline siliciclastic fringe. Twenty-three facies have been identified and grouped into three main depositional environments: outer, mid, and inner ramp. The last include barrier (shoal), lagoon, carbonate tidal-flat and shoreface sub-environments. The more prominent biogenic components show a mixture of sunlight-dependent phototrophic organisms (mainly large benthic foraminifera) and nutrient-dependent heterotrophic organisms (mainly rudists), with a remarkable rare occurrence of corals. Nutrients supplied from the emergent mainland probably promoted the development of heterozoan organisms. The vertical evolution of the ramp shows: a basal transgressive stage with facies retrogradation; a maximum flooding stage, and a regressive stage with aggradation and progradation of a distally steepened ramp. The presence and distribution of siliciclastics are problematic, since sands coming into the basin are likely to be rapidly and widely redistributed along the basin, taking into account the common storm, wave, and tidal processes preserved by the sedimentary facies. The presence of a clockwise NW-flowing longshore current is postulated to account for this distribution, which was likely induced by both dominant external currents around Iberia and wind-driven currents. These clockwise gyres facilitated the invertebrate dispersion into this enclosed basin and the local presence of upwelling. This could have been another source of episodic nutrient-rich waters from the deep ramp, which may have favored heterozoan development even in the more proximal and relatively shallower-water facies.

Environmental Variability of the Northern Caspian Sea during Khazarian Epoch (Based on Drilling Data)

The objective of our work was to reconstruct transgressive-regressive events in the Northern Caspian Sea during the Khazarian epoch of its Pleistocene history to determine the natural conditions of the development of the basins and to assess their response to climate change. This work is based onstudies of the sedimentary formations of the upper part of the Quaternary sequence in the Northern Caspian Sea. The borehole was drilled in the eastern part of the depression Shirotnaya, the sea depth was 11.9 m, and the depth of drilling was 56.4 m. The core was subjected to lithological, malacological, and palynological analysis. This paper discusses the results related to the interval 56.0–26.6 m. The structure, facies-lithological, and malacological composition of the core capture three transgressive stages of the Caspian, namely the Early Khazarian, Late Khazarian, and Hyrcanian stages, separated by regressive phases. The representative spore-pollen assemblages made it possible to carry out the climatic and stratigraphic subdivision of the studied Khazarian deposits and to suggest a provisional version of the history of climatic and phytocenotic events during the sedimentation period. The results of our studies have introduced new discussion points into the paleogeography of the Northern Caspian Sea.

Provenances of Upper Quaternary sediments in the Northern Caspian Sea

This paper presents a generalization of the clay minerals composition in the sediments of the KOR-3 core located in the central part of the Northern Caspian Sea (19 samples). The history of changes in the different river basins contribution to the sedimentary supply in the central part of the Northern Caspian Sea could be traced by changes in clay mineral ratios in the Upper Quaternary sediments. Changes in the ratio reflect differences in material sources during the first and second stages of the Khvalynian transgression of the Caspian Sea — one of the most controversial stages in the history of the Caspian Sea. Consequently, our conclusion is that there were different causes of sea-level uprise at different stages of the transgression. The subsequent events (deep Mangyshlak regression and New Caspian transgression) are not associated with abrupt changes in the inflow of rivers into the Northern Caspian Sea, according to the composition of clay minerals.

Holocene stable oxygen isotope record from the North Caspian Rybachya core

The first results of 22 δ18Oost measurements on ostracod shells from the Rybachya core in the North Caspian Sea are presented. The stable oxygen isotope record here confirms previous studies of Upper Quaternary and Holocene sediments of the Rybachya core area, where it was suggested that a paleovalley was formed during the Mangyshlak regressive stage and then gradually filled during the Holocene transgressive stage. We recorded three local isotopic stages, each characterized by δ18Oost values. The hydrological variability and sea level changes associated with the transgressive-regressive rhythm of the Caspian Basin and the climatic changes in the Northern Hemisphere led to the δ18Oost changes obtained in this work. Relatively depleted δ18Oost values from sediments filling the paleovalley allowed to interpret the isotope record in terms of freshwater influx. The average δ18O enrichment of ostracod shells is taken as evidence for the isolation of the paleovalley during the Caspian Sea level retreat. However, the higher δ18Oost values obtained for the last (modern) stage of Caspian history are under the strong control of host water salinity, which is higher than during the paleovalley filling episode.

THE EFFECT OF ANTHROPOGENIC FACTORS ON THE HOLOCENE BOTTOM SEDIMENT FORMATION OF THE SHALLOW WATER AREAS IN THE BLACK SEA NORTHWESTERN SHELF ON DIATOMS

Modern water ecosystems of the northwestern Black Sea shelf shallow coastal areas develop under the influence of both natural and anthropogenic factors. Diatoms, which are abundant in these waters, are sensitive organisms-­indicators of the ecologicaconditions of the aquatic environment. The purpose of the article is to reveal the effect of anthropogenic factors on the Late Holocene sedimentary conditions using the analysis of the ecological structure of diatom assemblages from the NovoeuksinianHolocene bottom sediments column sections. The research was carried out on the shelf areas within the Gulf of Odesa and near the Danube Delta. It was determined that the specifics of modern sedimentation and ecological conditions of the northwestern shelf shallow water areas depend on the flat bottom relief, a significant amount of river water and suspended sediments, regulated flow of the main tributaries, increased bioproductivity and a high level of anthropogenic load. The characteristic species and typical ratios of diatom ecological groups indicating the stages of Holocene transgression in the coastal zone were determined for the studied water areas. For the open Gulf of Odesa, the appearance of the littoral assemblage with the dominance of Paralia sulcata species in the plankton is a sign of the onset of the Middle-­Late Holocene marine conditions. Ihas been established that diatom assemblages from the surface sediments of the Odesa Bank and the Chilia Delta branch estuarine areas are distinguished by the appearance of planktonic and epiphytic species-­indicators of eutrophication ananthropogenic water pollution. According to the calculations, this sediment was formed during the second half of the 20th century, when significant anthropogenic changes in the hydrological regime of the Dnipro and Danube occurred. As a result, the vertical sequences of diatom composition made it possible to trace not only the natural transformations of sedimentation conditions due to the Holocene transgression, but also the response of biotopes to anthropogenic changes. The studies have shown that the nature of sedimentation on the shallow shelf has changed since the second half of the 20th century, which was the result of intense anthropogenic impact.

Early Cretaceous sedimentary records of the early-stage continental rifting in the Songliao Basin, NE China

Early Cretaceous sedimentary records of the early-stage continental rifting in the Songliao Basin, NE China

Distinct variations and mechanisms for terrestrial OC 14C-ages in the Eastern China marginal sea sediments since the last deglaciation

Distinct variations and mechanisms for terrestrial OC 14C-ages in the Eastern China marginal sea sediments since the last deglaciation

PALEOLITHIC SITE SUKHAYA MECHETKA (VOLGOGRAD) IN THE CONTEXT OF STRATIGRAPHY AND PALEOGEOGRAPHY OF THE LOWER VOLGA RIVER AREA

The Sukhaya Mechetka is the most important Middle Paleolithic site in the Eastern Europe. The paper presents the analysis of the section, including its cultural layer, in the context of stratigraphy and paleogeography of the Lower Volga River region. The structure of the section represents 11 main stages of sedimentation and paleogeographic evolution of the site territory, which are closely related to global and regional changes in the climate and the level of the Caspian Sea. The base of the section reflects the stage of a river existence on the territory of the site that flowed into the Volga River estuary, formed by the Early Khazarian transgression of the Caspian Sea at the end of the Middle Pleistocene (MIS 6). A long continental period of evolution under multi-directional climate fluctuations of different amplitudes from the Mikulino interglacial (MIS 5e) till the Late Valdai glacial epoch (MIS 2) is reflected in the middle part of the section. The Late Khazarian and Hyrcanian transgressive basins (MIS 5), as well as the Paleo-Khvalynian stage (MIS 3) of the Caspian Sea did not reach the latitude of Sukhaya Mechetka. The stage of Early Khvalynian transgression (MIS 2, 16 190 ± 200 years ago) during the degradation of Ostashkovo glaciation is expressed in the upper part of the section. Three paleosoils have been found that reflect the warming periods of the MIS 5 stage and are related to its 5e, 5c and 5a sub-stages. The middle paleosoil contains a cultural layer assigned to the Micoquian/KMG community of Northern Eurasia. The climate was moderately warm during the Neanderthal settlement of the territory, steppe landscapes dominated the interfluve, and the forests grew in the balka. The Hyrcanian transgression of the Caspian Sea with an estuary in the Volga River valley predetermined a high erosion basis and the formation of a balka with a wide bottom and gentle banks. A permanent fresh watercourse attracted mammals which were an object of hunting for ancient men. All this, obviously, became a basis for the organization of settlement in the Sukhaya Mechetka balka. Correlation of the section with detailed studied loess-soil sections of the Lower Volga River region makes it possible to determine the age of its cultural horizon in between 97-110 thousand years ago.

Dominance of Phycosiphon incertum vs Zoophycos in shelf environment: example from the transgressive Palermo Aike black shale deposits of the Austral-Magallanes Basin, Argentina

Dominance of Phycosiphon incertum vs Zoophycos in shelf environment: example from the transgressive Palermo Aike black shale deposits of the Austral-Magallanes Basin, Argentina

Types, characteristics and geological significance of event deposits of Chang 9 Member of Triassic Yanchang Formation in southwestern Ordos Basin, NW China

Through core observation, thin section identification, and logging and testing data analysis, the types and characteristics of event deposits in the ninth member of Yanchang Formation of Triassic (Chang 9 Member) in southwestern Ordos Basin, China, are examined. There are 4 types and 9 subtypes of event deposits, i.e. earthquake, gravity flow, volcanic and anoxic deposits, in the Chang 9 Member in the study area. Based on the analysis of the characteristics and distribution of such events deposits, it is proposed that the event deposits are generally symbiotic or associated, with intrinsic genetic relations and distribution laws. Five kinds of sedimentary microfacies with relatively developed event deposits are identified, and the genetic model of event deposits is discussed. Seismites are mainly developed in the lake transgression stage when the basin expands episodically, and commonly affected by liquefaction flow, gravity action and brittle shear deformation. Gravity flow, mainly distributed in the high water level period, sandwiched in the fine-grained sediments of prodelta or semi-deep lake, or creates banded or lobate slump turbidite fan. It is relatively developed above the seismites strata. The volcanic event deposits are only seen in the lower part of the Chang 9 Member, showing abrupt contact at the top and bottom, which reflects the volcanic activity at the same time. Anoxic deposits are mostly formed in the late stage of lake transgression to the highstand stage. Very thick organic-rich shales are developed in the highstand stage of Chang 9 Member, and the event deposits in the depositional period of these shales are conducive to potential reservoirs.

Holocene coastal sedimentary evolution and neolithic human adaption in response to sea level changes in the Palaeo-Taihu valley, East China

The neolithic human-land relationship in the eastern coastal plain of China, interlinked sedimentary evolution and sea level changes, has been central to numerous studies. Here, we focus on the evolution of the sedimentary environment and its impacts on human activity in the Palaeo-Taihu valley (PTV) within the northern wing of Hangzhou Bay in response to post-glacial sea level rise. We employ multi-proxy analyses (lithology, AMS 14C, grain size, elements, TOC, and microfossil) on a new sediment core (WZ05) of comparatively analyze a series of previous radiocarbon-dated cores. Our results indicate that since the last deglaciation, the sedimentary environment of Core WZ05 has undergone the floodplain, upper tidal flat, estuary, upper tidal flat, coastal marsh, and fluvial plain, respectively. Regional cores comparisons display three stages: (1) A late Pleistocene incised valley (older than 9600 cal yr BP) dominated by terrestrial deposits accompanied by pottery sherds and rice phytoliths revealing that human activity on the floodplain can be dated to ca. 10600-10300 cal yr BP; (2) An early-mid Holocene transgression stage (9600-7800 cal yr BP) resulted from rapid sea level rise, causing widespread seawater inundation of the floodplain which interrupted early human activity. The transgression maximum during 8000-7800 cal yr BP was marked by the highest sedimentation rates and the coexistence of deep and shallow water species of foraminifera, corresponding to the appearance of a semi-enclosed estuary; (3) Finally, mid-late Holocene terrestrial deposits are the response to a slower sea-level rise and the seaward migration of the coast as the sediment supply exceeded sea level rise at 7800 cal yr BP. Tidal flats and coastal marshes dominated the PTV until 6500 cal yr BP, thereafter sediments continue to fill the PTV such that it develops into a subaerial plain. The reappearance of a freshwater environment since ∼ 7600 cal yr BP allowed the recovery of rice cultivation in the region. Saltwater deposits interspersed with terrestrial deposits between 7000-6500 cal yr BP probably reflect marine inundation during storm events and would have depressed rice domestication.

Paleobiodiversity and paleoenvironments of the eastern Paratethys Pleistocene lacustrine-palustrine sequence in the Baklan Basin (SW Anatolia, Turkey)

The Lower-Upper Pleistocene sedimentary record of the Baklan Basin, a long-lived continental half-graben basin in southwestern Turkey, is characterized by shallow lacustrine and palustrine deposits. The paleoenvironmental changes recorded in the basin succession allow for a multiproxy approach in reconstructing the paleoclimatic, paleoecological, and paleobiogeographical evolution of southwestern Anatolia during the Early-Late Pleistocene. Based on sedimentological, paleontological, and geochemical data, three main types of depositional intervals have been identified, corresponding to different phases of a lake expansion cycle: The first interval is characterized by the perennial shallow lake environment (PSL deposits), which represents the very early stage of the Early Expansion System Tract (VEEST). This suggests a very early stage of lake transgression in arid climate conditions. The second interval is represented by the palustrine carbonate lake center environment (PLC deposits), which corresponds to the late stage of the Early Expansion System Tract (LEEST). This indicates a late early stage of lake transgression in semiarid to subhumid climates. The third interval is marked by the palustrine lake margin environment (PLM deposits), which represents the Late Expansion System Tract (LEST) under humid conditions. The Lower-Upper Pleistocene successions of the Baklan Basin provide an excellent example of lacustrine and palustrine deposition in a laterally extensive, low-gradient, shallow lake system in the semi-isolated Pontocaspian freshwater to slightly brackish water (oligohaline) long-lived lake. The presence of Pontocaspian ostracod and mollusc faunas in the studied successions indicates that the largest major Caspian transgression around 2.6 millon years ago extended to southwestern Anatolia. The studied successions represent a rich archive of landscape, climate, and biotic development in the eastern Paratethys region during the Early-Late Pleistocene. The biogeographic signature of fossil faunas (mammals, ostracods, molluscs, and fishes) and floras (Characeae) is predominantly modern Palearctic and Holarctic, with a minor amount of endemic Pontocaspian elements. This study presents the Pleistocene Pontocaspian species of the Anatolian lakes that may have served as refugia for the Palearctic taxa during adverse time intervals. Consequently, this study shows that Lower-Upper Pleistocene lacustrine to palustrine sedimentation in the Baklan Basin has been controlled by the combination of tectonics, climate changes, and the largest major Caspian Sea transgression. The findings of this study could be used to evaluate the impact of similar allocyclic factors on the sedimentological, hydrological, and geochemical development of other intermontane lake basins.

Depositional history of the subaqueous delta off the southeastern coast of Korea during the post-glacial period

Depositional history of the subaqueous delta off the southeastern coast of Korea during the post-glacial period

FLUCTUATIONS OF LAKE NERO DURING THE HOLOCENE<a href="#FN1"><sup>1</sup></a>

The paleohydrological condition in the Rostov depression (Yaroslavl region) has been the subject of many years of discussions. The ideas about the Holocene fluctuations of the Lake Nero level differ among researchers. We have studied the structure of bottom sediments and bottom topography in the deepest northeastern part of the lake. A bathymetric survey was carried out. Drilling with the selection of undisturbed columns, GPR profiling, radiocarbon dating and a set of lithological analyzes were performed. Stratigraphic unconformities in the structure of bottom sediments indicate a drop in the lake level during the Lateglacial and the early Holocene. The level dropped to 87 m asl, which is 7 m lower than the current water level in the lake. The size of the lake at this stage was reduced several times. From 9 to 6.5 ka BP a transgressive stage was established: the average level of the lake could have risen to 91–94 m asl, which is close to its modern level. From 6.5 to 2.4 ka BP a decrease in the level by 1–3 m below the current one is revealed, followed by a gradual increase in the level. The current level was reached 300–500 years ago. The main factor in the fluctuations in the level of Lake Nero in the Holocene is the change in the height of the runoff threshold, caused by the transformation of the Ustye, Veksa, and Kotorosl river sistems. This transformation was associated both with regional changes in fluvial activity and with the processes of self-development of river channels.