GEOLOGY, GEOMORPHOLOGY, AND LATE PLEISTOCENE/HOLOCENE HISTORY OF RELIEF IN THE LENA DELTA, EAST SIBERIA.
PAVLOVA, ELENA YU. State Research Center of Russian Federation Arctic and Antarctic Research Institute.
Dorozhkina, Marina V. State Research Center of Russian Federation Arctic and Antarctic Research Institute.
Several geomorphologic levels differing in height and age with a different spatial distribution are identified in the Lena delta: low floodplain with a complex of modern channel features of the present age; high floodplain of the Late Holocene age; first above the floodplain accumulative terrace of the Early-Middle Holocene age; second above the floodplain erosion terrace of the Late Pleistocene-Early Holocene age; third above the floodplain erosion terrace of the Late Pleistocene age; denudation relics worked out on the Paleozoic rock and pre-Pleistocene pebble-conglomerate rocks. An analysis of published data and the results of studies for the last 4 years allow us at present to outline the main development stages of the territory.
Late Pleistocene. The development of the territory occupied now by the Lena delta, was closely connected in the Late Pleistocene with the regressive-transgressive sea cycles (Romanovsky et al., 1997) that determined primarily the erosion basis and as a result the change of intensity of the accumulation-erosion processes on land.
During the period of a long sea regression during the Pleistocene, extensive shallow Laptev Sea shelf areas dried up to a depth of 100-140 m (Selivanov, 1996). Under the conditions of prevailing continental sedimentation the formation of Late Pleistocene syncryogenetic ice complex deposits occurred (Sher, 1997). At Sartan time, the coastline was located in 250-300 km north of the modern Lena delta coastline (Atlas). The main Lena runoff was in the northern and northwestern direction along the valleys coinciding with the modern Olenyokskaya and Bolshaya Tumatskaya branches. The fragments of these paleo-valleys are fixed at present on the Laptev Sea shelf. At this time the third above the floodplain erosion terrace formed within the modern delta territory due to intense entrenchment of the water flows. The plains of alluvial and lake genesis formed simultaneously in the western sector of the modern Lena delta (Gusev, 1961; Atlas).
End of the Late Pleistocene – Early Holocene. The relief formation at the end of the Late Pleistocene- Early Holocene occurred under the complicated paleogeographical conditions of the post-glacial sea transgression development and more active recent tectonic movements.
The first stage of the post-glacial transgression that began 18 kyr BP belongs to this period (Romanovsky et al., 1997). As noted by these authors, the most important features of the first transgression stage was a high rate of sea level rise, especially beginning from 13 kyr BP. Transgression, especially at the beginning was of an ingression character with mouth area of river paleo-valleys transforming to marine freshened bays. The average rate of the coastline displacement towards land comprised 40-60 m/year. By 11 kyr, the sea level raised to a 40 m isobath (Are, 1982).
At the end of the Late Pleistocene – Early Holocene, recent tectonic movements within the modern Lena delta become more active (Grigoriev, 1993; Pavlova and Dorozhkina, 2000, 2000a). The predominant stable arched block uplift of the western delta sector and a subsidence of the eastern sector caused a non-unidirectional development of the western and eastern delta sectors from the end of the Late Pleistocene. A divide zone of two delta areas that are characterized by a non-unidirectional tectonic regime has a submeridonal strike and passes between the Malaya and Bolshaya Tumatskaya branches. The difference in the tectonic regime of the western and eastern delta sectors resulted later in the formation of the entire terrace complex in the western delta sector whereas only two floodplain levels and relics of the third above the floodplain terrace were formed in the eastern sector. The formation of the second above the floodplain erosion terrace belongs to the end of the Late Pleistocene – Early Holocene.
Early Holocene – first half of the Middle Holocene. The formation of alluvial deposits of the first above the floodplain terrace in the Lena delta began as a minimum 8.5 kyr BP, i.e. in the Early Holocene rather than 4.5 kyr according to Korotayev (Korotayev, 1984, 1991). The formation of the first above the floodplain terrace occurred at the background of the second post-glacial transgression stage (Romanovsky et al., 1997) that began 8-7 kyr BP and was characterized by a slow level rise of the global ocean and climate warming. Stabilizing of the sea basin level belongs approximately to 6 kyr BP, which is in agreement with the Laptev Sea study data (Bauch et al., 1999).
The sea transgression and climate warming were the decisive factors of active thermal abrasion of sea shores. Lake thermokarst was widespread (Romanovsky et al., 1997). The formation of alasy basins confined to the surface of the third above the floodplain terrace continued.
The main Lena River runoff in the Early-Middle Holocene was along the Olenyokskaya branch in the northwestern direction and along the Malaya and Bolshaya Tumatskaya branches in the northern direction.
End of the Middle Holocene – Late Holocene. The formation of the high floodplain occurred during the end of the Middle Holocene – Late Holocene at a relative sea level close to a modern one (Are, 1982) or greater by not more than 1-3 m (Kaplin and Selivanov, 1999). The absence of a significant sea influence on the formation of delta deposits in the Late Holocene is confirmed by data of modern studies (Schwamborn, et. al., 2000a).
At this time the direction of the main runoff within the Lena delta changes to the north-east-eastward due to more active recent tectonic movements that began at the end of the Late Pleistocene-Early Holocene resulting in the arched block uplift of the western delta sector and a prevailing subsidence of the eastern sector. The main runoff was along the Trofimovsksya and Bykovskaya branches with decreasing runoff in the Olenyokskaya and Malaya and Bolshaya Tumatskaya branches. As a result of transformed main runoff, the most active accumulation of alluvial deposits begins from this time in the central area and in the northeastern delta sector. During the second half of the Holocene, the river and ravine thermoerosion and thermokarst processes actively developed contributing to the general dissection of the delta relief. The formation of alasy continued and bulgunyakhi formed confined to the first above the floodplain terrace and the bottoms of alasy basins of the third above the floodplain terrace.
Modern stage. At the modern stage, the formation of low and high floodplains widespread by area within the Lena delta occurs. Their formation is closely connected with the hydrological regime of the delta being primarily due to the changed Lena water content, distribution of the water runoff and sediment load and intense spreading of the flood flow below the main delta branching node by arms. The development of extensive areas throughout the entire northeastern delta sector annually flooded during the flood period is determined by the maximum water runoff and discharges of suspended sediment in the Trofimovskaya branch. The minimum of the water runoff and suspended sediment discharges in the Olenyokskaya and Tumatskaya branches (western delta sectors) governs a restricted by-area development of the low floodplain along the branches. The same tendency is preserved for the high floodplain.
The marine factors control the relief formation within the external margin of the Lena River delta. Among the group of marine factors, the most significant are surge sea level oscillations. Tidal phenomena play a noticeable relief-forming role only within the open external northern and northeastern margin of the Lena mouth area on the protrusion delta formation segments.
At the current stage the fluvial processes that govern the creation of accumulation relief features of the floodplain levels along the branches and erosion (thermoerosion) destruction of shores play the main role in the formation of the Lena delta relief. The processes of thermoerosion due to the impact of temporary water flows are most active within the third above the floodplain terrace and account for the development of a complicated branched system of deeply entrenched ravines in the ice complex deposits with a high ice content. The processes of thermoerosion are spread to a lesser extent at the surface of the second above the floodplain terrace. Here, a leading role in the relief formation belongs to thermokarst, which generates numerous thermokarst lakes. The thermokarst processes are less typical of the floodplain levels and the first above the floodplain terrace where shallow thermokarst lakes form. The processes of frost heaving result in the formation of the heaving hills - bulgunyakhi at the surface of the high floodplain and the third above the floodplain terrace. The processes forming a crack-polygonal micro-relief actively occur at practically all terrace levels except for the lowest near-channel and coastal-marine segments. The processes of eolian accumulation and deflation are most pronounced at the surface of sand relics of the second above the floodplain terrace and on the segments of near-channel shoals and islets not fixed by vegetation.
We note that the Lena delta is located within a seismically active zone to which the epicenters of earthquakes with a magnitude of up to M=3.5-6 (Imayev et al., 1996) are confined. The confining of the Lena delta to a seismically active zone is evidence of the continuing recent tectonic movements at the modern stage of the territory development.
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