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An Overview of the Lena River Delta Setting

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An Overview of the Lena River Delta Setting Journal of Coastal Research 1083-1093 West Palm Beach, Florida Fall 2000 An Overview of the Lena River Delta Setting: Geology, Tectonics, Geomorphology, and Hydrology Feliks Aret and Erk Reimnitz] t Petersburg State University :j: GEOMAR of Mea ns of Wischhofstra l3 e 1-3 Communications Geb.4 Moskovsky avo9 D-24148 Kiel, Germany St .-Petersburg, 190031 Russia ABSTRACT _ ARE, F. and Reimnitz, E., 2000. An overview of the Lena River Delta setting: geology, tectonics , geomorphology, and .tltllllll:. hydrology. Journal ofCoastal Research, 16(4), 1083-1093. West Palm Beach (Florida), ISSN 0749-0208. • • ~- ~ ~. The Lena River Delta , largest in the Arctic, occupies 32,000 km-. It has a complicated structure caused by neotectonic i!!EJ:il--- block-movements, which formed an island archipelago with elevatio n differences as large as 60 m of distinct geological --=_2_ unit s. The modern active delta occupies spaces between older islands of the archipelago, and is just beginning to -+ S-- protrude into the open sea. The hydrologic pattern in the delta also shows the influence of tectonism . Numerous earthquakes during last century with magnitudes as large as 6 suggest that tectonic movement is contin uing. Radio­ carbon dati ng shows that the modern delta has been built during the second half of the Holocene. The total advance of the delta during this time was 120-150 km. The Lena River is considered the main sediment source for the Laptev Sea. The latest investigations give the suspended sediment load in the lower Lena River at 21 Mt/y, but only < 30% of this load is thought to reach the sea . The bed load transport is considerable but its value is unknown. The active sub-aerial delta is bordered by a shallow platform as wide as 18 km, which turns into a relatively steep slope at the 2 m isobath. This feature, corresponding in dept h to the thickness of the seasonal ice cover, is observed only offArctic rivers and is not understood. Some sections of the modern delta have morphological features characteristic for ad­ vancing shores, other s show signs of retreat. However no measurements of delta shore dynamics are available. Thus the general direction of the process is disputable. The western part of the Lena Delta is formed by a large, 20-m-high sand island fringed by a unique lace coast formed by na rrow estuary-like bays deeply penetrating the lan d. This unique coast undergoes intensive erosion not only on promontori es but also inside of estuaries due to storm surges reaching to > 2 m height. The sand island is characterized by typical lake-thermokarst relief, but no volumes of underground ice large enough to explain this relief are known. The elongated lake depressions and lakes are oriented about 2_80 True. In the middle of generally 1- to 2-m-deep lakes are equally oriented hollows as deep as 25 m. The lakes are degrading because of erosion by stream channels draining them . The lake-thermokarst relief on the nort h slope of the island is partly or totally destroyed by erosion processes. Thus the Lena Delta is characterized by several unique features that are either poorly understood or unexplained. ADDITIONAL INDEX WO RDS: Siberia , Laptev Sea, Lena Ri ver, Arctic deltas, sedim ent budget, permafrost, ice com­ plex, lake thermoka rst, storm survey, tectonism . INTRODUCTION a nd SINGH, 1980) and 32,000 km2 (ANTONOV, 1967; MIKHAI­ LOV, 1997 Fi gure 1). On small-scale maps this huge bulge Concerns with global change resul ted in growing in terest truly appears like a delta protruding far into the sea, but of geoscientists in La nd-Ocean interactions in the Arctic, in­ remnants of Pre-Holoce ne plains and Devonian be drock out­ cluding coastal dynamics. Several large international pro­ liers actually occupy about 40 % of the area. The mo dern delt a gra ms a re devoted to this problem . Intensive a nd compre he n­ occupies only t he area betw een a group of olde r islands of sive studies of the Laptev Sea sys te m are being con ducted within the frame of a Ru ssian-German bilateral proj ect. The various origins and re latively small areas prograde into the Le na River Delta is a key area in these investigations be­ ope n sea. The pattern of main distributaries evi de ntly is con­ ca use its water discharge of 520 km'Vyear and se diment yield trolle d by the underlying geology. The de lta is characte rized of 21 Mt/year (ALABYAN et al., 1995) is the major terrestrial by high seismicity (AVETISOV, 197 5; KOZMIN, 1984 ; IMAYEV source of water and se dime nt for the Laptev Sea. et al., 1996). Alt hough t he se diment load has been measured This delta , the largest in the Arctic (WALKER, 1998), is for many years, the percentage actually reaching the sea is characterized by a com plicate d structure, a nd is poorly stud­ controversial. Intensive erosion and accretion is observed ied a nd uniqu e in many resp ects. The Lena River drains into with in the delt a (ARE, 1985), but whe the r it actually is ad­ the sea at a pronounce d protuberance in the coastline that vanc ing today is unknown. The or igin and age of olde r parts occupies an a rea estimated at between 28 ,000 km2 (REINECK of the "de ltaic bulge" a re debatable. One part-the Arga-Mu­ ora Sise Isl a nd , for brevity Arga Is land (Figure 1), is char­ 99006 received 10 February 1999; accepted in revision 27 September acterized by typical lake-thermoka rst re lief. But the under­ 1999. gro und ice need ed for formation of such relief still has not 1084 Are and Reimnitz 130 0 E Figure 1. The Lena River Delta, showing the location of the Arga Island water shed and topographical profile A-A ' (see Figure 9). Numbered items are : (1) Dunay Island polar station, (2) Olenyok skaya Channel, (3) Tumatskaya Channel, (4) Sardakhskaya Channel,(5) Bykovsky Peninsula,(6) Khardang Island, (7) Dashka Crossover,(8) Arynskaya Channel, (9) barrier islands. been observed. The depth of lakes is much larger than that East in the delta apex it lies 30 m above sea level. On By­ of typical thermokarst lakes. kovsky Peninsula (Figure 1) the base of unit B is depressed In this paper we review all available geologic and hydro­ at least 10 m below sea level (GRIGORYEV, 1993). A review logic data on the Lena Delta and show directions and prom­ of radiocarbon age determinations by GRIGORYEV (1993) in­ ising approaches for future investigations within the frame dicates that unit B in the delta was formed during th e Upper of Land-Ocean interaction studies. Pleistocene, most likely between 45 and 12 thousand years BP. GEOLOGICAL BACKGROUND Unit C is represented by several islands in the north-west­ ern part of the delta area (Figure 1). The largest of these is Three different, non deltaic geomorphological units are dis­ Arga Island (100 X 75 km) . The origin of unit C is debatable. tinguished within the delta area (KOROTAEV, 1991; GRIGO­ RYEV, 1993): A) Outcrops of Devonian bedrock, too small to According to prevailing viewpoints it is composed either of be shown in Figure 1, B) Remnants of a 20-60 m high coastal marine deposits (VASILENKO, 1963; IVANOV, 1970; LOMAcH­ plain composed of ice complex (perennially frozen fine­ ENKOV, 1971; KOROTAEV, 1991; MIKHAILOV, 1997) or alluvial grained Quaternary sediments with large ice-wedges and plain deposits (GUSEV, 1960; LUNGERSGAUZEN, 1966; GAL­ very high ice content), and C) A sand terrace 20-22 m-high ABALA, 1987; GRIGORYEV, 1993). The first viewpoint is based with low ice content (Figure 1). on geomorphologic considerations and has no geologic basis. The occurrence of unit B, crossing the delta in a zig-zagging The second one is supported by the facts that unit C is com­ chain of mesas of different sizes , is a striking visual phenom­ posed of freshwater sand lacking a marine microfauna, but enon when seen on satellite images or as shown in Figure 1. contains freshwater diatoms (IVANov, 1972; GALABALA, The usuallake-thermokarst relief covers the surface of these 1987). islands. The unit is underlain by sands of Mid-Quaternary GALABALA (1987) considers unit C typical for shallow del­ age (GALABALA, 1987). On Khardang Island (Figure 1), its taic deposits and emphasizes that its composition is very sim­ base lies slightly below sea level, while 100 km to the South- ilar to that of the Bestyakh series, widely distributed in the Journal of Coastal Research, Vol. 16, No.4, 2000 Lena River Delt a 1085 Khardang Island Arga Island 20 o lt1~~:::r=:=":==:::'1 -2°~;;J~~0~~~:F~~1~~§:---( Figure 2. Shallow Quaternary stratigraphy of the western Lena Delta, simplifi ed after GALABALA (1987). Numbered items are :(1) bedrock, (2) ice complex, (3) Muorinskaya series,(4) Turakhskaya series, (5) deltaic sediments ,(6) Holocene marine sediments, (7) ice wedge, (8) fault. lower and middle reaches of the Lena River. GRIGORYEV aerial pattern? To answer these questions, tectonics of the (1993) believes that unit C is of fluvial origin. Thus Arga area have to be considered. Island and other smaller sand islands of the same elevation ALEKsEEv (1961 ) noted that Olenyokskaya and Bykov­ most probably are remnants of a Lena River alluvial fan. skaya channels flow along a Cenozoic fault line. According to Knowing the age of units Band C is necessary for under­ KIRYUSHINA et al. (1961 ), the Lena Delta with its high seis­ standing the modern structure of the delta.
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