BISAC NAT010000 Ecology
BISAC NAT045050 Ecosystems & Habitats / Coastal Regions & Shorelines
BISAC NAT025000 Ecosystems & Habitats / Oceans & Seas
BISAC NAT045030 Ecosystems & Habitats / Polar Regions
BISAC SCI081000 Earth Sciences / Hydrology
BISAC SCI092000 Global Warming & Climate Change
BISAC SCI020000 Life Sciences / Ecology
BISAC SCI039000 Life Sciences / Marine Biology
BISAC SOC053000 Regional Studies
BISAC TEC060000 Marine & Naval
The paper considers the geological structure and evolution of large barrier-lagoon systems in the eastern and southeastern coasts of the Baltic Sea. The data available on some coastal deltaic plains in the Leningrad Region, Latvia and Lithuania are discussed in some details. The considered materials lead the authors to the conclusion about a unified mechanism of the systems’ development. A considerable rise of the sea level at the Littorina Sea transgression fostered large transgressive bars developing at the margins of deltaic plains and lagoons formation on the surface of these plains.
sea-level oscillation, coast, barrier-lagoon system, paleogeography
Introduction
The geological and geomorphological context of large barrier-lagoon systems have been recently studied in details in the SE Baltic region including the Curonian and Vistula Spits and the adjacent coasts of the central and eastern Poland [23]. The results strongly suggest the sea level fluctuations to be the leading factor in the barrier system appearance and evolution. The same factor was undoubtedly crucial in the evolution of large constructional landforms on the eastern coasts of the Baltic, including the Gulf of Finland, Latvian and Lithuania coasts. The field studies were confined to several key regions.
- Neva Lowland
The lowland is confined to the land between the Gulf of Finland and the
The Neva Lowland includes Sestroretsk and Lakhti depressions, together with the land adjoining the lowermost part of the
Sestroretsk basin and Sestroretsk Razliv Lake.
The
The Sestroretsk depression enclosing the lake is separated from the Gulf of Finland by a sandy barrier 1–3 km wide and 10 km long. The sandy accumulations are thickest in the western part of the barrier; there they form several ridges of dunes up to 10–12 m high [13] arranged en echelon along the sea coast. The prevalent winds determine the dune movement from west to east, with trunks of earlier buried trees exposed in deflation basins [20]. Similar processes are typical of the Curonian and
The dune formation on the
There are ridges 10–13 m a.s.l. overgrown with pines in the NE of Sestroretsk marsh. Judging from the map and description of them compiled in the 19th century, they may be interpreted as the coastline of the
Fig. 1. The
All the boreholes drilled on the Sestroretsk marsh penetrated a peat layer 1.5 to 3 m thick and reac17.hed a diatom gyttja attributable to the Littorina Sea at a depth of 4–5 m. The gyttja is 3–4 m thick, occasionally up to 10 m [3].
Similar deposits are found on the right bank of the Sestra Zavodskaya Canal, 200 m downstream from the railroad bridge; the diatom gyttja of the Littorina age occurs there at a depth of ~3 m under eolian sands. Such a sequence is traceable over a large area [22].
The above facts give grounds to suggest that the infilling of the ancient Sestra R. valley with sediments up to 50 m thick began at the time of Littorina transgression and proceeded along with the coastal plain flooding. The coastal plain was separated from the sea by bar, the first lagoon formed which presents a swamp at present. All the subsequent fluctuations of the sea level and the post-glacial isostatic uplift of the land surface resulted in the shoreline moving gradually seaward and in development of a series of barrier-lagoon systems. The Sestroretsk dunes are the latest large barrier.
Lakhti depression and Lakhtinsky Razliv Lake.
The
In the Primorsky district there is a very deep buried valley confined to a deep-seated regional tectonic fault of sublatitudinal trend. The Lakhti depression partly coincides with the buried valley (the side tributaries of the latter are found in the vicinities of Ol’gino village) and partly – with an active submeridional fault [14].
The Lakhti depression occupied for the most part with wetland has been studied since the early 20th century. Of particular interest are detailed works by K.K. Markov [17], the geological sections exposed by boreholes being described in his monograph (fig. 2 A, B). There are two peat horizons with a layer of sandy loam between them. Judging from diatoms assemblages and from pollen diagrams, the layer may be attributed to the Littorina time. Later multidisciplinary studies of the Lakhtinsky Razliv deposits permitted the sequence to be subdivided into series of the Yoldia Sea, the Ancylus Lake and the Littorina Sea sediments (fig. 2 C, D) [18]. A layer of gyttja from a depth of 2.6 to 2.9 m was dated by 14C to 9160±150 yr. BP (10 746–9891 cal. BP).
In the borehole C the deposits change upwards first into bluish-gray silty clay with occasional black hydrotroilite interlayers, then into light brown fine sands and clays of the Littorina stage. Still higher they are gradually replaced with dark brown peat. The radiocarbon date obtained for a sample taken at the base of the peat suggests the Littorina Sea regression and the peat accumulation in the region began according to 14С 2890±100 yr. BP (3268–2790 cal. BP [18].
Boreholes drilled near the Ol’gino settlement penetrated marine sands and reached buried peat layer of the Littorina age at a depth of 4–6 m [22]. As follows from the drilling results of the barrier has been gradually migrating landwards. Under conditions of the rising sea level the coastal plain was partly inundated, a lagoon being formed behind the bar. Such a mechanism of the barrier-lagoon system development is universal and has been observed in many coastal regions [1].
Several boreholes drilled within the Lakhtinsky Razliv Lake at different depths reached a poorly decomposed horizon of peat. The age of the latter was estimated at about 300 years old (314±100 14С yr BP). Therefore, the Baltic Sea level at that time was below this of the present-day. Geological and geomorphological analysis of the Curonian and Vistula Spits also provided evidence of the Baltic Sea level oscillations at the historical time. In particular, there are traces of a high stand of the sea level (0.5–1.0 m above that of today) datable to ~1700 yr ago and to the Viking epoch (the 9th – 10th centuries) [24].
The Neva Lowland within the limits of St.-Petersburg
Paleoenvironmental reconstructions in the eastern part of the
Detailed studies, with recent scientific approach and methods being applied, identified the
Fig. 3. Areas of the
The Ligovka Spit surface under the banked earth is at 7–8 m a.s.l., while the Littorina surface around the spit is 3 to 4 m lower. The spit is about 10 km long and 600 m to 2 km wide. It is composed of cross-bedded (diagonal-bedded) sands of Littorina age replaced upwards with coarse sands with gravel and pebbles. The total thickness of sands varies from 5 to 7 m. The entire spit body does not lie on marine sediments as it is most typical of such constructional landforms; it occurs directly on the continental deposits. The sands superpose peat beds over the entire length of the spit. The deposit of peat (changing occasionally into gyttja) “exceeds in size all the known buried peatlands of the Littorina age” [22]. Within the city boundaries the Littorina deposits up to 13.6 m thick are mostly bluish and gray sands, sandy silts and loams, occasionally they include vivianite particles and peat interlayers traceable along the strike. The gyttja clays, including those known as therapeutic muds in the
Judging from the morphological and lithological characteristics of the Ligovka Spit, that landform is not a marine formation emerged gradually to the surface in the course of the marine regression, though such is a viewpoint of a number of investigators [9, 17, 21, 22]. Rather than that, it may be considered to be a large barrier developed at the margin of the coastal plain against the background of rising sea level; in the process a lagoon was formed at the back of the barrier, while the bar itself grew in thickness and moved onto the lagoon. We have studied a similar scenario of the barrier-lagoon systems evolution on the Curonian and Vistula Spits. In fact, such a mechanism of the barrier formation is actually universal and may be observed on all the marine coasts [2, 23].
It may be concluded from the above that the lagoon developed at the initial stages of the Littorina transgression over a sizable part of the present day city area; it was separated from the Gulf of Finland by a large barrier. An undisturbed sedimentation proceeded in the lagoon and resulted in a series of thinly layered sands and silts; they are exposed at present at the confluence of the
- The Narva–Luga lowland
The lowland is situated at the mouth of the Narva and Luga rivers and is distinct for a rather complicated topography. Lakes and swamps occupy a considerable proportion of its surface, with constructional coastal landforms of various ages between them. The lowland includes three isolated plateau fragments known as Kurovitsky, Krikovsky and Kurgalsky, the latter forming a peninsula of the same name separating the Narva Bay from the Luga Bay.
The western slope of the peninsula drops towards the
Fig.4A. An overgrown downwind slope of the dune. Fig.4B. Fluvioglacial deposits
There are several generations of spits on the lowland varying widely in their size, age, and orientation. The largest of them – Riygiküla, Kudruküla, and Meriküla ones – were first described in details by K.K. Markov [17].
The Riygiküla Spit is the farthest from the sea. It forms an arc encircling the southern periphery of the lowland and is composed mostly of coarse littoral deposits. A radiocarbon date obtained on the archeological site – 5305–5040 yr BC (6212±48, Hela-2742) [7] permits to assign this ancient coastline to an early stage of the Littorina transgression.
A younger constructional landform – that is Kudruküla Spit – extends as an arc 25 km long and 0.2 to 1.5 km wide along the Narva Bay coast. A chain of dunes up to 15 m high marks its axis, though no traces of sands being recently wind-blown have been found on the surface composed mostly of coarse-grained sand and pebbles. In the central part of the arcuate spit there are archeological sites dated at 2215–2020 yr BC (37250±40, Hela-2744). Further on there are dunes up to 20 m high forming a scarp facing east, towards the swamped surface of the ancient lagoon.
To the north of it there is another archeological site dated at 1910 yr BC (3607±31, Hela-2516). The Kudruküla spit borders on the southern slope of the Kurgalsky plateau remnant ~20 m high composed of fluvioglacial deposits (Fig 4 B). There are archeological sites arranged along its edge, their age being estimated at 3970–3940 yr BC (5090±40, Hela-1945) [7].
The Kudruküla spit sites are localized on sandy ridges of a moderate height alternating with swamped hollows. Quite possibly the ridges are former foredunes having developed at the back of the beach under conditions of the slowly receding sea [25]. Similar eolian landforms steadily gaining in elevation landwards are found in other parts of the Narva-Luga Lowland. Both relative elevation and the altitude a.s.l. increase southwards and reach 4–6 m at the distance of 2 km from the sea.
At present the lowland is separated from the sea with a lengthy barrier
As follows from the drilling data given in the monograph by K.K. Markov [17] (Fig. 5), a series of lagoons developed successively on the coastal waterlogged plain during the Littorina. That suggestion is corroborated by the peat occurrence at the base of diatomaceous gyttja of the Littorina age. An abundance of sand in the coastal zone favored development of large bars and undisturbed deposition of diatom gyttja in lagoons.
Fig. 5. Boreholes on the Narva-Luga Lowland (according to [17]). 1 – peat, 2 – gyttja, 3 – sand with gravel and pebbles, 4 – sandy loam, 5 – sand
It should be noted in conclusion that facts established by drilling, along with dated archeological artefacts and the results of geomorphological analysis of the Narva-Luga Lowland allow a definite conclusion that the barrier-lagoon systems evolved there throughout the Littorina time which resulted in several generations of lagoons to have been formed on the coastal plain. The lagoons separated from the sea by sand barriers (Riygiküla, Kudruküla, and Meriküla Spits) developed in a successive order on the low deltaic plain advancing slowly seawards. The latter may be ascribed both to isostatic uplift of the Earth’s crust and to sediments accumulation in the nearshore zone. Another evidence in favor of the land advancing seawards is given by the age of archeological artefacts (the nearer is the archeological object to the coastline, the younger it is).
- The coasts of Latvia
The barrier-lagoon systems are widely spread over the Latvian and Lithuanian coasts. The largest and most thoroughly studied is Ventspils Lagoon 30 km long and up to 15 km wide crossed by the
Fig. 6. Boreholes in the Ventspils Lagoon (according to [11, 12]). 1 – soil, 2 – sand, 3 – peat, 4 – gyttja, 5 – clay, 6 – mollusk shells
- The coasts of Lithuania
An extensive barrier-lagoon system extends for more than 60 km southwards from
The coastal lowland is separated from the sea with a sandy barrier that includes occasional morainic hills forming minor cusps of the shoreline. The sandy barrier is occasionally up to 2 km wide and bears a series of dune ridges 20–30 m (and more) high. Locally the barrier is partly eroded and reduced in width and elevation.
The considered part of the coast is noted for a presence of a linear uplift in the offshore zone traceable almost from the water edge to a depth of 40 m (Fig. 7). It is known as Liepaja Swell and marked with boulder fields (eroded till) and outcrops of Pre-Quaternary rocks [5].
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Fig. 7. Underwater slope (foreshore) between Liepāja and Šventoji (according to [5]). 1 – sand, 2 – till (boulder loam), 3 – outcrops of solid rocks, 4 – clays, 5 – ridges.
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Fig. 8. Boreholes near Šventoji (according to [26]). 1 – beach, 2 – dunes, 3 – paleo-lagoon, 4 – boreholes, 5 – road, 6 – quarry
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There is a load deficiency in the coastal zone which accounts both for the barrier erosion and for its displacement landwards, the coastal wetland being overlain. The process is indicated by some rounded lumps of peat found on the beach and was observed directly. During strong storms (such as in 1967) the lower segments of the barrier are overwashed by waves, the sediments being moved into the lagoon [4]. The most detailed studies of the coastal lowland were performed in the vicinities of the Šventoji town, several boreholes having been drilled there (Fig. 8).
Fig. 9. Cores of sediments drilled in the Šventoji vicinities, according to [26]. 1 – sand, 2 – sand with gravel and pebbles, 3 – gyttja, 4 – sandy peat, 5 – peat, 6 – till
The material thus obtained was further thoroughly studied using palynological analysis and dating by radiocarbon and OSL techniques [26]. As can be seen in given sections, the Littorina transgression deposits are dominated by lagoon facies (Fig. 9).
They occur immediately above the deposits formed earlier on the coastal lowlands, namely fluvial sands, gyttja or peat. Beach facies of the Lttorina age penetrated by borehole 24816 were dated by 14C at 6218±63, which is indicative of the shoreline advance. Similar advancing coastlines have been observed on the Curonian and Vistula Spits [24], as well as on all the above listed segments of the low land.
- Conclusion
Large constructional landforms of barrier type have been studied for the purpose of better understanding of their structure and genesis. The studies were performed on the eastern Baltic coasts including the Gulf of Finland and the coasts of
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