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GEOLOGICA BALCANICA. 26.3, Sofia, Sept. 1996, p. 53-57
IGCP Contribution to Project 253 "TERMINATION OF PLEISTOCENE"
Neotectonic and geomorphological characteristics of the western part of the Aydemir Lowland, Silistra District
Dora Angelova
Geological Institute, BAS, 1113 Sofia, Acad G. Bonchev St, bL 24
(Submitted 9.03.1994; accepted 06.11.1995)
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Abstract: Neogene, Quaternary and recent characteristics of the western part of the Aydemir lowland are presented in this paper. The "Srebarna" swamp is situated in this region being a biosphere reserve under UNESCO protection as well as their results-relief and substrate. There are some· geodynamic development differences of various blocks of the investigated region found on the basis of correlative deposition age determinations. A paleoseismodislocation named "Srebarna" bound to the PleistoceneHolocene boundary and aged 10500 ± 250 yr. has been found for the frrst time. It has a length 2750 m, a width varying from 750 up to 1000 m and a vertical amplitude-20 m. The paleoseismodislocation is situated at a tectonic junction of a first-rank longitudinal tectonic zone and a third-rank meridional tectonic zone. It marks the epicentral zone of the earthquake. A block bas been broken ofT as a result of one-act seismic event ( intensity of 8-9 degree and supposed depth of 30-60 km). As a result of this event, the Srebarna a.nd .Kalnezhka River mouths have been naturally embanked by the gravitationally subsided block. It has been a natural barrage for the rivers' waters. The block bas caused water baffiing to its base, and thus their loss in the karstic underseams. The investigations presented in this paper are of practical importance due to the value of the biosphere reserve and the respective natural-geomorphological phenomenon.
Angelova, D. 1996. Neotectonic and geomorphological characteristics of the western parts of the Aydemir lowland, Si!istra District- Geologica Bale., 26, 3; 53-57.
Key words: neotectonics; Srebama Lake; Northern Bulgaria
The region studied covers a part of the Aydemir lowland that includes the Srebarna Lake -an international biosphere reserve (MH'ieB, H.!IHeB,
1982). It is naturally fed by karstic waters and river Danube waters. Because of anthropogenic environmental disturbance (the embankment of
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Ill f I ...
• J
Fig. I. Pre-Quaternary relief map of the western part of Aydemir lowland: J- "Srebama" swamp area;2- isohypses of absolute elevations; 3 - borehole location; 4.- faults
Danube), Srebarna Lake area continuously decreases and this process is quite troublesome. The Srebarna Lake is situated entirely in the val-1ey below the confluence ofSrebama and Kalnezhka rivers. The lake depth varies between 1,5 and 3 m (Fig.1 ).
The regional neotectonic situation has not been an object of special investigations. The region has been studied in geological respect by HHKOJIOB (1969) (Cretaceous rock complexes), TIOIJOB (1993) (Neogene sediments), <I>HJIHDOB, MHKOBa (1986) and Angelova (1995) (about the Quaternary deposits), and in geomorphologic respect- by MnHKOB (1968) and liO)KHJIOBa et al., (1985). Some data concerning the tectonics of NE Bulgaria are supplied by lleTKOB (1962), Boncev et al. (1982), liyHJ et al. (1982), ToToMaHoB, Bp'h6JI.RHCKH (1980), Mandrescu (1990), KocTa,nHHOB et al. (1992).
According to the existing ideas the valley where the Srebarna Lake has been formed is a result of erosion processes during Early and Middle Pleistocene time (MHmcoB, 1968). Common tectonic processes are a result of positive vertical movements to the South of Danube River and negative vertical movements to the North of it. The analysis of the marsh and alluvial sediments (lio)KHJIOBa et al., 1985; Angelova, 1995) shows that they have been formed in a situation of block movements with different intensity and they reflect the climatic changes on a planetary scale (Fig.2).
In structural and geomophological respect the region is situated in the Danube epiplatform plain (BannapoB, MHmea, 1977). Concerning its morphostructural features the lake is localized on the boundary between highly fragmented and slightly up-lifted Pre-Pliocene and Pliocene denudation plains over which a loess accumulative plain and Low-Danube lowland have been formed during Pleistocene. After contemporary
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geomorphological zoning the region belongs to the Low-Danube intermountain kettle (BannapoB et al., 1990).
It inherited a various relief which had been formed during Barremian time (HHKOJIOB et al., 1969). The region has been overflowed by the Pre-Carpathian basin waters during the Upper Pontian- the clays ofSarpovska Formation are their correlate (TionoB, 1993). Several separate shallow freshwater basins have been formed over the whole region as a result of the Pre-Carpathian basin bottom pulsations as well as its shallowing during the Dacian (a correlate- Aydemir Formation sands, TionoB, 1993) and the Romanian (correlate - Srebarna Formation limestones, llonoB, 1993). The existing regional geodynamic interpretation (Kojumdjieva, Popov, 1989) is within the framework of the reconstruction made by Balla (1984, 1987) - a depression as a result of concentric extension. Being an effect of the crust ~ectonic condition, the geomorphological data show a low amplitude normal fault manifestation (from several meters up to 20-30 m and more rarely 100- 120m) between accumulative and destructive parts of the lowland.
The tectonic phase preceding the Quaternary is characterized by vertical tectonic deformations. These deformations have a negative sign in the Sub-Carpathian region and exceed 2000 m (BannapoB, 1990), and on the Bulgarian territory they have a positive sign (+50 m ).
The Quaternary and recent tectonic evolution controlled by the regional tectonic setting on the boundary between North Bulgarian Swell and Lower Danube Lowland as well as by the local development of separate blocks that build the Aydemir lowland and the northern parts of Dobrudzha massif.
The beginning of the Quaternary tectonic movements is marked by Villafranchian erosion and erosion - accumulative surfaces situated at a relative altitude 70 - 100 m (EsnorHeB, 1988). Their absolute elevations in the region are +60 and + 100 m, respectively (Fig.l ). The correlates are fluvial and alluvial deposits: clays, pebbles, gravel sands. Their thickness vary from several centimeters up to 10 m. The age is proved by vertebrate fauna (<I>HJIBDOB, Mmcosa, 1986). These sediments are not established in the base of the bottoms of Danube River and Srebarna Lake. This fact proves their young age.
Pleistocene movements are marked by a complex of river terraces. After accomplished investigations it was found that the rivers valleys in the region (Danube, Srebarna and Karlezhka rivers) have been formed as a result of erosiontectonic processes which have begun during
~rotigrophic -~ i! Strat9qtlic -on ~ v ..c: Climatic o- S , .,..,. ,.... · · Oa. c: L ' th 1 v• 1100 o u B.P ~Subdivisl- -0c: 1S e Lithology V emf ~)'IIQr sUuuiVISI- ·;:;c: _,., e I o ogy em year l ~ periods f---- ons 11?~1£ o s ons lf~l~ ll 30 so ~~+-+-----~ ot-----i=~~~~~·~i-~-4~-----f~3~iO~~~~~MI~~~~~
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NEOGENE
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Fig. 2. Correlation scheme of Todoranka Clayey Formation and Tarlica Pebble-sand-clayey Formation and their sedimentation velocity: 1 - soil; 2 - clays; 3 - peat; 4 - goethite; 5 - sandy clay; 6 - sands; 7 -pebble gravel; 8 -boulders; 9- re-deposited loess; 10- Neogene sediments; 11 - wash-out
Early Pleistocene time and continued till the end of the Pleistocene.
The value of the total tectonic movements in the western part of Aydemir lowland is estimated at 55-60 m. The Holocene has begun by clear and differentiated vertical movements which determine the specificity of recent tectonic de-elopment of the local structures. The latter are
differentiated to a high degree due to tectonic neterogeneity of the investigated region. Their structure has originated as a result of a long dif-erentiation, and this process is still not com-leted. That fact could be well-illustrated by
recent and contemporary geodynamic processes. Their sediment correlates are: Todoranka Clayey Formation (swamp sediments) which has
thickness from 4 up to 20 m and Tarlica Peble-Sand-Clayey Formation (alluvial sediments) 'th a thickness from 1 up to 30m (Angelova,
1995) (Fig. 2). The swamp sediments fill up old karst forms in some localities (Fig.l ). That karst forms are developed in the limestones of the Russe Formation (Nikolov, 1969).
The average sedimentation velocity in theregion could be seen on the fig. 2. The sedimentation changes are controlled by a transition from peristrative to constrative lithodynamic phase. The total value of the negative Holocene movements is from -20 m to -40 m (Fig.l ), and those of the positive ones, from 4 up to 5 m.
The problem of the Srebama lake origin has not been seriously discussed up to now. The
model proposed in this paper is a result of the evidence found about a paleoseismodislocation. It is named after the name of the Srebama village where a residual relief deformation has been caused by one-act event at the Holocene-Pleistocene boundary. It has caused a bifurcation of Srebarna and Karlezhka rivers mouths and formation of the swamp (Fig.3). An arch-bent structure (river terrace) has been uplifted 10m over the contemporary swamp level. Its Pebble-sand alluvial deposits are deformed in such a manner that they form a pocket with a depth of 10-12 m near the normal fault. That fact as well as the presence of a gravitational deformation in front of the pocket have to be considered as an evidence about a sudden displacement on the fault plane with a short-term fault opening accompanied simultaneously by a subsidence or abatement of the superficial alluvium deposits, i. e. all these are evidences about seismogenic character of the considered rupture. The known maximum paleoseismodislocation dimensions are: length- 2 750 m, width- from 750 up to 1000 m and vertical displacement - 20 m. The paleoseismodislocation age is determined on the basis of analogies with rock units (Angelova, 1995) - I 0 500 ± 250 years (Figs. 2 and 3).
The "Srebarna" seismodislocation is localized within the confines of a big longitudinal firstrank tectonic zone which has an ESE strike, that includes also the Danube River delta {HHKoHOB, HHKOHOBa, 1990). It reflects the complex con-
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N s ~swamp Slllstro -1
Fig. 3. Geological-geomorphological profile of the "Srebarna" paleoseismodislocation: I- Tarlica Pebble-sand-clayey Formation (Upper Holocene); 2- Todoranka Clayey Formation (Lower-Upper Holocene); 3 -undivided Lower Holocene alluvial deposits; 4- Russe Formation (Baremian-Aptian); 5- accumulative pockets; 6- relief deformation; 7 - paleoseismodislocation area; 8 - boreholes locations; 9 - block displacement orientation; 10 - event localisation; II - earthquake influence area
figuration of longitudinal ( regional) and meridional (local) structures. The formed tectonic junction marks the epicentral area of the earthquake.
The geological structure of the shifted block, its geomorphological features, the one-act phenomenon as well as the gravitational deformation are controlled by the local tectonic activity with a possible intensity of 8-9 degree according to Solonenko classification. The problem about the focus depth remains open. This depth can be estimated at 30 - 60 km on the basis of the rupture length, vertical block displacement and the presumable intensity.
According to instrumental data, the region has been reactivated in 1864, 1892 and 1893 (rpuropoaa, rpuropoB, 1962; HHKOHOB, HHKOHOBa, 1990). Mter data published by HHKOHOB, HuKoHoBa (1990), an earthquake on 14 October 1892 which occurred in the Danube River delta had an intensity of VII - VIII degree, energy class of 9 degree and foci depth 60 ± 10 km.
According to geodetic data on the recent vertical displacements surveyed on a regional scale, the region belongs to a stable area having a displacement velocity of 0 mmfyr. (ToToMaHOB et al., 1978). The recent vertical local displacements of the smaller blocks show a differentiated character with different signs: Silistra + 0.10 mm/yr, Aydemir + 0.80 mmfyr, Popina + 0.40 mmfyr, etc. Recent swamping in the investigated region indicates in most cases a local tectonic subsidence.
The investigated region belongs to a low-seismicity zone of NE Bulgaria after different authors who have carried out seismotectonic zoning in this area (Oenos, 1962 - 4-5 degree; Bonchev et al., 1982 - 4- 5 degree; liYH3 et al.,
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1982 - 5-6 degree; KocTa,nnHOB et al., 1992 -4-5 degree). The mentioned prognostic estimations are a result of mathematic modelling, and they do not reflect the actual Neogene-Quaternary geological and geomorphological situation. An attention has to be paid in this case to the geodynamic processes and phenomena zoning made by Manadrescu (1990). According to this author, the Aydemir lowland belongs to a region of high geodynamic activity potential.
According to the characteristics of NeogeneQuaternary and recent activity obtained it can be concluded that:
-the "Srebarna" "Lake" has to be regarded as a typical swamp;
- its origin is defined as a result of an oneact event that has occurred at the PleistoceneHolocene boundary;
- as a result of this event, the Srebarna and Kalnezhka rivers mouths have been naturally embanked by a gravitationally subsided block. It has been a natural barrage for rivers waters. The block has caused waters baffiing to its base and thus their loss in the karstic underseams.
- "Srebarna" swamp marks a tectonic junction which reflects a complex configuration between a longitudinal ( regional) first-rank tectonic zone and a meridional (local) tectonic zone. That junction marks the earthquake epicentral area as well;
- a paleoseismodislocation has been found in Bulgaria and described in this paper for the first time as an event which has occurred at the Pleistocene-Holocene boundary on the basis of dated sediments and significant relief deformation;
- the region could be used as a reference region for the geodynamic Holocene history of
Europe because of its strategic situation and dated sediments;
- the swamp should be considered not only as a biosphere phenomenon but also as an geological and geomorphological phenomenon;
- an additional study of the Neogene-Quaternary and recent structures as well as their movements lead to a new geodynamic concept, more realistic seismic risk estimations, a new seismotectonic zoning in the investigated region, and preservation of the natural phenomenon "Srebarna".
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