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A preliminary stripped gravity map of the Pannonian Basin
Physics of the Earth and Planetary Interiors, 51 (1988) 185-189
185
Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands
A preliminary stripped gravity map of the Pannonian Basin M. Bielik Geophysical Institute, Slovak Academy of Sciences, Ditbravsk4 cesta, 842 28 Bratislava (Czechoslovakia) (Received September 12, 1986; revision accepted December 20, 1986)
Bielik, M., 1988. A preliminary stripped gravity map of the Pannonian Basin. Phys. Earth Planet. Inter., 51: 185-189. By removing the gravity effects of the known near-surface sources from the gravity map we obtain a ' stripped gravity map' that enables a better interpretation of the deep-seated geological structures of the Earth's crust. The total gravity effect of anomalous bodies is acquired by summation of the gravity effects of n-sided vertical prisms with horizontal bases. The stripping procedure has a more accurate physical foundation than any other mathematical method of gravity field analysis, provided that the geometry and density of the disturbing bodies are known with sufficient accuracy. This method was applied to the Pannonian Basin.
1. Introduction
Geophysical research into the deep parts of the Earth is now providing more and more data on geological structures located at various depths and in various regions of the Earth. This frequently enables qualitatively new maps to be constructed, which require a large amount of geophysical, geological, drilling and other information for their compilation. One of these is also the stripped gravity map of the Pannonian Basin. By the term 'stripping of gravity map' (Hammer, 1963) we understand a procedure of correcting the map of Bouguer anomalies for the gravity effects of known geological units with the aim to emphasize gravity anomalies caused by density inhomogeneities within the underlying bedrock. The purpose of this paper is to demonstrate the gravity effect of the Tertiary-Quaternary sediments of the Pannonian Basin and to show what the stripped gravity map of this area looks like. The method of constructing the stripped gravity map of the Pannonian Basin is based on the calculation of the gravity effect of the TertiaryQuaternary sediments of this important intra-A10031-9201/88/$03.50
© 1988 Elsevier Science Publishers B.V.
pine geotectonic structure, which is then subtracted from the values of the Bouguer anomalies. The total gravity effect of anomalous bodies is acquired by summation of the gravity effects of n-sided vertical prisms with horizontal bases. The stripping procedure has a more accurate physical foundation than any other mathematical method of gravity field analysis provided that the geometry and density of the disturbing bodies are known with sufficient accuracy. One of the main problems is to determine the geometrical and density characteristics of the model of Tertiary and Quaternary sediments.
2. Model of the sedimentary filling of the Pannonian Basin
The model of the relief of the underlying layers was constructed using the results obtained by Fusfin et al. (1971), Krrll and Wessely (1973), Mahel (1973), Sclater et al. (1980), Horvfith and Royden (1981), Kocfi_ket al. (1981), Mesk6 (1983), Royden et al. (1983), Kilrnyi and Rumpler (1984), Brezsnyfinszky and Haas (1986), Sefara et al. (1986) and is shown in Fig. 1. It implies that this v
186 16 o
18o
20 °
22 °
240
49 °
t.9o
z.8o
/,7 o
;7 a
46 °
,G°
45 °
5*
16°
18°
20°
22 °
Fig. 1. Map of pre-Tertiary basement relief of the Pannonian Basin. (1) pre-Tertiary formation at the surface; (2) isohypses of pre-Tertiary basement relief (km). (A) Vienna Basin; (B) Transcarpathian depression; (C) Little Carpathians; (D) Transdanubian middle mountains; (E) Little Hungarian Plain; (F) D a n u b e Lowland; (G) Great Hungarian Plain; (H) Drava trough; (I) Sava trough.
Tertiary-Quaternary depression is not a uniform sedimentary basin. It is rather a complex system of partial depressions of various depths and extents. The depressions are separated from each other by elevation of the underlying layers, which sometimes emerge at the surface. The central part is the Pannonian Basin, which is further surrounded by peripheral basins such as the Vienna Basin (A) and the Transcarpathian depression (B). The Pannonian Basin (Stegena and Horvfith, 1984) is separated by the Little Carpatians (C) from the Vienna Basin in the northwest. The Pannonian Basin can be further subdivided.
The basin area to the northwest of the Transdanubian middle mountains (D) is called Little Hungarian Plain (E) and the Slovakian part Danube Lowland (F). The southeastern part is the Great Hungarian Plain (G), northeast of it lies the Transcarpathian depression. Towards the southwest, two elongated depressions can be considered as parts of the Pannonian Basin: the Drava trough (H) and Sava trough (I). The Tertiary-Quaternary filling of the basin mainly consists of sands, clays, shales, sandstones with isolated limestones and evaporites, and also clays and marls in the layers closest to the surface.
187
TABLE Mean
Po
-Y
depth
intervals
used for the calculation of a stripped gravity map
l
Differential density (MG m -3)
Depth interval a (m) From
(Xi~ Yi/Zl }
( x i j y i ) z 2)
Geological body ~ t v divided \~!_ i ~nto layers 1~ 2,_ n
{
z Characteristics
I differential densities for the particular
at elementary
To
+ 150
- 500
- 500
- 1000
-0.62
- 1000
- 2000
-0.36
- 2000
- 3000
-0.23
- 0.47
- 3000
-4000
- 0.09
- 4000
- 5000
-0.03
- 5000
- 6000
- 0.01
- 6000
- 7000
- 0.00
a ( + ) = h e i g h t a b o v e s e a level; ( - ) = d e p t h b e l o w s e a level.
layers :
Layer no. Depth interval [m] Density I 1 Zl - Z 2 01
z2 7 z~
2
16°
~2
F i g . 2. P h y s i c a l - g e o l o g i c a l
18 o
20 °
model.
22"
I
21.o
i
i
t,9o
KOSICE MUKACEVO WIEN
~.8o
48o
1.7o
t,7 o
i
/.6 o
46
% t.5 o
0
F i g . 3. M a p o f t h e g r a v i t y e f f e c t o f t h e s e d i m e n t a r y
filling of the Pannonian
20 ~0 60kin
B a s i n . (1) i s o l i n e s i n /~m s
2.
188 The thickness of the sedimentary basin varies between 0 and 7 kin, with an average of 2.5 km. The density of the sedimentary filling of the Pannonian Basin varies vertically as well as horizontally. Its values depend mainly on the age of the sedimentary formation. The density pattern is relatively complicated, however, and for the actual computations a simple, but best-fit density pattern had to be adopted (Table I). The differential densities, which were referred to the density of 2.67 M G m-3, should be understood as mean values.
3. Calculation of gravity effect of the model and stripped gravity map
The gravity effect of the model of Tertiary sediments was treated as a three-dimensional direct gravimetric problem. The sedimentary filling was modelled by vertical prisms (Fig. 2). Individual geological bodies were divided into layers. Each layer was defined by a contour fine, upper and lower depth, and constant density. The total gravity effect of anomalous bodies was obtained by summation of the gravity effects of n-sided vertical prisms with horizontal bases. The computations were carried out using Smi~ek
Fig. 4. Simplifiedstripped gravity map of a part of the Pannonian Basin. (1) Positive Isolines; (2) negative isolines.
189
et al.'s relation (1970). The result of the computations, carried out in a square 10 × 10 km grid, was the map of the gravity effect of the sediments of the Pannonian Basin. This map was then corrected for the effect of the masses of N e o g e n e - Q u a t e r nary volcanism, which represents one of the most substantial features of the neotectonic evolution of this basin (Bielik, 1986). If the surface of the terrain (the upper boundary of the model) in this area is considered to be planar, no correction for its gravity effect is necessary. The upper boundary was taken as a plane at an elevation of 150 m above sea level. Figure 3 shows the resultant map of the gravity effect of the sedimentary filling of the Pannonian Basin. On the basis of the computed gravity effect, we then corrected the map of complete Bouguer anomalies using the relation Agsgm = A g b - Vz, where Agsgm are the values of the stripped gravity map, A g b the values of the complete Bouguer anomalies (Ibrmajer, 1963) and V~ the gravity effect of the sedimentary filling. Since data from the whole of the Pannonian Basin were not available, the stripped gravity map was only compiled for a part of the territory of this basin. The stripped gravity map of the mentioned region corrected for the gravity effect of the boundary between the lithosphere and the asthenosphere of central Europe for the density contrast of 0.1 M G m -3 (Babu~ka et al., 1986) is contained in Fig. 4.
4. Conclusion The stripped gravity map can offer a better insight into the crustal density structure than the Bouguer gravity map and represents new possibilities of gravity map application to the interpretation of unknown bodies. In the most immediate future, this map will be subjected to detailed qualitative and quantitative, geological and geophysical interpretation. Naturally, the degree of understanding of the structure and density conditions of the sediments varies considerably from one part of the basin to another. Also, the accuracy of the model will vary from one area to another and should by no means be considered as definitive. Consequently, it will be necessary to improve the accuracy of the model
as further data become available, on a detailed and regional scale.
References Babu~ka, V., Plomerov~t, J., Vysko~il, V., Burda, M. and Hiibner, M., 1986. Depth and density characteristics of the lithosphere-asthenosphere transition in central Europe. In: Research of Deep Geological Structure of Czechoslovakia. Geofyzika, Brno, pp. 51-57 (in Czech). Bielik, M., 1988. Construction of the stripped gravity map of the Pannonian Basin. Contrib. Geophys. Inst. Slovak Acad. Sci., in press. Brezsny/mszky, K. and Haas, J., 1986. Main features of the pre-Tertiary basement of Hungary. Geologica Carpathica, 37: 297-303. Fus~n, O., Ibrmajer, J., Plan~tr, J., Sl~vik, J. and Smi~ek, M., 1971. Geological structure of the basement of the covered parts of southern part of inner West Carpathians. Z~padn6 Karpaty, 15: 1-173. Hammer, S., 1963. Deep gravity interpretation by stripping. Geophysics, 28: 369-378. Horv~tth, F. and Royden, L., 1981. Mechanism for the formation of the intra-Carpathian basins: a review. Earth Evol. Sci., 1: 307-316. lbrmajer, J., 1963. Gravimetric map of Czechoslovakia on 1 : 200 000 scale. Stud. Geophys. Geod., 7: 303-308. Kilrneyi, E. and Rumpler, J., 1984. Pre-Tertiary basement relief map of Hungary. Geophys. Trans., 30: 425-428. Kochk, A., Mayer, S. and Pgen~ikova, M , 1981. Structural Model of the Vienna Basin at the Neogene Base. MS Geofond, Praha. (in Czech). Kr~511, A. and Wessely, G., 1973. Neue Ergebnisse beim Tiefenaufschluss in Wiener Becken. Erdol, Erdgas Z., 89: 400-413. Mahel v, M., 1973. Tectonic map of the Carpathian-Balkan Mountain system and adjacent areas. D. Stfir's Geol. Inst., Bratislava and UNESCO. Meskr, A., 1983. Regional Bouguer gravity maps of Hungary. Acta Geodaet., Geophys. Montanist., 18: 187-200. Royden, L., Horv~th, F. and Rumpler, J., 1983. Evolution of the Pannonian Basin system: 1. Tectonics. Tectonics, 2: 63-90. Sclater, J.G., Royden, L., Horv~th, F., Burchfiel, B.C., Semken, S. and Stegena, L., 1980. The formation of the lntraCarpathian basins as determined from subsidence data. Earth Planet. Sci. Lett., 51: 139-162. ~efara, J., Pospi~il, L., Sutora, A., Bodnfir, J., Bielik, M., Velich, R., Kurkin, M. and Miku~ka, J., 1986. Stripped gravity map of the Inner Carpathians (in Slovak). In: M. BfiLkovsk2~ (Editor), Geophysical Model of the Lithosphere. Geofyzika, Brno, pp. 105-119 (in Czech). Smi~ek, M., Plan~r, J. and Kr~hk, J., 1970. Computation of the gravity effect three-dimensional bodies of arbitrary shape. Contrib. Geophys. Inst. Slovak Acad. Sci., 2: 13-23. Stegena, L. and Horv~th, F., 1984. Review of the Pannonian Basin. Acta Geodaet., Geophys. Montanist., 19:153-160.
185
Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands
A preliminary stripped gravity map of the Pannonian Basin M. Bielik Geophysical Institute, Slovak Academy of Sciences, Ditbravsk4 cesta, 842 28 Bratislava (Czechoslovakia) (Received September 12, 1986; revision accepted December 20, 1986)
Bielik, M., 1988. A preliminary stripped gravity map of the Pannonian Basin. Phys. Earth Planet. Inter., 51: 185-189. By removing the gravity effects of the known near-surface sources from the gravity map we obtain a ' stripped gravity map' that enables a better interpretation of the deep-seated geological structures of the Earth's crust. The total gravity effect of anomalous bodies is acquired by summation of the gravity effects of n-sided vertical prisms with horizontal bases. The stripping procedure has a more accurate physical foundation than any other mathematical method of gravity field analysis, provided that the geometry and density of the disturbing bodies are known with sufficient accuracy. This method was applied to the Pannonian Basin.
1. Introduction
Geophysical research into the deep parts of the Earth is now providing more and more data on geological structures located at various depths and in various regions of the Earth. This frequently enables qualitatively new maps to be constructed, which require a large amount of geophysical, geological, drilling and other information for their compilation. One of these is also the stripped gravity map of the Pannonian Basin. By the term 'stripping of gravity map' (Hammer, 1963) we understand a procedure of correcting the map of Bouguer anomalies for the gravity effects of known geological units with the aim to emphasize gravity anomalies caused by density inhomogeneities within the underlying bedrock. The purpose of this paper is to demonstrate the gravity effect of the Tertiary-Quaternary sediments of the Pannonian Basin and to show what the stripped gravity map of this area looks like. The method of constructing the stripped gravity map of the Pannonian Basin is based on the calculation of the gravity effect of the TertiaryQuaternary sediments of this important intra-A10031-9201/88/$03.50
© 1988 Elsevier Science Publishers B.V.
pine geotectonic structure, which is then subtracted from the values of the Bouguer anomalies. The total gravity effect of anomalous bodies is acquired by summation of the gravity effects of n-sided vertical prisms with horizontal bases. The stripping procedure has a more accurate physical foundation than any other mathematical method of gravity field analysis provided that the geometry and density of the disturbing bodies are known with sufficient accuracy. One of the main problems is to determine the geometrical and density characteristics of the model of Tertiary and Quaternary sediments.
2. Model of the sedimentary filling of the Pannonian Basin
The model of the relief of the underlying layers was constructed using the results obtained by Fusfin et al. (1971), Krrll and Wessely (1973), Mahel (1973), Sclater et al. (1980), Horvfith and Royden (1981), Kocfi_ket al. (1981), Mesk6 (1983), Royden et al. (1983), Kilrnyi and Rumpler (1984), Brezsnyfinszky and Haas (1986), Sefara et al. (1986) and is shown in Fig. 1. It implies that this v
186 16 o
18o
20 °
22 °
240
49 °
t.9o
z.8o
/,7 o
;7 a
46 °
,G°
45 °
5*
16°
18°
20°
22 °
Fig. 1. Map of pre-Tertiary basement relief of the Pannonian Basin. (1) pre-Tertiary formation at the surface; (2) isohypses of pre-Tertiary basement relief (km). (A) Vienna Basin; (B) Transcarpathian depression; (C) Little Carpathians; (D) Transdanubian middle mountains; (E) Little Hungarian Plain; (F) D a n u b e Lowland; (G) Great Hungarian Plain; (H) Drava trough; (I) Sava trough.
Tertiary-Quaternary depression is not a uniform sedimentary basin. It is rather a complex system of partial depressions of various depths and extents. The depressions are separated from each other by elevation of the underlying layers, which sometimes emerge at the surface. The central part is the Pannonian Basin, which is further surrounded by peripheral basins such as the Vienna Basin (A) and the Transcarpathian depression (B). The Pannonian Basin (Stegena and Horvfith, 1984) is separated by the Little Carpatians (C) from the Vienna Basin in the northwest. The Pannonian Basin can be further subdivided.
The basin area to the northwest of the Transdanubian middle mountains (D) is called Little Hungarian Plain (E) and the Slovakian part Danube Lowland (F). The southeastern part is the Great Hungarian Plain (G), northeast of it lies the Transcarpathian depression. Towards the southwest, two elongated depressions can be considered as parts of the Pannonian Basin: the Drava trough (H) and Sava trough (I). The Tertiary-Quaternary filling of the basin mainly consists of sands, clays, shales, sandstones with isolated limestones and evaporites, and also clays and marls in the layers closest to the surface.
187
TABLE Mean
Po
-Y
depth
intervals
used for the calculation of a stripped gravity map
l
Differential density (MG m -3)
Depth interval a (m) From
(Xi~ Yi/Zl }
( x i j y i ) z 2)
Geological body ~ t v divided \~!_ i ~nto layers 1~ 2,_ n
{
z Characteristics
I differential densities for the particular
at elementary
To
+ 150
- 500
- 500
- 1000
-0.62
- 1000
- 2000
-0.36
- 2000
- 3000
-0.23
- 0.47
- 3000
-4000
- 0.09
- 4000
- 5000
-0.03
- 5000
- 6000
- 0.01
- 6000
- 7000
- 0.00
a ( + ) = h e i g h t a b o v e s e a level; ( - ) = d e p t h b e l o w s e a level.
layers :
Layer no. Depth interval [m] Density I 1 Zl - Z 2 01
z2 7 z~
2
16°
~2
F i g . 2. P h y s i c a l - g e o l o g i c a l
18 o
20 °
model.
22"
I
21.o
i
i
t,9o
KOSICE MUKACEVO WIEN
~.8o
48o
1.7o
t,7 o
i
/.6 o
46
% t.5 o
0
F i g . 3. M a p o f t h e g r a v i t y e f f e c t o f t h e s e d i m e n t a r y
filling of the Pannonian
20 ~0 60kin
B a s i n . (1) i s o l i n e s i n /~m s
2.
188 The thickness of the sedimentary basin varies between 0 and 7 kin, with an average of 2.5 km. The density of the sedimentary filling of the Pannonian Basin varies vertically as well as horizontally. Its values depend mainly on the age of the sedimentary formation. The density pattern is relatively complicated, however, and for the actual computations a simple, but best-fit density pattern had to be adopted (Table I). The differential densities, which were referred to the density of 2.67 M G m-3, should be understood as mean values.
3. Calculation of gravity effect of the model and stripped gravity map
The gravity effect of the model of Tertiary sediments was treated as a three-dimensional direct gravimetric problem. The sedimentary filling was modelled by vertical prisms (Fig. 2). Individual geological bodies were divided into layers. Each layer was defined by a contour fine, upper and lower depth, and constant density. The total gravity effect of anomalous bodies was obtained by summation of the gravity effects of n-sided vertical prisms with horizontal bases. The computations were carried out using Smi~ek
Fig. 4. Simplifiedstripped gravity map of a part of the Pannonian Basin. (1) Positive Isolines; (2) negative isolines.
189
et al.'s relation (1970). The result of the computations, carried out in a square 10 × 10 km grid, was the map of the gravity effect of the sediments of the Pannonian Basin. This map was then corrected for the effect of the masses of N e o g e n e - Q u a t e r nary volcanism, which represents one of the most substantial features of the neotectonic evolution of this basin (Bielik, 1986). If the surface of the terrain (the upper boundary of the model) in this area is considered to be planar, no correction for its gravity effect is necessary. The upper boundary was taken as a plane at an elevation of 150 m above sea level. Figure 3 shows the resultant map of the gravity effect of the sedimentary filling of the Pannonian Basin. On the basis of the computed gravity effect, we then corrected the map of complete Bouguer anomalies using the relation Agsgm = A g b - Vz, where Agsgm are the values of the stripped gravity map, A g b the values of the complete Bouguer anomalies (Ibrmajer, 1963) and V~ the gravity effect of the sedimentary filling. Since data from the whole of the Pannonian Basin were not available, the stripped gravity map was only compiled for a part of the territory of this basin. The stripped gravity map of the mentioned region corrected for the gravity effect of the boundary between the lithosphere and the asthenosphere of central Europe for the density contrast of 0.1 M G m -3 (Babu~ka et al., 1986) is contained in Fig. 4.
4. Conclusion The stripped gravity map can offer a better insight into the crustal density structure than the Bouguer gravity map and represents new possibilities of gravity map application to the interpretation of unknown bodies. In the most immediate future, this map will be subjected to detailed qualitative and quantitative, geological and geophysical interpretation. Naturally, the degree of understanding of the structure and density conditions of the sediments varies considerably from one part of the basin to another. Also, the accuracy of the model will vary from one area to another and should by no means be considered as definitive. Consequently, it will be necessary to improve the accuracy of the model
as further data become available, on a detailed and regional scale.
References Babu~ka, V., Plomerov~t, J., Vysko~il, V., Burda, M. and Hiibner, M., 1986. Depth and density characteristics of the lithosphere-asthenosphere transition in central Europe. In: Research of Deep Geological Structure of Czechoslovakia. Geofyzika, Brno, pp. 51-57 (in Czech). Bielik, M., 1988. Construction of the stripped gravity map of the Pannonian Basin. Contrib. Geophys. Inst. Slovak Acad. Sci., in press. Brezsny/mszky, K. and Haas, J., 1986. Main features of the pre-Tertiary basement of Hungary. Geologica Carpathica, 37: 297-303. Fus~n, O., Ibrmajer, J., Plan~tr, J., Sl~vik, J. and Smi~ek, M., 1971. Geological structure of the basement of the covered parts of southern part of inner West Carpathians. Z~padn6 Karpaty, 15: 1-173. Hammer, S., 1963. Deep gravity interpretation by stripping. Geophysics, 28: 369-378. Horv~tth, F. and Royden, L., 1981. Mechanism for the formation of the intra-Carpathian basins: a review. Earth Evol. Sci., 1: 307-316. lbrmajer, J., 1963. Gravimetric map of Czechoslovakia on 1 : 200 000 scale. Stud. Geophys. Geod., 7: 303-308. Kilrneyi, E. and Rumpler, J., 1984. Pre-Tertiary basement relief map of Hungary. Geophys. Trans., 30: 425-428. Kochk, A., Mayer, S. and Pgen~ikova, M , 1981. Structural Model of the Vienna Basin at the Neogene Base. MS Geofond, Praha. (in Czech). Kr~511, A. and Wessely, G., 1973. Neue Ergebnisse beim Tiefenaufschluss in Wiener Becken. Erdol, Erdgas Z., 89: 400-413. Mahel v, M., 1973. Tectonic map of the Carpathian-Balkan Mountain system and adjacent areas. D. Stfir's Geol. Inst., Bratislava and UNESCO. Meskr, A., 1983. Regional Bouguer gravity maps of Hungary. Acta Geodaet., Geophys. Montanist., 18: 187-200. Royden, L., Horv~th, F. and Rumpler, J., 1983. Evolution of the Pannonian Basin system: 1. Tectonics. Tectonics, 2: 63-90. Sclater, J.G., Royden, L., Horv~th, F., Burchfiel, B.C., Semken, S. and Stegena, L., 1980. The formation of the lntraCarpathian basins as determined from subsidence data. Earth Planet. Sci. Lett., 51: 139-162. ~efara, J., Pospi~il, L., Sutora, A., Bodnfir, J., Bielik, M., Velich, R., Kurkin, M. and Miku~ka, J., 1986. Stripped gravity map of the Inner Carpathians (in Slovak). In: M. BfiLkovsk2~ (Editor), Geophysical Model of the Lithosphere. Geofyzika, Brno, pp. 105-119 (in Czech). Smi~ek, M., Plan~r, J. and Kr~hk, J., 1970. Computation of the gravity effect three-dimensional bodies of arbitrary shape. Contrib. Geophys. Inst. Slovak Acad. Sci., 2: 13-23. Stegena, L. and Horv~th, F., 1984. Review of the Pannonian Basin. Acta Geodaet., Geophys. Montanist., 19:153-160.