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Acoustic reflectivity and shallow sedimentary structure in the Sea of Galilee, Jordan Valley
Marine Geology, 70 (1986) 175--189
175
Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands
ACOUSTIC REFLECTIVITY AND SHALLOW SEDIMENTARY STRUCTURE IN THE SEA OF GALILEE, JORDAN VALLEY
ZVI BEN-AVRAHAM', GABRIEL SHALIV 2 and AMOS NUR 3
'Faculty o f Exact Sciences, Department o f Geophysics and Planetary Sciences, Tel Aviv University, Tel Aviv (Israel) 2TAHAL -- Water Planning for Israel Ltd., Tel Aviv (Israel) 3Department o f Geophysics, Stanford University, Stanford, CA 94305 (U.S.A.) (Received October 17, 1984; revised and accepted July 12, 1985)
ABSTRACT Ben-Avraham, Z., Shaliv, G. and Nur, A., 1986. Acoustic reflectivity and shallow sedimentary structure in the Sea of Galilee, Jordan Valley. Mar. Geol., 70: 175--189. Mapping the floor of the Sea of Galilee (Lake Kinneret) with a shallow seismic system of 3.5 kHz resulted in interesting data that were not obtained previously with standard single-channel seismic systems. Over most of the lake acoustic penetration is not possible, probably because of the high gas content in the top sedimentary sequence. However, in a few areas, excellent penetration of about 20 m was achieved. One area is a terrace in the southern part of the lake, south of a small bathymetric escarpment at depths of 13--21 m along Israel latitudinal Grid 238. It is unclear whether the existence of gas in the sediment or other parameters are responsible for the marked difference in acoustic penetration on both sides of the scarp. Another area with acoustic penetration is in the vicinity of hot and salty submarine springs. Although there is no difference in the composition of the upper sedimentary layers between these areas and neighbouring areas, there is a marked difference in the acoustic penetration. The contact between areas with acoustic penetration to areas without acoustic penetration is very sharp. The craters of the submarine springs are usually located on the borders of the areas with acoustic penetration or even at some distance away from them. It is possible that the activity of the hot and salty submarine springs controls the acoustic penetration. However, determination of the exact mechanism for the existence of the zones of acoustic penetration must await further studies of the sediments, especially for measurements of various parameters that control the seismic response of the rock. Another discovery made with the shallow seismic profiles is the existence of some bathymetric irregularities on the floor of the Sea of Galilee. In view of the high sedimentation rate in the lake, which tends to smooth the floor, a bathymetric irregularity such as a linear bathymetric step could be a surface expression of an active fault. INTRODUCTION T h e S e a o f G a l i l e e ( L a k e K i n n e r e t ) is o n e o f several m o r p h o t e c t o n i c depressions along the Dead Sea Rift. In terms of plate tectonics this rift is c o n s i d e r e d t o b e a p l a t e b o u n d a r y o f t h e t r a n s f o r m t y p e ( F r e u n d e t al., 1 9 7 0 ) , b e t w e e n t h e S i n a i P l a t e a n d A r a b i a n P l a t e . I t c o n n e c t s t h e R e d Sea, 0025-3227/86/$03.50
© 1986 Elsevier Science Publishers B.V.
176 where seafloor spreading occurs, with the Zagros--Taurus zone of continental collision. The area around the Sea of Galilee underwent a tectonic evolution that started in the Upper Cretaceous (Michelson, 1973). The history of the inland basins in this region started in the Neogene. The present lake is relatively young, not more than 20,000 years old (Horowitz, 1973). The lake is a body of fresh water whose surface is at about 210 m below MSL. Its length is about 20 km, its maximum width about 12 km, and its maximum depth 42 m. Because of the considerable sedimentation rate of 2--7 mm yr -I (Serruya, 1973; Stiller and Assaf, 1973), the floor of the lake is quite smooth. A notable exception is a small WNW-trending bathymetric scarp at depths 13--21 m, along Israel latitudinal Grid 238 {32°45'N) from the western coast to about Israel longitudinal Grid 206(35°36'E). The lake is bounded by faults along some parts of its margins, as is evidenced by the steep bathymetric slopes along these parts of the coast. As a result of the complex geological history of the Sea of Galilee area, the structure under the lake is not well understood. Two fault systems intersect in the lake's area. The main fault system trends north--south and is part of the Dead Sea transform, and the secondary fault system trends NW--SE on the western side of the main fault and extending into the Galilee. Previous studies of the sub-bottom structure and stratigraphy of the Sea of Galilee included seismic reflection and refraction measurements using various methods (Ben-Avraham et al., 1981). Magnetic (Ben-Avraham et al., 1980) and heat flow measurements (Ben-Avraham et al., 1978) have also been carried out. In general, the results of the seismic profiles are poor, both in resolution and penetration. The reason may be the high gas content in the upper sediments. This gas originates from the high concentration of biomass on the lake's floor. Similar phenomena have been observed in other shallow lakes. In the previous experiments with seismic profiling in the Sea of Galilee, several instruments were used with various frequency bands. The range was from a few hundreds to a few tens of Hz. The seismic reflection profiles obtained during these experiments gave some details about the uppermost sediments. Although the results are poor, they clearly indicate that the lake area is tectonically active. Folded structures and active faults have been detected in the uppermost sediments, mainly along the margins, but also at the center of the lake in its deepest portion (Ben-Avraham et al., 1981). The average heat flow value measured in the Sea of Galilee is quite high, 2.23 ~cal cm -2 s-~ (Villinger, 1977), which also indicates tectonic activity. The main difficulty with the measurements in the Sea of Galilee is the shallow water which causes relatively large fluctuations in the b o t t o m water temperature. The determination of heat flow in the lake was possible because there is a detailed temperature record of the entire water body since 1963 at several permanent stations. To study the tectonic processes in the Sea of Galilee and the structure of the top few meters of sediments, further experiments in seismic profiling have been conducted using a 3.5 kHz seismic system (Figs.1 and 2). This is
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180 BOTTOM SEDIMENTS The upper layer of the b o t t o m sediments of the Sea of Galilee is composed of a black, jelly-like low-density material with thickness of 2--5 cm. The underlying sediments are grey and compact (Serruya, 1978). Below 12 m of water, the sediments are composed of fine-grained material. The fraction smaller than 20 pm represents 90--95% of the sediments (Banin et al., 1972). The b o t t o m sediments are also characterized by the abundance of calcium carbonate (40--55%) and the dominance of montmorillonite in the clay fraction. The montmorillonite constitutes approximately 60% of the total silicates and accounts for the very high specific surface of the lake sediments. There are indirect indications for a high biogenic gas content within the top sedimentary layers. First, the percentage of organic matter in the b o t t o m sediments is quite high, 2--4%, and decreases with depth (Serruya, 1978; J. Rounick, unpubl, manuscript, 1985). The gas content of the sediment itself was not measured so far. However, recent measurements of the methane (CH4) content of the water just above the b o t t o m in the deepest part of the lake during late winter and spring yielded values of 0.10--0.66 mg 1-1 . These values apparently vary seasonally and probably are much higher during the summer (J. Rounick, unpubl, manuscript, 1985). The average thermal conductivity of the b o t t o m sediments of the Sea of Galilee is 1.78 mcal cm -1 s -1 C -1 (Ben-Avraham et al., 1978). This value is lower than the average value for the b o t t o m sediments of the Dead Sea {1.81 mcal cm -1 s-1 C -1) and the Gulf of Elat {2.67 mcal cm -~ s -~ C -1). The reason for this may be the higher percentage of fine-grained material in the b o t t o m sediments of the Sea of Galilee as well as the presence of gas. RESULTS The profiles obtained with the 3.5 kHz system have led to several important discoveries. Over most of the lake there was no acoustic penetration, and the only arrivals seen on the records are those of the b o t t o m and b o t t o m multiples. The reason for this is probably the high gas content within the top sedimentary layers. However, in several areas excellent acoustic penetration was achieved. In these areas no b o t t o m multiples have been recorded. It should be noted that the 3.5 kHz profiles were obtained without any adjustments to the recording instrument controls. Gaseous sediments such as the ones in the Sea of Galilee should be acoustically absorptive. Furthermore, their surface could be much more reflective than normal sediments due to the very low compressional velocity associated with even but small volume of gas present. The situation over most areas of the Sea of Galilee is quite consistant with the gas effects in that over most of the lake there is no acoustic penetration, but b o t t o m multiples are usually present and strong. Such a situation is also possible where the b o t t o m is veneered with a layer of higher acoustic impedance {e.g. sand over clay)
181
or basalt flows on the floor (Davis, 1982). B o t t o m sampling (Thompson et al., 1985) and magnetic anomalies (Ben-Avraham et al., 1980; Ben-Avraham and Ginzburg, 1986}, indicate that none of the above exist in the deeper portions of the Sea of Galilee. One area in which acoustic penetration was achieved is in the southern part of the lake, south of the small bathymetric escarpment at depths of 13--21 .m along Israel latitudinal Grid 238 (Figs.4 and 8). This scarp starts at a b o u t Israel longitudinal Grid 206 and extends westwards into the coast. It steepens from east to west. This steepening can be seen very clearly on the 3.5 kHz profiles (Fig.4). The contact between areas with no acoustic penetration, north of the scarp, and areas with excellent acoustic penetration, south of the scarp, is very sharp. Clearly there must be large differences in the composition of the top sedimentary layers north and south of the scarp. Several compositional differences were indeed mapped on both sides of the scarp (8erruya, 1971). They are shown clearly by the distribution of the detrital elements phosphorus, iron and manganese in the surface sediments. The highest concentrations of these elements are in the terrace south of the scarp, where acoustic penetration exists {Serruya, 1971). It is unclear as y e t whether the contrast in acoustic reflectivity on both sides of the scarp is just a function o f the a m o u n t of gas in the sediments or whether other parameters are also involved. Previously obtained seismic profiles with lower frequency ranges across this scarp indicate a possible difference in the nature of the t w o sediment types across the transparent/non-transparent boundary (Fig. 3, profile 5p). It has been suggested that this scarp is a surface expression of a fault (Golani, 1962; Ben-Arieh, 1965; Neev, 1978; Ben-Avraham et al., 1981). The results of the 3.5 kHz profiles (Fig.4) support this idea. Over most of the terrace south of the scarp the sedimentary layers dip steeply and are cut b y the floor. A very interesting p h e n o m e n o n observed in this area is the existence of a layer with a high acoustic reflectivity that in some locations cut the bedding (Fig.5). This layer may be a gas front, salt, or basalt. In Zemach 1 well, which is located 2 km south of the lake, a large sequence, over 2000 m, of salt and basalt has been drilled. Several gas-bearing layers have also been detected (Markus and $1ager, 1985). A similar p h e n o m e n o n has been observed off central California where a gas front exists. The terrace, south of the scarp, is probably where layers of the Lisan Formation are exposed (Ben-Avraham et al., 1981). These layers were deposited by the salty Lisan Lake which predated the Sea of Galilee and occupied a large portion of the Dead Sea Rift. Its northernmost extension was probably to where the southern terrace of the Sea of Galilee is now located as is evidenced by the exposure of the Lisan Formation on land on both sides of this portion of the lake. The deformation seen in the 3.5 kHz profiles where penentration exists (Figs.4 and 5) indicates, in accordance with the results of previous studies {Ben-Avraham et al., 1978, 1980, 1981), a rather complex s u b b o t t o m
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structure as the result of active tectonic processes in this area. The folds on profile 7 are probably due to the intersection of two faults (Fig.8). A clear fault is seen on profile 8. The complexity of the Sea of Galilee area is due to the interaction of two fault systems, the north-~south Dead Sea transform system and the northwest--northeast fault system which is composed of branching faults along which a diffusion of the slip from the main transform takes place.
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185
Other areas where acoustic penetration was achieved are in the vicinity of h o t and salty submarine springs (Fig.6). These springs exist mainly along the western coast, b u t some are k n o w n also along the eastern coast. This is quite surprising because unlike the terrace south of the bathymetric escarpment, there are no major differences in the composition of the top sedimentary layers between the zones of acoustic penetration near the h o t springs and the surrounding areas. The boundary between areas of acoustic penetration near the springs and areas w i t h o u t acoustic penetration is very sharp. The h o t and salty springs are usually associated with a small crater. It turned o u t that 0
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186
large zones of acoustic penetration exist in the vicinity of these craters. The craters are usually located on the borders of these areas of acoustic penetration or even at some distance away from them. The zones of acoustic penetration appear to be related to the activity of the h o t and salty submarine springs, although none of the craters has been found inside the zones of acoustic penetration. Another discovery made with the shallow seismic profiles is the existence of some bathymetric irregularities on the floor of the Sea of Galilee (Fig.7). In view of the high sedimentation rate in the lake, which tends to smooth the floor, a bathymetric irregularity such as a linear bathymetric step could be the result of active faulting and other tectonic processes. Available data indicate t h a t most of the irregularities are elongated; they can be detected on several profiles. Previous bathymetric mapping of the floor of the lake was done with such large spacing between lines of measurement that small bathymetric irregularities on an order of less than 1 m were not mapped. DISCUSSION
Mapping the floor of the Sea of Galilee with a shallow seismic system operating at 3.5 kHz has proved to be an efficient means of studying the lake's floor. Instruments working at this frequency range apparently are sensitive to the presence of gas in the sediment. The conditions in the Sea of Galilee at present are such that the large a m o u n t of gas in the top sedimentary layers prevents acoustic penetration of a source operating at 3--4 kHz. Preliminary experiments have shown t h a t seismic waves of higher frequencies do not penetrate the lake's floor, whereas seismic waves with lower frequencies penetrate all areas, including those with the gas (Fig.3). Where the 3- to 4-kHz seismic waves could penetrate the sediments, t h e y reveal many details about the structure of the top 10 m of sediment; however, in most cases, probably because of the presence of gas, penetration could n o t be achieved, and thus no information was obtained(Fig. 8). Several studies have been made in the oceans to map the composition and bedforms of the top sedimentary layers with seismic systems operating at several kHz (Damouth and Hayes, 1977). The results can usually be related to the grain size of the sediments. Mapping the floor of the Sea of Galilee with such systems provides an opportunity to study the effect of gas in the sediments. Detailed studies are needed so as to better understand the acoustic response of gas-bearing sediments in lake and ocean floors. Such studies include experiments with sound sources of various frequencies accompanied by a sampling program to study physical and chemical parameters that affect the seismic response of the sediments. Beyond the potential of mapping sediment types and composition, the use of a 3.5-kHz seismic system in the Sea o f Galilee gives new insights into the mechanism of h o t and salty springs in the lake. The existence of a zone with acoustic penetration near the crater of every spring suggests t h a t these zones are somehow related to the salty water. If seismic penetration
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are areas in w h i c h a 3 . 5 - k H z
is indeed uniquely related to h o t spring activity, it may provide an easy and efficient tool for detection of hydrothermal activity in the Sea of Galilee and elsewhere. However, determination of the exact mechanism for the formation of the zones of acoustic reflectivity must await further studies of the sediments, especially for measurements of various parameters that control the seismic response of the rock. Possible mechanisms responsible for the different seismic behaviour of the sediments near the h o t springs are grain cementation caused by h o t brine, removal of gas generating organic matter, and chemical and mineralogical modification of the sediments. One of the enigmatic features is the fact that the craters formed by the submarine springs are always located outside the zone of acoustic penetration. This study also highlighted the importance of a detailed b a t h y m e t r y even in areas of high sedimentation rate such as the Sea of Galilee. The large
188 sequence of s e d i m e n t here serves, in effect, as a filter. O n l y t h e m o s t active and p r o n o u n c e d fault z o n e s p e n e t r a t e t h e f l o o r o f the lake and d e f o r m it. Thus, detailed b a t h y m e t r y can give a great deal o f i n f o r m a t i o n a b o u t the fault p a t t e r n in this area. ACKNOWLEDGMENTS This s t u d y was s u p p o r t e d b y Oil E x p l o r a t i o n ( I n v e s t m e n t ) Ltd., and t h e H y d r o l o g y Division, Tahal, Water Planning f o r Israel Ltd. We gratefully a c k n o w l e d g e the Marine G e o l o g y and G e o m a t h e m a t i c s Division o f the Israel Geological Survey f o r allowing us t o use their 3.5-kHz seismic s y s t e m during this study. We also t h a n k the staff o f t h e Israel O c e a n o g r a p h i c and L i m n o logical Research Ltd. for their technical assistance, and especially the captain o f R / V " H e r m o n a " and the staff of. t h e K i n n e r e t L i m n o l o g i c a l L a b o r a t o r y .
REFERENCES Banin, A., Singer, A. and Gal, M., 1972. Amounts, composition and physico-chemical properties of clays in Lake Kinneret sediments. Teeh. Rep. Fac. Agric., Hebrew Univ., Jerusalem, 78 pp. (in Hebrew). Ben-Arieh, 1965. Central Jordan Valley, Negev Kinnarot. Hakibbutz Hamehuhad Pb., 292 pp. (in Hebrew). Ben-Avraham, Z. and Ginzburg, A., 1986. The structure of the Sea of Galilee graben, Jordan Valley, Israel, from magnetic measurements. Tectonophysics, in press. Ben-Avraham, Z., H~inel, R. and Villinger, H., 1978. Heat flow through the Dead Sea rift. Mar. Geol., 28: 253--269. Ben-Avraham, Z., Shoham, Y., Klein, E., Michelson, H. and Serruya, C., 1980. Magnetic survey of Lake Kinneret -- Central Jordan Valley, Israel. Mar. Geophys. Res., 4: 257--276. Ben-Avraham, Z., Ginzburg, A. and Yuval, Z., 1981. Seismic reflection and refraction investigations of Lake Kinneret -- Central Jordan Valley, Israel. Tectonophysics, 80: 165--181. Damouth, J.E. and Hayes, D.E., 1977. Echo character of the east Brazilian continental margin and its relationship to the dispersal and distribution of terrigenous sediments. Mar. Geol., 18: 17--45. Davis, E.E., 1982. Evidence for extensive basalt flows on the sea-floor. Geol. Soc. Am. Bull., 93: 1023--1029. Freund, R., Garfunkel, Z., Zak, I., Goldberg, M., Weissbrod, T. and Derin, B., 1970. The shear along the Dead Sea Rift. Philos. Trans. R. Soc. London, Ser. A, 267: 107--130. Golani, U., 1962. The Geology of Lake Tiberias Region and the Hydrogeology of the Saline Springs. Water Planning for Israel Ltd. (TAHAL), Geotech. Dept. Rep. No.19. Horowitz, A., 1973. Development of the Hula Basin, Israel. Israel J. Earth-Sci., 22: 107--139. Markus, E. and Slager, J., 1985. The sedimentary-magmatic sequence of the Zemah 1 Well (Dead Sea rift, Israel) and its emplacement in time and space. Israel J. EarthSci., 34: 1--10. Michelson, H., 1973. Geology of the Kinneret Area. In: T. Berman (Editor), Lake Kinneret General Background. Israel Nat. Counc. Res. Dev., 12--73: 15--19. Neev, D., 1978. The geology of Lake Kinneret. Kinneret-assemblage of Scientific Articles, Publ. Lake Kinneret Auto., Zemah, Israel (in Hebrew), 114 pp. Serruya, C., 1971. Lake Kinneret: The nutrient chemistry of the sediments. Limnol. Oceanogr., 16: 510--521.
189 Serruya, C., 1973. Sediments. In: T. Berman (Editor), Lake Kinneret Dada Record, Israel Nat. Counc. Res. Dev., 13--73: 39--45. Serruya, C., 1978. Sediments. In: C. Serruya (Editor), Lake Kinneret, Monographiae Biologicae. Junk, The Hague, pp.205--215. Stiller, M. and Assaf, G., 1973. Sedimentation and transport of particles in Lake Kinneret traced by ~3~Cs. In: Hydrology o f Lakes. Symposium IAHS-AISH, Publ. 109:397--403. Thompson, R., Turner, G.M., Stiller, M. and Kaufman, A., 1985. Near-East paleomagnetic secular variation recorded in sediments from the Sea of Galilee (Lake Kinneret). Quat. Res., 23: 175--188. Villinger, H., 1977. Untersuchungen tiber die M6glichkeitvonW~mestromdichtemessungen in Flachen Gewassern. Unpubl. Thesis, Tech. Univ. Berlin, Berlin, 83 pp.
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Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands
ACOUSTIC REFLECTIVITY AND SHALLOW SEDIMENTARY STRUCTURE IN THE SEA OF GALILEE, JORDAN VALLEY
ZVI BEN-AVRAHAM', GABRIEL SHALIV 2 and AMOS NUR 3
'Faculty o f Exact Sciences, Department o f Geophysics and Planetary Sciences, Tel Aviv University, Tel Aviv (Israel) 2TAHAL -- Water Planning for Israel Ltd., Tel Aviv (Israel) 3Department o f Geophysics, Stanford University, Stanford, CA 94305 (U.S.A.) (Received October 17, 1984; revised and accepted July 12, 1985)
ABSTRACT Ben-Avraham, Z., Shaliv, G. and Nur, A., 1986. Acoustic reflectivity and shallow sedimentary structure in the Sea of Galilee, Jordan Valley. Mar. Geol., 70: 175--189. Mapping the floor of the Sea of Galilee (Lake Kinneret) with a shallow seismic system of 3.5 kHz resulted in interesting data that were not obtained previously with standard single-channel seismic systems. Over most of the lake acoustic penetration is not possible, probably because of the high gas content in the top sedimentary sequence. However, in a few areas, excellent penetration of about 20 m was achieved. One area is a terrace in the southern part of the lake, south of a small bathymetric escarpment at depths of 13--21 m along Israel latitudinal Grid 238. It is unclear whether the existence of gas in the sediment or other parameters are responsible for the marked difference in acoustic penetration on both sides of the scarp. Another area with acoustic penetration is in the vicinity of hot and salty submarine springs. Although there is no difference in the composition of the upper sedimentary layers between these areas and neighbouring areas, there is a marked difference in the acoustic penetration. The contact between areas with acoustic penetration to areas without acoustic penetration is very sharp. The craters of the submarine springs are usually located on the borders of the areas with acoustic penetration or even at some distance away from them. It is possible that the activity of the hot and salty submarine springs controls the acoustic penetration. However, determination of the exact mechanism for the existence of the zones of acoustic penetration must await further studies of the sediments, especially for measurements of various parameters that control the seismic response of the rock. Another discovery made with the shallow seismic profiles is the existence of some bathymetric irregularities on the floor of the Sea of Galilee. In view of the high sedimentation rate in the lake, which tends to smooth the floor, a bathymetric irregularity such as a linear bathymetric step could be a surface expression of an active fault. INTRODUCTION T h e S e a o f G a l i l e e ( L a k e K i n n e r e t ) is o n e o f several m o r p h o t e c t o n i c depressions along the Dead Sea Rift. In terms of plate tectonics this rift is c o n s i d e r e d t o b e a p l a t e b o u n d a r y o f t h e t r a n s f o r m t y p e ( F r e u n d e t al., 1 9 7 0 ) , b e t w e e n t h e S i n a i P l a t e a n d A r a b i a n P l a t e . I t c o n n e c t s t h e R e d Sea, 0025-3227/86/$03.50
© 1986 Elsevier Science Publishers B.V.
176 where seafloor spreading occurs, with the Zagros--Taurus zone of continental collision. The area around the Sea of Galilee underwent a tectonic evolution that started in the Upper Cretaceous (Michelson, 1973). The history of the inland basins in this region started in the Neogene. The present lake is relatively young, not more than 20,000 years old (Horowitz, 1973). The lake is a body of fresh water whose surface is at about 210 m below MSL. Its length is about 20 km, its maximum width about 12 km, and its maximum depth 42 m. Because of the considerable sedimentation rate of 2--7 mm yr -I (Serruya, 1973; Stiller and Assaf, 1973), the floor of the lake is quite smooth. A notable exception is a small WNW-trending bathymetric scarp at depths 13--21 m, along Israel latitudinal Grid 238 {32°45'N) from the western coast to about Israel longitudinal Grid 206(35°36'E). The lake is bounded by faults along some parts of its margins, as is evidenced by the steep bathymetric slopes along these parts of the coast. As a result of the complex geological history of the Sea of Galilee area, the structure under the lake is not well understood. Two fault systems intersect in the lake's area. The main fault system trends north--south and is part of the Dead Sea transform, and the secondary fault system trends NW--SE on the western side of the main fault and extending into the Galilee. Previous studies of the sub-bottom structure and stratigraphy of the Sea of Galilee included seismic reflection and refraction measurements using various methods (Ben-Avraham et al., 1981). Magnetic (Ben-Avraham et al., 1980) and heat flow measurements (Ben-Avraham et al., 1978) have also been carried out. In general, the results of the seismic profiles are poor, both in resolution and penetration. The reason may be the high gas content in the upper sediments. This gas originates from the high concentration of biomass on the lake's floor. Similar phenomena have been observed in other shallow lakes. In the previous experiments with seismic profiling in the Sea of Galilee, several instruments were used with various frequency bands. The range was from a few hundreds to a few tens of Hz. The seismic reflection profiles obtained during these experiments gave some details about the uppermost sediments. Although the results are poor, they clearly indicate that the lake area is tectonically active. Folded structures and active faults have been detected in the uppermost sediments, mainly along the margins, but also at the center of the lake in its deepest portion (Ben-Avraham et al., 1981). The average heat flow value measured in the Sea of Galilee is quite high, 2.23 ~cal cm -2 s-~ (Villinger, 1977), which also indicates tectonic activity. The main difficulty with the measurements in the Sea of Galilee is the shallow water which causes relatively large fluctuations in the b o t t o m water temperature. The determination of heat flow in the lake was possible because there is a detailed temperature record of the entire water body since 1963 at several permanent stations. To study the tectonic processes in the Sea of Galilee and the structure of the top few meters of sediments, further experiments in seismic profiling have been conducted using a 3.5 kHz seismic system (Figs.1 and 2). This is
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180 BOTTOM SEDIMENTS The upper layer of the b o t t o m sediments of the Sea of Galilee is composed of a black, jelly-like low-density material with thickness of 2--5 cm. The underlying sediments are grey and compact (Serruya, 1978). Below 12 m of water, the sediments are composed of fine-grained material. The fraction smaller than 20 pm represents 90--95% of the sediments (Banin et al., 1972). The b o t t o m sediments are also characterized by the abundance of calcium carbonate (40--55%) and the dominance of montmorillonite in the clay fraction. The montmorillonite constitutes approximately 60% of the total silicates and accounts for the very high specific surface of the lake sediments. There are indirect indications for a high biogenic gas content within the top sedimentary layers. First, the percentage of organic matter in the b o t t o m sediments is quite high, 2--4%, and decreases with depth (Serruya, 1978; J. Rounick, unpubl, manuscript, 1985). The gas content of the sediment itself was not measured so far. However, recent measurements of the methane (CH4) content of the water just above the b o t t o m in the deepest part of the lake during late winter and spring yielded values of 0.10--0.66 mg 1-1 . These values apparently vary seasonally and probably are much higher during the summer (J. Rounick, unpubl, manuscript, 1985). The average thermal conductivity of the b o t t o m sediments of the Sea of Galilee is 1.78 mcal cm -1 s -1 C -1 (Ben-Avraham et al., 1978). This value is lower than the average value for the b o t t o m sediments of the Dead Sea {1.81 mcal cm -1 s-1 C -1) and the Gulf of Elat {2.67 mcal cm -~ s -~ C -1). The reason for this may be the higher percentage of fine-grained material in the b o t t o m sediments of the Sea of Galilee as well as the presence of gas. RESULTS The profiles obtained with the 3.5 kHz system have led to several important discoveries. Over most of the lake there was no acoustic penetration, and the only arrivals seen on the records are those of the b o t t o m and b o t t o m multiples. The reason for this is probably the high gas content within the top sedimentary layers. However, in several areas excellent acoustic penetration was achieved. In these areas no b o t t o m multiples have been recorded. It should be noted that the 3.5 kHz profiles were obtained without any adjustments to the recording instrument controls. Gaseous sediments such as the ones in the Sea of Galilee should be acoustically absorptive. Furthermore, their surface could be much more reflective than normal sediments due to the very low compressional velocity associated with even but small volume of gas present. The situation over most areas of the Sea of Galilee is quite consistant with the gas effects in that over most of the lake there is no acoustic penetration, but b o t t o m multiples are usually present and strong. Such a situation is also possible where the b o t t o m is veneered with a layer of higher acoustic impedance {e.g. sand over clay)
181
or basalt flows on the floor (Davis, 1982). B o t t o m sampling (Thompson et al., 1985) and magnetic anomalies (Ben-Avraham et al., 1980; Ben-Avraham and Ginzburg, 1986}, indicate that none of the above exist in the deeper portions of the Sea of Galilee. One area in which acoustic penetration was achieved is in the southern part of the lake, south of the small bathymetric escarpment at depths of 13--21 .m along Israel latitudinal Grid 238 (Figs.4 and 8). This scarp starts at a b o u t Israel longitudinal Grid 206 and extends westwards into the coast. It steepens from east to west. This steepening can be seen very clearly on the 3.5 kHz profiles (Fig.4). The contact between areas with no acoustic penetration, north of the scarp, and areas with excellent acoustic penetration, south of the scarp, is very sharp. Clearly there must be large differences in the composition of the top sedimentary layers north and south of the scarp. Several compositional differences were indeed mapped on both sides of the scarp (8erruya, 1971). They are shown clearly by the distribution of the detrital elements phosphorus, iron and manganese in the surface sediments. The highest concentrations of these elements are in the terrace south of the scarp, where acoustic penetration exists {Serruya, 1971). It is unclear as y e t whether the contrast in acoustic reflectivity on both sides of the scarp is just a function o f the a m o u n t of gas in the sediments or whether other parameters are also involved. Previously obtained seismic profiles with lower frequency ranges across this scarp indicate a possible difference in the nature of the t w o sediment types across the transparent/non-transparent boundary (Fig. 3, profile 5p). It has been suggested that this scarp is a surface expression of a fault (Golani, 1962; Ben-Arieh, 1965; Neev, 1978; Ben-Avraham et al., 1981). The results of the 3.5 kHz profiles (Fig.4) support this idea. Over most of the terrace south of the scarp the sedimentary layers dip steeply and are cut b y the floor. A very interesting p h e n o m e n o n observed in this area is the existence of a layer with a high acoustic reflectivity that in some locations cut the bedding (Fig.5). This layer may be a gas front, salt, or basalt. In Zemach 1 well, which is located 2 km south of the lake, a large sequence, over 2000 m, of salt and basalt has been drilled. Several gas-bearing layers have also been detected (Markus and $1ager, 1985). A similar p h e n o m e n o n has been observed off central California where a gas front exists. The terrace, south of the scarp, is probably where layers of the Lisan Formation are exposed (Ben-Avraham et al., 1981). These layers were deposited by the salty Lisan Lake which predated the Sea of Galilee and occupied a large portion of the Dead Sea Rift. Its northernmost extension was probably to where the southern terrace of the Sea of Galilee is now located as is evidenced by the exposure of the Lisan Formation on land on both sides of this portion of the lake. The deformation seen in the 3.5 kHz profiles where penentration exists (Figs.4 and 5) indicates, in accordance with the results of previous studies {Ben-Avraham et al., 1978, 1980, 1981), a rather complex s u b b o t t o m
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structure as the result of active tectonic processes in this area. The folds on profile 7 are probably due to the intersection of two faults (Fig.8). A clear fault is seen on profile 8. The complexity of the Sea of Galilee area is due to the interaction of two fault systems, the north-~south Dead Sea transform system and the northwest--northeast fault system which is composed of branching faults along which a diffusion of the slip from the main transform takes place.
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185
Other areas where acoustic penetration was achieved are in the vicinity of h o t and salty submarine springs (Fig.6). These springs exist mainly along the western coast, b u t some are k n o w n also along the eastern coast. This is quite surprising because unlike the terrace south of the bathymetric escarpment, there are no major differences in the composition of the top sedimentary layers between the zones of acoustic penetration near the h o t springs and the surrounding areas. The boundary between areas of acoustic penetration near the springs and areas w i t h o u t acoustic penetration is very sharp. The h o t and salty springs are usually associated with a small crater. It turned o u t that 0
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186
large zones of acoustic penetration exist in the vicinity of these craters. The craters are usually located on the borders of these areas of acoustic penetration or even at some distance away from them. The zones of acoustic penetration appear to be related to the activity of the h o t and salty submarine springs, although none of the craters has been found inside the zones of acoustic penetration. Another discovery made with the shallow seismic profiles is the existence of some bathymetric irregularities on the floor of the Sea of Galilee (Fig.7). In view of the high sedimentation rate in the lake, which tends to smooth the floor, a bathymetric irregularity such as a linear bathymetric step could be the result of active faulting and other tectonic processes. Available data indicate t h a t most of the irregularities are elongated; they can be detected on several profiles. Previous bathymetric mapping of the floor of the lake was done with such large spacing between lines of measurement that small bathymetric irregularities on an order of less than 1 m were not mapped. DISCUSSION
Mapping the floor of the Sea of Galilee with a shallow seismic system operating at 3.5 kHz has proved to be an efficient means of studying the lake's floor. Instruments working at this frequency range apparently are sensitive to the presence of gas in the sediment. The conditions in the Sea of Galilee at present are such that the large a m o u n t of gas in the top sedimentary layers prevents acoustic penetration of a source operating at 3--4 kHz. Preliminary experiments have shown t h a t seismic waves of higher frequencies do not penetrate the lake's floor, whereas seismic waves with lower frequencies penetrate all areas, including those with the gas (Fig.3). Where the 3- to 4-kHz seismic waves could penetrate the sediments, t h e y reveal many details about the structure of the top 10 m of sediment; however, in most cases, probably because of the presence of gas, penetration could n o t be achieved, and thus no information was obtained(Fig. 8). Several studies have been made in the oceans to map the composition and bedforms of the top sedimentary layers with seismic systems operating at several kHz (Damouth and Hayes, 1977). The results can usually be related to the grain size of the sediments. Mapping the floor of the Sea of Galilee with such systems provides an opportunity to study the effect of gas in the sediments. Detailed studies are needed so as to better understand the acoustic response of gas-bearing sediments in lake and ocean floors. Such studies include experiments with sound sources of various frequencies accompanied by a sampling program to study physical and chemical parameters that affect the seismic response of the sediments. Beyond the potential of mapping sediment types and composition, the use of a 3.5-kHz seismic system in the Sea o f Galilee gives new insights into the mechanism of h o t and salty springs in the lake. The existence of a zone with acoustic penetration near the crater of every spring suggests t h a t these zones are somehow related to the salty water. If seismic penetration
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are areas in w h i c h a 3 . 5 - k H z
is indeed uniquely related to h o t spring activity, it may provide an easy and efficient tool for detection of hydrothermal activity in the Sea of Galilee and elsewhere. However, determination of the exact mechanism for the formation of the zones of acoustic reflectivity must await further studies of the sediments, especially for measurements of various parameters that control the seismic response of the rock. Possible mechanisms responsible for the different seismic behaviour of the sediments near the h o t springs are grain cementation caused by h o t brine, removal of gas generating organic matter, and chemical and mineralogical modification of the sediments. One of the enigmatic features is the fact that the craters formed by the submarine springs are always located outside the zone of acoustic penetration. This study also highlighted the importance of a detailed b a t h y m e t r y even in areas of high sedimentation rate such as the Sea of Galilee. The large
188 sequence of s e d i m e n t here serves, in effect, as a filter. O n l y t h e m o s t active and p r o n o u n c e d fault z o n e s p e n e t r a t e t h e f l o o r o f the lake and d e f o r m it. Thus, detailed b a t h y m e t r y can give a great deal o f i n f o r m a t i o n a b o u t the fault p a t t e r n in this area. ACKNOWLEDGMENTS This s t u d y was s u p p o r t e d b y Oil E x p l o r a t i o n ( I n v e s t m e n t ) Ltd., and t h e H y d r o l o g y Division, Tahal, Water Planning f o r Israel Ltd. We gratefully a c k n o w l e d g e the Marine G e o l o g y and G e o m a t h e m a t i c s Division o f the Israel Geological Survey f o r allowing us t o use their 3.5-kHz seismic s y s t e m during this study. We also t h a n k the staff o f t h e Israel O c e a n o g r a p h i c and L i m n o logical Research Ltd. for their technical assistance, and especially the captain o f R / V " H e r m o n a " and the staff of. t h e K i n n e r e t L i m n o l o g i c a l L a b o r a t o r y .
REFERENCES Banin, A., Singer, A. and Gal, M., 1972. Amounts, composition and physico-chemical properties of clays in Lake Kinneret sediments. Teeh. Rep. Fac. Agric., Hebrew Univ., Jerusalem, 78 pp. (in Hebrew). Ben-Arieh, 1965. Central Jordan Valley, Negev Kinnarot. Hakibbutz Hamehuhad Pb., 292 pp. (in Hebrew). Ben-Avraham, Z. and Ginzburg, A., 1986. The structure of the Sea of Galilee graben, Jordan Valley, Israel, from magnetic measurements. Tectonophysics, in press. Ben-Avraham, Z., H~inel, R. and Villinger, H., 1978. Heat flow through the Dead Sea rift. Mar. Geol., 28: 253--269. Ben-Avraham, Z., Shoham, Y., Klein, E., Michelson, H. and Serruya, C., 1980. Magnetic survey of Lake Kinneret -- Central Jordan Valley, Israel. Mar. Geophys. Res., 4: 257--276. Ben-Avraham, Z., Ginzburg, A. and Yuval, Z., 1981. Seismic reflection and refraction investigations of Lake Kinneret -- Central Jordan Valley, Israel. Tectonophysics, 80: 165--181. Damouth, J.E. and Hayes, D.E., 1977. Echo character of the east Brazilian continental margin and its relationship to the dispersal and distribution of terrigenous sediments. Mar. Geol., 18: 17--45. Davis, E.E., 1982. Evidence for extensive basalt flows on the sea-floor. Geol. Soc. Am. Bull., 93: 1023--1029. Freund, R., Garfunkel, Z., Zak, I., Goldberg, M., Weissbrod, T. and Derin, B., 1970. The shear along the Dead Sea Rift. Philos. Trans. R. Soc. London, Ser. A, 267: 107--130. Golani, U., 1962. The Geology of Lake Tiberias Region and the Hydrogeology of the Saline Springs. Water Planning for Israel Ltd. (TAHAL), Geotech. Dept. Rep. No.19. Horowitz, A., 1973. Development of the Hula Basin, Israel. Israel J. Earth-Sci., 22: 107--139. Markus, E. and Slager, J., 1985. The sedimentary-magmatic sequence of the Zemah 1 Well (Dead Sea rift, Israel) and its emplacement in time and space. Israel J. EarthSci., 34: 1--10. Michelson, H., 1973. Geology of the Kinneret Area. In: T. Berman (Editor), Lake Kinneret General Background. Israel Nat. Counc. Res. Dev., 12--73: 15--19. Neev, D., 1978. The geology of Lake Kinneret. Kinneret-assemblage of Scientific Articles, Publ. Lake Kinneret Auto., Zemah, Israel (in Hebrew), 114 pp. Serruya, C., 1971. Lake Kinneret: The nutrient chemistry of the sediments. Limnol. Oceanogr., 16: 510--521.
189 Serruya, C., 1973. Sediments. In: T. Berman (Editor), Lake Kinneret Dada Record, Israel Nat. Counc. Res. Dev., 13--73: 39--45. Serruya, C., 1978. Sediments. In: C. Serruya (Editor), Lake Kinneret, Monographiae Biologicae. Junk, The Hague, pp.205--215. Stiller, M. and Assaf, G., 1973. Sedimentation and transport of particles in Lake Kinneret traced by ~3~Cs. In: Hydrology o f Lakes. Symposium IAHS-AISH, Publ. 109:397--403. Thompson, R., Turner, G.M., Stiller, M. and Kaufman, A., 1985. Near-East paleomagnetic secular variation recorded in sediments from the Sea of Galilee (Lake Kinneret). Quat. Res., 23: 175--188. Villinger, H., 1977. Untersuchungen tiber die M6glichkeitvonW~mestromdichtemessungen in Flachen Gewassern. Unpubl. Thesis, Tech. Univ. Berlin, Berlin, 83 pp.