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Seismic reflection profiles across the southern Dead Sea basin
Tectonophysics 346 (2002) 61 – 69 www.elsevier.com/locate/tecto
Seismic ref lection profiles across the southern Dead Sea basin Abdallah Al-Zoubi a, Haim Shulman b, Zvi Ben-Avraham b,* b
a Natural Resources Authority, P.O. Box 7, Amman, Jordan Department of Geophysics and Planetary Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel
Abstract The main structural and stratigraphic elements that form the Southern Dead Sea basin (SDSB), which is part of the Dead Sea transform fault system, have been broadly delineated with various degrees of certainty by a number of researchers. The recent exchange of seismic data between Israel and Jordan enable us to fill in some of the dashed lines and eliminate some of the question marks that appear on the previously published geological cross-sections. Two east – west seismic time sections, one from each side of the international border that dissect the basin, have been selected, composed and interpreted. A remarkable correlation of the three major sedimentary sequences that form the basin-fill (Miocene clastics, Pliocene salt and Pleistocene – Holocene clastics and evaporites) has been established between the lines and, hence, across the entire width of the SDSB. The Miocene and Pliocene series are cut off in the east by the Ghor – Safi fault that is the equivalent of the western Sedom fault, thus, indicating that all three major structural steps, known to exist in the west: (1) rim block, (2) intermediate block and (3) deep block, occurs also in the east, and that the SDSB is indeed a full graben. The Ghor Safi fault is the northern extension of the major Arava strike-slip fault. The very thin Miocene (if at all) and lack of Pliocene salt in the eastern intermediate block, unlike the western one, suggests that the Ghor – Safi fault was initiated earlier then the Sedom fault. Salt is present over the entire width of the basin but does not exceed the 900 m that were penetrated by the Sedom Deep-1 borehole. The salt is conformable with the overlying thick Pleistocene and except for the Sedom diapir, no halokinesis phenomena have been recognized to be associated with both units. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Dead Sea; Miocene; Pliocene
1. Introduction The Dead Sea basin is located within the Dead Sea rift which is a transform type plate boundary separating the Arabian and Sinai plates and connecting the spreading zone of the Red Sea in the south to the Taurus collision in the north. The Dead Sea transform is generally attributed to a left lateral shear, which started in the Miocene, with a cumulative lateral dis*
Corresponding author. Tel.: +972-3-640-8528; fax: +972-3640-9282. E-mail address: [email protected] (Z. Ben-Avraham).
placement of 105 km (Quennell, 1958; Freund et al., 1970; Garfunkel, 1981) (Fig. 1). The basin is a large pull-apart of about 150 km long and about 20 km wide. It is composed of two main segments. The northern one is covered by a lake, while the southern is subareal. In the latter, two large salt diapirs have been formed. To the west is the Sedom diapir and to the north is the much larger Lisan diapir (Fig. 1). The basin, in particular the southern Dead Sea basin (SDSB), has been explored and investigated intensively over the last four decades. Numerous gravity, magnetic and seismic surveys were carried out on both sides of the Israeli – Jordanian border that dissects the
0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 1 ) 0 0 2 2 8 - 1
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A. Al-Zoubi et al. / Tectonophysics 346 (2002) 61 – 69
Fig. 1. (A) Outline of the Dead Sea – Jordan transform and the main strike-slip faults. The area of study, in the insert, is the Southern Dead Sea Basin (SDSB) (modified after Gardosh et al., 1997). (B) Seismic lines location, main tectonic elements and key exploration wells, superimposed on a portion of a Digital Shaded Relief Map of Israel and Environs, prepared by John K. Hall of the Geological Survey of Israel (1994).
A. Al-Zoubi et al. / Tectonophysics 346 (2002) 61 – 69
SDSB as part of oil exploration in the region. The geophysical data supplemented by boreholes provide valuable information about the complex architecture of the SDSB and the tectonic processes that formed it. However, the interpretation and synthesis of these data have been handicapped by the fact that all of the surveys on both sides had to be terminated short of the international border, and neither a tie between them has been established nor raw data from one side was made available to the counter research team. The recent exchange of seismic profiles between the two countries helps in bridging this gap of information. Two fair-to-good quality seismic timesections located close to the depocenter of the SDSB, one from each side of the border, have been selected, composed and interpreted (Fig. 1). Although they were acquired by different techniques, and although a gap of about 2 km exists between them, the basin fill reflections can be accurately carried across the gap. A review of the main tectonic elements of the SDSB and the basin fill stratigraphy—as reflected from the interpreted composite seismic line—is presented. Although outlined in previous publications, the current interpretation of the composite section confirm features that have only been assumed before.
2. Geophysical data 2.1. Seismic data acquisition Acquiring good-quality seismic data over the Southern Dead Sea basin is rather difficult because of various factors, such as (1) rough-terrain (badlands and the elevated Mt. Sedom diapir), (2) near-surface conditions (up to several hundreds of meters of unconsolidated gravel) and (3) man-made and industrial obstacles (such as the evaporation pans of the Dead Sea works). However, perhaps the most harmful factors in the regard to data quality is the narrow geometry of the SDS, which governs the direction and length of the lines. The average length of the cross-lines between the rim escarpment and the international border is about 7 –8 km on the Israeli side and 6 –7 km on the Jordanian side. These limited cables and geophones spreads are not sufficient to undershoot effectively the salt diapirs or to extract reliable stacking-velocities.
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Imaging the salt bodies on the time-sections is handicapped also by the fact that the measured interval velocity of the lower basin fill section ( ± 4500 m/s) is similar to that measured in the salt. The western line (A) was acquired by a 96-channel (48-fold) dynamite crew using a split-spread geometry. Group interval was 45 m, the near offset 450 m and far offset 2565 m. Five charges, 1.2 kg each, were buried at a depth of about 3 m or placed on the surface where drilling was prohibited or extremely difficult. The final version of the time section presented here has been migrated. The eastern line (B) was recorded by a 96-channel Vibroseis crew using an offend spread configuration, with a far offset of 2985 m. The 30-m geophones group interval and vibration points at 60-m intervals yielded a 24-fold section. A 6 –60-Hz sweep has been used for a length of 10 s. A migrated version of the line has not been made available to us. Both sections have been processed to a Datum of 395 m BMSL.
3. Interpretation of seismic data The geological interpretation of the various seismic events on line (A) is supported by reliable borehole information that include synthetic seismograms and velocity surveys (check-shots). The identification of the seismic reflections on line (B), however, relies mainly on correlation with line (A) and the characteristics of specific reflectors. Line (A) commences in the west on the rugged and highly faulted exposed Cretaceous carbonate rocks of the rim block, that is topographically elevated 100 – 300 m above from the Dead Sea level. The deepest coherent seismic event that can be reliably identified in this locality is the Cretaceous –Jurassic unconformity interface (Fig. 2). Further to the east, the line crosses a major normal vertical fault, which is part of a series of discontinuous subparallel faults with a large vertical throw. Referred to as the graben border-fault (Neev and Emery, 1967) or boundary fault (Kashai and Croker, 1987), they extend along the basin margin and delineate the rim block to the east. On the surface, they are recognized by a spectacular cliff escarpment (Fig. 1). On the seismic line, the Western Border – Fault separates coherent reflections of young sedimentary
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A. Al-Zoubi et al. / Tectonophysics 346 (2002) 61 – 69
fill in the east from less coherent Mesozoic and Paleozoic reflections in the west. In the down thrown side, known as the Intermediate Block (Kashai and Croker, 1987), the Upper Cretaceous rocks have been encountered by Amiaz-1 (Figs. 1 and 2) at 3300 m below the Dead Sea level under thick sediments of the basin fill, indicating that the border faults have been active from the early phase of basin formation through recent time, and that they are the main tectonic features along which the enormous subsidence of the Dead Sea basin took place (Kashai and Croker, 1987). The intermediate structural step extends along the entire western part of the SDSB. In the area where it is crossed by line (A), it is 4 km wide. The thick basin fill consists of three main units, as defined by Zak and Freund (1981). These units have been penetrated by Amiaz-1 and subsequently identified on line (A). (1) The basal unit is a fluviatile – lacustrian series of Miocene age consisting mainly of quartz sandstones derived mostly from distant sources south and east of the basin. The coherent strong amplitude reflections represent here 630 m of the Miocene sequence. (2) Overlying the Miocene are 1300 m of massive Pliocene salt. The Pliocene salt basin extended over the entire area currently occupied by the Dead Sea lake and reached about 15 km south of the Amazayahu fault (Fig. 1). At Mt. Sedom and Lisan peninsula, this salt emerges to the surface in large piercing diapirs. (3) The upper part of the basin fill that overlies the seismically chaotic appearance of the Pliocene salt, is the thickest series of sediments in the SDS. This sequence of marl, clay, sand, gravel and some evaporites of Pleistocene – Holocene age are over 4000 m in thickness and its top is exposed in the SDSB along most of the basin floor. Here, in the intermediate block, the 1700 m of this sequence that have been penetrated by Amiaz-1 appear as a series of high- and low-amplitude coherent reflections. The seismic data reveal also that the intermediate block, like the rim block, is internally deformed by sets of steep normal faults forming down-to-the-east step blocks. Further to the east, line (A) traverses the
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surface expression of Mt. Sedom diapir (Figs. 1 and 2). The diapir is an elongated body measuring 11 km (N– S) by 1– 1.5 km (E – W) and rising 250 m above the Dead Sea. The Sedom diapir exposes an approximately 2-km thick section consist of about 80% halite. The salt beds in the upper part of the diapir are steeply to vertically inclined and slightly deformed, mainly along the diapir borders. It is mostly covered by a 5 – 50-m thick anhydrite cap rock. The initiation of the complex Sedom salt body is related to faulting, increased overburden and subsequent upward flow along a major intrabasinal fault—the Sedom fault (Zak, 1997; Kashai and Croker, 1987; ten-Brink and BenAvraham, 1989). On the eastern flank of the Sedom diapir, almost the entire Pleistocene fill, from about 0.5 to 3.0 s is pushed upward, suggesting that in this area salt flowed continuously westward and upward from the deep basin throughout the Pleistocene (Gardosh et al., 1997). West of the Sedom diapir, in the intermediate block, the young sedimentary fill shows very little deformation. The Sedom Fault, beneath the diapir, is a subsurface normal fault separating the intermediate block from the Deep Sunken Block (Figs. 1 and 2). It is interpreted on line (A), where it separates coherent events at Miocene –cretaceous levels on the upthrown side from the chaotic reflections of the Sedom salt body. On the downthrown side, it separates Miocene continuous parallel reflections from the incoherent events of Triassic and Paleozoic age. The identification of these events on both sides of the Sedom fault and the fault location have been established by the Sedom deep-1 borehole (Fig. 1). While drilling through about 600 m of Miocene clastics of the downthrown side, the drill deviated to the west and penetrated at about 5500 m, Middle – Lower Triassic carbonate section of the upthrown side (the drill was subsequently directed to the east and penetrated additional 1000 m of Miocene). It should be noted here that the Sedom Deep-1 drilled some 5 km south of line (A) was projected into the line (Fig. 2) to a location that reflects the stratigraphy of the Deep Sunken Block
Fig. 2. Upper: Uninterpreted seismic time lines (A) and (B). Lower: Interpreted seismic lines (A) and (B) showing the main structural elements on both sides of the southern Dead Sea basin: Rim Blocks, Intermediate Blocks, Deep Sunken Block and the faults that separate them. Note that the Sedom Deep-1 is projected into line A to a position that fit the stratigraphy of the deep block (for actual location, see Fig. 1). Colour code: yellow, Pleistocene; purple, Pliocene salt; orange, Miocene; green, Cretaceous; blue, Jurassic.
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rather than its actual geographical and structural position (Fig. 1). The considerable increase in thickness of the Miocene across the Sedom fault from 650 m in Amiaz-1 to at least 1650 m in Sedom Deep-1 indicates syntectonic deposition and dates the initial activity of the fault to the Miocene. The Pliocene salt is also offset by the fault (Fig. 2) indicating that the Sedom fault was active through Lower Pleistocene time (Gardosh et al., 1997). A significant increase in thickness of the entire basin fill is interpreted on line (A), in the Deep Sunken Block. Its base, however, cannot be reliably recognized on any of the many seismic sections that cross it. The identification of the main basin fill units is confirmed by a tie to the Sedom Deep-1 (the Ghor Safi-1 (Figs. 1 and 2) was bottomed at 2783 m in Pleistocene). The 3870 m of Pleistocene that have been penetrated in Sedom Deep-1 appear on the seismic section as a series of subparallel high-and low-amplitude and frequencies reflections that extend to about 3.0 s. Beneath is a 0.4-s interval of chaotic appearance that corresponds with the 900 m of massive salt encountered by the drill. The underlying Miocene is characterized by high amplitude continuous reflections. Sedom Deep-1 drilled 1675 m below the salt and bottomed at 6445 m in what is estimated to be the lowermost part of the Miocene. Three distinguished high-amplitude events within the Miocene and the overlying salt interval can be carried across and tie into line (B) (Fig. 2). These Miocene events that appear to wedge out slightly to the east, and the salt sequence, come to an abrupt termination on the east indicating the presence of a fault. This fault, Ghor Safi, seems to be the eastern equivalent of the Sedom Fault that bounds an Intermediate Block to the west. A strong amplitude reflector at 0.8-s is picked as the top of the block. Stratigraphically, this event is assumed to be within the Upper Cretaceous and, therefore, the wedge-out of the Miocene on the downthrown block, marks the eastern edge of the Miocene basin. Nevertheless, the existence of some Miocene on top of the eastern intermediate block, cannot be ruled out. The absence of Pliocene salt and the thin section of Miocene, if at all, dates the initial activity of the eastern intrabasinal fault to the very early Miocene. The Ghor Safi Fault probably initiated before the Sedom Fault. When the Miocene clastics and the Pliocene salt were deposited,
the separation between the Deep Sunken Block and the Intermediate Block already existed. The eastern intermediate block is separated from the rim block by the eastern border fault. The fact that the line did not cross the surface expression of the fault but barely touched it, coupled by the rapid decrease in CDP fold as the line approached the surface escarpment makes the identification of the fault on the seismic profile rather difficult. It seems, however, that the abrupt change in frequencies and amplitudes on the right hand side of the section, between 0.4 and 2.0 s, marks the possible location of the border fault. The composite seismic line indicates clearly that the area between the western and eastern intermediate blocks is occupied mainly by thick Pleistocene sediments (over 4000 m) that exhibit excellent correlation across the entire deep sunken block. Except for a few minor listric faults, that can be attributed to movements resulting from differential overburden across the basin, no deformation or lateral facies change is noticed. The underlying Pliocene salt maintains uniform thickness throughout the entire deep block. It is a remnant of the original salt rock that occupied the entire SDS and was squeezed toward the west and north to form the Sedom and Lisan diapirs, respectively. At the locality of the composed line, this salt interval is conformable with the surrounding sediments and no swelling or any other halokinesis elements other than the Sedom diapir are evident. Based on Sedom Deep-1 and the seismic data, the base of the basin-fill in the western part of the deep sunken block is calculated to be 6 –7 km below the Dead Sea level. In the eastern part, the basin fill is deeper and is estimated to be at 7– 8 km below the Dead Sea level. The existence of all major structural steps on both sides of the SDS and the flat, undeformed horizontal strata of the sediments in fill indicate a full graben geometry, as suggested by ten-Brink and Ben-Avraham (1989) and ten-Brink et al. (1993).
4. Gravity A 30-km long gravity profile over the interpreted merged seismic lines and beyond them was constructed in an attempt to match the results obtained
A. Al-Zoubi et al. / Tectonophysics 346 (2002) 61 – 69
by both methods (Fig. 3). The profile was compiled from the Bouguer gravity map of the Dead Sea rift (ten-Brink et al., 1998) using a 2.5-dimensional modeling program. A Bouguer correction with a uniform reduction density of 2.67 g/cm3 was applied to the measured data. The model shown in Fig. 3 was constructed in stages using published estimates of the depth to different density interfaces from seismic, magnetic and deep drilling data. The upper part of the profile was constructed by applying density values derived from the interpreted seismic reflection profiles, that were tied to various boreholes, shown in Fig. 2. Density values for the deeper levels were derived from seismic refraction results obtained from Jordan and Isra-
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el (El-Isa et al., 1987; Ginzburg and Ben-Avraham, 1987, 1997; Hofstetter et al., 1991). After several iterations, the fit between the observed and calculated gravity values was found to be satisfactory and the standard deviation of the differences is about 2 mgal. Fig. 3 shows only the upper part of the crustal section. The gravity profile broadly supports the interpreted seismic full-graben model (Fig. 3). In the deep-sunken block, the depth to the basement complex, with a density of 2.7 g/cm3 is estimated to be at about 9500 m. The thickness of the overlying Paleozoic is about 1000 m with a density of 2.62 g/cm3. The Mesozoic reaches a thickness of about 1800 m with a density of 2.47 g/cm3. The Miocene sequence has a density of
Fig. 3. An observed and calculated gravity profiles over a geological model derived from the interpreted seismic sections (A) and (B).
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A. Al-Zoubi et al. / Tectonophysics 346 (2002) 61 – 69
2.37 g/cm3 and is about 2500 m thick and the overlying Plio-Pleistocene series are about 4000 m thick.
parallel, overlying Pleistocene strata show any significant deformation associated with halokinesis.
5. Concluding remarks
Acknowledgements
The foregoing interpretation of the composite seismic profile is generally in accordance with previously published geological models. However, some essential structural and stratigraphic elements, indicated on these models have been assumed in the past and were not based on direct geological –geophysical evidence. This has been especially the case in the less explored, eastern part of the SDSB. The current interpretation allows, for the first time, a direct tie of this part of the basin with the Sedom Deep-1 which is the sole reliable source of information as far as the deep block is concerned. As a result, an accurate identification of the major stratigraphic sequences could be made and the existence of an intermediate step block in the east could be established indicating that the SDS is a full graben. An eastern intermediate block is indicated on a geological cross-section constructed by Garfunkel (1997, Fig. 4.4D). However, in the very same paper he claims that ‘‘such a block is absent on the east.’’ He also concluded that the SDSB is asymmetric, with the deepest part extending along its east side. This conclusion is in agreement with a previous model by Zak and Freund (1981, Fig. 2a) that exhibit in fact a number of eastern step blocks with thicker basin-fill sediments in the east. The current interpretation confirm these observations, only that it indicates that the geometry of the sedimentary fill within the SDSB is that of a full graben. The current interpretation seems to resolve the controversy over the distribution and amount of rock salt present in the eastern part of the SDSB. Zak and Freund (1981) and Zak (1997) estimate the thickness of the Pliocene evaporites there, to be in the order of 4– 5 km and suggest that they are highly deformed. Garfunkel (1997), on the other hand, shows that most of the salt was squeezed to form Mt. Sedom and Lisan diapirs, and that only very thin salt interval, if at all, is present in the east. The seismic section clearly show that considerable sequence of bedded salt extends over the entire width of the SDS but do not exceed the 900 m penetrated by Sedom Deep-1. It also indicates that neither the salt nor the conformable, sub-
We are indebted to the Dead Sea Research Center at the University of Tel Aviv and the Natural Resources Authority of Jordan for supporting our cooperation; special thanks to the Director General of the NRA for his permission to carry out the study.
References El-Isa, Z., Mechie, J., Prodehl, C., Macris, J., Rihm, R., 1987. A crustal study of Jordan derived from seismic refraction data. Tectonophysics 138, 235 – 253. Freund, R., Garfunkel, Z., Zak, I., Goldberg, M., Weissbrod, T., Derin, B., 1970. The shear along the Dead Sea Rift. Philosophical Transaction of the Royal Society, London, Series A 267, 107 – 130. Gardosh, M., Kashai, E., Salhov, S., Shulman, H., Tannenbaum, E., 1997. Hydrocarbon exploration in the southern Dead Sea area. In: Niemi, T.M., Ben-Avraham, Z., Gat, J.R. (Eds.), The Dead Sea, the Lake and its Setting. Oxford Monographs on Geology and Geophysics, vol. 36, pp. 57 – 72. Garfunkel, Z., 1981. Internal structure of the Dead Sea leaky transform (rift) in relation to plate tectonics. Tectonophysics 80, 81 – 108. Garfunkel, Z., 1997. The history and formation of the Dead Sea basin. In: Niemi, T.M., Ben-Avraham, Z., Gat, J.R. (Eds.), The Dead Sea, the Lake and its Setting. Oxford Monographs on Geology and Geophysics, vol. 36, pp. 36 – 56. Ginzburg, A., Ben-Avraham, Z., 1987. The deep structure of the central and southern levant continental margin. Annales Tectonicae 1, 105 – 115. Ginzburg, A., Ben-Avraham, Z., 1997. A seismic refraction study of the north basin of the Dead Sea. Geophysical Research Letters 24, 2063 – 2066. Hofstetter, A., Feldman, L., Rotstein, Y., 1991. Crustal structure of Israel, constraints from teleseismic and gravity data. Geophysical Journal International 104, 371 – 379. Kashai, E.L., Croker, P.F., 1987. Structural geometry and evolution of the Dead Sea—Jordan rift system from new subsurface data. Tectonophysics 141, 36 – 60. Neev, D., Emery, K.O., 1967. The Dead Sea depositional processes and environments of evaporites, Jerusalem. Geological Survey of Israel Bulletin 41, 147. Quennell, A.M., 1958. The structure and geomorphic evolution of the Dead Sea rift. The Quarterly Journal of the Geological Society of London 64, 1 – 24. ten-Brink, U., Ben-Avraham, Z., 1989. The anatomy of a pull-apart basin; seismic reflection observations of the Dead Sea basin. Tectonics 8, 330 – 350.
A. Al-Zoubi et al. / Tectonophysics 346 (2002) 61 – 69 ten-Brink, U., Ben-Avraham, Z., Bell, R.E., Hassounah, M., Coleman, D.F., Andreason, G., Tibor, G., Coakley, B., 1993. Structure of the Dead Sea pull-apart basin from gravity analysis. Journal of Geophysical Research 98, 21887 – 21894. ten-Brink, U., Rybakov, M., Al-Zoubi, A., Hassounah, M., Batayneh, A., Frieslander, U., Goldshmidt, V., Daoud, M., Rotstein, Y., 1998. Bouguer gravity anomaly map of the Dead Sea transform plate boundary in Israel and Jordan, Scale 1:250,000, USGS Open File Report 98 – 516.
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Zak, I., 1997. Evolution of the Dead Sea brines. In: Niemi, T.M., Ben-Avraham, Z., Gat, J.R. (Eds.), The Dead Sea, the Lake and its Setting. Oxford Monographs on Geology and Geophysics, vol. 36, pp. 57 – 72. Zak, I., Freund, R., 1981. Asymmetry and basin migration in the Dead Sea rift. Tectonophysics 80, 27 – 38.
Seismic ref lection profiles across the southern Dead Sea basin Abdallah Al-Zoubi a, Haim Shulman b, Zvi Ben-Avraham b,* b
a Natural Resources Authority, P.O. Box 7, Amman, Jordan Department of Geophysics and Planetary Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel
Abstract The main structural and stratigraphic elements that form the Southern Dead Sea basin (SDSB), which is part of the Dead Sea transform fault system, have been broadly delineated with various degrees of certainty by a number of researchers. The recent exchange of seismic data between Israel and Jordan enable us to fill in some of the dashed lines and eliminate some of the question marks that appear on the previously published geological cross-sections. Two east – west seismic time sections, one from each side of the international border that dissect the basin, have been selected, composed and interpreted. A remarkable correlation of the three major sedimentary sequences that form the basin-fill (Miocene clastics, Pliocene salt and Pleistocene – Holocene clastics and evaporites) has been established between the lines and, hence, across the entire width of the SDSB. The Miocene and Pliocene series are cut off in the east by the Ghor – Safi fault that is the equivalent of the western Sedom fault, thus, indicating that all three major structural steps, known to exist in the west: (1) rim block, (2) intermediate block and (3) deep block, occurs also in the east, and that the SDSB is indeed a full graben. The Ghor Safi fault is the northern extension of the major Arava strike-slip fault. The very thin Miocene (if at all) and lack of Pliocene salt in the eastern intermediate block, unlike the western one, suggests that the Ghor – Safi fault was initiated earlier then the Sedom fault. Salt is present over the entire width of the basin but does not exceed the 900 m that were penetrated by the Sedom Deep-1 borehole. The salt is conformable with the overlying thick Pleistocene and except for the Sedom diapir, no halokinesis phenomena have been recognized to be associated with both units. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Dead Sea; Miocene; Pliocene
1. Introduction The Dead Sea basin is located within the Dead Sea rift which is a transform type plate boundary separating the Arabian and Sinai plates and connecting the spreading zone of the Red Sea in the south to the Taurus collision in the north. The Dead Sea transform is generally attributed to a left lateral shear, which started in the Miocene, with a cumulative lateral dis*
Corresponding author. Tel.: +972-3-640-8528; fax: +972-3640-9282. E-mail address: [email protected] (Z. Ben-Avraham).
placement of 105 km (Quennell, 1958; Freund et al., 1970; Garfunkel, 1981) (Fig. 1). The basin is a large pull-apart of about 150 km long and about 20 km wide. It is composed of two main segments. The northern one is covered by a lake, while the southern is subareal. In the latter, two large salt diapirs have been formed. To the west is the Sedom diapir and to the north is the much larger Lisan diapir (Fig. 1). The basin, in particular the southern Dead Sea basin (SDSB), has been explored and investigated intensively over the last four decades. Numerous gravity, magnetic and seismic surveys were carried out on both sides of the Israeli – Jordanian border that dissects the
0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 1 ) 0 0 2 2 8 - 1
62
A. Al-Zoubi et al. / Tectonophysics 346 (2002) 61 – 69
Fig. 1. (A) Outline of the Dead Sea – Jordan transform and the main strike-slip faults. The area of study, in the insert, is the Southern Dead Sea Basin (SDSB) (modified after Gardosh et al., 1997). (B) Seismic lines location, main tectonic elements and key exploration wells, superimposed on a portion of a Digital Shaded Relief Map of Israel and Environs, prepared by John K. Hall of the Geological Survey of Israel (1994).
A. Al-Zoubi et al. / Tectonophysics 346 (2002) 61 – 69
SDSB as part of oil exploration in the region. The geophysical data supplemented by boreholes provide valuable information about the complex architecture of the SDSB and the tectonic processes that formed it. However, the interpretation and synthesis of these data have been handicapped by the fact that all of the surveys on both sides had to be terminated short of the international border, and neither a tie between them has been established nor raw data from one side was made available to the counter research team. The recent exchange of seismic profiles between the two countries helps in bridging this gap of information. Two fair-to-good quality seismic timesections located close to the depocenter of the SDSB, one from each side of the border, have been selected, composed and interpreted (Fig. 1). Although they were acquired by different techniques, and although a gap of about 2 km exists between them, the basin fill reflections can be accurately carried across the gap. A review of the main tectonic elements of the SDSB and the basin fill stratigraphy—as reflected from the interpreted composite seismic line—is presented. Although outlined in previous publications, the current interpretation of the composite section confirm features that have only been assumed before.
2. Geophysical data 2.1. Seismic data acquisition Acquiring good-quality seismic data over the Southern Dead Sea basin is rather difficult because of various factors, such as (1) rough-terrain (badlands and the elevated Mt. Sedom diapir), (2) near-surface conditions (up to several hundreds of meters of unconsolidated gravel) and (3) man-made and industrial obstacles (such as the evaporation pans of the Dead Sea works). However, perhaps the most harmful factors in the regard to data quality is the narrow geometry of the SDS, which governs the direction and length of the lines. The average length of the cross-lines between the rim escarpment and the international border is about 7 –8 km on the Israeli side and 6 –7 km on the Jordanian side. These limited cables and geophones spreads are not sufficient to undershoot effectively the salt diapirs or to extract reliable stacking-velocities.
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Imaging the salt bodies on the time-sections is handicapped also by the fact that the measured interval velocity of the lower basin fill section ( ± 4500 m/s) is similar to that measured in the salt. The western line (A) was acquired by a 96-channel (48-fold) dynamite crew using a split-spread geometry. Group interval was 45 m, the near offset 450 m and far offset 2565 m. Five charges, 1.2 kg each, were buried at a depth of about 3 m or placed on the surface where drilling was prohibited or extremely difficult. The final version of the time section presented here has been migrated. The eastern line (B) was recorded by a 96-channel Vibroseis crew using an offend spread configuration, with a far offset of 2985 m. The 30-m geophones group interval and vibration points at 60-m intervals yielded a 24-fold section. A 6 –60-Hz sweep has been used for a length of 10 s. A migrated version of the line has not been made available to us. Both sections have been processed to a Datum of 395 m BMSL.
3. Interpretation of seismic data The geological interpretation of the various seismic events on line (A) is supported by reliable borehole information that include synthetic seismograms and velocity surveys (check-shots). The identification of the seismic reflections on line (B), however, relies mainly on correlation with line (A) and the characteristics of specific reflectors. Line (A) commences in the west on the rugged and highly faulted exposed Cretaceous carbonate rocks of the rim block, that is topographically elevated 100 – 300 m above from the Dead Sea level. The deepest coherent seismic event that can be reliably identified in this locality is the Cretaceous –Jurassic unconformity interface (Fig. 2). Further to the east, the line crosses a major normal vertical fault, which is part of a series of discontinuous subparallel faults with a large vertical throw. Referred to as the graben border-fault (Neev and Emery, 1967) or boundary fault (Kashai and Croker, 1987), they extend along the basin margin and delineate the rim block to the east. On the surface, they are recognized by a spectacular cliff escarpment (Fig. 1). On the seismic line, the Western Border – Fault separates coherent reflections of young sedimentary
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fill in the east from less coherent Mesozoic and Paleozoic reflections in the west. In the down thrown side, known as the Intermediate Block (Kashai and Croker, 1987), the Upper Cretaceous rocks have been encountered by Amiaz-1 (Figs. 1 and 2) at 3300 m below the Dead Sea level under thick sediments of the basin fill, indicating that the border faults have been active from the early phase of basin formation through recent time, and that they are the main tectonic features along which the enormous subsidence of the Dead Sea basin took place (Kashai and Croker, 1987). The intermediate structural step extends along the entire western part of the SDSB. In the area where it is crossed by line (A), it is 4 km wide. The thick basin fill consists of three main units, as defined by Zak and Freund (1981). These units have been penetrated by Amiaz-1 and subsequently identified on line (A). (1) The basal unit is a fluviatile – lacustrian series of Miocene age consisting mainly of quartz sandstones derived mostly from distant sources south and east of the basin. The coherent strong amplitude reflections represent here 630 m of the Miocene sequence. (2) Overlying the Miocene are 1300 m of massive Pliocene salt. The Pliocene salt basin extended over the entire area currently occupied by the Dead Sea lake and reached about 15 km south of the Amazayahu fault (Fig. 1). At Mt. Sedom and Lisan peninsula, this salt emerges to the surface in large piercing diapirs. (3) The upper part of the basin fill that overlies the seismically chaotic appearance of the Pliocene salt, is the thickest series of sediments in the SDS. This sequence of marl, clay, sand, gravel and some evaporites of Pleistocene – Holocene age are over 4000 m in thickness and its top is exposed in the SDSB along most of the basin floor. Here, in the intermediate block, the 1700 m of this sequence that have been penetrated by Amiaz-1 appear as a series of high- and low-amplitude coherent reflections. The seismic data reveal also that the intermediate block, like the rim block, is internally deformed by sets of steep normal faults forming down-to-the-east step blocks. Further to the east, line (A) traverses the
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surface expression of Mt. Sedom diapir (Figs. 1 and 2). The diapir is an elongated body measuring 11 km (N– S) by 1– 1.5 km (E – W) and rising 250 m above the Dead Sea. The Sedom diapir exposes an approximately 2-km thick section consist of about 80% halite. The salt beds in the upper part of the diapir are steeply to vertically inclined and slightly deformed, mainly along the diapir borders. It is mostly covered by a 5 – 50-m thick anhydrite cap rock. The initiation of the complex Sedom salt body is related to faulting, increased overburden and subsequent upward flow along a major intrabasinal fault—the Sedom fault (Zak, 1997; Kashai and Croker, 1987; ten-Brink and BenAvraham, 1989). On the eastern flank of the Sedom diapir, almost the entire Pleistocene fill, from about 0.5 to 3.0 s is pushed upward, suggesting that in this area salt flowed continuously westward and upward from the deep basin throughout the Pleistocene (Gardosh et al., 1997). West of the Sedom diapir, in the intermediate block, the young sedimentary fill shows very little deformation. The Sedom Fault, beneath the diapir, is a subsurface normal fault separating the intermediate block from the Deep Sunken Block (Figs. 1 and 2). It is interpreted on line (A), where it separates coherent events at Miocene –cretaceous levels on the upthrown side from the chaotic reflections of the Sedom salt body. On the downthrown side, it separates Miocene continuous parallel reflections from the incoherent events of Triassic and Paleozoic age. The identification of these events on both sides of the Sedom fault and the fault location have been established by the Sedom deep-1 borehole (Fig. 1). While drilling through about 600 m of Miocene clastics of the downthrown side, the drill deviated to the west and penetrated at about 5500 m, Middle – Lower Triassic carbonate section of the upthrown side (the drill was subsequently directed to the east and penetrated additional 1000 m of Miocene). It should be noted here that the Sedom Deep-1 drilled some 5 km south of line (A) was projected into the line (Fig. 2) to a location that reflects the stratigraphy of the Deep Sunken Block
Fig. 2. Upper: Uninterpreted seismic time lines (A) and (B). Lower: Interpreted seismic lines (A) and (B) showing the main structural elements on both sides of the southern Dead Sea basin: Rim Blocks, Intermediate Blocks, Deep Sunken Block and the faults that separate them. Note that the Sedom Deep-1 is projected into line A to a position that fit the stratigraphy of the deep block (for actual location, see Fig. 1). Colour code: yellow, Pleistocene; purple, Pliocene salt; orange, Miocene; green, Cretaceous; blue, Jurassic.
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rather than its actual geographical and structural position (Fig. 1). The considerable increase in thickness of the Miocene across the Sedom fault from 650 m in Amiaz-1 to at least 1650 m in Sedom Deep-1 indicates syntectonic deposition and dates the initial activity of the fault to the Miocene. The Pliocene salt is also offset by the fault (Fig. 2) indicating that the Sedom fault was active through Lower Pleistocene time (Gardosh et al., 1997). A significant increase in thickness of the entire basin fill is interpreted on line (A), in the Deep Sunken Block. Its base, however, cannot be reliably recognized on any of the many seismic sections that cross it. The identification of the main basin fill units is confirmed by a tie to the Sedom Deep-1 (the Ghor Safi-1 (Figs. 1 and 2) was bottomed at 2783 m in Pleistocene). The 3870 m of Pleistocene that have been penetrated in Sedom Deep-1 appear on the seismic section as a series of subparallel high-and low-amplitude and frequencies reflections that extend to about 3.0 s. Beneath is a 0.4-s interval of chaotic appearance that corresponds with the 900 m of massive salt encountered by the drill. The underlying Miocene is characterized by high amplitude continuous reflections. Sedom Deep-1 drilled 1675 m below the salt and bottomed at 6445 m in what is estimated to be the lowermost part of the Miocene. Three distinguished high-amplitude events within the Miocene and the overlying salt interval can be carried across and tie into line (B) (Fig. 2). These Miocene events that appear to wedge out slightly to the east, and the salt sequence, come to an abrupt termination on the east indicating the presence of a fault. This fault, Ghor Safi, seems to be the eastern equivalent of the Sedom Fault that bounds an Intermediate Block to the west. A strong amplitude reflector at 0.8-s is picked as the top of the block. Stratigraphically, this event is assumed to be within the Upper Cretaceous and, therefore, the wedge-out of the Miocene on the downthrown block, marks the eastern edge of the Miocene basin. Nevertheless, the existence of some Miocene on top of the eastern intermediate block, cannot be ruled out. The absence of Pliocene salt and the thin section of Miocene, if at all, dates the initial activity of the eastern intrabasinal fault to the very early Miocene. The Ghor Safi Fault probably initiated before the Sedom Fault. When the Miocene clastics and the Pliocene salt were deposited,
the separation between the Deep Sunken Block and the Intermediate Block already existed. The eastern intermediate block is separated from the rim block by the eastern border fault. The fact that the line did not cross the surface expression of the fault but barely touched it, coupled by the rapid decrease in CDP fold as the line approached the surface escarpment makes the identification of the fault on the seismic profile rather difficult. It seems, however, that the abrupt change in frequencies and amplitudes on the right hand side of the section, between 0.4 and 2.0 s, marks the possible location of the border fault. The composite seismic line indicates clearly that the area between the western and eastern intermediate blocks is occupied mainly by thick Pleistocene sediments (over 4000 m) that exhibit excellent correlation across the entire deep sunken block. Except for a few minor listric faults, that can be attributed to movements resulting from differential overburden across the basin, no deformation or lateral facies change is noticed. The underlying Pliocene salt maintains uniform thickness throughout the entire deep block. It is a remnant of the original salt rock that occupied the entire SDS and was squeezed toward the west and north to form the Sedom and Lisan diapirs, respectively. At the locality of the composed line, this salt interval is conformable with the surrounding sediments and no swelling or any other halokinesis elements other than the Sedom diapir are evident. Based on Sedom Deep-1 and the seismic data, the base of the basin-fill in the western part of the deep sunken block is calculated to be 6 –7 km below the Dead Sea level. In the eastern part, the basin fill is deeper and is estimated to be at 7– 8 km below the Dead Sea level. The existence of all major structural steps on both sides of the SDS and the flat, undeformed horizontal strata of the sediments in fill indicate a full graben geometry, as suggested by ten-Brink and Ben-Avraham (1989) and ten-Brink et al. (1993).
4. Gravity A 30-km long gravity profile over the interpreted merged seismic lines and beyond them was constructed in an attempt to match the results obtained
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by both methods (Fig. 3). The profile was compiled from the Bouguer gravity map of the Dead Sea rift (ten-Brink et al., 1998) using a 2.5-dimensional modeling program. A Bouguer correction with a uniform reduction density of 2.67 g/cm3 was applied to the measured data. The model shown in Fig. 3 was constructed in stages using published estimates of the depth to different density interfaces from seismic, magnetic and deep drilling data. The upper part of the profile was constructed by applying density values derived from the interpreted seismic reflection profiles, that were tied to various boreholes, shown in Fig. 2. Density values for the deeper levels were derived from seismic refraction results obtained from Jordan and Isra-
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el (El-Isa et al., 1987; Ginzburg and Ben-Avraham, 1987, 1997; Hofstetter et al., 1991). After several iterations, the fit between the observed and calculated gravity values was found to be satisfactory and the standard deviation of the differences is about 2 mgal. Fig. 3 shows only the upper part of the crustal section. The gravity profile broadly supports the interpreted seismic full-graben model (Fig. 3). In the deep-sunken block, the depth to the basement complex, with a density of 2.7 g/cm3 is estimated to be at about 9500 m. The thickness of the overlying Paleozoic is about 1000 m with a density of 2.62 g/cm3. The Mesozoic reaches a thickness of about 1800 m with a density of 2.47 g/cm3. The Miocene sequence has a density of
Fig. 3. An observed and calculated gravity profiles over a geological model derived from the interpreted seismic sections (A) and (B).
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2.37 g/cm3 and is about 2500 m thick and the overlying Plio-Pleistocene series are about 4000 m thick.
parallel, overlying Pleistocene strata show any significant deformation associated with halokinesis.
5. Concluding remarks
Acknowledgements
The foregoing interpretation of the composite seismic profile is generally in accordance with previously published geological models. However, some essential structural and stratigraphic elements, indicated on these models have been assumed in the past and were not based on direct geological –geophysical evidence. This has been especially the case in the less explored, eastern part of the SDSB. The current interpretation allows, for the first time, a direct tie of this part of the basin with the Sedom Deep-1 which is the sole reliable source of information as far as the deep block is concerned. As a result, an accurate identification of the major stratigraphic sequences could be made and the existence of an intermediate step block in the east could be established indicating that the SDS is a full graben. An eastern intermediate block is indicated on a geological cross-section constructed by Garfunkel (1997, Fig. 4.4D). However, in the very same paper he claims that ‘‘such a block is absent on the east.’’ He also concluded that the SDSB is asymmetric, with the deepest part extending along its east side. This conclusion is in agreement with a previous model by Zak and Freund (1981, Fig. 2a) that exhibit in fact a number of eastern step blocks with thicker basin-fill sediments in the east. The current interpretation confirm these observations, only that it indicates that the geometry of the sedimentary fill within the SDSB is that of a full graben. The current interpretation seems to resolve the controversy over the distribution and amount of rock salt present in the eastern part of the SDSB. Zak and Freund (1981) and Zak (1997) estimate the thickness of the Pliocene evaporites there, to be in the order of 4– 5 km and suggest that they are highly deformed. Garfunkel (1997), on the other hand, shows that most of the salt was squeezed to form Mt. Sedom and Lisan diapirs, and that only very thin salt interval, if at all, is present in the east. The seismic section clearly show that considerable sequence of bedded salt extends over the entire width of the SDS but do not exceed the 900 m penetrated by Sedom Deep-1. It also indicates that neither the salt nor the conformable, sub-
We are indebted to the Dead Sea Research Center at the University of Tel Aviv and the Natural Resources Authority of Jordan for supporting our cooperation; special thanks to the Director General of the NRA for his permission to carry out the study.
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Zak, I., 1997. Evolution of the Dead Sea brines. In: Niemi, T.M., Ben-Avraham, Z., Gat, J.R. (Eds.), The Dead Sea, the Lake and its Setting. Oxford Monographs on Geology and Geophysics, vol. 36, pp. 57 – 72. Zak, I., Freund, R., 1981. Asymmetry and basin migration in the Dead Sea rift. Tectonophysics 80, 27 – 38.