NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
TECTONIC INTERPRETATION OF GRAVITY FIELD DATA OF WEST OF NILE DELTA REGION, EGYPT By: Ahmed A. ElGalladi , Hosni H. Ghazala, Ismael M. Ibraheem(*) and ElKhedr H. Ibrahim
Department of Geology, Fac. of Science, Mansoura Univ., Mansoura 35116, Egypt. (*) E-mail:
[email protected] ABSTRACT
The area of study is located to the west of Nile Delta on both sides of the CairoAlexandria desert road, between latitudes 30° 00′ and 31° 00′ N, and longitudes 29° 40′ and 30° 40′ E. The available Bouguer gravity data of the study area were subjected to critical interpretation through the application of gravity filters, Euler deconvolution and modeling techniques. Gravity filters include spectral analysis, separation of regional and residual anomalies, and vertical derivatives. Power spectrum results show that the average depth to the granitic layer is about 5.5 km and is about 13 km to the basaltic layer of the earth's crust in this area. Modeling results show that the depths of basement surface range from 2 km to 6.5 km from the ground surface. The dominant fault trends in the study area are NW-SE, NE-SW and E-W. Structural-tectonic map was constructed taking into consideration all deduced and available information. The results of this study may be considered as basic information to understand the deep-seated basement structures that may affect the distribution of the relatively shallower groundwater aquifers in the study area. INTRODUCTION The area of study (Fig. 1) is located to the west of Nile Delta on both sides of the Cairo-Alexandria desert road, between latitudes 30° 00′ and 31° 00′ N, and longitudes 29° 40′ and 30° 40′ E. El Nubariya, Wadi El Natrun, El Sadat El Tahrir, and El Bustan are the main cities in the study area. The available Bouguer gravity map, scale 1: 100,000, of the study area were compiled and conducted in 1977 by the General Petroleum Company. The main target of this study is to emphasize the structural configuration of the basement rocks that affecting on the overlying sediments. The applied filters include spectral analysis, derivatives and separation of regional and residual components using lowpass and bandpass filters. The application of these filters was done in the wave number domain using MAGMAP software package (Geosoft, 1999). Euler deconvolution method was used for estimating depths and contact locations of the faults. The results of filtering and Euler deconvolution in addition to the available -1-
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
borehole data in and around the study area were used to construct the initial models used in 2D forward modeling that has been performed along selected gravity profiles using the GM-SYS modeling package that established by NGA (1999). The results of applied filters in addition to 2D-forward modeling and all previous information were used to construct a detailed basement tectonic map to the study area. The results of this study can be considered as basic information to understand the effect of the deepseated structures on the groundwater distributions in the study area. Geomorphology, Geology and Structural Setting The study area is characterized by low relief and mild topography with elevations varying from -23 m below MSL (at Wadi El Natrun depression) to +233 m above MSL (at Qaret El Haddadein).
Fig. (1): Location map of the study area. Generally, the investigated area slopes gently towards the north and east directions. The geomorphologic studies of the area west of the Nile Delta were dealt by many authors as Said (1962), El Fayoumi (1964), Shata and El Fayoumi (1967), Abu El-Izz (1971), Sanad (1973), El Shazly et al. (1975), El Ghazawi (1982), and Embaby (1995 and 2003). Most of those Authors classified the western fringe of the Nile Delta into six significant landforms. These units are arranged from north to south as follows: 1- The coastal plains, 2- The northern tableland, 3- The alluvial plains, 4The structural plain, 5- The southern tableland, and 6- The shifting sand. -2-
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
The surface geology of the study area (Fig. 2) consists of Cenozoic sediments. Tertiary sediments (Oligocene, Miocene and Pliocene) are generally dominating the southern and southwestern parts of the study area. They composed of sand and sandstone with clay and limestone intercalations. The Oligocene basalt sheets are the only exposed volcanic rocks to the southeast of the area. The Quaternary sediments are governing the northern part of the study area; they are mainly clastic with essential sand facies and occasional gravel and clay intercalations. Sand dunes are also detected towards the southern part of the area. To the southeast of the study area, Cretaceous and Eocene sediments are the oldest exposed rocks at Abu Roash area. In the subsurface, the sedimentary rocks overlying the basement complex have a thickness of about 4000 m, as recorded from Wadi El Natrun-1 test well. The sedimentary sequence starts from base by Triassic rocks that lies unconformably on the basement rocks and ends at top with Quaternary sediments. The northern Egypt represents the northeast African passive continental margin of African craton. Structurally, Egypt can be divided into Nubian-Arabian Shield and shelf area. The Shelf area is divided into three units: the Stable Shelf, the Gulf of Suez taphrogeosyncline, and the Unstable Shelf. The Unstable Shelf, which includes the study area, is highly deformed and covers a large part of Egypt (Said, 1962).
Fig. (2): Geological map of west Nile Delta (after CONOCO, 1987). -3-
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
Meshref (1982) suggested that, Northern Egypt was affected by three tectonic events. The oldest resulted in NW-SE or WNW-ESE structural trend, which was followed by another event that resulted in ENE -WSW (Syrian arc) structural trend. The third tectonic event resulted in the E-W, NW-SE (Suez) and NNE-SSW (Aqaba) structural trend. The study area is located within the unstable shelf of northern parts of the Western Desert so it is affected by two regional structural elements: the taphrogeosynclinal trough of the Nile Delta due to east and the northward dipping homoclinal slope of the Western Desert (Shata and El Fayoumy, 1970). Data Processing and Interpretation The Bouguer contour map was digitized then Forward Fast Fourier Transform (FFT) was applied on the data using Geosoft Package (1999) to explore the frequency content of these data and apply the gravity filters in the wave number domain. Inverse FFT is then applied to the resultant data to map the results of filters in time domain. The power spectral analysis was applied to delineate the regional and the local components of the Bouguer anomaly. 2D forward modeling is then performed on four preselected profiles cover the study area using the GM-SYS modeling package. All deduced and available information were employed in the initial model. The gravity contour map (Fig. 3) is characterized by the presence of five main gravity belts of varying shapes and amplitudes, these gravity belts are formed of a number of gravity anomaly closures of variable areal extension, directions, and shapes. The central gravity high belt coincided with Wadi El Natrun anticline is composed of a complete elongated anomaly oriented nearly NW-SE with a maximum gravity value of -9 mGal. The northeastern gravity low belt is incomplete belt trending in NW-SE direction with a minimum gravity value of -34 mGal. The northwestern gravity high belt is incomplete gravity high belt with a maximum gravity value of -4 mGal. The western gravity low belt is trending in NE-SW direction with a minimum gravity value of -21 mGal. Finally, the southern gravity high belt is oriented nearly in E-W direction with a maximum gravity value of +3 mGal. It composes of two complete anomalies and two incomplete anomalies. Spectral Analysis Spectral analysis technique was explained by several authors (Bhattacharyya, 1966, Spector and Grant, 1970, Cassano and Rocca, 1975, Gerard and Debeglia, 1975, Hahn et al, 1976, Bhattacharyya, 1978, Fedi et al, 1997, and others) for depths of magnetic or gravity anomalies. Energy spectral analysis provides a technique for quantitative studies of large and complex aeromagnetic or gravity data sets. The logarithm of the radial average of the energy spectrum (the square of the Fourier amplitude spectrum) is plotted versus the radial frequency. The slopes of the linear segments of the spectrum correspond to separate depth ensembles and provide -4-
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
parameters used for the design of numerous filters (Kivior and Boyd, 1998). Spector and Grant (1970) have developed a depth determination method that conforms twodimensional power spectrum calculated from gridded field data to spectrum values obtained from theoretical model. The radially averaged power spectrum of the gravity field data was computed (Fig. 4) and best-fit straight lines were drawn on the spectra. Three best fit segments are obtained representing deep (Reg. 1), intermediate (Reg. 2) and shallow (Res.) sources. The (slop/4π) of each segment gives the average depth of gravity sources correspondent to each segment. Depth computations performed on these segments show that the average depth values are 12.6, 5.5 and 2.2 km from ground level for deep, intermediate and shallow gravitational sources respectively. Regional-residual separation The Bouguer anomaly of West Nile Delta was separated into residual and regional gravitational anomalies according to the results of the power spectral analyses using lowpass and bandpass filtering techniques. Lowpass filtered gravity contour map (k≤ 0.04): This map (Fig. 5) reflects the verydeep seated causative sources. It is obtained by applying lowpass filter on the gridded gravity data where the wave number (k) ≤ 0.04 km-1 (wavelength ≥ 157 km). This regional map is similar to the Bouguer anomaly map in the anomaly trend. The discrepancies between this map and the Bouguer map may be observed in the absence of the local anomalies. Lowpass filtered gravity contour map (k≤ 0.125): Lowpass filter is applied on the gravity data to get the deep causative sources where k≤ 0.125 km-1 (wavelength ≥ 50 km). The appearance of the contours of this regional map (Fig. 6) shows close agreement with the Bouguer gravity map but it reveals much smoother contours than the Bouguer map. Bandpass filtered gravity contour map (0.04 ≤ k ≤ 0.125): Intermediate causative bodies producing gravity anomalies are described in this residual map (Fig. 7). The anomalies are trending in NE-SW, NW-SE, and E-W directions. Most of the anomalies have semicircular, elongated and open shapes with gravity values between 3.2 and +3.2 mGal. Bandpass filtered gravity contour map (0.125 ≤ k ≤ 0.86): A high frequency band lies between 0.125 and 0.86 km-1 representing the shallow (residual) gravity sources (Fig. 8). Vertical Derivatives Vertical derivatives enhance the shorter wavelength anomalies relative to longer wavelength anomalies. Unfortunately, noises could be amplified within the data. In the present study, a lowpass filter with a wave number equals 0.86 km-1 is applied to -5-
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
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Fig. (3): Bouguer gravity map of the study area.
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Fig. (4): 2-D radially averaged power spectrum showing average depths of the gravity sources, west Nile Delta, Egypt. -6-
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
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Fig. (6): Lowpass filtered gravity map (k≤ 0.125) -7-
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
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Fig. (8): Bandpass filtered gravity map (0.125 ≤ k ≤ 0.86) -8-
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
remove the effect of noise. First and second vertical derivatives are then calculated, Fig. 9 and 10 respectively. In first derivative map (Fig. 9), the main features in the Bouguer anomaly map are clearly identified and local anomalies became more prominent than that of Bouguer map to correspond shallower depths. In Fig. 10, which represents the SVD, it was noticed that the zero contour line is separating the main deduced features in the study area. Euler deconvolution Euler deconvolution has been applied to estimate depth and location of the gravity source anomalies. The 3D equation of Euler deconvolution by Reid et al. (1990) is given as:
Where (x0, y0, z0) is the position of a source whose total gravity (g) is detected at (x, y, z), β is the regional value of the gravity, and N is the structural index (SI) which can be defined as the rate of attenuation of the anomaly with distance. The structural index (SI) must be chosen according to prior knowledge of the source geometry. For example, SI=2 for a sphere, SI=1 for a horizontal cylinder, and SI=0 for a fault (FitzGerald et al., 2004). Reid et al. (1990 and 2003) and Reid (2003) presented a structural index equal to zero for the gravity field for detecting faults. The horizontal (∂g/∂x, ∂g/∂y) and vertical (∂g/∂z) derivatives are used to compute anomalous source locations. In the present study, Euler Deconvolution solution was applied on the gravity data where the depths, location and trends of the faults in the area of study were determined (Fig. 11). The NW-SE, NE-SW and E-W trends characterize the structure setting of the study area. The estimated depth ranges of the interpreted structural elements are obtained as shown in this figure. 2D-Modeling of gravity data In constructing the initial geological model to be used in 2D forward modeling of gravity data; many input criteria should be taken into consideration. Such criteria essentially include; the prevailing structure in the study area according to inferred and previous geological information, densities of different expected lithologic units and the expected depths of each unit especially basement rocks in the study area. 2D forward modeling has been performed on four preselected profiles (Fig. 12), passing through Wadi El Natrun-1 Borehole, using GM-SYS modeling package; which was established by NGA (1999). The methods used to calculate the gravity model response is based on the methods of Talwani et al. (1959), and Talwani and Heirtzler (1964), and make use of the algorithms described in Won and Bevis (1987). GM-SYS uses a two-dimensional, flat-earth model for the gravity calculations; that is, each structural unit or block extends to plus and minus infinity in the direction perpendicular to the profile. -9-
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
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Figure (9): First Vertical Derivative (FVD) of the gravity field data of the study area. 29°40'
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Figure (10): SVD map of the gravity field data of the study area. (contour 0 is shown in the map) - 11 -
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009) 29 40' 31 00'
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Figure (11): Euler deconvolution solution of the gravity data, west Nile Delta, Egypt (S.I= 0). According to Said (1962), the crustal thickness in the Western Desert is about 34 km. Makris et al. (1979) calculated the crustal thickness along two seismic profiles; the crustal thickness obtained along Siwa-Sidi Barrani-Crete section is 34 km in the south, 30 to 32 km at the Egyptian coast, 26 to 28 km at the thinnest part of the Mediterranean Sea and 30 to 32 km below Create. Between Cairo and Baharia the crustal thickness obtained is 33 to 34 km. Riad et al. (1983) using gravity data, concluded that the crustal thickness of Egypt and Sinai Peninsula varies from 32 to 35 km with noticeable crustal thinning towards the Mediterranean coast. Marzouk (1988) used seismic and gravity data to determine the crustal model in Egypt. He concluded that the Moho depth is varying between 32 and 35 km in the Western Desert and decreasing towards the Mediterranean Sea where it reaches 26 - 11 -
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
km. He also used a density of 2.42 g/cm3for the sediments, 2.82 g/cm3 for the upper crust, 2.9 g/cm3 for the lower crust and 3.3 g/cm3 for the upper Mantle. Omran (2000) showed that the crustal thickness of the Nile Delta and Nile Cone varies between 20 and 32 km with markedly thinning and stretching at the Mediterranean coast and extending northerly off-shore. Salem et al. (2004) studied the Nile Delta and its surroundings using gravity data. They found that the thickness of the sedimentary layer is between 2 and 12 km, the depth to Conrad discontinuity is 22 to 13 km with visible thinning and stretching northerly, and the depth to Moho discontinuity ranges between 23 and 33 km. They used a density of 3.33 g/cm3 for the Upper Mantle layer, 2.93 g/cm3 for the lower crustal layer, 2.7 g/cm3 for the upper crustal layer, 2.5 g/cm3 for the lower sedimentary layer (Miocene to Recent), and 2.1 g/cm3 for the upper sedimentary layer (Pre-Miocene). Saleh (2007) using P-wave spectral ratios of 108 earthquakes concluded that the crustal column in the northern part of Egypt is ranging from 30 to 34.5 km. In the present study, the density model is defined through the comparison and correlation of rock densities used in several studies, in addition to the correlation with the lithology of Wadi El Natrun-1 well. A density value of 2.41 g/cm3 is used for the sedimentary cover, 2.68 g/cm3 for the upper crustal layer, 2.92 g/cm3 for the lower crustal layer, and 3.32 g/cm3 for the upper Mantle. The results of modeled profiles are shown in Fig. (13, 14, 15, and 16). Profile P1: The profile P1 (Fig. 12) has been taken along S-N direction. It’s 110.4 km long. Three boreholes are detected along this profile; NWD 343-1, Wadi El Natrun-1, and Hosh Isa-1. NWD 343-1 well is located 2 km to the west of the profile and reached the Lower Jurassic rocks at depth of 3.527 km. Wadi El Natrun-1well is located on the profile and reached the basement at depth of 4.054 km. Hosh Isa-1 well is located to the west of the profile with a distance of 2.7 km and penetrated the Upper Cretaceous rocks at depth of 2.092 km. The gravity modeling of this profile (Fig. 13) indicates that the depth to the basement surface occurs at depth of 2.8 km at the south (with an exception of a noticeable uplift can be reached at depth of 2 Km) and reaches 6.5 Km at the north where the sedimentary cover thickness increases seaward as basement surface dips northward. The upper crustal thickness (including sediments) ranges from 22 km at the south and 21.5 km at the north. Meanwhile, the depth to Moho discontinuity varies from 32 km at the south to 31.5 km at the north. Profile P2: This profile is an E-W, 96.2 km long profile, passing through Wadi El Natrun-1 borehole. The inspection of the gravity model of this profile (Fig. 14) shows that the thickness of the sediments ranges from 4 to 5.7 km. The Conrad discontinuity depth is about 22 km while the depth to Moho discontinuity is about 32 km. - 12 -
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
29 40' 31 00'
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Fig. (13): Gravity model of profile P1, west Nile Delta, Egypt. - 13 -
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
Fig. (14): Gravity model of profile P2, west Nile Delta, Egypt. Profile P3: This profile has been constructed along NW-SE direction passing a distance of 124 km. Shaltut-1 borehole is reached a depth of 4.126 km in the Jurassic rocks. The gravity model of this profile (Fig. 15) indicates that the thickness of the sedimentary cover varies from 2 to 6 km. The depth to Conrad discontinuity ranges between 21.5 and 22 km. Moreover, the thickness of the crust varies from 31.5 to 32 km. Profile P4: The SW- NE oriented profile (P4) with length of 105.8 km has been established passing through E. Tiba-1 and Wadi El Natrun-1 Boreholes. E. Tiba-1 borehole has a depth of 4.828 km cutting the Upper Cretaceous rocks (Alam El Bueid member). The investigation of the gravity model of this profile (Fig. 16) shows that the depth to the basement varies from 4 to 6.5 km. Moreover, the depth to the base of the upper crustal layer ranges between 17 and 21 km. Furthermore, the depth to the base of the lower crustal layer varies from 31.5 to 31.7 km. In general, as a result of inspection of the previous gravity models, the sedimentary cover in the study area varies from 2 to 6.5 km where the thickness of the sediments increases northward and eastward. Also the depth to Conrad discontinuity ranges between 17 and 22 km, whereas the thickness of the crust varies from 31.5 to 32 km. Deduced Structural-Tectonic map Many authors studied the faulting system in the area west of Nile Delta, among them; Sigaev (1959), Shata et al. (1962), El Fayoumy (1964), Idris (1970), Sanad - 14 -
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
(1973), El Shazly et al. (1975), El Gazawi (1982), and El Sabagh (1992). A structure map compiled after those authors is shown in (Fig. 17). In the present study, a deduced structural-tectonic map (Fig. 18) is constructed based on the previous geological information, filtered maps, derivative maps, modeled gravity profiles and Euler solutions.
Fig. (15): Gravity model of profile P3, west Nile Delta, Egypt.
Fig. (16): Gravity model of profile P4, west Nile Delta, Egypt. - 15 -
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
Fig. (17): Structural map of west Nile Delta (after different authors). Deep faults can be detected at the northeastern and northwestern parts of the study area whereas the shallow and intermediate faults are located in the south and southwestern parts. Two horizontal displacements are found in the north and southeast of Wadi El Natrun anticline. Summary and Conclusion The Bouguer gravity data of the study area were subjected to critical interpretation through the application of gravity filters, Euler deconvolution and modeling techniques. Gravity filters include spectral analysis, separation of regional and residual anomalies, and vertical derivatives. The following can be summarized and concluded: Depth computations performed on power spectrum segments show that the average depth values are 12.6, 5.5 and 2.2 km from ground level for deep, intermediate and shallow gravitational sources respectively. The filtering technique was applied based on the results of the power spectrum curve where the anomalous contribution is distributed through three frequency bands: the short wavelength spectrum band (7-50 km) which represents the near surface causative bodies, the intermediate wavelength band (50-157 km) that reflects the intermediate causative bodies, and the long wavelength band (≥ 157 km) represented the deep-seated features. - 16 -
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
Fig. (18): Interpreted structural-tectonic map of the study area. Euler Deconvolution solution was applied on the gravity data where the depths, locations and trends of the faults in the study area were determined. NW-SE, NESW and E-W trends characterize the structure setting of the study area. Deep faults characterize the northeastern and northwestern parts of the study area whereas the shallow and intermediate faults discriminate the south and southwestern parts of the study area. The density model for 2D forward modeling is defined through the comparison and correlation of rock densities used in several studies, in addition to the correlation with the lithology of Wadi El Natrun-1 well. A density value of 2.41 g/cm3 is used for the sedimentary cover, 2.68 g/cm3 for the upper crustal layer, 2.92 g/cm3 for the lower crustal layer, and 3.32 g/cm3 for the upper Mantle. 2D forward modeling has been performed on four preselected profiles passing through Wadi El Natrun-1 Borehole. The results of inspection of the gravity models show that the sedimentary cover in the study area varies from 2 to 6.5 km and increases northward and eastward. Also the depth to Conrad discontinuity ranges between 17 and 22 km whereas the thickness of the crust varies from 31.5 to 32 km. A deduced structural-tectonic map is constructed based on the geological information, filtered maps, derivative maps, modeled gravity profiles and Euler solutions. - 17 -
NRIAG Journal of Geophysics, Special Issue PP. 487-508, (2009)
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