Bathymetry, Gravity and Magnetic Images of the Mediterranean water of the Nile Delta, Egypt, using GIS Technique, Part IV Interpretation of Gravity Data of the Northern Mediterranean Sea Coast of Egypt Morad B. Awad Professor of Marine Geophysics, National Institute of Oceanography and Fisheries, NIOF Presentation submitted to the Conference “Egyptian Coasts,…Problems and Solutions”, “As National Awareness Meeting” held in Ismaeillia, University of Suez Canal on 10th -11th, December 2003, Suez Canal University, in corporation with Egyptian Society for Coastal Protection. Organized by

STUDY AREA The study area lies at northern Mediterranean Sea coast of Egypt opposite to the Nile Delta, Egypt. It includes four small water bodies represented by the Lakes (Maryut, Idku, Burullus and Manzalah) that distributed along the northern shoreline of the Egypt, northern Nile Delta, Fig. 1 Location Map of the Study Area

Qualitative Interpretation of Bouguer Gravity Anomaly Maps :

Two negative anomalies occupying the southern part of the study area and two major positive gravity closures occupying the northern part of the study area, to the sea side between latitude 31.5 o and 32 o N. These negative and positive gravitational field anomalies may reflect the variation in depth and composition of the causative masses Qualitative Interpretation of Bouguer Gravity Anomaly Maps : Bouguer anomalies are ranging from -20 m.gal, at the south to +48 m.gal at its north and characterized by a sharp gradient near the shore line, that trending east-west direction parallel to a normal fault system, i.e. parallel to the shoreline. Contour lines have an abrupt change near Lake Manzala which indicates a strike slip fault crossing the expected normal fault and younger than it The general model of the fault system of the study describes that in the first stage, the stratigraphical unit is affected by a fault system, that striking along the east-west direction, and in a second stage, it is affected by the strike slip fault striking along the north east direction.

 Delineable interpretation of the Bouguer anomaly mapa and the assumed fault System.  Regional and residual gravity anomaly distribution is described by 4 th order best fitting polynomial mathematical model that representing the anomaly trend in the study area.  The positive closures may be due to either the existence of submerged island of high density material, or to a sort of uplifting in basement rocks.  The negative closures are expected to be due to sediments of low density that had been loaded by Nile River; through Pliocene/Pleistocene and recent times.

 The regional anomaly is ranging between 7 to 31 m.gal and increase toward north, which may indicate that the basement is uplifted toward the north with a gentle slope, i.e. this is one of the Mediterranean plate characterizing the north Africa tectonics.  The residual gravity anomaly map indicates gravity anomalies that ranging between -8 m.gal, in the south, to 20 m.gal in the north. Its gradient shows an abrupt change near the shoreline and also at Lake Manzala, as mentioned on Bouguer gravity anomaly map. These anomalies are probably due to surface and near surface causative bodies in addition to geomorphologic features.  The distribution of positive and negative gravity closures, their shapes, features and structures, may have their implications on the shore line and its associated geomorphology (i.e. gulfs, heads, and lakes). Geomorphological Implications  The distribution of northern lakes, Idku, Maryut and Manzalah is associated with negative closures since the lakes have a low density material loaded by the Nile River and its branches.

Fig. (8) Residual Bouguer anomaly map of southern Mediterranean Sea coast of Egypt. C.I. = 2 m.gal Fig. (7) Regional Bouguer anomaly map of southern Mediterranean Sea coast of Egypt. C.I. = 2 m.gal

 There is a normal fault associating the shoreline and crossing the area from east to west which considered as older one.  There is a strike slip fault affecting the area, crossing the normal fault and is covered by sediments.  Lake Burullus is affected with a high level gradient anomalies which is observed in both of Bouguer- and residual- maps.  The positive closure appears in the residual map has a larger area when compared with that one in the Bouguer map. This means that in this area the effects of the basement and the sediment sections are clear which reaches approximately to the residual effect.  The negative closures associated with lakes, appear in the residual map, are due to the low density of sediments.  The basement takes a gentle slope and is uplifting towards the north direction. Geomorphological Implications (Cont.)

Qualitative Interpretation of Gravity Anomaly Profiles

Qualitative Interpretation of Gravity Anomaly Profiles (Cont.) Interpretation of profile AA1, illustrates the sub-surface structures,\It shows the main normal fault affecting the study area. The sloping of the basement surface is indicated in relation with the North African plate slope, which is uplifted north-ward.

Interpretation of profile BB1, illustrates the sub-surface structures,\It shows the main normal fault affecting the study area. The sloping of the basement surface is indicated in relation with the North African plate slope, which is uplifted north-ward. Qualitative Interpretation of Gravity Anomaly Profiles (Cont.)

Interpretation of profile CC1, illustrates the sub-surface structures,\It shows the main normal fault affecting the study area. The sloping of the basement surface is indicated in relation with the North African plate slope, which is uplifted north-ward. Qualitative Interpretation of Gravity Anomaly Profiles (Cont.)

Quantitative Interpretation of Gravity Anomaly Profiles (Cont.) Spectral analysis along gravity profile AA’ Depth of the 3rd segment (shallow seated), KM Depth of the 2nd segment (intermediate seated),KM Depth of the 1st segment (deep seated), KM Length of profile, KM pr ofil e AA1AA BB1BB CC1CC1

Quantitative Interpretation of Gravity Anomaly Profiles (Cont.) Spectral analysis along gravity profile BB’ Depth of the 3rd segment (shallow seated), KM Depth of the 2nd segment (intermediate seated),KM Depth of the 1st segment (deep seated), KM Length of profile, KM pro file AA1AA BB1BB CC1CC1

Quantitative Interpretation of Gravity Anomaly Profiles (Cont.) Spectral analysis along gravity profile CC’ Depth of the 3rd segment (shallow seated), KM Depth of the 2nd segment (intermediate seated),KM Depth of the 1st segment (deep seated), KM Length of profile, KM profi le AA BB CC 1

Two Dimensional Model study Along profile AA’ Quantitative Interpretation of Gravity Anomaly Profiles (Cont.)  The computed structural model  along the profile AA1 shows that  the study area is affected by a set  of normal faults namely; F1/, F2  /, and F3/ from north to south.  The normal fault zones are  separating alternating horsts and  grabens which are defined by  high and low gravity belts. The  thickness of the sedimentary  section varies along the profile  from 1 Km to 3 Km, from north to  south, where the basement is  uplifted from south to north.  This is one of the Africa basement  configurations.

Two Dimensional Model study Along profile BB’ Quantitative Interpretation of Gravity Anomaly Profiles (Cont.)  The computed structural models along the profiles BB1, CC1 show that the study area is effected by a series of normal faults namely; F4/, and F5/ in the first profile and; F6/, F7/, and F8/ in the second one from north to south. The thickness of the sedimentary section is varying from 1 Km to 3 Km from north to south along these profiles.

Two Dimensional Model study Along profile CC’ Quantitative Interpretation of Gravity Anomaly Profiles (Cont.)  Accordingly it is concluded that  the area is suffering from a set of  normal faults, one of them is found to be delineated from east to west parallel to the shoreline, while the basement is dipping up towards the north direction. The eastern and western sides of the shoreline are underlain by a thick sedimentary section.

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