Many authors have attained different conclusions about the age of Malha Formation; Jurassic based upon the presence of Middle Jurassic miospores; Neocomian based upon well preserved palynomorph assemblages, which consist mainly of spores and pollen grains in the kaolin-bearing Malha Formation, and Aptian-Albian based upon the stratigraphic relationships [1-9]. In addition, the studied ferruginous sandstone is considered by the Geological Survey of Egypt [10] and Moustafa [11] on their maps as Triassic Qisaib Formation. On the contrary, the Egyptian General Petroleum Corporation and Conoco [12] drew these successions as Malha Formation of Aptian-Albian age (Figure 1). The present study tries to solve the problem in age controversy of these successions by a paleomagnetic investigation to determine the correct age assignments.

Figure 1: Location and Geological map of the study area (modified after the Egyptian General Petroleum Corporation and Conoco [12]).

Nubian sandstone ranges in age from the Cambrian to Upper Cretaceous eras below the first major marine transgressive during the Upper Campanian Duwi Formation [7]. It consists of mainly continental sandstones with thin beds of marine limestones, and marls. The Nubia sandstone now is classified into different rock units, where the Aptian-Alpian in Western Desert occurs within upper part of Sabaya Formation and lower part of Maghrabi Formation based upon palynology styudies [13]. The Malha and the Risan Aneiza Formations are part of the Nubian Sandstone group that rests unconformably on the basement rock units [14].

The term “Malha Formation” was first introduced by Abdallah et al. [5] for the multicolored and fluvio-marine sandstone with interbeds of clay at Wadi Malha, at the western side of the Gulf of Suez. According to Said [15], the thickness of the Malha Formation in the north Eastern Desert varies greatly from one place to another, Formation varies from 40 to 100 m in West Sinai, to 250 m in East Sinai, and to more than 1000 m in the subsurface of North Sinai.

This formation unconformably overlies different rock units, according to the paleotopographic and geologic settings. It overlies the Lower Paleozoic Naqus Formation in East Sinai, whereas in West Sinai and the Gulf of Suez, it unconformably overlies either the Carboniferous, as in the present study, or the Jurassic deposits. The Malha Formation underlies unconformably the Galala Formation and the contact between them is marked by the transition zone from varicolored sandstone to pale grey siltstone of the Galala Formation.

Samples have been collected from 6 sites of different lithologies at the surface outcrops of Malha Formation at Gebel Sarbut el- Gamal at latitude 29°08’15”N and longitude 33°13’25”E (Figure 2). The number of samples differs from one site to another, ranging between 3 and 5 hand blocks. Hand block samples have been cut into total of 125 oriented core specimens using a drilling machine at the Paleomagnetism Laboratory of the National Research Institute of Astronomy and Geophysics (NRIAG), Helwan, Egypt. Samples were oriented using magnetic compass.

Figure 2: geology-geosciences-Lithologic-Malha-Formation

Several rock magnetic experiments, as the Thermomagnetic (Js- T) curves, hysteresis loops, Isothermal Remanent Magnetization (IRM) acquisition curves and the back-field coercivity curves, using MicroMag, were done, in order to identify the main carriers of remnant magnetization.

The natural remanent magnetization (NRM) was measured in two different laboratories using 2G-SQUID (in the Paleomagnetic Laboratory at the Institute of Geophysics, Warsaw, Poland) and the JR-6 spinner magnetometer (in the Paleomagnetism Laboratory at NRIAG). Demagnetization steps were done, using both alternating field (AF) and thermal (Th) techniques. The AF- demagnetization was carried out using a magnetically shielded Degausser attached with the SQUID, with 5-10 mT steps, up to 120 mT. Thermal demagnetization, on the other hand, was done using non-magnetic thermal demagnetizer of Magnetic Measurements (MMTD80). During the thermal demagnetization, the samples were heated in steps of 50°C - up to 350°C and then the steps reduced to be 15°C up to 700°C in most cases.

The magnetic susceptibility was measured before the demagnetization process and also after the selected heating steps, using Bartington Susceptibility meter (MS2B), in order to monitor any mineralogical alteration in the specimen during thermal demagnetization.

Analysis of the demagnetization data was carried out visually; using the stereographic and orthogonal projections and statistically; using the Principal Component Analysis (PCA) for each specimen to determine the characteristic magnetic component(s) [16,17]. The obtained components, from each site, were gathered and the respective site-mean direction was calculated. The overall mean directions, as well as the virtual geomagnetic pole (VGP) position were computed using Fisher statistics for each locality [18]. Data processing was done using Rema soft program.

Rock magnetic experiments showed that, hematite is the main magnetic mineral in almost all the studied specimens:

Figure 3 represents an example for the IRM curves. As shown, the magnetization in the studied specimen did not reach the saturation state until 1000 mT, indicating the presence of hard magnetic mineral(s). The same specimen was subjected to back field curves experiment, in order to identify the coercivity of remanence. As we can see in Figure 4, the existed magnetic mineral is of high coercivity value, of about 400 mT. This hard magnetic mineral of high coercivity could be hematite and/or goethite. Hysteresis loop curve in Figure 5 shows that, no saturation state was reached, indicating the presence of hematite as the main magnetic carrier. Studying the magnetization against temperature (thermomagnetic curves) Figure 6 reveals that, the Curie the temperature value of the existed magnetic mineral is about 650°C. Such a high value of Curie temperature characterizes the hematite mineral. This mineral could carry a stable magnetization within the investigated specimens [20-25].

Figure 3: Isothermal remanent magnetization curve for a specimen from Malha Formation. As seen, the progressive steps of increasing applied magnetic field failed to reach the saturation magnetization.

Figure 4: Back field curve for a specimen from Malha Formation. As seen, the progresive steps of increasing applied magnetic field in the opposite direction to the field applied for IRM experiment resulted in losing magnetization at about 400 mT.

Figure 5: Hysteresis loop curve for a specimen from Malha Formation. No saturation state is reached.

Figure 6: Thermomagnetic curve for a specimen from Malha Formation. The specimen lost magnetization at about 650°C, where the Y axis represents normalized magnetization.

Demagnetization process relied mainly on the thermal demagnetization, as the AF demagnetization failed to isolate the characteristic magnetic component, due to the presence of hematite, as the main magnetic mineral. As expected in the study of sedimentary rocks, the intensity of remanence was low of around 2.0 × 10-3 A/m. Also, the values of magnetic susceptibility for the studied specimens were low around 5 × 10-6 SI units. As shown in Figure 7, the intensity of NRM decreased gradually during successive demagnetization steps till around 650°C, where about 90% of the remanence was removed. Zijderveld diagram showed that a straight line pointed toward the origin, indicating single component of stable magnetization. Also, an abrupt change in the susceptibility values was noticed, indicating the change in the mineralogy during demagnetization steps (Figure 7). The site-mean values of the obtained components that resulted from the demagnetization process of the studied specimens were calculated and tabulated in Table 1. In the Geologic Time Scale Figure 8; the base of Aptian age is found to be reverse magnetization and this agree with results of the present work shown in Table 1 where the component of the site (M1, M2 and M3) are (-ve) at the base of Malha Formation and the upper three sites are of (+ve) components indicating that the Malha Formation started deposition at the very beginning of Aptian age [19].

Figure 7: Thermal demagnetization plots [Stereonet, Zijderveld diagram (Zijderveld [16]), intensity decay curve and suscebtibility values curve] for a representative specimen from Malha Formation.

Figure 8: Modified after Geologic Time Scale (F. M. Gradstein et al. [19]).

The paleopole position obtained in this study, agrees with the Cretaceous paleomagnetic poles obtained in Egypt (Table 2 and Figure 9) [26-29].

Figure 9: Previous obtained Cretaceous poles of Egypt with the pole of the present study.

The present study is an attempt to throw some light on the age of the ferruginous sandstone successions, which is a controversial point in the study area. Our conclusion, based upon paleomagnetic investigations agrees with the Egyptian General Petroleum Corporation and Conoco [12], and contradicts with the term Triassic Qisaib Formation, which introduced in other studies by different authors [10,11]. The presence of anti-parallel directions is an evidence for the primary origin of the resultant components, as well as the reversal of the geomagnetic field during the deposition of Malha Formation (Table 1 and Figure 8). To the knowledge of the authors; the present work is the first paleomagnetic study of Malha Formation in Sinai, so a comparison with relevant studies on Nubia Sandstone in other places in Egypt was done in Table 2 and Figure 9, such as: Schult et al. [20] studied some Nubia Sandstone rocks in Aswan, they obtained VGP lies at Lat.=80°N and Long.=227°E [Table 2, pole no. 10], Hassain et al. [21] carried out a paleomagnetic study of the Nubia Sandstone at Qena, They found that the VGP lies at Lat.=76°S and Long.=265°E [Table 2, pole no. 14], Hussein & Aziz [22] studied the paleomagnetism and magnetic mineralogy of Nubia Sandstone in East Owienat, The pole position obtained from that study lies at Lat.=77°N and Long.=258°E [Table 2, pole no. 18], in their paleomagnetic studies on Nubia Sandstone at Eastern Desest; El-Hemaly et al. [23] showed that the VGP lies at Lat.=74°N and Long.=244°E [Table 2, pole no. 22] and El-Shayeb et al. [24] studied the paleomagnetism of some Nubia Sandstone formations in the Western Desert and obtained pole position at Lat.=78°S and Long.=294°E [Table 2, pole no. 27]. The paleopole position obtained in this study, agrees with the Cretaceous paleomagnetic poles obtained in Egypt (Table 2 and Figure 9). The results show obviously the presence of a single and stable component as revealed from the demagnetization process [25-29].

Table 1: Paleomagnetic results of the Malha Formation.

Table 2: Selected Cretaceous paleomagnetic poles of Egypt, with the pole of the present work.