ElKhedr H. Ibrahim, Nasser L. ElAgami, Hatem H. Odah

Tertiary diagenetic remnant magnetization in the Cretaceous Nubian
Sandstone, North Sinai, Egypt
ElKhedr H. Ibrahim, Nasser L. ElAgami^¹, Hatem H. Odah^²

Geology Dep., Fac. of Sciences, Monsouro Univ., Egypt; ^¹Nuclear Materials Authority, Cairo, Egypt; ^²National Res. Inst. Astronomy and Geophysics, HeIwan, Egypt.

The origin of Fe oxides in the Cretaceous Nubian Sandstone, and the origin of magnetization and its timing relative to age of sediments are here dealt with. For this, the petrography and mineralogy of 22 samples from the Cretaceous Nubian Sandstone sections in Gebels ElMinsherah and ElHalal, north Sinai are studied.

The age of magnetization is verified by comparing the paleomagnetic results of the Egyptian Nubian Sandstones with the mean paleomagnetic poles of Africa (spanning the interval from Cretaceous to Holocene).

The mineralogy revealed that hematite is the main carrier of the natural remnant magnetization in the Nubian Sandstone. Hematite was formed authigenically through a variety of diagenetic processes including ferric hydroxide dehydration and silicate alteration. Other sources such as volcanic activity phases are not excluded. These processes formed a chemical remnant magnetization CRM in the Nubian Sandstones.

The paleomagnetic evidences indicate that this CRM is a regional phenomenon present in all Egyptian Nubian Sandstones. This secondary magnetization was either too stable to obscure all pre-existing magnetizations or it represents sum of several discrete magnetizations acquired through different diagenetic processes over a long period of time. Comparing the paleomagnetic results of the Nubian Sandstones of Egypt with the mean paleomagnetic poles of Africa (spanning the interval from Cretaceous to Holocene) gave a strong evidence that the CRM was acquired during the Oligocene - Miocene time.

So, the paleomagnetic results of the Nubian Sandstones of Egypt which once were considered reliable are in fact the result of remagnetization, and as a consequence the paleomagnetic pole positions of Nubian Sandstones should undergo rapid and thorough revision.