ESR studies and dating of Egyptian gypsum at Ras Mala'ab, Sinai, Egypt

Abstract

A gypsum sample from the famous gypsum-anhydrite evaporitic deposit composing the Ras Mala'ab Formation, Upper Miocene, occurring at Ras Mala'ab, on the east coast of the Gulf of Suez, was subjected to electron spin resonance (ESR) dosimetric studies. ESR was further used to date the formation or most recent recrystallization of the gypsum. The gypsum derivative ESR spectrum is characterized by the large broad Fe2+ signal (g=2.50) and hf-sixtet Mn2+ signals. Only the characteristic gypsum signal (G1, g=2.0040) was detected between the third and fourth lines of the hf-Mn2+ which is attributed to the electron-center SO. This signal was sensitive to artificial γ-irradiation and showed significant enhancement using a γ-dose of 550 Gy. Additionally, the signal was very stable up to 400 °C. The gypsum sample with a total dose of 1500 Gy, determined graphically by extrapolating the linear relationship between defect concentration and the artificial γ-doses for (G1, g=2.0040) and an annual dose (D) due to cosmic rays (0.3 mGy), yielded an age of 5.00 Ma. This could mean the age of formation or latest recrystallization of this gypsum deposit. The geological age assignment of the Ras Mala'ab Group, including the evaporitic gypsum unit, is Middle to Late Miocene. It is directly overlain by the Pliocene clastics at the locality of Ras Mala'ab. This might suggest that these evaporitic gypsum facies represent the top of the Serravallian (mid–late) Miocene in the Gulf of Suez area. Since the Serravallian period was between 14.8 and 11.2 Ma ago; therefore, the ESR age of the Ras Mala'ab gypsum represents the latest recrystallization event of these gypsum deposits.