DEPOSITIONAL ENVIRONMENTS OF SOME MIOCENE ROCKS IN NORTHERN WESTERN DESERT, EGYPT

Abstract

The present work deals with the study of the depositional environments and diagenetic processes which affected the Miocene rocks exposed at Siwa and Salum in the northern part of the Western Desert of Egypt. For this reason, three lithostratigraphic successions were collected; two. from Siwa Oasis (Gebel El-Takrur and Gebel El-Gari) and one from Salum area. Petrologic and diagenetic observations were based on both field and laboratory investigations of about 150 samples. Age assignments of the Middle Miocene strata considered here are based largely on faunal assemblage. This assemblage includes Neoalveolina melo, Amphistegina sp., Ostrea frondosa,• 0. plicatula, Pecten cristato­ costatus, Chlamys zitteli and others. The northern part of the Western Desert of Egypt situated between the Mediterranean Sea and the Qattara Depression, is a vast plateau which rises steadily towards the south to reach its highest elevation of 250 meter on the northern edge of the Qattara Depression. This plateau is made up of marine limestone sequences belonging to the Marmarica Formation. These carbonate rocks were probably deposited in a shallow marine environment, fairly calm and practically sheltered from terrigenous agents. From field studies, the terrigenous contents in the Marmarica Formation increase towards the south and east of the Western Desert. In the studied Salum section, there is one shale bed, whereas in Siwa sections, shale and marl beds are more represented, especially in the lower part of the successions.