Journal of Conference Abstracts

Comparative Study of the Middle-Upper Jurassic Carbonate Sediments of the Amran Group (Yemen Republic) and of the Antalo Group (Ethiopia)

Khalid A. Al-Thour (Fax.: (967) 1-416 295)

Geology Department, Sana'a University, Sana'a, P.O. Box: 2027, Yemen Republic

The middle-upper Jurassic carbonates of the Amran Group, Yemen Republic (southwestern Arabia) and of the Antalo Group, Ethiopia (eastern Africa), display a systematic vertical change in thickness and facies according to their position within a classic shallow marine depositional environment. The Amran Group is about 900 m (Callovian-Tithonian) and the Antalo Group is about 1400 m (Oxfordian-Kimmeridgian). Sea-level changes and tectonics controls the deposition of deeper water facies over shallow ones. These facies are widely distributed and the whole sequence reflects deposition on a broad platform upon which shoals separated platform carbonates from basin sedimentation and an open marine environment. The repetition and interfingering of both fining- and shallowing-upward cycles within the study areas suggest that deposition occurred within the same basin with slightly different conditions in different places.Both groups can be subdivided into several depositional sequences. They are physically correlatable owing to the presence of prominent carbonate sediments of specific lithologies, and according to the abundant foraminiferal fauna (Alveosepta Jaccardi, Pseudocyclamina mayrci, Alveosepta Powersi, Krunbia Palastiniensis, ..., among others). This correlation confirms that flooding of the entire southwestern Arabia and eastern Africa during Callovian-Tithonian time was dictated by some regional geodynamic events.

A physically visible angular unconformity separates the middle-upper Jurassic Amran and Antalo Groups from the lower Cretaceous Tawilah Group and Amba Aradam sandstone successions. Regionally speaking, the Amran and Antalo Groups thickens progressively toward the Red Sea. However, the thickness of this shallow-water succession correlatable is around 250-300 m. The data on thickness trends show that, for the entire Jurassic, the present Red Sea region has been a strongly subsiding trough, a failed arm of the multibranched rift system dissecting a large part of Gondwana. This ancestral structure was reactivated in Tertiary times, giving rise to the Red Sea.

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