The Oligo-Miocene of Eil (NE Somalia): a prograding coral-Lepidocyclina system

Abstract

The Oligo-Miocene succession of Eil is the product of a depositional regression and constitutes a 120–150 m thick depositional sequence that prograded seaward for at least 20–25 km. Its time-transgressive stratigraphy is documented physically by well exposed tangential clinoforms (previously considered as evidence of a tectonic coastal flexure) and biostratigraphically by the occurrence of calcareous nannoplankton, planktonic and benthonic foraminifera, and a rich coral fauna. The upper boundary of the sequence is indicated by a reefal toplap, which constitutes the flat surface of the Nogal Plateau. Age (Chattian to Burdigalian) and toplap relationships of the sequence indicate clearly that progradation took place after the Late Oligocene flooding which followed the strong fall of sea-level during the Chattian.

Because of the horizontal geometry of the entire sedimentary system, it has been possible to make a clear environmental reconstruction and a facies model with original water depths. A worldwide Tertiary facies—the Lepidocyclina beds— was confined to the front of the reef, at depths ranging from 35–40 to 120–130 m.