Pre-rift doming, peneplanation or subsidence in the southern Red Sea? Evidence from the Medj-zir Formation (Tawilah Group) of western Yemen

The behaviour of the lithosphere prior to rifting and rift- related volcanism in the southern Red Sea area is recorded in the stratigraphy underlying the Yemen Volcanic Group. The Medj-zir Formation is dated as Paleocene to Oligocene in age and is the upper part of the Cretaceous to Paleogene Tawilah Group of Yemen. The Medj-zir Formation is well exposed over a large area of western Yemen, forming a unit which varies in thickness from 30–60 m in the west to 60–75 m in the east. The Yemen Volcanic Group overlies the Medj-zir with a sharp, but conformable boundary. The lowest volcanics are dated as 30–32 Ma. The Medj-zir Formation consists of clastic and subordinate carbonate facies which indicate deposition in fluvial, shallow marine and lacustrine-lagoonal environments. Three members are recognized: the basal Zijan Member comprises fluvial facies and shallow marine sandstones and mudstones; the Kura Member is made up of fluvial channel and overbank facies, the latter including well-developed ferruginous paleosols; the Lahima Member, at the top, is fine-grained, including gastropod-rich limestones deposited in a lacustrine or brackish lagoonal environment. These facies and transitions between them represent deposition in a coastal plain to shallow marine shelf setting which was subjected to minor fluctuations in sea level. It is noteworthy that there are no angular unconformities or erosional hiatuses within the Medj- zir Formation anywhere in western Yemen. Deposition- al hiatuses of unknown duration are represented by well-developed paleosols indicating long periods of tectonic stability. The contact with the overlying volcanics is conformable in all examined sections and there are no volcanic horizons or clasts in the Medj-zir Formation.

The evidence from the sedimentology and stratigraphic relationships in the Medj-zir Formation shows that the Paleocene to Oligocene period prior to volcanism and rifting in the southern Red Sea the region was tectonically stable. During the Paleogene western Yemen was part of a very extensive alluvial to coastal plain extending out from a source area in central Africa to the Tethyan margins of North Africa, southern Arabia and the Horn of Africa. The topography of this alluvial plain was not affected by a prevolcanic phase of doming as has been predicted in current models for plume-related rifting.