Lekhwair Fm

Yamama Fm; contact apparently conformable; placed where grey, argillaceous, pellety limestone of the basal Ratawi overlies pure, fine-grained carbonates of the upper Yamama. Regionally, there is evidence of possible cut-out of beds at this boundary which could, therefore, be unconformable.

Kharaib Fm; contact apparently conformable but regional evidence suggests the presence of a considerable sedimentary break between the Ratawi and Kharaib Formations over Qatar. The boundary is a sharp one between pure, porous limestone of the basal Kharaib Fm and argillaceous limestone and marls of the underlying Lekhwair Fm.

Recognised throughout Qatar. An equivalent is known from Bahrain, whence it may be traced through eastern Saudi Arabia to the type locality in south Iraq. Eastwards from Qatar, the argillaceous Ratawi Formation is rapidly replaced by shallow-water limestones.

Correlation of the Ratawi Formation of Qatar with the better-studied cyclical sequence recognised in Abu Dhabi suggests that a considerable section known in the latter area is absent from the upper part of the Ratawi Formation over the Qatar arch.

The most widely recognised member of the formation is unit 4 of the reference section. This is the lateral equivalent of the “Chrysalidina Zone” of the Buwaib Formation of Saudi Arabia. Similar limestones are present in the type section, in south Iraq. In Abu Dhabi, the porous Zone F of the Thamama Group correlates with this limestone bed. Units 6 and 8 can also be traced over a wide area.

The Ratawi Formation of Qatar is the approximate equivalent of the Ratawi of the type section in south Iraq. It is a predominantly argillaceous unit interbedded limestone which underlies limestone containing Orbitolina discoidea and overlies pure carbonates of the Yamama Formation.

Accurate correlation with Saudi Arabia is not possible. It appears probable that the Ratawi of Qatar is the equivalent of the Buwaib Formation of nearby subsurface sections. Cyclammina greigi, which is typical of the lower Buwaib Formation in nearby areas occurs in unit 3 of the Qatar reference section. The form Daxia which apparently replaces it in the upper Buwaib has not been recognised in Qatar. It is probable that fossils recorded as Choffatella decipiens in Qatar have been named Daxia in Saudi Arabia. The range of C. decipiens cannot be applied as a criterion in correlation. The lowest occurrence of Orbitolina and Dictyoconus arabicus in the basal part of the Kharaib Formation and of the Biyadh Formation is probably more significant.

The lowest part of the Ratawi Formation in Qatar is an argillaceous limestone which may be the equivalent of the upper part of the “Mid Thamama Limestone” of Saudi Arabia. It was originally named the Rakan Formation by Q.P.C.

It is probable that the fossil recorded as Pseudocyclammina kelleri from the Ratawi Formation of Qatar is not the true P. kelleri of Iraq. In appearance it has some affinity with Cyclammina and could be similar to Everticyclammina hensoni Redmond from the Buwaib Formation of Saudi Arabia.

Current usage in Q.P.C. assigns the name Ratawi to the combined Hawar, Kharaib and Ratawi Formations described herein. This reflects the fact that the unit between the base Shu’aiba and top Yamama Formations is the lateral equivalent of the Zubair-Ratawi clastic sequence of areas to the north. The system of nomenclature applied in this Lexicon will be re-adopted in Qatar.