Thamama Gr
Type Locality and Naming
The name Thamama Group is applied in Qatar to a clearly defined sequence of rock units which overlie the Hith Anhydrite Formation and underlie the terrigenous clastics of the Nahr Umr Formation. This usage conforms to normal practise in much of Arabia, and is retained in spite of recently published evidence which could invalidate the customary application of the name (R.W. Powers et al., 1966). The name continues to be used for comparable subsurface sections in Saudi Arabia (R.W. Powers, 1968).
Synonym:
Reference Section:
Lithology and Thickness
In 1956, the Thamama Group of Qatar was subdivided into the following formations:
Shu’aiba Fm; Hawar Shale Fm; Kharaib Fm.
The Shu’aiba Fm is a widespread, transgressive carbonate unit of Aptian age which can be recognised over much of Arabia, but does not reach the outcrop area of Saudi Arabia. Its inclusion in the Thamama Group is based upon its apparent conformable relationship and close age affinity with the underlying units, together with its common erosional contact with the overlying clastics of the Nahr Umr Fm.
In 1961, it was shown that the Lekhwair Fm of south Iraq has a diachronous, laterally gradational contact with the Zubair Fm. A situation obtained in some areas where shales of the Lekhwair Fm have wholly replaced the Zubair, and include an appreciable proportion of limestone. The Ratawi in such developments closely resembles the combined Hawar, Kharaib and Ratawi Formations as originally defined in Qatar. For this reason, the Hawar and Kharaib were reduced by Q.P.C. to the status of members within a Lekhwair Fm including the predominantly argillaceous sequence which underlies the Shu’aiba and overlies the Yamama Formations.
The units assigned to the Yamama and Sulaiy Formations are believed to be very like rock sequences similarly named in neighbouring parts of Saudi Arabia. It is probable that a break in sedimentation occurs at the sharp contact between the Yamama and Ratawi Formations.
Relationships and Distribution
Lower contact
Overlie the Hith Anhydrite Fm. The chronological list of current rock units indicates the next older unit as Shu’aiba Fm
Upper contact
Underlie the terrigenous clastics of the Nahr Umr Fm.
Regional extent
The Thamama Group of Qatar contains lithologic units comparable to those described from the subsurface of the Eastern Province of Saudi Arabia. Exact correlation being impossible on the basis of available evidence, different formation names have been applied to parts of the succession. Even where names from Saudi Arabia have been applied, it is possible that formation boundaries are slightly different from those in the type area.
GeoJSON
Fossils
Faunal evidence originally believed to prove Jurassic age up to the top of the Yamama has been discredited.
Age
Depositional setting
Additional Information
Since 1961 the Thamama Gr has attained great economic importance as a source of oil and has, consequently, been closely studied. One result is evidence of the widespread regional value of a formation closely comparable to the original Kharaib Fm. The name Kharaib Fm is now being applied in Abu Dhabi and Oman. It is proposed to reinstate it in Q.P.C. terminology. The Hawar Fm will be reintroduced, while the name Ratawi will be applied, in the original sense, to the predominantly argillaceous unit which occurs between the Kharaib and Yamama Formations. The Lekhwair Fm so defined approximates closely in lithology and age to the Ratawi of the type area.