Simsima Fm
Type Locality and Naming
The Simsima Fm was originally described by Sugden from Dukhan Well No. 28, but the type section here designated is Q.P.C. Well Dukhan No. 55, lat. 25°24’0” N, long. 50°45’46” E, elevation 7 m (23 ft), completed 17.8.1956, between drilled depths 353 and 502 m (1158 and 1646 ft). W. Sugden (unpublished report, 1956).
Synonym: “Tayarat Formation”, Owen and Nasr, 1958. “Simsima Formation”, Dominguez, 1965. “Simsima Formation”, Dunnington, 1967. “Upper Chalk”, Hajash, 1967. “Aruma Upper Limestone”, Fox and Brown, 1968.
Reference Section:
Lithology and Thickness
(After W.O. Gigon and P.J.C. Hoogkamer, 1969, unpublished report).
Top. 1. Limestone, medium grey, slightly chalky, lime packstone, becoming slightly argillaceous towards the base. Particles consist of larger and smaller foraminifera, ostracoda, coral, echinoid and gastropod fragments with dasycladacean algae. 18 m (58 ft). 2. Shale, greenish-grey, soft, calcareous shale with some pyrite and mainly smaller foraminifera and ostracoda. 2.75 m (9 ft). 3. Limestone. Alternations of brown-grey, partly dolomitised, porous lime wackestones and light grey, often chalky lime packstone. Lower 9m slightly argillaceous. Particles consist of large and small foraminifera and broken fossils including rudists and echinoids. 98 m (319 ft). 4. Limestone, medium brown, porous, dolomitised wackestone. Particles which are not dolomitised consist of mainly broken fossils, e.g. lamellibranchs, etc. 20 m (66 ft). 5. Limestone, light grey, partly chalky and in the upper part slightly dolomitised lime packstone. Particles consist of broken fossils and larger and smaller foraminifera. 10 m (34 ft). Its thickness is 148 m (486 ft). The Simsima Formation has recently been subdivided into an upper, Salwa Member (units 1 and 2) and a lower, Jana’an Member.
The Simsima Fm is the most lithologically uniform and widespread rock unit of the Aruma Gr.
Shales occur in the Salwa Member. The uniformity of development of the Member in Qatar allows easy differentiation between the Simsima and the basal Umm er Radhuma shale. In other areas, the latter can immediately overlie shales of the Salwa Member, making difficult the accurate selection of the formation boundary on lithologic grounds.
Relationships and Distribution
Lower contact
Ruilat Fm; contact conformable. Placed where white, chalky foraminiferal wackestone of the basal Simsima overlies white recrystallised, dolomitised, spicular lime mudstone of the Ruilat Fm. A minor unconformity is believed to occur beneath the Salwa Member over certain structures, but this has not been demonstrated in Qatar.
Upper contact
Umm Er Radhuma Fm; contact probably disconformable. At boundary of white, chalky, algal lime packstone of the upper Simsima with dark brown dolomite of the basal Umm er Radhuma Formation. There is an abrupt change from Maastrichtian to Paleocene fossils at the junction.
This contact is a typical as the Simsima Fm is normally overlain by the basal shale-marl member of the Umm Er Radhuma Fm.
Regional extent
Encountered in all deep wells drilled in Qatar. Recognized in offshore Qatar, Abu Dhabi, and Oman. The equivalent rock unit is recorded from Iraq, Kuwait and Saudi Arabia. In the latter area it outcrops as part of the Aruma Fm. The two members can be traced eastwards from Qatar as far as Oman.
The Salwa Member is the equivalent of the informal Lina Member of the Aruma Fm of Saudi Arabia, while the Jana’an Member correlates with the upper two units of the Atj Member of that formation.
GeoJSON
Fossils
In 1. Loftusia gr. minor-morgani, Elphidiella multiscissurata Smout, Fissoelphidium operculiferum Smout, Omphalocyclus macroporus (Lamarck), dasycladacean algae. In 3. Lepidorbitoides socialis Leymerie, Siderolites calcitrapoides Lamarck, O. macroporus (Lamarck). In 4 and 5. Orbitoides media (d’Archiac), O. macroporus.
A large number of foraminifera, including many new forms, were described from the Simsima Formation by F.R.S. Henson, who undertook detailed work on core material from Well Dukhan No. 1. They include Elphidiella multiscissurata Smout, Fissoelphidium operculiferum Smout, Rotalia cf. trochidiformis Lamarck, Loftusia gr. minor-morgani, Omphalocyclus macroporus (Lamarck), Pseudorbitolina marthae Douville, Lepidorbitoides socialis Leymerie, Archaecyclus mid-orientalis Eames and Smout, Siderolites calcitrapoides Lamarck, Rotalia cf. skourensis Pfender, Orbitoides media (d’Archiac), Loftusia coxi Henson, Lituonella cf. douvillei Davies, Dictyoconus cf. arietinus Silvestri, Dictyoconella complanata Henson, Dicyclina schlumbergeri Munier Chalmas, Broeckinella arabica Henson, Lituonelloides compressus Henson, Simplorbites gensacicus (Leymerie), Pseudedomia multistriata Henson.
Jana’an Member Includes the Lepidorbitoides socialis and the Orbitoides media faunizones in Qatar.
Age
Depositional setting
Additional Information
Early miscorrelation resulted in the complete “Upper” Cretaceous carbonate succession found in Dukhan being equated with the Tayarat Fm of Iraq. The upper, Maestrichtian, part of this limestone is the true equivalent of the Tayarat and was so named for a short while. Difficulties of exact correlation with the Iraq succession led to the proposal of a new, local, name, the Simsima Fm. This name has been adopted by several organizations working in S.E. Arabia. Long usage has established it as a valid formation name which is retained in spite of its demonstrable synonymy with the earlier published Tayarat Fm.
Salwa Member is informally known as the “Loftusia Zone”.