Hamlah Fm
Type Locality and Naming
Q.P.C. Well Dukhan No. 65, lat. 25°27’38” N, long. 50°47’13” E. elevation 42.6 m (140 ft), completed 21.10.1960, between drilled depths 2367 and 2436.5 m (7764 and 7992 ft). The formation takes its name from a locality on the west coast of Qatar. A.J. Standring (unpublished report)
Synonym:
Reference Section:
Lithology and Thickness
Top. 1. Dolomite, grey and brown to dark grey, dense to porous, saccharoidal. Stringers and nodules of anhydrite in the upper part. Becoming argillaceous downwards. 5 m (16 ft). 2. Shale, dark grey-green, with subordinate marl, grey, and some green glauconitic sandstone and dolomite. 6 m (20 ft). 3. Dolomite, dark grey, calcareous, with anhydrite streaks, interbedded with green shale, and grey marl. 9 m (30 ft). 4. Dolomite, grey-brown, interbedded with grey, dense limestone and some anhydrite. 41 m (135 ft). 5. Sandy marl with beds of shale, dark grey. 8 m (27 ft). Its thickness is 69.5 m (228 ft).
Relationships and Distribution
Lower contact
Gulailah Fm; contact unconformable. At contact of sandy marl and shale of the basal Hamlah, above, with dolomites of the Gulailah, below.
Upper contact
Regional extent
Encountered only in the Dukhan Field in Qatar. Possibly represented by part or all of the “Gulailah Formation” of Dominguez, which underlies the Izhara Formation in offshore areas of Qatar
GeoJSON
Fossils
None
Age
Depositional setting
Additional Information
Originally included in the Gulailah Formation (s.s.) but difficulties arose since this attribution resulted in an appearance of continuous sedimentation from the Triassic to the Middle Jurassic with no indication of the widespread pre-Toarcian unconformity known to exist in neighbouring areas. Discovery of a marked change in structural configuration at a drilled depth of 7992 ft in Well Dukhan No. 65 made it necessary to separate the unit from the Gulailah.
The Hamlah Formation could be considered to be an additional, lower unit of the Izhara Formation not present in the type locality. The Izhara can however, be mapped as a distinct rock unit over a wide area. The lithology of the Hamlah is sufficiently different for it to be considered a valid formation.
The Izhara Formation is conventionally correlated with the whole Lower Dhruma. For this reason the Hamlah is correlated with the Marrat Formation and assigned a possibly Lower Jurassic, Toarcian age.
Although the Hamlah Formation is known only from one part of Qatar, its presence over a large area which is structurally lower than the present and Mesozoic crests of the Qatar arch can be reasonably assumed. The thickness of the Hamlah Formation at different localities will depend upon the degree of structural activity during the late Triassic and early Jurassic.