Jilh Fm

Column: Oman Subsurface, Saudi Arabia Outcrop, Saudi Arabia Subsurface, Bahrain, Kuwait. Type section is Jilh al’Ishar, Saudi Arabia (see Powers, 1968). Oman subsurface reference sections are Yibal-85 and Lekhwair-70, both in North Oman.

Synonym:

In Oman, the Jilh Formation is a sequence of dolomites with subordinate red and grey green shales with a few thin interbeds of anhydrite. The shale content of the unit seems to increase from north to south but is difficult to assess because of progressive truncation. The finely crystalline dolomites are hard and tight and poorly fossiliferous; fossils are only rarely seen in cuttings. It contains no limestones.

In Saudi Arabia, it has three to four subdivisions:

In Upper Jilh Mbr, it is fine to medium-grained quartz sandstone, and, green to purple silts and shales; several prominent layers of yellow to golden-brown hard, slabby, in part sandy, dense, finely molds of marine fossils.

In Jilh Salt Mbr, it is presumably evaporite (full description needed).

In Middle Jilh Mbr, it is Limestones, varying from gray chalky to yellow-brown slabby dense, with interbedded fine quartz sandstone and marl. Rare poorly-preserved marine fossils.

In Lower Jilh Mbr, it is green shale, in part gypsiferous, with a few thin, hard limestone layers. Rare poorly preserved marine fossils and a few amphibian bone fragments. Gray to buff and reddish, fine to medium-grained quartz sandstone, locally ferruginous, in places with many small spherical concretions, interbedded with an about equal amount of light to dark-green shale. The basal portion is poorly exposed, but, probably mainly sandstone.

"The Jilh Fm encompasses the 2255 (Bartenia communis) and 2370 (Triadispora crassa) Palynozones, with no micro- palaeontological zones defined due to the overall poor microfaunal recovery. Work by Forke (2009) on outcrop samples (Al Jabal Al Akhdar) only yielded rare, very low diversity microfaunas and rare bioclasts, neither of which provided meaningful age determinations.

Mabillard et al. (1985) placed the Jilh Fm entirely within their Anisian Palynozone 2370, although in all but one well in their study recovery comes from the middle to lower part of the Jilh Fm. Palynological evidence from the Lekhwair area indicates that the upper Jilh can range as young as Norian, lower 2255 Palynozone (Jacovides et al., 1998). At this stage it is unknown whether the two zones are indeed mutually exclusive, are in part age equivalent (possible facies effect) or have been wrongly differentiated by two separate generations of palynologists. The Mabillard et al. (1985) age interpretation of Palynozone 2370 is based as much on absent post- Anisian markers as those present, and relates to generally poor data coverage, limited recovery and low diversity assemblages. Essentially the maximum age of the 2370 Palynozone is open to interpretation, as the forms recorded are all general Middle to Late Triassic taxa (Anisian – Norian).

In summary, the Upper Jilh is considered to be Norian in age (Palynozone 2255), whilst the lower Jilh is of uncertain age but may be as old as Anisian (Palynozone 2370). The implication is that the Jilh Fm could represent over 40 My of time, and it is felt that the maximum age of the Jilh Fm has yet to be conclusively defined, i.e. it may range only to the Ladinian rather than the Anisian. Clearly our knowledge of the Jilh is inadequate in terms of age calibration, sequence development and related breaks." (Forbes et al., 2010)

"The environment was probably marginal-marine to tidal-flat, although virtually no shelly fossils are recorded, and relict ooidal-pelletoidal textures are present. As in the Sudair Fm, the depositional setting is marginal marine to possibly coastal plain, indicating a persistent shallow setting. Deposition of the Jilh Fm is followed by uplift and erosion associated with extensive weathering (Triassic laterites and bauxites of the Minjur Fm are observed in the Al Huqf area and locally preserved in the subsurface)." (Forbes et al., 2010)