Sudair Fm

Column: Oman Subsurface, Saudi Arabia Outcrop, Saudi Arabia Subsurface, Bahrain. Type section is at the Al Arid escarpment, Saudi Arabia (see Powers, 1968). Oman subsurface reference sections are Yibal-85 and Lekhwair-70, both in North Oman.

Synonym:

Sandy claystone. In lower part, it is brick to dark-red, massive shale, with occasional pale greenish-white layers, many of which are silty. The lower part includes a number of thin beds of soft, fine sandstone. In upper part, it is a pale greenish-white unit several meters thick, in which calcareous siltstones and silty, impure limestones are predominant.

In Oman, the formation is less shaley than the type Sudair section in Saudi Arabia. In Oman, "The Sudair Formation consists mainly of anhydritic, finely crystalline dolomites, but with a number of beds, some ca. 6–10 m thick, of red and green-grey shales, which is the most characteristic lithological feature of the succession. The shale packages are concentrated at the Top and Base of the Formation, with the Middle part being an almost continuous dolomite. In general, there seems to be a trend of decreasing shale content from the south to the north. Relict textures show evidence of original grain-supported sediments. The dolomites tend to be more anhydritic, resulting in slightly higher densities than those of the Jilh Fm. Shales predominate in the northwest of South Oman. A distinctive limestone (mudstone) occurs in the lower part above the basal interbedded shales and dolomite in the western part of North and Central Oman (e.g. Yibal-85, Lekhwair-70).

The Sudair Fm has no formal subdivisions, but clear lithological subdivision of the unit can be recognized by: (1) an upper sequence of claystones with inter-bedded dolomites; (2) a middle massive anhydritic dolomite; and (3) a lower sequence of claystones with inter-bedded dolomites." (Forbes et al., 2010)

Sequence stratigraphy: Sharland et al. (2001) place their MFS Tr30 and MFS Tr20 within the Sudair Fm. The latter is suggested to occur within the limestone at the base of the Sudair Fm. However, as already noted, correlations across the Arabian Plate during the Triassic are extremely difficult due to either the lack of, or the imprecise nature of biostratigraphic calibration."

"The Sudair Fm is characterized by Palynozone 2351 (Densoisporites nejburgii with Endosporites papillatus) and, in the middle to (more typically) lower claystone part, by an influx of small marine acritarchs, which defines Palyno-subzone 1095 (Veryhachium spp.). This Veryhachium-Micrhystridium acritarch bloom, together with the associated miospores, appears to be of worldwide significance, occurring in Induan – Early Olenekian-aged rocks. The zone ranges into the Upper Khuff, of interpreted Induan age. As in the Jilh Fm, no micropaleontological zones are defined due to the overall poor recovery. This reflects the general restricted environment of deposition. Outcrop and subsurface work by Vachard and separately by Forke (2009) at outcrop only yielded rare, very low-diversity microfaunas and rare bioclasts. Virtually none of the forms observed provided meaningful age determinations. A single specimen of Hemigordiellina tenuifistula noted in Vachard (2007) may suggest an Olenekian age at the base of the Sudair Fm outcrop equivalent, see also Koehrer et al. (2010)." (Forbes et al., 2010)

The Sudair Fm has only scattered shelly fossil debris (molluscs) and some marine palynomorphs. The environment spans low-energy marginal-marine, tidal-flat and perhaps coastal- plain settings; settings that persist into the overlying Jilh Fm.