Rus Fm

Type locality: Umm Er Ru’us, Saudi Arabia (Powers, 1968; see also Weijermars, 1999). Columns: Oman Subsurface, S Iraq, Saudi Arabia Outcrop, Saudi Arabia Subsurface, Bahrain, Qatar, Kuwait, United Arab Emirates; Middle formation in Hasa Gr in Kuwait. Middle formation in Hadhramaut Gr in Oman. Subsurface reference sections in Oman are Dhulaima-2 and Barik-8. The previous reference section in Oman was well Biladi-1 (Hughes Clarke, 1988).

Synonym:

"The Rus Fm comprises a widespread succession of evaporites and carbonates. Within the Hadhramaut Gr it represents a median evaporitic phase separating the carbonates of the overlying Dammam Fm and the underlying Umm er Radhuma Fm, one of the most important shallow aquifers in Interior Oman. . . . Dominantly evaporite, gypsum or anhydrite, with associated variable dolomite and dolomitic marl. Silica geodes and secondary silicification are common in South Oman. . . . Whilst drilling the top of the Rus is marked by the first appearance of dolomite. It has a characteristic thick anhydrite (gypsum) layer, generally near the base."

Locally, it can consist of white, soft, chalky porous limestone, with one or more calcarenite beds at the top. Light-colored marls with local irregular masses of crystalline gypsum and occasional thin harder limestone beds; geodal quartz at several levels. In other areas this unit is highly variable, including as common equivalents - (a) white, compact, finely crystalline anhydrite with interbedded green shales and minor amounts of dolomitic limestone, or, (b) gray marls with coarsely crystalline calcite and interbedded shale and limestone. Gray to buff compact crystalline limestone commonly partly dolomitized, with minor amounts of soft limestone made porous by the leaching of small organic remains. Quartz geodes occur rarely in the lower part, and, are typical of the uppermost part.

The sequence is barren of age diagnostic fauna.

Generally barren of fossils suggesting deposition in a restricted lagoon, playa or sabkha setting. By its widespread occurrence, it must reflect a regional sea-level fall from a stable shelf..