Umm er Radhuma Fm
Type Locality and Naming
Type locality: Near Umm Radmah, Saudi Arabia (Powers, 1968). Column: Oman Subsurface, S Iraq, Saudi Arabia Outcrop, Saudi Arabia Subsurface, Bahrain, Qatar, Kuwait, United Arab Emirates; Subsurface reference section in Oman is in Dhulaima-2. The previous reference section in Oman was well Biladi-1 (Hughes Clarke, 1988). Lowermost formation in Hasa Gr in Kuwait. Lowermost formation in Hadhramaut Gr in Oman
Synonym:
Lithology and Thickness
Light tan to gray, hard, dolomitic limestone and dolomite. In Oman, it is a succession of variably chalky wackestones/packstones, rich in larger foraminifera, that is subdivided into Upper, Middle and Lower (Shammar Mbr, a basal grey shale informally referred to as the Shammar Shale). The dolomitic uppermost unit and permeable zones within the middle of the Formation combine to make this the most important aquifer in Interior Oman. A thick, continuous carbonate sequence with only minor argillaceous breaks, but with an important shale/marl unit forming the base, the Shammar Member. Sandstone occurrences are known from the Middle Umm Er Radhuma in the Qarn Alam area (Osterloff and Al- Rhamah, 2001). The carbonates are slightly dolomitic limestones except for the top 15–30 m where dolomitization can be considerable, giving sucrosic and very porous beds. The upper dolomitic unit is often, and characteristically, split by a thin anhydrite. The Middle Umm Er Radhuma limestones are often vuggy. The vugginess of the Middle Umm Er Radhuma limestones makes this an important aquifer. The more localized basal Middle Umm Er Radhuma sandstones also represent a good aquifer. The sandstones range from fine to coarse, with common lithic grains and pyrite.
Relationships and Distribution
Lower contact
The lower boundary in Oman is a hiatus or disconformity onto the Aruma Gr (Simsima Fm). The latest Maastrichtian through Danian are generally absent. Where the basal Shammar Mbr shales lie upon Fiqa Fm shales recognition of the boundary may require age determination by fossils, which is also the case where the Shammar is absent, e.g. to the east, and the unit overlies older carbonates. [However, the Shammar Shales Mbr are grey only and differ in this aspect from the often underlying varicoloured shales of the Nahr Umr Fm and Natih Fm. It is impossible to distinguish the Shammar shales from the undifferentiated Aruma Gr shales of South Oman and the Shargi Fm shales of North Oman.] In Kuwait, it overlies the Tayarat Fm. An absence of Late Maastrichtian – Danian (Early Paleocene) sediments across much of the Arabian Plate is noted by Sharland et al. (2001).
Upper contact
The boundary between the Rus Fm and the Umm er Radhuma Fm is impossible to distinguish on lithology as the upper few metres of the Umm Er Radhuma are dolomites similar to the Rus Fm. Post- drilling, it can be placed at the base of the deepest significant anhydrite bed in the Rus Fm
Regional extent
Oman: Umm er Radhuma Fm is a relatively uniform and widespread unit throughout Interior Oman, but is cut out beneath the Fars to the southeast. Correlation shows a thickening in all units from southeast to northwest. No differences in the Umm Er Radhuma occurs between North, Central and South Oman with the exception of the development of the Shammar Mbr shale at its base. The Shammar is thinner in South and Central Oman and interbedded with thin limestones only in North Oman.
GeoJSON
Fossils
Biozones F74 (3 subzones), F73 (3 subzones), F72 and F71 (2 subzones) (see Group discussion, Figures 4.8 to 4.12). The Early Paleocene (Danian) age of Biozone F71 requires confirmation..
Age
Depositional setting
Common marine fossils indicate a shallow-marine environment, probably intertidal in part.
Additional Information