Wasia Gr

The Wasia Group (used in Oman) and its correlative Wasia Fm (elsewhere) is widely used throughout the Gulf area to cover the package of sediments between two regional hiatus/disconformities; i.e. those between the Shu’aiba Fm and Nahr Umr Fm, and a more marked break beneath the Upper Cretaceous, usually shaly, Aruma Gr sediments. The Wasia is divided into several units over most of the Gulf area, but in Oman only the lowest of those formations, the Nahr Umr Fm, is maintained as a characteristic argillaceous basal unit to the Group, whereas the upper, mainly carbonate, sequence is assigned to a single unit, the Natih Fm. The type section for the Wasia (the Wasia Fm version) is not specified, but implied to be in southern Iraq and Kuwait (van Bellen et al., 1959/2005). Reference sections occur in Oman in southern Al Jabal Al Akhdar wadis, e.g. Wadi Al Muaydin. Columns: Oman Subsurface, Oman Outcrop.

Synonym:

Cyclic carbonates of the Natih Formation overlie the Nahr Umr sediments, some of which have argillaceous source rock bearing intervals. Sequence Stratigraphy: Upper part of Megasequence AP8 (Sharland et al., 2001). Sharland et al. (2001) correlate their MFS K140 to K90 surfaces into the Wasia Group in Oman (see formational discussions).

"Biozone F62 (Dohaia planata, Dictyoconella minima) to F57, Sub-biozones F573/571 (Cytherelloidea IRR29/Orbitolina texana, Spineloberis IRK20), are recognised. The zonation and ages are based on Sikkema (1991). Packer (2004) revised Sub-biozones F575 and F573 and defined Sub-biozone 571. Potential Middle Turonian Natih Fm and latest Aptian Nahr Umr Fm ages are not reflected in the current zonation. See formational discussions."

"Following uplift along eastern Oman, probably associated with a Neo- Tethys rift shoulder, the exposed area was eroded, successively cutting deeper towards the southeast. Oman, at that time, was geographically located in the humid equatorial belt and erosion was associated with karstification of the underlying Shu’aiba Fm carbonates. There followed a major supply of siliciclastics into the northern Arabian Gulf areas (Beydoun, 1995), where (reservoir) sandstones were deposited. These are represented in Oman by the distal shale/marl deposits of the Nahr Umr Fm. The (up to 150 m) thick Nahr Umr shales provide a world-class regional seal for the hydrocarbons in the Shu’aiba Fm and older formations. Closer to the shelf-margin settings, towards the Neo-Tethys Ocean in the northeast (the area of the present-day Al Hajar Mountains) and south east (Masirah Trough and Dhofar regions, the Nahr Umr Fm sequence consists of marly and argillaceous carbonates. Cyclic carbonates of the Natih Fm overlie the Nahr Umr Fm sediments, some of which have argillaceous source rock bearing intervals. Sedimentation was terminated by a regional emergence causing erosion and leaching of the Natih Fm, and locally older carbonates."