Natih Fm

The Natih Formation is recognized across most of the Arabian Gulf and had formed the interior part of an extensive carbonate platform that covered the eastern part of the Arabian Shield during the middle Cretaceous. It is the upper formation in the Wasai Gr of Oman. Type and reference sections: Fahud-3, drilled as Fahud North-3 in North Oman. The surface reference section is in Wadi Al Muaydin, with a thickness of 344 m. Additional reference sections are Suwaihat-1 in Central Oman, and Nimr-1 in South Oman. Column: Oman Subsurface, Oman Outcrop. The Formation consists of a number of repetitive sedimentary cycles ranging from several tens to 150 m in thickness and these have been used to subdivide seven informal members designated by the letters A–G from top to base (Hughes Clarke, 1988; Scott, 1990). Philip et al. (1995) proposed a correlation of these informal members to the outcrop sections in the Oman Mountains.

Synonym:

An interbedded sequence of chalky limestones and calcareous shales subdivided into members, each comprising an upper limestone overlying a basal shale. The shales are distinctive, varicolored olive brown and green, which is unlike other shales but identical to the Nahr Umr in general fauna and lithology. Source-rock facies (bituminous limestones) occur in the B and E members in the North and north Central Oman (van Buchem et al., 1996; also Homewood et al., 2008). The repetitive sedimentary cycles ranging from several tens to 150 m in thickness and these have been used to subdivide seven informal members designated by the letters A–G from top to base (Hughes Clarke, 1988; Scott, 1990). Philip et al. (1995) proposed a correlation of these informal members to the outcrop sections in the Oman Mountains. Each member represents a sedimentary cycle: from basal, generally thin, variably argillaceous mud- supported units followed by thick shallowing-upward carbonates ending with shell-rich or pelletal-skeletal, grain-supported units. Superimposed on these cycles two to five, third-order, generally shoaling-upward sequences have been distinguished (Scott, 1990; van Buchem et al., 2002; Droste and van Steenwinkel, 2004; Homewood et al., 2008).

Members: G = basal thin carbonate; .F = relatively thin shale to carbonate, E = thick lower carbonate (latest Albian) and upper medium-thin shale (earliest Cenomanian), D-C (upper Cenomanian) and B-A (Cenomanian/Turonian transition) = relatively thin cycles of shale to carbonate.

Ammonites, microfossils, nannofossils, etc. Biozones F62 (Dohaia planata, Dictyoconella minima) to F57 (Orbitolina concava), Sub- biozone F577 (Hedbergella washitensis).

It forms the interior part of an extensive carbonate platform that covered the eastern margin of the Arabian Shield during the mid Cretaceous. The Natih Fm records the transition of the Arabian Gulf from a tectonically stable shelf margin to a flexural basin, subsiding in response to the westward obduction of an accretionary prism (Semail/Hawasina). In the Oman area the depositional setting of the Natih Formation alternated between an extremely broad and flat, submerged-to-emergent platform setting (G, F, D and C members), and moderately deeper-water, intra-shelf basins, with anoxic conditions allowing deposition of source rocks (E, B and A members) (van Buchem et al., 1996, 2002; Homewood et al., 2008). Droste and van Steenwinkel (2004) provide a detailed seismostratigraphic underpinned model for the development of the Natih intra-shelf carbonates.

Further studies focussing on the Oman outcrops were published by Schwab et al. (2005), building a seismic model based on outcrop data, and Grélaud et al. (2006) documenting incision surfaces in the Natih. The outcrop sedimentology in Oman is extensively documented by Homewood et al. (2008).