Mafraq Fm

The mixed clastic-carbonate Mafraq Formation (Early – Middle Jurassic) is the lowermost unit (1 of 5) of the Sahtan Gr and comprises a fluvial to shallow-marine succession that onlaps the tilted and eroded Akhdar Gr (Permian – Triassic) from the northwest to the southeast. Column: Oman Subsurface, Oman Outcrop. Type and reference sections: Lekhwair-70 in North Oman is proposed as a new subsurface type section. Mafraq-1 (the original type section of Hughes Clarke, 1988) is now interpreted to contain the new Minjur Fm (‘Oman Soil’), overlain by probably only Upper Mafraq. It is used here as an additional reference section, together with Saih Rawl-27 in North Oman and Zauliyah-5 in Central Oman. The oldest, Early Jurassic, Mafraq sediments occur in the northwest (basinward) in the vicinity of Lekhwair where it is also thickest (ca. 150 m). In this area the Mafraq Formation can be subdivided into a carbonate Upper Mafraq Mbr and a clastic Lower Mafraq Mbr.

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In northwest Oman the Mafraq is very thick and can be subdivided into the Lower and Upper Mafraq. The Lower Mafraq consists of flood-plain to shallow-marine sandstones and shales, and is usually clay-dominated. The ease of well-to-well correlation varies and these sands probably represent both amalgamated and single fluvial channel fills. The Upper Mafraq is more carbonate-dominated, capped by distinctive, mainly red shales which are interpreted to be soils. On platform areas where the Mafraq cannot be subdivided, it is a thin unit with clastics, subordinate carbonates and soils.

The Mafraq Fm spans Biozones F45 (Pfenderina salernitana, P. trochoidea) to F30 (Haurania deserta-Orbitopsella praecursor-Pseudocyclammina liassica) and Palynozones 2262 (Spheropollenites scabratus) and 2242 (Corollina spp.). The Upper Mafraq section may also yield Palynozone 4191 (Dichadogonyaulax spp.).

"The Lower Mafraq is a mixed continental-marine unit with an overall strong terrestrial character and is virtually restricted to the Lekhwair-Dhulaima area. It contains a significant marine pulse of Early Toarcian age (Jacovides et al., 1998). Rousseau et al. (2006) describe the Lower Mafraq as a wedge of floodplain deposits that onlaps to the east and southeast. Based on reinterpretation of Jacovides et al. (1998) data it appears that Lower Mafraq deposits do not usually extend onto platform areas south of the greater Lekhwair-Dhulaima area. Thin pockets of deposition may occur elsewhere, e.g., possible occurrence in the Al Huwaisah area. In this respect a better understanding of the Lower Mafraq soils has been of key importance, prompting the reassignment of the Lowermost Mafraq as previously used in the subsurface to the Minjur Formation of the Akhdar Group (van Steenwinkel, 2009). In the final Early Toarcian Lower Mafraq package (e.g. the high Gamma shale in Lekhwair-70, Figure 8.13) influxes of marine taxa suggest significant marine influence. The presence of associated amorphous kerogen highlights bottom water anoxia and limited current activity.

The Upper Mafraq is a shallow- marine sequence with significant macro- and microfossil content. Interestingly the dominantly red shales at the top of the Upper Mafraq occasionally yield very low-diversity miospore assemblages with high counts of fungal debris. This supports the interpretation that these may be soil related.

Rousseau et al. (2006) discuss at length the geometries and development of the Lower and Upper Mafraq units, however, Jacovides et al. (1998) suggest that the base Upper Mafraq should be placed at the base of the dolomite unit in their key well Lekhwair-319. This would place the Lower Mafraq Limestone Marker of Rousseau et al. (2006) within the Upper Mafraq and negate their correlation of this unit across the region. In this interpretation a south-easterly directed onlapping geometry still applies but there is also a clear expansion in the Upper Mafraq section, in particular, towards the northwest."