Rahab Shale Fm
Type Locality and Naming
"The final deposits of the Al Khlata Fm are mostly shales in southeast Oman (the Rahab shales; separated here as the Rahab Shale Fm), usually with a content of rhythmically laminated (varve-like) shales. Although strictly a subunit of the P1 Member, the P1 Rahab is worthy of separate consideration by virtue of its unique and distinctive character." Never formally published. Referred to as the Rahab Shale in Levell et al. (1988) and as the Rahab Shale Member by Kharusi et al. (1995). Type section is Hajal-1 in South Oman. Column: Oman Subsurface, Oman Outcrop
Synonym: Al Khlata P1 Mbr, Rahab Mbr, Rahab Fm
Lithology and Thickness
The P1 Rahab is dominated by glaciolacustrine mudrock. "The Rahab shales are dark grey and lithologically similar to the shales of the Lower Gharif Fm and may contain plant remains, but they are restricted to Palyno-subzone 2141B. The shales are locally rhythmically laminated and contain dropstones with the abundance of dropstones typically declining upwards (Aitken and Clark, 2007). Interbedded siltstones, sandstones and diamictites occur locally. In Hajal-1 and the northern Eastern Flank, the interbedded sandstone or pebbly siltstone/ diamictite is sufficiently thick and continuous to define two discrete lobes, which can be considered to represent a genuine ‘Upper’ and ‘Lower’ Rahab Shale (Kharusi et al., 1995; Aitken and Clark, 2007)." On the Eastern Flank and in the South Oman Salt Basin the P1 Rahab is recognised in shaly facies. Further to the south it has been interpreted as a sandier facies with a Rahab sandstone overlying the Rahab Shale (Kharusi et al., 1995; Stephenson and Osterloff, 2002), but this cannot be conclusively supported without much more rigorous biostratigraphical control. In the type well, and in the northern Eastern Flank, interbedded sandstones and siltstones/diamictites thicken sufficiently to separate two distinct shaly units (Osterloff et al., 2004b).
Relationships and Distribution
Lower contact
The Rahab Shale unit is technically the uppermost sub-member of the Al Khlata Fm.
Upper contact
The boundary with the Gharif Fm is at the top of this shale unit, beneath Gharif basal sands. Work by Stephenson and Osterloff (2002) and Osterloff et al. (2004a) suggested that sediments in the Lower Gharif Member (specifically the coarse- grained clastics) may be, in part, coeval with the Rahab Shale. In this interpretation, glacially-influenced sedimentation may continue into units that lithostratigraphically are part of the Lower Gharif Member. This scenario is debatable and reworking and/or truncation may also be responsible. Furthermore, some of Stephenson and Osterloff (2002) interpretations may be based on erroneous lithostratigraphical assignments (Penney, personal communication).
Regional extent
The P1 Rahab is confined to the Eastern Flank and the South Oman Salt Basin. The Rahab lacustrine shale facies has not been recorded in Central and North Oman, where the differentiation of upper Al Khlata sands from initial non-marine Basal Gharif sands becomes difficult. Recognition of Palyno-subzone 2141B is critical in this respect, but sufficient data is often lacking.
GeoJSON
Fossils
Palyno-subzone 2141B (Cycadopites cymbatus Palynozone). This distinctive Palyno- subzone is characterized by an influx of Microbaculispora spp. associated with reduced numbers of Horriditriletes spp. and C. cymbatus.
Age
Depositional setting
Additional Information