CHAPTER 11 MISFAR GROUP
Published:January 01, 2010
The geological history of the large time gap between deposition of the Haima Supergroup and the Haushi Group in which the Misfar Group sediments occur is largely unresolved. Little is known about sedimentation in the Oman realm during the Devonian – early Carboniferous as hardly any sedimentary record is preserved. The distribution of the Misfar sediments is patchy along the eastern and northeastern flanks of the South Oman Salt Basin, with rare occurrences in the Ghaba Basin of North Oman. The thickness, distribution and preservation of Misfar sediments were probably controlled at least in part by Ara salt withdrawal and dissolution.
Konert et al. (2001) discuss the distribution of the Silurian – Devonian sediments on the Arabian Plate. Lower Devonian sediments are missing in Iraq and Turkey whereas a complete Devonian section is present in Saudi Arabia. Devonian sediments were therefore also probably more widespread in Oman and subsequently eroded during the Carboniferous, as also suggested by Faqira et al. (2009). Near the present-day east coast of Oman, erosion cuts down to Huqf sediments and even basement. In the north (Al Jabal Al Akhdar, Saih Hatat) gently folded Neoproterozoic – Cambrian sediments are unconformably overlain by Permian carbonates.
Devonian sediments were first interpreted by Pokorny and Hopping (1964). The Misfar itself was first distinguished in well Misfar-1 (Figure 11.2), which was drilled in 1973. A Middle to Late Devonian age was assigned to the Formation based on palynomorph assemblages (Winkler, 1975). In a review of the Palaeozoic palynology of Oman, Potter (1982) refined the age range of the Misfar and placed it within the Emsian to Givetian. Schuurman (1984) carried out the first regional Misfar study, which concluded that the unit was originally probably aerially extensive, as supported by Devonian reworked palynomorphs within the Permian – Carboniferous Al Khlata Formation. Its age was refined to the Early Devonian (Emsian). Hughes Clarke (1988) subsequently assigned Group status to the Misfar. Oil exploration in the eastern flank of the South Oman Salt Basin in rocks of the Misfar Group added additional well penetrations and prompted further industry studies in the 1990s supported by the University of Sheffield and the British Geological Survey (respectively Owens, 1999; Owens et al., 1999). A review of this and other well-related work by Osterloff (2000b) supported the presence of younger, Late Devonian – early Carboniferous (Famennian – Tournaisian), Misfar sequences in addition to the Emsian – Eifelian succession recognised previously. His work also indicates that correlations and interpreted depositional sequences (Verstralen and Partington, 2000) should be considered with great care until better data become available.
Lithology: The Misfar Group is a sequence of (often organic) shales, quartzose sandstones (in the Misfar area only) and sandy limestones. The limestones contain ostracods and stromatolite-like structures, whilst the fine clastics yield palynomorphs.
Subsurface recognition: Due to the lithological variation within the Misfar and the, in part, lithological similarities with the overlying Al Khlata Formation, identification and picking of the sequence on logs can be difficult.
Whilst drilling, the Misfar is recognised by a marked change from the coarse sublithic and often conglomeratic sandstones of the overlying Al Khlata to varicoloured, dark grey, brown-grey, green-grey frequently organic shales/siltstones and fine, quartz-rich sandstones of the Misfar (Figure 11.4). Carbonates (wackestone and packstones with common ostracods and some algal mats) are found in the upper part in the Misfar area.
The lower boundary is probably not so clear during drilling and may be difficult to assess due to the similarity of the Misfar sandstones with the underlying sand-prone formations of the Haima Supergroup. The predominantly grey shales of the Misfar contrast with the red and green shales of the Ghudun (grey shales are absent from the Ghudun).
Hotshot samples should confirm the presence of Palynozone 2121 (Devonian) or the younger unzoned Famennian – Tournaisian (Late Devonian – early Carboniferous) assemblages. In situ identification of these can be hampered by the presence of reworked Devonian sediments in the Al Khlata.
Post-drilling the presence of the Misfar can be confirmed by further lithological and palynological analysis, and by the stratigraphic position relative to underlying formations with more diagnostic log patterns, e.g. the Ghudun and the Amin.
Boundaries: Everywhere the Misfar Group is unconformably overlain by the Al Khlata.
It unconformably overlies the Ghudun Formation, Amin Formation (Hadh-1, Figure 11.3) and Ara Group.
Distribution: The Misfar has a very limited geographical extent. It is present in some 20 wells in the subsurface of southeast Central Oman. It is also potentially present, as only the younger Famennian – Tournaisian sequence, in a small number of wells in the Ghaba Basin of North Oman (see Osterloff, 1999).
The Formation was probably more aerially extensive, as indicated by the frequent occurrences of reworked Devonian palynomorphs within the younger Permian – Carboniferous Al Khlata Formation. Stratigraphic evidence from oil-wells has an in-built bias in being sited on structurally-high areas, where erosion is greatest. There is therefore a possibility that the Devonian units could be more widely encountered in structurally-low areas. All the evidence to date points to the preserved units of the Devonian only occurring in salt-related depocentres. Due to its separated stratigraphic position, this Devonian sequence is deemed sufficiently important to warrant Group status, but no further subdivision is supported until a more consistent data set is available.
No Misfar equivalent or other Devonian – early Carboniferous sediments are known from outcrop in Oman.
Deposition:Osterloff (2000b) reports coastal plain and shoreface sandstones that are capped by a marginal-marine unit that may represent a flooding event in deposits tentatively assigned to an Emsian – Eifelian sequence.
Marine, ostracod-rich carbonates are also recorded in the upper part of the section from the Misfar area. A younger Famennian – Tournaisian sequence is dominated by fluvial and flood-plain deposits, grading up to possible marginal marine facies.
No obvious log characteristics allow these ‘sequences’ to be more widely recognised or correlated. Reworking of the earlier Middle – Late Devonian sediments into successively younger sequences also hampers an accurate age assessment. With these restrictions it is difficult to establish systematic facies successions or correlate the available preserved sections.
Subdivision: Attempts have been made to subdivide the Misfar, but with the limitations discussed above this has to be seriously questioned until better data become available. Osterloff (2000b) suggested two distinct stratigraphic units that can be differentiated based on palynological data originally interpreted by Owens (1999) and Owens et al. (1999). The same age considerations discussed by Osterloff (2000b) negate earlier lithostratigraphic correlation (Verstralen and Partington, 2000).
Age: Early – Middle Devonian, Emsian – Eifelian, ca. 407.0–391.8 Ma and Late Devonian – early Carboniferous, Famennian – Tournaisian, ca. 374.5–345.3 Ma (see Osterloff, 2000b). Sharland et al. (2001) place their Emsian, MFS D20 and Pragian MFS D10 in marine mudstones towards the top and base, respectively, of the Misfar section (sensuVerstralen and Partington, 2000). Osterloff (2000b) questions the marine character and correlatability of the lower mudstone. The younger Famennian – Tournaisian section could encompass the latest Famennian MFS D30 of Sharland et al. (2001).
Biostratigraphy: The Emsian – Eiffelian palynomorph assemblages are assigned to Palynozone 2121 (Grandispora velata). The Famennian – Tournaisian assemblages reviewed by Osterloff (2000b) have not been assigned to a palynozone. Miospores recorded in these younger Misfar sections include Aratrisporites saharaensis, Vallatisporites pusillites, V. vallatus and Retispora lepidophyta. The latter species is essentially restricted to the Famennian, but is commonly reworked into the Carboniferous.
A stratigraphical footnote to the ‘Misfar story’ is provided by Penney (1998), who recorded distinctly younger miospore assemblages of a general Tournaisian? – Visean age in one well from the Ghaba Basin. Unconformably overlain by Upper Gharif this early Carboniferous ‘remnant’ is a succession of sands and clays, with dolomite and limestone streaks and has not been formally assigned to any lithostratigraphical unit. A 10-m thick dolomite in the upper part of the section could potentially contain the late Visean MFS C10 surface of Sharland et al. (2001).
Figures & Tables