5.27 Kahmah Group (Zones VI and VII) Hajar Super-Group

Author:
New name, here proposed.

Synonym:
The 'Thamama Group' as previously used by Morton (1959), Tschopp (1967a,b), Wilson (1969) and in unpublished oil company reports on Oman surface geology. The name Thamama is maintained in Oman for the time being in subsurface work. In its type locality in Saudi Arabia, the Thamama Group is in part defined by the presence of the underlying Hith Anhydrite (Powers et al., 1966). Since the latter is absent in the Oman Mountains, the name Thamama cannot be used.

General description:
A thick carbonate sequence ranging progressively from deeper-marine pelagic porcellanites and cherts at the base to shallow-marine limestones at the top. Age range from Tithonian to Aptian (Zones VI to VII).

Type locality:
Wadi Kahmah and other wadis draining the Jebel Akhdar area. The best studied section is that in Wadi Mi'aidin (Fig. 5.17.2).

Thickness:
The thickness of the unit is subject to differences in the erosion prior to the deposition of younger strata. There is a general trend in its outcrop area, however, which indicates a stratigraphic thinning to the east and northeast, and a thickening in the opposite direction towards the Rub al Khali. A similar trend is observed in the subsurface (Ziegler, unpublished KSEPL report). The observed thickness ranges from zero in northeast Saih Hatat to ca. 750 m in the southwest (Wadi Mi'aidin).

Lithology:
The unit consists of a lower part of thin- to thick-bedded, microbioclastic mudstone and wackestone with very fine, thin-bedded, porcellaneous mudstones ('porcellanites') containing black chert nodules at the base; and an upper part of predominantly thick-bedded, bioclastic and pelletal wackestone and packstone, alternating with thinner-bedded, bioclastic mudstone and thicker-bedded, skeletal, oolitic and pelletal grainstone, the latter being commonly fossiliferous (e.g. Orbitolina discoidea). In the upper part of the sequence the wackestone contains rudists (monopleurids and caprinids), while foraminiferal grainstone bands and 'coquina' beds of thick-shelled molluscs become increasingly abundant toward the top. This was especially noted in the Jebel Nakhl area.
The succession of lithologies reflects an overall shallowing sequence, from starved porcellanites deposited on a deeply submerged shelf at the base, to shallow, open-marine shelf limestones at the top. The upper part in particular shows a repetition of small-scale sedimentary cycles of mudstone grading into grainstone. [A subdivision of the formation into three megacycles as described from the subsurface (Ziegler, unpublished KSEPL report) could not be observed in outcrop].
It is possible to subdivide the Kahmah Group in the field into smaller lithostratigraphic units, especially along the southern flank of Jebel Akhdar where these units attain a certain lateral extent. The variations in lithology are reflected by differences in the morphology of the units. Towards the north, across Jebel Akhdar, these units become inseparable in the field. No attempt has been made to define or name formational units in the field and the Kahmah Group is not subdivided on the map (Encl. 2).

Fossils:
Microfauna, particularly foraminifera, are common in the upper part. Rare tintinnids occur near the base. The fauna of the upper part of the formation (e.g. Orbitolina discoidea, Bacinella, Choffatella decipiens) indicates a Zone VII, Barremian-Aptian age. Nannoplankton and tintinnids of Zone VI, Tithonian to lowest Cretaceous, age were found in the Porcellanites.
Megafossils collected by PD (Oman) geologists also indicate a general early Cretaceous age; in particular the following corals:

• Thamnasteria duparci KOBY

• Thamnasteria regularis DE FROMMENTEL

• Actinastrea bernensis-pseudominima KOBY

• Coccophyllum acanthophorum FRECH

Age:
Zones VI and VII, Tithonian to Aptian.

Boundaries:
The underlying contact is stratigraphic and conformable over the shallow marine carbonates at the top of the Sahtan Group.
The upper contact is stratigraphic and conformable but is a widely recognisable disconformity corresponding to emergence before transgression of the marls at the base of the Wasia Group (Nahr Umr Formation).

Distribution:
The Kahmah Group is mainly known in the outcrop areas around Jebel Akhdar and Saih Hatat.

Variations in other sections:
The age equivalent upper Musandam Group of the Ruus al Jibal is lithologically very similar. Where present, the Kahmah Group shows little variation in lithofacies; only a general northeasterly coarsening and shallowing of the sequence can be observed across Jebel Akhdar and onto the north flank of Jebel Nakhl. Along the northern and eastern sides of Saih Hatat, the Kahmah Group was eroded prior to the deposition of the Muti Formation and/or Tertiary. In Wadi Mijlas, the sequence is too recrystallised for its facies to be studied.
Two exposures of the Kahmah Group were observed at Jebel Madar and in the Bani Khalid area, south of the mountain belt. Here, the Kahmah Group contains a more grainy facies in its upper part, above the 'Porcellanites', consisting predominantly of thick- and well-bedded, skeletal, fossiliferous pelletal packstone and grainstone with rudists and Orbitolina spp. In the Bani Bu Khalid area, the Kahmah Group is overlain by sheared shales and silts with some limestones, assigned to the Muti Formation. This is the only exposure of the Kahmah Group observed south of Jebel Akhdar and Saih Hatat, apart from that in the core of Jebel Madar, south of the main belt of Hawasina rocks.

Remarks:
The sediments of the Kahmah Group were deposited on the Arabian Continental Margin. They evidence a regional deepening in the late Jurassic shown by the sharp change from the shallow-marine Sahtan sediments to the pelagic porcellanites at the base of the Kahmah. There was then a steady shoaling-up sequence (sedimentation > subsidence) to very shallow-marine conditions in the Aptian (rudistid limestones), followed by emergence before an Albian transgression (the Nahr Umr Formation).

GLENNIE K.W., BOEUF M.G.A., HUGHES CLARKE M.W., MOODY-STUART M., PILAAR W.F.H., & REINHARDT B.M. [1974].- Geology of the Oman Mountains. Verhandelingen van het Koninklijk Nederlands geologisch minjbouwkundig Genootschap [Transactions of the Royal Dutch Geological and Mining Society], Deel 31, part one (text): 423 p., part two (tables and illustrations), part three (enclosures).
From pages 140 to 142.
KNGMG © 1974
Reprinted by permission of the KNGMG whose permission is required for further use.