Kahmah Gr
Type Locality and Naming
The Kahmah Group comprises a thick carbonate sequence. Column: Oman Subsurface. Type section is Al Jabal Al Akhdar outcrops, named after Wadi Kahmah on the south side of the massif. In subsurface use, the Group is divided into an upward succession of seven formations: (1) Mesozoic Clastics Fm, (2) Rayda Fm, (3) Salil Fm, (4) Habshan Fm, (5) Lekhwair Fm, (6) Kharaib Fm, and (7) Shu’aiba Fm. The Mesozoic Clastic Fm is confined to South Oman. Additional reference sections covering the Shu’aiba Fm, Kharaib Fm and Lekhwair Fm are Lekhwair-258, Al Huwaisah-2, and Lekhwair-7 in North Oman, Hazar-2 and Hasirah-1 in Central Oman, and Jazal-1 in South Oman. Reference sections covering the Habshan Fm, Salil Fm and Rayda Fm are Dhulaima-4 and Natih-124 in North Oman. The reference sections for the Mesozoic Clastics Formation are in South Oman: Dimeet-1, Al Burj-4, Barah-1, and Marmul-287.
Synonym:
Lithology and Thickness
The Kahmah Gr comprises a thick carbonate sequence, ranging from deeper marine pelagic porcellanites, cherts and marls in the lower part to shallow-marine shelfal carbonates in the upper part. Partly time-equivalent clastics, ranging from shales to conglomerates, represent the Mesozoic Clastics Fm. The upper and lower boundaries of the Mesozoic Clastics Fm represent major truncation surfaces. Sequence stratigraphy: The Kahmah Gr represents the lower part of the AP8 Megasequence of Sharland et al. (2001). Sharland et al. (2001) correlate their MFS K80 to K10 surfaces into the Kahmah Gr of Oman (see formational discussions for detail).
Relationships and Distribution
Lower contact
The base is almost everywhere a major sedimentary break, typically from the shallow- marine Sahtan Fm carbonates to pelagic facies of the basal Kahmah Gr.
Upper contact
Regional extent
The sequence thins from north to south, mainly related to progressively deeper erosion associated with the overlying base Wasia Gr unconformity. The lower Kahmah (Rayda Fm, Salil Fm and Habshan Fm) is limited to areas north of ca. 21°N latitude. The upper Kahmah (Lekhwair Fm, Kharaib Fm and Shu’aiba Fm) extends further south, but south of ca. 20°N, it is progressively cut out beneath the basal Wasia Gr, and south of ca. 19°30 ́N only sporadic thin remnants occur, until a thicker Lower Cretaceous sequence is developed west of Salalah. Well Jazal-1 represents the thickest Kahmah Gr subsurface section in South Oman, delineated as undifferentiated Lekhwair Fm / (Kharaib Fm). A trend of decreasing argillaceous content to the northeast, towards the shelf margin, reduces the potential to split the Upper Kahmah Gr into its constituent formations. In the Jabal Al Akhdar outcrop sections, the Lekhwair Fm, Kharaib Fm and Shu’aiba Fm equivalents are difficult to differentiate and form a single, continuous shallow-marine carbonate unit.
The Mesozoic Clastic Fm is confined to South Oman in an approximately 300 km long NE-SW trending depositional low. This is associated with salt withdrawal along the Eastern Flank of the South Oman Salt Basin..
GeoJSON
Fossils
Biozones F56 (Orbitolina (M.) parva) to F51 (Calpionella alpina, C. elliptica) are documented by Sikkema (1991). The older sediments can usually only be analyzed using thin-sections, the younger may be analyzed with conventional washed samples. These zones are fully defined by Sikkema (1991) but subsequent published and unpublished reports suggest that a major update of these zones is merited. See formational discussions below for detail. Nannoplankton analyses have been applied to a lesser degree. No comprehensive zonation has been documented, but this fossil group perhaps provides the greatest potential for calibration of parts of the Kahmah Gr to well documented worldwide oceanic events. See formational discussions.
Late Jurassic – Early Cretaceous Palynomorph zones include those of Clarke (1968). This scheme has effectively fallen into disuse due to the dominance of conventional micro- paleontological based studies in these carbonate sections. Even more so than the micropaleontological zonation these zones are in need of revision. Recent work suggests that Middle East palynological knowledge has matured to the extent that the discipline can make a significant contribution to our understanding in parts of the Kahmah Gr (e.g., Simon Petroleum Technology, 1995; Shaw, 2009).
Age
Depositional setting
Additional Information