Akhdar Gr

The Akhdar Group was established by Glennie et al. (1974) in the Al Jabal Al Akhdar area of the Al Hajar Mountains of northeast Oman to include a sequence of Permian – Triassic carbonates. Column: Oman Subsurface. Type section is the Al Jabal Al Akhdar, Al Hajar Mountains, where it comprises the Permian limestone Saiq Fm, overlain by the Triassic dolomite Mahil Fm (Glennie et al., 1974). In the subsurface, the Akhdar Group comprises the upward succession of the Khuff Fm, Sudair Fm, Jilh Fm and Minjur Fm. This four-fold subdivision was originally established in the outcrop area of Saudi Arabia and it has become standard lithostratigraphic terminology of the subsurface Permian – Triassic succession on the Arabian side of the Gulf region (Al Aswad, 1997; Sharland et al., 2001).

Synonym:

Going downward: The Minjur Fm consists mainly of clastic sediments, including soils. The Jilh Fm and Sudair Fm comprise dolomitic carbonates with subordinate red and grey-green shales and anhydrites, whereas the Khuff Fm is limestone dominated with variable dolomite development (which tends to be greatest in the Middle Khuff, where specific levels of anhydritic carbonates are also seen). Moving south- and eastwards the Khuff Fm generally thins (Upper Khuff is absent in South Oman) and increases in clastic content with red claystones dominating the Lower and to a lesser extent the Middle Khuff. Oolitic grainstones in the upper part of the Khuff Fm are the main reservoir facies in northwest Oman. Sequence stratigraphy: The Akhdar Gr represents most of the Sharland et al. (2001) AP6 Megasequence. Sharland et al. (2001) correlate their MFS TR70 to MFS P20 surfaces through the Jilh Fm, Sudair Fm and Khuff Fm (see relevant formational sections). Sharland et al. (2004) revise the ages of the Tr10, P40 and P30 MFS surfaces.

"The Akhdar Gr is recognized by the presence of Triassic to Permian Palynozones 2255-2103/?2252. The Permian micropaleontological FZA Biozone (Hemigordius sp.-Schwagerina sp.) is currently used, but ongoing subsurface and outcrop work suggests the potential for several regionally recognized biozones within the Khuff Fm in particular. The original palynological scheme set up by Mabillard et al. (1985), has remained relatively unchanged and has been applied rather sporadically. The 2255 Palynozone was originally applied to the Mafraq Fm by Kharusi (1989), and is herein reassigned to the Minjur Fm. The reinterpretation of data in Jacovides et al. (1998) indicates that the Minjur Fm should now be considered of Rhaetian age and the upper Jilh Fm of the Lekhwair area is of probable Norian age (see discussions in those formation entries).

Osterloff et al. (2004a) suggest a shift in definition in terms of marker species used to recognise the 2103-2252 transition. As a consequence, they restrict Palynozone 2252 to the Gharif Fm with Palynozone 2103 ranging down to the base of the Khuff. However, much uncertainty remains and the position of top Palynozone 2252 still requires confirmation.

In the Khuff Fm in particular, ongoing work has changed the previously assigned ages. Data from outcrop (Angiolini et al., 2004; Forke, 2009; Koehrer et al., 2010) implies a Wordian age for the base of the Khuff Fm. Recognition of the end Permian extinction event (Vachard and Forbes, 2009; Forke, 2009; Koehrer et al., 2010) and other lines of evidence indicates that the Permian – Triassic boundary can be placed at the Middle – Upper Khuff transition. The Palynozones of Mabillard et al. (1985) and Kharusi (1989) are in much need of revision (see discussions at formational level), with respect to definition, ages assigned and the formations in which they are recorded. Ongoing micropaleontological work will greatly improve Khuff Fm resolution."

"Deposition of the Akhdar Gr followed the initial phases of the breakup of the Permian super- continent Pangaea (Konert et al., 2001). The subdivisions of the Akhdar Gr are depositional sequences that can be directly related to transgressions and regressions imposed on an overall stable and low-relief shelf setting. A major transgression during Lower Khuff Fm times established a wide epeiric carbonate platform with low relief under dominantly arid conditions and dominated by carbonate sedimentation. The worldwide Permian – Triassic extinction event in Outcrop occurs at the Saiq Fm – Mahil Fm transition, associated with the almost complete disappearance of macrofossils. In the subsurface, the same level at the transition of the Middle to Upper Khuff Fm Member is associated with the virtual disappearance of all marine microfauna.

Renewed uplift, most likely of the rift shoulders caused a northwestward tilting and deep erosion at the end of the Triassic. For Oman this period is represented by the fluvial to nearshore marine clastics of the Minjur Fm, which are well developed only in the northwest of North Oman and only patchily preserved elsewhere in North Oman. The Minjur Fm was followed by a hiatus of some 13 My duration associated with extensive soil formation.

Most of the Akhdar Gr sediments were deposited under dominantly arid conditions on an extensive flat and shallow carbonate platform setting from proximal (landward) in South Oman to more distal (seaward) in North Oman. The succession reflects a series of transgressions and regressions that are associated with eustatic sea-level changes that have a direct response in this shallow, wide, relatively stable carbonate platform setting (Al Aswad, 1997; Al-Jallal, 1995; Angiolini et al., 1998; Osterloff et al., 2004a). Clastic sedimentation, recorded throughout the Arabian Platform as the Rhaetian Minjur Fm, and equivalents, relates to a rifting phase associated with the break up of Pangaea (Al-Husseini, 2008)."