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GROUPFORMATIONMEMBER
AkhdarMinjur 
Jilh 
Sudair 
KhuffUpper Khuff
Middle Khuff
Lower Khuff
GROUPFORMATIONMEMBER
AkhdarMinjur 
Jilh 
Sudair 
KhuffUpper Khuff
Middle Khuff
Lower Khuff

Introduction

The Akhdar Group was established by Glennie et al. (1974) in the Al Jabal Al Akhdar area of the Al Hajar Mountains to include a sequence of Permian – Triassic carbonates. A two-fold subdivision into Saiq and Mahil formations is recognised in this outcrop area. In the subsurface, the Akhdar Group comprises in ascending order, the Khuff, Sudair, Jilh and Minjur formations. 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). Renaming the original Lower Mafraq (Hughes Clarke, 1988) as the Minjur Formation of the Akhdar Group, as proposed by van Steenwinkel (2009), is formalised herein.

Deposition of the Akhdar Group followed the initial phases of the break up of the Permian supercontinent Pangaea (Konert et al., 2001). A major transgression during Lower Khuff times established a wide epeiric carbonate platform with low relief under dominantly arid conditions and dominated by carbonate sedimentation. The subdivisions of the Akhdar Group are depositional sequences that can be directly related to transgressions and regressions imposed on an overall stable and low-relief shelf setting.

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 Formation, which are well developed only in the northwest of North Oman and only patchily preserved elsewhere in North Oman. The Minjur was followed by a hiatus of some 13 My duration associated with extensive soil formation.

The worldwide Permian – Triassic extinction event in outcrop occurs at the Saiq-Mahil transition, associated with the almost complete disappearance of macrofossils. In the subsurface, the same level at the transition of the Middle to Upper Khuff Member is associated with the virtual disappearance of all marine microfauna.

Type and reference sections: Al Jabal Al Akhdar, Al Hajar Mountains, where it comprises the Permian limestone Saiq Formation, overlain by the Triassic dolomite Mahil Formation (Glennie et al., 1974). The subdivision in the subsurface is different with the use of the Khuff, Sudair, Jilh (Hughes Clarke, 1988) and Minjur formations. For subsurface reference sections see discussions at Formation level.

Lithology: The Minjur Formation consists mainly of clastic sediments, including soils. The Jilh and Sudair formations comprise dolomitic carbonates with subordinate red and grey-green shales and anhydrites, whereas the Khuff Formation 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 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 Formation are the main reservoir facies in northwest Oman.

Boundaries: Over most of Oman, the lower boundary of the Akhdar Group is a conformable contact with the Haushi Group. In the Al Hajar Mountains the base unconformably overlies pre-Haushi sediments. Everywhere, the top of the Group is separated from Jurassic or younger sediments by an unconformity.

Distribution: The Akhdar Group is present throughout Oman, except in areas of southeast Oman where it is removed by Cretaceous erosion, with the Upper Cretaceous or Tertiary resting on older units. It thins from northwest to southeast, partly by erosion of upper units and partly by onlapping older units.

In particular the Minjur, Jilh and Sudair formations are restricted to the western areas of North, Central and South Oman and to the Al Hajar Mountains (Mahil Formation).

Deposition: Most of the Akhdar Group 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 Formation, and equivalents, relates to a rifting phase associated with the break up of Pangaea (Al-Husseini, 2008).

Subdivision: The Akhdar Group comprises the Khuff, Sudair, Jilh and Minjur formations.

Sequence stratigraphy: The Akhdar Group 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, Sudair and Khuff formations (see relevant formational sections). Sharland et al. (2004) revise the ages of the Tr10, P40 and P30 MFS surfaces.

Age: Middle Permian – Late Triassic, Wordian – Rhaetian, ca. 267–199.6 Ma.

Biostratigraphy: The Akhdar Group is recognised 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 recognised biozones within the Khuff Formation 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 Formation by Kharusi (1989), and is herein reassigned to the Minjur Formation. The original Ahkdar scheme is as follows:

ZoneSubzoneMarker speciesRelative age (original assignment)Formation
2255 Bartenia communisCarnian – ?RhaetianJilh – Minjur
2247Vesicaspora schemili(Norian – ?Rhaetian)Minjur
2241Partitisporites verrucosus(Carnian – Norian)Jilh – Minjur
2370 Triadispora crassaAnisianJilh
2351 Densoisporites nejburgiiScythian (Olenekian – Induan*)Upper Khuff – Sudair
1095Veryhachium spp.(‘Early’ Scythian (?Induan*))Upper Khuff – Lower Sudair
  Unzoned interval**Basal Upper Khuff
2103 Lueckisporites virkkiaeKazanian-Tatarian (Roadian – Capitanian*)Middle Khuff
2252 Hamiapollenites spp.Artinskian – KungurianUpper Gharif – ?Middle Khuff
ZoneSubzoneMarker speciesRelative age (original assignment)Formation
2255 Bartenia communisCarnian – ?RhaetianJilh – Minjur
2247Vesicaspora schemili(Norian – ?Rhaetian)Minjur
2241Partitisporites verrucosus(Carnian – Norian)Jilh – Minjur
2370 Triadispora crassaAnisianJilh
2351 Densoisporites nejburgiiScythian (Olenekian – Induan*)Upper Khuff – Sudair
1095Veryhachium spp.(‘Early’ Scythian (?Induan*))Upper Khuff – Lower Sudair
  Unzoned interval**Basal Upper Khuff
2103 Lueckisporites virkkiaeKazanian-Tatarian (Roadian – Capitanian*)Middle Khuff
2252 Hamiapollenites spp.Artinskian – KungurianUpper Gharif – ?Middle Khuff
*

Updated stage terminology.

**

Assumed post end-Permian extinction section.

The reinterpretation of data in Jacovides et al. (1998) indicates that the Minjur Formation should now be considered of Rhaetian age and the upper Jilh Formation of the Lekhwair area is of probable Norian age (see discussions below).

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 Formation 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 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 Formation. 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 micropalaeontological work will greatly improve Khuff Formation resolution. The current understanding of the Akhdar Group bio- and chronostratigraphy can be summarised as follows:

FormationMicropalaeontological zonePalynological zoneRelative age
MinjurUnzoned2255**Rhaetian
Jilh?2255**Norian
2370Anisian?/Ladinian – Norian
Sudair – Upper Khuff*2351 (with Subzone 1095)Olenekian
Upper Khuff*UnzonedInduan (Triassic)
Middle Khuff*FZA2103-?2252late Capitanian – late Changhsingian (Permian)
Lower KhuffWordian – Capitanian
FormationMicropalaeontological zonePalynological zoneRelative age
MinjurUnzoned2255**Rhaetian
Jilh?2255**Norian
2370Anisian?/Ladinian – Norian
Sudair – Upper Khuff*2351 (with Subzone 1095)Olenekian
Upper Khuff*UnzonedInduan (Triassic)
Middle Khuff*FZA2103-?2252late Capitanian – late Changhsingian (Permian)
Lower KhuffWordian – Capitanian
*

Ongoing work suggests several zones are possible in the Khuff.

**

Palynozone 2255 is in need of revision.

Minjur Formation

Authors: Bramkamp (unpublished, 1950), see Powers (1968), updated definition for Oman by van Steenwinkel (2009).

Introduction

The Minjur Formation was not previously recognised as a separate unit in the subsurface stratigraphy of Oman and the sediments were assigned to the Lower Mafraq Formation as part of the Sahtan Group. Kharusi (1989), Al Mauly and Stacher (1990), Jacovides et al. (1998) and Osterloff and Penney (1999) recognised the Triassic age of sediments in the Lekhwair and Qarn Nihayda areas, but these were assumed to be deposited following uplift and erosion of the Akhdar Group carbonates. Van Steenwinkel (2009) demonstrates a consistent correlation of the unit in the Lekhwair area of coastal-plain Minjur deposits, with palaeosols at the top of the unit. These are most likely correlatable with at least part of the ‘Oman soil’ that developed further to the south and east. On platform areas the ‘Oman soil’ may represent a composite Triassic (Minjur) – Jurassic (Mafraq) deposit. In the absence of biostratigraphy any such soil is here assigned to the Minjur Formation. The Minjur correlates well with the clastic Minjur Formation in adjacent UAE and Saudi Arabia (see Al Aswad and Al Harbi, 2000). Palynological data indicates a time gap of ca. 13 My following deposition of the Minjur Formation before sedimentation continued in the Late Pliensbachian with the onlapping Mafraq clastics. The Minjur Formation in Oman therefore represents a wedge of early-syntectonic deposits associated with renewed uplift along the Neo-Tethys rift shoulder in Oman during Late Triassic times. The post-Minjur unconformity represents a protracted period of emergence involving much of the Arabian foreland.

Type and reference section: The type section extends from the back slope formed by the top of the Jilh Formation up to the base of the Marrat Formation in the face of Khasm al Khalta, Saudi Arabia (Powers, 1968). The Oman subsurface reference section is Lekhwair-70 in North Oman (Figure 8.13).

Lithology: The Minjur Formation in Oman is represented by a coastal-plain siliciclastic unit in the northwest (Lekhwair area) and by lateritic palaeosols to the southeast. In the Al Huqf outcrops it is a relatively thin unit comprising deep red-coloured lateritic palaeosols, shales and sands of Triassic age, called the ‘Oman Soil’. The ‘Oman Soil’ has been penetrated by wells, such as Mafraq-1 (Figure 8.14).

The main depocentre of the Minjur Formation in Oman is in the Lekhwair area of northwestern Oman, where it can reach up to ca. 100 m thickness. In the subsurface it is characterised at the base by a thin transgressive sandstone, overlain by an overall shallowing coastal plain succession. Lagoonal shales are overlain by coastal-plain estuarine and fluvial sandstones. The estuarine sandstones, 10–20 m thick, are cross-bedded and contain gravels with darcy-range permeabilities. They have a characteristic blocky low Gamma signature, which is correlatable as a distinct belt in all the wells of the Lekhwair area. They are overlain by more heterolithic, tidally infuenced fluvial channel sands and floodplain/marginal-marine shales. The Formation is capped by a distinctive set of three palaeosols (Figure 8.13). As the Minjur thins onto the high towards the southeast, these seem to coalesce, potentially together with younger palaeosols, to ultimately form the ‘Oman Soil’.

Figure 9.1:

Location map: Akhdar Group.

Figure 9.1:

Location map: Akhdar Group.

Subsurface recognition: The clastics of the Minjur Formation are easily distinguished from the underlying carbonates of the Jilh and Sudair formations. They are very difficult to distinguish from the overlying clastics of the Mafraq Formation.

Post-drilling, the Minjur in the Lekhwair area has a typical Gamma response of the shallowing upward coastal-plain sequence. The soils at the top of the Formation have a characteristic high Neutron and particularly high Gamma response. The high uranium content that is evident in the spectral Gamma log can help to differentiate the palaeosols.

The Minjur Formation is characterised by Palynozone 2255.

Boundaries: The lower boundary in Saudi Arabia appears conformable, but becomes unconformable towards the east (Oman). The upper boundary is a significant hiatus to the onlapping Mafraq Formation of the Sahtan Group.

The top of the newly defined Minjur Formation in Oman is within what was previously called Lower Mafraq. In the Lekhwair area the succession is capped by three layers of palaeosols, within a shale-dominated sequence. Palynozone 2255 is recorded in this section. The uppermost soil usually has the highest Gamma reading and is defined as the top of the Minjur Formation.

On platform areas the ‘Oman soil’ may include both Jurassic (Mafraq) and Triassic (Minjur) deposits. In the absence of biostratigraphy any such soil is here assigned to the Minjur Formation.

Distribution: The main depocentre of the Minjur Formation in Oman is in the Lekhwair area of northwestern Oman, with a patchy distribution related to accommodation space created by salt withdrawal or by faulting across the rest of North Oman. On platform areas of North Oman it is represented, at least in part, by the thin ‘Oman soil’. These platform soils are here placed in the Minjur Formation, but as previously discussed, they may actually be an amalgamation of soils representing Triassic through to Middle Jurassic time. Further study is required.

The onlap of the Minjur towards the platform areas to the southeast is clearly visible on seismic. Within the flat-lying Lekhwair area, truncations and onlaps are more subtle.

Deposition: The Minjur Formation starts with a thin transgressive sand at the base, which disappears in a seaward direction towards the northwest. The basal sand is overlain by lagoonal shales, with a maximum flooding surface near the base. This flooding is correlatable throughout the Lekhwair area. From there, the Formation shallows upward into estuarine and fluvial sandstones, capped with palaeosols. The estuarine deposits contain gravelly cross-bedded sandstones with high-permeabilities. These sandstones are present in all the wells that currently penetrated the Minjur Formation in the Lekhwair area. They consist of two sub-units, with a distinct low Gamma spike in between that corresponds to a thin layer of coal or coal clasts. The lateral continuity of these sands suggests deposition as coastal-plain bars, rather than local channels. The presence of the coal layer in all the wells is probably related to sea-level fluctuation at parasequence scale, depositing swamp deposits on top of the bars during relative sea-level fall, followed by another sand during subsequent sea-level rise. The unit is overlain by more heterolithic fluvial sandstones and flood-plain shales. Tidal influence is indicated by alternating high and low energy deposits, represented by sands with clay drapes (sometimes stylolitised) and flaser bedding. Mud clasts are present at the high-energy bases of the cross-bedded sands. Small rootlets and plant remains indicate a swampy environment. The succession culminates in three layers of palaeosols.

Age: Late Triassic, Rhaetian, ca. 203.6–199.6 Ma.

Biostratigraphy: The uppermost soil, dominantly red claystone section has yielded Palynozone 2242 miospore assemblages, with significant fungal debris, but these are now considered to be facies dependent and not age specific in this context, as they also occur higher up within the Lower Mafraq section. The Late Triassic Palynozone 2255 has been recorded in the shales towards the base of this ‘red’ shale package and is particularly well developed (as Palyno-subzone 2241) in the more marine shaly section below the gravely sand.

Kharusi (1989) assigned a Carnian – Norian age (Palyno-subzone 2241) to the lowermost Mafraq (Minjur) in the Lekhwair-Dhulaima regions, but Jacovides et al. (1998) were unable to replicate this degree of interpretation, even when working on the same wells. They assigned only general Late Triassic age. Wells Lekhwair-85 and Dhulaima-7 seem of key importance in reconciling the two studies. Kharusi (1989) again forwards a case for a Carnian – Norian miospore assemblage in the lowermost Mafraq (Minjur) whereas dinocyst data from Jacovides et al. (1998) provides a compelling case for an uppermost Jilh Formation that is likely to be no older than Norian (influx of Rhaetogonyaulax wigginsii and presence of rare Wanneria listeri). Significantly, in well Lekhwair-85, Jacovides et al. (1998) also record the dinocyst Dapcodinium priscum in the basal Mafraq (Minjur), which restricts the age to no older than Rhaetian, thereby suggesting that Palyno-subzone 2241 should be extended up into the Rhaetian.

As a final biostratigraphical footnote, Osterloff and Penney (1999) report Norian – Carnian, ?Ladinian and even Anisian – Scythian ages for ostensibly Lower Mafraq clastics in two wells from the Qarn Nihayda area (in a salt withdrawal setting). Such ages are difficult to reconcile with other data.

Jilh Formation

Authors: Bramkamp (unpublished, 1945), see Powers (1968).

Introduction

In general, the Jilh Formation in Oman is poorly documented and poorly understood both in the subsurface and in outcrop, where it is generally truncated or eroded completely. As in the Sudair Formation, the depositional setting is marginal marine to possibly coastal plain, indicating a persistent shallow setting. Deposition of the Jilh Formation is followed by uplift and erosion associated with extensive weathering (Triassic laterites and bauxites of the Minjur Formation are observed in the Al Huqf area and locally preserved in the subsurface).

Type and reference sections: Jilh al’Ishar, Saudi Arabia (see Powers, 1968). Oman subsurface reference sections are Yibal-85 (Figure 9.2) and Lekhwair-70 (Figure 9.3), both in North Oman.

Figure 9.2

(facing page): Composite electrical logs, lithology and lithological description of the Jilh and Sudair formations, Akhdar Group, in well Yibal-85, North Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.2

(facing page): Composite electrical logs, lithology and lithological description of the Jilh and Sudair formations, Akhdar Group, in well Yibal-85, North Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.3:

Composite electrical logs, lithology and lithological description of the Jilh and Sudair formations, Akhdar Group, in well Lekhwair-70, North Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.3:

Composite electrical logs, lithology and lithological description of the Jilh and Sudair formations, Akhdar Group, in well Lekhwair-70, North Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Lithology: The Jilh Formation is a sequence of dolomites with subordinate red and grey green shales with a few thin interbeds of anhydrite. The shale content of the unit seems to increase from north to south but is difficult to assess because of progressive truncation. The finely crystalline dolomites are hard and tight and poorly fossiliferous; fossils are only rarely seen in cuttings. It contains no limestones (Figure 9.4).

Figure 9.4:

Ditch cuttings from the Akhdar Group: (a) Dolomite from the Jilh Formation in Lekhwair-70; (b) Dolomite from the Sudair Formation in Al Bashair-1; and (c) Shale from the Sudair Formation in Al Huwaisah-56 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 9.4:

Ditch cuttings from the Akhdar Group: (a) Dolomite from the Jilh Formation in Lekhwair-70; (b) Dolomite from the Sudair Formation in Al Bashair-1; and (c) Shale from the Sudair Formation in Al Huwaisah-56 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Subsurface recognition: Whilst drilling, the lithology change from the clastics of the Mafraq or the Minjur formations to dolomites is characteristic. However, this change could reflect penetration of Jilh, Sudair or Upper Khuff sediments as the dolomites of all three formations/members are lithologically similar.

Post-drilling the Formation can be recognised by log correlation, with supporting palynological dating. The Jilh Formation has a fairly uniform log character and the densities tend to be slightly lower than those of the Sudair Formation (Yibal-85, Lekhwair-70; Figures 9.2 and 9.3).

Boundaries: The lower boundary has been taken as conformable with the Sudair Formation, however, age uncertainties relating to the base Jilh indicates the presence of a potentially significant hiatus. The upper boundary is a disconformity (probably a 3rd-order sequence boundary), placed below the basal transgressive lag of the Minjur Formation, however, the time gap may be minimal in the Lekhwair area where Norian Jilh is overlain by Rhaetian Minjur. The marine Minjur is truncated by the Early Jurassic (Mafraq) unconformity southeast of the Lekhwair area and either Lower Mafraq or the Oman soil (potentially in part Minjur) lies directly on Jilh carbonates.

Distribution: In Saudi Arabia, the Jilh is a clastic unit in type section outcrop, but passes into carbonates in the subsurface, particularly the Lower Jilh of Powers (1968). It occurs only in wells in the west of Interior Oman, being progressively truncated to the south and east. The most complete sections are in the Lekhwair area. Its lateral equivalents are part of the Mahil Formation in Al Jabal Al Akhdar and the Ghail and Elphinstone units in Ruus Al Jibal outcrops of Musandam (see Glennie et al., 1974).

Deposition: The environment was probably marginal-marine to tidal-flat, although virtually no shelly fossils are recorded, and relict ooidal-pelletoidal textures are present.

Age: Middle – Late Triassic: Anisian?/Ladinian – Norian, ca. 245.9–203.6 Ma. As discussed in Sharland et al. (2001), Middle to Late Triassic age calibration is problematical throughout the Arabian Plate. They tentatively place their MFS Tr70, MFS Tr60 (both Carnian) and MFS Tr50 (Ladinian) surfaces in Upper, Middle and Lower Jilh clean limestone sections, respectively. It is possibly significant that they cannot recognise their Anisian MFS Tr40 outside Jordan, Syria and Iraq, as they imply a significant time gap between the Jilh and Sudair formations across the region.

Biostratigraphy: The Jilh Formation encompasses the 2255 (Bartenia communis) and 2370 (Triadispora crassa) Palynozones, with no micropalaeontological zones defined due to the overall poor microfaunal recovery. Work by Forke (2009) on outcrop samples (Al Jabal Al Akhdar) only yielded rare, very low diversity microfaunas and rare bioclasts, neither of which provided meaningful age determinations.

Mabillard et al. (1985) placed the Jilh Formation entirely within their Anisian Palynozone 2370, although in all but one well in their study recovery comes from the middle to lower part of the Jilh Formation. Palynological evidence from the Lekhwair area indicates that the upper Jilh can range as young as Norian, lower 2255 Palynozone (Jacovides et al., 1998). At this stage it is unknown whether the two zones are indeed mutually exclusive, are in part age equivalent (possible facies effect) or have been wrongly differentiated by two separate generations of palynologists.

The Mabillard et al. (1985) age interpretation of Palynozone 2370 is based as much on absent post-Anisian markers as those present, and relates to generally poor data coverage, limited recovery and low diversity assemblages. Essentially the maximum age of the 2370 Palynozone is open to interpretation, as the forms recorded are all general Middle to Late Triassic taxa (Anisian – Norian).

In summary, the upper Jilh is considered to be Norian in age (Palynozone 2255), whilst the lower Jilh is of uncertain age but may be as old as Anisian (Palynozone 2370). The implication is that the Jilh Formation could represent over 40 My of time, and it is felt that the maximum age of the Jilh Formation has yet to be conclusively defined, i.e. it may range only to the Ladinian rather than the Anisian. Clearly our knowledge of the Jilh is inadequate in terms of age calibration, sequence development and related breaks.

Sudair Formation

Authors: Gierhart and Dell’oro (unpublished, 1957), as ‘Sudair Shale’, see Powers (1968).

Introduction

This Formation is less shaly than the type Sudair section in Saudi Arabia. But it is consistently recognised by its position above the Khuff carbonates, the presence of reddened shales and by its assigned age.

In general, the Sudair Formation (like the overlying Jilh Formation), is poorly documented and poorly understood.

Type and reference sections: Al Arid escarpment, Saudi Arabia (see Powers, 1968). Oman subsurface reference sections are Yibal-85 (Figure 9.2) and Lekhwair-70 (Figure 9.3), both in North Oman.

Lithology: The Sudair Formation consists mainly of anhydritic, finely crystalline dolomites, but with a number of beds, some ca. 6–10 m thick, of red and green-grey shales, which is the most characteristic lithological feature of the succession. The shale packages are concentrated at the top and base of the Formation, with the middle part being an almost continuous dolomite (Figure 9.4). In general there seems to be a trend of decreasing shale content from the south to the north.

Relict textures show evidence of original grain-supported sediments. The dolomites tend to be more anhydritic, resulting in slightly higher densities than those of the Jilh Formation. Shales predominate in the northwest of South Oman.

A distinctive limestone (mudstone) occurs in the lower part above the basal interbedded shales and dolomite in the western part of North and Central Oman (e.g. Yibal-85, Lekhwair-70, Figures 9.2 and 9.3).

Subsurface recognition: Whilst drilling it is impossible to recognise the Formation on lithologies alone, as these are similar to those of the Jilh and Upper Khuff.

Post-drilling the Formation can be recognised using log character, supported by palynological work. The densities of the Sudair Formation sediments tend to be slightly higher than those of the Jilh Formation (Yibal-85, Lekhwair-70, Figures 9.2 and 9.3), because they tend to be more anhydritic.

Boundaries: The lower boundary is defined at the base of a persistent red shale unit lying conformably on Khuff carbonates. The upper boundary is at the top of a mottled green-grey and red shale overlain conformably by continuous bedded dolomites of the Jilh Formation.

Distribution: The Sudair Formation occurs only in the western and northern subsurface of Interior Oman, and is truncated by Jurassic and Cretaceous units to the south and east. The Sudair correlates with the middle part of the Mahil Formation of the Al Hajar Mountains and the Hagil Formation of the Musandam Peninsula (see Glennie et al., 1974), both of these having a still further reduced shale content.

Deposition: The Sudair Formation has only scattered shelly fossil debris (molluscs) and some marine palynomorphs. The environment spans low-energy marginal-marine, tidal-flat and perhaps coastal-plain settings; settings that persist into the overlying Jilh Formation.

Subdivision: The Sudair Formation has no formal subdivisions, but clear lithological subdivision of the unit can be recognised by:

  • (1) an upper sequence of claystones with with inter-bedded dolomites;

  • (2) a middle massive anhydritic dolomite; and

  • (3) a lower sequence of claystones with inter-bedded dolomites.

Age: Early Triassic, Olenekian, ca. 249.5–245.9 Ma. Sharland et al. (2001) place their MFS Тr30 and MFS Тr20 within the Sudair Formation. The latter is suggested to occur within the limestone at the base of the Sudair Formation. However, as already noted, correlations across the Arabian Plate during the Triassic are extremely difficult due to either the lack of, or the imprecise nature of biostratigraphical calibration.

Biostratigraphy: The Sudair Formation is characterised by Palynozone 2351 (Densoisporites nejburgii with Endosporites papillatus) and, in the middle to (more typically) lower claystone part, by an influx of small marine acritarchs, which defines Palyno-subzone 1095 (Veryhachium spp.). This Veryhachium-Micrhystridium acritarch bloom, together with the associated miospores, appears to be of worldwide significance, occurring in Induan – Early Olenekian-aged rocks. The zone ranges into the Upper Khuff, of interpreted Induan age.

As in the Jilh no micropalaeontological zones are defined due to the overall poor recovery. This reflects the general restricted environment of deposition. Outcrop and subsurface work by Vachard and separately by Forke (2009) at outcrop only yielded rare, very low-diversity microfaunas and rare bioclasts. Virtually none of the forms observed provided meaningful age determinations. A single specimen of Hemigordiellina tenuifistula noted in Vachard (2007) may suggest an Olenekian age at the base of the Sudair Formation outcrop equivalent, see also Koehrer et al. (2010).

Khuff Formation

Authors: Steineke (unpublished, 1937), see Powers (1968).

Introduction

Deposition of the Khuff Formation followed the break up of the Permian supercontinent Pangaea (Konert et al., 2001). It was deposited as post-rift cover on a passive continental margin of the newly formed Neo-Tethys Ocean during a period of relative tectonic quiescence and steady subsidence (Stampfli, 2000). Thus the Khuff Formation represents the product of a regional marine transgression over the margin of the Arabian continent. This was caused partially by a major tectono-eustatic event related to the onset of rapid subsidence of the early Neo-Tethys passive margin and the drowning of its rift shoulders and partially in response to the eustatic sea-level rise caused by deglaciation after the Carboniferous – Permian glacial phase. This transgression was over a stable shelf of very low relief with minimal clastic supply.

The Formation is characterised by layer-cake type epeiric carbonate-evaporite platform deposition. Markers can be correlated over several 100s of kilometres and the unit extends over more than 2,500 kilometres in a southeast-northwest strike direction and more than 1,500 kilometres in a southwest-northeast dip direction (Al-Jallal, 1995). The correlative succession on the Iran side of the rift is well described by Insalaco et al. (2006).

In outcrops of Al Jabal Al Akhdar in North Oman of the Upper Saiq Formation and overlying Mahil Formation (lower part) are time equivalent to respectively the Lower/Middle and the Upper Khuff Formation. These outcrops are described by Glennie et al. (1974), Montenat et al. (1976), Rabu et al. (1986), Baud et al. (2001), Weidlich and Bernecker (2003) and Koehrer et al. (2010). The Lower Khuff is well exposed in the Al Huqf area of Central Oman (Dubreuilh et al., 1992a; Angiolini et al., 1998, 2004).

The Khuff Formation was reviewed in Osterloff et al. (2004a), who recognised 11 depositional sequences representing transgressive-regressive cycles.

The mass extinction event at the Permian – Triassic boundary is recorded at the boundary between the Middle and Upper Khuff (Saiq to Mahil transition in the outcrops of Al Jabal Al Akhdar in North Oman).

Type and reference sections: Near Ayn Khuff, Saudi Arabia (Powers, 1968). Oman subsurface reference sections are Lekhwair-70 in North Oman (Figure 9.5), Hasirah-1 in Central Oman (Figure 9.6), and Qaharir-4 (Figure 9.7) and Amal-9 (Figure 9.8), both in South Oman.

Figure 9.5:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Lekhwair-70, North Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.5:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Lekhwair-70, North Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.6:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Hasirah-1, Central Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.6:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Hasirah-1, Central Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.7:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Qaharir-4, South Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.7:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Qaharir-4, South Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.8:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Amal-9, Central Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.8:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Amal-9, Central Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Lithology: The Khuff Formation is a sequence of dolomites and limestones with minor amounts of thin, shaly beds and one main interval (Middle/Top D Anhydrite elsewhere) with disseminated and nodular anyhydrite. Shelly fossils occur throughout the Middle to Lower carbonates (crinoids, brachiopods, corals, fusulinids, etc.), but are most common and best-preserved in the lower part over Central Oman. Towards the southeast, this virtually continuous carbonate sequence becomes interspersed with, and finally dominated by fine clastics, mainly in red-bed facies. In the extreme southeast truncation leaves only equivalents of the Lower Khuff present, developed in red-bed facies. In summary the Khuff Formation is predominantly represented by a continental red-bed claystone sequence (with occasional thin limestones) in South Oman, and a shallow-marine carbonate sequences in Central and North Oman (Figures 9.9 and 9.10).

Figure 9.9:

Ditch cuttings from the Khuff Formation, Akhdar Group: (a) Limestone-grainstone from the Upper Khuff Member in Al Huwaisah-56; (b) Limestone-mudstone/wackestone from the Lower Khuff Member in Al Huwaisah-56; and (c) Shale from the Khuff Formation in Rajaa-2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 9.9:

Ditch cuttings from the Khuff Formation, Akhdar Group: (a) Limestone-grainstone from the Upper Khuff Member in Al Huwaisah-56; (b) Limestone-mudstone/wackestone from the Lower Khuff Member in Al Huwaisah-56; and (c) Shale from the Khuff Formation in Rajaa-2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 9.10:

Ditch cuttings from the Khuff Formation, Akhdar Group: (a) Shale from the Middle Khuff Member in Al Huwaisah-56 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 9.10:

Ditch cuttings from the Khuff Formation, Akhdar Group: (a) Shale from the Middle Khuff Member in Al Huwaisah-56 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

In the upper part of the Lower Khuff in South and Central Oman a ‘Khuff Marker Limestone’ (KML) bed is recognised. It loses its character northwards towards Lekhwair as the overall limestone development increases.

Subsurface recognition: Whilst drilling it may not be possible to pick the top of the Upper Khuff accurately, since the dolomites of the Upper Khuff are essentially identical to those of the Jilh and Sudair (tight, hard and poorly fossiliferous). However, the more porous and fossiliferous dolomites and limestones of the Middle and Lower Khuff may be identified. Hotshot samples may identify Palynozone 2103, and the incoming of the Permian faunas, for the Middle Khuff.

Post-drilling identification of the Khuff Formation relies on log correlation and supporting biostratigraphical analysis.

Boundaries: The top of the Upper Khuff is conformable with the overlying Sudair Formation and is defined at the base of a persistent red shale unit above the Khuff carbonates.

The top of the Middle Khuff coincides with the first downhole occurrence of Permian faunas (benthonic foraminifera, algae) and is routinely picked at the base of a grey-green shale unit, with a very distinctive log character. This shale is ubiquitous in the subsurface carbonate sections but cannot be correlated to outcrop sections in Al Jabal Al Akhdar, possibly due to facies variation or strong dolomitisation.

The Lower/Middle Khuff boundary in North Oman is extrapolated by log correlation from the South. It is not a clear boundary in the North. It is identified in the South by the first appearance of red/pale yellow/grey mottled claystones.

In Interior Oman, the Khuff Formation conformably overlies the Gharif Formation. The base of the Khuff Formation is taken everywhere at the base of the last recognisable limestone above the ‘Upper Gharif Shale’. Therefore situations occur where the base Khuff can be picked at the base of the Khuff Marker Limestone (KML, see Amal-9, Figure 9.8) and even at base Middle Khuff carbonates (Qaharir-4, Figure 9.7).

This ‘Upper Gharif Shale’, below the base Khuff limestones and above the top ‘Upper Gharif Sandstone’ has also been called Lower Khuff Red Beds in South Oman and the Eastern Flank. Heward (2004) indicates that the previous use of the term Lower Khuff Red Beds is inappropriate in Central Oman, i.e. away from the Eastern Flank area. This shale unit was previously placed in the Khuff Formation and the top Gharif was then picked at the first sand encountered downhole. Osterloff et al. (2004a) discuss this transition further. They suggest a cycle-based log correlation strategy and thereby illustrate the diachronous nature of the base of the Khuff carbonates. The carbonate pick gets younger moving southwards (see their figures 27 and 28). Biostratigraphical control is unfortunately lacking at this time and it is not possible to fully confirm the time equivalences inferred. The reader needs to be aware of the dual terminology applied and the difficulties of correlation in this diachronous setting. A strict lithostratigraphical interpretation is applied herein (i.e. Khuff limestones versus Gharif shales).

In South Oman, the top Khuff/Upper Gharif redbed facies boundary is usually abrupt and formed by an unconformity overlain by Jurassic or younger units.

Distribution: The Khuff carbonates are uniform and widely correlatable on wireline logs, showing only gentle thinning onto the Al Huqf axis and southward facies change into continental red-beds. At outcrop in the Al Hajar Mountains, the Saiq Formation (Glennie et al., 1974) is the lateral equivalent of the Permian part of the Khuff (specifically the Lower and Middle Khuff). It differs in being a more uniformly marine facies (more diverse fauna, no shales or evaporites) and generally lies with angular unconformity on Lower Palaeozoic and Proterozoic sediments and basement. The Saiq Formation passes upwards conformably into Triassic Mahil dolomites, locally with a conglomerate marking the boundary (see Glennie et al., 1974). The Upper Khuff is time equivalent to the lower part of the Mahil Formation.

Deposition: The faunas and the sediments of the Khuff Formation indicate environments ranging from shallow-marine through tidal-flat to mature coastal-plain of very low relief, all within a generally arid climatic setting. The floral recovery, however, suggests affinity with Upper Gharif vegetation and the potential for a wetter/seasonal climatic influence.

Subdivision: The Khuff has been subdivided on the basis of wireline logs, palaeontology and lithology into three members: Upper, Middle and Lower.

During Lower Khuff times facies varied from fully carbonate in the North, via a full range of mixing, to fully clastic in the very south, where in strict lithostratigraphical terms an Upper Gharif Member designation is applied.

Age: Middle Permian – Early Triassic, Wordian – Induan, ca. 267–249.5 Ma. Sharland et al. (2001) place their Early Triassic MFS Tr10 within a clean carbonate, near the top Upper Khuff, as illustrated in well Hasirah-1 (their figure 4.29). Sharland et al. (2004) revised the age of this MFS to intra-Induan. They position their late Permian MFS P40, MFS Р30 and MFS P20 surfaces within the Middle Khuff of the same well. Sharland et al. (2004) made minor revisions to the ages of the MFS P40 and P30 surfaces. Perhaps the best correlative unit across the entire Arabian Plate is represented by the significant anhydritic unit seen within the Middle Khuff. This interpreted late Capitanian unit is recorded across the region and variably known as the Middle, Median, Nar or Top D Anhydrite. There is some debate about the exact age of this unit, e.g. even ranging up to possible mid-Wuchiapingian (Al-Husseini, 2006), but on a regional scale it can certainly be viewed as a significant isochron, albeit inconclusively dated.

Biostratigraphy: The Early Triassic Palynozone 2351 (Densoisporites nejburgii) with marine Palyno-subzone 1095 (Veryhachium spp.) is recorded in the Upper Khuff, whilst late – middle Permian Palynozone 2103 (Lueckisporites virkkiae) occurs in the Middle – Lower Khuff. Palynozone 2252 (Hamiapollenites spp.) may also be recorded in Khuff sections, although Osterloff et al. (2004a), would restrict its occurrence to the Gharif Formation. Consensus on how to consistently differentiate Palynozone 2252 from 2103 and their spatial and temporal relationships has yet to be resolved (see Osterloff et al., 2004a).

Angiolini et al. (2004) describe an abundant and diverse Wordian marine fauna in the Lower Khuff Formation of the Al Huqf outcrop area, dominated by brachiopods, bivalves, gastropods and cephalopods. The macrofauna also includes bryozoans, echinoderms, trilobites, corals, conularids and vertebrate remains. Foraminifera, algae conodonts and ostracods are also present.

An ongoing micropalaeontological study of several outcrop (Al Jabal Al Akhdar) and subsurface sections has already yielded significant improvements in the ability to subdivide the Khuff Formation. Previously, only one long-ranging Biozone (FZA, Hemigordius sp.-Schwagerina sp.) was recognised, covering the Middle – Lower Khuff members. Data and initial interpretations for both outcrop and subsurface sections have been supplied by Daniel Vachard (Université de Lille) and for outcrop only by Holger Forke (Naturkundemuseum Berlin). From this data set it is evident that several key events within often rich microfaunal, and macrofaunal, assemblages provide a good opportunity for Khuff Formation subdivision, biozonation and calibration. The results complement and build on the comprehensive work on the Khuff equivalent microfaunas of Saudi Arabia (Vaslet et al., 2005 and Vachard et al., 2005) and Iran (Insalaco et al., 2006).

The recovery and interpretation of assemblages from the Saiq and Mahil formations (Khuff Formation equivalents) in the Al Jabal Al Akhdar outcrops is hampered by extensive dolomitisation. To date some of the best recovery has come from the subsurface and in particular well Yibal-192 (Vachard and Forbes, 2009, see also Koehrer et al., 2010). Key downhole events and potential zones can be summarised as follows in this well:

MemberRelative ageZone (working/informal name)Additional key taxaComments
Upper KhuffInduanRectocornuspira kahlori/Spirorbis phlyctaena mid Upper Khuff occurrence only
Upper KhuffInduan (inferred)Unzoned Post end Permian extinction ‘zone’, rare metazoans only.
uppermost Middle Khufflate ChanghsingianParadagmarita monodiAgathammina ovata, Brunsispirella linaeTaxa range up to, but not into or above, basal Upper Khuff shale.
Middle KhuffWuchiapingian – early ChanghsingianNeodiscus milliloides/Ichthyofrondina latilimbataNeomillerella mirabilis, Charliella aff. altineri 
Middle Khufflatest Capitanian – ?early WuchiapingianUnzoned This covers a ca. 35 m thick anhydritic level (i.e. ‘Middle Khuff Anhydrite’)
Middle Khufflatest CapitanianShanita amosi  
Middle Khufflate CapitanianUnzoned  
Middle – Lower Khufflate CapitanianMonodiexodina - BBCDunbarula? sp., Schubertella? sp. Levenella? sp.BBC stands for a rich and very characteristic metazoan assemblage dominated by Bryozoans, Brachiopods and Crinoids.
Lower Khuffearly? CapitanianUnzonedBBC assemblage continues, Langella ocarinaSeveral taxa recorded are thought not to range below the Capitanian.
MemberRelative ageZone (working/informal name)Additional key taxaComments
Upper KhuffInduanRectocornuspira kahlori/Spirorbis phlyctaena mid Upper Khuff occurrence only
Upper KhuffInduan (inferred)Unzoned Post end Permian extinction ‘zone’, rare metazoans only.
uppermost Middle Khufflate ChanghsingianParadagmarita monodiAgathammina ovata, Brunsispirella linaeTaxa range up to, but not into or above, basal Upper Khuff shale.
Middle KhuffWuchiapingian – early ChanghsingianNeodiscus milliloides/Ichthyofrondina latilimbataNeomillerella mirabilis, Charliella aff. altineri 
Middle Khufflatest Capitanian – ?early WuchiapingianUnzoned This covers a ca. 35 m thick anhydritic level (i.e. ‘Middle Khuff Anhydrite’)
Middle Khufflatest CapitanianShanita amosi  
Middle Khufflate CapitanianUnzoned  
Middle – Lower Khufflate CapitanianMonodiexodina - BBCDunbarula? sp., Schubertella? sp. Levenella? sp.BBC stands for a rich and very characteristic metazoan assemblage dominated by Bryozoans, Brachiopods and Crinoids.
Lower Khuffearly? CapitanianUnzonedBBC assemblage continues, Langella ocarinaSeveral taxa recorded are thought not to range below the Capitanian.

Koehrer et al. (2010) place the Permian – Triassic boundary towards the top of the basal Upper Khuff shale. However, we retain the top Permian for subsurface use at the base of this shale. Significantly the base of this shale exhibits an uphole decrease in uranium alongside decreases in thorium and potassium, as seen in the Spectral Gamma wireline log. All three shifts are quite marked and the ‘Uranium event’ in particular is a recognised worldwide, end Permian phenomenon (see Insalaco et al., 2006). If ever cored, this claystone (unfortunately not seen at outcrop in Oman) could add considerably to our knowledge of events at the end of the Permian.

The occurrence of Shanita amosi (Late Capitanian) prior to the Middle Anhydrite is again a well-documented, regionally recognisable correlative event, associated with the so-called Mid-Permian faunal crisis.

In addition to the above, Vachard (2007) working on outcrop from Al Jabal Al Akhdar, recognises an Induan Biozone (sporadic Hemigordiellina sinensis in uppermost Khuff), an early Changhsingian Biozone (with Glomomidiellopsis uenoi), a general Wuchiapingian Biozone (Neodiscopsis ambiguus) with several potential subzones, and an early Capitanian Biozone (Sphaerionia sikuoides). Forke (2009) ranges G. uenoi up into the late Changhsingian.

No proof of Wordian-age carbonates was seen in Yibal-192, however, Koehrer et al. (2010) cite evidence for a lowermost Khuff (Saiq Formation) middle to late Wordian age at outcrop, noted as a zone with Neoschwagerina schuberti. Forke (2009) cites additional supporting evidence and also interprets a middle-late? Wordian assemblage in outcrop, recognised by a Chusenella-Schubertella assemblage. This suggests that a degree of base Khuff diachronism may also be present in North Oman.

Figures & Tables

Figure 9.1:

Location map: Akhdar Group.

Figure 9.1:

Location map: Akhdar Group.

Figure 9.2

(facing page): Composite electrical logs, lithology and lithological description of the Jilh and Sudair formations, Akhdar Group, in well Yibal-85, North Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.2

(facing page): Composite electrical logs, lithology and lithological description of the Jilh and Sudair formations, Akhdar Group, in well Yibal-85, North Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.3:

Composite electrical logs, lithology and lithological description of the Jilh and Sudair formations, Akhdar Group, in well Lekhwair-70, North Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.3:

Composite electrical logs, lithology and lithological description of the Jilh and Sudair formations, Akhdar Group, in well Lekhwair-70, North Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.4:

Ditch cuttings from the Akhdar Group: (a) Dolomite from the Jilh Formation in Lekhwair-70; (b) Dolomite from the Sudair Formation in Al Bashair-1; and (c) Shale from the Sudair Formation in Al Huwaisah-56 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 9.4:

Ditch cuttings from the Akhdar Group: (a) Dolomite from the Jilh Formation in Lekhwair-70; (b) Dolomite from the Sudair Formation in Al Bashair-1; and (c) Shale from the Sudair Formation in Al Huwaisah-56 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 9.5:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Lekhwair-70, North Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.5:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Lekhwair-70, North Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.6:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Hasirah-1, Central Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.6:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Hasirah-1, Central Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.7:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Qaharir-4, South Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.7:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Qaharir-4, South Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.8:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Amal-9, Central Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.8:

Composite electrical logs, lithology and lithological description of the Khuff Formation, Akhdar Group, in well Amal-9, Central Oman (Mohammed et al., 1997). See Figure 9.1 for location.

Figure 9.9:

Ditch cuttings from the Khuff Formation, Akhdar Group: (a) Limestone-grainstone from the Upper Khuff Member in Al Huwaisah-56; (b) Limestone-mudstone/wackestone from the Lower Khuff Member in Al Huwaisah-56; and (c) Shale from the Khuff Formation in Rajaa-2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 9.9:

Ditch cuttings from the Khuff Formation, Akhdar Group: (a) Limestone-grainstone from the Upper Khuff Member in Al Huwaisah-56; (b) Limestone-mudstone/wackestone from the Lower Khuff Member in Al Huwaisah-56; and (c) Shale from the Khuff Formation in Rajaa-2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 9.10:

Ditch cuttings from the Khuff Formation, Akhdar Group: (a) Shale from the Middle Khuff Member in Al Huwaisah-56 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 9.10:

Ditch cuttings from the Khuff Formation, Akhdar Group: (a) Shale from the Middle Khuff Member in Al Huwaisah-56 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

GROUPFORMATIONMEMBER
AkhdarMinjur 
Jilh 
Sudair 
KhuffUpper Khuff
Middle Khuff
Lower Khuff
GROUPFORMATIONMEMBER
AkhdarMinjur 
Jilh 
Sudair 
KhuffUpper Khuff
Middle Khuff
Lower Khuff
ZoneSubzoneMarker speciesRelative age (original assignment)Formation
2255 Bartenia communisCarnian – ?RhaetianJilh – Minjur
2247Vesicaspora schemili(Norian – ?Rhaetian)Minjur
2241Partitisporites verrucosus(Carnian – Norian)Jilh – Minjur
2370 Triadispora crassaAnisianJilh
2351 Densoisporites nejburgiiScythian (Olenekian – Induan*)Upper Khuff – Sudair
1095Veryhachium spp.(‘Early’ Scythian (?Induan*))Upper Khuff – Lower Sudair
  Unzoned interval**Basal Upper Khuff
2103 Lueckisporites virkkiaeKazanian-Tatarian (Roadian – Capitanian*)Middle Khuff
2252 Hamiapollenites spp.Artinskian – KungurianUpper Gharif – ?Middle Khuff
ZoneSubzoneMarker speciesRelative age (original assignment)Formation
2255 Bartenia communisCarnian – ?RhaetianJilh – Minjur
2247Vesicaspora schemili(Norian – ?Rhaetian)Minjur
2241Partitisporites verrucosus(Carnian – Norian)Jilh – Minjur
2370 Triadispora crassaAnisianJilh
2351 Densoisporites nejburgiiScythian (Olenekian – Induan*)Upper Khuff – Sudair
1095Veryhachium spp.(‘Early’ Scythian (?Induan*))Upper Khuff – Lower Sudair
  Unzoned interval**Basal Upper Khuff
2103 Lueckisporites virkkiaeKazanian-Tatarian (Roadian – Capitanian*)Middle Khuff
2252 Hamiapollenites spp.Artinskian – KungurianUpper Gharif – ?Middle Khuff
*

Updated stage terminology.

**

Assumed post end-Permian extinction section.

FormationMicropalaeontological zonePalynological zoneRelative age
MinjurUnzoned2255**Rhaetian
Jilh?2255**Norian
2370Anisian?/Ladinian – Norian
Sudair – Upper Khuff*2351 (with Subzone 1095)Olenekian
Upper Khuff*UnzonedInduan (Triassic)
Middle Khuff*FZA2103-?2252late Capitanian – late Changhsingian (Permian)
Lower KhuffWordian – Capitanian
FormationMicropalaeontological zonePalynological zoneRelative age
MinjurUnzoned2255**Rhaetian
Jilh?2255**Norian
2370Anisian?/Ladinian – Norian
Sudair – Upper Khuff*2351 (with Subzone 1095)Olenekian
Upper Khuff*UnzonedInduan (Triassic)
Middle Khuff*FZA2103-?2252late Capitanian – late Changhsingian (Permian)
Lower KhuffWordian – Capitanian
*

Ongoing work suggests several zones are possible in the Khuff.

**

Palynozone 2255 is in need of revision.

MemberRelative ageZone (working/informal name)Additional key taxaComments
Upper KhuffInduanRectocornuspira kahlori/Spirorbis phlyctaena mid Upper Khuff occurrence only
Upper KhuffInduan (inferred)Unzoned Post end Permian extinction ‘zone’, rare metazoans only.
uppermost Middle Khufflate ChanghsingianParadagmarita monodiAgathammina ovata, Brunsispirella linaeTaxa range up to, but not into or above, basal Upper Khuff shale.
Middle KhuffWuchiapingian – early ChanghsingianNeodiscus milliloides/Ichthyofrondina latilimbataNeomillerella mirabilis, Charliella aff. altineri 
Middle Khufflatest Capitanian – ?early WuchiapingianUnzoned This covers a ca. 35 m thick anhydritic level (i.e. ‘Middle Khuff Anhydrite’)
Middle Khufflatest CapitanianShanita amosi  
Middle Khufflate CapitanianUnzoned  
Middle – Lower Khufflate CapitanianMonodiexodina - BBCDunbarula? sp., Schubertella? sp. Levenella? sp.BBC stands for a rich and very characteristic metazoan assemblage dominated by Bryozoans, Brachiopods and Crinoids.
Lower Khuffearly? CapitanianUnzonedBBC assemblage continues, Langella ocarinaSeveral taxa recorded are thought not to range below the Capitanian.
MemberRelative ageZone (working/informal name)Additional key taxaComments
Upper KhuffInduanRectocornuspira kahlori/Spirorbis phlyctaena mid Upper Khuff occurrence only
Upper KhuffInduan (inferred)Unzoned Post end Permian extinction ‘zone’, rare metazoans only.
uppermost Middle Khufflate ChanghsingianParadagmarita monodiAgathammina ovata, Brunsispirella linaeTaxa range up to, but not into or above, basal Upper Khuff shale.
Middle KhuffWuchiapingian – early ChanghsingianNeodiscus milliloides/Ichthyofrondina latilimbataNeomillerella mirabilis, Charliella aff. altineri 
Middle Khufflatest Capitanian – ?early WuchiapingianUnzoned This covers a ca. 35 m thick anhydritic level (i.e. ‘Middle Khuff Anhydrite’)
Middle Khufflatest CapitanianShanita amosi  
Middle Khufflate CapitanianUnzoned  
Middle – Lower Khufflate CapitanianMonodiexodina - BBCDunbarula? sp., Schubertella? sp. Levenella? sp.BBC stands for a rich and very characteristic metazoan assemblage dominated by Bryozoans, Brachiopods and Crinoids.
Lower Khuffearly? CapitanianUnzonedBBC assemblage continues, Langella ocarinaSeveral taxa recorded are thought not to range below the Capitanian.

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