Skip to Main Content
GROUPFORMATIONINFORMAL MBRINFORMAL UNIT
HaushiGharifUpper Gharif 
Middle Gharif 
Lower GharifHaushi Limestone
 Basal Sands
Al KhlataP1Rahab Shale
P5 
P9 
GROUPFORMATIONINFORMAL MBRINFORMAL UNIT
HaushiGharifUpper Gharif 
Middle Gharif 
Lower GharifHaushi Limestone
 Basal Sands
Al KhlataP1Rahab Shale
P5 
P9 

Authors:Hudson and Sudbury (1959) as Haushi Formation, raised to Group status by Winkler (1975). The Haushi Group as used here is defined by Hughes Clarke (1988).

Introduction

Following the widespread ‘Hercynian’ uplift and erosion in the Carboniferous (Konert et al., 2001; Al-Husseini, 2004b; Faqira et al., 2009) the northern Gondwanan terranes rifted with the opening of the Neo-Tethys Ocean in the latest Carboniferous – earliest Permian (Konert et al., 2001). The earliest sediments above the ‘Hercynian’ unconformity in Oman are represented by the glacially-influenced sediments of the Al Khlata Formation that form the base of the Haushi Group. Palaeogeographic reconstructions show that the Arabian Peninsula at that time was situated close to the South Pole, at high latitudes above some 40–45°S. The glacials unconformably overlie the older sediments of the Misfar Group, Haima or Huqf Supergroups. Clastic sediments of the Gharif Formation conformably overlie the Al Khlata Formation, deposited in the subsequent deglaciation period with the Arabian Plate rapidly moving some 20° northwards to middle latitudes. Following an initial marine transgression these sediments are dominated by fluvial/alluvial clastics deposited under progressively dryer conditions, from cold, through temperate to arid and ultimately a wetter, tropical seasonal climate. During Gharif times Oman was situated on the eastern flank of the Rub’ Al-Khali Basin, an intracontinental basin at the edge of Neo-Tethys, with a limited sediment source area from the Al Huqf High to the east. In late Gharif times there is a notable increase in the clastic supply from the east, probably in response to uplift in the source areas along the shoulders of the Neo-Tethys rifting system.

The stratigraphy of the Haushi Group was reviewed by Osterloff et al. (2004a, b) as part of a regional Carboniferous to Early Triassic review (Al-Husseini, 2004a).

Type and reference sections: Surface sequences in the northern Al Huqf outcrop area, named after the Haushi anticline in the Al Huqf area (see Formation discussions). No subsurface reference sections have been defined for the Group, as Hughes Clarke (1988) only proposed subsurface reference sections at formational level. Safiq South-1 (Figure 10.2) is herein proposed as a suitable reference section at Group level.

Figure 10.1:

Location map: Haushi Group.

Figure 10.1:

Location map: Haushi Group.

Figure 10.2:

Composite electrical logs, lithology and lithological description of the Gharif and Al Khlata formations, Haushi Group, in well Safiq South-1, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.2:

Composite electrical logs, lithology and lithological description of the Gharif and Al Khlata formations, Haushi Group, in well Safiq South-1, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Lithology: The Haushi Group is a largely siliciclastic succession, glacially-influenced in its lower part, the Al Khlata Formation, then becoming shallow marine shifting to alluvial/fluvial in its upper part, the Gharif Formation. The only carbonates are associated with a marine transgression of the Lower Gharif Member (Haushi Limestone). The clastics in the Haushi Group are characterised throughout by much higher lithic and arkosic content than in the underlying clastics of the Haima Supergroup.

Subsurface recognition: The clastics can be easily distinguished from the overlying Akhdar Group carbonates. Igneous and metamorphic rock fragments in the Al Khlata Formation distinguish them from the clastics of the underlying Haima Supergroup, although problems can occur when Haima clastics are significantly reworked into the Al Khlata, e.g. Al Khlata sands with a dominant Amin sand grain content. It is more difficult to separate the Haushi clastics from the clastics of the Misfar Group, which requires detailed litho- and biostratigraphic analyses.

Boundaries: The upper boundary of the Haushi Group clastics is conformable with the overlying Khuff Formation carbonates of the Akhdar Group. The diachronous nature of this lithostratigraphical boundary is discussed in the previous Khuff Formation section. The lower boundary is everywhere unconformable on top of the older Haima and Huqf Supergroups or, rarely, the Misfar Group.

Distribution: The Haushi clastics occur in the Al Huqf outcrop area, are present throughout the subsurface of Interior Oman and are widespread across the Arabian Plate (e.g. Al-Laboun, 1988; Alsharhan et al., 1993; Le Heron et al., 2009). They are truncated over specific highs in Oman, mainly along the Eastern Flank of the South Oman Salt Basin, where the Gharif or Al Khlata is overlain by the Nahr Umr Formation.

Haushi sediments are absent in the Al Hajar Mountains, except in a small outcrop inlier in the Saih Hatat area (eastern Al Hajar Mountains), where Al Khlata sediments have recently been recognised and dated by PDO internal work.

Deposition: The depositional environment ranges from marginal marine to coastal/alluvial plain environments for the Gharif Formation and glaciolacustrine, glaciodeltaic and glaciofluvial settings for the Al Khlata Formation.

Subdivision: The Haushi Group is divided into the Al Khlata and Gharif formations.

Sequence stratigraphy: For the most part the Haushi Group represents the AP5 Megasequence of Sharland et al. (2001, revised 2004). They place the Upper Gharif clastics in the basal part of their AP6 Megasequence, as representing the initial transgressive unit prior to Khuff carbonate development. The pre-Khuff unconformity seen in Saudi Arabia (Al-Husseini, 2008) is taken to mark the Arabia-Cimmeria ‘break up unconformity’, as discussed by Sharland et al. (2001), but this break is not clearly recognised in Oman.

Age: Late Carboniferous – early Permian, late Moscovian? – Wordian, ca. 310?–267 Ma. The base Al Khlata age is problematical and may range below the late Moscovian into the Bashkirian (see formational discussion). Sharland et al. (2001) place their MFS P10 within Lower Gharif, immediately below the Haushi Limestone.

Biostratigraphy: The Haushi Group can be recognised by the presence of Palynozones 2252 (Hamiapollenites spp.) to 2159 (Anapiculatisporites concinnus, redefined as Punctatisporites spp.). More than any other part of the column the Haushi Group has been subject to extensive biostratigraphical, in this case palynological, study reflecting the economic importance of these sediments as primary oil reservoirs. This is particularly so for the Al Khlata, where palynological content often provides the only differentiating criteria when dealing with the complex geometries and extreme lateral variation seen in these glaciogenic rock units.

Key age calibration points have been provided by plant, fusilinid and brachiopod megafossil recovery. Broutin et al. (1995), Berthelin et al. (2003) and Berthelin et al. (2006) describe a rich and varied plant assemblage from the very top of the Upper Gharif in the northern Al Huqf outcrop region. This ‘Gharif Paleoflora’ has been interpreted to be of late Roadian – early Wordian in age. Angiolini et al. (2006) indicate a Sakmarian age for the Haushi Limestone based on fusilinid recovery with supporting brachiopod evidence.

The applied palynological zonation is summarised as follows:

ZoneMarker speciesRelative ageFormation/Unit
2252Hamiapollenites spp.Roadian – WordianUpper Gharif
2190Kingiapollenites subcircularisArtinskian – Kungurian*Middle/Lower Gharif
2105Alisporites indarraensislate SakmarianLower Gharif
1115**Ulanisphaeridium omanensislate SakmarianLower Gharif
2141Cycadopites cymbatus?Sakmarian*Lower Gharif-Al Khlata (P1)
2165Microbaculispora spp.?Asselian – Sakmarian*Al Khlata (P5)
2159Punctatisporites spp.?late Moscovian – Gzhelian*Al Khlata (P9)
 Barren zoneUnknownbasal Al Khlata
ZoneMarker speciesRelative ageFormation/Unit
2252Hamiapollenites spp.Roadian – WordianUpper Gharif
2190Kingiapollenites subcircularisArtinskian – Kungurian*Middle/Lower Gharif
2105Alisporites indarraensislate SakmarianLower Gharif
1115**Ulanisphaeridium omanensislate SakmarianLower Gharif
2141Cycadopites cymbatus?Sakmarian*Lower Gharif-Al Khlata (P1)
2165Microbaculispora spp.?Asselian – Sakmarian*Al Khlata (P5)
2159Punctatisporites spp.?late Moscovian – Gzhelian*Al Khlata (P9)
 Barren zoneUnknownbasal Al Khlata
*

Large uncertainties apply to these ages in particular.

**

Marine zone coeval with part of 2105 Miospore Zone.

The zonation is primarily built on downhole quanitative changes in the total miospore assemblage and the marker species listed can range above and below the zone which they name. Penney et al. (2008) place the 2165–2159 boundary at the Asselian – Gzhelian (Permian – Carboniferous) boundary and do not range the Al Khlata below a tentative late Moscovian. Previously Penney and Stephenson (2006) had, albeit tentatively, argued that Palynozone 2159 could range as old as Moscovian – Bashkirian. Osterloff et al. (2004b) also believe that the Al Khlata could range down to a Namurian equivalent age by comparison with European assemblages (i.e. ca. Bashkirian). This uncertainty arises from the need to ‘jump’ correlate these non-marine sequences to comparable spore and pollen recovery associated with marine fauna in Western Australia, which only then in turn can be compared to the marine fauna in the type Russian Carboniferous. As applied in Penney et al. (2008), the glaciogenic nature of the Al Khlata sediments, with multiple reworking episodes, dictates the need for a particular quantitative based approach when dealing with the palynological content. Detailed age definitions can only be tentative within a general late Carboniferous – early Permian range.

Correlation with sequences in Saudi Arabia is provided by comparison with the OSPZ6 to OSPZ1 palynological zones of Stephenson et al. (2003) (see also Stephenson, 2004). These are further documented in Osterloff et al. (2004b) and Stephenson (2006) for the Gharif and Penney et al. (2008) for the Al Khlata.

Gharif Formation

Introduction

The mainly clastic sediments of the Gharif Formation were deposited during the early – middle Permian, covering a period in the region of 25 million years. This was a time of gentle subsidence in which the Arabian Plate rapidly drifted northwards to lower latitudes, moving from a cold to a tropical climate zone.

Oman was situated on the eastern flank of a wider intra-continental (Rub’ Al-Khali Basin) at the edge of Neo-Tethys.

The overall thickness of the Gharif and its individual members is remarkably constant (some 230 m over Central Oman). The unit can be broadly correlated with the Unayzah A Member in Saudi Arabia (Sharland et al., 2001; Stephenson et al., 2003).

The Gharif overlies the Al Khlata in all sections, except in the northeast, where it directly overlies Haima sediments. It is absent in the Al Hajar Mountain outcrops.

The sedimentologically immature composition of the Gharif clastics show that they are most probably derived from the varied material supplied by the Al Khlata glacial phase. The fluviomarine reworking of this material allows the clastics to become more mature from south to north. They are probably continuous with, or laterally equivalent to, the pre-Khuff clastics known elsewhere in the Gulf region (e.g. the Faraghan Formation in Iran; Szabo and Kheradpir, 1978; Unayzah Formation of Saudi Arabia; Melvin and Sprague, 2006; Al-Husseini, 2006; Sprague et al., 2008).

As summarised by Heward (2004) in his Central Oman study the Gharif in the Al Huqf region is exposed, or occurs at shallow depths, in the southeast and is buried to progressively greater depths in the northwest. Porosity, permeability, oil characteristics, degree of aquifer support and recovery are all influenced by this Central Oman burial trend.

In mapping the Gharif outcrops in the Al Huqf area Dubreuilh et al. (1992a) observed that the Haushi Limestone is folded more intensely than the overlying Gharif and Khuff and interpreted a significant unconformity. Based on this they introduced the Saiwan Formation (equivalent to the Lower Gharif Basal Sands and the Haushi Limestone) as the final formation of the Haushi Group. Their Gharif Formation, Hughes Clarke’s (1988) Middle and Upper Gharif, thus became the basal unit of the Akhdar Group (Le Métour et al., 1995; Angiolini et al., 2001).

There is no evidence from the subsurface of Oman for a regional unconformity as implied by Dubreuilh et al. (1992a). The tectonics in the outcrop are probably associated with local deformation along the Maradi fault zone (locally known as the Haushi-Nafun fault, Enclosure 2a) in Middle to Upper Gharif times, possibly associated with the rifting of Gondwana. Consequently, it is preferred not to place the Middle and Upper Gharif in the Akhdar Group (cf. Angiolini et al., 2004). The ‘Saiwan Formation’ can be considered to be a local, outcrop term for only the Haushi Limestone unit of the Lower Gharif Member. This correlation is detailed in Angiolini et al. (2006), where they also identify a Basal Sand equivalent section at outcrop. Unfortunately, the Dubreuilh et al. (1992a) outcrop stratigraphy from the Saiwan area has caused confusion in the published literature (e.g. Le Heron et al., 2009; Wopfner and Jin, in press).

Type and reference sections: Wadi Gharif area in western Al Huqf and outcrops in Haushi area, northern Al Huqf. Note that these outcrop sections are much thinner than those recorded in the subsurface.

The principal subsurface reference section is Safiq South-1 in Southern Oman (Figure 10.2). Additional reference sections are Saih Rawl-2 (Figure 10.3) and Al Bashair-1 (Figure 10.4) in North Oman, Wafra-6 (Figure 10.5) and Bahja-1 (Figure 10.6) in Central Oman and Qaharir-4 in South Oman (Figure 10.7).

Figure 10.3

(facing page): Composite electrical logs, lithology and lithological description of the Gharif Formation, Haushi Group, in well Saih Rawl-2, North Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.3

(facing page): Composite electrical logs, lithology and lithological description of the Gharif Formation, Haushi Group, in well Saih Rawl-2, North Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.4:

Composite electrical logs, lithology and lithological description of the Gharif and Al Khlata formations, Haushi Group, in well Al Bashair-1, North Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.4:

Composite electrical logs, lithology and lithological description of the Gharif and Al Khlata formations, Haushi Group, in well Al Bashair-1, North Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.5:

Composite electrical logs, lithology and lithological description of the Gharif Formation, Haushi Group, in well Wafra-6, Central Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.5:

Composite electrical logs, lithology and lithological description of the Gharif Formation, Haushi Group, in well Wafra-6, Central Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.6:

Composite electrical logs, lithology and lithological description of the Gharif and Al Khlata formations, Haushi Group, in well Bahja-1, Central Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.6:

Composite electrical logs, lithology and lithological description of the Gharif and Al Khlata formations, Haushi Group, in well Bahja-1, Central Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.7:

Composite electrical logs, lithology and lithological description of the Gharif Formation, Haushi Group, in well Qaharir-4, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.7:

Composite electrical logs, lithology and lithological description of the Gharif Formation, Haushi Group, in well Qaharir-4, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Lithology: The Gharif Formation is a sequence of sands, shales and, except in the southeast, a limestone unit. Over most of the area, the Formation shows some correlative cyclicity of sands and shales and contains marine fossils in the lower part. An informal subdivision into Lower, Middle and Upper members is applied. The Lower Gharif can be further subdivided into the Haushi Limestone and Basal Sands units.

The sands are less lithic than the Al Khlata, but are still relatively arkosic, although becoming more mature and quartzose to the north.

Shales/claystones of the Upper and Middle Gharif members are dominantly red and purple, whilst those of the Lower Gharif Member are usually grey. There are no differences between the sandstones of the Upper, Middle and Lower Gharif as they are subfeldspathic throughout (Figures 10.8, 10.9 and 10.10), except those of the Basal Sands, which can be more quartzitic.

Figure 10.8:

Ditch cuttings from the Haushi Group: (a) Shale from the Upper Gharif Member in Saih Rawl-2; (b) Shale from the Middle Gharif Member in Barik-10; and (c) Shale from the Lower Gharif Member in Nimr-13 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.8:

Ditch cuttings from the Haushi Group: (a) Shale from the Upper Gharif Member in Saih Rawl-2; (b) Shale from the Middle Gharif Member in Barik-10; and (c) Shale from the Lower Gharif Member in Nimr-13 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.9:

Ditch cuttings from the Haushi Group: (a) Sandstone from the Middle Gharif Member in Karim West-55; (b) Sandstone from the Lower Gharif Member in Nimr-13; and (c) Sandstone from the Lower Gharif Member in Rajaa-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.9:

Ditch cuttings from the Haushi Group: (a) Sandstone from the Middle Gharif Member in Karim West-55; (b) Sandstone from the Lower Gharif Member in Nimr-13; and (c) Sandstone from the Lower Gharif Member in Rajaa-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.10:

Ditch cuttings from the Lower Gharif Member, Haushi Group: (a) Haushi Limestone; and (b) Basal Sandstone from Saih Rawl-5 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.10:

Ditch cuttings from the Lower Gharif Member, Haushi Group: (a) Haushi Limestone; and (b) Basal Sandstone from Saih Rawl-5 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

The Upper and Middle Gharif are everywhere characterised by a clastic sequence. Thin calcrete horizons occur locally within the claystones and siltstones. Most Upper and Middle Gharif sands are only a few metres thick and lenticular, unless stacked vertically or laterally.

The upper part of the Middle Gharif is characterised by the presence of a continuous red-brown claystone, known as the Middle Gharif Shale. In most of North Oman the Lower Gharif Haushi Limestone Bed (wackestone) is capped by a thin dark grey shale, which separates it from the subfeldspathic sandstones of the overlying Middle Gharif (e.g. Al Bashair-1, Figure 10.4).

Subsurface recognition: The top of the Gharif Formation is recognised as the first clastics below limestones of the Khuff Formation. It is often marked by an increase in Rate of Penetration, i.e. a positive drillbreak. The Gharif shales/claystones differ from the Nahr Umr shales by colour. The sandstones differ from the Mesozoic Clastics by lacking chert and from the Al Khlata by a much reduced lithic component.

The shales of the Lower Gharif generally differ from those of the Upper and Middle Gharif by colour (more grey shales as opposed to red/purple dominance respectively). The occurrence of limestones indicates penetration of the Lower Gharif (Haushi Limestone).

The sands of the Basal Sands Bed are generally quartzitic rather than feldspathic, but this is very difficult to distinguish in cuttings.

Post-drilling the Gharif Formation is recognised by its distinct log pattern and palynological analysis. The upper part of the Middle Gharif is characterised by a continuous red-brown claystone (the ‘Playa Shales’), which can be easily recognised on logs by its wide Density/Neutron separation. Locally the claystone loses its characteristics due to interbedding with siltstone/sandstone (Kharusi et al., 1995). Palynological analysis will confirm Palynozones 2252, 2190, 2105 (with 1115) and uppermost 2141.

Boundaries: The lower boundary is at the base of a transgressive, potentially erosive, basal sandstone lying on the Rahab Shale of the Al Khlata Formation. The boundary is generally considered to be conformable with potential to be locally erosive and/or unconformable, with only a minor time gap. In North Oman where the Rahab Shale is absent, the lower boundary of the Gharif Formation is more difficult to pick and has been previously interpreted to lie immediately below the first occurrence of marine indicators. Palynological analysis can assist this boundary recognition.

The upper boundary in northern areas is at the conformable base of the almost continuous carbonates of the Khuff Formation. To the south, the lower Khuff passes laterally into a red-bed facies, and the separation of the largely continental sands and shales of the Gharif from the red shales, silts and calcretes of the lower Khuff may be less clear without correlation through intermediate sections (see Khuff discussion, Figure 10.11). The Gharif may also be overlain unconformably by the Kahmah Group and shales of the Nahr Umr Formation.

The upper boundary of the Middle Gharif is taken at the top of the red brown ‘Playa Shales’ unit, underlying the sands of the Upper Gharif. In North Oman (Greater Saih Rawl area) the boundary is tentatively picked either below very thin sands or below silty sands which are overlain by thick high Gamma ray claystones of the Upper Gharif (Saih Rawl-2, Figure 10.3) (Kharusi et al., 1995).

The top of the Lower Gharif is defined by and taken at the top of the first marine grey shale or dark grey, micaceous lagoonal shale below Middle Gharif red sediments.

In North and Central Oman and the west of South Oman the Lower Gharif is characterised by the presence of the Haushi Limestone overlying Basal Sands. To the southeast these pass gradually into more clastic dominated sections, making the upper boundary and any subdivision more difficult to identify. Generally an undifferentiated Lower Gharif Member is assigned and subdivision only occurs where distinct Haushi Limestone and the Basal Sands units can be recognised. Recognition and subdivision of the Lower Gharif in South Oman is based on log correlation underpinned by palynology (Penney et al., 2006).

Distribution: The Gharif Formation occurs over the whole of the subsurface in Oman. The exceptions are areas in South Oman where salt movement occurred and the Al Khlata is overlain by the Nahr Umr Formation. The Gharif is also missing in the Al Hajar Mountains in the north but it crops out in the Al Huqf area in central East Oman. The distinctive Haushi Limestone marker level occurs in North and Central Oman, including Al Huqf outcrops, and in the west of South Oman.

Deposition: The Gharif Formation is dominantly continental with shallow-marine deposits in the Lower Gharif. The continental deposits represent river channel belts, palaeosols and lakes, and the marine deposits, coasts, lagoons and offshore shallow-water shoals. The variety of marine characteristics decreases from north to south, and in the southeast the sequence seems entirely continental, being largely red-bed facies.

The Lower Gharif Basal Sands comprise an irregular blanket of shallow-marine shoreface sands which may in places intercalate with possible pro/paraglacial deposits. A marineflooding shale then follows in South Oman (Palynozone 1115). This is also called the Maximum Flooding Shale by Guit et al. (1995) and may correlate with a shale immediately below the Haushi Limestone in North and Central Oman, where the presence of Palynozone 1115 has yet to be conclusively proven (Stephenson et al., 2006). Stephenson et al. (2006) suggest that the sands above the Maximum Flooding Shale in South Oman broadly correlate with the Haushi Limestone elsewhere.

Sheetlike sands in the Middle Gharif of western Oman appear similar to the marine deposits of the Basal Sands. Above this the Middle Gharif sands are channel sands that may develop into multilateral stacks. Calcrete-bearing palaeosols and reworked calcrete conglomerates are common.

The Middle Gharif Shale is a distinctive thick sequence of weakly developed palaeosols (interpreted as aridisols), locally containing substantial sand bodies.

Multi-storey and multi-lateral channel belts occur in the Upper Gharif of northern Oman. The Upper Gharif shale consists mainly of palaeosols (interpreted as red-grey gleyed vertisols) with channel sands and subaqueous shales. The latter, in particular are considered to represent the initial transgressive pulse, which ultimately leads to Khuff carbonate deposition. Ostracods and fish scales and teeth occur occasionally in lake deposits in the Upper Gharif (and even in the Middle Gharif; Heward, 2004). The change from aridisols in the Middle Gharif to gleyed vertisols in the Upper Gharif reflects a climatic change from arid to more tropical, seasonally wet.

The base of the Upper Gharif has been interpreted to be erosional or unconformable, but in his report on Central Oman, Heward (2004) classifies the base Upper Gharif as a rejuvenation of sand supply without either significant erosion or unconformity.

Subdivision: The lithostratigraphic subdivision of the Gharif was first established by van Vliet (1983), see Hughes Clarke (1988). The Gharif Formation can be divided into three sedimentary sequences, which correspond to the Upper, Middle and Lower Gharif members. These together with the Haushi Limestone and Basal Sands units of the Lower Gharif represent the best workable subdivisions. Additional historical (internal PDO) units like the Fringe Clastics and the Amal Sand have not been widely applied and their inconsistent use tends to hinder regional understanding, especially if applied without any palynological control.

In South Oman, the lower part of the Lower Gharif Member is a complex of fluvial and fluviodeltaic clastics succeeded by marginal-marine clastics toward the top; while in North Oman similar lower clastics (Basal Sands) give way to the bioclastic limestone, known as the Haushi Limestone. Guit et al. (1995) identified a Maximum Flooding Shale within the Lower Gharif of South Oman (for overview see Figure 10.11).

Figure 10.11:

Simplified stratigraphic column of the Haushi Group (after Kharusi et al., 1995).

Figure 10.11:

Simplified stratigraphic column of the Haushi Group (after Kharusi et al., 1995).

Osterloff et al. (2004a), divided the Lower Gharif into three sub-members; two thicker sandy units, separated by the thin Maximum Flooding Shale and associated bioturbated sandstones (which corresponds to MFS P10 of Sharland et al., 2001).

Penney et al. (2006) distinguish an upper marine-influenced section and a lower transitional sequence in the Lower Gharif of South Oman, which correspond to Palyno-subzones 2105B and 2105A (and top 2141C) respectively. The Maximum Flooding Shale falls within the 2105B Palyno-subzone and is delineated by the 1115 Palynozone when developed in marine facies. Their upper marine influenced section interdigitates and grades into the Haushi Limestone further north.

The Middle Gharif Member is a sequence of marginal-marine clastics overlain by lacustrine and fluvial units, capped by stacked palaeosols (the ‘Playa Shale’ sensuGuit et al., 1995), deposited in a semi-arid climate. The unit is barren of palynomorphs in its upper part (Osterloff et al., 2004a).

The Upper Gharif Member, comprising a palaeosol-dominated sequence (red-grey, gleyed vertisols) with variable scale subaqueous shales and channel sands, overlies the Middle Gharif Member.

Osterloff et al. (2004a) proposed a finer subdivision of the Gharif into eight cycles which were considered to reflect periods of increased and then decreased sediment influx. Many of these picks are placed ‘sequence stratigraphically’ in the middle of shales and others at the bases of sandstones. This resulted in the cycles being of limited practical use and this stratigraphic scheme offers no advantage over the basic lithostratigraphy outlined above and has not been widely adopted.

Age: Early – middle Permian, Sakmarian – Wordian, ca. 292–267 Ma.

As already mentioned two key calibration points exist within the Gharif Formation. The Upper Gharif ‘Gharif Paleoflora’ has been interpreted to be late Roadian – early Wordian in age (see discussion in Berthelin et al., 2006). Angiolini et al. (2006) indicate a Sakmarian age for the Haushi Limestone based on fusilinid recovery with supporting brachiopod evidence. Palynological work broadly supports this age range.

Sharland et al. (2001) place their MFS P10 within the bioturbated shales immediately below the Haushi Limestone (as illustrated in their figure 4.24, Safiq South-1).

Biostratigraphy: Five Palynozones with 4 Palyno-subzones are recognised, as follows:

ZoneSubzoneMarker speciesRelative ageFormation/Unit
2252 Hamiapollenites spp.Roadian – WordianUpper Gharif
2190 Kingiapollenites subcircularisArtinskian – Kungurian*Middle-uppermost Lower Gharif
2105 Alisporites indarraensislate SakmarianLower Gharif
BAlisporites indarraensisLower Gharif (includes Haushi Limestone and Maximum Flooding Shale)
A(relative decrease A. indarraensis and K. subcircularis. Persistent C. cymbatus)Lower Gharif (includes Basal Sands)
1115** Ulanisphaeridium omanensislate SakmarianLower Gharif (Maximum Flooding Shale, ?ranging to Haushi Limestone)
2141 Cycadopites cymbatus?Sakmarian*Lower Gharif-Al Khlata (P1 Rahab)
CCycadopites cymbatuslower Lower Gharif
B(influx Microbaculispora spp. Decrease Horriditriletes spp. and C. cymabtus)lowermost Lower Gharif/P1 Rahab
ZoneSubzoneMarker speciesRelative ageFormation/Unit
2252 Hamiapollenites spp.Roadian – WordianUpper Gharif
2190 Kingiapollenites subcircularisArtinskian – Kungurian*Middle-uppermost Lower Gharif
2105 Alisporites indarraensislate SakmarianLower Gharif
BAlisporites indarraensisLower Gharif (includes Haushi Limestone and Maximum Flooding Shale)
A(relative decrease A. indarraensis and K. subcircularis. Persistent C. cymbatus)Lower Gharif (includes Basal Sands)
1115** Ulanisphaeridium omanensislate SakmarianLower Gharif (Maximum Flooding Shale, ?ranging to Haushi Limestone)
2141 Cycadopites cymbatus?Sakmarian*Lower Gharif-Al Khlata (P1 Rahab)
CCycadopites cymbatuslower Lower Gharif
B(influx Microbaculispora spp. Decrease Horriditriletes spp. and C. cymabtus)lowermost Lower Gharif/P1 Rahab
*

Large uncertainties apply to these ages in particular.

**

Marine zone coeval with part of 2105B Palyno-subzone.

Both zonal and subzonal identification rely on the recognition of quantitative changes in miospore assemblages. Unlike the previous zonal definitions no species names are applied to the Palyno-subzones, however, where they differ from zonal definitions the main (downhole) defining criteria are noted above. Elements of the scheme are discussed or illustrated in Stephenson and Osterloff (2002), Osterloff et al. (2004a,b), Stephenson et al. (2003) and Penney et al. (2008). Further detail and key definitions are provided in Penney et al. (2006) and Penney and Stephenson (2006).

A full understanding of the distribution of Palynozones 2252 and 2190 in the Upper and Middle Gharif respectively is hindered by the dominance of palynologically barren facies (red beds, soils, clean sands). The Palynozone 2141 to 2105 shift from C. cymbatus and Microbaculispora/Horriditriletes dominated to bisaccate and monosaccate pollen-dominated assemblages reflects both the post-glacial warming and the overall transgressive nature of the Lower Haushi Member.

Palaeobotany and climate: Spore and pollen assemblage changes through the Lower Gharif indicate a rapid warming of climate following the melting of the last Al Khlata ice sheet (Stephenson and Osterloff, 2002). This warming and associated sea-level rise culminated in the deposition of the Haushi Limestone within a temperate climatic zone (Angiolini et al., 2003; Stephenson et al., 2008). Cold-climate lowland ferns of the Al Khlata were replaced by Glossopterid-type floras producing bisaccate pollen (gymnosperms or seed plants). Gymnosperms, possibly primitive conifers, dominated the Middle Gharif, even though the soil types encountered suggest that this seems to be the most arid climatic period of Gharif deposition. A change in the vegetation occurred in the Upper Gharif probably associated with a seasonally wetter (?monsoonal) climate. Lowland vegetation seems to have been dominated by pteridophytes, with some lycopsids, and conifers grew on the better-drained uplands.

Plant beds are known from two levels in the Upper Gharif of the outcrops in the northern Al Huqf (Broutin et al., 1995; Berthelin et al., 2003, 2006) and are significant in containing a mix of Gondwanan (Glossopteris), Euramerian (conifer and ginkgo) and Cathaysian floras (Tingia). Silicified wood occurs widely in Gharif outcrops in the Al Huqf area in channel sands and is thought to be from large conifers (Broutin et al., 1995; Angiolini et al., 2001). Silicified wood also occurs quite abundantly as smaller, disseminated pieces in the transgressive and shallow marine Lower Gharif sands.

Al Khlata Formation

Authors:Levell et al. (1982). See also Hughes Clarke (1988) and Levell et al. (1988). It is named after the outcrops in Wadi Al Khlata in the southern Al Huqf. Prior to 1982, the Al Khlata Formation was known, informally, as the Lower Haushi or the Marmul Formation in South and Central Oman and the Ghaba Formation in North Oman. These names appear in some early publications (e.g. as illustrated in Gorin et al., 1982).

Introduction

The Al Khlata represents the oldest rocks deposited after the Mid-Carboniferous (Al-Husseini, 2008) or base Haushi, ‘Hercynean’ unconformity and comprises the lower part of the Haushi Group. The unit corresponds with the Arabian Plate Glaciation 3 in Sharland et al. (2001) and amongst others in Al-Husseini (2004). It is defined to include all glacially-related deposits of the Permian – Carboniferous glaciations in Oman. Work by Stephenson and Osterloff (2002), Osterloff et al. (2004a) and Penney et al. (2006) suggests that glacial-influenced sedimentation, even very localised diamictites, could continue into units that lithostratigraphically are part of the lowermost Gharif.

The earliest published account of rocks of the Al Khlata Formation in the Al Huqf outcrops of Central Oman (but not identified as glacial deposits) is by Thesiger (1948, reported by Game, 1950) as part of his extensive travels through Arabia. He states, “the infrequent ridges were chiefly composed of limestone, but here, for the first time we found outcrops of red granite and of gabbro, and many fragments of porphyry, jasper, and rhyolite”. Possible glacial sediments had been noted in the 1950s by geologists of the Iraq Petroleum Company (e.g. Hudson, 1958). They interpreted the outcropping rocks as diamictites with a glaciogenic origin as subaquous rain-out deposits. In the subsurface, the Al Khlata Formation was discovered in 1956, by the Dhofar Cities Services Petroleum Company (DCSPC), in well Marmul-1. The heavy oil encountered discouraged further exploration. It was not until the 1980s when renewed interest associated with a successful exploration boom of the Eastern Flank of the South Oman Salt Basin resulted in further studies. The remote Al Huqf outcrops were ‘rediscovered’ and the rocks unmistakably identified as glacial deposits soon thereafter by Braakman et al. (1982) and subsequently others. Levell et al. (1988), Al-Belushi et al. (1996) and Martin et al. (2008) more fully discuss the nature, composition and origin of the Formation.

The Al Khlata was initially subdivided into four sedimentary sequences by Levell et al. (1988). Osterloff et al. (2004b) illustrates the lithostratigraphical scheme that subsequently developed in PDO based on palynological calibration. The four informal units, in ascending order are: P9, P5 and P1, where the P1 is split further by the recognition of an upper Rahab Shale unit. This naming convention originates from Al Khlata Production Unit names, e.g. AK P9. The numbers highlight the link to, and the importance of, palynology as they relate directly to the database codes of the zonal marker miospore species (from Palynozones 2159, 2165 and 2141).

Although strictly a subunit of the P1 ‘Member’ the P1 Rahab is worthy of separate consideration by virtue of its unique and distinctive character and it is described separately below.

Type and reference sections: Wadi Al Khlata, north and south branches, Al Huqf outcrop area. Note that only ca. 50 m of section is seen at outcrop (P5 and P1 members only) compared with the thicker, stratigraphically more complete subsurface reference section, Rahab-2 well in South Oman (Figure 10.12).

Figure 10.12:

Composite electrical logs, lithology and lithological description of the Al Khlata Formation, Haushi Group, in well Rahab-2, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.12:

Composite electrical logs, lithology and lithological description of the Al Khlata Formation, Haushi Group, in well Rahab-2, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Additional reference sections are Safiq South-1 (Figure 10.2), and Hajal-1 in South Oman (Figure 10.13), Saih Rawl-27 in North Oman (Figure 10.14) and Bahja-1 in Central Oman (Figure 10.6).

Figure 10.13:

Composite electrical logs, lithology and lithological description of the Rahab shale, Al Khlata Formation, Haushi Group, in well Hajal-1, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.13:

Composite electrical logs, lithology and lithological description of the Rahab shale, Al Khlata Formation, Haushi Group, in well Hajal-1, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.14:

Composite electrical logs, lithology and lithological description of the Al Khlata Formation, Haushi Group, in well Saih Rawl-27, North Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.14:

Composite electrical logs, lithology and lithological description of the Al Khlata Formation, Haushi Group, in well Saih Rawl-27, North Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Lithology: The Al Khlata Formation consists of a complex mix of clastic lithologies, involving considerable lateral and vertical changes in facies, lithology and thickness. In South Oman, these changes are most pronounced and occur over short distances. The lithologies range from coarse- to finegrained diamictites, conglomerates, gravels, pebbly sands, sands, silts, silty shales and shales. There are also some very distinctive glacial facies associated with the diamictites such as rythmically laminated (varve-like) shales with dropstones and climbing ripple cross-laminated sands. There is also a lack of bioturbation in the shales and macroscopic plant debris except very near the top of the Formation. The pebbles, cobbles and boulders (up to metre-size in some diamictites) may be faceted and striated and are of a very varied composition. The diamictites are grey, argillaceous/silty/slightly sandy with a variety of rock fragments. Huqf-like lithologies are often seen, but a suite of acidic, plutonic igneous rocks, mainly granites and granophyres is the dominant rock fragment type. Volcanic clasts are also common, including feldspar porphyries, trachytes and various chertified devitrified lithologies. Similarly, the grains in the sands are varied, with very common lithic and arkosic sands. Pure quartz sands also occur, but micas, other than in lithic grains, are rare. From south to north in Oman, the coarser-grained elements tend to disappear and the sediments become generally more mature (more quartzose), better-sorted and less laterally-variable.

Subsurface recognition: The presence of lithic rock fragments in the cuttings is diagnostic for the presence of the Al Khlata Formation. Whilst drilling, the Al Khlata is characterised by the absence of red clastics (Figures 10.15 and 10.16). The diamictites are grey and appear in cuttings as a loose mixture of fine- to very coarse-grained quartz sand and rock fragments. The latter show signs of having been fractured from larger pebbles. Hot-shot samples for palynological analyses help to confirm the age of the sediments.

Figure 10.15:

Ditch cuttings from the Haushi Group: (a) Shale from the Rahab in Rahab-1; (b) Shale from the Al Khlata Formation in Rahab-1; and (c) Diamictite from the Al Khlata Formation in Thuleilat-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.15:

Ditch cuttings from the Haushi Group: (a) Shale from the Rahab in Rahab-1; (b) Shale from the Al Khlata Formation in Rahab-1; and (c) Diamictite from the Al Khlata Formation in Thuleilat-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.16:

Ditch cuttings from the Al Khlata Formation, Haushi Group: (a) Diamictite from well Rahab-1; (b and c) Sandstone from Haima-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.16:

Ditch cuttings from the Al Khlata Formation, Haushi Group: (a) Diamictite from well Rahab-1; (b and c) Sandstone from Haima-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Post-drilling Al Khlata sediments can be recognised by detailed lithological analysis combined with log pattern. Diamictites have typical log patterns (cf. Levell et al., 1988):

  • (a) Generally a highly serrate, trendless moderate Gamma-ray profile.

  • (b) Marked bows/blocks in the Neutron-Density-curves with either positive or negative separations, although positive separations are more common.

  • (c) Hardness, reflected in high Density and low Neutron log response, giving a distinctive ‘kick’ to the right, moderate to low Interval Transit times, and moderate to high Resistivities. Sonic and Resistivity logs typically reveal spikes representing individual clasts/boulders in the diamictites (particularly on Micro-Resistivity tools).

  • (d) Chaotic Dipmeter.

Muddy diamictite displays similar open-hole responses to the sandy diamictites, but generally with higher Gamma-ray values and with a wider, positive Neutron-Density separation.

Bounding formations often have more characteristic and predictable log patterns than the Al Khlata, e.g. the Ghudun or Amin.

The Al Khlata diamictites are more argillaceous and friable than the diamictites of the Neoproterozoic Ghadir Manqil.

Detailed palynological analysis should confirm the presence of Palynozones 2141, 2165 and 2159.

Where the Al Khlata rests on the Mahwis, there is a noticeable change in colour of shale from grey in the Al Khlata to distinctive green-grey in the Mahwis.

Boundaries: The Al Khlata may lie unconformably upon the Misfar Group, and Haima or Huqf Supergroups, often with an angular, contact. The characteristic lithologies in the Al Khlata are easily recognisable, but the basal beds can sometimes be quartzose sands very similar to (and directly reworked from) Haima sands. In such cases, the boundary can generally be confirmed by palynological means, although there can be the complication of a palynologically barren section at the base of the Al Khlata.

The upper boundary is essentially a transgressive surface, with the Al Khlata overlain by the Gharif Formation, which leads to a maximum flooding near base Haushi Limestone. The boundary is considered mainly conformable with potential to be erosive to unconformable, but with only a minor time gap. The final deposits of the Al Khlata are mostly shales in southeast Oman (the Rahab shales), usually with a content of rhythmically laminated (varve-like) shales. The boundary with the Gharif is at the top of this shale unit, beneath Gharif basal sands. Work by Stephenson and Osterloff (2002) and Osterloff et al. (2004a) suggested that sediments in the Lower Gharif Member (specifically the coarsegrained clastics) may be, in part, coeval with the Rahab Shale. In this interpretation, glacially-influenced sedimentation may continue into units that lithostratigraphically are part of the Lower Gharif Member. This scenario is debatable and reworking and/or truncation may also be responsible. Furthermore, some of Stephenson and Osterloff (2002) interpretations may be based on erroneous lithostratigraphical assignments (Penney, personal communication).

Where the Rahab Shale is missing, over most of northern and western Oman, the top of the Al Khlata is difficult to assign whilst drilling. Whether the absence of the Rahab Shale is due to truncation or facies change has yet to be fully resolved. In such cases post-drill biostratigraphic work is required to support correlations.

The Al Khlata can be unconformably overlain by younger units, notably the Nahr Umr Formation.

Distribution: The Al Khlata Formation is found throughout South, Central and North Oman. It has been observed in the eastern Al Hajar Mountains outcrops, in Wadi Dayqah, as rain-out diamictites, which have been dated as Palyno-subzone 2159A (basal Al Khlata). Similar glaciogenic facies are recorded from the Carboniferous – Permian of southwestern Saudi Arabia. This includes the subsurface Unayzah B/C (Stephenson et al., 2003; Melvin and Sprague, 2006; Sprague et al., 2008; Al-Husseini, 2008); the lower part of the Juwayl Member of the Wajid Formation in outcrop in southwest Saudi Arabia (McClure 1980; Kellogg et al., 1986; Evans et al., 1991; Al-Husseini, 2008), the Jawb Member and Ghazal Member of the subsurface Juwayl Formation (see Al-Husseini, 2008), ‘Al Khlata equivalent’ deposits (Al-Hajiri and Owens, 2000). The Kooli Formation in western Yemen (El-Nakhal et al., 2002) and the Akbra Formation in north Yemen (Kruck and Thiele, 1983) are also considered to be time equivalent. Appropriate references and the fuller significance of the sedimentary development in Oman are given by Braakman et al. (1982), Levell et al. (1988) and Osterloff et al. (2004b).

Deposition: The Al Khlata deposits are highly heterogeneous with a wide range of thicknesses. Pinching out over highs, the Al Khlata thickness can exceed 800 m in localised depocentres (Osterloff et al., 2004b). Sediments are dominated by glaciodeltaic and glaciolacustrine lithofacies associations, with subordinate glaciofluvial deposits. These were all deposited in proglacial environments, probably during deglaciation phases, based on the absence of features that can be readily attributed to the direct action of ice (Aitken et al., 2004; Aitken and Clark, 2007). During periods of glacial (re-)advance, much of the previously deposited sediments were scoured, either directly by the action of ice or, more probably, by proglacial meltwaters. As such the Al Khlata represents numerous glacial and warm interglacial periods. Deposition primarily occurred during the shift from glacial to interglacial periods and was dominated by ice melt-out deposition. Subsequent glacial advances severely eroded previous deposits. Only a fraction of the sediments initially laid down are preserved in the subsurface, with extensive ‘missing time’. Striated glacial pavements occur in the Al Huqf outcrops (Braakman et al., 1982), with crag and tail features indicating ice movement towards the northeast. These may reflect the earliest, most extensive and coldest glacial periods, with little or no deposits preserved.

Detrital zircons, striae, palaeocurrent and diamictite provenance data from the Al Huqf outcrops and selected subsurface sections (Martin et al., 2008) indicate that the oldest deposits (P9) are sourced from far away, most likely carried by a relatively large-scale ice cap/sheet. A characteristic feature of this succession in the subsurface of southeast Oman is the high frequency of erosional valleys observed within it. Valleys have also been observed in younger deposits and one should be careful assigning age without a detailed palynological check. Detrital zircons from younger sediments (P5) suggest more local source areas, possibly from glaciers on local highs. In the outcrops of the Al Huqf this sequence is found in erosional valleys, but in the subsurface this P5 unit is often completely missing (Aitken et al., 2004; Aitken and Clark, 2007). It does seem that P5 represents the most fragmented and variably distributed of the three Al Khlata units, particularly on the Eastern Flank (Penney et al., 2006 and Osterloff et al., 2004b). Renewed glaciation in the early Permian (P1) resulted in the widespread expansion of ice sheets across Gondwana. Detrital zircons suggest sediments derived from Yemen and Saudi Arabia in southern Oman and local sources in northern Oman. This could also explain other observations made in outcrops of the Al Huqf area reported by Heward (1986), Al Belushi (1996) and Al Belushi et al. (1996) suggesting that ice sheets may have moved from highlands in the northeast towards the southwest in addition to the overall picture of a glaciation from the southwest. A blanketing diamictite unit (of glaciolacustrine origin) can be recognised on a regional scale in southeast Oman at the base of the youngest Al Khlata (P1) level and it is overlain by at least two large-scale lacustrine cycles (the Rahab Shale) comprising mainly siltstones, which are locally rhythmically laminated. Dropstones observed in these siltstones decrease upwards in frequency. These lacustrine cycles were deposited during the final stages of deglaciation.

Subdivision:Levell et al. (1988) distinguished four rock units. From oldest to youngest these are: (1) lower Al Khlata deposits; (2) upper Al Khlata extensive glaciolacustrine deposits; (3) Al Khlata regional ice advance deposits; (4) deglaciation deposits characterised by the Rahab Shale. Typically in the subsurface, the only lithostratigraphic subdivision of the Al Khlata Formation that has been used is the ‘Rahab Member’ (noted as such in Mohammed et al., 1997). This ‘Rahab Member’ has been used consistently on a regional scale in southeast Oman. Osterloff et al. (2004b) illustrates the lithostratigraphical scheme that has developed in PDO via necessary palynological calibration of these production zones. The four informal units, in ascending order are: P9, P5 and P1, where the P1 is split further by the recognition of an upper Rahab Shale unit.

The Al Khlata sensu stricto was defined as a separate member by Mohammed et al. (1997), but this subdivision is discontinued as this member essentially covered the whole Al Khlata Formation except for the ‘Rahab Member’.

Age: Late Carboniferous – early Permian, late Moscovian? – Sakmarian, ca. 310?–292 Ma. This age assignment is based on palynological evidence (Penney and Stephenson, 2006; Penney et al., 2008; see also Osterloff et al., 2004b). Osterloff et al. (2004b) believe that the Al Khlata could range down to a Namurian equivalent age by comparison with European assemblages (i.e. ca. Bashkirian).

Biostratigraphy: Three Palynozones with six Palyno-subzones and two transitional assemblages are applied as follows:

ZoneSubzone**Marker speciesRelative ageFormation/Unit
  Cycadopites cymbatus Lower Gharif-P1
2141BCycadopites cymbatus?SakmarianLowermost Gharif/P1 Rahab
A(decrease Microbaculispora spp., increase Horriditriletes spp)P1
  Microbaculispora spp. P5
2165BMicrobaculispora spp.?Asselian – SakmarianUpper P5
 A(decrease Horriditriletes spp. and taeniate bisaccates) Lower P5
2159B/2165A (acme monosaccate pollen) Lower P5/Upper P9
  Punctatisporites spp.* P9
 BPunctatisporites spp.* Upper P9
2159B/A(decrease monosaccates, increase Punctatisporites spp.)?late Moscovian – GzhelianMiddle P9
 A(further decrease monosaccates and increase (>95%) Punctatisporites spp.) Lower P9
  Barren zoneUnknown 
ZoneSubzone**Marker speciesRelative ageFormation/Unit
  Cycadopites cymbatus Lower Gharif-P1
2141BCycadopites cymbatus?SakmarianLowermost Gharif/P1 Rahab
A(decrease Microbaculispora spp., increase Horriditriletes spp)P1
  Microbaculispora spp. P5
2165BMicrobaculispora spp.?Asselian – SakmarianUpper P5
 A(decrease Horriditriletes spp. and taeniate bisaccates) Lower P5
2159B/2165A (acme monosaccate pollen) Lower P5/Upper P9
  Punctatisporites spp.* P9
 BPunctatisporites spp.* Upper P9
2159B/A(decrease monosaccates, increase Punctatisporites spp.)?late Moscovian – GzhelianMiddle P9
 A(further decrease monosaccates and increase (>95%) Punctatisporites spp.) Lower P9
  Barren zoneUnknown 
*

Previously Anapiculatisporites concinnus.

Penney et al. (2008) place the 2165-2159 boundary at the Asselian – Gzhelian (Permian – Carboniferous) boundary and do not range the Al Khlata below a tentative late Moscovian. Previously Penney and Stephenson (2006) and Osterloff et al. (2004b) have argued that the Al Khlata could range as old as Bashkirian (see Group age discussion). The basal barren zone adds further uncertainty to the maximum interpreted age of the Al Khlata. Such large uncertainties apply to these ages because there is no direct palynological correlation possible with the type Russian sections. Also no other fossil groups have been identified in the Al Khlata to corroborate these zonal age assignments. It is significant that no marine fossils are found in the Al Khlata, which is consistent with a terrestrial, fluvio-lacustrine setting. Reworked Devonian (Misfar Group) sporomorphs occur regularly, especially in Palynozone 2159.

The Al Khlata palynological succession is detailed in Penney and Stephenson (2006), and Penney et al. (2008) and discussed in Osterloff et al. (2004b). The latter also summarises the warming climatic story inherent in the Al Khlata with reference to the miospore content, e.g. potentially cold, dry tundra conditions at P9 ending in the melt out, warmer, more diverse vegetation, conditions associated with the P1 Rahab lacustrine phase.

Correlation with sequences in Saudi Arabia is provided by comparison with the OSP palynological Zones OSPZ2 to OSPZ1 of Stephenson et al. (2003) (see also Penney et al., 2008).

P1 Rahab

Author: Never formally published. Referred to as the Rahab Shale in Levell et al. (1988) and as the Rahab Shale Member by Kharusi et al. (1995).

Introduction

Although strictly a subunit of the P1 Member, the P1 Rahab is worthy of separate consideration by virtue of its unique and distinctive character. On the Eastern Flank and in the South Oman Salt

Basin the P1 Rahab is recognised in shaly facies. Further to the south it has been interpreted as a sandier facies with a Rahab sandstone overlying the Rahab Shale (see Figures 10.11; Kharusi et al., 1995; Stephenson and Osterloff, 2002), but this cannot be conclusively supported without much more rigorous biostratigraphical control. In the type well, and in the northern Eastern Flank, interbedded sandstones and siltstones/diamictites thicken sufficiently to separate two distinct shaly units (Osterloff et al., 2004b).

Type section: Hajal-1 in South Oman (Figure 10.13). Not seen at outcrop.

Lithology: The Rahab shales are dark grey and lithologically similar to the shales of the Lower Gharif and may contain plant remains, but they are restricted to Palyno-subzone 2141B. The shales are locally rhythmically laminated and contain dropstones with the abundance of dropstones typically declining upwards (Aitken and Clark, 2007). Interbedded siltstones, sandstones and diamictites occur locally. In Hajal-1 (Figure 10.13) and the northern Eastern Flank, the interbedded sandstone or pebbly siltstone/diamictite is sufficiently thick and continuous to define two discrete lobes, which can be considered to represent a genuine ‘Upper’ and ‘Lower’ Rahab Shale (Kharusi et al., 1995; Aitken and Clark, 2007).

Subsurface recognition: Whilst drilling it is impossible to differentiate between the shales of the Lower Gharif and the Rahab on the basis of lithology alone.

Post-drilling the log pattern is diagnostic for the Al Khlata in general, especially where diamictites are seen. The P1 Rahab is a high Gamma interval, occasionally seen as two distinct cycles. Palynological analysis confirms the presence of Palyno-subzone 2141B.

Boundaries: The P1 Rahab is overlain by the Gharif (predominantly) and occasionally by the Nahr Umr or the Kahmah Group. When sands overlie the Rahab they are generally the subfeldspathic sandstones of the Lower Gharif.

A problem occurs in southeast Oman where progradational lacustrine shoreface deposits, in places associated with diamictites, may either represent post Rahab or laterally equivalent Rahab facies (Rahab Sandstone, Aitken and Clark, 2007). Truncation and reworking may further complicate the picture.

Distribution: The P1 Rahab is confined to the Eastern Flank and the South Oman Salt Basin. The Rahab lacustrine shale facies has not been recorded in Central and North Oman, where the differentiation of upper Al Khlata sands from initial non-marine Basal Gharif sands becomes difficult. Recognition of Palyno-subzone 2141B is critical in this respect, but sufficient data is often lacking.

Deposition: The P1 Rahab is dominated by glaciolacustrine mudrock and reservoir facies are typically rare. Glaciodeltaic, glaciolacustrine shoreline and turbidite reservoirs all occur locally. It represents the final glacial melt out phase as the climate begins to change significantly prior to Gharif deposition.

Age: ?early Sakmarian, ca. ?294.6–292 Ma.

Biostratigraphy: Palyno-subzone 2141B (Cycadopites cymbatus Palynozone). This distinctive Palyno-subzone is characterised by an influx of Microbaculispora spp. associated with reduced numbers of Horriditriletes spp. and C. cymbatus.

Figures & Tables

Figure 10.1:

Location map: Haushi Group.

Figure 10.1:

Location map: Haushi Group.

Figure 10.2:

Composite electrical logs, lithology and lithological description of the Gharif and Al Khlata formations, Haushi Group, in well Safiq South-1, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.2:

Composite electrical logs, lithology and lithological description of the Gharif and Al Khlata formations, Haushi Group, in well Safiq South-1, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.3

(facing page): Composite electrical logs, lithology and lithological description of the Gharif Formation, Haushi Group, in well Saih Rawl-2, North Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.3

(facing page): Composite electrical logs, lithology and lithological description of the Gharif Formation, Haushi Group, in well Saih Rawl-2, North Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.4:

Composite electrical logs, lithology and lithological description of the Gharif and Al Khlata formations, Haushi Group, in well Al Bashair-1, North Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.4:

Composite electrical logs, lithology and lithological description of the Gharif and Al Khlata formations, Haushi Group, in well Al Bashair-1, North Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.5:

Composite electrical logs, lithology and lithological description of the Gharif Formation, Haushi Group, in well Wafra-6, Central Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.5:

Composite electrical logs, lithology and lithological description of the Gharif Formation, Haushi Group, in well Wafra-6, Central Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.6:

Composite electrical logs, lithology and lithological description of the Gharif and Al Khlata formations, Haushi Group, in well Bahja-1, Central Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.6:

Composite electrical logs, lithology and lithological description of the Gharif and Al Khlata formations, Haushi Group, in well Bahja-1, Central Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.7:

Composite electrical logs, lithology and lithological description of the Gharif Formation, Haushi Group, in well Qaharir-4, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.7:

Composite electrical logs, lithology and lithological description of the Gharif Formation, Haushi Group, in well Qaharir-4, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.8:

Ditch cuttings from the Haushi Group: (a) Shale from the Upper Gharif Member in Saih Rawl-2; (b) Shale from the Middle Gharif Member in Barik-10; and (c) Shale from the Lower Gharif Member in Nimr-13 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.8:

Ditch cuttings from the Haushi Group: (a) Shale from the Upper Gharif Member in Saih Rawl-2; (b) Shale from the Middle Gharif Member in Barik-10; and (c) Shale from the Lower Gharif Member in Nimr-13 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.9:

Ditch cuttings from the Haushi Group: (a) Sandstone from the Middle Gharif Member in Karim West-55; (b) Sandstone from the Lower Gharif Member in Nimr-13; and (c) Sandstone from the Lower Gharif Member in Rajaa-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.9:

Ditch cuttings from the Haushi Group: (a) Sandstone from the Middle Gharif Member in Karim West-55; (b) Sandstone from the Lower Gharif Member in Nimr-13; and (c) Sandstone from the Lower Gharif Member in Rajaa-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.10:

Ditch cuttings from the Lower Gharif Member, Haushi Group: (a) Haushi Limestone; and (b) Basal Sandstone from Saih Rawl-5 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.10:

Ditch cuttings from the Lower Gharif Member, Haushi Group: (a) Haushi Limestone; and (b) Basal Sandstone from Saih Rawl-5 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.11:

Simplified stratigraphic column of the Haushi Group (after Kharusi et al., 1995).

Figure 10.11:

Simplified stratigraphic column of the Haushi Group (after Kharusi et al., 1995).

Figure 10.12:

Composite electrical logs, lithology and lithological description of the Al Khlata Formation, Haushi Group, in well Rahab-2, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.12:

Composite electrical logs, lithology and lithological description of the Al Khlata Formation, Haushi Group, in well Rahab-2, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.13:

Composite electrical logs, lithology and lithological description of the Rahab shale, Al Khlata Formation, Haushi Group, in well Hajal-1, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.13:

Composite electrical logs, lithology and lithological description of the Rahab shale, Al Khlata Formation, Haushi Group, in well Hajal-1, South Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.14:

Composite electrical logs, lithology and lithological description of the Al Khlata Formation, Haushi Group, in well Saih Rawl-27, North Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.14:

Composite electrical logs, lithology and lithological description of the Al Khlata Formation, Haushi Group, in well Saih Rawl-27, North Oman (Mohammed et al., 1997). See Figure 10.1 for location.

Figure 10.15:

Ditch cuttings from the Haushi Group: (a) Shale from the Rahab in Rahab-1; (b) Shale from the Al Khlata Formation in Rahab-1; and (c) Diamictite from the Al Khlata Formation in Thuleilat-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.15:

Ditch cuttings from the Haushi Group: (a) Shale from the Rahab in Rahab-1; (b) Shale from the Al Khlata Formation in Rahab-1; and (c) Diamictite from the Al Khlata Formation in Thuleilat-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.16:

Ditch cuttings from the Al Khlata Formation, Haushi Group: (a) Diamictite from well Rahab-1; (b and c) Sandstone from Haima-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 10.16:

Ditch cuttings from the Al Khlata Formation, Haushi Group: (a) Diamictite from well Rahab-1; (b and c) Sandstone from Haima-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

GROUPFORMATIONINFORMAL MBRINFORMAL UNIT
HaushiGharifUpper Gharif 
Middle Gharif 
Lower GharifHaushi Limestone
 Basal Sands
Al KhlataP1Rahab Shale
P5 
P9 
GROUPFORMATIONINFORMAL MBRINFORMAL UNIT
HaushiGharifUpper Gharif 
Middle Gharif 
Lower GharifHaushi Limestone
 Basal Sands
Al KhlataP1Rahab Shale
P5 
P9 
ZoneMarker speciesRelative ageFormation/Unit
2252Hamiapollenites spp.Roadian – WordianUpper Gharif
2190Kingiapollenites subcircularisArtinskian – Kungurian*Middle/Lower Gharif
2105Alisporites indarraensislate SakmarianLower Gharif
1115**Ulanisphaeridium omanensislate SakmarianLower Gharif
2141Cycadopites cymbatus?Sakmarian*Lower Gharif-Al Khlata (P1)
2165Microbaculispora spp.?Asselian – Sakmarian*Al Khlata (P5)
2159Punctatisporites spp.?late Moscovian – Gzhelian*Al Khlata (P9)
 Barren zoneUnknownbasal Al Khlata
ZoneMarker speciesRelative ageFormation/Unit
2252Hamiapollenites spp.Roadian – WordianUpper Gharif
2190Kingiapollenites subcircularisArtinskian – Kungurian*Middle/Lower Gharif
2105Alisporites indarraensislate SakmarianLower Gharif
1115**Ulanisphaeridium omanensislate SakmarianLower Gharif
2141Cycadopites cymbatus?Sakmarian*Lower Gharif-Al Khlata (P1)
2165Microbaculispora spp.?Asselian – Sakmarian*Al Khlata (P5)
2159Punctatisporites spp.?late Moscovian – Gzhelian*Al Khlata (P9)
 Barren zoneUnknownbasal Al Khlata
*

Large uncertainties apply to these ages in particular.

**

Marine zone coeval with part of 2105 Miospore Zone.

ZoneSubzoneMarker speciesRelative ageFormation/Unit
2252 Hamiapollenites spp.Roadian – WordianUpper Gharif
2190 Kingiapollenites subcircularisArtinskian – Kungurian*Middle-uppermost Lower Gharif
2105 Alisporites indarraensislate SakmarianLower Gharif
BAlisporites indarraensisLower Gharif (includes Haushi Limestone and Maximum Flooding Shale)
A(relative decrease A. indarraensis and K. subcircularis. Persistent C. cymbatus)Lower Gharif (includes Basal Sands)
1115** Ulanisphaeridium omanensislate SakmarianLower Gharif (Maximum Flooding Shale, ?ranging to Haushi Limestone)
2141 Cycadopites cymbatus?Sakmarian*Lower Gharif-Al Khlata (P1 Rahab)
CCycadopites cymbatuslower Lower Gharif
B(influx Microbaculispora spp. Decrease Horriditriletes spp. and C. cymabtus)lowermost Lower Gharif/P1 Rahab
ZoneSubzoneMarker speciesRelative ageFormation/Unit
2252 Hamiapollenites spp.Roadian – WordianUpper Gharif
2190 Kingiapollenites subcircularisArtinskian – Kungurian*Middle-uppermost Lower Gharif
2105 Alisporites indarraensislate SakmarianLower Gharif
BAlisporites indarraensisLower Gharif (includes Haushi Limestone and Maximum Flooding Shale)
A(relative decrease A. indarraensis and K. subcircularis. Persistent C. cymbatus)Lower Gharif (includes Basal Sands)
1115** Ulanisphaeridium omanensislate SakmarianLower Gharif (Maximum Flooding Shale, ?ranging to Haushi Limestone)
2141 Cycadopites cymbatus?Sakmarian*Lower Gharif-Al Khlata (P1 Rahab)
CCycadopites cymbatuslower Lower Gharif
B(influx Microbaculispora spp. Decrease Horriditriletes spp. and C. cymabtus)lowermost Lower Gharif/P1 Rahab
*

Large uncertainties apply to these ages in particular.

**

Marine zone coeval with part of 2105B Palyno-subzone.

ZoneSubzone**Marker speciesRelative ageFormation/Unit
  Cycadopites cymbatus Lower Gharif-P1
2141BCycadopites cymbatus?SakmarianLowermost Gharif/P1 Rahab
A(decrease Microbaculispora spp., increase Horriditriletes spp)P1
  Microbaculispora spp. P5
2165BMicrobaculispora spp.?Asselian – SakmarianUpper P5
 A(decrease Horriditriletes spp. and taeniate bisaccates) Lower P5
2159B/2165A (acme monosaccate pollen) Lower P5/Upper P9
  Punctatisporites spp.* P9
 BPunctatisporites spp.* Upper P9
2159B/A(decrease monosaccates, increase Punctatisporites spp.)?late Moscovian – GzhelianMiddle P9
 A(further decrease monosaccates and increase (>95%) Punctatisporites spp.) Lower P9
  Barren zoneUnknown 
ZoneSubzone**Marker speciesRelative ageFormation/Unit
  Cycadopites cymbatus Lower Gharif-P1
2141BCycadopites cymbatus?SakmarianLowermost Gharif/P1 Rahab
A(decrease Microbaculispora spp., increase Horriditriletes spp)P1
  Microbaculispora spp. P5
2165BMicrobaculispora spp.?Asselian – SakmarianUpper P5
 A(decrease Horriditriletes spp. and taeniate bisaccates) Lower P5
2159B/2165A (acme monosaccate pollen) Lower P5/Upper P9
  Punctatisporites spp.* P9
 BPunctatisporites spp.* Upper P9
2159B/A(decrease monosaccates, increase Punctatisporites spp.)?late Moscovian – GzhelianMiddle P9
 A(further decrease monosaccates and increase (>95%) Punctatisporites spp.) Lower P9
  Barren zoneUnknown 
*

Previously Anapiculatisporites concinnus.

Contents

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal