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SUPERGROUPGROUPFORMATION
HuqfNimrHaradh
Karim
AraDhahaban
Al Noor
Athel
U
Birba
NafunBuah
Shuram
Khufai
Masirah Bay
Hadash
Abu MaharaGhadir Manqil
SUPERGROUPGROUPFORMATION
HuqfNimrHaradh
Karim
AraDhahaban
Al Noor
Athel
U
Birba
NafunBuah
Shuram
Khufai
Masirah Bay
Hadash
Abu MaharaGhadir Manqil

Authors: Kassler (unpublished, 1966); refer to Gorin et al. (1982) and Hughes Clarke (1988) for fuller discussions. First published by Glennie (1977) as the Huqf Group. Elevated to supergroup level by Teyssen (1990). Droste (2001) first placed the Nimr Group (previously in Haima Supergroup) within the Huqf Supergroup.

Introduction

The Huqf Supergroup comprises a Neoproterozoic to ‘Early’ Cambrian succession of continental to marine clastics, carbonates and evaporites, overlying metamorphic basement and overlain unconformably by the Haima Supergroup or younger units.

It comprises the clastics, volcanics and diamictites of the Abu Mahara Group, the fine clastics and carbonates of the Nafun Group, the carbonates and evaporites of the Ara Group and the varied clastics of the Nimr Group.

Tectonism and sedimentation in this period is associated with the convergence and subsequent transpressional/transtensional continent-continent collision between East Gondwanaland (eastern Arabia, India and Antarctica) and West Gondwanaland (western Arabia, Africa and South America) (Immerz et al., 2000; Grotzinger et al., 2002; van den Berg et al., 2008) rather than with rifting and extensional tectonics as published by Husseini (1988, 1989), Loosveld et al. (1996), Oterdoom et al. (1999). The early basins that originated in Oman during this time are filled with thick and very variable sequences of Ghadir Manqil clastic sediments of the Abu Mahara Group. The base of the overlying Nafun Group is marked by a transgressive post-glacial carbonate, which initiates an overstepping of basement-cored structural highs and the deposition of an extensive blanket of carbonates (Khufai and Buah formations) and siliciclastics (Masirah Bay and Shuram formations), see also Rabu (1988). Subsequent, very rapid, subsidence, resulted in the deposition of thick successions of evaporites and carbonates (Ara Group) in the giant salt basins of Interior Oman, the Arabian Gulf (Hormuz), Iran and Pakistan. These were followed by progradational alluvial to marginal-marine sediments of the Nimr Group. It should be noted that continental depositional settings and sediment composition were not influenced at all by land plants. These did not develop until Late Ordovician or more effectively Silurian times (Wellman et al., 2003).

The end of Huqf Supergroup sedimentation in the ‘Early’ Cambrian corresponds to a hiatus dated at about 520 Ma, associated with the Angudan unconformity in South Oman. A number of PhD and postdoctoral studies since the mid-1990s have greatly contributed to our understanding of the Huqf Supergroup. These and other findings are further documented in McCarron (2000), Schröder, (2000), Schröder et al. (2003, 2004, 2005), Leather (2001), Leather et al. (2002), Allen and Leather (2006), Cozzi and Al Siyabi (2004, 2005), Cozzi et al. (2004a,b), Le Guerroué (2006), Le Guerroué et al. (2005, 2006a,b,c), Rieu (2006), Rieu et al. (2006, 2007a,b), Fike (2007), Fike et al. (2006), Fike and Grotzinger (2008, in press), Nicholas and Gold (in preparation). Overview publications include Brasier et al. (2000), Al-Husseini et al. (2003), Bowring et al. (2007) and Allen (2007).

Many of the publications listed above highlight the role of isotope studies in developing a better constrained stratigraphy (e.g. Bowring et al. (2007) and Fike and Grotzinger (2008, in press). This is particularly with respect to δ13С and δ34S data where key levels include the base Cambrian and Shuram negative δ13C excursions and the marked shift in δ34S towards the base of the Ara (ca. A0 level, Figure 15.1). This data set has resulted in many new interpretations and several conflicts have yet to be fully resolved. Where appropriate isotope data exists these are shown in subsequent sections and any such conflicts or interpretative issues are noted.

Figure 15.1.

Huqf Supergroup Litho-, Chrono- and Isotope Stratigraphy. * U/Pb absolute dates. Modified after Amthor et al. (2003); Le Guerroué et al. (2006a); Bowring et al. (2007); Fike (2007) and Allen (2007).

Figure 15.1.

Huqf Supergroup Litho-, Chrono- and Isotope Stratigraphy. * U/Pb absolute dates. Modified after Amthor et al. (2003); Le Guerroué et al. (2006a); Bowring et al. (2007); Fike (2007) and Allen (2007).

Type and reference sections: Al Jabal Al Akdhar, Al Huqf outcrop area, Mirbat Area southeast Oman, and subsurface of South Oman Salt Basin. See Group and formational sections for specific detail.

Boundaries: The Huqf Supergroup lies unconformably on basement rocks of various ages. It is overlain by the clastic sediments of the Haima Supergroup or younger formations, from which it is separated by the Angudan unconformity.

Distribution: The Huqf Supergroup has been found in the subsurface and at surface in South and Central Oman and as far north as the Al Jabal Al Akhdar outcrops. Sparse well data and seismic evidence indicates that it is likely to be fully developed under northern Oman as an at least part equivalent of, and perhaps continuous with, the Hormuz series of the Southern Gulf and Iran (Al Husseini and Husseini, 1990).

The original configuration of the depositional basins is not known. The present distribution is limited by younger uplifts where the Huqf may have been totally eroded. Internal facies changes show the influence of the old Arabian basement lineaments (north-south and north-northeast to south-southwest, as shown in Stuart-Smith and Romine, 2003).

Subdivision: The Huqf Supergroup embraces the following groups (Figure 15.1); from top to base: Nimr (varied clastics), Ara (dominantly carbonates and evaporites), Nafun (carbonates and fine clastics series), and Abu Mahara (clastics and diamictites).

Compared to Hughes Clarke (1988), his Huqf Group has been elevated to Supergroup level and constituent formations to Group level. Droste (1997) assigned the Nimr clastics as a Group to the Haima Supergroup. In terms of tectonostratigraphic development it is a continuation and completion of the convergence of East and West Gondwanaland, with clastic sedimentation diachronously appearing already in the upper Ara Group. The Nimr Group is therefore included within the Huqf Supergroup in this lexicon, following Droste (2001) and Amthor and Cozzi (2006).

Sequence stratigraphy: The Huqf Supergroup comprises the AP1 Megasequence, as defined in Sharland et al. (2001). However, subsequent datings within the Abu Mahara Group indicate ages beyond the lower limit of 610 Ma for their AP1 Megasequence.

Age: Late Precambrian (Neoproterozoic: Cryogenian) – ‘Early’ Cambrian, ca. 720–520 Ma. Age assumptions are partly constrained and based on geochronological data by Brasier et al. (2000) and Bowring et al. (2007). They are also consistent with globally correlated carbon isotope data (Amthor et al., 2003; Le Guerroué, 2006a, b,c; Cozzi and Siyabi, 2004, 2005; Bowring et al., 2007) as well as sulphur isotope work (Fike, 2007; Fike et al., 2006; Fike and Grotzinger, 2008, in press). See Figure 15.1.

Biostratigraphy:Love et al. (2009) relate that specific steroids analysed from hydrocarbon extracts within the Huqf Supergroup (Abu Mahara Group) are indicative of the presence of Demospongiae (primitive sponges). They cite this as the earliest form of animal life yet recorded, at a pre-635 Ma age. Evidence of other forms of early life are found as acritarchs, filaments and general Amorphous Organic Matterial in the Abu Mahara and Nafun groups (Knoll, 1994; Butterfield, 2001; Butterfield and Grotzinger, in preparation). Fossil diversity is low throughout, typically dominated by simple sphaeromorphic acritarchs (leiospheres), which lack any real biostratigraphic potential. Amorphous Organic Matterial particularly dominates palynological recovery in the Nafun and Ara. The Nimr Group is palaeontologically barren. Rare and sporadic occurrences of potentially useful palynomorphs within the Supergroup are discussed in the relevant sections. Evidence of algal sediment trapping (stromatolites and thrombolites) is found abundantly in the Nafun and Ara groups. The Precambrian (Ediacaran) part of the Ara Group also contains early metazoan life in the form of Cloudina and probable Namacalathus (Amthor et al., 2003, see Figure 15.1).

HUQF SUPERGROUP NIMR GROUP

GROUPFORMATIONMEMBER‘SUB-UNIT’
NimrHaradh  
KarimRunibUpper Runib
Middle Runib
Lower Runib
IradIrad Shale
Irad Sandstone
KhaleelUpper Khaleel
Lower Khaleel
GROUPFORMATIONMEMBER‘SUB-UNIT’
NimrHaradh  
KarimRunibUpper Runib
Middle Runib
Lower Runib
IradIrad Shale
Irad Sandstone
KhaleelUpper Khaleel
Lower Khaleel

Authors: Priebe and Kapellos (unpublished, 1993). The Nimr Group equates to the informal ‘Lower Haima of South Oman’ of Hughes Clarke (1988). Priebe and Kapellos (1993) placed the Nimr Group within the Haima Supergroup, see also Droste (1997). Following Droste (2001) and Amthor and Cozzi (2006) the Nimr Group is herein formally assigned to the Huqf Supergroup, in accordance with tectonostratigraphic evidence and context (below the Angudan unconformity). Reviewed and updated by Romine et al. (2008).

Introduction

The Nimr Group is a sequence of continental clastics, best known from the Eastern Flank area in South Oman, where it has been extensively penetrated by wells. This also introduces a bias in our knowledge base, with relatively few penetrations in North Oman where the Nimr is restricted to the deeper part of the basins. The Nimr Group is separated from the overlying Mahatta Humaid Group or younger units by the Angudan unconformity, which is developed as a clear angular unconformity in southwest Oman (Droste, 1997). On a larger scale this is related to the final locking of East and West Gondwana, (the East African Orogeny) as described by Koopman et al. (2007) and van den Berg et al. (2008). The Nimr Group represents a phase of fluvial-lacustrine siliciclastic deposition following the complete cessation of marine influence. This is a direct consequence of the development of a major orogeny-associated mountain belt and foreland in East Africa. Runoff and erosional products from these mountains may have flowed northward (foreland parallel) and into the basins of Oman. Romine et al. (2008) similarly postulate that sandstones entering the mini-basins arrived not from uplifts shedding coarse-grained clastics into these basins from the west, but from axial flow along the salt mini-basins from the south and southwest. These deposits ponded, loaded and mobilised the underlying salt resulting in the development of mini-basins in South Oman.

It appears that initial sedimentation in South Oman, where the Nimr is best constrained, is blanketing, which suggests minimal topographic relief. Subsequent large thickness differences indicate sedimentation became strongly affected by synsedimentary salt dissolution and withdrawal of the underlying Ara salt with the thickest deposits in the sinks between elongate salt pillows and salt walls (Heward, 1990). Additional complexities arise from the overall convergent plate tectonic setting. In the absence of a reliable correlation framework in these barren clastics and with the complex interaction of salt halokinesis and thrust tectonics it has proved difficult to reconstruct the detail of the sedimentary succession. Unsurprisingly it is difficult to distinguish the finer grained Nimr Group clastics from underlying similar clastics in the Ara Group, particularly where the development of the Dhahaban Formation in the Ara Group is essentially clastic as well. Nimr-type continental clastic sedimentation (red bed, fine grained) commenced in the Ara Group and in the context of the complex overall tectonic setting it is very likely that Nimr style sedimentation is time-transgressive and transitional in nature, possibly repetitive in facies, across the various basins and mini-basins that developed on top of the Ara Group. Early studies noted ‘tilted’ and ‘chaotic’ complexes in the Nimr Group.

The underlying Dhahaban Formation ranges from what can be described as the last clear Ara evaporite-carbonate cycle to a possible salt dissolution and clastic sedimentation mix of sediments. Thereby possibly reflecting the same tectonic salt halokinesis and convergent basin-scale tectonic setting of subsequent Nimr Group times, albeit still in an overall evaporitic setting.

The Nimr Group consists of two formations, the Haradh and Karim that can be recognised throughout Oman (Romine et al., 2008). The Karim is shown to be mainly a fine-grained unit, with few if any true conglomerates. The shales that typify the Karim appear to be very widespread.

The switch to relatively coarse-grained sand deposition in the overlying Haradh, plus indications of erosion at the Haradh-Karim boundary suggest that the contact between the Haradh and Karim may be a disconformity or unconformity in places (e.g. Eastern Flank).

The Angudan unconformity represents the effects of major tectonism directly reaching the interior of Oman. The sedimentary effects of this uplift are recorded in some of the more remote and restricted conglomerates of the Haradh, and much more widely in the syntectonic conglomerates of the Amin Formation. Movement of salt continued during and following the Angudan unconformity, which as a consequence dips into the mini-basins.

Type and reference sections: The general type area is the Eastern Flank of South Oman (Figure 15.2). Type wells for the Nimr Group in South Oman are formalised herein: for the Group it is Nimr-4 (Figure 15.3); Haradh Formation: Amal-5 (Figure 15.6); and Karim Formation: Karim North-2 (Figure 15.11). For additional reference sections see formational discussions and for Group reference wells in North Oman see: Mabrouk-4 (Figure 15.4) and Saih Nihayda-33H2 (Figure 15.5).

Figure 15.2:

Location map: Nimr Group.

Figure 15.2:

Location map: Nimr Group.

Figure 15.3.

Composite electrical logs, lithology and lithological description of the Nimr Group, in well Nimr-4, South Oman. See Figure 15.2 for location.

Figure 15.3.

Composite electrical logs, lithology and lithological description of the Nimr Group, in well Nimr-4, South Oman. See Figure 15.2 for location.

Figure 15.4.

Composite electrical logs, lithology and lithological description of the Nimr Group, in well Mabrouk-4, North Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.4.

Composite electrical logs, lithology and lithological description of the Nimr Group, in well Mabrouk-4, North Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.5.

Composite electrical logs, lithology and lithological description of the Nimr Group, in well Saih Nihayda-33H2, North Oman. See Figure 15.2 for location.

Figure 15.5.

Composite electrical logs, lithology and lithological description of the Nimr Group, in well Saih Nihayda-33H2, North Oman. See Figure 15.2 for location.

Figure 15.6.

Composite electrical logs, lithology and lithological description of the Haradh Formation, Nimr Group, in well Amal-5, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.6.

Composite electrical logs, lithology and lithological description of the Haradh Formation, Nimr Group, in well Amal-5, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Lithology: The Nimr Group is a thick sequence of chert-rich, often micaceous brown-grey, pinkish-grey and occasionally reddish-brown argillaceous sandstones and reddish shales and siltstones (Figures 15.9 and 15.10). The sandstones display a range of grain sizes from fine-through to very coarse-grained. The chert fragments are subangular to locally well rounded, commonly laminated (laminations are clearly visible in the coarser fractions but difficult to see in finer fractions) and tend to occur in localised or discrete layers (a feature which can easily be seen in cores). Many of the conglomerates formerly assigned to the Nimr in the south of South Oman have been re-assigned to the ‘Amin Conglomerate’ by Romine et al (2008).

Subsurface recognition: The sandstones contain generally rounded to subrounded/subangular, reworked and oxidised cherts derived from older Huqf Supergroup sediments.

The red-coloured shales and siltstones are diagnostic by comparison with Al Khlata clastics, which are generally grey.

The upper boundary is difficult to pick when the Nimr is (a) overlain by the Lower Amin Member as both lithologies are similar and (b) locally conglomeratic in the uppermost sections and overlain by Amin conglomerates. In such instances only a Dipmeter log may help to indicate the stratigraphic break.

There may be a positive drill break at the upper boundary, although this is not always the case (see Mabrouk-4 and Saih Nihayda-33H2, Figures 15.4 and 15.5).

Post-drilling, the Nimr is usually more compact than the overlying post-Angudan units such as the Amin, Ghudun or Al Khlata, which is clearly reflected by a higher density (see e.g. Nimr-4, Figure 15.3). There is an overall serrate Gamma trace character. A large scale coarsening-upward trend can often be seen on the Gamma log (see Nimr-4, Figure 15.3). However, in many wells it can be quite indistinct. In addition, in some wells the sequence can be seen to be composed of a series of stacked coarsening-upward packages.

The Density-Neutron log patterns are variable in the Karim but are often can be fairly uniform in the Haradh, with minimal positive separation.

There are Dipmeter log breaks at both upper and lower boundaries. In some places the Nimr Group sediments appear to have been rotated tectonically, or due to salt movement and solution-collapse. In such wells the Dipmeter will show a sharp increase in dip below the contact of the Nimr and overlying units. Caution must be used however with this criterion, because crossbedding in the Nimr may also show as non-horizontal dips in some East Oman wells.

Usually, the log profiles of the Amin and Nimr are quite different. The Nimr is typically argillaceous, more so on average than the Lower Amin and very much more so than the upper Amin, which is normally very clean. Generally the Haradh appears as a gently coarsening-upward sequence, with a ‘dirty’ Gamma log compared to the much cleaner Upper Amin. Where the Lower Amin rests on the Haradh there is a clear break between Lower Amin, more variable, sand-shale-siltstone sequences and the classic Haradh smooth, gently coarsening-upward profile that typifies the Haradh in both South and North Oman.

The lower Nimr (Karim) has many fine-very fine-grained sandstones, siltstones and shales. In general, the Karim is finer grained and much more clay-rich than the Lower Amin and typically has soft claystones.

Boundaries: The Nimr is unconformably overlain everywhere by the Al Khlata, Mahwis, Amin or Ghudun. Over most of the Eastern Flank in South Oman, the Nimr unconformably overlies the Ara Group, but near the western margin of the South Oman Salt basin the transition may be conformable and gradual, with respect to the clastic influenced development of the Dhahaban Formation.

Distribution: The Nimr Group is widespread in North, Central and South Oman, clearly infilling preexisting, and developing Huqf topography. Romine et al. (2008) recognise both Haradh and Karim formations throughout Oman.

In South Oman the thickness of the Nimr Group is extremely variable. This is a direct result of syndepositional salt movement, resulting in salt withdrawal mini-basins. Mini-basin fill can reach up to two kilometres, with the largest thickness in the northwest of the South Oman Salt Basin (Droste 1997; Romine et al., 2008).

On seismic the Nimr Group is a sequence of onlapping wedges between salt highs. Several depocentres can be seen, separated by basement and salt highs. The infill shows thickness changes over short distances caused by erosion and truncation on the salt highs and shifting of depocentres by salt withdrawal (Heward, 1990; Bell et al., 2004; Al Barwani and McClay, 2008).

Reduced thicknesses seen in North Oman reflect apparent erosion of the Nimr on highs, with a broad thick in the Ghaba Salt Basin. Seismic data indicate that during Nimr deposition, minibasins like those of South Oman did not form in the Ghaba Salt Basin, thus sediment thicknesses are interpreted to be more evenly distributed (Romine et al., 2008).

The Nimr Group is relatively thin or absent in the Fahud Salt Basin.

Hughes Clarke (1988) suggested that the Nimr Group did not occur at outcrop. Roger et al. (1992) and Dubreuilh (1992b) defined an informal 170–200 m thick Thumaylah Formation in the Al Huqf outcrops, comprising fine- to coarse-grained sandstones that they considered to be equivalent to the Nimr Group of the subsurface, a view which was supported by Millson et al. (1996a). However, further discussions in Buckley and Harbury (1996), Millson et al. (1996b) and Buckley (1997), indicate that no definitive Nimr Group deposits occur at outcrop.

Deposition: The Nimr Group was deposited as a series of continental clastics. A muddy, silty playa environment was widespread in Oman during Karim time. The Haradh Formation of medium- to coarse-grained sands represents a widespread braided fluvial system, within a low-gradient alluvial plain setting, which covered much of Oman (Romine et al., 2008). Large thickness differences suggest the onset of salt withdrawal at this time. Seismic geometries of pod infills indicate that intra-salt pods developed progressively from the west, in agreement with depositional geometries seen in cores (Bell et al., 2004; Al Barwani and McClay, 2008).

Subdivision: The Group can be subdivided into two formations; the Karim (lower) and the Haradh (upper) formations. The Karim Formation is particularly well developed, and documented, in the Eastern Flank area where further subdivision is possible.

Previously, outside the Eastern Flank area the Nimr has been undifferentiated due to a perceived lack of correlatable shale deposits (Hartkamp-Bakker, 1997; Hartkamp-Bakker and Oterdoom, 1997). Romine et al. (2008) however, consistently differentiate a lower, fine-grained, Karim from an upper, sandy Haradh in North Oman (see Figures 15.4 and 15.5).

Sequence stratigraphy: The Nimr Group equates to the uppermost part of the AP1 Megasequence of Sharland et al. (2001).

Age: ‘Early’ Cambrian, ca. 538–520 Ma. No fossil evidence or reliable radiometric data is available to help accurately date this Group. It is older than undated Mahatta Humaid Group sediments (e.g. estimate of 520 Ma for base of the Amin Formation) and younger than (radiometrically dated) Birba Formation sediments of the Ara Group (with a youngest mid-Ara date of 541 Ma, Bowring et al., 2007).

Biostratigraphy: Barren of fauna and flora.

Haradh Formation

Author: Wiemer (unpublished, 1981), see Droste (1997).

Introduction

The Haradh is widely distributed across South and North Oman.

The Haradh Formation was originally defined in the Eastern Flank of the South Oman Salt Basin, where it may unconformably overlie the Karim Formation. In complete section the Haradh represents an overall coarsening-upward sequence of sandstones and rare shales. Jagger and Kelly (2005) and Romine et al. (2008) interpret a low-relief, low-sinuosity braided-river or stream model for the Haradh, sourced from the south. Compared to the more uniform underlying Karim Formation the Haradh Formation features significant thickness changes over short distances suggesting active syndepositional salt withdrawal, and a greater susceptibility to truncation. More limited penetrations in Central and North Oman indicate similar lithologies, where Romine et al. (2008) have been able to differentiate the Haradh from a finer grained Karim Formation.

Type and reference sections: Amal-5 in South Oman (Figure 15.6). Additional subsurface reference sections in South Oman are Simsim-2 (Figure 15.7) and Karim North-2 (Figure 15.8).

Figure 15.7.

Composite electrical logs, lithology and lithological description of the Haradh Formation, Nimr Group, in well Simsim-2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.7.

Composite electrical logs, lithology and lithological description of the Haradh Formation, Nimr Group, in well Simsim-2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.8.

Composite electrical logs, lithology and lithological description of the Haradh Formation, Nimr Group, in well Karim North-2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.8.

Composite electrical logs, lithology and lithological description of the Haradh Formation, Nimr Group, in well Karim North-2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Lithology: The Haradh Formation is a sequence of chert rich red-grey sandstones with subordinate reddish shales and siltstones (Figures 15.9 and 15.10). Rarely the shales are green.

Figure 15.9.

Ditch cuttings from the Nimr Group; (a and b) sandstone from the Haradh Formation, well Lablab-1; and (c) shale from the Karim Formation from Karim North-2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 15.9.

Ditch cuttings from the Nimr Group; (a and b) sandstone from the Haradh Formation, well Lablab-1; and (c) shale from the Karim Formation from Karim North-2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 15.10.

Ditch cuttings from the Nimr Group; (a) sandstone from Tarfa-2; and (b) shale/siltstone from Tarfa-2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 15.10.

Ditch cuttings from the Nimr Group; (a) sandstone from Tarfa-2; and (b) shale/siltstone from Tarfa-2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

The lower part consists of fine-grained, well-sorted, light red to grey quartzose sandstones with streaks of red-grey micaceous siltstones and reddish shales. These sediments grade upwards into lithic and sublithic cross-bedded, relatively well sorted, porous sandstones that are red-grey, medium to coarse grained and frequently rich in chert fragments.

Romine et al. (2008) suggest that in most places the sands and siltstones are red with grey colouration only occurring due to hydrocarbon-related reduction.

Chert fragments are abundant in the coarser fraction and can constitute up to 60% of the rock. The cherts are thought to be derived older Huqf Supergroup sediments. They appear not to have been reworked over long distances as the chert fragments still tend to be angular to subangular, retaining many primary features such as the banding or laminations so distinctive in Huqf cherts.

Locally in South Oman conglomerates may occur, although most previously reported ‘conglomerates’ appear to be pebbly sandstones supported by a sand matrix, with additional dolomite clasts.

Many of the conglomerates formerly assigned to the Nimr in the south of South Oman have been re-assigned to the ‘Amin Conglomerate’ by Romine et al. (2008).

Subsurface recognition: Whilst drilling, the Haradh can be recognised because the sandstones tend to be slightly more argillaceous than the sandstones of the overlying Amin. However, when a very thick section of the Amin is preserved the sandstones in the lower parts of the Amin are sometimes slightly argillaceous (serrate Gamma log, locally with thin beds/clasts of shale and siltstone) and then differentiation with the Nimr can be problematic.

The Haradh differs from the Amin in having more angular cherts. Cherts of the Amin are generally well rounded.

The Haradh sandstones differ from those of the underlying Runib and Irad members of the Karim Formation in being slightly coarser (fine grained as opposed to silt/very fine grained).

Runib and Irad sandstones are predominantly reddish-brown (Figure 15.10), whereas the Haradh may locally be more grey. The finer fraction sandstones of the Haradh are similar to the sandstones of the Khaleel Member of the underlying Karim Formation. The red shales and siltstones of the Haradh are similar to those of the Karim, but differ from the essentially grey colours of the Al Khlata Formation.

Post-drilling the Formation can be identified by the marked increase in Gamma log from Amin to Haradh.

The Gamma-ray response is very spiky and overall funnel shaped with values between 50 and 120 API. The spiky response reflects the micaceous nature of the sands, and intercalated shales shown by thin (2–5 m) Density and Neutron separations. The Haradh has higher Density and lower Neutron readings than the Amin. It can also have a distinctive, correlatable high Gamma (double peak) shale marker horizon, occurring approximately midformation (Figures 15.7 and 15.8).

In thick and complete sections very distinct large scale coarsening-upward trends are recognisable.

Boundaries: The Haradh Formation is unconformably overlain by the Al Khlata or the Amin formations.

The lower boundary with the Karim Formation is generally marked by an increase in Gamma (Figures 15.6, 15.7 and 15.8). Hartkamp-Bakker and Oterdoom (1997) observed an unconformity at the Haradh/Karim boundary in the Eastern Flank of the South Oman Salt Basin. Whether this holds on a more regional scale is unclear.

Distribution: The Haradh is widely distributed across South and North Oman (Romine et al., 2008). Previous authors (including Droste, 1997) confined the Haradh to South Oman and in particular to the Eastern Flank of the South Oman Salt Basin. The distribution is patchy in South Oman due to erosional truncation and syndepositional salt movement (Wiemer, 1981; see also Heward, 1990). Although well data is limited in the north seismic data suggest that the Ghaba Basin was a significant depocenter for the Haradh (Romine et al., 2008).

Deposition: The Haradh Formation has been interpreted to comprise braided river channel and sheetflood deposits in a prograding alluvial apron (Heward, 1990; Droste, 1997). Jagger and Kelly (2005) propose, and Romine et al. (2008) support, a low-relief, low-sinuosity braided-river or stream model for the Haradh with little evidence of alluvial fan deposition. A southerly or perhaps southwesterly source for sands of the Haradh fits well with overall grainsize reduction trends and supporting evidence from the Karim.

Subdivision: None proposed. A marker shale horizon is evident and correlatable in parts of South Oman, e.g. Eastern Flank. Often displayed as a double peak (high Gamma) it occurs approximately mid-formation (see Simsim-2, Karim North-2; Figures 15.7 and 15.8).

Age: ‘Early’ Cambrian, ca. 530–520 Ma. This age assignment is very tentative and lacks calibration.

Biostratigraphy: No fossils have been retrieved from this Formation.

Karim Formation

FORMATIONMEMBER‘SUB-UNIT’
KarimRunibUpper Runib
Middle Runib
Lower Runib
IradIrad Shale
Irad Sandstone
KhaleelUpper Khaleel
Lower Khaleel
FORMATIONMEMBER‘SUB-UNIT’
KarimRunibUpper Runib
Middle Runib
Lower Runib
IradIrad Shale
Irad Sandstone
KhaleelUpper Khaleel
Lower Khaleel

Author: Wiemer (unpublished, 1981), see Droste (1997).

Introduction

The Karim Formation consists of a lower predominantly sandstone interval overlain by a shale unit with interbeds of silt and sandstones.

The status and stratigraphy of the Karim Formation has been reviewed by Hartkamp-Bakker (1995, 1997), who proposed a threefold member subdivision. Mohammed et al. (1997) designated Karim North-2 as the type section. The data from the Karim Formation has a strong Eastern Flank bias as many wells elsewhere in South Oman are of limited extent (partial penetrations) and there are only a very few North Oman wells that have penetrated the Karim.

Type and reference sections: Karim North-2 in South Oman (Figure 15.11). Additional subsurface reference sections are Mabrouk-4 in North Oman (Figure 15.4), and Nimr-4 (Figure 15.3), Amal-1 (Figure 15.12), Simsim-18H2 (Figure 15.13), Karim-4 (Figure 15.14) and Runib South-3 (Figure 15.15), all in South Oman.

Figure 15.11.

Composite electrical logs, and lithology of the Karim Formation, Nimr Group, in well Karim North-2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.11.

Composite electrical logs, and lithology of the Karim Formation, Nimr Group, in well Karim North-2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.12.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Amal-1, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.12.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Amal-1, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.13.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Simsim-18H2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.13.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Simsim-18H2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.14.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Karim-4, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.14.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Karim-4, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.15.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Runib South-3, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.15.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Runib South-3, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Lithology: The Karim Formation is a sequence of micaceous shales and siltstones, and quartzose to sublithic sandstones. Sandstones dominate the lower section (Priebe and Kapellos, 1993) and have scattered chert, occasionally dolomite, fragments. The Runib and Irad sandstones are silty and red, the Khaleel sandstone is fine and grey. The basal sandstones consist predominantly of fine- to very fine-grained, well to moderately sorted, light grey, occasionally red sandstones. They are interbedded with micaceous red, occasionally green, silts and shales. Locally large lithoclasts of carbonates and chert, derived from the underlying Huqf sediments occur at the base, but hardly any true conglomerates are observed in the Karim (Romine et al., 2008).

The sediments show a variety of structures: low-angle laminations, convolute lamination and trough cross-bedding in the sandstones, ripple and horizontal laminae and mudcracks in the mudstones. Soft sediment deformation features such as sand dikes, ball and pillow structures are common in the finer-grained lithologies. Erosionally-based packages up to a metre thick and sometimes fining-upward can be recognised. Intra-formational siltstone clasts may occur especially at the base of these units. A variable amount of dolomite cement has been encountered in these sediments (Droste, 1997). Shales are predominantly red, occasionally greyish-green, and micaceous with interbeds of red-brown micaceous siltstones and sandstones (Droste, 1997).

Subsurface recognition: Penetration of the Karim Formation is difficult to identify at the well site. The general increase in shales of the ditch cuttings, the decrease in grain size and the change from predominantly red-grey sandstones to reddish-brown sandstones are all indicators.

The subdivision of the Formation into members and sub-units is impossible at the wellsite as subdivision is based on log patterns.

The shales/siltstones are micaceous and predominantly red (Figure 15.9), rarely green. Karim Formation sandstones are frequently silty and argillaceous, often with carbonate/dolomite cement in the lower parts. The sandstones/siltstones of the Runib and Irad members are generally silty/very fine grained and predominantly reddish-brown. The sandstones of the Khaleel Member are fine grained and light to brown-grey and very similar to the sandstones of the Haradh (finer fraction).

It is almost impossible to differentiate between the sandstones of the Haradh and Karim while drilling on lithology alone. The red shales and siltstones of the Karim are similar to those of the Haradh, and the Dhahaban and Al Noor formations of the Ara Group. Often a negative drill break is observed as the shales are penetrated. The clasts and pebbles of chert, and more rarely dolomite, occur near the base (reworked from the Huqf Supergroup). The presence of chert and small amounts of dolomite fragments in the cuttings from the lower part of the Karim is an indication of the proximity of the top of the Ara Group.

Post-drilling the Formation is characterised by the increased Gamma log values relative to the Haradh, accompanied by a wider positive separation of the Density-Neutron logs (although this depends largely on the development of shales/siltstones relative to sandstones, see Figures 15.13, 15.14 and 15.15).

The subdivision of the Formation (into members and sub-units) is based on recognition of the sequence of log patterns.

Boundaries: The Karim Formation is overlain everywhere by the Haradh Formation. Hartkamp-Bakker and Oterdoom (1997) interpretated an unconformity at this boundary in the Eastern Flank of the South Oman Salt Basin. This may not be the case regionally. It is unconformably underlain by anhydrites and dolomites of the Huqf Supergroup.

Distribution: The Karim Formation is widely distributed throughout Oman, where it has not been eroded by later uplift. Previous authors had confined it to the Eastern Flank of South Oman but the regional review of the Nimr Group by Romine et al. (2008) recognises and correlates a lower Nimr, finegrained Karim Formation over much of Oman. In South Oman several depocentres can be recognised and the internal stratigraphy of the Karim Formation is variable. Extreme differences in thickness are associated with salt withdrawal during deposition. Its main depocentre is in the northwest of the South Oman Salt Basin. It does not appear to be as thick, nor as widespread in the southern half of the South Oman Salt Basin. This region corresponds to an area of extensive pre-existing platform carbonates.

In North Oman the Karim also appears to be widespread, but never reaches the thicknesses seen in South Oman, i.e. can exceed 1,000 m in the south but generally less than 400 m in the north.

It would appear that low-energy lacustrine or playa conditions prevailed over a wide area of Oman following the end of Ara time. Romine et al. (2008) suspect that this playa extended across the Central Oman High at the time, but that the sediments have been subsequently eroded.

Deposition: The interbedded and overlying shale intervals with interbeds of silts and sandstones are thought to be lacustrine/playa lake in origin (Hartkamp-Bakker and Oterdoom, 1997; Droste, 1997). The fine- to very fine-grained sandstones of the Karim Formation have been interpreted as sheet sands and channel fill deposits in an alluvial fan setting (Hartkamp-Bakker and Oterdoom, 1997; Droste, 1997). Romine et al. (2008) found no evidence in either core, or completion logs for true alluvial fan deposits as the sediments are simply not coarse enough for such an interpretation. They therefore interpret Karim deposition to represent low-angle sheetflood and relatively unconfined channel flow.

Romine et al. (2008) note an increase in sand content to the south of the South Oman Salt Basin, supporting a dominant southerly sediment source. They find no sedimentological evidence for uplift along the western bounding high of the South Oman Salt Basin, based on the available well control.

Subdivision: The Formation is subdivided into three members: Runib (upper), Irad (middle) and Khaleel (lower) based on sequential log patterns (fining and coarsening upward, and aggradational log sequences, Hartkamp-Bakker, 1995; 1997a; Hartkamp-Bakker and Oterdoom, 1997). In general, these subdivisions are field specific and difficult to apply consistently on a wider scale, particularly outside the Eastern Flank producing area of South Oman.

The Khaleel is defined as serrated sandstones fining upwards into silty shales. The Irad on top of the Khaleel is defined as a generally fining-upward sequence with a few sands at the base. The Irad Shale is often referred to as the Mid-Karim Shale. The top of the Irad Shale is a sharp transition from shales to the sandstones and siltstones of the Runib Member. The Runib is a generally coarsening-upward sequence of sandstones and silty shales with a package of serrated sandstones at the top.

Age: ‘Early’ Cambrian, ca. 538–530 Ma. Again this is a very tentative age assignment, with no available calibration.

Biostratigraphy: No fossils have been retrieved from this Formation.

Figures & Tables

Figure 15.1.

Huqf Supergroup Litho-, Chrono- and Isotope Stratigraphy. * U/Pb absolute dates. Modified after Amthor et al. (2003); Le Guerroué et al. (2006a); Bowring et al. (2007); Fike (2007) and Allen (2007).

Figure 15.1.

Huqf Supergroup Litho-, Chrono- and Isotope Stratigraphy. * U/Pb absolute dates. Modified after Amthor et al. (2003); Le Guerroué et al. (2006a); Bowring et al. (2007); Fike (2007) and Allen (2007).

Figure 15.2:

Location map: Nimr Group.

Figure 15.2:

Location map: Nimr Group.

Figure 15.3.

Composite electrical logs, lithology and lithological description of the Nimr Group, in well Nimr-4, South Oman. See Figure 15.2 for location.

Figure 15.3.

Composite electrical logs, lithology and lithological description of the Nimr Group, in well Nimr-4, South Oman. See Figure 15.2 for location.

Figure 15.4.

Composite electrical logs, lithology and lithological description of the Nimr Group, in well Mabrouk-4, North Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.4.

Composite electrical logs, lithology and lithological description of the Nimr Group, in well Mabrouk-4, North Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.5.

Composite electrical logs, lithology and lithological description of the Nimr Group, in well Saih Nihayda-33H2, North Oman. See Figure 15.2 for location.

Figure 15.5.

Composite electrical logs, lithology and lithological description of the Nimr Group, in well Saih Nihayda-33H2, North Oman. See Figure 15.2 for location.

Figure 15.6.

Composite electrical logs, lithology and lithological description of the Haradh Formation, Nimr Group, in well Amal-5, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.6.

Composite electrical logs, lithology and lithological description of the Haradh Formation, Nimr Group, in well Amal-5, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.7.

Composite electrical logs, lithology and lithological description of the Haradh Formation, Nimr Group, in well Simsim-2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.7.

Composite electrical logs, lithology and lithological description of the Haradh Formation, Nimr Group, in well Simsim-2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.8.

Composite electrical logs, lithology and lithological description of the Haradh Formation, Nimr Group, in well Karim North-2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.8.

Composite electrical logs, lithology and lithological description of the Haradh Formation, Nimr Group, in well Karim North-2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.9.

Ditch cuttings from the Nimr Group; (a and b) sandstone from the Haradh Formation, well Lablab-1; and (c) shale from the Karim Formation from Karim North-2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 15.9.

Ditch cuttings from the Nimr Group; (a and b) sandstone from the Haradh Formation, well Lablab-1; and (c) shale from the Karim Formation from Karim North-2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 15.10.

Ditch cuttings from the Nimr Group; (a) sandstone from Tarfa-2; and (b) shale/siltstone from Tarfa-2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 15.10.

Ditch cuttings from the Nimr Group; (a) sandstone from Tarfa-2; and (b) shale/siltstone from Tarfa-2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 15.11.

Composite electrical logs, and lithology of the Karim Formation, Nimr Group, in well Karim North-2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.11.

Composite electrical logs, and lithology of the Karim Formation, Nimr Group, in well Karim North-2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.12.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Amal-1, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.12.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Amal-1, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.13.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Simsim-18H2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.13.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Simsim-18H2, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.14.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Karim-4, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.14.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Karim-4, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.15.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Runib South-3, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

Figure 15.15.

Composite electrical logs, lithology and lithological description of the Karim Formation, Nimr Group, in well Runib South-3, South Oman (Mohammed et al., 1997). See Figure 15.2 for location.

SUPERGROUPGROUPFORMATION
HuqfNimrHaradh
Karim
AraDhahaban
Al Noor
Athel
U
Birba
NafunBuah
Shuram
Khufai
Masirah Bay
Hadash
Abu MaharaGhadir Manqil
SUPERGROUPGROUPFORMATION
HuqfNimrHaradh
Karim
AraDhahaban
Al Noor
Athel
U
Birba
NafunBuah
Shuram
Khufai
Masirah Bay
Hadash
Abu MaharaGhadir Manqil
GROUPFORMATIONMEMBER‘SUB-UNIT’
NimrHaradh  
KarimRunibUpper Runib
Middle Runib
Lower Runib
IradIrad Shale
Irad Sandstone
KhaleelUpper Khaleel
Lower Khaleel
GROUPFORMATIONMEMBER‘SUB-UNIT’
NimrHaradh  
KarimRunibUpper Runib
Middle Runib
Lower Runib
IradIrad Shale
Irad Sandstone
KhaleelUpper Khaleel
Lower Khaleel
FORMATIONMEMBER‘SUB-UNIT’
KarimRunibUpper Runib
Middle Runib
Lower Runib
IradIrad Shale
Irad Sandstone
KhaleelUpper Khaleel
Lower Khaleel
FORMATIONMEMBER‘SUB-UNIT’
KarimRunibUpper Runib
Middle Runib
Lower Runib
IradIrad Shale
Irad Sandstone
KhaleelUpper Khaleel
Lower Khaleel

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