CHAPTER 17 NAFUN GROUP (HUQF SUPERGROUP)
Published:January 01, 2010
Authors: Originally defined by Teyssen (unpublished, 1990), who elevated the Huqf from Group to Supergroup and included the Khufai, Shuram and Buah formations, as defined by Hughes Clarke (1988), within the Nafun Group. The Masirah Bay Formation, originally defined by Mohammed et al. (1997) as part of the Abu Mahara Group, is included in the Nafun Group. The Basal Carbonate Member (Bell, 1993) of the Masirah Bay Formation is defined herein as the Hadash Formation, following McCarron (2000) and Leather (2001).
The formations of the Nafun Group (Buah, Shuram, Khufai, Masirah Bay and Hadash) can be widely correlated. They are characterised by gradual lateral changes in facies and thickness. The rocks comprise a rather similar and relatively predictable variety of lithologies, i.e. carbonates, shales, organic shales, sandstones and (minor) cherts. Leather (2001) recognised that the pronounced basin partitioning that characterises the Abu Mahara Group had ended with the Masirah Bay Formation and its basal carbonate unit, which he distinguished separately as the Hadash Formation. Final amalgamation of the Arabian-Nubian Shield terranes and the Oman microcontinent was completed by the beginning of the Nafun Group (ca. 635 Ma).
The Nafun depositional phase in Oman was a tectonically quiescent period as indicated by uniform sedimentation patterns and regular ‘tramline’ patterns on seismic. The topography that had developed in Oman during the previous tectonic phase became the first-order control on facies distribution patterns in the Nafun Group. West- and southwest-directed progradation in siliciclastic units (Allen, 2007), and facies distribution patterns indicate sediment provenance from the east and northeast. Slab pull-apart forces (or possibly thermal sag post-Abu Mahara extension) may have been sufficient to engineer accommodation space for the Nafun Group formations (Grotzinger et al., 2002). No significant extension-related growth faulting has been observed on seismic.
The Nafun Group has been the focus of a number of PhD and research studies starting with McCarron (2000) and Leather (2001) at Group level, and subsequently in more detail on the Buah (Cozzi et al., 2004a and 2004b; Cozzi and Al Siyabi, 2004 and 2005) and Shuram formations (Le Guerroué, 2006; Le Guerroué et al., 2006a and 2006b). See also Fike et al. (2006), Fike (2007) and Fike and Grotzinger (2008).
Type and reference sections: Miqrat-1 in North Oman (Figure 17.2).
Outcrop based type sections are listed for the Buah, Shuram, Khufai and Hadash formations. See Formation sections for details of these and additional subsurface reference wells. The only well type section for a Nafun Group formation is the Masirah Bay Formation, i.e. well Masirah-1 in North Oman (Figure 17.14).
Lithology: The first descriptions of rocks that are now considered to be part of the Nafun Group were given by Morton (1959) and Beydoun (1964). They use the terminology of First and Second Clastic Groups (respectively, ‘Abu Mahara/Masirah Bay’ and Shuram formations) and of First and Second Dolomite Groups (respectively, Khufai and Buah formations). These two successive cycles of clastic and carbonate deposition are still the main divisions of the Nafun Group. The lower units, the Masirah Bay (clastics) and the Khufai (shelf carbonates) formations are overlain by the Shuram Formation with mixed carbonate-siliciclastic shelf deposits and the Buah Formation carbonates. The Hadash Formation, a transgressive post-glacial ‘cap carbonate’, marks the base of the Nafun Group.
Boundaries: The Nafun Group overlies conformably or unconformably either the older clastics of the Abu Mahara Group or basement. It is overlain by the carbonates and evaporites of the Ara Group in the subsurface salt basins or by much younger successions separated by a major unconformity in the Jabal Al Akhdar or Al Huqf outcrops. Both carbon and sulphur isotopes can be used to help distinguish between the carbonates of the Ara Group and the Nafun Group (Figure 15.1).
Distribution: The Nafun Group is recognised across Oman and crops out in the Jabal Al Akhdar and Al Huqf areas.
Deposition: The fine clastics and carbonates of the Nafun Group represent deposition of extensive blankets of carbonate and siliciclastic rock units. Two siliciclastic to carbonate couplets (Masirah Bay-Khufai and Shuram-Buah) are interpreted to correspond to broadly shallowing-up marine successions that start with fine clastics and pass gradationally upwards into progradational carbonate ramp deposits. Based on sulphur isotope data, Fike et al. (2006) postulate Shuram-Buah deposition in a significantly more oxidising environment compared to the older Masirah Bay-Khufai deposition.
At the base of the Group, the deposition of the transgressive shallow-marine carbonate Hadash Formation took place in a regime of post glacial, globally rising sea-level.
Subdivision: The Nafun Group is subdivided into five formations. Listed from top to bottom these are the Buah, Shuram, Khufai, Masirah Bay and Hadash.
Tschopp (1967) correlated these formations with the sequence exposed in the core of the Al Hajar Mountains, comprising respectively from top to bottom the Kharus (Buah), Miaidin (Shuram), Hajir (Khufai) and Mistal formations (largely Abu Mahara Group). See also Glennie et al. (1974) and Rabu (1988).
Sequence stratigraphy: The Nafun Group correlates with the middle of the AP1 Megasequence of Sharland et al. (2001). However, subsequent radiometric datings suggest that the AP1 Megasequence is in much need of revision, e.g. their illustrated Lower Abu Mahara Formation would now entirely pre-date AP1. The sequence stratigraphic interpretation of the Nafun Group hinges strongly on the recognition of two major maximum flooding surfaces located at the base of the Masirah Bay and in the Shuram formations. They correspond to the MFS Pc10 and Pc20 of Sharland et al. (2001) and define two 2nd-order sequences. The Masirah Bay Formation is composed of two 3rd-order depositional sequences (sequences 1 and 2), as proposed by Leather (2001) and Cozzi and Al Siyabi (2004), the highstand parts of these corresponding to the two shallow water prograding sand bodies of the Masirah Bay Formation. The next sequence is marked by renewed transgression and a maximum flooding that corresponds to the green shales at the top of the Masirah Bay Formation (Leather, 2001). The overlying Khufai carbonates were deposited during the highstand of the third sequence, terminated by a sequence boundary that would be marked by a possible exposure surface on the top of the Khufai limestone in the shallow water sections (Al Huqf area). The subsequent lowstand deposits are found in the Jabal Al Akhdar sections where re-sedimented poorly to well sorted quartz sandstones are found (McCarron, 2000; Cozzi et al., 2004b; Le Guerroué et al., 2006a). The following transgressive systems tract and maximum flooding are contained within the Shuram Formation, which includes the Pc20 maximum flooding surface of Sharland et al. (2001).
Age: Ediacaran, ca. 635–547 Ma. There are no radiometric age dates from the Nafun Group that directly constrain depositional ages.
Brasier (1999) summarises all attempts and techniques used to date the Huqf Supergroup in Oman, and in particular the pre-Ara (Nafun) sections. As with many other authors since then, he stresses the need to integrate biostratigraphy, isotopic data, radiometric ages and the recognition of correlatable glacial events when considering this time period.
Two tie points are available to estimate the duration and depositional ages and these are: (i) the ca. 547 Ma age for the base of the overlying Ara Group (Bowring et al., 2007), and (ii) the presumed age of the underlying glaciogenic Fiq Formation and Hadash Formation cap carbonate at ca. 635 Ma. This constraint assumes that the Fiq glaciation ends at the same time as equivalents in South China (Condon et al., 2005) and Namibia (Hoffmann et al., 2004). See discussion in Bowring et al. (2007).
Using these tie points, Le Guerroué et al. (2006a,b,c and 2009) interpolated the approximate depositional ages for the Masirah Bay and Khufai at 635 to 600 Ma, and at 600 to 548 for the Shuram and Buah, on the basis that there are no major stratigraphic gaps in the Nafun succession. This would place the Khufai/Shuram boundary and the associated prominent negative carbon isotope anomaly at ca. 600 Ma, which is not consistent with the most recent global Neoproterozic chronostratigraphic framework (Ogg et al., 2008, see their figure 3.5).
Bowring et al. (2007) determined sediment accumulation rates for parts of the Ara and Buah formations using high-precision geochronology and carbon isotope stratigraphy from Oman, China, and Namibia. These rates were extrapolated to provide an age model indicating a duration of ca. 7 to 14 My for the accumulation of the Shuram and Buah formations and an estimated age of between ca. 554 to ca. 562 Ma for the base of the Shuram Formation. This scenario places the Khufai/Shuram boundary and the associated prominent negative carbon isotope anomaly broadly at ca. 558 Ma, which is more consistent with Ogg et al. (2008). In addition, Bowring et al. (2007) place a stratigraphic gap at the Khufai-Shuram boundary, and estimate the depositional age of the Masirah Bay to Khufai cycle at ca. 635 to ca. 590 Ma.
Debate will continue but the current best estimate for the base of the Shuram Formation appears to be ca. 558 Ma. However, as stated in Bowring et al. (2007); “until more direct temporal constraints become available, use of the carbon isotopic record in the Shuram as a global chemostratographic reference invites caution”.
Biostratigraphy: Knoll (2000), Grey and Calver (2007) and Butterfield and Grotzinger (in preparation) discuss and illustrate the potential use of marine acritarchs in resolving Ediacaran stratigraphy. Unfortunately numerous attempts in Oman have failed to yield really meaningful assemblages. These include routine, single well studies over many years and two focussed multiwell projects, i.e. Knoll (1994) and Butterfield (2001). The latter is built on in Butterfield and Grotzinger (in preparation). Only about 45% of samples analysed yielded recognisable palynomorphs. The samples from the Shuram Formation provide the best and most consistent recovery (Butterfield and Grotzinger, in preparation). The assemblages generally contain few taxa and none of the forms occur in significant numbers, nor do they have any real biostratigraphic use. The overwhelmingly dominant fossils are thin-walled leiospheres (smooth, simple, long-ranging acritarchs). Rare ornamented acritarchs include Germinosphaera sp. but the age ranges of this and associated forms are poorly constrained. Fragments of filamentous microfossils may also be recorded but again they add little to our understanding of the stratigraphy of the Group. Samples may be rich in amorphous organic material but such dark organic-rich facies are usually targeted for palynological study. Key early–middle Ediacaran acritarch assemblages are known to contain large, often fragile ‘spiney’ (acanthomorphic) forms and any future success in liberating such fossils will require careful and specialised laboratory processing techniques. Butterfield and Grotzinger (in preparation) further discuss this perceived lack of Ediacaran acanthomorphic acritarchs in the Nafun Group (notably Shuram Formation) and the potential significance of this absence. They favour an environmental/habitat control. See formational discussions for further detail.
Stromatolite development is ubiquitous in the carbonates of this Group, but as yet no biostratigraphical differentiation has been proposed.
The Buah Formation is the second and uppermost carbonate unit of the Nafun Group. The Formation name is derived from the Buah anticline in the Al Huqf outcrop area (E 57°43′15″, N 22°47′16″), where the sequence is now known to be incomplete.
It has been described extensively from outcrop and subsurface data (Tschopp, 1967; Gorin et al., 1982; Hughes Clarke, 1988; Rabu, 1988; Wright et al., 1990; Dubreuilh et al., 1992a: McCarron, 2000; Cozzi and Al Siyabi, 2004; and Cozzi et al., 2004a, b). Although the lithostratigraphic correlation between the different outcrops and subsurface was recognised early on, it only became possible to build reliable detailed correlations using chemostratigraphy, notably carbon isotopes (McCarron, 2000; Cozzi et al., 2004a).
These data confirm overall widespread and gradual subsidence, but with variations in subsidence that have a notable and direct control on facies development. Slow subsiding areas such as the Al Huqf developed a gently deepening carbonate ramp extending over tens of kilometres. Differential subsidence resulted in steeper ramps in the Jabal Al Akhdar area, with shallow-water carbonates and sediment starved organic-rich outer ramp deposits co-existing within distances of less than 10 kilometres. Similar relationships are observed from wells in the South Oman Salt Basin.
Type and reference sections: Mukhaibah anticline, Al Huqf outcrop area (E 57°40′50″, N 19°51′20″, Figure 17.1). The thickness in the type section is ca. 180 m, following the definition of the Buah Formation by Cozzi and Al Siyabi (2004, 2005). Additional subsurface reference sections are Miqrat-1 in North Oman (Figure 17.2), Mukhaizna South-2 (Figure 17.3), and Saba-1 (Figure 17.4), both in Central Oman and Thamoud-6 in South Oman (Figure 17.5). The latter three wells illustrate particularly shale dominated sections (see below).
Lithology: The Buah Formation is a carbonate unit showing a range of facies types. In the type outcrop area the Formation can be subdivided into a lower part with thin, flaggy bedding and an upper part with more massive-beds with chert nodules. The carbonate is largely dolomite, but silty lime mudstones (Figure 17.6) and sandstones can occur near the base of the lower part. Stratiform and domal stromatolites are everywhere common, as are intra-formational conglomerates, solution breccias, some associated with gypsum/anhydrite, and lime grainstones of pellets/ooids/oncoids. Deeper water settings from Jabal Al Akhdar outcrops include finely laminated organic-rich mudstones and shaly dolomites. Following Cozzi and Al Siyabi (2004), Romine et al. (2008) recognise a widespread starved basinal setting with organic-rich ‘argillites’ and very shaley dolomites in the northern part of the South Oman Salt Basin. More typical shelfal carbonates occur elsewhere in the South Oman Salt Basin (e.g. Figures 17.3 and 17.5). Generally carbonates are common in the upper part, shales and siltstones in the lower.
Subsurface recognition: The Buah Formation generally differs from the overlying Birba in the presence of clastics. The exception is, and difficulties occur where A0 shales are present in the basal Birba Formation. Dolomites in the upper part are lithologically very similar to those of the Birba.
The shales in the Buah are predominantly dark grey. Shales/siltstones are not common in North Oman but when they do occur they are generally red or reddish-brown (e.g. Miqrat-1, Figure 17.2).
The Buah is sand prone in the lower parts (south Central and South Oman only). It generally shows a positive drill break at the upper boundary.
Post drilling, the Formation can be picked by log correlation with nearby wells. It has a distinct, uniform Dipmeter character (unlike the overlying Birba, which has a ‘Bag of Nails’ character). Carbon isotope data (Cozzi and Al Siyabi, 2004, 2005) show an increasing trend from negative values of ca. -6‰ at the base, increasing up-section to 0‰ and positive values of +2–4‰ at the top, approximately the same as for the overlying Birba carbonates (Figures 15.1 and 17.2). Recent work by Fike (2007) has shown that sulphur isotopes may be used to distinguish Ara from Nafun Group carbonates. Nafun Group carbonates are characterised by δ34S values of 20–25‰, whereas Ara Group carbonates are significantly enriched, with δ34S values of over 35–40‰ (Figure 15.1).
Boundaries: The lower boundary is everywhere transitional from the red clastics of the Shuram Formation into the Buah Formation. The upper boundary in outcrop is a hiatus, where the Buah is overlain by Haima Group clastics, with associated karstification. In the Al Huqf type area the top is sharply overlain by thick carbonates of possible Ara Group surface equivalence (Nicholas and Brasier, 2000). Previous studies (Gorin et al., 1982; Wright et al., 1990; McCarron, 2000) incorporated these potential Ara cycles into the Buah Formation, distinguishing a lower and upper Buah. Cozzi and Al Siyabi (2004) redefined the top of the Buah Formation outcrops as the base of the possible Ara equivalents, indicated by the marked appearance of lithic arenites and evaporites. Nicholas and Gold (in preparation) define this succession as the Sarab Formation.
In the Jabal Al Akhdar the Buah is overlain (with no apparent angular break) by the Fara Formation (Rabu, 1988), comprising pelagic resedimented carbonates, cherts, finegrained turbidites and volcaniclastics, such as welded ignimbrites. Here the upper part of the Buah Formation contains several m-thick mega-breccia intervals (Cozzi and Al Siyabi, 2004; Cozzi et al., 2004a,b). These breccias with m-scale shallow-water-derived clasts, reflect catastrophic collapse of portions of the Buah ramp during the last stages of its development, marking the beginning of Ara style basin compartmentalisation.
Elsewhere, in uplifted areas, the Buah is unconformably overlain by much younger rocks, e.g. in the Oman Mountains there is an angular unconformity between Permian sediments and the Buah.
In the subsurface, the top of the Buah is a disconformity overlain by the carbonates, anhydrites and salts of the Ara Formation. Seismic data also shows clear evidence for a top Buah unconformity associated with interbasinal highs.
Where the carbonates and volcaniclastics of the Ara A0C sequence overlie the Buah carbonates, the boundary is relatively easy to define.
If these volcaniclastic deposits are lacking, and given the similar carbonates lithologies of the Buah Formation and the lower Birba Formation of the Ara Group it is often not easy to define the boundaries. Geochemical methods, e.g. carbon and sulphur isotope analysis, or Dipmeter data can be employed to assist.
Distribution: The Buah Formation crops out extensively in the Al Huqf area. In the Jabal Al Akhdar area it is equivalent to the Kharus Formation (Glennie et al., 1974; Gorin et al., 1982; Rabu, 1988). Possible Buah equivalents are also found in the Saih Hatat erosional window, as suggested by Mattes and Conway Morris (1990) and Mann and Hanna (1990), where a dolomitic unit called the Hiyam Dolomite (Glennie et al., 1974), later renamed Hiyam Formation (Le Métour, 1988; Rabu et al., 1993), is found below the sub-Permian unconformity.
The Buah Formation is widespread in the subsurface of Oman’s interior basins, although it appears to be absent over much of the Central Oman High (Romine et al., 2008).
Deposition: The Buah Formation represents an overall shallowing-upward sequence from basinal mudstones and black shales to shallow marine/supratidal carbonate ramp/shelf deposits with strong lateral progradation (Cozzi and Al Siyabi, 2004; Cozzi et al., 2004a and b). Romine et al. (2008) recognise a more basinal argillite facies in the northern part of the South Oman Salt Basin.
Buah deposition was controlled in facies and thickness by areas of low subsidence where basement highs served as nucleation points for Buah ramp progradation. In areas where subsidence was much greater, thicker mid- to inner ramp Buah sequences were deposited. Where a slope break formed, the Buah developed into a distally steepened carbonate ramp composed of mid- and outer ramp facies (Cozzi et al., 2004a and b).
Subdivision: The Formation is informally subdivided into lower and upper corresponding to deeper and shallower facies of the overall upward shallowing and prograding sequence. Future work may further detail the upper part of the Buah Formation and its transition into the Birba Formation in the subsurface and determine the stratigraphic position of the Sarab Formation in the Al Huqf area.
Sequence Stratigraphy: The general shallowing-upward character of the Buah Formation and its lateral ramp progradation indicate deposition during a highstand systems tract (McCarron, 2000; Sharland et al., 2001; Cozzi and Al Siyabi, 2004).
Age: Ediacaran, ca. 552–547 Ma. Geochronologic data from both outcrops (Fara Formation, Jabal al Akhdar; Brasier et al., 2000; Bowring et al., 2007), and subsurface (Ara cycles, South Oman Salt Basin; Amthor et al., 2003; Bowring et al., 2007), constrain the top of the Buah at about 547 Ma. The age of the base of the Buah is speculative and can only be assigned to be ca. 552 Ma, based on a worldwide carbon isotope correlation (cautiously applied by Bowring et al., 2007), and overall matching with typical sedimentation rates for carbonate ramps in slowly subsiding tectonic settings. Critical to this estimate is the suggestion that the top of the globally recognised ‘Shuram’ negative carbon isotope excursion approximates to 550.5 Ma, based on correlation with China (Condon et al., 2005). In well Miqrat-1 (Figure 17.2) this occurs in the lower Buah Formation and applying the potential sediment accumulation rates of Bowring et al. (2007) would indicate a possible range of between ca. 552.5 to 551.5 Ma for the age of the base of the Buah. Much of the reasoning here can be viewed as speculative and almost certainly the debate amongst specialists will continue until direct, or more compelling, evidence is uncovered.
Biostratigraphy: See Group discussion. Fossil recovery has been minimal and non-age specific.
Authors: First described by Kassler (unpublished, 1966), subsequently described in Gorin et al. (1982). The Shuram Formation was first formally published as a lithostratigraphic unit by Hughes Clarke (1988).
The Shuram Formation is a sequence of interbedded red-brown shales, carbonates, and dark grey organic-rich shales. It is named after Wadi Shuram in the Al Huqf area of Central Oman where it is well exposed. The Shuram Formation represents the mainly siliciclastic phase of the uppermost of two clastic-carbonate ‘cycles’ in the Nafun Group of Oman (McCarron, 2000; Allen et al., 2005; Le Guerroué, 2006; Allen, 2007).
The carbonates at the base of the Shuram Formation are associated with a very strong negative δ13C isotope excursion, as first documented in Oman by Kapellos et al. (1992), based on the subsequently published work of Burns and Matter (1993). This ‘Shuram excursion’ has been interpreted as an unprecedented shift in global ocean values, possibly associated with the rise in atmospheric oxygen at the end of the Neoproterozoic. As such it has been used to support correlation with Ediacaran strata elsewhere (Fike, 2007; Le Guerroué et al., 2006a).
Type and reference sections: Wadi Shuram, to the southwest of the Khufai anticline in the Al Huqf outcrop area (E 57°34′00″, N 20°02′20″). Thickness in the type section is ca. 250 m. Equivalent to the Mu’aydin Siltstone Formation described by Kapp and Llewelyn (1965) in the Jabal Al Akhdar area, with its type section in Wadi Al Mu’aydin and a thickness of ca. 700 m (see also Glennie et al., 1974; Gorin et al., 1982; Rabu, 1988; Le Guerroué, 2006).
Lithology: In the type area and nearby wells, the Shuram is a reddish and greenish, silty shale to variably micaceous siltstone, with finely laminated carbonate beds, which increase in thickness and number from the mid-point of the sequence towards the top. Hummocky, cross-stratified sandstones and oolitic and cross-stratified carbonates are also seen in Wadi Shuram and Wadi Aswad in the Al Huqf area.
In the Al Hajar Mountains conglomeratic quartz-litharenites and organic-rich mudstones occur at the base of the Mu’aydin Formation.
In the subsurface the sequence is most commonly represented by red-brown shales and siltstones interbedded in the upper part with limestones and dolomites. In terms of the siliciclastic dominated lower Shuram, argillites and dark grey, organic-rich (euxinic) shales occur in south Central and South Oman, specifically in the Eastern Flank region.
Subsurface recognition: The organic shales are dark grey and the non-organic shales of the Shuram are predominantly alternating reddish-brown and green, whereas shales in the Buah are predominantly dark grey (south Central and South Oman only; reddish in North Oman - see Buah).
The carbonates in the Shuram are mainly limestones (Figure 17.6), carbonates in the Buah are mainly dolomites. The limestones can be ooidal.
There may be a negative drilling break at the upper boundary.
Post drilling, the variable, generally high Gamma is characteristic. The lower contact of the Shuram with the Khufai is usually placed in outcrop at the first red siltstones above the Khufai that are not interbedded with carbonate.
Boundaries: The lower boundary is with the clean carbonates of the Khufai. The nature of this boundary is controversial. Bowring et al. (2007, 2009) interpret a significant unconformity, whereas Le Guerroué et al. (2009) argue for continuous sedimentation. In addition, McCarron (2000) places a sequence boundary just below the top the Khufai Formation. In the subsurface, the contact with the Khufai Formation is usually picked as an unconformable contact. The upper boundary can be problematic and varied, ranging from the change from clean Buah dolomites to the limestones and shales of the Shuram to the much easier pick, where the lower Buah has clastic development.
Distribution: The relationship between the southern euxinic facies and the red clastics of the type area is not yet properly understood. The former facies can reach over 1,000 m in thickness.
Laminated cherts (silliceous argillites) are confined to south Central and South Oman. There are no organic shales in eastern Central Oman, including the Al Huqf outcrops.
The Shuram Formation is equivalent to the Mu’aydin Formation in Jabal Al Akhdar (Rabu, 1988).
Deposition: Overall the fine-grained sediments of the Shuram indicate low-energy deposition. In outcrops of the Al Huqf area the sediments are interpreted as deposited on a storm-dominated siliciclastic shelf, becoming more distal and deepening towards Jabal Al Akhdar and into the salt basins. Subsurface interbasinal highs (e.g. Makarem) witnessed shallow-water deposition. Le Guerroué (2006) interpreted the depositional patterns of the Shuram Formation to be those of a carbonate-ramp setting without sharp slope breaks. Subsurface penetrations of the Shuram in South Oman are dominated by fine-grained facies (argillite) indicating deeper-water deposition.
The increasing carbonate in the upper part of the Shuram Formation into the Buah Formation suggests progressive upward shoaling.
Subdivision: From subsurface wells an Upper ‘Carbonate’ Member and a Lower Shuram Member of argillaceous siltstones and silty shales was proposed by Kassler (1966). McCarron (2000) and Le Guerroué (2006) used a threefold subdivision based on facies associations in the outcrops of the Al Huqf and Jabal Al Akhdar. The lowest of these subdivisions consists of very fine-grained sediments, with organic-rich laminae at the base, interpreted as deepwater slope sediments. The middle part consists of monotonous siltstones and shales, deposited in deep to shallow middle slope settings. The upper part of the Shuram Formation is characterised by siliciclastic/carbonate parasequences that progressively thicken upwards, interpreted as shallowing-up parasequences of storm-dominated siltstones capped by shallow-shelf carbonates.
Sequence Stratigraphy:Sharland et al. (2001) and Cozzi and Al Siyabi (2004) interpret the lower part of the Shuram Formation as a transgressive to maximum flooding sequence resulting in their MFS Pc20. Le Guerroué (2006) links his Lower Shuram Member with the initial transgression and the base of the Middle Member as the maximum flooding. The Upper Member represents the subsequent highstand culminating in the overlying Buah Formation.
Age: Ediacaran, ca. 558–552 Ma (see Bowring et al., 2007). No age indicators occur within the Formation. Depositional ages for the Shuram Formation are inferred, based on the downward sediment accumulation rate extrapolation as discussed previously. The base of the Shuram and the associated carbon isotope excursion is estimated at 562–554 Ma by Bowring et al. (2007) and at around 600 Ma by Le Guerroué et al. (2006). This discrepancy remains unresolved (Le Guerroué et al., 2009 and Bowring et al., 2009) until more reliable age constraints become available.
The current, poorly constrained, estimate for the age of the Shuram is 558–552 Ma, following Bowring et al. (2007; 2009) as this is more consistent with the most recent global Neoproterozic chronostratigraphic framework (Ogg et al., 2008). Here we simply select the age for the base of the Shuram as the mid-point of the 562–554 Ma age range suggested by Bowring et al. (2007).
Biostratigraphy: See Group discussion. Butterfield and Grotzinger (in preparation) indicate widespread and good palynological recovery from Shuram mudstones. However, the assemblages consist almost entirely of sphaeroidal acritarchs (Leiosphaeridia spp., Pterospermopsimorpha sp. and possible Germinosphaera sp.). Filaments and, typical Ediacaran, large ornamented (acanthomorphic) acritarchs are absent. They associate this absence and the general and uniform nature of Shuram recovery with deeper water conditions.
Authors: The Khufai was first described by Beydoun (1960, 1964) as the ‘First Dolomite Group’. Kapp and Llewellyn (1965) described the unit in the Al Hajar Mountains, defining a type section in Wadi al Hajir. Kassler (1966) defined the type section in the Khufai dome in the Huqf area. The Khufai was subsequently described by Gorin et al. (1982), Rabu et al. (1986) and Hughes Clarke (1988).
The Khufai Formation is a carbonate unit that, in outcrop sections in the type area and in nearby well sections, shows a wide range of carbonate facies types. Carbonates of variable development are interbedded with shale. Dolomites and shales characterise the upper parts, rarely with sandstones, and limestone or dolomite, locally fetid (McCarron, 2000) are present in the lower part.
Type and reference sections: Rim of the Khufai anticline, Al Huqf outcrop area (E 57°37′20″, N 20°05′30″). The thickness in the type section is ca. 250 m (McCarron, 2000). The subsurface reference sections are Miqrat-1 in North Oman (Figure 17.2) and Dhahir-1 in Central Oman (Figure 17.11).
Lithology: The Khufai consists of light grey, cryptocrystalline dolomites and limestones (Figure 17.12), both often fetid, grainstones (ooidal, oncoidal) and grey, micaceous shales. The carbonates are predominantly dolomitised in Central and southeastern North Oman (Romine et al., 2008), but genetic textures and structures are retained; i.e. cross-bedding, oncoid-ooid-pelletoid grain types, stratiform stromatolites and collapse breccias. Silicification and typically yellowish-brown chert nodules occur. In the Oman Mountains and most of South Oman the Khufai is predominantly limestone.
Outcrop sections in the Al Huqf region (McCarron, 2000) and Jabal Al Ahkdar region (according to Le Guerroué, 2006 and Le Guerroué et al., 2006a) show that quartz sand beds may occur, particularly in the upper part of the Formation.
Subsurface recognition: Whilst drilling the most obvious feature is the change from the red-brown and green shales and siltstones of the Shuram to the grey dolomites and limestones of the Khufai Formation. The limestones of the Khufai and Shuram are similar.
The Rate of Penetration log may show a positive or negative drill break depending on the overlying lithology.
Post drilling the top of the Khufai can be picked using the Gamma log and isotope data trends (if available).
Isotope studies indicate an identifiable and regionally correlatable change in the carbon isotope distribution pattern (the negative Shuram excursion), which can be correlated across wells and outcrops in the Al Huqf and Jabal Al Akhdar regions of East and North Oman (Figure 17.13) (Kapellos et al., 1992; Burns and Matter, 1993; Cozzi and Al Siyabi, 2004). The Khufai Formation is characterised by a positive carbon isotope signature with values up to 5‰ in the Al Huqf area and up to 8‰ in the Al Hajar Mountains (Kapellos et al., 1992; Burns and Matter, 1993; McCarron, 2000).
Boundaries: In continuous sections, the upper boundary at outcrop is sharp, and seen as a hardground on a dolomitised grainstone overlain by softer-weathering silty carbonates of the basal Shuram. In the Oman Mountains the boundary is associated with sandstone development (Le Guerroué, 2006; Le Guerroué et al., 2006a).
The Khufai has a transitional lower boundary from dolomitic (thin interbeds) clastics of the Masirah Bay Formation into continuous Khufai dolomites (McCarron, 2000).
Subsurface sections suggest an unconformable upper boundary. Bowring et al. (2007, 2009) argue for a possible stratigraphic break, between the Khufai and Shuram formations, whereas Le Guerroué et al. (2009) argue for continuous sedimentation. As discussed above, additional reliable age dates are required to better resolve this. Younger unconformities can lead to the Khufai being overlain by Haima, Haushi and younger units.
Distribution: The Khufai Formation is widespread in both North and South Oman. It crops out in the Al Hajar Mountains, where it is known as the Hajir Formation (Rabu, 1988), and in the Al Huqf area. The Khufai is thicker generally in North Oman where it reaches 300 m along a northwest to southeast axis that crosses the Huqf uplift into the Masirah Graben to the east (Romine et al., 2008).
Like the Masirah Bay Formation below it, the Khufai Formation thins to the southwest and west as it crosses the Central Oman High.
In South Oman the Khufai has a thickness range of 50–100 metres.
Deposition: Khufai sediments were deposited in a carbonate-ramp environment throughout most of Oman (Wright et al., 1990; Leather, 2001; McCarron, 2000). Facies distribution patterns reflect underlying topography, with proximal/shallow-water facies being deposited on highs (e.g. Al Huqf High, upper Khufai in outcrop). The Central Oman High was not prominent at this time, with the Khufai probably extending across it. Deposition in the Al Hajar Mountains is interpreted to be in a deeper, outer-ramp environment (McCarron, 2000). Le Guerroué et al. (2006a) note the probability that what has been called lower Shuram in Jabal Al Akhdar may be the lateral, distal facies equivalent of the last parasequence in the Khufai of the Al Huqf region, i.e. the lithostratigraphical boundary is diachronous..
Age: Ediacaran, ca. 615–590 Ma. Age determinations are approximate and depend on extrapolation and assumptions from limited volcanic tuff based radiometric age determinations in the Ara Group above and Abu Mahara Group below. Detrital zircons from the upper Khufai Formation provide a maximum age of deposition within a broad, probable range of 600–580 Ma (see Bowring et al., 2007, 2009 and Le Guerroué et al., 2006a,b,c and 2009 for extensive discussion). The absence of any carbon isotope excursion associated with the 582 Ma Gaskiers glaciation may be significant and a speculative age of ca. 590 Ma is applied here to the uppermost Khufai. Bowring et al. (2007, 2009) place a stratigraphic gap at the Khufai/Shuram boundary, whereas Le Guerroué et al. (2006a, 2009) assume continuous sedimentation across the boundary and estimate the age of the top Khufai to be ca. 600 Ma. The base of the unit is roughly estimated at 615 Ma based on extrapolation (see Bowring, 2007).
Biostratigraphy: See Group discussion. One Khufai sample in Butterfield (2001), further documented in Butterfield and Grotzinger (in preparation) yielded relatively well-preserved material from an early diagenetic chert nodule, comprising constituents of integrated microbial mats. Compared to many microbial-mat biotas, the Khufai assemblage is relatively diverse, including coiled Obruchevella, at least two species of Siphonophycus, and occasional large Myxococcoides-type spheroids. Such forms are typical of Proterozoic peritidal microbial mat biotas. Similar environments are known to yield classic, large, ornamented (acanthomorphic) Ediacaran acritarchs. The Khufai may therefore have the best potential to yield significant and correlatable acritarch assemblages.
Masirah Bay Formation
Authors: Bell (unpublished, 1993) placed the Masirah Bay Formation in the Abu Mahara Group. It was subsequently proposed to be assigned to the Nafun Group by McCarron (2000) and Leather (2001), which is followed in this Lexicon.
The Masirah Bay Formation represents the lowest clastic sequence of the two major cycles of siliciclastic-to-carbonate deposition that have been recognised in the Nafun Group (Masirah Bay-Khufai and Shuram-Buah). It was originally not distinguished as a separate rock unit within the ‘First Clastic Group’ of Beydoun (1960, 1964) and the Abu Mahara Formation of Hughes Clarke (1988). Bell (1993) elevated the Abu Mahara to Group level and distinguished the Masirah Bay at Formation level, with a subdivision into three members; Transitional, Clastics and Basal Carbonate. The Basal Carbonate (now Hadash Formation) represents rapidly rising sea level during post-Marinoan deglaciation on a low-relief (ramp) margin. This was followed by the marine siliciclastics of the Masirah Bay Formation over much of Oman (Allen and Leather, 2006). These deposits mark a significant change in the tectonic style and depositional environments compared to the underlying Abu Mahara Group. As such they are possible representatives of the rebound to warmer conditions following the Marinoan glacial period. McCarron (2000), followed by Leather (2001), proposed elevating the Basal Carbonate Member to the Hadash Formation. The Hadash Formation and the Masirah Bay Formation strongly overstep the underlying basement-cored Abu Mahara Group basins, with the Masirah Bay Formation passing up gradationally into the prograding carbonate ramp of the Khufai Formation (Allen and Leather, 2006).
Type and reference sections: Masirah-1, offshore North Oman (Figure 17.14). Additional subsurface reference sections are Miqrat-1 in North Oman (Figures 17.2 and 17.15) and Zafer-1 in Central Oman (Figure 17.16).
Lithology: The Masirah Bay Formation is a mainly clastic succession. The shales and siltstones are dark grey (organic), or varicoloured dark grey, green and dark brown (non-organic) (Figure 17.17). The sandstones are very fine to medium grained, light grey and glauconitic near the top of the sequence (Figure 17.18). In a number of sections the very top (ca. 18 - 25+ m) consists of a succession of interbedded shales and dolomites (upper Member 3 of Allen and Leather, 2006).
Subsurface recognition: Whilst drilling, an increased proportion of shale and a decrease in the proportion of carbonates generally confirm penetration of the Masirah Bay Formation.
The shales and siltstones are varicoloured dark grey, grey-green, grey-brown, dark brown and red (minor); when rich in organic material they are solely dark grey.
There are no visible differences between the organic-rich shales of the Masirah Bay and the Shuram formations. Dolomites and/or limestones in the uppermost part of the Masirah Bay Formation are similar to those of the Khufai and the Shuram.
The clastic sediments from the Masirah Bay Formation can be lithologically similar to those of the underlying Ghadir Manqil Formation (although the Ghadir Manqil may have a slightly higher incidence of reddish colours). Generally it is difficult to distinguish the Masirah Bay from the Ghadir Manqil if the Hadash Formation is absent or not recognised.
The Rate of Penetration log may show a positive or negative drill break dependent on the overlying lithology.
Post drilling the Masirah Bay Formation can be recognised using log correlation with offset wells and detailed lithological analysis.
Boundaries: The Masirah Bay Formation is generally overlain conformably by the Khufai Formation. It is underlain by the Hadash Formation or the Ghadir Manqil Formation. The lower contact with the Hadash Formation seems to be conformable and gradual in the Jabal Al Akhdar outcrops. The absence of the Hadash Formation, for example in the type well Masirah-1, suggests at least locally nondeposition, probably related to topographic highs.
Distribution: The Masirah Bay is widespread and correlatable across Oman. Exposures of the Masirah Bay Formation in the Al Huqf area of east Central Oman (Kassler, 1966) and in the core of the Jabal Al Akhdar (Kapp and Llewellyn, 1965, mapped as the Amq Member of the Mistal Formation by Rabu, 1988) have been correlated on the basis of lithology (Tschopp, 1967; Gorin et al., 1982) and the stable isotopic signatures of the overlying carbonate deposits of the Khufai Formation (Burns and Matter, 1993; Burns et al., 1994; McCarron, 2000; Leather, 2001; Cozzi et al., 2004a,b; Cozzi and Al Siyabi, 2004; Allen and Leather, 2006). A correlation with the possible Neoproterozoic in the Mirbat area has remained speculative despite detailed studies (Rieu, 2006; Rieu et al., 2006, 2007a,b; Rieu and Allen, 2008).
Coarser (shallow) clastics are best developed in east Central Oman, with finer clastics (deeper water) towards the north, west and southwest. This matches with a marked thickening to the east and thinning to the west, with a speculative north-south trending pattern of thickening and thinning interpreted as depositional highs and lows (Allen and Leather, 2006).
Deposition: The Masirah Bay Formation represents an overall shallowing trend from open-marine shales to proximal tidal and estuarine clastics and ultimately to marginal-marine carbonates. It transgresses basin margins followed by the progradation of a clastic system to the west before flooding and the growth of a carbonate-ramp system (the Khufai Formation). Most of Oman’s interior basins as well as the Al Hajar Mountains remained in relative deep-water offshore settings, with a palaeo-coastline postulated further to the east. In central-eastern Oman (including the Al Huqf area) shallow water coarser clastics indicate close proximity to the continental source area, as described from the upper Masirah Bay Formation by Allen and Leather (2006). Palaeocurrent and progradation directions indicate easterly provenance for Masirah Bay sediments.
Subdivision:Bell (1993) subdivided the Masirah Bay into three members: Transition (upper), Clastics (lower) and a Basal Carbonate Member. The latter has been given formational status as the Hadash Formation. The Transitional Member was identified in only a small number of wells, of which some may be re-assigned to the Khufai Formation, as in Miqrat-1. Consequently, his threefold subdivision can no longer be supported. Allen and Leather (2006) divide the upper part of the Masirah Bay Formation in the Al Huqf area into three members with the lower two essentially representing coarsening and shallowing-up shoreface to tidal/estuarine clastics and the upper, third, member representing the transition to the Khufai carbonates, where the uppermost ca. 18 - 25+ m has interbedded silts and carbonates. These three units of Allen and Leather (2006) cannot be recognised in the more complete section and uniformly finer, deeper water deposits of Jabal Al Akhdar.
Sequence stratigraphy: The Masirah Bay Formation, together with the underlying Hadash Formation oversteps the underlying Abu Mahara Group basins, indicating an overall transgression. Allen and Leather (2006) indicate two progradational pulses of shallow-marine sandstone in the upper Masirah Bay of central-east Oman. Sand deposition is terminated by a final transgressive phase, which culminates in what likely equates to MFS Pc10 of Sharland et al. (2001). The final progradational highstand phase (upper Member 3 of Allen and Leather, 2006) incorporates the interbedded carbonates that precede the fully developed shallowing-upward carbonate ramp of the Khufai Formation (McCarron, 2000; Cozzi and Al Siyabi, 2004; Allen and Leather, 2006).
Age: Ediacaran, ca. 635–615 Ma. The base of the Masirah Bay Formation is constrained by the presumed age of the underlying (Marinoan) glacials and specifically the Hadash Formation cap carbonate at ca. 635 Ma (Bowring et al., 2007). This assumes that the last glaciation in the Neoproterozoic of Oman ends at the same time as equivalents in South China (Condon et al., 2005) and Namibia (Hoffmann et al., 2004). Bowring et al. (2007) extracted a U-Pb date of 645 Ma from glacial deposts just below a cap dolomite horizon,in well Lahan-1 (see Abu Mahara Group discussions).
The age of the top of the Masirah Bay Formation is unconstrained but within the current framework of the Nafun Group it is estimated at ca. 615 Ma.
Biostratigraphy: See Group discussion. The acritarchs Germinosphaera sp. and Pterospermopsimorpha sp. have has been recorded in one well, but the age ranges of these genera are poorly constrained (Butterfield, 2001; Butterfield and Grotzinger, in preparation).
Authors: The Hadash sediments were first mentioned as a unit by Bell (unpublished, 1993), as the Basal Carbonate Member of the Masirah Bay Formation. McCarron (2000) and Leather (2001) proposed an upgrade to Formation status, which is followed in this Lexicon.
The Hadash is a key marker horizon of variable thickness but of great lateral extent across Oman. It separates the major clastic-carbonate sequences of the Nafun Group from the Abu Mahara Group below. The unit has been elevated to Formation status, in recognition of its regional significance, as documented by McCarron (2000), see also Allen (2007). Its widespread occurrence, overstepping underlying Abu Mahara basins, marks the change from structurally confined Abu Mahara Group sedimentation to the more unconfined extensive sedimentation of the Nafun Group, hence its assignment to the latter. On a global scale the Hadash Formation represents the termination of the last (Marinoan) glaciation of the Neoproterozoic.
Type and reference sections: Outcrop section proposed by McCarron (2000) near the village of Hadash, above Wadi Mistal in Jabal Al Akhdar. Additional subsurface reference sections are Miqrat-1 in North Oman (Figure 17.15) and Zafer-1 in Central Oman (Figure 17.16).
Lithology: The Hadash Formation consists of stromatolitic dolomite, light grey to brown and finely to crypto-crystalline, with minor shales and (in outcrops in the Oman Mountains) siltstones and rare, localised, immature sands. Five metres of a cap dolomite of assumed Hadash equivalence was cored in well Lahan-1 (see Bowring et al., 2007). It comprises heavily fractured and diagenetically altered greenish-grey, bedded to finely laminated dolomite.
Subsurface recognition: The Hadash is thin (maximum thickness of 20 m) and can be easily missed while drilling. It only forms a small percentage of the cuttings. Cavings are likely to confuse interpretation. It has a negative drill break at the top.
Post drilling the Hadash can be recognised from a clear log response and a characteristic negative δ13C excursion (within a general range of -4‰ to -8‰, McCarron, 2000; Leather, 2001). The Lahan-1 dolostone yielded -2 to -4.5‰ readings (Bowring et al., 2007).
Boundaries: The sharp contact of the Hadash Formation with underlying Ghadir Manqil Formation sediments in some places appears to be conformable, in others, erosional. It is marked by the first occurrence of carbonates above Abu Mahara glacial clastics. The upper boundary is transitional to the shales and fine clastics of the Masirah Bay Formation. This transition can appear quite abrupt in the subsurface but occurs over 3–4 parasequences in the Oman Mountains (Grotzinger, personal communication). Leather (2001) notes the prevalence of siltstone at the top of the Hadash Formation in the west of Jabal Al Akhdar, but maintains the upper boundary at the top of the highest carbonate bed.
Distribution: Although not very thick, the Hadash Formation is nevertheless widespread and correlative across Oman. It may be absent and such locations are interpreted to represent relative highs with non-deposition, although deposition and subsequent erosion cannot be excluded. Its great lateral extent, overstepping Abu Mahara basin margins, onlapping onto basin highs, suggests it was deposited during a marine transgression, possibly corresponding to post-glacial eustatic sea-level rise.
Deposition: The carbonates probably represent shallow-marine conditions and mark the rebound to warmer climatic conditions following the Marinoan glaciation. The deposition of the Hadash Formation took place in a regime of globally rising sea level (Leather, 2001; Leather et al., 2002; Le Guerroué et al., 2005; Allen et al., 2004; Rieu et al., 2007b; Allen, 2007). These so-called cap carbonates are thought to be deposited due to oversaturation of global sea waters with carbonate following deglaciation, warming, and rise of sea level (Allen and Leather, 2006). The facies, diagenetic textures and isotopic content (δ13C to -4‰ or more for the Hadash Formation) are characteristic for other carbonates that cap, Marinoan-type, Neoproterozoic glacial successions globally (Kennedy, 1996; Hoffman et al., 1998; Hoffman and Schrag, 2002).
Sequence stratigraphy: The widespread Hadash Formation represents a rapid transgression and maximum flooding, possibly associated with post-glacial eustatic sea-level rise.
Although inherently part of the the AP1 Megasequence of Sharland et al. (2001), i.e. at the base of their illustrated Upper Abu Mahara Formation/Mistal Formation, new radiometric age dates require a redefinition and realignment of the AP1 Megasequence. At ca. 635 Ma the Hadash Formation predates the base AP1, which is placed at 610 Ma.
Age: Basal Ediacaran, ca. 635 Ma. The negative δ13C (to -4‰ or more) excursion of the Hadash Formation, when compared to the ‘global’ composite carbon isotope curve of Halverston et al. (2005) supports a correlation with the end phase of the Marinoan Glaciation. An ash layer 9 m below a cap dolomite cored in well Lahan-1 in South Oman has been dated to a maximum depositional age of 645 Ma by Bowring et al. (2007). This cap dolomite has been correlated with the Hadash Formation, therefore indicating an approximate age of ca. 635 Ma, which is compatible with a post-Marinoan assignment. The base of the Ediacaran, dated at 635 Ma, is taken as the base of the Marinoan cap carbonate (Ogg et al., 2008).
No age range is indicated for this thin unit, but almost certainly the Hadash Formation is of very short duration.
Biostratigraphy: Stromatolitic structures are common. No other fossils are known to have been recovered from this Formation.
Figures & Tables