CHAPTER 18 ABU MAHARA GROUP (HUQF SUPERGROUP)
Published:January 01, 2010
|Abu Mahara||Ghadir Manqil|
|Abu Mahara||Ghadir Manqil|
Authors: The Abu Mahara was first defined as a rock unit by Kassler (unpublished, 1966), see also Gorin et al. (1982).
Sediments of what is herein defined as the Abu Mahara Group were originally included in the ‘First Clastic Group’ by Morton (1959) and Beydoun (1960, 1964). Kassler (1966) describes an Abu Mahara Clastic Formation as part of the Huqf Series. Gorin et al. (1982) then published the Abu Mahara Formation in their Huqf Group and following Kassler (1966) defined it as the sediments between the basement and the Khufai Formation. This subdivision was followed by Hughes Clarke (1988). In the Al Hajar Mountains the unit was originally included in the ‘Mistal conglomerate’ by Kapp and Llewellyn (1965) and as part of the Mistal Formation by Rabu (1988). The Abu Mahara Formation was elevated to Group status by Teyssen (1990). Bell (1993) defined the Abu Mahara Group comprising the Ghadir Manqil Formation and the Masirah Bay Formation. This is amended here to include only the sediments between the basement and the Hadash Formation of the overlying Nafun Group, following McCarron (2000), Leather (2001), Leather et al. (2002), Allen and Leather (2006) and Allen (2007).
Note that much of the following discussion is based mainly on outcrop data as well related material is limited and can be equivocal.
Studies, mainly on outcrops in Al Jabal Al Akhdar and Al Huqf (Figure 18.1), in combination with sparse well data (mainly in the South Oman Salt Basin) show confinement to narrow, roughly north-south trending basins (possibly grabens), with rapid lateral variations in thickness and facies. This is very different from the much more unconfined distribution of the overlying Nafun Group.
To assist the reader Figure 18.2 illustrates the different unit names discussed below and mainly shows the proposed correlative surface rock unit scheme of Rieu and Allen (2008, after their figure 2). The figure should be viewed with caution because, as discussed below, much remains to be confirmed with respect to the age relationships of these units, both from region to region and into the subsurface, e.g. the Ayn Formation in Mirbat could be considerably younger than the 722 Ma that the detrital zircon assemblage suggests and the age of the Shareef Formation is as yet unconstrained.
The stratigraphic subdivisions of the Abu Mahara Group in the publications of Leather (2001) and Cozzi and Al Siyabi (2004) have led to a degree of confusion. Le Guerroué et al. (2005) and Allen (2007) have subsequently attempted to rationalise this. The stratigraphy as defined for the subsurface has become mixed with the stratigraphy as defined in the Al Hajar Mountain outcrops by Rabu (1988). Le Guerroué et al. (2005) proposed to elevate the outcrop members, of Rabu’s Mistal Formation, to Formation status. In the absence of a rigid framework of reliable absolute datings in the subsurface linked to the outcrops, it is difficult to subdivide and correlate the Abu Mahara Group. It is clear that there are different subsurface successions of diamictites and intercalated volcanics. Whether and how these correlate with the outcrops in the Al Hajar Mountains is, as yet, unclear. Assigning the Saqlah and Fiq members (defined as part of the Mistal Formation in the Al Hajar Mountains by Rabu (1988)) as part of the Ghadir Manqil Formation as proposed by Leather (2001) and Cozzi and Al Siyabi (2004) may be tempting, but too many uncertainties remain to confidently apply such a comparison to the subsurface.
In the Al Huqf area, at Al Jobah, a ca. 40 m thick section of shallow marine to fluvial (pyroclastic) clastics and associated volcaniclastics (ignimbrites) is recorded (Halfayn Formation of Dubreuilh et al. 1992a) between the basement granodiotite and an interpreted Hadash cap carbonate (Leather, 2001). Note that the volcaniclastic sections yield detrital (?primary) zircons as young as ca. 802 Ma, which almost certainly relate directly to the Al Jobah basement rocks (Figure 18.2). Leather (2001) and Bowring et al. (2007) discuss the implications of the U-Pb ages at Al Jobah.
In the subsurface only the Ghadir Manqil Formation has been distinguished below the Hadash Formation (Bell, 1993). In view of the remaining uncertainties with respect to surface to subsurface correlation the approach of Bell (1993) is followed here and consequently the Abu Mahara Group comprises only the Ghadir Manqil Formation until further data suggests otherwise.
Sediments of the subsurface Abu Mahara Group comprise clastics with diamictites of the Ghadir Manqil Formation (see Figure 18.2, probably equivalent to the the Fiq and possibly Gubrah members of the Mistal Formation) and associated volcanics (possibly equivalent to the Saqlah Member of the Mistal Formation in Al Hajar Mountains). No other formations have been differentiated in the subsurface and much additional work is required to do so and clarify any comparison with surface geology.
Rieu and Allen (2008) attempt to correlate the Mirbat Group of South Oman, Dhofar region to the Jabal Al Ahkdar sections, recognising two glacial sections in both areas. They define the Mirbat Group, consisting of, in ascending order, the Ayn (glaciomarine/fluvio-deltaic), Arkahawl (distal marine/turbiditic), Marsham (shallow marine/fluvial) and Shareef (glaciomarine) formations (Figure 18.2). They propose a Ghubrah to Ayn and a Shareef to basal Fiq equivalence, albeit in the absence of relevant constraining age data.
Glacial sediments are widely known across the globe in this period of the Neoproterozoic, which has led to the ‘Snowball Earth’ hypothesis documented by Hoffman et al. (1998) and Hoffman and Schrag (2002). Oman has emerged as one of the few localities in the world that can provide direct constraints on both the Sturtian (ca. 760–700 Ma) and Marinoan (terminating at ca. 635 Ma) glacial episodes (Brasier et al., 2000; Leather et al., 2002; Kilner et al., 2005; Rieu et al., 2007b; Rieu and Allen, 2008). The Oman evidence weakens the case of a ‘Snowball Earth’ as a totally frozen earth (see discussions in, amongst others Leather et al., 2002; Allen et al., 2004; Allen, 2007), but like elsewhere the evidence hinges crucially on too few datings.
Type and reference sections: Khufai anticline, northern Al Huqf (Kassler, 1966). Subsurface reference sections are Hathnar-1 (Figure 18.3), in South Oman (as type section of the Ghadir Manqil Formation), Farha-1 in North Oman (Figure 18.4), Suwaihat-5H2 in Central Oman (Figure 18.5) and Ghadir Manqil-1 in South Oman (Figure 18.6). Additional reference sections include outcrops at Jabal Al Akhdar in Wadi Mistal, Wadi Bani Kharus and Wadi Bani Awf in North Oman, the Mukhaibah and Buah anticlines of the Al Huqf region and in the Mirbat area of South Oman (Figure 18.1).
Lithology: The Abu Mahara Group consists of sandstones, variably quartz cemented, argillaceous siltstones and silty shales. Conglomerates and diamictites with pebbles and boulders of igneous or metamorphic rock are widespread. Volcanics occur in north-northeast trending occurrences from South Oman into the Al Hajar Mountains.
Potential surface equivalents, specifically in the Jabal Al Akhdar area (Figure 18.2), provide additional insight into the possible range of rock types that could be expected in the subsurface.
The Ghubrah Member consists of glacial and interglacial sediments in the form of grey and purple diamictites interbedded with basalts and rare tuffaceous sandstones, siltstones, mass-flow conglomerates and turbiditic sandstones and shales.
The overlying Saqlah Member consists of a basalt section, up to 60 m thick, with associated volcaniclastics and lithic sandstones and carbonates.
The youngest Fiq Member is composed of an assemblage of glacial rain-out diamictites, massflow deposits, turbidites, slope and shelf sediments. See Leather et al. (2002) for a discussion of the four main facies associations noted in the Fiq Member (distal and proximal glaciomarine, non-glacial gravity flow and non-glacial shallow-marine). These provide a wide range of sediments, which include; massive diamictites, dropstone laminites, conglomerates, pebbly sands, sands, silts, massive and laminated mudstones and localised carbonate development.
Broadly comparable lithofacies are present in Dhofar (Mirbat), see Rieu and Allen (2008), where the minor development of fluvial and fluvio-deltaic sands and conglomerates is also noted.
Boundaries: The basal contact is commonly erosional on basement. The top of the Abu Mahara Group is usually conformably overlain by the cap carbonate Hadash Formation in the outcrops of Jabal Al Akhdar. In a similar manner, the Ayn Formation in the Mirbat Group is also overlain by a ‘cap carbonate’. Correlation within the Abu Mahara Group remains conjectural in the absence of reliable age dating (Rieu, 2006; Rieu et al., 2006; Rieu and Allen, 2008). In other places the upper boundary with the Hadash Formation appears disconformable, probably due to transgressive deposition during postglacial sea-level rise (McCarron, 2000; Leather, 2001). In the subsurface, seismic lines show erosion at top Ghadir Manqil Formation due to basin inversion in Central Oman (van Marle et al., 1994). Uplift associated with thrusting is observed in the Ghaba Salt Basin and folding in the Fahud Salt Basin, both truncated by the base Nafun unconformity. Within the outcrops of the Al Hajar Mountains (Mistal Formation) there is a significant ‘mid group’ angular unconformity between the Ghubrah diamictites and overlying Saqlah volcanics.
Distribution: Present throughout Oman. Full sections are rare, but thickness varies from ca. 150 m to over 2,000 m (Romine et al., 2008). Present thickness variations may represent, at least in part, original depositional thickness as basins were filled by glaciogenic sediment.
Deposition: Abu Mahara sediments, in most places in Oman, have a marine-glaciogenic character, as demonstrated by the outcrops in Jabal Al Akhdar (Leather, 2001; Leather et al., 2002; Allen et al., 2005). In the Dhofar area of South Oman (Mirbat), Rieu and Allen (2008) illustrate a comparable range of environments but with fluvial and fluvio-deltaic elements also present (within the Marsham Formation and the Ayn Formation, respectively).
Subdivision: The Abu Mahara Group, as defined in the subsurface, comprises only the Ghadir Manqil Formation. In the Al Hajar Mountains the unit has been mapped as the Mistal Formation, with, from top to bottom, the Fiq (?with Amq), Saqlah and Gubrah members (Rabu, 1988). Rieu (2006), Rieu et al. (2007a) and Rieu and Allen (2008) defined the Mirbat Group in the Dhofar area of South Oman and correlate this with the Abu Mahara Group in Jabal Al Akhdar. The Mirbat Group comprises, in descending order, the Shareef, Marsham, Arkahawl and Ayn formations.
Sequence stratigraphy: The Abu Mahara has been placed in the lowermost part of their AP1 Megasequence by Sharland et al. (2001, illustrated as Lower Abu Mahara). 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. In the absence of any reliable stratigraphic subdivisions and dating it is impossible to provide a meaningful sequence stratigraphic discussion other than highlighting some of the main tectonostratigraphic interpretations.
Sediment distribution suggests deposition in narrow north-south trending basins (possibly grabens) interpreted as extensional basins (Loosveld et al., 1996). Sediments representing two glacial intervals (Sturtian dated at ca. 720–700 Ma and Marinoan at ca. 645–635 Ma), are preserved in Oman (Brasier et al., 2000; Leather et al., 2002; Rieu and Allen, 2008). The oldest strata belong to the Ghubrah Member of the Mistal Formation in the Al Hajar Mountains (ca. 720–660 Ma; see Allen, 2007). They are separated from the overlying Saqlah volcanics (no definitive age date) and the Fiq Member (ca. 660–635 Ma; see Allen, 2007) by an unconformity that has been attributed to rifting. Romine et al. (2008) use seismic evidence and basement information to support a subsequent compressive deformation phase in North Oman, including west-verging thrusts and inversion of the Abu Mahara basins in northeastern Oman. The end of this phase is marked by the base Nafun unconformity dated at ca. 635 Ma by Bowring et al. (2007). Rieu and Allen (2008) propose that the Mirbat Group Arkahawl and Marsham formations in Dhofar may represent sedimentation during much of the time interval between the deposition of the Ghubrah and Saqlah formations. A thin ‘cap carbonate’ occurs at the base of the Arkahawl Formation, above the glaciogenic Ayn Formation, which Rieu and Allen (2008) infer to be related to the Sturtian glaciation (ca. 720–700 Ma) of earlier Cryogenian times.
Age: Cryogenian, ca. 720–635 Ma. Bowring et al. (2007), also summarised in Allen (2007), provide an approximate framework of absolute U-Pb age dates based on outcrops in the Al Hajar Mountains and the Lahan-1 well in South Oman. These range from two tuff-related dates of 713 Ma for the Ghubrah Formation to 645 Ma in the glaciogenic Ghadir Manqil of well Lahan-1 (just below a cap carbonate, Hadash Formation, Figure 18.2).
Biostratigraphy: Rare marine assemblages of palynomorphs, including simple acritarchs (leiospheres) and filamentous bodies have been found in these rocks (see Ghadir Manqil Formation section and Butterfield and Grotzinger, in preparation). The forms recorded are long-ranging and consistent with a general Cryogenian to Ediacaran age.
Ghadir Manqil Formation
Author: Bell (unpublished, 1993).
The Ghadir Manqil Formation, as defined for the subsurface in Oman in this Lexicon, is the only stratigraphic division of the subsurface Abu Mahara Group. Consequently much of the discussion at Group level applies equally here. In outcrops further subdivisions have been made based on dominant lithologies (clastics, diamictites and volcanics). Sections of similar lithological character may be recognised in the subsurface, but these cannot be easily correlated with the surface in the absence of reliable calibration data, particularly primary U-Pb datings. In Teyssen (1990) the Abu Mahara Group included the Masirah Bay Formation. This Formation has been re-assigned, together with the Hadash Formation (previously known as the Basal Carbonate Member of the Masirah Bay Formation) to the base of the Nafun Group, hence the subsurface Abu Mahara Group now comprises only the Ghadir Manqil Formation. To avoid confusion with the outcrop stratigraphy as used in McCarron (2001), Leather (2000), Le Guerroué et al. (2005), Rieu and Allen (2008) and explained in Allen (2007) a strict separation with the outcrop stratigraphy is retained in this Lexicon. This will allow flexibility in approach to the subsurface if any new stratigraphic subdivisions can be differentiated, particularly as more age dates become available.
Type and reference sections: Hathnar-1 in South Oman (Figure 18.3). Additional subsurface reference sections are Farha-1 in North Oman (Figure 18.4), Suwaihat-5H2 in Central Oman (Figure 18.5) and Ghadir Manqil-1 in South Oman (Figure 18.6).
Lithology: The Ghadir Manqil Formation includes clastics, volcanics and diamictites (Figure 18.7). The clastics are dominated by immature fine to coarse-grained sandstones (see also Abu Mahara Group discussion).
The volcanics are intermediate to acidic in composition. Ignimbrites are found in the north of the Al Huqf area where they overlie granodioritic basement (Al Jobah outcrop, Halfayn Formation in Figure 18.2). Volcanics are also known from the Jabal Al Akhdar outcrops (Saqlah Member of the Mistal Formation) and from the Saih Hatat area (correlation uncertain). The volcanism in Jabal Al Akhdar erupted under generally submarine conditions (including pillow lavas). Geochemically the Jabal Al Akhdar volcanics are typical intra-plate basalts. Clasts observed within the widespread diamictites comprise predominatly metamorphosed basement rocks with minor amounts of volcanic or intrusive igneous material. The diamictites are commonly interbedded with immature sandstones. The clastics comprise shales, silts and a full range of fine to coarse grained, even pebbly, immature sandstones. Clasts of feldspar, lithics, and even fragments of welded tuffs and ignimbrites have been noted in these sands (Bell, 1993).
Subsurface recognition: Whilst drilling, it is necessary to identify the Hadash Formation, when present. This carbonate is often very thin and may easily be missed in ditch cuttings.
The diamictites have a sericitic/chloritic greenish matrix and various exotic clasts (Figure 18.7).
The wireline log character of the Ghadir Manqil Formation is rather variable, as one can expect for this mixed lithology unit. An intermediate level Gamma response is characteristic for thick diamictite-dominated units. Intrusive rocks are generally difficult to distinguish from basement in the absence of core and follow-up studies.
Boundaries: Lies unconformably on basement and is overlain conformably or uncomformably, by the Hadash and Masirah Bay formations, see Abu Mahara Group discussion.
Distribution: The Ghadir Manqil Formation is widespread, but with only few well penetrations.
The volcanics are restricted to even fewer wells, and outcrops in the Al Huqf area and outcrops in the Jabal Al Akhdar and Saih Hatat areas of North Oman. The clastics are best developed in the Al Huqf outcrop above the Al Jobah basement (Dubreuilh et al., 1992a; Platel et al., 1992c) and are seen in a small number of wells in Central and South Oman. Diamictites are common in both outcrop and well sections.
Deposition: The Ghadir Manqil sediments were deposited in a marine-glaciogenic shallow-marine to fluvio-deltaic environment (Leather, 2001; Allen et al., 2005; Rieu and Allen 2008). The interbedding of volcanics and diamictites in the Mistal Formation of Jabal Al Akhdar shows that active volcanism was at least partially contemporaneous with glacial conditions.
Subdivision:Bell (1993) distinguished three members, essentially corresponding to three different lithological assemblages: Diamictite, Clastics and Volcanics, based on the dominant lithology. In the absence of clear stratigraphic relationships between the main lithological groups, this subdivision is difficult to retain with any degree of consistency. From available dates in the outcrops of the Oman Mountains it is known that the succession includes at least two glaciations so assignment to one single ‘diamictite’ member (e.g. Ghubrah or Fiq) would be stratigraphically risky in this context. The majority of the diamictites and clastics in well sections are likely to be younger than the volcanics.
Age: Cryogenian, ca. 720–635 Ma (see Bowring et al., 2007; Allen, 2007), based on U-Pb age dates in the older parts of the Ghadir Manqil Formation and an age of ca. 635 Ma related to the base of the Nafun Group. Specifically these are two tuff-related dates of 713 Ma for the Ghubrah Formation and 645 Ma in the glaciogenic Ghadir Manqil of well Lahan-1 (just below a cap carbonate, Hadash Formation, Figure 18.2). See also Group discussion.
Biostratigraphy:Knoll (1994), further discussed in Brasier (1999), describes a palynological assemblage from the pre-Hadash Formation shales in well Suwaihat-5H2 (see also Butterfield, 2001 and Butterfield and Grotzinger, in preparation). Recovery is dominated by thin walled, simple, spherical acritarchs (leiospheres) but significant numbers of filamentous microfossils are also present. Similar filaments were recorded by Butterfield (2001) from the same well, also documented in Butterfield and Grotzinger (in preparation). Taxa listed by Knoll (1994) include Leiosphaeridia minutissima, L. crassa, Germinosphaera tadasii, Stictosphaeridium sinapticuliferum, Siphonophycus spp., Eosynnechoccus sp. and Trachysphaeridium sp. cf. T. laufeldi. A precise age implication is lacking but the range of potential ages discussed in these documents appears consistent with a general late Cryogenian – Ediacaran age. Butterfield and Grotzinger (in preparation) document two other poor/moderate assemblages from different wells, but these do not add to our stratigraphical understanding.
Love et al. (2009) presented geochemical evidence for the world’s oldest form of animal life yet seen in the fossil record. This resulted from analysis of extracted hydrocarbons of rocks from the Ghadir Manqil Formation (pre-635 Ma). Specific steroids analysed from these extracts are indicative of Demospongiae (primitive marine sponges).
Figures & Tables
|Abu Mahara||Ghadir Manqil|
|Abu Mahara||Ghadir Manqil|