CHAPTER 14 MAHATTA HUMAID GROUP (HAIMA SUPERGROUP)
Published:January 01, 2010
|Mahatta Humaid||Miqrat||Upper Miqrat|
|Lower Amin (‘Amin Conglomerate’)|
|Mahatta Humaid||Miqrat||Upper Miqrat|
|Lower Amin (‘Amin Conglomerate’)|
Authors: Priebe and Kapellos (unpublished, 1993) first defined the Mahatta Humaid as a Group, revising the original, outcrop-based, Mahatta Humaid Sandstone Formation of Kassler (1966) and the Mahatta Humaid Formation of Winkler (1975) and Oprinsen (1986). Revised and published by Boserio et al. (1995) and Droste (1997). The Mahatta Humaid Group is amended here to exclude the Ghudun and Andam formations, which have been redefined as part of the new Andam Group.
The Mahatta Humaid Group is a siliciclastic succession of ‘Early – Middle’1 Cambrian age. The base of the Group is a major, commonly angular, unconformity, known as the Angudan unconformity (Loosveld et al., 1996), which is related to ‘Early’ Cambrian (ca. 540–520 Ma) transpression that marked the termination of Pan-African accretion (Stuart-Smith and Romine, 2003; Romine et al., 2004). This unconformity separates the Mahatta Humaid Group from the underlying Neoproterozoic – ‘Early’ Cambrian Huqf Supergroup, either resting on clastics of the Nimr Group or carbonates and evaporites of the Ara Group. The upper boundary is also unconformable. In North Oman, the Group is overlain by the finegrained, marginal to fully marine clastics of the Andam Group whilst in South Oman, the Andam Group is generally truncated and the group is erosively overlain by the Al Khlata or younger formations. The Mahatta Humaid Group comprises barren continental clastics of the Amin, Miqrat and Mahwis formations, all of which are significant hydrocarbon reservoirs in both South and North Oman.
Type sections: The outcrops in the Qarn Mahatta Humaid and Buah Anticline/Wadi Sumaynah areas of the northern and central Al Huqf (Millson et al., 1996a; Buckley, 1997). See Formation level discussions for appropriate subsurface reference sections.
Lithology: The Mahatta Humaid Group contains a wide range of lithologies including chert-rich conglomerates, quartzose sandstones, argillaceous sandstones, siltstones and mudstones. Typically the coarsest grained lithologies (conglomerates) occur towards the base of the Group. Lithologies are detailed at Formation level.
Subsurface recognition: Whilst drilling, the Mahatta Humaid Group may be difficult to identify on lithology alone, especially in North and Central Oman, as the lithologies of all formations of the overlying Andam Group and of the underlying Nimr Group are for the most part similar. The Mahatta Humaid Group does not contain carbonates; hence the disappearance of limestone downhole is a key indicator. For practical purposes (and prior to wireline logging) the upper boundary is often picked at the base of the last downhole occurrence of limestone. In South Oman the top of the Group is generally easier to pick. The clastics of the Mahatta Humaid Group are commonly overlain by the Permian – Carboniferous Al Khlata Formation. The Al Khlata sandstones are generally coarser grained and the Al Khlata shales are dark grey as opposed to the red shales of the Mahatta Humaid Group.
Where overlain by the Cretaceous Nahr Umr Formation, the pick is straightforward, occurring at the downhole change from fossiliferous grey-green marls to red argillaceous sandstones and mudstones.
In the rare instances where the Mahatta Humaid Group is overlain by the Ghudun Formation, the boundary is difficult to pick as lithologically they are very similar.
Post drilling, the top of the Mahatta Humaid Group is sometimes marked by a Gamma-ray log break to higher values of the Miqrat Formation. In many wells it corresponds to a thin calcareous sandstone bed overlying the Mahatta Humaid Group. Further details of subsurface recognition are listed under the respective formations below.
Boundaries: The base of the Mahatta Humaid Group is a prominent unconformity (the Angudan; Loosveld et al., 1996). It is disconformably overlain by the Andam Group.
Distribution: The Mahatta Humaid Group is widely distributed across Oman. In North Oman the depocentres follow a south-west to north-east trend along the axis of the Ghaba Salt Basin, with its thickest development in the central part. It is thinner in the Fahud Salt Basin. In South Oman the Group is generally thinner, and only occurs on the eastern side of the South Oman Salt Basin. The Group is truncated to the east and west beneath the Al Khlata and younger formations. The biostratigraphically barren nature of the Mahatta Humaid Group can hinder detailed as well as regional correlation.
Deposition: The Mahatta Humaid Group is an entirely continental, arid succession containing a variety of depositional environments, including alluvial/fluvial fans, aeolian dune and interdune settings, sabkhas, playa/lacustrine environments and sheetflood deposits. Environments of deposition are detailed at Formation level. It is important to stress that these continental depositional settings and sediment composition were not influenced at all by the presence of, yet to evolve, land plants.
Subdivision: In North Oman, the Mahatta Humaid Group consists of two formations: the Amin and the Miqrat. In South Oman the Mahwis Formation may be the lateral equivalent of the Miqrat (and, perhaps, lower parts of the Andam Group; Boserio et al., 1995; Droste, 1997), the two having developed independently in separate basins. Without biostratigraphic constraints this is difficult to prove.
Sequence stratigraphy: The Mahatta Humaid Group represents the lower part of the AP2 Megasequence of Sharland et al. (2001).
The sequence stratigraphy of the Mahatta Humaid Group has not been published in detail. The base of the Mahatta Humaid Group is a major unconformity (the Angudan unconformity) that represents a significant, probably 2nd order, sequence boundary. It is likely that the overlying continental clastics of the Amin, Mahwis and Miqrat formations represent a lowstand sequence set, possibly with a minor (?non-marine) flooding event at the base of the Miqrat Formation. The base of the overlying Andam Group represents a transgressive surface.
Age: ‘Early – Middle’ Cambrian, ca. 520–499 Ma. There are no direct age indicators and therefore the numeric age estimate is tentative and based on the age of the overlying unit and position above the AP1/AP2 boundary (Angudan unconformity in Oman).
Biostratigraphy: The Miqrat, Mahwis and Amin formations are barren.
The Miqrat Formation only occurs in North and Central Oman. It was deposited in inland sabkha and playa settings in a low-relief continental environment, continuing the arid depositional setting from the underlying Amin Formation. It comprises red-brown siltstones intercalated with very fine- to fine-grained, commonly micaceous and feldspathic sandstones, the latter probably deposited by flash floods. To the north the sandstones are very thin or absent and are replaced by a siltstone-dominated succession. The Miqrat Formation is an important gas reservoir in North Oman. A number of reservoir units are identified in the Saih Nihayda, Saih Rawl, Burhaan West and Khazzan-Makarem fields (Vroon ten Hove et al., 1996b; Droste, 1997).
Type and reference sections: The Haima outcrops in the Qarn Mahatta Humaid and Buah Anticline areas of the Al Huqf (Millson et al., 1996a; Buckley, 1997). Subsurface reference sections are Saih Rawl-54 (Figure 14.2) and Burhaan West-1 (Figure 14.3), both in North Oman.
Lithology: The Miqrat Formation consists of mottled red-brown shales and siltstones intercalated with light grey to grey-brown, commonly argillaceous, very fine- to fine-grained sandstones (Figure 14.4). The sandstones are quartzose to feldspathic, micaceous and moderately hard to friable. Locally very thin beds, lenses and laminae of coarser grained, medium-grained sand to granule-sized grains are present. Sandstones occur in cm- to dm-scale beds that stack into dm- to m-scale bedsets that may either fine- or coarsen-upwards. Internally sandstone beds may be massive, (sub) horizontally laminated, current and wave ripple-laminated (sometimes draped by thin siltstones) and more rarely cross-laminated. Sandstone beds are typically highly disrupted. Deformed muddy siltstone clasts are locally common, mainly associated with massive or well-stratified beds. Very locally, sandstones may be bimodally sorted with reverse graded pin-stripe lamination and contain frosted ‘millet seed’ grains.
Subsurface recognition: Whilst drilling the Miqrat Formation is very difficult to differentiate. There is little difference between the dominant lithologies of the Al Bashair and Miqrat formations, although the Miqrat lacks limestones. For practical purposes (and prior to wireline logging) the upper boundary is often picked at the base of the last downhole occurrence of limestone. The lower boundary of the Miqrat is more distinct, marked by the change to coarser grained and cleaner, lithic (cherty) sandstones of the Amin Formation. The Miqrat sandstones, in contrast to the Amin, are generally fine-grained, quartzose to feldspathic and micaceous.
The Rate of Penetration trend is variable between wells but overall is typically lower in the upper and basal parts, and higher in the sandier middle part. However, the Rate of Penetration is not a reliable indicator as it is largely dependent on the degree of cementation within the sandstones. The trend is not so well defined where the Miqrat is thin.
Post drilling a serrate log response with high Gamma values (100–150 API, occasionally to +200 API) is typical. The lithology and various logs can be used to distinguish Upper, Middle and Lower Miqrat members (see below).
Boundaries: The Miqrat Formation is generally overlain by the Al Bashair Formation of the Andam Group and rarely by the Al Khlata Formation, mainly along the eastern flank of the Ghaba Salt Basin. It is underlain everywhere by the Amin Formation. Both of these boundaries have been reported to be seismic onlap surfaces by Droste (1997), which suggests an unconformable contact with the over- and underlying units. The upper boundary with the Al Bashair Formation is a transgressive surface along which ravinement may have occurred. The lower boundary with the Amin Formation is usually sharp. In the outcrops of the Al Huqf area at the Buah Anticline location, the base of the Miqrat is a thick (ca. 20 m), dark grey, siltstone package that contains coarsening-upwards parasequences and possible bio-turbation. Other possible trace fossils occur at the top of the Amin Formation in Al Huqf area outcrops and in the subsurface close to the eastern flank of the Ghaba Salt Basin. This suggests that the base of the Miqrat Formation, at least on the eastern flank of the Ghaba Salt Basin, may represent a short-lived marine incursion, although there is a great deal of uncertainty associated with this interpretation.
Distribution: The Miqrat Formation is confined to North and north Central Oman. It is well developed in the Ghaba Salt Basin with a thickness of up to 350 m, and trends southwest to northeast parallel with the axis of the basin, and to the east on the Al Huqf Flank. It is much thinner (<150 m) to the west in the Umm As Samim area. The Miqrat Formation is truncated beneath the base Haushi unconformity to the east, and beneath the base Barakat unconformity to the west and south. It does not extend any further south than Central Oman. The Lower Miqrat Member is restricted to the central and northwestern part of the Ghaba Salt Basin, the Makarem High and the Fahud Salt Basin (Vroon ten Hove et al., 1996b).
It has been suggested that the Miqrat Formation is the lateral equivalent of the Mahwis Formation of South Oman and possibly part of the Andam Group (Oprinsen, 1986; Priebe and Kapellos, 1993; Droste and Mohammed, 1994; Droste, 1997). The Miqrat and Mahwis formations are probably broadly contemporaneous, although biostratigraphic control to constrain correlations is absent. It is possible that they developed independently in separate basins, an assertion that may be supported by their different depositional environments (see below).
Deposition: The Miqrat Formation comprises alluvial and playa lacustrine deposits that are interstratified at a variety of scales and typically bedded at a dm to m-scale, resulting in a complex and heterogeneous stratigraphic architecture. The alluvial intervals display variable degrees of channelling but dominantly comprise laterally extensive, low-relief sand sheets. These were deposited by unconfined flows, interstratified with mud-prone abandonment intervals. The alluvial plain passes transitionally and/or laterally into playa lake deposits through distributary systems and sabkhas. During fluctuating alluvial discharge and/or climate change the playa lake(s) expanded and contracted, with the latter phase characterised by abandonment, desiccation and the formation of extensive sabkhas. Sabkhas include a minor component of aeolian-derived material but extensive aeolian deposits have not been recorded (Adamson et al., 2003). Aeolian deposits occur in the Al Huqf outcrops in the Upper Miqrat at Qarn Mahatta Humaid and in the Middle Miqrat at the Buah Anticline, as described by Millson et al. (1996a) and Buckley (1997).
Subdivision:Droste (1997) divided the Miqrat Formation into Upper and Lower members in the western Ghaba Salt Basin. Wireline log response allows an informal subdivision into Lower, Middle and Upper members. It is extremely difficult to recognise the three units without the aid of logs, although the Middle Miqrat Member has a higher clean sandstone content than the other two members. The Middle Miqrat has previously been referred to as the Miqrat Sandstone.
The Upper Miqrat consists of siltstones, claystones and rare, thin, commonly argillaceous, very fine- to fine-grained sandstones. It is characterised by an overall increasing upwards Gamma ray. Borehole dimension expands slightly due to washout. Other logs exhibit relatively flat profiles with limited ‘shale’ character Neutron-Density separation.
The Middle Miqrat comprises intercalated sandstone and siltstone, with better and thicker sand development than the Upper or Lower Miqrat nearly everywhere. The upper boundary is picked at the top of the first good sandstone unit. The base coincides with the top of the micaceaous Mudstone Marker Bed (see below). In wireline, the Middle Miqrat sands are identified by lower Gamma values and a negative Neutron-Density separation.
The Lower Miqrat is similar to the Upper Miqrat, consisting of siltstones, shales, and claystones with some sandstones (typically highly micaceous), although locally it is sandier than the Upper Miqrat. The upper boundary is the top of a very micaceous shale/siltstone (impossible to detect from cuttings) marked by very high Gamma ray values (ca. 200 API) and a positive (shale-type) Neutron-Density separation. This top Lower Miqrat unit is also known as the micaceous Mudstone Marker Bed and is regionally correlatable.
Age: ‘Middle’ Cambrian, ca. 510–499 Ma. The age is based on geological context and should be viewed as tentative. Sharland et al. (2001) find no evidence for their ‘Middle’ Cambrian Cm20 MFS in the Miqrat of Oman (see their figure 4.8) or in the time-equivalent Saq Formation of Saudi Arabia.
Biostratigraphy: The Miqrat Formation is barren of fossils.
The Mahwis Formation is a succession of micaceous to highly micaceous siliciclastics, unconformably overlying the Amin Formation. It is generally considered to be the South Oman time equivalent of the North Oman Miqrat Formation. It shows a distinctive fining-upwards sequence over thicknesses reaching several hundred metres. The Mahwis represents a change in sedimentation and depositional differentiation between the different salt basins with the transition from the overall clean Amin sands to the dirty shaly sands of the Mahwis Formation. Conglomerates in the Mahwis Formation have a similar composition to those of the Amin Formation and the renewed influx may reflect reactivation of the same source areas, mainly affecting the southernmost parts of the South Oman Salt Basin. Seismic evidence shows the presence of wedge-shaped geometries within the Mahwis Formation, with numerous erosional and onlap surfaces, probably related to syndepositional salt movement. The Mahwis Formation is a significant oil reservoir in South Oman.
Type and reference sections: Qaharir-1 in South Oman (Figure 14.5). Additional subsurface reference sections are Karim-4 (Figure 14.6), Murshid-1 (Figure 14.7) and Jazal-1 (Figure 14.8), all in South in Oman and Misfar-1 in Central Oman (Figure 14.9).
Lithology: The Mahwis Formation is a highly micaceous, very fine- to fine-grained (rarely medium- to coarse-grained) sandstone-dominated succession. It has subordinate, typically thin, red, reddish grey, greenish-grey and green siltstones and heterolithics (Figures 14.10 and 14.11). Sandstones typically contain low-angle cross-bedding or are massive, with subordinate planar and trough cross-bedding and minor ripple cross-lamination (very rarely climbing or with muddy drapes) and wavy lamination. Intra-formational mudrock rip-up clasts are locally abundant. Sandstone beds are typically less than 1 metre in thickness and are rarely capped by thin siltstone intervals. Thicker siltstone and heterolithic units occur up to several metres thick and are generally laminated and may display mottled fabrics. These and even thinner siltstone and heterolithic units appear to be laterally continuous and of value for correlation. Soft sediment deformation, including dewatering pipes, dish structures, desiccation cracks or simply highly disturbed laminae, occurs in both the sandstone and siltstone lithofacies. The facies stack into metre-scale bedsets that may themselves stack into larger fining-upwards units of 25–50 m thickness (Heward, 1989).
Conglomerates are largely restricted to the southern and western parts of the South Oman Salt Basin, in beds up to 10 m thick. They contain igneous, metamorphic, chert and dolomite clasts and are lithologically similar to the Amin Conglomerate.
Subsurface recognition: The Mahwis Formation has an overall serrate, high (80–100 API) Gamma ray response, with the highest spiky readings reflecting the siltstone intervals. It has higher minimum values than the overlying Ghudun and underlying Amin formations. It typically displays minimal Neutron-Density separation. The conglomerates have low Sonic log values, high Density and low to variably high Gamma.
Boundaries: The lower boundary can be conformable and transitional with the Amin Formation, but is also often abrupt and assumed to be unconformable. The upper boundary is a significant break where the Mahwis is overlain by Ghudun clastics or younger formations.
Distribution: The Mahwis Formation is restricted to the South Oman Salt Basin, where it is an erosional remnant beneath the base Haushi unconformity, with a thickness exceeding 600 m. The Mahwis is largely restricted to the Eastern Flank. Further south it extends out to the west. The Miqrat and Mahwis formations are probably broadly contemporaneous, although biostratigraphic control to constrain correlations is absent. It is possible that they developed independently in separate basins, an assertion that may be supported by their different depositional environments.
Deposition: The Mahwis Formation is interpreted to have been deposited in semi-arid alluvial fan settings that grade northwards into alluvial plains (Heward, 1989) with a variety of subenvironments: sheetflood-dominated fluvial sandstones (dominantly characterised by low-angle/horizontal lamination with common mud rip-up clasts), rare cross-bedded sandstones that appear to be weakly channelled fluvial sandstones (transitional sheetflood/braided systems?), grey-green muddy ephemeral lacustrine/sabkha units, some of which form subregional marker horizons.
Overall, the Mahwis Formation appears to fine northwards and to a lesser extent eastwards. These regional facies trends from proximal southwest to a distal northeast suggest that the sediments were derived from the southwestern and southern margins of the South Oman Salt Basin. Periodic tectonic rejuvenation is assumed, based on the stratigraphic distribution of conglomerate beds, that caused variation in sediment input and may have been the driving force behind the 25–50 m thick fining-upwards units (Heward, 1989).
Subdivision:Droste (1997) and Priebe and Kapellos (1993; partly published in Boserio et al., 1995), subdivided the Mahwis Formation into a ‘Lower’ Sandstone/Conglomerate Member, consisting of fine- to coarsegrained, micaceous sandstones with interbeds of conglomerate, and an ‘Upper’ Sandstone/Siltstone Member, comprising predominantly finegrained, micaceous to strongly micaceous shaly sandstones. The boundary between these two members was considered to be transitional and related to a proximal-distal facies change (Boserio et al., 1995; Droste, 1997).
This subdivision has not been widely adopted and has proved to be of little practical use. Additional well data have shown that conglomerate and shaly units occur locally both vertically and laterally throughout the Mahwis Formation. At field scale the Mahwis Formation may be subdivided into informal upper and lower units based on the occurrence of sub-regionally correlatable shaly intervals.
Age: ‘Middle’ Cambrian, ca. 510–499 Ma. The age is based on geological context, including assumed lateral equivalence with the Miqrat Formation, and should be viewed as tentative.
Biostratigraphy: The Mahwis Formation is barren of fossils.
Authors: Winkler and Rácz (unpublished, 1978) based on subsurface data, revised by Oprinsen (1986), published by Hughes Clarke (1988), revised by Droste (1997), who recognised that the ‘Amin Unit 3’ of Oprinsen (1986) and Hughes Clarke (1988) was the lower part of the Miqrat Formation.
The Amin Formation, at the base of the Mahatta Humaid Group, is a succession of sandstones with variously developed siltstones and conglomerates, the latter dominantly in the lower or lowermost part. The Amin clastics overlie the truncated Nimr Group, separated by the regional Angudan unconformity, overstepping older sediments of the Huqf Supergroup onto the flanks of the Salt Basins. Sediments of the Amin Formation are widespread across southern and northern Oman with a variable thickness resulting from synsedimentary salt withdrawal and thinning onto the basin flanks. The onset of deposition of the Amin Formation is characterised by the influx of coarse clastics into the basins. Conglomerates interbedded with sandstones are well developed, exceeding several hundred metres in thickness along the western flank of the South Oman Salt Basin. They also occur along the southeastern margin of the Ghaba Salt Basin and in the Fahud Salt Basin, but here they are much thinner. Basinward and upwards in the Amin succession, these conglomerates grade into mixed alluvial and aeolian sandstones. The Amin Formation is a significant oil reservoir in South Oman and a major gas reservoir in North Oman.
Type and reference sections: Farha-1 in North Oman (Figure 14.12). Additional subsurface reference sections are Al Bashair-1 (Figure 14.13) and Saih Nihayda-33, both in North Oman (Figure 14.14), Sadad-1 in Central Oman (Figure 14.15) and Runib South-3 (Figure 14.16), Marmul NW-7 (Figure 14.17) and Amin-1 (Figure 14.18), all in South Oman.
Lithology: The Amin Formation is dominated by quartzose, commonly chert-rich, sandstones with conglomerates (Figure 14.19, known as the ‘Amin Conglomerate’) towards its base and relatively minor amounts of siltstone. Sandstones are sublithic arenites to quartz arenites and are generally very fine- or fine- to medium-grained. More rarely they are coarse-grained. The lithics are mainly chert and metamorphic fragments. Sandstones are most commonly horizontally laminated or can be irregularly laminated with massive and current ripple laminated units and rare cross-bedding. Sandstones commonly have erosive bed bases and may be slightly argillaceous. In finer grained silty intervals adhesion ripples occur. Elsewhere, very clean sandstones, with abundant, frosted, rounded to subrounded grains, occur that display inclined (5° to 30°), sharply defined, reversely graded, tabular, cm-scale foresets with locally abundant interbedded mm-scale, inversely graded ‘pinstripe’ laminae.
Siltstone units are typically red-brown or grey and occur as thin (typically a maximum of 1–2 m thick) packages. They display horizontal lamination, ripple cross-lamination or are massive and commonly contain desiccation cracks. Siltstone units appear to be most common in the Fahud Salt Basin.
The conglomerates are poorly sorted and polymict, containing pebbles and granules of chert (usually the most abundant lithology), quartz, metamorphic rock and occasionally dolomite. They occur in metre-scale beds interbedded with white to light grey, fine- to medium-grained sandstones, medium- to very coarse-grained conglomeratic sandstones, with a sand to silt matrix and red to reddish-brown shales. These sandstones and conglomerates show various scales of trough and tabular cross-bedding. Droste (1997) reported large-scale fining upward trends (over more than 100 m) with an increasing number of mudstone intercalations upwards.
Subsurface recognition: Whilst drilling, the Amin Formation is characterised by clean, predominantly light grey fine- to medium-grained sandstones with very rare to no argillaceous component. The grains are commonly well-rounded quartz and chert grains. In thick, well-developed Amin successions the sandstones of the lower part (serrate Gamma) may be brown, orange or pink (Figures 14.11 and 14.19). On the Eastern Flank of South Oman where the Amin occurs at shallow depths the sandstones are generally loose and unconsolidated. In North Oman where the Amin Formation occurs at depth the sandstones are generally recovered as consolidated aggregates of grains.
The Amin Formation is generally coarser grained than the Miqrat Formation, which has a higher argillaceous content, consequently, the Amin/Miqrat boundary is usually very clear from cuttings. It is also coarser grained than the Mahwis Formation (very fine) in the northern Eastern Flank. To the southwest in the Eastern Flank, the transition from the Mahwis to the Amin may be more difficult to identify due to the range of variable grain sizes in the Mahwis Formation.
The chert fragments of the Amin Formation are generally more rounded because of longer reworking than those of the underlying Haradh Formation (Nimr Group). The Haradh cherts are more angular (subangular) with primary features more visible e.g. banding of the chert fragments. The chert conglomerates (of the ‘Amin Conglomerate’) are very distinctive in ditch cuttings. The chert ‘pebbles’ are recovered, due to bit action, as coarse, broken up angular fragments (Figure 14.19).
In deep gas wells of North Oman the top of the Amin may be characterised by an interval (10–15 m thick) of slow drilling due to the presence of strongly cemented (quartz overgrowths) sandstones. But elsewhere the top is marked by a positive drill break.
The Amin may be difficult to identify when overlain by clean, well-rounded sandstones of the Al Khlata Formation, as the base of the Al Khlata Formation may comprise reworked Amin sandstone. However, the first indication in the ditch cuttings of lithics composed solely of cherts (Amin) and diminishing quantities of exotic lithics (Al Khlata) indicates penetration of the Amin Formation. Recognition of the boundary is easier if shales and/or diamictites of the Al Khlata overlie the Amin sandstones.
The lower boundary is recognised by a distinctive lithology change when overlying evaporites and carbonates of the Ara Group or the shales/siltstones of the Nimr Group (see Runib South-3, Figure 14.16). Problems of recognition arise where the sandstones of the Amin Formation overlie sandstones of the Nimr Group (North and north Central Oman only) as both lithologies are similar. The Nimr Group sandstones may have higher and more serrate Gamma log trends than the sandstones of the Amin Formation. If such a distinction is not apparent then a Dipmeter log may be the only means of separating the formations.
Post drilling, the Upper Amin can be recognised by the relatively much lower Gamma log character (ca. 30 API) than under- and overlying formations (with the exception of the Ara Carbonates). The Gamma ray pattern may be uniform or serrate. In well-developed, thick sections a uniform or blocky Gamma in the upper part (pure clean sandstone) and a serrate Gamma log in the lower part (increase in argillaceous content, e. g. Marmul NW-7, Figure 14.17) is normal. Usually the Amin Formation has a Dipmeter break at the lower boundary, with the Nimr Group, which often has much higher structural dips than the Amin Formation. Even where a significant dip break is not observed, a subtle dip break, commonly with more random azimuths in the Nimr Group, marks the base of the Amin Formation.
Boundaries: The Amin Formation is overlain by the Miqrat Formation in North and Central Oman and by the Mahwis Formation and, more rarely, the Al Khlata Formation in South Oman. Both contacts are considered to be mainly unconformable.
The lower boundary is a hiatus or angular unconformity (the Angudan unconformity) over the Nimr Group or older units of the Huqf Supergroup.
Distribution: The Amin Formation is widespread throughout North, Central and South Oman. In North Oman it is well developed in the Ghaba Salt Basin with a thickness of up to 700 m. Its depocentre trends southwest to northeast parallel with the basin axis. The Amin Formation is truncated along the western margin by the base Haushi unconformity. It thins to the northwest into the Fahud Salt Basin, where, except in isolated salt-related sub-basins, the Amin is less than 200 m thick. To the west it is truncated beneath the base Barakat unconformity. Across Central Oman it is truncated beneath the base Barakat and base Haushi unconformities. The Amin Formation is largely restricted to the Eastern Flank in the South Oman Salt Basin, but it extends out into the basin, further than the overlying Mahwis. It is confined to salt withdrawal basins and has a large variation in thickness (up to 1,000 m).
The ‘Amin Conglomerate’ occurs towards the base of the Formation. It is best developed along the western margin and the north to northwestern parts of the South Oman Salt Basin (where it can be many hundreds of metres thick). The ‘Amin Conglomerate’ is also well developed in the southern Ghaba Salt Basin, possibly extending into the central parts of the basin, and the Fahud Salt Basin. Droste (1997) suggests that the conglomerates infill palaeo-topography on the Angudan unconformity. The conglomerates grade laterally and down-depositional dip into sandier facies (e.g. Buckley, 1997; Droste, 1997).
Deposition: The Amin Formation reflects a period of arid, continental desert conditions. The distribution of the different facies is poorly understood in detail, with a complex and laterally variable stack of depositional environments. Previous authors (e.g. Oprinsen, 1986; Hughes Clarke, 1988; Droste, 1997; Knight and Mugheiry, 1999) have suggested or implied that the Amin Formation is a dominantly aeolian succession. This is not supported by extensive core data. It may be more appropriate to consider the Amin Formation to be an arid fluvial system with an aeolian component. Within the sandy intervals of the Amin Formation several sub-environments can be recognised. Fluvial sheet/streamflood environments may be the most common depositional setting of the Amin. Others include channelled to weakly channelled braided systems, aeolian sand sheets (interdunes) and dunes, wet and dry sabkhas and ephemeral lakes/playas. Typically, individual aeolian dunes are small (1–2 m maximum thickness), although they occasionally stack into thicker intervals. In South Oman, the preserved dunes appear to be larger than in North Oman. Although the detailed distribution of the sub-environments is poorly constrained in general terms the Fahud Salt Basin appears to be dominated by fluvial facies with a minor aeolian component. In the Ghaba Salt Basin and South Oman aeolian deposits are more common, although interbedded with fluvial, sabkha and playa deposits.
Conglomerates in the Amin have traditionally been interpreted to represent the deposition of alluvial fans (e.g. Hughes Clarke, 1988; Droste, 1997). However, in the outcrop (e.g. Buckley, 1997) the conglomerates are well organised with distinct bar form development, clast imbrication locally and bar top deposits suggesting a bed-load dominated braided river environment, although these could be downdip of true alluvial fan deposits.
(1) a Sandstone Member defined as a uniform package of clean quartz-rich sandstones, broadly conforming to the Amin Unit-2 of Hughes Clarke (1988, his figure 6);
(2) a Conglomeratic Sandstone Member, defined as conglomerates and conglomeratic sandstones with some interbedded mudstones, probably equivalent to the lower part of the Amin Unit-1 (‘gravel beds’) of Hughes Clarke (1988, his figure 6); and
(3) an Interbedded Siltstone and Sandstone Member defined as interbedded fine, occasionally up to coarse-grained quartz sandstones and red-brown mudstones, broadly conforming to the upper part of the Amin Unit-1 of Hughes Clarke (1988, his figure 6).
The Sandstone Member was the uppermost unit overlying the other two members which were considered to be partly laterally equivalent. This subdivision updated the three-fold subdivision of the Formation by Hughes Clarke (1988), in particular removing the Amin Unit-3 of Hughes Clarke from the Amin Formation and placing it within the Miqrat Formation.
The above subdivision has been loosely used but is of limited practical value, as within the lower part of the Amin conglomerates may occur at the base, in the middle or at the top. It is now more common to simply split the Amin Formation into informal Upper and Lower members based on lithology and wireline log response (Fryberger, 2009). The Upper Member corresponds to the Sandstone Member of Droste (1997) and comprises clean quartz-rich sandstones that have a blocky Gamma log signature and good positive (sand-type) Neutron-Density separation. The Lower Member consists of conglomerates, siltstones and sandstones and is best identified in wireline logs by a higher shale line in the Gamma and a more highly serrate character than the Upper Member. The informal term ‘Amin Conglomerate’ is retained because it is widely used as a sub-unit within the Lower Member, although the conglomerates may occur at different stratigraphic levels within the Lower Amin Member.
Age: ‘Early’ Cambrian, ca. 520–510 Ma. This numeric age range is tentative and purely based on geological context, in particular the position above the Angudan unconformity. The Lalun Formation in Iran (Stöcklin and Setudehnia, 1972) could be a lateral equivalent of the Amin Formation (Hughes Clarke, 1988) as could the Siq Sandstone Formation of Saudi Arabia (as illustrated by Al-Husseini, 2008).
Biostratigraphy: The Amin Formation is barren of fossils.
Figures & Tables
|Mahatta Humaid||Miqrat||Upper Miqrat|
|Lower Amin (‘Amin Conglomerate’)|
|Mahatta Humaid||Miqrat||Upper Miqrat|
|Lower Amin (‘Amin Conglomerate’)|