A series of lithofacies associations are defined for the Permian Umm Irna Formation indicating deposition in a fluvial regime characterised by low-sinuosity channels with deposition on point bars, and as stacked small-scale braided channels. Umm Irna Formation floodplain interfluves were characterised by low-energy sheet-flood deposits, shallow lakes and ponds, and peaty mires. Floodplain sediments, where not waterlogged, are generally pedogenically altered red-beds with ferralitic palaeosols, indicating a fluctuating groundwater table and humid to semi-arid climate. The Dead Sea outcrop provides a field analogue for similar fluvial and paralic depositional environments described for the upper Gharif Formation alluvial plain ‘Type Environment P2’ in the subsurface in Oman and the upper the basal clastics of the Khuff Formation at outcrop and in the subsurface in Central Saudi Arabia. Coarse-grained clasts within channel sandstones are mineralogically immature; their palaeocurrent directions and new evidence of glaciogenic sediments from Central Saudi Arabia suggests derivation from Pennsylvanian–Early Permian glaciofluvial outwash sandstones located to the east-southeast.
The palynology of the Umm Irna Formation is remarkably varied. Samples from argillaceous beds of fluvial origin appear to contain a palynomorph representation of the wider hinterland of the drainage basin of the river including floodplain plants and more distant communities. In restricted water bodies like oxbow lakes or other impermanent stagnant floodplain ponds and peaty mires (immature coals), a higher proportion of purely local palynomorphs appear to be preserved in associated sediments. One of the assemblages representing local plant communities displays a Cathaysian palaeophytographic affinity, while others from similar levels within the Umm Irna Formation present a Gondwanan affinity. This indicates the risk of generalisation from single borehole or limited outcrop studies.
The presence of Protohaploxypinus uttingii suggests an age range of Wordian–Capitanian to early Wuchiapingian (Middle to early Late Permian) for the Umm Irna Formation. The quantitative character of the Umm Irna Formation assemblages is very close to those of the basal Khuff clastics in the Central Saudi Arabian wells Dilam-1, Nuayyim-2 and Haradh-51. The lithological character and palynology of the transition between the Sa’ad and Arqov formations in the West Bank, west of the Dead Sea are similar to those of the transition between the Umm Irna Formation and overlying Ma’in Formation in Jordan.
The predominantly siliciclastic succession unconformably overlying Cambrian sandstones along the northern margins of the Dead Sea, Jordan, was first assigned a Triassic age by Cox (1924, 1932). The biostratigraphy of these rocks was later refined by Huckriede and Stoppel (in Bender, 1968, 1974) who recognised Scythian conodonts within the Triassic sequence. A Late Permian age was assigned to the lower part of this succession by Bandel and Khoury (1981) who formalised the lithostratigraphy and further refined the biostratigraphy. The sedimentology and structure of the lower part of the Permian–Triassic sequence (Umm Irna Formation) was studied in detail by Makhlouf (1987), Makhlouf et al. (1990, 1991) and Powell and Moh’d (1993). More recent work has focussed on the remarkably well-preserved plant fossils in the Umm Irna Formation and their depositional environments (Kerp et al., 2006; Uhl et al., 2007; Abu Hamad et al., 2008; Dill et al., 2010).
In this paper we describe well-preserved and diverse palynomorph assemblages within these alluvial sediments, which further refines the biostratigraphy and palaeoenvironments of this formation, and better enables correlation across the Arabian Platform (e.g. Saudi Arabia, Oman, Turkey), which was located at the southern margin of the Neo-Tethys Ocean (Stampfli and Borel, 2002). Through detailed sedimentological lithofacies analyses of fluvial, paralic and red-bed palaeosol lithofacies we ascribe palynological assemblages to localised palaeoenvironmental niches on the alluvial plain. Three-dimensional analyses of these sequences at outcrop provide an insight into the depositional environments and sand-body stacking patterns of the extensive, broadly coeval upper parts of the Gharif Formation and the basal Khuff clastics proved in the subsurface in Oman and Saudi Arabia, respectively, where they are hydrocarbon reservoir rocks. We relate the sedimentary sequences and alluvial architecture in the Umm Irna Formation to major sequence stratigraphical events (depositional sequences) across the Arabian Platform in the Middle Permian.
Organic-rich shales and immature coals in the Umm Irna Formation succession may provide suitable hydrocarbon source rocks in the subsurface basins of the Arabian Platform. Furthermore, the presence of fluvial sand-bodies, interbedded with these source rocks, present hydrocarbon reservoirs within extensionally faulted pre-Cretaceous half-grabens (Powell and Moh’d, 1993).
Earlier workers (Wetzel and Morton, 1959; Bender, 1974) noted that the Permian–Triassic sequence thins southward below the overstepping, unconformable Lower Cretaceous Kurnub Sandstone along the Dead Sea shore and, furthermore, that the sequence was intruded by dolerite dykes and sills that do not cut the overlying Cretaceous rocks. It was also recognised that the Permian–Triassic and Jurassic sequence becomes more complete when traced northwards along the outcrop, below the Cretaceous unconformity. Bandel and Khoury (1981) and Powell and Moh’d (1993) suggested that the relative completeness of the early Mesozoic sequence in north Jordan, as compared to the Dead Sea area (this study), was due to the general step-like, northerly down-faulting of the sequence in pre-Cretaceous times (probably latest Jurassic) rather than a result of northward tilting and subsequent erosion of the sequence prior to deposition of the fluvial Kurnub Sandstone in the Late Cretaceous.
This paper focuses on the type area (including the type section in Wadi Himara) of the Umm Irna Formation, where it unconformably overlies Cambrian sandstones (Umm Ishrin Sandstone Formation; Powell, 1989) along the northeastern shore of the Dead Sea, and in adjacent ephemeral wadis that drain into the lake (Figure 1). The Umm Irna Formation is generally overlain by the Ma’in Formation.
The type section of the Umm Irna Formation was defined by Bandel and Khoury (1981) in Wadi Himara, located about 2 km east of the Dead Sea (Figure 1, Locality 1; N 31°38′24.2″; E 35°35′05″) where it is about 68 m thick. They sub-divided the formation into six informal units (upward-fining cycles) comprising pebbly, coarse-grained sandstone, siltstone and claystone with irregularly developed ferruginous glaebule horizons.
Some of the plant fossil-rich carbonaceous siltstones near the base of the formation yielded palynomorphs indicating a ‘Late Permian’ age according to Brugman (inBandel and Khoury, 1981); this age was supported in similar studies of samples from higher in the Umm Irna Formation (Armstrong, inMakhlouf, 1987). Makhlouf et al. (1991) recognised an informal Lower Member (Facies 1), about 10 m thick, consisting of sandstones and silty shales in upward-fining sequences, which they attributed to a distal braidplain setting. Their Upper Member (Facies 2) comprises five fining-upward cycles with elements of both braided and meandering stream deposits, with silty beds deposited in abandoned channels. Palaeosols with ferruginous glaebules are developed in the middle and upper part of the formation (Makhlouf et al., 1991; Powell and Moh’d, 1993).
Organic-rich lenses and beds comprising both dispersed mega-flora and disseminated finely comminuted plant fossil material, charcoalified wood and immature coals are present, especially in the Lower Member (Uhl et al., 2007; Abu Hamad et al., 2008; Dill et al., 2010), but as we show here – also throughout the formation. The plant material occurs in grey to brownish-black claystone and siltstone as in-situ palaeosol coals and as drifted material within grey claystone and dark grey organic-rich siltstones in abandoned channel lenses and within laterally accreted point-bar units. The plant macrofossils are more common and better preserved in the latter depositional setting.
Permian rocks (Hudayb Group; Andrews, 1992) have also been proven in hydrocarbon exploration wells in north and northeast Jordan (North Highlands-2 and Ajlun-1 inAndrews, 1992). An Early to Mid–Late Permian age was assigned to these rocks in north Jordan in the sub-surface, constrained in some of the wells to Kungurian (late Early Permian) to Kazanian (Middle Permian; Keegan inAndrews, 1992). The lowermost unit (the Anjara Formation of Andrews) comprises terrestrial, interbedded sandstone and dark siltstone. It is overlain by a middle carbonate unit, the Huwayra Formation, and an upper siliciclastic unit, the Buwayda Formation, that contains lignite and woody plant material (Andrews, 1992), clean, fine- to medium-grained sandstone (probably channel-fill deposits) and red-brown, ferruginous and lignitic fine-grained sandstone interbedded with black to dark grey siltstone (probably coaly mire and red-bed pedogenic floodplain deposits). Based on the similarity of its lithology and gamma-log response, the Buwayda Formation is interpreted as equivalent to the Umm Irna Formation at outcrop. Since the latter rests unconformably on the Cambrian Ram Sandstone Group along the Dead Sea shore it seems likely that the middle (Huwayra) and lower (Anjara) formations in the subsurface in north Jordan are absent at outcrop.
Abu Hamad et al. (2008) and Kerp et al. (2006) regard the upper boundary of the Umm Irna Formation as unconformable with the overlying Ma’in Formation (Himara Member). Low diversity palynological assemblages from approximately 10 m above the unconformity were dated as Early Triassic (Abu Hamad, 2004; Kerp et al., 2006), and higher parts of the formation are dated as ‘mid to late Scythian’ (probably Induan to Olenekian) based on bivalve determinations by Cox (1932), and on conodonts (Huckriede and Stoppel in Bender, 1968, 1974). West of the Dead Sea, Permian sediments were deposited in a shallow-marine environment with inter-fingering fluvial siliciclastics representing the palaeoshoreline in the Negev area (Eshet and Cousminer, 1986; Eshet, 1990).
During the deposition of the Umm Irna Formation (Figures 2a, b), Jordan lay about 15 to 20 degrees south of the palaeo-equator at the northern margin of the Arabian Platform in a continental to marginal-marine setting with the Neo-Tethys Ocean located to the north (Stampfli and Borel, 2002). The palaeogeographical location, together with the diverse and abundant macro- and microfloras, and ferralitic palaeosols described from this formation indicate a humid-tropical climate. During relative high sea level, marine transgressions advanced to the south and southeast.
MATERIALS AND METHODS
Measured sections were logged in detail at nine localities in the Umm Irna Formation adjacent to the Dead Sea (Figures 1 and 2a). These were recorded with a high-resolution digital camera and GPS. Palaeocurrent measurements were taken from trough cross-bedded, ‘rib and furrow’ channel-fill sandstones where these are exposed in three dimensions to maximise accuracy of palaeocurrent azimuth measurements. A total of 25 samples were taken at various localities for palynological analysis. Palynological samples were extracted by deep excavation of the claystone, siltstone and coaly beds using a pick-axe to avoid near-surface contamination or the present-day oxidation of palynomorphs. In addition, samples were taken, where possible, from shaded north-facing exposures to avoid as far as possible, oxidized areas. The preparation of strew mounts for palynological analysis comprised crushing, followed by hydrochloric and hydrofluoric acid treatments (Wood et al., 1996). The post-hydrofluoric acid organic residues were oxidized using Schulze’s solution and dilute nitric acid. The palynological slides bear the British Geological Survey code prefix ‘MPA’ and are curated in the BGS collections in Keyworth, Nottingham. Plant fossils were also collected from organic-rich siltstones and claystones in abandoned channel lithofacies.
SEDIMENTOLOGY OF THE UMM IRNA FORMATION
Stratigraphical and sedimentological studies of the Umm Irna and Ma’in formations (Bandel and Khoury, 1981; Makhlouf, 1987; Makhlouf et al., 1990, 1991; Powell and Moh’d, 1993; Dill et al., 2010) have demonstrated depositional environments typified by predominantly fluvial, braided to low-sinuosity sandstone channels together with finer grained, red-bed interfluve sandstone and siltstone with intermittent ferruginous palaeosol horizons. Organic-rich siltstones and thin immature coals (some with seatearths) are occasionally present. The main sampled localities, lithofacies and depositional environments are summarised here, together with new data from the current study that provide a framework for the palynological analysis described in the next section.
Locality 1: Wadi Himara South Side
This composite 9 m section (Figures 2 and 3; Plate 1), which includes the lower part of the Umm Irna Formation and its unconformable boundary with the Cambrian Umm Ishrin Formation, represents the type area and the type section described by previous authors (Bandel and Khoury, 1981; Makhlouf et al., 1991; Abu Hamad et al., 2008). It is located along the southern branch of Wadi Himara, upstream from a major waterfall, where a small side wadi bifurcates from this branch. The location is N31°38′24.2″ and E35°35′05.8″. The section is dominated by thick, lensoid trough cross-bedded sandstone units with erosional bases. Foresets are lined by small pebbles and granules of quartz, with some sub-rounded, pink alkali-feldspar clasts. Carbonaceous siltstone laminae also drape some of the foresets. The lowermost sandstone is colour-mottled mauve-grey with vertical and subvertical vertisols which include yellow-brown limonite-goethite glaebules. Some of the sandstone beds are capped by thin carbonaceous lamina with downward penetrating vertical plant rootlets. The relative proportion of yellow fluvial channel sandstone lithofacies compared to red-bed alluvial floodplain sediments in the Umm Irna Formation is illustrated in Plates 1 and 2, the latter plate showing an inaccessible vertical face on the opposite, northern side of the wadi.
A few thin carbonaceous siltstone and claystone beds are present about 4.5 m above the base of the formation (Plate 1): an upper bed with plant macrofossils and a lower one split by a thin sandstone lamina.
Locality 2: Panorama Road (East)
This section is on the eastern side of the road cutting of the recently constructed Panorama Road (Plate 3a). The section is between points N31°39′11.5″, E35°34′41.9″ and N31°39′08.9″, E35°34′43″, and includes four separate productive palynological sample points (Figure 4). The section lies within a faulted block, so its exact stratigraphical position is uncertain, although we believe it to lie within the lower part of the formation on account of its lithofacies being closely comparable with the lower part of the Umm Irna Formation in the type locality at Wadi Himara, located 1.5 km to the southeast. Locality 9 (see below; Plate 3b), located on the opposite (western) side of the road cutting, did not yield palynomorphs. The lowermost bed at Locality 2 (represented by palynological sample MPA 62244) comprises dark grey claystone and siltstone with plant macrofossils, and is overlain by pale grey siltstone. This is overlain by a thin dark grey siltstone with disseminated plant fragments (represented by sample MPA 62243), with laminae which are oxidised to pale brown colour. The overlying beds comprise pale grey and red siltstone with ca. 1.5 m thick beds of trough cross-bedded sandstone. The uppermost palynological sample points were within two thin dark grey claystone beds (MPA 62242 and 62241), again with disseminated plant fragments, underlying a thin yellow channel sandstone (Figure 3).
Locality 3: Wadi Zerqa Ma’in South
This roadside section located (N31°36′46.1″, E35°33′57.5″) south of Wadi Zerqa Ma’in is approximately 5 m thick, and is dominated by claystone and siltstone with several thin trough cross-bedded, medium grained sandstone beds, pebbly in part (Figure 5; Plate 4). Some sandstone beds are penetrated by vertical and sub-vertical plant rootlets to a depth of up to 25 cm below overlying thin carbonaceous siltstone laminae. The uppermost sandstone (1.7 m thick) shows repeated plant colonisation at three levels, indicating periods of low fluvial flow-regime. A distinctive pale grey medium-grained sandstone bed, 0.20 m thick, present about 1.5 m above the base of the section, contains a network of circular, horizontal burrows (Plate 4). Thin, low-angle laterally-accreted sandstone units with carbonaceous claystone drapes are present between sample levels MPA 62249 and MPA 62250. Dark grey carbonaceous siltstone beds above and below the burrowed horizon yielded a diverse assemblage of palynomorphs (MPA 62249). Samples from the carbonaceous claystone higher in the section (MPA 62250) and from siltstones (MPA 62251 and 62252) also yielded diverse assemblages.
Locality 4: Side Wadi South of Wadi Zerqa Ma’in
This north-facing section (N31°36′44.4″, E35°33′56.2″) represents part of the upper Umm Irna Formation, and is exposed in a small unnamed wadi, south of Wadi Zerqa Ma’in, and about 800 m south of Locality 3. The 3 m thick section comprises, yellow-grey trough cross-bedded sandstone with thin carbonaceous siltstone lenses with plant macrofossils and rootlets; the upper part of the sandstone has rootlets penetrating to 60 cm (Figure 6). The upper part of the section comprises 2 m of siltstone intercalated with thin yellow and pale red sandstone beds. Lower and upper claystones provided samples MPA 62253 and MPA 62254, which yielded diverse and well-preserved assemblages (Figure 6). A thin sandstone bed, about 2 m above the base of the section contains a network of horizontal circular burrows similar to those seen nearby at Locality 3.
Locality 5: Roadside Section South of Wadi Zerqa Ma’in
This short 6 m thick section (Figure 7) is preserved in a small gully (N31°36′31.0″, E35°33′49.2″) a few metres immediately east of the Dead Sea road. The section represents the upper part of the Umm Irna Formation and is dominated by yellow and grey, medium-grained, trough cross-bedded sandstone with a thin claystone bed and siltstone bed about 2 m above the base of the section. Carbonaceous plant rootlets penetrate up to 75 cm from the claystone to the lower sandstone. Two samples of the siltstone yielded well-preserved diverse assemblages (MPA 62255 and 62256).
Locality 6: Wadi Autun
The upper part of the Umm Irna Formation is exposed to the east of the Dead Sea road in the narrow Wadi Autun (N31°32′40.1″, E35°33′31.7″). The north-facing, measured section (6 m thick) lies about 15 m below the top of the formation (Figure 8; Plate 5). It consists of intercalated yellow, pink and grey medium-grained sandstone and grey siltstones with black carbonaceous laminae. A lower sample (MPA 62257) was taken from a grey siltstone overlain by black carbonaceous, almost coaly, siltstone that is overlain by trough cross-bedded sandstone with an erosive base (Figure 8). The upper sample (MPA 62258) was from a carbonaceous siltstone overlying a claystone- and siltstone-filled abandoned channel, but is itself overlain by sandstone with an erosional base. The two samples yielded moderately well preserved assemblages which are quite distinct from each other (despite their stratigraphic proximity), and from other assemblages recovered from the Umm Irna Formation (see Palynology section).
Localities 7 and 8: Dyke Plateau
Locality 7 is 1.65 km south of Wadi ad Dab (Figure 1). The upper part of the Umm Irna Formation and its boundary with the overlying Ma’in Formation is well exposed along a cliff marking a former cliff line of the Dead Sea and in a number of east-draining side wadis, above a wave-cut platform (Figure 9). The beds are mostly horizontal, but dip gently to the north in the northern part of the section. The well-exposed section is cut by a sub-vertical dolerite dyke, which may be coeval with the Tayasir Volcanics in Galilee, the base of which gives a probable late Jurassic to early Cretaceous age (144 ± 5 and 148 ± 7 Ma); see Lang and Mimran (1985) and Powell and Moh’d (1993) for discussion. The section, illustrated in the panoramic photographs (Plates 6, 7 and 8) extends from N31°32′13.0″, E35°33′27.2″ where the boundary with the overlying Ma’in Formation is exposed, southwards to N31°31′56.5″, E35°33′25.6″ and includes a small westward-orientated side wadi, Locality 8 (N31°38′24.2″, E35°35′05.8″; Figure 9).
The cliff section includes a laterally accreted channel sandstone-siltstone-claystone complex with evolved palaeosols, which was previously figured by Powell and Moh’d (1993, Figure 8). The northern part of the cliff section is dominated by an erosively-based, fluvial sandstone channel which is exposed in three dimensions at N31°32′09.2″, E35°33′26.4″. The overall channel is low-sinuosity and comprises stacked channel-fill sandstones with lower, middle and upper sections. Palaeocurrent measurements (100 in total) on “rib and furrow” trough cross-beds indicate an overall palaeocurrent direction to the west-northwest to west-southwest (Figure 9). However, the lower section of the channel is generally unidirectional to the north-northwest. A feature of the lower section is deep scouring of the channel into the surrounding pedogenetically-altered level-bedded floodplain deposits. The basal channel-ill shows an upward change from pebble-to granule-size clasts comprising reworked brown and black limonitic and goethitic palaeosol clasts derived from the proximal floodplain, along with sub-rounded granules of pink feldspar and quartz, from a more distant source (Plate 9). The middle and upper sections of the channel have more divergent palaeocurrent directions, dominantly to the west-southwest, but with considerable divergence in the uppermost sinuous channel (Figure 9). The waning flow regime in the upper section is also illustrated by the presence of claystone and organic-rich siltstone drapes on foresets (Plate 7).
To the south of the sandstone channel axis, a number of well-exposed, laterally accreted sandstone-siltstone-claystone units form multi-stacked, lensoid sand-bodies (Plates 6, 7 and 8). These first-order units are internally structured by second-order laterally accreted units (Plate 8). Many of the low-angle lateral accretion surfaces are draped by thin beds and laminae of grey claystone and black organic-rich siltstone. In places, penecontemporaneous slumped beds and microfaults can be seen at channel margins. The grey claystone beds include well-preserved drifted plant macrofossils (Abu Hamad et al., 2008; Mustafa, 2003) and coalified logs. Black organic-rich siltstones were sampled (Plate 7; MPA 62259 to MPA 62261) and yielded diverse assemblages.
As the section is traced southwards, the laterally-accreted channel thins and wedges out to be replaced by level-bedded, floodplain red-beds comprising red to red-brown fine-grained sandstone and siltstone with occasional rippled surfaces and faint surface burrows (Plate 10). At some levels desiccation cracks are present and there are deep vertisol palaeosol horizons. The latter are characterised by red, mauve, yellow, and brown colour-mottling and the presence of granule to pebble-size limonite and goethite soil glaebules. Some of the highly evolved palaeosol horizons are over 3 m thick, often located within abandoned, clay-filled channels (Plate 11).
Locality 8 lies in a small east-west side wadi that exposes the upper part of the Umm Irna Formation succession (see Sample 62263 locality, Figure 9). Here, evolved palaeosol beds are overlain by a series of stacked, laterally accreted channel sandstones. The upper channel overlies an abandoned channel infilled with grey to black organic-rich siltstone and claystone (Plate 11), which provided samples MPA 62262 to 62264. Sample MPA 62263 yielded a diverse assemblage.
Locality 9: Panorama Road (West)
This locality (N31°39′08.8″, E35°34′43.3″) is opposite Locality 2 on the western side of the new Panorama Road (Figure 1). Two thin carbonaceous siltstones overlie leached grey white fine-grained quartz sandstone, with rootlet traces (Plate 3b). The underlying sandstones represent leached evolved, seatearth palaeosols. The carbonaceous siltstones were sampled but no palynomorphs were recovered.
UMM IRNA FORMATION LITHOFACIES ASSOCIATIONS
The principal lithofacies associations in the Umm Irna Formation are described below and summarised in Table 1.
Channel-fill Sandstone (Cfs)
Channel-fill sandstones are composed predominantly of medium- to coarse-grained, feldspathic sandstone with granules and small pebbles of quartz and pink feldspar. The sandstones comprise upward-fining units with channelised, scoured basal contacts (Plate 9). Medium- to large-scale, trough cross-bedding with graded foresets is the most common bedform within the channelised units (Plate 9). In the lower part of the formation in the Al Mamalih area (Powell and Moh’d, 1993) that represents the southernmost outcrop (Figure 1), and in the northern outcrops (Makhlouf et al., 1991) the palaeocurrent trend within channels is predominantly to the northwest. However, 100 palaeocurrent measurements taken from trough cross-bedding on multiple, present-day erosion surfaces within a major channel in the upper part of the formation (Dyke Plateau Locality 7; Figure 9) reveal a predominantly unimodal west-northwest to west-southwest palaeoflow depending on the position of the channel branch and stacking level within the main channel complex. This sediment dispersal pattern is in broad agreement with the WSW trend of the major channels in the upper part of the formation of the northern outcrops recorded by Makhlouf et al. (1991). The stacked channel sandbodies at Locality 7 show increasingly divergent palaeoflow directions in the upper part of the channel (Figure 9; Plates 6 and 9); these sandstones are fine- to medium-grained, in contrast to the underlying coarse-grained, granule-rich sandstone, and include grey claystones with plant macrofossils and dark grey organic-rich siltstones deposited as channel lag drapes on trough cross-bed foresets. These fine-grained siltstone beds are also common in point-bar lithofacies association (see below).
The major ‘winged’ channels are up to 35 m wide and up to 7 m in height (Plates 1 and 2). They pass laterally into small-scale, trough cross-bedded units at channel margins. Channel bases are often incised up to 3 m depth into surrounding floodplain sediments and often contain a basal conglomerate (channel-lag) composed of rounded and sub-rounded rip-up clasts such as pedogenic ferralitic iron glaebules derived from the erosion of local red-bed palaeosols (Plate 9; see below). These locally sourced clasts pass up, by contrast, to sub-rounded granules and pebbles of quartz and pink feldspar derived from a more distant source (Plate 9). Individual channel sand-bodies are vertically stacked and show low-sinuosity with minor deviation of the principal channel axes (Figure 9).
Laterally Accreted Channels (Point Bars) (Lac)
Near the Dead Sea foreshore (Localities 7 and 8, Dyke Plateau; Locality 3; Plates 6 to 9) laterally accreted, fine- to medium-grained sandstones with grey, macroplant-rich claystones and dark grey organic-rich siltstones and immature coals infill broad, shallow channels cut into the overbank/floodplain red sandstone and siltstones (Plates 6b and c). Lateral point-bar migration is shown by multiple-stacked, low-angle laterally accreted units (Allen, 1965; Miall, 1977). The plant-rich and organic-rich siltstones and claystones were deposited during low flow regime conditions, and the exceptional preservation of much of the macroflora (Plate 11) indicates settlement from suspension in abandoned minor channels within the point-bar complex. Similar laterally accreted channel fills often associated with well-preserved, drifted plant beds within point-bar units (c.f. Dyke Plateau; Plate 7) have been described from the broadly coeval Unayzah Formation at outcrop in Central Saudi Arabia (El-Khayal and Wagner, 1985; Senalp and Al-Duaiji, 1995), and in the Gharif plant beds in the Gharif Formation, Oman (Broutin et al., 1995). In places, the upper part of the Umm Irna Formation shows evidence of horizontal sand-filled burrow networks (Plate 4). This suggests estuarine or brackish conditions within estuarine embayment during periods of relative high sea-level stands.
Interfluve Sediments (Is)
Sediments between, and adjacent to, the major channel sandstones consist of red-mauve, planar-laminated to ripple cross-laminated siltstone and fine-grained sandstone, with occasional silty mudstone, ranging in colour from red to red-mauve to yellow-grey (Plates 1, 2 and 6). The intense red coloration is a result of oxidation and brunification of these well-drained, flood-plain sediments in a seasonal humid climate (Besly and Fielding, 1989; Glover and Powell, 1996). Occasional desiccation cracks and indeterminate surface burrows are present in the red siltstones (Plate 11) indicating periodic emergence of the floodplain. These sediments make up the bulk of the Umm Irna Formation and are interpreted as heterolithic overbank sheet-flood deposits with localised shallow lakes and peaty mires (see Coals and Organic-rich Soils (Cors), below). Secondary pedogenesis is common in these sediments.
Ferralitic Palaeosols (Fps)
Red silty claystone and siltstone sediments that make up the bulk of the floodplain interfluve sediments between the yellow-grey channel sandstones contain pedogenic ferralitic palaeosol horizons (Duchafour, 1982; Wright, 1986; Retallack, 1988). These are present throughout the formation, but are more common in the upper part. They comprise yellow-brown, ferruginous (goethite-haematite) soil glaebules ranging in diameter from a few millimetres to a few centimetres within vari-coloured (red, yellow, mauve and grey) siltstone (Plates 10 and 11). Some of the palaeosol beds are associated with polygonal desiccation cracks, penetrating up to 0.20 m deep, with a yellow-brown weathered zone up to 0.01 m either side of the crack. Palaeosol horizons range in thickness from about 0.20 to 0.40 m in the lower part of the formation to deep, highly evolved palaeosols including vertisols up to 1.5 m thick in the upper part of the formation (Plate 11).
The palaeosols are interpreted as having formed diagenetically within the vadose zone of fine-grained, interfluvial floodplain deposits, in a tropical savanna climate, characterised by alternating wet and dry seasons, and strong leaching of iron (Makhlouf, 1987; Makhlouf et al., 1991; Powell and Moh’d, 1993).
The presence of deep polygonal desiccation cracks in some of the palaeosol horizons suggests that the climate was seasonally arid. Iron glaebules are characteristic of ferralitic palaeosols (Duchafour, 1982) in which concentric haematite (or in some cases goethite and/or limonite) forms as a result of intense cycles of wetting and drying in the ‘B’ horizon of well-drained oxidised soils (Retallack, 1988; Besly and Fielding, 1989; Glover and Powell, 1996). Although ferralitic soils commonly form in equatorial and humid tropical climates with a very short dry season, they also form in drier climatic zones, even savannas (Mohr et al., 1972). Haematite forms in preference to goethite in dry climates when organic matter is present in only small amounts (Duchafour, 1982; Wright, 1986).
Caliche soil horizons, which typically develop in semi-arid and arid climates with fluctuating groundwater levels, are not present in the Umm Irna Formation, but have been described in the broadly co-eval upper part of the Unayzah Formation of Central Saudi Arabia (Senalp and Al-Duaiji, 1995). Thick (deeper) palaeosol horizons in the Umm Irna Formation indicate a long residence time for palaeosol development on the floodplain, perhaps over hundreds or thousands of years, with low vertical sedimentation rates as compared to the thin poorly evolved palaeosol horizons in the lower part of the formation. Deep soil profiles showing red-brown to grey-yellow horizonation, together with both haematite glaebules and desiccation cracks indicate more evolved palaeosols similar to present-day lateritic soils, which develop in regions with seasonally fluctuating groundwater levels (Mohr et al., 1972).
Coals and Organic-rich Soils (Cors)
Although much of the Umm Irna Formation comprises oxidised red-beds and associated palaeosol horizons, it also contains a variety of beds rich in organic-matter formed in anaerobic or dysaerobic conditions. These include in-situ, poorly developed thin coals (ca. 0.05 m thick) some with downward-penetrating carbonaceous rootlets (up to 0.5 m depth) and others underlain by hard, pale grey seatearth sandstone beds (Plate 3b) interpreted as leached siliceous soil horizons underlying the peaty mire (Besly and Fielding, 1989; Glover and Powell, 1996). These thin coal beds developed as localised, short-lived peaty mires in the interfluves, and although they are more common in the lower part of the formation (Dill et al., 2010) (see Locality 1, type section Wadi Himara) they are also present in the upper part of the formation (Locality 6, Wadi Autun; Locality 8, Dyke Plateau), but here they are associated with, or in close proximity, to the larger fluvial channels. Samples of the plant debris-rich, coaly beds from Locality 9 (Plate 3b) proved to be barren of palynomorphs; this may be due to destruction of organic matter during pedogenesis.
Organic-rich Mudstones (Orm)
These comprise grey, grey-black and black claystones and siltstones with a high proportion of organic-matter in the form of palynomorphs and drifted, disseminated plant fragments, including some exceptionally well-preserved plant fossil assemblages (Kerp et al., 2006; Abu Hamad et al., 2008; Plates 7 and 11). In contrast to the in-situ coals and organic-rich soils described above, these are most commonly found as thin lag deposits draping laterally accreted foresets within point bar units (Plates 6 and 7), and within ponded-up abandoned channels either within the point bar complex or in abandoned channels (ox-bow channels) (Plate 11).
Tidal-estuarine Sediments (Tes)
Sediments deposited in tidal and estuarine palaeoenvironments in the Umm Irna Formation are uncommon (Figures 5 and 6; Plates 4 and 10). They comprise thin beds of red, yellow and grey siltstone and fine-grained sandstone beds, generally horizontally bedded with small-scale bedforms such as wave ripples, tidal bundles, and networks of horizontal circular burrows (Plate 4) (the latter often destroying any original sedimentary fabric), and surface burrow traces (Plate 10). Marine macrofossils have not been identified in these beds, but the bedforms, ichnofossils and absence of large-scale cross-bedding indicates deposition in tidal or estuarine embayments, during periods of sea-level rise (Figures 2 and 10).
DEPOSITIONAL SETTING AND COMPARISON WITH THE GHARIF AND UNAYZAH FORMATIONS
The depositional environments of the Umm Irna Formation have been described previously (Makhlouf et al., 1991; Powell and Moh’d, 1993; Uhl et al., 2007; Abu Hamad et al., 2008; Dill et al., 2010). The general consensus, supported by the current study, suggests deposition in a fluvial regime characterised by low-sinuosity channels with deposition on point bars, and as stacked small-scale braided channels (Figure 10). The depositional environments are similar to those described for the Gharif Formation alluvial plain ‘Type Environment P2’ in the subsurface in Oman (Osterloff et al., 2004) and the upper part of the Unayzah Formation at outcrop and in the subsurface in Central Saudi Arabia (Senalp and Al-Duaiji, 1995).
Umm Irna Formation floodplain interfluves were characterised by low-energy sheet-flood deposits, shallow lakes and ponds. Where there was no standing water, the floodplain sediments underwent primary reddening (‘brunification’) in an oxidising humid climate to semi-arid climate characterised by seasonal ground-water fluctuation, which resulted in the development of poorly- to highly-evolved ferralitic palaeosols depending on the residence time of the soil horizon (Figure 10).
In areas adjacent to the main channel axes, organic matter was preserved in shallow ponds and small lakes as drifted plant remains. The large size and exceptional preservation of much of the plant fossil material suggests that the plant communities thrived in, or adjacent to, these wetter areas (in contrast to the more oxidising condition on the interfluves). Plant fragments commonly preserved in yellow-grey, thin claystone and siltstone beds are interpreted as representing localised hydromorphic, waterlogged conditions (ponds) on the flood-plain. During wetter climatic phases and in wetter ecological niches adjacent to the ephemeral river channels, plant communities thrived to produce thin peaty mires with a reasonably long residence time, sufficient to produce thin immature coals or organic-rich mudstones with underlying rootlets beds and, in places, leached seatearth sandstones. Climatic and/or local microclimate conditions on the floodplain were sufficiently waterlogged and anaerobic/dysaerobic at times to preserve the organic matter.
There is evidence (see below) that the palynomorph communities were highly specialized and occupied ecological niches adjacent to the floodplain ponds (high abundance – low diversity communities). In other organic-rich claystone and siltstone beds the palynomorph and macroplant communities are more diverse, suggesting transport down-stream from a variety of plant community niches adjacent to the main river channels or in the broader river hinterland (Figure 10).
The fluvial red-bed sequences in the Umm Irna Formation (Jordan) and the upper Unayzah Formation at outcrop (Central Saudi Arabia) are similar, but there are notable differences in the development of palaeosol horizons in the interfluves. In addition to reddened (oxidised) palaeosol horizons in the outcrop upper Unayzah Formation (Delfour et al., 1982; El-Khayal and Wagner, 1985; Al-Laboun, 1987; Senalp and Al-Duaiji, 1995) the formation contains both caliche (calcrete) and nodular anhydrite/gypsum attributed to deposition in an arid to semi-arid environment.
These carbonate and sulphate pedogenic phases are absent from the Umm Irna Formation in Jordan. By contrast, the pedogenic processes are dominated in Jordan by localised deep vertical leaching (varicoloured vertisols), and iron (limonite/goethite) glaebules, often developed below coaly and or plant-rich siltstone beds. We attribute these variations in palaeosol type to the Umm Irna Formation depositional area being located further north, closer to the palaeoequator (Stampfli and Borrel, 2002) in a more humid climatic regime than that of the upper Unayzah where evaporation of groundwater was less intense, so that caliche, gypsum and anhydrite were not precipitated in the soil profile. Vertisols, locally developed up to 3 m depth (Plate 11a), in the Umm Irna Formation also indicate a humid, tropical climate with relatively long residence times for palaeosols to develop. Similar variations in vertisol and caliche palaeosol development attributed to climatic change on Pennsylvanian to Early Permian floodplains were noted by Glover et al. (1993) and Glover and Powell (1996). These variations in palaeosol development and the presence of horizontal burrows at some horizons in the Umm Irna Formation (Plate 4) might, however, reflect a broad depositional setting, with the burrowed, coaly succession in the Jordan succession having been deposited in a coastal, low gradient alluvial plain and the Central Saudi Arabia succession representing a more upland area with a drier climate. The burrowed horizons are associated with thin tidal clinoforms (Figure 10) and thin carbonaceous claystones and siltstones with abundant palynomorphs (Localities 3 and 4; Figures 5 and 6). We interpret these sequences as representing high sea-level stands that resulted in shallow-marine or brackish incursions across the low-gradient alluvial plain, probably within lower relief areas of abandoned channels. There is evidence that peaty mires develop preferentially on the floodplain during sea-level highs as a result of marine flooding and raised groundwater levels during high stands (Wright and Marriott, 1993; Glover and Powell, 1996).
A significant sea-level rise, heralding the marine Ma’in Formation, is marked by a sequence boundary (SB in Plate 6) near the top of the Umm Irna Formation. Above this boundary, channel sandstones are absent, and furthermore, level-bedded red-beds show evidence of wave ripples, faint burrow traces and occasional desiccation cracks; these characteristics suggest brief episodes of estuarine or brackish marine flooding in a marginal-marine embayment. Rising sea-level had the effect of lowering the geomorphological gradient, thus inhibiting channel incision. The sequence boundary overlies the most extensive low-sinuosity channels (Locality 7, Dyke Plateau) rather than braided channels, again indicating increasing sinuosity of the rivers during rising sea level (Wright and Marriott, 1993).
PALAEOFLOW AND PROVENANCE OF THE UMN IRNA FORMATION SANDSTONES
Palaeocurrent azimuths taken from the channel sandstones during this study, combined with those from the Al Mamalih area to the south (Powell and Moh’d, 1993) and in the north of the outcrop (Makhlouf et al., 1991) indicate sediment dispersal to the northwest and west-northwest (see above). This indicates a variable sediment dispersal pattern throughout the formation with a spread of about 60 degrees (Figure 9). Previous studies (Makhlouf et al., 1991; Powell and Moh’d, 1993; Dill et al., 2010) suggested that the likely provenance is a granitoid, igneous source since the coarse-grained channel sandstones include fresh, sub-rounded, pink feldspar granules. The immature, unweathered nature of the feldspar clasts suggests that, (a) the source area was not distant, and (b) the climate in the source area was semi-arid or arid in contrast to the more humid depositional floodplain basin. Heavy mineral analysis (Dill et al., 2010) of the channel sandstones indicates a zircon- and thorium-enriched monazite suite that they correlated with the alkaline granite suite exposed about 200 km to the south in the Arabian-Nubian Shield (McCourt and Ibrahim, 1990). Dill et al. (2010) concluded that the preponderance of stable to ultrastable heavy mineral assemblages suggest a rather long transport or, in contrast, chemical disintegration of labile minerals. They favoured the decomposition of labile constituents (presumably under humid climatic conditions) rather than a very distal source of zircon. However, this hypothesis does not account for the very fresh, unaltered feldspar granules (Plate 9) in some of the Umm Irna Formation channel sandstones which must have been deposited rapidly and not far distant from the presumed granitoid source area.
Furthermore, during the Permian any ‘Precambrian’ granitoid terrain (i.e. potential source area) in central Jordan located to the east and southeast of the outcrop as indicated by the palaeocurrent flow, would be overlain by a thick succession of Lower Palaeozoic sandstones (Ram Group; Cambrian to Ordovician in age; Powell, 1989) of predominantly fluvial origin, and further to the southeast in the Tabuk Basin these, in turn, are overlain by Ordovician to Silurian marine and glaciogenic siliciclastic sediments (Khreim Group; Powell, 1989). Consequently, near the Jordan-Saudi Arabia border in the Wadi Sirhan area, the granitoid basement rocks are buried to more than 5,000 m below this siliciclastic sedimentary cover.
The unweathered nature of the coarse-grained detrital feldspars in the Umm Irna Formation channel sandstones, which are considered to have been deposited in low-sinuosity channels in a humid to semi-arid climatic regime, suggests that the Umm Irna Formation sediments were not secondarily derived from mature siliciclastic Lower Palaeozoic cover rocks. Dill et al., (2010) favoured a provenance of Umm Irna channel sands from the granitoid, cratonic shield rocks which extend from Aqaba, in south Jordan, (Aqaba Complex; Ibrahim and McCourt, 1995) to the Midyan (Nadj) Terrain of Saudi Arabia. Sedimentary cover rocks may have been absent from parts of these shield areas due to non-deposition, or more probably, as a result of rapid erosion of the isostatically-buoyant, Arabian-Nubian granitoid terrain in mid- to late Palaeozoic times. The present-day disposition of these granitoid basement rocks indicates fluvial transport of at least 200 km. If the climate had been equatorial humid (see evidence above), then the feldspars might be expected to be extensively altered by chemical weathering. Furthermore, the palaeocurrent measurements from this study (Figure 9) and earlier publications (Makhlouf et al., 1991; Powell and Moh’d, 1993) indicate a pronounced westward flow with a source area located somewhere to the east or southeast, in current-day Central Saudi Arabia rather than a source area located to the south i.e. the present-day Aqaba granitoid shield. The source area may have been the Neoproterozoic granitoid rocks exposed on the Ha’il Arch located to the east or southeast (Figure 2b).
An alternative hypothesis envisages the immature pebbly, coarse-grained channel sandstone clasts being secondarily derived from pre-existing, relatively unaltered Pennsylvanian-Early Permian glaciofluvial outwash sandstones (e.g. distal outwash sandur-plain sediments) located somewhere to the east-south east. Extensive glaciogenic and glaciofluvial siliciclastics of Early Permian age (lower part of the Unayzah Formation) have been described from Central Saudi Arabia (Melvin and Sprague, 2006). They describe quartzose sandstones from the Unayzah B member with well-rounded very coarse-grained quartz sand and granules and pebbles including granite, quartz and feldspar laid down on an extensive glaciofluvial outwash braidplain upon the pre-Unayzah unconformity, which were later overidden by morainic deposits deposited by advancing ice-sheets associated with the end-Carboniferous–Early Permian glaciation. Thus, a source area located to the east (in relative terms prior to Arabian Plate rotation) is a better fit for the predominantly westerly palaeocurrent data.
In Central Saudi Arabia similar quartz-rich pebble sandstones are present in the Unayzah Formation, again unconformably overlying Lower Palaeozoic sandstones that, in turn, overlie granitoid basement rocks of the Arabian Shield (Senalp and Al-Duaiji, 1995). Here, north and northeast palaeoflow indicates dispersal away from the Neoproterozoic Shield.
Twenty-five samples were processed and of these only one was barren of palynomorphs. A few samples yielded less than ten palynomorphs, but generally samples yielded large populations of palynomorphs, which were moderately- to well-preserved (Plates 12 to 16). The assemblages are very variable but in general contain common non-taeniate bisaccate pollen (mainly fragmentary or too poorly preserved to be identified); those that are determinable include Falcisporites stabilis, Alisporites nuthallensis, A. indarraensis, and Cedripites priscus. The most common taeniate bisaccate pollen is Protohaploxypinus uttingii and P. limpidus. Monosaccate pollen is rare. The colpate pollen Pretricolpipollenites bharadwaji is common in a few samples. A group of ornamented monolete spores is very common in one sample.
Palynological Sample Descriptions
Locality 1: Wadi Himara South Side
Palynological samples from the lower claystone (Figure 3) yielded rather sparse assemblages dominated by indeterminate bisaccate pollen, Alisporites nuthallensis, Cedriptes priscus and Falcisporites stabilis. The assemblages also contain a few spores including the monolete spore taxa Laevigatosporites spp. and Thymospora cf. opaqua, and Playfordiaspora cancellosa. A single specimen of Pretricolpipollenites bharadwaji occurs in the lower sample.
Locality 2: Panorama Road (East)
The two lower samples (Figure 4) yielded rather sparse assemblages dominated by indeterminate bisaccate pollen but with recognisable Alisporites nuthallensis and Falcisporites stabilis. The upper samples were more diverse with indeterminate bisaccate pollen, Alisporites nuthallensis and Falcisporites stabilis, but also with the coarsely ornamented triangular spore Camptotriletes warchianus and the distally-taeniate pollen Distriatites insolitus.
Locality 3: Wadi Zerqa Ma’in South
The claystone and siltstone beds yielded four palynological samples (Figure 5), of which two yielded moderately well preserved diverse assemblages (at 3.0 and 3.8 m below the top of the section) containing Alisporites indarraensis, A. nuthallensis, bisaccate indeterminate pollen and Falcisporites stabilis. The lower of the two samples is the more diverse, containing Cristatisporites spp., Distriatites insolitus, Laevigatosporites cf. callosus and common Protohaploxypinus uttingii.
Locality 4: Side Wadi South of Wadi Zerqa Ma’in
A 2 m thick siltstone yielded two diverse and well-preserved assemblages (Figure 6), which contain common indeterminate bisaccate pollen, Alisporites nuthallensis and Falcisporites stabilis. The upper sample was unusual in relation to the Umm Irna Formation in containing common monosaccate pollen which, were in general too poorly preserved to allow determination, but probably consisted mainly of the genera Cannanoropollis and Potonieisporites. The lower sample contains a rather unusual assemblage of common probable algal cysts and spores (Leiosphaeridia spp. and Peroaletes khuffensis) as well as very abundant Protohaploxypinus uttingii. Both samples contain the distally-taeniate pollen Distriatites insolitus.
Locality 5: Roadside Section South of Wadi Zerqa Ma’in
A siltstone sampled twice (Figure 7) yielded well-preserved diverse assemblages dominated by indeterminate bisaccate pollen, Alisporites nuthallensis, Cedripites priscus and Falcisporites stabilis. The upper sample (at 3.7 m below the top of the section) contains common Leiosphaeridia spp., as well as the probable algal palynomorph Peroaletes khuffensis. The enigmatic, probable algal palynomorph Reduviasporonites chalastus occurs in small numbers in both samples, as does the monolete spore Thymospora opaqua.
Locality 6: Wadi Autun
Two carbonaceous siltstone beds, separated by about 5 m of sandstone and siltstone (Figure 8), yielded moderately well-preserved assemblages, which are quite distinct from each other (despite their stratigraphical proximity), and from other assemblages recovered from the Umm Irna Formation. The lower sample yielded an assemblage containing common bisaccate pollen (Cedripites priscus and Falcisporites stablis) but also the probable algal palynomorph Leiosphaeridia, and the trisulcate pollen Pretricolpipollenites bharadwaji. The latter is very well preserved. The upper sample, 4.35 m above, is quite different in composition, and slightly more diverse, but contains no specimens of Pretricolpipollenites bharadwaji, and is dominated by well-preserved spores, tentatively assigned to the ’Torispora-Thymospora complex’. This sample is the most spore-dominated of all the Umm Irna samples. Other spores in this assemblage include Punctatisporites spp. and the probable lycopsid spore Playfordiaspora cancellosa.
Localities 7 and 8: Dyke Plateau
The cliff section (Locality 7; Figure 9) includes a laterally accreted channel sandstone-siltstone-claystone complex with evolved palaeosols. Low-angle lateral accretion surfaces in the complex are draped by thin beds and laminae of grey claystone and black organic-rich siltstone which contain well-preserved macrofossils and coalified logs. Black organic-rich mudstones were sampled (Plate 7; MPA 62259 to MPA 62261) which yielded diverse very similar assemblages dominated by indeterminate bisaccate pollen, Alisporites nuthallensis and Falcisporites stabilis. However these samples are amongst the most diverse of those recorded from the Umm Irna Formation and are interesting in that they derive essentially from the same horizon, being separated by approximately 5 m laterally along the cliff (Plate 6). Other taxa present include common Protohaploxypinus spp., and other indeterminate taeniate bisaccate pollen, Lueckisporites virkkiae and similar forms, Reduviasporonites chalastus, Potonieisporites spp., Thymospora opaqua and Playfordiaspora cancellosa. In all three samples, non-taeniate bisaccate indeterminate pollen make up more than 75% of the assemblages, and taeniate bisaccate pollen is dominant amongst the rest of the assemblages.
Locality 8 lies in a small east-west side wadi approximately 300 m south of Locality 7. Here, an abandoned channel infilled with grey to black organic-rich siltstone and mudstone (Figure 9) provided samples MPA 62262 to 62264. MPA 62262 and 62264 were very sparse, containing only a few poorly preserved bisaccate pollen but MPA 62263 yielded a diverse assemblage containing very common Cedripites priscus, Pretricolpipollenites bharadwaji and Reduviasporonites chalastus as well as the Torispora-Thymospora complex (as seen in the upper sample of Locality 6, Wadi Autun). Also present are Leiosphaeridia spp., Taeniaesporites spp., and Thymospora opaqua.
Variation in the Palynology of the Umm Irna Formation
The most common type of palynological assemblage in the Umm Irna Formation is one dominated by non-taeniate bisaccate pollen (usually indeterminate due to fragmentary preservation), as well as by Alisporites nuthallensis, A. indarraensis and Falcisporites stabilis. Five out of the 25 samples processed yielded this assemblage. However, surprising variation was also observed between samples separated by only a few metres stratigraphically and geographically. Taxa that might be considered to be stratigraphically significant: Alisporites nuthallensis, Cedripites priscus, Distriatites insolitus, Falcisporites stabilis, Playfordiaspora cancellosa, Pretricolpipollenites bharadwaji, Protohaploxypinus uttingii, Reduviasporonites chalastus, and Thymospora opaqua, show no particular confinement to parts of the Umm Irna Formation (Table 2), and thus at present it seems futile to attempt a biozonation of the unit. However the variation that is present does require an explanation.
The 25 palynological samples were taken from thin (usually less than 2 m) often laterally impersistent argillaceous units in a sandstone-dominated sequence. Sandstones do not preserve palynological assemblages, thus the sequence of argillaceous units that preserved palynomorphs represents a series of snapshots of the evolving palaeoenvironments of a fluvial regime characterised by low-sinuosity channels with deposition on point bars, and in stacked small-scale braided channels, as well as localised peaty mires on the floodplain.
These snapshots show surprising variability. An example is that from Locality 7, Dyke Plateau, where two argillaceous units were sampled, both from close to the top of the Umm Irna Formation. Samples MPA 62259-62261 were from a single argillaceous horizon (Plate 7) containing abundant plant fossils, which is part of a laterally accreted channel sandstone- siltstone-mudstone complex. The unit, which is approximately 5 m wide and < 0.5 m thick, was deposited adjacent to the main channel axes, and the organic matter was preserved in shallow ponds and small lakes as drifted plant remains. Plant fragments commonly preserved in yellow-grey, thin claystone and mudstone beds are interpreted as representing localised hydromorphic, waterlogged conditions (ponds) on the floodplain. Assemblages from MPA 62259-62261 are almost identical in terms of the range of taxa and their relative abundances, as might be expected from samples taken a few metres apart at the same level in the same unit. However MPA 62263, from an argillaceous unit a few metres higher in the formation, and from 300 m to the south (Figure 9), yielded a very different assemblage containing very common Cedripites priscus, Pretricolpipollenites bharadwaji and Reduviasporonites chalastus.
A similar example of upsection variation between samples separated by only a few metres is shown in Locality 6, Wadi Autun, where two otherwise similar argillaceous units stratigraphically separated by 4.35 m yielded completely different palynological assemblages. The upper assemblage is made up almost entirely of ornamented monolete spores assigned here to the Torispora-Thymospora complex, the lower is dominated by Pretricolpipollenites bharadwaji. Similarly Locality 4 yielded two samples both from thin argillaceous horizons 1 m apart. The lower assemblage contains a rather unusual assemblage of common probable algal cysts and spores (Leiosphaeridia spp. and Peroaletes khuffensis) as well as very abundant Protohaploxypinus uttingii (a pollen grain recorded only sporadically from the other Umm Irna Formation samples, and rather rarely from Arabian and Pakistan sections (e.g. Stephenson and Filatoff, 2000; Jan et al., 2009); the upper contains a more conventional Umm Irna Formation assemblage with common Alisporites nuthallensis and Falcisporites stabilis.
Variation of this type could be a result of evolutionary or taphonomic processes and changing palaeoenvironment; or could relate to the way that sedimentary processes and local sedimentary environments reflect differing parts of the regional plant community. Variation due purely to evolutionary change is unlikely to be the cause because the time elapsed during and between the deposition of the argillaceous beds is likely to have been small. In addition most of the taxa concerned are known to range throughout the Mid- and Late Permian period and so their local appearances and disappearances need another explanation. The assemblages from Localities 4, 6 and 7 are also well preserved, without obvious signs of preservational bias. Rapid changes in palaeoenvironment are possible, and perhaps could have generated the observed variation. Kerp et al. (2006) considered that the lower part of the Umm Irna Formation had a Gondwanan affinity, because of its Dicroidium fossils, whereas the upper part may have a Cathaysian affinity (based on plant fossils described by Mustafa, 2003). However it seems unlikely that repeated radical palaeonvironmental changes could occur in a timescale consistent with the stratigraphic distribution of the argillaceous beds.
A more likely explanation for variation is sedimentary process and local sedimentary environment (Figure 10). It is known that rivers are the most important transporters of palynomorphs (see for example Muller, 1959). The diverse low-sinuosity channel, point bar, and small-scale braided channel environments of the Umm Irna Formation are likely to have been similarly diverse in terms of water and nutrient availability and substrate type, which may have led to numerous small and varied plant communities across the floodplain. Larger water flows in channels would likely contain a palynomorph representation of the wider hinterland of the drainage basin of the river including these floodplain plant communities and more distant communities, and sediments deposited with this assemblage of palynomorphs would reflect in their variety something of the variety of the full drainage basin. However where water is cut off from the river system as in water bodies like oxbow lakes or other impermanent stagnant floodplain ponds, a higher proportion of purely local palynomorphs would be preserved in associated sediments. These would be the spores and pollen of plants growing very close to the water body. Taking Locality 6, Wadi Autun, as an example, the lower sample is from a black carbonaceous, almost coaly siltstone that is overlain by trough cross-bedded sandstone with an erosive base (Figure 8). This unit may have been deposited in a stagnant environment isolated from water flow that encouraged the plant that produced the pollen Pretricolpipollenites bharadwaji, possibly a cycad. Thus its pollen is well represented in the sediment. The upper sample from a carbonaceous siltstone overlying a mudstone- and siltstone-filled abandoned channel, may represent a similarly restricted local environment perhaps surrounded by a stand of plants producing ornamented monolete spores, probably tree ferns.
Numerically the most important taxa reported from the assemblages are indeterminate bisaccate pollen, Falcisporites stabilis and Alisporites nuthallensis. The first category probably consists mainly of poorly preserved specimens of the latter two taxa. Falcisporites stabilis, the most common identifiable palynomorph in the assemblages, is considered by Kerp et al. (2006) to have been produced by the corystosperm Dicroidium, the most common plant fossil from the Wadi Himara macroflora (Kerp et al., 2006). The Wadi Himara macroflora as a whole was considered by Kerp et al. (2006) to be extrabasinal, but Abu Hamad (2004) considered that Dicroidium of Wadi Himara which are preserved often as large delicate frond fragments, were probably not transported far, and therefore grew in around the rivers and ponds of the floodplain.
Alisporites, still less A. nuthallensis, is more difficult to relate to a single plant genus or even family, but like Dicroidium it was probably gymnospermous and may have occupied a similar palaeoenvironment. Mustafa (2003) recorded Lobatannularia, Gigantonoclea and Pecopteris from the upper Umm Irna Formation. Balme (1995) does not list the plant-palynomorph relationships of Lobatannularia and Gigantonoclea, but it is likely that at least a few species of Pecopteris produced monolete spores. Taken as a whole the varied and rich Umm Irna Formation palynomorph assemblages could have been produced by a range of plants including Glossopteris (which produced taeniate bisaccate pollen, see Playford, 1990), Cyclodendron (which produced Indotriradites, see Beeston, 1990), and Walchia (Potonieisporites, see Balme, 1995). A tentative reconstruction of the palaeoevironments and plant communities is shown in Figure 10.
On plant fossil evidence Kerp et al. (2006) considered that the lower part of the Umm Irna Formation has a Gondwanan affinity, while the upper part is of Cathaysian affinity. The present study has also shown this variation of affinity in palynological assemblages. For instance, the upper palynological sample of Locality 6, Wadi Autun, which represents the upper part of the Umm Irna Formation is dominated by monolete spores and therefore might be considered to have a Cathaysian affinity (e.g. Gao, 1985), while the lower Wadi Autun sample containing common Pretricolpipollenites bharadwaji is of unknown affinity. Other assemblages of the upper part of the Umm Irna Formation from Dyke Plateau are Gondwanan in aspect (following Kerp et al., 2006). This serves to indicate the dangers of theorizing widely from core or single outcrop studies.
The phenomenon of sedimentary setting influencing palynological assemblages has also been identified in Oman. Stephenson (2011) identified similarly distinct assemblages in sequences of Subunit B of the Upper Member of the Gharif Formation of similar age and depositional environment to the Umm Irna Formation. The palaeoenvironment of Subunit B in Oman consisted of a system of coalescent and sandy estuarine point-bars (Berthelin et al., 2006), on which a rich flora of Gondwanan glossopterids and lycopsids, Euramerican conifers, and Cathaysian gigantopterids, lycopsids and Noeggerathiales grew. The argillaceous parts of Subunit B were considered by Broutin et al. (1995) to represent ‘swamp claystone’ and crevasse splay deposits suggesting impersistent and stagnant water bodies. These contain unusual assemblages of spores as well as freshwater non-haptotypic palynomorphs (probably the zygospores of zygnemataceaen alga) quite unlike more typical Upper Gharif Member assemblages (see Stephenson, 2011). This may be because the ‘swamp claystones’ were formed in impersistent and stagnant water bodies more likely to represent a local flora than a regional flora.
The assemblages contain the following distinctive taxa that have been recorded from well-dated sections and allow possibilities for correlation within the region and with the standard Permian stages: Distriatites insolitus, Lueckisporites virkkiae, Playfordiaspora cancellosa, Protohaploxypinus uttingii and Thymospora opaqua. Of these the most important is the distinctive bitaeniate bisaccate pollen Lueckisporites virkkiae, which occurs first in the lower part of the Kazanian (Wordian) in its type area in the Russian Platform (Utting et al., 1997).
In Europe, where independent palaeontological evidence is available, its last occurrence is close to the equivalent of the Permian/Triassic Boundary (Visscher and Brugman, 1986). Independent confirmation of a Wordian first occurrence for Lueckisporites virkkiae in the Gondwana palaeophytogeographic province comes from radioisotopic dating of the Argentinian Striatites Biozone (Archangelsky and Vergel, 1996), at the base of which that taxon makes its first appearance. Melchor (2000) reported a radioisotopic date of 266.3 ± 0.8 Ma (Wordian) for the base of the Striatites Biozone.
The presence of Distriatites insolitus, Playfordiaspora cancellosa and Thymospora opaqua allows correlation with the Arabian Oman and Saudi Arabia Palynological Zones (OSPZ) biozones because all three first appear at the base of OSPZ5 (Stephenson et al., 2003; Stephenson, 2006, 2008), however Florinites? balmei, the zone indicator for the succeeding OSPZ6 biozone does not occur in the samples, suggesting either that the Umm Irna Formation assemblages correlate with OSPZ5 which is Roadian–Wordian in age, or that Florinites? balmei is absent for some reason. However, the sometimes abundant presence of the multitaeniate pollen Protohaploxypinus uttingii suggests that the assemblages probably extend in age above the Roadian–Wordian. Protohaploxypinus uttingii occurs first in OSPZ6 (Stephenson et al., 2003; Stephenson, 2006). It is entirely absent from the Gharif Formation in Oman, occurring only in the overlying Khuff Formation (unpublished Petroleum Development Oman reports). In Central Saudi Arabia, the taxon is sometimes common, and is very distinctive, being the smallest multi-taeniate pollen in the Arabian Permian sequence with a characteristically shrunken intexinal body (see Plate 12). It occurs in the Central Saudi Arabian Dilam-1, Nuayyim-2 and Haradh-51 wells in the basal Khuff clastics (Stephenson and Filatoff, 2000). Extensive subsurface studies in Saudi Arabia have established the upper limit of the range of Protohaploxypinus uttingii as immediately above the Middle Khuff Anhydrite unit (Khuff D Anhydrite) (personal communication, Nigel Hooker, 2012), which suggests an age range for Protohaploxypinus uttingii of Wordian–Capitanian to early Wuchiapingian (Mid- to early Late Permian). This suggests a similar total age range for the Umm Irna Formation.
The assemblages of the Dilam-1, Nuayyim-2 and Haradh-51 wells are also similar in quantitative and qualitative terms to those of the Umm Irna Formation. Non-taeniate bisaccate pollen is the most common form in all the sections, and Alisporites nuthallensis, Lueckisporites virkkiae, Distriatites insolitus, Playfordiaspora cancellosa, Thymospora opaqua and Reduviasporonites chalastus occur in Dilam-1 and Nuayyim-2 (Stephenson and Filatoff, 2000). Haradh-51 contains all but Playfordiaspora cancellosa and Thymospora opaqua. Quantitative data for key taxa in Dilam-1 are shown in Figure 11.
The geographically closest sequences to those of the Umm Irna Formation studied in detail for palynology are from boreholes in the eastern and southern West Bank, west of the Dead Sea. Eshet and Cousminer (1986) and Eshet (1990) studied assemblages from eleven boreholes including the Makhtesh Qatan-1 Well, from which the only Permian core material came (Eshet and Cousminer, 1986). The Makhtesh Qatan-1 Well is situated approximately 100 km southwest of the southern limit of the Dead Sea, but the restored position before movement of the Dead Sea Fault would put the well rather close to the Umm Irna Formation outcrop on the east shore of the Dead Sea. Eshet and Cousminer (1986) processed samples from cores 10 to 19 within what they regarded as the Permian sequence. These cores span the Sa’ad and Arqov formations. According to Eshet (1990), the Sa’ad Formation in Makhtesh Qatan-1 Well consists of 80 m of sandstones, coaly shales with plant remains, and rare limestone and dolostone. The Arqov Formation consists of 194 m of sandstone, shale and carbonate units interpreted to have been deposited in a near-shore marine environment.
Within the lower part of the section in Makhtesh Qatan-1 Well, Eshet and Cousminer (1986) give palynological details only of cores 14 and 15 just above the top of the Sa’ad Formation. Cores 16 to 19 from within the Sa’ad Formation appear to have been barren or contained no notable palynomorphs (see Eshet and Cousminer, 1986; their text-figure 3). Within the upper part of the Arqov Formation two levels appear to have yielded palynomorphs (at depths of 2,099 m and 2,084 m). Cores 14 and 15 yielded amongst others Potonieisporites novicus, Falcisporites (Alisporites) nuthallensis, Hamiapollenites(Distriatites) insolitus, Vittatina spp., Lueckisporites virkkiae and Falcisporites stabilis, suggesting that this level in the lowest Arqov Formation is palynologically similar to the Umm Irna Formation. Wireline log and lithological interpretations by Eshet and Cousminer (1986; their text-figure 2) further suggest close similarity between the transition between the Sa’ad and Arqov formations in the West Bank and the transition between the Umm Irna and Ma’in formations in Jordan.
Correlations with the Salt Range sequence in Pakistan are hampered by the lack of coverage through the sequence due to the great thickness of limestone (which is inimical to the preservation of palynomorphs). Balme (1970) mainly documented samples from the Changhsingian (latest Permian) Chhidru Formation, and the underlying Wargal and Amb formations (Wuchiapingian and Wordian respectively, see Wardlaw and Pogue, 1995), were represented by only five samples. Similarly Hermann et al. (2012) documented samples only from the Chhidru Formation. Jan et al. (2009) documented ?Wordian assemblages from the Sardhai Formation underlying the Amb Formation in the Salt and Khisor ranges and correlated the formation with the Oman basal Khuff clastics and with part of the Saudi Arabian basal Khuff clastics, suggesting also that the basal Khuff clastics was diachronous, younging to the north through the Arabian Plate (see Al-Jallal, 1995; Jan et al., 2009; Figure 5; Le Nindre and Lasseur, 2012).
The assemblages of the Sardhai Formation are similar to those of the Umm Irna Formation in containing common non-taeniate bisaccate pollen and taxa such as Alisporites indarraensis, A. nuthallensis, Camptotriletes warchianus, Distriatites spp., Lueckisporites virkkiae, Protohaploxypinus uttingii and Thymospora opaqua; however Florinites? balmei, which is common in the Sardhai Formation does not occur in the Umm Irna Formation. The converse is true of Falcisporites stabilis. The variation in quantitative character seen in parts of the Umm Irna Formation seems also to be present in the Sardhai Formation with taxa such as Florinites? balmei and Protohaploxypinus uttingii being particularly variable in number.
Palynological determinations (Keegan, unpublished reports quoted in Andrews, 1992) reported a late Early to Mid-Permian age (Kungurian to Kazanian) for the Buwayda Formation proved in Well ER-1-A in north Jordan. It was not possible to obtain these unpublished reports; however another unpublished NRA report (Gupta, 1986) indicates the presence of Lueckisporites virkkiae, taxa similar to Thymospora opaqua, Alisporites nuthallensis and Camptotriletes warchianus; as well as Laevigatosporites spp. and Taeniaesporites spp. in the same well. This suggests that the Permian rocks of ER-1-A are of similar age to the Umm Irna Formation. The paralic, siliciclastic Buwayda Formation, overlain unconformably by Lower Triassic rocks is therefore considered to be broadly coeval with the Umm Irna Formation at outcrop. If this correlation is correct, the Early Permian marine and siliciclastic formations underlying the Buwayda Formation in north Jordan are absent along the Dead Sea outcrops. Tentative relationships between formations within Jordan and the wider Tethyan area are shown in Figure 12.
The Permian Umm Irna Formation of the eastern Dead Sea margin represents a northerly extension of the hydrocarbon-bearing Unayzah and Khuff formation sequences of the Arabian Plate. Until now this formation has not been studied in the context of the better known sequences of Central and southern Arabia, which are constrained by a comprehensive and rigorously-tested biostratigraphical framework. Sedimentological field studies and subsequent palynological analysis of the Umm Irna Formation has defined a series of lithofacies associations and confirmed the suggestion of previous authors that the formation represents deposition in a fluvial regime stacked small-scale braided channels (lower part) and by low-sinuosity channels with deposition on point bars. We interpret the upward gradational transition from low- to high-sinuosity channels and the higher proportion of laterally accreted channels with drifted plant beds and clay-filled abandoned channels, as a response to increasing accommodation space on the low-gradient alluvial plain, during Late Permian sea-level rise.
The dimensions and internal architecture of Umm Irna Formation channel-sands and their overall depositional environments are similar to those described for the Gharif Formation alluvial plain ‘Type Environment P2’ in the subsurface in Oman and the Unayzah Formation at outcrop in Central Saudi Arabia. Umm Irna Formation floodplain interfluves were characterised by low-energy sheet-flood and crevasse splay deposits with shallow lakes and ponds. Where there was no standing water, the floodplain sediments underwent primary reddening (‘brunification’) in an oxidising humid to semi-arid climate characterised by seasonal ground-water fluctuation, which resulted in the development of ferralitic palaeosols.
The presence of immature pebbly, coarse-grained channel sandstone clasts was previously explained by erosion of Neoproterozoic granitoid hinterlands (e.g. Dill et al., 2010). However palaeocurrent directions and new evidence from Central Saudi Arabia (Melvin and Sprague, 2006) suggests the possibility of secondary derivation from pre-existing, relatively unaltered Pennsylvanian–Early Permian glaciofluvial outwash sandstones located to the east-southeast.
The palynology of the Umm Irna Formation is remarkably varied, and this may reflect depositional processes and the varied palaeocommunity patterns on the floodplain. Samples from argillaceous beds associated with flowing water (laterally accreted point bars and channel sands) appear to contain a palynomorph representation of the wider hinterland of the drainage basin of the river including drifted, floodplain plants and more distant communities. However, where water is cut off from the river system as in standing water bodies such as oxbow lakes or other impermanent stagnant floodplain ponds, where peaty mires developed, a higher proportion of purely local palynomorphs appear to be preserved in associated sediments.
The abundant and well-preserved palynomorphs allow improved correlation and dating of the Umm Irna Formation. The presence of Protohaploxypinus uttingii suggests a total age range of Wordian–Capitanian to early Wuchiapingian (Mid- to early Late Permian) for the Umm Irna Formation. The quantitative character of the Umm Irna Formation assemblages is very close to those of the basal Khuff clastics in the Central Saudi Arabian wells Dilam-1, Nuayyim-2 and Haradh-51. The lithological character and palynology of the transition between the Sa’ad and Arqov formations in the West Bank, west of the Dead Sea are similar to those of the transition between the Umm Irna Formation and overlying Ma’in Formation in Jordan.
We would like to thank Issa Makhlouf, Basem Kahlil Moh’d and Ahmed Smaadi for their help and companionship in the field. We are grateful to Bassam Tarawneh, Natural Resources Authority (NRA) for making available unpublished reports of the NRA. Palynological samples were prepared by Jane Flint of BGS. Thanks go to Robert Knox and Colin Waters (BGS) for their comments on an early version of the text, and to three anonymous reviewers. The authors thank GeoArabia’s Production comanager Nestor “Nino” Buhay IV for designing the manuscript for press. Mike Stephenson and John Powell publish with the permission of the Executive Director of the British Geological Survey (NERC).
LIST OF PALYNOMORPH TAXA RECORDED
?Kingiacolpites subcircularis Tiwari & Moiz 1971
Alisporites cf. nuthallensis Clarke 1965
Alisporites indarraensis Segroves 1969
Alisporites nuthallensis Clarke 1965
Indeterminate bisaccate pollen
Camptotriletes warchianusBalme 1970
Cedripites priscusBalme 1970
Densipollenites indicus Bharadwaj 1962
?Dictyotriletes aules Rigby 1977
Distriatites insolitus Bharadwaj & Salujah 1964
Falcisporites stabilisBalme 1970
?Guttulapollenites hannonicus Goubin 1965
Laevigatosporites cf. callosusBalme 1970
Lueckisporites virkkiae Potonié & Klaus emend.
Lueckisporites cf. virkkiae Potonié & Klaus emend.
Indeterminate monosaccate pollen
Nuskoisporites sp. R (of Stephenson unpublished PhD thesis)
Peroaletes khuffensisStephenson and Filatoff 2000
Pilasporites calculus Balme & Hennelly 1956
Platysaccus queenslandi de Jersey 1962
Playfordiaspora cf. cancellosa (Playford & Dettmann)
Maheshwari & Banerji 1975
Playfordiaspora cancellosa (Playford & Dettmann)
Maheshwari & Banerji 1975
Plicatipollenites malabarensis (Potonié & Sah) Foster 1975
Potonieisporites novicus Bharadwaj 1954
Protohaploxypinus uttingiiStephenson and Filatoff 2000
Protohaploxypinus amplus (Balme & Hennelly) Hart 1964
Protohaploxypinus microcorpus (Schaarschmidt)
Protohaploxypinus limpidus (Balme & Hennelly)
Balme & Playford, 1967
Punctatisporites gretensis forma minor Hart 1965
Reduviasporonites chalastus (Foster) Elsik, 1999
Reduviasporonites sp. B (of internal Petroleum Development Oman reports)
Spinose acritarch species
Taeniaesporites pellucidus (Goubin) Balme 1970
Indeterminate taeniate bisaccate pollen Thymospora cf. opaqua Singh 1964
Thymospora opaqua Singh 1964
Pretricolpipollenites bharadwajiBalme 1970
ABOUT THE AUTHORS
Mike Stephenson is Head of Science (Energy) at the British Geological Survey (BGS), Nottingham, United Kingdom. His education has included a BSc, MSc and PhD from Imperial College and University of Sheffield (UK), and various postgraduate teaching qualifications. Mike is an expert on the stratigraphy of the Middle East, and he has published around 30 papers on this region as well as working extensively as a consultant for oil companies in the area. He is a Fellow of the Geological Society, sits on the Petroleum Group Committee of the Geological Society and is a member of the Petroleum Exploration Society of Great Britain (PESGB). He was Secretary-General of the Commission Internationale de Microflore du Paléozoique (CIMP) between 2002 and 2008, and is presently Editor-in-Chief of the Elsevier science journal Review of Palaeobotany and Palynology. Mike Stephenson is an Honorary Professor at the universities of Nottingham and Leicester.
John H. Powell was formerly Chief Geologist, England, with the British Geological Survey (BGS). He gained his BSc and PhD at the University of Newcastle upon Tyne, UK. John has over 30 years professional experience in sedimentology, applied geology and geological mapping in the UK and internationally. He has worked with the Natural Resources Authority, Jordan, on mapping, sedimentology and basin analysis of the Phanerozoic succession, especially the Lower Palaeozoic and Cretaceous – Eocene sequences. John was BGS Regional Geologist for the Middle East and Africa from 1998 to 2000, and has worked in Syria, Morocco, Mauritania and Mozambique. He serves on the Geological Society of London Accreditation Committee and GSL Stratigraphy Commission. He was recently awarded the John Phillips Medal by the Yorkshire Geological Society for stratigraphic research.