Palynology and Alluvial Architecture in the Permian Umm Irna Formation, Dead Sea, Jordan

GeoArabia, 2013, v. 18, no. 3, p. 17-60 Gulf PetroLink, Bahrain Palynology and alluvial architecture in the Permian Umm Irna Formation, Dead Sea, Jordan Michael H. Stephenson and John H. Powell ABSTRACT 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. INTRODUCTION 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). 17 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/18/3/17/4567630/stephenson.pdf by guest on 26 September 2021 Stephenson and Powell 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). GEOLOGICAL SETTING 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 (in Bandel and Khoury, 1981); this age was supported in similar studies of samples from higher in the Umm Irna Formation (Armstrong, in Makhlouf, 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 18 18 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/18/3/17/4567630/stephenson.pdf by guest on 26 September 2021 Permian Umm Irna Formation, Jordan 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 in Andrews, 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 in Andrews, 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

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