Palynology and alluvial architecture in the Permian Umm Irna Formation, Dead Sea, Jordan

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.