Ediacaran Araba Complex of Jordan

Abstract

The Ediacaran Araba Complex in Jordan is defined and described for the first time in lexicon style, with an emphasis on the sedimentary, volcanic and volcaniclastic units outcropping adjacent to Wadi Araba, and from seismic and deep exploration well data. The Araba Complex ranges in age from ca. 605 to 550 Ma and comprises a major cycle of sedimentary, volcanic and volcaniclastic, and igneous rocks emplaced in an overall extensional tectonic regime that followed intrusion and amalgamation of the granitoid and metamorphic Aqaba Complex, a part of the Gondwanan Arabian-Nubian Shield (ANS; ca. 900 to 610 Ma). The Araba Complex is bounded by two major erosional unconformities, the newly defined Ediacaran Araba Unconformity (ca. 605 Ma) at its base, underlain by the Aqaba Complex, and the post-extensional, regional lower Cambrian Ram Unconformity (ca. 530 Ma) that is marked by the widespread deposition of thick alluvial and marginal-marine siliciclastics (Ram Group). Two sub-cycles can be recognised in the Araba Complex mega-cycle. The earliest (Safi Group) followed suturing and extensional rifting of the Aqaba Complex that resulted in rapid basinal subsidence and the deposition of coarse-grained, polymict conglomerates (Saramuj Formation) in predominantly proximal, but evolving to more distal, alluvial fan settings. The early extensional basin appears to have been orientated approximately north-south (depocentre to the west) and can be traced from north Sinai to Lebanon, approximately parallel to the present-day Dead Sea Transform. Rounded clasts, up to boulder-size, include a variety of local to regionally-derived basement lithologies, including granites, diorites, metamorphic rocks; doleritic and rhyolitic dyke rocks. Rapid isostatic uplift and weathering of the granitoid basement resulted in high sediment flux that kept pace with rapid basin subsidence; this, in turn, led to erosion and partial peneplanation of the hinterland ANS. Regional detrital zircon ages from the conglomerate clasts and matrix indicate age ranges from ca. 650 to 600 Ma with a minor cluster between 750 to 700 Ma, indicating mostly a local or, at least, near-field provenance. Subsequent to this early, rapid basin-fill, continued crustal extension resulted in tapping of rhyolitic and basaltic effusive volcanics and volcaniclastics (Haiyala Volcaniclastics and Museimir Effusives, ca. 598–595 Ma), including flow-banded rhyolitic lavas and air-fall tuffs, the latter deposited in a lacustrine or shallow-water environments. The second Araba sub-cycle (595–586 Ma) is characterised by renewed basinal subsidence, very low burial metamorphism to about 6 km depth, and associated stock-like intrusion of the Qunaia Monzogabbro (595 ± 2 Ma) that resulted in thermal contact metamorphism of the Saramuj conglomerate, as well as granite plutons (e.g. Feinan-Humrat intrusions) and dolerite dykes. The second cycle is characterised by renewed extension, rifting and the deposition of volcanic rocks, agglomerates (Aheimir Volcanics) and monomict conglomerates (Umm Ghaddah Formation) that were sourced, locally, from volcanic rocks on the rift margins. To the east, in the sub-surface of south-central Jordan, the early Safi sub-cycle is absent. Deep exploration wells and seismic data in the Jafr area demonstrate that the Araba Complex comprises terrestrial lavas (Ma’an Formation) with weathered soil horizons, unconformably overlying weathered Aqaba Complex granitic basement (Araba Unconformity). Seismic data in the Jafr region records the eruption of lavas in north-south trending graben and half-graben settings, and possible northwest-trending bounding faults similar to the Najd basins in Saudi Arabia. Again, in contrast to the outcrop areas to the west, the upper part of the Araba Complex, hereabouts, consists of fine-grained, in part carbonate-cemented sandstone and claystone, together with anhydrite (Jafr Formation) suggesting a shallow-marine or coastal sabkha setting, and a possible link to similar shallow-marine extensional basin-fills that developed widely within NW-trending Najd basins across the ANS in Saudi Arabia (e.g. Jibalah and Antaq basins). To date, no Ediacaran biotas have been described from the Araba Complex, but the Jafr Formation, which post-dates the appearance of soft-bodied faunas around 579 Ma, and which was probably deposited in marginal-marine environments, is a potential candidate for these enigmatic fossils. Subsequent to the final Araba extensional rifting phase, renewed regional uplift, far to the south, of the ANS hinterland during the early Cambrian, led to widespread deposition of alluvial and shallow-marine siliciclastics as a progradational ‘sand-sea’ (Ram Group) that blanketed the now peneplained Aqaba Complex in south Jordan and surrounding countries (Ram Unconformity). However, the younger Ediacaran Araba Complex outcrops adjacent to Wadi Araba remained, in places, as a relatively immature palaeotopography. It was not until early mid-Cambrian times (ca. 509 Ma), during the Burj marine transgression that this late Ediacaran palaeotopography was finally buried. The Araba Complex in Jordan with its multi-cycle development provides an insight to the regional development of Ediacaran extensional basins in the Arabian-Nubian Shield, an important phase in the evolution and transition from Neoproterozoic to Phanerozoic crustal tectonics and associated basin-fill.