Cambrian stratigraphy of Jordan

ABSTRACT The lower and middle Cambrian succession (Ram Group) in Jordan is described in lexicon-style format to document an important phase of Earth history following the uplift and erosion of the Arabian-Nubian Shield (Aqaba Complex) during the late Neoproterozoic, and younger, but more localised, intrusive and volcanic/volcaniclastic activity that formed the Araba Complex. The early Cambrian Ram Unconformity (ca. 530 Ma) marks the base of a predominantly fluvial siliciclastic succession derived from rapidly eroding Neoproterozoic (including Ediacaran) basement rocks, but includes a brief, but biostratigraphically significant, sequence of marine siliciclastics and carbonates, the early mid-Cambrian Burj Formation. Rapid uplift and erosion of the granitoid basement (Arabian-Nubian Shield or ANS) resulted in a peneplanation of the Aqaba Complex over millions of years duration (latest Neoproterozoic to Cambrian) in the Southern Desert of Jordan. Early Cambrian pebbly sandstones and locally derived conglomerates (Salib Formation) were deposited on an alluvial plain by high velocity-high discharge, northward flowing (NNE to NNW) braided rivers, characterised by trough cross-bedding and erosive tabular sets. Brief, and rare, marine influence is represented, locally, by thin Skolithos-burrowed sandstones. A regional sea-level rise in the early mid-Cambrian marks a major marine transgressive-regressive cycle and southward thinning carbonate-siliciclastic wedge (Burj Formation) widely present in the subsurface across the Arabian Platform. During deposition of this transgressive marine sequence the palaeoshoreline was oriented WNW-ESE in southern Jordan. The transgressive phase (TST) is represented by tidal-dominated siltstones and fine-grained sandstones (Tayan Member) containing a diverse Cruziana/Rusophycus ichnofaunal assemblage. The overlying carbonate unit (Numayri Member) represents the highstand (HST) and maximum marine flooding surface (MFS), and comprises a carbonate ramp sequence of shelly wackestone, packstone and grainstone with ooids and oncolites, and a diverse shelly fauna including trilobites, brachiopods and hyolithids. A return to regressive tidal-influenced sandstone and siltstone (along with thin carbonates in central Jordan) (Hanneh Member) represents a regressive wedge (RST) deposited in response to renewed uplift of the ANS. Trilobites, represented by the Kingaspis campbelli and Redlichops faunules, suggest a biostratigraphical age of early mid-Cambrian for the carbonate MFS, which equates approximately to the base of the Cambrian Series 3 (Stage 5). This event probably represents the Cambrian marine flooding surface Cm20 (approximate geochronological age of 509 to 505 Ma). South of Feinan, in the Wadi Araba, the carbonates pass laterally to marine sandstone (Abu Khusheiba Sandstone) with extensive Skolithos burrows and Cruziana/Rusophycus traces. Traced southwards (palaeohinterland) the marine influence diminishes, so that the Burj/Abu Khusheiba units are absent in the Southern Desert. Ediacaran intrusives, together with extrusive volcanic and volcaniclastic rocks (Araba Complex) are associated with rifting and half-graben formation in the Feinan-Petra area. This later tectonic activity produced a younger (Ediacaran to early Cambrian), immature palaeotopography, in marked contrast to the Neoproterozoic Aqaba Complex peneplain in the Southern Desert. Consequently, early and mid-Cambrian fluvial and shallow-marine siliciclastics (Salib and Abu Khusheiba formations) onlap progressively onto this immature palaeotopography that was subsequently buried by mid-Cambrian time. Increased basinal subsidence to the north of the Araba Complex ‘high’ provided increased accommodation space that resulted in the deposition of a thick sandstone succession in north Jordan. The Feinan-Petra region seems to have acted as an east-west hinge-line with greater subsidence of the Arabian Platform to the north; similar thickness trends are seen in the Burj and Umm Ishrin formations. Renewed uplift and erosion of the ANS to the south led to deposition of a thick succession of fluvial-dominated sands, again deposited by large-scale braided rivers (Umm Ishrin Formation). Fluvial sedimentation continued through mid to late Cambrian times and also the Ordovician (Disi and Umm Sahm formations), but episodic shallow-marine or estuarine flooding of the low-gradient alluvial plain resulted in colonisation, locally, by arthropods and annelid worms that produced a diverse and abundant Cruziana/Rusophycus/Planolites assemblage of tentative Floian (Arenig) age (upper Disi Formation). Overall the Cambrian to Ordovician Ram Group siliciclastics (Salib-Umm Ishrin-Disi-Umm Sahm formations) show an upward increase in sand maturity from arkose (Salib) to orthoquartzite (Disi); heavy-mineral signatures (ZTR), specifically datable zircons, indicate provenance from a predominantly distant Neoproterozoic granitoid source rock area located to the south (ANS) that was undergoing intensive weathering. However, a small zircon component was derived from older pre-Neoproterozoic rocks, consistent with the general trend in the Levant. The highly permeable Cambrian siliciclastics of Jordan and surrounding countries provide an important regional aquifer, the Ram (formerly Disi) Aquifer. In a suitable setting these reservoir rocks might have potential for hydrocarbon exploration where source rocks of Neoproterozoic, Silurian or Permian age are faulted and in proximity, at depth, in the central Arabian Platform.