The stratigraphy and sedimentation of Upper Cambrian, Permo- Triassic and Lower Triassic rocks along the North Eastern margin of the Dead Sea basin, Jordan

Abstract

A thick sequence of predominantly terrigenous clastic sediments ranging from Cambrian to Cretaceous in age is exposed along-the northeastern margin of the Dead Sea, Jordan. The present study is confined to rocks of Upper Cambrian, Permo - Triassic (Um Irna Formation)-and Lower Triassic (Ma'in Formation) age, -which attain a total thickness of about 150 m. The lithology, stratigraphy, sedimentary structures, lithofacies and depositional environments of these sediments have been studied in detail. Early After deposition of the'Middle Cambrian marine Burj Formation, a major regressive event occured punctuated by minor transgressive phases, during which the Upper Cambrian sediments were laid down. These lie conformably on the Burj Formation, and are unconformably overlain by the Permo - Triassic Um Irna Formation. The Upper Cambrian succession consists of a medium - to coarse - grained quartzarenite facies and a subordinate heterolithic facies of siltstone and mudstone with sandy lenses. Internally the arenites are structured by trough cross - bedding arranged in cosets or solitary sets. Foreset dip directions show a unidirectional northwesterly mode. Most of the trough cross - beds are deformed giving rise to penecontemporaneous overturning of the foresets in the down-current direction. This is attributed to shear stress exerted by a dense sediment - laden current of water moving over the top of seismically liquefied cross - bedded sand. The Upper Cambrian sediments were, deposited on a braidplain and adjacent tidal flats due to periodic shifting of the active part of the braidplain. Reworking of the inactive part of the braidplain by tidal currents and subsidence led to marine incursions and the devel - opment of tidal flats. The Permo - Triassic Um Irna Formation is conformably overlain by interbedded sandstone, siltstone and mudstone of the Lower Triassic Ma in Formation. Um Irna Formation can be divided into a lower and upper member according to grain size and the proportion of sandstone to siltstone and shale in succession. The lower member attains a thickness of 10 m and is characterised by the presence of five sandstone-dominated fining - upward sequences from 0.75 to 2.15 m thick comprising an erosively - based coarse-to fine-grained sandstone overlain by, and laterally intertonguing with, maroon silt - stone and shale. The sandstones are internally structured by small - scale (< 20 cm thick) trough cross - bedding. Foreset azimuths show a unidirectional trend towards the north - northwest. The upper member also consists of five fining - upward sequences from .4 to 14.5 m thick, each sequence comprising an erosively - based pebbly sandstone grading up through medium-and fine-grained sandstone into maroon siltstone and silty - shale. Trough cross - bedding is the dominant internal structure with foresets directed towards the north - northwest. A characteristic feature of the silty - shale is the presence of abundant ferruginous concretions (pisoliths). Both members were deposited by fluvial processes. The lower member is attributed to deposition mainly by shallow, low sinuosity sand bed channels draining the distal reaches of a low gradient alluvial plain. The upper member contains a higher proportion of overbank fines (silt and mud) consistent with deposition by a mean - dering fluvial system, while the dominance of pebbly components in the sandy facies and their multilateral and multistorey nature suggests deposition by low sinuosity channels on the more proximal reaches of the braidplain. The nature of the concretions (Fe- pisoliths)- suggests that the diagenetic (pedogenic) environment was complex with alternating episodes of leaching, cementation and fracturing during their growth. The Lower Triassic Ma'in Formation is overlain by the shallow marine Dardur Formation. Two facies are recognised within the Ma'in Formation. The lower Himara Member comprises two facies which are deep maroon in colour and highly bioturbated: a lower sandy facies and an upper heterolithic facies (sand, silt and mud). This member is thought to have been deposited on a-tidal flat as evidenced by rhythmic beds of sand, silt and clay, flaser, wavy and lenticular bedding, herringbone cross - bedding, mudcracks, raindrop imprints, superimposed ripple marks showing interference, and ladder back forms reflecting shallowing and late stage emergence run off features. The depositional model proposed is one of a microtidal to mesotidal coastline, with a palaeotidal range of 0.45 m to 2.35 m. The coastline is inferred to have been interacting with a braided fluvial plain, which fed in quartzitic sediment from the southeast as indicated by the palaeocurrent pattern. The upper, more sandy, Nimra Member is cream in colour, more fossiliferous and more calcareous. The proposed depositional model is that of a shallow subtidal shelf receiving clastic sediment by way of river mouths extending seawards as subtidal channels. The shallow open marine nature of the environment is indicated by the presence of oold'si ,foraminifers, lamellibranchs, echinoids, bryozoars and gastropod fragments cemented mainly by dolomite. The overall model proposed for the Ma'in Formation is that of an intertidal flat (Hirnara Member) located between a braided alluvial plain (Um Irna Formation) and a subtidal marine shelf (Nimra Member).