Structure and sedimentation of Permo-Triassic and Triassic rocks exposed in small-scale horsts and grabens of pre-Cretaceous age: Dead Sea margin, Jordan

Abstract

Small-scale horsts and grabens, formed prior to the deposition of the Lower Cretaceous Kurnub Sandstone, are described for the first time from remote exposures near the eastern shore of the Dead Sea, Jordan. The grabens preserve a sequence of Permo-Triassic and Triassic rocks (Umm Irna and Ma'in Formations), flanked by horsts of Cambrian sandstone (Umm Ishrin Formation). The pre-Cretaceous faults are mostly orientated NNE or ENE, with a maximum vertical displacement of 130 m. Rotation of a joint system, formed during the late Palaeozoic, in the Cambrian sandstone indicates that the horst blocks were rotated anti-clockwise during block faulting. Local and regional stratigraphical evidence suggest that faulting occurred during an extensional tectonic regime in late Jurassic to early Cretaceous times. The horsts and grabens did not affect sedimentation of the basal Kurnub Sandstone and they are preserved without relief below the unconformity, suggesting a period of erosional peneplanation prior to deposition of the fluvial Kurnub Sandstone. The structures probably represent an extension of the Central Naqab-Sinai fault zone which was displaced by sinistral shear along the Dead Sea-Gulf of Aqaba Rift in Tertiary (Neogene) times. Lithofacies sequences in the Umm Irna Formation exposed in the grabens comprise, in ascending sequence, braided to low-sinuosity or meandering fluvial siliciclastic facies, derived from the Arabo-Nubian Shield to the south and south-east. Lithological characteristics, bedforms and pedogenic features suggest a fluctuating, seasonal, semi-arid climate. Transgression of the Tethys Ocean in early Triassic times resulted in deposition of shallow-marine siliciclastics and carbonates (Ma'in Formation) in subtidal to intertidal environments.