Diagnosis of marine and non-marine environments from the Cambro-Ordovician sandstones of Jordan

Abstract

The Precambrian shields of the Sahara and Arabia are overlain by a thick laterally extensive Cambro-Ordovician sandstone blanket. This is generally divisible into a lower coarse pebbly cross-bedded facies overlain by a finer-grained facies with a characteristic suite of trace fossils. This paper describes the sedimentology of the Gambro-Ordovician sequence where it crops out in the Southern Desert of Jordan. The lower coarse pebbly facies consists of a complex of channels up to 300 m wide and 15 m deep with occasional shale-filled channels of similar dimensions. Three formations of this facies occur. The lowest overlies a mature pediment with residual inselbergs cut in the Precambrian basement. The succeeding two formations have channelled bases. This facies is attributed to three cycles of pedimentation and sedimentation on braided alluvial outwash plains. The overlying facies consists of an alternation of medium grained well sorted sandstones and rare shales. The sandstones are devoid of channelling being composed of laterally extensive tabular-planar cross-bedding. The interbedded shales have sheet geometries, in contrast to the shoestring shaped shales of the facies beneath. They consist of laminated silts and microcross-laminated fine sands with a diverse suite of trace fossils. This upper facies is attributed to deposition on a marine shelf by the alternation of migrating sand shoals and tidal flats.