Evolution of a gradually abandoned Oligocene fluvial palaeovalley fill: Abu‐Roash area, north Western Desert, Egypt

Abstract

AbstractLittle is known about the evolution and gradual abandonment of the fluvial channel and associated sedimentary architecture preserved in the ancient sedimentary rock record. This research aims to analyze the changes and evolution of a gradually abandoned fluvial valley and associated within‐channel deposits and architecture from the rock record. This is achieved through detailed photogrammetry, outcrop sedimentology and quantification of palaeochannel parameters for the Oligocene palaeovalley fill of the north Western Desert basin of Egypt. The three‐dimensional vertical Oligocene outcrops and trenches of the study area provide the opportunity to analyze sedimentary facies, ichnology, architectural elements and evolution of the gradually abandoned fluvial channel belt. This paper presents a detailed model for the gradual abandonment and evolution of the fluvial palaeovalley through the stepwise transition from braided to meandering channel systems. This includes the transition from a bedload‐dominated, low sinuosity, braided channel that prominently features downstream‐accreted, mid‐channel and compound bars, to a small, mixed and suspended load‐dominated intermediate sinuosity (underfit) channel that is characterized by heterolithic‐dominated point bars, floodplain and mud‐plugs via a large, sand‐dominated, intermediate sinuosity channel with laterally accreted bars of transitional stage. The gradual abandonment of the fluvial palaeovalley is also associated with a successive decrease of the channel width and depth from >800 to 166 m and from 12 to 5 m, respectively. The successive abandonment is also associated with a gradual increase in the diversity and abundance of Scoyenia–Termitichnus ichnofacies groups, and vice versa in the Skolithos ichnofacies group. The successive decrease in channel width with gradual abandonment of the Oligocene palaeovalley is probably attributed to autogenic gradual avulsion, rather than allogenic. Comparison between the Oligocene palaeovalley fill and the modern analogues indicate the similarities in channel pattern changes and channel dimension through filling stages and differ from most modern counterparts because it is aggradational. This research enhances the understanding of the sedimentological characteristics and stratigraphic architecture of the gradually abandoned fluvial channels in the rock record and introduces insights for prediction of within‐channel facies and architectural heterogeneities of the fluvial channel reservoirs.