Palaeofluvial estimates from exhumed meander scrolls, Taref Formation (Turonian), Dakhla Region, Western Desert, Egypt

Abstract

An array of meander scroll sets (point bar complexes) is exposed over a land surface eroded across the dip slope of sandstones of the Nubia Group, Taref Formation (Turonian) around the southern fringe of Dakhla Oasis, Egypt (25.5°N, 29°E). These sandstones are thus interpreted as fluvially deposited. Using published equations, bankfull channel width, depth, as well as sinuosity are estimated from radii of meander curvature. With a reasonable assumption of velocity, bankfull discharge is then estimated. This is found to be comparable with bankfull discharge values estimated from published plots of hydraulic geometry (width and depth) against discharge. Both approaches result in mean bankfull discharge estimates of 4.4–7.0×10 3 m 3s −1. Using mean radius of meander curvature, sinuosity is estimated at 1.66. In this hyperarid climatic region aeolian erosion has carved yardangs into the Taref sandstone, the flanks of which expose uniform, tabular cross-beds of coarse, clean quartz sand. From the palaeofluvial estimates, from other features of the exhumed meander scrolls, and from the character of the sandstone, channels in which the Taref sands were deposited are inferred to have been wide and shallow, of low sinuosity, rapidly migrating laterally, with minimal overbank deposition of muds, and rapid vertical accretion. Published indications of a tropical monsoon climate over Egypt in the Turonian are harmonized with the absence of fossil flora from the Taref with reference to indications of a rapidly changing fluvial landscape.