Field, Mega- and Microscopic Description of the Volcaniclastic Red Beds and the Associated Scoriaceous Basalt of Wadi Al Roaian, Ablah Area, Assir Terrain

Abstract

The study area is located in the entrance of Wadi Girshah in Ablah area, Assir terrain, southwestern Saudi Arabia. The present study aims to shed light on the field, mega- and microscopic description of the volcaniclastic red beds and the associated scoriaceous basalts of Wadi Al Roaian. It is based mainly up on the field works augmented by petrographic description. The present study revealed that, the succession of the upper part of Girshah Formation comprises three main horizons: 1) lower unit of epidotized and silicified basaltic and andesitic tuffs; 2) middle unit of volcaniclastic red beds-scoriaceous basalts of successive cycles (each of these cycles begins by reddish tuffaceous mudstone and sandstone and is terminated by the calcite-bearing scoriaceous basalt). This unit indicated the deposition in lacustrine environments and the red iron oxyhydroxides minerals were formed either by the direct hematitization of the deposited tuffs or by the diagenetic hematitization of the green clays formed instead of the precursor tuffaceous materials; 3) upper scoriaceous basalt unit composed from hematitized and calcitized basalt. The unit was formed by basic volcanic eruptions in subaerial condition which was predominated by the formation of calcium carbonate lakes associated with the progressive and subsequent calcitization of the Ca-plagioclase minerals of the basalt and the associated glassy tuffaceous material. The scoriaceous basalts of Wadi Al Roaian area represent subaerial basic volcanic eruption in continental situation and are associated with subsequent events of mineral alteration and formation of secondary minerals i.e . calcite, hematite and goethite.