Abstract
Turabah area is located in the southwestern Arabian Shield of Saudi Arabia. The present study aims to shed light on the origin and mechanism of formation of the ironstones of the Tertiary sedimentary succession of the Umm Himar Formation in Turabah area. These ironstones are enclosed within mudstones and carbonates with less frequent sandstone. These lithologies show lateral and vertical facies changes and thickness variations and are stacked in cyclic nature within seven sedimentary units. The studied ironstones are present in two main horizons within the lower part of the succession as follows. (1) Lower massive and oolitic hematitic ironstones which are present as thin continuous and discontinuous bands and laminae within the vari-colored tuffaceous mudstone unit of the lower part of the succession. The oolitic ironstones are present in successive small cycles. The lower parts of these cycles are composed of ferruginous friable mudstones. The middle parts are composed of hard ferruginous mudstones with abundant vertical and inclined burrows. The upper parts of these cycles are composed of indurated oolitic hematitic ironstones. The iron ooids of these ironstones were formed during successive and subsequent stages of diagenetic ferruginization of the precursor tuffaceous materials. (2) Upper bedded peloidal hematitic-magnetitic ironstones which are present in the hematitic-magnetitic ironstone unit no. 6 just overlying the bedded basalt unit no. 5. These ironstones were formed during subsequent stages of diagenetic ferruginization of precursor tuffaceous materials. These stages include the formation of green clays, devitrification, and finally ferruginization of the tuffaceous materials. This ironstone type is laterally changed into the penecontemporaneous bedded tuffaceous basalt.