Stratigraphic setting, facies types, depositional environments and mechanism of formation of Ash Shumaysi ironstones, Wadi Ash Shumaysi, Jeddah district, West Central Saudi Arabia

Abstract

The ironstones of Wadi Ash Shumaysi are mainly enclosed within the middle part of the fluvio-lacustrine siliciclastic succession of Ash Shumaysi Formation (Oligo-Miocene). This succession is composed mainly of conglomerates, sandstones, siltstones, muddy, sandy and glauconitic ironstones, fresh water carbonates, tuffaceous mudstone and basalts in descending order. The study aims to determining the stratigraphic setting, facies types, depositional environments, and mechanism of formation of the succession and the associated ironstones, through field and petrographic investigations. The succession comprises the following four main facies associations arranged according to their predominance from the lower to the upper parts of the succession of Ash Shumaysi Formation: I: Proximal (Conglomerate Facies) Braided Streams, II: Distal (Sandstone Facies) Braided Streams, III: Floodplain facies association, including ironstones, IV: Lacustrine delta, including ironstones. The ironstones of facies associations III and IV represent the following types: 1. Sandy ironstones (ironstone type 1 (IST 1)): which formed by the deposition of water laid detrital quartz grains and Fe-bearing clays under different water depth and currents. 2. Glauconitic muddy ironstones (ironstone type 2 (IST 2)): which formed by the deposition of Fe-bearing clays within slightly agitated starved lakes. During periods of low clastic input, the deposited Fe-bearing clays become converted into green glauconitic clays as a result of the high Fe, Mg, K, Al, and Si activities. During ultimate stages of oxidized diagenetic alterations, the green glauconitic clays become hematitized into glauconitic ironstones. 3. Oolitic muddy ironstones-oolitic ferruginous sandstone (IST 3): which is represented by lacustrine successive shallowing-upward cycles. Each of these cycles begins by friable ferruginous in situ oolitized muddy ironstone which grades upward into oolitic sandy ironstone/oolitic ferruginous sandstone. The second interval is recorded overlying the fining-upward cycle of facies F5. 4. Flaser laminated silty ironstone/ferruginous siltstone (IST 4): which consists of small-scale shallowing-upward cycles of parallel laminated hematitic siltstone and flaser laminated silty ironstone/ferruginous siltstone. In this ironstone type, the inputted depositional components are composed mainly from silt-sized detrital quartz grains embedded within Fe-bearing clays. These components are vertically and laterally distributed within the depositional environments by the impact of the currents and wave actions. The diagenetic processes play an important role in the petrographic and mineralogic evolution of ironstone type IST 1. These processes includes: (a) the diagenetic recrystallization and dehydration of the precursor amorphous Fe-bearing clays and the intimately associated Fe-oxyhydroxies and formation of ferrihydrites, goethite. and hematite; (b) corrosion and embayment of the quartz grains by the enclosing different Fe mineral phases; and (c) in situ pellitization and oolitization throughout the tangential growth of the Fe-oxhyhdroxides during their dehydration and recrystallization and formation of pseudo-ooids. The diagenetic processes in ironstone type IST 2 includes glauconitization of the remained unglauconitized clays during the depositional stages, hematitization and the related oolitization, and pelletization processes. The main diagenetic processes involved in the formation of ironstone type IST 3 are: (a) hematitization of the amorphous Fe-clays and formation of goethite and hematite cement, (b) in situ pelletization and oolitization. The most important diagenetic process in ironstone type IST 4 is the dehydration and recrystallization of the amorphous Fe-clays and formation of goethite and hematite cement.