COATED GRAINS FROM WELSH BASIN IRONSTONE AND ORDOVICIAN SEAWATER CHEMISTRY

Phosphatic ironstone of the Welsh Basin (~467 Ma) provides new insight into the paleoenvironmental significance of coated grains and Ordovician ocean chemistry. Deposition occurred in a back-arc basin on the margin of Avalonia as the Iapetus Ocean closed to the north. Lithofacies associations indicate ironstone accumulated as part of an aggradational parasequence on a storm-dominated shelf during upwelling. Its base is a laminated pyritic mudstone that grades upward into variably bioturbated mudstone and coated grain-rich, hummocky cross-stratified ironstone, which in turn is overlain by cross-stratified grainstone composed entirely of coated grains. Coarser, clastic-rich parasequences above suggest the top contact is a maximum flooding surface marking the onset of highstand deposition.

Large, ferruginous coated grains (granule size) are the most conspicuous feature of this ironstone and provide an unparalleled window into the physical and chemical processes controlling its deposition. Grain cortices are composed of concentric and discontinuous layers of francolite, hematite, and chamosite. Cortices nucleated on chamositic mud clasts, sponge spicules and foraminifera. Bacterial degradation of sedimentary organic matter released P to pore water, producing francolite layers. Fe-(oxyhydr)oxide cortical laminae (now hematite) formed in sediment above the Fe-redox interface, whereas chamosite precipitated at this boundary from pore-water with sufficient Si and Al. Discontinuous cortical layers record storm reworking, exhumation and abrasion of grains, followed by reburial into the zone of precipitation. Facies containing coated grains are enriched in V and Cr, corroborating sedimentologic evidence that indicates ironstone deposition was stimulated by the delivery of upwelling-related, ferruginous seawater.

Results support an emerging model of Ordovician ironstone that relies on upwelling of anoxic, Fe-rich bottom water onto an oxygenated shelf. This supports recent research suggesting the early Paleozoic ocean was not yet fully ventilated. Thus, Phanerozoic ironstone is not simply a geologic novelty, but an intelligible record of oceanic change through important Earth events.