Middle Ordovician Upwelling-Related Ironstone of North Wales: Coated Grains, Ocean Chemistry, and Biological Evolution

Middle Ordovician phosphatic ironstone of the Welsh Basin provides new insight into paleoenvironmental significance and ocean chemistry. Deposition occurred in a back-arc basin along southern margin Avalonia as Rheic Ocean opened to south. Ironstone is interpreted have accumulated part an aggradational parasequence on storm-dominated shelf with coastal upwelling. This has laminated pyritic mudstone base that grades upward variably bioturbated coated grain-rich, intraclastic ironstone, which overlain turn by cross-stratified grainstone composed entirely Fe grains. A coarser clastic more proximal lithofacies rests conformably above suggests contact between two parasequences maximum flooding surface marking onset highstand conditions. Lithofacies associations suggest sustained upwelling created wedge nutrient-rich, ferruginous seawater middle stimulated high productivities. Large, grains (granule size) discontinuous concentric carbonate fluorapatite, hematite, chamosite cortical layers record fluctuations pore water E h are been related changes intensity intermittent storm reworking seafloor. Results support emerging model for underpinned development bottom was periodically tapped Expanding, semi-restricted seaways such were ideal locations ponding this anoxic, hydrothermally enriched seawater, especially during early Paleozoic when deep inconsistently oxygenated. The coincidence depositional episodes graptolite diversification events that, addition Fe, supply upwelling-related P may driven radiation some planktonic ecosystems Great Biodiversification Event. Concomitant minor extinctions benthic trilobites these waters impinged shelf.