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

Abstract

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

Early Paleozoic Cenerian (Sardic) geodynamic relationships of peripheral eastern north Gondwana affinities: revisiting the Ordovician of the Getic/Kučaj nappe (eastern Serbia)

“…The Supragetic base ment unit of east ern Ser bia cor re lates with re gional an alogues, such as at West Kraishte, and the "Morava nappe" of west ern Bul garia (for mer "Vlasina unit"; Antiae et al, 2016;Žák et al, 2021). This unit com prises a lower greenschist-fa cies base ment unit (Popoviae, 1993;Vaskoviae, 2002;Kräutner and Krstiae, 2003;Spahiae et al, 2019a), which is the car rier of phosphates as in di ca tors of a re duc ing en vi ron ment (Pavloviae, 1975(Pavloviae, , 1977 for a phos phatic iron stone en vi ron ment see Dunn et al, 2021). The Neoproterozoic-Lower Or do vi cian age of these rocks is con firmed by stratigraphically low er most gra phitic schists (fos sil ves i cles of the alga Archaeofavosina sim plex Naum; Kaleniae et al, 1975;Ferretti et al, 2022).…”

“…The most com mon oc cur rence of chamosite and sid er ite is in banded iron for ma tions, rep re sent ing the prin ci pal iron-bear ing min er als in iron stones, of ten as so ci ated with under ly ing fine-grained lithified claystone se quences (Deer et al, 2013a). In ad di tion to the Or do vi cian iron stone max i mum produc tion stage (Oggiano and Mameli, 2006;Dunn et al, 2021), banded iron for ma tions are doc u mented across most Pre cambrian con ti nen tal re gions, to gether be ing a prin ci pal source of iron and phos phates (e.g., ¯elaŸniewicz et al, 2009;Dunn et al, 2021), such as that of the Supragetic base ment unit. Oolitization is a sed i men tary pro cess of ac cre tion de vel oped in a quiet en vi ron ment (Guerrak, 1988) with low sed i men ta tion rates, and thus has of ten been de scribed as part of a transgressive ini tial de posit above an un con formity (Young, 1992).…”

“…Oolitization is a sed i men tary pro cess of ac cre tion de vel oped in a quiet en vi ron ment (Guerrak, 1988) with low sed i men ta tion rates, and thus has of ten been de scribed as part of a transgressive ini tial de posit above an un con formity (Young, 1992). The iron stone sed i ment is also as cribed to the for ma tion of aggradational parasequence on a storm-dom i nated shelf char ac ter ized by re cur rent coastal upwelling (Dunn et al, 2021). Or do vi cian iron stones may also be cor re lated with the bot tom of fin ing-up wards se quences on shal low shelves (transgressive con di tions ;Guerrak, 1988;Pufahl et al, 2019).…”

“…Thus, the fre quent oc cur rence of Mid dle-Up per Or do vi cian chamosite-and sid er ite-bear ing iron stones within the Getic/Kuèaj/Svoge nappes of the Carpathian-Bal kan belt (Veselinoviae, 1975;Krstiae and Maslareviae, 1998;Gutierrez-Marco et al, 2003;Yanev et al, 2006;Figs. 2 and 3) is al ter natively used as a proxy for (i) un con formi ties (transgressive ini tial de posit above an un con formity; Young, 1992); and (ii) to gether with ev i dence of Or do vi cian mafic magmatism sup ply ing ferruginous wa ter as a pos si ble source of iron (e.g., Mücke and Farshad, 2005;Oggiano et al, 2006;Pufahl et al, 2021;Matheson et al, 2022), as an al ter na tive palaeogeographic and tectonic re con struc tion of the Getic/Kuèaj nappe in fur ther por traying a nar row palaeoceanic sea way.…”

Middle Jurassic ooidal ironstones (southern Tibet): Formation processes and implications for the paleoceanography of eastern Neo-Tethys

“…In iron ooids of the FOF, francolite occurs mainly associated with high organic-matter content (Figure 6C), suggesting that the remobilization of P from organic matter is responsible for its formation. Increased P supply to the surface ocean may have been fostered by coastal upwelling, a process inferred to have led to peaks of phosphorite deposition during Ordovician and Miocene icehouse stages (Follmi, 1993;Dunn et al, 2021) as well as Cretaceous greenhouse stages (Donaldson et al, 1999). Generally, Because P retention is promoted by oxic bottomwaters whereas organic P delivery to the sediment is enhanced by benthic oxygen deficiency.…”