LE3 .A278 2020

2020

Pufahl, Peir
Murphy, Brendan

Acadia University

Master of Science

Masters

Geology

Earth & Environmental Sciences

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 on the margin of Avalonia and the Rheic Ocean. 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, intraclastic 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. Upwelling from a ferruginous water mass provided a source of nutrients stimulating surface productivity, which governed bottom water redox conditions in middle and distal shelf environments. Large, ferruginous coated grains (granule size) composed of concentric and discontinuous layers of carbonate fluorapatite, hematite, and chamosite record alternating redox conditions and periodic hydrodynamic reworking. Facies containing coated grains are enriched in redox-sensitive trace metals V, Cr, and Zn corroborating sedimentologic evidence that indicates ironstone deposition was stimulated by the delivery of ferruginous seawater to the shelf. Results support a new model of Ordovician ironstone accumulation that relies on upwelling of anoxic, Fe-rich bottom water supporting the early Paleozoic ocean was not yet fully ventilated. Thus, ironstone is a robust window into understanding important Paleozoic events that governed future environmental change.

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https://scholar.acadiau.ca/islandora/object/theses:3493