Clinton ironstone revisited and implications for Silurian Earth system evolution

Ironstone is a marine biochemical sedimentary rock with syndepositional Fe enrichment that has long been viewed as a geologic curio. It is widespread in the Silurian Clinton Group and coeval strata across the Appalachian Foreland Basin (the ?Clinton ironstones?), where it has been the focus of extensive historical study aimed at elucidating its enigmatic origin. Recent interpretation of early Paleozoic ironstone as the consequence of up welling directly tied to ocean circulation provides the impetus to reinvestigate the global significance of the Clinton ironstones. To achieve this, review of 175 years of literature is combined with detailed sedimentologic, stratigraphic, and petrographic reinvestigation of the Clinton ironstone in its type area of New York State. Results are interpreted in the context of modern understanding of the Fe biogeochemical cycle and early Paleozoic Earth system. Two distinct ironstone types exist. Granular ironstone contains Fe coated grains with quartz sand nuclei. Fossiliferous ironstone contains calcareous skeletal grains with Fe mineral coatings and cements. Fe minerals in both are predominantly hematite with subordinate chamosite. Ironstone is found in discrete time-specific stratigraphic units that alternate with siliciclastic facies interpreted as accumulating in fluvial to storm wave base environments. Most ironstone formed during transgression as the indirect result of upwelling of ferruginous deep water off the Laurentian margin. Supply of this water into the Appalachian Foreland Basin led to ironstone genesis where ferruginous and shallow oxygenated waters mixed; this zone generally stretched from storm wave base to the lower shoreface. Particulate ferric oxyhydroxide that precipitated in this zone accumulated across an otherwise sediment starved seafloor during transgression. Its burial and subsequent reduction formed a zone of authigenic Fe oxyhydroxide and silicate precipitation within a few decimeters of the seafloor. The complete ferruginization of beds was the result of movement of the zone of precipitation combined with periods of exhumation and reworking. The cyclicity of the Clinton ironstones records discrete periods of ferruginous water upwelling and/or incursion. These periods correlate with Silurian marine biodiversity crises, notably the Ireviken Event. The depositional model presented herein supports that carbon cycle perturbations and extinction events such as the Ireviken were caused by the expansion of anoxic and euxinic conditions on continental shelves, the direct product of increased upwelling of anoxic basinal waters that were also the source of the Fe in the Clinton and other early Paleozoic upwelling-associated ironstone. Review of the Clinton ironstones in light of both the increased understanding of ironstone and the refined view of early Paleozoic atmospheric, oceanic, and biotic perturbations thus suggests that ironstone is an underutilized proxy of oceanic and biotic instability in the early Paleozoic Earth system.

Automated summary

Ironstone is a marine biochemical sedimentary rock with syndepositional Fe enrichment, consisting of two distinct types: granular ironstone and fossiliferous ironstone. The formation process involves the precipitation and reduction of iron, and is related to the modern understanding of the Fe biogeochemical cycle and early Paleozoic Earth system.