56-4 A Microscopic View of Global Events: Iron Sequestration during Silurian Oceanographic Disturbances Revealed by Mineral Mapping

Abstract:

The Silurian Clinton Ironstones of central New York are a distinct, time-specific facies widely interpreted as a proxy for ancient marine anoxia and sea-level variability. However, traditional macroscopic classifications of these deposits as simple hematite-chamosite facies obscure a complex diagenetic landscape, reflecting the localized processes that controlled iron sequestration. To resolve this, we utilized high-resolution Scanning Electron Microscopy-Automated Mineralogy (SEM-AM) to quantify modal mineralogy, micro-textures, and mineral-specific grain size across a proximal-to-distal transect of the Appalachian Foreland Basin.

Analysis of four ironstone horizons from the Clinton Group type area, alongside a laterally equivalent phosphatic lag, reveals a compositional spectrum. Proximal, high-energy shoals (Westmoreland) are hematite-dominated (>59%), recording aggressive physical ravinement and oxic exhumation. Conversely, lower-energy facies (Wolcott Furnace) preserve un-winnowed berthierine matrices (8.5%) driven by localized microbial decay of peloids. Critically, mid-ramp and starved hardground environments sequestered massive portions of the upwelled iron budget into early-burial carbonate sinks, yielding ironstones dominated by carbonates (10–21% in Furnaceville and 46–48% in Basal Dawes, respectively).

Micro-textural mapping captures shifting fluid fronts recorded by concentric laminae of apatite, chamosite, hematite, and other early authigenic minerals. We interpret these zoned grains as the physical record of a fluctuating pycnocline operating during prolonged sequence-boundary sediment starvation. Moving basinward, the transition to a euxinic, pyrite-apatite dominated trough confirms a shared, basin-wide genetic mechanism. These SEM-AM maps demonstrate that ironstones and phosphatic lags are biogeochemically coupled products of an oscillating redox boundary, contextualizing ironstones as a diagenetic continuum rather than a monolithic facies.

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