264-7 REE-Enriched Phosphate and Ironstone Deposition in the U.S. Midcontinent Basin During the Late Ordovician Elkhorn Biogeochemical Event

Abstract:

Marine basins provide regional controls on the expression of global perturbations to the ocean-atmosphere system. While several examples are well established in the literature for Mesozoic oceanic anoxic events, details of similar Paleozoic events are still emerging. In the Upper Ordovician the Guttenberg and Hirnantian events have received the most attention. Between the two, the little-known Elkhorn event is particularly insightful for illuminating the processes controlling formation of critical mineral-enriched phosphorites and ironstones.     

A new regional Upper Ordovician chronostratigraphic interpretation is derived from sequence stratigraphic analysis that incorporates extensive bio- and chemostratigraphic data sets and facies mapping. The starting point was revision of the Cincinnati reference area where strata of the Elkhorn event cap the succession. This work required refining the definitions of regional stage boundaries based on biostratigraphic criteria. It included chitinozoan studies that generated new biozonation, complementing previous conodont and graptolite work. Building on this foundation, carbonate carbon isotope chemostratigraphy provided increased chronostratigraphic resolution and spatial coverage. This integrated approach was expanded across the U.S. midcontinent to provide a robust regional time-rock framework within which the temporal and spatial extent of facies, including critical mineral-enriched phosphorites and ironstones, could be examined.

The regional analysis reveals that the Elkhorn biogeochemical event is associated with a carbon cycle record featuring two paired positive and negative excursions. The positive excursions are linked with carbonate deposition and capped by widespread paleokarst surfaces. By contrast the negative excursions are associated with transgressive phosphorites, black shales, and ironstones. Mineralization is most extensive in the second set of excursions, including basin center development of the Graf phosphorite (15 m-thick, phosphate and black shale with TOC 20%) and basin margin ironstones mapped over 700 km. These patterns provide a textbook example of a marine chemocline mineral system driven by perturbations to the Late Ordovician ocean-atmosphere system.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-22461

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