EPICONTINENTAL IRONSTONE ACCUMULATION DURING THE END-ORDOVICIAN GLACIATION AND EXTINCTION

The Upper Ordovician Neda Formation, Midwest, USA, is composed of hematitic dolostone and ironstone that rest conformably on crinoid and bryozoan wackestone to packstone of the underlying Maquoketa Formation. Deposition occurred within the Maquoketa Sea south of the Transcontinental Arch between 15 and 20°S. Where not eroded, the top of the Neda Formation is a subaerial ferricrete that formed as sea level fell during the Hirnantian Glaciation. This unconformity is the Ordovician-Silurian boundary and records a regression of at least 100 m and nearly two million years of exposure.

Lithofacies associations indicate deposition began on a storm-dominated ramp as sea level fell, and impingement of an anoxic water mass shut down the carbonate factory. Coastal upwelling of ferruginous and euxinic bottom waters derived from the Iapetus Ocean produced depauperate, Fe-rich mudstone and granular ironstone. Mixing with oxygenated surface water is interpreted to have precipitated Fe-(oxyhydr)oxide in the water column that accumulated as Fe-rich mud on the seafloor. In shallower environments, advection of Fe away from the upwelling front and Fe-redox pumping beneath the sediment-water interface produced granular ironstone. Granular ironstone is composed of abundant coated Fe grains preserving both concentric, redox-aggraded cortical layers and those that are erosively truncated, which record periods of exhumation, reworking, and reburial beneath the seafloor.

It is becoming clear that Paleozoic ironstones are the product of upwelling rather than Fe derived from continental weathering. An increase in the equator-to-pole temperature gradient and concomitant reorganization of thermohaline circulation during Hirnantian Glaciation likely intensified coastal upwelling and drew ferruginous seawater into the Maquoketa Sea to produce ironstone. Such expansion of anoxic conditions supports recent research suggesting the onset of shallow anoxia in other basins contributed to the second pulse of the end-Ordovician mass extinction. Widespread anoxia probably persisted into the early Silurian.