Distribution of abundant Ordovician and Jurassic oolitic ironstones provides a test for current ideas about factors controlling their origin. These include warm climate and deep weathering, dispersed continents, and highstand of global sea level.

Most of the Jurassic ironstones developed in middle northern latitudes on the unstable European part of assembled Laurasia at a time of low global sea level, mild moist climate, and abundant vegetation. Most of the Ordovician ironstones accumulated around the northwestern margin of assembled Gondwana and Armorica during highstand of sea level and an absence of land plants, and in high southern latitudes that supported an ice cap by the end of the period.

These contrasts demonstrate that neither dispersed continents nor major highstand of sea level was a necessary factor. Moreover, requisite weathering in Ordovician time may have been induced by adequate soil-air CO2 pressure maintained by elevated atmospheric Pco2 in tne absence of land plants.

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