The margins of Gondwana are generally considered to be the major sites of oolitic ironstone production during the Ordovician, and appear to be linked with global eustatic sea-level rise. Occurrences of oolitic ironstones within the North American craton are less well documented, but provide important supplementary data. The low latitude of Laurasia contrasted with Gondwana allows useful comparisons of climatic and temporal patterns of Ordovician ironstone formation. Middle Ordovician ironstones occur in siliciclastic sequences in the American mid-continent and appear to become progressively younger as the epicontinental sea advanced from the southwest across a predominantly carbonate terrain. In northeastern Kansas, the regional distribution pattern of primary, syndiagenetic goethite iron oolites within the St. Peter Sandstone indicate deposition peripheral to a north-northeast-trending chain of islands underlain by predominantly granitic rocks, located along an ancestral Nemaha uplift. Detailed compositional mapping in the subsurface was made possible by the distinctive petrophysical properties of the goethite zone and the extensive regional control of wireline-logged exploration wells. Petrographic data from ironstone core- and drill-cuttings both validate log analysis and give insights on possible modes of genesis. We propose that eustatic changes in sea level were the primary factor governing the formation and observed distribution patterns of the oolite bed(s). The relationship of the observed occurrence patterns to major rift-related faults of the Central North American Rift system suggests that synsedimentary tectonism also influenced this process. The most likely source of iron appears to be by derivation from intensive, humid weathering of granite exposed extensively on the ancestral Nemaha uplift archipelago. ?? 1992.