Calcareous Foraminifers from the Humboldt Oolite of Iowa: Key to Early Osagean (Mississippian) Correlations between Eastern and Western North America

Abstract

Tuberculate foraminifers dominate the calcareous microfauna of the early Osagean Humboldt Oolite in north-central Iowa. This assemblage, previously unknown east of the Transcontinental Arch, occurs commonly in western North America, thus facilitating foraminiferal correlations between that area and the Midcontinent. Analysis of early Osagean microfossil occurrences within the Cordillera permits definition of an informal Zone of Tuberculate Foraminifers (ZTF) composed, in ascending order, of the subzones (1) Tuberendothyra tuberculata, (2) Spinoendothyra spp., (3) Inflatoendothyra spp., and (4) Eoforschia moelleri group-Paradainella spp. Comparison of the Humboldt microfauna to this succession indicates that an hiatus probably separates the oolite from the underlying Gilmore City Limestone, which contains a pre-ZTF assemblage. This break, which lies at the Kinderhookian-Osagean boundary, can be recognized widely in the western-shelf seaway where ZTF assemblages generally have been misinterpreted as late rather than early Osagean. Faunal gaps higher in the ZTF suggest that shelf sedimentation at some localities in the Cordillera may have been interrupted again later in the early Osagean. In Eurasia, the Humboldt assemblage best correlates to the late Tournaisian Kizel horizon and equivalent beds of the Russian Platform, Urals, and Ukraine. Newly named taxa include the alga Cylindrofolia glenisteri and the problematic Asphaltina? macadami.