Marine and terrestrial palynofloras from transitional Devonian–Mississippian strata, Illinois Basin, U.S.A.

Abstract

Diverse and reasonably well preserved palynoforal assemblages are described from a 12.9 m-thick section of the Upper Devonian Saverton Shale and a 17.0 m-thick section of the Lower Mississippian Hannibal Shale exposed along a bluff at Atlas South, Pike County, Illinois, U.S.A. The microphytoplankton assemblage, consisting of acritarchs and prasinophytes, comprises 17 genera and 38 species, including two new species (Cymatiosphaera scitula and Gorgonisphaeridium savertonense) and one new combination (Puteoscortum sprucegrovense). The miospore assemblage contains 14 species - one new (Punctatisporites hannibalensis) and one new combination (Vallatisporites hystricosus) - distributed among 13 genera. The overwhelming majority of microphytoplankton and miospore taxa occur in the Saverton Shale. The Saverton microphytoplank-ton assemblage indicates a latest Devonian (Strunian) age and is most similar in composition to previously described Late Devonian assemblages from North America and China. There is a low to moderate degree of similarity between the Saverton microphytoplankton assemblage and those reported elsewhere in the world. The miospore assemblage further corroborates a latest Devonian age (LN miospore Zone), as signifed by its content of Retispora lepidophyta, Verrucosisporites nitidus, Indotriradites explanatus, and Vallatisporites hystricosus. Based on the significant drop in diversity of the microphytoplankton, the presence of morphologically simple morphotypes, and several taxa, whose range extends into the Early Mississippian, a Kinderhookian date for the Hannibal Shale is reasonable. The Hannibal miospore palynofora is even more impoverished than the associated microphytoplankton, and its few named taxa are consonant with, albeit not independently corroborative of, an Early Mississippian age. Sedimentologic and paleontologic-palynologic evidence indicates that the Saverton and Hannibal shales were both deposited in a low energy, somewhat offshore, normal marine environment within the Illinois Basin