Palaeoenvironmental analysis of Late Cretaceous dinoflagellate cyst assemblages using high-resolution sample correlation from the Western Interior Basin, USA

The Cretaceous geological record of the Western Interior Seaway of North America is one of the most thoroughly studied in the world. Extensive work on the lithostratigraphy, biostratigraphy, and geochemistry has resulted in a high-resolution chronostratigraphic framework, allowing precise, bed-by-bed correlation between sections hundreds of kilometres apart. Late Cenomanian to Early Turonian marine strata from five sites along a transect from Arizona to Kansas have been analysed for their palynological assemblages. At the time of deposition, the sites are thought to have been palaeoenvironmentally very different in terms of shoreline proximity, bathymetry, and salinity. Precise correlation of samples between the sites using the high-resolution stratigraphy has provided a valuable testing ground for a comparison of the dinoflagellate cyst assemblages. A number of species are determined to have palaeoenvironmental preferences in terms of salinity and shoreline proximity and this is supported by integration with the lithostratigraphy, macropalaeontology, micropalaeontology, and geochemistry. Previous palaeoenvironmental interpretations suggest that one Kansas site had comparatively much higher levels of primary productivity and this is reflected in the high ratio at this site of peridinioid to gonyaulacoid cysts. The high-resolution correlation also shows that a number of the biostratigraphically useful dinoflagellate cyst taxa had synchronous or near-synchronous range tops and bases across the basin. This integrated, high-resolution study with tight chronostratigraphic control supports the use of dinoflagellate cysts, not only as biostratigraphic tools but also for palaeoenvironmental and palaeoecological interpretation.