Abstract: Late Cretaceous Floral Changes: Evidence for Pre-impact Terrestrial Instability?

THOMPSON, ANNA, and NAN CRYSTAL ARENS, Department of Integrative Biology and Museum of Paleontology, University of California at Berkeley, Berkeley, CA

Climate during the latest Cretaceous is thought to have been stable and equable. This should result in a stable and homogeneous late Maastrichtian flora that was only significantly disturbed by the terminal Cretaceous catastrophe. Biostratigraphically, this flora is recognized as the Aquilapollenites - Wodehousia palynological zone. However, preliminary palynological work at a finer stratigraphic scale (10 cm) of a single section in the Upper Hell Creek Formation in McCone County, Montana, indicates that Aquilapollenites sp. were rare, and Wodehousia sp. were absent in the latest Maastrichtian. Instead, the palynofloras in the 2 m below the boundary are characterized by extremely small grains (10-15 micrometers), large turnover of taxa, and declining palynological taxa richness (from 177 to 35 taxa). This suggests that floral change and loss of species (by extinction, extirpation or shift of range) was already underway before the terminal-Cretaceous catastrophe. Furthermore, the large number of extremely small palynomorphs could indicate that the plants were environmentally stressed.

To interpret floristic changes from the palynological record, a nonbiostratigraphic method of placing the K-T boundary is needed, especially in sections without an iridium/shocked quartz boundary clay. A -2‰ carbon isotopic excursion (recorded in land plant tissue), which is correlated with the marine record, allows us to place the K-T boundary precisely in a variety of terrestrial facies. This combination of chemostratigraphy and quantitative palynology from more sections of the Hell Creek and Tullock Formations in eastern Montana will clarify patterns of floral change across the K-T boundary on finer stratigraphic and regional scales and will allow us to tease apart Earth-based and terrestrial mechanisms of floral extinction and recovery.

AAPG Search and Discovery Article #90931©1998 AAPG Foundation Grants-in-Aid