EXCEPTIONAL DIVERSITY IN LATE EOCENE FRESHWATER DIATOMS FROM THE FLORISSANT FORMATION, TELLER COUNTY, COLORADO

Preliminary examination of lake shales of the late Eocene Florissant Formation indicates an unusual diversity of diatom flora when compared with other fossil freshwater sites of similar or earlier age. The diatoms occur in submillimeter-thick white laminae intercalated with finely laminated shale that is interbedded with generally thin beds of volcanic tuff. This complex subunit of diatomite, shale, and tuff alternates with thicker-bedded massive gray mudstone.

Three locally recognized lacustrine deposits (the upper shale, middle shale, and lower shale) within the Florissant Formation have been interpreted as representing a chronological sequence of lake formation in which an ancient river drainage was dammed by lahars from the nearby Thirty-nine Mile volcanic field (Evanoff, Gregory-Wodzicki, and Johnson, 2001). For more than 130 years, these lake deposits have yielded exquisitely preserved leaf and insect fossils, as well as spiders, birds, fish, mollusks, ostracods, and pollen. Mammals have been found in the associated fluvial beds. This current study is the first in which the diatoms - their taxonomy and diversity - are documented. The initial findings of this investigation will be presented.

Stratigraphically controlled samples were collected from the lacustrine unit known as the lower shale exposed in a private quarry. The diatomite laminae and diatomaceous shales were processed with conventional methods for extracting diatoms. SEM images and photomicrographs of petrographic thin sections were also utilized for diatom identification. Comparisons were made between the Florissant diatom assemblages and those reported from the middle Eocene Okanagan Highlands in British Columbia, Canada, and the Wagon Bed Formation of Wyoming. While the centric genus Aulacoseira is known from as early as late Cretaceous, many of the pennate taxa observed here are the earliest of their kind known in the world.

Future work with the Florissant diatoms will address taphonomic and ecologic relationships of the diatoms with other fossil groups in an effort to better understand the history and paleoecology of continental interior lakes during a period of warm global climate in the Eocene.