USING THE REPUBLIC FLORA TO UNDERSTAND AQUATIC ECOSYSTEMS OF THE EOCENE

Early to Middle Eocene strata preserved in the Republic Basin of northeastern Washington State and southern British Columbia represent a complex scenario of volcanism, hydrothermal activity and extensional tectonism influencing lacustrine systems. In general, the fossiliferous Tom Thumb Tuff Member, or Lower member, of the middle Eocene Klondike Mountain Formation, Ferry County, Washington, has been interpreted as “lake beds”. However, finding conformable contacts among these sections is extraordinarily difficult and the greatest potential for interpreting the depositional environment rests on the fossil fish and plant assemblages present. Based on fossil fish taxa shared with other Eocene localities and the number of species present, Wilson (1996) interpreted Republic as representing a small basin interconnected with other basins in Colorado, Montana and British Columbia. Fossil plant assemblages at Republic support this interpretation, but can also provide additional information regarding the aquatic habitats. The presence of some aquatic plants can be diagnostic of water depth. The waterlily Nuphar, well represented in the flora by seeds, is today is capable of inhabiting aquatic environments at water depths of 0.5-2.0 meters. Fossil plants also can document the influence of lake bottom sediments. Within the fossiliferous layers in the vicinity of Republic, most of the localities occur in lithologies that appear to be derived from the Sanpoil volcanics. Only Mt. Elizabeth, which occurs in a sub-basin of the Republic graben sometimes referred to as the Curlew Basin, has sediments derived from the metamorphic complex, and Salvinia is found only at this locality. Also, the presence of fossil mosses, associated with some aquatic plant remains such as Azolla, creates an unusual case where the pH of past aquatic systems can be assessed. Although paleolimonologists have harvested a wealth of data from Quaternary aged aquatic fossil plant assemblages, paleobotanists examining Paleogene deposits have yet to fully use these kinds of data in making paleoecological reconstructions. The Republic flora represents an ideal situation for using aquatic plants to interpret abiotic attributes of much older aquatic systems