2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 4
Presentation Time: 1:30 PM-5:30 PM


PASSMORE, Susan M., JOHNSON, Kirk R., REYNOLDS, Michele, SCOTT, Michon and MEADE-HUNTER, Dena, Department of Earth Sciences, Denver Museum of Nature & Sci, 2001 Colorado Boulevard, Denver, CO 80205, smpass@aol.com

The Middle Eocene (48-50 Ma) upland fossil lakebeds of Republic, Washington have yielded over 67 families and 141 genera of plants, including dicots, monocots, conifers, Ginkgo, cycads, and ferns. The dominant vegetation is that of a mixed/coniferous upland forest. This beautifully preserved flora from the Tom Thumb Member of the 900m thick Klondike Mountain Formation has been collected for 100 years and is taxonomically well understood but little is known about the relative abundance of species within the deposit or their distribution through the section. In 1997, 2000, and 2001, we undertook detailed excavations to examine relative abundance and species diversity over time. All specimens encountered were collected and sorted to species using leaf architectural analysis and a morphotyping system. To assess short-term variation, we excavated eight contiguous 20cm levels from a 1.6m section at Upper Boot Hill in Republic. This high-resolution approach was designed to generate results that would be comparable both to analyses of pollen from Quaternary lakes and to the flora of the Parachute Creek Member of the Green River Formation.

Based on preliminary analysis of five of the 20cm levels, the dominant Republic taxa are Sequoia sp., Alnus parvifolia, Metasequoia occidentalis, Pinus sp., and Amentotaxus sp., all of which are present in all five levels. Other abundant taxa include Cercidiphyllum obtritum, Ulmus sp., and Ginkgo adiantoides. The great diversity of dicots at Republic has overshadowed the fact that conifers are by far the most common fossils. The domination by conifers is considerably different from the Green River flora, which rarely preserves conifers. There is noticeable floral variation between the 20cm levels suggesting that the Republic flora experienced change over short time scales. These results allow a quantitative comparison to be made between coeval Eocene lakebed floras from different elevations.