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Fossil Plants in the Denver Basin Provide Insight into Climate, Local Habitat, Extinction, and Rainfall Patterns Related to Uplift of the Front Range
JOHNSON, Kirk R., Department of Earth and Space Sciences, Denver Museum of Nature and Science, 2001 Colorado Boulevard, Denver, CO 80205-5798, [email protected].

Plant megafossils, primarily leaves but also wood, fruits, seeds, and stems, are abundant in the bedrock formations that overlie the Fox Hills Sandstone in the Denver Basin. Although they have been collected and studied since late in the 19th century, there has never been a basin-wide approach to integrating fossil floras with a synorogenic stratigraphic framework. Extensive construction since 1990 and a rejuvenated interest in Denver Basin fossils have facilitated and energized the discovery and collection of more than 90 new fossil plant sites. Data from these new sites is much more complete and useful than data from historical collections which are located at Harvard University, Yale University, and the Smithsonian Institution. At present, DMNS houses extensive collections from the Laramie Formation and the Late Cretaceous (late Maastrichtian) and early Paleocene (Puercan) portions of D1 synorogenic strata (Arapahoe, Denver, and lower Dawson formations). Smaller collections from the early Eocene (Wasatchian) of D2 synorogenic strata (upper Dawson Formation) indicate some potential for future work. Several patterns are now apparent. Late Cretaceous floras are diverse and dominated by angiosperms with small often lobed leaves. Palms and other monocots and ferns are relatively common but conifers are rare to absent. Earliest Paleocene floras (early Puercan, 65.5-65 mya) are also dominated by dicot angiosperms, palms and ferns but these floras are species poor, often with less than a dozen taxa. Despite their low diversity, these floras appear to exhibit a compositional gradient from the coarse clastic depositional systems of the synorogenic basin margin to the distal peat swamps of the eastern basin. The Paleocene floras of uppermost D1 (apparently 64.1 mya), particularly the Castle Rock and Scotty’s Palm sites, exhibit the high diversity and foliar physiognomy modern tropical rainforests suggesting that orography of the uplifted Front Range was the cause of locally high levels of rainfall. The floras of D2 synorogenic strata are poorly known but are composed of relatively smaller leaves than the upper D1 floras, suggesting that elevational gradients had changed in the nine million years between the deposition of the two floras.