Evidence for an Early Paleocene Rainforest From Castle Rock, Colorado
ELLIS, Beth, Denver Museum of Nature and Science, 2001 Colorado Boulevard, Denver, CO 80205-5798, [email protected] , and JOHNSON, Kirk R., Denver Museum of Nature and Science, 2001 Colorado Boulevard, Denver, CO 80205-5798, [email protected]

In 1994, a markedly atypical Denver Basin fossil leaf site was discovered in the uppermost Denver Formation on the east side of Interstate I-25 in Castle Rock, Colorado. Extensive excavation of four quarries along 52 m of outcrop in this 18-cm-thick bed yielded an unusually diverse, warm rainforest flora composed largely of previously unknown taxa. The bed represents rapid and parautochthonous burial of leaf litter by overbank flooding. This interpretation is supported by the presence of leaves at all angles to bedding, a complete cycad plant buried in growth position, and by high floral heterogeneity between quarries. Subsequent dating of the site, using pollen analysis, radiometric dating, and analysis of sedimentation rate from two cores, places it in the Paleocene and earlier than 61.4 MYA. The flora is still being analyzed, but the first 522 specimens studied represent 114 species (107 angiosperms, 3 ferns, 2 cycads, 2 conifers). Dicot leaves from this site are generally very large, and many have smooth margins and drip tips. Leaf-margin analysis of the flora estimates a mean annual temperature (MAT) of 23.7 +/- 2 degrees C and leaf-area analysis estimates a mean annual precipitation (MAP) of 213 cm (standard error range of 149-306 cm). This flora is more diverse and has a greater estimated MAT and MAP than any other early Paleocene flora known from the Western Interior.