- UW Department of Geology and Geophysics
A very diverse, early Paleocene (63.8 ± 0.3 Ma) fossil leaf site located in Castle Rock, Colorado represents nearly autochthonous burial of a rainforest floor. This is an unusual fossil flora preserved in an unusual manner. The site, on the western margin of the Denver Basin in synorogenic sediments associated with the rise of the Laramide Front Range, is dated using multiple methods. Leaves are preserved in three distinct units overlying a poorly developed paleosol that contains in situ tree trunks. Fossil-bearing units are continuous along 150 m of outcrop. The leaves were apparently preserved as a result of rapid deposition of sand and mud onto the floor of a mature rainforest via overbank flooding. Five quarries were excavated and the leaves from these quarries were segregated by morphotype and scored for leaf area and margin type. From 1030 specimens, we document 93 unique dicotyledonous angiosperm leaf types, three cycads, three ferns, two conifers, and seven seed types. There is little taxonomic variation among leaf-bearing units of a single quarry, but the taxonomic composition varies significantly among laterally spaced quarries, suggesting that the fossil leaf litter reflets the original growth positions of the source trees. We compare the fossil leaf litter to leaf litter of modern forests and show that the Castle Rock flora has numerous features in common with extant equatorial rainforests, including dominance by angiosperms, high species richness, large leaves that often have smooth margins and drip tips, and high spatial heterogeneity from quarry to quarry.