Overview of the Late Cretaceous, early Paleocene, and early Eocene megafloras of the Denver Basin, Colorado Kirk R. Johnson 1 *, Michele L. Reynolds 1 , Kevin W. Werth 1 , and Joseph R. Thomasson 2 l Denver Museum of Nature & Science, 2001 Colorado Boulevard, Denver, CO 80205, U.S.A. department of Biological Sciences, Sternberg Museum of Natural History, Fort Hays State University, Hays, KS 67601, U.S.A. * Author to whom correspondence should be addressed: kjohnson@dmns.org. ABSTRACT Late Cretaceous and Paleogene plant fossils collected at 149 localities in the Denver Basin, Colorado, are placed into a stratigraphic framework based on palynostratigraphy, magnetostratigraphy, vertebrate paleontology, geochronology, sequence stratigraphy, electric well logs, and two cored wells. Between 69 and 54 Ma, the Denver Basin accumulated sedimentary rocks that recorded the withdrawal of a seaway, the uplift of a mountain range, and evidence of the Cretaceous-Tertiary and Paleocene-Eocene boundary events. Fossil floras deposited in the Denver Basin record these events as variations of floral composition, species diversity, and leaf margin and size (used to estimate mean annual temperature and precipitation, respectively). Attention to these details and to the position of the floras relative to the basin margins and sedi- mentary facies allows for the recognition of six megafloral associations (K-L, K-Dl, P-Dl-West, P-Dl-Central, P-Dl-East, and E-D2). Preliminary comparison of these assemblages documents: floral change at the K-T boundary; a strong paleoenvironmental gradient probably associated with increased topographic relief along the basin margin in the early Paleocene; and a warmer, drier Eocene vegetation. KEY WORDS: Paleobotany, paleoclimate, Cretaceous, Paleocene, Eocene, Denver Basin, Lara- mie Formation, Dl sequence, D2 sequence. INTRODUCTION The Denver Basin is a Rocky Mountain foreland basin lying to the east of the Colorado Front Range and underlying Denver and Colorado Springs (Fig. 1). Front Range uplift and basin formation com- menced in the latest Cretaceous, shortly after the withdrawal of the Western Interior Seaway, and continued through the Cretaceous-Tertiary (K-T) boundary event into the early Paleocene. Sediment accumulation ceased in the middle of the Paleo- cene, then commenced at the end of the Paleocene and recorded the global thermal maximum of the early Eocene. The preorogenic and synorogenic rocks of the Denver Basin contain abundant fossil plants that document environmental and climatic changes, response to the K-T boundary event, and the effects of increasing topographic gradients on the distribution of orographic rainfall and the result- ing plant communities. Unlike today, Late Cretaceous Colorado was moist, warm, and heavily forested. As a result, fossil leaves are abundant in Upper Cretaceous and lower Tertiary strata of the Denver Basin. This was recog- nized as early as 1867 when F. V. Hayden collected fossil leaves from the Laramie Formation near Mar- shall, Colorado (Le Conte, 1868; Knowlton, 1922). Extensive research on these fossils was undertaken by paleontologists working for the Surveys of the Western Territories and the U.S. Geological Survey (USGS). Their work resulted in a series of papers and monographs by Lesquereux (1878,1888), Ward (1887), Knowlton (1896, 1922, 1930), and Brown (1943, 1962). Beginning in the 1890s, considerable atten- tion was focused on what became known as the "Laramie Problem." This was a dispute between paleobotanists and vertebrate paleontologists con- cerning the correct stratigraphic position of the Rocky Mountain Geology, v. 38, no. 1, p. 101-120, 10 figs., 2 tables, May, 2003 101