Overview of the Late Cretaceous, early Paleocene, and early Eocene megafloras of the Denver Basin, Colorado

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 sedimentary facies allows for the recognition of six megafloral associations (K-L, K-D1, P-D1-West, P-D1-Central, P-D1-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.