THE UPPER CRETACEOUS WILLIAMS FORK FORMATION (LATE CRETACEOUS: CAMPANIAN–MAASTRICHTIAN), NORTHWESTERN COLORADO, AND ITS PALEOECOLOGICAL IMPLICATIONS FOR LARAMIDIA

While the Williams Fork Formation (WFF) of the “Mesaverde Group” in northwestern Colorado has not been as extensively studied as other units of similar age across Laramidia, more focused efforts in the past 16 years offer a better understanding of its paleoenvironment. The WFF contains plant mega- & microfossils, leaves with insect feeding traces, freshwater mollusks, & a diverse vertebrate assemblage, including dinosaurs, giant crocodilians, lizards, turtles, amphibians, fishes, & mammals, similar to other coeval deposits including Wyoming’s Mesaverde Formation & Alberta’s Horseshoe Canyon Formation. Of the half a dozen most productive sites recently surveyed, each has yielded new taxa, including a new metatherian mammal (Heleocola piceanus), or extended the previously known geographic distribution of taxa to the region, spanning the transitional zone between northern & southern faunas during the late Campanian to Maastrichtian. Many of the sites are associated with a particular channel sandstone (Sandstone C) dated to a maximum depositional age of 71.7 +/-1.8 Ma based on detrital zircons, placing this section of the WFF in the basal Maastrichtian.

The WFF fossil assemblage contains terrestrial, freshwater, estuarine & marine taxa, representing a fauna that inhabited a nearshore marine to deltaic setting. Vertebrate microsites reflect different environmental influences, with some sites presumably near estuaries containing abundant & diverse chondrichthyans, whereas others are dominated by gars & contain few if any chondrichthyans. Dinosaur fossils are fragmentary, but include the ceratopsian Pentaceratops, a kritosaurin hadrosaur, neornithischians, ankylosaurs, ornithomimosaurs, dromaeosaurs, oviraptorosaurs, & tyrannosaurs. Plant megafossils reflect a terrestrial flora with 16 morphotypes of angiosperm dicot leaves, 7 conifer taxa, including Metasequoia & Geinitzia, & 6 types of ferns. Palynology & water lily leaves recovered from sites below the dated channel sandstone also document freshwater habitats. Two-thirds of the palynofloral assemblage consist of a single species of aquatic fern spore as well, with one-quarter angiosperm pollen & a much lower proportion of gymnosperm pollen. The palynostratigraphy most closely resembles that of the slightly older Judith River Formation.