USING PALEOSOLS AND ICHNOFOSSILS TO INTERPRET THE CHANGING PALEOECOLOGY, PALEOENVIRONMENTS, AND PALEOCLIMATE OF THE EOCENE-OLIGOCENE WHITE RIVER FORMATION, NORTHEASTERN COLORADO

Exposures of the Upper Eocene to Middle Oligocene White River Formation in Logan County, Colorado, contain paleosols and ichnofossils of an alluvial system. The paleoenvironmental and paleoecological significance of vertical changes in these paleosols and ichnofossil assemblages were examined in relation to the initial stages of cooling in the North American midcontinent.

Four distinct paleosol types occur in the Logan County locality. Type I paleosols, interpreted as entisols, occur in silty, tuffaceous mudstone characterized by shallow, dense networks of calcareous rhizoliths; platy and subangular blocky peds; vertical, mudstone-filled burrows identified as Scoyenia; horizontal, interconnected, mudstone- and sandstone-filled burrows identified as Steinichnus; ovoid, thin-walled, cells identified as Celliforma; and carnivorous vertebrate coprolites. Type II paleosols, interpreted as inceptisols, occur in clay-rich, tuffaceous mudstone characterized by elongate, branching, calcareous rhizocretions; angular blocky peds; branching, horizontal and vertical burrows filled with textured mudstone balls identified as Coprinisphaera; and vertebrate tracks. Type III paleosols, interpreted as inceptisols, occur in silty, tuffaceous mudstone characterized by elongate, branching rhizocretions; large, angular blocky peds; Coprinisphaera; and horizontal to vertical, elliptical pellet-filled burrows assigned to Edaphichnium. Type IV paleosols, interpreted as alfisols, occur in clay-rich siltstone characterized by subangular, blocky peds; rhizocretions up to 1 m long and 10 cm in diameter; horizontal to vertical burrows assigned to Scoyenia; and ovoid, mudstone cells interpreted as insect cocoons.

The vertical transition from less-developed (Type I) to well-developed (Type IV) paleosols record changes in rates of sedimentation, pedogenesis, and erosion as well as a transition from open grasslands in a semiarid climate to woodlands in a warm-humid climate.