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