PALEONTOLOGICAL TREASURES OF PICKET WIRE CANYONLANDS, A GLIMPSE INTO THE PURGATOIRE RIVER VALLEY, COMANCHE NATIONAL GRASSLAND, SOUTHEASTERN COLORADO

Picket Wire Canyonlands is a deeply carved, rugged valley of the Purgatoire River drainage in southeastern Colorado. Managed since 1992 by the USDA Forest Service, the canyon is well known for its extensive dinosaur tracksite, which preserves evidence of gregarious behavior in Upper Jurassic sauropods. The Forest Service carefully monitors the tracksite, protects it through the maintenance of erosion control structures, and hosts an aggressive educational and research program to ensure that this important paleontological site is preserved in perpetuity.

Until recently, significant paleontology in the canyon other than the tracksite was poorly known. But presently, volunteer survey efforts are revealing a number of important sites, including several partial dinosaur skeletons within the Morrison Formation. Exposures of the Morrison are pervasive, but are generally masked by colluvial material, leading to the past misconception that few dinosaur remains occur in this part of the Morrison basin. Partial skeletons of Camarasaurus and Allosaurus have been recovered, and three additional sauropod partial skeletons have been located. Another intriguing aspect of Morrison paleoecology in Picket Wire, as in other parts of the Morrison basin, is the common occurrence of polished silicic cobbles (?gastroliths). These stones represent a sedimentological paradox, as the maximum particle size for any unit in the Morrison is coarse sand. In some cases, these exotic, polished stones can be more convincingly argued to represent true gastroliths, as they are intimately associated with dinosaur remains.

Extensive exposures of Permian, Triassic, and Middle Jurassic geologic layers hold significant paleontological resources but are inadequately studied. Permian strata contain a distinctive algal-laminated dolomite bed-set useful for regional correlation. The Triassic section includes shallow marine beds bearing abundant ripples and salt casts that will be targeted to search for trackways. A Middle Jurassic conglomerate down-cut into the Entranda Sandstone bears theropod and crocodylian teeth, and abundant osteoderms. Cretaceous marine units bear abundant invertebrate traces and molds of molluscs. The Cretaceous Dakota Sandstone holds abundant theropod and ornithopod dinosaur tracks.