USING STRATIGRAPHY TO FIND THE GEOLOGIC AGE OF A NEW MORRISON BONEBED IN NORTHWESTERN, COLORADO

The Morrison Formation has been a treasure trove of dinosaur discovery for over a century. This is in part due to the vast numbers of bonebeds, including the wall in Dinosaur National Monument. Nearby, northwestern Colorado is a unique geologic area and is an untapped place of paleontological as well as geological study. The area has the potential to connect the incredibly well studied Colorado Plateau to the southwest, and the lesser known Green River Basin of Wyoming to the north. The Morrison Fm. has not been well studied in northwestern Colorado, but a new dinosaur bonebed to the northeast of Dinosaur National Monument has yielded promising results including an interesting taphonomic signature. Using stratigraphy and petrology, the bonebed has not only been placed in the Late Jurassic Morrison Fm., but in the older Salt Wash Member. To the author’s knowledge, this marks the first time a bonebed has been discovered within a member other than the Brushy Basin. As such, this locality warrants attention and increased study.

Consistent with other Morrison bonebeds, many of the fossilized specimens are articulated including nearly 50% of a Dryosaurus with cranial material, ribs and cervical vertebrae of a diplodocid sauropod, a complete phalange of a small to medium-sized theropod tentatively assigned to Tanycolagreus and a minimum of five caudal vertebrae of a Brachiosaurus. Many individual elements have also been recovered in the quarry. These include three Brachiosaurus ribs, and a possible pelvic bone, two large theropod ischiums tentatively identified as Ceratosaurus, a dorsal vertebra from a medium-sized dinosaur, and a few isolated phalanxes. To add to the unique assemblage, this quarry has also yielded several microfossils from crocodilian to mammal.

Geologically the site consists of a green-gray calcareous medium grained sandstone with pebbled green mud clast. The sandstone gradually grades above into a limestone. Below it is bounded by a gray calcareous bentonite mudstone. A large concretion is present within as well as a gypsum vein. This intriguing combination of lithologic features is interpreted as a river channel entering into a primarily wet large lacustrine environment which periodically alternated to dry.