The Lance Formation is notoriously difficult to decipher stratigraphically, as most sedimentary features are transient and the most apparent horizons are diagenetically altered beds that are not reliably traceable over distance. While mapping a series of rich bonebeds in the upper Lance Formation in the eastern limb of the Powder River Basin of northeastern Wyoming, we encountered a two meter thick bed of tan sandstone that consistently revealed characteristics of a seismite. The unit showed pervasive meter scale chaotically displaced bedding, flame structures and features characteristic of rapid dewatering. The overlying and underlying units of similar crossbedded sandstone showed no alteration of original bedding structure. This bed was traced continuously in outcrop for distances of up to a kilometer at several locations over 50 square km. This has enabled us to derive an accurate regional dip for the Lance in this area.
Using this seismite as a datum, we were able to successfully develop a stratigraphic framework for the area containing the bonebeds of interest. One of these bonebeds, 15 meters below the seismite horizon thus far has yielded Edmontosaurus, Struthiomimus and Triceratops bones, as well as abundant gar scales, numerous teeth of theropod and ornothopod dinosaurs and turtle fragments and crocodile scutes typical of Lancian “microsites”. Our major bonebed, occurring 29 meters below the seismic horizon, contains the disarticulated remains of more than 15,000 Edmontosaurus in a normally graded bed. This bed can now be mapped with confidence over a wide area. A third prolific bonebed, 38 meters stratigraphically below the seismite, has thus far yielded no remains of Edmontosaurus, but contained disarticulated remains of Pachycephalosaurus, Nodosaurus, Tyrannosaurus, Triceratops, and other dinosaurs and reptiles, including numerous turtles. Other known bonebeds in the area can now be explored in a stratigraphic context.
