Marilyn D. Wegweiser¹

(1) Ball State University, Muncie, IN

ABSTRACT: Dinosaur Trackways In The Upper Cretaceous Two Medicine Formation: Dinosaur Ichnofacies As A Tool To Improve Sequence Stratigraphic Interpretation, An Aid In Identifying Reservoir Rock

Recent reconnaissance geological mapping in northwestern Montana revealed an occurrence of abundant dinosaur tracks in silty sandstone, in the Upper Cretaceous Two Medicine Formation. Dinosaur trackways preserve a unique depositional record in the basal Two Medicine Formation of Montana. The trackways provide new information about the unique backshore to nearshore paleodepositional environment of the Upper Cretaceous inner continental seaway, in an otherwise largely unfossiliferous stratigraphic interval. Mapping the distribution of this unusual dinosaur trackway and the associated invertebrate fauna provides a tool for modeling a higher resolution paleoenvironmental reconstruction for this portion of the Upper Cretaceous and identifying position within sequence stratigraphic intervals. Invertebrate fossils associated with the dinosaur trackways include gastropod body fossils and arthropod tracefossils.

The Upper Cretaceous terrestrial substrate of the basal Two Medicine Formation is heavily bioturbated by a sparse fauna. Evidence of soft sediment deformation during track formation suggests tracks made both subaerially and subaqueously. Casts and molds of dinosaur tracks suggest the following tracemakers were present: Theropods (Carnosaurs; Tyrannosaurids?), Ornithopods (Hadrosaurians), and putatively Ceratopsians and Anklyosaurids. Biostratinomical analysis of this unusually preserved assemblage provides information suggesting relative abundance of adult to juvenile tracemakers and predator to prey ratios. In addition, it provides new implications for Tyrannosaurid hunting behavior. The Two Medicine dinosaur ichnofacies provides a useful sequence stratigraphic interval recognition tool that could be used for higher resolution of mapping potential reservoir rock intervals in the backshore environment of the inner Upper Cretaceous Seaway.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado