Ichnology and sedimentology of the Coniacian non-marine Frontier Formation, western Centennial Mountains, southwestern Montana, USA

ABSTRACT

The Coniacian non-marine portion of the Frontier Formation in southwestern Montana has been primarily studied stratigraphically, whereas its trace fossil record and relative sedimentological significance have remained overlooked so far. We present the first description of a trace fossil assemblage reported from Frontier Formation deposits exposed in the western Centennial Mountains, Beaverhead County, Montana. Three main lithofacies groups with distinctive trace fossil assemblages have been recognised at the locality of Price Creek: (1) coarse-grained, thin and laterally extensive cross-bedded sandstone channel bodies characterised by rare vertical, passively filled burrows; (2) fine-grained overbank sandstones and siltstones, with abundant horizontal, meniscated, and unwalled burrows, sinusoidal trails, vertical burrows, and dinosaur tracks, and (3) fining-upward, planar cross-bedded silty sandstones, associated with carbonaceous plant material and bone fragments. The trace fossil assemblage is ascribed to the pre-desiccation suit of the Scoyenia ichnofacies. The occurrence of theropod, ornithopod, and thyreophoran dinosaur tracks, and invertebrate trace fossils suggests considerable diversity of terrestrial and aquatic fauna. Tracks described here represent the first dinosaurian record reported from Coniacian deposits in Montana and one of the few fossil records reported from non-marine Coniacian deposits in North America.

KEYWORDS:

• Invertebrate trace fossils
• dinosaur tracks
• early Upper Cretaceous
• North America

Acknowledgments

We are thankful to the Montana Geological Society for the scholarship grant provided; the authors express their gratitude to all the BLM staff for their help, availability, and for the opportunity they give us to work on federal land; special thanks goes to the crew members that during field work greatly helped in excavation and data collection; we would also like to thank J. Scannella, S. Williams, A. Atwater, and D. Bowen for his advices and useful discussions on the paper. Finally, we thank the three anonymous reviewers whose suggestions considerably helped improving this article.

Disclosure statement

No potential conflict of interest was reported by the authors.

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Funding

This work was supported by the Motana Geological Society