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Facies

, 65:15 | Cite as

Anatomy of the Book Canyon conglomerate: a sequence boundary at the top of the Bear Gulch Limestone in the Big Snowy Trough

  • Amy SingerEmail author
  • George D. Stanley
  • Nancy W. Hinman
Original Article
  • 138 Downloads

Abstract

The Serpukhovian Book Canyon Conglomerate is an unpublished limestone conglomerate contained within the Tyler Formation in central Montana. It overlies and contains clasts of the Bear Gulch Limestone, a plattenkalk deposit yielding amazing paleontological detail. The Book Canyon Conglomerate is up to 2 m thick, markedly lensoid, and laterally discontinuous in its outcrop for a distance of 2 km but likely extends beyond the study area. Well logs and cores indicate its presence in the subsurface between the underlying Bear Gulch Limestone (Serpukhovian) and overlying Tyler Formation (Serpukhovian–Morrowan). This conglomerate provides new information regarding the transition of the marine-dominated Bear Gulch Limestone to the overlying Tyler Formation. The Book Canyon Conglomerate is dominantly clast-supported and includes sedimentary structures including pebble and cobble imbrication, tangential cross-stratification, and an overall upward-fining character. The basal surface of the conglomerate is sharp and irregular where the top of the underlying Bear Gulch Limestone has been erosionally removed. The Book Canyon Conglomerate is therefore interpreted as a fluvial channel deposit formed on a surface of subaerial exposure at the top of the Bear Gulch Limestone. In areas where the conglomerate is thin or absent, the top of the Bear Gulch Limestone is interpreted as a paleosol developed in a semi-arid climate where it is preserved between channel incisions of the Book Canyon Conglomerate. The paleosol contains microcrystalline silica cement and limestone clasts that have weathered to a black color. Freshwater exposure and channelized fluvial erosion early in the post-depositional history of the Bear Gulch Limestone explain variations in the contacts between units. These variations contribute to the debate concerning stratigraphic relationships. Further analysis of unconformities and fauna at the base and top of the unit clarifies the Bear Gulch Limestone’s position in time, and its relationship to the Heath and Tyler Formations.

Keywords

Plattenkalk Lagerstätte Bear Gulch Book Canyon Heath Tyler Serpukhovian Montana Conglomerate 

Notes

Acknowledgements

The authors are indebted to The Cox Ranch at Rose Canyon for access and generous field support as well as abundant good cheer. Robert Rader, Heather Hart, and Pamela Lavering all made key contributions in labor and lab work. This study has been supported by The Montana Geological Society, the University of Montana Foundation, The National Science Foundation EAGER grant, and the Evolving Earth Foundation. Additional thanks to Kevin McCarthy, Richard Lund, Peter Isaacson, Richard Bottjer, and George W. Grader for their helpful suggestions.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  • Amy Singer
    • 1
    Email author
  • George D. Stanley
    • 2
  • Nancy W. Hinman
    • 1
  1. 1.Department of GeosciencesThe University of MontanaMissoulaUSA
  2. 2.The University of Montana Paleontology CenterMissoulaUSA

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