Reinterpretation of the palynology and age of Laramide syntectonic deposits, southwestern Montana, and revision of the Beaverhead Group

Research Article| February 01, 1985 Reinterpretation of the palynology and age of Laramide syntectonic deposits, southwestern Montana, and revision of the Beaverhead Group D. J. Nichols; D. J. Nichols 1U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar W. J. Perry, Jr.; W. J. Perry, Jr. 1U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar J. C. Haley Johns J. C. Haley Johns 2Hopkins University, Baltimore, Maryland 21218 Search for other works by this author on: GSW Google Scholar Geology (1985) 13 (2): 149–153. https://doi.org/10.1130/0091-7613(1985)13<149:ROTPAA>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation D. J. Nichols, W. J. Perry, J. C. Haley Johns; Reinterpretation of the palynology and age of Laramide syntectonic deposits, southwestern Montana, and revision of the Beaverhead Group. Geology 1985;; 13 (2): 149–153. doi: https://doi.org/10.1130/0091-7613(1985)13<149:ROTPAA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract New palynological data from syntectonic deposits in southwestern Montana have major temporal implications for Laramide structural relations in the northern Rocky Mountains. Conglomerate and associated sandstones representing the bulk of material eroded from the thrust-faulted margin of the Blacktail-Snowcrest foreland massif are middle Campanian (Aquilapollenites senonicus Interval Zone; estimated absolute age 78–81 Ma). As a major consequence, the foreland buttress responsible for the shape of the southwestern Montana recess of the Cordilleran thrust belt was uplifted and in place by middle Campanian time. Palynomorph assemblages from the Campanian deposits include reworked spores of Paleozoic age that are evidence of provenance. Other less precisely dated syntectonic units in the area differ in age and provenance as well as lithology.Formerly, all these deposits were assigned to the Beaverhead Formation, which is raised to group status herein. The stratigraphic unit shed from the Blacktail-Snowcrest uplift is raised to formation status as the Lima Conglomerate. The term "Beaverhead Group" is restricted to the Lima Conglomerate and to contemporaneous and younger conglomeratic units that differ in lithology and probably in provenance; older sandstone units are removed from the Beaverhead Group. Deposits at the original type section of the Beaverhead Formation are not precisely dated but are no older than Campanian and probably no younger than early Maestrichtian. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.