Link to USGS home page.
Professional Paper 1696
  About USGS /  Science Topics /  Maps, Products & Publications /  Education / FAQ

40Ar/39Ar and K-Ar Geochronology and Tectonic Significance of the Upper Cretaceous Adel Mountain Volcanics and Spatially Associated Tertiary Igneous Rocks, Northwestern Montana

By Stephen S. Harlan, Lawrence W. Snee, Mitchell W. Reynolds, Harald H. Mehnert, R.G. Schmidt, Steve D. Sheriff, and Anthony J. Irving

thumbnail image of report's coverpage

We report new 40Ar/39Ar and K-Ar dates from the Upper Cretaceous Adel Mountain Volcanics of northwestern Montana and spatially related Tertiary igneous rocks. The Adel Mountain volcanic field consists of about 900 square kilometers of lavas, associated volcaniclastic strata, and intrusions that lie astride the easternmost folds of the Montana disturbed belt of the Cordilleran fold and thrust belt. The Adel Mountain volcanic rocks have been intensely deformed by folds and thrust faults along their southwestern margin but are essentially undeformed to the east. Prior to isotopic dating, the age of the Adel Mountain Volcanics was the subject of debate, with age assignments ranging from Late Cretaceous to early Tertiary. Isotopic dates reported here demonstrate that the Adel Mountain Volcanics are clearly Late Cretaceous and that the volcanic rocks were probably emplaced during an approximately 2- to 3-million-year interval between about 76 to 73 mega-annum (Ma). The new dates from the Adel Mountain Volcanics are significant in that they provide a more refined and reliable age for the Late Cretaceous cratonic paleomagnetic reference pole for North America. The dates from the Adel Mountain Volcanics, as well as those from spatially related younger intrusions, also provide important constraints on the age of fold and thrust-belt deformation along the eastern margin of the Montana disturbed belt. Syntectonic deformation of the Adel Mountain Volcanics, as well as apparent folding and faulting of Tertiary quartz monzonite sills, indicates that contractional deformation clearly spanned the Late Cretaceous and may have extended to as young as the Paleocene/Eocene boundary at about 55.5 Ma. Elsewhere, posttectonic field relationships indicate that deformation may have ended prior to 60 Ma. Complexities in field relationships with respect to folds and faults shown by the early Tertiary intrusions as well as complications in the argon systematics indicate that these interpretations must be considered preliminary. Further field work and structural studies and additional high-precision geochronology are needed in order to place limits on the cessation of contractional deformation in this part of Montana. Unambiguously posttectonic dikes (47.5 Ma) that cut all deformed rocks and structures in the area indicate that disturbed belt deformation had clearly ceased by the early middle Eocene prior to the onset of widespread crustal extension in this part of the northern Cordillera.

Version 1.0

Posted August 2005

Part or all of this report is presented in Portable Document Format. The latest version of Adobe Acrobat Reader or similar software is required to view it. If you wish to download the latest version of Acrobat Reader free of charge, click here.

High-quality printed paper copies of this book are available for purchase at Information Services, U.S. Geological Survey (1-888-ASK-USGS).

For viewing and printing.
(This version of the report is accessible as defined in Section 508.)
Downloading Suggestion:

It is best to download a large PDF file to your hard drive rather than open it inside your browser. (A standard click may automatically open the PDF file inside the browser but doing so will result in a very slow load.) For guidance on how to do this, go to [].Downloading the PDF file may take several moments but will be worth the wait. Once it is downloaded, open the PDF from your hard drive using Adobe Acrobat—it will open in a fraction of the time it would take to open the PDF over the Internet. logo  Take Pride in America button