February Talk: Structural inheritance for the Laramide, Central Montana Uplift: A wrench-fault tectonic model related to Proterozoic Orgogenesis in the Foreland of the North American Cordillera By Jeff Bader

Structural inheritance for the Laramide, Central Montana Uplift: A wrench-fault tectonic model related to Proterozoic Orgogenesis in the Foreland of the North American Cordillera

By Mr. Jeffrey W. Bader, North Dakota Geological Survey

ABSTRACT

The Central Montana uplift of eastern Montana lies adjacent to the Northern Rocky Mountains on the western edge of the Northern Great Plains Physiographic Province, and just north of the Laramide belt of the Central Rockies. The origins of this deformed region have received little attention relative to Laramide tectonism and Precambrian basement interrelations, particularly within the overall context of plate tectonic evolution of the North American Cordillera. The Central Montana uplift is characterized at the surface by six well-defined fault zones with general trends to the WNW and NE. Previous work attributes development of these fault zones to transcurrent motion on basementrooted faults that deformed the sedimentary cover during Laramide orogenesis. A Paleoproterozoic origin for these fault zones is proposed, with the faults initially forming in cratonic basement as pure-shear conjugates during SW–NE convergence at the northeastern margin of the Wyoming Province. It is further proposed that the conjugate shears were re-activated as simple shears during the Laramide orogeny under similar SW–NE stress conditions as those in the Precambrian. Structural analysis of basement fabrics of the western Beartooth Mountains and Laramide surface structures (compiled from previously published data) indicate that these major, and likely regional, crustal anisotropies (conjugate shears) may have guided Laramide deformation across eastern Montana. Isostatic gravity data support these interpretations. Precambrian development of conjugate shears was likely related to Trans-Hudson (1.78-1.74 Ga) orogenesis along the northeastern Wyoming Province boundary. Data also indicate that these structures were re-activated as wrench faults during the Laramide orogeny, forming the Central Montana uplift. The structural style of the Laramide orogenic system, as exemplified by the Central Montana uplift, and relation to basement anisotropy has significant implications for the many petroleum systems of the entire Williston Basin of North America.

BIOGRAPHY

Jeffrey W. Bader, North Dakota Geological Survey, 2835
Campus Rd., Stop 8156, Grand Forks, ND 58202
email: jwbader@nd.gov.

Jeff Bader has been a geologist for over 40 years and is a Professional Geologist in Wyoming and Utah. He has a BA in Geology from the University of Colorado and a MS in Petroleum Geology from San Jose State University. He spent the first 8 years of his career at the USGS, Branch of Petroleum Geology. He then was a consultant for more than 20 years and for the last 6 years has served with the North Dakota Geological Survey where he is presently Director of the Wilson M. Laird Core and Sample Library. He also is an Adjunct Professor of Geology at the University of North Dakota. His research interests center on cratonic origins/development and resulting influences on the Phanerozoic tectonic, structural, stratigraphic, and sedimentological evolution of sedimentary basins. Jeff has authored over 50 publications on these topics, mostly in the Rocky Mountain region.

TIME: Please login to meeting a little before 5:30pm.

• 5:30pm – 6:00pm attendees will be randomly assigned to break out rooms for three, ten-minute sessions
• 6:00pm – 6:15pm back to meeting for society business announcements and introductory remarks
• 6:15pm – 7:15pm Talk by Mr. Jeff Bader and Q&A

LOCATION: Your own home! This is a Zoom meeting hosted by FLC.

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