AAPG PSGD Webinar: Wrench-Fault Tectonic Map of Eastern Montana, USA by Jeffery Bader

Wrench-Fault Tectonic Map of Eastern Montana, USA
Presented by Jeffrey W. Bader
Bader Enterprises
Abstract:
Three prominent west-northwest-striking, basement-rooted, sinistral wrench faults have historically been interpreted to be present in eastern Montana: Nye Bowler, Lake Basin, and Cat Creek fault zones. North-northeast-striking, conjugate fault zones are also present, but historical documentation for dextral shift on these probable basement-weakness zones is minimal. They include the Fromberg and Weldon/Brockton-Froid fault zones. The fault zones combine to form the Central Montana uplift. These likely Precambrian anisotropies were most recently reactivated during the Laramide orogeny, when NE-SW-directed contraction dominated the North American Cordilleran foreland from Arizona to northern Montana. Slip on the buried basement faults has created characteristic simple-shear wrench deformation in the sedimentary cover rocks across eastern Montana and into northeastern Wyoming.
The Wrench-Fault Tectonic Map (WFTM) of Eastern Montana presents a simple-shear interpretation of surface structures related to wrench deformation on the previously mentioned fault zones, as well as a proposed fourth sinistral zone; herein referred to as the Poplar Dome fault zone. The approximate location of buried master-fault zones is inferred based on the orientation of surface structures as related to the simple-shear model used to identify subsidiary wrench structures developed in sedimentary cover rocks. This allows for the preliminary definition of wrench structures such as en échelon folds and faults (R, R’, P, normal, and reverse/thrust) on the WFTM based solely on the orientation of such structures prior to field and/or subsurface confirmation. Restraining bends forming rhomboid-shaped uplifts (e.g., Pryor Mountains) and releasing-sidesteps forming box-graben pull-apart basins (e.g., Bull Mountains and Crazy Mountains) may be explained by localized transpression and transtension. Other interpretations related to wrench-fault recognition will also be presented.
These structures have significant and fundamental implications related to the origin of the Archean Wyoming Province and subsequent Phanerozoic structural and tectonic evolution of the central and northern Rocky Mountains. In addition, the Precambrian age of these structures, convergent nature of the deformations, perhaps along with younger Proterozoic provinces (e.g., Yavapai, Mazatzal, Grenville), may provide insights into crustal inheritance for major Rocky Mountain Phanerozoic tectonic events (Ancestral Rocky Mountains and Sevier/Laramide orogens). Finally, as these zones continue to be investigated, their breadth across the Rocky Mountain corridor and entire Laramide belt grows, creating new structural plays that will be exploited with continued growth of new horizontal drilling techniques and targets (e.g., Niobrara), along with CCUS, hydrogen, and helium reservoir delineation.
Bio:
Jeff Bader has been a geologist for nearly 45 years and is a Professional Geologist in Wyoming. 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 working for numerous firms. He spent the last 8 years of his working career serving the North Dakota Geological Survey where he was Director of the Wilson M. Laird Core and Sample Library for 5 years. In September of 2022, Jeff decided to semi-retire, so he could focus his research on specific geological areas of interest from his home in Mexico, where he started Bader Enterprises. This research includes cratonic origins/development and resulting influences on the Phanerozoic tectonic, structural, stratigraphic, and sedimentological evolution of sedimentary basins. Jeff’s most recent work since semi-retiring has centered on analysis of potential Precambrian anisotropies and their role in crustal inheritance during Laramide orogenesis in the North American Cordilleran foreland of Montana, Wyoming, and Colorado. Jeff has authored over 50 publications on these topics, mostly in the Rocky Mountain region.