A Tectonic Context for Basement Linears in Northeast Wyoming
Ginger S. Dodson
Dodson Exploration, Evergreen, CO
Wrench fault zones mapped where the Rocky Mountain province meets the Great Plains of North America indicate scaled lateral offsets in interior cratonic movement. In northeast Wyoming and surrounding areas the tectonic terrain is influenced by the intersection of two separate thousand mile long mega scale features. The westerly trending Nye-Bowler segment of a mega wrench fault zone connects the north ends of the Big Horn Mountains and the Black Hills. This linear shear also bounds the northern limit of the Powder River Basin between the two uplifts. The northerly trending Front Range fault system splits at the north end of the Laramie Range extending east and west into a linear system of basin-bounding faults. These faults form the southern margin of the Powder River Basin.
Surface mapped faults in this region are either vertical north-trending, or lateral east-west trending. Wrench elements in regional terrain north and west of the Powder River Basin have left-lateral orientations. East and south of the basin, wrench elements in the regional tectonic terrain usually have right-lateral orientations. Linear subsurface basement shear zone mapping from aeromagnetic data for the Powder River Basin shows that the synclinal axis occurs in an area where the right- and left-lateral styles intersect.
There is a strong correlation between linear basement shear zones and elongate oil and gas fields in the Powder River Basin. The wider flatter east side of the basin originally had stratal east dip and now has west dip. The steeper west side of the basin now dips more steeply to the east than it did during deposition of the Cretaceous strata. Surface tilting from lateral adjustments may influence deposition and migration similar to isostatic adjustments. These types of scaled tectonic movements influence the ongoing evolution of in-situ fracturing and faulting.
AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado