Basement Fault Control of Offshore Cretaceous Sandbars in the Powder River Basin, Wyoming

Gay, S. Parker
Applied Geophysics, Inc, Salt Lake City, UT

     Cutting a broad 25 mile wide NW-trending swath across the Powder River Basin are a series of oil fields that occur in Upper Cretaceous offshore sandbars. Stratigraphic units involved include Shannon, Sussex, Ferguson, Parkman, Tecla and Teapot. At first glance these fields would seem to fall in the “purely stratigraphic” category. However, of the 20 fields studied 13 lie over well-mapped basement faults , several of which I will show. The remaining 7 probably lie over basement faults that are not easily mappable with the magnetic methods employed.
     Two depositional mechanisms have been proposed to explain the relationship of sandbars to basement faults. Swift and Rice (l984) suggested that fault movement created long, linear seafloor highs on which the winnowing action of bottom currents deposited porous sands. More recently, Denver geologists Horne and Inden have proposed instead that sands deposited as lowstand shorelines were reworked and preserved on the downthrown sides of seabed fault scarps following sealevel rise. Either mechanism calls on faulting as the control, so these bars cannot properly be considered “purely stratigraphic.”
     Basement control on deposition of offshore sand bars is just one facet of “reactivation tectonics,” which is currently revising many long-standing, outmoded, or poorly-explained concepts of structural geology. Although these new geological revelations have been slow in acceptance, they will be accepted when it is realized how many geological phenomenon are better explained by reactivation of preexisting basement faults.

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah