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