Abstract: Overpressured Shale Masses and Growth Faulting, New Tectonic Style in Cretaceous Foreland Basin, Colorado

Robert J. Weimer, Tom L. Davis

Basement-controlled fault systems have been regarded as the dominant structural style of the Rocky Mountain petroleum-producing basins. Recurrent movement on major fault systems influenced sedimentation and the location of local unconformities during the Paleozoic and again in the Late Cretaceous-Cenozoic. New observations suggest fault movement under the Cretaceous sea floor which influenced bathymetry, fracturing, thickness of strata, marine-sand distribution, and possibly source beds in the Benton and Niobrara Formations and lower Pierre Formation.

An associated new tectonic style has been recognized in the uppermost Cretaceous rocks along the west side of the Denver basin. Geologic and geophysical data show a growth-fault system associated with deltaic sedimentation and overpressured shale masses similar to the tectonic style of many Cenozoic sequences along continental margins. The zone of unusual faulting is about 10 mi wide and 25 mi long (16 × 65 km), extending northeast from near Boulder, Colorado. Numerous high angle normal faults form complicated horst-graben patterns with throws ranging from a few feet to more than 400 ft (122 m). From seismic data, nearly all of the dominantly northeast-trending listric normal faults do not appear to extend below a depth of 5,000 ft (1,523 m). Near-surface growth-fault movement is indicated by a 4-fold thickening of the Fox Hills Sandstone from a normal 75 ft to over 300 ft (23 to 91 m), and the presence of thicker mineable coal seams in the Laramie Formation within downthrown fault-blocks. A causitive feature associated with this shallow growth-fault system is a localized low-density, low-velocity, overpressured shale mass within the upper 4,000 ft (1,219 m) of the Pierre Shale.

The recognition of the interrelation between tectonics and sedimentation in the Cretaceous foreland basin is important to exploration for petroleum and coal in the Rocky Mountain region.

AAPG Search and Discovery Article #90971©1976 AAPG-SEPM Rocky Mountain Sections 25th Annual Meeting, Billings, Montana