A LATE CRETACEOUS POLYGONAL FAULT SYSTEM IN CENTRAL NORTH AMERICA

Polygonal fault systems (PFS) have been interpreted worldwide using three dimensional seismic data recorded to image sedimentary strata. The initially simple listric normal faults grow vertically and can intersect laterally, forming regional-scale PFS that are difficult to image without the use of 3-D seismic data. PFS are formed in fine-grained sedimentary muds and chalks that undergo listric normal faulting soon after deposition. The faulting is initiated during sediment dewatering and mud particle consolidation that can be independent of external stresses. In the last 20 years, hundreds of basins worldwide have been interpreted to contain or partly contain polygonal faults.

This paper presents a PFS interpretation for siliceous muds and chalks deposited in the Late Cretaceous Western Interior Seaway of North America. The faulted strata have been observed as a PFS at depths ranging from ~2750 m subsurface to outcrop. Seismic dataset interpretations and borehole analyses corroborate with previously published outcrop analyses and seismic interpretations by others. The larger observed faults are mesoscale in size, with throws up to 80 m and mean strike lengths of ~1.5 Km. Potentially encompassing over 2,000,000 Km², observational averages imply 10⁷ or more mesoscale size faults with a magnitude greater number of smaller faults. At shallow depth and outcrop the PFS model can explain Late Cretaceous shale fracturing attributed to other causes such as tectonic faulting and simple sediment slumping. In the subsurface, a PFS model can help map fracture geometries for hazard assessment and hydrocarbon or water production.