A LATE CRETACEOUS POLYGONAL FAULT SYSTEM IN CENTRAL NORTH AMERICA

Abstract

Polygonal fault systems (PFS) have been interpreted worldwide using seismic data imaging sedimentary strata. Normal faults initiate over a large area in fine-grained subaqueous strata soon after deposition. As they continue to grow laterally and vertically, their fault traces intersect to form polygons in plan view. These polygons are difficult to image without three-dimensional (3-D) seismic data. The faulting is initiated during sediment dewatering and mud particle consolidation that can be independent of external stresses. In the past 20 years, hundreds of basins worldwide have been interpreted to contain polygonal faults. This paper presents a PFS interpretation for fine-grained sediments deposited in the Late Cretaceous Western Interior Seaway of the Great Plains 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 previously published outcrop analyses and seismic interpretations. The larger observed faults are mesoscale in size, with throws up to 80 m, and strike lengths up to ∼1.5 km. Potentially encompassing over 2,000,000 km 2 , observational averages imply 10 7 or more mesoscale-size faults, with an order of magnitude greater number of smaller faults. At shallow depth and outcrop, the PFS model of extensive normal faulting could help to explain Late Cretaceous shale faulting attributed to other causes such as deeper sediment dissolution or glacial processes. In the subsurface, faulting and fracturing consistent with a PFS model can help to explain fault geometries observed in well control and 3-D seismic data.