An extensive polygonal fault system (PFS) within fine-grained Upper Cretaceous sediments beneath the Great Plains of North America has implications for geotechnical engineering. Geological well control, outcrop, and three-dimensional seismic data from southeast Saskatchewan exemplify the fault characteristics typically observed within the PFS. The deepest faults are sparse, offset a seismic reflection identified from the Niobrara Formation Govenlock member, and have vertical offsets <2 m. The deformation increases in fault density and vertical offset at shallower depths, reaching 6 faults/km2 with up to 30 m of vertical offset. Upper Cretaceous strata throughout the Great Plains area are at or near outcrop, and the extensive PFS faulting and weathering have weakened the rock. This faulting and weakness have been observed and attributed to other factors such as glacial erosion, overconsolidation, swelling bentonite beds, or landslides from toe erosion at topographic slopes. The PFS faulting should be recognized as an extensive process to be considered when undertaking geotechnical analysis on the Great Plains where underlying Upper Cretaceous rocks exist. Engineering implications include road cuts, dam impoundments, building foundations, and natural slumping.
An examination of Plesiosauria (Diapsida: Sauropterygia) from the Niobrara Formation (Upper Cretaceous) of central North America