FAULT ZONE ARCHITECTURE OF POLYGONAL FAULT SYSTEMS IN THE NIOBRARA CHALK OF THE CENTRAL GREAT PLAINS

We are researching faulting in the Niobrara Chalk in the Central Great Plains. Recent work indicates that the normal faults found in the Niobrara are polygonal in nature. Polygonal faults and shear fractures are recognized as a common occurrence during diagenesis; the general mechanism for the faulting is diagenesis, but the specific are unclear. Polygonal fault zones are not uncommon in Cretaceous chalks; other studies include the Khoman Formation in Egypt. The Khoman is well studied with elevated pore pressures playing a notable role in fault initiation and subsequent activity. Our work and previous literature suggest common distinctive fault traits throughout the Niobrara and equivalent chalk formations.

This study focuses on fault zone architecture. We analyzed samples collected from in and around fault zones. Textures and structural elements within fault zones were observed to identify the mechanics of movement and the evolution of fault conditions. Analysis of microstructures on samples shows a lot of significant geometric angular relationships between features. The most prominent feature is calcite veining. The varying thickness in veins indicate changes in fault movement rate. Crystal size and orientation of veins change over time, an abundance of cross-cutting relationships help determine relative timing of various features. The fault architecture observed in samples is particularly well organized in comparison to most fault breccias found in fault zones. Another notable feature is corrugated slip surfaces; changes in the degree of corrugation represent changes in the rocks internal strength with time. After comparing the relative orientations between the slip planes and veins we concluded that pressurized fluids play an important role during faulting. These features raise interesting questions about the interplay between diagenetic and structural processes in chalks. Further studies are currently being done on thin sections to analyze faulting at a microscopic scale. This significance of this study is to better understand; fault architecture/mechanics in chalks, the role of pore pressure and the significance of fluid flow through fault and surrounding fractures, and the processes involved with structural diagenesis.