Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt

Although polygonal fault systems and related features are common in fine-grained sediments in modern submarine basins and have been studied in basins worldwide using three-dimensional (3-D) seismic data, extensive on-land exposures have remained elusive. We report here on the discovery of a polygonal fault system occurring in nearly continuous surface exposure over similar to 900 km(2) in chalk of the Cretaceous Khoman Formation near Farafra Oasis, Egypt. Field exposures reveal polygon boundaries defined by clusters of dozens of normal faults with strongly grooved fault surfaces and coarse calcite veins along faults with evidence for multiple fluid flow events. Geometric patterns and fault intersections reveal that mechanically interacting normal faults with multiple orientations were active contemporaneously in a horizontal strain field that was essentially isotropic and extensional. We interpret the very steep dips (similar to 80 degrees) to reflect fault initiation in response to elevated pore fluid pressures. In the uppermost part of the Khoman Formation, a terrain of isolated circular structures displaying shallow inward dips overlies the polygonal fault network. The spatial relationship to the underlying faults is consistent with these small circular basins having formed as fluid escape structures as the polygonal fault system evolved. Outcrops in the Khoman Formation provide an unprecedented look into the 3-D geometry of a polygonal fault system, providing context for the analysis of analogous systems in marine basins and other on-land exposures.