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 threedimensional (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 ~900 km2 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 (~80o) 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.
B. J. Tewksbury et al., "Polygonal Faults in Chalk: Insights From Extensive Exposures of the Khoman Formation, Western Desert, Egypt," Geology, vol. 42, no. 6, pp. 479 - 482, Geological Society of America (GSA), Jun 2014.
The definitive version is available at https://doi.org/10.1130/G35362.1
Geosciences and Geological and Petroleum Engineering
Keywords and Phrases
Geology; Circular Structures; Continuous Surface; Fine-Grained Sediment; Geometric Patterns; Multiple Orientations; Spatial Relationships; Three-Dimensional (3-D) Seismic Data; Western Desert , Egypt; Limestone; Chalk; Cretaceous; Dip; Fault Geometry; Fault Zone; Fluid Flow; Normal Fault; Strain; Egypt; Farafra Oasis; New Valley; Western Desert
International Standard Serial Number (ISSN)
Article - Journal
© 2014 Geological Society of America (GSA), All rights reserved.
01 Jun 2014