Polygonal Faults in Chalk: Insights From Extensive Exposures of the Khoman Formation, Western Desert, Egypt

Abstract

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.

Department(s)

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

Geographic Coverage

Egypt

International Standard Serial Number (ISSN)

0091-7613

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2014 Geological Society of America (GSA), All rights reserved.

Publication Date

01 Jun 2014

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