Insights About Polygonal Faults and Related Structures from Extensive Exposures of the Cretaceous Khoman Formation, Western Desert, Egypt

Barbara Tewksbury, Hamilton College
John Hogan, Missouri University of Science and Technology
Simon Kattenhorn, University of Idaho
Charlotte Mehrtens, University of Vermont
Elhamy Tarabees, Damanhour University

Seismic investigations over the last 15 years in marine basins around the world have revealed the common occurrence of sets of extensional faults that intersect to form networks of large polygons, each hundreds of meters to more than a kilometer across. Although polygonal faults been studied remotely in over 100 basins worldwide, extensive on-land exposures that lend themselves to field study have remained elusive. Our work has revealed a polygonal fault system in chalk of the Khoman Formation near Farafra Oasis, Egypt that is almost continuously exposed over an area of ~1000 km2 by a unique combination of regional structure, topography, and climate. Polygonal faults in the Khoman are expressed as a complex network of thousands of low, narrow ridges outlining polygonal areas 500-1000 m across. The polygonal ridge network consists of normal faults that occur in clusters, with the ridges held up by multi-phase calcite veins formed along faults in the chalk, along with subsidiary iron sulfide veins now altered to iron oxides. Grooves in the host chalk have rakes of 75-90°, and offsets indicate small amounts of normal slip on faults with steep dips (70° to nearly 90°). Fault geometries indicate that mechanically interacting, multiple fault orientations were active contemporaneously and that the horizontal strain field was essentially isotropic and extensional. The absence of any evidence for silica or clay indicates that models of polygonal fault initiation proposed by others involving volume loss due to diagenesis of biogenic silica or smectite is neither applicable in the Khoman nor necessary for polygonal faulting. We interpret the very steep dips to reflect fault initiation in response to elevated pore fluid pressures.

A terrain of isolated basins overlies the polygonal fault network. The basins range from ~50-200 m in diameter and have layering with very shallow inward dips. The polygonal fault system cuts and locally offsets the basins in the oldest parts of this terrain, and basins are spatially associated both with faults and radial veins. We interpret these isolated basins as fluid escape structures formed as the polygonal fault system evolved. This interpretation is consistent with multi-phase calcite veins along the polygonal faults.

Polygonal fault systems should be common in fine-grained sedimentary sequences in the rock record. Our field observations provide new insights into features of polygonal fault systems that lie below the resolution of seismic studies and that may assist in the recognition of on-land exposures elsewhere.