ACCOMMODATING LAYER SHRINKAGE AND FAULT SLIP IN A POLYGONAL FAULT SYSTEM: MESO-SCALE STRUCTURES IN THE KHOMAN FORMATION, WESTERN DESERT, EGYPT

Polygonal fault systems consist of an array of layer-bound normal faults intersecting in broadly polygonal patterns. Faults of different orientations in a polygonal system form synchronously in an essentially isotropic horizontal stress field (S_hmax = S_hmin). Polygonal faults form early during burial of fine-grained sediments. As a layer shrinks radially, polygonal networks of normal faults develop with polygons 500-1000 m or more across. Although strains are radially isotropic and extensional, layers containing polygonal faults exhibit no net extension because host rocks undergo simultaneous volume loss accompanied by fluid expulsion. Polygonal fault systems are common in modern marine basins and have been studied almost exclusively in the subsurface using 3D seismic data. Chalk of the Cretaceous Khoman Formation in the Western Desert of Egypt displays the first extensive exposure of polygonal faults to have been recognized on land. Polygonal faults occur in continuous surface exposure over an area of ~700 km² and extend, partly mantled by aeolian sand, over at least 1800 km². The Khoman provides an opportunity to study meso-scale features that develop to accommodate layer shrinkage and to investigate how normal slip is accommodated at the base of a layer-confined fault system.

Virtually all polygon interiors in the Khoman display partial to complete sets of arcuate normal fault segments. Where well-developed, these faults occur as sets that are concentric in map view and conical in 3D, forming nested, outward dipping normal faults marked by multi-phase calcite veins. Some polygons have as many as a dozen or more nested sets of faults. We interpret these nested conical faults as accommodating continued shrinkage of polygon interiors after development of the main network of polygonal faults.

Formation of a layer-bound set of steeply dipping normal faults creates a room problem at the layer base. High resolution satellite imagery reveals irregular, low-amplitude folds near the base of the Khoman that we interpret as accommodating layer-confined fault slip by flow and folding of adjacent sediment. Such an interpretation is consistent with our previous work, which suggests that high transient pore fluid pressures may have triggered polygonal fault development in the Khoman when the sediment was poorly consolidated.