Paper No. 8
Presentation Time: 9:00 AM-6:00 PM
POLYGONAL FAULTS AND RELATED STRUCTURES IN CRETACEOUS CHALK OF THE KHOMAN FORMATION, WESTERN DESERT, EGYPT
TEWKSBURY, Barbara J., Dept of Geosciences, Hamilton College, 198 College Hill Rd., Clinton, NY 13323-1218, KATTENHORN, Simon A., ConocoPhillips Company, 600 N. Dairy Ashford, Houston, TX 77079, MEHRTENS, Charlotte, Geology, University of Vermont, 180 Colchester Avenue, Burlington, VT 05401, HOGAN, John P., Geological Sciences and Engineering, University of Missouri-Rolla, 127 McNutt Hall, Rolla, MO 65401 and EL-HADDAD, Abd Elaziz A., Department of Geology, Sohag University, Naser City Street, Sohag, 42888, Egypt, btewksbu@hamilton.edu
Chalks of the Cretaceous Khoman Formation near Farafra Oasis, Egypt, display a complex polygonal 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. Slickenlines, slip fibers, and grooves in both the host chalk and the calcite veins have rakes of 75-90°, and offsets indicate small amounts of normal slip on steeply dipping faults. The multi-phase calcite veins suggest multiple fluid events with high fluid pressures
. Fault geometries indicate that mechanically interacting, multiple fault orientations were active contemporaneously and that the horizontal strain field was essentially isotropic and extensional. All of these features indicate a polygonal fault system rather than sets of intersecting faults of different ages and orientations. These polygonal faults occur in continuous surface exposure over an area of about 700 km
2 and extend, partly mantled by aeolian sand, over at least 2500 km
2.
A terrain of isolated basins overlies the polygonal fault network across the entire Farafra region. 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. In the youngest part of the terrain, the isolated basins occur as eye-shaped mesas capped by inward dipping silicified limestone of the latest Cretaceous Dakhla Formation. We interpret these isolated basins as fluid escape structures formed as the polygonal fault system evolved. Active polygonal fault systems are commonly overlain by fluid escape features that include seafloor pockmarks draped with sediment, creating locally inward dipping layers.
Although polygonal fault systems and related features are common in modern submarine basins in fine-grained muds and chalks, and have been studied in basins worldwide using 3D seismic, the polygonal faults in the Khoman Formation represent the first extensive exposure of polygonal faults and associated high fluid pressure features to have been recognized on land.
