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Deformation Bands in Poorly Sorted, Poorly Lithified and Shallowly Buried Sandstone along The Seiyal Fault, Western Desert, Egypt

Steven Gohlke1, Charlotte Mehrtens1, and Barbara Tewksbury2
1University of Vermont
2Hamilton College

Deformation bands in the Taref Member of the Nubian Sandstone are associated with cataclastic deformation adjacent to the Seiyal Fault, a major EW basement fault in south central Egypt. Deformation bands, which cut across bedding, are limited to a particular rock type, a buff-colored quartz arenite with variable amounts of late poikilotopic calcite cement. Cathodoluminescence imaging reveals that the sandstone was poorly lithified at the time when deformation bands formed. Over 1600 grain size measurements indicate that the country rock unaffected by cataclasis is poorly sorted, with values ranging from 2.66 to 4.01 phi. By contrast, deformation bands studied elsewhere in sedimentary rocks typically (but not exclusively) occur in well-sorted sandstones.

The lithologies previously overlying the Taref Member were differentially decompacted and backstripped using a 1-D Airy model. Through use of the computer program OSX Backstrip v3.2, it is estimated that the Taref Member was buried to a maximum depth of 844 m with an error of +/- 340 m at 49 Ma. This large error range is due to uncertainties about the amount of erosion during formation of unconformities, the variations in porosity of chalk within the section, and the facies changes affecting stratigraphic thickness of the overburden. The estimated maximum burial depth is at the lower end of depths typically quoted for the formation of cataclastic deformation bands in consolidated rock (~1-3 km). The burial curve suggests that the deformation bands likely formed between 49 and 23 Ma, when the sandstone was buried to ~1 km.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013