DEFORMATION BANDS IN POORLY SORTED, POORLY LITHIFIED AND SHALLOWLY BURIED SANDSTONE ALONG THE SEIYAL FAULT, WESTERN DESERT, EGYPT

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 that propagated upwards through overlying Mesozoic and Cenozoic sedimentary cover in south central Egypt. Deformation bands, which cut across bedding within the Taref Member, are limited to a particular rock type, a buff-colored quartz arenite with variable amounts of poikilotopic calcite cement. Cathodoluminescence imaging reveals that early cement formation was limited to kaolinite and minor amounts of calcite. There is no evidence of silica cementation. These results indicate 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 a medium to fine-grained sandstone with poor sorting 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.

Differentially decompacting and backstripping the previously overlying lithologies using a 1-D Airy model allowed a range of burial conditions to be calculated for the Taref Member from the Late Cretaceous to Middle Eocene. The minimum and maximum thicknesses, as well as percentages of the lithologies present in the overlying stratigraphy, were determined through a literature search of local geology reports containing stratigraphic sections. Three unconformities were identified in the stratigraphy, two of which occurred during regional uplift and were therefore erosional.

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.