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Format:
Print
Author:
Gohlke, Steven A.
Dept./Program:
Geology
Year:
2014
Degree:
M.S.
Abstract:
The purpose of this study is to use deformation bands as a tool to aid in basin analysis in south central Egypt from the Late Cretaceous to the Middle Eocene. Deformation bands are micro-faults that occur in porous granular material, such as sandstone. They form clear overprinting relationships with other features, preserve a specific episode of fault slip, are an impermeable barrier to fluid flow, and the physical conditions under which they form are well constrained. Thus they present a valuable opportunity to document the effect of regional tectonic events on sequences of sedimentary rocks in this region.
The goal of this study is to constrain the timing of deformation, burial, and diagenesis for a structural dome consisting of folded bedding in the Taref Member of the Nubian Sandstone. This dome is cut by the Seiyal Fault, major EW basement fault that propagated upwards through overlying Mesozoic and Cenozoic sedimentary cover in south central Egypt. Combined sets of data from field mapping, petrography, back stripping, and stable isotope geochemistry were used.
Deformation bands in the Seiyal Fault damage zone cut across bedding within the Taref Member and 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. Silica cement is limited to the cataclastic matrix within deformation bands. These results indicate that the sandstone was poorly lithified at the time when deformation bands formed. Over 1,600 grain size measurements indicate that the country rock unaffected by cataclasis is a medium to fine-grained sandstone with poor sorting valutes ranging from 2.66 to 4.01 (phi). By contrast, deformation bands studies elsewhere in sedimentary rocks typically (but not exclusively) occur in well-sorted sandstones.
The minimum and maximum thicknesses for each of Late Cretaceous to Middle Eocene rock units overlying the Taref Member were determined through a literature search of local geology reports containing stratigraphic sections. These rock units were then delithified according to the percentages of the lithologies present in each and backstripped using a one-dimensional Airy model. Three unconfomities were identified in the stratigraphy, two of which occurred during regional uplift and were therefore erosional.
Through use of the standard 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 40 Ma. This large error range is due to uncertainties in 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 maximum burial depth range calculated for the Taref Member 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.