Microstructures in calcite veins along polygonal faults in the Khoman Formation, Western Desert, Egypt

Tyler Ramey, Sonoma State University
Aubrey Coon, Hamilton College
Barbara Tewksbury, Hamilton College
Matty Mookerjee, Sonoma State University

Polygonal faults are layer-bound normal faults that are arranged in broadly polygonal networks in fine-grained sedimentary rocks. Polygonal fault systems have been recognized in over 100 marine basins worldwide. The only extensive on-land exposure of polygonal faults occurs in Cretaceous chalk of the Khoman Formation near Farafra Oasis in the Western Desert of Egypt. The polygonal network consists of thousands of low-slip normal faults that occur in clusters and that are marked by low ridges formed by multi-phase calcite veins that occur along faults in the chalk. The origin of polygonal faults remains the subject of controversy, although most models involve formation during early during diagenesis in association with layer shrinkage and fluid expulsion. Previous work proposed that the calcite veins in the Khoman accompanied formation and evolution of the polygonal fault network and are a manifestation of repeated cycling of pore fluid pressure, failure, and associated fluid expulsion. This study tests that hypothesis by examining the microstructures of the veins using standard petrography analysis along with Electron Backscatter Diffraction (EBSD), Electron Dispersive Spectrometry (EDS), Cathodoluminescence (CL) and Darkfield (DF) imaging.

In a thin sections cut parallel to calcite fibers and perpendicular to vein walls, a crack-seal structure is very clear and marked by prominent arcuate dark bands consisting of inclusions of fine chalk fragments and tiny slivers of chalk wallrock. These features are characteristic of shear veins formed by repeated cracking, opening of the vein in the shear direction at the vein/wallrock interface, inclusion of wallrock fragments at the site of cracking, and precipitation of an increment of optically continuous calcite, which seals the vein again.

The presence of shear veins showing crack-seal structure indicates that calcite vein formation accompanied repeated slip along the normal faults in the Khoman. Although we cannot rule vein formation during a later episode of unrelated fault activity, the simplest explanation is that the veins and polygonal faults are contemporaneous. These veins may reflect something fundamental about polygonal fault formation and fluid expulsion and deserve further detailed study.


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