Brittle Fracture and Quartz Cementation in Deformation Bands, Cambrian Hickory Sandstone (Cambrian), Central Texas

By

MILLIKEN, KITTY L.

Department of Geological Sciences, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX,

Reed, Robert M.

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

    Scanned cathodoluminescence (CL) imaging allows for the unequivocal identification of detrital and authigenic quartz in sandstones. In porous quartzose sandstone from the Cambrian Hickory Sandstone in central Texas, deformation bands and their surrounding host sandstones are revealed in CL to have roughly similar amounts of compaction, cementation, and porosity. CL imaging also shows that grain volume loss due to pressure solution is not present (either inside or outside the bands). Thus, contrasting rock properties, such as the degree of lithification and permeability, arise primarily as a result of particle size reduction within the deformation bands and not from differential compaction or cementation.

CL traverses across deformation bands reveal a layered structure. Marginal zones have an abrupt outer boundary with prominently less-deformed sandstone. Within the marginal zone grains are clearly crushed, but grain fragments are separated by discrete intragranular fractures such that particles derived from the same grain can be readily identified. The interior zone of larger bands (> 5-10 grain widths) shows an extreme degree of grain crushing, producing a bimodal particle size distribution in which a few, large, unfractured ‘survivor grains’ are surrounded by finely comminuted grain debris (most in the size range of 1s0 to 30 microns) within which particles derived from different grains have been highly mixed. A model of progressive band development is proposed in which bands initiate at a single crushed grain and evolve from a planar zone dominated by simple grain-to-grain crushing to a two-zoned band characterized by significant grain boundary sliding within the interior zone.