Chemical bleaching: An indicator for fluid flow in sandstone deformation bands
Marjorie A. Chan, W. T. Parry, and Brenda Beitler
Department of Geology and Geophysics, 135 S. 1460 E. University of Utah, Salt Lake City, UT 84112-0111
e-mail: machan@mines.utah.edu
Iron is a sensitive indicator of fluid flow in Jurassic sandstones on the Colorado Plateau that have been variably bleached through interaction with hydrocarbon bearing solutions. Deformation bands associated with monocline uplifts in the Navajo Sandstone display a variety of colors in comparison with the host rock color that indicate the timing of bleaching relative to deformation band formation. This diagenesis of deformation bands is a valuable record of fluid-rock interactions, fluid pathways through more porous zones, and timing of fluid movement. White deformation bands in red sandstone indicate that deformation bands were likely permeable at an early dilatant stage in their development history. Field characteristics, petrography, bulk rock chemistry, and clay mineralogy show that the coloration differences in some deformation bands is not simply a matter of grain diminution. It is clear that bleached deformation bands experienced an episode of chemical reduction where fluids removed some iron and left the remaining iron as pyrite and magnetite. The hydrologic properties of deformation band structures may be partially quantified by estimating the pore volumes required to do the bleaching and removal of iron. Geochemical modeling and mass balance calculations show that as much as 10 kg of chemically reducing fluid per 100 grams of rock (1,500 pore volumes of fluid) are necessary to remove 0.1 wt % Fe (0.14 wt % Fe2O3) from a deformation band. These large pore volumes suggest that moving, hydrocarbon-solutions regionally bleached the sandstone white, and bleached deformation bands result where deformation bands provided fluid access to unbleached, red sandstone. Color and chemical composition are valuable indices to mechanical and hydrologic properties through regional structures of host rocks as well as small-scale deformation bands.