COLORADO PLATEAU EXAMPLES OF GEOCHEMICAL AND MINERALOGIC PATTERNS OF REGIONAL STRATIGRAPHICALLY-CONTROLLED SANDSTONE BLEACHING BY MIGRATING REDUCED FLUIDS

In many parts of the Colorado Plateau sandstone units stratigraphically bounded by relatively impermeable lithologies show distinctive top-down white to red/brown color gradients. Bleaching of early-diagenetic ferric-iron grain coatings in the upper parts of these units, likely by migration of buoyant reduced fluids, provides one plausible explanation for this common pattern. Such sections may be useful for understanding subsurface paleofluid-rock systems, but few systematic observations have been made bearing on the geochemical, mineralogic, or geochronologic characteristics of these sections. Here I describe new observations from bleached sandstone sections in Jurassic or Permian sandstones from Canyonlands and Sedona regions. Three geologically similar sections show broadly similar patterns, with upper intervals characterized by extensive quartz overgrowths, pyrite pseudomorphs, very low concentrations of most trace elements, and large fractional iron loss compared with lower unbleached intervals. Between the upper and lower intervals is a zone of abundant calcite cement and strong enrichments in most trace elements. In some sections, concentric or pipe-like concretions of dense iron-oxide cement are present between the upper and intermediate intervals. Taken together these observations are consistent with lateral migration of buoyant reducing fluids along the tops of permeable sandstone sections, and dissolution and mobilization of iron, with only a small fraction reprecipitating as pyrite. Accumulations of dense iron-oxide cement in upper sections may represent localized saturation zones, possibly associated with dense downwelling plumes transporting solutes to intermediate depths where they precipitated with calcite and clay-rich cement. An entirely different type of bleaching is also seen in the Navajo sandstone in at least one region where vertically oriented dense iron-oxide cemented pipes show evidence for reduced iron transport to an oxidizing boundary at the Jurassic paleosurface. In all cases, (U-Th)/He dates of iron-oxides in these sections record only minimum ages of iron mobilization and redeposition, as indicated by clear evidence for recrystallization of reduced precursors (sulfide or carbonate) and/or late-stage U-uptake.