Origin of Variegated Redbeds in the Cathedral Bluffs Tongue of the Wasatch Formation (Eocene), Wyoming

Abstract

ABSTRACT Variegated redbeds in the Cathedral Bluffs Tongue of the Wasatch Formation are the result of syndepositional chemical and hydrological conditions in alluvial-plain sediments that were favorable for both the formation of hematite in clay-rich mud rock and the removal of iron oxides from silt-rich mud rock. These mud rocks consist of depositional couplets of green cross-laminated clayey siltstone overlain by red horizontally laminated silty claystone, and were formed by processes active in subhumid tropical savanna floodplain alluvium. Red color of silty claystone is produced by hematite in the matrix and hematite coatings of grains, whereas green color in clayey siltstone is caused by interstratified illite-montmorillonite and chlorite. Hematite probably formed from the dehydration of ree iron hydroxides that coated clays in the muddy alluvium. Green layers of couplets, although rich in clay, contain little iron oxide, indicating that iron has been removed and (or) concentrated in red layers. Sedimentologic evidence suggests that a combination of mechanisms produced patterns of red pigment in mud rock of the depositional couplets. These mechanisms are: (1) original depositional variation in iron concentration associated with textural differences in the floodplain sediment, and (2) early postdepositional depletion of iron in the lower silt-rich and iron-poor part of the couplet and vertical migration of iron from green to red layers. This process took place when the siltstone part of the couplet was saturated with water, but before the sediment was buried to sufficient depth to be below the regional groundwater table. Stratigraphic and sedimentologic evidence also requires that the iron distribution that defines present green and red pigment patterns was established soon after deposition and before the next couplet was deposited. In addition, movement of ground water, once the sediment was buried below the regional water table, must have been negligible, because variegated patterns would have been destroyed by vertical and lateral fluid movement. Variegated redbeds in the Cathedral Bluffs Tongue were deposited on a gently sloping alluvial plain adjacent to Lake Gosiute. The beds consist of upward-fining couplets of green cross-laminated clayey siltstone overlain by horizontally laminated red silty claystone. Each couplet represents an episode of deposition from decelerating sediment-laden flood waters, followed by desiccation and a period of nondeposition. The floods, which covered parts of the alluvial plain between stream channels, were associated with a seasonally wet and dry subhumid climate. Successive depositional couplets are not necessarily annual deposits, and may be separated by long intervals of time.