Copyright © 1974, The Society of Economic Paleontologists Itnd Mineralogists SIGNIFICANCE OF COLOR IN RED, GREEN, PURPLE, OLIVE, BROWN, AND GRAY BEDS OF DIFUNTA GROUP,

ABSTRACT: Varicolored rocks of the Difunta Group (Upper Cretaceous-Paleocene) are composed of detritus derived from a relatively uniform terrane of volcanic rocks and de-posited in fluvial, deltaic, and shelf environments. Red, green and purple rocks are restricted to delta-plaln facies, whereas the dark colors are present in all facies. The color of claystone is a function of color mixing of red hematite, green illite and chlorite, and black organic matter; and possibly of grain size of hematite (purple color). Red and purple rocks owe their color to pervasive hematite grain coatings and crystals intergrown with clay; brown rocks owe their color to faint or localized iron-oxide grain coatings; and gray rocks to organic matter and authigenic iron sulfide. Green rocks owe their color to chlorite and illite and to the absence of hematite, organic matter and sulfides. Olive and yellow claystone colors are imparted by color mixing of green clay and black organic matter. Field relations and petrographic studies indicate that red and purple colors originated through post-depositional reddening of sediment, in part in soil zones on the delta plain, in a sub-humid to semi-arid climate that had seasonal wet and dry periods. Reddening occurred both by aging of hydrous ferric oxides plus staining of grains by hematite pigment forme