Significance of Color in Red, Green, Purple, Olive, Brown, and Gray Beds of Difunta Group, Northeastern Mexico

ABSTRACT Varicolored rocks of the Difunta Group (Upper Cretaceous-Paleocene) are composed of detritus derived from a relatively uniform terrane of volcanic rocks and deposited in fluvial, deltaic, and shelf environments. Red, green and purple rocks are restricted to delta-plain 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 formed by oxidation of detrital iron oxide and mafic grains. Some brown siltstone beds were pigmented in a manner similar to red beds, but other siltstone beds developed brown color upon weathering. Green beds formed by bleaching of red (or proto-red) beds by interstratal percolation of reducing water derived largely from fluvial channels overlying the green beds. Olive and gray claystone are present predominantly in marine facies that contain abundant organic matter and in some delta-plain facies where destruction of organic matter was incomplete. Total Fe content of claystone samples is essentially the same regardless of color, except that gray claystone has significantly less total Fe than other colors; 67% of the samples have total Fe between 3 and 4%. Iron reduced in red beds was not removed in solution but resides in chlorite in green strata, and some iron reduced in gray beds resides in sulfides.