Abstract

Clastic sedimentary rocks of the Duchesne River Formation are largely red or reddish brown. Red pigment (hematite) is present as very fine-grained iron oxide, as staining on grain surfaces, and in cement. Apparently most of the pigment was derived originally from red sedimentary source rocks of the Uinta Mountains. Fine-grained red material and red-stained grains survived erosion and transportation from the source area and deposition over large areas of the Uinta basin, but much of the red pigment has been mobilized subsequently and removed from some of the deposits.

The types and colors of cements are related to the color of sandstones. Red, reddish-brown, and brown sandstones contain predominantly silica cement. Sandstones of orange or yellowish brown color are cemented with both silica and carbonate cements, and most of the carbonate cement is stained. Green, yellow, and gray sandstones contain mainly carbonate cement, most of which is not stained. Petrographic relations indicate that the earliest cement was silica. Later carbonate cementation in many sandstones mobilized some of the red pigment derived from the source area. Additional red pigment was derived from the partial oxidation of iron-bearing detrital grains. In some sandstones another stage of carbonate cementation again mobilized pigment, reducing or removing most of it. As a result, most of the rocks retain the reddish color inherited from red sedimentary source material, either in its original form or redistributed in cement. Postdepositional red pigment has further stained many of the rocks, but red pigment has been removed from some after deposition.

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