Bleaching of Jurassic Navajo Sandstone on Colorado Plateau Laramide highs:

evidence of exhumed hydrocarbon supergiants?

Abstract

The Jurassic Navajo Sandstone is an ancient cross-bedded dune sand unit of rock which is famous for its spectacular outcrops and color variations. Navajo Sandstone covers a large area ranging from northern Arizona all the way to central Utah. At one time the Navajo Sandstone was the largest and most extensive erg (sand sea) to ever exist in Earth’s history. The spectacular color variations in the Lower Jurassic Navajo Sandstones reflect stratigraphic and structural control on spatial distribution of fluid-driven alteration. The most extensive regional bleaching of the Navajo Sandstone occurs on eroded crests of Laramide uplifts on the Colorado Plateau in southern Utah. Alteration patterns suggest that blind reverse faults which core the eastern monoclines associated with these uplifts were carriers for hydrocarbons. Buoyant hydrocarbons were trapped at the crests on monoclines and anticlines where they bleached the sandstone within structural and stratigraphic traps. The extent of bleaching indicates that the Navajo Sandstone may have been one of the largest hydrocarbon reservoirs known. Rapid scoring and breaching of this hydrocarbon reservoir during Tertiary uplift and erosion of the Colorado Plateau could have released enough carbon into the atmosphere to significantly contribute to global fluxes and possibly influence the climate of the era.

Figure 1. Generalized stratigraphic column

of Jurassic sedimentary units on Colorado

Plateau in southern Utah. Ss—sandstone;

Fm—formation.

(Beitler et.al), 2003

Figure 2. Structural contours showing Laramide uplifts on Colorado Plateau

in southern Utah. Contours are on top of Permian Kaibab Limestone (modified

after Hunt, 1956). Contour interval =1000 ft. Lightest area in Uinta Basin

= -1500 ft; darkest area in Monument upwarp = +7000 ft. Field localities indicate

regions where sandstone color was investigated. Shaded regions—Glen Canyon

Group surface exposure (modified after Hintze et al., 2002). Inset isopach map

is of Navajo Sandstone; contour interval= 100 m (modified after Verlander, 1995).

Outline of Painter Oil Field location, modern gas reservoir in

Navajo-Aztec-Nugget Sandstone (after Lamb, 1980), is shown as gray oval.

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