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Type Section Information

The Pony Trail Group is a sequence of volcanic and volcaniclastic rocks named by Muffler (1964) along the eastern flank of the Cortez Mountains, in the Frenchie Creek Quadrangle. Muffler (1964) separated the group into three formations. In ascending order, these are the Big Pole Formation, Sod House Tuff, and Frenchie Creek Rhyolite. The Big Pole Formation was named for Big Pole Creek, the Sod House Tuff after Sod House Creek, and the Frenchie Creek Rhyolite after Frenchie Creek. All three units have type localities along Pony Trail Canyon in the Frenchie Creek Quadrangle (Muffler, 1964).

Geologic Age

The Pony Trail Group has been dated at 151 +/- 3 Ma. All three formations are considered Upper Jurassic in age (Smith and Ketner, 1976).

General Lithology

The three formations included in the Pony Trail Group, the Big Pole Formation, Sod House Tuff and Frenchie Creek Rhyolite are discussed in ascending order here.

The Big Pole Formation is about 6,000 feet of fine to medium-grained, light brown-gray to green, poorly sorted, volcaniclastic sandstones, with both current and graded bedding. Subequal amounts of volcanic fragments, and plagioclase and quartz, are in an argillaceous matrix composed of very fine-grained quartz and volcanic fragments (Muffler, 1964). The sandstones are recrystalized as much as 300 feet outward from plutons in the Cortez Mountains. Rhyodacitic flows and flow breccias are interlayered with the sandstones in the upper 500 to 1,000 feet of the formation.

The Sod House Tuff is about 1,000 feet of silicic ash-flow tuff which is commonly gray to white, and overlies the Big Pole Formation with at least local unconformity (Muffler, 1964). The tuffs have been extensively recrystallized and commonly contain about 20 percent quartz and 10 percent sanidine in an aphanitic matrix. Some samples are composed of as much as 35 percent plagioclase (Muffler, 1964). Locally the tuffs have been extensively sericitized and are green in outcrop. Pumice is abundant in some of the tuff. Very thin beds of current bedded sandstone are interbedded and form a very minor percentage of the unit (Muffler, 1964).

Unconformably overlying the Sod House Tuff is the Frenchie Creek Rhyolite. The Frenchie Creek Rhyolite is about 2,500 feet thick in the type section and may, according to Smith and Ketner (1976), be 7,000 feet thick in the Carlin-Pinon Range. It is composed primarily of maroon or black, flow-banded and devitrified, fine-grained and porphyritic rhyolite and rhyodacite flows with lesser amounts of interbedded green and white flow breccias and volcanic sandstones. Phenocrysts of plagioclase, quartz, and biotite make up 5 to 30 percent of most flows, which are commonly sericitized and chloritized (Muffler, 1964). Fine to medium-grained brown, green, and black sandstones are present in the lower portion of the formation. These sands are composed of plagioclase, quartz, and volcanic grains and fragments up to 1.5 inches long (Smith and Ketner, 1976). On the eastern flank of the Pinon Range (Sec. 15, T. 28 N., R. 53 E.), Smith and Ketner (1976) have assigned a small exposure of altered dacitic and andesitic flows to the Frenchie Creek Rhyolite.

Highly altered hypabyssal plugs intrude all three formations of the Pony Trail Group and appear to have been a feeder for the Frenchie Creek Rhyolite. These plugs are dominantly highly altered rhyolite with sericitized and chloritized plagioclase, biotite, and quartz phenocrysts in a trachytic matrix (Muffler, 1964).

Average Thickness

The Pony Trail Group is about 3,500 feet thick along the west flank of the Cortez Range at Pony Trail Canyon, and perhaps as much as 10,000 feet in the Frenchie Creek and Big Pole areas on the eastern flank of the Cortez Range (Muffler, 1964).

Areal Distribution

The Pony Trail Group is exposed within the Cortez Mountains, and Dry Hills, and along the eastern flank of the Carlin-Pinon Range. Smith and Ketner (1976) suggest that the original distribution of the unit may have once been much larger.

Depositional Setting

Muffler (1964) suggested that the depositional environment for the Big Pole Formation was cryptic but may be in part related to turbidity currents since some of the sands are graded and cross-bedded.

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Last modified: 09/12/06