The Tertiary Rhyolite Formations

(OLIGOCENE / MIDDLE TERTIARY)

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**Note: Formation descriptions are under heavy construction.**

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At Brian Head and northward along the rim of the Markagunt Plateau, at Yankee Meadows, and in places along Bowery and Parowan Canyons, the Claron formation is covered by a sequence of volcanic debris, breccia, siliceous limestone, and coarse sandstones-a group of distinctive rocks for which the name Brian Head formation has been adopted.

Generalized Section of the Brian Head formation at Brian Head

Glacial Deposits Feet
Rhyolitic lavas in thin sheets; forms top of Brian Head formation Unconformity; surface of erosion
    5. Volcanic ash; red, highly compacted, includes, tuff and ash, thin sheets of acidic lavas.
6-18

    Unconformity
    4. Breccia, angular blocks of acidic lava, variable in texture, color tone, and composition; commonly 6 to 12 inches, some as much as 6 feet, in diameter. Near the top includes broken sheets f black scoriaceous andesite and lenticular aggregates of ash and lapilli. Base concealed but at one place below the breccia dense acidic rock with strong parallel horizontal joints suggests an intrusion.
8-40
    3. Lava, acidic, porphyritic, glassy; thin regular sheets marked by flow structures; breaks into sharp angular steps.
0-6
    2. Volcanic ash in irregular beds, 2 to 8 inches thick; finer grained and more thinly and evenly bedded toward the top; includes lenses of coarse sandstone, siliceous limestone, clay, and chalcedony and is marked by black bands, chiefly magnetite; much of the rock is porous and in places resembles travertine and geyserite.
265
    1. Volcanic ash; with subordinate calcareous sandstone; gray, yellow, white, unevenly bedded; abundantly implanted with pink, brown, white, and black chalcedony, distributed as nodules and lenses; near the middle continuous beds of chalcedony are 2 to 5 feet thick and as much as 500 feet long.
210
Total:   489-539
Unconformity ?  
Limestone-normal type of Wasatch age.  

The Brian Head formation is conspicuously capped by sheets of acidic_ lava, but its base is generally concealed by talus and disintegration pro ucts. Though in mass it is readily distinguishable from the underlying Wasatch formation, the division plane between them is difficult to define. At exposures examined, the topmost beds in the Wasatch and the bottom beds in the overlying Brian Head are substantially conformable and somewhat similar in arrangement and thus seemingly confirm the observation of Dutton:(1)

Around the western and southern borders of the Markagunt ... sands and marls derived from the decay of igneous rock . . . rest everywhere upon the Eocene limestones [Wasatch], frequently shading downward into sandstones undistinguishable in composition and texture from ordinary sediments derived from ordinary materials.

However, within 50 feet above and below the assumed contact the formations have little in common: igneous debris in contrast with fluviatile and lacustrine limestone. In places east of the area here described, beds assigned to the Brian Head formation unconformably overlie the Wasatch formation.

The source of the materials in the Brian Head formation is unknown. No rocks from which they might have been derived are exposed in eastern. Southwest Utah or in adjacent regions. Furthermore, in the absence of fossils, diligently sought for, the age of the sediments can be but approximately fixed. They lie above rocks of Eocene, possibly middle or even late Eocene, age and beneath rocks known as the Tertiary volcanics, which in turn underlie Quaternary basalts and glacial deposits. In stratigraphic position they correspond with the green sandstones, the pumice conglomerates, and the variegated volcanic ash and clay shales that are exposed beneath the lava cap of the Sevier Plateau in Casto and Limekiln Canyons and beneath the Miocene (?) rhyolites along the northwestern rim of the Markagunt Plateau in Red and Little Canyons. The sediments are obviously older than the Sevier River formation, which also is composed chiefly of volcanic debris, but which overlies several thousand feet of igneous rocks assigned to the early Tertiary and late Tertiary periods and contains diatoms of upper Pliocene or lower Pleistocene age. In its original distribution, the Brian Head formation may have extended far south, east, and west of its present outcrops. In addition to the prominent exposures at Brian Head and Yankee Meadows, the Brian Head formation crops out in Bowery Canyon and other places where parts of it have been preserved by downfaulting.



NOTES

1. Dutton, C. E., op. cit., p. 202-203.


SOURCE
-from Geology of Eastern Iron County, Utah by Herbert E. Gregory