Geological Survey Water-Supply Paper 1475-M Ground-Water Resources of the Bryce Canyon National Park, Utah

STRATIGRAPHY

The oldest rock formation known to underlie the Bryce Canyon area is the Redwall limestone of Mississippian age. This formation was penetrated at a depth of 10,189 feet below the surface in an oil test in Johns Valley (table 4) about 12 miles northeast of the lodge and headquarters area (off pl. 24). This same test hole passed through 779 feet of Pennsylvanian strata comparable to the Molas and Hermosa formations. Rocks of Permian age were penetrated in the Lion Oil Co. test hole, 4 miles northwest of the lodge and headquarters area (pl. 24; table 4). These rocks were the Coconino sandstone, the Toroweap formation, and the Kaibab limestone.

TABLE 4.—Logs of wells and auger holes in the Bryce Canyon National Park area, Utah
[See pls. 24, 25A for location. All water levels in this table are in feet below land surface]

The thickness and the geologic and hydrologic character of the Mesozoic and Cenozoic rocks underlying Bryce Canyon National Park are given in table 5. Of these formations the oldest shown on the geologic map (fig. 55) is the Winsor formation of Late Jurassic age.

FIGURE 55—Geologic map of the northern part of Bryce Canyon National Park area, Utah. Modified from Gregory (1951). (click on image for an enlargement in a new window)

The Mesozoic rocks in table 5 are predominantly sandstone and shale with only minor limestone or conglomerate beds. Except for the Navajo sandstone, most formations have gypsum as a common cementing agent. This mineral is easily soluble, and thus may be present in much of the ground water contained in these beds. Gypsum is not so common in rocks of Cenozoic age, which are mostly limestone or sand and gravel.

TABLE 5.—Generalized section of the geologic formations in the Bryce Canyon National Park area, Utah
[Based on information obtained from several sources, including Gregory (1951), Gregory and Moore (1931), and Harshbarger and Repenning (1934)]

STRUCTURE

The principal structural features of the Bryce Canyon National Park area are gently northeastward-dipping beds and several major normal faults. The regional dip of the beds ranges from 1/2° to 3° north, northeast, and east. Departures from the regional dip are found only in the immediate vicinity of faults and in gentle folds. A gently north-plunging anticline in the vicinity of Willis and Heward Creeks was described by Hager (1957). The axis of this anticline is west of the Pink Cliffs escarpment in the Heward Creek area, but it crosses the escarpment in the vicinity of Sheep Creek.

Faulting is restricted to a few large-scale normal faults; two of these faults—the Paunsaugunt and Sevier—are regional in extent. The Sevier fault is about 7 miles west of the area covered by this report. These two faults trend northward, and the east block of each fault is upthrown in relation to the west block. The Paunsaugunt fault does not coincide with the Pink Cliffs escarpment and, in fact, is not noticeable as a topographic feature of any kind. Two cross faults that trend generally eastward, connecting the Sevier and Paunsaugunt faults, are evident as topographic features—faultline scarps that form the borders of Emery Valley.

Each of these structural features exercises some control on the occurrence and movement of ground water in the bedrock aquifers of the area. Ground water moves through the rocks in the general direction of the regional dip, which accounts for the many springs along the eastern scarp of the Paunsaugunt Plateau. Along the Paunsaugunt fault, springs occur where the eastward movement of ground water is blocked by upfaulted relatively impervious beds, which force the water to rise to the surface. Tension fractures along the axis of the Bryce Canyon anticline (fig. 55) might create secondary permeability and afford an opportunity to obtain successful water supplies from wells penetrating the bedrock.