Geological Survey Bulletin 1229 Geology of the Circle Cliffs Area, Garfield and Kane Counties, Utah

Structurally, most of the Circle Cliffs area consists of the breached Circle Cliffs anticline, whose shape and trend have been the major influences on the formation of the local topography. The anticline is about 65 miles long and plunges southward to the Colorado River, where it is coaxial with the smaller, northward-plunging Beaver anticline. To the north, the anticline crest is en echelon with the Teasdale anticline. The Circle Cliffs anticline is flanked on the east by the Henry Mountains syncline and on the west by the Kaiparowits basin and the Boulder Mountain segment of the Aquarius Plateau. The relief of folding, from the crest of the Circle Cliffs anticline to the trough of the Henry Mountains syncline, is almost 1-3/4 miles vertically in 9-10 miles horizontally. The general configuration of the anticline is shown by sections A—A' and B—B' on plate 1 (Sheet 1).

In the northwest corner of the area a small segment of the trough of the Harris syncline crosses the mapped area. This syncline is almost 50 miles long and can be traced 4-5 miles north and about 45 miles south of the segment shown.

METHOD OF REPRESENTING STRUCTURE

Attitudes of strata, faults, and throughgoing joints are shown by appropriate symbols on plate 2. The shape of the strata is shown by structure contours drawn through points of equal altitude (in reference to mean sea level) on the top of the White Rim Sandstone Member of the Cutler Formation.

The structure contours were determined by field mapping of the contoured horizon where that horizon is exposed at the surface, and by field mapping of other key beds where the contoured horizon is not exposed or has been removed by erosion. Points of equal altitude on the key beds were adjusted to the approximate altitude of the contoured horizon by subtraction of the appropriate stratigraphic thickness. Most of the contours at the southwest side of the mapped area were constructed on formations of the Glen Canyon Group, and those on the northeast side of the area were drawn on beds of Cretaceous age. Contours along the Waterpocket fold were constructed by drawing numerous cross sections across the fold, using the appropriate stratigraphic thickness of the formations in the section, and assuming that the folding is of the parallel, or concentric, type in which the stratigraphic thickness of the beds remains unchanged in the fold. This assumption is probably invalid, because thinning and thickening in parts of the fold are very likely; but the general picture across the fold is reasonable.

The crest and trough axes shown on plates 1 and 2 represent the intersections between the axial planes of the folds and the strata cropping out at the surface. The dip of the axial planes was assumed to be vertical for purposes of structural contouring. If, instead of being vertical, the axial plane of the Circle Cliffs anticline were to dip southwest, the intersection between the axis and the contoured horizon would be displaced to the southwest from the crestline. The amount of displacement would be dependent on the interval from the surface to the top of the White Rim Sandstone Member, which is the contoured horizon. The same condition is true of the other major anticlines and synclines shown on the structure map.

Nearly all faults were observed in the field, and except where otherwise noted, they are probably vertical or within about 10° of vertical. The joints shown on plate 2 are mainly in the Wingate and Navajo Sandstones and in the Salt Wash Sandstone Member of the Morrison Formation, and they were transferred to the map from aerial photographs.

CIRCLE CLIFFS ANTICLINE

The Circle Cliffs anticline, the dominant structural feature in the mapped area, trends slightly west of north and is doubly plunging. The west flank dips gently west; but the east flank, which is known as the Waterpocket fold, dips steeply into the Henry Mountains syncline (pl. 2, fig. 9). Closure on the crest of the anticline is about 1,200 feet, embracing an oval area about 30 miles long and 9 miles wide.

The anticline is fairly smooth and uniform. The few irregularities or minor folds superimposed on the major structure include a nose that is about 200 feet higher than the average curvature of the anticline in the northwest corner of the mapped area, and a small shelf or collapse structure in the southernmost part of the area. The other irregularities are anomalies on the steep east flank, where closely spaced contours indicate a local steepening of dip and widely spaced contours indicate a shallowing of the dip.

HENRY MOUNTAINS SYNCLINE

The broad Henry Mountains syncline, which was mapped and described in detail by Hunt, Averitt, and Miller (1953), is east of the Circle Cliffs anticline it trends and generally plunges north to northwest. Several minor folds occur in the syncline, one of which, the Muley Creek anticline, was pierced by an oil-test well (Alexander and Clark, 1954). The anticline trends north 6-7 miles and roughly parallels the trough of the Henry Mountains syncline. Another minor fold in the syncline is a small rise, or "saddle," in the trough-line where the strata are about 200-300 feet higher than the same strata in the structural basins north and south of the rise.

FAULTS

Many small faults, some traceable for about 5 miles, offset strata on the west flank of the Circle Cliffs anticline. Faults are uncommon in the steeply folded east flank. Nearly all the faults on the west flank apparently resulted from caving of the rock strata. Where faults are abundant, most of the fault blocks are grabens, and the overall pattern of faulting indicates a foundering of the west-central part of the anticline. The faults are not in general alinement with the dominant joint patterns, probably because the jointing reflects separate diastrophic events, which Kelley suggests (1955, p. 49-53) might be both early and late Tertiary in age. Stream directions in the Circle Cliffs area are independent of fault trends and commonly cross rather than follow fault breaks.

All faults are high angle and, where the dip may be measured, normal faults. Apparently little or no strike-slip movement has occurred in the mapped area. Most faults show some drag, which is expressed by folds subparallel to the fault plane, and many die out along their strike into small monoclinal, anticlinal, and synclinal features. Thus, many faults show considerably more displacement along their central parts than elsewhere. The maximum throw of the mapped faults is about 50 feet.

JOINTS

Most joint systems in the Colorado Plateaus are regional in extent, and those in the Circle Cliffs are no exception. The dominant joint trends—northeast and north to northwest—continue far away from the Circle Cliffs anticline (Kelley, 1955, fig. 2).

Some of the joints were almost certainly associated with the folding of the Circle Cliffs anticline. Examples of these are the joints that curl around so that they are perpendicular to the strike of strata in the southeastern part of the area, and the profusion of north-, northwest-, and east-trending joints, many of which are cemented with calcium carbonate, that are on a structural anomaly east of the crest of the anticline. Much of the present drainage parallels joints rather than dip slopes, especially in the northwestern part of the area.

Although some joint systems probably formed during the folding of the Circle Cliffs anticline, others evidently formed later, possibly during the mid-Tertiary tectonic episode in the Basin and Range province to the west and south.

PERIODS OF DEFORMATION

The tectonic history of the rocks exposed in the Circle Cliffs area is intimately allied with the history of the Colorado Plateau as a whole. The history of development of the Colorado Plateau is treated comprehensively by Hunt (1956), and most of the following discussion is of diastrophic events documented by unconformities and tectonic features in the Circle Cliffs area.

Information regarding Paleozoic and Precambrian deformation is meager; Heylmun (1958, p. 1800-1801) suggested that regional westward tilting occurred before deposition of the Devonian rocks and again after deposition of the Mississippian rocks. Local thinning of Pennsylvanian and Permian rocks (Wolfcamp age) about 18 miles west of the Circle Cliffs anticline delineates a domal high, which Heylmun (1958, p. 1791, 1802) referred to as an ancestral Circle Cliffs uplift.

Northwestward tilting at the close of the Paleozoic Era resulted in an erosional unconformity between the Kaibab Limestone and overlying Triassic rocks that is angularly discordant to the east (McKee, 1954a, p. 35). The tilting was also responsible for the northwestward thickening of the Moenkopi Formation (McKee, 1954a, p. 23).

The erosional unconformity between the Moenkopi Formation and the overlying Chinle Formation probably is due to additional northwestward or northward regional tilting. The unconformity is not angular, as the Chinle rests on nearly the same horizon of the Moenkopi throughout the Circle Cliffs area (except in the Shinarump channel systems) and appears also to be concordant over a larger area (Spieker, 1954, p. 9).

Contacts of formations of the Glen Canyon Group are disconformable both within the group and with formations overlying and underlying the group, but none of these local disconformities have much regional structural significance. The disconformable contacts were caused mainly by changes in the depositional environment rather than by large-scale regional tilting or hiatuses in deposition.

The contact between the Entrada Sandstone and the overlying Curtis(?) or Summerville Formation displays marked local angularity throughout the Hall Creek area (fig. 10B) and to the north, but this discordance probably represents local slumping and folding within the Entrada itself. Regionally, this contact is of tectonic significance because a geosyncline that had existed to the west since Cambrian time was uplifted in the period between the deposition of the Entrada Sandstone and the deposition of the overlying Curtis(?) Formation (Wright and Dickey, 1958). The Entrada Sandstone and the underlying Carmel Formation were a part of the geosynclinal pile, whereas the Curtis(?) Formation and, later, the Morrison Formation were deposited by streams flowing northeastward from the site of the geosyncline.

The Dakota Sandstone-Morrison Formation contact appears conformable in the Circle Cliffs area. Although the Cedar Mountain Formation of Early Cretaceous age could not be distinguished at the top of the Morrison and may be absent, the Dakota-Morrison contact probably does not reflect a major depositional break or period of folding, at least in the mapped area. The contact of the Dakota with the Mancos Shale, the member contacts within the Mancos, and the Mancos Shale-Mesaverde Formation contact are gradational or conformable and do not represent major tectonic events. The folding of the Circle Cliffs anticline postdates the deposition of the Mesaverde Formation in the area.

The Circle Cliffs anticline is the most clearly exposed tectonic feature in the area, but information to date its origin is lacking. The lower members of the Mancos Shale—up to and including the Emery Sandstone Member—are folded in the Circle Cliffs anticline and do not display any marked lithologic or thickness changes in the area of folding such as would be expected if they had been deposited while the Circle Cliffs area was being uplifted. The Masuk Member was eroded from the more tightly folded areas, but its lithology, too, does not reflect intense nearby diastrophism during deposition. It may be concluded with reasonable certainty only that the folding must have occurred after the deposition of the Emery Sandstone Member of the Mancos Shale. On the Aquarius Plateau, a few miles west of the Circle Cliffs, flat-lying beds tentatively correlated by Spieker (1954, p. 10-11) with the Flagstaff Limestone of late Paleocene and early Eocene (?) age rest unconformably on the folded Navajo Sandstone; therefore, rock relations in and near the mapped area indicate that folding of the Circle Cliffs anticline occurred some time between the depositions of the Emery Sandstone Member of the Mancos Shale and the Flagstaff Limestone, or between the middle of Late Cretaceous and late Paleocene time.

At least three unconformities representing periods of folding in the Late Cretaceous and Paleocene are known in the Wasatch Plateau region north of the Circle Cliffs area. The oldest is between the Ferron Sandstone Member of the Mancos Shale and the Star Point Sandstone, the lowest unit of the Mesaverde Group in that area; the second is between the Blackhawk and Price River Formations of the Mesaverde Group; and the third is between the North Horn Formation and the Flagstaff Limestone. West of the Circle Cliffs area, at Bryce Canyon, an unconformity presumably equivalent to that under the Flagstaff Limestone separates the Kaiparowits Formation and strata of probable Wasatch age (Spieker, 1954, p. 10). The lower part of the Flagstaff is late Paleocene in age, the North Horn is Late Cretaceous to middle Paleocene in age (Spieker, 1949, p. 27), and the Kaiparowits is Late Cretaceous in age (Gregory, 1951, p. 45; Katich, 1954, p. 53).

Indirect relations indicate that the unconformities between the Ferron and Star Point (Walton, 1954, p. 81) and between the Blackhawk and Price River (Spieker, 1946; 1949; 1954, p. 10) may predate the Circle Cliffs folding. The upper part of the Price River is equivalent to the lower part of the Kaiparowits, and hence the Kaiparowits postdates the second unconformity. About 30 miles west of the Circle Cliffs, the Kaiparowits is folded over the East Kaibab monocline. This monocline, the Circle Cliffs anticline, and the intervening Kaiparowits basin are considered by myself and other workers to be closely related in age and origin, and their relations to the Kaiparowits at East Kaibab monocline indicate that they are younger than that unit. Thus, if the unconformity between the Kaiparowits and the sequence at Bryce Canyon is the same as that between the North Horn and Flagstaff, the folding of the East Kaibab monocline, and hence of the Circle Cliffs anticline, is middle to late Paleocene in age.

The Colorado Plateau was uplifted and tilted in Tertiary, and perhaps in Quaternary, time (Hunt, 1956, p. 57-64; Walton, 1954, p. 81-82; Spieker, 1954, p. 13). Deformation of this age is not reflected in the consolidated rocks of the Circle Cliffs area, except possibly by the joint systems.