Geological Survey Bulletin 1393 The Geologic Story of Arches National Park

PERHAPS THE GREATEST geologic contrast between these two closely adjacent parks lies in their different geologic structure—the kind and amount of bending and breaking of the once nearly flat lying strata. Consolidated rocks, particularly brittle types, are subject to two types of fracturing by Earth forces. Joints are fractures along which no movement has taken place. Faults are fractures along which there has been displacement of the two sides relative to one another (fig. 6). As noted in the report on Canyonlands National Park (Lohman, 1974), the strata there, particularly along the valley of the Green River, are virtually flat lying or have only very gentle dips. Along the Colorado River above the confluence with the Green, however, the slightly dipping strata are interrupted by several gentle anticlinal and synclinal folds (fig. 5) and by at least one fault (fig. 6). The largest of these folds—the Cane Creek anticline, which crosses the Colorado River north of Canyonlands—has yielded oil in the past and is now yielding potash by solution mining of salt beds in the Paradox Member of the Hermosa Formation.

COMMON TYPES OF ROCK FOLDS. Top, Anticline, or upfold; closed anticlines are called domes. Bottom, Syncline, or down-fold; closed synclines are called basins. From Hansen (1969, p. 31, 108). (Fig. 5)

COMMON TYPES OF FAULTS. Top, Normal, or gravity fault, resulting from tension in and lengthening of the Earth's crust. Bottom, reverse fault, resulting from compression in and shortening of the Earth's crust. Low-angle reverse faults generally are called overthrusts or overthrust faults. In both types, note amount of displacement and repetition of strata. Displacements may range from a few inches or feet to many thousands of feet. From Hansen (1969, p. 116). (Fig. 6)

In strong contrast to Canyonlands, Arches National Park contains three northwesterly trending major folds and is bordered on the southwest by a fourth. The largest and most important are the collapsed Salt Valley and Cache Valley anticlines, which separate the two most scenic groups of arches and other erosional forms—Eagle Park, Devils Garden, Fiery Furnace, and Delicate Arch on the northeast, and Klondike Bluffs, Herdina Park, and The Windows section on the southwest. Farther southwest is the Courthouse syncline, containing the attractive group of erosional forms called Courthouse Towers (fig. 1). Finally, near the southwest edge of the park, is the Seven Mile—Moab Valley anticline (also known as the Moab—Spanish Valley anticline), whose southwest limb is cut off by the Moab fault (figs. 7, 23). The folds just named and the sharply contrasting geologic structures of the two parks are well shown on sheet 2 of the geologic map of the Moab quadrangle (Williams, 1964), and the geologic formations are shown in color on sheet 1.

PARADOX BASIN, in southeastern Utah and southwestern Colorado, showing the extent of common salt and major potash deposits in the Paradox Member of the Hermosa Formation, and the salt anticlines. Adapted from Hite (1972, fig. 16). (Fig. 7)

Arches National Park and most of nearby Canyonlands National Park lie within what geologists have termed the "Paradox basin," which contains a remarkable assemblage of sediments called the Paradox Member of the Hermosa Formation. These deposits were laid down in shallow seas and lagoons during Middle Pennsylvanian time, roughly 300 million years ago (fig. 59). As indicated in figure 4, the Paradox Member contains, in addition to shale and limestone, minerals deposited by the evaporation and concentration of sea water—common salt, gypsum, anhydrite, and potash salts. For this reason such deposits are collectively called evaporites. Figure 7 also shows that the northeastern part of the Paradox basin, which is the deepest part, contains a series of partly alined anticlines which have cores of salt and, hence, are called salt anticlines. As might be expected, roughly alined synclines intervene between the anticlines, but are not shown because of space limitations. According to Cater (1970, p. 50): "The salt anticlines of Utah and Colorado are unique in North America both in structure and in mode of development." To this may be added that they also are relatively rare in the world.

A section across the Salt Valley anticline and the Courthouse syncline in the northwestern part of the park is shown in figure 8, and the axes of these structures are shown in figure 9.

A GEOLOGIC SECTION ACROSS NORTHWEST END OF ARCHES NATIONAL PARK, showing strata beneath Courthouse syncline and Salt Valley anticline. For line of section, see figure 9. Caprock consists of gypsum and shale, from which common salt has been leached by ground water, covered by alluvium. Heavy slanted lines near crest of anticline are faults. Adapted from Hite and Lohman (1973, fig. 13). (click on image for an enlargement in a new window) (Fig. 8)

INDEX MAP OF NORTHWESTERN PART OF ARCHES NATIONAL PARK, showing axes of Courthouse syncline and Salt Valley anticline, line of section A—A' in figure 8 and line of section B—B' in figure 10. Open circles along line of section are sites of test wells for oil, gas, or potash. Adapted from Hite and Lohman (1973, fig. 12). (Fig. 9)

Normally, a series of roughly parallel northwestward-trending folds would result from shortening of a segment of the Earth's crust by compressive forces from the northeast and the southwest, but such does not seem to be the origin of these folds. The folds occur in a relatively narrow belt along the northeastern part of the Paradox basin, the deepest part, which was broken by a series of northwesterly trending normal faults (fig. 6) that cut the deep-lying Precambrian and older Paleozoic rocks (fig. 8) prior to the deposition of the salt-bearing Paradox Member of the Hermosa Formation. Movement along these faults continued intermittently during and after deposition of the Paradox, however, and resulted in the formation of a series of northwesterly trending ridges and troughs. Following Paradox time, normal sediments derived from a rising landmass to the northeast began to fill the basin. These sediments accumulated most rapidly and to greater thicknesses in the fault-derived troughs. Salt differs from normal sediments in two properties critical to the development of salt anticlines: first, salt is considerably lighter (fig. 10), and, second, salt under pressure will flow slowly by plastic deformation, much like ice in a glacier flows slowly downstream. Thus, salt in the troughs underlying the thicker and heavier masses of sediments was squeezed into the adjoining ridges, causing them to rise. Once started, this process tended to be self-perpetuating, as the flow of salt from beneath the thick masses of sediments in the troughs made room for the accumulation of still greater thicknesses of normal sediments. Consequently, the troughs receiving most of the sediments began to form downfolds, or synclines, and the ridges receiving little or no normal sediments began to form huge salt rolls that later were to become the cores of the salt anticlines when finally the ridges too were buried by sediments. Thus, the cross section (fig. 8) shows about 12,000 feet of the Paradox Member beneath the crest of the Salt Valley anticline and only about 2,000 feet beneath the Courthouse syncline. Near the middle of these structures farther to the southeast, all the Paradox Member has been squeezed out from beneath the bordering synclines.

GRAVITY ANOMALIES OVER SALT VALLEY, along line B—B' shown in figure 9, and relative densities and shapes of rock bodies beneath. Densities are in grams per cubic centimeter. Gravity values are in milligals, as shown. The standard acceleration of gravity is 980.665 centimeters per second per second; 1 gal is equal to 1 centimeter per second per second, and 1 milligal is one thousandth of a gal. Modified from Case and Joesting (1972, fig. 2). (Fig. 10)

The general shape of the Salt Valley anticline is shown also by cross-section B—B' (fig. 10), taken along the northeast-southwest line B—B' in figure 9, which is based upon so-called gravity anomalies over Salt Valley. The lighter Paradox Member, having an average density of 2.20, has a lower gravitational attraction than the heavier rocks on each side, which have an average density of 2.55.

By this time you are doubtless wondering why prominent upfolds of the rocks, such as the Salt Valley anticline and associated Cache Valley anticline and the Seven Mile—Moab Valley anticline, now underlie relatively deep valleys bordered by prominent ridges. The formation of these valleys was not simple and involved many steps extending over a considerable amount of geologic time, as portrayed by Cater (1970, fig. 13; 1972, fig. 4). For a part of the story, let us reexamine the cross section (fig. 8); the rest of the story will be told in the section on "Uplift and Erosion."

Figure 8 shows that the unnamed upper member of the Hermosa Formation and the overlying Cutler and Moenkopi Formations are thickest beneath the Courthouse syncline but wedge out against the flanks of the anticline. Although the Chinle Formation and younger rocks appear to extend across the fold, and may have extended across this part of the fold, in Colorado all rocks older than the Jurassic Morrison wedge out against the flanks of the salt anticlines (Cater, 1970, p. 35) and also in the widest part of the Salt Valley anticline southwest of the section in figure 8. The salt anticlines were uplifted in a series of pulses so that some formations either were not deposited over the rising structures or were removed by erosion before deposition of the next younger unit. By Morrison time the supply of salt beneath the synclines seems to have become used up; hence, the anticline stopped rising, and the Morrison and younger formations were deposited across the structures. Thus, in figure 4, the minimum thickness of all units older than the Morrison is given as zero. Figure 4 shows the marine Mancos Shale to be the youngest rock unit exposed in the park, but the Mesaverde Group of Late Cretaceous age and possibly the early Tertiary (fig. 59) Wasatch Formation may have been deposited and later removed by erosion.