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). (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)
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].
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.