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.
[Illustration: COMMON TYPES OF ROCK FOLDS. Top, Anticline, or
upfold; closed anticlines are called domes. Bottom, Syncline, or
downfold; closed synclines are called basins. From Hansen (1969, p.
31, 108). (Fig. 5)]
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.
[Illustration: 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)]
[Illustration: 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. 1B). (Fig. 7)]
[Illustration: 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)]
[Illustration: 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.
[Illustration: 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.