The Geologic Story of Arches National Park 5

The Geologic Story of Arches National Park 5

Comparison of the rock columns for the two parks also reveals other

differences. Both parks contain exposures of rocks as old as the

Pennsylvanian Paradox Member of the Hermosa Formation. However, only in

the Horseshoe Canyon Detached Unit of Canyonlands are rocks as young as

the Jurassic Entrada Sandstone, whereas all the spectacular natural

arches that make Arches famous were formed in the Entrada Sandstone, and

Arches also contains several younger formations of Jurassic and

Cretaceous age (fig. 4).

 

A commonly asked question is “Why are most of the rocks so red,

particularly those in which the arches were formed?” This can be

answered with one word—iron, the same pigment used in rouge and in paint

for barns and boxcars. Various oxides of iron, some including water,

produce not only brick red but also pink, salmon, brown, buff, yellow,

and even green or bluish green. This does not imply that the rocks could

be considered as sources of iron ore, for the merest trace, generally

only 1 to 3 percent, is enough to produce even the darkest shades of

red. The white or nearly white Navajo Sandstone and the Moab Member of

the Entrada Sandstone contain little or no iron.

 

As pointed out by Stokes (1970, p. 3), microscopic examination of the

colored grains of quartz or other minerals shows the pigment to be

merely a thin coating on and between white or colorless particles. Sand

or silt weathered from such rocks soon loses its color by the scouring

action of wind or water, so that most of the sand dunes and sand bars

are white or nearly so.

 

 

 

 

Bending And Breaking of The Rocks

 

 

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.