The Geologic Story of Arches National Park 4

The Geologic Story of Arches National Park 4

Deposition of The Rock Materials

 

 

The vivid and varied colors of the bare rocks and the fantastic buttes,

spires, columns, alcoves, caves, arches, and other erosional forms of

Arches National Park result from a fortuitous combination of geologic

and climatic circumstances and events unequalled in most other parts of

the world.

 

First among these events was the piling up, layer upon layer, of

thousands of feet of sedimentary rocks under a wide variety of

environments. Sedimentary rocks of the region are composed of clay,

silt, sand, and gravel carried and deposited by moving water; silt and

sand transported by wind; and some materials precipitated from water

solutions, such as limestone (calcium carbonate), dolomite (calcium and

magnesium carbonate), gypsum (calcium sulfate with some water),

anhydrite (calcium sulfate alone), common salt (sodium chloride), potash

minerals, such as potassium chloride, and a few other less common types.

Some of the beds were laid down in shallow seas that once covered the

area or in lagoons and estuaries near the sea. Other beds were deposited

by streams in inland basins or plains, a few were deposited in lakes,

and the constituents of deposits like the Navajo Sandstone, were carried

in by the wind. The character and thickness of the exposed sedimentary

rocks and the names and ages assigned to them by geologists are shown in

the rock column (fig. 4) and in the cross section (fig. 8). The history

of their deposition is summarized on pages 98-102. Figure 4 was compiled

mainly from generalized sections given by A. A. Baker (1933), Dane

(1935), McKnight (1940), and Wright, Shawe, and Lohman (1962), and, in

part, from Hite and Lohman (1973).

 

[Illustration: ROCK COLUMN OF ARCHES NATIONAL PARK. Average

thickness of units 250-1,000 feet is exaggerated two times; those

less than 250 feet, four times. 1 foot = 0.305 meter. (Fig. 4)]

 

 

AGE (millions of yrs ago)

GEOLOGIC AGE

NAME OF ROCK UNIT

KIND OF ROCK AND HOW IT IS SCULPTURED BY EROSION

THICKNESS (feet)

NAMED FOR OCCURRENCE AT OR NEAR

 

 

100

Late Cretaceous

Mancos Shale

Lead-gray fossiliferous marine shale. Forms slopes.

?

Mancos, Colo.

Dakota Sandstone

Conglomeratic sandstone, gray shale, carbonaceous shale, and

coal. Forms ledge.

100

Dakota, Nebr.

Unconformity

Late Jurassic

Morrison Fm.

700

Morrison, Colo.

Brushy Basin Member

Variegated shale, some sandstone and conglomerate, petrified

wood, chert, and dinosaur bones. May contain some beds

of Burro Canyon (Early Cretaceous) age.

Salt Wash Member

Crossbedded white and gray conglomeratic sandstone beds and

lenses, locally carnotite bearing, and red and gray

sandy mudstone. Forms slopes.

Unconformity

160

San Rafael Group

(San Rafael Swell, Utah)

Summerville Fm.

Thin bedded red sandstone and shale. Some cherty limestone

concretions. Forms slopes.

0-40

Summerville Point, Utah

Entrada Ss.

(Entrada Point, Utah)

Moab Member

White, crossbedded fine-grained sandstone. Caps Slick Rock

Member north of Devils Garden and Fiery Furnace and on

Klondike Bluffs.

0-100

Moab, Utah

Slick Rock Member

Salmon-colored to pink and white fine-grained generally

crossbedded sandstone, containing some medium- to

coarse-grained sand. Generally forms cliffs or narrow

fins many of which contain arches or windows.

0-240

Slick Rock, Colo.

Dewey Bridge Member

Red muddy sandstone and sandy mudstone, with contorted

bedding. Forms easily eroded bases to arches in

Windows Section, hence aided in their development.

0-175

Dewey Bridge, Utah

Unconformity

190

Jurassic and Triassic(?);

Glen Canyon Group

Navajo Sandstone

Massive crossbedded buff, gray, and white fine-grained

sandstone, and local beds of gray limestone. Forms

cliffs along Colorado River, floors Windows Section.

0-350

Navajo Country, Four Corners (Glen Canyon, U.)

Late Triassic(?)

Kayenta Formation

Lavender, gray, and white lenses of sandstone, red sandy

shale, and conglomerate. Contains some freshwater

shells. Caps and protects cliffs of Wingate Sandstone.

0-250

Kayenta, Ariz.

Late Triassic

Wingate Sandstone

Massive, horizontally bedded and crossbedded reddish buff

fine-grained sandstone. Forms vertical cliffs along

Colorado River, Cache Valley, Salt Wash, and

Courthouse Wash.

0-350

Fort Wingate, N. Mex.

200

Chinle Formation

Irregularly bedded buff to red sandstone, red mudstone,

limestone, and conglomerate. Lenticular sandstone and

conglomerate (Moss Back Member) locally at base.

Freshwater shells, petrified wood, reptile bones.

Forms slopes.

0-700

Chinle Valley, Ariz.

Moss Back Ridge, Utah Unconformity

Middle(?) and Early Triassic

Moenkopi Formation

Thin-bedded brown shale, gray and brown sandstone, arkosic

grit, and conglomerate. Crops out on southwest side of

Moab Valley and in several places in Salt and Cache

Valleys. Forms slopes.

0-1,300

Moenkopi Wash, Ariz.

Unconformity

250

Permian

Cutler Formation

Chocolate brown and red sandy shale, maroon and pinkish-gray

arkose and conglomerate. Lower part probably

equivalent in age to Rico Formation in areas to south

and east. Crops out in Moab Canyon west of Moab fault.

Forms slopes.

0-2,500

Cutler Creek, Colo.

Pennsylvanian

Hermosa Formation

Unnamed upper member

Gray marine fossiliferous sandy limestone, gray and

greenish-gray sandstone and sandy shale, and red sandy

shale. Exposed in ledges southwest of Moab fault in

highway cut west of park entrance.

0-1,500

Hermosa Creek, Animas River Valley, Colo.

300

Paradox Member

Salt, gypsum, and anhydrite, with black and gray shale and

limestone. Few exposures in Salt and Cache Valleys.

Forms slopes.

0-11,000

Paradox Valley, Colo.

Unconformity

Pennsylvanian(?)

Unnamed conglomerate

Yellow sandstone with boulders of limestone and chert

containing Mississippian fossils. Exposed at two

places in Salt Valley.

?

 

 

Not exposed in the area but present far beneath the sedimentary cover

and exposed in several places a few miles to the northeast are examples

of the other two principal types of rocks—(1) igneous rocks, solidified

from molten rock forced into or above preexisting rocks along cracks,

joints, and faults, and (2) much older metamorphic rocks, formed from

other preexisting rock types by great heat and pressure at extreme

depths. Igneous rocks of Tertiary age (fig. 59) form the nearby La Sal

Mountains. The particles comprising the sedimentary rocks in the area

were derived by weathering and erosion of all three types of rocks in

various source areas.

 

Arches National Park and nearby Canyonlands National Park are both in

the heart of the Canyon Lands Section of the Plateau; therefore, it is

only reasonable to wonder why the differences in their general character

seemingly outweigh their similarities. First, let us consider the

similarities. Both parks are underlain by dominantly red sedimentary

rocks, both parks feature unusual erosional forms of sandstone, and both

contain beautiful natural arches, although the arches in Canyonlands are

restricted almost entirely to the southeastern part of The Needles

section and are in much older rocks than those in Arches.

 

To be sure, differences in the rocks themselves play a part in the

dissimilarity of the two parks, and these differences are of two types.

First, there are lateral changes in the character of the strata, known

to geologists as facies changes, brought about by differences in the

environment, in the type of materials, and in the mode of deposition

even within relatively short distances. Thus, during parts of the

Permian Period while sand, later to be known as the Cedar Mesa and White

Rim Sandstone Members of the Cutler Formation, was being deposited in

the southern part of Canyonlands, red mud, silt, and sand of the Cutler

were laid down farther north in Canyonlands (Lohman, 1974, fig. 9), and

similar, though somewhat coarser, beds of the Cutler were laid down at

Arches (fig. 4). Further comparisons of the rock columns in the two

parks show that while limestones of the Rico Formation were being

deposited in a shallow sea in the southern part of Canyonlands,

additional red mud, silt, and sand of the Cutler were being laid down

above sea level in areas to the northeast. The source of the coarser

materials was the ancient Uncompahgre Highland, which stood above sea

level from Late Pennsylvanian time to Late Triassic time (figs. 7, 59).

Although wider and longer, it occupied about the same position as the

present Uncompahgre Plateau between Grand Junction and Gateway, Colo.

Streams eroded the hard igneous and metamorphic rocks from this ancient

landmass and dumped the material into basins to the northeast and

southwest. The basin to the southwest, now called the Paradox basin

(after Paradox Valley, Colo.), at intervals contained shallow seas and lagoons, which I will discuss later.