Following the wet interval when the Kayenta Formation was deposited over
wide areas of the Colorado Plateau by streams, the Plateau once again
became a vast desert, and this time the dry climate persisted from the
Late Triassic into the Jurassic. The howling winds piled up enormous
sand dunes, layer upon layer, to a total thickness of more than 2,200
feet at Zion National Park, and as much as 500 feet remains in eastern
Utah and parts of southwestern Colorado. This immense sandpile
eventually was cemented by calcite into the Navajo Sandstone.
[Illustration: KAYENTA FORMATION, showing lenses of hard channel
sandstones and wedge of red siltstone and mudstone. Along road cut
of Rim Rock Drive near head of main stem of Ute Canyon. Vertical
grooves remain from drill holes used in blasting roadcut. (Fig. 14)]
Beautifully sculptured remains of the Navajo are featured attractions at
Zion, Capitol Reef, and Arches National Parks, Rainbow Bridge, Navajo,
and Dinosaur National Monuments; border many miles of beautiful Lake
Powell; and form the eastern flank of the San Rafael Swell. For reasons
to be explained, this sandstone thins to the northeast, and is absent
entirely at about the Utah-Colorado State line, some 35 miles southwest
of the Monument. Thus, in the Monument, the Navajo, most of the Kayenta,
and the lower part of the Entrada Sandstone are missing at another gap
in the rock record, as shown in figure 15.
[Illustration: GAP IN THE ROCK RECORD, between Kayenta Formation
below 3½- × 6-inch green notebook and Slick Rock Member of Entrada
Sandstone above. The reasons for this gap are given in the text on
page 38. That this is an erosional unconformity is clearly indicated
by the uneven top of the Kayenta, particularly to the left of the
notebook. Note solution pits and openings in the Entrada near top of
photograph. (Fig. 15)]
How is it possible that the Navajo Sandstone is more than 2,200 feet
thick in Zion National Park, is several hundred feet thick in much of
the Plateau in Utah and parts of southwestern Colorado, yet is absent
entirely, together with a considerable thickness of younger rocks in and
near Colorado National Monument? How much of the missing strata once
were present in the Monument is not known, but it seems clear that at
least part was present but was eroded away before the Entrada Sandstone
was deposited. There is evidence[21] that following the deposition and
consolidation of the Navajo Sandstone the Plateau and adjacent areas
were uplifted, tilted gently westward, and eroded for a considerable
period of time. Erosion naturally was most pronounced in the eastern
areas, including the Monument, where the uplift was greatest. Thus, in
the northeastern part of the Plateau all the Navajo and most of the
Kayenta were eroded away, and erosion continued there while the lowest
member of the Entrada, the Dewey Bridge Member, and the lower part of
the overlying Slick Rock Member were being laid down in the Moab, Utah,
area.[22] This old erosion surface is clearly visible in many places
along the cliff wall on the southwest side of Rim Rock Drive between the
Visitor Center and Kissing Couple.
The reduction in thickness of the Navajo Sandstone from southwest to
northeast and absence of the Navajo and some younger rocks in and near
the Monument are shown on an isometric (three dimensional) block diagram
prepared by artist John R. Stacy and me, which is displayed in the
Museum of the Visitor Center. This block diagram portrays the surface
and subsurface rocks from Zion National Park, Utah, to Black Canyon of
the Gunnison National Monument, Colo., via Capitol Reef National Park,
the Henry Mountains, and Colorado National Monument. Throughout the
Plateau and parts of adjacent areas, the erosion surface on top of the
Navajo Sandstone is covered by scattered pebbles of chert—a hard variety
of silica (SiO₂) derived from cherty beds of freshwater limestone in the
Navajo.[23] Where the Navajo has been completely eroded away and the
ancient erosion surface is on the Kayenta Formation, as in Colorado
National Monument, scattered pebbles (some of which are chert) derived
from the conglomerate lenses in the Kayenta are found locally on the old
surface.[24]
Because of this gap in the rock record we will continue part of our
story farther west, where the rock record is more nearly complete.