Sunday, April 21, 2013

Powell Point at the Top of the Grand Staircase

Traveling north on the unpaved Cottonwood Canyon Road in south-central Utah, this overlook oversees the appropriately-named Kodachrome Basin State Park. A National Geographic expedition "Motoring into Escalante Land" first penned the colorful park name in a September 1949 article. Initially, Kodak objected to the magazine's usage of the product name of the film, still in its infancy, without permission, but later recanted after recognizing the obvious marketing value.

Notice the tilted sedimentary beds. Widely-spaced faults and monoclines punctuate the region. The flat-topped summit on the horizon is Powell Point, our destination, about 12 miles as the crow flies.

View looking north from Slick Rock Bench (near Wiggler Wash) at the Kodachrome Basin. Powell Point (center) is atop the Table Cliffs Plateau to the left of the gap on the horizon, while Canaan Peak is off to the right. Note the steeply dipping rocks (Entrada through Straight Cliffs Formation) that form the tail of a monocline between a branch of the Kaibab anticline and the Hackberry Canyon syncline.

The "picture perfect" vista is brought to you courtesy of the geological San Rafael Group. The multi-member, stratal package formed when a finger-like incursion of the former Panthalassic Ocean and the now-named Pacific Ocean invaded the land from the north and extended into the shallow Utah-Idaho trough during the Middle Jurassic. The depression of the trough was induced subsequent to the formation of an orogenic belt from the west in Nevada. The tectonic event was the Nevadan orogeny, the first of three major mountain-building episodes that completely transformed western North America during the Mesozoic. The mountain belt and its foreland basin are clearly visible on the Middle Jurassic paleo-map below.

The elongate marine embayment, also referred to as the Sundance seaway, deposited alternating sequences of terrestrial and shallow marine deposits. Depending on where you're travelling in the southeast quarter of Utah and western Colorado, you'll see the San Rafael's red and brown mudstones and and shales interjected with light-colored beds of evaporites and eolian sandstones of the Page and Entrada Sandstones, and the Carmel, Curtis and Summerville Formations.

Middle Jurassic Paleography of Western North America
Ron Blakey, Colorado Plateau Geosystems, Inc.

The following spectacular western scene is encountered further north along the Cottonwood Road. We're barely three miles from Cannonville, Utah, situated on Scenic Byway 12. The San Rafael Group's various layers are illuminating the landscape. Drawing ever near, Powell Point anoints the summit of the Grand Staircase's Pink Cliffs at 10,188 feet.

Envision the Grand Staircase as a multi-stepped, geological layercake that begins above the North Rim of the Grand Canyon in Arizona to the south and extends 150 miles to the north into southern Utah. It's subdivided into 6,000 vertical feet of cliffs, the risers of the stairs that are named by color, and intervening terraces or benches. The alternating cliff/slope and bench/terrace configuration is related to varied erosion rates of the various rock types. The cliffs are comprised of harder rocks that are more resistant to erosion (such as sandstone and limestone); whereas, the benches possess softer rocks that erode more readily (with shale and siltstone). 

Modified from

The Grand Staircase is the westernmost member of the 1.9 million-acre Grand Staircase-Escalante National Monument, created by President Bill Clinton in 1996. The other two geographical sections are the Kaiparowits Basin and, furthest east, Escalante Canyons.

Powell Point is the highest point on the geological cake, the icing if you will, residing on top of the Pink Cliffs. Below it is a bench, then a step, then another bench, and so on. This photo was taken from the Skutumpah Terrace, below the second riser of the Staircase called the Gray Cliffs to the north, somewhat hidden in the photo. The riser below us, to the south, is the White Cliffs of glistening Navajo Sandstone. The colorful terrace is built on softer, more erodible deposits of the Carmel and the overlying Entrada Sandstone, both related to the advance of the aforementioned seaway.  

Powell Point, seen from atop the Skutumpah Road just to the west of Kodachrome Basin, was named in 1879 by the geologist Clarence Dutton in honor of his famous contemporary colleague John Wesley Powell, the iconic geologist and explorer of the American West. The Point is held up by the white and pink limey cliffs of the Claron Formation, deposited during the Eocene around 55 million years ago in a vast system of freshwater shallow lakes and streams. The lower pink stratum is colored by oxides of the mineral hematite. They are the same formations that have eroded into the ghostly spires, badlands and hoodoos of Bryce Canyon and Cedar Breaks just to the west.

Middle Jurassic Paleography of Western North America
Ron Blakey, Colorado Plateau Geosystems, Inc.

Intertonguing Carmel Formation and Page Sandstone deposits of the San Rafael Group occupy the foreground that became interbedded as the sea level of the Sundance sea transgressed and regressed on land. And in the middle distance, gray badlands and slopes of the Gray Cliffs luxuriate below the high plateau of Powell Point, our next stop.

Powell Point is atop the white and pink Pink Cliffs seen from the Skutumpah Road
that traverses multi-colored strata of the San Rafael Group.
The various benches, slopes and cliffs of the Gray Cliffs are below.

We've arrived near the top of the Grand Staircase! Powell Point is about four miles to the west of paved Highway 12, where this photo was taken. At one time, like the other concordant high plateaus (consisting of the same strata) of the region, Table Cliffs Plateau was capped by resistant basalt during the Oligocene, which served to protect the underlying Claron Formation from erosion.

The Claron, being weakly-lithified (less rigid and erosion-susceptible), assaulted by frequent freeze-thaw cycles at this lofty elevation, and winnowed away by headward erosion of the Paria River system, has caused Powell Point to retreat as its cliffs are inexorably excavated away. On a grander scale, the Table Cliffs Plateau is situated on the east, high-side of the Paunsaugant fault, a Basin and Range extensional feature that threatens the demise of the other high plateaus, and possibly (likely) the entire Colorado Plateau. It’s only a matter of time.

The vegetated slopes directly below the Table Cliffs Plateau consist of the Pine Hollow and Canaan Peak Formations deposited in the Paleocene of the Cenozoic Era by streams and rivers. Together, they straddle the boundary between the latest Mesozoic's Cretaceous Period and the earliest Cenozoic, the deposits of which hold up Powell Point. Immediately below, the Cretaceous blue-gray badlands are eroding into the base of the Pink Cliffs.

The Cretaceous Period was a time of tectonic activity, elevated sea level and climate change in western North America. It is estimated that one-third of the world's landmass at the time was submerged during this unprecedented rise in sea level. The units of the Cretaceous record the marine filling of an immense foreland basin that formed as the Sevier orogeny deformed the continent's interior. The Sevier was the second Mesozoic orogeny to transform western North America.

Sevier-induced deformation of the continent’s western interior (related to compression on North America's western margin) and high global eustasy (elevated sea level related to the formation of new oceanic crust) acted in concert to drown the craton (continental interior) in a vast, north-south inland sea that reached from the Arctic to the Gulf of Mexico, and divided the newly-formed North American continent in two. The vertical and horizontal oscillations of this Western Interior seaway blanketed its bottom with mud, while its shoreline was marked with swamps fed by sediments from east-flowing rivers that originated from mountains of the Sevier orogenic belt to the west. That explains why you can find sea shells in Kansas, sharks teeth in South Dakota and beach sands throughout the Great Plains.

Late Cretaceous Paleography of Western North America
Ron Blakey, Colorado Plateau Geosystems, Inc.

“The Blues” situated below the white and pink Claron cliffs of Powell Point, consist of drab, blue-gray fluvial and floodplain sequences of the highly fossiliferous Kaiparowits Formation. The descending strata include the Wahweap, Straight Cliffs, Tropic Shale and Dakota Formations. The Staircase's Gray Cliffs are a series of low cliffs formed from hard sandstones with several intervening benches of softer sandstones and shales. These deposits formed during the great epeirogenic (continental) flood of the Western Interior Seaway, biblical in proportions but not in origin. Amen.

The following diagram of the Grand Staircase illustrates the direction we travelled north from the Skutumpah Terrace to Powell Point atop the Pink Cliffs. As we gained altitude and crossed from each successive bench and riser, we also rose stratigraphically into deposits that were laid down earlier, from the Mesozoic through the Cenozoic.

At its maximum development at about 90 million years ago, the continental sea inundated two-thirds of the eastern portions of Utah and Arizona. The sea's two-major transgressions and regressions (advances and retreats) left a stratigraphic record of largely sandstones and shales that blanketed the landscape, covering the entire Grand Staircase. Its deposits, originally at sea level, now reside at an elevation of two miles!

Stated another way, if standing on Powell Point today looking south down the Grand Staircase in the direction of the Grand Canyon, the lower benches and risers of the White, Vermilion and Chocolate Cliffs have lost their overlying Cretaceous strata from erosion, referred to by geologists as "unroofing." This is a consequence of the uplift of the entire Colorado Plateau on which the Grand Staircase is a part. 

During the Late Cretaceous, the Colorado Plateau is thought to have initiated a "gentle" bouyant ascent due to the Laramide orogeny, the third mountain-building event to transform western North America. Rather than creating a range of mountains, as it did with the Rockies, the Laramide created the Colorado Plateau, a largely un-deformed and yet two-mile uplifted-block of continental crust. That event carried sediments formed at sea level to the various cliffs and benches of the Grand Staircase. With all that we know, the timing and precise mechanism of this and subsequent uplift has remained a major enigma in geology for almost 150 years.

Modified from Geologic Road Guides to Grand Staircase-Escalante National Monument, Utah,
Utah Geological Association Publication 29

Our ascent of the upper portion of the Grand Staircase has taken us from the Middle Jurassic period of the Mesozoic (about 170 million years ago) through the Eocene epoch of the early Cenozoic (about 55 million years ago). Within that time frame, a Middle Jurassic seaway invaded the region from the west and deposited the sediments of the San Rafael Group, seen on the Skutumpah Terrace. Tectonic collisions along North America's west coast beginning in the latest Jurassic formed the Western Interior seaway, whose Late Cretaceous sediments are seen within the Gray Cliffs. And during the Eocene, pink and gray limey deposits of the Claron Formation were deposited within a system of freshwater streams and lakes, seen in the Pink Cliffs.

Boston Strong