Saturday, December 24, 2011

Erosive Magic at the Devil’s Garden in the Canyons of the Escalante

“And so castles made of sand
fall into the sea
From Castles Made of Sand  by Jimi Hendrix, 1967 

Step back into childhood and let your wildest imaginations take over. Stone goblins. Grotesque gnomes. Fanciful hoodoos. Psychedelic mushrooms. Daliesque arches. Sinister trolls. They’re all there, at Devil’s Garden...part geological excursion and part bedtime story.

Devil’s Garden, all 10 acres of it, is in the Escalante Canyons section of the Grand Staircase-Escalante National Monument in south-central Utah. It's not to be confused with the garden of the same name in Arches National Park, east of Moab, Utah. Leaving Route 12, drive south from Escalante, Utah, on the Hole-in-the-Rock Road which follows the trend of the Straight Cliffs on the west and the pioneer Mormon's Hole-in-the-Rock trail on the east. After about 13 miles, there’s a turnoff for Devil’s Garden.

A few miles to the north of the Garden, along the base of the Straight Cliffs, a small drainage course began a gradual descent to the east as it cut through layers of the Tropic Shale and the Morrison Formation. Turning south to parallel the road, it carved a serpentine track through the undulating plain of the desert downward through the Entrada Sandstone. At Devil’s Garden, erosion sculpted the sandstone into a vast array of whimsical and grotesque shapes.

Simply stated, the Garden represents a region of rock that is more resistant to erosion. Its features all formed from resistant deposits at the top of the slickrock Gunsight Butte Member of the Entrada Sandstone, an orange-brown to red-orange, wind-blown deposit laid down in dunes. To the west toward the cliffs, it is overlain by the brown to red-brown Cannonville Member of the Entrada, a bedded, earthy-weathering, slope-forming sandstone. Above the Cannonville, the Entrada's Escalante Member forms light yellow, are rock-sandstone outcrops at the foot of the Straight Cliffs. The Cannonville Member, containing more clay and silt than the underlying Gunsight Butte and overlying Escalante Members, erodes more readily. Erosion has stripped the Cannonville Member away and exposing the more resistant Gunsight Butte Member.

Interestingly, at the "other" Devil's Garden in Arches National Park, its arches are also formed of Entrada Sandstone, but involve a different stratigraphic member of the Entrada, a contact with an underlying formation, and a different tectonic scenario.

The region of the Colorado Plateau during the Mesozoic contains perhaps the best exposed, best documented and highest percentage of desert-sediments in the stratigraphic record of the Earth known as ergs or sand seas. It is believed that the great Jurassic eolian deposits of the Plateau are associated with large-scale tectonic events. Beginning in the Middle Paleozoic to the Early Cenozoic, a continuum of eastward, progressive, and punctuated, yet continuous deformational pulses contributed to the growth of the western margin of Laurentia and the Cordillera. 

During the Jurassic, the generation of shortening events of the Cordillera flexed the continental interior downward. That created a wide topographic depression in direct association with the mountain belt to the west capable of exerting a rain-shadow effect and an environment of prolonged aridity to the east.  Flexural basin subsidence in such a retro-forearc environment provided accommodation space for the preservation of the enormous sand-ergs (and non-eolian deposits) that evolved.

The Early Jurassic Glen Canyon Group of ergs (Wingate and Navajo, and lateral correlatives of Aztec and Nugget Sandstones) and the Middle Jurassic San Rafael Group of ergs (Page and Entrada) are generally assumed to be associated with the dynamic subsidence generated by the onset of oceanic Farallon slab subduction beneath the North American plate. Note that both groups possess fluviatile and marine components as well.

The Middle Jurassic Utah-Idaho Trough in Utah has been interpreted as a foreland basin system. It has been speculated to be along the leading edge of the Elko Orogenic belt. Its foredeep was flooded from the north via communication with the Pacific Ocean and facilitated by rising global eustatic sea levels. As sea level fluctuated within the epeiric marine incursion, known as the Sundance Sea (or Zuni Sea for the name of the global transgression), a complex interfingering of marine, marginal marine and non-marine beds formed. These deposits are collectively represented by the San Rafael Group. The Carmel Formation was deposited near the south margin of the sea while the Entrada Sandstone formed from desert dunes to beach and back-beach sands. 

Geologists (Dickinson and Gehrels, 2011) have found that detrital grains in Jurassic eolianites of the Glen Canyon and San Rafael Groups were derived mostly from Precambrian and Paleozoic granitoid basement provinces in eastern and central Laurentia, and some later (after 285 Ma) from rock assemblages of the nearby Cordilleran orogen. Most age populations reflect derivation from Paleozoic, Neoproterozoic and even Grenvillian sources within the Appalachian orogen or its sedimentary cover. One hypothesis involves the transport of sediment upwind to the north of the Colorado Plateau by a transcontinental Jurassic paleoriver, dispersal by paleowinds confirmed by analyses of eolian cross-bedding, and recyclization by regional depositional systems.

The generation of enormous volumes of sand during the arid conditions of the Jurassic, in association with persistant paleowinds, resulted in a massive sand budget that accumulated in a tectonically-formed accommodation space.

This Middle Jurassic (160 Ma) paleographic map (modified from Ron Blakey, NAU Geology) shows the Sundance Sea having invaded Utah from the north. The red dot represents the approximate location of the Devil's Garden in south-central Utah. The sea blanketed the region of the Garden (and at times well beyond through Utah to the Utah-Arizona border and eastward onto the west flank of the Ancestral Rocky Mountain's Uncompahgre Uplift) with sediments of the San Rafael Group. The stratal components of the group vary with the geography across the region as the seaway expanded and contracted. This has greatly  contributed to the complexity of the strata and their inter-relationships, an ongoing source of re-interpretation amongst geologists and stratigraphers.

Very basically, the San Rafael Group is comprised of marine and terrestrial deposits in and around the sea consisting of mudstones, sandstones, limestones and gypsum: the Carmel Formation (siltsone, mudstone, sandstone, limestone and interbedded gypsum deposited near the southern margin of the shallow sea); the Page Sandstone (lies unconformably on the Navajo Sandstone); the Entrada Sandstone (beach and back-beach sands, frequent sabkha); Curtis Formation (marine sandstones and mudstones): Summerville Formation (siltstone, sandstone and gypsum).

 It is the Entrada Sandstone that is integral to the genesis of Devil's Garden. At one time, the Entrada Sandstone covered most of southern Wyoming, Utah, Colorado, northern Arizona and New Mexico, and was almost as widespread as the Navajo erg. 

Eventually, the sedimentary sequences of the foredeep were uplifted and the sea withdrew. During the Late Jurassic, the deposits of the San Rafael Group were covered by those of the Morrison Formation from anastomosing and meandering streams on broad floodplains from the rising highlands to the west.


This is Metate Arch, perhaps the most photographed feature at Devil's Garden.

Doesn't this look like someone you know?

The erosive magic of Devil's Garden is created by weathering, both mechanical and chemical. Agents of erosion, endless cycles of rain, wind, snow and ice, enter cracks and fissures within the rock. Repeated freezing and thawing breaks the rock at the surface by frost wedging. Rainwater containing absorbed atmospheric carbon dioxide dissolves the calcium carbonate cement. Summer thunderstorms carry away the accumulating debris. Softer rock erodes more readily than resistant rock. In time, isolated fins, ridges and pedestals begin to appear, slowly sculpting the rock into the hoodoos of the Garden.

This thin fin has eroded into a window. Perhaps one day a delicate arch will begin to appear.

An inedible Entrada mushroom

Various stratal horizons within the Entrada are evident and are reflected in its patterns of erosion.

Sunday, December 11, 2011

Memorable Places Here and There on the Colorado Plateau: The “Wide, Open Spaces” of Escalante

For me, there’s nothing like “the wide, open spaces.” Perhaps it comes from growing up in Central New York State. The landscape there is quite striking, but the horizon is usually only as far as the next glacial drumlin.

This expansive vista looks down the long escarpment of the Straight Cliffs, the cliff-face of Fiftymile Mountain in central-south Utah. Follow the cliffs to the horizon through the rugged, wash-punctuated, desert benchland, and you end up at a spectacular drop-off at Lake Powell, the man-made reservoir created by the flooding of Glen Canyon
by the controversial Glen Canyon Dam. That’s exactly the route that the early pioneers of the Church of the Latter Day Saints, better known as Mormons, followed in 1878 called the Hole-in-the-Rock Road.

Their mission was to establish a new settlement in the region of the San Juan River in southeastern Utah. They did so by forging a road through the desert from Escalante, negotiating the sheer cliff at the Hole-in-the-Rock and crossing the untamed Colorado River with 234 men, women and children, 83 wagons, livestock and all their worldly possessions. Their epic “shortcut” was only used for one year before being abandoned. We drove on the Hole-in-the-Rock Road, the contemporary version, that essentially parallels their trail through the region of the Escalante Canyons.

The Straight Cliffs rise 1,110 feet or more, and as their name implies, extend for 50 miles to the southeast. The cliffs form the eastern escarpment of Fiftymile Mountain called “mountain lying down” by Native American Paiutes and the “Fifty” by local Mormons. Its rock face is a long, nearly continuous wall from the town of Escalante, Utah, southward to the Colorado River. Largely free from side canyons or protruding spurs, only two canyons throughout its length break its otherwise straight line of cliffs, greened with clumps of juniper, sagebrush and piñon.

At the foot of the cliffs runs Fiftymile Bench, a platform or broad terrace with a line of smaller, lower cliffs of its own, and more well-developed in the southern reaches of the Straight Cliffs. At various intervals, a succession of cusps juts out from the long bench.

And below the bench, lies the sagebrush, blackbrush, rabbitbrush and cacti decorated desert through which we traveled. The area is remote, isolated and majestically beautiful.

The Straight Cliffs and Fiftymile Mountain form the geological boundary between two of three sections of the 1.9 million acre Grand Staircase-Escalante National Monument (GSENM), the central Kaiparowits Basin section, and the easternmost Escalante Canyons section. The desert to the east of the Straight Cliffs, on which we traveled, marks the beginning of the Escalante Canyons region. It includes the winding Escalante River and the canyons it has dissected largely through the Glen Canyon Group’s sandstones.

The uppermost diagram (below) shows the centrally-located, dissected-mesa of the Kaiparowits section of the GSENM, viewed from a northern perspective. Fiftymile Mountain and the Straight Cliffs (circled) form the natural boundary with the easternmost section of the monument, Escalante Canyons (lowermost diagram). The Escalante Canyons section is viewed from a southern perspective.   

The Straight Cliffs are one of the boldest expressions of the Gray Cliffs of the Grand Staircase, the GSENM's westernmost section. The Staircase is a series of topographic benches and cliffs, and that, as its name implies,  progressively steps up in elevation from south to north, from northern Arizona into southern Utah. The Straight Cliffs lies to the east and well outside the Grand Staircase section but are composed of the same durable Cretaceous sandstones that form the second highest riser of the Grand Staircase’s Gray Cliffs.

The relentless drought of the Early Jurassic that dominated the vicinity of the future Colorado Plateau  brought the Wingate and Navajo wind-driven sand seas of the Glen Canyon Group to the region. Likewise in the Middle Jurassic, the Page and Entrada Sandstone eolian ergs inundated the region, but this time in association with the Sundance Sea, a narrow restricted arm of the ocean that entered from Wyoming into the subsidence space of the Utah-Idaho Trough, a foreland basin. The complex and varied deposits of the sea's fluctuating shoreline left the sediments of the San Rafael Group (such as the region's Carmel, Page and Entrada silt and sandstones).

During the Late Jurassic the region experienced the widespread, fluvially-generated Morrison Formation in the wake of the Nevadan Orogeny to the west, a mountain-building event that contributed to the formation of the Cordilleran Arc. Cordilleran Orogenesis in the western United States spanned at least 120 million years from the Middle Jurassic into early Eocene time. It comprised numerous mountain-building events that culminated with the formation of an enormous, elongate mountain range from Alaska to southern Mexico, with a complex and diverse stratigraphy.

Beginning in the latest Jurassic, the subduction of the oceanic Farallon Plate beneath the North American Plate along western North America resulted in the formation of the Sevier fold and thrust belt. Siliciclastic sediments were shed from the west carried by rivers into a seaway that formed in the immense, flooded  foreland basin that developed in the center of the continent. During the Cretaceous, the Western Interior Seaway inundated most of the interior of North America including the GSENM area in southern Utah, leaving a vast array of sediments as its shoreline changed and sealevel rose and fell with deposits such as the region's Dakota Formation deposited in coastal areas ahead of the encroaching sea. The Tropic Shale represents muds deposited at the bottom of the sea. The Straight Cliffs, Wahweap and Kaiparowits Formations represent sediments that were deposited on a piedmont belt between the mountains and the sea, after the sea retreated to the east.

These events left their deposits after which Paleogene uplift and dissection painted the finishing touches on the canvas of the landscape. On the map below at about 85 Ma, note the location of the Sevier Highlands, its associated thrust fault, and the future landform of the Kaiparowits Plateau. The Straight Cliffs-Fiftymile Mountain boundary between it and the Escalante Canyons section to the east formed after Laramide uplift of the Colorado Plateau and the subsequent dissection of the region locally by tributaries of the Colorado River System into the deposits of the Glen Canyon Group.

The northwest to southeast-trending Straight Cliffs looks somewhat like the east to west-trending Book Cliffs located further to the north, formed under similar depositional conditions and time frames. The former’s trend is roughly parallel and the latter’s trend is roughly perpendicular to the ancient shoreline of the Western Interior Seaway where they were deposited nearly 90 million years ago during the Late Cretaceous. The Straight Cliffs are composed of dark gray, massive marine shales interbedded with tan sandstones. They contain a diverse fluvial and marine architecture of offshore, shoreface, coastal plain, paludal and fluvial facies that reflect the transgressive-regressive whim of the seaway's fluctuations in sealevel. With the mind's eye staring at the Straight Cliffs, you can see layer after layer of ancient beaches and barrier islands formed by the shifting shoreline similar to the Atlantic Coast of today.

The majority of the cliffs are composed of the Straight Cliffs Formation's John Henry Member, a slope and ledge-former, representing thick beach sandstone beds separated by muddy sandstones deposited in a shoreline environment. The John Henry also contains thick, lagoonal coal deposits. Two members form the base of the cliffs, the Smoky Hollow Member (back-beach and lagoonal deposits) and resting on the Tibbett Canyon Member (offshore sandstones).

Fiftymile Bench is built on the Late Jurassic Morrison Formation, much of which is covered by colluvium and landslide debris derived from the overlying Straight Cliffs Formation. Beneath the bench’s debris, windows of the underlying gray, muddy Tropic Shale and thin Dakota Formation are present. In places, Pleistocene-age, mass wasting deposits have cascaded over the bench’s lower cliffs and ledges in the Morrison Formation's Salt Wash Member to the desert-flats below. In one particular locale, seen from a distance, the erosional process of a flow is evident in the formation of hoodoos (below).

The Hole-in-the-Rock Road basically follows a strike valley in the Middle Jurassic Carmel Formation. Below Escalante the road is near the top of the Paria River Member of the Carmel but soon enters the Winsor Member. Prominent landforms projecting above the Carmel desert (such as Sooner Rocks where we made camp), are composed of the orange-brown Gunsight Butte Member of the Entrada Sandstone. As the road undulated with the terrain, it weaved on and off of benches of unconsolidated Pleistocene and Holocene mixed eolian and alluvial deposits, and Entrada Sandstone. The overlying, softer, slope-forming Cannonville Member of the Entrada contains more clay and silt. To the east of the road, washes, slots and narrow intricate canyons exhibit the contact with the Windsor Member of the Carmel Formation. These dissections merge into larger corridors that eventually fuse with the Escalante River gorge, which ultimately joins the Colorado River, today drowned by Lake Powell.

This map of the GSENM illustrates the relationship of its three sections and the geological boundary that the Straight Cliffs and Fiftymile Mountain form between the Kaiparowits and Escalante sections. Also notice the Hole-in-the-Rock Road paralleling the strike of the cliffs from Escalante to the southeast towards the Colorado River and Lake Powell. The Kaiparowits Plateau is roofed with marine, fluvial and floodplain deposits; whereas, the Escalante Canyons have been unroofed of such until east of the Waterpocket Fold. 

Here's a bedrock map at the Straight Cliffs-Fiftymile Mountain boundary zone. Note the Hole-in-the-Rock Road running from Escalante to Lake Powell. 

Stratigraphic column in the region of the Straight Cliffs (circled)

Having departed from the town of Escalante, we headed south on the Hole-in-the-Rock Road. This view looks back to the north toward Escalante with the Straight Cliffs located to the west. The photo was taken while literally standing on the Mormon's Hole-in-the Rock trail through the desert. Our SUV, off to the right, is parked on the Hole-in-the-Rock Road. After having been constructed over 130 years ago and having been used for only one year by Mormon pioneers, you can still see the swales created by the Conestoga wagon wheels through the red Carmel soil.

Continuing our journey south on the Hole-in-the-Rock Road for fifty miles or so, we turned off towards the cliffs and took a scenic detour on Fiftymile Bench Road. The road uses a landslide over Morrison cliffs to gain access onto Fiftymile Bench. Seen below, we ascended numerous switchbacks on a rugged road that led us to the top of the debris flow that came off the bench. The view of the desert flats far below (at about 4,300 feet above sea level) in the Carmel formation, and the Escalante Canyons and watershed of the Escalante River in the distance was quite spectacular.

Unbeknownst to us at the time, we were to make camp that night at Sooner Rocks, the barely visible, rocky outcrop in the center of the photo. The debris flow (at about 5,600 feet) on which we were ascending is comprised of an unconsolidated rocky mix largely from the Straight Cliffs above. You can spot the array of boulders in the foreground that have cascaded down from the cliffs and the road snaking upward from below. Also notice a cusp of the Fiftymile Bench extending off to the left in the Morrison Formation. About 50 miles away, the Henry Mountain laccolithic complex is faintly discernible at the horizon to the right (north-northeast)  beyond the Waterpocket Fold, while Boulder Mountain capped in Tertiary lava flows is far to the left (north-northwest). 

With sunset rapidly approaching and the wind picking up, we began an earnest search for a good spot for camp. Every desirable campsite with a sheltered wind-break seemed taken. Eventually we settled on Sooner Rocks for camp just off the Hole-in-the-Rock Road, built of smooth, slick, red Entrada Sandstone in the form of a cluster of resistant, domed, bare-rock outcrops. At sundown the temps began to drop and the wind began to gust at 40 mph plus. We set up camp, staked and secured our tents, opened a fine bottle of wine and watched the sun set, while bracing for a storm at night that never really came.

“Goodnight stars, goodnight air, goodnight noises everywhere.”
From Goodnight Moon by Margaret Wise Brown

In the morning, we awakened to a light rain and a spectacular cloud break at sunrise that ignited the Sooner Rocks in brilliant Entrada-orange. Like the Navajo Sandstone, the Entrada Sandstone exhibits large-scale eolian cross-bedding and weathers to smooth surfaces. Notice its swirly, undulating cross-beds which have been selectively etched-out by erosion. It reminded me of graded fields back home in the northeast that had been harvested of corn. Also, notice the criss-crossing, wavy and anastomosing, whitish network of deformation bands on the sandstone-dome. Probably of tectonic origin, they are indicative of accommodation to normal slip-movements, many exhibiting offsets. Their lighter, white color is likely attributable to variable bleaching through interaction with hydrocarbon-bearing solutions or other reducing agents, and indicative of the host sandstone's permeability early in its developmental history. Geochemical modeling implies the removal of some iron by fluids after chemical reduction, further contributing to their color.

As the rising sun illuminated the nearer bench portion of the escarpment to the west, we noticed a dusting of snow along the top of the Straight Cliffs, highly atypical for late May. Exquisite!

After investigating a few slot canyons in the area, we returned to the Hole-in-the-Rock Road and headed north, back to Escalante. This view nicely shows the banded stratigraphy of the Straight Cliffs, the Fiftymile Bench in the multi-colored Morrison Formation, and the heavily vegetated, Carmel desert on which we've been traveling.

“Now...Bring me that horizon" (Pirates of the Caribbean)

Highly Suggested Reading: Geology of Utah's Parks and Monuments by the Utah Geological Association and Bryce Canyon Natural History Association, Second Edition, 2003.