Geology is all around us, scarcely thought of as we go about our lives. Yet, it affects everything we do as a civilization, as a society and as individuals. While barely appearing to change from day to day, it works to alter the course of evolution. Preserving a record of creatures and landscapes both ancient and forgotten, the story of our past is written in stone and waiting to be read. I offer a view of how I see our world and its inhabitants, both past and present, as seen through my lens.
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 eventually.” 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.
WHERE IS THE DEVIL'S GARDEN?
Devil’s Garden, all 10 acres of it, is in the Escalante Canyons section of the GrandStaircase-EscalanteNational Monument in south-central Utah. It's not to be confused with the garden of the same name in ArchesNational 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.
HOW DID THE GARDEN GROW?
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 FORMATION OF THE ENTRADA SANDSTONE: TECTONICS, ACCOMMODATION SPACE, WIND AND LOTS OF SAND ERG TECTONO-GEODYNAMICS 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.
TWO GROUPS AND FOUR ERGS 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 SUNDANCE SEA 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.
SOURCES OF THE SAND Geologists (Dickinson and Gehrels, 2011) have found that detrital grains in Jurassic eolianites of the GlenCanyon 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 BIG PICTURE SUMMARY 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.
THE SAN RAFAEL GROUP AND THE ENTRADA SANDSTONE 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.
LET YOUR IMAGINATION GET THE BEST OF YOU
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.